U.S. patent application number 13/651003 was filed with the patent office on 2013-04-18 for dead-volume management in compressed-gas energy storage and recovery systems.
This patent application is currently assigned to SUSTAINX, INC.. The applicant listed for this patent is SustainX, Inc.. Invention is credited to Joel Berg, Benjamin R. Bollinger, Troy O. McBride.
Application Number | 20130091836 13/651003 |
Document ID | / |
Family ID | 48085026 |
Filed Date | 2013-04-18 |
United States Patent
Application |
20130091836 |
Kind Code |
A1 |
McBride; Troy O. ; et
al. |
April 18, 2013 |
DEAD-VOLUME MANAGEMENT IN COMPRESSED-GAS ENERGY STORAGE AND
RECOVERY SYSTEMS
Abstract
In various embodiments, coupling losses between a cylinder
assembly and other components of a gas compression and/or expansion
system are reduced or eliminated via valve-timing control.
Inventors: |
McBride; Troy O.; (Norwich,
VT) ; Bollinger; Benjamin R.; (Windsor, VT) ;
Berg; Joel; (Bolton, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SustainX, Inc.; |
Seabrook |
NH |
US |
|
|
Assignee: |
SUSTAINX, INC.
Seabrook
NH
|
Family ID: |
48085026 |
Appl. No.: |
13/651003 |
Filed: |
October 12, 2012 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61547353 |
Oct 14, 2011 |
|
|
|
61569528 |
Dec 12, 2011 |
|
|
|
61620018 |
Apr 4, 2012 |
|
|
|
Current U.S.
Class: |
60/410 |
Current CPC
Class: |
F01B 1/01 20130101; F04B
49/22 20130101; Y02E 60/15 20130101; F01K 13/02 20130101; F01K
27/00 20130101; Y02E 60/16 20130101; F01K 7/00 20130101; F02G 1/043
20130101; F04B 41/02 20130101 |
Class at
Publication: |
60/410 |
International
Class: |
F04B 49/22 20060101
F04B049/22 |
Goverment Interests
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH
[0002] This invention was made with govenment support under
DE-OE000231 awarded by the DOE. The government has certain rights
in the invention.
Claims
1. An energy storage and recovery system comprising: a cylinder
assembly for at least one of expanding gas to recover energy or
compressing gas to store energy; a high-side component, selectively
fluidly connected to the cylinder assembly, for at least one of (i)
supplying gas to the cylinder assembly for expansion therein or
(ii) accepting gas from the cylinder assembly after compression
therein; a low-side component, selectively fluidly connected to the
cylinder assembly, for at least one of (i) supplying gas to the
cylinder assembly for compression therein or (ii) accepting gas
from the cylinder assembly after expansion therein; and a control
system for operating the cylinder assembly to perform at least one
of (i) a pre-compression of gas therewithin prior to admission
therein of gas for expansion, thereby reducing coupling loss
between the cylinder assembly and the high-side component, or (ii)
a pre-expansion of gas therewithin prior to admission therein of
gas for compression, thereby reducing coupling loss between the
cylinder assembly and the low-side component.
2. The system of claim 1, further comprising a sensor for sensing
at least one of a temperature, a pressure, or a position of a
boundary mechanism within the cylinder assembly to generate control
information, the control system being responsive to the control
information.
3. The system of claim 2, wherein the control system is configured
to operate the cylinder assembly, during at least one of (i)
pre-compression of gas therewithin or (ii) expansion of gas
therewithin, based at least in part on control information
generated during at least one of (i) a previous gas expansion
within the cylinder assembly or (ii) a previous pre-compression of
gas within the cylinder assembly.
4. The system of claim 2, wherein the control system is configured
to operate the cylinder assembly, during at least one of (i)
pre-expansion of gas therewithin or (ii) compression of gas
therewithin, based at least in part on control information
generated during at least one of (i) a previous gas compression
within the cylinder assembly or (ii) a previous pre-expansion of
gas within the cylinder assembly.
5. The system of claim 1, wherein the high-side component comprises
a compressed-gas storage reservoir.
6. The system of claim 1, wherein the high-side component comprises
a second cylinder assembly for at least one of compressing gas or
expanding gas within a pressure range higher than a pressure range
of operation of the cylinder assembly.
7. The system of claim 1, further comprising a second cylinder
assembly for at least one of compressing gas or expanding gas
within a pressure range higher than a pressure range of operation
of the cylinder assembly, wherein the high-side component comprises
a mid-pressure vessel for containing gas at a pressure within both
of or between pressure ranges of operation of the cylinder assembly
and the second cylinder assembly.
8. The system of claim 1, wherein the low-side component comprises
a vent to atmosphere.
9. The system of claim 1, wherein the low-side component comprises
a second cylinder assembly for at least one of compressing gas or
expanding gas within a pressure range lower than a pressure range
of operation of the cylinder assembly.
10. The system of claim 1, further comprising a second cylinder
assembly for at least one of compressing gas or expanding gas
within a pressure range lower than a pressure range of operation of
the cylinder assembly, wherein the low-side component comprises a
mid-pressure vessel for containing gas at a pressure within both of
or between pressure ranges of operation of the cylinder assembly
and the second cylinder assembly.
11. The system of claim 1, further comprising a load mechanically
coupled to the cylinder assembly and at least one of (i) driven by
the cylinder assembly during gas expansion or (ii) driving the
cylinder assembly during gas compression.
12. The system of claim 11, wherein the load comprises at least one
of a mechanical crankshaft or a hydraulic pump/motor.
13. The system of claim 1, further comprising a heat-transfer
subsystem for thermally conditioning gas during at least one of
compression or expansion thereof.
14. The system of claim 13, wherein the heat-transfer subsystem
comprises a mechanism for introducing heat-transfer fluid into the
gas.
15. The system of claim 14, wherein the mechanism comprises at
least one of a spray head or a spray rod.
16. The system of claim 13, wherein the heat-transfer subsystem
comprises a heat exchanger for thermally conditioning at least one
of gas from the cylinder assembly or heat-transfer fluid.
17. The system of claim 13, wherein the heat-transfer subsystem
comprises at least one of (i) a mixing chamber for forming foam
from gas and heat-transfer fluid or (ii) a screen for altering at
least one of average bubble size or bubble-size variance of foam
comprising gas and heat-transfer fluid.
Description
RELATED APPLICATIONS
[0001] This application claims the benefit of and priority to U.S.
Provisional Patent Application No. 61/547,353, filed Oct. 14, 2011,
U.S. Provisional Patent Application No. 61/569,528, filed Dec. 12,
2011, and U.S. Provisional Patent Application No. 61/620,018, filed
Apr. 4, 2012. The entire disclosure of each of these applications
is hereby incorporated herein by reference.
FIELD OF THE INVENTION
[0003] In various embodiments, the present invention relates to
pneumatics, hydraulics, power generation, and energy storage, and
more particularly, to systems and methods using pneumatic,
pneumatic/hydraulic, and/or hydraulic cylinders for energy storage
and recovery.
BACKGROUND
[0004] Storing energy in the form of compressed gas has a long
history and components tend to be well tested and reliable, and
have long lifetimes. The general principle of compressed-gas or
compressed-air energy storage (CAES) is that generated energy
(e.g., electric energy) is used to compress gas (e.g., air), thus
converting the original energy to pressure potential energy; this
potential energy is later recovered in a useful form (e.g.,
converted back to electricity) via gas expansion coupled to an
appropriate mechanism. Advantages of compressed-gas energy storage
include low specific-energy costs, long lifetime, low maintenance,
reasonable energy density, and good reliability.
[0005] If a body of gas is at the same temperature as its
environment, and expansion occurs slowly relative to the rate of
heat exchange between the gas and its environment, then the gas
will remain at approximately constant temperature as it expands.
This process is termed "isothermal" expansion. Isothermal expansion
of a quantity of high-pressure gas stored at a given temperature
recovers approximately three times more work than would "adiabatic
expansion," that is, expansion where no heat is exchanged between
the gas and its environment--e.g., because the expansion happens
rapidly or in an insulated chamber. Gas may also be compressed
isothermally or adiabatically.
[0006] An ideally isothermal energy-storage cycle of compression,
storage, and expansion would have 100% thermodynamic efficiency. An
ideally adiabatic energy-storage cycle would also have 100%
thermodynamic efficiency, but there are many practical
disadvantages to the adiabatic approach. These include the
production of higher temperature and pressure extremes within the
system, heat loss during the storage period, and inability to
exploit environmental (e.g., cogenerative) heat sources and sinks
during expansion and compression, respectively. In an isothermal
system, the cost of adding a heat-exchange system is traded against
resolving the difficulties of the adiabatic approach. In either
case, mechanical energy from expanding gas must usually be
converted to electrical energy before use.
[0007] An efficient and novel design for storing energy in the form
of compressed gas utilizing near isothermal gas compression and
expansion has been shown and described in U.S. Pat. No. 7,832,207,
filed Apr. 9, 2009 (the '207 patent) and U.S. Pat. No. 7,874,155,
filed Feb. 25, 2010 (the '155 patent), the disclosures of which are
hereby incorporated herein by reference in their entireties. The
'207 and '155 patents disclose systems and techniques for expanding
gas isothermally in staged cylinders and intensifiers over a large
pressure range in order to generate electrical energy when
required. Mechanical energy from the expanding gas may be used to
drive a hydraulic pump/motor subsystem that produces electricity.
Systems and techniques for hydraulic-pneumatic pressure
intensification that may be employed in systems and methods such as
those disclosed in the '207 and '155 patents are shown and
described in U.S. Pat. No. 8,037,678, filed Sep. 10, 2010 (the '678
patent), the disclosure of which is hereby incorporated herein by
reference in its entirety.
[0008] In the systems disclosed in the '207 and '155 patents,
reciprocal mechanical motion is produced during recovery of energy
from storage by expansion of gas in the cylinders. This reciprocal
motion may be converted to electricity by a variety of techniques,
for example as disclosed in the '678 patent as well as in U.S. Pat.
No. 8,117,842, filed Feb. 14, 2011 (the '842 patent), the
disclosure of which is hereby incorporated herein by reference in
its entirety. The ability of such systems to either store energy
(i.e., use energy to compress gas into a storage reservoir) or
produce energy (i.e., expand gas from a storage reservoir to
release energy) will be apparent to any person reasonably familiar
with the principles of electrical and pneumatic machines.
[0009] In order to reduce overall pressure ranges of operation,
various CAES systems may utilize designs involving multiple
interconnected cylinders. In such designs, trapped regions of "dead
volume" may occur such that pockets of gas remain in cylinders
before and after valve transitions. Such volumes may occur within
the cylinders themselves and/or within conduits, valves, or other
components within and interconnecting the cylinders. Bringing
relatively high-pressure gas into communication (e.g., by the
opening of a valve) with relatively low-pressure gas within a dead
volume tends to lead to a diminishment of pressure of the
higher-pressure gas without the performance of useful work, thereby
disadvantageously reducing the amount of work recoverable from or
stored within the higher-pressure gas. Air dead volume tends to
reduce the amount of work available from a quantity of
high-pressure gas brought into communication with the dead volume.
This loss of potential energy may be termed a "coupling loss." For
example, during a compression stage a volume of gas that was
compressed to a relatively high pressure may remain inside the
compression cylinder, or conduits attached thereto, after the
movable member of the cylinder (e.g., piston, hydraulic fluid, or
bladder) reaches the end of its stroke. The volume of compressed
air that is not pushed onto the next stage at the end of stroke
constitutes "dead volume" (also termed, in compressors, "clearance
volume"). If the volume of high-pressure gas within the cylinder is
then brought into fluid communication (e.g., by the opening of a
valve) with a section of intake piping, a portion of the
high-pressure gas will tend to enter the piping and mix with the
contents thereof, equalizing the pressure within the two volumes.
This equalization of pressure entails a loss of exergy (i.e.,
energy available as work). In a preferred scenario, the gas in the
dead volume is allowed to expand in a manner that performs useful
work (e.g., by pushing on a piston), equalizing in pressure with
the gas in the piping prior to valve opening. In another scenario,
the gas in the dead volume is allowed to expand below the pressure
of the gas within the intake piping, and pressure equalization
takes place during valve transition (opening).
[0010] In another example of formation of dead space in a CAES
system during an expansion procedure, if gas is to be introduced
into a cylinder through a valve for the purpose of performing work
by pushing against a piston within the cylinder, and a chamber or
volume exists adjacent to the piston that is filled with
low-pressure gas at the time the valve is opened, the high-pressure
gas entering the chamber is immediately reduced in pressure during
free expansion and mixing with the low-pressure gas and, therefore,
performs less mechanical work upon the piston than would have been
possible without the pressure reduction. The low-pressure volume in
such an example constitutes air dead volume. Dead volume may also
appear within the portion of a valve mechanism that communicates
with the cylinder interior, or within a tube or line connecting a
valve to the cylinder interior, or within other components that
contain gas in various states of operation of the system. It will
be clear to persons familiar with hydraulics and pneumatics that
dead volume may also appear during compression procedures, and that
energy losses due to pneumatically communicating dead volumes tend
to be additive.
[0011] Moreover, in an expander-compressor system operated to
expand or compress gas near-isothermally (i.e., at approximately
constant temperature) within a cylinder, gas that escapes the
cylinder to become dead volume (e.g., by displacing an
incompressible fluid) in a hydraulic subsystem may, as pressures
change within the system, expand and compress adiabatically (i.e.,
at non-constant temperature), with associated energy losses due to
heat transfer between the gas and materials surrounding the dead
volume. These thermal energy losses will tend to be additive with
losses that are due to non-work-performing expansion of gas to
lower pressures in dead volumes.
[0012] Therefore, in various compressor-expander systems, including
isothermal compressor-expander systems, preventing the formation of
dead volume will generally enable higher system efficiency.
Attempts to minimize dead volume frequently involve reducing the
sizes and lengths of conduits interconnecting cylinders and other
components. However, such efforts may not eliminate all dead volume
and tend to constrain the overall geometry and placement of
individual system components. Therefore, there is a need for
alternative or additional approaches to reducing dead volume and/or
the deleterious effects of dead volume in pneumatic components in
order to reduce coupling losses and improve efficiency during
compression and/or expansion of gas.
SUMMARY
[0013] Embodiments of the invention reduce the impact of dead
volume in pneumatic cylinders and/or pneumatic chambers of
pneumatic/hydraulic cylinders during compression and/or expansion
in CAES systems. The impact of dead volume is reduced by
time-coordinated matching of gas pressures within system components
that would, absent such matching, suffer coupling losses and
potential equipment damage. Herein, a space within any component of
a CAES system is termed a "dead volume" or "dead space" if its
volume cannot, in some or all states of operation, be reduced to
zero due to mechanical constraints (e.g., imperfect fit of a piston
to the interior face of a cylinder head when the piston is at top
dead center, forming an ineradicable, residual chamber volume) and
if in some states of system operation the space contains gas at a
pressure that can be brought into fluid communication with gas at a
significantly different pressure (e.g., through a valve
transition).
[0014] In cylinders, the time-coordinated matching of pressures may
be accomplished using actuated valves that are selectively closed
and opened in a manner that yields approximately matched pressures
within system components about to be brought into fluid
communication with each other. To reduce loss of exergy due to
non-work-performing expansion of gas when components containing gas
at relatively high pressure (e.g., 3,000 psi) are brought into
fluid communication with components containing gas at relatively
low pressure (e.g., 300 psi or atmospheric pressure), the gas in
one or more potential dead spaces is pre-compressed to a pressure
approximately equal to that of the higher-pressure gas before the
higher- and lower-pressure gas volumes are brought into fluid
communication with each other. In other embodiments, the gas in one
or more potential dead spaces is pre-expanded to a pressure
approximately equal to that of the lower-pressure gas before the
higher- and lower-pressure volumes are brought into fluid
communication with each other. Such pre-compression and
pre-expansion produce specific target pressures (e.g., 3,000 psi)
at specific times or in specific states of system operation (e.g.,
when a cylinder piston reaches top-dead-center position and is
poised to begin an expansion stroke). Both target pressure and
timing of pressure matching may be altered adaptively during the
course of system operation based on measurements of pressures
within various parts of the CAES system and/or other aspects of
system state. An actuated valve may be operated (i.e., opened or
closed) at specific times in order to reduce the effect of dead
space, e.g., a valve may be opened only when a pre-compression
condition is met. The timing of actuated valve operation may,
furthermore, be conditioned on feedback in order to provide
increased system energy efficiency and/or other advantages. Herein,
an "actuated" valve is a valve whose opening or closing occurs at a
time that may be altered, either arbitrarily or within limits, by a
system operator or control mechanism, as distinct from a passive
"check"-type valve whose opening and closing are determined by
differential pressures or a "cam-driven" valve whose times of
opening or closing are dictated mechanically. Actuated valves may
improve performance by opening at times different than would be
entailed by operation of check valves by differential pressures.
Variable-timing cam-driven valves may be considered actuated valves
and are within the scope of this invention.
[0015] In certain embodiments of the invention, the CAES system may
include or consist essentially of a cylinder assembly (or plurality
of cylinder assemblies, e.g., multiple stages) that features a
movable internal member (e.g., piston) or other boundary mechanism
such as hydraulic fluid or a bladder. The internal boundary
mechanism of the cylinder divides the interior of the cylinder into
two chambers that may contain distinct bodies of fluid, and these
may be at different pressures in various states of operation of the
system. The system may further include a first control valve in
communication with a high-pressure storage reservoir and the
cylinder assembly, a second control valve in communication with the
cylinder assembly and a vent to atmosphere, a heat-transfer
subsystem in fluid communication with the cylinder assembly, an
electric motor/generator in mechanical communication with a drive
mechanism (e.g., crankshaft, hydraulic pump, linear generator
mover) configured to drive the movable member disposed within the
cylinder assembly, and a control system configured to operate the
first and second control valves based on various information
characterizing various aspects of the cylinder assembly and/or
other components of the system (e.g., pressure, temperature, piston
position, piston velocity).
[0016] One aspect of the invention relates to a method for reducing
coupling losses and improving system performance during an
expansion stage of a CAES system. In various embodiments, gas
within a first chamber of a cylinder is pressurized to
approximately some relatively high pressure (e.g., 3,000 psi) at or
near the beginning of an expansion stroke of the cylinder. In this
state of operation, the piston of the cylinder is at or near its
top dead center position and the first chamber constitutes dead
volume. A first control valve is then operated to place a volume of
high-pressure gas (e.g., air at 3,000 psi) from an external source
(e.g., a pressurized gas storage reservoir) in fluid communication
with the first chamber. Because the gas within the first chamber is
at approximately the same pressure as the high-pressure source
placed in communication with the first chamber by the opening of
the first control valve, gas from the high-pressure source does not
tend to expand into the first chamber suddenly and without
performing useful work. Coupling losses during the connection of
the source to the cylinder are thus reduced or eliminated. In
short, system performance may be improved by forestalling events of
rapid pressure equalization of connecting spaces.
[0017] During a subsequent cylinder expansion stroke (also herein
termed a "downward stroke"), useful work is recovered from
high-pressure gas during both (1) admission of high-pressure gas to
the first cylinder chamber while the boundary mechanism moves
downward so as to allow the first cylinder chamber to enlarge, a
phase of operation herein termed a "direct-drive phase" or "direct
drive," and (2) a subsequent expansion phase (i.e., after the first
control valve is closed) during which the boundary mechanism
continues to move downward and a fixed mass of high-pressure gas
expands in the enlarging first chamber). As shown and described in
the '207 and '155 patents and the '128 application, gas expansion
may be maintained as substantially isothermal by introducing a
certain volume of liquid (e.g., a quantity of foam or spray) at an
appropriate temperature into the cylinder prior to and/or during
the expansion.
[0018] At or near the end of the expansion stroke, when the gas
reaches a lower pressure (e.g., 300 psi), a second control valve is
operated to begin to exhaust the gas (e.g., to a vent and/or to a
mid-pressure vessel and second cylinder assembly) as an upward
stroke of the movable member within the piston occurs. During the
first portion of the second half of the cylinder stroke (e.g., the
upward stroke), the gas is exhausted through the second control
valve (e.g., into a mid-pressure vessel and second cylinder
assembly) by translating the movable member (e.g., piston) or other
boundary mechanism to reduce the volume of the first chamber in the
cylinder assembly. During a second portion of the second half of
the cylinder stroke (e.g., the upward stroke), prior to the movable
member reaching the end of stroke (e.g., top of stroke) inside the
cylinder, the second control valve is closed and a "pre-compression
stroke" is performed to compress the remaining volume of air (dead
volume) and/or liquid inside the cylinder.
[0019] The time of closure of the second control valve, relative to
the sequence of states of operation just described, is not
arbitrary. Premature closure of the second valve will typically
tend to trap an excessive quantity of gas in the first chamber,
resulting in overpressurization of the gas in the first chamber
when the volume of the first chamber attains a minimum (i.e., at
top dead center of stroke). When this occurs, opening of the first
valve at the start of the next expansion cycle will result in
energy loss through non-work-performing expansion of the gas within
the first chamber into the high-pressure storage reservoir and
other components (e.g., piping) in fluid communication
therewith.
[0020] On the other hand, tardy closure of the second valve will
tend to trap an inadequate quantity of gas in the first chamber,
resulting in underpressurization of the gas in the first chamber
when the volume of the first chamber attains a minimum (i.e., at
top dead center of stroke). When this occurs, opening of the first
valve at the start of the next expansion cycle will generally
result in energy loss through non-work-performing expansion of gas
from the high-pressure storage reservoir and other components
(e.g., piping) in fluid communication therewith into the first
chamber.
[0021] Therefore, in certain embodiments of the invention the
optimal time of actuation of the second control valve is based at
least in part on sensed conditions in one or more portions of the
CAES system, e.g., pressure in the first chamber, pressure in the
high-pressure storage reservoir, piston position, piston velocity,
etc. The principles on which the time of actuation of the second
control valve is determined will be made clear hereinbelow.
[0022] When liquid is introduced into the first chamber to enable
approximately isothermal expansion, a quantity of liquid will
generally accumulate in the first chamber (e.g., on top of the
piston or other movable member) during an expansion stroke. In an
ideal case, i.e., if all of the gas compressed in the first chamber
is passed to the storage reservoir or to a higher-pressure cylinder
stage by the time the volume of the first chamber is at a minimum,
the remaining volume of the first chamber (i.e., the volume between
the movable member and the interior face of the upper end-cap of
the cylinder) will be occupied entirely by liquid; all gas will
have been expelled and there will be no gas-filled dead volume in
the first chamber at the commencement of a new expansion stroke.
However, in practice, the first chamber volume at the commencement
of a new expansion stroke will tend to contain both liquid and gas
remaining from the previous expansion stroke. The gas fraction of
this volume may constitute dead volume at the start of the new
stroke. During the pre-compression stroke, therefore, as already
described hereinabove, the effective coupling loss due to this dead
volume is minimized by compressing the remaining air to a pressure
substantially equal to the pressure of the air in the storage
reservoir (or the pressure of the gas to be introduced into the
cylinder for expansion, if such gas is not arriving directly from
the storage reservoir). Thus, when additional high-pressure gas is
admitted to the cylinder for expansion, no or minimal pressure
difference exists between the two sides of the first control valve.
This allows the first control valve to be operated with a lower
actuation energy, further improving system efficiency.
[0023] Most or even substantially all of the work done upon the air
in the first chamber during a pre-compression stroke is typically
recovered as the air re-expands during the subsequent expansion
stroke. Furthermore, if the pressure of the air in the dead volume
is approximately equal to the pressure of the air in the storage
vessel or the next higher-pressure stage, then there will be
substantially no coupling loss when the first valve to the storage
reservoir or next-higher-pressure stage is opened. The higher
pressure within the dead volume entails less gas flow from the
storage reservoir or next-higher-pressure stage to the cylinder
when the valve is opened during an expansion stage, thereby
reducing coupling loss and improving efficiency. Moreover, the
longevity of some system components may be increased because
transient mechanical stresses caused by high-pressure air rushing
suddenly into dead volume are minimized or eliminated.
[0024] Employing measurements of pressures within various
components (e.g., lines and chambers) allows the timing of
actuated-valve closings and openings (i.e., valve transition
events) to be optimized for specific system conditions. For
example, CAES systems constructed according to similar designs may
differ in pipe lengths and other details affecting potential dead
space. In such a case, with actuated valves, valve transition
events may be tuned to optimize efficiency of an individual system
by minimizing dead-space coupling losses. For example, if an
overpressure is detected in a pre-pressurized cylinder chamber,
closure of the valve that permits gas to exit the chamber during a
return stroke may be retarded to reduce the amount of
pre-pressurized gas. A CAES system in which valve transition event
tuning is performed by a computerized system controller may be
considered, in this respect, a self-tuning system. For another
example, as the pressure within a gas storage reservoir declines as
the gas within the reservoir cools or is exhausted (or increases as
the gas within the reservoir warms or is augmented), valve
transition event timing may be tuned in a manner that adaptively,
continuously maximizes the energy efficiency of the CAES system.
Thus, a CAES system in accordance with various embodiments of the
invention may adaptively self-tune its valve transition events so
as to minimize dead-space coupling losses in response both to
idiosyncrasies of system construction and to changing conditions of
operation.
[0025] Furthermore, to minimize the impacts of dead space, the
timing of valve actuations may be chosen in light of the non-ideal
features of actual valves. Non-instantaneous valve transitions tend
to entail tradeoffs between system capacity (amount of air
compressed or expanded per stroke) and system efficiency (partly
determined by energy losses due to dead space). The impact of
non-ideal valve actuation on CAES system operation is considered
for certain embodiments of the invention hereinbelow.
[0026] Every compression or expansion of a quantity of gas, where
such a compression or expansion is herein termed "a gas process,"
is generally one of three types: (1) adiabatic, during which the
gas exchanges no heat with its environment and, consequently, rises
or falls in temperature, (2) isothermal, during which the gas
exchanges heat with its environment in such a way as to remain at
constant temperature, and (3) polytropic, during which the gas
exchanges heat with its environment but its temperature does not
remain constant. Perfectly adiabatic gas processes are not
practical because some heat is always exchanged between any body of
gas and its environment (ideal insulators and reflectors do not
exist); perfectly isothermal gas processes are not practical
because for heat to flow between a quantity of gas and a portion of
its environment (e.g., a body of liquid), a nonzero temperature
difference must exist between the gas and its environment--e.g.,
the gas must be allowed to heat during compression in order that
heat may be conducted to the liquid. Hence real-world gas processes
are typically polytropic, though they may approximate adiabatic or
isothermal processes.
[0027] The Ideal Gas Law states that for a given quantity of gas
having mass m, pressure p, volume V, and temperature T, pV=mRT,
where R is the gas constant (R=287 J/Kkg for air). For an
isothermal process, Tis a constant throughout the process, so pV=C,
where C is some constant.
[0028] For a polytropic process, as will be clear to persons
familiar with the science of thermodynamics, pV.sup.n=C throughout
the process, where n, termed the polytropic index, is some constant
generally between 1.0 and 1.6. For n=1, pV.sup.n=pV.sup.l=pV=C,
i.e., the process is isothermal. In general, a process for which n
is close to 1 (e.g., 1.05) may be deemed approximately
isothermal.
[0029] For an adiabatic process, pV.sup.n=C, where y, termed the
adiabatic coefficient, is equal to the ratio of the gas's heat
capacity at constant pressure Cp to its heat capacity at constant
volume, C.sub.V, i.e., .gamma.=C.sub.P/C.sub.V. In practice,
.gamma. is dependent on pressure. For air, the adiabatic
coefficient y is typically between 1.4 and 1.6.
[0030] Herein, we define a "substantially isothermal" gas process
as one having n.ltoreq.1.1. The gas processes conducted within
cylinders described herein are preferably substantially isothermal
with n.ltoreq.1.05. Herein, wherever a gas process taking place
within a cylinder assembly or storage reservoir is described as
"isothermal," this word is synonymous with the term "substantially
isothermal."
[0031] The amount of work done in compression or expansion of a
given quantity of gas varies substantially with polytropic index n.
For compressions, the lowest amount of work done is for an
isothermal process and the highest for an adiabatic process, and
vice versa for expansions. Hence, for gas processes such as
typically occur in the compressed-gas energy storage systems
described herein, the end temperatures attained by adiabatic,
isothermal, and substantially isothermal gas processes are
sufficiently different to have practical impact on the operability
and efficiency of such systems. Similarly, the thermal efficiencies
of adiabatic, isothermal, and substantially isothermal gas
processes are sufficiently different to have practical impact on
the overall efficiency of such energy storage systems. For example,
for compression of a quantity of gas from initial temperature of
20.degree. C. and initial pressure of 0 psig (atmospheric) to a
final pressure of 180 psig, the final temperature T of the gas will
be exactly 20.degree. C. for an isothermal process, approximately
295.degree. C. for an adiabatic process, approximately 95.degree.
C. for a polytropic compression having polytropic index n=1.1 (10%
increase in n over isothermal case of n=1), and approximately
60.degree. C. for a polytropic compression having polytropic index
n=1.05 (5% increase in n over isothermal case of n=1). In another
example, for compression of 1.6 kg of air from an initial
temperature of 20.degree. C. and initial pressure of 0 psig
(atmospheric) to a final pressure of approximately 180 psig,
including compressing the gas into a storage reservoir at 180 psig,
isothermal compression requires approximately 355 kilojoules of
work, adiabatic compression requires approximately 520 kilojoules
of work, and a polytropic compression having polytropic index
n=1.045 requires approximately 375 kilojoules of work; that is, the
polytropic compression requires approximately 5% more work than the
isothermal process, and the adiabatic process requires
approximately 46% more work than the isothermal process.
[0032] It is possible to estimate the polytropic index n of gas
processes occurring in cylinder assemblies such as are described
herein by empirically fitting n to the equation pV.sup.n=C, where
pressure p and volume V of gas during a compression or expansion,
e.g., within a cylinder, may both be measured as functions of time
from piston position, known device dimensions, and
pressure-transducer measurements. Moreover, by the Ideal Gas Law,
temperature within the cylinder may be estimated from p and V, as
an alternative to direct measurement by a transducer (e.g.,
thermocouple, resistance thermal detector, thermistor) located
within the cylinder and in contact with its fluid contents. In many
cases, an indirect measurement of temperature via volume and
pressure may be more rapid and more representative of the entire
volume than a slower point measurement from a temperature
transducer. Thus, temperature measurements and monitoring described
herein may be performed directly via one or more transducers, or
indirectly as described above, and a "temperature sensor" may be
one of such one or more transducers and/or one or more sensors for
the indirect measurement of temperature, e.g., volume, pressure,
and/or piston-position sensors.
[0033] All of the approaches described above for converting
potential energy in a compressed gas into mechanical and electrical
energy may, if appropriately designed, be operated in reverse to
store electrical energy as potential energy in a compressed gas.
Since the accuracy of this statement will be apparent to any person
reasonably familiar with the principles of electrical machines,
power electronics, pneumatics, and the principles of
thermodynamics, the operation of these mechanisms to both store
energy and recover it from storage are not necessarily described
for each embodiment. Such operation is, however, contemplated and
within the scope of the invention and may be straightforwardly
realized without undue experimentation.
[0034] The systems described herein, and/or other embodiments
employing foam-based heat exchange, liquid-spray heat exchange,
and/or external gas heat exchange, may draw or deliver thermal
energy via their heat-exchange mechanisms to external systems (not
shown) for purposes of cogeneration, as described in U.S. Pat. No.
7,958,731, filed Jan. 20, 2010 (the '731 patent), the entire
disclosure of which is incorporated by reference herein.
[0035] The compressed-air energy storage and recovery systems
described herein are preferably "open-air" systems, i.e., systems
that take in air from the ambient atmosphere for compression and
vent air back to the ambient atmosphere after expansion, rather
than systems that compress and expand a captured volume of gas in a
sealed container (i.e., "closed-air" systems). The systems
described herein generally feature one or more cylinder assemblies
for the storage and recovery of energy via compression and
expansion of gas. The systems also include (i) a reservoir for
storage of compressed gas after compression and supply of
compressed gas for expansion thereof, and (ii) a vent for
exhausting expanded gas to atmosphere after expansion and supply of
gas for compression. The storage reservoir may include or consist
essentially of, e.g., one or more one or more pressure vessels
(i.e., containers for compressed gas that may have rigid exteriors
or may be inflatable, that may be formed of various suitable
materials such as metal or plastic, and that may or may not fall
within ASME regulations for pressure vessels), pipes (i.e., rigid
containers for compressed gas that may also function as and/or be
rated as fluid conduits, have lengths well in excess (e.g.,
>100.times.) of their diameters, and do not fall within ASME
regulations for pressure vessels), or caverns (i.e., naturally
occurring or artificially created cavities that are typically
located underground). Open-air systems typically provide superior
energy density relative to closed-air systems.
[0036] Furthermore, the systems described herein may be
advantageously utilized to harness and recover sources of renewable
energy, e.g., wind and solar energy. For example, energy stored
during compression of the gas may originate from an intermittent
renewable energy source of, e.g., wind or solar energy, and energy
may be recovered via expansion of the gas when the intermittent
renewable energy source is nonfunctional (i.e., either not
producing harnessable energy or producing energy at
lower-than-nominal levels). As such, the systems described herein
may be connected to, e.g., solar panels or wind turbines, in order
to store the renewable energy generated by such systems.
[0037] In one aspect, embodiments of the invention feature a method
of increasing efficiency of an energy-recovery process performed in
a cylinder assembly in which gas is expanded from an initial
pressure to a final pressure. Gas is pre-compressed in the cylinder
assembly to approximately the initial pressure, and, following
pre-compression, compressed gas is admitted at the initial pressure
into the cylinder assembly. The pre-compression reduces coupling
loss during the admission of compressed gas. Gas is expanded in the
cylinder assembly to the final pressure, and the expansion cycle is
completed by exhausting only a portion of the expanded gas out of
the cylinder assembly. The foregoing steps may be repeated at least
once, thereby performing at least one additional expansion
cycle.
[0038] Embodiments of the invention may include one or more of the
following in any of a variety of combinations. The gas may be
thermally conditioned with heat-transfer fluid during expansion.
The thermal conditioning may render the expansion substantially
isothermal. The heat-transfer fluid may be sprayed into the gas
and/or may form a foam with the gas. After expansion of the gas, at
least a portion of the heat-transfer fluid may be exhausted out of
the cylinder assembly. The compressed gas may be admitted into the
cylinder assembly from a storage reservoir containing gas at the
initial pressure. The compressed gas may be admitted into the
cylinder assembly from a second cylinder assembly in which gas is
expanded to the initial pressure from a pressure greater than the
initial pressure. The portion of the expanded gas may be exhausted
to the ambient atmosphere or to a second cylinder assembly in which
gas is expanded from the final pressure to a pressure lower than
the final pressure. Admitting compressed gas into the cylinder
assembly may include or consist essentially of actuating a valve to
establish a connection between the cylinder assembly and a source
of the compressed gas. The pre-compression may reduce the actuation
energy required to actuate the valve. At least a portion of the gas
that is pre-compressed may be within dead volume of the cylinder
assembly. Exhausting only a portion of the expanded gas out of the
cylinder assembly may include or consist essentially of exhausting
substantially all of the expanded gas in the cylinder assembly that
is not within dead volume of the cylinder assembly. The amount of
the gas that is pre-compressed may be substantially less than an
amount of the gas expanded in the cylinder assembly.
[0039] A temperature, a pressure, and/or a position of a boundary
mechanism within the cylinder assembly may be monitored during gas
expansion and/or gas exhaustion, thereby generating control
information. The control information may be utilized in a
subsequent expansion cycle to control at least one of the
pre-compression, expansion, or exhaustion steps. The gas expansion
may drive a load connected to the cylinder assembly, e.g., a
mechanical crankshaft or a hydraulic pump/motor. The gas expansion
may generate electricity. Exhausting only a portion of the expanded
gas out of the cylinder assembly may include or consist essentially
of (i) monitoring a temperature, a pressure, and/or a position of a
boundary mechanism within the cylinder assembly, thereby generating
control information, and (ii) based at least in part on the control
information, discontinuing the gas exhaustion, thereby trapping a
remnant portion of the expanded gas within the cylinder assembly.
The remnant portion of the expanded gas may be determined such that
a pre-compression step of a subsequent expansion cycle compresses
the remnant portion to approximately the initial pressure.
[0040] In another aspect, embodiments of the invention feature a
method of increasing efficiency of an energy-storage process
performed in a cylinder assembly in which gas is compressed from an
initial pressure to a final pressure. Gas is pre-expanded in the
cylinder assembly to approximately the initial pressure. Following
the pre-expansion, gas is admitted at the initial pressure into the
cylinder assembly. The pre-expansion reduces coupling loss during
the admission of gas. (During pre-expansion, work is being
recovered via the expansion of the gas, e.g., via movement of a
piston or other boundary mechanism, and valves to the cylinder
assembly are closed, in contrast to a case, e.g., where gas expands
and is free to exhaust from the cylinder assembly to atmosphere,
and no work is recovered therefrom.) The gas is compressed in the
cylinder assembly to the final pressure, and a compression cycle is
completed by exhausting only a portion of the compressed gas out of
the cylinder assembly. The foregoing steps may be repeated at least
once, thereby performing at least one additional compression
cycle.
[0041] Embodiments of the invention may include one or more of the
following in any of a variety of combinations. The gas may be
thermally conditioned with heat-transfer fluid during compression.
The thermal conditioning may render the compression substantially
isothermal. The heat-transfer fluid may be sprayed into the gas
and/or form a foam with the gas. After compression of the gas, at
least a portion of the heat-transfer fluid may be exhausted out of
the cylinder assembly. The gas may be admitted into the cylinder
assembly from a vent to atmosphere, and the initial pressure may be
approximately atmospheric pressure. The gas may be admitted into
the cylinder assembly from a second cylinder assembly in which gas
is compressed to the initial pressure from a pressure less than the
initial pressure. The portion of the compressed gas may be
exhausted to a compressed-gas storage reservoir. The portion of the
expanded gas may be exhausted to a second cylinder assembly in
which gas is compressed from the final pressure to a pressure
higher than the final pressure. Admitting gas into the cylinder
assembly may include or consist essentially of actuating a valve to
establish a connection between the cylinder assembly and a source
of the gas. The pre-expansion may reduce the actuation energy
required to actuate the valve. At least a portion of the gas that
is pre-expanded may be within dead volume of the cylinder assembly.
Exhausting only a portion of the compressed gas out of the cylinder
assembly may include or consist essentially of exhausting
substantially all of the compressed gas in the cylinder assembly
that is not within dead volume of the cylinder assembly. The amount
of the gas that is pre-expanded may be substantially less than an
amount of the gas compressed in the cylinder assembly.
[0042] A temperature, a pressure, and/or a position of a boundary
mechanism within the cylinder assembly may be monitored during gas
compression and/or gas exhaustion, thereby generating control
information. The control information may be utilized in a
subsequent compression cycle to control at least one of the
pre-expansion, compression, or exhaustion steps. The gas
compression may be driven by a load connected to the cylinder
assembly, e.g., a mechanical crankshaft or a hydraulic pump/motor.
Exhausting only a portion of the compressed gas out of the cylinder
assembly may include or consist essentially of (i) monitoring a
temperature, a pressure, and/or a position of a boundary mechanism
within the cylinder assembly, thereby generating control
information, and (ii) based at least in part on the control
information, discontinuing the gas exhaustion, thereby trapping a
remnant portion of the compressed gas within the cylinder assembly.
The remnant portion of the compressed gas may be determined such
that a pre-expansion step of a subsequent expansion cycle expands
the remnant portion to approximately the initial pressure.
[0043] In yet another aspect, embodiments of the invention feature
an energy storage and recovery system that includes or consists
essentially of a cylinder assembly for expanding gas to recover
energy and/or compressing gas to store energy, a high-side
component selectively fluidly connected to the cylinder assembly, a
low-side component selectively fluidly connected to the cylinder
assembly, and a control system. The high-side component supplies
gas to the cylinder assembly for expansion therein and/or accepts
gas from the cylinder assembly after compression therein. The
low-side component supplies gas to the cylinder assembly for
compression therein and/or accepts gas from the cylinder assembly
after expansion therein. The control system operates the cylinder
assembly to perform (i) a pre-compression of gas therewithin prior
to admission therein of gas for expansion, thereby reducing
coupling loss between the cylinder assembly and the high-side
component, and/or (ii) a pre-expansion of gas therewithin prior to
admission therein of gas for compression, thereby reducing coupling
loss between the cylinder assembly and the low-side component.
[0044] Embodiments of the invention may include one or more of the
following in any of a variety of combinations. A sensor may sense a
temperature, a pressure, or a position of a boundary mechanism
within the cylinder assembly to generate control information, and
the control system may be responsive to the control information.
The control system may be configured to operate the cylinder
assembly, during (i) pre-compression of gas therewithin and/or (ii)
expansion of gas therewithin, based at least in part on control
information generated during (i) a previous gas expansion within
the cylinder assembly and/or (ii) a previous pre-compression of gas
within the cylinder assembly. The control system may be configured
to operate the cylinder assembly, during (i) pre-expansion of gas
therewithin and/or (ii) compression of gas therewithin, based at
least in part on control information generated during (i) a
previous gas compression within the cylinder assembly and/or (ii) a
previous pre-expansion of gas within the cylinder assembly. The
high-side component may include or consist essentially of a
compressed-gas storage reservoir. The high-side component may
include or consist essentially of a second cylinder assembly for
compressing gas and/or expanding gas within a pressure range higher
than a pressure range of operation of the cylinder assembly. The
system may include a second cylinder assembly for compressing gas
and/or expanding gas within a pressure range higher than a pressure
range of operation of the cylinder assembly, and the high-side
component may include or consist essentially of a mid-pressure
vessel for containing gas at a pressure within both of or between
pressure ranges of operation of the cylinder assembly and the
second cylinder assembly. The low-side component may include or
consist essentially of a vent to atmosphere. The low-side component
may include or consist essentially of a second cylinder assembly
for compressing gas and/or expanding gas within a pressure range
lower than a pressure range of operation of the cylinder assembly.
The system may include a second cylinder assembly for compressing
gas and/or expanding gas within a pressure range lower than a
pressure range of operation of the cylinder assembly, and the
low-side component may include or consist essentially of a
mid-pressure vessel for containing gas at a pressure within both of
or between pressure ranges of operation of the cylinder assembly
and the second cylinder assembly.
[0045] A load may be mechanically coupled to the cylinder assembly.
The load may be driven by the cylinder assembly during gas
expansion and/or drive the cylinder assembly during gas
compression. The load may include or consist essentially of at
least one of a mechanical crankshaft or a hydraulic pump/motor. The
system may include a heat-transfer subsystem for thermally
conditioning gas during compression and/or expansion thereof. The
heat-transfer subsystem may include or consist essentially of a
mechanism for introducing heat-transfer fluid into the gas, e.g., a
spray head and/or a spray rod. The heat-transfer subsystem may
include a heat exchanger for thermally conditioning gas from the
cylinder assembly and/or heat-transfer fluid. The heat-transfer
subsystem may include (i) a mixing chamber for forming foam from
gas and heat-transfer fluid and/or (ii) a screen for altering
average bubble size and/or bubble-size variance of foam comprising
gas and heat-transfer fluid.
[0046] In a further aspect, embodiments of the invention feature a
method of increasing efficiency of an energy-recovery process
performed in a cylinder assembly in which gas is expanded. The
cylinder assembly is selectively fluidly connected to a high-side
component by a high-side valve and selectively fluidly connected to
a low-side component by a low-side valve. A first valve transition
is performed by opening the high-side valve to allow compressed gas
to enter the cylinder assembly from the high-side component, and
the cylinder assembly contains gas at a first pressure prior to the
first valve transition. A second valve transition is performed by
closing the high-side valve, and the gas within the cylinder
assembly expands thereafter. A third valve transition is performed
by opening the low-side valve to allow a portion of the expanded
gas to enter the low-side component from the cylinder assembly, and
(i) a remnant portion of the gas remains in the cylinder assembly
after the third valve transition and (ii) the expanded gas is at a
second pressure prior to the third valve transition. A fourth valve
transition is performed by closing the low-side valve, and the
remnant portion of the gas within the cylinder assembly is
compressed thereafter to approximately the first pressure. A
transition restriction is enforced, where the transition
restriction includes or consists essentially of (i) performing the
first valve transition only when the first pressure is
approximately equal to a pressure of the high-side component and/or
(ii) performing the third valve transition only when the second
pressure is approximately equal to a pressure of the low-side
component.
[0047] Embodiments of the invention may include one or more of the
following in any of a variety of combinations. The high-side
component may include or consist essentially of a compressed-gas
storage reservoir. The high-side component may include or consist
essentially of a second cylinder assembly for compressing gas
and/or expanding gas within a pressure range higher than a pressure
range of operation of the cylinder assembly. The high-side
component may include or consist essentially of a mid-pressure
vessel for containing gas at a pressure within both of or between
pressure ranges of operation of the cylinder assembly and a second
cylinder assembly for compressing gas and/or expanding gas within a
pressure range higher than a pressure range of operation of the
cylinder assembly. The low-side component may include or consist
essentially of a vent to atmosphere. The low-side component may
include or consist essentially of a second cylinder assembly for
compressing gas and/or expanding gas within a pressure range lower
than a pressure range of operation of the cylinder assembly. The
low-side component may include or consist essentially of a
mid-pressure vessel for containing gas at a pressure within both of
or between pressure ranges of operation of the cylinder assembly
and a second cylinder assembly for compressing gas and/or expanding
gas within a pressure range lower than a pressure range of
operation of the cylinder assembly.
[0048] The high-side valve and/or the low-side valve may be
actuated valves. Each of the high-side valve and the low-side valve
may be a hydraulically actuated valve, a variable cam actuated
valve, an electromagnetically actuated valve, a mechanically
actuated valve, or a pneumatically actuated valve. The second valve
transition may be timed to admit an amount of gas into a volume of
the cylinder assembly that is expandable to the second pressure in
the cylinder assembly. A temperature within the cylinder assembly,
a pressure within the cylinder assembly, a position of a boundary
mechanism within the cylinder assembly, the pressure of the
high-side component, and/or the pressure of the low-side component
may be monitored during an expansion cycle including or consisting
essentially of the first, second, third, and fourth valve
transitions, thereby generating control information. The control
information may be utilized in a subsequent expansion cycle to
control timing of the first, second, third, and/or fourth valve
transitions of the subsequent expansion cycle. The timing may be
controlled to maximize efficiency of the subsequent expansion
cycle. Gas may be thermally conditioned with heat-transfer fluid
during at least a portion of an expansion cycle including or
consisting essentially of the first, second, third, and fourth
valve transitions. The thermal conditioning may render the gas
expansion substantially isothermal.
[0049] In yet a further aspect, embodiments of the invention
feature a method of increasing efficiency of an energy-recovery
process performed in a cylinder assembly in which gas is expanded.
The cylinder assembly is selectively fluidly connected to a
high-side component by a high-side valve and selectively fluidly
connected to a low-side component by a low-side valve. A plurality
of expansion cycles are performed within the cylinder assembly.
Each expansion cycle includes or consists essentially of (i)
performing a first valve transition by opening the high-side valve
to allow compressed gas to enter the cylinder assembly from the
high-side component, (ii) performing a second valve transition by
closing the high-side valve, the gas within the cylinder assembly
expanding thereafter, (iii) performing a third valve transition by
opening the low-side valve to allow a portion of the expanded gas
to enter the low-side component from the cylinder assembly, a
remnant portion of the gas remaining in the cylinder assembly after
the third valve transition, and (iv) performing a fourth valve
transition by closing the low-side valve, the remnant portion of
the gas within the cylinder assembly being compressed thereafter.
During each expansion cycle, the timing of the first, second,
third, and/or fourth valve transitions is altered to maximize
efficiency of the expansion cycle. The timing may be altered based
at least in part on control information generated during a previous
expansion cycle. The control information may include or consist
essentially of a temperature within the cylinder assembly, a
pressure within the cylinder assembly, a position of a boundary
mechanism within the cylinder assembly, the pressure of the
high-side component, and/or the pressure of the low-side
component.
[0050] In another aspect, embodiments of the invention feature a
method of increasing efficiency of an energy-storage process
performed in a cylinder assembly in which gas is compressed. The
cylinder assembly is selectively fluidly connected to a high-side
component by a high-side valve and selectively fluidly connected to
a low-side component by a low-side valve. A first valve transition
is performed by opening the low-side valve to allow gas to enter
the cylinder assembly from the low-side component, and the cylinder
assembly contains gas at a first pressure prior to the first valve
transition. A second valve transition is performed by closing the
low-side valve, and the gas within the cylinder assembly is
compressed thereafter. A third valve transition is performed by
opening the high-side valve to allow a portion of the compressed
gas to enter the high-side component from the cylinder assembly,
and (i) a remnant portion of the gas remains in the cylinder
assembly after the third valve transition and (ii) the compressed
gas is at a second pressure prior to the third valve transition. A
fourth valve transition is performed by closing the high-side
valve, and the remnant portion of the gas within the cylinder
assembly expands thereafter to approximately the first pressure. A
transition restriction is enforced, where the transition
restriction includes or consists essentially of (i) performing the
first valve transition only when the first pressure is
approximately equal to a pressure of the low-side component or (ii)
performing the third valve transition only when the second pressure
is approximately equal to a pressure of the high-side
component.
[0051] Embodiments of the invention may include one or more of the
following in any of a variety of combinations. The high-side
component may include or consist essentially of a compressed-gas
storage reservoir. The high-side component may include or consist
essentially of a second cylinder assembly for compressing gas
and/or expanding gas within a pressure range higher than a pressure
range of operation of the cylinder assembly. The high-side
component may include or consist essentially of a mid-pressure
vessel for containing gas at a pressure within both of or between
pressure ranges of operation of the cylinder assembly and a second
cylinder assembly for compressing gas and/or expanding gas within a
pressure range higher than a pressure range of operation of the
cylinder assembly. The low-side component may include or consist
essentially of a vent to atmosphere. The low-side component may
include or consist essentially of a second cylinder assembly for
compressing gas and/or expanding gas within a pressure range lower
than a pressure range of operation of the cylinder assembly. The
low-side component may include or consist essentially of a
mid-pressure vessel for containing gas at a pressure within both of
or between pressure ranges of operation of the cylinder assembly
and a second cylinder assembly for compressing gas and/or expanding
gas within a pressure range lower than a pressure range of
operation of the cylinder assembly.
[0052] The high-side valve and/or the low-side valve may be
actuated valves. Each of the high-side valve and the low-side valve
may be a hydraulically actuated valve, a variable cam actuated
valve, an electromagnetically actuated valve, a mechanically
actuated valve, or a pneumatically actuated valve. The second valve
transition may be timed to admit an amount of gas into a volume of
the cylinder assembly that is compressible to the second pressure
in the cylinder assembly. A temperature within the cylinder
assembly, a pressure within the cylinder assembly, a position of a
boundary mechanism within the cylinder assembly, the pressure of
the high-side component, and/or the pressure of the low-side
component may be monitored during a compression cycle including or
consisting essentially of the first, second, third, and fourth
valve transitions, thereby generating control information. The
control information may be utilized in a subsequent compression
cycle to control timing of the first, second, third, and/or fourth
valve transitions of the subsequent compression cycle. The timing
may be controlled to maximize efficiency of the subsequent
compression cycle. Gas may be thermally conditioned with
heat-transfer fluid during at least a portion of a compression
cycle including or consisting essentially of the first, second,
third, and fourth valve transitions. The thermal conditioning may
render the gas compression substantially isothermal.
[0053] In yet another aspect, embodiments of the invention feature
a method of increasing efficiency of an energy-storage process
performed in a cylinder assembly in which gas is compressed. The
cylinder assembly is selectively fluidly connected to a high-side
component by a high-side valve and selectively fluidly connected to
a low-side component by a low-side valve. A plurality of
compression cycles are performed within the cylinder assembly. Each
compression cycle includes or consists essentially of (i)
performing a first valve transition by opening the low-side valve
to allow gas to enter the cylinder assembly from the low-side
component, (ii) performing a second valve transition by closing the
low-side valve, the gas within the cylinder assembly being
compressed thereafter, (iii) performing a third valve transition by
opening the high-side valve to allow a portion of the compressed
gas to enter the high-side component from the cylinder assembly, a
remnant portion of the gas remaining in the cylinder assembly
thereafter, and (iv) performing a fourth valve transition by
closing the high-side valve, the remnant portion of the gas within
the cylinder assembly expanding thereafter. During each compression
cycle, the timing of the first second, third, and/or fourth valve
transitions are altered to maximize efficiency of the compression
cycle. The timing may be altered based at least in part on control
information generated during a previous compression cycle. The
control information may include or consist essentially of a
temperature within the cylinder assembly, a pressure within the
cylinder assembly, a position of a boundary mechanism within the
cylinder assembly, the pressure of the high-side component, and/or
the pressure of the low-side component.
[0054] In an aspect, embodiments of the invention feature an energy
storage and recovery system including or consisting essentially of
a cylinder assembly for expanding gas to recover energy and/or
compressing gas to store energy, a high-side component selectively
fluidly connected to the cylinder assembly, a low-side component
selectively fluidly connected to the cylinder assembly, and a
control system. The high-side component supplies gas to the
cylinder assembly for expansion therein and/or accepts gas from the
cylinder assembly after compression therein. The low-side component
supplies gas to the cylinder assembly for compression therein
and/or accepts gas from the cylinder assembly after expansion
therein.
[0055] The control system may operate the cylinder assembly to (i)
perform a first valve transition by opening the low-side valve to
allow gas to enter the cylinder assembly from the low-side
component, the cylinder assembly containing gas at a first pressure
prior to the first valve transition, (ii) perform a second valve
transition by closing the low-side valve, the gas within the
cylinder assembly being compressed thereafter, (iii) perform a
third valve transition by opening the high-side valve to allow a
portion of the compressed gas to enter the high-side component from
the cylinder assembly, where (a) a remnant portion of the gas
remains in the cylinder assembly after the third valve transition
and (b) the compressed gas is at a second pressure prior to the
third valve transition, (iv) perform a fourth valve transition by
closing the high-side valve, the remnant portion of the gas within
the cylinder assembly expanding thereafter to approximately the
first pressure, and (v) enforce a transition restriction including
or consisting essentially of (a) performing the first valve
transition only when the first pressure is approximately equal to a
pressure of the low-side component and/or (b) performing the third
valve transition only when the second pressure is approximately
equal to a pressure of the high-side component.
[0056] The control system may operate the cylinder assembly to (i)
perform a first valve transition by opening the low-side valve to
allow gas to enter the cylinder assembly from the low-side
component, the cylinder assembly containing gas at a first pressure
prior to the first valve transition, (ii) perform a second valve
transition by closing the low-side valve, the gas within the
cylinder assembly being compressed thereafter, (iii) perform a
third valve transition by opening the high-side valve to allow a
portion of the compressed gas to enter the high-side component from
the cylinder assembly, where (a) a remnant portion of the gas
remains in the cylinder assembly after the third valve transition
and (b) the compressed gas is at a second pressure prior to the
third valve transition, (iv) perform a fourth valve transition by
closing the high-side valve, the remnant portion of the gas within
the cylinder assembly expanding thereafter to approximately the
first pressure, and (v) enforce a transition restriction including
or consisting essentially of (i) performing the first valve
transition only when the first pressure is approximately equal to a
pressure of the low-side component and/or (ii) performing the third
valve transition only when the second pressure is approximately
equal to a pressure of the high-side component.
[0057] In another aspect, embodiments of the invention feature an
energy storage and recovery system including or consisting
essentially of a cylinder assembly for expanding gas to recover
energy and/or compressing gas to store energy, a high-side
component selectively fluidly connected to the cylinder assembly, a
low-side component selectively fluidly connected to the cylinder
assembly, and a control system. The high-side component supplies
gas to the cylinder assembly for expansion therein and/or accepts
gas from the cylinder assembly after compression therein. The
low-side component supplies gas to the cylinder assembly for
compression therein and/or accepts gas from the cylinder assembly
after expansion therein.
[0058] The control system may operate the cylinder assembly to
perform, within the cylinder assembly, a plurality of expansion
cycles each including or consisting essentially of (i) performing a
first valve transition by opening the high-side valve to allow
compressed gas to enter the cylinder assembly from the high-side
component, (ii) performing a second valve transition by closing the
high-side valve, the gas within the cylinder assembly expanding
thereafter, (iii) performing a third valve transition by opening
the low-side valve to allow a portion of the expanded gas to enter
the low-side component from the cylinder assembly, a remnant
portion of the gas remaining in the cylinder assembly after the
third valve transition, and (iv) performing a fourth valve
transition by closing the low-side valve, the remnant portion of
the gas within the cylinder assembly being compressed thereafter.
The control system may also, during each expansion cycle, alter a
timing of the first, second, third, and/or fourth valve transitions
to maximize efficiency of the expansion cycle.
[0059] The control system may operate the cylinder assembly to
perform, within the cylinder assembly, a plurality of compression
cycles each including or consisting essentially of (i) performing a
first valve transition by opening the low-side valve to allow gas
to enter the cylinder assembly from the low-side component, (ii)
performing a second valve transition by closing the low-side valve,
the gas within the cylinder assembly being compressed thereafter,
(iii) performing a third valve transition by opening the high-side
valve to allow a portion of the compressed gas to enter the
high-side component from the cylinder assembly, a remnant portion
of the gas remaining in the cylinder assembly thereafter, and (iv)
performing a fourth valve transition by closing the high-side
valve, the remnant portion of the gas within the cylinder assembly
expanding thereafter. The control system may also, during each
compression cycle, alter a timing of the first, second, third,
and/or fourth valve transitions to maximize efficiency of the
compression cycle.
[0060] These and other objects, along with advantages and features
of the invention, will become more apparent through reference to
the following description, the accompanying drawings, and the
claims. Furthermore, it is to be understood that the features of
the various embodiments described herein are not mutually exclusive
and can exist in various combinations and permutations. Note that
as used herein, the terms "pipe," "piping" and the like shall refer
to one or more conduits that are rated to carry gas or liquid
between two points. Thus, the singular term should be taken to
include a plurality of parallel conduits where appropriate. Herein,
the terms "liquid" and "water" interchangeably connote any mostly
or substantially incompressible liquid, the terms "gas" and "air"
are used interchangeably, and the term "fluid" may refer to a
liquid, a gas, or a mixture of liquid and gas (e.g., a foam) unless
otherwise indicated. As used herein unless otherwise indicated, the
terms "approximately" and "substantially" mean .+-.10%, and, in
some embodiments, .+-.5%. A "valve" is any mechanism or component
for controlling fluid communication between fluid paths or
reservoirs, or for selectively permitting control or venting. The
term "cylinder" refers to a chamber, of uniform but not necessarily
circular cross-section, which may contain a slidably disposed
piston or other mechanism that separates the fluid on one side of
the chamber from that on the other, preventing fluid movement from
one side of the chamber to the other while allowing the transfer of
force/pressure from one side of the chamber to the next or to a
mechanism outside the chamber. At least one of the two ends of a
chamber may be closed by end caps, also herein termed "heads." As
utilized herein, an "end cap" is not necessarily a component
distinct or separable from the remaining portion of the cylinder,
but may refer to an end portion of the cylinder itself. Rods,
valves, and other devices may pass through the end caps. A
"cylinder assembly" may be a simple cylinder or include multiple
cylinders, and may or may not have additional associated components
(such as mechanical linkages among the cylinders). The shaft of a
cylinder may be coupled hydraulically or mechanically to a
mechanical load (e.g., a hydraulic motor/pump or a crankshaft) that
is in turn coupled to an electrical load (e.g., rotary or linear
electric motor/generator attached to power electronics and/or
directly to the grid or other loads), as described in the '678 and
'842 patents. As used herein, "thermal conditioning" of a
heat-exchange fluid does not include any modification of the
temperature of the heat-exchange fluid resulting from interaction
with gas with which the heat-exchange fluid is exchanging thermal
energy; rather, such thermal conditioning generally refers to the
modification of the temperature of the heat-exchange fluid by other
means (e.g., an external heat exchanger). The terms "heat-exchange"
and "heat-transfer" are generally utilized interchangeably herein.
Unless otherwise indicated, motor/pumps described herein are not
required to be configured to function both as a motor and a pump if
they are utilized during system operation only as a motor or a pump
but not both. Gas expansions described herein may be performed in
the absence of combustion (as opposed to the operation of an
internal-combustion cylinder, for example).
BRIEF DESCRIPTION OF THE DRAWINGS
[0061] In the drawings, like reference characters generally refer
to the same parts throughout the different views. Cylinders, rods,
and other components are depicted in cross section in a manner that
will be intelligible to all persons familiar with the art of
pneumatic and hydraulic cylinders. Also, the drawings are not
necessarily to scale, emphasis instead generally being placed upon
illustrating the principles of the invention. In the following
description, various embodiments of the present invention are
described with reference to the following drawings, in which:
[0062] FIG. 1 is a schematic drawing of a compressed-gas energy
storage system in accordance with various embodiments of the
invention;
[0063] FIG. 2 is a schematic drawing of various components of a
compressed-gas energy storage system in accordance with various
embodiments of the invention;
[0064] FIG. 3 is a schematic drawing of the major components of a
compressed air energy storage and recovery system in accordance
with various embodiments of the invention;
[0065] FIG. 4 is a schematic drawing of various components of a
multi-cylinder compressed-gas energy storage system in accordance
with various embodiments of the invention;
[0066] FIG. 5 is a schematic drawing of a cylinder assembly with
apparatus for the generation of foam external to the cylinder in
accordance with various embodiments of the invention;
[0067] FIG. 6 is a schematic drawing of a cylinder assembly with
apparatus for the generation of foam external to the cylinder and
provision for bypassing the foam-generating apparatus in accordance
with various embodiments of the invention;
[0068] FIG. 7 is a schematic drawing of a cylinder assembly with
apparatus for the generation of foam in a vessel external to the
cylinder in accordance with various embodiments of the
invention;
[0069] FIG. 8 is a schematic drawing of a cylinder assembly with
apparatus for the generation of foam internal to the cylinder in
accordance with various embodiments of the invention;
[0070] FIG. 9 is a schematic drawing of a compressed-air energy
storage system employing multiple pairs of high- and low-pressure
cylinders in accordance with various embodiments of the
invention;
[0071] FIG. 10 is an illustrative plot of pressure as a function of
time for four different expansion scenarios in accordance with
various embodiments of the invention;
[0072] FIG. 11 is a graphical display of experimental test data in
accordance with various embodiments of the invention;
[0073] FIG. 12 is an illustrative plot of the ideal pressure-volume
cycle in a cylinder operated as either a compressor or
expander;
[0074] FIG. 13 is an illustrative plot of cylinder chamber pressure
as a function of cylinder chamber volume for three different
expansion scenarios in an illustrative CAES system in accordance
with various embodiments of the invention;
[0075] FIGS. 14A-14C are illustrative plots of cylinder chamber
pressure as a function of cylinder chamber volume for different
expansion scenarios in an illustrative CAES system in accordance
with various embodiments of the invention;
[0076] FIG. 15 is an illustrative plot of cylinder chamber pressure
as a function of cylinder chamber volume for three different
compression scenarios in an illustrative CAES system in accordance
with various embodiments of the invention; and
[0077] FIG. 16 is an illustrative plot of cylinder chamber pressure
as a function of cylinder chamber volume for three different
compression scenarios in an illustrative CAES system in accordance
with various embodiments of the invention.
DETAILED DESCRIPTION
[0078] FIG. 1 depicts an illustrative system 100 that may be part
of a larger system, not otherwise depicted, for the storage and
release of energy. Subsequent figures will clarify the application
of embodiments of the invention to such a system. The system 100
depicted in FIG. 1 features an assembly 101 for compressing and
expanding gas. Expansion/compression assembly 101 may include or
consist essentially of either one or more individual devices for
expanding or compressing gas (e.g., turbines or cylinder assemblies
that each may house a movable boundary mechanism) or a staged
series of such devices, as well as ancillary devices (e.g., valves)
not depicted explicitly in FIG. 1.
[0079] An electric motor/generator 102 (e.g., a rotary or linear
electric machine) is in physical communication (e.g., via hydraulic
pump, piston shaft, or mechanical crankshaft) with the
expansion/compression assembly 101. The motor/generator 102 may be
electrically connected to a source and/or sink of electric energy
not explicitly depicted in FIG. 1 (e.g., an electrical distribution
grid or a source of renewable energy such as one or more wind
turbines or solar cells).
[0080] The expansion/compression assembly 101 may be in fluid
communication with a heat-transfer subsystem 104 that alters the
temperature and/or pressure of a fluid (i.e., gas, liquid, or
gas-liquid mixture such as a foam) extracted from
expansion/compression assembly 101 and, after alteration of the
fluid's temperature and/or pressure, returns at least a portion of
it to expansion/compression assembly 101. Heat-transfer subsystem
104 may include pumps, valves, and other devices (not depicted
explicitly in FIG. 1) ancillary to its heat-transfer function and
to the transfer of fluid to and from expansion/compression assembly
101. Operated appropriately, the heat-transfer subsystem 104
enables substantially isothermal compression and/or expansion of
gas inside expansion/compression assembly 101.
[0081] Connected to the expansion/compression assembly 101 is a
pipe 106 with a control valve 108 that controls a flow of fluid
(e.g., gas) between assembly 101 and a storage reservoir 112 (e.g.,
one or more pressure vessels, pipes, and/or caverns). The storage
reservoir 112 may be in fluid communication with a heat-transfer
subsystem 114 that alters the temperature and/or pressure of fluid
removed from storage reservoir 112 and, after alteration of the
fluid's temperature and/or pressure, returns it to storage
reservoir 112. A second pipe 116 with a control valve 118 may be in
fluid communication with the expansion/compression assembly 101 and
with a vent 120 that communicates with a body of gas at relatively
low pressure (e.g., the ambient atmosphere).
[0082] A control system 122 receives information inputs from any of
expansion/compression assembly 101, storage reservoir 112, and
other components of system 100 and sources external to system 100.
These information inputs may include or consist essentially of
pressure, temperature, and/or other telemetered measurements of
properties of components of system 101. Such information inputs,
here generically denoted by the letter "T," are transmitted to
control system 122 either wirelessly or through wires. Such
transmission is denoted in FIG. 1 by dotted lines 124, 126.
[0083] The control system 122 may selectively control valves 108
and 118 to enable substantially isothermal compression and/or
expansion of a gas in assembly 101. Control signals, here
generically denoted by the letter "C," are transmitted to valves
108 and 118 either wirelessly or through wires. Such transmission
is denoted in FIG. 1 by dashed lines 128, 130. The control system
122 may also control the operation of the heat-transfer assemblies
104, 114 and of other components not explicitly depicted in FIG. 1.
The transmission of control and telemetry signals for these
purposes is not explicitly depicted in FIG. 1.
[0084] The control system 122 may be any acceptable control device
with a human-machine interface. For example, the control system 122
may include a computer (for example a PC-type) that executes a
stored control application in the form of a computer-readable
software medium. More generally, control system 122 may be realized
as software, hardware, or some combination thereof. For example,
control system 122 may be implemented on one or more computers,
such as a PC having a CPU board containing one or more processors
such as the Pentium, Core, Atom, or Celeron family of processors
manufactured by Intel Corporation of Santa Clara, Calif., the 680x0
and POWER PC family of processors manufactured by Motorola
Corporation of Schaumburg, Ill., and/or the ATHLON line of
processors manufactured by Advanced Micro Devices, Inc., of
Sunnyvale, Calif. The processor may also include a main memory unit
for storing programs and/or data relating to the methods described
above. The memory may include random access memory (RAM), read only
memory (ROM), and/or FLASH memory residing on commonly available
hardware such as one or more application specific integrated
circuits (ASIC), field programmable gate arrays (FPGA),
electrically erasable programmable read-only memories (EEPROM),
programmable read-only memories (PROM), programmable logic devices
(PLD), or read-only memory devices (ROM). In some embodiments, the
programs may be provided using external RAM and/or ROM such as
optical disks, magnetic disks, or other storage devices.
[0085] For embodiments in which the functions of controller 122 are
provided by software, the program may be written in any one of a
number of high-level languages such as FORTRAN, PASCAL, JAVA, C,
C++, C#, LISP, PERL, BASIC or any suitable programming language.
Additionally, the software can be implemented in an assembly
language and/or machine language directed to the microprocessor
resident on a target device.
[0086] As described above, the control system 122 may receive
telemetry from sensors monitoring various aspects of the operation
of system 100, and may provide signals to control valve actuators,
valves, motors, and other electromechanical/electronic devices.
Control system 122 may communicate with such sensors and/or other
components of system 100 (and other embodiments described herein)
via wired or wireless communication. An appropriate interface may
be used to convert data from sensors into a form readable by the
control system 122 (such as RS-232 or network-based interconnects).
Likewise, the interface converts the computer's control signals
into a form usable by valves and other actuators to perform an
operation. The provision of such interfaces, as well as suitable
control programming, is clear to those of ordinary skill in the art
and may be provided without undue experimentation.
[0087] System 100 may be operated so as to compress gas admitted
through the vent 120 and store the gas thus compressed in reservoir
112. For example, in an initial state of operation, valve 108 is
closed and valve 118 is open, admitting a quantity of gas into
expansion/compression assembly 101. When a desired quantity of gas
has been admitted into assembly 101, valve 118 may be closed. The
motor/generator 102, employing energy supplied by a source not
explicitly depicted in FIG. 1 (e.g., the electrical grid), then
provides mechanical power to expansion/compression assembly 101,
enabling the gas within assembly 101 to be compressed.
[0088] During compression of the gas within assembly 101, fluid
(i.e., gas, liquid, or a gas-liquid mixture) may be circulated
between assembly 101 and heat-exchange assembly 104. Heat-exchange
assembly 104 may be operated in such a manner as to enable
substantially isothermal compression of the gas within assembly
101. During or after compression of the gas within assembly 101,
valve 108 may be opened to enable high-pressure fluid (e.g.,
compressed gas or a mixture of liquid and compressed gas) to flow
to reservoir 112. Heat-exchange assembly 114 may be operated at any
time in such a manner as alter the temperature and/or pressure of
the fluid within reservoir 112.
[0089] That system 100 may also be operated so as to expand
compressed gas from reservoir 112 in expansion/compression assembly
101 in such a manner as to deliver energy to the motor/generator
102 will be apparent to all persons familiar with the operation of
pneumatic, hydraulic, and electric machines.
[0090] FIG. 2 depicts an illustrative system 200 that features a
cylinder assembly 201 (i.e., an embodiment of assembly 101 in FIG.
1) in communication with a reservoir 222 (112 in FIG. 1) and a vent
to atmosphere 223 (120 in FIG. 1). In the illustrative system 200
shown in FIG. 2, the cylinder assembly 201 contains a piston 202
slidably disposed therein. In some embodiments the piston 202 is
replaced by a different boundary mechanism dividing cylinder
assembly 201 into multiple chambers, or piston 202 is absent
entirely, and cylinder assembly 201 is a "liquid piston." The
cylinder assembly 201 may be divided into, e.g., two pneumatic
chambers or one pneumatic chamber and one hydraulic chamber. The
piston 202 is connected to a rod 204, which may contain a
center-drilled fluid passageway with fluid outlet 212 extending
from the piston 202. The rod 204 is also attached to, e.g., a
mechanical load (e.g., a crankshaft or a hydraulic system) that is
not depicted. The cylinder assembly 201 is in liquid communication
with a heat-transfer subsystem 224 that includes or consists
essentially of a circulation pump 214 and a spray mechanism 210 to
enable substantially isothermal compression/expansion of gas.
Heat-transfer fluid circulated by pump 214 may be passed through a
heat exchanger 203 (e.g., tube-in-shell- or parallel-plate-type
heat exchanger). Spray mechanism 210 may include or consist
essentially of one or more spray heads (e.g., disposed at one end
of cylinder assembly 201) and/or spray rods (e.g., extending along
at least a portion of the central axis of cylinder assembly 201).
In other embodiments, the spray mechanism 210 is omitted and a
foam, rather than a spray of droplets, is created to facilitate
heat exchange between liquid and gas during compression and
expansion of gas within the cylinder assembly 201, as described in
U.S. patent application Ser. No. 13/473,128, filed May 16, 2012
(the '128 application), the entire disclosure of which is
incorporated by reference herein. Foam may be generated by foaming
gas with heat-exchange liquid in a mechanism (not shown, described
in more detail below) external to the cylinder assembly 201 and
then injecting the resulting foam into the cylinder assembly 201.
Alternatively or additionally, foam may be generated inside the
cylinder assembly 201 by the injection of heat-exchange liquid into
cylinder assembly 201 through a foam-generating mechanism (e.g.,
spray head, rotating blade, one or more nozzles), partly or
entirely filling the pneumatic chamber of cylinder assembly 201. In
some embodiments, droplets and foams may be introduced into
cylinder assembly 201 simultaneously and/or sequentially. Various
embodiments may feature mechanisms (not shown in FIG. 2) for
controlling the characteristics of foam (e.g., bubble size) and for
breaking down, separating, and/or regenerating foam.
[0091] System 200 further includes a first control valve 220 (108
in FIG. 1) in communication with a storage reservoir 222 and
cylinder assembly 201, and a second control valve 221 (118 in FIG.
1) in communication with a vent 223 and cylinder assembly 201. A
control system 226 (122 in FIG. 1) may control operation of, e.g.,
valves 222 and 221 based on various system inputs (e.g., pressure,
temperature, piston position, and/or fluid state) from cylinder
assembly 201 and/or storage reservoir 222. Heat-transfer fluid
(liquid or circulated by pump 214 enters through pipe 213. Pipe 213
may be (a) connected to a low-pressure fluid source (e.g., fluid
reservoir (not shown) at the pressure to which vent 223 is
connected or thermal well 242); (b) connected to a high-pressure
source (e.g., fluid reservoir (not shown) at the pressure of
reservoir 222); (c) selectively connected (using valve arrangement
not shown) to low pressure during a compression process and to high
pressure during an expansion process; (d) connected to
changing-pressure fluid 208 in the cylinder 201 via connection 212;
or (e) some combination of these options.
[0092] In an initial state, the cylinder assembly 201 may contain a
gas 206 (e.g., air introduced to the cylinder assembly 201 via
valve 221 and vent 223) and a heat-transfer fluid 208 (which may
include or consist essentially of, e.g., water or another suitable
liquid). When the gas 206 enters the cylinder assembly 201, piston
202 is operated to compress the gas 206 to an elevated pressure
(e.g., approximately 3,000 psi). Heat-transfer fluid (not
necessarily the identical body of heat-transfer fluid 208) flows
from pipe 213 to the pump 214. The pump 214 may raise the pressure
of the heat-exchange fluid to a pressure (e.g., up to approximately
3,015 psig) somewhat higher than the pressure within the cylinder
assembly 201, as described in the '409 application. Alternatively
or in conjunction, embodiments of the invention add heat (i.e.,
thermal energy) to, or remove heat from, the high-pressure gas in
the cylinder assembly 201 by passing only relatively low-pressure
fluids through a heat exchanger or fluid reservoir, as detailed in
U.S. patent application Ser. No. 13/211,440, filed Aug. 17, 2011
(the '440 application), the entire disclosure of which is
incorporated by reference herein. Heat-transfer fluid is then sent
through a pipe 216, where it may be passed through a heat exchanger
203 (where its temperature is altered) and then through a pipe 218
to the spray mechanism 210. The heat-transfer fluid thus circulated
may include or consist essentially of liquid or foam. Spray
mechanism 210 may be disposed within the cylinder assembly 201, as
shown; located in the storage reservoir 222 or vent 223; or located
in piping or manifolding around the cylinder assembly, such as pipe
218 or the pipes connecting the cylinder assembly to storage
reservoir 222 or vent 223. The spray mechanism 210 may be operated
in the vent 223 or connecting pipes during compression, and a
separate spray mechanism may be operated in the storage reservoir
222 or connecting pipes during expansion. Heat-transfer spray 211
from spray mechanism 210 (and/or any additional spray mechanisms),
and/or foam from mechanisms internal or external to the cylinder
assembly 101, enable substantially isothermal compression of gas
206 within cylinder assembly 201.
[0093] In some embodiments, the heat exchanger 203 is configured to
condition heat-transfer fluid at low pressure (e.g., a pressure
lower than the maximum pressure of a compression or expansion
stroke in cylinder assembly 201), and heat-transfer fluid is
thermally conditioned between strokes or only during portions of
strokes, as detailed in the '440 application. Embodiments of the
invention are configured for circulation of heat-transfer fluid
without the use of hoses that flex during operation through the use
of, e.g., tubes or straws configured for non-flexure and/or pumps
(e.g., submersible bore pumps, axial flow pumps, or other in-line
style pumps) internal to the cylinder assembly (e.g., at least
partially disposed within the piston rod thereof), as described in
U.S. patent application Ser. No. 13/234,239, filed Sep. 16, 2011
(the '239 application), the entire disclosure of which is
incorporated by reference herein.
[0094] At or near the end of the compression stroke, control system
226 opens valve 220 to admit the compressed gas 206 to the storage
reservoir 222. Operation of valves 220 and 221 may be controlled by
various inputs to control system 226, such as piston position in
cylinder assembly 201, pressure in storage reservoir 222, pressure
in cylinder assembly 201, and/or temperature in cylinder assembly
201.
[0095] As mentioned above, the control system 226 may enforce
substantially isothermal operation, i.e., expansion and/or
compression of gas in cylinder assembly 201, via control over,
e.g., the introduction of gas into and the exhausting of gas out of
cylinder assembly 201, the rates of compression and/or expansion,
and/or the operation of the heat-exchange subsystem in response to
sensed conditions. For example, control system 226 may be
responsive to one or more sensors disposed in or on cylinder
assembly 201 for measuring the temperature of the gas and/or the
heat-exchange fluid within cylinder assembly 201, responding to
deviations in temperature by issuing control signals that operate
one or more of the system components noted above to compensate, in
real time, for the sensed temperature deviations. For example, in
response to a temperature increase within cylinder assembly 201,
control system 226 may issue commands to increase the flow rate of
spray 211 of heat-exchange fluid 208.
[0096] Furthermore, embodiments of the invention may be applied to
systems in which cylinder assembly 201 (or a chamber thereof) is in
fluid communication with a pneumatic chamber of a second cylinder
(e.g., as shown in FIG. 4). That second cylinder, in turn, may
communicate similarly with a third cylinder, and so forth. Any
number of cylinders may be linked in this way. These cylinders may
be connected in parallel or in a series configuration, where the
compression and expansion is done in multiple stages. The fluid
circuit of heat exchanger 203 may be filled with water, a coolant
mixture, an aqueous foam, or any other acceptable heat-exchange
medium. In alternative embodiments, a gas, such as air or
refrigerant, is used as the heat-exchange medium. In general, the
fluid is routed by conduits to a large reservoir of such fluid in a
closed or open loop. One example of an open loop is a well or body
of water from which ambient water is drawn and the exhaust water is
delivered to a different location, for example, downstream in a
river. In a closed-loop embodiment, a cooling tower may cycle the
water through the air for return to the heat exchanger. Likewise,
water may pass through a submerged or buried coil of continuous
piping where a counter heat-exchange occurs to return the fluid
flow to ambient temperature before it returns to the heat exchanger
for another cycle.
[0097] In various embodiments, the heat-exchange fluid is
conditioned (i.e., pre-heated and/or pre-chilled) or used for
heating or cooling needs by connecting the fluid inlet 238 and
fluid outlet 240 of the external heat-exchange side of the heat
exchanger 203 to an installation such as a heat-engine power plant,
an industrial process with waste heat, a heat pump, and/or a
building needing space heating or cooling, as described in the '731
patent. Alternatively, the external heat-exchange side of the heat
exchanger 203 may be connected to a thermal well 242 as depicted in
FIG. 2. The thermal well 242 may include or consist essentially of
a large water reservoir that acts as a constant-temperature thermal
fluid source for use with the system. Alternatively, the water
reservoir may be thermally linked to waste heat from an industrial
process or the like, as described above, via another heat exchanger
contained within the installation. This allows the heat-exchange
fluid to acquire or expel heat from/to the linked process,
depending on configuration, for later use as a heating/cooling
medium in the energy storage/conversion system. Alternatively, the
thermal well 242 may include two or more bodies of energy-storage
medium, e.g., a hot-water thermal well and a cold-water thermal
well, that are typically maintained in contrasting energy states in
order to increase the exergy of system 200 compared with a system
in which thermal well 242 includes a single body of energy-storage
medium. Storage media other than water may be utilized in the
thermal well 242; temperature changes, phase changes, or both may
be employed by storage media of thermal well 242 to store and
release energy. Thermal or fluid links (not shown) to the
atmosphere, ground, and/or other components of the environment may
also be included in system 200, allowing mass, thermal energy, or
both to be added to or removed from the thermal well 242. Moreover,
as depicted in FIG. 2, the heat-transfer subsystem 224 does not
interchange fluid directly with the thermal well 242, but in other
embodiments, fluid is passed directly between the heat-transfer
subsystem 224 and the thermal well 242 with no heat exchanger
maintaining separation between fluids.
[0098] FIG. 3 is a schematic of the major components of an
illustrative system 300 that employs a pneumatic cylinder 302 to
efficiently convert (i.e., store) mechanical energy into the
potential energy of compressed gas and, in another mode of
operation, efficiently convert (i.e., recover) the potential energy
of compressed gas into mechanical work. The pneumatic cylinder 302
may contain a slidably disposed piston 304 that divides the
interior of the cylinder 302 into a distal chamber 306 and a
proximal chamber 308. A port or ports (not shown) with associated
pipes 312 and a bidirectional valve 316 enables gas from a
high-pressure storage reservoir 320 to be admitted to chamber 306
as desired. A port or ports (not shown) with associated pipes 322
and a bidirectional valve 324 enables gas from the chamber 306 to
be exhausted through a vent 326 to the ambient atmosphere as
desired. In alternate embodiments, vent 326 is replaced by
additional lower-pressure pneumatic cylinders (or pneumatic
chambers of cylinders). A port or ports (not shown) enables the
interior of the chamber 308 to communicate freely at all times with
the ambient atmosphere. In alternate embodiments, cylinder 302 is
double-acting and chamber 308 is, like chamber 306, equipped to
admit and exhaust fluids in various states of operation. The distal
end of a rod 330 is coupled to the piston 304. The rod 330 may be
connected to a crankshaft, hydraulic cylinder, or other mechanisms
for converting linear mechanical motion to useful work as described
in the '678 and '842 patents.
[0099] In the energy recovery or expansion mode of operation,
storage reservoir 320 is filled with high-pressure air (or other
gas) 332 and a quantity of heat-transfer fluid 334.
[0100] The heat-transfer fluid 334 may be an aqueous foam or a
liquid that tends to foam when sprayed or otherwise acted upon. The
liquid component of the aqueous foam, or the liquid that tends to
foam, may include or consist essentially of water with 2% to 5% of
certain additives; these additives may also provide functions of
anti-corrosion, anti-wear (lubricity), anti-biogrowth (biocide),
freezing-point modification (anti-freeze), and/or surface-tension
modification. Additives may include a micro-emulsion of a
lubricating fluid such as mineral oil, a solution of agents such as
glycols (e.g. propylene glycol), or soluble synthetics (e.g.
ethanolamines). Such additives tend to reduce liquid surface
tension and lead to substantial foaming when sprayed. Commercially
available fluids may be used at an approximately 5% solution in
water, such as Mecagreen 127 (available from the Condat Corporation
of Michigan), which consists in part of a micro-emulsion of mineral
oil, and Quintolubric 807-WP (available from the Quaker Chemical
Corporation of Pennsylvania), which consists in part of a soluble
ethanolamine. Other additives may be used at higher concentrations
(such as at a 50% solution in water), including Cryo-tek 100/Al
(available from the Hercules Chemical Company of New Jersey), which
consists in part of a propylene glycol. These fluids may be further
modified to enhance foaming while being sprayed and to speed
defoaming when in a reservoir.
[0101] The heat-transfer fluid 334 may be circulated within the
storage reservoir 320 via high-inlet-pressure,
low-power-consumption pump 336 (such as described in the '731
patent). In various embodiments, the fluid 334 may be removed from
the bottom of the storage reservoir 320 via piping 338, circulated
via pump 336 through a heat exchanger 340, and introduced (e.g.,
sprayed) back into the top of storage reservoir 320 via piping 342
and spray head 344 (or other suitable mechanism). Any changes in
pressure within reservoir 320 due to removal or addition of gas
(e.g., via pipe 312) generally tend to result in changes in
temperature of the gas 332 within reservoir 320. By spraying and/or
foaming the fluid 334 throughout the storage reservoir gas 332,
heat may be added to or removed from the gas 332 via heat exchange
with the heat-transfer fluid 334. By circulating the heat-transfer
fluid 334 through heat exchanger 340, the temperature of the fluid
334 and gas 332 may be kept substantially constant (i.e.,
isothermal). Counterflow heat-exchange fluid 346 at near-ambient
pressure may be circulated from a near-ambient-temperature thermal
well (not shown) or source (e.g., waste heat source) or sink (e.g.,
cold water source) of thermal energy, as described in more detail
below.
[0102] In various embodiments of the invention, reservoir 320
contains an aqueous foam, either unseparated or partially separated
into its gaseous and liquid components. In such embodiments, pump
336 may circulate either the foam itself, or the separated liquid
component of the foam, or both, and recirculation of fluid into
reservoir 320 may include regeneration of foam by apparatus not
shown in FIG. 3.
[0103] In the energy recovery or expansion mode of operation, a
quantity of gas may be introduced via valve 316 and pipe 312 into
the upper chamber 306 of cylinder 302 when piston 304 is near or at
the top of its stroke (i.e., "top dead center" of cylinder 302).
The piston 304 and its rod 330 will then be moving downward (the
cylinder 302 may be oriented arbitrarily but is shown vertically
oriented in this illustrative embodiment). Heat-exchange fluid 334
may be introduced into chamber 306 concurrently via optional pump
350 (alternatively, a pressure drop may be introduced in line 312
such that pump 350 is not needed) through pipe 352 and directional
valve 354. This heat-exchange fluid 334 may be sprayed into chamber
306 via one or more spray nozzles 356 in such a manner as to
generate foam 360. (In some embodiments, foam 360 is introduced
directly into chamber 306 in foam form.) The foam 360 may entirely
fill the entire chamber 306, but is shown in FIG. 3, for
illustrative purposes only, as only partially filling chamber 306.
Herein, the term "foam" denotes either (a) foam only or (b) any of
a variety of mixtures of foam and heat-exchange liquid in other,
non-foaming states (e.g., droplets). Moreover, some non-foamed
liquid (not shown) may accumulate at the bottom of chamber 306; any
such liquid is generally included in references herein to the foam
360 within chamber 306.
[0104] System 300 is instrumented with pressure, piston position,
and/or temperature sensors (not shown) and controlled via control
system 362. At a predetermined position of piston 304, an amount of
gas 332 and heat-transfer fluid 334 have been admitted into chamber
306 and valve 316 and valve 354 are closed. (Valves 316 and 354 may
close at the same time or at different times, as each has a control
value based on quantity of fluid desired.) The gas in chamber 306
then undergoes free expansion, continuing to drive piston 304
downward. During this expansion, in the absence of foam 360, the
gas would tend to decrease substantially in temperature. With foam
360 largely or entirely filling the chamber, the temperature of the
gas in chamber 306 and the temperature of the heat-transfer fluid
360 tend to approximate to each other via heat exchange. The heat
capacity of the liquid component of the foam 360 (e.g., water with
one or more additives) may be much higher than that of the gas
(e.g., air) such that the temperature of the gas and liquid do not
change substantially (i.e., are substantially isothermal) even over
a many-times gas expansion (e.g., from 250 psig to near atmospheric
pressure, or in other embodiments from 3,000 psig to 250 psig).
[0105] When the piston 304 reaches the end of its stroke (bottom
dead center), the gas within chamber 306 will have expanded to a
predetermined lower pressure (e.g., near atmospheric). Valve 324
will then be opened, allowing gas from chamber 306 to be vented,
whether to atmosphere through pipe 322 and vent 326 (as illustrated
here) or, in other embodiments, to a next stage in the expansion
process (e.g., chamber in a separate cylinder), via pipe 322. Valve
324 remains open as the piston undergoes an upward (i.e., return)
stroke, emptying chamber 306. Part or substantially all of foam 360
is also forced out of chamber 306 via pipe 322. A separator (not
shown) or other means such as gravity separation is used to recover
heat-transfer fluid, preferably de-foamed (i.e., as a simple liquid
with or without additives), and to direct it into a storage
reservoir 364 via pipe 366.
[0106] When piston 304 reaches top of stroke again, the process
repeats with gas 332 and heat-transfer fluid 334 admitted from
vessel 320 via valves 316 and 354. If additional heat-transfer
fluid is needed in reservoir 320, it may be pumped back into
reservoir 320 from reservoir 364 via piping 367 and optional
pump/motor 368. In one mode of operation, pump 368 may be used to
continuously refill reservoir 320 such that the pressure in
reservoir 320 is held substantially constant. That is, as gas is
removed from reservoir 320, heat-transfer fluid 334 is added to
maintain constant pressure in reservoir 320. In other embodiments,
pump 368 is not used or is used intermittently, the pressure in
reservoir 320 continues to decrease during an energy-recovery
process (i.e., involving removal of gas from reservoir 320), and
the control system 362 changes the timing of valves 316 and 354
accordingly so as to reach approximately the same ending pressure
when the piston 304 reaches the end of its stroke. An
energy-recovery process may continue until the storage reservoir
320 is nearly empty of pressurized gas 332, at which time an
energy-storage process may be used to recharge the storage
reservoir 320 with pressurized gas 332. In other embodiments, the
energy-recovery and energy-storage processes are alternated based
on operator requirements.
[0107] In either the energy-storage or energy-compression mode of
operation, storage reservoir 320 is typically at least partially
depleted of high-pressure gas 332, as storage reservoir 320 also
typically contains a quantity of heat-transfer fluid 334. Reservoir
364 is at low pressure (e.g., atmospheric or some other low
pressure that serves as the intake pressure for the compression
phase of cylinder 302) and contains a quantity of heat-transfer
fluid 370.
[0108] The heat-transfer fluid 370 may be circulated within the
reservoir 364 via low-power-consumption pump 372. In various
embodiments, the fluid 370 may be removed from the bottom of the
reservoir 364 via piping 367, circulated via pump 372 through a
heat exchanger 374, and introduced (e.g., sprayed) back into the
top of reservoir 364 via piping 376 and spray head 378 (or other
suitable mechanism). By spraying the fluid 370 throughout the
reservoir gas 380, heat may be added or removed from the gas via
the heat-transfer fluid 370. By circulating the heat-transfer fluid
370 through heat exchanger 374, the temperature of the fluid 370
and gas 380 may be kept near constant (i.e., isothermal).
Counterflow heat-exchange fluid 382 at near-ambient pressure may be
circulated from a near-ambient-temperature thermal well (not shown)
or source (e.g., waste heat source) or sink (e.g., cold water
source) of thermal energy. In one embodiment, counterflow
heat-exchange fluid 382 is at high temperature to increase energy
recovery during expansion and/or counterflow heat-exchange fluid
382 is at low temperature to decrease energy usage during
compression.
[0109] In the energy-storage or compression mode of operation, a
quantity of low-pressure gas is introduced via valve 324 and pipe
322 into the upper chamber 306 of cylinder 302 starting when piston
304 is near top dead center of cylinder 302. The low-pressure gas
may be from the ambient atmosphere (e.g., may be admitted through
vent 326 as illustrated herein) or may be from a source of
pressurized gas such as a previous compression stage. During the
intake stroke, the piston 304 and its rod 330 will move downward,
drawing in gas. Heat-exchange fluid 370 may be introduced into
chamber 306 concurrently via optional pump 384 (alternatively, a
pressure drop may be introduced in line 386 such that pump 384 is
not needed) through pipe 386 and directional valve 388.
[0110] This heat exchange fluid 370 may be introduced (e.g.,
sprayed) into chamber 306 via one or more spray nozzles 390 in such
a manner as to generate foam 360. This foam 360 may fill the
chamber 306 partially or entirely by the end of the intake stroke;
for illustrative purposes only, foam 360 is shown in FIG. 3 as only
partially filling chamber 306. At the end of the intake stroke,
piston 304 reaches the end-of-stroke position (bottom dead center)
and chamber 306 is filled with foam 360 generated from air at a low
pressure (e.g., atmospheric) and heat-exchange liquid.
[0111] At the end of the stroke, with piston 304 at the
end-of-stroke position, valve 324 is closed. Valve 388 is also
closed, not necessarily at the same time as valve 324, but after a
predetermined amount of heat-transfer fluid 370 has been admitted,
creating foam 360. The amount of heat-transfer fluid 370 may be
based upon the volume of air to be compressed, the ratio of
compression, and/or the heat capacity of the heat-transfer fluid.
Next, piston 304 and rod 330 are driven upwards via mechanical
means (e.g., hydraulic fluid, hydraulic cylinder, mechanical
crankshaft) to compress the gas within chamber 306.
[0112] During this compression, in the absence of foam 360, the gas
in chamber 306 would tend to increase substantially in temperature.
With foam 360 at least partially filling the chamber, the
temperature of the gas in chamber 306 and the temperature of the
liquid component of foam 360 will tend to equilibrate via heat
exchange. The heat capacity of the fluid component of foam 360
(e.g., water with one or more additives) may be much higher than
that of the gas (e.g., air) such that the temperature of the gas
and fluid do not change substantially and are near-isothermal even
over a many-times gas compression (e.g., from near atmospheric
pressure to 250 psig, or in other embodiments from 250 psig to
3,000 psig).
[0113] The gas in chamber 306 (which includes, or consists
essentially of, the gaseous component of foam 360) is compressed to
a suitable pressure, e.g., a pressure approximately equal to the
pressure within storage reservoir 320, at which time valve 316 is
opened. The foam 360, including both its gaseous and liquid
components, is then transferred into storage reservoir 320 through
valve 316 and pipe 312 by continued upward movement of piston 304
and rod 330.
[0114] When piston 304 reaches top of stroke again, the process
repeats, with low-pressure gas and heat-transfer fluid 370 admitted
from vent 326 and reservoir 364 via valves 324 and 388. If
additional heat-transfer fluid is needed in reservoir 364, it may
be returned to reservoir 364 from reservoir 320 via piping 367 and
optional pump/motor 368. Power recovered from motor 368 may be used
to help drive the mechanical mechanism for driving piston 304 and
rod 330 or may be converted to electrical power via an electric
motor/generator (not shown). In one mode of operation, motor 368
may be run continuously, while reservoir 320 is being filled with
gas, in such a manner that the pressure in reservoir 320 is held
substantially constant. That is, as gas is added to reservoir 320,
heat-transfer fluid 334 is removed from reservoir 320 to maintain
substantially constant pressure within reservoir 320. In other
embodiments, motor 368 is not used or is used intermittently; the
pressure in reservoir 320 continues to increase during an
energy-storage process and the control system 362 changes the
timing of valves 316 and 388 accordingly so that the desired ending
pressure (e.g., atmospheric) is attained within chamber 306 when
the piston 304 reaches bottom of stroke. An energy-storage process
may continue until the storage reservoir 320 is full of pressurized
gas 332 at the maximum storage pressure (e.g., 3,000 psig), after
which time the system is ready to perform an energy-recovery
process. In various embodiments, the system may commence an
energy-recovery process when the storage reservoir 320 is only
partly full of pressurized gas 332, whether at the maximum storage
pressure or at some storage pressure intermediate between
atmospheric pressure and the maximum storage pressure. In other
embodiments, the energy-recovery and energy-storage processes are
alternated based on operator requirements.
[0115] FIG. 4 depicts an illustrative system 400 that features at
least two cylinder assemblies 402, 406 (i.e., an embodiment of
assembly 101 in FIG. 1; e.g., cylinder assembly 201 in FIG. 2) and
a heat-transfer subsystem 404, 408 (e.g., subsystem 224 in FIG. 2)
associated with each cylinder assembly 402, 406. Additionally, the
system includes a thermal well 410 (e.g., thermal well 242 in FIG.
2) which may be associated with either or both of the heat-transfer
subsystems 404, 408 as indicated by the dashed lines.
[0116] Assembly 402 is in selective fluid communication with a
storage reservoir 412 (e.g., 112 in FIG. 1, 222 in FIG. 2) capable
of holding fluid at relatively high pressure (e.g., approximately
3,000 psig). Assembly 406 is in selective fluid communication with
assembly 402 and/or with optional additional cylinder assemblies
between assemblies 402 and 406 as indicated by ellipsis marks 422.
Assembly 406 is in selective fluid communication with an
atmospheric vent 420 (e.g., 120 in FIG. 1, 223 in FIG. 2). System
400 may compress air at atmospheric pressure (admitted to system
400 through the vent 420) stagewise through assemblies 406 and 402
to high pressure for storage in reservoir 412. System 400 may also
expand air from high pressure in reservoir 412 stagewise through
assemblies 402 and 406 to a low pressure (e.g., approximately 5
psig) for venting to the atmosphere through vent 420.
[0117] As described in U.S. Pat. No. 8,191,362, filed Apr. 6, 2011
(the '362 patent), the entire disclosure of which is incorporated
by reference herein, in a group of N cylinder assemblies used for
expansion or compression of gas between a high pressure (e.g.,
approximately 3,000 psig) and a low pressure (e.g., approximately 5
psig), the system will contain gas at N-1 pressures intermediate
between the high-pressure extreme and the low pressure. Herein each
such intermediate pressure is termed a "mid-pressure." In
illustrative system 400, N=2 and N-1=1, so there is one
mid-pressure (e.g., approximately 250 psig during expansion) in the
system 400. In various states of operation of the system,
mid-pressures may occur in any of the chambers of a
series-connected cylinder group (e.g., the cylinders of assemblies
402 and 406) and within any valves, piping, and other devices in
fluid communication with those chambers. In illustrative system
400, the mid-pressure, herein denoted "mid-pressure P1," occurs
primarily in valves, piping, and other devices intermediate between
assemblies 402 and 406.
[0118] Assembly 402 is a high-pressure assembly: i.e., assembly 402
may admit gas at high pressure from reservoir 412 to expand the gas
to mid-pressure P1 for transfer to assembly 402, and/or may admit
gas at mid-pressure P1 from assembly 406 to compress the gas to
high pressure for transfer to reservoir 412. Assembly 406 is a
low-pressure assembly: i.e., assembly 406 may admit gas at
mid-pressure P1 from assembly 402 to expand the gas to low pressure
for transfer to the vent 420, and/or may admit gas at low pressure
from vent 420 to compress the gas to mid-pressure P1 for transfer
to assembly 402.
[0119] In system 400, extended cylinder assembly 402 communicates
with extended cylinder assembly 406 via a mid-pressure assembly
414. Herein, a "mid-pressure assembly" includes or consists
essentially of a reservoir of gas that is placed in fluid
communication with the valves, piping, chambers, and other
components through or into which gas passes. The gas in the
reservoir is at approximately at the mid-pressure which the
particular mid-pressure assembly is intended to provide. The
reservoir is large enough so that a volume of mid-pressure gas
approximately equal to that within the valves, piping, chambers,
and other components with which the reservoir is in fluid
communication may enter or leave the reservoir without
substantially changing its pressure. Additionally, the mid-pressure
assembly may provide pulsation damping, additional heat-transfer
capability, fluid separation, and/or house one or more
heat-transfer sub-systems such as part or all of sub-systems 404
and/or 408. As described in the '362 patent, a mid-pressure
assembly may substantially reduce the amount of dead space in
various components of a system employing pneumatic cylinder
assemblies, e.g., system 400 in FIG. 4. Reduction of dead space
tends to increase overall system efficiency.
[0120] Alternatively or in conjunction, pipes and valves (not shown
in FIG. 4) bypassing mid-pressure assembly 414 may enable fluid to
pass directly between assembly 402 and assembly 406. Valves 416,
418, 424, and 426 control the passage of fluids between the
assemblies 402, 406, 412, and 414.
[0121] A control system 428 (e.g., 122 in FIG. 1, 226 in FIG. 2,
362 in FIG. 3) may control operation of, e.g., all valves of system
400 based on various system inputs (e.g., pressure, temperature,
piston position, and/or fluid state) from assemblies 402 and 406,
mid-pressure assembly 414, storage reservoir 412, thermal well 410,
heat transfer sub-systems 404, 408, and/or the environment
surrounding system 420.
[0122] It will be clear to persons reasonably familiar with the art
of pneumatic machines that a system similar to system 400 but
differing by the incorporation of one, two or more mid-pressure
extended cylinder assemblies may be devised without additional
undue experimentation. It will also be clear that all remarks
herein pertaining to system 400 may be applied to such an
N-cylinder system without substantial revision, as indicated by
elliptical marks 422. Such N-cylinder systems, though not discussed
further herein, are contemplated and within the scope of the
invention. As shown and described in the '678 patent, N
appropriately sized cylinders, where N.gtoreq.2, may reduce an
original (single-cylinder) operating fluid pressure range R to
R.sup.1/N and correspondingly reduce the range of force acting on
each cylinder in the N-cylinder system as compared to the range of
force acting in a single-cylinder system. This and other
advantages, as set forth in the '678 patent, may be realized in
N-cylinder systems. Additionally, multiple identical cylinders may
be added in parallel and attached to a common or separate drive
mechanism (not shown) with the cylinder assemblies 402, 406 as
indicated by ellipsis marks 432, 436, enabling higher power and
air-flow rates.
[0123] FIG. 5 is a schematic diagram showing components of a system
500 for achieving approximately isothermal compression and
expansion of a gas for energy storage and recovery using a
pneumatic cylinder 502 (shown in partial cross-section) according
to embodiments of the invention. The cylinder 502 typically
contains a slidably disposed piston 504 that divides the cylinder
502 into two chambers 506, 508. A reservoir 510 contains gas at
high pressure (e.g., 3,000 psi); the reservoir 510 may also contain
a quantity of heat-exchange liquid 512. The heat-exchange liquid
512 may contain an additive that increases the liquid's tendency to
foam (e.g., by lowering the surface tension of the liquid 512).
Additives may include surfactants (e.g., sulfonates), a
micro-emulsion of a lubricating fluid such as mineral oil, a
solution of agents such as glycols (e.g., propylene glycol), or
soluble synthetics (e.g., ethanolamines). Foaming agents such as
sulfonates (e.g., linear alkyl benzene sulfonate such as Bio-Soft
D-40 available from Stepan Company of Illinois) may be added, or
commercially available foaming concentrates such as firefighting
foam concentrates (e.g., fluorosurfactant products such as those
available from ChemGuard of Texas) may be used. Such additives tend
to reduce liquid surface tension of water and lead to substantial
foaming when sprayed. Commercially available fluids may be used at
an approximately 5% solution in water, such as Mecagreen 127
(available from the Condat Corporation of Michigan), which consists
in part of a micro-emulsion of mineral oil, and Quintolubric 807-WP
(available from the Quaker Chemical Corporation of Pennsylvania),
which consists in part of a soluble ethanolamine. Other additives
may be used at higher concentrations (such as at a 50% solution in
water), including Cryo-tek 100/Al (available from the Hercules
Chemical Company of New Jersey), which consists in part of a
propylene glycol. These fluids may be further modified to enhance
foaming while being sprayed and to speed defoaming when in a
reservoir.
[0124] A pump 514 and piping 516 may convey the heat-exchange
liquid to a device herein termed a "mixing chamber" (518). Gas from
the reservoir 510 may also be conveyed (via piping 520) to the
mixing chamber 518. Within the mixing chamber 518, a
foam-generating mechanism 522 combines the gas from the reservoir
510 and the liquid conveyed by piping 516 to create foam 524 of a
certain grade (i.e., bubble size variance, average bubble size,
void fraction), herein termed Foam A, inside the mixing chamber
518.
[0125] The mixing chamber 518 may contain a screen 526 or other
mechanism (e.g., source of ultrasound) to vary or homogenize foam
structure. Screen 526 may be located, e.g., at or near the exit of
mixing chamber 518. Foam that has passed through the screen 526 may
have a different bubble size and other characteristics from Foam A
and is herein termed Foam B (528). In other embodiments, the screen
526 is omitted, so that Foam A is transferred without deliberate
alteration to chamber 506.
[0126] The exit of the mixing chamber 518 is connected by piping
530 to a port in the cylinder 502 that is gated by a valve 532
(e.g., a poppet-style valve) that permits fluid from piping 530 to
enter the upper chamber (air chamber) 506 of the cylinder 502.
Valves (not shown) may control the flow of gas from the reservoir
510 through piping 520 to the mixing chamber 518, and from the
mixing chamber 518 through piping 528 to the upper chamber 506 of
the cylinder 502. Another valve 534 (e.g., a poppet-style valve)
permits the upper chamber 506 to communicate with other components
of the system 500, e.g., an additional separator device (not
shown), the upper chamber of another cylinder (not shown), or a
vent to the ambient atmosphere (not shown).
[0127] The volume of reservoir 510 may be large (e.g., at least
approximately four times larger) relative to the volume of the
mixing chamber 518 and cylinder 502. Foam A and Foam B are
preferably statically stable foams over a portion or all of the
time-scale of typical cyclic operation of system 500: e.g., for a
120 RPM system (i.e., 0.5 seconds per revolution), the foam may
remain substantially unchanged (e.g., less than 10% drainage) after
5.5 seconds or a time approximately five times greater than the
revolution time.
[0128] In an initial state of operation of a procedure whereby gas
stored in the reservoir 510 is expanded to release energy, the
valve 532 is open, the valve 534 is closed, and the piston 504 is
near top dead center of cylinder 502 (i.e., toward the top of the
cylinder 502). Gas from the reservoir 510 is allowed to flow
through piping 520 to the mixing chamber 518 while liquid from the
reservoir 510 is pumped by pump 514 to the mixing chamber 518. The
gas and liquid thus conveyed to the mixing chamber 518 are combined
by the foam-generating mechanism 522 to form Foam A (524), which
partly or substantially fills the main chamber of the mixing
chamber 518. Exiting the mixing chamber 518, Foam A passes through
the screen 526, being altered thereby to Foam B. Foam B, which is
at approximately the same pressure as the gas stored in reservoir
510, passes through valve 532 into chamber 506. In chamber 506,
Foam B exerts a force on the piston 504 that may be communicated to
a mechanism (e.g., an electric generator, not shown) external to
the cylinder 502 by a rod 536 that is connected to piston 504 and
that passes slideably through the lower end cap of the cylinder
502.
[0129] The gas component of the foam in chamber 506 expands as the
piston 504 and rod 536 move downward. At some point in the downward
motion of piston 504, the flow of gas from reservoir 510 into the
mixing chamber 518 and thence (as the gas component of Foam B) into
chamber 506 may be ended by appropriate operation of valves (not
shown). As the gas component of the foam in chamber 506 expands, it
will tend, unless heat is transferred to it, to decrease in
temperature according to the Ideal Gas Law; however, if the liquid
component of the foam in chamber 506 is at a higher temperature
than the gas component of the foam in chamber 506, heat will tend
to be transferred from the liquid component to the gas component.
Therefore, the temperature of the gas component of the foam within
chamber 506 will tend to remain constant (approximately isothermal)
as the gas component expands.
[0130] When the piston 504 approaches bottom dead center of
cylinder 502 (i.e., has moved down to approximately its limit of
motion), valve 532 may be closed and valve 534 may be opened,
allowing the expanded gas in chamber 506 to pass from cylinder 502
to some other component of the system 500, e.g., a vent or a
chamber of another cylinder for further expansion.
[0131] In some embodiments, pump 514 is a variable-speed pump,
i.e., may be operated so as to transfer liquid 512 at a slower or
faster rate from the reservoir 510 to the foam-generating mechanism
522 and may be responsive to signals from the control system (not
shown). If the rate at which liquid 512 is transferred by the pump
514 to the foam-mechanism 522 is increased relative to the rate at
which gas is conveyed from reservoir 510 through piping 520 to the
mechanism 522, the void fraction of the foam produced by the
mechanism 522 may be decreased. If the foam generated by the
mechanism 522 (Foam A) has a relatively low void fraction, the foam
conveyed to chamber 506 (Foam B) will generally also tend to have a
relatively low void fraction. When the void fraction of a foam is
lower, more of the foam consists of liquid, so more thermal energy
may be exchanged between the gas component of the foam and the
liquid component of the foam before the gas and liquid components
come into thermal equilibrium with each other (i.e., cease to
change in relative temperature). When gas at relatively high
density (e.g., ambient temperature, high pressure) is being
transferred from the reservoir 510 to chamber 506, it may be
advantageous to generate foam having a lower void fraction,
enabling the liquid fraction of the foam to exchange a
correspondingly larger quantity of thermal energy with the gas
fraction of the foam.
[0132] All pumps shown in subsequent figures herein may also be
variable-speed pumps and may be controlled based on signals from
the control system. Signals from the control system may be based on
system-performance (e.g., gas temperature and/or pressure, cycle
time, etc.) measurements from one or more previous cycles of
compression and/or expansion.
[0133] Embodiments of the invention increase the efficiency of a
system 500 for the storage and retrieval of energy using compressed
gas by enabling the surface area of a given quantity of
heat-exchange liquid 512 to be greatly increased (with
correspondingly accelerated heat transfer between liquid 512 and
gas undergoing expansion or compression within cylinder 502) with
less investment of energy than would be required by alternative
methods of increasing the surface of area of the liquid, e.g., the
conversion of the liquid 512 to a spray.
[0134] In other embodiments, the reservoir 510 is a separator
rather than a high-pressure storage reservoir as depicted in FIG.
5. In such embodiments, piping, valves, and other components not
shown in FIG. 5 are supplied that allow the separator to be placed
in fluid communication with a high-pressure gas storage reservoir
as well as with the mixing chamber 518, as shown and described in
the '128 application.
[0135] FIG. 6 is a schematic diagram showing components of a system
600 for achieving approximately isothermal compression and
expansion of a gas for energy storage and recovery using a
pneumatic cylinder 604 (shown in partial cross-section) according
to embodiments of the invention. System 600 is similar to system
500 in FIG. 5, except that system 600 includes a bypass pipe 638.
Moreover, two valves 640, 642 are explicitly depicted in FIG. 6.
Bypass pipe 638 may be employed as follows: (1) when gas is being
released from the storage reservoir 610, mixed with heat-exchange
liquid 612 in the mixing chamber 618, and conveyed to chamber 606
of cylinder 604 to be expanded therein, valve 640 will be closed
and valve 642 open; (2) when gas has been compressed in chamber 606
of cylinder 604 and is to be conveyed to the reservoir 610 for
storage, valve 640 will be open and valve 642 closed. Less friction
will tend to be encountered by fluids passing through valve 640 and
bypass pipe 638 than by fluids passing through valve 642 and screen
626 and around the foam-generating mechanism 622. In other
embodiments, valve 642 is omitted, allowing fluid to be routed
through the bypass pipe 638 by the higher resistance presented by
the mixing chamber 618, and valve 640 is a check valve preventing
fluid flow when gas is being released in expansion mode. The
direction of fluid flow from chamber 606 to the reservoir 610 via a
lower-resistance pathway (i.e., the bypass pipe 638) will tend to
result in lower frictional losses during such flow and therefore
higher efficiency for system 600.
[0136] In other embodiments, the reservoir 610 is a separator
rather than a high-pressure storage reservoir as depicted in FIG.
6. In such embodiments, piping, valves, and other components not
shown in FIG. 6 are supplied that allow the separator to be placed
in fluid communication with a high-pressure gas storage reservoir
as well as with the mixing chamber 618 and bypass pipe 638.
[0137] FIG. 7 is a schematic diagram showing components of a system
700 for achieving approximately isothermal compression and
expansion of a gas for energy storage and recovery using a
pneumatic cylinder 702 (shown in partial cross-section) according
to embodiments of the invention. System 700 is similar to system
500 in FIG. 5, except that system 700 omits the mixing chamber 518
and instead generates foam inside the storage reservoir 710. In
system 700, a pump 714 circulates heat-exchange liquid 712 to a
foam-generating mechanism 722 (e.g., one or more spray nozzles)
inside the reservoir 710. The reservoir 710 may, by means of the
pump 714 and mechanism 722, be filled partly or entirely by foam of
an initial or original character, Foam A (724). The reservoir 710
may be placed in fluid communication via pipe 720 with a
valve-gated port 744 in cylinder 702. Valves (not shown) may govern
the flow of fluid through pipe 720. An optional screen 726 (or
other suitable mechanism such as an ultrasound source), shown in
FIG. 7 inside pipe 720 but locatable anywhere in the path of fluid
flow between reservoir 710 and chamber 706 of the cylinder 702,
serves to alter Foam A (724) to Foam B (728), regulating
characteristics such as bubble-size variance and average bubble
size.
[0138] In other embodiments, the reservoir 710 is a separator
rather than a high-pressure storage reservoir as depicted in FIG.
7. In such embodiments, piping, valves, and other components not
shown in FIG. 7 will be supplied that allow the separator to be
placed in fluid communication with a high-pressure gas storage
reservoir as well as with the cylinder 702. In other embodiments, a
bypass pipe similar to that depicted in FIG. 6 is added to system
700 in order to allow fluid to pass from cylinder 702 to reservoir
710 without passing through the screen 726.
[0139] FIG. 8 is a schematic diagram showing components of a system
800 for achieving approximately isothermal compression and
expansion of a gas for energy storage and recovery using a
pneumatic cylinder 802 (shown in partial cross-section) according
to embodiments of the invention. System 800 is similar to system
500 in FIG. 5, except that system 800 omits the mixing chamber 518
and instead generates foam inside the air chamber 806 of the
cylinder 802. In system 800, a pump 814 circulates heat-exchange
liquid 812 to a foam-generating mechanism 822 (e.g., one or more
spray nozzles injecting into cylinder and/or onto a screen through
which admitted air passes) either located within, or communicating
with (e.g., through a port), chamber 806. The chamber 806 may, by
means of the pump 814 and mechanism 822 (and by means of gas
supplied from reservoir 810 via pipe 820 through a port 844), be
filled partly or substantially entirely by foam. The reservoir 810
may be placed in fluid communication via pipe 820 with valve-gated
port 844 in cylinder 802. Valves (not shown) may govern the flow of
fluid through pipe 820.
[0140] FIG. 9 is a schematic drawing of an illustrative CAES system
900 employing pairs of high- and low-pressure cylinders in which
air is compressed and expanded. Half of the cylinders are
high-pressure cylinders (HPCs, indicated in FIG. 900 by block 902)
and half of the cylinders are low-pressure cylinders (LPCs,
indicated in FIG. 900 by block 904), resulting in a two-stage
compression process. Block 902 represents some number N of
high-pressure cylinders (not shown) and block 904 represents an
equal number N of low-pressure cylinders (not shown). The HPCs and
LPCs jointly drive a crankshaft that in turn drives an electric
generator or, in some states of operation of system 900, is driven
by an electric motor. Systems employing principles of operation
similar to those of 900 but including other subsystems, other
mechanisms, other arrangements of parts, other numbers of stages
(i.e., a single stage or more than one stage), and unequal numbers
of high- and low-pressure cylinders, are also contemplated and
within the scope of the invention.
[0141] Separating the LPCs from the HPCs is a mid-pressure vessel
(MPV) 906 that buffers and decouples the HPCs 902 and LPCs 904
during either compression or expansion processes. This allows each
cylinder assembly (i.e., each high- or low-pressure cylinder and
the valves that control the entry or exit of gas from the cylinder)
to operate independently from all the other cylinder assemblies
within system 900. Independent operation of cylinder assemblies
allows, in turn, for optimization of the performance (e.g.,
optimization of valve timing) of each cylinder assembly. A system
controller (not shown), e.g., a computerized controller,
coordinates the operation of individual cylinders with each other
and with the other pneumatic components and processes within system
900.
[0142] In addition to the cylinders 902, 904 and MPV 906, system
900 also includes a spray reservoir 908 that holds a heat-transfer
fluid (e.g., treated water) at low (e.g., atmospheric) pressure, a
low-pressure spray chamber 910 that creates foam and/or spray at
atmospheric pressure for intake into the LPCs for compression, and
a high-pressure spray chamber 912 that creates foam and/or spray at
storage pressure (i.e., the pressure at which gas and/or
heat-transfer fluid is stored after compression and/or before
expansion) for expansions in the HPCs. Finally, system 900 includes
one or more storage reservoirs (not shown) that are connected to
the HPCs 902 via the high-pressure spray chamber 912. The storage
reservoirs typically contain compressed air, e.g., air compressed
by system 900 and stored for future expansion to drive electricity
generation.
[0143] Each of the cylinder assemblies in the HPC group 902 and LPC
group 904 typically includes a cylinder similar to cylinder 201 in
FIG. 2, a high-side valve similar to valve 220 in FIG. 2, and a
low-side valve assembly similar to valve 221 in FIG. 2. Each
high-side valve includes or consists essentially of one or more
poppet elements that open out of the cylinder, connecting the
expansion/compression chamber of the cylinder to a volume that is
generally at higher pressure than the chamber. For a low-pressure
cylinder, the high-side valve connects the cylinder's
expansion/compression chamber to the MPV 906; for a high-pressure
cylinder, the high-side valve connects the cylinder's
expansion/compression chamber to the high-pressure spray chamber
912. Because these high-side valves open out of the cylinder rather
than into the cylinder, they passively check open under an
over-pressure condition in the cylinder, reducing the risk of a
hydrolocking event with possible attendant damage to system
components or interference with system operation.
[0144] Each low-side valve includes or consists essentially of one
or more poppet elements that open into the cylinder, connecting the
expansion/compression chamber of the cylinder to a volume that is
generally at lower pressure than the chamber. For a low-pressure
cylinder, the low-side valve connects the cylinder's
expansion/compression chamber to the spray reservoir 908; for a
high-pressure cylinder, the low-side valve connects the cylinder's
expansion/compression chamber to the MPV 906. All of the valves,
both low-side and high-side, may be hydraulically actuated. Other
actuated valves such as variable cam-driven valves,
electromagnetically actuated valves, mechanically actuated valves,
and pneumatically actuated valves are also considered and may be
utilized.
[0145] The system 900 may cyclically perform a normal compression
process (or "compression cycle") or a normal expansion process (or
"expansion cycle"). In a normal compression process, each
low-pressure and high-pressure cylinder progresses through a series
of four conditions or phases, each of which has an associated a
valve configuration. The four phases are (1) compression stroke,
(2) direct fill, (3) regeneration or expansion stroke, and (4)
breathe, intake, or auxiliary stroke. The numbering of the phases
is arbitrary in the sense that when the phases are performed in a
repeating cycle, no one phase is "first" other than by convention.
It is assumed in this description that all high-side and low-side
valves in system 900 activate instantaneously and ideally; the
implications of non-ideal valve actuation will be described
subsequently. The four phases are described in detail below.
[0146] The compression stroke begins with the cylinder's piston at
the bottom of its stroke range. The cylinder's
expansion/compression chamber (herein also referred to simply as
"the chamber") is filled with air at relatively low pressure (e.g.,
atmospheric). For example, the cylinder may have previously drawn
in air through its low-side valve from a source on its low side
(e.g., an HPC draws air in from the MPV 906, or an LPC draws air in
from the ambient intake/exhaust port). With the piston at bottom of
stroke, the cylinder's low-side valve and high-side valve both
close, if they were not already closed, and the piston begins to
move upward, compressing the air within the chamber: i.e., the
compression stroke begins. The compression stroke continues as the
piston moves up from bottom of stroke with both valves closed,
compressing the air inside. The compression phase nominally ends
when the pressure in the cylinder is approximately equal to the
pressure in the component (e.g., MPV 906 or HP spray chamber and
thence to high-pressure storage reservoir) to which the cylinder is
connected on its high side. At this point, the direct-fill stroke
or phase begins.
[0147] The direct-fill stroke or phase occurs while the piston is
still moving upward and involves pushing the compressed air within
the chamber out of the cylinder and into the high-side component.
Direct fill begins when the pressures in the cylinder and the
high-side vessel are approximately equal and the high-side valve is
actuated to open. The low-side valve remains closed. Once the
high-side valve is open, the cylinder pushes compressed air from
its chamber into the high-side component as the piston continues to
travel toward top of stroke. Direct fill ends when the cylinder
reaches top of stroke, whereupon the high-side valve is closed.
[0148] The regeneration stroke occurs with both valves closed as
the cylinder piston moves downward, away from top of stroke. Each
cylinder has some amount of clearance volume, which is the physical
space within the cylinder--above the piston and below the valves
and in all the connections and crevices--that is present when the
piston is at top of stroke. Moreover, in a CAES system that
utilizes a liquid/water mixture to effect heat transfer within the
cylinder (e.g., system 900), some fraction of the clearance volume
will be occupied by liquid and some by air. That portion of the
clearance volume occupied by air during a particular state of
operation of the cylinder is the air dead volume (also herein
termed simply the "dead volume") of the cylinder in that state of
operation. This is the portion of air that was compressed during
the compression stroke that was not then subsequently pushed into
the high-side component. This compressed air contains energy (i.e.,
both thermal and elastic potential), and the regeneration stroke
allows this energy to be recaptured. The regeneration stroke starts
at top of stroke with both valves closed and continues as the
piston moves downward, expanding the dead volume air. The
regeneration stroke ends when the pressure in the cylinder has
dropped to the low-side vessel pressure and the low-side valve is
commanded open.
[0149] As the cylinder piston is moving downward, once the low-side
valve is opened the intake stroke begins. The intake stroke
continues, drawing in new air to be compressed on the next stroke,
until the piston reaches bottom of stroke. At this point, the
low-side valve is closed and the next compression stroke may
begin.
[0150] Each of the four compression stages is separated from
preceding and subsequent stages by a valve transition event, i.e.,
the opening or closing of one or more valves. In the descriptions
of the stages above, the valve transition points were clearly
defined as top of stroke, bottom of stroke, or pressure
equalization, but this assumes that the valves of system 900
respond ideally and instantaneously. However, because of finite
valve response time, each valve transition event is an opportunity
for system optimization.
[0151] The first valve transition event mentioned above is the
transition from intake stroke to compression stroke, which
nominally occurs at bottom dead center (BDC; the condition where
the piston is at its nethermost point of motion). With finite
(nonzero) valve response time, the low-side valve will need to be
commanded closed slightly before BDC and will likely be full seat
closed slightly after BDC. Transitioning this valve too early means
that less air is drawn in than could have been, resulting in less
air to be compressed during the following compression stroke
(reduced capacity). Transitioning the valve too late means that
some of the air drawn in will be re-exhausted before the valve
fully seats, also resulting in reduced capacity.
[0152] The second valve event is at the end of compression,
transitioning into direct-fill, where the high-side valve is opened
to end admission of air into the chamber and start pushing air into
the high-side vessel. Nominally, the high-side valve opens when the
pressure in the chamber is equal to the pressure in the high-side
component. However, because of finite valve response time, if the
valve is commanded open when the pressures are equal, then the
pressure in the chamber will spike significantly as the flow from
the chamber is limited and throttled through the high-side valve
while the valve is transitioning open. The pressure spike may be
avoided by commanding the high-side valve to open before pressures
are equal in the chamber and the high-side component. However, if
the high-side valve starts to open when the chamber pressure is
still lower than that within the high-side component, then some
amount of back-flow into the chamber will occur as fluid from the
high-side component flows backward through the high-side valve into
the chamber. (This backflow will be throttled, since the high-side
valve is partially open.) If the high-side valve is opened too
early, then the pressure within the chamber will jump quickly to
the high-side component pressure, and the piston will need to
perform additional work to push the air back out of the chamber
into the high-side component. Thus, this valve transition entails a
tradeoff between backflow and pressure-spike, both of which impact
the pressure profile and the work that needs to be performed by the
piston upon the air.
[0153] The valve transition closing the high-side valve at the end
of the direct fill impacts system capacity. If the high-side valve
is closed too early, then less air is pushed into the high-side
component than could have been, and the pressure will momentarily
spike before dropping. If the high-side valve is closed too late,
then some of the air pushed into the high-side component will be
pulled back out again as the piston moves away from top dead center
(TDC; the condition where the piston is at its uppermost point of
motion), and as the high-side valve finishes closing the flow will
be throttled so the energy of the air flowing back into the
cylinder performs less work on the piston. Potential work lost to
throttling is not, in general, recovered.
[0154] Finally, the transition at the end of the regeneration
stroke that opens the low-side valve to start the intake stroke
impacts the work done on the piston. If the low-side valve is
opened too early, then the remaining air at higher pressure from
the dead volume will no longer expand, doing work on the piston,
but will expand (throttled) through the opening low-side valve. If
the low-side valve opens too late, then the pressure in the chamber
will drop below the low-side component pressure and the piston will
have to do additional work to pull the piston down and pull air
through the partially open low-side valve.
[0155] Similarly, in a normal expansion process, each low-pressure
and high-pressure cylinder progresses through a series of four
conditions or sub-processes, each of which has an associated a
valve configuration. The four conditions are (1) vent (or exhaust,
or auxiliary) stroke, (2) pre-compression stroke, (3) direct drive,
and (4) expansion stroke. The numbering of the phases is arbitrary
in the sense that when the phases are performed in a repeating
cycle, no one phase is "first" other than by convention. It is
assumed in this description that all high-side and low-side valves
in system 900 activate instantaneously and ideally; the
implications of non-ideal valve actuation will be described
subsequently. The four phases are described in detail below.
[0156] An expansion cycle begins at the bottom of stroke with the
piston beginning to move upward from BDC and the low-side valve
open, commencing a vent stroke. As the piston moves upward away
from BDC, air in the chamber (e.g., air that was expanded in a
previous cycle) is exhausted through the low-side valve to the
low-side component (e.g., exhausted from an HPC to the MPV 906 or
from an LPC to the ambient intake/exhaust port). A vent stroke ends
when the low-side valve is closed and a pre-compression stroke
begins. In general, the piston continues to move upward without
interruption as a vent stroke ends and a pre-compression stroke
begins.
[0157] As the cylinder piston continues to move upward, before it
reaches TDC the low-side valve is closed to begin the
pre-compression stroke. In pre-compression, both the high-side
valve and low-side valve are closed and the air volume trapped
within the chamber is compressed. In controlling the events of
pre-compression, a goal is to close off the low-side valve at such
a time that the air trapped in the chamber is compressed as nearly
as possible to the pressure of the cylinder's adjoining high-side
component when the piston reaches TDC. This allows the high-side
valve to open at top of stroke with equal pressures on either side,
resulting in approximately zero throttled flow through that valve
and approximately zero loss of exergy due to non-work-performing
loss of pressure of a quantity of gas. Timing of the start of
pre-compression (closure of the low-side valve) greatly impacts the
achievement of the equal-pressure goal.
[0158] Once the piston is at TDC and the high-side valve is opened,
the direct-drive stroke begins. In direct drive, the piston is
moving down, away from TDC, and air in the high-side component is
expanding and flowing through the high-side valve into the chamber,
directly driving the piston downward. The direct drive stroke
continues until an appropriate mass of air has been added to the
cylinder, at which point the high-side valve closes and the
expansion stroke begins. In general, the piston continues to move
downward without interruption as a direct-drive stroke ends and an
expansion stroke begins.
[0159] In an expansion stroke, both valves are closed, and the air
that was admitted to the chamber during the direct-drive phase
expands, continuing to perform work upon the piston as it drives it
downward. If a correct mass of air was added to the cylinder during
direct drive, then the air pressure at the end of the expansion
stroke will be approximately equal to the end-of-stroke target
pressure at the moment when the piston reaches BDC. For a
high-pressure cylinder, the end-of-stroke target pressure is the
pressure in the MPV; for a low-pressure cylinder, the end-of-stroke
target pressure is the vent pressure, which is typically slightly
higher than atmospheric pressure. Once the piston reaches BDC, the
low-side valve is opened and the cylinder begins to move up in the
next vent stroke.
[0160] Valve actuation timings during a compression or expansion
cycle may have a significant impact on the efficiency of the cycle.
Such impact tends, in some embodiments, to be greater during an
expansion cycle than during a compression cycle. During an
expansion cycle, the first valve transition, as described
hereinabove, is the closing of the low-side valve to begin the
pre-compression. Incorrect or suboptimal timing of this valve
transition may have significant consequences for the cycle. First,
this valve transition is associated with the need for rapid
high-side valve transition (short actuation time): as the valve is
closing, the pressure in the chamber is rising quickly, resulting
in throttled flow through the valve during the transition. Thus,
the slower the transition, the greater the flow losses. Second, if
the valve is closed later than is ideal, there will be less air in
the cylinder to compress and less stroke length during which to
compress it, resulting in a pressure at TDC less than the pressure
in the high-side vessel.
[0161] If this difference is large enough, then the pressure will
be below the minimum coupling pressure and the high-side valve will
be physically unable to open (i.e., the actuator will not be able
to provide enough force to open the valve against the pressure
difference) and the cylinder will not be able to complete the
expansion cycle. If the pressure at TDC is above the minimum
coupling pressure but below the pressure within the high-side
component, then the high-side valve will be able to open, but gas
will flow into the cylinder during the opening event without
performing useful work on the piston. Contrariwise, if the low-side
valve closing at the start of the pre-compression occurs too early,
then the pressure within the chamber will reach the pressure within
the high-side component before the piston reaches TDC, and surpass
the pressure within the high-side component by the time the piston
reaches TDC. In this case, more air would have been re-compressed
than necessary (and less air would have been exhausted), resulting
in a capacity reduction. If the pressure in the chamber reaches the
high-side vessel before TDC, then the high-side valve should open
when the pressures are equal rather than waiting to TDC, in order
to prevent over-pressurization of the cylinder.
[0162] Once the high-side valve is open at TDC and the cylinder is
in direct drive, with the piston moving down, the next transition
is the closing of the high-side valve at the end of direct drive.
This transition may impact both capacity and efficiency. Under
perfect valve timing (i.e., closure of the high-side valve occurs
at a time such that the exactly correct mass of air is drawn into
the cylinder), the pressure will decrease during expansion phase
and be equal to the target pressure approximately at BDC. If the
valve is closed too early, the chamber will reach target pressure
before BDC and will have expanded less air during the stroke than
it could have (i.e., there will be a loss of capacity). If the
high-side valve is closed too late, then the chamber pressure will
still be above target at BDC, and the pressure difference will
entail a loss once the valve is opened.
[0163] The last valve transition is at the end of the expansion
stroke, when the low-side valve is opened. Ideally, the pressure at
the end of the expansion stroke is equal to the target pressure
exactly at BDC. If the chamber pressure drops to the target
pressure before BDC, then the valve is opened to prevent the
crankshaft from having to work to pull the piston down. If the
piston reaches BDC before the pressure has decreased to the target,
then the valve should also open. (An exception may occur if the
chamber pressure is above the maximum allowable vent pressure.)
This valve opening event is also impacted by the finite valve
response time, in a manner similar to that described hereinabove
for other valve actuation events.
[0164] It will be evident to persons familiar with pneumatics and
hydraulic devices that similar considerations apply to the timing
and non-ideality of valve actuation events during a compression
mode of system 900. For example, at the transition between a
compression stroke and a direct-fill stroke, as described
hereinabove, the high-side valve of a cylinder opens. If the
high-side valve is opened too late, the pressure within the chamber
will exceed that within the high-side component (e.g.,
high-pressure storage vessel), and gas will expand from the chamber
into the high-side component in a non-work-performing manner upon
valve actuation. If the high-side valve is opened too soon, the
pressure within the chamber will be less than that within the
high-side component, and gas will expand from the high-side
component into the chamber in a non-work-performing manner upon
valve actuation. Valve actuation timing is similarly constrained at
other transitions between the other phases of cylinder operation
during a compression process.
[0165] FIG. 10 is an illustrative plot of cylinder pressure as a
function of time for four different expansion scenarios in an
illustrative CAES system similar or even identical to the system
200 shown in FIG. 2. Points A, B, C, D, and E in FIG. 10, marked by
dots, correspond to operating states of one or more components of
system 200, or to changes in such operating states, as described
below. In the illustrative plot shown, Point A represents an
initial state of the pneumatic cylinder assembly (201 in FIG. 2)
during which the piston slidably disposed therein (202 in FIG. 2)
is at top dead center and the high-side valve (220 in FIG. 2)
between the storage reservoir (222 in FIG. 2) and the
lower-pressure cylinder assembly 201 is opened. Point B represents
the end of a direct-drive phase of operation during which the
high-side valve 220 between the storage reservoir 222 and the
lower-pressure cylinder assembly 201 is closed and the pressure
inside the cylinder assembly 201 is approximately equal to the
bottle pressure P.sub.b (i.e., the pressure of gas inside storage
reservoir 222 in FIG. 2 (e.g., 300 psi)). Point C represents the
end of an expansion stage or phase, during which the quantity of
gas admitted into the cylinder assembly 201 performs work on the
piston 202 slidably disposed therein, from top dead center at a
bottle pressure P.sub.b to bottom dead center at an exhaust
pressure P .sub.exhaust. Point C also represents the opening
actuation of the low-side valve (221 in FIG. 2) between the vent to
atmosphere (223 in FIG. 2) and the cylinder assembly 201 when the
piston 202 is at bottom dead center. Points A, B, and C represent
approximately the same operating states in all four scenarios of
operation of system 200 described hereinbelow.
[0166] Four versions of Point D (labeled D.sub.1, D.sub.2, D.sub.3,
and D.sub.4 to correspond to the four different valve-actuation
scenarios) represent the end of the exhaust stage and beginning of
a pre-compression stage; event D thus corresponds to the closing
actuation of the low-side valve 221 between the vent 223 (or a
lower-pressure stage) and the cylinder assembly 201. Four versions
of event E (E.sub.1, E.sub.2, E.sub.3, and E.sub.4 for the four
different valve-actuation scenarios) represent the end of the
pre-compression stage, at which time the piston 202 is again at top
dead center and the pressure inside the cylinder assembly 201 is
approximately equal to the bottle pressure P.sub.b. If the system
is operated cyclically, Point E (any version) immediately precedes
Point A and the expansion cycle may be repeated. The pressure
inside the cylinder assembly 201 at the end of the pre-compression
stroke (i.e., at any version of Point E) is determined by the
relative timing of the closing of the low-side valve 221 (i.e., at
any version of Point D).
[0167] In an idealized expansion scenario (Scenario 1, represented
by a solid line in FIG. 10), there is no dead volume in the
cylinder assembly 201 and all valve actuations occur
instantaneously. The low-side valve 221 closes at Point D.sub.1
when the piston 202 is at top dead center, instantaneously followed
by the opening of the high-side valve 220 at Point E.sub.1. In
Scenario 1, pressurization of the cylinder assembly 201 occurs
immediately and with no coupling loss, as there is no volume to
pressurize at the top of stroke: this instantaneous pressurization
of cylinder assembly 201, simultaneous with D.sub.1 and E.sub.1, is
represented by the perfect verticality of the Scenario 1 line in
FIG. 10 at Point D.sub.1/E.sub.1. Scenario 1 is shown as a
reference line in FIG. 10.
[0168] In a second scenario (Scenario 2, represented by a bold
dashed line in FIG. 10), dead volume exists in the cylinder
assembly 201. Between Point C and Point D.sub.1, the piston 202
performs a return stroke within the cylinder assembly 201, with the
low-side valve 221 open to allow venting of gas from the upper
chamber of cylinder assembly 201. At Point D.sub.1, the low-side
valve 221 is closed so that the gas remaining in the upper chamber
of cylinder assembly 201 will be pressurized during the remainder
of the return stroke. However, in Scenario 2 the low-side valve 221
is closed too late, trapping an insufficient amount of gas in the
upper chamber of cylinder assembly 201, so when the piston 202
reaches top dead center the gas inside the upper chamber of
cylinder assembly 201 is at a pressure P.sub.2 that is lower than
the storage bottle pressure P.sub.b. In other words, operational
Point D.sub.2 occurs too late in time (or piston position) to allow
adequate pre-compression by Point E.sub.2 of the gas remaining in
the cylinder assembly 201. Adequate pre-compression would be to
approximately reservoir pressure P.sub.b. When the high-side valve
is opened at Point E.sub.2, a coupling loss occurs when pressurized
gas at reservoir pressure P.sub.b is admitted from the storage
reservoir 222 (or, in some other embodiments, the previous cylinder
stage) to the lower-pressure cylinder assembly 201.
[0169] In a third scenario (Scenario 3, represented by a short-dash
line in FIG. 10), point D.sub.3, i.e., closure of the valve 221
between the vent 223 and the cylinder assembly 201, is timed
correctly to enable the gas in the upper chamber of cylinder
assembly 201 to reach a pressure P.sub.3 when the piston 202 is at
top dead center (point E.sub.3), where P.sub.3 substantially equal
to the stored bottle pressure P.sub.b. In other words, operational
Point D.sub.3 is at the correct time (or piston position) so that
pre-compression of the gas remaining in the cylinder assembly 201
is to a pressure P.sub.3 approximately equal to the reservoir
pressure P.sub.b at Point E.sub.3. When the pressure P.sub.3 inside
the upper chamber of the cylinder assembly 202 is approximately
equal to the pressure P.sub.b of gas from the storage reservoir 222
(or previous cylinder stage), then there will be little or no
coupling loss when valve 220 is opened, and overall system
efficiency and performance will be improved.
[0170] In a fourth scenario (Scenario 4, represented by a bold
dotted line in FIG. 10), the valve 221 between the vent 223 and the
cylinder assembly 201 is closed (point D.sub.4) too soon and the
pressure inside the upper chamber of cylinder assembly 201 reaches
a pressure P.sub.4 higher than the stored bottle pressure P.sub.b
when the piston 202 reaches top dead center (Point E.sub.4). In
other words, operational Point D.sub.4 is too early in time (or
piston position) and the pre-compression of the remaining air in
the upper chamber of the cylinder assembly 201 exceeds reservoir
pressure P.sub.b at Point E.sub.4. When the valve 220 between the
storage vessel 222 (or earlier cylinder stage) and the cylinder
assembly 201 is opened (Point E.sub.4), the difference in pressure
between results in a coupling loss.
[0171] The system controller (226 in FIG. 2) may be programmed in
such a manner as to receive feedback (e.g., information from
measurements) from previous expansion cycles and the present state
of system 200. Such feedback, whether informational or mechanical,
may be used to adjust the timing of Point D, the closing of the
low-side valve 221 to commence pre-compression. For example, a
lookup table may be employed to set valve actuation times in
response to measurements of conditions in the system. In one
embodiment, the controller 226 utilizes timing information from
previous expansion strokes and pressure measurements of the
cylinder 201 and reservoir 222 at the completion of the
pre-compression process (Point E) to set the next time of closure
of valve 221. Such feedback may provide optimal performance of the
expander/compressor, improving efficiency, performance, and system
component lifetime. The time of opening of valve 220 and other
events in the expansion cycle may also be adjusted by the system
controller 226 based on feedback.
[0172] Thus, in accordance with embodiments of the invention,
efficiency is maximized during a gas compression or expansion by a
combination of feedforward and feedback control of the valve timing
where either early or late actuation of the valves would reduce
overall efficiency of the compression or expansion process. This
efficiency of the valve timing may be calculated mathematically by
comparing the work required with ideal valve timing to the actual
measured work with the experimental or sub-optimal valve timing.
Other factors that are measurable or may be calculated via
measurable values and impact efficiency are the rate of pressure
decrease of the storage system during the expansion process, the
rate of mass storage during the compression process, and the degree
of under- or over-pressurization during either process. For both
the compression and expansion processes, there is typically a known
ideal pressure profile that may be approached by optimizing valve
timing. The ideal pressure profile may be approached by determining
valve timing that minimizes or maximizes the integrated work about
key points in the pressure-volume curve. Deriving and subjecting
the system to such timing values constitutes the feedforward
component of the valve timing controller. Correcting for modeling
uncertainties, system disturbances, quickly occurring system
changes, or longer-term system drift is performed by incorporating
representative measurements in the valve timing controller, and
this constitutes the feedback component of the valve timing
controller. Each valve transition event may be optimized for
efficiency as described herein. For example, in opening a high-side
valve at the end of a compression stroke to begin direct fill, an
early actuation would cause gas to travel backwards from the
high-side reservoir into the cylinder, reducing efficiency of the
compression process, and late actuation would result in a pressure
spike, increasing work required to complete the compression and
causing a loss of useful energy when the valve opens and the air in
the cylinder pressure equalizes with storage. Thus, in short, as
utilized herein, "maximizing efficiency" of a compression or
expansion process entails valve-timing optimization to minimize or
eliminate lost work during an expansion of a particular amount of
gas or minimizing or eliminating additional work required to
compress a particular amount of gas.
[0173] FIG. 11 is a graphical display of experimental test data
from an expansion process involving expansion of gas first in a
high-pressure pneumatic cylinder and then in a low-pressure
pneumatic cylinder. That is, in the physical system from which the
data in FIG. 11 were drawn, a first, high-pressure cylinder
expanded gas from a high pressure P.sub.II to an intermediate
pressure P.sub.I while a second, low-pressure cylinder either (a)
did not pre-compress the gas in its expansion chamber from a
preexisting default pressure P.sub.I, (PLOT A in FIG. 11) or (b)
pre-compressed the gas in its expansion chamber from the
preexisting default pressure P.sub.L to the intermediate pressure
P.sub.I (PLOT B in FIG. 12).
[0174] In PLOT A, the pressure in the expansion chamber of the
first, high-pressure cylinder as a function of time during
expansion of the chamber's contents from P.sub.H to P.sub.I is
indicated by curves 1100 and 1104, and the pressure in the
expansion chamber of the second, low-pressure cylinder as a
function of time is indicated by curves 1102, 1106, and 1108.
Expansion of the gas in the high-pressure cylinder is indicated by
curve 1100. The approximately constant pressure of the gas in the
low-pressure cylinder, being exhausted by a return stroke
concurrently with the expansion recorded by curve 1100, is
indicated by curve 1102.
[0175] At the moment corresponding to labeled point A.sub.1, a
valve is opened to place the expansion chamber of the first,
high-pressure cylinder in fluid communication with the expansion
chamber of the second, low-pressure cylinder. Because the two
chambers are at different pressures at that time (i.e., the gas in
the high-pressure cylinder chamber is at P.sub.I and the gas in the
low-pressure cylinder chamber is at P.sub.L), after point A.sub.I
(valve opening) the pressure within the chamber of the
high-pressure cylinder decreases rapidly to an intermediate
pressure P.sub.I2 (curve 1104) while the pressure within the
chamber of the low-pressure cylinder increases rapidly to the
intermediate pressure P.sub.I2 (curve 1106). By point A.sub.2,
shortly after A.sub.1, the pressures in the two cylinder chambers
have equilibrated. The rapid expansion indicated by curve 1104
performs no work on any mechanical component of the system and
therefore entails a loss of available energy (i.e., a dead-volume
loss). At point A.sub.2, an expansion occurs in the expansion
chamber of the low-pressure cylinder, from P.sub.I2 to some low end
pressure P.sub.E1 (curve 1108).
[0176] In PLOT B, the pressure in the expansion chamber of the
first, high-pressure cylinder as a function of time during
expansion of the chamber's contents from P.sub.H to I.sub.2 is
indicated by curve 1110, and the pressure in the expansion chamber
of the second, low-pressure cylinder as a function of time during
pre-compression of the gas in the low-pressure cylinder chamber
from P.sub.L to approximately P.sub.I is indicated by curve 1112.
Prior to labeled point B, the low-pressure cylinder is performing
an exhaust stroke, and gas is being expelled from the expansion
chamber of the low-pressure chamber at approximately constant
pressure P.sub.L through an open exhaust valve. At the moment
corresponding to point B, the valve permitting gas to exit the
expansion chamber of the low-pressure cylinder is closed, trapping
a fixed quantity of gas within the chamber at pressure P.sub.L.
This quantity of gas is then compressed to pressure P.sub.I as
indicated by curve 1112.
[0177] At point C, the pressure in the expansion chamber of the
low-pressure cylinder is approximately equal to the pressure
P.sub.L in the expansion chamber of the high-pressure cylinder and
a valve is opened to place the two chambers in fluid communication
with each other. Because the two chambers are at approximately
equal pressures, there is no significant equilibration upon valve
opening (i.e., there is no curve in PLOT B corresponding to the
expansion of gas in the high-pressure cylinder indicated by curve
1104 in PLOT A) and thus little or no energy loss due to
equilibration. Subsequent to point C, an expansion occurs in the
expansion chamber of the low-pressure cylinder, from P.sub.L to
some low end pressure P.sub.E2 (curve 1114). In FIG. 11, end
pressure P.sub.E2 is not equal to end pressure P.sub.EI, but this
is not a necessary result of the pre-compression process
illustrated in FIG. 11 and end pressure P.sub.E2 may have any of a
range of values in accordance with embodiments of the present
invention.
[0178] FIG. 12 is an illustrative plot of the ideal pressure-volume
cycle in a cylinder operated as either a compressor or expander.
FIG. 12 provides explanatory context for subsequent figures.
Instantaneous and perfectly timed valve actuations are presumed for
the system whose behavior is represented in FIG. 12. The horizontal
axis represents volume (increasing rightward) and the vertical axis
represents pressure (increasing upward). In FIG. 12, the volume
represented by the horizontal axis is the volume of the
expansion/compression chamber of a cylinder assembly that is
similar or identical to cylinder 201 in FIG. 2 and is being
operated as either an expander or compressor. The four curves in
FIG. 12 (labeled 1, 2, 3, and 4) form a cyclic loop; each curve
represents one of the four distinct phases of operation described
above for both compression and expansion. For a cylinder operating
as a compressor, Curves 1 through 4 are traversed counterclockwise
(e.g., in order 1, 4, 3, 2), where Curve 1 represents the
direct-fill phase; Curve 2 represents the compression stroke; Curve
3 represents the intake stroke; and Curve 4 represents the
regeneration stroke. For a cylinder operating as an expander,
Curves 1 through 4 are traversed clockwise (e.g., in order 1, 2, 3,
4). Curve 1 represents the direct-fill phase; Curve 2 represents
the expansion stroke; Curve 3 represents the exhaust stroke; and
Curve 4 represents the precompression stroke. Points A, B, C, and D
in FIG. 12 represent valve transition events. As each Event (A, B,
C, or D) is traversed during cyclic operation of the cylinder, the
valve actuations that occur at each point depend on whether the
cylinder is being operated as an expander or compressor.
Specifically, if the cylinder is being operated as a compressor, at
Event A a high-side valve V1 (220 in FIG. 2) is closed and a
low-side valve V2 (221 in FIG. 2) remains closed; at Event D, V1
remains closed and V2 is opened; at Event C, V1 remains closed and
V2 is opened; and at Event B, V1 is opened while V2 remains closed.
If the cylinder is being operated as an expander, at Event A, V1 is
opened and V2 remains closed; at Event B, V1 is closed and V2
remains closed; at Event C, V1 remains closed and V2 is opened; at
Event D, V1 remains closed and V2 is closed.
[0179] As will be made clear by subsequent figures, the effects of
finite (non-ideal, nonzero) actuation times for valves V1 and V2 at
all valve transitions in FIG. 12 tend to decrease system capacity
and/or efficiency and to alter the shapes of Curves 1, 2, 3, and 4.
Also, mistiming of valve transitions in FIG. 12 may decrease system
capacity and/or efficiency. An optimal actuation timing exists for
each valve actuation event under any given conditions of operation;
this optimal time will, in general, change as the conditions under
which the system is being operated change (e.g., as the pressure in
a high-pressure gas storage reservoir gradually increases or
decreases).
[0180] FIG. 13 is an illustrative plot of cylinder chamber pressure
as a function of cylinder chamber volume for three different
expansion scenarios in an illustrative CAES system similar or even
identical to the system 200 shown in FIG. 2. FIG. 13 shows the
effects of early, correctly timed, and tardy closure of valve V2 in
the transition (Event D in FIG. 12; not shown in FIG. 13) from
intake phase to pre-compression phase (i.e., from Curve 3 to Curve
4 in FIG. 12). The three scenarios depicted in a pressure-volume
plot in FIG. 13 greatly resemble the scenarios depicted in a
pressure-time plot in FIG. 10, as shall be explained below.
[0181] The region of the expander's pressure-volume cycle portrayed
in FIG. 13 corresponds to Point A in FIG. 12 as defined for an
expansion process. (In a non-ideal system, events occurring during
a valve transition event do not occur without changes of pressure
and volume, and so cannot be represented by a single point in a
pressure-volume plot. The instant that an actuated valve is
commanded to transition is one representation of the start of a
valve transition.) The dashed curve 1302 represents the
pressure-volume history of the gas within the cylinder chamber
during the latter part of the pre-compression phase (Curve 4 in
FIG. 12) for a scenario (the Correct V2(D) Closure scenario) in
which closure of V2 (the low-pressure valve, 221 in FIG. 2) in the
transition from vent phase to pre-compression phase occurs at an
optimal time. The Correct V2(D) Closure scenario corresponds to the
curve passing through points D.sub.2 and E.sub.2 in FIG. 10.
[0182] The thick solid curves 1304, 1306 represent the
pressure-volume history of the gas during the latter part of the
pre-compression phase for a scenario (the Late V2(D) Closure
scenario) in which closure of V2 in the transition from vent phase
to pre-compression phase is tardy. The Late V2(D) Closure scenario
corresponds to the curve passing through points D.sub.3 and E.sub.3
in FIG. 10. The thin solid curves 1308, 1310 represent the
pressure-volume history of the gas during the latter part of the
pre-compression phase for a scenario (the Early V2(D) Closure
scenario) in which closure of V2 in the transition from vent phase
to pre-compression phase occurs too early. The Early V2(D) Closure
scenario corresponds to the curve passing through points D.sub.4
and E.sub.4 in FIG. 10.
[0183] All curves in FIGS. 13-16 are traversed, in time, in the
sense shown by the arrowheads attached to each curve.
[0184] In the systems whose behavior is partly represented by FIGS.
13-16, the gas volume of the high-side component (e.g.,
high-pressure storage reservoir 222 in FIG. 2) that is connected to
the cylinder through V1 is presumed to be sufficiently large that
exchanges of air between the high-side component and the cylinder
chamber do not substantially change the pressure P.sub.H of the gas
within the high-side component.
[0185] In the Correct V2(D) Closure Scenario, closure of V2 traps
the correct amount of air in the cylinder chamber to produce at TDC
a chamber pressure approximately equal to that the pressure P.sub.H
within the high-side component. In FIG. 13, P.sub.H is
approximately 21.5 megapascals (MPa). At point 1312, when the gas
in the chamber approximates pressure P.sub.H, V1 is opened; since
both the gas in the high-side component and the gas in the chamber
are at or near to P.sub.H, the pressure of the gas in the chamber
does not change significantly. Subsequently, gas is transferred
during direct-drive phase at approximately constant P.sub.H from
the high-side component to the chamber as the piston descends in
the cylinder (solid curve 1314). The effects of dead volume during
the transition from pre-compression phase to direct-drive phase are
minimized or even nonexistent in the Correct V2(D) Closure
Scenario.
[0186] In the Late V2(D) Closure Scenario, closure of V2 traps
insufficient air in the chamber to produce at TDC a chamber
pressure approximately equal to P.sub.H. Instead, the gas in the
chamber achieves some lower pressure P.sub.m; in FIG. 13, P.sub.H2
is approximately 15 MPa. At point 1316, V1 opens; since the gas in
the high-side component is at a higher pressure (P.sub.H) than the
gas in the chamber (P.sub.H2), gas from the high-side component
rapidly enters the chamber, raising the pressure of the gas in the
chamber while the volume of the chamber does not change
significantly. This pressure-volume change at near-constant volume
is represented by curve 1306. Potentially useful pressure energy is
lost during this non-work-performing expansion of gas from the
high-pressure component into the chamber, i.e., a dead-volume loss
occurs. At the end of curve 1306, the gas in the chamber reaches
point 1312, after which the Late V2(D) Closure Scenario coincides
with the Correct V2(D) Closure Scenario (curve 1314).
[0187] In the Early V2(D) Closure Scenario, closure of V2 traps
more air in the chamber than is needed to produce at TDC a pressure
approximately equal to P.sub.H. Instead, the gas achieves some
higher pressure P.sub.H3; in FIG. 13, P.sub.H3 is approximately 25
MPa. At point 1318, V1 opens; since the gas in the high-side
component is at a lower pressure (P.sub.H) than the gas in the
chamber (P.sub.H3), gas from chamber rapidly enters the high-side
component, lowering the pressure of the gas in the chamber while
its volume does not change significantly. This pressure-volume
change at near-constant volume is represented by curve 1310.
Potentially useful pressure energy is lost during this
non-work-performing expansion of gas from the chamber into the
high-pressure component, i.e., a dead-volume loss occurs. At the
end of curve 1318, the gas in the chamber is at point 1312, after
which the Early V2(D) Closure Scenario coincides with the Correct
V2(D) Closure Scenario (curve 1314).
[0188] FIGS. 14A-14C are illustrative plots of cylinder chamber
pressure as a function of cylinder chamber volume for two different
expansion scenarios in an illustrative CAES system similar or even
identical to the system 200 shown in FIG. 2. The two scenarios
depicted in FIGS. 14A-14C are the Correct V1(A) Opening Scenario
and the Late V1(A) Opening Scenario. The region of the expander's
pressure-volume cycle portrayed in FIGS. 14A-14C corresponds to
Point A in FIG. 12 as defined for an expansion process. Valves V1
and V2 are defined as for FIG. 13. FIGS. 14A-14C show the effects
of correctly timed and tardy opening of valve V1 in the transition
from pre-compression phase to direct-drive phase (i.e., from Curve
4 to Curve 1 in FIG. 12). It is presumed that in the scenarios
depicted in FIGS. 14A-14C, the previous valve transition (Event D
in FIG. 12) was optimally made. Three separate figures, FIGS.
14A-14C, depicting curves 1402, 1404, and 1406 respectively, are
employed to avoid partial obscuration of curve 1402 by curve
1406.
[0189] Curve 1402 of FIG. 14A represents the volume-pressure
history of both scenarios until point 1408 is reached. Thereafter,
the two scenarios diverge. In the Correct V1(A) Opening Scenario,
V1 is opened at point 1408, just as the cylinder reaches TDC.
Because pre-compression was optimally performed, the pressure in
the chamber approximates P.sub.H and there is little or no gas
exchange between the chamber and the high-pressure component when
V1 is opened, and subsequent to point 1408 gas is vented during a
direct-drive phase at approximately constant P.sub.H from the
high-side component to the chamber as the piston descends in the
cylinder (curve 1404 in FIG. 14B). In FIGS. 14A-14C, P.sub.H is
approximately 1.82 MPa. The effects of dead volume during the
transition from pre-compression phase to direct-drive phase are
minimal or nonexistent in the Correct V1(A) Opening Scenario.
[0190] In the Late V1(A) Opening Scenario, V1 is not opened at
point 1408, when the piston is at TDC, but remains closed for a
time thereafter. As the piston descends, the pressure-volume state
of the gas in the chamber thus begins to retrace curve 1402 in the
opposite direction (left-hand portion of curve 1406, FIG. 14C):
that is, the gas trapped in the chamber simply begins to re-expand.
At point 1410 (FIG. 14C), at which the gas in the chamber has
achieved some pressure P.sub.H4 significantly lower than P.sub.H,
V1 is opened. Gas then enters the chamber from the high-side
component, raising the pressure of the gas in the chamber to
P.sub.H while the volume of the chamber is increasing (rising
portion of curve 1406). Potential energy is lost and less work is
done during this non-work-performing expansion of gas from the
high-pressure component into the chamber, i.e., a dead-volume loss
occurs.
[0191] FIG. 15 is an illustrative plot of cylinder chamber pressure
as a function of cylinder chamber volume for three different
compression scenarios in an illustrative CAES system similar or
even identical to the system 200 shown in FIG. 2. FIG. 15 shows the
effects of early, correctly timed, and tardy opening of valve V1 in
the transition (Event B in FIG. 12, defined for compression mode)
from compression phase to direct-fill phase (i.e., from Curve 2 to
Curve 1 in FIG. 12). The region of the expander's pressure-volume
cycle portrayed in FIG. 15 corresponds to Point B in FIG. 12 as
defined for a compression process.
[0192] The dotted curve 1502 (partly obscured by thick solid curve
1504) represents the pressure-volume history of the gas within the
cylinder chamber during the latter part of the compression phase
(Curve 2 in FIG. 12) for a scenario (the Correct V1(B) Opening
scenario) in which opening of V1 in the transition from compression
phase to direct-fill phase occurs at an optimal time. The thick
solid curve 1504 represents the pressure-volume history of the gas
in the chamber during the latter part of the compression phase for
a scenario (the Late V1(B) Opening scenario) in which opening of V1
in the transition from compression phase to direct- fill phase is
tardy. The thin solid curve 1506 represents the pressure-volume
history of the gas during the latter part of the compression phase
for a scenario (the Early V1(B) Opening scenario) in which opening
of V1 in the transition from compression phase to direct- fill
phase occurs too early.
[0193] In the system whose behavior is partially depicted in FIGS.
15, V1 and V2 have nonzero actuation times. Therefore, the optimal
time of opening of V1 (i.e., the timing of V1 opening for the
Correct V1(B) Opening scenario) occurs at point 1508, before the
pressure in the chamber reaches P.sub.H. At point 1508 the gas in
the cylinder chamber has not yet achieved the pressure P.sub.H of
the gas in the high-pressure component, but only a small amount of
gas is throttled through the partly-open valve into the chamber as
the pressure-volume state of the gas in the chamber evolves from
point 1508 to point 1510, at which time the pressure in the chamber
approximates P.sub.H. A small amount of pressure overshoot may
occur (represented by the small hump in curve 1502); subsequently,
gas is exhausted during direct- fill phase at approximately
constant P.sub.H from the cylinder chamber to the high-side
component as the piston continues to ascend in the cylinder (curve
1512). The effects of dead volume during the transition from
pre-compression phase to direct-drive phase are minimal or
nonexistent in the Correct V1(B) Opening Scenario.
[0194] In the Late V1(B) Opening scenario, V1 is opened at point
1514, by which time the gas in the chamber has reached a pressure
P.sub.H5, significantly higher than P.sub.H. After the tardy
opening of V1, gas in the chamber transfers into the high-pressure
component as the pressure in the chamber decreases (left-hand side
of large hump in curve 1514). Energy is lost during this
non-work-performing expansion of gas from the chamber into the
high-pressure component, i.e., a dead-volume loss occurs.
[0195] A similar curve would be traced even if the valve started to
transition open at point 1510 when the pressures are equal, due to
the nonzero time to open the valve and pressure rise that occurs
with a partially open valve. Notably, in systems employing a
pressure-driven check valve for V1 rather than an actuated valve, a
pressure-volume history similar to that of the Late V1(B) Opening
scenario (curve 1504), although not necessarily so extreme,
typically occurs in every compression cycle, as an overpressure
(e.g., P.sub.H5 or some other pressure significantly higher than
P.sub.H) must be achieved on the chamber side of V1 with respect to
the high-pressure-component side of V1 in order for V1 to be
actuated. The use of actuated valves rather than check valves in
CAES systems is thus advantageous in this, as well as in other,
respects.
[0196] In the Early V1(B) Opening scenario, V1 is opened at point
1516, by which time the gas in the chamber has reached a pressure
of only P.sub.H6, significantly lower than P.sub.H. After the early
opening of V1, gas in the high-pressure component flows into the
chamber as the pressure in the chamber increases (right-hand side
of curve 1506). Energy is lost during this non-work-performing
expansion of gas from the high-pressure component into the chamber,
i.e., a dead-volume loss occurs.
[0197] FIG. 16 is an illustrative plot of cylinder pressure as a
function of cylinder volume for three different compression
scenarios in an illustrative CAES system similar or even identical
to the system 200 shown in FIG. 2. FIG. 16 shows the effects of
early, correctly timed, and tardy opening of valve V2 in the
transition (Event D in FIG. 12, defined for compression mode) from
regeneration phase to intake phase (i.e., from Curve 4 to Curve 3
in FIG. 12). The region of the expander's pressure-volume cycle
portrayed in FIG. 16 corresponds to Point D in FIG. 12 as defined
for a compression process.
[0198] The dotted curve 1602 represents the pressure-volume history
of the gas within the cylinder chamber during the latter part of
the regeneration phase (Curve 4 in FIG. 12) for a scenario (the
Correct V2(D) Opening scenario) in which opening of V2 in the
transition from regeneration phase to intake phase occurs at an
optimal time. The thick solid curve 1604 represents the
pressure-volume history of the gas in the chamber during the latter
part of the compression phase for a scenario (the Late V2(D)
Opening scenario) in which opening of V2 in the transition from
regeneration phase to intake phase is tardy. The thin solid curve
1606 represents the pressure-volume history of the gas during the
latter part of the compression phase for a scenario (the Early
V2(D) Opening scenario) in which opening of V2 in the transition
from regeneration phase to intake phase occurs too early.
[0199] In the system whose behavior is partially depicted in FIGS.
16, V1 and V2 have nonzero actuation times. Therefore, the optimal
time of opening of V2 (the timing of V2 opening for the Correct
V2(D) Opening scenario) occurs at point 1608, before the pressure
in the chamber approximates P.sub.L (the pressure of the
low-pressure component that communicates with the cylinder through
V2, e.g., vent 223 in FIG. 2). The gas in the cylinder chamber has
not yet decreased to the pressure P.sub.L of the gas in the
low-pressure component, but only a small amount of gas is throttled
through the partly-open valve V2 into the chamber as the
pressure-volume state of the gas in the chamber evolves from point
1608 to point 1610, at which time the pressure in the chamber
approximates P.sub.L. A small amount of pressure overshoot may
occur (dip in curve 1602); subsequently, gas is admitted to the
chamber during the intake phase at approximately constant P.sub.L
from the low-side component to the chamber as the piston continues
to descend in the cylinder. The effects of dead volume during the
transition from regeneration phase to intake phase are minimal or
nonexistent in the Correct V2(D) Opening Scenario.
[0200] In the Late V2(D) Opening scenario, V2 is opened at point
1612, by which time the gas in the chamber has decreased to a
pressure of P.sub.L2, significantly lower than P.sub.L. After the
tardy opening of V2, gas from the low-pressure component flows into
the chamber as the pressure in the chamber increases (right-hand
side of large dip in curve 1604). Energy is lost during this
non-work-performing expansion of gas into the chamber from the
low-pressure component: i.e., a dead-volume loss occurs.
[0201] Notably, in systems employing a pressure-driven check valve
for V2 rather than an actuated valve, a pressure-volume history
similar to that of the Late V2(D) Opening scenario (curve 1604)
typically occurs in every compression cycle, as an underpressure
(e.g., P .sub.L2 or some other pressure significantly lower than
P.sub.L) must be achieved on the chamber side of V2 with respect to
the low-pressure-component side of V2 in order for V2 to be
actuated. The use of actuated valves rather than check valves in
CAES systems is thus advantageous in this, as well as in other,
respects.
[0202] In the Early V2(D) Opening scenario, V2 is opened at point
1614, by which time the gas in the chamber has only declined to a
pressure of P .sub.L3, significantly higher than P.sub.L. After the
early opening of V2, gas in the chamber exits to the low-pressure
component as the pressure in the chamber decreases (left-hand side
of curve 1606). Energy is lost during this non-work-performing
expansion of gas from chamber into the low-pressure component,
i.e., a dead-volume loss occurs.
[0203] It will be clear to persons familiar with the sciences of
hydraulics and pneumatics that considerations similar to those
described above with reference to FIGS. 10 and 13-16 also pertain
to optimal, early, and late valve actuation at valve transition
events not explicitly depicted herein, for both compression and
expansion modes of operation of a CAES system. Optimization by the
means described of valve actuations at all valve transition events
in a CAES system is contemplated and within the scope of the
invention. In brief, wherever two or more volumes of gas are to be
brought into fluid communication with each other in the course of
operating a CAES system, optimally timed valve actuations will
generally be those that occur at moments calculated to bring the
volumes of gas into fluid communication with each other when their
pressures are approximately equal.
[0204] Generally, the systems described herein may be operated in
both an expansion mode and in the reverse compression mode as part
of a full-cycle energy storage system with high efficiency. For
example, the systems may be operated as both compressor and
expander, storing electricity in the form of the potential energy
of compressed gas and producing electricity from the potential
energy of compressed gas. Alternatively, the systems may be
operated independently as compressors or expanders.
[0205] Embodiments of the invention may, during operation, convert
energy stored in the form of compressed gas and/or recovered from
the expansion of compressed gas into gravitational potential
energy, e.g., of a raised mass, as described in U.S. patent
application Ser. No. 13/221,563, filed Aug. 30, 2011, the entire
disclosure of which is incorporated herein by reference.
[0206] The terms and expressions employed herein are used as terms
of description and not of limitation, and there is no intention, in
the use of such terms and expressions, of excluding any equivalents
of the features shown and described or portions thereof, but it is
recognized that various modifications are possible within the scope
of the invention claimed.
* * * * *