U.S. patent application number 13/677241 was filed with the patent office on 2013-05-16 for thermal energy storage system.
This patent application is currently assigned to TERRAJOULE CORPORATION. The applicant listed for this patent is Terrajoule Corporation. Invention is credited to Stephen James BISSET, Robert Charles MIERISCH.
Application Number | 20130118170 13/677241 |
Document ID | / |
Family ID | 48279328 |
Filed Date | 2013-05-16 |
United States Patent
Application |
20130118170 |
Kind Code |
A1 |
MIERISCH; Robert Charles ;
et al. |
May 16, 2013 |
THERMAL ENERGY STORAGE SYSTEM
Abstract
A variety of energy storage and retrieval systems are described.
Generally "hot" and "cold thermal reservoirs are provided. The
"hot" reservoir holds both liquid and saturated vapor phase working
fluid. The "cold" reservoir holds working fluid at a lower
temperature than the hot reservoir. A heat engine/heat pump unit:
(a) extracts energy from vapor passing from the hot reservoir to
the cold reservoir via expansion of the vapor in a manner that
generates mechanical energy to facilitate retrieval of energy; and
(b) compresses vapor passing from the cold reservoir to the hot
reservoir to facilitate the storage of energy. In some embodiments,
the heat engine/heat pump takes the form of a reversible positive
displacement heat engine that can act as both an expander and a
compressor. To facilitate the storage and retrieval of electrical
energy, an electric motor/generator unit may be mechanically
coupled to the heat engine/heat pump unit.
Inventors: |
MIERISCH; Robert Charles;
(Menlo Park, CA) ; BISSET; Stephen James; (Palo
Alto, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Terrajoule Corporation; |
Redwood City |
CA |
US |
|
|
Assignee: |
TERRAJOULE CORPORATION
Redwood City
CA
|
Family ID: |
48279328 |
Appl. No.: |
13/677241 |
Filed: |
November 14, 2012 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61559318 |
Nov 14, 2011 |
|
|
|
Current U.S.
Class: |
60/659 |
Current CPC
Class: |
F01K 3/12 20130101; F01K
3/006 20130101 |
Class at
Publication: |
60/659 |
International
Class: |
F01K 3/12 20060101
F01K003/12 |
Claims
1. An energy storage and retrieval system comprising: a first
thermal reservoir arranged to hold water and saturated steam in a
first state; a second thermal reservoir arranged to hold water and
steam in a second state having a lower temperature than the first
state; and a reversible positive displacement steam engine arranged
to, (a) extract energy from steam passing from the first thermal
reservoir to the second thermal reservoir via expansion of the
steam in a manner that generates mechanical energy to facilitate
retrieval of energy from the energy storage and retrieval system,
and (b) compress steam passing from the second thermal reservoir to
the first thermal reservoir to facilitate the storage of energy in
the energy storage and retrieval system, whereby water and steam
serve as a working fluid for the energy storage and retrieval
system.
2. An energy storage and retrieval system as recited in claim 1
further comprising an electric motor/generator mechanically coupled
to the steam engine, the electric motor/generator being arranged to
drive the steam engine when the steam engine is operated as a heat
pump and arranged to generate electricity when the steam engine is
operated as a heat engine.
3. An energy storage and retrieval system as recited in claim 1
wherein the steam engine is selected from the group consisting of a
unaflow steam engine, a universal unaflow steam engine and a
counter-flow steam engine.
4. An energy storage and retrieval system as recited in claim 1
wherein the first thermal reservoir includes a pressure vessel
arranged to hold the working fluid in the first thermal reservoir
at a pressure substantially above ambient atmospheric pressure.
5. An energy storage and retrieval system as recited in claim 1
wherein the second thermal reservoir is arranged to hold
unpressurized working fluid and includes a sub-atmospheric pressure
chamber that facilitates sub-atmospheric flashing of liquid water
to steam and/or sub-atmospheric condensation of steam to a liquid
water state.
6. An energy storage and retrieval system as recited in claim 1
wherein the second thermal reservoir includes a pressure vessel
arranged to hold steam at sub-atmospheric pressures and facilitates
sub-atmospheric flashing of liquid water to steam and/or
sub-atmospheric condensation of steam to a liquid water state.
7. An energy storage and retrieval system as recited in claim 1
wherein the steam engine includes a crankshaft and at least one
working chamber and each working chamber has an associated
reciprocating piston coupled to the crankshaft and a plurality of
associated valves that facilitate the introduction of steam into
the working chamber and the exhaustion of steam from the working
chamber and wherein the timing of the opening and closing of the
valves is variable such that: (a) the steam engine can be operated
in both an expansion mode and a compression mode with the
crankshaft rotating in the same direction; and (b) the timing of
the opening and closing of the valves relative to the crankshaft
angle may be varied to facilitate altering an expansion/compression
ratio of the steam engine.
8. An energy storage and retrieval system as recited in claim 1
wherein the steam engine includes a water injector for adding water
to steam passing through the steam engine for compression before
the compressed steam is exhausted from the steam engine to the
first thermal reservoir.
9. An energy storage and retrieval system as recited in claim 1
wherein the steam engine includes: a plurality of sequential
expansion stages; and a steam separator for removing water from
partially expanded steam between an associated pair of the
expansion stages.
10. An energy storage and retrieval system as recited in claim 1
wherein a Round Trip Efficiency of the energy storage and retrieval
system is at least 70 percent.
11. An energy storage and retrieval system as recited in claim 1
wherein the second thermal reservoir includes first and second
stages, wherein the first stage receives and condenses steam
exhausted from the steam engine after expansion by the steam engine
and the second stage operates as a source of steam for compression
by the steam engine.
12. An energy storage and retrieval system as recited in claim 11
further comprising: a heat source arranged to directly or
indirectly heat working fluid in the second stage of the second
thermal reservoir; and a cooling unit for removing heat from
working fluid in the first stage of the second thermal
reservoir.
13. An energy storage and retrieval system as recited in claim 1
further comprising a heater for at least one of: superheating steam
drawn from the first thermal reservoir before such steam is passed
through the steam engine; and reheating steam between expansion
stages of the steam engine.
14. An energy storage and retrieval system as recited in claim 1
further comprising an electric motor/generator mechanically coupled
to the steam engine, the electric motor/generator being arranged to
drive the steam engine when the steam engine is operated as a heat
pump and arranged to generate electricity when the steam engine is
operated as a heat engine, and wherein: the steam engine is
selected from the group consisting of a unaflow steam engine and a
universal unaflow steam engine; the first thermal reservoir
includes a pressure vessel arranged to hold the working fluid in
the first thermal reservoir at a pressure substantially above
ambient atmospheric pressure; and the second thermal reservoir
includes a pressure vessel arranged to hold steam at
sub-atmospheric pressures and facilitates sub-atmospheric flashing
of liquid water to steam and sub-atmospheric condensation of steam
to a liquid water state.
15. An energy storage and retrieval system as recited in claim 14
wherein the steam engine includes a crankshaft and at least one
working chamber and each working chamber has an associated
reciprocating piston coupled to the crankshaft and a plurality of
associated valves that facilitate the introduction of steam into
the working chamber and the exhaustion of steam from the working
chamber and wherein the timing of the opening and closing of the
valves is variable such that: (a) the steam engine can be operated
in both an expansion mode and a compression mode with the
crankshaft rotating in the same direction; and (b) the timing of
the opening and closing of the valves relative to the crankshaft
angle may be varied to facilitate altering an expansion/compression
ratio of the steam engine.
16. An energy storage and retrieval system as recited in claim 2
wherein the electric motor/generator includes at least one motor
and at least one generator that is separate from the motor.
17. An energy storage and retrieval system comprising: a first
thermal reservoir arranged to hold working fluid in a first state
that includes liquid phase and saturated vapor phase work fluid; a
second thermal reservoir arranged to hold working fluid in a second
state having a temperature that is lower than the temperature of
the working fluid in first thermal reservoir; and a heat
engine/heat pump unit arranged to, (a) extract energy from working
fluid vapor passing from the first thermal reservoir to the second
thermal reservoir via expansion of the working fluid in a manner
that generates mechanical energy to facilitate retrieval of energy
from the energy storage and retrieval system, and (b) compress
working fluid vapor passing from the second thermal reservoir to
the first thermal reservoir to facilitate the storage of energy in
the energy storage and retrieval system.
18. An energy storage and retrieval system as recited in claim 17
further comprising an electric motor/generator arranged to drive
the heat engine/heat pump unit when the heat engine/heat pump unit
is operated in a manner that conveys working fluid vapor from the
second thermal reservoir to the first thermal reservoir and for
generating electricity when the heat engine/heat pump unit is
operated in a manner that conveys working fluid vapor from the
first thermal reservoir to the second thermal reservoir.
19. An energy storage and retrieval system as recited in claim 17
wherein the first thermal reservoir includes a pressure vessel
arranged to hold working fluid in the first thermal reservoir at a
pressure substantially above ambient atmospheric pressure.
20. An energy storage and retrieval system as recited in claim 17
wherein the second thermal reservoir is arranged to facilitate
sub-atmospheric flashing of liquid working fluid to a vapor state
and/or sub-atmospheric condensation of vapor working fluid to a
liquid state.
21. An energy storage and retrieval system as recited in claim 17
wherein the working fluid is selected from the group consisting of:
(a) a mixture that includes water; (b) a fluorocarbon or a mixture
that includes a fluorocarbon; (c) ammonia or a mixture that
includes ammonia; and (d) a hydrocarbon or a mixture that includes
a hydrocarbon.
22. An energy storage and retrieval system comprising: a first
thermal reservoir arranged to hold working fluid in a first state
that includes liquid phase and saturated vapor phase work fluid; a
low temperature thermal energy source arranged to provide vapor
phase working fluid in a second state having a lower temperature
than the first state; a condenser arranged to condense vapor phase
working fluid; and a heat engine/heat pump unit arranged to, (a)
extract energy from working fluid vapor passing from the first
thermal reservoir to the condenser via expansion of the working
fluid vapor in a manner that generates mechanical energy to
facilitate retrieval of energy from the energy storage and
retrieval system, and (b) compress working fluid vapor passing from
the low temperature thermal energy source to the first thermal
reservoir to facilitate the storage of energy in the energy storage
and retrieval system.
23. An energy storage and retrieval system as recited in claim 22
wherein the heat engine/heat pump unit is a reversible positive
displacement steam engine and the working fluid is water.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application No. 61/559,318, filed Nov. 14, 2011, which is hereby
incorporated by reference in its entirety for all purposes.
BACKGROUND
[0002] The present invention relates generally to thermal energy
storage systems. More particularly, it relates to systems that
store thermal energy making use of a phase change such as liquid
water/steam.
[0003] There are many circumstances in which it is desirable to
store electrical energy in order to resolve temporal mismatches
between energy supply and demand. By way of example, the
availability of some alternative energy sources such as solar
energy and wind energy can vary significantly over the course of a
day and the output of devices that harvest such energy, (e.g.,
photovoltaic collectors or wind turbines) is often mismatched with
the demand for the electrical energy that such devices produce.
Another example of the mismatch between supply and demand is
embodied in baseline electricity generating facilities such as
nuclear powerplants which are generally designed to provide a
substantially steady electrical output while the demand for
electricity tends to vary significantly over time.
[0004] The mismatch between supply and demand can make bulk
electrical energy storage desirable and over the years, a wide
variety of systems and devices have been proposed and/or used to
facilitate such storage and retrieval electrical energy. One of the
most familiar examples of a device designed to store electrical
energy is an electrical battery. Although batteries tend to work
well for relatively small-scale energy storage applications, they
tend to be cost prohibitive in larger scale energy storage systems.
An example of a larger scale energy storage system that has been
used in grid scale electrical energy storage applications is pumped
hydro-electric power and storage. In a pumped hydro-electric power
storage system, power is generated by a turbine which extracts the
potential energy of water flowing from an upper reservoir to a
lower reservoir. When demand for power is low, and there is an
excess of supply from other power sources at low cost, the turbine
can be reversed to pump water from the lower reservoir to the upper
reservoir, thereby storing energy as gravitational potential
energy. At a later time, when the demand for power is higher, the
water stored in the upper reservoir can be used to drive the
turbine to generate electrical power.
[0005] Other medium and large scale energy storage systems have
utilized mechanisms such as thermal energy, compressed air,
flywheels, electrical capacitors and chemical energy as the energy
storage mechanisms. Although such conventional energy storage
systems have a number of benefits, there are continuing efforts to
develop cost effective energy storage systems having relatively
high round trip energy recovery efficiencies. Such devices can make
it practical to acquire or purchase electricity and store the
associated energy when energy availability is higher than demand
and its price and/or value is low, and to retrieve and utilize or
sell such energy when energy availability is less than demand and
its price and/or value is high.
SUMMARY OF THE INVENTION
[0006] A variety of energy storage and retrieval systems are
described. Generally "hot" and "cold thermal reservoirs are
provided. The "hot" thermal reservoir is arranged to hold a working
fluid in both a liquid phase and a saturated vapor phase state. The
"cold" thermal reservoir is arranged to hold the working fluid in a
second state having a lower temperature than the working fluid in
the hot thermal reservoir. A heat engine/heat pump unit is arranged
to: (a) extract energy from working fluid passing from the hot
reservoir to the cold reservoir via expansion of the working fluid
in a manner that generates mechanical energy to facilitate
retrieval of energy from the energy storage and retrieval system;
and (b) compress working fluid passing from the cold thermal
reservoir to the hot thermal reservoir to facilitate the storage of
energy in the energy storage and retrieval system.
[0007] A variety of different materials can be used as the working
fluid. In some preferred embodiments, water is used as the working
fluid. When water is used as the working fluid, it passes through
the heat engine/heat pump unit as steam. In such embodiments, the
heat engine/heat pump unit may take the form of a steam engine.
[0008] The heat engine/heat pump may take the form of a reversible
heat engine that can act as both an expander and a compressor or
separate devices may be used as the expander and compressor. In
some embodiments, a reversible positive displacement heat engine
(e.g. a piston steam engine) is used as the heat engine/heat pump.
By way of example, unaflow, universal unaflow and counter-flow
steam engines work well. In some preferred implementations, the
heat engine includes adjustable valves and a controller arranged to
vary the timing of the opening and closing of the valves relative
to a crankshaft angle.
[0009] In order to facilitate the storage and retrieval of
electrical energy, an electric motor/generator unit may be
mechanically coupled to the heat engine/heat pump unit. The
electric motor/generator is arranged to drive the heat engine/heat
pump unit during the compression of working fluid and to generate
electricity during the expansion of the working fluid. The electric
motor/generator may be implemented as a single reversible unit or
as separate motor and generator devices.
[0010] The hot and cold reservoirs may take the form of pressure
vessels or unpressurized reservoirs. When an unpressurized
reservoir is used, a sub-atmospheric pressure chamber may be
provided to facilitate sub-atmospheric flashing of working fluid to
vapor (e.g., liquid water to steam) and/or sub-atmospheric
condensation of vapor to a liquid. When a pressurized vessel is
used as the hot reservoir, the working fluid in the hot reservoir
may be stored at a pressure substantially above ambient atmospheric
pressure.
[0011] In some alternative embodiments, the cold store may be
replaced by the combination of a condenser arranged to condense
steam and a separate low temperature thermal energy source arranged
to provide steam having a lower temperature than the hot
reservoir.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The invention and the advantages thereof, may best be
understood by reference to the following description taken in
conjunction with the accompanying drawings in which:
[0013] FIG. 1 is a schematic diagram of a heat engine/heat pump
based energy storage and retrieval system in accordance with one
embodiment of the invention.
[0014] FIG. 2(a) is a schematic diagram of an energy storage and
retrieval system having a pressurized hot reservoir and an
unpressurized cold reservior with a sub-atmospheric pressure
flashing chamber operating in an energy storage mode.
[0015] FIG. 2(b) is a schematic diagram of the energy storage
system of FIG. 2(a) operating in an energy retrieval mode.
[0016] FIG. 3 is a schematic diagram of an energy storage and
retrieval system having unpressurized hot and cold reservoirs.
[0017] FIG. 4 is a schematic diagram of an energy storage and
retrieval system having pressurized hot and cold reservoirs.
[0018] FIG. 5 is a schematic diagram of an energy storage and
retrieval system having a low temperature source and a separate low
temperature sink
[0019] In the drawings, like reference numerals are sometimes used
to designate like structural elements. It should also be
appreciated that the depictions in the figures are diagrammatic and
not to scale.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0020] A number of thermal energy based energy storage and
retrieval systems are described. In general, the described storage
systems utilize a working fluid having a substantial heat capacity
and make use of a phase change to help improve the storage capacity
of the system (e.g. a liquid water/steam phase change).
[0021] Referring next to FIG. 1, a basic architecture of a heat
engine/heat pump based electrical energy storage and retrieval in
accordance with one embodiment of the invention will be described.
The storage system 100 includes a "hot" thermal reservoir 105 and a
"cold" thermal reservoir 110. A heat engine/heat pump 120 is used
to convey a working fluid between the two thermal reservoirs 105
and 110. In the illustrated embodiment, the heat engine/heat pump
120 is coupled to a generator/motor 130 which in turn is coupled to
an electric source and the electric load as appropriate. When
operating the system 100 in a manner that stores energy,
electricity from an electrical source powers motor 130 which in
turn drives heat pump 120. The heat pump compresses a vapor form of
the working fluid drawn from the cold thermal reservoir 110 which
inherently heats the working fluid as well. The compressed working
fluid is then stored in the hot thermal reservoir 105.
[0022] When operating system 100 in a manner that retrieves energy,
the heat engine/heat pump 120 is operated in a heat engine mode.
The heat engine 120 drives generator 130 which in turn generates
electricity which can be used to power an electrical load,
delivered to the power grid, or utilized in any other desired
manner. Although a system that receives and delivers electrical
energy is illustrated, it should be appreciated that in alternative
embodiments, mechanical energy (such as a rotating shaft driven by
a wind turbine or other device) could drive the heat pump and/or
the mechanical output of the heat engine could be used directly as
desired.
[0023] The hot thermal reservoir 105 holds working fluid at a
higher temperature than the cold thermal reservoir 110 although the
actual temperatures of the reservoirs may vary widely. Thus, as
used herein, the labels "hot" and "cold" are merely intended to
indicate the relative temperatures of the reservoirs as opposed to
the specific temperatures of the working fluid within the
reservoirs or their temperatures relative to ambient temperatures
or any specific reference temperature. As will be described in more
detail below, the thermal reservoirs 105 and 110 may take a wide
variety of forms and may store either pressurized or unpressurized
working fluid.
[0024] In the described embodiments, steam is used as the working
fluid within the heat engine/heat pump 120 and therefore
water/steam is stored in the thermal reservoirs 105 and 110.
However, it should be appreciated that in alternative embodiments,
a wide variety of other working fluids could be used in place of
water. By way of example, various water based mixtures (e.g.,
water/ammonia mixtures), fluorocarbons, ammonia, hydrocarbons and
other refrigerants and/or mixtures that include any of these fluids
can be used in alternative embodiments. In general, it is desirable
to store the working fluid primarily in a liquid phase so that a
phase change can be used advantageously to improve the storage
capacity of the system.
[0025] To facilitate a working explanation of the energy storage
and retrieval system 100, FIGS. 2(a) and 2(b) schematically
illustrate the operation of one suitable configuration of system
100. In this embodiment, the cold thermal reservoir 110 (also
referred to as a "cold store") holds unpressurized water, whereas
hot thermal reservoir 105 ("hot store") holds pressurized water.
FIG. 2(a) schematically illustrates the operation of an energy
storage system in an energy storage mode, whereas FIG. 2(b)
illustrates the operation of the system in an energy retrieval
mode. In the energy storage mode illustrated in FIG. 2(a), steam is
drawn from the cold store 110 and compressed by heat pump 120(a).
The pressurized steam is injected into the hot store 105 where it
is condensed by its contact with water within the hot store. The
introduction and condensation of the compressed steam has the
effect of warming the hot store thereby facilitating the storage of
thermal energy. As will be appreciated by those familiar with art,
an adiabatic compression of a gas (e.g. steam) has the effect of
significantly warming the gas and therefore, the compressed steam
will generally be much warmer than the steam from the cold store
110 that served as the source for the heat pump 120(a).
[0026] Since the hot store 105 is pressurized, there will
inherently be both saturated steam and pressurized water within the
hot store. Although there may be some temperature stratification
within the reservoir, there will generally be a thermodynamic
equilibrium between the saturated steam and the liquid water at the
liquid/vapor boundary (i.e., the water surface). As compressed
steam is introduced to the hot store, both the temperature and the
pressure of the hot store will increase while generally maintaining
a thermodynamic equilibrium at the liquid/vapor boundary. To help
reduce temperature stratification within the hot store, it is often
desirable to inject the steam near the bottom of the hot store as
illustrated in FIG. 2(a) so that the injected steam will contact
the liquid water as it rises through the reservoir, thereby
facilitating condensation of the injected steam. Although the
illustrated injection of incoming steam into the liquid water works
well to facilitate condensation and reduce temperature
stratification, it is not required as other mechanisms can be used
to accomplish the same function(s).
[0027] Since the hot thermal reservoir is effectively a closed
system, the actual operating pressure of the hot store at any time
will be highly dependent on its current temperature (i.e., higher
temperatures=higher pressures). As more compressed steam is added
to the hot store, both its temperature and pressure will continue
to rise. However, it is desirable to set some limit as to the
maximum operational temperature/pressure for the hot store. In
practice, the maximum operational temperatures/pressures tend to be
dictated by economic constraints, with a primary constraint being
the cost of the vessel(s) used as the hot thermal reservoir.
Currently, there are a number of pressure vessels (tanks)
commercially available that are rated to pressures of about 17 or
18 bar gauge (i.e., about 17 or 18 atmospheres). At a pressure of
17 bar gauge (about 250 psig) the temperature of the water and
steam within the hot store would be about 207.degree. C. Since such
pressure vessels are commercially available at relatively low
costs, it may be desirable to design a system such that the maximum
operational pressure of the hot store 105 is in that range.
However, that is by no means a requirement and the maximum
operational pressure of the hot store may be widely varied to meet
the needs of any particular application.
[0028] As indicated above, the cold store 110 serves as a source of
steam for the heat pump 120(a). In the embodiment of FIG. 2(a), the
cold store 110 is unpressurized. Although the bulk of the water in
the cold store 110 is unpressurized, a column 112 is provided to
facilitate sub-atmospheric flashing (boiling) of steam. As is well
known, water boils at a temperature of about 100.degree. C. at
normal atmospheric pressures. However, at lower pressures, water
will flash (boil) into steam at lower temperatures (and of course
water boils at higher temperatures at higher pressures). For
example, at a pressure of 0.1 bar absolute, water will boil at a
temperature of about 46.degree. C., at a pressure of 0.05 bar
absolute, water will boil at a temperature of about 33.degree. C.
and at a pressure of 0.02 bar absolute, water will boil at a
temperature of about 17.5.degree. C. This property of water can be
used to generate steam at temperatures well below 100.degree. C.
even when the water within an associated store is
unpressurized.
[0029] In the embodiment illustrated in FIG. 2(a), a flashing
column 112 is provided in conjunction with the cold thermal
reservoir. The flashing column 112 opens at a level below the
waterline within the cold thermal reservoir 110 and extends above
the surface of the water within the reservoir 110. When air is
evacuated from the column, a vacuum (relative to ambient pressure)
is generated within the column, which has the effect of drawing the
surface of the water within the column to a higher level than the
surrounding water at ambient pressure. If the height of the column
is sufficient and the evacuation of air complete, some of the water
within the column will flash (boil) such that saturated steam fills
the column above the waterline. The chamber formed by the portion
of the column above the waterline is sometimes referred to herein
as a flashing chamber 113. The saturated steam within the flashing
chamber will equalize to a temperature and pressure that is in
equilibrium with the water at the water surface within the column
112. Therefore, like in the hot store 105, the temperature and
pressure of the steam within flashing chamber 113 will vary as a
function of the temperature of the adjacent water. The actual
height of the flashing column 112 may be widely varied based on the
design goals of any particular thermal storage system, but when
steam is used as the working fluid, column heights of at least
about 9.4 meters are generally preferred to generate the desired
sub-atmospheric steam pressures within the flashing chamber
113.
[0030] In the energy storage mode, steam is drawn from the flashing
chamber 113 and compressed by the heat pump 120(b) as illustrated
in FIG. 2(a). When steam is drawn from the flashing chamber, the
pressure within the flashing chamber drops, which causes more water
to vaporize (flash) to bring the chamber 113 back into equilibrium.
Since the latent heat of vaporization of water (and other working
fluids) is relatively high, vaporization of some of the water into
steam extracts heat from the surrounding water thereby effectively
cooling the surrounding water. Therefore, as steam is drawn from
the flashing chamber 113, the temperature of the water within the
cold thermal reservoir 110 will gradually drop.
[0031] When a flashing column is used, the actual design of the
column may be widely varied in order to meet the needs of any
application. Typically, it will be important to provide sufficient
water surface area within the flashing chamber(s) to ensure that
steam can be generated at a rate high enough to supply the heat
pump 120(b) at the desired flow rates. In some applications it will
also be desirable to provide a mechanism for circulating water
around the cold thermal reservoir 110 and the flashing column 112.
This mixing of the water helps reduce the risk of temperature
stratification within the flashing column 112. Specifically, it
should be appreciated that when a significant amount of steam is
being generated, the water at the flashing surface will cool
relatively quickly and thermosiphoning alone may not be sufficient
to maintain the temperature of the water at the flashing surface at
close to the same temperature as the main body of water within the
cold store 110--especially if the diameter or width of the column
(and thus the flashing surface area) is small relative to the
column height. It is generally undesirable for water at the surface
of the flashing chamber to be at a temperature that is
significantly below the temperature of underlying body of water
since that would cause a reduction in the temperature and pressure
of the steam being supplied to the heat pump 120(b) which would
reduce the overall efficiency of the system.
[0032] A variety of mechanism can be used to enhance circulate of
the water within the cold store and flashing chamber. For example,
impellers, propellers and other mixing devices can be appropriately
positioned within the cold store (e.g. in the column 112) and used
to enhance circulation and mixing. In various other embodiments,
two or more columns may be used to enhance circulation.
[0033] Referring next to FIG. 2(b), operation of the energy storage
system 100 in an energy retrieval mode will be described. During
energy retrieval, steam is drawn from the hot store 105 and passed
through steam engine 120(b) as illustrated in FIG. 2(b). The steam
engine expands the steam extracting energy from the steam in a
manner that produces useful mechanical work such as the rotation of
a drive shaft. The drive shaft can then be used in any desired
manner, as for example, to power a generator 130(b) that generates
electricity, to drive a pump or to drive other machinery. The steam
supplied to the steam engine 120(b) is typically drawn from a
location near the top of the hot store, although this is not a
requirement.
[0034] When steam is drawn from the hot store 105, the pressure
within the store will decrease, which in turn causes more water to
vaporize (flash) to bring the hot store 105 back towards
equilibrium. Such vaporization of some of the liquid water into
steam extracts heat from the surrounding water thereby effectively
cooling the surrounding water. Therefore, as steam is drawn from
the hot store 105, the temperature and pressure of the water/steam
within the hot store will gradually drop.
[0035] As will be appreciated by those familiar with heat engines,
the adiabatic expansion of a gas will cause the temperature of the
gas to drop significantly during the expansion process. Thus, the
expanded steam exhausted by the steam engine 120(b) will be
substantially cooler than the input steam and may be returned to
the cold store 110. In order for the steam engine to get the most
work out of the steam (and therefore to operate most efficiently),
it is generally desirable to exhaust the steam at as low of a
pressure as possible. As such, it is often desirable to exhaust
steam from the steam engine at a pressure that is below atmospheric
pressure. Since the pressure within the flashing chamber 113 is
below atmospheric pressure, the flashing chamber serves as a good
location to receive the exhaust from the steam engine.
[0036] Conceptually, when steam is delivered to the flashing
chamber 113, the pressure within the flashing chamber increases,
which causes some of the steam within chamber 113 to condense to
bring the chamber 113 back into equilibrium. Thus, in the energy
retrieval mode, flashing chamber 113 effectively acts as a
sub-atmospheric condensation chamber. The condensation of some of
the steam back into liquid water adds heat to the cold store
thereby effectively warming the surrounding water. Therefore, as
steam is introduced to the flashing chamber 113, the temperature of
the water within the cold thermal reservoir 110 will gradually
rise.
[0037] As steam is injected into the cold store, precautions are
preferably taken to help insure efficient condensation of the steam
and good thermal mixing of the condensate with the water in the
cold store. One way to enhance condensation within the chamber 113
is to spray water droplets into the chamber as steam is introduced
to the chamber. The water droplets enhance condensation of the
steam since steams tends to condense on the droplets. Such spraying
can be accomplished by a sprayer 114 which draws water from the
cold store and sprays a shower of water from the top (or near the
top) of the column, thereby effectively creating a shower of water
within the condensation chamber 113. A simple pump (not shown) may
be used to draw water from any suitable location in the cold store
and spray it into the condensation chamber to thereby enhance
condensation. Other conventional condensation enhancing mechanisms
such as drip trays and Raschig rings, trays or other structures
arranged to enhance the exposed surface area of water (not shown)
can be used as well.
[0038] It is also desirable to insure that there is good mixing
between water at the surface of the flashing chamber and the
underlying water in the cold store. Without good mixing, the
temperature at the surface will rise relative to the underlying
pool as the energy from the condensing steam heats the water near
the surface. Higher temperatures at the water/vapor boundary have
the effect of increasing the pressure within chamber 113. The
increased pressure within chamber 113 means the steam can be
expanded less in the steam engine, which reduces the work that can
be extracted from the steam, thereby reducing the overall Round
Trip Efficiency of the energy storage and retrieval process. The
same mixing mechanisms used to enhance circulate of the water
within the cold store during steam generation can also be used to
circulate water during condensation.
[0039] The described energy storage and retrieval system 100 is
well suited to facilitate thermal storage of energy. When excess
electrical or mechanical energy is available, such energy can be
used to drive the heat pump 120(a) in a manner that compresses and
heats working fluid (e.g. steam) drawn from the cold thermal
reservoir 110 for storage in the hot thermal reservoir 105. When it
is desirable to retrieve energy from the storage system, hot, high
pressure steam is drawn from the hot store and expanded in heat
engine 120(b) to extract useful work from the steam that can be
used to generate electricity or for any other desired purposes. In
an ideal system, in which the heat engine/heat pump 120 and the
motor/generator 130 do not have any losses and there are no thermal
or pressure losses (or gains) in the thermal reservoirs, the Round
Trip Efficiency of the energy storage can be 100%. That is, the
amount of useful energy that could be retrieved from the system
would theoretically be the same as the amount of energy used to
drive the system. Of course, a 100% round trip storage/retrieval
efficiency (RTE) is not possible in a practical system, however
even when the electricity is used to power the system and
electricity is the form of the power ultimately output by the
system, round trip storage/retrieval efficiencies on the order of
60-80% are believed to be readily obtainable using existing
technology (e.g., using a positive displacement steam
engine/compressor in conjunction with a motor/generator as
described in more detail below) and even higher round trip
efficiencies may be possible. Higher efficiencies are also possible
in systems that would directly supply and/or utilize the mechanical
power that drives and/or is output by the heat engine/heat pump
since any inefficiencies of the motor/generator 130 would be
eliminated. As will be apparent to those familiar with grid scale
energy storage applications, there are a number of applications
where round trip storage and retrieval efficiencies in the 60-80%
range are economically viable.
Other Thermal Reservoir Configurations
[0040] In the embodiment illustrated in FIGS. 2(a) & 2(b), the
hot thermal reservoir 105 is pressurized, whereas the cold thermal
reservoir is not. However, that is not a requirement and both
reservoirs may utilize either pressurized or unpressurized
reservoirs. By way of example, FIG. 3 illustrates an arrangement in
which both the hot store 105(a) and the cold store 110(a) are
unpressurized vessels containing a quantity of water and space for
humid air. In this embodiment, the hot store 105(a) also includes a
mechanism (e.g. column 107) that facilitates the generation of
steam at sub-atmospheric pressures like the steam generating column
112 described above with respect to FIG. 2. The cold store may
operate in the same fashion as described above with respect to FIG.
2. Thus, both the hot and cold stores have mechanisms for
generating steam at sub-atmospheric pressures.
[0041] An advantage of the type of system illustrated in FIG. 3 is
that the cost of an unpressurized water vessel is potentially much
cheaper than the cost of pressurized vessel which can help reduce
overall system costs. A disadvantage is that the temperature and
pressure difference between the hot and cold stores will be
substantially less in the configuration of FIG. 3. With a smaller
temperature/pressure differential the volume of steam that must
pass through the heat engine/heat pump 120 is much larger for a
given system energy storage capacity. Thus a substantially larger
heat engine/heat pump and larger thermal reservoirs would typically
be required, both of which involve additional expense. Which
configuration is more-cost effective in any particular
implementation will be a function of the relative costs of the
various components, the available space, etc.
[0042] Yet another embodiment is illustrated in FIG. 4. In this
embodiment, both the hot store 105(b) and the cold store 110(b) are
pressure vessels. The hot store 105(b) works in the same manner
described above with respect to the pressurized hot store 105 of
FIG. 2. The cold store 110(b) operates in a generally similar
manner, but there is typically no longer a need for a separate
flashing column Thus, during the storage of energy, steam for the
heat pump can be drawn from the steam chamber region 111 of the
cold store 110(b) and during the retrieval of energy, steam
exhausted from the steam engine can be directed back into the steam
chamber region 111. Like with the previously described embodiments,
it will typically be desirable to spray water into the steam
chamber region 111 when steam is being added to the cold store to
enhance the condensation of steam.
[0043] From the foregoing, it should be apparent that the size,
geometry and layout of the thermal reservoirs 105 and 110 may be
widely varied within the scope of the invention. In practice, many
tanks and vessels have a circular cross section although this is
not required. In a pressurized hot stores, it is often preferable
to orient the tank(s) in a generally horizontal manner as
illustrated in FIGS. 2-4 (i.e., such that their length is greater
than their height). This can be desirable because it gives more
water surface area, which enhances transitions between the gas and
liquid phases of the water. The horizontal orientation also
potentially reduces the pressure differential between the top and
bottom of the tank, which can be useful when steam is injected into
liquid water near the bottom of a tank, as illustrated for example,
in FIG. 2(a). It should be appreciated that the pressure within the
water column will increase with depth such that there is a higher
pressure at the bottom of the tank than the pressure within the
steam chamber at the top of the tank. Thus, injecting steam lower
in the tank within the water column requires the input steam to be
compressed more than if the steam is injected into the steam
chamber region 111 above the waterline. Over-compression has the
drawback of reducing the overall Round Trip Efficiency of the
system and therefore an advantage of using shallower broader tanks
is that less over-compression is needed it inject steam near the
bottom of a shallower tank. Of course in other embodiments, the
compressed steam can be introduced into the steam chamber region
111 and other mechanisms can be used as necessary to insure good
condensation of the steam and to minimize thermal stratification
within the tank.
[0044] When a sealed tank is used as the cold store, the tank
should be capable of withstanding the vacuum pressure of the
flashing chamber (e.g., up to 1 atmosphere of negative pressure).
In some applications, the use of such vacuum pressure resistant
tanks may be more cost effective than providing a non-pressurized
tank with a flashing column 112. By way of example cylindrical
tanks having a height of at least 10 meters (as for example 12
meters to match shipping container length) work well for many
applications.
[0045] In most of the examples given above, the thermal reservoirs
are described primarily in the context of various tanks and
pressure vessels. Although tanks and pressure vessels work quite
well, it should be appreciated that a wide variety of different
structures can be used as the thermal reservoirs when appropriate.
By way of example, it available, a lake, pond or other defined body
of water could be used as an unpressurized thermal reservoir--and
particularly as the cold store. When readily available, the use of
such bodies of water may have several potential advantageous. For
example, if the total mass of water in a lake or pond it very
substantially more than is used by the energy storage system, then
the overall temperature of a lake used as a cold reservoir may not
fluctuate significantly through the course of an energy storage and
retrieval cycle. A cold store that maintains a relatively constant
temperature tends to facilitate more efficient storage. In other
embodiments cisterns and various in-ground containments can be used
as one or both of the thermal reservoirs.
[0046] In most of the illustrated embodiments each of the thermal
reservoirs takes the form of a single containment. However it
should be appreciated that either or both of the reservoirs may be
formed from any number of individual containment structures.
Indeed, in some applications, modular containments may be
preferable. For example, in one specific application a number of
modular tanks each sized to fit within a shipping container may be
used to form one of the thermal reservoirs (e.g., a cold
store).
[0047] Typically it is desirable to insulate the hot store in order
to avoid thermal losses in the primary energy store. Depending on
the location and operating conditions, it may, or may not, be
desirable to insulate the cold store. The reason that insulation
may be less desirable on the cold store is that inefficiencies in
the storage and retrieval process will typically add heat to the
system and such heat tends to migrate to the low temperature store.
More specifically, during a charge/discharge cycle, a portion of
the energy lost due to inefficiencies of the compressor and heat
engine will appear as a net heat flow into the cold store. This
amount of heat typically needs to be removed from the system to
prevent the temperature of the cold store from rising to an
inefficient level over time. A variety of conventional mechanisms
may be used to regulate the temperature of the cold store 110. As
such, some natural cooling of the cold store may be desirable to
help maintain system balance.
[0048] Referring next to FIG. 5, yet another alternative energy
storage system configuration will be described. In this embodiment,
the functionality of the cold store 110 described in the previous
embodiments is effectively divided into two separate components, a
low temperature sink 175 and a low temperature energy source 185.
This type of configuration can be advantageous when there is an
available source of heat, such as waste heat from an industrial
process or a thermal power plant. In other aspects, the system may
be substantially the same as any of the previously described
embodiments.
[0049] In the illustrated configuration, the hot store 105(c) is an
insulated pressure vessel containing water and a saturated steam
space that operates as described above. The low temperature energy
source may take the form of an unpressurized water vessel heated by
any suitable heat source. The low temperature sink 175 is a
condenser cooled by an available heat sink The heat sink may take
any suitable form and the most appropriate heat sink may vary by
location. By way of example, a stream or other body of water,
evaporative coolers, fin-fan coolers or a variety of other heat
exchangers may be used to cool the condenser.
[0050] In general, the low temperature energy source 185 is
maintained at a temperature that is higher than the sink 175. When
storing energy, water in the low temperature energy source 185 is
flashed into steam as previously discussed with respect to the cold
store 110. As before, a column or other suitable vessel structure
may be used to facilitate sub-atmospheric flashing of the steam.
The temperature of the water and thus the saturated steam supplied
by the source 185 is generally higher than it would be if a cold
store was used. Since the temperature of the water is higher, the
pressure of the steam input to the heat pump 120(a) is higher which
means that less energy is required to compress the steam for
storage in the hot store 105.
[0051] When retrieving energy, the steam engine 120(b) exhausts
steam into the low temperature sink 175 (as opposed to the source
185). The sink 175 condenses the steam and is cooled by the
available heat sink Like the cold store, the sink 175 includes a
sub-atmospheric condenser 177 that facilitates condensation of the
exhaust steam at sub-atmospheric pressures. In general, the
temperature of the sink 175 is cooler than the temperature of the
source 185. Since the temperature of water within the sink's
condensation chamber is lower than the temperature of the source,
the pressure within the condenser 177 (and thus the pressure at
which condensation occurs) is lower as well. Therefore, steam can
be exhausted from the steam engine 120(b) at a lower pressure than
it could if the steam was returned to source 185. This allows the
steam engine to extract more energy from the expansion of the
steam, thereby improving the efficiency of the storage and
retrieval system.
[0052] It should be appreciated that in an ideal system operating
with the arrangement of FIG. 5, more energy is available for
extraction from the steam in the hot store 105 than is required to
compress the steam during energy storage. Thus, from the standpoint
of an electrical system that drives the heat pump and utilizes
electrical energy from the steam engine, the electrical system is
theoretically able to withdraw more electricity during retrieval
than was used during storage which suggests a Round Trip Efficiency
of greater than 100% from the standpoint of the electrical system.
Of course, this isn't free energy, rather, the system effectively
converts the thermal energy from the low temperature heat source
185 to electrical energy. However, if such thermal energy is waste
heat and therefore essentially free, it can be put to use in a
manner that improves the apparent Round Trip Efficiency of the
energy storage system 100.
[0053] Optionally, condensate from the condenser in sink 175 can be
recirculated to low temperature heat source 185 where it is heated
before being used to generate steam. An advantage of such
recirculation is that it potentially reduces the system's overall
consumption of water.
[0054] It should be apparent that there are a number of industrial
and power generation processes that generate waste heat which could
be utilized in the type of system illustrated in FIG. 5. If such
waste heat is carried by water, such water can potentially be
stored directly in the low temperature heat source 185.
Alternatively, one or more appropriate heat exchangers may be used
to warm the water stored in the source 185.
[0055] The size of the reservoirs that are suitable for use as the
hot and cold thermal reservoirs 105, 110 can vary with the desired
energy storage capacity of the system, the pressure ratings of the
vessels, tanks or other structures used as the reservoirs and/or
flashing columns, the operating temperatures ranges (which may be
dictated in part by the permissible operating pressure ranges),
etc. By way of example, to give a rough scale of a suitable system
size, a system designed to deliver 1 Megawatt (MW) of power for 10
hours (and thus has over 10 MW hours of storage capacity) could be
implemented using a 170,000 gallon hot store that can be
pressurized to about 17 bar (about 250 psi) and a 500,000 gallon
unpressurized cold store. In this particular example, the cold
store has about 3 times the volume of the cold store. However, it
should be appreciated that the actual relative volumes of the hot
and cold store may vary widely and that the relative prices of the
vessels that are available for use as the thermal reservoirs may
strongly influence the ultimate reservoir sizes selected for any
particular power storage capacity.
[0056] It should be appreciated that the size of the hot store can
generally be reduced if the vessel used as the hot store can
withstand higher operating temperatures and pressures. Conversely,
the size of the hot store will have to increase if the vessel used
can only withstand lower operating temperatures and pressures or if
the hot store is unpressurized. Furthermore, it should be
appreciated that the volume of the cold reservoir can significantly
affect the required volume of the hot reservoir as well. More
specifically, as indicated above, during operation in the energy
storage mode, thermal energy is withdrawn from the cold store 110
thereby cooling the cold store and thermal energy is added to the
hot store 105 thereby heating the hot store. Thus, the temperatures
of the thermal reservoirs will diverge during energy storage.
Conversely, during energy retrieval, thermal energy is withdrawn
from the hot store 105 thereby cooling the hot store and thermal
energy is added to the cold store 110 thereby heating the cold
store. Thus the temperatures of the thermal reservoirs will
converge during energy retrieval. As the temperatures converge, the
amount of energy that can be retrieved from steam drawn from the
hot store will decrease.
[0057] It should be appreciated that the temperature of a cold
store that is the same size as the hot store will vary
significantly more than the temperature of a cold store that has
three times (3.times.) the volume of the hot store. Thus, given
identical hot stores a system that has a 3.times. cold store will
have the ability to extract more energy than a system with a cold
store that is the same size as the hot store. By extension, if a
still larger cold store is use (e.g. a 10.times. cold store) it is
possible to extract even more energy from a hot store of the same
size. If a relatively large body of water such as a lake that has
many, many times the volume of the hot store, then the cold store
may stay substantially the same temperature throughout an energy
storage and retrieval cycle. Such a system would have the ability
to extract even more energy given an identical hot store which
provides a larger usable energy storage capacity. Thus, it should
be apparent that selection of the relative sizes of the hot and
cold reservoirs will often be based primarily on the cost benefit
analysis given available resources, tanks and pressure vessels.
[0058] Either of the hot and cold thermal reservoirs may be
implemented as single tank, vessel or reservoir, or may be
implemented as multiple tanks, vessels and/or reservoirs. By way of
example, a 500,000 gallon hot store could be implemented as
seventeen 30,000 gallon pressure vessels which might each be about
40 feet long and 12 feet in diameter.
Steam Engine
[0059] The heat engine/heat pump 120 can be implemented in a
variety of different manners. Although a variety of heat pumps and
heat engines including steam turbines may be used, one class of
heat engines that is particularly well suited for use as the heat
engine/heat pump 120 are positive displacement piston steam
engines. One advantage that piston steam engines have over turbines
and other conventional heat pumps and heat engines is their
relatively high operating efficiencies over a wide range of inlet
and exhaust pressures and temperatures. This is particularly useful
in the described energy storage and retrieval systems because it
allows the temperatures and pressures of both the hot and cold
thermal reservoirs to vary significantly over the course of an
energy storage and retrieval cycle without drastically reducing the
system's round trip energy storage and retrieval efficiency.
[0060] Another advantage of piston steam engines is that they can
operate relatively efficiently over a range of steam mass flow
rates which allows the system to efficiently draw or deliver energy
even in the face of variable energy supply and demand. This
capacity meshes well in applications where the energy available for
storage at any given time may vary significantly during the course
of a day, such as is inherent in solar and wind farms. It also fits
extremely well in applications that have variable demand for energy
during retrieval (which are many).
[0061] Many piston steam engines are reversible in that they may be
operated as both as a heat engine (expander) and as a heat pump
(compressor). This can be advantageous because it allows a single
machine to be used as both the heat pump and the heat engine.
Although a reversible device is useful in many applications, it is
not required. Rather, when desired, separate devices can be used as
the heat pump and the heat engine and the heat pumps/heat engines
shown in the drawings are intended to represent both approaches.
This may be desirable, for example, when separate expanders and
compressors are less expensive and/or more efficient than a
reversible device. Furthermore, it should be appreciated that more
than one steam engine (or other devices) may be used in parallel as
the heat engine/heat pump in order to deliver the desired
throughput.
[0062] It should also be appreciated that despite the fact that
positive displacement piston steam engines are not currently
popular, piston steam engines are a very mature technology. Indeed,
piston steam engines were the dominant source of power well into
the 20.sup.th century. Accordingly, there are a number of existing
piston steam engine designs that may be used as the heat
engine/heat pump. These include single stage steam engines and
multi-stage (multiple expansion stages) steam engines, etc. In some
specific applications, a single or multi-stage Unaflow,
Universal-Unaflow or counter-flow piston steam engine may be used.
One such engine has been developed by the Applicant and is based on
the Skinner Universal-Unaflow steam engine from the 1930s.
[0063] The actual number of serial stages that are desirable and
appropriate for any particular application will vary in accordance
with a number of factors including the expected temperature
differential between the thermal reservoirs, the availability of
steam separators and water injectors, costs and other factors. In
some applications, a single stage of expansion may be preferred
whereas in other multiple stages (typically in the range of 2-5
serial stages) would be preferred.
[0064] Unaflow steam engines and Universal-Unaflow steam engines
are piston based steam engines that utilize poppet valves to
control the introduction of steam into the cylinder that serves as
an expansion chamber. Conventionally, the timing of the valves has
been controlled by a camshaft. However, it should be appreciated
that other known valve timing control mechanisms can be used as
well. For example, electronic solenoids may be used to open and
close the valves which facilitates electronic control the timing of
the opening and closing of the valves if desired. Regardless of the
mechanism used, there are several advantages to providing a wide
range of control over the relative timing of the opening and
closing of the valves. For example, in order to maximize
efficiency, it is generally preferable for the compression ratio of
the steam engine/heat pump to relatively closely match the pressure
ratio between the pressures of the hot and cold stores. Varying the
timing of the opening and closing of the intake and exhaust valves
is one good way to accurately control the compression ratio of the
engine. Varying the intake valve cutoff timing also provides a good
mechanism for controlling the mass flow rate of steam through the
steam engine which allows good control of the power generated by
the steam engine during expansion.
[0065] Adjustable valve timing can also be very useful in
facilitating reversible operation of the steam engine such that the
steam engine may be operated as either an expander (a traditional
steam engine) or as a compressor (heat pump). Conceptually, many
steam engines, including piston and other positive displacement
steam engines may be operated as either an expander or a
compressor. The primary difference is that in the expander mode,
high pressure steam is drawn into the expansion chamber and is used
to drive a piston that delivers useful work to a crankshaft,
whereas in the compressor mode, an electric motor (or other
suitable power source) powers the crankshaft which drives the
pistons to compress steam within the chamber (which now acts as a
compression chamber instead of an expansion chamber). If the timing
of the opening and closing of the valves remains the same (relative
to crank angle), then to change from expansion to compression, the
rotational direction of the crankshaft must be reversed. This
requires stopping the steam engine and reversing its direction.
This works fine in energy storage systems where rapid shifts
between power consumption and power generation are not
required--such as when storage is expected to occur primarily at
night when electricity rates are low and retrieval is expected to
occur primarily during other parts of the day when electricity
rates are higher. However, there are also a number of applications,
such as load balancing, where the ability to quickly shift between
power storage and power delivery modes is quite valuable (i.e.,
shifting modes in seconds rather than minutes). In a piston based
steam engine, this type of rapid shifting between power generation
and power storage modes is possible if the crankshaft can continue
to rotate in the same direction when shifting modes. This can be
accomplished by altering the relative timing of the opening and
closing of the valves--whose functions (intake and exhaust) are, of
course, reversed when the operational mode is reversed.
[0066] As is well known, anything close to the adiabatic
compression of steam will cause the superheating of the steam. Too
much superheating of the steam may be undesirable since it can lead
to thermal losses in the system. Therefore, in some embodiments,
one or more water injectors are provided to spray water into the
steam before or during compression so that some of the thermal
energy generated during compression is absorbed by vaporization of
the injected water. The injector(s) may be arranged to inject water
directly into each cylinder during compression or they may be
arranged to inject the water into the steam prior to its
introduction into a compression cylinder. In general it is
desirable to keep the compressed steam that exits the steam engine
during compression at a state that is close to saturation so that
excess superheating does not occur. However, that is not a
requirement. It should be appreciated that the desired amount of
water to inject will vary as a function of several different
factors such as the compression ratio currently in use, the mass
flow rate of steam passing through the cylinder(s), the entrance
temperature and pressure, etc. Therefore, it is preferable to
control the volume of water injected at any given time based on the
operating state of the engine so that a point close to saturation
can be achieved in the compressed steam exiting the steam engine.
If more than one compression stage is used, then water injection
may be desired for each compression stage, although again, this is
not a requirement.
[0067] Conversely, adiabatic expansion of saturated steam can cause
some of the steam to condense during the expansion. It is generally
undesirable to introduce wet steam (i.e., steam that includes water
droplets) into a cylinder because the condensation can erode inlet
valve ports. Therefore, in multi-stage steam engines, it is often
desirable to pass the steam through a steam separator (not shown)
between sequential stages in order to remove most of the water
droplets from the steam. It may also be desirable to pass the steam
through a steam separator before it enters the first (or only)
expansion chamber (cylinder). Any water condensed by the steam
separators can be returned to one of the thermal reservoirs
(typically the cold reservoir) to help capture the energy contained
therein. A variety of different conventional steam separators can
be used to remove water droplets from the steam. By way of example,
mesh steam separators work well.
[0068] In other embodiment it may be desirable to superheat the
steam before it enters the steam engine and/or to reheat the steam
between sequential stages. This can be accomplished through the use
of an optional superheater that heats steam drawn from the hot
store before it enters the steam engine, and/or an optional
reheater that heats steam between sequential stages of the steam
engine. Although the use of superheaters and/or reheaters is
possible, they are often not cost effective unless the heat
source(s) used to power the superheaters and/or reheaters are waste
heat sources or other very low cost energy sources.
Additional Features
[0069] It should be apparent that the various described energy
storage systems are well suited for the delivering low cost bulk
energy storage and retrieval with a high Round Trip Efficiency.
Storage and retrieval intervals can be as short as a few seconds
and as long as hours or days. Intermittent power sources such as
wind and solar tend to exacerbate the mismatch between energy
supply and demand, thus increasing the need for such energy storage
and retrieval systems.
[0070] Although only a few embodiments have been described in
detail, it should be appreciated that the invention may be
implemented in many other forms without departing from the spirit
or scope of the invention. For example, the specific construction
and layout of the hot and cold thermal reservoirs and the specific
devices used as the heat pumps and heat engine may be widely
varied. The hot and cold stores may each be implemented as a single
tank or as multiple separate tanks. When multiple tanks are used
for one of the stores, the working fluid within the different tanks
in the same store may be a substantially the same temperature or at
different temperatures. As previously mentioned, one or more
reversible heat engine(s)/heat pump(s) may be used as the
expander/compressor or separate devices may be used for expansion
and compression. Similarly, in electricity storage and retrieval
applications, a reversible motor/generator may be used to drive/be
driven by the heat engine/heat pump, or separate motor(s) and
generator(s) may be used. The described systems are readily
scalable and may be used in a variety of different energy storage
applications, including grid scale energy storage and retrieval
applications.
[0071] It should also be appreciated that if sources of low cost
heat are readily available, such heat can be used to help improve
the efficiency of the system. One such arrangement was illustrated
in FIG. 5 where waste heat was used to warm (or provide) the water
that serves as the source of the low temperature steam used for
compression. In other embodiments appropriate heaters or heat
exchangers could be used to either superheat steam from the hot
store 105 before it is introduced to the heat engine 120(b) or to
reheat steam between expansion stages. As will be appreciated by
those familiar with the art, superheating steam before expansion
and/or reheating steam between expansion stages helps avoid
problems and inefficiencies that can be induced by condensation of
steam during the expansion process and can reduce or eliminate the
need for the use of steam separators. Of course, the best use of
the available heat will depend in part on the temperatures that can
be obtained. Thus, when desired and appropriate, a superheater (not
shown) may be provided between the hot store 105 and the heat
engine to superheat steam before it enters the steam engine 120(b)
and/or one or more reheaters (not shown) may be provided to reheat
steam at appropriate stage in the expansion process.
[0072] Effective Round Trip Efficiencies of at least 60, 70 and
even 80% can be attained using the described approach. Although
higher Round Trip Efficiencies are generally desirable, cost
considerations may dictate the system design and thus the
attainable Round Trip Efficiency. However, systems having 60-80%
(or higher) Round Trip Efficiencies are believed to be economically
viable in a number of specific applications.
[0073] The described embodiments should be considered illustrative
and not restrictive and the invention is not to be limited to the
details given herein, but may be modified within the scope and
equivalents of the appended claims.
* * * * *