U.S. patent application number 15/993285 was filed with the patent office on 2018-12-06 for thermal energy storage systems and methods.
This patent application is currently assigned to Combined Power LLC,dba Hyperlight Energy. The applicant listed for this patent is Combined Power LLC,dba Hyperlight Energy. Invention is credited to John D. H. King, Nicholas Aaron Kramer, Gregory S. Mungas.
Application Number | 20180347913 15/993285 |
Document ID | / |
Family ID | 64455569 |
Filed Date | 2018-12-06 |
United States Patent
Application |
20180347913 |
Kind Code |
A1 |
Mungas; Gregory S. ; et
al. |
December 6, 2018 |
THERMAL ENERGY STORAGE SYSTEMS AND METHODS
Abstract
Thermal energy storage systems and methods are provided
including a bed, a blend of aggregates packed in the bed, and a
high-density heat transfer fluid flowing through the blend of
aggregates. The blend of aggregates includes rock materials and may
also include non-rock materials. The heat transfer fluid flows
through the blend of aggregates such that heat is transferred
between the heat transfer fluid and the blend of aggregates. The
porosity of the aggregates increases heat transfer and the high
density of the heat transfer fluid reduces the pressure gradient of
the heat transfer fluid. In exemplary embodiments, the heat
transfer fluid is a liquid comprised of carbon-based molecules.
Methods of safely storing and releasing energy are provided in
which axial thermal conductivity of the bed is minimized and
inadvertent pressure release failures are mitigated.
Inventors: |
Mungas; Gregory S.; (Sun
City West, AZ) ; King; John D. H.; (La Jolla, CA)
; Kramer; Nicholas Aaron; (San Diego, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Combined Power LLC,dba Hyperlight Energy |
Lakeside |
CA |
US |
|
|
Assignee: |
Combined Power LLC,dba Hyperlight
Energy
Lakeside
CA
|
Family ID: |
64455569 |
Appl. No.: |
15/993285 |
Filed: |
May 30, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62512998 |
May 31, 2017 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F28D 20/0056 20130101;
F28D 20/023 20130101; F28D 20/021 20130101; F24D 2200/12 20130101;
Y02E 60/14 20130101; F24D 2220/08 20130101; F28D 20/025 20130101;
F24D 2220/10 20130101 |
International
Class: |
F28D 20/02 20060101
F28D020/02 |
Claims
1. A thermal energy storage system comprising: a bed; a blend of
aggregates packed in the bed, the blend including rock materials;
and a high-density heat transfer fluid flowing through the blend of
aggregates such that heat is transferred between the heat transfer
fluid and the blend of aggregates; wherein porosity of the
aggregates increases heat transfer and the high density of the heat
transfer fluid reduces the axial pressure gradient of the heat
transfer fluid.
2. The system of claim 1 wherein the heat transfer fluid is a
liquid with a fluid density of at least 0.4 g/cc at at least one
location in the packed bed
3. The system of claim 1 wherein the blend of aggregates further
includes non-rock materials.
4. The system of claim 1 wherein the porosity is less than about
30%.
5. The system of claim 4 wherein the porosity is about 10%.
6. The system of claim 1 wherein the blend of aggregates comprises
one or more of: sand, pebbles, washed rock, quartz, and
magnetite.
7. The system of claim 1 wherein the packed bed stores energy at
temperatures less than about 325.degree. C.
8. The system of claim 7 wherein the packed bed stores energy at
temperatures less than about 250.degree. C.
9. The system of claim 8 wherein the packed bed stores energy at
temperatures less than about 150.degree. C.
10. The system of claim 1 wherein the packed bed stores energy at
temperatures less than about 425.degree. C.
11. The system of claim 1 wherein the bed is a vertical
cylinder.
12. The system of claim 1 further comprising filtering media at a
base or top of the bed.
13. The system of claim 1 wherein the bed comprises means to
facilitate the management of rock particulate contamination.
14. The system of claim 1 wherein the surface area of aggregate per
unit volume is about 215 m.sup.-1.
15. The system of claim 1 wherein the heat transfer fluid has a
thermal conductivity of less than about 10% of the aggregates.
16. A thermal energy storage system comprising: a bed including a
fluid inlet and a fluid outlet; a blend of aggregates packed in the
bed, the blend including rock materials and non-rock materials; and
a heat transfer fluid flowing through the blend of aggregates such
that heat is transferred between the heat transfer fluid and the
blend of aggregates, the heat transfer fluid being a liquid
comprised of carbon-based molecules.
17. The system of claim 16 wherein the heat transfer fluid does not
freeze at temperatures above 100.degree. C.
18. The system of claim 16 wherein the fluid viscosity of the heat
transfer fluid ensures fluid pressure drop from the fluid inlet to
the fluid outlet is less than about 5 psid under nominal conditions
and less than about 100 psid under cold start-up conditions.
19. The system of claim 16 wherein the vapor pressure of the heat
transfer fluid is less than twice atmospheric pressure at the
highest design temperature of any portion of the bed in direct
contact with the heat transfer fluid.
20. The system of claim 16 wherein the vapor pressure of the heat
transfer fluid is less than about 300 psia at the highest design
temperature of any portion of the bed in direct contact with the
heat transfer fluid.
21. A method of storing and releasing energy, comprising: packing a
blend of aggregates in a bed to minimize axial thermal conductivity
of the bed, the blend including rock materials and non-rock
materials; providing a liquid organic heat transfer fluid having a
thermal conductivity of less than about 10% of the thermal
conductivity of the blend of aggregates; and directing the heat
transfer fluid through the blend of aggregates such that heat is
transferred between the heat transfer fluid and the blend of
aggregates; wherein over 85% of stored energy can be extracted in a
charge/discharge cycle in less than eighteen hours.
22. The method of claim 21 wherein over 85% of stored energy can be
extracted in a charge/discharge cycle in less than six hours.
23. The method of claim 21 further comprising slowing the rate of
release of stored energy by at least one order of magnitude.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to and benefit of U.S.
Patent Application Ser. No. 62/512,998, filed May 31, 2017, which
is hereby incorporated by reference in its entirety.
FIELD
[0002] The present disclosure relates to thermal energy storage
systems and methods. The present disclosure relates to systems and
methods of storing and releasing energy using organic heat transfer
fluids and energy storage media such as pelletized rock
BACKGROUND
[0003] There has been a longstanding need to develop a low-cost,
cost-effective energy storage solution for grid scale utility
energy storage, particularly for renewable energy production
wherein the timeframes available for energy production typically do
not occur or align well with grid scale energy demand. Currently,
over the timespan of a day, the U.S. electric grid primarily
provides energy on demand with the ability to effectively store
<<5% of the energy produced over the course of the day. Much
of this storage is currently associated with the use of existing
hydroelectric facilities (e.g. dams) that allow one to pump water
into reservoirs during "energy storage" events. This form of grid
scale energy storage is not a cost-effective energy storage
solution for new facilities typically not being the primary reason
for damn construction and is already near capacity in the U.S.
Current attempts to develop low cost energy storage solutions have
results in levelized costs of energy for just the energy storage
component that exceed the current levelized costs for energy
production.
[0004] The cost of storage of energy in the form of electricity is
typically much higher than the cost of storage of energy in the
form of heat. This is due primarily to the much higher relative
product lifecycle cost per unit energy stored for materials that
support electrochemical reactions compared to the product lifecycle
cost per unity energy stored for abundant low-cost materials that
can store heat. Two primary renewable energy sources that could
potentially support a large fraction of global energy demand,
concentrated solar thermal heat and geothermal heat, are abundant
renewable thermal energy sources that would benefit greatly from a
low cost thermal energy storage solution (i.e. "thermal battery")
that would enable energy storage and energy delivery during times
of peak and/or substantive energy demand.
[0005] In conventional storage of thermal energy using passive
thermal mass, several different architectures currently exist. One
example is storing heat in a higher temperature working fluid such
as molten salt by heating and/or cooling the working fluid. In this
scenario, thermal energy is stored in the form of sensible heat
through a difference in temperature of a unit mass of fluid between
two temporal conditions (e.g. an initial state and an end state). A
portion of the stored heat may also be stored in the phase change
(e.g. enthalpy of fusion/melting) that may occur over a period of
time within a volume of the storage medium when heat is added or
removed. In other conventional thermal storage architectures,
storage of heat has been demonstrated with solid thermal mass,
e.g., heating concrete or bulk rock. One of the challenges in
heating and cooling solid mass is the longer timeframes typically
associated with adding and removing heat that limit how quickly
thermal mass can be recharged and discharged, particularly in
utility scale battery applications.
[0006] An additional factor that must be considered, particularly
with sensible heat storage solutions, is the evolution of the
temperature gradient (a.k.a. thermocline) that must exist in the
thermal storage medium in order to allow heat to be transferred
into or out of the energy storage medium. This temperature gradient
and its spatial distribution and temporal evolution typically
effectively reduces the energy storage capacity of the battery as
well as negatively impacts the roundtrip storage efficiency. This
occurs due to the fact that a certain quantity of energy may
typically be required to be deposited to establish and/or overcome
an existing temperature gradient in order to allow energy to flow
in a desired direction. Unlike electrochemical energy storage
solutions, the "quality," i.e., the temperature, of the delivered
heat may typically change which can impact the downstream
conversion efficiency and/or ability of a downstream process to
fully utilize energy that was previously produced. Therefore, for
at least the reasons outlined herein, careful management of the
temperature distributions in a thermal energy reservoir are
critical for ensuring high roundtrip efficiencies of thermal
batteries while simultaneously minimizing the volume and cost
associated with the energy storage medium of the thermal
battery.
[0007] Most of the concentrating solar power industry is currently
focused on inorganic heat transfer fluids, (e.g. molten salt) that
freeze at temperatures much higher than ambient. These high
freezing points can become a major operational and/or risk
mitigation issue to make sure that freezing doesn't happen
inadvertently (e.g. several days of cloud cover that results in the
system cooling off) which in most cases would render the solar
plant partially or fully inoperable for extended periods of time
particularly in areas of the fluid system that cannot be readily
heated. Furthermore, the molten salts can be very challenging to
work with in terms of corrosion of materials.
[0008] Water has also been used as a "low cost" heat transfer fluid
for concentrating solar; however, given that water dissolves so
many compounds that change the behavior of corrosion with the
metals that contain it, water can actually be very difficult to
work with in terms of guaranteeing the longevity and integrity of
the wetted fluid system over its operational life. It also can
place a huge cost constraint on systems due to the fact that any
system operating with over 15 psig of water vapor pressure
typically requires a steam boiler operator, the OPEX cost of which
over a typical lifetime of power purchase agreements can exceed the
CAPEX installation cost of the hardware for smaller (.about.acre
scale) systems.
[0009] Organic compound-based heat transfer fluids can resolve the
freezing point, corrosion, and other cost issues associated with
water and salts. Designing for organic compound-based heat transfer
fluids, however, potentially introduces their own unique
challenges. Organic compound-based heat transfer fluids can be
flammable and/or create a pressure release hazard due to the
natural vapor pressure of the fluid and/or additional gas pressure
the fluid can generate if it decomposes into gas or combusts.
Furthermore, in considering explosive energy potential due to
inadvertent off-nominal interactions with the thermal battery (e.g.
excess heating, excessive mechanical shock), the energy storage
medium should be robust enough to contain stored energy without an
inadvertent rapid energy release that creates a substantive
operations hazard.
[0010] Energy release from a thermal battery can occur in many
different forms, but typically for inadvertent structural damage to
occur on very short timescales (<<10 seconds) all such
explosive energy releases typically involve the inadvertent buildup
of pressure. For example, in one scenario, rapid chemical reactions
that may be initiated due to heating or mechanical impact, may
convert some and/or all of the energy storage media in the battery
into very low-density gases compared to the liquid and/or solid
densities that initially comprised the battery. To keep these
low-density gases contained in the same volume initially occupied
by the battery, very large pressures may be generated.
Alternatively, inadvertent rapid energy release from the battery
may cause runaway heating to occur. This inadvertent heating may
result in thermal damage or initiation of fire hazards that while
less hazardous than explosive energy releases, may still result in
substantive property damage and/or loss.
[0011] Candidate thermal battery energy storage media must address
the constraints identified above while also being fundamentally
very low cost over the lifetime of the use of the battery. For many
utility scale energy applications, the associated product lifetimes
can be quite long and may typically be equal to or greater than 20
years. These long timescales for utility batteries are much longer
than conventional electrochemical batteries are typically designed
to survive, much less operate. The thermal energy storage media for
these utility scale energy applications should minimize maintenance
requirements over these extended product lifetimes to help ensure
the lifecycle cost of the thermal battery remains low.
[0012] In the design of any energy storage solution, three metrics
that are considered important are: 1) the roundtrip
charge/discharge efficiency; 2) the peak charging/discharging rate;
and 3) the hazard potential for inadvertent rapid energy release.
Metric 1 effectively determines what the energy cost is for
effectively time-shifting energy delivery. High efficiency energy
storage mechanisms are desirable in order to mitigate additional
energy storage cost for time-shifting energy delivery. The metric 2
peak charging/discharging rate is very important to ensure that the
energy storage solution can effectively support both the charging
and discharging rates that would be required particularly in a
utility scale energy operation where ramp rates for discharge and,
in some cases, recharge as well can be very substantial. In
California, the "duck curve" utility power demand profile places
major constraints on the ramp rates for powerplants that can be
quite challenging to meet for traditional batteries. Finally, the
metric 3 is a metric typically derived in the form of a MIL-SPEC
hazard classification from MIL-STD testing that determines the
worst case high rate of energy release (and associated hazard) that
can be produced when the energy storage system is exposed to
extreme environments and/or threats that may be experienced
particularly when operating in off-nominal conditions or
scenarios.
SUMMARY
[0013] The present disclosure alleviates the problems of existing
energy storage solutions by providing thermal energy storage
systems and methods which use pelletized rock as the majority of
the energy storage media and high-density heat transfer fluids,
e.g., liquids, that do not have the high pressure drop issues
across a packed bed that can more commonly be associated with low
density heat transfer fluids such as gases (e.g. air, CO.sub.2,
etc.) and vapors (e.g. steam, etc.). Disclosed systems and methods
use very low porosity, e.g., less than about 30%, and in many cases
much lower, beds to get the surface area per unit volume high. This
advantageously provides rapid charge and discharge rates for
utility scale power and reduced cost to minimize the volume of
expensive heat transfer fluid that needs to be used.
[0014] Exemplary embodiments provide a lower temperature, e.g.,
<325.degree. C., thermal battery for lower temperature process
heat applications in that temperature range, and in alternate
exemplary embodiments, at temperatures less than about 250.degree.
C. or less than about 150.degree. C. Most reasonable-cost heat
transfer fluids that aren't toxic and can be readily handled have
vapor pressures that rapidly climb above 300.degree. C. Also, the
costs associated with rated fittings and components start to climb
rapidly at the higher pressures and temperatures above 300.degree.
C. Energy storage systems and methods disclosed herein can
advantageously operate at a cost less than about $615/kWh while
supporting a >35% efficient power cycle, a 90% efficient
receiver panel, 4-14 hours of thermal energy storage with up to 99%
energetic efficiency and up to 95% exergetic efficiency. In some
cases, with slightly higher rated pressure vessels and associated
cost for these system, the energy storage systems can be designed
to operate at temperatures up to .about.425.degree. C. near the
breakdown limit of high temperature, synthetic hydrocarbon compound
heat transfer fluids.
[0015] Exemplary embodiments of a thermal energy storage system
comprise a bed, a blend of aggregates packed in the bed, and a
high-density heat transfer fluid flowing through the blend of
aggregates such that heat is transferred between the heat transfer
fluid and the blend of aggregates. In exemplary embodiments, the
bed is a vertical cylinder. The blend of aggregates includes rock
materials. The porosity of the aggregates increases heat transfer
and the high density of the heat transfer fluid reduces the
pressure gradient of the heat transfer fluid. In exemplary
embodiments, the porosity of the aggregates is less than about 30%
and the porosity may be about 10%. The heat transfer fluid may have
a thermal conductivity of less than about 10% of the aggregates. In
exemplary embodiments, the heat transfer fluid is a liquid with a
fluid density of at least 0.4 g/cc at at least one location in the
packed bed.
[0016] In exemplary embodiments, the blend of aggregates comprises
one or more of sand, pebbles, washed rock, quartz, and magnetite.
The blend of aggregates may further include non-rock materials. The
rock-based material constituent may comprise about 80% or more of
the blend of aggregates, and the non-rock materials about 20% or
less of the blend of aggregates. The surface area of aggregate per
unit volume may be about 215 m.sup.2/m.sup.3. The packed bed may
store energy at temperatures less than about 325.degree. C., and in
exemplary embodiments, at temperatures less than about 250.degree.
C. or less than about 150.degree. C. In some higher temperature
applications, the packed bed may store energy at temperatures less
than about 425.degree. C. In exemplary embodiments, there is coarse
filtering media at a base of the bed. In other embodiments, the
heat transfer fluid is directed vertically up through the bed
during charge/discharge cycles. The bed may include means to
facilitate the management of rock particulate contamination.
[0017] Exemplary embodiments of a thermal energy storage system
comprise a bed including a fluid inlet and a fluid outlet, a blend
of aggregates packed in the bed, and a heat transfer fluid flowing
through the blend of aggregates such that heat is transferred
between the heat transfer fluid and the blend of aggregates. The
blend of aggregates includes rock materials, and in some cases
non-rock materials. The heat transfer fluid may be a liquid
comprised of carbon-based molecules and, in exemplary embodiments,
the heat transfer fluid is an oil. In exemplary embodiments, the
heat transfer fluid does not freeze at temperatures above
100.degree. C. In exemplary embodiments, the fluid viscosity of the
heat transfer fluid ensures fluid pressure drop from the fluid
inlet to the fluid outlet is less than about 5 psid under nominal
conditions and less than about 100 psid under cold start-up
conditions. In exemplary embodiments, the vapor pressure of the
heat transfer fluid is less than about 300 psia at the highest
design temperature of any portion of the bed in direct contact with
the heat transfer fluid. In exemplary embodiments, the vapor
pressure of the heat transfer fluid is less than twice atmospheric
pressure at the highest design temperature of any portion of the
bed in direct contact with the heat transfer fluid.
[0018] Exemplary methods of storing and releasing energy comprise
packing a blend of aggregates in a bed, the aggregate particle
sizes and distributions of which are designed to minimize porosity
and temperature differences between the particles and the heat
transfer fluid as well as minimize axial thermal conductivity of
the bed, providing a liquid organic heat transfer fluid having a
thermal conductivity of less than about 10% of the thermal
conductivity of the individual aggregate particles, and directing
the heat transfer fluid through the blend of aggregates such that
heat is readily transferred between the heat transfer fluid and the
blend of aggregates. The blend of aggregates includes rock
materials. Over 85% of the stored thermal energy in the bed can be
extracted in a charge/discharge cycle in less than 18 hours. In
exemplary embodiments, over 85% of stored energy can be extracted
in a charge/discharge cycle in less than six hours. Exemplary
methods further include bed/heat transfer fluid design to minimize
inadvertent explosive release of built-up gas pressure in the
Thermal Battery in extreme operational scenarios. Exemplary methods
may further comprise slowing the rate of release of inadvertent gas
pressure buildup during off-nominal operations by at least one
order of magnitude.
[0019] Accordingly, it is seen that systems and methods of storing
and releasing energy are provided. These and other features and
advantages will be appreciated from review of the following
detailed description, along with the accompanying figures in which
like reference numbers refer to like parts throughout.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] The above-mentioned features and objects of the present
disclosure will become more apparent with reference to the
following description taken in conjunction with the accompanying
drawings wherein like reference numerals denote like elements and
in which:
[0021] FIG. 1 is a front cut-away view of an exemplary embodiment
of a thermal energy storage system in accordance with the present
disclosure in this case illustrating a downward directed thermal
battery charge heat transfer fluid flow;
[0022] FIG. 2 is a table listing exemplary candidate thermal
storage media and energy capacity cost for 100.degree. C. delta T
(sensible heat storage);
[0023] FIG. 3 is a graph showing exemplary pressure drop
characteristics of a packed bed of uniform spherical particles as a
function of fluid density (e.g. air and oil) and particle diameter;
and
[0024] FIG. 4 is a schematic depicting an exemplary embodiment of a
packed bed thermal energy storage system in accordance with the
present disclosure.
DETAILED DESCRIPTION
[0025] In the following detailed description of exemplary
embodiments of the disclosure, reference is made to the
accompanying drawings in which like references indicate similar
elements, and in which is shown by way of illustration specific
embodiments in which disclosed systems and devices may be
practiced. These embodiments are described in sufficient detail to
enable those skilled in the art to practice the embodiments, and it
is to be understood that other embodiments may be utilized and that
logical, mechanical, functional, and other changes may be made
without departing from the scope of the present disclosure. The
following detailed description is, therefore, not to be taken in a
limiting sense, and the scope of the present disclosure is defined
by the appended claims. As used in the present disclosure, the term
"or" shall be understood to be defined as a logical disjunction and
shall not indicate an exclusive disjunction.
[0026] With reference to FIGS. 1 and 4, a thermal energy storage
system 10, or thermal battery, comprises a bed 12 packed with
energy storage media composed of a blend 14 of aggregates and a
heat transfer fluid 20 that flows through the blend of aggregates
so heat is transferred between the heat transfer fluid 20 and the
blend 14 of aggregates. In exemplary embodiments, the aggregate
blend 14 is comprised of pelletized rock materials 16 and non-rock
materials 18, which are not in the form of a rock constituent. The
rock materials can be cheap, abundant, non-reactive,
environmentally benign solid packing media such as sand, pebbles,
washed rock, quartz, magnetite (iron oxide) and/or combinations of
the above. One of the lowest cost sensible thermal energy storage
media currently available is these types of rock in manually
crushed or naturally pelletized form. More complex thermal storage
media (e.g. concrete) may rapidly increase in cost to over 300% of
the cost of the pelletized rock bed design discussed herein.
[0027] There are several ways to select and utilize the blend 14 of
aggregates to increase efficiency of the system, reduce its cost,
and/or reduce the volume of the packed bed 12. Because rocks
typically have both a much lower cost than heat transfer fluids
(see FIG. 2) as well as a higher heat capacity, reducing the void
fraction is an effective way to reduce the total cost and reduce
the total volume of the thermal battery. FIG. 2 lists several
candidate sensible storage materials along with relative cost
impacts assuming an average achievable 100.degree. C. usable delta
T on the store over the duration of the storage cycle.
[0028] Another factor in the selection of particles for the bed is
the ability of the particles to naturally resist what is known as
thermal ratcheting which is a process that breaks down the
particles over many cycles of heating/cooling. Particles can be
selected to be more robust to thermal ratcheting by, for example,
using particles with material/geometric properties including, but
not limited to, relatively smooth surfaces (e.g. wash rock/pebbles)
and minimum internal void structure to minimize stress
concentration factors, low thermal expansion coefficients, and/or
high tensile strengths.
[0029] In exemplary embodiments and as discussed in more detail
herein, the maximum particle size for the packed bed may be
selected such that the Biot number (effective measure of
temperature gradient in particle compared to temperature gradient
across heat transfer fluid boundary layer) is less than about 0.5
when considering the heat transfer fluid as described below. The
charging/discharging rate of the packed bed 12 is strongly
influenced by the average Biot number of the particles in the bed.
To meet the ramp rate requirements for peak discharge during "Duck
Curve" evening hours in CA, average particle sizes less than
.about.2 cm may likely be required. Although this increases
pressure drop in the bed, <1 bar/m pressure drop appears readily
feasible with any porosities greater than about 2%. FIG. 3 is a
graph showing pressure drop characteristics. In exemplary
embodiments, the particle size distributions in the bed may be
selected such that the largest particle is less than about 10 cm in
mean diameter. In exemplary embodiments, about 50% or greater of
the particles in the bed 12 may be less than about 5 cm in mean
diameter.
[0030] Porosity in packed beds with uniformly sized and distributed
particles are typically in the range of 30%-40%. However, it is
possible to achieve smaller porosities by using a range of particle
sizes--for example, by using smaller particles 22 to fill-in-the
gaps between larger particles. In exemplary embodiments, a
distribution of rock sizes is selected to ensure relatively poor
sorting (i.e. small size rocks fill in the pore sizes of larger
rocks) such that a total bed porosity of less than about 15% is
achieved. In exemplary embodiments and as discussed in more detail
herein, the porosity of the blend 14 of aggregates can increase
heat transfer between the energy storage media and the heat
transfer fluid 20. In exemplary embodiments, the porosity is less
than about 30% and can be as low as 10%. In exemplary embodiments,
the porosity is greater than 2%. In exemplary embodiments the
make-up of the rock-based energy storage media in the bed 12 may
comprise more than about 30% by mass or volume of the non-rock
materials 18 (e.g. concrete).
[0031] In exemplary embodiments, the packing structure is
approximately uniform in the bulk of the packed bed. However, the
presence of the containment wall may disrupt the packing structure
such that the porosity approaches 1 at the wall surface. The
porosity typically decreases to near its bulk value in
approximately five particle diameters. The high porosity at the
wall typically reduces frictional resistance to the flow and may
lead to higher flow rates (also known as `by-pass` flow) near the
walls which would create a non-uniform flow distribution and affect
the charge/discharge efficiency of the battery. In exemplary
embodiments, carefully designing the radial distribution of
particle sizes and packing structure particularly in the vicinity
of support/retaining structure can reduce these undesirable effects
near containment/retaining structures. In exemplary embodiments,
the rock material and particle size distributions may be selected
such that the bed porosity changes by less than about 5% absolute
after 5,000 cycles due to additional thermal stress fracturing/bed
settling during charging/discharging operations.
[0032] In the embodiments described above, low porosity packed beds
are desirable to reduce the cost of the thermal battery. However,
low porosities can substantially increase fluid velocity and
pressure drop for fluid flow across the packed bed (FIG. 3). The
cost driven objective to achieve lower porosity packed beds tends
to emphasize the need to reduce pressure drop losses and
fluid-velocity-related bed degradation by using a high-density heat
transfer fluid rather than gas/vapor heat transfer fluid. A
suitable density for a high-density heat transfer fluid would be in
the range of about 0.4 g per cc to about 1.5 g per cc. In exemplary
embodiments, the heat transfer fluid is a liquid that is operated
to ensure it does not vaporize at the higher operating temperatures
of the bed.
[0033] Incorporating encapsulated Phase Change Materials (PCMs)
into the packing may provide further benefits, such as increasing
the energy density. The low conductivity of PCMs is currently a
challenge that may be overcome by enhancing heat transfer by mixing
the PCM with the higher conductivity packing material or using
encapsulated PCMs with very small particle diameters (<<1
cm). Encapsulation of PCMs prevents the PCM material from melting
and mixing into the heat transfer fluid system. In exemplary
embodiments, encapsulated PCM's particles are non-rock constituents
included as part of the aggregate blend. In exemplary embodiments,
encapsulated PCM's may consist of up to 50% of the mass or volume
of the aggregate materials in the packed bed. The cost of
encapsulated phase change materials is typically more expensive per
unit of energy stored than rock. Therefore, in a very low-cost bed
design that incorporates encapsulated PCMs, typically the PCMs
would be confined to helping buffer and control output temperatures
versus providing the primary mechanism for bulk thermal energy
storage.
[0034] In exemplary embodiments, the overall bed porosity and
particle size distributions are designed to minimize the axial
thermal conductivity of the bed. Typically, the heat transfer fluid
has a thermal conductivity less than about 10% of the rock
constituent. By working with a higher surface area per unit volume
rock bed, the heat flow path through the bed may be further
disrupted and the bulk thermal conductivity of the overall bed may
be reduced. In some implementations, this type of bed design may
help minimize variations in the temporal evolution of the
thermocline in the bed due to heat redistribution inside the bed
itself.
[0035] In some implementations, this type of bed design may
increase the roundtrip efficiency of the thermal battery over a
charge/discharge cycle. In exemplary embodiments, a majority of the
stored energy can be extracted in a relatively short period of
time. In some cases, over 85% of the stored energy can be extracted
in a charge/discharge cycle in less than twelve hours and in some
instances, less than six hours. In exemplary embodiments, the
surface area of rock per unit volume is approximately 215
m.sup.2/m.sup.3. In another implementation, the surface area of
rock per unit volume is higher. In yet another implementation, the
surface area of rock per unit volume is lower.
[0036] For sizing a solar thermal battery recharge rate, daily
recharge rates will follow the solar cycle and will typically vary
between 4-8 hrs depending on site and season. For sizing a solar
thermal battery discharge rate, utility power discharge rates
typically require discharge in as short as four hours to meet
utility peaking power demands. Commercial operations may tend to
spread the accumulated solar heat over a 24/7 operations cycle and
therefore have even longer storage requirements that can approach
18 hours in some cases. The energy storage size and corresponding
rates of recharge/discharge are dependent on the scale of the solar
field. For an approximately one acre system, the peak rates of
solar thermal battery charging/discharging will be of order 1 MW
and the storage capacity would be up to .about.10 MWhr. Larger
solar fields would require larger individual thermal batteries
(.about.GW rate of charge/discharge for 10 GWhr storage) or banks
of the smaller 10 MWhr thermal batteries.
[0037] The container or bed 12 could be designed in various ways to
improve reliability and performance of the thermal energy storage
system. Processed metals (e.g. steel, aluminum) typically
substantially (>500%) increase in cost per unit volume relative
to pelletized rock due to the significant processing steps
necessary to extract and refine these materials into their usable
forms. These costs for metal increase even more when these metals
are processed into usable geometric forms (e.g. pipe, pressure
vessel walls, etc.). Therefore, in the design of the container for
the pelletized rock bed, the bed 12 can be designed to minimize the
loads and pressures that the structural elements of the thermal
battery are exposed to as well as the surface area and resultant
volume of structural materials that must be deployed in the bed.
For example, as shown in FIG. 1, the bed 12 may be designed as a
vertical cylinder. In exemplary embodiments, the bed 12 has a
height-to-diameter ratio of approximately one. In some
implementations, this ratio may be greater than or less than one.
In an exemplary embodiment, the bed is designed with manifolding
and/or filtering to facilitate the management of rock particulate
contamination that may be generated from thermal cycle stress
fatigue and associated micro-fracturing of the rock bed media
particularly over the charge/discharge cycles associated with a
thermal battery designed for decade long or greater battery
operations.
[0038] Pressure vessels to contain the packed bed and safely manage
pressure inside the thermal battery can be a very large component
of the cost of the overall thermal battery. To ensure the heat
transfer fluid remains liquid (i.e. liquid doesn't start locally
boiling), the operating pressure inside the thermal battery needs
to be greater than the vapor pressure of the warmest heat transfer
fluid element in the thermal store. This constraint defines a
minimum pressure for the packed bed above which additional design
margin should be included depending on certifying organization for
the pressure vessel. The vapor pressure of heat transfer fluids is
typically very sensitive to temperature also typically increasing
very rapidly at temperatures at and above where the heat transfer
fluid vapor pressure exceeds local atmospheric conditions. In some
embodiments, the maximum operating temperature inside the thermal
battery is selected such that the pressure vessel can be rated as
an atmospheric pressure vessel. In other embodiments, the heat
transfer fluid and the pressure vessel are optimally designed for a
specified set of operating temperatures or temperature ranges in
order to minimize the cost of the overall thermal battery.
[0039] In operation of exemplary systems and methods of energy
storage and release, the pelletized rock bed 12 functions with a
liquid heat transfer fluid 20 that percolates through the bed 12
under Darcy Flow conditions during heat transfer operations (i.e.
during thermal battery charging/discharging events). This liquid
heat transfer fluid 20 through the pelletized bed 12 with rock size
distribution as described above provides a means for rapidly
transferring heat between the liquid heat transfer fluid 20 and the
blend 14 of aggregates of the pelletized rock bed while
simultaneously minimizing the fluid pressure drop (in exemplary
embodiments less than about 5 psid) through the bed due primarily
to the very low superficial fluid velocities (e.g., less than about
1 cm/s) of the liquid heat transfer fluid 20 through the bed 12
that are factored in the overall bed design.
[0040] In exemplary embodiments, coarse media 22 is placed at the
base of a cylindrical bed to provide a means of approximately
uniformly manifolding fluid across the base of the bed while
simultaneously supporting the weight of the bed. The manifolding is
designed to minimize radial pressure drop gradient relative to the
normal axial pressure gradient in the bed to help ensure the axial
mass flux of fluid across the packed bed is approximately uniform.
The axial pressure gradient is derived by the difference between
the inlet and outlet pressure divided by the length of the bed and
is represented by the equations shown in FIG. 4. In other exemplary
embodiments, this base of coarse media 22 may be separated by a
screen to help ensure the porosity of the base manifold is
approximately maintained over the operation of the thermal
battery.
[0041] In exemplary embodiments, the top of the bed may include a
fluid gap and/or coarser media in order to provide a means of
approximately uniformly manifolding fluid across the top of the
bed. In other exemplary embodiments, this top manifolding zone may
be separated by a screen to help minimize particle contamination in
the heat transfer fluid stream.
[0042] The pore velocity of the fluid may be designed sufficiently
low such that coarser micro-fracture particles are retained in the
bed due to the higher ballistic coefficients of these larger
particles in the presence of gravity and only fine filtering is
conducted at the exit at the top of the bed for scenarios where
fluid flow is nominally directly vertically upward. This
implementation may help facilitate the reduction in cost associated
with filters and/or filter replacements over the extended life
operations of a thermal battery contemplated herein.
[0043] The heat transfer fluid typically has a cost per unit volume
that is greater than about at least 500% of the cost per unit
volume of the pelletized rock, although other lower cost heat
transfer fluids may be used. The heat transfer fluid 20 is selected
with the following basic characteristics: 1) The heat transfer
fluid does not freeze at temperatures above about 100.degree. C.;
2) the heat transfer fluid has a fluid viscosity low enough to
ensure the fluid pressure drop from the fluid inlet to the fluid
outlet of the bed is less than about 5 psid under nominal
conditions; 3) the heat transfer fluid has a fluid viscosity low
enough to ensure the fluid pressure drop from the fluid inlet to
the fluid outlet of the bed is less than about 100 psid under cold
start-up conditions; 4) the heat transfer fluid does not chemically
break down and/or alter its chemical constituency by greater than
about 5% due to operations over at least 5,000 charge/discharge
cycles and/or 7 years (whichever happens first) when exposed to
both the temperatures and chemistry of the rock bed; 5) the heat
transfer fluid has a vapor pressure of less than about 300 psia at
the highest design temperature of any portion of the bed in direct
contact with the heat transfer fluid. In other exemplary
embodiments the heat transfer fluid vapor pressure at the highest
design temperature of any portion of the bed in direct contact with
the heat transfer fluid is near atmospheric pressure to allow for a
very low cost pressure vessel design. In exemplary embodiments, the
high density of the heat transfer fluid minimizes drops in pressure
of the heat transfer fluid.
[0044] In exemplary embodiments, the heat transfer fluid 20 is a
liquid comprised of carbon-based molecules, and the heat transfer
fluid may be an oil. However, organic compound-based heat transfer
fluids can be flammable and/or create a pressure release hazard
(due to the natural vapor pressure of the fluid and/or additional
gas pressure the fluid can generate if it decomposes into gas or
combusts). Advantageously, disclosed systems and methods mitigate
this effect and minimize this pressure release hazard such as an
explosion by slowing the rate of pressure generation and associated
energy release down by several orders of magnitude, e.g., from
sub-10's milliseconds to sub-10's of seconds or less. Furthermore,
to mitigate explosive hazard and/or inadvertent energetic releases,
the heat transfer fluid and rock chemistry are jointly designed
and/or selected to ensure that under worst case excessive heating
conditions, less than about 1% by mass per second of either the
fluid and/or rock will chemically react with one another.
[0045] Another important consideration in the design of the packed
bed is the behavior of the thermocline. There is necessarily an
axial temperature gradient across the packed bed. As the packed bed
is charged, the thermal gradient progresses along the length of the
packed bed. Eventually, it reaches the end of the fluid exit of the
packed bed and the packed bed exit temperature begins to increase.
Disclosed systems and methods may place a limit on the maximum
amount by which this outlet temperature can increase during
charging. For instance, if the exit fluid goes back into the solar
field then the fluid will be heated up to higher temperatures in
the solar receivers. The fluid's maximum operation temperature then
sets a limit on how much the packed bed exit temperature can
increase during charge. This may effectively mean that the packed
bed cannot be fully charged, and therefore, in exemplary
embodiments, is slightly oversized in order to provide the required
energy. Incorporating PCMs (which melt at a fixed temperature) into
the packed bed packing can be used to control these exit
temperature variations.
[0046] There are other practical aspects that can make packed bed
operation challenging. For instance, the thermal expansion and
contraction of the particles can damage the particles and the
containment, which is known as thermal ratcheting. These effects
can be reduced to some extent by careful design of the storage
container--such as by using a conical store, or by installing
internal structures to support the particles and keep them in the
correct place. The particles also require some treatment before
they can be used. For instance, pressure losses are increased if
the particles are not washed before the bed is constructed. In some
embodiments, the packed bed has a variable cross-sectional geometry
of particles that may have undergone some additional preparatory
steps to help reduce pressure drop and bed degradation.
[0047] Furthermore, to mitigate structural damage due to storage
vessel pressure related failures, the heat transfer fluid
thermophysical/heat transfer properties and rock pore size
distributions are jointly designed/selected to ensure that under a
worst case pressure vessel casing failure that the pressure decay
profile of the actual system is such that at the same point in time
where P/P.sub.0 of an ideal isentropic expansion of the just vapor
component of the heat transfer fluid is 1/e (.about.36.7%), the
pressure of the actual system at the same point in time is less
than approximately 10% of the ideal isentropic expansion. In some
implementations where the vapor pressure of the system is lower,
this design point may be less than approximately 50%. In still
other implementations where the vapor pressure of the system is not
a substantial operations concern, this design point may be less
than approximately 90%.
[0048] Additional beneficial mechanisms for mitigating and/or
slowing the release of pressure during inadvertent pressure release
under pressure vessel failure scenarios may be factored into a bed
design. These include large fluid frictional losses associated with
much lower density gases rapidly trying to propagate through the
packed bed, the momentum transfer between low density gas and very
high density particles that limits the rate of expansion of the
energetic bed volume during failure, sonic velocity choking of
gas/vapor trying to escape limited surface area-to-packed bed
volume pore spaces that limits the volumetric generation of high
velocity gases, and/or thermal quenching of runaway chemical
decomposition reactions that slows and/or stop the rates of
inadvertent chemical energy release.
[0049] While the apparatus, systems, and methods have been
described in terms of what are presently considered to be the most
practical and preferred embodiments, it is to be understood that
the disclosure need not be limited to the disclosed embodiments. It
is intended to cover various modifications and similar arrangements
included within the spirit and scope of the claims, the scope of
which should be accorded the broadest interpretation to encompass
all such modifications and similar structures. The present
disclosure includes any and all embodiments of the following
claims.
[0050] Thus, it is seen that thermal energy storage systems and
methods and solar concentrating power plants are provided. It
should be understood that any of the foregoing configurations and
specialized components or chemical compounds may be interchangeably
used with any of the systems of the preceding embodiments. Although
illustrative embodiments are described hereinabove, it will be
evident to one skilled in the art that various changes and
modifications may be made therein without departing from the
disclosure. It is intended in the appended claims to cover all such
changes and modifications that fall within the true spirit and
scope of the disclosure.
* * * * *