U.S. patent application number 16/302798 was filed with the patent office on 2020-06-18 for methods and systems for thermal energy storage and recovery.
The applicant listed for this patent is Kansas State University Research Foundation. Invention is credited to Hitesh Bindra, Jacob Edwards, Dan Gould.
Application Number | 20200191500 16/302798 |
Document ID | / |
Family ID | 60326185 |
Filed Date | 2020-06-18 |
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United States Patent
Application |
20200191500 |
Kind Code |
A1 |
Bindra; Hitesh ; et
al. |
June 18, 2020 |
METHODS AND SYSTEMS FOR THERMAL ENERGY STORAGE AND RECOVERY
Abstract
Thermal energy storage and recovery methods and systems are
provided herein, which utilize a thermal energy storage vessel. The
vessel comprises a packed bed of chemically inert particulates
exhibiting high thermal conductivity. A gaseous heat transfer fluid
(e.g., steam) is fed to the vessel, whereby at least a portion of
the fluid condenses on the particulates and transfers latent heat
to the particulates. During a heat recovery step, a heat recovery
fluid (e.g., air) is fed to the vessel, whereby sensible heat
transfers from the particulates to the heat recovery fluid. The
warmed heat recovery fluid may then be used to provide required
heat for a variety of applications.
Inventors: |
Bindra; Hitesh; (Manhattan,
KS) ; Edwards; Jacob; (Lenexa, KS) ; Gould;
Dan; (Manhattan, KS) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kansas State University Research Foundation |
Manhattan |
KS |
US |
|
|
Family ID: |
60326185 |
Appl. No.: |
16/302798 |
Filed: |
May 19, 2017 |
PCT Filed: |
May 19, 2017 |
PCT NO: |
PCT/US2017/033566 |
371 Date: |
November 19, 2018 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
62339576 |
May 20, 2016 |
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F28D 20/02 20130101;
F28D 2021/0063 20130101; Y02E 60/142 20130101; F28D 2021/0064
20130101; F28D 20/0056 20130101; F28D 17/005 20130101; F22B 33/18
20130101; Y02E 60/145 20130101; F28D 20/021 20130101; Y02E 20/14
20130101 |
International
Class: |
F28D 17/00 20060101
F28D017/00; F28D 20/02 20060101 F28D020/02 |
Claims
1. A method of storing thermal energy contained within a
condensable fluid for later use comprising: feeding a gaseous heat
transfer fluid into a vessel comprising a packed bed of inert
particulates; contacting said gaseous heat transfer fluid with said
inert particulates and condensing at least a portion of said heat
transfer fluid on said inert particulates, wherein said contacting
and condensing step transfers at least a portion of the latent heat
contained within said gaseous heat transfer fluid to said
particulates; and storing said portion of the latent heat within
said particulates for a period of time until at least a portion of
the stored latent heat can be recovered from said particulates.
2. The method of claim 1, wherein said heat transfer fluid is
steam.
3. The method of claim 2, further comprising generating said steam
using energy from a renewable steam generator.
4. The method of claim 1, further comprising the step of recovering
said portion of the stored latent heat from said particulates, said
recovering step comprising: feeding a heat recovery fluid to said
vessel; contacting said heat recovery fluid with said inert
particulates and transferring thermal energy from said particulates
to said heat recovery fluid in the form of sensible heat, thereby
forming a warmed heat recovery fluid.
5. The method of claim 4, further comprising directing said warmed
heat recovery fluid to a heat exchange unit.
6. The method of claim 5, wherein said heat exchange unit comprises
a component of an absorption refrigeration system.
7. The method of claim 5, wherein said heat exchange unit comprises
a component of an indoor heating system.
8. The method of claim 4, wherein said heat recovery fluid is
air.
9. The method of claim 4, wherein said recovering step occurs over
a greater period of time than said contacting and condensing
step.
10. A thermal energy storage system comprising: an evaporator
adapted for vaporizing a fluid stream; a vessel comprising at least
one packed bed of inert particulates and having at least one fluid
inlet and at least one fluid outlet; and a conduit configured to
direct the vaporized fluid stream from the evaporator to said at
least one fluid inlet, said at least one fluid outlet configured to
remove a condensate of the vaporized fluid stream from said
vessel.
11. The system of claim 10, wherein said evaporator is a steam
generator.
12. The system of claim 11, wherein said steam generator is a
renewable steam generator.
13. The system of claim 10, wherein said vaporized fluid stream
comprises steam.
14. The system of claim 10, further comprising: a heat exchange
unit; and a second conduit configured to direct a heat recovery
fluid from said one or more fluid outlets to said heat exchange
unit.
15. The system of claim 14, wherein said heat recovery fluid is
air.
16. The system of claim 14, wherein said heat exchange unit
comprises a component of an absorption refrigeration system.
17. The system of claim 14, wherein said heat exchange unit
comprises a component of an indoor heating system.
18. The system of claim 10, wherein said at least one packed bed of
inert particulates comprises a cylindrical chamber and a plurality
of solid, spherical particles having an average diameter of less
than 3 millimeters.
19. The system of claim 18, wherein said particles have an average
diameter of from about 1 mm to 3 mm.
20. The system of claim 10, wherein said particulates comprise at
least one of alumina, graphite, silica, quartz, ceramic, or rock.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
[0001] The present invention is generally directed to methods and
systems for thermal energy storage and recovery.
Description of the Prior Art
[0002] Due to growing energy production from intermittent energy
sources such as solar and wind, and due to the increasing
skewedness in energy demand patterns, there have been several
studies for understanding thermal energy storage systems. Recently,
there have been numerous studies where the thermal behavior of the
storage systems was studied during storage or recovery processes.
These studies have been performed for solid as well as liquid
storage media. However, in all of these studies the heat carrier or
heat transfer fluid was in a single phase (i.e., in a gas or liquid
state for all scenarios). Systems utilizing single phase heat
transfer fluids have experienced unacceptably slow thermal energy
gain, and thus the total energy storage achieved by such systems
has been similarly unacceptable. Additionally, these systems have
generally exhibited less predictable heat dispersion as the single
phase fluid passes through the energy storage vessel non-uniformly,
resulting in variable warm and cool spots with less efficient
energy storage. Therefore, improved thermal energy storage methods
and systems are needed.
SUMMARY OF THE INVENTION
[0003] In one embodiment of the present invention, there is
provided a method of storing thermal energy contained within a
condensable fluid for later use. The method comprises feeding a
gaseous heat transfer fluid into a vessel comprising a packed bed
of inert particulates. The method further comprises contacting the
gaseous heat transfer fluid with the inert particulates and
condensing at least a portion of the heat transfer fluid on the
inert particulates. The contacting and condensing step transfers at
least a portion of the latent heat contained within the gaseous
heat transfer fluid to the particulates. The method also comprises
storing the portion of the latent heat within the particulates for
a period of time until at least a portion of the stored latent heat
can be recovered from the particulates.
[0004] In another embodiment, there is provided a thermal energy
storage system comprising an evaporator adapted for vaporizing a
fluid stream, a vessel comprising at least one packed bed of inert
particulates and having at least one fluid inlet and at least one
fluid outlet, and a conduit configured to direct the vaporized
fluid stream from the evaporator to the at least one fluid inlet.
The at least one fluid outlet is configured to remove a condensate
of the vaporized fluid stream from the vessel.
BRIEF DESCRIPTION OF THE DRAWINGS
[0005] FIG. 1 is a schematic drawing of one embodiment of the
present invention, wherein solid lines represent fluid flow during
normal system operation and dashed lines represent fluid flow
during heat recovery operation.
[0006] FIG. 2 is a schematic drawing of another embodiment of the
present invention, wherein solid lines represent fluid flow during
normal system operation and dashed lines represent fluid flow
during heat recovery operation.
[0007] FIG. 3 is a is a schematic drawing of another embodiment of
the present invention, wherein solid lines represent fluid flow
during energy storage operation and dashed lines represent fluid
flow during heat recovery operation.
[0008] FIG. 4 is a schematic drawing of an experimental setup of a
packed bed heat sink.
[0009] FIG. 5 is a series of images showing the experimental packed
bed vessel (top left), as well as X-ray images of the internal
fluid flow (top) and thermal images of the external surface of the
vessel (bottom) during the energy storage process.
[0010] FIG. 6 is a temperature plot showing the results for the
slow injection steam experiments.
[0011] FIG. 7 is a temperature plot showing the results for the
fast injection steam experiments.
[0012] FIG. 8 is a temperature plot showing the results for the air
injection experiments.
[0013] FIG. 9 is a schematic drawing of drawing of one embodiment
of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0014] Inventive methods and systems for storing thermal energy are
provided herein. The inventive methods and systems utilize a
thermal energy storage vessel comprising a packed bed of inert
particulates. The particulates are capable of storing thermal
energy transferred from a hot gaseous heat transfer fluid. Thermal
energy can then be recovered from the particulates by passing a
heat recovery fluid through the packed bed. The warmed heat
recovery fluid can then be utilized in a number of applications
requiring a source of heat.
[0015] One embodiment of the present invention is shown
schematically in FIG. 1. As shown in FIG. 1, an evaporator 110,
thermal energy storage vessel 120, and heat exchange unit 130 are
provided in a system. These components are described in greater
detail below. During normal operation, a heat transfer fluid stream
112 is fed to evaporator 110, which provides gaseous heat transfer
fluid streams 114a and 114b. Stream 114b is fed to heat exchange
unit 130 to provide necessary thermal energy for normal system
operation. For example, in certain embodiments, heat exchange unit
130 may form a part of a building heating system or an absorber
refrigeration system. The spent heat transfer fluid exits unit 130
as outlet stream 132. Stream 114a is fed to vessel 120, wherein
thermal energy from the heat transfer fluid is transferred to solid
particulates contained within vessel 120. As explained in greater
detail below, at least a portion of the gaseous heat transfer fluid
is condensed within vessel 120 thereby transferring latent heat to
the particulates contained therein. The condensed fluid exits
vessel 120 via outlet stream 122. This heat is stored by the
particulates until it is desired to recover that energy for use
within heat exchange unit 130, such as when evaporator 110 is
offline. During heat recovery operation, the output of gaseous heat
transfer fluid from evaporator 110 is reduced or ceased. In order
to provide the necessary thermal energy to heat exchange unit 130,
a heat recovery fluid stream 126 is fed to vessel 120, wherein
thermal energy stored in the solid particulates is transferred to
the heat recovery fluid, thereby forming a warmed heat recovery
fluid stream 128. Warmed heat recovery fluid stream 128 is then fed
to heat exchange unit 130 thereby ensuring a continuous supply of
heat energy to the process or system of which unit 130 is a
part.
[0016] Another embodiment of the present invention is shown in FIG.
2. The embodiment of FIG. 2 operates similarly to the embodiment of
FIG. 1, except for the arrangement of process equipment. During
normal operation, a heat transfer fluid stream 212 is fed to
evaporator 210, which generates gaseous heat transfer fluid stream
214. Stream 214 is fed to thermal energy storage vessel 220,
wherein thermal energy from the heat transfer fluid is transferred
to solid particulates contained within vessel 220 resulting in the
at least partial condensation of stream 214. Although a certain
amount of thermal energy is transferred to the particulates and
stored within vessel 220, a substantial amount of useful thermal
energy may remain in the heat transfer fluid and be passed through
vessel 220. The heat transfer fluid exits vessel 220 and is fed to
heat exchange unit 230 via stream 222 to provide necessary thermal
energy for normal system operation. The spent fluid exits heat
exchange unit via stream 232. During heat recovery operation (i.e.,
when evaporator 210 is offline), the necessary thermal energy
required by heat exchange unit 230 is provided by feeding a heat
recovery fluid stream 226 to vessel 220. Similar to the process of
FIG. 1, the thermal energy stored in the solid particulates is
transferred to the heat recovery fluid, thereby forming a warmed
heat recovery fluid stream 228. Warmed heat recovery fluid stream
228 is then fed to heat exchange unit 230.
[0017] Yet another embodiment of the present invention is shown in
FIG. 3. The embodiment of FIG. 3 operates similarly to the
embodiment of FIG. 2, except that the arrangement of the thermal
energy storage vessel and the heat exchange unit are reversed.
During normal operation, a heat transfer fluid stream 312 is fed to
evaporator 310, which generates gaseous heat transfer fluid stream
314. Stream 314 is fed to heat exchange unit 330 to provide
necessary thermal energy for normal system operation. Although a
certain amount of thermal energy may be removed from the fluid in
heat exchange unit 330, a substantial amount of useful thermal
energy may remain in the heat transfer fluid, particularly if the
heat load of unit 330 is relatively low. The heat transfer fluid is
then passed through heat exchange unit 330 and exits via stream
332. Stream 332 is fed to thermal energy storage vessel 320,
wherein thermal energy from the heat transfer fluid is transferred
to the particulates and stored within vessel 320, resulting in at
least partial condensation of the gaseous heat transfer fluid,
which exits vessel 320 via outlet stream 322. During heat recovery
operation, the necessary thermal energy required by heat exchange
unit 330 is provided by feeding a heat recovery fluid stream 326 to
vessel 320, transferring thermal energy stored in the solid
particulates to the heat recovery fluid (thereby forming a warmed
heat recovery fluid stream 328), and feeding warmed heat recovery
fluid stream 328 to heat exchange unit 330. The spent heat recovery
fluid exits unit 330 via stream 334.
[0018] Although certain preferred embodiments are described above,
it should be understood that these embodiments are not intended to
be limiting and that other process equipment and arrangements may
be utilized within the scope of the present invention. That is, one
of ordinary skill in the art would understand that the thermal
energy storage vessel (or multiple thermal energy storage vessels)
and heat exchange unit (or multiple heat exchange units) may be
provided in parallel or in series in a variety of combinations. It
is also understood that the system equipment described here is
operably connected using various conduits for transporting the
working fluids.
[0019] The heat transfer fluid may be any of a number of fluids
capable of conducting and transferring thermal energy through the
system. As described in more detail below, preferred heat transfer
fluids are capable of being vaporized in the evaporator and at
least partially condensed when contacted with the particulates in
the thermal energy storage vessel in order to take advantage of
latent heat energy released during condensation. Preferred heat
transfer fluids will have a normal boiling point above about
75.degree. C. In certain preferred embodiments, the heat transfer
fluid has a normal boiling point temperature from about 75.degree.
C. to about 200.degree. C., preferably from about 85.degree. C. to
about 150.degree. C., more preferably from about 95.degree. C. to
about 125.degree. C., and most preferably about 100.degree. C. In
certain embodiments, the heat transfer fluid may comprise
refrigerants, such as R134a or others that undergo phase change,
organic volatile solvents including glycol-based fluids, such as
ethylene glycol and propylene glycol, and steam, with steam being
particularly preferred. Depending upon system configuration (e.g.,
thermal storage occurring upstream or downstream of the heat load)
the steam may be in the form of superheated steam or saturated
steam. However, it is preferred that steam is supplied to the
thermal energy storage vessel as saturated steam.
[0020] The heat recovery fluid may similarly be any of a number of
fluids capable of conducting and transferring thermal energy
through the system. In certain embodiments, the heat recovery fluid
may be the same as or different from the heat transfer fluid.
However, in particularly preferred embodiments, the heat recovery
fluid is different from the heat transfer fluid. In certain aspects
of the present invention, it is important to utilize a heat
recovery fluid that will not undergo a phase change when brought
into contact with the heated inert particulates. Thus, the heat
transferred to the heat recovery fluid is sensible heat. In certain
embodiments, the heat recovery fluid is selected such that it will
enter and leave the vessel entirely in the gaseous state, but
should not be taken as excluding the use of liquid heat recovery
fluids. In particularly preferred embodiments, the heat recovery
fluid is air. However, it is also within the scope of the present
invention that the heat recovery fluid be any other heat transfer
fluid that can recover energy in the form of sensible heat, latent
heat, or a combination of both, such as water, DOWTHERM.TM.,
heating oils, refrigerants, and others.
[0021] The evaporator is capable of supplying sufficient energy to
the heat transfer fluid so as to vaporize the fluid. For example,
in preferred embodiments, the evaporator is a renewable steam
generator, meaning that the energy required to vaporize the fluid
is obtained from a renewable source. Renewable steam generators
include, for example, solar steam generators, biomass steam
generators, and wind-powered steam generators.
[0022] However, it is understood that non-renewable evaporators may
also be used if desired or required by a particular application.
For example, the heat transfer fluid may be heated using a product
stream from an exothermic reaction process or a heated flue gas
stream. The heat exchange unit is capable of supplying heat to any
number of processes. For example, in particularly preferred
embodiments, the heat exchange unit may be a component in an indoor
heating system, absorption refrigeration system, or any other
domestic or industrial process heat, power generation, or cooling
application.
[0023] The thermal energy storage vessel may be constructed in a
variety of geometries, using a variety of materials. The vessel
generally comprises an outer housing, one or more fluid inlets, and
one or more fluid outlets. In certain embodiments, one or more of
the fluid inlets may also be utilized as a fluid outlet, so long as
the vessel is configured such that the heat transfer fluid may be
passed through the vessel during normal operation and that the heat
recovery fluid may be passed through the vessel during heat
recovery operation. The vessel further comprises at least one
packed bed containing the inert particulates. In certain
embodiments, the packed bed comprises a cylindrical chamber and the
inert particulates comprise a plurality of solid particles. The
particles may be a variety of shapes, including, for example,
spherical, cylindrical, elliptical, irregularly shaped, or
combinations thereof. In preferred embodiment, the particles have
an average diameter of from about 0.1 mm to about 8 mm, preferably
from about 0.5 mm to about 6 mm, and more preferably from about 1
mm to 3 mm. Regardless of the particle shape, as used herein the
particle diameter refers to the linear distance across the particle
as taken across its largest dimension. In particularly preferred
embodiments, the particles are spherical particles having an
average diameter of less than 3 mm. The particles may comprise a
variety of materials. However, the materials selected should
generally be chemically inert and exhibit high thermal
conductivity. In certain embodiments, the particles comprise at
least one of alumina, graphite, silica, quartz, ceramic, or rock
(e.g., pea gravel). In particularly preferred embodiments, the
particles comprise alumina and/or pea gravel. The vessel may also
be insulated using a variety of methods and materials known in
order to minimize heat loss to the surrounding environment during
energy storage. Exemplary thermal energy storage vessels are
described in U.S. Application Publication No. 2014/0202157 and U.S.
Application Publication No. 2014/0299306, both of which are
incorporated by reference in their entireties, herein.
[0024] Embodiments of the present invention demonstrate improved
thermal energy transfer, storage, and recovery over prior art
methods and systems due to the unique heat transfer phenomena
taking place within the thermal energy storage vessel (and
particularly within the packed bed portion of the vessel) during
normal system operation and subsequent heat recovery operation. In
contrast to prior art methods which used single-phase heat transfer
fluids to transfer thermal energy to storage systems, the present
invention preferably uses a condensable fluid that exists in both
the vapor and liquid phase (e.g., saturated steam) during transport
through the packed bed. It has been discovered that the use of such
condensable fluids as the heat transfer fluid during thermal the
initial energy transfer phase provides faster and more predictable
heat transfer conditions. Without being bound by any theory, it is
believed that the improved heat transfer behavior is due to the
combination of two types of thermal energy transfer occurring
within the vessel during this operation. First, when the vaporized
heat transfer fluid condenses on the packed bed particulates,
latent heat is released by the fluid and transferred to the
particulates. The term "latent heat" is generally the change in
internal energy experienced by a body or thermodynamic system with
no change in temperature. Thus, the latent heat transfer aspect is
due to the phase change (condensation) without a change of fluid
temperature. This transfer of latent heat provides a rapid and more
predictable transfer of a greater quantity of heat to the solid
particulates as compared to a transfer of mere sensible heat from
the heat transfer fluid. Second, when the hot vapor or liquid
contacts the cooled particulates, sensible heat is transferred from
the fluid to the particulates due to the difference in temperature.
The term "sensible heat" is generally the change in internal energy
experienced by a body or thermodynamic system as measured by the
temperature change. Thus, the sensible heat transfer aspect is due
to the temperature difference between the fluid and the
particulates.
[0025] Based on the above-described principles, embodiments of the
present invention comprise contacting a gaseous heat transfer fluid
with inert particulates within the packed bed. During the initial
energy transfer operation, the particulates are generally cooler
than the fluid. Due to this temperature difference, sensible heat
is transferred from the fluid to the particulates, and thus the
fluid temperature decreases. When the fluid temperature reaches the
boiling point temperature within the packed bed, at least a portion
of the fluid condenses on the particulates, and latent heat is
transferred from the fluid to the particulates. While the fluid
condensation does not change the fluid temperature, the temperature
of the particulates and the packed bed increases significantly
during this step. After condensation occurs, some additional
sensible heat transfer may continue to occur if there is a
temperature difference between the fluid and the particulates.
Advantageously, it has been discovered that the use of a
condensable heat transfer fluid (e.g., steam) in the packed bed
configurations of the present invention allows for improved
cross-sectional uniformity as the fluid is fed through the packed
bed and energy is transferred to the particulates. Thus, an
observable and predictable thermal front is exhibited as the heat
transfer fluid is fed into the packed bed and heat is being stored.
Accordingly, during as heat transfer fluid is being fed to the
energy storage vessel, a sufficiently steep temperature gradient is
maintained along the flow direction, which discourages exergy
losses due to thermal dispersion. The rate at which the gaseous
heat transfer fluid is fed to the packed bed will influence that
rate at which energy is transferred to the particulates. At higher
feed rates, faster energy transfer is typically observed. However,
the feed rate is limited by the fact that the heat transfer fluid
should have a sufficient residence time within the packed bed such
that the fluid can condense and release latent heat. Generally, the
residence time (and related flow rate) should be selected based on
other design parameters (e.g., particulate size, particulate
material, and type of heat transfer fluid) such that the
particulates have corresponding Biot numbers less than 0.1. The
heat transferred to the packed bed particulates is then stored
within the particulates for a period of time until recovery is
desired (e.g., during times of reduced heated vapor
generation).
[0026] During heat recovery operation of the system, the heat
recovery fluid is fed into the thermal energy storage vessel and
passed through the packed bed containing the heated particulates.
The heat recovery fluid should have a cooler temperature than the
heated particulates but should also be fed to the vessel in a
single, preferably gaseous, phase. In contrast to the heat transfer
fluid, the heat recovery fluid preferably passes through the packed
bed in a single phase. For example, a gaseous heat recovery fluid
is fed into the vessel and the fluid remains in the gaseous phase
throughout the heat recovery operation in the vessel with little or
no phase change. As the recovery fluid does not experience a change
in phase, there is no energy transfer due to latent heat. Rather,
thermal energy is transferred to the heat recovery fluid only in
the form of sensible heat. This has the advantage of providing a
longer duration of heat supply, as the particulates do not lose the
stored thermal energy as quickly as they obtained it. Therefore, in
particularly preferred embodiments, the heat recovery operation
occurs over a greater period of time than the initial heat transfer
operation.
[0027] An exemplary energy storage and recovery system is shown in
FIG. 9, and its operation utilizing steam as the heat transfer
fluid, in accordance with one embodiment of the present invention,
is described below. The embodiment described herein uses water as
the heat recovery fluid in order to recover steam, but it should be
understood that other heat transfer fluids and heat recovery fluids
can also be used within the scope of the present invention. During
energy storage operation, valve 21 opens to allow steam from an
evaporator (not shown) to flow through fluid inlet 22 and into
vessel 20. As the steam contacts the particulates 29 within vessel
20, the steam condenses, thereby transferring latent heat energy to
the particulates 29. The condensed steam (water) then exits vessel
20 through fluid outlet 24 and valve 25, and the heat is stored in
the packed bed of particulates 29 until energy recovery is desired.
Preferably, vessel 20 is insulated or otherwise configured to
minimize or eliminate heat loss through the outer surface of the
vessel during heat storage. During energy recovery operation,
valves 21 and 25 are closed, and water is introduced into vessel 20
through water inlet 26 and valve 27. When the water contacts the
hot particulates 29, heat is transferred from the particulates 29
to the water, thereby vaporizing the water into steam. The steam
then exits vessel 20 through fluid outlet 28 and valve 23 as
recovered thermal energy.
Example
[0028] The following example sets forth radially and azimuthally
symmetric steam condensation heat transfer experiments using a
packed bed of alumina particles. It is to be understood, however,
that these examples are provided by way of illustration and nothing
therein should be taken as a limitation upon the overall scope of
the invention.
Introduction
[0029] Conventional thermal energy storage systems have been
largely ineffective due to complete mixing in the storage medium as
the heat transfer fluid is introduced. When heat transfer fluid is
introduced via mixed flow, the temperature of the energy storage
medium increases; however, the temperature profile throughout the
energy storage medium and storage vessel at any particular point in
this process is relatively uniform. Such mixed flow methods require
that in order to bring the temperature in entire bed from cold to
hot will require the discharge of warm to hot fluid during the
entire storage process (i.e., wasting large amounts of energy). As
described in more detail below, a more preferred scenario is to
achieve a plug flow (i.e. no mixing downstream) which enables steep
temperature gradient. This is because bringing the entire bed from
cold to hot temperature in such a scenario would not require any
discharge of warm or hot fluid, and thus any fluid leaving the
storage vessel during the storage process is always cold.
[0030] Another physical process which limits the ability to achieve
plug flow is temperature diffusion within the storage medium. This
occurs, for example, when heat transfers within the storage medium
ahead of the heat transfer fluid. Although some prior art storage
solutions have been able to achieve some gradient in the
temperature profile of the storage medium during storage operation,
this gradient is relatively gradual from inlet to outlet.
Accordingly, these solutions have been unable to achieve the steep
temperature profiles associated with plug flow due to the
temperature diffusion within the storage medium. Such solutions
have been shown to achieve thermal energy storage efficiencies not
more than 50-60%.
Design Objectives
[0031] Previous studies assumed that a packed bed configuration
enables the uniform flow and temperature distribution in the radial
direction (i.e., in the direction normal to the flow). However, due
to wall heat losses, the temperature of the wall and the bed were
different from each other. Thus, accurate analysis required
simultaneous wall temperature measurements. One of the primary
objectives is to understand the axial dispersion of the temperature
front upon steam injection, temperature must be measured at various
locations in the axial direction without interfering significantly
with the system behavior. This experiment focuses on steam
condensation at atmospheric pressure, which simplifies the design
of the vessel and fittings. One of the difficulties in
understanding the steam condensate process is associated with the
uncertainties in the flow conditions and thickness of the two phase
(liquid-vapor) zone inside the packed bed. Moreover, effective
thermal conductivity inside the bed is strongly dependent upon
instantaneous liquid hold-up and interface location. Therefore, the
condensate flow rate was measured or estimated throughout the
experiment to correlate with the temperature response.
Experimental Setup
[0032] All experiments were performed in a cylindrical quartz tube
randomly packed with spherical particles. The size of the
cylindrical vessel was 6 inches (15.24 cm) tall and 2.5 inches
(6.35 cm) diameter. The cylinder diameter was the only size
limitation because of the standard ceramic flanges used to seal the
ends. The diameter of the spherical alumina particles were 3-mm in
order to provide the largest available ratio between the sphere
diameters and the cylinder tube diameter. Having a ratio of tube to
particle diameter greater than 20 also allowed for the plug-flow
assumption or radially uniform dispersion of the thermal front.
[0033] The spherical alumina particles were procured from
Saint-Gobain NorPro under the commercial name Denstone.RTM. 99.
These commercially available particles were chosen because they
allowed for uniform isotropic heating and have been previously
tested for chemical inertness and robust thermo-mechanical behavior
with steam. Alumina particles have high heat capacity, high thermal
conductivity, and chemical inertness, which allows the rapid
localized equilibration of thermal energy between the fluid phase
and solid phase. Alumina is also non-degradable, allowing it to
last a relatively long time and to remain stable through multiple
heating and cooling cycles. Large heat transfer surface area was
achieved due to considerably smaller particle or packing size as
compared to the overall bed dimensions. This made the thermal front
propagation more predictable and along the flow direction. With
saturated steam as the heat transfer fluid, the rate of heat
rejection is much faster during the condensation process.
Therefore, if the media has sluggish response to absorb heat, this
will lead to very complex energy balance in three phases.
Therefore, using alumina particles achieves the crucial design
objective of performing reproducible thermal behavior tests.
[0034] The quartz tube was sealed with ceramic flanges at both ends
to provide lowest possible energy dispersion effects from the
boundaries. The quartz tube was chosen because it provided for
visual inspection of movement of the liquid-vapor interface and the
known constant emissivity value of quartz material in temperature
range of 25.degree. C.-100.degree. C. allowed for easy measurement
of wall temperatures with an IR camera.
[0035] The measurement setup to attain the temperature values along
the outside wall of the heat sink vessel was a forward looking
infrared (FLIR) camera. The packed bed temperatures were measured
with an Omega.RTM. multi-point thermocouple tube to record the
temperature at six axial locations in the bed. The multi-point
thermocouple was positioned as close to the center of the bed as
possible using a fitting screwed into the top flange of the vessel.
Each thermocouple was numbered respective to its position from the
inlet of the test chamber.
[0036] In-house steam supply was used in the experiments. Before
starting the experiments, it was ensured that steam supply pressure
and flow did not change for the duration of the individual
experiments. This was done by allowing the steam to condense in a
cylindrical flask with cold water and monitoring the change in the
level with time during steam flow. In addition, during the actual
experiments of steam injection in the packed bed condensate, flow
was monitored to take into account the uncertainties, if any. Steam
was supplied from the top of the test chamber after passing through
a pressure regulator holding the back pressure constant for all
experiments. A globe valve was situated just before the entrance to
the test vessel to allow control of steam after the supply valve
was opened. This combination allowed the evaluation of the system's
response to a step input of constant pressure steam. The downstream
end of the test chamber was connected to a tube-in-tube heat
exchanger where any remaining vapor was condensed. This extra step
allowed for the total mass of steam that passed through the chamber
to be collected and measured, enabling a value for the total amount
of energy input into the system to be obtained.
[0037] A simplified schematic of the experimental setup is shown in
FIG. 4, with vessel 10 comprising a packed bed of particulates 12,
fluid inlet 14, fluid outlet 16, and multi-point thermocouple 18
having thermocouples (TC-1-TC-6).
Experimental Procedure
[0038] The experiments were performed in two sets. The first set of
experiments were the slow-injection tests, wherein the steam was
slowly and gradually injected into the packed bed. In these
experiments, a throttled steam supply was further regulated by a
globe valve to nearly atmospheric pressure before being injected
into the top of test section. This throttled condition was
confirmed by the fact that the maximum temperature of the steam at
the top of the cylinder did not rise significantly above
100.degree. C. Prior to each test run, the system was flushed with
dry cold air to ensure uniform temperature and no vapor content in
the system. Steam was then continuously injected through the
cylinder until such point as the thermographic camera registered
that the wall of the cylinder had achieved steady state
conditions.
[0039] The second set of experiments were the fast-injection tests.
In these experiments, the globe valve was left completely open and
flow injection was initiated with a butterfly gate valve. After the
correct initial conditions were achieved (i.e., the alumina
particles were both dry and at room temperature), and after setting
the desired steam inlet conditions, the butterfly gate valve was
quickly opened to inject the steam into the system. The steam was
continuously injected into the system at the set pressure (i.e.,
atmospheric pressure) until the thermographic camera displayed a
uniformly heated outer wall. While substantially all of the steam
was condensed in the tube before being discharged in the slow
injection tests, in the fast injection experiments the steam was
allowed to flow through the packed bed and exit out the bottom into
a discharge pipe.
[0040] The injection flow rate of the steam for each case was
determined by condensing the steam, collecting the condensate, and
timing how long the valve was open. Multiple experiments were run
to ensure repeatability and consistent flow rate values for both
the slow and fast injection cases. The average condensate
collection flow rates for the slow and fast were measured to be
1.25 cm.sup.3/s and 45 cm.sup.3/s. In the following section,
instead of exact flow rates, discussion will be made using the
terms slow and fast injection. For each experiment it was found
that uncertainty in the measurement of condensation collection flow
rate is within 5% of the numbers stated above.
[0041] The condensate was collected and measured for the
experiments using an air supply to remove all of the liquid before
and after the run and a beaker to collect all of the condensation
during and after a run. The velocity was assumed constant, and thus
only flow rates were measured for experiments where the bed was
completely filled. After observing the experimental results trends
for the slow-injection case, it was decided to only fill the bed
partially and measure the different condensing flow rates. After
testing this, it was found that the velocity did change.
Specifically, it was noticed that the velocity had a negative
linear slope as the bed was being heated. Using this varying
velocity data, the accuracy of the models' solutions was improved,
as presented in the results sections. To compare the steam
injection experiments to a single phase heat transfer fluid, air
was heated and sent through the randomly packed bed. The flow rate
of the air was controlled with an air compressor and its outlet
nozzle. The air was allowed to reach a steady state temperature by
bleeding out through a valve located just before the packed bed,
and once it had reached the steady temperature, the inlet and bleed
valves were slowly opened and closed, respectively. This allowed
the pressure to remain relatively unchanged so that the steady
state air would enter the packed bed without any uncertainties.
This allowed the experiments to be easily reproduced and led to
accurate data and results.
Results
[0042] Thermal Imaging.
[0043] To observe the uniform heat rejection process in the bed
with radial symmetry, thermal images of the cylindrical quartz
walls were captured with frequency 6.25 fps. X-ray images of the
interior of the vessel and thermal images along the wall of the
packed bed for one of the experiments are shown in FIG. 5. Without
the solid media in the vessel, the steam would circulate through
the containment and condense randomly along the walls. With the
solid media present, the thermal front propagation images at
different times shown in FIG. 5 reveal that the steam condensation
over the packed bed is radially uniform, indicating plug flow.
Therefore, qualitative and quantitative analyses using the data
obtained can be done in axial direction (1-D) i.e. direction of
steam flow.
[0044] Vapor Flow Modeling.
[0045] Steam or vapor flow in hot porous media has been studied
previously using convection diffusion models. Radially symmetric
time dependent one-dimensional convection-diffusion equation can be
written as
.differential. T .differential. t + .kappa. v .differential. T
.differential. x = .alpha. .differential. 2 T .differential. x 2 (
1 ) .kappa. = { .rho. C p } f { .rho. C p } b + f ( 2 )
##EQU00001##
[0046] where T is the local temperature of bed and fluid stream, x
and t are the axial dimension and time variable respectively,
{.rho.C.sub.p} is the energy density per unit time, subscripts f
and b denote fluid and bed respectively, v is the fluid stream
velocity and a is thermal diffusivity of the bed. Although at inlet
condition, the fluid stream is saturated steam and possesses latent
heat, for simplified discussion in mathematical form this can be
considered as specific heat spread over the small temperature
difference around the evaporation temperature. This
convection-diffusion equation can be used to provide a simplified
qualitative analysis of the vapor flow. The
.kappa. v .differential. T .differential. x and .alpha.
.differential. 2 T .differential. x 2 ##EQU00002##
terms in Equation 1 are the advection and conduction terms,
respectively. Assuming the temperature is zero at the exit or
bottom of the bed, in the direction of the motion of the fluid, the
temperature gradient,
.differential. T .differential. x , ##EQU00003##
is negative, and the term
.differential. 2 T .differential. x 2 ##EQU00004##
is positive. Therefore at any axial location in the bed, both of
these terms will lead to positive rate of change of temperature. In
the following subsections, the impact of slow and fast injections
of steam on the temperature front progression at the points of
measurement is described.
[0047] Slow Steam Injection.
[0048] The characteristic thermal response of the packed bed system
at different times upon slow injection of steam is highlighted in
this discussion with explanation of results. In the case of slow
injection, there are two thermal transport mechanisms--advection
and conduction modes at different spatial locations and different
time frames. Near the steam entry port, the temperature of the bed
and fluid streams became almost equal to the steam inlet
temperature or saturation temperature in a very short period of
time. As steam supply was continuously available, irrespective of
injection rate, the constant temperature conditions at the inlet
implies that the bed temperature at the top was always maintained
at saturation temperature. This constant bed temperature at the top
(inlet) conducts heat from the top to bottom of the bed due to
non-negligible thermal conductivity of the alumina particles and
water in the bed (i.e., conduction mechanism). Simultaneously, the
steam injected into the bed was also carrying some amount of energy
as it moved in the bed (i.e., advection mechanism).
[0049] Due to the slow injection rate, an initial rise in the
temperature at axially farther locations was dominated by the
conduction mechanism. As the steam or two phase mixture front,
which is at a temperature near the saturation temperature, reaches
those regions located far away from injection point, there was a
sudden change in the temperature, as shown in FIG. 6, with
temperature measurements at different times and different
locations. The rate of increase of temperature at different
locations is divided into two distinct regimes with two distinct
slopes for the last four (3-6) locations. The initial regimes,
which show lower slope, are governed by conduction mechanism. The
later regimes with higher slope are governed by advection. These
conclusions are substantiated with the observation that conduction
dominated for longer times for the locations farther from the
injection point.
[0050] Fast Steam Injection.
[0051] Based on the explanations provided in the previous
subsection, it was expected that the advection term would be much
higher as compared to conduction term throughout the fast injection
experiment. The higher advection term implies that total amount of
influx enthalpy carried by the steam or two-phase mixture is much
higher and thus, as the fluid stream moves through the bed it is
equilibrating the bed to the saturation temperature at almost
constant rate at all spatial locations. Due to much higher rate of
enthalpy injection in the bed due to advection term, the effects of
conduction were not expected have much impact on rate of
temperature increase in the bed. The results in FIG. 7, for fast
injection, confirm this. In the slow steam injection case, a short
steep injection front was observed between the infiltrating steam
and the run off condensation. While this phenomena is prominent in
the slow injection case, fast injection showed simultaneous heat
transfer from both advection and diffusion throughout the bed. As
the steam front traveled through the bed at much quicker pace, the
conduction in the bed did not play a major role in transporting
this heat to points in the bed farther downstream.
[0052] The bottom thermocouple in the fast case was unusable
because of how fast it responds to temperature change compared to
the other five thermocouples. The reason for this is not clear, so
it was left out of the plots intentionally for the fast temperature
cases. But for the slow cases, the temperature change at this
location was slow enough that this effect was unnoticeable. In
comparing slow injection and fast injection, the cases showed a
considerable difference in the heat transfer and thermal front
propagation through the packed bed. The most noticeable difference
was the reduced amount of time for the bed to reach peak
temperature throughout the bed in the fast injection case.
[0053] Air Injection.
[0054] Only one set of results are shown for the case wherein air
was used as the heat transfer fluid, as the difference in flow rate
caused no profound difference in the thermal front propagation in
the packed bed. The slow injection air case is similar to the fast
injection air case because no significant difference was
demonstrated using the experimental setup. However, one noticeable
difference was that the slower case had a slightly higher
temperature due to more heat transferred to the slower moving air.
This allowed the air to reach higher temperature before entering
the packed bed. The top thermocouple was bypassed by the air
stream, and thus this reading was unreliable for assessing the
temperature data and was not used. The packed bed at each vertical
position was at a lower temperature going from the inlet to exit of
the vessel. This results in a more elongated thermal front as
compared to the case with slow steam injection. The experimental
data from an air injection case is shown in FIG. 8.
[0055] Comparing the results of the experiments using air to those
using steam as the heat transfer fluid, there are some very
distinct observations which can be made. Due to the low energy
density of air, it requires a greater amount of time to heat the
packed bed. Even in the steady state situation, the rate of heat
loss through the bed walls were comparable to the rate of heat
input injected in the bed. This led to temperature gradients within
the bed, even after a continuous steady state was achieved, as can
be seen in the time series plots of different thermocouples. The
plots show that lower end thermocouples remain at lower
temperatures with steady state condition. In contrast, the high
energy density of steam (because of the latent heat) allows it to
saturate the packed bed to top temperature more quickly, as shown
in FIGS. 6 and 7.
Conclusion
[0056] The IR camera images of experimental runs show that steam
condenses with cross-sectional uniformity over the packed bed of
spherical particles in the directional plane normal to the steam
flow, justifying the design basis. The experiments were conducted
with two modes of steam injection--fast and slow mode. Thermal
response of the bed was found to be distinct in both of these
cases. In case of slow injection mode, the temporal behavior of the
bed was found to be divided into two spatial zones, advection
driven temperature rise in the bed near the steam injection point
and conduction driven temperature rise for the regions far from the
injection point. As the slow moving steam front reached the far
zone, a steeper rise in the temperature was seen during later
stages of the experiment. Fast injection mode involved high
enthalpy flux penetrating and equilibrating the bed quickly, and
thus only advection driven temperature rise was observed at all
spatial locations. Notably, as the steam condensed with
cross-sectional uniformity and the latent heat was transferred to
the particulates for storage, the packed bed design was able to
maintain a sufficiently steep temperature gradient along the flow
direction to discourage energy losses due to thermal dispersion.
These tests show that efficiency of energy storage is close to 99%.
Advantageously, the system can achieve an overall efficiency of
energy storage and recovery (i.e., round trip efficiency) between
95% and 98%, depending upon the type of recovery fluid and duration
of storage.
[0057] As shown in the results above, different heat transfer
fluids also exhibited distinct heat transfer behavior. The use a
condensable fluid (steam) demonstrated much faster heat transfer to
the packed bed due to the latent heat transferred during
condensation of the steam. In contrast, the use of air as a heat
transfer fluid exhibited much slower heat transfer, as the heat
transfer in that case was only achieved using sensible heat
transfer.
* * * * *