U.S. patent application number 15/311774 was filed with the patent office on 2017-03-23 for thermal energy storage facility.
The applicant listed for this patent is Stellenbosch University. Invention is credited to Paul Gauche, Andre du Randt Louw.
Application Number | 20170082380 15/311774 |
Document ID | / |
Family ID | 58276991 |
Filed Date | 2017-03-23 |
United States Patent
Application |
20170082380 |
Kind Code |
A1 |
Gauche; Paul ; et
al. |
March 23, 2017 |
THERMAL ENERGY STORAGE FACILITY
Abstract
A thermal energy storage facility is provided that consists of a
packed bed with an essentially unconstrained outer region and a
duct having a heat exchange end region located within a central
lower region of the packed bed and a fluid supply end opposite the
heat exchange end region. The heat exchange end region of the duct
permits heat transfer fluid at an elevated temperature to pass from
the duct to the packed bed during a charge cycle to heat the packed
bed from an inner region outwards. The heat transfer fluid is drawn
from the packed bed into the duct during a discharge cycle.
Inventors: |
Gauche; Paul; (Stellenbosch,
ZA) ; Louw; Andre du Randt; (Cape, ZA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Stellenbosch University |
Western Cape Province |
|
ZA |
|
|
Family ID: |
58276991 |
Appl. No.: |
15/311774 |
Filed: |
May 12, 2015 |
PCT Filed: |
May 12, 2015 |
PCT NO: |
PCT/IB2015/053471 |
371 Date: |
November 16, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
F28D 20/0056 20130101;
F28F 2265/26 20130101; Y02E 60/14 20130101; Y02E 60/142 20130101;
F28F 21/04 20130101; F28F 21/08 20130101 |
International
Class: |
F28D 20/00 20060101
F28D020/00; F28F 21/08 20060101 F28F021/08; F28F 21/04 20060101
F28F021/04 |
Foreign Application Data
Date |
Code |
Application Number |
May 16, 2014 |
ZA |
2014/0355 |
Claims
1. A thermal energy storage facility comprising a packed bed having
an essentially unconstrained outer region and a duct communicating
with an interior of the packed bed and having an external fluid
supply end wherein the duct has a heat exchange end region located
within a central lower region of the packed bed and has apertures
over the heat exchange end region so as to permit heat transfer
fluid at an elevated temperature in excess of 500.degree. C.
travelling into the duct via the fluid supply end to pass from the
duct to an interior of the packed bed to heat the packed bed from
an inner region of the packed bed outwards during a charge cycle,
and, during a discharge cycle, permitting heat transfer fluid to be
drawn through a charged packed bed into the heat exchange end
region of the duct at an elevated temperature such that it exits
the fluid supply end of the duct in a heated condition.
2. The thermal energy storage facility as claimed in claim 1,
wherein the fluid supply end is located at or near a top region of
the packed bed or underneath the packed bed, and wherein the heat
transfer fluid exits the duct via the fluid supply end during the
discharge cycle.
3. The thermal energy storage facility as claimed in claim 1,
wherein an external surface of the packed bed is substantially
exposed to the environment.
4. The thermal energy storage facility as claimed in claim 1, which
includes one or more protecting structures for protecting the
packed bed from rain or wind.
5. The thermal energy storage facility as claimed in claim 1,
wherein the duct is defined by a pipe of substantially rigid
material having the heat exchange end region and the fluid supply
end opposite the heat exchange end region, the duct extending at
least partially and generally vertically through a central region
of the packed bed with the heat exchange end region located within
a lower region of the packed bed.
6. The thermal energy storage facility as claimed in claim 1,
wherein the apertures span between about 20% to about 50% of the
height of the packed bed.
7. The thermal energy storage facility as claimed in claim 5,
wherein the duct is manufactured from one or more high temperature
alloys, preferably steel, and/or concrete or for the duct to
consist of element-filled steel gabion bags, and for the elevated
temperature to be between 500.degree. C. and 1500.degree. C.
8. The thermal energy storage facility as claimed in claim 1
wherein the fluid supply end of the duct is located at or near a
top region of the packed bed in which instance elements are packed
such that the packed bed and duct together to have a frustum-like
shape.
9. The thermal energy storage facility as claimed in claim 1
wherein the fluid supply end of the duct is located beneath the
packed bed in which instance the packed bed is packed such that the
packed bed and duct together have a generally conical or pyramidal
shape.
10. The thermal energy storage facility as claimed in claim 1
wherein the packed bed is in the form of an elongate mound having
at least an upper region of triangular or truncated triangular
shape in cross-section wherein the heat exchange end region of the
duct is located in an inner lower region of the packed bed.
11. The thermal energy storage facility as claimed in claim 1
wherein elements of the packed bed are packed in a generally
undisturbed pile, the pile sloping downwards at its natural angle
of repose which ranges between 25.degree. and 65.degree..
12. The thermal energy storage facility as claimed in claim 1
wherein elements of the packed bed are natural rock in the form of
granite, gneiss or dolerite.
13. The thermal energy storage facility as claimed in claim 1
wherein the height of the packed bed is between 1 m and 60 m, the
diameter of the duct is between 0.2 m and 15 m, and the packed bed
has a volume of from 1 m.sup.3 to 300000 m.sup.3.
14. The thermal energy storage facility as claimed in claim 1,
including a thermal energy source and, a thermal energy load in
fluid communication with the thermal energy source and wherein the
thermal energy storage facility is for storing thermal energy
produced by the thermal energy source so as to supply the thermal
energy load during times in which the thermal energy source may
have a reduced output.
15. (canceled)
16. The thermal energy storage facility as claimed in claim 14,
wherein the thermal energy source is a concentrating solar power
plant.
17. The thermal energy storage facility as claimed in claim 14 in
which a blower or fan is provided for urging heat transfer fluid
towards the duct during the charge cycle and/or for urging heat
transfer fluid towards the thermal energy load during the discharge
cycle.
18. A method of constructing a thermal energy storage facility
according to claim 1 comprising the steps of erecting a duct having
a heat exchange end region and a fluid supply end opposite the heat
exchange end region and packing a packed bed around the duct
without substantially constraining the packed bed, the packed bed
sloping downwards at its natural angle of repose wherein the heat
exchange end region of the duct is located in an inner lower region
of the packed bed.
Description
CROSS-REFERENCE(S) TO RELATED APPLICATIONS
[0001] This application claims priority from South African
provisional patent application number 2014/03555 filed on 16 May
2014, which is incorporated by reference herein.
FIELD OF THE INVENTION
[0002] This invention relates to thermal energy storage. More
particularly, the invention relates to a thermal energy storage
facility wherein energy absorbing material is provided by a packed
bed.
BACKGROUND TO THE INVENTION
[0003] The generation of power from sources using conventional
fossil fuels is increasingly being replaced by the use of renewable
energy of one or other type. As far as the present patent
application is concerned, the invention is especially appropriate
for use in association with concentrating solar power plants or
combined cycle power plants, although it is not limited to these
applications.
[0004] The use of solar energy is associated with the need for
storing the energy collected for use at a later time so that energy
is available at night time or when the sun is obscured, typically
by cloud. One practical way of storing energy is in the form of
heat (thermal energy) that can be used subsequently for the
generation of electricity, by way of a steam generating cycle and
an associated turbine and generator.
[0005] Different thermal energy storage facilities have been
proposed and are currently in use, at least to some extent. These
include the storage of thermal energy in molten salts or
alternatively, primarily as latent heat in the case of phase change
materials. Although these are successful to a greater or lesser
extent, there is considerable scope for improvement, particularly
as regards reduction of cost.
[0006] Packed rock beds are also used for thermal energy storage at
high temperatures. A pile of rocks is typically housed in a
substantially sealed enclosure and used to absorb thermal energy
from a heat transfer fluid in order to discharge the thermal energy
at a later stage. The heat transfer fluid typically enters the
sealed enclosure though a suitable inlet and heats the pile of
rocks contained in the enclosure from the outside thereof during a
charge cycle. During a discharge cycle, the cooling of the rock bed
results in heat transfer fluid passing through the rock bed and
exiting by way of the outside of the rock bed and then through the
enclosure to an outlet therefrom.
[0007] In instances in which the rock bed is located within a
container, a problem referred to as "the ratcheting effect" may
occur. An increase in the temperature of particles in rocks
constituting the rock bed leads to expansion, which in turn may
cause some rocks to be urged against other rocks or walls of the
container if the thermal expansion coefficient of the packed bed is
greater than that of the container. Upon discharge, the temperature
is reduced and the particles and container contract. If, however,
the thermal expansion coefficient of the container is greater than
that of the particles, the particles will settle during the
charging process and upon discharge the container will contract
more than the particles. In this way, the particles are constrained
by the container and are subjected to stress, thereby being reduced
to smaller rock pieces. This may damage walls of the container
and/or lead to a top region of the rock bed being gradually lowered
to an unsatisfactory extent.
[0008] A further problem is that the cost of a sealed enclosure may
be substantial, particularly in the case of large energy storage
facilities.
[0009] The present invention aims to address these and other
problems, at least to some extent.
[0010] The preceding discussion of the background to the invention
is intended only to facilitate an understanding of the present
invention. It should be appreciated that the discussion is not an
acknowledgment or admission that any of the material referred to
was part of the common general knowledge in the art as at the
priority date of the application.
[0011] In this specification that follows, the term "packed bed"
will be used to include not only elements made of naturally
occurring rock, but also elements made of ceramic material,
concrete, mining or industrial by-products and any other elements
having appropriate heat capacity and other thermal properties.
SUMMARY OF THE INVENTION
[0012] In accordance with the invention there is provided a thermal
energy storage facility comprising a packed bed having an
essentially unconstrained outer region and a duct communicating
with an interior of the packed bed and having an external fluid
supply end, the thermal energy storage facility being characterised
in that the duct has a heat exchange end region located within a
central lower region of the packed bed so as to permit heat
transfer fluid at an elevated temperature travelling into the duct
via the fluid supply end to pass from the duct to an interior of
the packed bed to heat the packed bed from an inner region of the
packed bed outwards during a charge cycle, and, during a discharge
cycle, permitting heat transfer fluid to be drawn through a charged
packed bed into the heat exchange end region of the duct at an
elevated temperature such that it exits the fluid supply end of the
duct in a heated condition.
[0013] Further features of the invention provide for the fluid
supply end to be located at or near a top region of the packed bed
or, alternatively, underneath the packed bed; for an external
surface of the packed bed to be substantially exposed to the
environment; or alternatively, for the facility to be provided with
one or more protecting structures such as a roof for protecting the
packed bed from rain or wind.
[0014] Still further features of the invention provide for the duct
to be defined by a pipe of substantially rigid material having a
heat exchange end region and a fluid supply end opposite the heat
exchange end region; for the duct to extend at least partially and
generally vertically through a central region of the packed bed
with the heat exchange end located within a lower region of the
packed bed; for the heat exchange end region of the duct to have
apertures for permitting heat transfer fluid at an elevated
temperature travelling into the duct via the fluid supply end to
pass from the duct to the packed bed and heat the packed bed during
a charge cycle, and permitting heat transfer fluid at an elevated
temperature to be drawn from the packed bed into the duct such that
it is capable of exiting the duct during a discharge cycle.
[0015] Yet a further feature of the invention provides for the
apertures to be provided at a heat exchange region of the duct
substantially spanning between about 20% to about 50% of the height
of the packed bed from the bottom or near the bottom thereof.
[0016] Still further features of the invention provide for the duct
to be a hollow, elongate shaft; for the pipe to be manufactured
from one or more high temperature alloys; for the pipe to be
manufactured substantially from steel, and/or concrete or for the
pipe to be constructed of element-filled steel gabion bags; and for
the elevated temperature to be in excess of 500.degree. C. and even
possibly in excess of 1000.degree. C.
[0017] Still further features of the invention provide for the
packed bed to be a pile of elements packed around at least the heat
exchange end of the pipe; for the fluid supply end of the pipe to
be located at or near the top region of the packed bed in which
instance the elements are packed such that the packed bed and pipe
together to have a frustum-like shape; alternatively, for the fluid
supply end of the pipe to be located beneath the packed bed in
which instance for the packed bed may be packed such that the
packed bed and pipe together have a generally conical or pyramidal
shape and the fluid supply end of the pipe is located underneath
the packed bed; or as a further alternative for the packed bed to
be in the form of an elongate mound having at least an upper region
of triangular or truncated triangular cross-section wherein the
heat exchange end region of the duct is located in an inner lower
region of the packed bed; for the elements to be packed in a
generally undisturbed pile; for the pile of elements to slope
downwards at its natural angle of repose; for the angle of repose
to range between 25.degree. and 65.degree., preferably between
34.degree. and 42.degree.; and for the element s to be natural rock
selected from granite, gneiss or dolerite.
[0018] The size of the facility depends on the required thermal
energy storage capacity. The height of the packed bed may be
between 1 m and 60 m, and the diameter of the duct may be between
0.2 m and 15 m. The packed bed may have a volume of about 1 m.sup.3
to about 300000 m.sup.3.
[0019] The invention extends to a thermal energy storage system
comprising a thermal energy source, a thermal energy load in fluid
communication with the thermal energy source and a thermal energy
storage facility for storing thermal energy produced by the thermal
energy source so as to supply the thermal energy load during times
in which the thermal energy source may have a reduced output, the
thermal energy storage facility including a packed bed having an
essentially unconstrained outer region and a duct in fluid
communication with the thermal energy source and the thermal energy
load, the duct having a heat exchange end region and a fluid supply
end opposite the heat exchange end region, characterised in that
the heat exchange end region is located within a central lower
region of the packed bed, the heat exchange end region permitting
heat transfer fluid at an elevated temperature travelling from the
thermal energy source into the duct via the fluid supply end to
pass from the duct to the packed bed and heat the packed bed during
a charge cycle, and permitting heat transfer fluid at an elevated
temperature to be drawn from the packed bed into the duct such that
it is capable of exiting the duct during a discharge cycle to
supply thermal energy to the thermal energy load.
[0020] Further features of this aspect of the invention provide for
the duct to be defined by a pipe of substantially rigid material
having a heat exchange end region and a fluid supply end opposite
the heat exchange end region; for the duct to extend at least
partially and generally vertically through a central region of the
packed bed with the heat exchange end region located within a lower
region of the packed bed, the duct having apertures at least at or
near the heat exchange end region, the apertures permitting heat
transfer fluid at an elevated temperature travelling from the
thermal energy source into the duct via the fluid supply end to
pass from the duct to the packed bed and heat the packed bed during
a charge cycle, and permitting heat transfer fluid at an elevated
temperature to be drawn from the packed bed into the duct such that
it is capable of exiting the duct during a discharge cycle to
supply thermal energy to the thermal energy load. According to one
aspect of the invention, the thermal energy source is a
concentrating solar power plant, the thermal energy load is a steam
turbine, and the heat transfer fluid is a gas.
[0021] The system may include a blower or fan for urging heat
transfer fluid towards the duct during the charge cycle and/or for
urging heat transfer fluid towards the thermal energy load during
the discharge cycle.
[0022] The invention extends to a method of constructing a thermal
energy storage facility comprising the steps of erecting a duct
having a heat exchange end region and a fluid supply end opposite
the heat exchange end region and packing a packed bed without
substantially constraining the packed bed, the packed bed sloping
downwards at its natural angle of repose, characterised in that the
packed bed is packed around at least the heat exchange end region
of the duct.
[0023] Further features of this aspect of the invention provide for
a step of erecting a duct of substantially rigid material in a
generally vertical orientation, the duct having apertures along the
heat exchange end region; and for a step of packing a packed bed
around at least the heat exchange end region of the pipe so that
the packed bed and pipe together form a required shape wherein the
heat exchange end region of the duct is located in an inner lower
region of the packed bed.
[0024] The method may include the step of erecting one or more
protecting structures for protecting the packed bed from rain or
wind, preferably a roof.
[0025] In order for the invention to be more fully understood,
implementations thereof will now be described with reference to the
accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026] The invention will now be described, by way of example only,
with reference to the accompanying drawings in which:
[0027] FIG. 1 is a schematic sectional view illustrating an
embodiment of a thermal energy storage facility according to the
invention;
[0028] FIG. 2 is a schematic sectional view illustrating a second
embodiment of a thermal energy storage facility according to the
invention;
[0029] FIG. 3 is a schematic illustration of an embodiment of
thermal energy storage system according to the invention;
[0030] FIG. 4A is an illustration of a simulation output indicating
temperature in a thermal energy storage facility according to the
invention during a charge cycle;
[0031] FIG. 4B is an illustration of a simulation output indicating
temperature in the thermal energy storage facility of FIG. 4A near
the end of a discharge cycle;
[0032] FIG. 5 is a simulation output illustrating discharge
temperature over time in a thermal energy storage facility
according to the invention;
[0033] FIG. 6 is a simulation output illustrating a pressure drop
in different regions of a thermal energy storage facility according
to the invention;
[0034] FIG. 7 is a schematic sectional view illustrating an
embodiment of a thermal energy storage facility according to the
invention, wherein the facility includes a roof
[0035] FIG. 8 is a schematic sectional elevation illustrating an
alternative embodiment of thermal energy storage facility according
to the invention; and,
[0036] FIG. 9 is a cross-sectional view of the embodiment
illustrated in FIG. 8 taken along line IX-IX.
DETAILED DESCRIPTION WITH REFERENCE TO THE DRAWINGS
[0037] An embodiment of a thermal energy storage facility (1)
according to the invention is shown in FIG. 1. The thermal energy
storage facility (1) comprises a duct defined by a rigid pipe (3)
extending generally vertically through a central region of a packed
bed (5). The packed bed (5) is packed around the pipe (3) such that
its outer region (7) is essentially unconstrained. The pipe (3) has
a heat exchange end (9) located within a lower region of the packed
bed (5) and a fluid supply end (11) opposite the heat exchange end
(9) located at a top region of the packed bed (5).
[0038] The fluid supply end (11) is configured to act as an inlet
for heat transfer fluid at an elevated temperature travelling into
the pipe (3) during a charge cycle, and to act as an outlet for
heat transfer fluid at an elevated temperate travelling out of the
pipe (3) during a discharge cycle.
[0039] The pipe (3) has a heat exchange end region (13) adjacent
the heat exchange end (9) along which it is provided with apertures
(15). In this embodiment of the invention, the heat exchange end
region (13) is a perforated region, in which apertures are defined
in the pipe, spanning about a lower quarter of the pipe (3) and
thus also about 25% of the height of the packed bed (5). The
remainder of the wall of the pipe (3) is substantially continuous
or solid, as schematically illustrated in FIG. 1.
[0040] During a charge cycle, heat transfer fluid passes through
the apertures and heat is transferred from the heat transfer fluid
to the packed bed (5) in order to store heat in the packed bed (5).
This process is reversed during a discharge cycle when heat is
transferred from the packed bed (5) to a heat transfer fluid that
then flows through the apertures to recover heat stored in the
packed bed (5) by absorbing it into the heat transfer fluid.
[0041] An external surface of the packed bed (5) may be exposed to
the environment and is substantially unimpeded so that the packed
bed (5) can expand and contract as it becomes heated and cooled.
The packed bed (5) consists of a pile of elements packed around the
pipe (3) such that the packed bed (5) and pipe (3) together have a
frustum-like shape, as shown in FIG. 1.
[0042] The facility (1) can be constructed by erecting the pipe (3)
or other duct in a generally vertical orientation, and packing the
packed bed (5) around the pipe without substantially constraining
the packed bed (5). The packed bed (5) may be packed in a generally
undisturbed pile, in other words, the pile of elements may be
permitted to slope downwards at its natural angle of repose. The
natural angle of repose of elements may vary depending on the type
and size of the elements used, for example, between 25.degree. and
65.degree., and is preferably between 34.degree. and 42.degree.. In
this embodiment, the type of rock used is a naturally occurring
rock that is dolerite having a natural angle of repose of
38.degree., the height of the packed bed (5) and the pipe (3) is
about 50 m, and the diameter of the pipe is about 10 m.
[0043] The thermal energy storage facility (1) is capable of
storing thermal energy produced by a thermal energy source so as to
supply a thermal energy load during times in which the thermal
energy source may have a reduced or no output.
[0044] A further embodiment of a thermal energy storage facility
(61) according to the invention is shown in FIG. 2. The thermal
energy storage facility (61) includes a rigid pipe (63), a portion
of which extends from the bottom (77) of the packed bed (65) into a
central region thereof. The packed bed (65) is packed around the
heat exchange end (69) of the pipe (63) such that the outer region
(67) is essentially unconstrained and such that the packed bed and
pipe together have a generally conical or pyramidal shape. The heat
exchange end (69) and heat exchange end region (73) which includes
apertures (75) is located in a bottom region of the packed bed (65)
and a fluid supply end (71) opposite the heat exchange end (69) is
located underneath the packed bed (65). In this embodiment, the
fluid supply end (71) of the pipe (63), located underneath the
packed bed, extends through a bottom support or the ground on which
the packed bed (65) rests so as to protrude into a central lower
region of the packed bed (65). The heat exchange end region (73) is
a perforated region, in which apertures are defined in the pipe,
spanning about 25% of the height of the packed bed (65). The
remainder of the wall of the pipe (63) extending below the support
or ground is substantially continuous.
[0045] FIG. 3 illustrates an embodiment of a thermal energy storage
system (17) in which the thermal energy storage facility (1) as
described with reference to FIG. 1 is in fluid communication with a
thermal energy source (19) and a thermal energy load (21). In this
embodiment, the thermal energy source (19) is a concentrating solar
power plant and the thermal energy load (21) is a steam
turbine.
[0046] The system (17) further includes a blower (23) or fan, a
first charge cycle valve (25), a second charge cycle valve (27), a
first discharge cycle valve (29), and a second discharge cycle
valve (31). The blower (23) is in fluid communication with the
thermal energy source (19), the thermal energy load (21) and the
thermal energy storage facility (1), and is configured to urge heat
transfer fluid into the pipe (3) of the thermal energy storage
facility (1) during a charge cycle and to urge heat transfer fluid
towards the thermal energy load (21) during the discharge cycle. In
this embodiment, the heat transfer fluid is air which is heated to
a temperature in the region of 500.degree. C. to 650.degree. C.
[0047] In other embodiments of the invention, the heat transfer
fluid may be heated to temperatures in excess of 500.degree. C. or
even 1000.degree. C.
[0048] Numerous different arrangements are possible within the
scope of the invention and, as illustrated in FIGS. 8 and 9, the
packed bed (81) may assume the form of an elongate mound generally
having a triangular or truncated triangular shape in cross-section,
as shown in FIG. 9. Also, in such an instance, the inlet pipe (82)
may be located on, or somewhat above, the surface (83) on which the
packed bed is supported in which instance the inlet region (84)
would be heat insulated in order to retain the heat within the heat
transfer fluid flowing either into the inner region of the packed
bed during a charging cycle or out of it during a discharge cycle.
As in the instances described above, the packed bed could be
enclosed in a rain and wind protective enclosure (86) having an
outlet/inlet (87) for heat transfer fluid flowing out of the
enclosure during a charging cycle and into the enclosure during a
discharge cycle.
[0049] In use, during a charge cycle, the first and second charge
cycle valves (25, 27) are open to permit heat transfer fluid at an
elevated temperature to enter the pipe (3) via the fluid supply end
(11). The first discharge cycle valves (29) must be closed such
there is no fluid communication to the thermal energy load (21) and
the second discharge valve (31) may be open or closed depending on
the demand or otherwise of the thermal energy load (21). The
apertures (15) in the pipe (3) permit heat transfer fluid in the
pipe (3) to pass from the pipe (3) to the packed bed (5) and heat
the packed bed (5). In this way, thermal energy from the thermal
energy source (19) is absorbed by the packed bed (5) in an inner
region thereof.
[0050] It should be appreciated that the second discharge cycle
valve (31) may typically be left open during the charge cycle to
permit a portion of heat transfer fluid at an elevated temperature
to flow directly to the thermal energy load (21), bypassing the
thermal energy storage facility (1).
[0051] During a discharge cycle, both the first and second
discharge cycle valves (29, 31) are open. Heat transfer fluid at an
elevated temperature is drawn through the packed bed (5) into the
pipe (3) via the apertures (15) and the open valves (29, 31) permit
the heat transfer fluid to exit the pipe (3) via the fluid supply
end (11) and supply thermal energy to the thermal energy load (21).
The first and second charge cycle valves (25, 27) are closed during
a discharge cycle so that there is no fluid communication between
the thermal energy source (19) and the packed bed.
[0052] A numerical model of a thermal energy storage facility
according to the invention was created and simulated using ANSYS
FLUENT, a commercial computational fluid dynamics (CFD) software
product.
[0053] In this model, the thermal energy source used to charge a
packed rock bed was selected as exhaust gas from a gas turbine
power cycle. One charge cycle and one discharge cycle was
simulated. The duration of the charge cycle was set to 8 hours and
the duration of the discharge cycle was set to 30 hours. Parameters
used in the simulation are provided in Table 1 below.
TABLE-US-00001 TABLE 1 Numerical model parameters Temperature of
heat transfer fluid supplied 560.degree. C. from rock bed Ambient
temperature 25.degree. C. Mass flow rate of heat transfer fluid
2272 kg/s Global porosity of rock bed 0.44 Rock density (dolerite)
2819 kg/m.sup.3 Heat capacity of rock 839 J/kgK Conductivity of
rock 2 W/mK Rock bed height 48.14 m Rock pile angle 38.degree. Rock
pile volume 248231 m.sup.3 Particle size in rock bed 0.05 m Energy
required by load* 100 MW.sub.e *Final electrical output power from
a steam cycle
[0054] Temperature contours at the end of the charge cycle and the
discharge cycle taken along a single plane through the thermal
energy storage facility were analysed and illustrations of the
simulation results are respectively shown in FIGS. 4A and 4B.
[0055] As illustrated in FIG. 4A, a high temperature region (41)
forms in an inner region of the rock bed, while a low temperature
region (43) remains at the outer region of the rock bed. According
to the simulation, the temperature in the high temperature region
(41) is about 560.degree. C., while the temperature in the outer
region (43) is about 25.degree. C. A thermocline region (45) is
present between the high temperature region (41) and the outer
region (43), which is a transitional region between hot and cold
zones in the rock bed.
[0056] FIG. 4B illustrates temperature contours near the end of the
discharge cycle. Most of the thermal energy is extracted from the
rock bed near the end of the 30 hour discharge cycle. The
simulation indicated that a relatively small high temperature
region (47) of about 560.degree. C. is present in the zone where
heat exchange between the rock bed and pipe takes place, with lower
temperatures existing in the majority of the pipe (49). The
majority of the rock bed is at a low temperature (51) of about
25.degree. C.
[0057] It is envisaged that, in actual operation, the facility may
stop discharging when the outlet temperature falls below a certain
temperature or may reduce the flow rate to conserve available
energy. Several charge and discharge cycles may be needed before
the facility reaches steady state and is able to supply 30 hours of
full load storage.
[0058] The simulation output of FIG. 5 illustrates discharge
temperature of the heat transfer fluid over time in the numerical
model described above. The temperature of fluid exiting the pipe
drops to below 800K (about 530.degree. C.) at approximately 19
hours of discharging.
[0059] An illustration of a pressure drop along the modelled
thermal energy storage facility in a charge cycle is given in FIG.
6. The pressure drop is shown along a single plane taken through a
central region of the packed bed. The pressure gradient is higher
in the region near the heat exchange region provided by the
apertures in the pipe, and decreases significantly as fluid flows
to the outer regions of the packed bed.
[0060] As fluid flows from the central region of the packed bed to
the outer regions thereof, the flow area through the packed bed
increases and results in lower flow rates. In this way, a lower
pressure drop is obtained. It is envisaged that parameters such as
the length of the heat exchange region and the radius of the pipe
may be adapted to obtain a desired pressure drop. For example, the
length of the heat exchange region and the radius of the pipe may
both be increased in order to lower the pressure drop. The particle
size of the elements in the packed bed may also be altered to
obtain a desired pressure drop.
[0061] The duct may have any suitable shape and configuration
provided that its heat exchange end region is located within a
central lower region of the packed bed. The duct may extend
vertically through a central region of a packed bed or may enter
the packed bed from below a support on which the packed bed rests.
Alternatively, the duct may be configured so as to enter the packed
bed from a side thereof and extend through the side towards a
central lower region within the packed bed provided that the entry
part of the duct is adequately insulated to prevent or minimize
heat transfer to an outer region of the packed bed.
[0062] The duct may be defined by a pipe, which in turn may be any
suitable shaft of hollow, elongate form manufactured from a
substantially rigid material so as to withstand the heat of the
heat transfer fluid and packed bed, and forces applied thereto by
the packed bed. The pipe may, for example, be manufactured from a
high temperature alloy such as steel or concrete, or from both
steel and concrete. Alternatively, the pipe may consist of filled
steel gabion bags, preferably element-filled stainless steel gabion
bags, stacked so as to form a duct or passageway for the flow of
heat transfer fluid therethrough.
[0063] The apertures in the heat exchange region of the pipe may
span any suitable portion of the pipe and/or any suitable height of
the packed bed. The heat exchange region may, for example, span
between about 20% to about 50% of a total height of the packed
bed.
[0064] The heat transfer fluid may be a gas or a liquid. The gas
may be air, carbon dioxide, or other non-flammable gas. The liquid
may be molten salt, thermal oil, or other heat transfer liquid. Any
suitable rock type may be employed, for example but not limited to
granite, dolerite or gneiss.
[0065] It is envisaged that the thermal energy storage facility or
system of the present invention may include a protecting structure
or structures which serve to protect the packed bed from rain, wind
or other elements. Preferably, the protecting structure prevents
rain from reaching the packed bed while not substantially
insulating it from the environment such that air is capable of
escaping from the packed bed.
[0066] An embodiment of a thermal energy storage facility (53)
according to the invention is illustrated in FIG. 7. The embodiment
shown in FIG. 7 is similar to the embodiment shown in FIG. 1, and
like reference numerals represent like components. In this
embodiment, the facility (53) further includes a protecting
structure in the form of a roof (55). The roof (55) slopes downward
from an apex region (57), terminating at eaves (59).
[0067] The roof (55) is impermeable to water and thus prevents the
packed bed (5) from being exposed to rain. The downward slope
causes water to run down the roof (55) and falls from its eaves
(59). In this way, rain is prevented from reaching the packed bed
(5). The roof (55) is permeable to air to permit air to escape from
the packed bed (5).
[0068] The roof (55) may, for example, include air vents (60) that
are configured not to permit rain or water to flow inwards but
allow air to flow through it may be provided. Alternatively, a
waterproof and breathable membrane may be employed especially in
smaller installations. Polytetrafluoroethylene (PTFE) based
materials such as Gore-Tex.TM. may be used to ensure that the
protecting structure is waterproof while allowing air to pass
through it.
[0069] It is also foreseen that a supporting structure may be
provided to support a lower outer region of the packed bed. The
supporting structure may, for example, be a wall located around
lower outer edges of the packed bed. A support or ground on which
the packed bed rests may require foundation treatment to enable it
to withstand the elevated temperatures at the central lower region
of the packed bed. Similarly, it may require moisture collection
monitoring.
[0070] The size of the facility depends on the required thermal
energy storage capacity. The height of the packed bed may, for
example, be between 1 m and 60 m, and the diameter of the pipe may,
for example, be between 0.2 m and 15 m. The packed bed may, for
example, have a volume of about 1 m.sup.3 to about 300 000 m.sup.3.
The packed bed is preferably sized large enough so that the
elements are substantially self-insulating, thereby protecting
inner regions of the packed bed against environmental elements such
as wind and rain, at least to some degree. Smaller packed beds,
such as a packed bed having a volume of 1.0 m.sup.3 may also be
used.
[0071] The dimensions provided above are exemplary and it should be
appreciated that the facility may be substantially larger in cases
where a higher thermal energy storage capacity is required.
[0072] The invention therefore provides a thermal energy storage
facility and system and a method of constructing such a facility.
The facility disclosed can, in particular, be implemented with
concentrating solar power plants or combined cycle power plants,
although it is not limited to these applications.
[0073] Thermal energy is stored at a central lower region of the
packed bed, thereby substantially alleviating the need for further
insulation around the packed bed, such as a sealed container. This
feature may translate into a significant cost reduction in the
construction and maintenance of the thermal energy storage
facility.
[0074] The inclined, substantially unimpeded sides of the packed
bed may overcome the problem of ratcheting experienced when a
packed bed is housed in a container, whereby particles expand and
contract with heating and cooling, packing together more tightly
and exerting a force on the container. The design of the facility
may also ensure relatively low pressure drops along a majority of
the packed bed.
[0075] The thermal energy storage facility can be constructed in a
relatively simple manner, for example, by simply erecting the pipe
as herein described and packing a pile of elements around the pipe
at its natural angle of repose. This may reduce components required
as well as construction and/or operating costs in order to make
thermal energy storage systems economically viable. For example, it
may be easier to replace the packed bed at the end of its lifetime
if the packed bed is not contained.
[0076] Throughout the specification and claims unless the contents
requires otherwise the word `comprise` or variations such as
`comprises` or `comprising` will be understood to imply the
inclusion of a stated integer or group of integers but not the
exclusion of any other integer or group of integers.
* * * * *