U.S. patent application number 12/280690 was filed with the patent office on 2009-01-22 for method of heat accumulation and heat accumulation system.
Invention is credited to Yoshio Morita, Motohiro Suzuki.
Application Number | 20090020264 12/280690 |
Document ID | / |
Family ID | 38474806 |
Filed Date | 2009-01-22 |
United States Patent
Application |
20090020264 |
Kind Code |
A1 |
Morita; Yoshio ; et
al. |
January 22, 2009 |
METHOD OF HEAT ACCUMULATION AND HEAT ACCUMULATION SYSTEM
Abstract
A thermal storage method according to the present invention
includes the steps of: (A) putting a first composition A, including
an n-hydrate (where n is a hydration number) of an inorganic salt
in solid phase and having a phase change temperature Tm of
100.degree. C. or less, in a thermal storage material container;
(B) heating the first composition A to a temperature T2 that is
higher than the phase change temperature Tm and removing water
contained in the first composition A from the thermal storage
material container, thereby obtaining a second composition B in
which an m-hydrate (where m is also a hydration number and m<n)
of the inorganic salt in solid phase and an aqueous solution of the
inorganic salt are both included; (C) stopping removing the water
from the thermal storage material container on sensing that the
second composition B has been obtained; (D) reserving the second
composition B; and (E) mixing the second composition B with water,
thereby recovering at least a part of the heat that is stored in
the second composition B.
Inventors: |
Morita; Yoshio; (Osaka,
JP) ; Suzuki; Motohiro; (Osaka, JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
600 13TH STREET, NW
WASHINGTON
DC
20005-3096
US
|
Family ID: |
38474806 |
Appl. No.: |
12/280690 |
Filed: |
February 28, 2007 |
PCT Filed: |
February 28, 2007 |
PCT NO: |
PCT/JP2007/053784 |
371 Date: |
August 26, 2008 |
Current U.S.
Class: |
165/104.12 |
Current CPC
Class: |
F28D 20/003 20130101;
Y02E 60/14 20130101; F28D 20/021 20130101; C09K 5/063 20130101;
Y02E 60/145 20130101; Y02E 60/142 20130101; F28D 20/02
20130101 |
Class at
Publication: |
165/104.12 |
International
Class: |
F28D 15/00 20060101
F28D015/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 1, 2006 |
JP |
2006-054536 |
Mar 29, 2006 |
JP |
2006-091180 |
Claims
1. A thermal storage method comprising the steps of: (A) putting a
first composition, including an n-hydrate (where n is a hydration
number) of an inorganic salt in solid phase that has a phase change
temperature of 100.degree. C. or less, in a thermal storage
material container; (B) heating the first composition to a
temperature that is higher than the phase change temperature and
removing water contained in the first composition from the thermal
storage material container, thereby obtaining a second composition
in which an m-hydrate (where m is also a hydration number and
m<n) of the inorganic salt in solid phase and an aqueous
solution of the inorganic salt are both included; (C) stopping
removing the water from the thermal storage material container on
sensing that the second composition has been obtained; (D)
reserving the second composition; and (E) mixing the second
composition with water, thereby recovering at least a part of the
heat that is stored in the second composition.
2. The thermal storage method of claim 1, wherein the step (E)
includes using a composition, obtained by mixing the second
composition with the water, as the first composition.
3. The thermal storage method of claim 1, wherein the volume
fraction of solid in the second composition is greater than 0 vol %
but equal to or smaller than 90 vol %.
4. The thermal storage method of claim 1, wherein the step (E)
includes mixing the second composition with water in liquid
phase.
5. The thermal storage method of claim 1, wherein the step (E)
further includes running heat recovery water onto a water channel,
which is arranged so as to contact with either the surface of the
thermal storage material container or a composition obtained by
mixing the second composition with water, thereby making a heat
exchange between the surface or the composition and the heat
recovery water.
6. The thermal storage method of claim 1, wherein the inorganic
salt is magnesium sulfate.
7. The thermal storage method of claim 1, wherein the inorganic
salt is calcium chloride.
8. The thermal storage method of claim 1, wherein the inorganic
salt is calcium bromide.
9. A thermal storage system comprising: a thermal storage material
container that contains a thermal storage material as a first
composition, including an n-hydrate (where n is a hydration number)
of an inorganic salt in solid phase that has a phase change
temperature of 100.degree. C. or less; a thermal storage material
heating section for heating the thermal storage material to a
temperature higher than the phase change temperature; a water
removal passage for removing water in gas phase from the thermal
storage material container; a sensor for sensing the amount of the
water that has been removed from the thermal storage material
container through the water removal passage; a control section for
stopping the removal of the water through the water removal passage
based on a result of detection done by the detector on sensing that
the thermal storage material is a second composition in which an
m-hydrate (where m is also a hydration number and m<n) of the
inorganic salt in solid phase and an aqueous solution of the
inorganic salt are both included; a water supply channel for
supplying water into the thermal storage material container; and a
heat recovery section for recovering at least a part of the heat
that has been stored in the thermal storage material.
10. The thermal storage system of claim 9, wherein the water supply
channel is used to supply water in liquid phase into the thermal
storage material container.
11. The thermal storage system of claim 9, further comprising a
condensing section for condensing the water in gas phase, which has
been removed from the thermal storage material container through
the water removal passage, into liquid phase, and a water reservoir
section for reserving the water that has been condensed by the
condensing section.
12. The thermal storage system of claim 11, wherein the thermal
storage material heating section includes first heating means for
heating a heat medium and a first heat exchange section for making
a heat exchange between the heat medium that has been heated by the
first heating means and the thermal storage material, and wherein
the condensing section includes a second heat exchange section for
making a heat exchange between the water in gas phase, which has
been removed from the thermal storage material container through
the water removal passage, and the heat medium, on which a heat
exchange has already been done by the first heat exchange section
with the thermal storage material, thereby recovering a part of the
heat of condensation of the water in gas phase.
13. The thermal storage system of claim 11, wherein the sensor is
provided for the water reservoir section.
14. The thermal storage system of claim 9, wherein the heat
recovery section has a heat recovery water channel for running heat
recovery water, and wherein the heat recovery water channel is
connected to a heat exchange tube that is arranged in contact with
either the surface of the thermal storage material container or the
thermal storage material itself.
Description
TECHNICAL FIELD
[0001] The present invention relates to a method and system for
thermal storage.
BACKGROUND ART
[0002] Various thermal storage techniques for conserving thermal
energy have been used effectively to save energies. Meanwhile,
recently, hot water supply units that use a CO.sub.2 heat pump or a
fuel-cell cogeneration system (which will be referred to herein as
a "fuel-cell cogene") have attracted a lot of attention in the art.
In order to reduce the sizes of those units and install them more
efficiently, development of high-density thermal storage technology
is awaited.
[0003] Conventional thermal storage techniques are roughly
classifiable into sensible heat storage, latent heat storage, and
chemical heat storage techniques. According to the sensible and
latent heat storage techniques, some loss such as heat exchange
loss is inevitable in practice but there is no need to newly apply
any thermal energy during the heat release process. That is why
theoretically speaking, almost the same quantity of heat as what
was applied during the heat storage process can be released as
needed.
[0004] According to the sensible and latent heat storage
techniques, the heat storage performance depends on specific heat
and phase change heat, which are physical properties of a thermal
storage material, and the storable heat quantity of a latent heat
storage material becomes equal to the sum of the sensible heat and
the phase change heat. However, since the phase change heat of a
thermal storage material is generally smaller in a low temperature
range than in a high temperature range, the thermal storage density
decreases in the low temperature range. Also, according to the
latent heat storage technique, the phase change temperature range
is determined by the temperature of a heat source used, and a
latent heat material with such a phase change temperature is
selected. For example, as a latent heat storage material for use in
a reservoir for a hot water supply unit, the hydrate or inorganic
substance disclosed in Non-Patent Document No. 1 may be used to
store the heat of around 80 to 110.degree. C. Meanwhile, Patent
Document No. 1 suggests the use of a mixed phase change substance
as a latent heat storage material in such a temperature range.
[0005] On the other hand, the chemical heat storage is a thermal
storage technique that uses the heat generated as a result of a
reversible chemical reaction. The chemical heat storage may use the
chemical reaction of an adsorptive material, a hydrogen occlusion
alloy, an organic reaction system or an inorganic reaction system.
No matter which of these chemical reactions is used, the
conventional chemical heat storage utilizes either a gas-liquid
reaction or a gas-solid reaction. If the heat were stored as a gas
as it is, then the volume of the stored gas would be so great that
the thermal storage density would be rather low. That is why the
gas generated during the thermal storage process is either
condensed or turned into a solid compound such as a metal hydride,
thereby trying to decrease the volume. However, the heat generated
as a result is released into the external air. For that reason, in
using the heat (i.e., in the heat release process), external
thermal energy needs to be newly applied to trigger a reaction
opposite to what happened during the thermal storage process by
producing a gas from either a liquid or a solid compound.
Consequently, if a conventional chemical heat storage system is
used, the thermal energy that can be stored is obtained by
subtracting the thermal energy (i.e., heat loss) needed to get the
heat release reaction done from the one stored during the heat
storage process as a matter of principle. Patent Document No. 2
discloses a chemical heat storage method that uses a reaction in
which a solid hydrate is produced as a result of the reaction
between a solid inorganic anhydride and water vapor. The document
also discloses that the heat of formation (i.e., heat of
condensation) generated by condensing water vapor as a gas is
recovered in a heat pump, which is a thermal storage heat source.
According to this method, however, an external heat source is also
newly used during the heat release process.
[0006] Thus, according to the conventional sensible and latent heat
storage techniques, the types of usable thermal storage materials
are limited by the temperature of the heat source and a high
thermal storage density cannot be ensured in a broad temperature
range, which is a problem. Also, if the conventional chemical heat
storage is adopted, external energy should be applied during the
heat release process and therefore, the amount of substantial heat
stored would decrease, which is also a problem.
[0007] Meanwhile, chemical heat storage techniques that eliminate
the need for the external application of thermal energy during the
heat release process by utilizing the dehydration reaction of a
hydrate are disclosed in Patent Documents Nos. 3 and 4.
[0008] Specifically, Patent Document No. 3 teaches storing heat by
turning a hydrate (such as a calcium chloride hexahydrate) into an
anhydrate (such as calcium chloride) and also teaches turning the
anhydrate into a hydrate and recovering the heat of formation
generated as a result of the reaction during the heat release
process. More specifically, during the heat storage process, a
hydrate in a thermal storage material container is heated and
dehydrated so as to turn into an anhydrate. In the heat release
process, on the other hand, water vapor is supplied into the
thermal storage material container, thereby turning the anhydrate
in the thermal storage material container into a hydrate.
[0009] Meanwhile, Patent Document No. 4 discloses how to produce a
calcium chloride tetrahydrate by heating and drying a calcium
chloride hexahydrate during the thermal storage process and how to
generate heat by making the calcium chloride tetrahydrate absorb
water during the heat release process. Patent Document No. 4 also
proposes that a thermal storage material be held by a holding
member such as a ceramic sheet to dry the thermal storage material
in a shorter time during the thermal storage process and to make
the material absorb water more quickly during the heat release
process.
[0010] The quantity of thermal energy (or simply the heat quantity)
that can be stored through a heat cycle including the thermal
storage process and the heat release process will be referred to
herein as a "quantity of thermal storage" and the quantity of
thermal storage of in a thermal storage material per unit volume
(or unit weight) will be referred to herein as a "thermal storage
density". [0011] Patent Document No. 1: PCT International
Application Japanese National Phase Publication No. 2003-507524
[0012] Patent Document No. 2: Japanese Patent Gazette for
Opposition No. 7-6708 [0013] Patent Document No. 3: Japanese Patent
Application Laid-Open Publication No. 2004-3832 [0014] Patent
Document No. 4: Japanese Patent Application Laid-Open Publication
No. 3-244998 [0015] Non-Patent Document No. 1: Bulletin of the
Institute of Electrical Engineers of Japan, p. 15, Vol. 101,
1981
DISCLOSURE OF INVENTION
Problems to be Solved by the Invention
[0016] However, the present inventors discovered via experiments
that according to the thermal storage method disclosed in Patent
Document No. 3, to release heat, water should be supplied, in the
form of vapor or droplets, into a thermal storage material
container, including an anhydrate in solid phase, but only a part
of the anhydrate in the container reacted with the water and the
rest of the anhydrate remained unreacted at the bottom of the
container. As a result, the heat that had been stored in the
anhydrate could not be recovered sufficiently and the quantity of
the thermal output (or released) decreased. Also, if one tried to
make the whole anhydrate in the thermal storage material container
react with the water, then the layer of the anhydrate should have a
reduced thickness. In that case, however, the size of the thermal
storage material container should be increased too much and the
effective thermal storage density of the thermal storage apparatus
would decrease. As used herein, the "effective thermal storage
density" refers to the quantity of heat stored in a thermal storage
apparatus or a thermal storage system per unit volume (which is a
theoretical value). Furthermore, according to this method, when the
heat should be released, either water vapor or droplets of water
are supplied to the anhydrate (in solid phase) in the thermal
storage material container. However, the mixture of the anhydrate
and the water thus obtained includes a high percentage of solid and
the area of contact between the mixture (i.e., thermal storage
material) and the thermal storage material container is small. That
is why the heat released from the mixture cannot be transferred
efficiently to the thermal storage material container. As a result,
the thermal output rate decreases, which is also a problem.
[0017] On the other hand, according to the thermal storage method
disclosed in Patent Document No. 4, the thermal storage material
that stores some heat (such as a calcium chloride tetrahydrate
salt) is held in a sheet of ceramic, and therefore, the whole
material can be allowed to react with water uniformly in a short
time. However, if the amount of the thermal storage material is
increased to ensure a sufficient quantity of thermal storage, then
the ceramic sheet should have an increased area. In that case, the
size of the thermal storage apparatus also increases accordingly,
and the effective thermal storage density might decrease.
Furthermore, according to this method, when heat needs to be
released, the heat is output from the thermal storage material,
wrapped in the ceramic sheet, by way of a heat exchange plate. That
is why the thermal conductivity between the thermal storage
material and the heat exchange plate decreases and eventually the
thermal output rate decreases, too.
[0018] In order to overcome the problems described above, the
present invention has an object of providing a thermal storage
method that uses a hydrate, which can increase both the quantity of
thermal output and a thermal output rate without decreasing the
effective thermal storage density in a thermal storage system.
Means for Solving the Problems
[0019] To overcome the problems of the prior art described above, a
thermal storage method according to the present invention includes
the steps of: (A) putting a first composition, including an
n-hydrate (where n is a hydration number) of an inorganic salt in
solid phase that has a phase change temperature of 100.degree. C.
or less, in a thermal storage material container; (B) heating the
first composition to a temperature that is higher than the phase
change temperature and removing water contained in the first
composition from the thermal storage material container, thereby
obtaining a second composition in which an m-hydrate (where m is
also a hydration number and m<n) of the inorganic salt in solid
phase and an aqueous solution of the inorganic salt are both
included; (C) stopping removing the water from the thermal storage
material container on sensing that the second composition has been
obtained; (D) reserving the second composition; and (E) mixing the
second composition with water, thereby recovering at least a part
of the heat that is stored in the second composition.
[0020] According to this method, in the step (B), the first
composition is heated and water is removed from the thermal storage
material container. Thus, the heat can be stored by not just
sensible and latent heat storage techniques but also a chemical
heat storage technique based on a difference in concentration of
the inorganic salt. Also, in the step (E), heat is released by
mixing the second composition with water. That is why there is no
need to newly apply any thermal energy during the heat release
process. Consequently, the thermal storage density can be increased
significantly compared to the conventional latent or chemical heat
storage technique.
[0021] In addition, according to this method, before the
composition in the thermal storage material container turns into a
solid phase simple substance, the removal of water from the thermal
storage material container is stopped to obtain a second
composition in solid-liquid mixed phase. That is why while heat is
released by performing the step (E), the solid (i.e., the
m-hydrate) included in the second composition is allowed to react
with water uniformly, and the decrease in the quantity of thermal
output, which would be caused if a part of the solid remained
unreacted, can be minimized. As a result, the quantity of thermal
output can be increased without making a bulky thermal storage
apparatus.
[0022] Furthermore, in the step (E), water is further added to the
second composition in the solid-liquid mixed phase. That is why the
mixture includes a lot of liquid and the area of contact between
the mixture (i.e., the thermal storage material) and the thermal
storage material container increases. As a result, the thermal
conductivity between the thermal storage material and its
container, and eventually the thermal output rate while the heat is
used, can be increased.
EFFECTS OF THE INVENTION
[0023] According to the present invention, when a thermal storage
process is carried out using a hydrate, the second composition and
water can be mixed together uniformly during the heat release
process, and therefore, the quantity of thermal output can be
increased. In addition, the thermal conductivity between the
mixture of the second composition and water and the thermal storage
material container can be increased. As a result, the thermal
output rate can be increased eventually.
[0024] On top of that, the quantity of thermal output can be
increased without decreasing the effective thermal storage density
in a thermal storage system. Consequently, a smaller thermal
storage system than a conventional one can be provided.
BRIEF DESCRIPTION OF DRAWINGS
[0025] FIG. 1 schematically shows how thermal storage and heat
release processes may be performed according to a first preferred
embodiment of the present invention.
[0026] FIG. 2 schematically shows how thermal storage and heat
release processes may be performed according to a second preferred
embodiment of the present invention.
[0027] FIGS. 3(a) and 3(b) are schematic representations
illustrating the results of composition-water mixing experiments in
a specific example of the present invention and in a comparative
example, respectively.
[0028] FIG. 4 is a graph showing the conductivities of respective
aqueous solutions that were obtained in the composition-water
mixing experiments in the specific example of the present invention
and in the comparative example.
[0029] FIG. 5 is a graph showing how the thermal storage density
changes with the solute concentration of magnesium sulfate.
[0030] FIG. 6 is a graph showing how the thermal storage density
changes with the solute concentration of sodium thiosulfate.
[0031] FIG. 7 is a graph showing how the thermal storage density
changes with the solute concentration of calcium chloride.
[0032] FIG. 8 is a graph showing how the thermal storage density
changes with the solute concentration of calcium bromide.
[0033] FIG. 9 is a graph showing how the thermal storage density
changes with the solute concentration of zinc nitrate.
[0034] FIG. 10 illustrates a general arrangement of a thermal
storage system for use in a specific example of the present
invention.
[0035] FIG. 11 is a schematic cross-sectional view illustrating the
configuration of a thermal storage vessel included in the thermal
storage system shown in FIG. 10.
[0036] FIG. 12 is a phase diagram of magnesium sulfate and water
showing how to perform a thermal storage method in a first specific
example of the present invention.
[0037] FIG. 13 is a phase diagram of calcium chloride and water
showing how to perform a thermal storage method in a second
specific example of the present invention.
[0038] FIG. 14 is a phase diagram of magnesium sulfate and water
showing how to perform a thermal storage method in a third specific
example of the present invention.
[0039] FIG. 15 is a phase diagram of zinc nitrate and water showing
how to perform a thermal storage method in a fourth specific
example of the present invention.
DESCRIPTION OF REFERENCE NUMERALS
[0040] T1 temperature (room temperature) of first composition
before thermal storage [0041] T2 temperature of second composition
during thermal storage [0042] Tm phase change temperature of
n-hydrate [0043] Tm' liquidus temperature of first composition
[0044] c1 solute concentration of first composition [0045] c2
solute concentration of second composition [0046] A, D, E, H first
composition [0047] B, F second composition [0048] 10, 11, 14, 20,
24 thermal storage process [0049] 12, 16, 22, 26 heat release
process [0050] 51 thermal storage vessel [0051] 51a thermal storage
material [0052] 52 heating section [0053] 53 external heat source
[0054] 54 first condenser [0055] 55 second condenser [0056] 56
water tank [0057] 57 vacuum pump [0058] 58 pump [0059] 59 valve
[0060] 60 water removal passage [0061] 62 water supply channel
[0062] 64 heat medium passage [0063] 65 heat recovery water channel
[0064] 66 thermal storage material container [0065] 68, 69 space in
the thermal storage material container [0066] 70 partition [0067]
72 water passage hole [0068] 80 heat exchange tube [0069] 100
thermal storage system
BEST MODE FOR CARRYING OUT THE INVENTION
[0070] Hereinafter, a thermal storage method according to the
present invention will be outlined.
[0071] First of all, a first composition, including an n-hydrate
(where n is a hydration number) of an inorganic salt in solid
phase, is put in a thermal storage material container. The
n-hydrate has a phase change temperature of 100.degree. C. or less.
Next, this first composition is heated to a temperature that is
higher than the phase change temperature of the n-hydrate and water
contained in the first composition is removed from the thermal
storage material container, thereby obtaining a second composition
in which an m-hydrate (where m is also a hydration number that is
smaller than that of the n-hydrate) of the inorganic salt and an
aqueous solution of the inorganic salt are both included. In this
process step, the n-hydrate of the inorganic salt is heated to a
temperature exceeding the phase change temperature, thereby storing
a quantity of heat corresponding to the sensible heat and latent
heat. At the same time, by removing water from the thermal storage
material container, a quantity of heat generated due to a
difference in the concentration of the solute can be stored.
Subsequently, on sensing that the second composition has been
obtained, the removal of the water from the thermal storage
material container is stopped. For example, the volume of water
removed from the thermal storage material container may be
detected, and when the volume reaches a predetermined value, the
removal of the water may be stopped.
[0072] Next, this second composition is stored. And when some heat
needs to be used, the second composition is mixed with water in
liquid phase, thereby releasing the quantity of heat that has been
stored due to the difference in the concentration of the solute.
Furthermore, by decreasing the temperature of the composition,
already containing the water mixed, to equal to or lower than the
phase change temperature of the n-hydrate, the quantity of heat
corresponding to the sensible heat and latent heat, which has been
stored in the second composition, can also be released. In this
case, if heat recovery water is run so as to contact with the
thermal storage material container, for example, and if a heat
exchange is made between that water and the thermal storage
material container, part of the heat quantity released can be
recovered. After the heat exchange, the water may be used for a
heat-assisted unit such as a hot water supply unit.
[0073] When the heat is used, the same amount of water as that of
the water removed from the thermal storage material container
during the thermal storage process (which will be referred to
herein as "reaction water") is preferably supplied into the thermal
storage material container. In this case, the reaction water may be
supplied into the thermal storage material container either at
entirely a time or little by little a number of times. When it gets
the first composition (i.e., in solid-liquid mixed phase or in
simple liquid phase) again with the reaction water all supplied,
this first composition has its temperature decreased to equal to or
lower than the phase change temperature again, thereby releasing
the reaction heat and the sensible and latent heat. If the reaction
water has been supplied entirely at a time, the sensible heat will
be used after the hydration reaction is complete. That is why the
thermal output rate can be increased irrespective of the rate of
the hydration reaction. Also, even if the reaction water needs to
be supplied little by little a number of times, part of the
reaction water is supplied first to impregnate the solid (i.e., the
m-hydrate) with the water and then the rest of the reaction water
is supplied. Consequently, the rate of the hydration reaction can
also be increased and the thermal output rate can be increased
eventually.
[0074] According to the present invention, by removing water from
the thermal storage material container during the thermal storage
process, the water is partially removed from the reaction system
between the inorganic salt and the water. Such a removal of water
from the reaction system will sometimes be referred to herein as
"separation of water" in order to distinguish such a removal from
the dehydration of a hydrate.
[0075] In this manner, according to the present invention, not only
a latent heat process but also a chemical heat storage process
based on a concentration difference are adopted in combination.
That is why compared to the conventional thermal storage method
that uses only a latent heat process or a chemical heat storage
process, the quantity of thermal storage can be increased
significantly and a thermal storage density that is higher than a
latent heat storage density, including a sensible heat, can be
achieved. Furthermore, according to the chemical heat storage
process based on the concentration difference, there is no need to
apply any thermal energy externally during the heat release process
unlike the conventional chemical heat storage method. As a result,
the heat loss that is unavoidable according to the principle of the
conventional chemical heat storage method can be reduced according
to the present invention.
[0076] Furthermore, according to the method described above, when
it is sensed that the second composition in the solid-liquid mixed
phase has been obtained, the removal of the water from the thermal
storage material container is stopped. Thus, it is possible to
prevent the water from being separated so much as to make the
composition in the thermal storage material container assume solid
phase. In the second composition in the solid-liquid mixed phase,
the m-hydrate in solid phase is dispersed in the liquid, and can
react uniformly to the water supplied into the thermal storage
material container during the heat release process. Consequently,
the decrease in the quantity of thermal output (or heat released)
due to the residue of an unreacted solid at the bottom of the
thermal storage material container can be minimized. Such effects
will be described in detail later based on the results of
experiments the present inventors carried out.
[0077] Besides, according to the method described above, water in
liquid phase is mixed with the second composition in solid-liquid
mixed phase during the heat release process, and therefore, the
mixture includes a lot of liquid and the heat can be conducted
efficiently to the thermal storage material container and a heat
exchanger by taking advantage of convection. Consequently, the
thermal output rate can be increased.
[0078] The first composition according to the present invention may
be either in solid phase with an n-hydrate of an inorganic salt at
room temperature or in solid-liquid mixed phase with an n-hydrate
and an aqueous solution of the inorganic salt. Also, if necessary,
the first and second compositions may include some additive such as
an anticorrosion agent, a phase separation prevention agent or a
supercooling prevention agent.
[0079] As the inorganic salt, a substance that will generate
positive reaction heat during a hydration reaction for producing an
n-hydrate based on its m-hydrate and water needs to be used. Also,
a substance that has a high thermal storage density based on a
concentration difference is preferably used. Specifically, at least
one inorganic salt selected from the group consisting of chlorides,
bromides, iodides, hydroxides, nitrates, sulfates, thiosulfates,
phosphates, borates and acetates may be used. Among other things,
magnesium sulfate, calcium chloride and calcium bromide are
particularly beneficial in terms of thermal storage density,
environment friendliness and cost.
[0080] A method for removing water from the thermal storage
material container is not particularly limited. For example, water
in gas phase may be vaporized off outside of the thermal storage
material container by a vaporization technique. In that case, the
latent heat vaporized is preferably recovered in the same thermal
storage unit through heat exchange.
EMBODIMENT 1
[0081] Hereinafter, a Thermal Storage Method as a First preferred
embodiment of the present invention will be described with
reference to the accompanying drawings.
[0082] FIG. 1 illustrates an example of thermal storage and release
processes according to this preferred embodiment. In FIG. 1, the
abscissa represents the concentration of the composition and the
ordinate represents the temperature, and a phase diagram, showing
what phases are present in an inorganic salt and water, is
shown.
[0083] In this example, Composition A (with an inorganic salt
concentration c1) to be an n-hydrate in solid phase at a
temperature T1 is used as the first composition. The temperature T1
may be equal to room temperature, for example.
[0084] In the thermal storage process, first, when the temperature
of the n-hydrate is increased from the temperature T1 to a
temperature T2, which is higher than the phase change temperature
Tm, as indicated by the arrow 10, an m-hydrate in solid phase is
produced through the dehydration reaction of the n-hydrate. As a
result, Composition A enters a solid-liquid mixed phase in which
the m-hydrate and an aqueous solution of the inorganic salt are
both included. Thereafter, with the temperature T2 of the thermal
storage material maintained as indicated by the arrow 11, water is
removed from the thermal storage material container. And when it is
sensed that a predetermined amount of water has been removed, the
removal of the water is stopped. In this manner, Composition B
(with a concentration c2 (>c1)), which includes the inorganic
salt at a higher concentration than Composition A, is obtained. The
composition B thus obtained is in a solid-liquid mixed phase
including both the m-hydrate and the aqueous solution of the
inorganic salt.
[0085] In the heat release process, Composition B at the
temperature T2 is mixed with water to obtain Composition A in a
solid-liquid mixed phase as indicated by the arrow 12, and then the
temperature of Composition A is decreased to the temperature T1
that is lower than the phase change temperature Tm of the
n-hydrate, thereby turning the composition back into an n-hydrate.
The amount of the water to be mixed with the second composition is
preferably substantially equal to that of the water removed from
the thermal storage material container during the thermal storage
process. As described above, when the heat is used, the heat is
released in the form of sensible heat and latent heat, and
therefore, the thermal output rate does not depend on the rate of
reaction between the m-hydrate and the water. That is why compared
to the conventional chemical heat storage method, the thermal
output rate can be increased.
[0086] In this preferred embodiment, the total quantity of heat
.DELTA.H that can be stored is the sum of the sensible heat .DELTA.
H.sub.S1 to raise the temperature of the n-hydrate from the
temperature T1 to the phase change temperature Tm of the n-hydrate,
the latent heat of fusion .DELTA.H.sub.L1 of the n-hydrate at the
temperature Tm, the sensible heat .DELTA.H.sub.S2 to raise the
temperature of Composition A in the solid-liquid mixed phase from
Tm to T2 and the reaction heat .DELTA.H.sub.R1 generated by
increasing the concentration of the inorganic salt with the
separation of water from Composition A:
.DELTA.H(total quantity of heat
stored)=.DELTA.H.sub.S1+.DELTA.H.sub.L1+.DELTA.H.sub.S2+.DELTA.H.sub.R1
This total quantity of heat stored .DELTA.H is changeable with the
temperature T1 of Composition A before thermal storage, the
temperature T2 of Composition B after the thermal storage, and the
concentrations c1 and c2 of the inorganic salt.
[0087] A thermal storage process indicated by the arrows 10 and 11
has been described with reference to FIG. 1. However, the path of
obtaining Composition B at the temperature T2 from Composition A at
the temperature T1 is not particularly limited. For example, the
water may be separated while Composition A is being heated.
Likewise, the heat release process is not limited to the path
indicated by the arrow 12, either.
[0088] The temperature T2 of Composition B after the thermal
storage and the concentration c2 of the inorganic salt may be
appropriately determined such that Composition B can assume the
solid-liquid mixed phase. Preferably, the temperature T2 and the
concentration c2 are defined so as to fall within the following
ranges.
[0089] The temperature T2 of Composition B is preferably higher
than room temperature, e.g., within the range of 30.degree. C. to
100.degree. C., but at least needs to be determined to be higher
than the phase change temperature Tm. More preferably, the
temperature T2 is less than 100.degree. C. Then, in a chemical heat
storage process based on the concentration difference, the water
separation reaction heat of a reversible liquid can be used. Also,
if a CO.sub.2 heat pump is used to heat Composition A, it is
effective to set the temperature T2 equal to or lower than
90.degree. C. (e.g., in the vicinity of 80.degree. C.). The reason
is that by storing the reaction heat, generated by the hydration
reaction, as sensible heat, the thermal storage material in the
thermal storage material container can be heated to around
100.degree. C. and the heat can be used according to the hot water
supply rate.
[0090] In this preferred embodiment, if the temperature T2 were
defined to be less than 100.degree. C., water should be removed
from Composition A at less than 100.degree. C. during the thermal
storage process. In that case, if the pressure inside the thermal
storage material container is reduced with a vacuum pump and if the
water included in Composition A in the thermal storage material
container is boiled at that temperature less than 100.degree. C.,
water in gas phase can be removed from the thermal storage material
container.
[0091] The concentration c2 of the inorganic salt is preferably
controlled such that Composition B includes solid at 90 vol % or
less. If the solid volume fraction is 90 vol % or less, then
Composition B has flowability (which defines a flowability limit of
a solid-liquid mixed slurry). That is why during the heat release
process, the m-hydrate included in Composition B can be mixed with
water efficiently. The volume fraction described above is more
preferably 78 vol % or less, and even more preferably 74 vol % or
less. Then, the m-hydrate included in Composition B and water can
be mixed together more uniformly, thus further increasing the
quantity of heat released. Meanwhile, the volume fraction just
needs to be at least greater than 0 vol %. In a situation where
calcium chloride is used as the inorganic salt, for example, even
if the calcium chloride concentration c2 of Composition B is a
concentration that allows a solid to precipitate, heat can still be
stored due to a difference in solute concentration. On the other
hand, if magnesium sulfate is used as the inorganic salt, the
volume fraction described above is preferably at least equal to 68
vol %. Then, the concentration difference (c2-c1) between
Compositions A and B can be wide enough to increase the quantity of
heat stored.
[0092] If a heat pump is used as an external power supply to heat
Composition A, for example, the phase change temperature Tm of the
n-hydrate of the inorganic salt is preferably equal to or higher
than 30.degree. C. but equal to or lower than the heating
temperature of the heat pump. Also, if a heat pump is used as an
external power supply and if Composition A is heated by using water
at a normal pressure as a heat medium, then the phase change
temperature Tm is supposed to be lower than 100.degree. C.
Considering the preferred range of the temperature T2 described
above, the phase change temperature Tm is preferably lower than
90.degree. C.
[0093] Also, if the temperature of Composition B being mixed with
water exceeded 100.degree. C. due to the mixture, then heat of
vaporization would be lost. That is why the loss of the quantity of
heat stored can be reduced by stopping supplying water to the
thermal storage material container before the temperature reaches
100.degree. C. In that case, after the temperature of the mixture
in the thermal storage material container has been decreased to a
predetermined temperature (of 70.degree. C., for example) or less,
part or all of the rest of the water may be supplied.
[0094] In the example illustrated in FIG. 1, Composition A that is
in solid phase (i.e., n-hydrate) at room temperature is used as the
first composition. Alternatively, Composition D that includes both
the n-hydrate and the aqueous solution at room temperature may also
be used. Such composition D may be prepared by mixing the n-hydrate
with water.
[0095] Even when Composition D is used, the thermal storage and
release processes may also be performed just as described above.
Specifically, in the thermal storage process, the temperature of
Composition D is increased from T1 (which may be equal to room
temperature, for example) to a temperature T2, which is higher than
the phase change temperature Tm of the n-hydrate as indicated by
the arrow 14. After that, water is separated from Composition D to
obtain Composition B in solid-liquid mixed phase including an
m-hydrate, which includes a smaller number of water molecules than
the n-hydrate, and an aqueous solution. On the other hand, in the
heat release process, Composition B is mixed with water to obtain
Composition D and then the temperature of Composition D is
decreased to the temperature T1 as indicated by the arrow 16.
[0096] Even when Composition D is used, the total quantity of heat
.DELTA.H that can be stored is also calculated as described above.
That is to say, the total quantity of heat stored .DELTA.H is the
sum of the sensible heat .DELTA.H.sub.S3 to raise the temperature
of Composition D from the temperature T1 to the phase change
temperature Tm of the n-hydrate, the latent heat of fusion
.DELTA.H.sub.L2 of the n-hydrate at the temperature Tm, the
sensible heat .DELTA.H.sub.S4 to raise the temperature of
Composition D in the solid-liquid mixed phase from Tm to T2 and the
reaction heat .DELTA.H.sub.R2 generated by increasing the
concentration of the inorganic salt with the separation of water
from Composition D:
.DELTA.H(total quantity of heat
stored)=.DELTA.H.sub.S3+.DELTA.H.sub.L2+.DELTA.H.sub.S4+.DELTA.H.sub.R2
EMBODIMENT 2
[0097] Hereinafter, a thermal storage method as a second preferred
embodiment of the present invention will be described with
reference to the accompanying drawings. This preferred embodiment
is different from the preferred embodiment described above in that
the first composition becomes the second composition by going
through a simple liquid phase in the thermal storage process. More
specifically, in the preferred embodiment described above, a
solid-liquid mixed phase is produced by heating the first
composition to the temperature T2 and the second composition is
obtained by separating water from the first composition in the
solid-liquid mixed phase. Meanwhile, according to this preferred
embodiment, when the first composition is heated to the temperature
T2, an aqueous solution is produced, and the second composition is
obtained by separating water from that aqueous solution.
[0098] FIG. 2 illustrates an example of thermal storage and release
processes according to this preferred embodiment. In FIG. 2, the
abscissa represents the concentration of the composition and the
ordinate represents the temperature, and a phase diagram, showing
what phases are present in an inorganic salt and water, is
shown.
[0099] In this example, Composition E (with an inorganic salt
concentration c1) to be an n-hydrate in solid phase at a
temperature T1 is used as the first composition. The temperature T1
may be equal to room temperature, for example.
[0100] In the thermal storage process, first, the temperature of
the n-hydrate is increased from the temperature T1 to a temperature
T2, which is higher than the phase change temperature Tm, as
indicated by the arrow 20. Then, Composition E will have a simple
liquid phase. Thereafter, with the temperature of Composition E
kept equal to T2, water is removed from the thermal storage
material container. And when it is sensed that a predetermined
amount of water has been removed, the removal of the water is
stopped. In this manner, Composition F (with a concentration c2
(>c1)), which includes the inorganic salt at a higher
concentration than Composition E, is obtained. The composition F
thus obtained is in a solid-liquid mixed phase including both an
m-hydrate, which has a smaller hydration number than the n-hydrate,
and the aqueous solution of the inorganic salt.
[0101] In the heat release process, Composition F at the
temperature T2 is mixed with water to obtain Composition E in
liquid phase as indicated by the arrow 22, and then the temperature
of Composition E is decreased to the temperature T1 that is lower
than the phase change temperature Tm of the n-hydrate, thereby
turning the composition back into an n-hydrate. The amount of the
water to be mixed with Composition F is preferably substantially
equal to that of the water removed from the thermal storage
material container during the thermal storage process. As described
above, when the heat is used, the heat is released in the form of
sensible heat and latent heat, and therefore, the thermal output
rate does not depend on the rate of reaction between the m-hydrate
and the water. That is why compared to the conventional chemical
heat storage method, the thermal output rate can be increased
according to this preferred embodiment.
[0102] In this preferred embodiment, a composition including
calcium chloride and water at a molar ratio of one to six may be
used as Composition E. In that case, in the thermal storage
process, calcium chloride hexahydrate is heated from the
temperature T1 (which may be equal to room temperature) to the
temperature T2 and water is separated from the composition, thereby
obtaining Composition F including calcium chloride dihydrate and an
aqueous solution of calcium chloride. This thermal storage process
is represented by the following reaction formula:
CaCl.sub.2.6H.sub.2O(solid)aCaCl.sub.2/(4.03 to 4.43)H.sub.2O
(aqueous solution)+bCaCl.sub.2.2H.sub.2O(solid)+2H.sub.2O
where the values a and b are changeable with the temperature T2
after the thermal storage.
[0103] As in the preferred embodiments described above, it is also
possible according to this preferred embodiment to store the
sensible heat .DELTA.H.sub.S5 to raise the temperature of the
n-hydrate from the temperature T1 to the phase change temperature
Tm of the n-hydrate, the latent heat of fusion A H.sub.L3 of the
n-hydrate at the temperature Tm, the sensible heat .DELTA.H.sub.S6
to raise the temperature of Composition E in liquid phase from Tm
to T2 and the reaction heat .DELTA.H.sub.R3 generated by increasing
the concentration of the inorganic salt with the separation of
water from Composition E:
.DELTA.H(total quantity of heat
stored)=.DELTA.H.sub.S5+.DELTA.H.sub.L3+.DELTA.H.sub.S6+.DELTA.H.sub.R3
This total quantity of heat stored .DELTA.H is changeable with the
temperature T1 of Composition E before thermal storage, the
temperature T2 of Composition F after the thermal storage, and the
concentrations c1 and c2 of the inorganic salt.
[0104] It should be noted that the thermal storage process does not
always follow the path indicated by the arrow 20. Alternatively,
the thermal storage process may take any other path as long as
Composition F at the temperature T2 can be obtained from
Composition E at the temperature T1. For example, Composition F may
also be obtained by removing water from the thermal storage
material container while at the same time heating Composition E
during the thermal storage process. Likewise, the heat release
process does not always have to follow the path indicated by the
arrow 22 but may take any other path as long as Composition F at
the temperature T2 can be turned back into Composition E.
[0105] The concentration c2 of the inorganic salt in Composition F
and the temperature T2 after the thermal storage may be
appropriately determined such that the second composition is in the
solid-liquid mixed phase. Their preferred ranges are the same as
the ones of the preferred embodiment described above. Also, the
preferred solid volume fraction range of Composition F is also the
same as the one described above.
[0106] In the example illustrated in FIG. 2, Composition E, which
is in solid phase (i.e., n-hydrate) at room temperature, is used as
the first composition. Alternatively, a composition that includes
two types of hydrates at room temperature may also be used. Still
alternatively, Composition H that includes both the n-hydrate and
an aqueous solution at room temperature may also be used. Such a
composition H may be prepared by mixing the n-hydrate with
water.
[0107] In a situation where Composition H is used, the thermal
storage process is performed as indicated by the arrow 24.
Specifically, Composition H is heated from T1 (which may be equal
to room temperature, for example) to a temperature T2, which is
higher than the liquidus temperature Tm' of Composition H, to make
an aqueous solution. Then, water is separated from Composition H,
thereby obtaining Composition F in a solid-liquid mixed phase
including an m-hydrate, which has a smaller number of water
molecules than the n-hydrate, and an aqueous solution. On the other
hand, in the heat release process, Composition F in the
solid-liquid mixed phase is mixed with water and the temperature is
decreased to T1 as indicated by the arrow 26.
[0108] Even when Composition H is used, the total quantity of heat
.DELTA.H that can be stored is also calculated as described above.
That is to say, the total quantity of heat stored .DELTA.H is the
sum of the sensible heat .DELTA.H.sub.S7 to raise the temperature
of Composition H from the temperature T1 to the phase change
temperature Tm of the n-hydrate, the latent heat of fusion
.DELTA.H.sub.L4 of the n-hydrate at the temperature Tm, the
sensible heat .DELTA.H.sub.S8 to raise the temperature of
Composition E in the liquid phase from Tm to T2 and the reaction
heat .DELTA.H.sub.R4 generated by increasing the concentration of
the inorganic salt with the separation of water from Composition
H:
.DELTA.H(total quantity of heat
stored)=.DELTA.H.sub.S7+.DELTA.H.sub.L4+.DELTA.H.sub.S8+.DELTA.H.sub.R4
[0109] As described above, according to the thermal storage methods
of the first and second preferred embodiments of the present
invention, a higher thermal storage density than a latent heat
storage material is achieved particularly in a temperature range in
which a heat pump can operate. In addition, since there is no need
to apply any energy externally during the heat release process, the
quantity of heat stored can be increased substantially. The thermal
storage density varies with the type of an inorganic salt included
in the first composition and with the temperature T1 before the
thermal storage and the temperature T2 after the thermal storage.
In any case, however, the first and second compositions are
preferably controlled so as to double the thermal storage density
of sensible heat storage that uses the sensible heat of water.
[0110] On top of that, not just can such a high thermal storage
density be achieved but also can the thermal output quantity be
increased by making the second composition enter the solid-liquid
mixed phase during the thermal storage. As a result, the thermal
storage system can be smaller than a conventional one. If the
conventional thermal storage system is applied to a CO.sub.2 heat
pump or a hot water supply unit that uses fuel cell cogeneration,
then it is difficult to reduce the size of the reservoir
drastically enough to install it conveniently. However, according
to the thermal storage method of the present invention, a thermal
storage system that uses a thermal storage vessel with a smaller
volume than such a reservoir instead is realized, thus providing a
small hot water supply unit that can be installed conveniently.
[0111] <Experiment of Mixing Second Composition and
Water>
[0112] According to the present invention, by mixing water with the
second composition in the solid-liquid mixed phase during the heat
release process, the solid (i.e., m-hydrate) included in the second
composition and the water can be mixed together more uniformly than
a situation where water is mixed with a composition in solid phase.
The present inventors carried out comparative experiments to
confirm this effect. The method and results of those experiments
will be described below.
[0113] In those experiments, calcium chloride was used as the
inorganic salt. As a specific example of the present invention, a
composition in the solid-liquid mixed phase, including both calcium
chloride dihydrate and an aqueous solution of calcium chloride, was
prepared. On the other hand, a composition in solid phase,
including calcium chloride dihydrate, was prepared as a comparative
example. Then, experiments of mixing those compositions with water
were carried out.
[0114] As a specific example of the present invention, a
solid-liquid mixed composition (with a weight of 75.183 g),
including calcium chloride dihydrate in solid phase and an aqueous
solution of calcium chloride, was prepared by mixing together
45.614 g of calcium chloride anhydride and 29.569 g of water. Next,
a beaker with 14.717 g of water was provided and the composition
representing the specific example of the present invention was
poured into that beaker. As a result, as schematically illustrated
in FIG. 3(a), a solution 31 that appeared homogenous to the naked
eyes was obtained. The conductivity of the solution 31 measured
71.5 mS/cm.
[0115] Next, based on the conductivity of the solution 31 measured,
the mass concentration of calcium chloride in the solution 31 was
calculated. The mass concentration in the solution 31 can be
obtained based on the results of measurements representing the
relation between the mass concentration of calcium chloride and the
conductivity as shown in FIG. 4. As can be seen from FIG. 4, it was
discovered that the conductivity of 71.5 mS/cm of the solution 31
obtained in this experiment corresponded with that of an aqueous
solution with a calcium chloride mass concentration of 48% (i.e.,
the solution 31 had a calcium chloride mass concentration of 48%).
This value is close to 51% that was an actual mass concentration of
calcium chloride in the beaker. Thus, the present inventors
confirmed that almost all of the calcium chloride dihydrate that
had been introduced into the beaker was mixed with the water to
produce a substantially homogenous calcium chloride aqueous
solution.
[0116] Meanwhile, 59.382 g of calcium chloride dihydrate was
prepared as a composition representing a comparative example and
was put into a beaker with 30.015 g of water. As a result, a
solution 35 was obtained as schematically illustrated in FIG. 3(b).
But there was a solid precipitate 33 at the bottom of the beaker.
The conductivity of the solution 35 measured 156.8 mS/cm. Also, the
solid precipitate 35 turned out to be calcium chloride dihydrate.
When the precipitate 35 was taken out and checked, it was
discovered that compared to the powder of calcium chloride
dihydrate in solid phase that had not yet been put into the beaker,
the particles had an increased size and a hardened surface.
[0117] As in the specific example of the present invention
described above, the mass concentration of calcium chloride in the
solution 35 was obtained based on the conductivity of the solution
35 measured for the comparative example by reference to the
relation between the mass concentration of calcium chloride and the
conductivity as shown in FIG. 4. As a result, it was discovered
that the conductivity of 156.8 mS/cm corresponded with that of an
aqueous solution with a calcium chloride mass concentration of 34%
(i.e., the solution 35 had a calcium chloride mass concentration of
34%). This value is much lower than 51% that was an actual mass
concentration of calcium chloride in the beaker. This result was
obtained probably because only the surface of the calcium chloride
dihydrate powder was hydrated and no water could penetrate deep
into the power, thus leaving a lot of calcium chloride dihydrate in
solid phase, which could not be dissolved in the water, and
eventually decreasing the concentration of calcium chloride in the
solution 35.
[0118] Based on the results of these mixture experiments, the
present inventors confirmed that by mixing the second composition
in the solid-liquid mixed phase with water, the percentage of the
hydrate included in the second composition, which had not been
reacted with the water but was left as a precipitate, could be
reduced significantly compared to mixing the same hydrate in solid
phase with water. Thus, we discovered that according to the present
invention, the decrease in thermal output quantity, which would
have been caused by the residue of the hydrate in the second
composition that had not reacted with water, could be
minimized.
[0119] <Screening of Thermal Storage Materials>
[0120] The present inventors selected thermal storage materials
that can be used effectively as the first composition in the
thermal storage method of the present invention. The method of
screening and the results will be described below.
[0121] First, as for various hydrates having melting points in the
range of room temperature to 100.degree. C., the reaction heat to
be generated by separating water from those hydrates was measured.
In making the measurements, each of those hydrates was heated to at
least its phase separation temperature, and water was separated
with that temperature maintained until the concentration of the
solute was doubled. After that, the composition obtained by
separating the water was mixed with water and its reaction heat was
measured.
[0122] The results of measurements are shown in the following Table
1. Among the various hydrates shown in Table 1, some hydrates with
zero or negative reaction heat cannot store heat even by separating
water from them, and therefore, cannot be used as thermal storage
materials according to the present invention. On the other hand,
hydrates with positive reaction heat can store heat by separating
water from them, and can be used as thermal storage materials
according to the present invention. Among these materials, hydrates
of magnesium sulfate, sodium thiosulfate, calcium chloride, calcium
bromide and zinc nitrate, all of which have reaction heat of
relatively great magnitudes and little corrosiveness and do not
require much caution in handling, were selected as particularly
preferred thermal storage materials.
TABLE-US-00001 TABLE 1 Composition of Temperature Reaction heat
.DELTA.H hydrates difference .DELTA.T (.degree. C.) (kJ/kg)
LiCl.cndot.2H.sub.2O 55 110 Na.sub.2CO.sub.3.cndot.10H.sub.2O 7 19
CaBr.sub.2.cndot.6H.sub.2O 60 150 CuCl.sub.2.cndot.2H.sub.2O 50 100
LiBr.cndot.2H.sub.2O 50 100 Zn(NO.sub.3).sub.2.cndot.6H.sub.2O 35
88 Mn(NO.sub.3).sub.2.cndot.4H.sub.2O 50 120
Na.sub.2HPO.sub.4.cndot.12H.sub.2O 1 3 MgHPO.sub.4.cndot.3H.sub.2O
20 40 FeCl.sub.3.cndot.6H.sub.2O 33 86 CaCl.sub.2.cndot.4H.sub.2O
60 144 Fe(NH.sub.4)(SO.sub.4).sub.2.cndot.12H.sub.2O 5 14
Cal.sub.2.cndot.6H.sub.2O 50 130 NaHPHO.sub.3.cndot.2.5H.sub.2O 15
27 K(CH.sub.3COO).cndot.1.5H.sub.2O 20 35
Ca(NO.sub.3).sub.2.cndot.4H.sub.2O 30 72
La(NO.sub.3).sub.3.cndot.6H.sub.2O 25 63
Na.sub.2SiO.sub.3.cndot.9H.sub.2O 12 34
K.sub.3PO.sub.4.cndot.7H.sub.2O 25 68
Zn(NO.sub.3).sub.2.cndot.4H.sub.2O 50 120
Fe(NO.sub.3).sub.3.cndot.9H.sub.2O 32 90 MgSO.sub.4.cndot.7H.sub.2O
40 110 CoBr.sub.2.cndot.6H.sub.2O 50 130
K.sub.2S.sub.2O.sub.3.cndot.H.sub.2O 5 8
Na.sub.2S.sub.2O.sub.3.cndot.5H.sub.2O 35 88
Na.sub.2HPO.sub.4.cndot.7H.sub.2O 0 0 FeBr.sub.2.cndot.6H.sub.2O 40
104 NaBr.cndot.2H.sub.2O 20 40 MnSO.sub.4.cndot.5H.sub.2O 40 100
Zn(NO.sub.3).sub.2.cndot.2H.sub.2O 70 140
Co(NO.sub.3).sub.2.cndot.6H.sub.2O 35 88 NaBO.sub.2.cndot.4H.sub.2O
25 55 Na(CH.sub.3COO).cndot.3H.sub.2O 10 35
MnCl.sub.2.cndot.4H.sub.2O 50 110 NaHSO.sub.4.cndot.H.sub.2O 15 27
FeSO.sub.4.cndot.7H.sub.2O 4 11 Fe(NO.sub.3).sub.3.cndot.6H.sub.2O
45 113 NaAl(SO.sub.4).sub.2.cndot.12H.sub.2O 0 0
NaBO.sub.3.cndot.4H.sub.2O -10 -22
NaH.sub.2PO.sub.4.cndot.2H.sub.2O 10 20
Na.sub.3PO.sub.4.cndot.12H.sub.2O -10 -30
SrCl.sub.2.cndot.6H.sub.2O 25 63
[0123] Next, for each of the inorganic salts included in those
hydrates selected, the relation between the solute concentration
(i.e., the concentration of the inorganic salt) and the thermal
storage density was analyzed. As used herein, the "thermal storage
density" means the quantity of heat stored per unit volume in a
situation where water is separated from a composition including an
inorganic salt and water to such a concentration as to turn it into
solid phase. The quantity of heat stored is represented as the sum
of the sensible and latent heat of the composition including the
inorganic salt and water and the reaction heat generated by the
separation of water. In this case, the quantity of heat stored was
calculated at each solute concentration based on the values
described in the documents as for the sensible and latent heat and
on the measured values as for the reaction heat, respectively.
[0124] FIGS. 5 through 9 are graphs each showing how the thermal
storage density per unit volume changed with the solute
concentration of one of the inorganic salts described above. In
FIGS. 5 through 9, a thermal storage density obtained in a
situation where only the sensible and latent heat of a hydrate of
that inorganic salt were used (which will be referred to herein as
a "comparative density") is indicated by the dashed line parallel
to the axis of abscissas of that graph for the purpose of
comparison. The comparative densities are obtained by heating a
heptahydrate of magnesium sulfate, a pentahydrate of sodium
thiosulfate, a hexahydrate of calcium chloride, a hexahydrate of
calcium bromide, and a hexahydrate of zinc nitrate from 15.degree.
C. to 90.degree. C. That is why if the solute concentration of each
inorganic salt is adjusted so as to achieve a thermal storage
density that is higher than the comparative density indicated by
the dashed line, a higher thermal storage density can be achieved
compared to a thermal storage method that uses only sensible heat
and latent heat.
[0125] Specifically, if the composition including magnesium sulfate
and water as shown in FIG. 5 is used as the first composition, the
concentration of magnesium sulfate is preferably within the range
of 36 mass % to 53 mass %. On the other hand, if the composition
including sodium thiosulfate and water as shown in FIG. 6 is used,
the concentration of sodium thiosulfate is preferably within the
range of 52 mass % to 75 mass %. If the composition including
calcium chloride and water as shown in FIG. 7 is used, the
concentration of calcium chloride is preferably within the range of
47 mass % to 64 mass %. If the composition including calcium
bromide and water as shown in FIG. 8 is used, the concentration of
calcium bromide is preferably within the range of 58 mass % to 74
mass %. And if the composition including zinc nitrate and water as
shown in FIG. 9 is used, the concentration of zinc nitrate is
preferably within the range of 51 mass % to 84 mass %.
[0126] The following Table 2 summarizes the properties of
compositions that achieve the maximum thermal storage densities
using the respective inorganic salts described above. The sensible
heats shown in Table 2 were calculated on the supposition that the
temperature difference was 75.degree. C.
TABLE-US-00002 TABLE 2 Specific Specific Thermal Thermal Thermal
heat heat Sensible Latent Reaction storage storage storage (kJ/kg
(kJ/kg heat heat heat density Specific density material K) solid K)
liquid (kJ/kg) (kJ/kg) (kJ/kg) (kJ/kg) gravity (kJ/L)
MgSO.sub.4.cndot.7H.sub.2O 1.7 2.8 174 155 182 511 1.62 828
CaBr.sub.2.cndot.6H.sub.2O 1.0 1.5 101 120 123 344 1.91 657
CaCl.sub.2.cndot.6H.sub.2O 1.6 2.4 168 192 92 452 1.47 664
Na.sub.2S.sub.2O.sub.3.cndot.5H.sub.2O 1.5 2.4 150 202 79 431 1.64
707 Zn(NO.sub.3).sub.2.cndot.6H.sub.2O 1.4 2.1 144 120 230 494 1.74
860
[0127] Hereinafter, specific examples of a thermal storage method
according to the present invention will be described.
[0128] (Configuration of Thermal Storage System)
[0129] First, a configuration for a thermal storage system for use
in the specific examples of the present invention to be described
below will be described with reference to the accompanying
drawings. FIG. 10 illustrates a general arrangement of the thermal
storage system and FIG. 11 is schematic cross-sectional view
illustrating the configuration of the thermal storage vessel
thereof.
[0130] As shown in FIG. 10, the thermal storage system 100
includes: a thermal storage vessel 51 with a good heat insulation
property; a heating section 52 and an external heat source 53 for
heating a thermal storage material in the thermal storage vessel
51; a heat medium passage 64 and a pump 58 for circulating a heat
medium (such as water) to heat the thermal storage material between
the thermal storage vessel 51 and the heating section 52; a water
removal passage 60 and a vacuum pump 57 for removing the water
vapor produced from the thermal storage vessel 51; first and second
condensers 54 and 55, which are arranged on the water removal
passage 60 to condense the water vapor, produced from the thermal
storage vessel 51, into water in liquid phase; a water supply
channel 62 for supplying water to the thermal storage material in
the thermal storage vessel 51; a valve 59 arranged on the water
supply channel 62; and a heat recovery water passage 65 for passing
heat recovery water through the inside of the thermal storage
vessel 51 and supplying it to a heat-assisted unit such as a hot
water supply unit. The water removal passage 60 is connected to a
water tank 56, which is provided with a sensor (water level sensor)
for detecting the water level in the water tank 56 (not shown).
Also, the thermal storage system 100 further includes a control
section (not shown, either) for stopping the removal of water from
the thermal storage vessel 51 by turning OFF the vacuum pump 57
based on the detection result obtained by the water level
sensor.
[0131] As shown in FIG. 11, the thermal storage vessel 51 is made
up of a plurality of thermal storage material containers 66, each
of which includes a thermal storage material 51a. In each of those
thermal storage material containers 66, the upper space over the
thermal storage material 51a is split by a partition 70 into a
space 68 for reducing the pressure inside the thermal storage
material container and a space 69 for supplying water. The vacuum
pump 57 described above is spatially connected to the respective
spaces 68 of these thermal storage material containers 66 by way of
the water removal passage 60. On the other hand, the water supply
channel 62 is connected to the spaces 69. Thus, the water that has
been supplied into the spaces 69 through the water supply channel
62 is poured onto the thermal storage material 51a through a water
passage port 72 that has been cut through the partition 70.
Furthermore, the thermal storage vessel 51 is provided with heat
exchange tubes 80 that directly contact with either the surface of
the thermal storage material containers 66 or the thermal storage
material itself. The heat exchange tubes 80 are also connected to
the heat medium passage 64 for passing a heat medium (i.e., water
in this example) to heat the thermal storage material 51a and to
the heat recovery water passage 65 for running water that recovers
heat from the thermal storage material 51a when the heat is used.
The water that has been supplied from the heating section 52 into
this thermal storage vessel 51 by way of the heat medium passage 64
exchanges heat with the thermal storage material 51a while running
down these heat exchange tubes 80 and then goes back to the heating
section 52. Meanwhile, the heat recovery water (such as hot water
supplied) is supplied into the thermal storage vessel 51 through
the heat recovery water passage 65, exchanges heat with the thermal
storage material 51a while running up the heat exchange tubes 80
and then is used by a heat-assisted unit (such as a hot water
supply unit).
[0132] In the specific examples of the present invention to be
described below, a CO.sub.2 heat pump, for example, was used as the
external heat source 53 and the heating section 52 used a CO.sub.2
heat medium (at a temperature of 110.degree. C.) supplied from the
external heat source 53. The CO.sub.2 heat medium supplied from the
external heat source 53 moves to the heating section 52, where the
CO.sub.2 heat medium runs against the flow of the heat medium
(water) that heats the thermal storage material. In this manner,
heat exchange is made between the CO.sub.2 heat medium and the
water.
[0133] During the thermal storage process, the heat medium (water)
to heat the thermal storage material is heated by the CO.sub.2 heat
medium in the heating section 52 to as high as 90.degree. C., runs
through the heat medium passage 64 from the heating section 52, and
then flows into the heat exchange tubes 80 of the thermal storage
vessel 51, where the heat medium exchanges heat with the thermal
storage material 51a, thereby heating the thermal storage material
51a. After having exchanged heat with the thermal storage material
51a, the water goes back through the heat medium passage 64 toward
the heating section 12 again, where the water exchanges heat with
the CO.sub.2 heat medium.
[0134] The water vapor produced from the thermal storage material
51a by vaporization is directed through the water removal passage
60 toward the first condenser 54, where the water vapor exchanges
heat with, and is condensed by, the heat medium (water) that has
passed through the heat exchange tubes 80 of the thermal storage
vessel 51 because they are mutually opposite flows. Thereafter, the
water vapor is forwarded to the second condenser 55, where the
water vapor exchanges heat with, and is further condensed by, the
CO.sub.2 heat medium of the CO.sub.2 heat pump because they are
mutually opposite flows. As a result, the water vapor turns into
water in liquid phase. And that water is reserved in the water tank
56.
[0135] On the other hand, when the heat needs to be used, the valve
59 is opened and water is supplied through the water supply channel
62 onto the thermal storage material 51a, thereby causing the
thermal storage material 51a to generate heat. Meanwhile, heat
recovery water is also supplied through the heat recovery water
passage 65 into the heat exchange tubes 80, thereby recovering part
of the heat released from the thermal storage material 51a.
[0136] Optionally, to reduce the volume of the water tank 56, the
water tank 56 may be designed so as to drain water once the amount
of water reserved there reaches a predetermined level. In that
case, temperature sensors for detecting the temperatures of the
thermal storage vessel 51 and the first and second condensers 54
and 55 may be provided instead of the water level sensor. Those
temperatures may be detected at intervals of several minutes, for
example, and the removal of water from the thermal storage material
containers 66 may be stopped based on the results of the detection.
Specifically, a relative humidity is calculated based on the
respective temperatures of the thermal storage vessel 51 and first
and second condensers 54 and 55, a water vaporization rate is
calculated based on the relative humidity, and then vaporization
rates are integrated with respect to time. In this manner, the
total amount of water removed from the thermal storage material
containers 66 can be calculated and the solute concentration of the
thermal storage material 51a can be estimated. And when the
estimated solute concentration becomes equal to or greater than a
predetermined value, the operation of the vacuum pump 57 is
stopped, thereby stopping the removal of water.
EXAMPLE 1
[0137] Hereinafter, specific examples of a thermal storage method
according to the present invention will be described with reference
to the accompanying drawings.
[0138] In a first specific example of the present invention, using
a hydrate including magnesium sulfate and water at a molar ratio of
one to seven, thermal storage and heat release processes are
carried out with the thermal storage system that has already been
described with reference to FIGS. 10 and 11. FIG. 12 is a phase
diagram of magnesium sulfate and water, where the thermal storage
process of this specific example is indicated by the arrow 90.
[0139] First, magnesium sulfate heptahydrate is put as a first
composition 91 into the thermal storage vessel 51. The temperature
of the magnesium sulfate heptahydrate is supposed to be room
temperature (of approximately 15.degree. C.).
[0140] Next, a thermal storage process is carried out by obtaining
a second composition 92 in solid-liquid mixed phase at
approximately 80.degree. C. from the magnesium sulfate heptahydrate
91 at room temperature as indicated by the arrow 90. The second
composition 92 is in solid-liquid mixed phase including both
magnesium sulfate monohydrate and an aqueous solution of magnesium
sulfate.
[0141] Hereinafter, the thermal storage process will be described
step by step. First, by running the pump 58, water is circulated as
a heat medium to heat the thermal storage material between the
thermal storage vessel 51 and the heating section 52. Next, by
operating the CO.sub.2 heat pump (external power supply) 53, the
heating section 52 is made to heat the water, thereby supplying hot
water at 90.degree. C. thus obtained to the heat exchange tubes 80
of the thermal storage vessel 51 through the heat medium passage
64. The heat exchange tubes 80 make a heat exchange between the
magnesium sulfate heptahydrate 91 and the hot water, thereby
raising the temperature of the magnesium sulfate heptahydrate 91
from room temperature (of 15.degree. C.). In this process, the
thermal storage material 51a contained in the thermal storage
material containers 66 comes to have a higher temperature in its
upper portion (i.e., surface portion) than in its lower portion
(i.e., bottom portion). Thus, the temperature at the lower portion
of the thermal storage material 51a is always monitored. And on
sensing the temperature exceed 48.degree. C., which is the phase
change temperature of magnesium sulfate heptahydrate, the vacuum
pump 57 is operated, thereby evacuating the inner space 68 of the
thermal storage material containers 66.
[0142] The first composition 91 in the thermal storage material
containers 66 has changed from magnesium sulfate heptahydrate in
solid phase into a solid-liquid mixed phase at a temperature of
48.degree. C. or more. That is why by evacuating the inner space 68
of the thermal storage material containers 66, the water contained
in the first composition 91 in solid-liquid mixed phase is
vaporized by vacuum evaporation. At the surface portion of the
thermal storage material 51a, the temperature is close to
90.degree. C., and therefore, the water is vaporized while boiling
at approximately 0.6 atmospheric pressure.
[0143] The water vaporized by this evacuation (i.e., water vapor)
passes through the water removal passage 60 and then enters the
first and second condensers 54 and 55 in this order. Specifically,
in the first condenser 54, heat exchange is made between the water
vapor and the water at approximately 40.degree. C., which has
passed through the heat exchange tubes 80 of the thermal storage
vessel 51 along the heat medium passage 64, as two opposite flows,
thereby condensing a portion of the water vapor. In the second
condenser 55, heat exchange is made between the remaining water
vapor and CO.sub.2 heat medium at approximately -10.degree. C. that
has come from the external heat source (i.e., CO.sub.2 heat pump)
53, thereby cooling the water vapor and turning it into water. The
water that has been condensed by these condensers 54 and 55 is once
stored in the water tank 56. In this manner, the water vapor is
condensed, the heat of condensation of the water vapor generated by
the first condenser 54 is recovered by the heat medium (i.e.,
water) running from the thermal storage vessel 51 into the heating
section 52, and the heat of condensation of the water vapor
generated by the second condenser 55 is recovered by the CO.sub.2
heat medium of the CO.sub.2 heat pump 53.
[0144] This series of operations is stopped on sensing that the
water in the water tank 56 has reached, or exceeded, a
predetermined level. Specifically, after the operation of the
vacuum pump 57 has been stopped, the piping (i.e., the water
removal passage 60) of the vacuum pump system is opened to the air,
thereby raising the pressure in the thermal storage material
containers 66 to the atmospheric pressure. In this manner, a second
composition 91 in solid-liquid mixed phase can be obtained. Since
the thermal storage vessel 51 is a heat insulating container, the
second composition 92 will have its temperature maintained until
its heat is used.
[0145] In this specific example, the second composition 92, from
which water is no longer removed, is supposed to consist of 83
parts by weight of magnesium sulfate and 17 parts by weight of
water. This is a solid-liquid mixed phase including both magnesium
sulfate monohydrate in solid phase and an aqueous solution
including 42 parts by weight of magnesium sulfate and 58 parts by
weight of water. Also, in the second composition 92 thus obtained,
the volume fraction of solid is approximately 78 vol %.
[0146] When the heat is used, the second composition 92 is turned
back into the first composition 91 by supplying reaction water
(e.g., 0.4 kg of water for 1 kg of thermal storage material) at
room temperature through the water supply channel 62. In this case,
magnesium sulfate included in the second composition 92 and the
water produce a hydration reaction, thereby generating heat. At the
same time, water to be supplied as hot water is run from the heat
recovery water channel 65 and passed through the heat exchange
tubes 80 of the thermal storage vessel 51 so as to have its
temperature raised to 40.degree. C. or more. Then the water with
the increased temperature is supplied to a heat assisted unit such
as a hot water supply unit.
[0147] According to the thermal storage method of this specific
example, the thermal storage density is 653 kJ/L, which is
approximately 2.1 times as high as 313 kJ/L that is the thermal
storage density achieved by storing heat in water with the upper
portion of the reservoir heated to 90.degree. C. but with the lower
portion thereof maintained at 15.degree. C. Optionally, the water
that has been removed from the thermal storage vessel 51 and then
condensed may be stored at a high temperature by using a heat
insulating container as the water tank 56. In that case, if the hot
water in the water tank 56 is supplied as reaction water to the
second composition 92 when the heat is used, then the thermal
storage density described above can be further increased by 126
kJ/L corresponding to the sensible heat. Also, in that case, the
water in the water tank 56 may be further heated by the CO.sub.2
heat pump and then supplied to the second composition 92.
[0148] The mass concentration of magnesium sulfate in the first
composition 91 is not particularly limited. In this specific
example, a composition that becomes solid (i.e., heptahydrate in
this case) at room temperature is used. Alternatively, a
composition that includes magnesium sulfate and water at a molar
ratio of one to six may also be used. Still alternatively, a
composition that assumes solid-liquid mixed phase, including a
solid (such as a hexahydrate or a heptahydrate) and an aqueous
solution of magnesium sulfate, at room temperature may also be
used.
[0149] Likewise, the mass concentration of magnesium sulfate in the
second composition 92 is not limited to the concentration of 83%
shown in FIG. 12, either, but may be appropriately determined such
that the second composition 92 assumes the solid-liquid mixed
phase. Suppose the first composition 91 is a composition including
magnesium sulfate and water at a molar ratio of one to seven and
the second composition 92 that has stored heat has a temperature of
80.degree. C., for example. In that case, the mass concentration of
magnesium sulfate in the second composition 92 has only to surpass
the range 93 shown in FIG. 12 (i.e., higher than the concentration
of the first composition 91) and to be lower than the concentration
at which it changes into solid phase.
[0150] The temperature of the second composition 92 that has stored
heat is not limited to 80.degree. C., either, but may be
appropriately determined within the range that is higher than the
phase change temperature but equal to or lower than 100.degree. C.
Preferably, it is equal to or higher than the phase change
temperature of its hexahydrate but equal to or lower than
90.degree. C. The reason is as follows. If the temperature of the
second composition is equal to or higher than the phase change
temperature of its hexahydrate, then the second composition 92 can
have its solute concentration increased and the difference in
solute concentration between the first and second compositions 91
and 92 can be widened. As a result, a greater quantity of heat can
be stored. On the other hand, if the temperature of the second
composition is equal to or lower than 90.degree. C., the reaction
heat generated by the hydration reaction can be stored as sensible
heat as described above.
[0151] If the temperature of the second composition 92 that has
stored heat is equal to or higher than the phase change temperature
of its hexahydrate but equal to or lower than 90.degree. C., the
mass concentration of magnesium sulfate in the second composition
92 is preferably 63% to 83%, more preferably 63% to 79%. The reason
is that if it is equal to or higher than 63%, the quantity of heat
stored due to the difference in solute concentration can be
increased. On the other hand, if it is equal to or lower than 79%,
then the volume fraction of solid included in the second
composition 92 that has stored heat can be decreased. As a result,
the second composition 92 and water can be mixed together more
uniformly in the heat release process.
EXAMPLE 2
[0152] A second specific example of the present invention is
different from the first specific example described above in that a
hydrate, including calcium chloride and water at a molar ratio of
one to six, is used in this example.
[0153] Hereinafter, a second specific example of a thermal storage
method according to the present invention will be described with
reference to the accompanying drawings. In the second specific
example of the present invention, thermal storage and heat release
processes are also carried out with the thermal storage system that
has already been described with reference to FIGS. 10 and 11. FIG.
13 is a phase diagram of calcium chloride and water, where the
thermal storage process of this specific example is indicated by
the arrow 94.
[0154] First, calcium chloride hexahydrate is put as a first
composition 95 into the thermal storage vessel 51. The temperature
of the calcium chloride hexahydrate is supposed to be room
temperature (of approximately 15.degree. C.).
[0155] In a thermal storage process, a second composition 96 in
solid-liquid mixed phase at approximately 80.degree. C. is obtained
from the calcium chloride hexahydrate at room temperature as
indicated by the arrow 94. The second composition 96 is in
solid-liquid mixed phase including both calcium chloride dihydrate
and an aqueous solution of calcium chloride.
[0156] Hereinafter, the thermal storage process will be described
step by step. First, the calcium chloride hexahydrate is heated by
the same method as the one already described for the first specific
example. On sensing that the temperature at the lower portion of
the thermal storage material containers 66 has exceeded 30.degree.
C., which is the phase change temperature of the calcium chloride
hexahydrate, the inner space 68 of the thermal storage material
containers 66 starts to be evacuated by the same method as the one
already described for the first specific example. The water vapor
that has been produced by this evacuation is condensed by the first
and second condensers 54 and 55 and then once stored in the water
tank 56 as in the first specific example described above.
[0157] This series of operations is stopped on sensing that the
water in the water tank 56 has reached, or exceeded, a
predetermined level. In this specific example, the second
composition 96, from which water is no longer removed, is supposed
to consist of 61 parts by weight of calcium chloride and 39 parts
by weight of water. This is a solid-liquid mixed phase including
both calcium chloride dihydrate in solid phase and an aqueous
solution including 60 parts by weight of calcium chloride and 40
parts by weight of water. On the other hand, the second composition
92 includes very little solid and its volume fraction is almost 0
vol %.
[0158] When the heat is used, the second composition 96 is turned
back into the first composition 95 by supplying reaction water
(e.g., 0.16 kg of water for 1 kg of thermal storage material) at
room temperature through the water supply channel 62. In this case,
calcium chloride included in the second composition 96 and the
water produce a hydration reaction, thereby generating heat. At the
same time, water to be supplied as hot water is run from the heat
recovery water channel 65 and passed through the heat exchange
tubes 80 of the thermal storage vessel 51 so as to have its
temperature raised to 40.degree. C. or more by making a heat
exchange between the water to be supplied as hot water and the
thermal storage material. Then the water to be supplied as hot
water is supplied to a hot water supply unit.
[0159] According to the thermal storage method of this specific
example, the thermal storage density is 617 kJ/L, which is
approximately twice as high as 313 kJ/L that is the thermal storage
density achieved by storing heat in water with the upper portion of
the reservoir heated to 90.degree. C. but with the lower portion
thereof maintained at 15.degree. C. Optionally, the water that has
been removed from the thermal storage vessel 51 and then condensed
may be stored at a high temperature by using a heat insulating
container as the water tank 56. In that case, if the hot water in
the water tank 56 is supplied as reaction water to the thermal
storage material 51a when the heat is used, then the thermal
storage density described above can be further increased by 74 kJ/L
corresponding to the sensible heat. Also, in that case, the water
in the water tank 56 may be further heated by the CO.sub.2 heat
pump and then supplied to the second composition 96.
[0160] The mass concentration of calcium chloride in the first
composition 95 is not particularly limited. In this specific
example, a composition that becomes calcium chloride hexahydrate at
room temperature is used. Alternatively, a composition that
includes calcium chloride, which becomes calcium chloride
tetrahydrate at room temperature, and water at a molar ratio of one
to four may also be used. Still alternatively, a composition that
assumes solid-liquid mixed phase, including a solid (i.e., a
hexahydrate) and an aqueous solution of calcium chloride, at room
temperature may also be used.
[0161] Likewise, the mass concentration of calcium chloride in the
second composition 96 is not limited to the concentration of 61%
shown in FIG. 13, either, but may be appropriately determined such
that the second composition 96 assumes the solid-liquid mixed
phase. Suppose the second composition 96 that has stored heat has a
temperature of 80.degree. C., for example. In that case, the mass
concentration of calcium chloride in the second composition 96 has
only to surpass the range 97 shown in FIG. 13 (i.e., higher than
the concentration on the liquidus line at the temperature after the
thermal storage) and to be lower than the concentration at which it
changes into solid phase.
[0162] The temperature of the second composition 96 that has stored
heat is not limited to 80.degree. C., either, but may be
appropriately determined within the range that is higher than the
phase change temperature but equal to or lower than 100.degree. C.
Preferably, it is equal to or higher than the phase change
temperature of its tetrahydrate but equal to or lower than
90.degree. C. The reason is as follows. If the temperature of the
second composition is equal to or higher than the phase change
temperature of its tetrahydrate, then the second composition 96 can
have its solute concentration increased and the difference in
solute concentration between the first and second compositions 95
and 96 can be widened. As a result, a greater quantity of heat can
be stored. On the other hand, if the temperature of the second
composition is equal to or lower than 90.degree. C., the reaction
heat generated by the hydration reaction can be stored as sensible
heat as described above.
[0163] If the temperature of the second composition 96 that has
stored heat is equal to or higher than the phase change temperature
of its tetrahydrate but equal to or lower than 90.degree. C., the
mass concentration of calcium chloride in the second composition 96
is preferably equal to or higher than the concentration on the
liquidus line but equal to or lower than 74%. The reason is that if
it is equal to or lower than 74%, then the volume fraction of solid
included in the second composition 96 that has stored heat can be
decreased. As a result, the second composition 96 and water can be
mixed together more uniformly in the heat release process.
EXAMPLE 3
[0164] In a third specific example of the present invention, a
composition, including magnesium sulfate and water at a molar ratio
of one to eight, is used as the first composition. This specific
example is different from the specific examples described above in
that this composition is not in single solid phase but in a
solid-liquid mixed phase, including magnesium sulfate heptahydrate
and an aqueous solution of magnesium sulfate, at room
temperature.
[0165] Hereinafter, a third specific example of a thermal storage
method according to the present invention will be described with
reference to the accompanying drawings. FIG. 14 is a phase diagram
of magnesium sulfate and water, where the thermal storage process
of this specific example is indicated by the arrow 104.
[0166] First, a first composition 105 (at a temperature of
approximately 15.degree. C., for example) in solid-liquid mixed
phase, including magnesium sulfate and water at a molar ratio of
one to eight, is heated to a temperature higher than 48.degree. C.,
which is the phase change temperature of magnesium sulfate
heptahydrate, by the same method as the one already described for
the first specific example. In this case, the latent heat of fusion
of the first composition 105 becomes smaller than that of magnesium
sulfate heptahydrate itself, but the sensible heat increases
because it includes a lot of water. Subsequently, by removing water
from the first composition 105, a second composition 106 at a
temperature of 80.degree. C., including 83 parts by weight of
magnesium sulfate and 17 parts by weight of water, is obtained as
in the first specific example described above. Also, as in the
first specific example, the latent heat of vaporization, generated
by the evaporation, is recovered as heat of condensation in the
heat medium (i.e., water) and in the CO.sub.2 heat medium of the
CO.sub.2 heat pump (external heat source) to heat the thermal
storage material.
[0167] In the heat release process, when the heat needs to be used,
the second composition 106 thus obtained is mixed with an amount of
water corresponding to that of the removed water (i.e., 0.44 kg of
water for 1 kg of thermal storage material), thereby turning the
second composition 106 back into the first composition 105. In this
manner, a quantity of heat corresponding to the reaction heat that
was generated during the thermal storage process is released.
Furthermore, the temperature of the first composition 105 thus
obtained is decreased to the phase change temperature of the
magnesium sulfate heptahydrate or less, thereby releasing a
quantity of heat corresponding to the latent heat of fusion and the
sensible heat. The released heat is recovered in the water to be
supplied as hot water by the same method as the one already
described for the first specific example.
[0168] According to the thermal storage method of this specific
example, the thermal storage density is 586 kJ/L if water at room
temperature is used as the reaction water to be mixed with the
second composition 106 during the heat release process but 726 kJ/L
if the hot water stored in the water tank is used as the reaction
water.
EXAMPLE 4
[0169] In a fourth specific example of the present invention, a
composition, including zinc nitrate and water at a molar ratio of
one to six, is used as the first composition. This composition is a
solid (i.e., zinc nitrate hexahydrate) at room temperature.
[0170] Hereinafter, a fourth specific example of a thermal storage
method according to the present invention will be described with
reference to the accompanying drawings. FIG. 15 is a phase diagram
of zinc nitrate and water, where the thermal storage process of
this specific example is indicated by the arrow 112.
[0171] First, zinc nitrate hexahydrate is heated to a temperature
higher than 36.degree. C., which is the phase change temperature of
zinc nitrate hexahydrate, thereby obtaining a first composition 113
as an aqueous solution. Subsequently, by removing water from the
first composition 113 while heating the first composition 113 in
the state of aqueous solution, a second composition 114 in
solid-liquid mixed phase is obtained. In the heat release process,
the second composition 114 thus obtained is mixed with an amount of
water corresponding to that of the water removed during the thermal
storage process, thereby turning the second composition 114 back
into the first composition 113 in the aqueous solution state. In
this manner, a quantity of heat corresponding to the reaction heat
that was generated during the thermal storage process is released.
Furthermore, the temperature of the first composition 113 in the
aqueous solution state is decreased to the phase change temperature
of the zinc nitrate hexahydrate or less, thereby releasing a
quantity of heat corresponding to the latent heat of fusion and the
sensible heat. In this specific example, as the zinc nitrate
hexahydrate has a phase change temperature of 36.degree. C., the
heat is used as having a temperature lower than 36.degree. C.
[0172] Alternatively, the first composition 113 described above may
be replaced with a composition that includes zinc nitrate and water
at a molar ratio of one to four and that becomes zinc nitrate
tetrahydrate (solid) at room temperature. Still alternatively, the
first composition 113 described above may also be replaced with a
composition 116 that includes zinc nitrate and water at a molar
ratio of one to seven and that assumes a solid-liquid mixed phase
at room temperature. Even when the composition 116 is used, the
composition 116 is also heated to a temperature higher than a
liquidus temperature of 33.degree. C., which is the phase change
temperature of the composition 116, in the thermal storage process.
Thereafter, water is removed from the composition 116, thereby
obtaining a second composition 114 in solid-liquid mixed phase. In
the heat release process, on the other hand, the second composition
114 is mixed with an amount of water corresponding to that of the
water removed during the thermal storage process, thereby obtaining
a first composition 116 as an aqueous solution. After that, the
temperature of the first composition 116 is decreased to 33.degree.
C. or less. In this specific example, as the thermal storage
material has a phase change temperature of 33.degree. C., the heat
is used as having a temperature lower than 33.degree. C.
INDUSTRIAL APPLICABILITY
[0173] According to the present invention, by adopting a chemical
heat storage process based on a difference in solute concentration
in combination with the conventional latent heat process, a thermal
storage method that ensures a high thermal storage density can be
provided. In the heat release process, there is no need to newly
apply any thermal energy externally to promote that process. On top
of that, the thermal output quantity and the thermal output rate
can be both increased compared to conventional ones.
[0174] The present invention is applicable particularly effectively
to a thermal storage process that uses a heat source operating in a
low temperature range (e.g., 110.degree. C. or less, among other
things, in the range of 40.degree. C. to 90.degree. C.). It is
particularly beneficial to apply the present invention to various
energy-saving units such as a CO.sub.2 heat pump and a fuel cell
cogenerator because the thermal storage vessel of those units can
have a reduced size in that case.
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