U.S. patent application number 12/402440 was filed with the patent office on 2010-09-16 for thermal storage system.
This patent application is currently assigned to TERRAFORE, INC.. Invention is credited to Rajan KASETTY, Anoop K. MATHUR.
Application Number | 20100230075 12/402440 |
Document ID | / |
Family ID | 42729745 |
Filed Date | 2010-09-16 |
United States Patent
Application |
20100230075 |
Kind Code |
A1 |
MATHUR; Anoop K. ; et
al. |
September 16, 2010 |
Thermal Storage System
Abstract
An apparatus for storing and retrieving thermal energy from a
phase change material including a plurality of heat exchangers.
Each one of the plurality of heat exchangers provides the means for
transferring energy between the phase change material on a primary
side of the heat exchanger and a fluid on a secondary side of the
heat exchanger. The phase change material is a mixture of two or
more inorganic salts.
Inventors: |
MATHUR; Anoop K.;
(Shoreview, MN) ; KASETTY; Rajan; (Riverside,
CA) |
Correspondence
Address: |
SHRAVI, LLC
P.O. BOX 44212
EDEN PRAIRIE
MN
55344-1212
US
|
Assignee: |
TERRAFORE, INC.
Riverside
CA
|
Family ID: |
42729745 |
Appl. No.: |
12/402440 |
Filed: |
March 11, 2009 |
Current U.S.
Class: |
165/104.21 ;
165/158 |
Current CPC
Class: |
Y02E 60/145 20130101;
Y02E 60/14 20130101; F28D 20/021 20130101 |
Class at
Publication: |
165/104.21 ;
165/158 |
International
Class: |
F28D 15/00 20060101
F28D015/00; F28F 9/00 20060101 F28F009/00 |
Claims
1. An apparatus for storing and retrieving thermal energy, the
apparatus comprising: a tank containing a phase change material; a
first heat exchanger fluidly connected in series with a second heat
exchanger, wherein the phase change material from the tank enters a
primary side of the second heat exchanger; the phase change
material flows through and exits the second heat exchanger and
enters a primary side of the first heat exchanger; the phase change
material flows through and exits the first heat exchanger and
enters the tank; a first fluid enters and flows through a secondary
side of the first heat exchanger wherein energy is transferred
between the phase change material and the first fluid; and the
first fluid exits the first heat exchanger and enters and flows
through a secondary side of the second heat exchanger wherein
energy is transferred between the phase change material and the
first fluid; and a third heat exchanger fluidly connected in series
with a fourth heat exchanger, wherein the fourth heat exchanger is
embedded within the phase change material within the tank; a second
fluid from an energy source enters and flows through a secondary
side of the third heat exchanger; the second fluid exits the third
heat exchanger and enters and flows through a secondary side of the
fourth heat exchanger wherein energy is transferred between the
second fluid and the phase change material surrounding the
secondary side of the fourth heat exchanger; the second fluid exits
the fourth heat exchanger and returns to the energy source; the
phase change material from the tank enters and flows through a
primary side of the third heat exchanger wherein energy is
transferred between the phase change material and the second fluid;
and the phase change material exits the third heat exchanger and
enters the tank.
2. The apparatus of claim 1, wherein the phase change material
comprises a composition in a thermodynamic equilibrium state of
liquid and solid.
3. The apparatus of claim 1, wherein each of the first, the second
and the third heat exchangers comprise shell and tube heat
exchangers; the primary side of each of the first, the second and
the third heat exchangers comprises a shell; and the secondary side
of each of the first, the second and the third heat exchangers
comprises a plurality of tubes.
4. The apparatus of claim 1, wherein the first fluid entering the
first heat exchanger includes feed water; the first fluid exiting
the first heat exchanger and entering the second heat exchanger
comprises saturated steam; and the first fluid exiting the second
heat exchanger comprises superheated steam.
5. The apparatus of claim 4, wherein the superheated steam is used
for generating electricity.
6. The apparatus of claim 1, wherein the secondary side of the
fourth heat exchanger comprises a plurality of tubes embedded
within the phase change material; and the second fluid flows
through the plurality of tubes.
7. The apparatus of claim 6, wherein the phase change material
comprises a mixture of at least two or more inorganic salts.
8. The apparatus of claim 7, wherein the mixture comprises 83.2
percent by weight sodium nitrate and 16.8 percent by weight sodium
hydroxide.
9. The apparatus of claim 1, wherein the phase change material
comprises a dilute eutectic composition.
10. The apparatus of claim 9, wherein the dilute eutectic
composition comprises a mixture of at least two or more inorganic
salts.
11. The apparatus of claim 10, wherein the dilute eutectic
composition comprises a dilute eutectic composition in sodium
nitrate including sodium nitrate in the range of 98 to 99 percent
by weight and sodium hydroxide in the corresponding range of 2 to 1
percent by weight.
12. The apparatus of claim 10, wherein the inside surface of the
shell for each one of the first, the second and the third heat
exchanger comprises a coating of a salt phobic compound; and the
outside surfaces of the plurality of tubes in each one of the
first, the second, the third and the fourth heat exchanger comprise
a coating of the salt phobic compound.
13. The apparatus of claim 1, wherein the phase change material
within the tank is thermally stratified.
14. The apparatus of claim 13, wherein the phase change material
entering the second heat exchanger comprises the phase change
material extracted from near a top of the tank; and the phase
change material exiting the first heat exchanger enters the tank
near a bottom of the tank.
15. The apparatus of claim 14, wherein the phase change material
entering the second heat exchanger comprises a liquid state of the
phase change material; and the phase change material entering the
tank near the bottom of the tank comprises thermodynamic
equilibrium state of liquid and solid.
16. The apparatus of claim 13, wherein the phase change material
entering the third heat exchanger comprises the phase change
material extracted from near the bottom of the tank; and the phase
change material exiting the third heat exchanger enters the tank
near the top of the tank.
17. The apparatus of claim 16, wherein the phase change material
entering the third heat exchanger comprises thermodynamic
equilibrium state of liquid and solid; and the phase change
material exiting the third heat exchanger comprises the phase
change material in the liquid state.
18. The apparatus of claim 13, wherein the phase change material
entering the third heat exchanger comprises the phase change
material in the liquid state; and the phase change material exiting
the third heat exchanger comprises the phase change material in a
saturated state.
19. The apparatus of claim 18, wherein the saturated state is a
thermodynamic equilibrium state of liquid and vapor.
20. The apparatus of claim 18, wherein the saturated state is
vapor.
21. The apparatus of claim 13, wherein the phase change material
entering the third heat exchanger comprises the phase change
material in the liquid state; and the phase change material exiting
the third heat exchanger comprises the phase change material in a
superheated vapor state.
22. The apparatus of claim 13, wherein the fourth heat exchanger is
positioned near the bottom of the tank.
23. The apparatus of claim 22, wherein the phase change material
near the bottom of the tank comprises thermodynamic equilibrium
state of liquid and solid.
24. The apparatus of claim 1, wherein the energy source includes a
heating source.
25. The apparatus of claim 1, wherein the energy source includes a
cooling source.
26. A method for storing and retrieving thermal energy from a phase
change material within a tank, the method comprising the steps of:
transferring energy from the phase change material to a first
fluid; and transferring energy from a second fluid to the phase
change material.
27. The method of claim 26, further comprising the steps of:
flowing the first fluid through a secondary side of a first heat
exchanger; flowing the first fluid exiting the first heat exchanger
through a secondary side of the second heat exchanger; extracting
the phase change material from the tank; flowing the phase change
material through a primary side of the second exchanger;
transferring energy from the phase change material flowing through
the second heat exchanger to the first fluid flowing through the
second heat exchanger; flowing the phase change material exiting
the second heat exchanger through a primary side of the first heat
exchanger; transferring energy from the phase change material
flowing through the first heat exchanger to the first fluid flowing
through the first heat exchanger; and returning the phase change
material exiting the first heat exchanger to the tank.
28. The method of claim 27, further comprising the steps of:
extracting the phase change material from the tank; flowing the
phase change material through a primary side of a third heat
exchanger; returning the phase change material from the third heat
exchanger to the tank; extracting the second fluid from an energy
source and flowing the second fluid through a secondary side of the
third heat exchanger; transferring energy from the second fluid
flowing through the third heat exchanger to the phase change
material flowing through the third heat exchanger; flowing the
second fluid exiting the third heat exchanger through a secondary
side of a fourth heat exchanger, wherein the secondary side of the
fourth heat exchanger is embedded within the phase change material
in the tank; transferring energy from the second fluid flowing
through the fourth heat exchanger to the phase change material
surrounding the secondary side of the fourth heat exchanger; and
returning the second fluid exiting the fourth heat exchanger to the
energy source.
29. The method of claim 28, further comprising the step of
maintaining the phase change material in a thermodynamic
equilibrium state of liquid and solid.
30. The method of claim 29, further comprising the step of
maintaining the phase change material in a thermally stratified
state.
31. The method of claim 30, wherein the step of flowing the phase
change material through the second heat exchanger further comprises
the step of extracting the phase change material from near a top of
the tank; and the step of returning the phase change material
exiting the first heat exchanger to the tank further comprises the
step of returning the phase change material near a bottom of the
tank.
32. The method of claim 31, wherein the step of flowing the phase
change material through the third heat exchanger further comprises
the step of extracting the phase change material from near the
bottom of the tank; and the step of returning the phase change
material exiting the third heat exchanger to the tank further
comprises the step of returning the phase change material near the
top of the tank.
Description
TECHNICAL FIELD
[0001] The instant invention relates to a thermal storage system.
More specifically, the disclosure pertains to energy storage and
retrieval from a phase change material.
BACKGROUND
[0002] Various types of thermal energy storage systems are well
known in the art. In some systems, energy is stored and/or
retrieved as sensible heat in either a solid or a liquid, or in a
phase change material such as salt, or in a thermally stratified
composition of a solid and a heat transfer fluid.
[0003] The storage and retrieval of sensible heat is typically
accomplished by inducing a temperature difference between a hot and
a cold source. As such, sensible energy storage systems are prone
to large volume requirements of the liquid or solid storage media
and are therefore expensive.
[0004] The transfer of energy to and from a phase change material
is typically at a constant temperature and in the form of the
latent heat of fusion of the material. Because the latent heat of
fusion of a material is typically greater than the specific heat
capacity of the same material used in a sensible storage system,
the amount of the phase change material required for storing an
equivalent amount of energy is typically less than the same
material required for storing sensible energy. Accordingly, a phase
change storage system is less costly than an equivalent sensible
energy storage system and, because of their reduced size, the
stand-by loses from a phase change storage system will also be less
than those for a sensible heat storage system. However, retrieving
the energy stored in a phase change material can be problematic in
that the phase change material solidifies onto the heat exchanger
surfaces when the heat of fusion is extracted from the liquid
state. This build-up of the solid acts as an insulating layer,
thereby reducing the heat transfer between the phase change
material and the energy transport fluid.
[0005] Accordingly, there exists a need for an efficient and cost
effective energy storage system comprising a phase change
material.
SUMMARY
[0006] In accordance with an embodiment of the invention, a thermal
energy storage system comprises a storage tank housing a phase
change material. Thermal energy is stored or retrieved from the
phase change material, respectively, by adding or removing energy
in the form of the latent heat of fusion of the phase change
material.
[0007] In an embodiment of the invention, the storage system
comprises a first and a second heat exchanger fluidly connected in
series such that energy from the phase change material flowing
through a primary side of the first and the second heat exchangers
is transferred to a first heat transfer fluid flowing through a
secondary side of the first and the second heat exchangers. The
storage system further comprises a third and a fourth heat
exchanger fluidly connected in series such that energy from a
second heat transfer fluid flowing through a secondary side of the
third and the fourth heat exchangers is transferred to the phase
change material on a primary side of the third and the fourth heat
exchangers.
[0008] In an embodiment of the invention, the phase change material
is a dilute eutectic composition comprising a mixture of two or
more inorganic salts in a thermodynamic equilibrium state of solid
and liquid.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a schematic of a thermal energy storage system in
accordance with an embodiment of the invention; and
[0010] FIG. 2 is a thermodynamic phase change diagram for a phase
change material in accordance with an embodiment of the
invention.
DETAILED DESCRIPTION
[0011] While the present invention is subject to various
modifications, embodiments illustrating the best mode contemplated
by the inventors for carrying out the invention are described in
detail herein below by way of examples with reference to the
included drawings. However, it should be clearly understood that
there is no intention to limit the invention in any form or manner
to the disclosed embodiments, forms, or examples. As such, all
alternatives are considered as falling within the scope, spirit and
intent of the invention as defined by the appended claims.
[0012] FIG. 1 is a schematic of thermal energy storage system 10 in
accordance with an embodiment of the invention. Thermal storage
system 10 includes phase change material 12 contained within tank
14, first heat exchanger 16 fluidly connected in series to second
heat exchanger 18 and third heat exchanger 20 fluidly connected in
series to fourth heat exchanger 22.
[0013] In an embodiment of the invention, each one of the first,
second and third heat exchangers 16, 18 and 20, respectively,
comprise a primary side and a secondary side. As illustrated in
FIG. 1, first heat exchanger 16 comprises primary side 24 and
secondary side 26. Similarly, second heat exchanger 18 comprises
primary side 28 and secondary side 30; and third heat exchanger 20
comprises primary side 32 and secondary side 34. Each one of first,
second and third heat exchangers, 16, 18 and 20, respectively, are
one of the various types of heat exchangers as are well known in
the art, including a shell-and-tube heat exchanger. In the
embodiment of thermal storage system 10, fourth heat exchanger 22
is illustrated in FIG. 1 as comprising plurality of tubes 36
embedded in phase change material 12 within tank 14. As such,
plurality of tubes 36 include multiple tubes in a parallel
arrangement connected by headers. Alternatively, plurality of tubes
36 form one or more coil of continuous tubing.
[0014] A portion of phase change material 12 extracted from tank 14
flows along path 38 and enters primary side 28 of second heat
exchanger 18. The phase change material flowing through primary
side 28 exits second heat exchanger 18 along path 40 and enters
primary side 24 of first heat exchanger 16. The phase change
material flowing through primary side 24 exits first heat exchanger
16 along path 42 and enters tank 14. A first fluid flows along path
44 and enters secondary side 26 of first heat exchanger 16. The
first fluid flowing through secondary side 26 exits first heat
exchanger 16 along path 46 and enters secondary side 30 of second
heat exchanger 18. The first fluid flowing through secondary side
30 exits second heat exchanger 18 along path 48.
[0015] A second fluid flows along path 50 and enters secondary side
34 of third heat exchanger 20. The second fluid flowing through
secondary side 34 exits third heat exchanger 20 along path 52 and
enters secondary side 36 of fourth heat exchanger 22. The second
fluid flowing through secondary side 36 exits fourth heat exchanger
22 along path 54. A portion of phase change material 12 extracted
from tank 14 flows along flow path 56 and enters primary side 32 of
third heat exchanger 20. The phase change material flowing through
primary side 32 exits third heat exchanger 20 along flow path 58
and enters tank 14.
[0016] As is well known to those skilled in the art, each of the
first and second heat exchangers 16 and 18, respectively, provide
the means for transferring energy between the phase change material
and the first fluid, respectively, flowing through the primary and
the secondary sides of these heat exchangers. Similarly, third heat
exchanger 20 provides the means for transferring energy between the
phase change material and the second fluid, respectively, flowing
through the primary and the secondary sides of this heat exchanger.
Also, fourth heat exchanger 22 provides the means for transferring
energy between the second fluid flowing through secondary side 36
and phase change material 12 encasing secondary side 36 of fourth
heat exchanger 22.
[0017] Whether energy is transferred to the phase change material
or from the phase change material flowing through first, second and
third heat exchangers 16, 18 and 20, respectively, is determined by
the temperatures and the thermodynamic states of the fluid streams
entering each primary sides 24, 28 and 32 and secondary sides 26,
30 and 34 of first, second and third heat exchangers 16, 18 and 20,
respectively. Similarly, the temperature and the thermodynamic
state of the second fluid entering secondary side 36 of fourth heat
exchanger 22 will determine whether energy is transferred to or
from phase change material 12 encasing secondary side 36.
[0018] In accordance with an embodiment of the invention, the first
fluid entering secondary side 26 of first heat exchanger 16 is feed
water. Within first heat exchanger 16, energy from the phase change
material flowing through primary side 24 is transferred to the
fluid flowing through secondary side 26, converting the feed water
into saturated steam. The saturated steam exits secondary side 26
along flow path 46 and enters secondary side 30 of second heat
exchanger 18. Within second heat exchanger 18, energy from the
phase change material flowing through primary side 28 is
transferred to the fluid flowing through secondary side 30,
converting the saturated steam into superheated steam. The
superheated steam exits secondary side 30 along flow path 48. In an
embodiment of the invention, the superheated steam flowing along
flow path 48 is used for operating a steam turbine connected to a
generator for generating electricity. In another embodiment, the
superheated steam is used for operating a cooling apparatus such as
an absorption chiller. In an alternate embodiment, the superheated
steam is used for heating an enclosure. In yet another embodiment,
the superheated steam is used in an industrial process. As can be
seen, the superheated steam can be used in various
applications.
[0019] As will be apparent to one skilled in the art, the phase
change material flowing through primary sides 24 and 28 of first
and second heat exchangers 16 and 18, respectively, will undergo a
change in its thermodynamic state as energy from the phase change
material is transferred to the fluid flowing through secondary
sides 26 and 30 of first and second heat exchangers 16 and 18,
respectively. In an embodiment of the invention, the phase change
material extracted from tank 14 along path 38 is liquid. In another
embodiment, the phase change material extracted from tank 14 along
path 38 is a slurry comprising liquid and solid in a thermodynamic
equilibrium state. Similarly, the phase change material exiting
primary side 28 of second heat exchanger 18 and entering primary
side 24 of first heat exchanger 16 along flow path 40 is liquid or
a slurry comprising liquid and solid in a thermodynamic equilibrium
state. In an embodiment of the invention wherein the phase change
material entering primary side 28 of second heat exchanger 18 is a
slurry, then the fraction of solid in the slurry exiting primary
side 28 and entering primary side 24 of first heat exchanger 16
will be relatively more than the fraction of solid in the slurry
entering primary side 28. Similarly, if the phase change material
entering primary side 24 of first heat exchanger 16 is a slurry,
then the fraction of solid in the slurry exiting primary side 24
and returning to tank 14 along path 42 will be relatively more than
the fraction of solid in the slurry entering primary side 24.
[0020] In accordance with an embodiment of the invention, the
second fluid entering secondary side 34 of third heat exchanger 20
is a liquid such as water or oil at a relatively high temperature.
In another embodiment, the second fluid entering secondary side 34
is a gas such as saturated or superheated steam. In an alternate
embodiment, the second fluid is from a heat source such as a
concentrating solar collector or a solar power tower collector.
[0021] In an embodiment of the invention, a portion of phase change
material 12 extracted from tank 14 flows along path 56 and enters
primary side 32 of third heat exchanger 20. The phase change
material exiting third heat exchanger 20 is returned to tank 14
along flow path 58. Within third heat exchanger 20, energy from the
second fluid flowing through secondary side 34 is transferred to
the phase change material flowing through primary side 32. In
accordance with an embodiment of the invention, the phase change
material enters primary side 32 as a slurry comprising liquid and
solid in a thermodynamic equilibrium state and exits as a liquid.
In another embodiment of the invention, the phase change material
enters and exits primary side 32 as a slurry comprising liquid and
solid in a thermodynamic equilibrium state. As such, the fraction
of solid in the slurry exiting primary side 32 will be less than
the fraction of solid in the slurry entering primary side 32.
[0022] As previously stated, fourth heat exchanger 22 is
illustrated in FIG. 1 as comprising plurality of tubes 36 embedded
in phase change material 12 within tank 14. In accordance with an
embodiment of the invention, the phase change material surrounding
plurality of tubes 36 is converted from solid to a slurry
comprising liquid and solid in a thermodynamic equilibrium state.
In another embodiment of the invention, the fraction of solid in
the slurry surrounding plurality of tubes 36 is reduced. In an
alternate embodiment of the invention, the slurry surrounding
plurality of tubes 36 is converted into liquid.
[0023] In an embodiment of the invention, the temperature of the
phase change material returned to tank 14 along path 58 is
relatively higher than the temperature of the phase change material
surrounding plurality of tubes 36. Similarly, the phase change
material returned to tank 14 along path 42 will be relatively
cooler than the phase change material extracted from tank 14 along
path 38. As such, phase change material 12 within tank 14 will be
thermally stratified. As will be apparent to one skilled in the
art, the relatively warmer phase change material along path 58 does
not necessarily have to be returned near the top of tank 14 for
initiating and/or maintaining tank 14 in a thermally stratified
state. Similarly, the relatively cooler phase change material along
path 42 does not necessarily have to be returned near the bottom of
tank 14 for initiating and/or maintaining tank 14 in a thermally
stratified state. In a thermally stratified tank, the phase change
material extracted along path 38 will be from near the top of tank
14
[0024] In accordance with an embodiment of the invention, phase
change material 12 is a mixture of two or more inorganic salts,
each of which inorganic salt undergoes a thermodynamic phase change
between its respective solid and liquid states. As is well known to
those skilled in the art, the temperature at which one or more of
the inorganic salts undergo thermodynamic phase change between its
respective solid and liquid states is dictated by whether the
mixture of the salts forms a eutectic composition or a dilute
eutectic composition. Whether the mixture of the two or more
inorganic salts forms a eutectic composition or a dilute eutectic
composition is defined by one of the weight, volumetric or mole
content of each of the two or more inorganic salts.
[0025] FIG. 2 shows simple phase change diagram 100 for phase
change material 12 comprising a binary mixture of two inorganic
salts sodium hydroxide (NaOH) and sodium nitrate (NaNO.sub.3) in
accordance with an embodiment of the invention. In an alternate
embodiment, the phase change material comprises a mixture of three,
or more, inorganic salts. For the binary mixture comprising
inorganic salts NaOH and NaNO.sub.3 in an embodiment of the
invention, temperature 102 is a function of the weight percent of
NaNO.sub.3 104. As illustrated, the binary mixture comprising about
83.2 percent-by-weight of NaNO.sub.3 and about 16.8
percent-by-weight of NaOH (numeral 106) forms eutectic composition
108 having eutectic temperature of about 246 degrees-C. As can be
seen, for the binary mixture comprising more than about 83.2
percent-by-weight of NaNO.sub.3, solidus state 110 is maintained at
the eutectic temperature of about 246 degrees-C. Accordingly, the
NaOH will always be in the liquid state and, therefore, the binary
mixture will be in the form of a slurry comprising liquid NaOH in
thermodynamic equilibrium with some NaNO.sub.3 in solid state. In
accordance with an embodiment of the invention, phase change
material 12 comprises a dilute eutectic composition in NaNO.sub.3
including about 99.0 percent-by-weight of NaNO.sub.3 and about 1.0
percent-by-weight of NaOH (numeral 112).
[0026] As is well know to those skilled in the art, salt is a
corrosive compound. In an embodiment of the invention, each
component of thermal energy storage system 10 is manufactured from
material naturally resistant to corrosion from salt. In another
embodiment, each component of thermal energy storage system 10 is
treated for preventing or minimizing corrosion from salt. In an
alternate embodiment, each component of thermal energy storage
system 10 is coated with a salt phobic compound for preventing or
minimizing corrosion.
[0027] Various modifications and additions may be made to the
exemplary embodiments presented hereinabove without departing from
the scope and intent of the present invention. For example, while
the disclosed embodiments refer to particular features, the scope
of the instant invention is considered to also include embodiments
having different combinations of features different from and/or in
addition to those described herein. Accordingly, the scope of the
present invention is intended to embrace all such alternatives,
modifications, and variations as falling within the scope and
intent of the appended claims, including all equivalents
thereof.
* * * * *