U.S. patent application number 13/005249 was filed with the patent office on 2011-05-05 for thermoelectric energy storage system and method for storing thermoelectric energy.
This patent application is currently assigned to ABB RESEARCH LTD. Invention is credited to Mehmet Mercangoez, Christian OHLER.
Application Number | 20110100611 13/005249 |
Document ID | / |
Family ID | 41550763 |
Filed Date | 2011-05-05 |
United States Patent
Application |
20110100611 |
Kind Code |
A1 |
OHLER; Christian ; et
al. |
May 5, 2011 |
THERMOELECTRIC ENERGY STORAGE SYSTEM AND METHOD FOR STORING
THERMOELECTRIC ENERGY
Abstract
A system and method are provided for thermoelectric energy
storage. A thermoelectric energy storage system having at least one
hot storage unit is provided. In an exemplary embodiment, each hot
storage unit includes a hot tank and a cold tank connected via a
heat exchanger and containing a thermal storage medium. The
thermoelectric energy storage system also includes a working fluid
circuit for circulating working fluid through each heat exchanger
for heat transfer with the thermal storage medium. Improved
roundtrip efficiency is achieved by minimizing the temperature
difference between the working fluid and the thermal storage medium
in each heat exchanger during heat transfer. Exemplary embodiments
realize this improved roundtrip efficiency through modification of
thermal storage media parameters.
Inventors: |
OHLER; Christian; (Baden,
CH) ; Mercangoez; Mehmet; (Baden-Daettwil,
CH) |
Assignee: |
ABB RESEARCH LTD
Zurich
CH
|
Family ID: |
41550763 |
Appl. No.: |
13/005249 |
Filed: |
January 12, 2011 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/EP2009/058475 |
Jul 6, 2009 |
|
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|
13005249 |
|
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Current U.S.
Class: |
165/104.28 ;
392/341; 60/652 |
Current CPC
Class: |
F01K 3/12 20130101; F01K
3/02 20130101; F01K 3/00 20130101 |
Class at
Publication: |
165/104.28 ;
392/341; 60/652 |
International
Class: |
F24H 7/04 20060101
F24H007/04; F28D 15/00 20060101 F28D015/00; F01K 27/00 20060101
F01K027/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 16, 2008 |
EP |
08160520.6 |
Claims
1. A thermoelectric energy storage system for providing thermal
energy to a thermodynamic machine for generating electricity, the
system comprising: a heat exchanger; a hot storage unit which is
connected to the heat exchanger and which contains a thermal
storage medium; and a working fluid circuit configured to circulate
a working fluid through the heat exchanger for heat transfer with
the thermal storage medium contained in the hot storage unit, and
wherein working fluid circuit is configured to minimize a
temperature difference between the working fluid and the thermal
storage medium in the hot storage unit during heat transfer.
2. The system according to claim 1, wherein the hot storage unit
comprises at least two hot storage units, wherein each of the at
least two hot storage units is connected to a respective heat
exchanger and contains a thermal storage medium.
3. The system according to claim 1, wherein the thermal storage
medium is a liquid, and the working fluid circuit is configured to
modify a flow rate of the thermal storage medium such that the
temperature difference between the working fluid and the thermal
storage medium in the hot storage unit is minimized during heat
transfer.
4. The system according to claim 1, wherein the working fluid
circuit is configured to modify the temperature of the thermal
storage medium at entry and exit points of the heat exchanger such
that the temperature difference between the working fluid and the
thermal storage medium in the hot storage unit is minimized during
heat transfer.
5. The system according to claim 2, wherein at least one of the hot
storage units contains a different type of thermal storage medium
such that the temperature difference between the working fluid and
the thermal storage medium in each hot storage unit is minimized
during heat transfer.
6. The system according to claim 1, wherein the temperature
difference between the working fluid and the thermal storage medium
in the hot storage unit is less than 50.degree. C. during heat
transfer.
7. A method for storing thermoelectric energy in a thermoelectric
energy storage system, comprising; charging a hot storage unit by
providing heat via a heat exchanger to a thermal storage medium by
compressing a working fluid; discharging the hot storage unit by
expanding the working fluid heated via the heat exchanger from the
thermal storage medium through a thermodynamic machine; and
modifying thermal storage media parameters to ensure that a
temperature difference between the working fluid and the thermal
storage medium is minimized during charging and discharging.
8. The method according to claim 7, wherein the step of modifying
the thermal storage media parameters comprises modifying the flow
rate of the thermal storage medium.
9. The method according to claim 7, wherein the step of modifying
the thermal storage media parameters comprises modifying an initial
temperature and final temperature of the thermal storage
medium.
10. The method according to claim 7, wherein the step of modifying
the thermal storage media parameters comprises modifying the type
of thermal storage medium.
11. The system according to claim 2, wherein the thermal storage
medium is a liquid, and the working fluid circuit is configured to
modify a flow rate of the thermal storage medium such that the
temperature difference between the working fluid and the thermal
storage medium in each hot storage unit is minimized during heat
transfer.
12. The system according to claim 11, wherein at least one of the
hot storage units contains a different type of thermal storage
medium such that the temperature difference between the working
fluid and the thermal storage medium in each hot storage unit is
minimized during heat transfer.
13. The system according to claim 2, wherein the working fluid
circuit is configured to modify the temperature of the thermal
storage medium at entry and exit points of each connected heat
exchanger such that the temperature difference between the working
fluid and the thermal storage medium in each hot storage unit is
minimized during heat transfer.
14. The system according to claim 13, wherein at least one of the
hot storage units contains a different type of thermal storage
medium such that the temperature difference between the working
fluid and the thermal storage medium in each hot storage unit is
minimized during heat transfer.
15. The system according to claim 3, wherein the working fluid
circuit is configured to modify the temperature of the thermal
storage medium at entry and exit points of each connected heat
exchanger such that the temperature difference between the working
fluid and the thermal storage medium in each hot storage unit is
minimized during heat transfer.
16. The system according to claim 15, wherein the temperature
difference between the working fluid and the thermal storage medium
in each hot storage unit is less than 50.degree. C. during heat
transfer.
17. The system according to claim 2, wherein the temperature
difference between the working fluid and the thermal storage medium
in each hot storage unit is less than 50.degree. C. during heat
transfer.
18. The method according to claim 8, wherein the step of modifying
the thermal storage media parameters comprises modifying an initial
temperature and final temperature of the thermal storage
medium.
19. The method according to claim 8, wherein the step of modifying
the thermal storage media parameters comprises modifying the type
of thermal storage medium.
20. The method according to claim 9, wherein the step of modifying
the thermal storage media parameters comprises modifying the type
of thermal storage medium.
Description
RELATED APPLICATIONS
[0001] This application claims priority as a continuation
application under 35 U.S.C. .sctn.120 to PCT/EP2009/058475, which
was filed as an International Application on Jul. 6, 2009,
designating the U.S., and which claims priority to European
Application 08160520.6 filed in Europe on Jul. 16, 2008. The entire
contents of these applications are hereby incorporated by reference
in their entireties.
FIELD
[0002] The present disclosure relates generally to the storage of
electric energy. More particularly, the present disclosure relates
to a system and method for storing electric energy in the form of
thermal energy in a thermal energy storage.
BACKGROUND INFORMATION
[0003] Base load generators such as nuclear power plants and
generators with stochastic, intermittent energy sources, such as
wind turbines and solar panels, generate excess electrical power
during times of low power demand. Large-scale electrical energy
storage systems are a means of diverting this excess energy to
times of peak demand and balancing overall electricity generation
and consumption.
[0004] In EP1577548, the applicant described the idea of a
thermoelectric energy storage (TEES) system. A TEES converts excess
electricity to heat, stores the heat, and converts the heat back to
electricity, when necessary. Such an energy storage system is
robust, compact, site independent and is suited to the storage of
electrical energy in large amounts. Thermal energy can be stored in
the form of sensible heat via a change in temperature or in the
form of latent heat via a change of phase or a combination of both.
The storage medium for the sensible heat can be a solid, liquid, or
a gas. The storage medium for the latent heat occurs via a change
of phase and can involve any of these phases or a combination of
the phases in series or in parallel.
[0005] All electric energy storage technologies inherently have a
limited round-trip efficiency. Thus, for every unit of electrical
energy used to charge the storage, only a certain percentage is
recovered as electrical energy upon discharge. The rest of the
electrical energy is lost. If, for example, the heat being stored
in a TEES system is provided through resistor heaters, it has
approximately 40% round-trip efficiency. The efficiency of
thermoelectric energy storage is limited for various reasons rooted
in the second law of thermodynamics. Firstly, the conversion of
heat to mechanical work is limited to the Carnot efficiency.
Secondly, the coefficient of performance of any heat pump declines
with increased temperature difference between the input level and
the output level. Thirdly, any heat flow from a working fluid to a
thermal storage and vice versa requires a temperature difference in
order to happen. This fact inevitably degrades the temperature
level and thus the capability of the heat to do work.
[0006] It is noted that many industrial processes involve provision
of thermal energy and storage of the thermal energy. Examples are
refrigeration devices, heat pumps, air conditioning and the process
industry. In solar thermal power plants, heat is provided, possibly
stored, and converted to electrical energy. However, all these
applications are distinct from TEES systems because they are not
concerned with heat for the exclusive purpose of storing
electricity.
[0007] It is known in the art that heat can be provided to the
thermal storage unit through a heat pump. For example, a Stirling
machine (for reference, see U.S. Pat. No. 3,080,706, column 2,
lines 22-30). Also, WO 2007/134466 discloses a TEES system having
an integrated heat pump.
[0008] A heat pump requires work to move thermal energy from a cold
source to a warmer heat sink. Since the amount of energy deposited
at the hot side is greater than the work required by an amount
equal to the energy taken from the cold side, a heat pump will
"multiply" the heat as compared to resistive heat generation. The
ratio of heat output to work input is called a coefficient of
performance, and the ratio is a value larger than one. In this way,
the use of a heat pump will increase the round-trip efficiency of a
thermoelectric energy storage system. The round-trip efficiency is
the amount of electricity provided from the storage divided by the
amount of electricity provided to the storage.
[0009] U.S. Pat. No. 4,089,744 discloses a method of thermal energy
storage by means of reversible heat pumping. Excess electrical
output is stored in the form of sensible heat by using it to raise
the temperature level of a heat storage fluid. In this scheme, the
source of low level heat is stored hot water, which also serves as
the working fluid in the heat pump and the turbine cycles. A
thermodynamic analysis, such as the type of analysis shown in FIG.
6 of the '744 patent, shows that the efficiency of schemes
equivalent to that of U.S. Pat. No. 4,089,744 is limited to about
50%.
[0010] In view of this background, exemplary embodiments of the
present disclosure provide an efficient thermoelectric energy
storage having a round-trip efficiency of, for example, greater
than 55%.
SUMMARY
[0011] An exemplary embodiment provides a thermoelectric energy
storage system for providing thermal energy to a thermodynamic
machine for generating electricity. The exemplary system includes a
heat exchanger, and a hot storage unit which is connected to the
heat exchanger and which contains a thermal storage medium. The
exemplary system also includes a working fluid circuit configured
to circulate a working fluid through the heat exchanger for heat
transfer with the thermal storage medium contained in the hot
storage unit. The working fluid circuit is also configured to
minimize a temperature difference between the working fluid and the
thermal storage medium in the hot storage unit during heat
transfer.
[0012] An exemplary embodiment also provides a method for storing
thermoelectric energy in a thermoelectric energy storage system.
The exemplary method includes charging a hot storage unit by
providing heat via a heat exchanger to a thermal storage medium by
compressing a working fluid. The exemplary method also includes
discharging the hot storage unit by expanding the working fluid
heated via the heat exchanger from the thermal storage medium
through a thermodynamic machine. In addition, the exemplary method
includes modifying thermal storage media parameters to ensure that
a temperature difference between the working fluid and the thermal
storage medium is minimized during charging and discharging.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] Additional refinements, advantages and features of the
present disclosure are described in more detail below with
reference to exemplary embodiments illustrated in the drawings, in
which:
[0014] FIG. 1 shows an excerpt of a Substation Automation (SA)
system, and a simplified schematic diagram of an exemplary
thermoelectric energy storage system according to an embodiment of
the present disclosure;
[0015] FIG. 2 is an enthalpy-pressure diagram of the heat pump
cycle and the turbine cycle in an exemplary TEES system according
to an embodiment of the present disclosure;
[0016] FIG. 3 is a schematic illustration of a cross-section
through a heat pump cycle portion of an exemplary TEES system
according to an embodiment of the present disclosure;
[0017] FIG. 4 is a schematic illustration of a cross-section
through a turbine cycle portion of an exemplary TEES system
according to an embodiment of the present disclosure;
[0018] FIGS. 5a-5f depict simplified enthalpy-temperature diagrams
of the working fluids and thermal storage fluids in exemplary heat
exchangers during charging and discharging, according to an
embodiment of the present disclosure;
[0019] FIG. 6 shows an enthalpy-temperature diagram of the heat
transfer from the cycles in an exemplary TEES system according to
an embodiment of the present disclosure; and
[0020] FIG. 7 shows an enthalpy-temperature diagram of the heat
transfer from the cycles in an optimized scenario in an exemplary
TEES system according to an embodiment of the present
disclosure.
[0021] For consistency, the same reference numerals are used to
denote similar elements or similarly functioning elements
illustrated throughout the drawings.
DETAILED DESCRIPTION
[0022] Exemplary embodiments of the present disclosure provide a
thermoelectric energy storage system for converting electrical
energy into thermal energy to be stored and converted back to
electrical energy with an improved round-trip efficiency. An
exemplary embodiment provides a thermoelectric energy storage
system for providing thermal energy to a thermodynamic machine for
generating electricity. Another exemplary embodiment provides a
method for storing thermoelectric energy in a thermoelectric energy
storage system.
[0023] According to an exemplary embodiment of the present
disclosure, a thermoelectric energy storage system is provided
which includes a hot storage unit which is in connection with a
heat exchanger and which contains a thermal storage medium. The
exemplary system also includes a working fluid circuit for
circulating a working fluid through the heat exchanger for heat
transfer with the thermal storage medium. The temperature
difference between the working fluid and the thermal storage medium
in the hot storage unit is minimized during heat transfer.
[0024] When the thermoelectric energy storage system is in a
charging (or "heat pump") cycle, the thermodynamic machine includes
a turbine, and when the thermoelectric energy storage system is in
a discharging (or "turbine") cycle, the thermodynamic machine
includes a compressor.
[0025] According to an exemplary embodiment, the hot storage unit
can include at least two hot storage units, where each hot storage
unit is in connection with a respective heat exchanger and contains
a thermal storage medium.
[0026] In accordance with an exemplary embodiment, the heat
exchanger or heat exchangers are common to both the charging and
discharging cycles. However, it is also possible that there are
separate heat exchangers for the charging and discharging cycles.
Two or more heat exchangers utilized in series can be connected
hydraulically, for example.
[0027] The thermal storage medium may be a liquid, and a flow rate
of the thermal storage medium may be modified such that the
temperature difference between the working fluid and the thermal
storage medium in each hot storage unit is minimized during heat
transfer.
[0028] According to an exemplary embodiment, the thermal storage
medium may be a solid or a liquid. The particular exemplary
embodiment illustrated in FIGS. 3 and 4 of the accompanying
description shows an example in which the thermal storage medium is
a liquid.
[0029] In accordance with an exemplary embodiment, a single working
fluid circuit containing a single type of working fluid can be
utilized for both the charging and discharging cycles. However, it
is also possible for there to be separate working fluid circuits
for the charging and discharging cycles. Further, each separate
working fluid circuit may contain a different type of working
fluid.
[0030] According to an exemplary embodiment, the temperature of the
thermal storage medium at entry and exit points of each connected
heat exchanger can be modified such that the temperature difference
between the working fluid and the thermal storage medium in each
hot storage unit is minimized during heat transfer.
[0031] In accordance with an exemplary embodiment, at least one of
the hot storage units may contain a different type of thermal
storage medium such that the temperature difference between the
working fluid and the thermal storage medium in each hot storage
unit is minimized during heat transfer.
[0032] In accordance with an exemplary embodiment, the hot storage
unit or units include a thermal storage medium for sensible heat
storage and a phase change storage medium for latent heat storage,
which are arranged such that the temperature difference between the
working fluid and the thermal storage medium in each heat exchanger
unit is minimized during heat transfer.
[0033] In accordance with an exemplary embodiment, the temperature
difference between the working fluid and the thermal storage medium
in each hot storage unit can be less than 50.degree. C. during heat
transfer, for example.
[0034] In accordance with an exemplary embodiment of the present
disclosure, a method is provided for storing thermoelectric energy
in a thermoelectric energy storage system. The exemplary method
includes charging a hot storage unit by providing heat via a heat
exchanger to a thermal storage medium by compressing a working
fluid. The exemplary method also includes discharging the hot
storage unit by expanding the working fluid heated via the heat
exchanger from the thermal storage medium through a thermodynamic
machine. In addition, the exemplary method includes modifying the
thermal storage media parameters to ensure the temperature
difference between the working fluid and the thermal storage medium
is minimized during charging and discharging.
[0035] In accordance with an exemplary embodiment, the step of
modifying the thermal storage media parameters can include
modifying the flow rate of the thermal storage medium.
[0036] In accordance with an exemplary embodiment, the step of
modifying the thermal storage media parameters can include
modifying the initial temperature and final temperature of the
thermal storage medium.
[0037] In accordance with an exemplary embodiment, the step of
modifying the thermal storage media parameters can include
modifying the type of thermal storage medium.
[0038] FIG. 1 depicts a schematic diagram of an exemplary TEES
system 10 in accordance with an embodiment of the present
disclosure. The TEES system 10 includes a hot storage 12 and a cold
storage 14 which are coupled to each other by means of a heat pump
cycle system 16 and a turbine cycle system 18. The hot storage 12
contains a thermal storage medium. According to an exemplary
embodiment, the cold storage 14 can be a heat sink, for example.
Both the heat pump cycle and the turbine cycle contain a working
fluid.
[0039] The heat pump cycle system 16 includes, in the flow
direction of the working fluid, an evaporator 20, a compressor
train 22, a heat exchanger 24, and an expansion valve 26. The
turbine cycle system 18 includes, in the flow direction of the
working fluid, a feed pump 28, a heat exchanger 30, a turbine 32,
and a condenser 34. The heat exchangers 24, 30 in both the heat
pump cycle system 16 and the turbine cycle system 18, respectively,
are arranged to exchange heat with the hot storage 12. The
evaporator 20 and the condenser 34 in the heat pump cycle system 16
and the turbine cycle system 18, respectively, are arranged to
exchange heat with the cold storage 14.
[0040] The cold storage 14 is a heat reservoir at any temperature
lower than the hot storage temperature. However, the cold storage
temperature may be higher or lower than the ambient temperature.
For example, the cold storage may be another heat sink such as
cooling water or air from the ambient. In an alternative
embodiment, the turbine and compressor trains 22, 32 may be
thermodynamic machines based on positive displacement such as
reciprocating or rotary expanders or compressors.
[0041] The compressor train 22 may include one or more individual
compressors with possible intercooling. The turbine 32 may include
one or more individual turbines with possible reheating. Similarly,
the evaporator 20, the condenser 34, the feed pump 28 and the
expansion valve 26 may include one or multiple units.
[0042] In operation, the working fluid flows around the TEES system
10 in the following manner. The working fluid in the compressor 22
is initially in vapor form, and surplus electrical energy is
utilized to compress and heat the working fluid. The working fluid
is fed through the heat exchanger 24 where the working fluid
discards heat into the storage medium of the hot storage 12. The
compressed working fluid exits the heat exchanger 24 and enters the
expansion valve 26. Here, the working fluid is expanded to the
lower pressure of the evaporator 20. The working fluid flows from
the expansion valve into the evaporator 20 where the working fluid
is heated to evaporation. This is realized using available heat
from the cold storage 14.
[0043] In the condenser 34, working fluid is condensed by
exchanging heat with the cold storage 14. The condensed working
fluid exits the condenser 34 via an outlet and is pumped into the
heat exchanger 30 at the hot storage 12 via the feed pump 28. Here,
the working fluid is heated, evaporated, and overheated from the
stored heat from the storage medium in the hot storage 12. The
working fluid exits the heat exchanger 30 and enters the turbine 32
where the working fluid is expanded to thereby cause the turbine to
generate electrical energy.
[0044] The expansion valve 26, the evaporator 20, and the
compressor 22 are in operation during a period of charging, or the
"heat pump cycle". Similarly, the turbine 32, the condenser 34 and
the feed pump 28 are in operation during a period of discharging,
or the "turbine cycle". The hot storage 12 is in operation at all
times, i.e., during charging, storage, and discharging. These two
cycles can be clearly shown in an enthalpy-pressure diagram, such
as FIG. 2.
[0045] The solid-line cycle shown in FIG. 2 represents the heat
pump cycle that is charging the hot storage, and the heat pump
cycle follows a counter-clockwise direction as indicated by the
arrows. The working fluid is assumed to be water for this exemplary
embodiment. The heat pump cycle starts in the evaporator at point A
where steam is evaporated to form vapor using heat from the cold
storage (transition A.fwdarw.B1 in FIG. 2). In the next stage of
the heat pump cycle, the vapor is compressed utilizing electrical
energy in two stages from point B1 to C1 and B2 to C2. Where
compression occurs in two stages, this is a consequence of the
compressor train 22 including two individual units. In between
these two compression stages, the working fluid is cooled from
point C1 to B2. The hot, compressed, overheated vapor exits the
compression train 22 at point C2 where it is cooled down to the
saturation temperature at D1, condensed at D2, and further cooled
down to point D3. This cooling down and condensation is realized by
transferring the heat from the working fluid into the hot storage
12 thereby storing the heat energy. The cooled working fluid is
returned to its initial low pressure state at point A via the
expansion valve 26.
[0046] The dotted-line cycle shown in FIG. 2 represents the Rankine
turbine cycle that is discharging the hot storage, and the cycle
follows a clockwise direction as indicated by the arrows. The
Rankine turbine cycle starts at point E, where the pump 28 is
utilized to pump the working fluid in its liquid state from point E
to F1. Next, from point F1 to point G, the working fluid receives
the heat from the thermal storage medium. In detail, the heat is
transferred from the thermal storage medium to the working fluid
causing the working fluid to heat up at F2, to boil at F3, and
attain a certain degree of superheat at G. The superheated working
fluid vapor at point G is expanded down to point H in a mechanical
device such as a turbine to generate electricity. Following the
expansion, the working fluid enters the condenser 34 where it is
condensed to its initial state at point E by exchanging heat with
the cold storage 14.
[0047] The roundtrip efficiency of the complete energy storage
process, that is the heat pump cycle and the Rankine turbine cycle,
is calculated in the following manner; the work provided by the
turbine expansion divided by the work used in the heat pump
compressor:
(h.sub.G-h.sub.H)/(h.sub.C2-h.sub.B2+h.sub.C1-h.sub.B1),
[0048] where the letter h denotes the enthalpy of the corresponding
point. For the exemplary conditions depicted in FIG. 2, the
roundtrip efficiency is 50.8%. It is not possible from the
enthalpy-pressure diagram alone to judge if this is a particularly
efficient TEES system, or how it could be improved in
efficiency.
[0049] With reference to the exemplary TEES system illustrated in
FIG. 1, the heat exchanger 24 in the heat pump cycle components 16
and the heat exchanger 30 in the turbine cycle components 18 may
include several individual heat exchangers arranged in series, as
illustrated in FIGS. 3 and 4, respectively.
[0050] FIG. 3 depicts a simplified schematic diagram of the heat
pump cycle components 16 in a thermoelectric energy storage system
10 according to an exemplary embodiment of the present disclosure.
Here, three individual hot storage units x, y, z are arranged in
series. Each hot storage unit x, y, z comprises a heat exchanger
36, 38, 40 in connection with a storage tank pair 42, 44, 46,
respectively. Each storage tank pair comprises a cold tank and a
hot tank wherein the flow of the thermal storage medium is from the
cold tank to the hot tank via the associated heat exchanger. The
three hot storage units in FIG. 3 are denoted x, y and z from left
to right in the diagram. In the present embodiment, the heat
exchangers are counterflow heat exchangers, and the working fluid
of the cycle is water.
[0051] In operation, the heat pump cycle components 16 of FIG. 3
perform essentially in a similar manner as the heat pump cycle
components 16 of the TEES system 10 described with respect to FIGS.
1 and 2. In addition, the working fluid flows through the
additional two separate heat exchangers. In the exemplary
embodiment shown in FIG. 3, in the direction of flow of the working
fluid, the initial and final temperatures of the working fluid as
it passes through heat exchanger 40 are 510.degree. C. and
270.degree. C., through heat exchanger 38 are 270.degree. C. and
270.degree. C., and through heat exchanger 36 are 270.degree. C.
and 100.degree. C. Thus, an overall temperature drop of 410.degree.
C. is achieved.
[0052] The characteristics of the working fluid (shown as a solid
line) and thermal storage medium (shown as a dashed line) of each
of the three heat exchangers 36, 38, 40 and associated storage tank
pair 42, 44, 46 during charging are shown in FIG. 5 in the
enthalpy-temperature graphs a), b) and c), respectively. The
temperature of the thermal storage medium in each stage is
increasing, while the temperature of the working fluid decreases
only in stages a) and c).
[0053] FIG. 4 depicts a simplified schematic diagram of the turbine
cycle components 18 in a thermoelectric energy storage system 10
according to an exemplary embodiment of the present disclosure.
Here, the arrangement of three individual hot storage units x, y,
z, arranged in series, are the same units shown in FIG. 3. Again,
each storage tank pair 42, 44, 46 includes a hot tank and a cold
tank. However, in the exemplary embodiment of FIG. 4, the flow of
the thermal storage medium is from the hot tank to the cold tank
via the heat exchanger.
[0054] In operation, the turbine cycle components 18 of FIG. 4
perform essentially in a similar manner as the turbine cycle
components of the TEES system described with respect to FIGS. 1 and
2. In addition, the working fluid flows through the additional two
separate heat exchangers. In the exemplary embodiment shown in FIG.
4, in the direction of flow of the working fluid, the initial and
final temperatures of the working fluid as it passes through heat
exchanger 36 are 80.degree. C. and 240.degree. C., through heat
exchanger 38 are 240.degree. C. and 240.degree. C., and through
heat exchanger 40 are 240.degree. C. and 490.degree. C. Thus, an
overall temperature increase of 410.degree. C. is achieved.
[0055] When the heat pump cycle components 16 are in operation,
then the working fluid conduit for the heat pump cycle is coupled
to the hot storage units x, y, z. When the turbine pump cycle
components 18 are in operation, then the working fluid conduit for
the turbine cycle coupled to the hot storage units x, y, z,
instead. In this way, the turbine cycle obtains thermal energy from
the hot storage units that was deposited by the heat pump
cycle.
[0056] The characteristics of the working fluid (shown as a solid
line) and thermal storage medium (shown as a dashed line) of each
of the three heat exchangers 36, 38, 40 and associated storage tank
pairs 42, 44, 46 during discharging are shown in FIG. 5 in the
enthalpy-temperature graphs d), e) and f), respectively. The
temperature of the thermal storage medium in each stage is
decreasing, while the temperature of the working fluid increases
only in stages d) and f).
[0057] FIG. 6 shows the isobars, e.g., lines of constant pressure,
from FIG. 5 a)-f) on a single temperature-enthalpy graph for a
particular exemplary embodiment of the present disclosure. Further,
the capital letters used are consistent with FIG. 2. Thus, FIG. 6
illustrates the heat transfer process at the three separate hot
storage units x, y, z during the charging and discharging of the
TEES system 10.
[0058] The solid line isobars C2 to D3 represent the heat pump
cycle, the dotted line isobars F1 to G represent the Rankine
turbine cycle, and the dashed line isobars X1 to X2, Y1 to Y2, Z1
to Z2 represent the thermal storage media in the three hot storage
units x, y, z, respectively.
[0059] Heat can only flow from a higher to a lower temperature.
Consequently, the characteristic isobars for the working fluid
during cooling in the heat pump cycle have to be above the
characteristic isobars for the thermal storage media, which in turn
have to be above the characteristic isobars for the working fluid
during heating in the turbine cycle. The slope of these
characteristic isobars is defined by the product of the massflow
(kg/s) and heat capacity (J/kg/K) of each thermal storage medium
relative to the massflow of the working fluid. This product is
different for each of the three heat transfer subsections:
heating/cooling of liquid water in hot storage unit x;
boiling/condensation in hot storage unit y; and
providing/extracting heat to the supersaturation region in hot
storage unit z.
[0060] The temperature profiles are stationary in time due to the
sensible heat storage in the thermal storage media. Thus, while the
volume of thermal storage media in each heat exchanger remains
constant, the volume of hot and cold thermal storage media stored
in the hot and cold tanks changes. Also, the temperature
distribution in the heat exchangers remains constant.
[0061] Importantly, exemplary embodiments of the present disclosure
determine that the smaller the average temperature difference
between the working fluid and the heat storage media during heat
transfer, the greater the efficiency of the TEES system. In an
enthalpy-temperature graph, this feature is observed as a
relatively closer positioning of the characteristic isobars of the
charging and discharging cycles, as shown in FIG. 7.
[0062] Exemplary embodiments of the present disclosure determine
that the thermal storage media may be the same or a different fluid
in each hot storage unit x, y and z. Further, exemplary embodiments
of the present disclosure determine that the thermal storage media
may be at a different temperature in each hot storage unit x, y and
z. Also, the flow-rate of the thermal storage media within each hot
storage unit may differ. Specifically, in order to achieve an
optimized roundtrip efficiency of the TEES system, various
combinations of the thermal storage media, the initial and final
temperature of the thermal storage media and the thermal storage
media flow-rates may be utilized.
[0063] In the improved efficiency scenario illustrated in FIG. 7,
the flow-rate of the thermal storage medium through heat exchanger
38 of hot storage unit y is increased by a factor of three in
comparison with the scenario in FIG. 6. (It should be noted that
the flow rate in heat exchanger 38, in FIG. 6, was set to an
arbitrary rate that was relatively larger than the flow rate in
heat exchangers 36 and 40, but the flow rate was not optimized as
in FIG. 7.) A decrease in average temperature differences between
the thermal storage medium and the working fluid during heat
transfer in heat exchanger 38 of hot storage unit y can be noted.
Consequently, a resultant TEES system design has a higher
saturation temperature in heat exchanger 38 in the turbine cycle
than before (denoted as F2' and F3' in FIG. 7 in comparison with F2
and F3 in FIG. 6). This equates to a temperature of 230.degree. C.
in FIG. 7, in comparison with 200.degree. C. in FIG. 6.
Consequently, the roundtrip efficiency of the TEES system in the
embodiment of FIG. 7 is 61.1% in comparison to an efficiency of
50.8% in FIG. 2.
[0064] In others words, exemplary embodiments of the present
disclosure require the temperature difference between the working
fluid of the heat pump cycle and the heat storage media, as well as
the temperature difference between the working fluid of the turbine
cycle and the heat storage media to be relatively small (for
example, smaller than 50.degree. C. on average). This is achieved
through modification of certain TEES parameters as specified
above.
[0065] In an accordance with an exemplary embodiment of the present
disclosure, the three thermal storage media are fluids. For
example, these may be three different liquid sensible heat storage
media such as water, oil, or molten salts. Also, in an accordance
with an exemplary embodiment of the present disclosure, the heat
exchangers are counterflow heat exchangers, having a minimal
approach temperature 10 K (e.g., the minimal temperature difference
between the two fluids exchanging heat is 10 K) and the expansion
device can be a thermostatic expansion valve.
[0066] In accordance with an exemplary embodiment, the heat at the
boiling/condensation heat exchanger 38 is transferred to the latent
heat of a phase transition of a storage medium enabling an even
closer match of the temperature profiles in the
boiling/condensation region. An exemplary embodiment uses steam as
the working fluid for both the heat pump cycle and the turbine
cycle.
[0067] In an alternative exemplary embodiment, there is no cold
storage reservoir, but the evaporator and condenser instead use
heat from the ambient as an (infinitely large) reservoir for the
cold side of the heat pump cycle and the turbine cycle. The cold
storage of FIG. 1, which is a second heat storage reservoir, has
latent heat storage at temperatures around 100.degree. C. at the
cold side of the heat pump cycle and the turbine cycle. Because of
the temperature dependence of the saturation pressure of working
fluids such as water, such an additional heat storage reservoir may
result in greater economy with respect to the compressor and the
turbine. It is envisaged that this economy would more than
compensate for the additional cost for this reservoir at moderately
long storage times.
[0068] The skilled person will appreciate that the exemplary TEES
system, as illustrated in FIGS. 1, 3 and 4, may be realized in
several different ways. For example, the hot storage can be
constituted by: [0069] A solid structure with embedded heat
exchangers equipped with appropriate means of handling the
expansion-contraction of the storage medium with changing
temperatures. [0070] A two-tank molten salt storage system with
heat exchangers between the tanks and flow of molten salt from the
cold to the hot tank during charging and from the hot to the cold
tank during discharging periods. [0071] A multiple-hot-tank
multiple-cold-tank molten salt and liquid heat storage media
cascaded at different temperatures between the evaporator operating
temperature and the temperature of the heat pump working fluid at
the exit of the compression processes. [0072] A phase change
material with a suitable phase change temperature below the
condensation temperature of the heat pump working fluid at the high
operating pressure and above the boiling point of the turbine cycle
working fluid at the high operating pressure.
[0073] Any combination of the above mentioned thermal storage
options in series and in parallel. [0074] Two, three (as shown in
FIGS. 3 and 4), four or more hot storage units in the hot
storage.
[0075] The skilled person will appreciate that the condenser and
the evaporator in the exemplary TEES system may be replaced with a
multi-purpose heat exchange device that can assume both roles,
since the evaporation for the heat pump cycle and the condensation
for the turbine cycle will be carried out in different periods.
Similarly, the turbine and the compressor roles can be carried out
by the same machinery, referred to herein as a thermodynamic
machine, capable of achieving both tasks.
[0076] In the exemplary embodiments described above, The working
fluid for is water, due, in part, to the higher efficiencies of a
water-based heat pump cycle and turbine cycle, and the amiable
properties of water as a working fluid, e.g., no global warming
potential, no ozone depletion potential, no health hazards, etc.
However, the present disclosure is not limited thereto. For the
operation of the present disclosure at ambient temperatures below
the freezing point of water, a commercial refrigerant can be chosen
as the heat pump working fluid, or a second bottoming heat pump
cycle can be cascaded with the water-based cycle to provide the
heat of evaporation, for example.
[0077] It will be appreciated by those skilled in the art that the
present invention can be embodied in other specific forms without
departing from the spirit or essential characteristics thereof. The
presently disclosed embodiments are therefore considered in all
respects to be illustrative and not restricted. The scope of the
invention is indicated by the appended claims rather than the
foregoing description and all changes that come within the meaning
and range and equivalence thereof are intended to be embraced
therein.
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