U.S. patent application number 13/687235 was filed with the patent office on 2013-04-11 for thermoelectric energy storage system and method for storing thermoelectric energy.
This patent application is currently assigned to ABB RESEARCH LTD. The applicant listed for this patent is ABB RESEARCH LTD. Invention is credited to Jaroslav Hemrle, Lilian Kaufmann, Hehmet Mercangoez, Christian Ohler.
Application Number | 20130087301 13/687235 |
Document ID | / |
Family ID | 43883202 |
Filed Date | 2013-04-11 |
United States Patent
Application |
20130087301 |
Kind Code |
A1 |
Hemrle; Jaroslav ; et
al. |
April 11, 2013 |
THERMOELECTRIC ENERGY STORAGE SYSTEM AND METHOD FOR STORING
THERMOELECTRIC ENERGY
Abstract
A thermoelectric energy storage system includes an intercooler
for intercooling a working fluid between two compression stages.
The intercooling may be carried out by flashing a portion of the
working fluid taken from the output of an expander in a flash
intercooler and/or by heating a secondary thermal storage with a
further heat exchanger.
Inventors: |
Hemrle; Jaroslav;
(Baden-Dattwil, CH) ; Kaufmann; Lilian;
(Birmenstorf, CH) ; Mercangoez; Hehmet; (Stein,
CH) ; Ohler; Christian; (Baden, CH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ABB RESEARCH LTD; |
Zurich |
|
CH |
|
|
Assignee: |
ABB RESEARCH LTD
Zurich
CH
|
Family ID: |
43883202 |
Appl. No.: |
13/687235 |
Filed: |
November 28, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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PCT/EP2011/057799 |
May 13, 2011 |
|
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13687235 |
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Current U.S.
Class: |
165/4 |
Current CPC
Class: |
F01K 25/10 20130101;
F01K 3/12 20130101; F28D 17/00 20130101 |
Class at
Publication: |
165/4 |
International
Class: |
F28D 17/00 20060101
F28D017/00 |
Foreign Application Data
Date |
Code |
Application Number |
May 28, 2010 |
EP |
10164288.2 |
Claims
1. A thermoelectric energy storage system for storing electrical
energy by transferring thermal energy to a thermal storage in a
charging cycle, and for generating electricity by retrieving the
thermal energy from the thermal storage in a discharging cycle, the
thermoelectric energy storage system comprising: a working fluid
circuit configured to circulate a working fluid; a first compressor
configured to, in the charging cycle, compress the working fluid
from a low pressure to an intermediate pressure; an intercooler
configured to, in the charging cycle, cool the working fluid at the
intermediate pressure; a second compressor configured to, in the
charging cycle, compress the working fluid from the intermediate
pressure to a high pressure; and a first heat exchanger configured
to, in the charging cycle, transfer heat from the working fluid at
the high pressure to the thermal storage and, in the discharging
cycle, transfer heat from the thermal storage to the working fluid
at the high pressure.
2. The system according to claim 1, wherein the intercooler
includes a flash intercooler.
3. The system according to claim 1, wherein the intercooler
includes a second heat exchanger configured to, in the charging
cycle, transfer heat from the working fluid at the intermediate
pressure to a second thermal storage and, in the discharging cycle,
transfer heat from the second thermal storage to the working fluid
at intermediate pressure.
4. The system according to claim 1, comprising: a first expander
configured to, in the charging cycle, expand the working fluid
after the first heat exchanger to the intermediate pressure,
wherein, in the charging cycle, a first portion of the working
fluid at the intermediate pressure is input into the
intercooler.
5. The system according to claim 4, comprising: a second expander
configured to, in the charging cycle, expand the working fluid at
the intermediate pressure to the low pressure.
6. The system according to claim 1, comprising: a third heat
exchanger configured to, in the charging cycle, transfer heat from
a third thermal storage to the working fluid at the low pressure
and, in the discharging cycle, transfer heat from the working fluid
at the low pressure to the third thermal storage.
7. The system according to claim 1, wherein: the intercooler
includes a flash intercooler and a third heat exchanger; and in the
charging cycle, the working fluid between the flash intercooler and
the third heat exchanger is compressed from a first intermediate
pressure to a second intermediate pressure.
8. The system according to claim 1, comprising: a first turbine
configured to, in the discharging cycle, expand the working fluid
from the high pressure to the intermediate pressure for generating
electrical energy; and a second turbine configured to, in the
discharging cycle, expand the working fluid from the intermediate
pressure to the low pressure for generating electrical energy.
9. The system according to claim 1, comprising: a pump configured
to, in the discharging cycle, pump the working fluid from the low
pressure to the high pressure during the discharging cycle.
10. A method for storing electrical energy in a charging cycle and
retrieving electrical energy in a discharging cycle, wherein, in
the charging cycle, the method comprises: compressing the working
fluid from a low pressure to an intermediate pressure for storing
electrical energy; cooling the working fluid at the intermediate
pressure; compressing the working fluid from the intermediate
pressure to a high pressure for storing electrical energy; and
transferring heat from the working fluid at the high pressure to
the thermal storage, and wherein, in the discharging cycle, the
method comprises: transferring heat from the thermal storage to the
working fluid at the high pressure; and expanding the working fluid
from the high pressure for generating electrical energy.
11. The method according to claim 10, wherein: in the charging
cycle, the method comprises transferring heat from the working
fluid at the intermediate pressure to a second thermal storage; and
in the discharging cycle, the method comprises: expanding the
working fluid from the high pressure to the intermediate pressure
for generating electrical energy in a first turbine; transferring
heat from the second thermal storage to the working fluid at the
intermediate pressure; expanding the working fluid from the
intermediate pressure to the low pressure for generating electrical
energy in a second turbine.
12. The method according to claim 10, wherein, in the charging
cycle, the method comprises: expanding the working fluid after the
heat exchanging at the high pressure to the intermediate pressure;
and using a first portion of the working fluid at intermediate
pressure after the heat exchanging at the high pressure for cooling
the working fluid before heat exchanging at the high pressure.
13. The method according to claim 10, wherein, in the charging
cycle, the method comprises: expanding the working fluid at the
intermediate pressure to the low pressure; and transferring heat
from a third thermal storage to the working fluid at the low
pressure, and wherein, in the discharging cycle, the method
comprises transferring heat from the working fluid at the low
pressure to the third thermal storage.
14. The method according to claim 10, wherein, in the charging
cycle, the method comprises compressing the working fluid from a
first intermediate pressure to a second intermediate pressure
between a flash intercooling with the working fluid at the first
intermediate pressure and heat exchanging with a second thermal
storage at the second intermediate pressure.
15. The method according to claim 10, wherein at least one section
of at least one of the charging cycle and the discharging cycle is
performed transcritically.
16. The system according to claim 2, wherein the intercooler
includes a second heat exchanger configured to, in the charging
cycle, transfer heat from the working fluid at the intermediate
pressure to a second thermal storage and, in the discharging cycle,
transfer heat from the second thermal storage to the working fluid
at intermediate pressure.
17. The system according to claim 16, comprising: a first expander
configured to, in the charging cycle, expand the working fluid
after the first heat exchanger to the intermediate pressure,
wherein, in the charging cycle, a first portion of the working
fluid at the intermediate pressure is input into the
intercooler.
18. The system according to claim 17, comprising: a second expander
configured to, in the charging cycle, expand the working fluid at
the intermediate pressure to the low pressure.
19. The system according to claim 16, comprising: a third heat
exchanger configured to, in the charging cycle, transfer heat from
a third thermal storage to the working fluid at the low pressure
and, in the discharging cycle, transfer heat from the working fluid
at the low pressure to the third thermal storage.
20. The system according to claim 16, wherein: the intercooler
includes a flash intercooler and a third heat exchanger; and in the
charging cycle, the working fluid between the flash intercooler and
the third heat exchanger is compressed from a first intermediate
pressure to a second intermediate pressure.
21. The system according to claim 16, comprising: a first turbine
configured to, in the discharging cycle, expand the working fluid
from the high pressure to the intermediate pressure for generating
electrical energy; and a second turbine configured to, in the
discharging cycle, expand the working fluid from the intermediate
pressure to the low pressure for generating electrical energy.
22. The system according to claim 16, comprising: a pump configured
to, in the discharging cycle, pump the working fluid from the low
pressure to the high pressure during the discharging cycle.
23. The method according to claim 10, wherein the compressing of
the working fluid from the low pressure to the intermediate
pressure includes storing the electrical energy for converting the
electrical energy into heat energy.
24. The method according to claim 11, wherein, in the charging
cycle, the method comprises: expanding the working fluid after the
heat exchanging at the high pressure to the intermediate pressure;
and using a first portion of the working fluid at intermediate
pressure after the heat exchanging at the high pressure for cooling
the working fluid before heat exchanging at the high pressure.
25. The method according to claim 24, wherein, in the charging
cycle, the method comprises: expanding the working fluid at the
intermediate pressure to the low pressure; and transferring heat
from a third thermal storage to the working fluid at the low
pressure, and wherein, in the discharging cycle, the method
comprises transferring heat from the working fluid at the low
pressure to the third thermal storage.
26. The method according to claim 25, wherein, in the charging
cycle, the method comprises compressing the working fluid from a
first intermediate pressure to a second intermediate pressure
between a flash intercooling with the working fluid at the first
intermediate pressure and heat exchanging with a second thermal
storage at the second intermediate pressure.
27. The method according to claim 25, wherein at least one section
of at least one of the charging cycle and the discharging cycle is
performed transcritically.
Description
RELATED APPLICATIONS
[0001] This application claims priority as a continuation
application under 35 U.S.C. .sctn.120 to PCT/EP2011/057799, which
was filed as an International Application on May 13, 2011
designating the U.S., and which claims priority to European
Application 10164288.2 filed in Europe on May 28, 2010. 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 a 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, etc. 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 balance the overall electricity
generation and consumption.
[0004] In EP-A 1577548, the concept of a thermoelectric energy
storage (TEES) system is disclosed. A thermoelectric energy storage
converts excess electricity to heat in a charging cycle, stores the
heat, and converts the heat back to electricity in a discharging
cycle, when necessary. Such an energy storage system may be robust,
compact, site independent and may be suited to the storage of
electrical energy in large amounts. Thermal energy may 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 may be a solid,
liquid, or a gas. The storage medium for the latent heat occurs via
a change of phase and may involve any of these phases or a
combination of them in series or in parallel.
[0005] The round-trip efficiency of an electrical energy storage
system may be defined as the percentage of electrical energy that
can be discharged from the storage in comparison to the electrical
energy used to charge the storage, provided that the state of the
energy storage system after discharging returns to its initial
condition before charging of the storage. Thus, in order to achieve
high roundtrip efficiency, the efficiencies of both modes need to
be maximized inasmuch as their mutual dependence allows.
[0006] The roundtrip efficiency of the thermoelectric energy
storage system is limited for various reasons rooted in the second
law of thermodynamics. The first reason relates to the coefficient
of performance of the system. When the system is in the charging
mode, its ideal efficiency may be governed by the coefficient of
performance (COP) of a heat pump. The COP depends on the
temperatures of the cold side (Tc) and the hot side (Th) as given
by
COP = T h T h - T c ##EQU00001##
[0007] Thus, it can be seen that the COP of a heat pump declines
with increased difference between input and output temperature
levels. Secondly, the conversion of heat to mechanical work in a
heat engine is limited by the Carnot efficiency. When the system is
in the discharging mode, the efficiency (.eta.) is given by
.eta. = T h - T c T h ##EQU00002##
[0008] Thus, it can be seen that efficiency increases when the cold
side temperature decreases.
[0009] 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.
[0010] It is noted that many industrial processes involve the
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 thermoelectric
energy storage systems because they are not concerned with heat for
the exclusive purpose of storing electricity.
[0011] In EP-A 2157317, the concept of a transcritical
thermoelectric energy storage is disclosed. In such a system, the
working fluid undergoes transcritical cooling during the charging
and transcritical heating during the discharging cycle as it
exchanges heat with the thermal storage medium.
[0012] U.S. Pat. No. 3,165,905 (Ware) describes a refrigerating
machine including an economizer with the aim of improving the
efficiency of the refrigerating cycle.
[0013] An article entitled "The Commercial Feasibility of the Use
of Water Vapor As a Refrigerant" by Lachner B. F., Nellis G. F.,
Reindl D. T. (2007) International Journal of Refrigeration 30,
699-708, describes the use of flash intercooling in between
compression stages in order to improve the coefficient of
performance of refrigeration systems.
[0014] However, in certain cases, it would be disadvantageous to
apply such techniques for improving the efficiency of refrigeration
cycles to a system having both charging and discharging cycles,
since in applying such techniques to such a system, an efficiency
improvement in one cycles could result in an efficiency reduction
in the other cycle.
SUMMARY
[0015] An exemplary embodiment of the present disclosure provides a
thermoelectric energy storage system for storing electrical energy
by transferring thermal energy to a thermal storage in a charging
cycle, and for generating electricity by retrieving the thermal
energy from the thermal storage in a discharging cycle. The
exemplary thermoelectric energy storage system includes a working
fluid circuit configured to circulate a working fluid, a first
compressor configured to, in the charging cycle, compress the
working fluid from a low pressure to an intermediate pressure, and
an intercooler configured to, in the charging cycle, cool the
working fluid at the intermediate pressure. The exemplary
thermoelectric energy storage system also includes a second
compressor configured to, in the charging cycle, compress the
working fluid from the intermediate pressure to a high pressure,
and a first heat exchanger configured to, in the charging cycle,
transfer heat from the working fluid at the high pressure to the
thermal storage and, in the discharging cycle, transfer heat from
the thermal storage to the working fluid at the high pressure.
[0016] An exemplary embodiment of the present disclosure provides a
method for storing electrical energy in a charging cycle and
retrieving electrical energy in a discharging cycle. In the
charging cycle, the exemplary method includes compressing the
working fluid from a low pressure to an intermediate pressure for
storing electrical energy, cooling the working fluid at the
intermediate pressure, compressing the working fluid from the
intermediate pressure to a high pressure for storing electrical
energy, and transferring heat from the working fluid at the high
pressure to the thermal storage. In the discharging cycle, the
exemplary method includes transferring heat from the thermal
storage to the working fluid at the high pressure, and expanding
the working fluid from the high pressure for generating electrical
energy.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] 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:
[0018] FIG. 1a shows a simplified schematic diagram of a charging
cycle of a thermoelectric energy storage system according to an
exemplary embodiment of the present disclosure;
[0019] FIG. 1b shows a simplified schematic diagram of a
discharging cycle of a thermoelectric energy storage system
according to an exemplary embodiment of the present disclosure;
[0020] FIG. 2a shows an enthalpy-pressure diagram of the heat
transfer in the charging cycle of a transcritical thermoelectric
energy storage system according to an exemplary embodiment of the
present disclosure;
[0021] FIG. 2b shows an enthalpy-pressure diagram of the heat
transfer in the discharging cycle of a transcritical thermoelectric
energy storage system according to an exemplary embodiment of the
present disclosure;
[0022] FIG. 3a shows an enthalpy-pressure diagram of the heat
transfer in the charging cycle of a thermoelectric energy storage
system according to an exemplary embodiment of the present
disclosure; and
[0023] FIG. 3b shows an enthalpy-pressure diagram of the heat
transfer in the discharging cycle of a thermoelectric energy
storage system according to an exemplary embodiment of the present
disclosure.
[0024] For consistency, in general, the same reference numerals are
used to denote identical or similarly functioning elements
illustrated throughout the drawings.
DETAILED DESCRIPTION
[0025] Exemplary embodiments of the present disclosure provide an
efficient thermoelectric energy storage having a high round-trip
efficiency, while minimizing the system costs involved.
[0026] Exemplary embodiments of the present disclosure provide a
thermoelectric energy storage system and method for storing
electrical energy in a charging cycle and retrieving electrical
energy in a discharging cycle.
[0027] In accordance with an exemplary embodiment of the present
disclosure, the thermoelectric energy storage system stores
electrical energy by transferring thermal energy to a thermal
storage in a charging cycle, and generates electricity by
retrieving the thermal energy from the thermal storage in a
discharging cycle.
[0028] According to an exemplary embodiment of the present
disclosure, the thermoelectric energy storage system includes a
working fluid circuit configured to circulate a working fluid, a
first compressor configured to, in the charging cycle, compress the
working fluid from a low pressure to an intermediate pressure (such
that the temperature of the working fluid is rising), and an
intercooler configured to, in the charging cycle, cool the working
fluid at the intermediate pressure (for lowering the temperature of
the working fluid). In addition, the exemplary thermoelectric
energy storage system includes a second compressor configured to,
in the charging cycle, compress the working fluid from the
intermediate pressure to a high pressure, a first heat exchanger
configured to, in the charging cycle, transfer heat from the
working fluid at the high pressure to the thermal storage and, in
the discharging cycle, transfer heat from the thermal storage to
the working fluid at the high pressure.
[0029] According to an exemplary embodiment of the present
disclosure, the working fluid may be compressed in two stages: from
the low pressure to the intermediate pressure in a first stage and
from the intermediate pressure to the high pressure in a second
stage.
[0030] According to an exemplary embodiment of the present
disclosure, the intercooler includes a flash intercooler and/or a
second heat exchanger. In other words, the intercooling may be
carried out by (a) flashing a portion of the working fluid (taken
from the output of a expander) in the flash intercooler and/or by
(b) heating a secondary thermal storage with the second heat
exchanger. This may have the advantage of (a) reducing the
compressor energy of the first stage without compromising the
thermal energy delivered to the main thermal storage and/or of (b)
carrying out a reheat in the discharging cycle by using the
secondary thermal storage to increase the power output.
[0031] This may mean that second heat exchanger, in the charging
cycle, transfers heat from the working fluid at the intermediate
pressure to a second thermal storage and, in the discharging cycle,
transfers heat from the second thermal storage to the working fluid
at intermediate pressure.
[0032] If there are multiple compressor stages, then for one stage
one can use the flash intercooler and for another one can use a
thermal storage heat exchanger. However, it is possible to use
flash intercoolers exclusively, for example, two flash
intercoolers. In this case, there may not be any reheat stages in
the discharging cycle. Further, it is possible to use thermal
storage heat exchangers for intercooling exclusively, for example,
two heat exchangers. In this case there may be more than one reheat
stage in the discharging cycle.
[0033] It is noted that the charging cycle of a thermoelectric
energy storage system may be referred to as a heat pump cycle and
the discharging cycle of a thermoelectric energy storage system may
be referred to as a heat engine cycle. In the thermoelectric energy
storage concept, heat needs to be transferred from a hot working
fluid to a thermal storage medium during the charging cycle and
back from the thermal storage medium to the working fluid during
the discharging cycle. 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, for example, the thermal
storage medium part of a thermoelectric energy storage, is greater
than the compression work by an amount equal to the energy taken
from the cold side, for example, the heat absorbed by the working
fluid at the low pressure, a heat pump deposits more heat per work
input to the hot storage than resistive heating. The ratio of heat
output to work input is called a coefficient of performance (COP),
and it 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.
[0034] The charging cycle of a thermoelectric energy storage system
may include a work recovering expander, an evaporator, a compressor
and a heat exchanger, all connected in series by a working fluid
circuit. Further, a cold storage tank and a hot storage tank, for
example, containing a fluid thermal storage medium may be coupled
together via the heat exchanger. Whilst the working fluid passes
through the evaporator, it absorbs heat from the ambient or from a
thermal bath and evaporates. The discharging cycle of a
thermoelectric energy storage system may include a pump, a
condenser, a turbine and a heat exchanger, all connected in series
by a working fluid circuit. Again, a cold storage tank and a hot
storage tank, for example, containing a fluid thermal storage
medium may be coupled together via the heat exchanger. Whilst the
working fluid passes through the condenser, it exchanges heat
energy with the ambient or the thermal bath and condenses. The same
thermal bath, such as a river, a lake or a water-ice mixture pool,
may be used in both the charging and discharging cycles.
[0035] Exemplary embodiments of the present disclosure overcome the
problem of an excessive temperature rise in the working fluid
during compression in the charging cycle. This problem occurs where
the ratio of the highest operating pressure of a transcritical
thermoelectric energy storage system to the evaporator pressure of
the charging cycle is relatively great. Specifically, this
excessive temperature rise is detrimental to the completion of the
compression process in a single stage unless the working fluid is
heated to an acceptably high temperature.
[0036] Thus, the skilled person will appreciate that the present
disclosure provides a thermoelectric energy storage system where
the charging and discharging cycles are designed to have
corresponding compressor intercooling and reheat sections,
respectively, with matching heat loads and temperature levels, and
where an intercooler may be used for cooling each of the additional
compression stages of the charging cycle. Such intercoolers may be
located at the corresponding compressor discharges and are fed with
partially expanded working fluid from the condenser exit, such that
the heat of compression is absorbed by the process of vaporizing
the liquid part of the working fluid.
[0037] In accordance with an exemplary embodiment, the present
disclosure provides a multi-stage compression system in which the
working fluid is cooled close to its saturation temperature as it
is output from each intermediate compression stage. The heat
released from the working fluid during the cooling is recovered and
utilized to improve roundtrip efficiency of the thermoelectric
energy storage system.
[0038] An exemplary embodiment of the present disclosure provides a
method for storing electrical energy in a charging cycle and
retrieving electrical energy in a discharging cycle.
[0039] According to an exemplary embodiment of the present
disclosure, in the charging cycle, the method includes compressing
the working fluid from a low pressure to an intermediate pressure
for storing electrical energy (for example, for converting
electrical energy into heat energy), cooling the working fluid at
the intermediate pressure, compressing the working fluid from the
intermediate pressure to a high pressure for storing electrical
energy, and transferring heat from the working fluid at the high
pressure to the thermal storage.
[0040] According to an exemplary embodiment of the present
disclosure, in the discharging cycle, the method includes
transferring heat from the thermal storage to the working fluid at
the high pressure, and expanding the working fluid from the high
pressure for generating electrical energy.
[0041] It is to be understood that features of the method as
described herein may be features of the system as described
herein.
[0042] If technically possible but not explicitly mentioned,
combinations of embodiments of the present disclosure described
herein may be embodiments of the method and the system.
[0043] FIGS. 1a and 1b show a simplified schematic diagram of a
thermoelectric energy storage system 10 according to an exemplary
embodiment of the present disclosure.
[0044] The charging cycle system 12 shown in FIG. 1a includes a
first compression stage with a compressor 14, a second compression
stage with a compressor 16, and a third compression stage with a
compressor 18. The charging cycle system 12 also includes a first
expansion stage with an expander 20 and a second expansion stage
with an expansion valve 22. A working fluid circulates through all
components of a working fluid circuit 24 as indicated by the solid
line with arrows.
[0045] Further, the charging cycle system 12 includes a stream
splitter 26 between the expander 20 and the expansion valve 22, a
flash intercooler 28 between the compressor 14 and the compressor
16 and a heat exchanger 30 between the compressor 16 and the
compressor 18.
[0046] At the high pressure side 32, the charging cycle system 12
includes a heat exchanger 34, and at the low pressure side 36, the
charging cycle system 12 includes a heat exchanger 38.
[0047] In operation, the charging cycle system 12 performs a
transcritical cycle and the working fluid flows around the
thermoelectric energy storage system 10 in the following
manner.
[0048] In the first expansion stage, the working fluid enters the
expander 20 where the working fluid is expanded from a high
pressure to a lower (intermediate) pressure. On exiting the
expander 20, the working fluid stream is split in two streams by
the stream splitter 26, with a first portion of the working fluid
flowing to the second expansion stage with expansion valve 22 and a
second portion passing directly to the flash intercooler 28.
[0049] After the second expansion stage, where the working fluid is
expanded by expansion valve 22 from the intermediate pressure to a
low pressure, the working fluid passes to the heat exchanger 38
where the working fluid absorbs heat from the ambient or from a
cold storage 40 and evaporates. For example, the heat exchanger 38
is a counter flow heat exchanger 38 and a cold storage medium
circulates from a first cold storage tank 42 to a second cold
storage tank 44 for exchanging heat with the working fluid.
[0050] The vaporised working fluid is circulated to a first
compression stage in which surplus electrical energy is utilized to
compress and heat the working fluid in a compressor 14 from the low
pressure to the intermediate pressure. On exiting the compressor
14, this first portion of working fluid is mixed with the
relatively cooler, second portion of working fluid in the flash
intercooler 28.
[0051] The mixed working fluids pass to a second compression stage
which includes the compressor 16. In the second compression stage,
further surplus electrical energy is utilized to compress the
working fluid from the intermediate pressure to a higher second
intermediate pressure. The working fluid mass flow through the
second compression stage is greater than the working fluid mass
flow through the first compression stage.
[0052] Next, the working fluid passes through the heat exchanger 30
where it is cooled as heat energy is transferred from the working
fluid to a thermal storage medium from a further heat storage 46.
For example, the heat exchanger 30 is a counter flow heat exchanger
30 and the storage medium circulates from a first storage tank 48
to a second storage tank 50 for exchanging heat with the working
fluid.
[0053] The working fluid is then directed to a third compression
stage where it passes through the compressor 18 before entering the
heat exchanger 34. In the third compression stage, again surplus
electrical energy is driving the compressor 18 for compressing the
working fluid from the second intermediate pressure to the (higher)
high pressure.
[0054] Again, in the heat exchanger 34 heat energy is transferred
from the working fluid into a thermal storage medium from a hot
storage 52. For example, the heat exchanger 34 is a counter flow
heat exchanger 34 and the storage medium circulates from a first
hot storage tank 54 to a second hot storage tank 56 for exchanging
heat with the working fluid.
[0055] Finally, the working fluid is again directed into the first
expansion stage.
[0056] In the exemplary embodiment of FIG. 1a, the flash
intercooler 28 is a spray intercooler 28. In accordance with other
exemplary embodiments, other types of flash intercoolers 28 may be
used.
[0057] Further, it should be noted that additional compression and
expansion stages may be added. However, It should also be noted
that at least one intercooler 28, 30 is required in the charging
cycle 12 in order to achieve improved efficiency of the system 10.
For example, there may be only two compression stages with a first
compressor and a second compressor and only the flash intercooler
28 or the heat exchanger 30 in between the two stages (in the
second case only one expansion stage may be needed).
[0058] In accordance with an exemplary embodiment, each compression
stage may be equipped with a flash intercooler 28, when reheat
options are not considered in the discharging cycle. It should be
noted that different working fluids may be used for the different
cycles, as long as the temperature levels for the heat load of the
heat pump, the heat storage and the heat engine are chosen
appropriately.
[0059] In an exemplary embodiment, in which the working fluid is
carbon dioxide and the thermal storage medium is water, the
charging cycle may operate in the temperature range of 5.degree. C.
and 120.degree. C. The intercooling occurs at a temperature levels
well distributed within this range.
[0060] Summarized, according to an exemplary embodiment, the system
10 includes a first expander 20 configured to, in the charging
cycle, expand the working fluid after the first heat exchanger 34
to the intermediate pressure. In the charging cycle, a first
portion of the working fluid at the intermediate pressure is input
directly into the flash intercooler 28.
[0061] According to an exemplary embodiment, the system 10 includes
a second expander 22 configured to, in the charging cycle, expand a
second portion of the working fluid at the intermediate pressure to
the low pressure.
[0062] According to an exemplary embodiment, the system 10 includes
a third heat exchanger 38 configured to, in the charging cycle,
transfer heat from a third thermal storage 40 to the working fluid
at low pressure and, in the discharging cycle, transfer heat from
the working fluid at low pressure to the third thermal storage.
[0063] According to an exemplary embodiment, the intercooler
includes a flash intercooler 28 and a third heat exchanger 30,
wherein, in the charging cycle, the working fluid between the flash
intercooler and the third heat exchanger is compressed from a first
intermediate pressure to a second intermediate pressure by a
further compressor 16.
[0064] With respect to FIG. 1b, the heat stored in the heat
storages 40, 46 and 52 is subsequently utilized in the discharging
cycle system 56 shown in FIG. 1b.
[0065] The working fluid in the discharging cycle system 58 coming
from the heat exchanger 38 is pumped from the low pressure to the
high pressure by pump 60. After that the working fluid is heated in
the heat exchanger 34 and enters a first turbine 62 for converting
the heat into mechanical and subsequently into electrical energy.
The working fluid is reheated again in heat exchanger 30 and enters
a second turbine 64 for generating further electrical energy. In
the first turbine 62 the working fluid is expanded from the high
pressure to the intermediate pressure and in the second turbine 64
to the low pressure. After that the working fluid is cooled in the
heat exchanger 38.
[0066] According to an exemplary embodiment, the system 10 includes
a first turbine 62 configured to, in the discharging cycle, expand
the working fluid from the high pressure to the intermediate
pressure for generating electrical energy and/or a second turbine
64 configured to, in the discharging cycle, expand the working
fluid from the intermediate pressure to the low pressure for
generating electrical energy.
[0067] According to an exemplary embodiment, the system 10 includes
a pump 60 configured to, in the discharging cycle, pump the working
fluid from the low pressure to the high pressure during the
discharging cycle.
[0068] FIGS. 2a and 3a show a charging cycle 12a, 12b, and FIGS. 2b
and 3b show a discharging cycle 58a, 58b of a transcritical
thermoelectric energy storage system 10. The cycles are depicted in
pressure-enthalpy diagrams.
[0069] In each of the diagrams, a vapor dome 66 is indicated. The
critical point 68 of the working fluid is shown on top of the vapor
dome. Left of the vapor dome 66, the working fluid is in liquid
phase, and right of the vapor dome 66, the working fluid is in gas
phase (wet steam phase). Under the vapor dome 66, the working fluid
is in a mixed liquid and gas phase. A phase change of the working
fluid only occurs, when a state change passes the limiting line of
the vapor dome 66. Thus, stated change over the vapor dome 66 and
over the critical point 68 do not contain phase changes and may be
called transcritical. As may be seen from the diagrams, nearly all
state changes of the working fluid in the charging cycle and the
discharging cycle are transcritical, and therefore the charging
cycle and the discharging cycle are referred to as
transcritical.
[0070] FIG. 2a illustrates the charging cycle 12 of a storage
system 10 which may include two heat exchangers 30 for intercooling
the working fluid. The charging cycle 12a follows a
counter-clockwise direction as indicated by the arrows. The
charging cycle 12a starts at point A where the working fluid is
first evaporated at a low pressure 70 by utilizing, for example a
low grade heat source such as ambient air or by a heat exchanger
38. This transition is indicated in FIG. 2a with the line from
point A to point B1.
[0071] In the next section of the charging cycle 12a, the resultant
vapor is compressed utilizing electrical energy in three stages
from point B1 to C1 to a first intermediate pressure 72, from B2 to
C2 to a second intermediate pressure 74, and from B3 to C3 to a
high pressure 76. Such compression occurring in three stages is a
consequence of the thermoelectric energy storage 10 having a
compressor train comprising three individual units, for example the
compressors 14, 16, 18. In between each of these compression stages
the working fluid is cooled from point C1 to B2 and point C2 to B3.
For example, the working fluid may be cooled by two heat exchangers
30.
[0072] The hot compressed working fluid exiting the compression
train at point C3 is cooled down at constant pressure 76 to point
D, for example in a heat exchanger 34. Since the cycle 12a is
supercritical between the points C3 and D, no condensation of the
working fluid takes place. The heat rejected between point C1 to
B2, C2 to B3, and C3 and D is transferred to a thermal storage
medium via heat exchangers 30, 34, thereby storing the heat energy.
After reaching point D, the cooled working fluid is returned to its
initial low pressure state 70 at point A via a thermostatic
expansion valve 22 or alternatively with an energy recovering
expander.
[0073] FIG. 2b illustrates the discharging cycle of a
thermoelectric energy storage system 10 with one turbine 62 that
follows a clockwise direction as indicated by the arrows. The
discharging cycle 58a starts with the compression of the working
fluid as it is pumped from point E to point F from low pressure 70
to high pressure 76, for example by pump 60. From point F to point
G, the working fluid is in contact with the thermal storage medium
in a direct or indirect manner, wherein stored heat is transferred
from the thermal storage medium to the working fluid. For example,
this may be done with a heat exchanger 34. The working fluid is in
a supercritical state between point F and point G, hence no
evaporation takes place.
[0074] The subsequent expansion of the working fluid in a turbine
62 from pressure 76 to pressure 70 in order to generate electricity
is represented between point G and point H. Finally, the working
fluid is condensed to its initial state by exchanging heat, for
example with a cooling medium such as ambient air or with a cold
storage 40 via a heat exchanger 38. This is represented from point
H to point E on FIG. 2b.
[0075] When both thermodynamic cycles 12a, 58a shown in FIGS. 2a
and 2b would use the same working fluid, it is noted that the total
heat energy generated in the charging cycle 12a is greater than the
heat energy requirement of the discharging cycle 58a. Specifically,
the total heat energy required for functioning of the discharging
cycle 58a, which is equal to the enthalpy difference from point F
to point G in FIG. 1b, can be provided solely by the heat energy
released during the charging cycle between point C3 and point D in
FIG. 1a.
[0076] Therefore, it would be beneficial to efficiently utilize the
excess heat resulting from compressor intercooling. However, this
excess heat cannot be used to increase the enthalpy content at
point G (which may be envisaged as pushing point G further to the
right in the cycle in FIG. 1b), because the temperature at which
this excess heat is available is lower than the temperature of
point G. Thus, according to an exemplary embodiment of the present
disclosure, a storage system 10 with a charging cycle 12a includes
a discharging cycle, wherein the heat stored during intercooling is
used for reheating the working fluid between the expansions in
turbines 62, 64.
[0077] Also, the excess heat generated by intercooling may not be
used to increase the power output of the discharging cycle 58a
through increasing the working fluid flow. Thus, according to an
exemplary embodiment of the present disclosure, a storage system 10
with a discharging cycle 58a includes a discharging cycle, wherein
a flash intercooler 28 is used for cooling the working fluid
between two compression stages.
[0078] FIG. 3a and FIG. 3b depict a charging cycle 12b and a
discharging cycle 58b, respectively, on a pressure-enthalpy
diagram, which may be performed by an exemplary embodiment of the
transcritical thermoelectric energy storage system 10 shown in
FIGS. 1a and 1b.
[0079] Referring first to FIG. 3a, the charging cycle 12b follows a
counter-clockwise direction as indicated by the arrows. The
charging cycle 12b starts with the expansion of the working fluid
which occurs in two stages, between point D and point A1 from
pressure 76 to pressure 72 (expander 20), and between point A1 and
point A2 from pressure 72 to pressure 70 (expansion valve 22). The
working fluid stream is divided at point A1 (stream splitter 26),
where a first portion is diverted to point B2 and the remaining
portion is expanded further to point A2 (expansion valve 22).
[0080] There is an increase in enthalpy in the remaining portion as
it reaches point B1 and in the first compression stage between B1
and C1 from pressure 70 to pressure 72 (compressor 14) and there is
an increase in both pressure and enthalpy. The discharge of this
first compression stage is cooled by intercooling (intercooler 28).
Specifically, point B2 represents the flash intercooler 28 where
the hot working fluid from point C1 is mixed with the expanded
working fluid from point A1.
[0081] The discharge from the second compression stage, between
point B2 and point C2 from pressure 72 to pressure 74, is directed
to a heat exchanger 30 where the thermal energy of the working
fluid is delivered to a thermal energy storage 46 between points C2
and B3.
[0082] The third compression stage from pressure 74 to pressure 76
occurs between points B3 and C3 (compressor 18).
[0083] Such compression occurring in three stages is a consequence
of the thermoelectric energy storage 10 having a compressor train
comprising three individual units 14, 16, 18. In between each of
these compression stages the working fluid is cooled from point C1
to B2 and point C2 to B3 at constant pressure.
[0084] Similarly, the hot compressed working fluid exiting the
compression train at point C3 is cooled down at constant pressure
76 to point D (heat exchanger 34). Since the cycle 12b is
supercritical between the points C3 and D, no condensation of the
working fluid takes place. The rejected heat energy between points
C3 and D is stored in a thermal storage medium (hot storage 52).
After reaching point D, the cooled working fluid is returned to its
initial low pressure state 70 at point A1 via a work recovering
expander 20/thermostatic expansion valve 22.
[0085] The flash intercooler 28 utilized in the charging cycle 12b
may be a direct-contact heat exchanger, where the liquid working
fluid from point A1 to be evaporated is injected or sprayed into
the compressed working fluid vapour flow at C1. Such a
direct-contact heat exchanger includes a shell filled with a
packing of a high specific surface area in order to increase the
wetted heat transfer area.
[0086] FIG. 3b illustrates the discharging cycle 58b of the
thermoelectric energy storage system 10 that follows a clockwise
direction as indicated by the arrows. The discharging cycle starts
with the compression (pump 60) of the working fluid from low
pressure 70 to high pressure 76 and this transition is indicated in
FIG. 3b with the line from point E to point F.
[0087] From point F to point G1, the working fluid is in contact
with the thermal storage medium in a direct or indirect manner,
wherein stored heat is transferred from the thermal storage medium
to the working fluid at constant pressure (heat exchanger 34). The
working fluid is in a supercritical state between point F and point
G1, hence no evaporation takes place.
[0088] The subsequent expansion of the working fluid in a turbine
62 in order to generate electricity is represented between point G1
and point H1. Between points H1 and G2 there is a reheat stage at
pressure 74, where the reheat energy is provided from the thermal
storage 46. Specifically, the thermal storage 46 is coupled to the
heat exchanger 30 corresponding to the second intercooling stage in
the charging cycle 12b.
[0089] The second expansion of the working fluid from G2 to H2 from
pressure 74 to pressure 70 occurs in a second turbine stage
(turbine 64). Finally, the working fluid is condensed to its
initial state at constant pressure by exchanging heat with a
cooling medium such as ambient air or with a heat exchanger 38.
This is represented from point H2 to point E on FIG. 3b.
[0090] It should be noted that, in accordance with an exemplary
embodiment in which reheat options are not utilized in the
discharging cycle, then every compressor stage in the charging
cycle can be equipped with a separate flash intercooler.
[0091] In accordance with an exemplary embodiment, different
working fluids may be utilized in the charging and discharging
cycles. However, the temperature levels for the charging cycle, the
heat storage and the discharging cycle must be adjusted to ensure
transfer of heat in the desired direction.
[0092] In accordance with an exemplary embodiment, water is used as
the working fluid in the charging cycle. Furthermore, another fluid
with a high boiling point may be utilized instead of water. In this
embodiment, the intercooling heat load is at a suitably high
temperature to be stored and used to drive a secondary discharging
cycle having a low boiling point working fluid (such as
hydrocarbon). In this embodiment, thermal energy stored during
intercooling can be efficiently recovered without utilizing a flash
intercooler.
[0093] The skilled person will be aware that the condenser and the
evaporator in the thermoelectric energy storage system may be
replaced with a multi-purpose heat exchange device that can assume
both roles, since the use of the evaporator in the charging cycle
and the use of the condenser in the discharging 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.
[0094] Further the temperatures, the pressures and the amount of
working fluid exiting the stream splitter 26 may be measured and
these values may be controlled by valves situated in the working
fluid circuit.
[0095] While the present disclosure has been illustrated and
described in detail in the drawings and foregoing description, such
illustration and description are to be considered illustrative or
exemplary and not restrictive. The present disclosure is not
limited to the disclosed embodiments. Other variations to the
disclosed embodiments can be understood and effected by those
skilled in the art and practicing the claimed disclosure, from a
study of the drawings, the present disclosure, and the appended
claims. In the claims, the word "comprising" or "including" does
not exclude other elements or steps, and the indefinite article "a"
or "an" does not exclude a plurality. A single processor or
controller or other unit may fulfill the functions of several items
recited in the claims. The mere fact that certain measures are
recited in mutually different dependent claims does not indicate
that a combination of these measures cannot be used to advantage.
Any reference signs in the claims should not be construed as
limiting the scope.
[0096] 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.
* * * * *