U.S. patent application number 13/720073 was filed with the patent office on 2014-03-06 for thermoelectric energy storage system.
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, Mehmet Mercangoez, Christian OHLER.
Application Number | 20140060051 13/720073 |
Document ID | / |
Family ID | 44627823 |
Filed Date | 2014-03-06 |
United States Patent
Application |
20140060051 |
Kind Code |
A1 |
OHLER; Christian ; et
al. |
March 6, 2014 |
THERMOELECTRIC ENERGY STORAGE SYSTEM
Abstract
A thermoelectric energy storage system and method are provided
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 includes a
working fluid circuit configured to circulate a working fluid
through a heat exchanger, and a thermal storage conduit configured
to transfer a thermal storage medium from a thermal storage tank
through the heat exchanger. The working fluid includes a zeotropic
mixture. The working fluid is in a mixed vapor and liquid phase and
has continuously rising or continuously falling temperature during
heat transfer due to the working fluid including the zeotropic
mixture.
Inventors: |
OHLER; Christian; (Baden,
CH) ; Hemrle; Jaroslav; (Baden-Dattwil, CH) ;
Mercangoez; Mehmet; (Stein, CH) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ABB RESEARCH LTD; |
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|
US |
|
|
Assignee: |
ABB RESEARCH LTD
Zurich
CH
|
Family ID: |
44627823 |
Appl. No.: |
13/720073 |
Filed: |
December 19, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
PCT/EP2011/060323 |
Jun 21, 2011 |
|
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13720073 |
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Current U.S.
Class: |
60/652 ;
60/659 |
Current CPC
Class: |
F01K 25/06 20130101;
F01K 3/12 20130101 |
Class at
Publication: |
60/652 ;
60/659 |
International
Class: |
F01K 3/12 20060101
F01K003/12; F01K 25/06 20060101 F01K025/06 |
Foreign Application Data
Date |
Code |
Application Number |
Jun 23, 2010 |
EP |
10167030.5 |
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 through a heat
exchanger; and a thermal storage conduit configured to transfer a
thermal storage medium from a thermal storage tank through the heat
exchanger, wherein the working fluid includes a zeotropic
mixture.
2. The thermoelectric energy storage system of claim 1, wherein the
zeotropic mixture is selected such that the temperature of the
working fluid in the heat exchanger changes from a first
temperature to a second temperature.
3. The thermoelectric energy storage system of claim 1, wherein the
heat exchanger includes a counter flow heat exchanger.
4. The thermoelectric energy storage system of claim 1, wherein the
flow of the working fluid through the heat exchanger is controlled
such that a temperature difference between the working fluid and
the thermal storage medium is less than 50.degree. C.
5. The thermoelectric energy storage system of claim 1, comprising:
a valve configured to control the flow in the working fluid
circuit.
6. The thermoelectric energy storage system of claim 1, wherein the
thermal storage tank includes a hot storage tank, and the thermal
storage conduit includes a hot storage conduit configured to
transfer a hot storage medium between a first hot storage tank and
a second hot storage tank through the heat exchanger.
7. The thermoelectric energy storage system of claim 1, wherein:
the storage tank includes an intermediate storage tank; and the
heat exchanger includes a stream splitter configured to divide the
flow of the thermal storage medium from a first storage tank into a
flow to the intermediate storage tank and to a second storage
tank.
8. The thermoelectric energy storage system of claim 1, comprising:
a compressor configured to compress the working fluid in vapor
phase from a lower pressure to a higher pressure during the
charging cycle, such that electrical energy for driving the
compressor is injected into the thermal energy storage system.
9. The thermoelectric energy storage system of claim 1, comprising:
an expansion device configured to expand the working fluid in
liquid phase from a higher pressure to a lower pressure during the
charging cycle.
10. The thermoelectric energy storage system of claim 1,
comprising: a pump configured to pump the working fluid from a
lower pressure to a higher pressure during the discharging
cycle.
11. The thermoelectric energy storage system of claim 1,
comprising: a turbine configured to expand the working fluid from a
higher pressure to a lower pressure level for generating electrical
energy during the discharging cycle.
12. A method for storing and retrieving electrical energy, the
method comprising: storing electrical energy by transferring
thermal energy to a thermal storage in a charging cycle; and
retrieving electrical energy by changing the thermal energy from
the thermal storage into mechanical energy in a discharging cycle,
wherein the charging cycle and the discharging cycle comprise
transferring heat between a working fluid and a thermal storage
medium, and wherein the working fluid is in a mixed vapor and
liquid phase and has continuously rising or continuously falling
temperature during heat transfer due to the working fluid including
a zeotropic mixture.
13. The method of claim 12, comprising: controlling the flow
through the heat exchanger such that a temperature difference
between the working fluid and the thermal storage medium is less
than at least one of 50.degree. C., 10.degree. C. and 3.degree.
C.
14. The method of claim 12, comprising: dividing the flow of the
thermal storage medium in the heat exchanger such that heat
transfer in the heat exchanger has two different rates.
15. The thermoelectric energy storage system of claim 2, wherein
the zeotropic mixture is selected such that a temperature of the
working fluid in the heat exchanger continuously changes from a
first temperature to a second temperature.
16. The thermoelectric energy storage system of claim 4, wherein
the flow of the working fluid through the heat exchanger is
controlled such that the temperature difference between the working
fluid and the thermal storage medium is less than 10.degree. C.
17. The thermoelectric energy storage system of claim 4, wherein
the flow of the working fluid through the heat exchanger is
controlled such that the temperature difference between the working
fluid and the thermal storage medium is less than 3.degree. C.
18. The thermoelectric energy storage system of claim 4,
comprising: a valve configured to control the flow in the working
fluid circuit.
19. The thermoelectric energy storage system of claim 1, wherein
the thermal storage tank includes a cold storage tank, and the
thermal storage conduit includes a cold storage conduit configured
to transfer a cold storage medium between a first cold storage tank
and a second cold storage tank through the heat exchanger.
20. The thermoelectric energy storage system of claim 6, wherein
the thermal storage tank includes a cold storage tank, and the
thermal storage conduit includes a cold storage conduit configured
to transfer a cold storage medium between a first cold storage tank
and a second cold storage tank through the heat exchanger.
21. The thermoelectric energy storage system of claim 20, wherein:
the storage tank includes an intermediate storage tank; and the
heat exchanger includes a stream splitter configured to divide the
flow of the thermal storage medium from a first storage tank into a
flow to the intermediate storage tank and to a second storage
tank.
22. The thermoelectric energy storage system of claim 20,
comprising: a compressor configured to compress the working fluid
in vapor phase from a lower pressure to a higher pressure during
the charging cycle, such that electrical energy for driving the
compressor is injected into the thermal energy storage system.
23. The thermoelectric energy storage system of claim 20,
comprising: an expansion device configured to expand the working
fluid in liquid phase from a higher pressure to a lower pressure
during the charging cycle.
24. The thermoelectric energy storage system of claim 20,
comprising: a pump configured to pump the working fluid from a
lower pressure to a higher pressure during the discharging
cycle.
25. The thermoelectric energy storage system of claim 20,
comprising: a turbine configured to expand the working fluid from a
higher pressure to a lower pressure level for generating electrical
energy during the discharging cycle.
26. The method of claim 12, comprising: joining the flow of the
thermal storage medium in the heat exchanger such that heat
transfer in the heat exchanger has two different rates.
27. The method of claim 13, comprising: dividing the flow of the
thermal storage medium in the heat exchanger such that heat
transfer in the heat exchanger has two different rates.
28. The method of claim 13, comprising: joining the flow of the
thermal storage medium in the heat exchanger such that heat
transfer in the heat exchanger has two different rates.
Description
RELATED APPLICATIONS
[0001] This application claims priority as a continuation
application under 35 U.S.C. .sctn.120 to PCT/EP2011/060323, which
was filed as an International Application on Jun. 21, 2011
designating the U.S., and which claims priority to European
Application 10167030.5 filed in Europe on Jun. 23, 2010. The entire
contents of these applications are hereby incorporated by reference
in their entireties.
FIELD
[0002] The present disclosure relates to the storage of electric
energy. More particularly, the present disclosure relates to a
thermoelectric energy storage system, a method for storing and
retrieving electrical energy with a thermoelectric energy storage
system, and usage of a zeotropic mixture as a working fluid.
BACKGROUND INFORMATION
[0003] With thermoelectric storage systems, the useful concept of
storing electrical energy is implemented by converting electric
energy into thermal energy that may be stored for a required time
(charging of the storage). The electric energy may be restored by
reversed conversion from thermal energy into mechanical work and
subsequently into electricity (discharging of the storage).
[0004] The storage of electrical energy may become more and more
important in the future. Base load generators such as nuclear power
plants and generators with stochastic, intermittent energy sources,
such as wind turbines and solar panels, may generate excess
electrical power during times of low power demand. Large-scale
electrical energy storage systems may be a means of diverting this
excess energy to times of peak demand and balance the overall
electricity generation and consumption.
[0005] There may be several possibilities of storing heat. 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.
[0006] One of the most important characteristics of a
thermoelectric storage system may be the round-trip efficiency. The
round-trip efficiency of a thermoelectric 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. The round-trip efficiency may be
increased when thermodynamic reversibility factors are maximized.
However, it may be important that all electric energy storage
technologies inherently have a limited round-trip efficiency due to
thermodynamic limitations. Thus, for every unit of electrical
energy used to charge the storage, only a certain percentage may be
recovered as electrical energy upon discharge. The rest of the
electrical energy is lost. If, for example, the heat being stored
in a thermoelectric storage system is provided through resistor
heaters, it has approximately 40% round-trip efficiency.
[0007] The charging cycle of a thermoelectric storage system may be
referred to as a heat pump cycle, and the discharging cycle of a
thermoelectric storage system may be referred to as a heat engine
cycle. In a thermoelectric storage system, heat may have be
transferred from a hot working fluid to a thermal storage medium
during the heat pump cycle and back from the thermal storage medium
to the working fluid during the heat engine cycle. A heat pump may
require work to move thermal energy from a cold source to a warmer
heat sink. Since the amount of energy deposited at the hot side may
be greater than the work required by an amount equal to the energy
taken from the cold side, a heat pump may multiply the heat as
compared to resistive heat generation. The ratio of heat output to
work input may be called a coefficient of performance (COP) with a
value larger than one. In this way, the use of a heat pump will
increase the round-trip efficiency of a thermoelectric storage
system.
[0008] The efficiency of a thermoelectric storage system is limited
for various reasons rooted in the second law of thermodynamics.
Firstly, the conversion of heat to mechanical work in a heat engine
is limited due to the Carnot efficiency. Secondly, the coefficient
of performance of any heat pump declines with increased difference
between input and output temperature levels. 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.
[0009] Usually, a thermoelectric storage system has a working fluid
circuit with which heat is transferred with a working fluid to a
thermal storage medium (or vice versa) via one or more heat
exchangers. The transfer of heat over large temperature differences
is a thermodynamic irreversibility factor. This means that the
larger the temperature differences between the working fluid and
the thermal storage medium in the heat exchangers are, the lower
the round-trip efficiency will be. In order to minimize the maximal
temperature difference, relatively large heat exchangers may be
constructed or phase change materials can be used for thermal
storage. However, these solutions may result in high costs and may
generally not be practical.
[0010] A reduction of heat transfer losses may be of particular
importance in the considered application of thermal energy storage
with charging based on a heat pump cycle. In such an application,
any increase of heat exchange temperature losses during charging
and discharging may directly translate into a loss of useful work
and reduction of the round trip efficiency of the system.
[0011] For example, one solution to overcome at least some of the
above mentioned problems may be the transcritical thermoelectric
storage system introduced by the applicant. When a heat pump system
runs transcritical, the working fluid on the high pressure side of
the system does not change its phase from vapor to liquid while
passing through the heat rejecting heat exchanger. Therefore, in a
transcritical cycle, the heat rejecting heat exchanger may operate
like a gas cooler, rather than as an isothermal condenser. This may
enable the storage of the rejected heat by means of sensible heat
storage (heat storage based on a temperature change rather than on
a phase change) through a known fluid-to-fluid heat exchanger. This
may be advantageous since the technology for fluid-to-fluid heat
exchangers is advanced and enables very small approach temperatures
in compact volumes resulting in high efficiencies at reduced
costs.
[0012] However, if the cooling effect of the heat pump operation on
the low pressure side is also desired to be utilized in parallel to
create a cool thermal storage, the transcritical cycle usually has
a known isothermal evaporator and therefore may have to be used to
charge a PCM (phase change material) thermal storage such as ice at
constant temperature. Ice is an excellent thermal storage medium
but ice storage systems have to use heat exchangers which have to
grow ice on the heat transfer surfaces (low heat transfer
efficiency) or have to limit ice formation per heat exchanger pass
(large flow rates) to prevent clogging.
[0013] Another disadvantage of known ice storage systems may be
that these systems usually cannot exceed an ice content of 50%,
which means that half of the thermal storage is unused, increasing
both the capital cost of the system and also its footprint.
[0014] Another solution to thermoelectric storage system design
without isothermal evaporation or condensation is to use the
reverse Brayton cycle for charging and the known Brayton cycle for
discharging of the thermal storage. The working fluid of the
Brayton cycle is always in the gas phase and therefore all heat
transfer steps of a "Brayton cycle thermoelectric storage system"
can be matched with heat transfer to a sensible heat thermal
storage. The downside is that due to its high back-work ratio, a
Brayton cycle thermoelectric storage system may suffer from
increased losses in the heat pump expansion and heat engine
compression steps compared to other thermoelectric storage system
designs. These losses can be counteracted by pushing the operating
temperatures of the cold side and hot side of the cycles
respectively to very low and very high values, which in turn may
make it necessary to store the sensible heat to solid materials
such as rocks or sand via special purpose contraptions eventually
losing the potential benefit of sensible heat storage through a
known fluid-to-fluid heat exchanger.
[0015] It is known that zeotropic refrigerant mixtures may increase
the energy efficiency of certain refrigeration and heat pump
equipment under optimized conditions.
[0016] For heat engine operation, the Kalina cycle was proposed to
be used in power stations. Because temperature differences across a
heat exchanger may be more uniform with the Kalina cycle compared
to classical pure working fluid Rankine cycle, the system
efficiency increases around 10% for a normal power station, but for
special low-temperature applications by more than 30%.
[0017] However, in thermoelectric energy storage systems, the heat
pump cycle and the heat engine cycle have to be optimized with
respect to each other. Thus, it may be problematic to apply the
optimization principles of a refrigerator system or a heat engine
system to a thermoelectric storage system, because the optimization
of the one cycle may degrade the efficiency of the other cycle.
[0018] A major hurdle in achieving high efficiencies in
thermoelectric storage system operation may be large temperature
differences between the hot side and cold side in heat
exchangers.
[0019] Minimizing temperature differences in heat exchangers may
become especially challenging when latent heat storage systems are
used and the heat transfer involves conduction through the solid
phase of the storage material which might be the case with the
above mentioned transcritical thermoelectric storage system.
However, such phase change material based thermal storage systems
are an optimal way to match temperature profiles of thermoelectric
storage system thermodynamic cycles involving isothermal
evaporation or condensation steps.
[0020] In view of the above, exemplary embodiments of the present
disclosure provide a thermoelectric storage system which does not
have to rely on isothermal evaporation or isothermal condensation
of the working fluid.
SUMMARY
[0021] 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
thermoelectric energy storage includes a working fluid circuit
configured to circulate a working fluid through a heat exchanger,
and a thermal storage conduit configured to transfer a thermal
storage medium from a thermal storage tank through the heat
exchanger. The working fluid includes a zeotropic mixture.
[0022] An exemplary embodiment of the present disclosure provides a
method for storing and retrieving electrical energy. The exemplary
method includes storing electrical energy by transferring thermal
energy to a thermal storage in a charging cycle, and retrieving
electrical energy by changing the thermal energy from the thermal
storage into mechanical energy in a discharging cycle. The charging
cycle and the discharging cycle include transferring heat between a
working fluid and a thermal storage medium. The working fluid is in
a mixed vapor and liquid phase and has continuously rising or
continuously falling temperature during heat transfer due to the
working fluid including a zeotropic mixture.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] 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:
[0024] FIG. 1a shows a simplified schematic diagram of a charging
cycle of a thermoelectric storage system according to an exemplary
embodiment of the present disclosure;
[0025] FIG. 1b shows a simplified schematic diagram of a
discharging cycle of a thermoelectric storage system according to
an exemplary embodiment of the present disclosure;
[0026] FIG. 2 shows a T-S-diagram of a thermoelectric storage
system according to an exemplary embodiment of the present
disclosure;
[0027] FIG. 3a shows a simplified diagram of a heat storage for a
thermoelectric storage system according to an exemplary embodiment
of the present disclosure during charging;
[0028] FIG. 3b shows a simplified diagram of the heat storage of
FIG. 3a during discharging, according to an exemplary embodiment of
the present disclosure; and
[0029] FIG. 4 shows a heat flow-temperature diagram of the heat
transfer in a heat exchange according to FIG. 3a or FIG. 4a in
accordance with an exemplary embodiment of the present
disclosure.
DETAILED DESCRIPTION
[0030] Exemplary embodiments of the present disclosure provide an
efficient thermoelectric energy storage having a high round-trip
efficiency and a minimal approach temperature, while minimizing the
amount of required thermal storage medium, and also minimizing the
cost.
[0031] 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.
[0032] 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
through a heat exchanger, and a thermal storage conduit configured
to transfer a thermal storage medium from a thermal storage tank
through the heat exchanger. The working fluid includes a zeotropic
mixture.
[0033] In other words, a zeotropic mixture is used as a working
fluid for the thermoelectric energy storage system. The zeotropic
mixture may be selected such that the temperature of the working
fluid in the heat exchanger is continuously changing (e.g., rising
or falling) from a first temperature to a second temperature.
[0034] For a zeotropic mixture the concentrations of the liquid
phase and the vapour phase are never equal at different
temperatures. This creates a temperature glide during phase change
(at which point the concentrations of the vapour and the liquid are
continually changing). Thus in a T-S-diagram equal pressure lines
are increasing and thus, the temperature difference between the
working fluid and the storage fluid in a heat exchanger can be made
very small. For example, the system may be controlled such that the
temperature of the working fluid in the heat exchanger is rising
like the temperature of the storage fluid in the heat
exchanger.
[0035] The advantage of increasing constant pressure lines in the
T-S-diagram may be better understood with respect to the Lorenz
cycle that also has two such processes with increasing slope.
Similar to the Carnot cycle that optimizes heat engines operating
between two constant-temperature sources, the Lorenz cycle
optimizes heat engines operating between two gliding-temperature
sources by adjusting the thermal capacity of the working fluid to
that of the finite-capacity sources. That is, the Lorenz cycle has
four processes like the Carnot cycle that for a heat engine are:
isentropic compression, heating at constant thermal capacity
matching that of the heat source (and its temperature variation),
isentropic expansion, and cooling at constant thermal capacity
matching that of the heat sink (and its temperature variation).
Similar inverse cycles apply for refrigeration and heat pumping.
However, for thermodynamic cycles operating at two constant
pressure levels with a change of phase, realization of a Lorenz
cycle with a pure working fluid is not possible.
[0036] From another perspective, an exemplary embodiment of the
present disclosure combines the sensible heat storage possibility
of a Brayton cycle thermoelectric storage system with the low
back-work ratio of a Rankine cycle thermoelectric storage system in
one thermodynamic machine.
[0037] If the working fluid is a zeotropic mixture, a constant
pressure phase change takes place with a variation of temperature,
with the temperature variation between the saturated vapor and the
saturated liquid conditions being a function of the composition of
the mixture and the components of the mixture.
[0038] In other words, the zeotropic mixture may have the advantage
that the temperature differences between the working fluid and the
thermal storage medium may be made very small. For example, the
flow of the working fluid through the heat exchanger may be
controlled (for examples through a valve before or after the heat
exchanger) such that the temperature difference at any point of
contact between the thermal storage medium and the working fluid in
the heat exchanger, and during both charging and discharging
cycles, is less than 50.degree. C. For example, this temperature
difference is below 10.degree. C., or even below 3.degree. C.
[0039] According to an exemplary embodiment of the present
disclosure, a reversible thermodynamic machine is used to store
electrical energy by utilizing an electrically driven vapor
compression heat pump cycle with a zeotropic mixture as the working
fluid to provide heat into a hot thermal storage while removing
heat from a cold thermal storage, where the heat provided to the
hot thermal storage is used to increase the temperature of the hot
thermal storage and the heat removed from the cold thermal storage
is used to reduce the temperature of the cold thermal storage.
[0040] According to an exemplary embodiment of the present
disclosure, the hot and cold thermal storage materials are fluids
such as liquids, which are pumped from their initial temperature
states into heat exchangers, for example for counter current heat
exchange, with the zeotropic working fluid mixture undergoing
condensation during heat provision to the hot storage fluid and
evaporation during heat removal from the cold storage fluid. For
retrieval of the electrical energy back from the hot and cold
thermal storage the thermodynamic machine may be operated in
reverse and heat is removed from the hot thermal storage and added
to the cold thermal storage.
[0041] According to an exemplary embodiment of the present
disclosure, the temperature profiles of both the hot and cold
thermal storage fluids are matched with the zeotropic working fluid
mixture. The matching may be done very closely by utilizing very
efficient counter flow heat exchangers hence reducing the
irreversibility and increasing the roundtrip efficiency of
electrical energy storage.
[0042] Carbon dioxide is by far the most benign working fluid
available today. Other candidates to add to carbon dioxide to
achieve the desired behavior described in this disclosure
disclosure are hydrocarbons. According to an exemplary embodiment
of the present disclosure, a zeotropic mixture may include carbon
dioxide and hydrocarbons, for example, 50% carbon dioxide and 50%
butane.
[0043] The constituent components of the working fluid mixture may
be chosen to realize different operating conditions but in general
the thermodynamic behavior of the two or more components may have
to be close enough to ensure proper solubility and to avoid
unwanted separation into multiple liquid phases while being
different enough to ensure a significant glide during the
evaporation and condensation phases. The higher the temperature
difference between the bubble and dew points of the mixture the
more significant the temperature glide and therefore the lower the
required flow of the thermal storage fluids. The composition of the
working fluid mixture may also impact on the thermodynamic
behavior. When two components are used together as a working fluid
mixture the glide is largest when equal mass fractions are used. As
the fraction of one of the components is increased the behavior of
the mixture will start approaching the behavior of the component
with the larger mass fraction eventually becoming a single
component working fluid when the fraction is increased to 1. These
guidelines are useful when the components of the working fluid
mixture are forming a near ideal mixture. The behavior of mixtures
may be unpredictable when they show non-ideal behavior but it may
be possible to realize a very efficient thermoelectric storage
system also with non-ideal mixtures.
[0044] According to an exemplary embodiment of the present
disclosure, the charging cycle and/or the discharging cycle may be
executed transcritically. This may mean that the heat exchanging
step at the hot storage side in the T-S-diagram may be above the
critical point dome of the phase envelope leading to a
transcritical cycle. This possibility may be used if higher
temperatures are desired at the hot side or if the glide on the hot
side is desired to be increased further to minimize the mass of
required storage fluid.
[0045] A large temperature range may be covered by thermal storage
materials in liquid phase. According to an exemplary embodiment of
the present disclosure, ammonia and water mixtures may be used as
the cool side thermal storage fluids, which, for example, may go
from as low as -100.degree. C. and can go as high as +50.degree. C.
Other possibilities for hot and cold storage media are water at
atmospheric pressure without additives, which can cover from 0 to
100.degree. C., thermal oils (e.g. Dowtherm J), which can cover a
very wide range -80 to 315.degree. C., and molten salt mixtures,
which can go as high as 566.degree. C.
[0046] According to an exemplary embodiment of the present
disclosure, the storage tank includes an intermediate storage tank,
wherein the heat exchanger has a stream splitter configured to
divide (or join) the flow of the thermal storage medium from a
first (or second) storage tank into a flow to the intermediate
storage tank and to a second (or first) storage tank. In this way,
even in the case when the temperature of the working fluid is not
continuously rising or falling in the heat exchanger, the
temperature difference, at any point of contact between the thermal
storage medium and the working fluid in the heat exchanger, and
during both charging and discharging cycles, may be adjusted to be
very small, for example below 3.degree. C.
[0047] An exemplary embodiment of the present disclosure provides a
method for storing and retrieving electrical energy.
[0048] According to an exemplary embodiment of the present
disclosure, the method includes the steps of storing electrical
energy by transferring thermal energy to a thermal storage in a
charging cycle, and retrieving electrical energy by changing the
thermal energy from the thermal storage into mechanical energy in a
discharging cycle.
[0049] According to an exemplary embodiment of the present
disclosure, the charging cycle and the discharging cycle include
the steps of transferring heat between a working fluid and a
thermal storage medium, wherein the working fluid is in a mixed
vapor and liquid phase and has continuously rising or continuously
falling temperature during heat transfer, because the working fluid
includes a zeotropic mixture.
[0050] It is to be understood that features of the method as
described in the above and in the following may be features of the
system as described in the above and in the following.
[0051] If technically possible but not explicitly mentioned,
combinations of exemplary embodiments of the present disclosure
described above and in the following may be embodiments of the
method and the system.
[0052] These and other aspects of the present disclosure will be
apparent from and elucidated with reference to the exemplary
embodiments described hereinafter.
[0053] In principle, identical parts are provided with the same
reference symbols in the drawings.
[0054] FIGS. 1a and 2a show exemplary embodiments of a
thermoelectric storage system 10. FIG. 1a shows the charging cycle
system or heat pump cycle system 12, and FIG. 1b the discharging
cycle system or heat engine cycle system 14.
[0055] The thermoelectric storage system 10 is adapted for
performing a charging cycle with the system 12, during which
electrical energy to be stored is converted into heat, and for
performing a discharging cycle with the system 14, during which the
thermal energy is retrieved from the storage and converted back
into electricity. Furthermore the charging cycle can be followed by
a storage period during which neither the charging nor the
discharging cycle has to take place.
[0056] With respect to FIG. 1a, the thermoelectric charging cycle
system 10 includes a working fluid circuit 16, which includes a
zeotropic mixture as working fluid. The working fluid is compressed
to a higher pressure by a compressor 18 and injected into a heat
exchanger 20 for exchanging heat with a hot storage 22. The heat
exchanger 20 is a counter flow heat exchanger in which a hot
storage medium (e.g., a liquid) is transferred in a thermal storage
conduit 24 from a first hot storage tank 26 to a second hot storage
tank 28. After leaving the heat exchanger 20, the working fluid is
expanded to lower pressure with expansion device 30 and enters a
heat exchanger 32 for heat exchange with a cold storage 32. The
heat exchanger 32 may be a counter flow heat exchanger in which a
cold storage medium (for example a liquid) is transferred in cold
storage conduit 36 from a first cold storage tank 38 to a second
cold storage tank 40.
[0057] With respect to FIG. 1a, except the storage tanks 22, 26,
38, 40 all other components of the discharging cycle system 14 may
be different from the components of the charging cycle system 12.
However, it may be a cost advantage to use as many of them as
possible in both systems 12 and 14. For example, the compressor 18
may be used as turbine 42.
[0058] The turbine 20 is used for generating electrical energy out
of the heated working fluid by expanding the working fluid to a
lower pressure. After leaving turbine 20, the working fluid is
condensed in heat exchanger 32 by discharging heat to the cold
storage system 34 and pumped to higher pressure by pump 44. After
that the working fluid is heated with heat exchanger 20 to be
injected into turbine 42.
[0059] During discharging, the working fluid, the hot storage
medium and the cold storage medium are circulating in the reverse
direction with respect to charging of the system 10.
[0060] The operation of the thermoelectric storage system 10 will
be explained more detailed with reference to the T-S-diagram of
FIG. 3. FIG. 3 shows a state change diagram of the thermoelectric
storage system 10 with a working fluid composition of 50% carbon
dioxide and 50% butane as zeotropic mixture.
[0061] In the diagram of FIG. 2, the vapor dome of the working
fluid is indicated by line 46. Left of the vapor dome 46 the
working fluid is in its liquid phase, right of the vapor dome 46
the working fluid is in its gas phase. Below curve 46, the working
fluid is in a mixed gas/liquid state. On top of the vapor dome, the
critical point 48 of the working fluid is indicated.
[0062] During the charging cycle, the charging cycle system 12 of
FIG. 1a follows the thermodynamic cycle 50 counter-clockwise.
[0063] For charging of the storages 22, 34 the zeotropic mixture at
its bubble point is first evaporated at constant pressure P1 (for
example 10 bar) in the counter current heat exchanger 32. This
corresponds to going from point A to point B in the T-S-diagram. As
may be derived from the diagram, the temperature of the working
fluid is continuously rising during phase change from about
-20.degree. C. to about 40.degree. C.
[0064] The zeotropic mixture vapor is then compressed with the
compressor 18 to a higher pressure level P2 (for example 35 bar).
The compression corresponds to going from point B to point C in the
T-S-diagram and it is the main point where electrical energy to
drive the compressor 18 is injected into the thermoelectric energy
storage system 10.
[0065] According to an exemplary embodiment, the thermoelectric
energy storage system 10 may include a compressor 18 for
compressing the working fluid (e.g., the zeotropic mixture) in
vapor phase from a lower pressure to a higher pressure during the
charging cycle, such that electrical energy for driving the
compressor is injected into the thermal energy storage system.
[0066] Following the compression, the zeotropic mixture is cooled
at constant pressure P2 in a the counter current heat exchanger 20
corresponding to going from point C to point D in the T-S-diagram.
During this cooling process, the superheated zeotropic mixture from
the compressor 18 is first cooled to its dew point 50, followed by
condensation and subcooling of the liquid phase. At the pressure P2
the temperature of the working fluid is continuously decreasing
during phase change from about 90.degree. C. to about 10.degree. C.
Point D is to the left of the boiling curve 46. The subcooled
zeotropic mixture in liquid phase is then expanded back to the
pressure level P1 via an expansion device 30, which can be a work
recovering expander or a thermostatic expansion valve. The
expansion process corresponds to going from point D back to point A
in the TS diagram. Point D and point A are very close to each other
because the liquid phase of the working fluid is nearly
incompressible.
[0067] According to an exemplary embodiment, the thermoelectric
energy storage system 10 may include an expansion device (e.g., an
expansion valve) 30 for expanding the working fluid (e.g., the
zeotropic mixture) in liquid phase from a higher pressure to a
lower pressure during the charging cycle.
[0068] It is important to note that the extent of subcooling has to
be controlled to prevent vapor formation or flashing at the end of
the expansion process to ensure reversible operation during the
discharging cycle. This controlling may be performed by a control
device 60 connected to a sensor for measuring the temperature of
the working fluid at the end of the heat rejection process after
heat exchanger 20 and adjusting the thermal storage flow rate. This
may be done by changing the pump speed if a variable speed pump 30
is used or a valve located on the thermal storage fluid flow
line.
[0069] During the operation of the charging cycle system 12,
external heat is provided to carry out the evaporation in the heat
exchanger 32, and heat is removed to carry out the cooling in the
heat exchanger 20. Since both the evaporation in the heat exchanger
32 and cooling (e.g., desuperheating-condensation-subcooling) in
the heat exchanger 20 are constant pressure but variable
temperature processes they can be closely matched with thermal
storage fluids undergoing cooling in the heat exchanger 32 and
heating in the heat exchanger 20. This may also be controlled by
the control device 60.
[0070] In FIG. 1a, the cold side thermal storage fluid is taken
from tank 38 at a temperature T1, passed through the heat exchanger
32 providing the heat of evaporation for the zeotropic working
fluid mixture, and placed at tank 40 at a temperature T2, where T1
is greater than T2. Similarly the hot side thermal storage fluid is
taken from tank 26 at a temperature T3, passed through the heat
exchanger 20 absorbing the heat released by the zeotropic working
fluid mixture, and placed at tank 28 at a temperature T4, where T4
is greater than T3. For heat exchange, the temperature difference
of the working fluid is a bit lower (for heating the working fluid)
or a bit higher (for cooling the working fluid) as the temperature
of the storage medium.
[0071] The temperature difference may be between 1.degree. C. to
3.degree. C., between 3.degree. C. and 10.degree. C. or much
larger. The temperature difference may be chosen by a cost-benefit
analysis. I.e. Small temperature differences such as 1.degree. C.
to 3.degree. C. may result in high efficiency (for example around
75%) but maybe also high costs due to large heat exchangers, a
larger temperature difference such as 3.degree. C. to 10.degree. C.
may result in low efficiency (for example around 40%) but maybe
also low costs due to small heat exchangers.
[0072] The discharging is the reversed operation of the charging
process. The discharging cycle system 14 of FIG. 1b follows the
cycle 50 in FIG. 3 clockwise.
[0073] The zeotropic working fluid mixture at its bubble point is
first pumped via a pump 44 from a pressure P3 (for example 10 bar)
to a higher pressure P4 (for example 35 bar), corresponding to
going from point A to point D in the T-S-diagram. Due to the
pumping there may be a minimal temperature rise. The zeotropic
mixture then enters the heat exchanger 20. Here the zeotropic
mixture is heated to its new bubble point at the elevated pressure
level P4, subsequently evaporated and superheated at constant
pressure P4. These processes correspond to going from point D to
point C in the T-S-diagram.
[0074] According to an exemplary embodiment, the thermoelectric
energy storage system 10 may include a pump 44 for pumping the
working fluid from a lower pressure to a higher pressure during the
discharging cycle.
[0075] Next, the superheated zeotropic vapor mixture is expanded in
the turbine 42 from the higher pressure level P4 to the initial
pressure level P3. This step corresponds to going from point C to
point B in the T-S-diagram and it is the main point where
electrical energy generated at the turbine 42 is retrieved from the
thermal energy storage 10. Finally, the expanded zeotropic vapor
mixture is condensed at constant pressure P3 in the heat exchanger
32 back to a liquid in its bubble point to be pumped again to
complete the heat engine cycle. This final step corresponds to
going from point B to point A in the T-S-diagram.
[0076] According to an exemplary embodiment, the thermoelectric
energy storage system 10 may include a turbine 42 for expanding the
working fluid from a higher pressure to a lower pressure level for
generating electrical energy during the discharging cycle.
[0077] During the operation of the discharging cycle system 14,
external heat is provided to carry out the heating, evaporation,
and superheating in the heat exchanger 20, and heat is removed to
carry out the condensation in the heat exchanger 32. Since the
heating, evaporation, and superheating in the heat exchanger 20 and
condensation in the heat exchanger 32 are constant pressure but
variable temperature processes, they can be closely matched with
thermal storage fluids undergoing cooling in the heat exchanger 20
and heating in the heat exchanger 32.
[0078] In FIG. 1b, the cold side thermal storage fluid is taken
from tank 40 at the temperature T2, passed through the heat
exchanger 32 absorbing the heat of condensation for the zeotropic
working fluid mixture, and placed at tank 38 at the temperature T1,
where T1 is greater than T2. Similarly the hot side thermal storage
fluid is taken from tank 28 at the temperature T4, passed through
the heat exchanger 20 providing the energy to heat, evaporate, and
superheat the zeotropic working fluid mixture, and placed at tank
26 at the temperature T3, where T4 is greater than T3.
[0079] Although depicted in FIG. 1 and FIG. 1b, a thermal storage
system without one of the storages 22 and 34 may also be an
exemplary embodiment of the present disclosure, for example the
thermal storage system with the hot storage 22 or the thermal
storage system with the cold storage 34. According to an exemplary
embodiment, the cold storage 34 can be replaced by a large thermal
reservoir such as a river or a lake, and according to another
exemplary embodiment, the hot storage can be replaced by a waste
heat source greater than the ambient.
[0080] To summarize, the system 10 may include a hot storage tank
22 and the thermal storage conduit includes a hot storage conduit
24 for transferring a hot storage medium between a first hot
storage tank 26 and a second hot storage tank 28 through the heat
exchanger 20. Further, the thermal storage tank of the system 10
may include a cold storage tank 34, and the thermal storage conduit
includes a cold storage conduit 36 for transferring a cold storage
medium between a first cold storage tank 38 and a second cold
storage tank 40 through the heat exchanger 32.
[0081] FIGS. 1a and 1b are simple versions of practical
implementations. For example, instead of the heat exchangers 20, a
split stream heat exchanger 70 as shown in FIGS. 3a and 3b may be
used. With such a heat exchanger 70, the varying heat capacity and
hence the slope of temperature variation of the zeotropic working
fluid mixture in the heat exchanger 70 may be compensated by
varying the flow rate of the thermal storage fluid via an
intermediate storage tank in addition to tanks 26 and 28.
Additionally or alternatively, also the heat exchanger 32 may be
replaced by heat exchanger 70.
[0082] The heat storage system 72 system includes a heat exchanger
70 having an internal stream splitter 72. For example, the heat
exchanger 70 may include two basic heat exchangers interconnected
by a stream splitter 72. The zeotropic mixture 74 as working fluid
circulates through these components as indicated by the solid line
with arrows in FIGS. 3a and 3b. Further, a cold-fluid storage tank
76, an intermediate storage tank 78 and a hot-fluid storage tank 80
containing a fluid thermal storage medium 82 are connected together
via the heat exchanger 70.
[0083] During heat transfer from the working fluid 74 to the
thermal storage medium 82, the thermal storage medium, represented
by the dashed line in FIG. 3a, is flowing from the cold-fluid
storage tank 76 through the heat exchanger 72 into the hot-fluid
storage tank 80 and partially into the intermediate storage tank
78. The temperature of the thermal storage medium is detected on
either side of the stream splitter 72 by temperature sensors such
as thermocouples or resistive sensors coupled to the control device
60. The heat discarded from the working fluid 74 into the thermal
storage medium 82 is stored in the form of sensible heat.
[0084] Following detection of the temperature of the thermal
storage medium 82 on either side of the stream splitter 72, the
flow rate of the thermal storage medium 82 into the intermediate
storage tank 78 and the hot-fluid storage tank 80 is adjusted. This
is achieved by means of appropriate piping and valve arrangements
controlled by control device 60. The initial valve openings are
determined according to the desired temperature profile and the
valve openings are fine-tuned during operation according to the
temperature measurements.
[0085] The heat exchanger system 72 is designed such that the
position of the stream splitter 72 within the heat exchanger 70
coincides with the point of the heat exchanger 70 at which the
storage medium 82 is at the temperature level of the intermediate
storage tank 78. The diverted stream is stored in the intermediate
tank 78. The second stream continues through the rest of the heat
exchanger 70 into the hot storage tank 80.
[0086] As shown in FIG. 3b, during heat transfer from the thermal
storage medium 82 to the working fluid 74, the thermal storage
medium 82 is pumped from the hot-fluid storage tank 80 and from the
intermediate storage tank 78 into the cold-fluid storage tank 76.
The temperature of the thermal storage medium 82 is detected and
monitored on either side of the internal stream splitter 18 by the
control device 60. The flow rate of the streams of the thermal
storage medium 82 within the heat exchanger 70 can be controlled
and modified to optimize round-trip efficiency of the
thermoelectric storage system 10 with the control device 60. This
will be explained more detailed with respect to FIG. 4.
[0087] Summarized, the thermoelectric energy storage system 10 may
include an intermediate storage tank 78, wherein the heat exchanger
70 has a stream splitter 72 adapted to divide or join the flow of
the thermal storage medium from a first or second storage tank 76,
80 into a flow to the intermediate storage tank 78 and to the
second or first storage tank 76, 80.
[0088] FIG. 4 shows a heat flow-temperature diagram of the heat
transfer in the heat exchanger 70 during heating and cooling of the
working fluid. The solid line 90 indicates the temperature profile
of the working fluid 74 during cooling. The dotted line 92
indicates the temperature profile of the working fluid 74 during
heating. The lines 90 and 92 are not straight lines, since in
general there is no linear interrelationship between the inner heat
and the temperature of the zeotropic mixture 74.
[0089] The dashed line 94 and dot-dashed line 96 indicate the
temperature profile of the thermal storage medium 82 during both
processes. The arrows indicate the flow directions in the heat
exchanger 70. Heat can only flow from a higher to a lower
temperature. Consequently, the characteristic profile 90 for the
working fluid 74 during cooling is above the characteristic profile
94 for the thermal storage medium 82, which in turn has to be above
the characteristic profile 92 for the working fluid 74 during
heating.
[0090] The stream splitting/joining point 98 (due to the stream
splitter 72) is indicated at a thermal storage medium temperature
of approximately 340.degree. C. On the right hand side of this
point on the graph, the gradient of the temperature profile 94, 96
increases. This relatively increased gradient is a consequence of a
different flow rate of the thermal storage medium 82 after the
stream splitting/joining point 98.
[0091] In this particular embodiment, the temperature of the
cold-fluid storage tank 76 is approximately 100.degree. C., the
temperature of the intermediate storage tank 78 is approximately
340.degree. C., and the temperature of the hot-fluid storage tank
80 is approximately 520.degree. C. A minimum approach temperature
of around 25.degree. C. is assumed (e.g., the minimum temperature
difference between the two fluids exchanging heat is 25.degree.
C.). In such an embodiment, the flow rates of the thermal storage
medium between the cold-fluid storage tank and the stream splitter,
and between the stream splitter and hot storage tank, are
controlled such that they have a ratio approximately 2:1.
[0092] It can be seen from FIG. 4 that the isobars 90, 92
representing the working fluid closely follow the form of the
isobar 94 representing the thermal storage liquid 82. Thus, the
temperature differences at any point of contact between the thermal
storage medium and the working fluid in the heat exchanger, and
during both charging and discharging cycles (denoted
.DELTA.T.sub.min and .DELTA.T.sub.max), are minimized.
Advantageously, a minimization of the temperature differences is
thereby facilitated with the heat storage system 72, maximizing the
roundtrip efficiency of the thermoelectric storage system 10,
regardless of the size of the heat exchanger.
[0093] The skilled person will be aware that, a heat
flow-temperature diagram of the heat transfer in the heat exchanger
may have a different form in an alternative embodiment of the
present disclosure. For example, it is possible the gradient of the
working fluid isobar on the left of the stream splitting point is
greater than the gradient of the working fluid isobar to the right
of the splitting point. This would indicate that the streams output
from the cold storage tank and the intermediate tank are joined at
the stream splitter during heating, and divided at the stream
splitter during cooling.
[0094] 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.
* * * * *