FIELD OF THE INVENTION
[0001] The present invention relates generally to the storage of electric energy. It relates
in particular to a system and method for storing electrical energy in the form of
thermal energy.
BACKGROUND OF THE INVENTION
[0002] Base load generators such as nuclear power plants and generators with stochastic,
intermittent energy sources such as wind turbines and solar panels, generate excess
electrical power during times of low power demand. Large-scale electrical energy storage
systems are a means of diverting this excess energy to times of peak demand and balance
the overall electricity generation and consumption.
[0003] In an earlier patent application
EP1577548 the applicant has described the concept of a thermoelectric energy storage (TEES)
system. A TEES 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 is robust, compact, site independent and is suited to
the storage of electrical energy in large amounts. Thermal energy can be stored in
the form of sensible heat via a change in temperature or in the form of latent heat
via a change of phase or a combination of both. The storage medium for the sensible
heat can be a solid, liquid, or a gas. The storage medium for the latent heat occurs
via a change of phase and can involve any of these phases or a combination of them
in series or in parallel.
[0004] The round-trip efficiency of an electrical energy storage system can 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 a high roundtrip efficiency, the efficiencies
of both modes need to be maximized inasmuch as their mutual dependence allows.
[0005] It is important to point out that all electric energy storage technologies inherently
have a limited round-trip efficiency. Thus, for every unit of electrical energy used
to charge the storage, only a certain percentage is recovered as electrical energy
upon discharge. The rest of the electrical energy is lost. If, for example, the heat
being stored in a TEES system is provided through resistor heaters, it has approximately
40% round-trip efficiency. The roundtrip efficiency of the TEES system is composed
of the charging efficiency and the discharging efficiency.
[0006] It is noted that the charging cycle of a TEES system is also referred to as a heat
pump cycle and the discharging cycle of a TEES system is also referred to as a heat
engine cycle. In the TEES 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 is greater than the compression work by an amount
equal to the energy taken from the cold side (that is 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 coefficient
of performance, 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 TEES system.
[0007] The charging cycle of a known TEES system comprises 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 containing a fluid
thermal storage medium are 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 known TEES system comprises 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 containing a fluid
thermal storage medium are 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, is used in both the charging and discharging cycles.
[0008] International patent application
WO2008148962 describes an energy storage system utilising a reverse Brayton cycle for charging
the system and a conventional Brayton cycle for discharging the system. The working
fluid of the Brayton cycle is always in the gas phase and therefore all heat transfer
steps of an energy storage system using a Brayton cycle can be matched with heat transfer
to a sensible heat thermal storage. Disadvantageously, due to its high back-work ratio,
such a TEES suffers from increased losses in the heat pump expansion step and the
heat engine compression step compared to other TEES 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 makes it necessary to
store the sensible heat in solid materials, such as rocks or sand via special purpose
(possibly pressurized) apparatus. At the low operating pressures of a TEES using a
Brayton cycle, a gas side heat transfer coefficient would limit the heat transfer
rates (even if a direct contact heat exchanger is used), thereby resulting in an increase
in the thermal storage footprint.
[0009] Figure 1 shows a temperature-entropy diagram of the heat transfer from the charging
and discharging cycles in a known TEES system. The charging cycle can be considered
to start at reference A and follows an anti-clockwise direction. The dashed curve
A to B represents heating a working fluid in a heat exchanger as it flows from a cool
heat exchanger to a compressor. B to C in Figure 1 represents a compression process
in which the temperature of the working fluid reaches a maximum and a relatively high
pressure P2. Solid line C to D indicates the working fluid flowing through the hot
storage heat exchanger where heat is transferred to a thermal storage medium and thus
the temperature of the working fluid drops. The dashed curve between point D and E
illustrates the working fluid passing through a heat exchanger where it delivers its
remaining thermal energy to the working fluid flowing countercurrent on the low pressure
side of the cycle. Solid line E to F represents the expansion of the working fluid
from state P2 to P1 and this recovered energy is transferred either mechanically or
electrically to the compressor. The working fluid is now at the lowest energy state
in the cycle as it enters the cool storage heat exchanger and harvests thermal energy
from the thermal storage medium illustrated by solid line F to A on Figure 1.
[0010] The discharging cycle of the present invention may be considered to be the reverse
of the above described charging cycle.
[0011] Importantly, the vertical arrow to the left hand side of the diagram shows the large
temperature span of the hot storage in a prior art system. In this example, between
130 °C and 570 °C. Similarly, the vertical arrow to the right hand side of the diagram
shows the large temperature span of the cold storage in a prior art system. In this
example, between 45 °C and 425 °C.
[0012] A disadvantage of the known art is that relatively large temperature spans are required
for both hot side and cold side heat storage materials to reach high efficiencies
in a TEES operation. One possibility to retain high efficiency with lower temperature
spans is to reduce the temperature differences and the corresponding thermodynamic
irreversibility in the heat exchangers and in order to reduce these temperature differences
fluid to fluid countercurrent-flow heat exchangers may be utilized. However, this
arrangement is only possible when the heat storage material can flow in a heat exchanger.
It can be considered that fine granulated solids such as sand can be made to flow
in a heat exchanger but the resulting wear out on heat exchanger walls will tremendously
reduce the lifetime of the heat exchangers. Additionally the energy required to transport
the solids are quite high and will cause excessive auxiliary losses, which will in
turn reduce the TEES efficiency. Thus solids or phase change materials are unsuitable
as thermal storage materials for TEES design.
[0013] It may be considered that water is a preferred thermal storage medium due to its
high heat capacity and other amiable physical properties water. However, the limited
operating temperature range of water would increase the footprint of TEES systems.
The skilled person may consider increasing the temperature range of this thermal storage
medium upwards by increasing the storage vessel pressure but such modification may
also reduce overall safety and increase system costs. Similarly, molten salt is a
known thermal storage medium, but at temperatures below 200°C use in a conventional
TEES becomes impossible due to freezing.
[0014] There is a need to provide an efficient thermoelectric energy storage having a high
round-trip efficiency, whilst minimising the storage footprint in m
3 per kWh stored energy.
DESCRIPTION OF THE INVENTION
[0015] It is an objective of the invention to provide a thermoelectric energy storage system
for converting electrical energy into thermal energy to be stored and converted back
to electrical energy with an improved round-trip efficiency. This objective is achieved
by a thermoelectric energy storage system according to claim 1 and a method according
to claim 8. Preferred embodiments are evident from the dependent claims.
[0016] According to a first aspect of the invention, a thermoelectric energy storage system
is provided having a charging cycle for providing thermal energy to a hot thermal
storage arrangement, and a discharging cycle for generating electricity by retrieving
the thermal energy from the hot thermal storage arrangement. The thermoelectric energy
storage system comprises a working fluid circuit adapted to circulate a gaseous working
fluid through the hot thermal storage arrangement and a cold thermal storage arrangement.
The hot thermal storage arrangement comprises a heat exchanger and at least two hot
storage tanks coupled via a hot storage heat exchanger. The cold thermal storage arrangement
comprises a heat exchanger and at least two cold storage tanks coupled via a cold
storage heat exchanger. A further heat exchanger is adapted to further cool the working
fluid at the output of the hot thermal storage arrangement during the charging cycle,
and the further heat exchanger is adapted to pre-heat the working fluid at the input
into the hot thermal storage arrangement during a discharging cycle. Importantly,
the working fluid is constantly in gas phase during the charging cycle and the discharging
cycle.
[0017] In a preferred embodiment, during the charging cycle, the further heat exchanger
comprises a first input from the first heat exchanger connected to a first output
leading to a turbine, and a second input from the cool thermal storage arrangement
connected to a second output leading to a compressor.
[0018] In a further preferred embodiment, during the discharging cycle, the further heat
exchanger comprises a first input from a compressor connected to a first output leading
to the hot thermal storage arrangement, and a second input from a turbine connected
to a second output leading to the cold thermal storage arrangement.
[0019] In a preferred embodiment of the invention at least one section of a charging cycle
or a discharging cycle runs supercritically.
[0020] The thermal storage medium in the hot storage heat arrangement is liquid and is preferably
molten salt. The thermal storage medium in the cold storage heat arrangement is a
liquid and is preferably water.
[0021] The working fluid of the present invention is preferably carbon dioxide.
[0022] Preferably, an external cooler is positioned in the discharging cycle system directly
after the cool thermal storage arrangement, and preferably the external cooler is
connected to an ambient heat sink.
[0023] Advantageously, the arrangement of the present invention provides flexibility to
choose and limit the temperature range at which the heat is stored on the hot and
cold sides of the system.
[0024] In a second aspect of the present invention a method is provided for storing and
retrieving energy in a thermoelectric energy storage system. The method comprising
circulating a gaseous working fluid through a hot thermal storage arrangement and
a cold thermal storage arrangement. The system is charged by compressing the working
fluid and heating a thermal storage medium circulating in the hot storage arrangement
and by cooling a thermal storage medium circulating in the cold storage arrangement.
The system is discharged by heating the working fluid circulating through the hot
thermal storage arrangement and expanding the working fluid through a turbine and
by cooling the working fluid circulating through the cold thermal storage arrangement.
The working fluid output after heat exchange with the hot thermal storage arrangement
during charging is further cooled. The working fluid input into the hot thermal storage
arrangement during discharging is pre-heated. Importantly, the working fluid is maintained
constantly in a gas phase during both charging and discharging.
[0025] In a preferred embodiment, the step of cooling further the working fluid output after
heat exchange with the hot storage during charging, comprises transferring heat from
the working fluid after heat exchange with the hot storage to the working fluid output
after heat exchange with the cold storage.
[0026] In a preferred embodiment, the step of pre-heating the working fluid input into the
first heat exchanger during discharging, comprises transferring heat from the working
fluid exiting the turbine to the working fluid input into the first heat exchanger.
[0027] Preferably, at least one section of a charging cycle or a discharging cycle is performed
supercritically.
[0028] Advantageously, the present invention enables the decoupling of the temperatures
of the hot side and cold side of the TEES during both charging and discharging cycles
by a "regenerative heat exchange" within the cycles. Whilst the charging and discharging
modes of operation need to match, the temperature span of the hot and cool storages
can be independently selected.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] The subject matter of the invention will be explained in more detail in the following
text with reference to preferred exemplary embodiments, which are illustrated in the
attached drawings, in which:
Figure 1 shows a temperature-entropy diagram of the heat transfer from the cycles
in a known TEES system;
Figure 2 shows a simplified schematic diagram of a charging cycle of a thermoelectric
energy storage system in accordance with the present invention;
Figure 3 shows a simplified schematic diagram of a discharging cycle of a thermoelectric
energy storage system in accordance with the present invention;
Figure 4 shows a temperature-entropy diagram of the heat transfer from the cycles
in a TEES system of the present invention having a regenerative heat exchanger.
[0030] For consistency, the same reference numerals are used to denote similar elements
illustrated throughout the figures.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0031] Figures 2 and 3 schematically depict a charging cycle system and a discharging cycle
system, respectively, of a TEES system in accordance with an embodiment of the present
invention.
[0032] The charging cycle system 10 shown in Figure 2 comprises a work recovering expander
12, a cool storage heat exchanger 14, a compressor 16, a hot storage heat exchanger
18, and a regenerative heat exchanger 20. A working fluid circulates through the components
as indicated by the solid line with arrows in Figure 2. Notably, both the output from
the cool storage heat exchanger 14 and the output from the hot storage heat exchanger
18 are passed countercurrent through the regenerative heat exchanger 20.
[0033] A first thermal storage tank 22 and a second thermal storage tank 24 containing a
liquid thermal storage medium are coupled together via the hot storage heat exchanger
18. The thermal storage liquid flows between the first thermal storage tank 22 and
the second thermal storage tank 24 as indicated by the dashed line with arrows. Further,
a third thermal storage tank 26 and a fourth thermal storage tank 28 containing a
liquid thermal storage medium are coupled together via the cool storage heat exchanger
14. The thermal storage liquid flows between the third thermal storage tank 26 and
the fourth thermal storage tank 28 as indicated by the dotted line with arrows.
[0034] In operation, the charging cycle system 10 performs a thermodynamic cycle and the
working fluid flows around the TEES system in the following manner. The vapor working
fluid exiting the cool storage heat exchanger 14 at a relatively low pressure P1 is
circulated to the compressor 16 via the regenerative heat exchanger 20. The surplus
electrical energy which is to be stored is utilized to compress and heat the working
fluid in the compressor 16. When exiting the compressor 16, the working fluid is at
the highest temperature and relatively high pressure P2 in the cycle. The working
fluid is fed through the hot storage heat exchanger 18 where the working fluid discards
heat into the thermal storage medium.
[0035] Specifically, the fluid thermal storage medium is pumped from the first thermal storage
tank 22 at T1 through the hot storage heat exchanger to the second thermal storage
tank 24 at T2, countercurrent to the working fluid passing through the hot storage
heat exchanger 18. The heat energy discarded from the working fluid into the thermal
storage medium is stored in the form of sensible heat, where T2 is greater than T1.
[0036] The compressed working fluid exits the hot storage heat exchanger 18 and enters the
regenerative heat exchanger 20, where the remaining heat is transferred out of the
compressed working fluid and into the working fluid which is flowing countercurrent
and into the compressor 16. After exiting the regenerative heat exchanger the working
fluid still has measurable energy due to its state of high pressure at P2. The cooled
working fluid then enters the expander 12 where it is expanded back to lower pressure
P1. Following this expansion the energy state of the working fluid is too low to be
heated solely by regenerative heat exchange. Therefore, working fluid flows from the
expander 12 into a cool storage heat exchanger 14 which is utilized to heat the working
fluid.
[0037] Specifically, the fluid thermal storage medium is pumped from the third thermal storage
tank 26 at T3 through the cool storage heat exchanger to the fourth thermal storage
tank 28 at T4. It is noted that T3 is greater than T4. The heat energy harvested from
the thermal storage medium heats the working fluid passing through the cool storage
heat exchanger 14.
[0038] The discharging cycle system 30 shown in Figure 3 comprises a compressor 32, a regenerative
heat exchanger 20, a hot storage heat exchanger 18, a turbine 34, a cool storage heat
exchanger 14, and an external cooler 36. A working fluid circulates through these
components as indicated by the solid line with arrows in Figure 3. It is noted that
the discharging cycle system 30 utilizes similar components to the charging cycle
system 10 with the additional feature of an external cooler 36 which is connected
to an ambient heat sink.
[0039] The apparatus of the discharging cycle system 30 further comprises a first thermal
storage tank 22 and a second thermal storage tank 24 containing a fluid thermal storage
medium coupled together via the hot storage heat exchanger 18. The thermal storage
medium, represented by the dashed line in Figure 3, is pumped from the second thermal
storage tank 24 through the hot storage heat exchanger 18 to the first storage tank
22. Further, a third thermal storage tank 26 and a fourth thermal storage tank 28
containing a fluid thermal storage medium are coupled together via the cool storage
heat exchanger 14. The thermal storage medium, represented by the dotted line in Figure
3, is pumped from the fourth thermal storage tank 28 through the cool storage heat
exchanger 14 to the third storage tank 26.
[0040] In operation, the discharging cycle system 30 performs a thermodynamic cycle reversing
the charging cycle and the working fluid flows around the TEES system in the following
manner. The working fluid in liquid form is compressed to a high pressure P2 by a
compressor 32. The working fluid then enters the regenerative heat exchanger 20, where
it is further heated by the countercurrent working fluid leaving the turbine 34. The
working fluid then continues to the hot storage heat exchanger 18 in which heat energy
is transferred from the thermal storage medium to the working fluid and the working
fluid reaches its highest temperature level in the discharging cycle. The working
fluid then exits the hot storage heat exchanger 18 and enters the turbine 34 where
the working fluid is expanded back to pressure P1 thereby causing the turbine 34 coupled
to a generator (not illustrated) to generate electrical energy.
[0041] As stated in the previous paragraph, the working fluid then passes through the regenerative
heat exchanger 20 and further thermal energy in the working fluid is recovered. Next,
the working fluid enters the cool storage heat exchanger 14 in which still further
remaining thermal energy is transferred from the working fluid to the thermal storage
medium. As the working fluid cools in the cool storage heat exchanger 14, the previously
cooled thermal storage medium contained in the fourth thermal storage tank 28 at a
temperature T4 flows countercurrent to the working fluid into the third thermal storage
tank 26 at temperature T3, where T3 is greater than T4.
[0042] The vapor working fluid then discards heat to the ambient via an external cooler
36, where the working fluid reaches the lowest temperature in the cycle, before entering
the compressor 32. This placement of external cooling before the compressor 32 of
the power generation cycle is thermodynamically more efficient compared to the cooling
carried out after the compressor 32 as described in the state of the art in TEES design.
This can be explained in two ways:
- (i) Cooling the compressor inlet reduces the required compressor 32 work which in
turn increases the net electric output of the power generation cycle.
- (ii) The exergy of the low pressure low temperature gas at the compressor inlet is
lower compared to the exergy of the compressed gas.
[0043] Whilst the charging cycle system of Figure 2 and the discharging cycle system of
Figure 3 have been illustrated separately, the regenerative heat exchanger 20, the
hot storage heat exchanger 18 with first and second thermal storage tanks 22, 24,
and the cool storage heat exchanger 14 with third and fourth thermal storage tanks
26, 28, are common to both. The compressor 16 of the charging cycle may be utilized
as the turbine 32 of the discharging cycle when operated in reverse (and may be referred
to as a turbomachine herein). The charging and discharging cycles may be performed
consecutively, not simultaneously. It should also be noted that the working fluid
is always in the vapor form in this TEES arrangement. The heat transfer limitations
of gases are minimized by the relatively high pressures in the system in operation.
[0044] In the present embodiment, each of the three heat exchangers 14, 18, 20 are counterflow
or near counterflow heat exchangers. Further, the cool thermal storage medium is a
liquid, and is preferably water. The hot storage medium is also a liquid and preferably
a molten salt, which is preferably a mixture of sodium nitrate and potassium nitrate.
The compressor 16 of the heat pump cycle 10 of the present embodiment is an electrically
powered compressor.
[0045] Figure 4 shows a temperature-entropy diagram of the heat transfer from the cycles
in a preferred embodiment of a TEES system of the present invention having a regenerative
heat exchanger 20. In this embodiment the thermal storage medium in the hot storage
heat exchanger 18 is molten salt and the thermal storage medium in the cold storage
heat exchanger 14 is hot water, both under atmospheric pressure. The diagram shows
both the charging and discharging cycles. The charging cycle can be considered to
start at reference A and follows an anti-clockwise direction. The dashed curve A to
B represents the heating the working fluid in the regenerative heat exchanger 20 as
it flows from the cool heat exchanger 14 to the compressor 16. B to C in Figure 4
represents the compression process in which the temperature of the working fluid reaches
a maximum and a relatively high pressure P2. Solid line C to D indicates the working
fluid flowing through the hot storage heat exchanger 18 where heat is transferred
to the thermal storage medium and thus the temperature of the working fluid drops.
The dashed curve between point D and E illustrates the working fluid passing through
the regenerative heat exchanger 20 where it delivers its remaining thermal energy
to the working fluid flowing countercurrent on the low pressure side of the cycle.
Solid line E to F represents the expansion of the working fluid from state P2 to P1
and this recovered energy is transferred either mechanically or electrically to the
compressor 16. The working fluid is now at the lowest energy state in the cycle as
it enters the cool storage heat exchanger 14 and harvests thermal energy from the
thermal storage medium illustrated by solid line F to A on Figure 4.
[0046] The discharging cycle of the present invention may be considered to be the reverse
of the above described charging cycle with the exception of the additional cooling
from an ambient heat sink 36. This discharging step dissipates the losses due to the
irreversibility in the TEES and is represented by the line F to F* in Figure 4. The
working fluid attains the low pressure P1 upon cooling by the external cooler connected
to an ambient heat sink prior to entering the compressor where the pressure is increased
to P2; indicated by dotted line F* to E.
[0047] Importantly, the vertical arrow to the left hand side of the diagram shows the relatively
small temperature span of the hot storage in the present system. In this example,
between 425°C and 570°C. Similarly, the vertical arrow to the right hand side of the
diagram shows the relatively small temperature span of the cold storage in the present
system. In this example, between 45 °C and 105 °C. Thus, the arrangement of the present
invention provides flexibility to choose and limit the temperature range at which
the heat is stored on the hot and cold sides of the system.
[0048] In this preferred embodiment, in which the working fluid is carbon dioxide, it is
noted that the relatively high density and relatively high heat transfer coefficient
of the carbon dioxide under the TEES cycle conditions also influences the choice of
pressures within the cycles. Specifically, the surfaces of the heat exchangers 14,
18, 20 and the size of the turbomachine may be minimized through manipulation of the
pressures within the cycles. The skilled person will also be aware that careful choice
of the system parameters may reduce the total required volume of the thermal storage
materials and may also reduce the sensitivity of the roundtrip efficiency to the approach
temperatures in the thermal storage heat exchangers 14, 18 (primarily due to the large
temperature differences within the system). Importantly, heat balancing with another
refrigeration cycle is not necessary and the required amount of cooling water or air
is minimal; only that which corresponds to the energy losses in the two cycles.
[0049] In a preferred embodiment, both water and molten salt are used as thermal storage
materials, which combination functions to reduce the footprint of the TEES system
whilst maintaining high system efficiency.
[0050] Advantageously, the present invention enables the decoupling of the temperatures
of the hot side and cold side of the TEES during both charging and discharging cycles
by a "regenerative heat exchange" within the cycles. Whilst the charging and discharging
modes of operation 10, 30 need to match, the temperature span of the hot and cool
storages can be independently selected. The skilled person will understand that the
cool storage should be colder then the hot storage but otherwise liquid nitrogen may
be used as cool thermal storage medium and hot water at 90°C may be used as hot thermal
storage medium. Alternatively, molten salt may be used as the hot thermal storage
medium and liquid nitrogen as the cool thermal storage medium thereby creating a very
large temperature difference in the cycle.
[0051] Also, in such a regenerative heat exchange arrangement, the pressure ratio (the ratio
of P1 and P2 in the above embodiment) is only weakly affected by the temperature difference
between hot and cold sides of the TEES and therefore it is possible to connect a cold
side at a very low temperature with a hot side at ambient temperature. Equally, it
is possible to connect a cold side at ambient temperature with a hot side at relatively
high temperatures. And due to the fact that the working fluid is always in the gas
phase, the heat transfer with both the hot and cold sides of the cycle takes place
via a change in temperature (and also possibly as an exchange of sensible heat on
both sides of the TEES). As indicated above, theoretically, the temperature range
within which both cycles can interact with the hot and cold sides may be freely chosen.
This results in particular advantage for TEES design because the temperature ranges
are limited in which appropriate thermal storage media can exist as a liquid. Thus,
by using the regenerative heat exchange cycle, any appropriate storage materials may
be used and then the cycle parameters adjusted accordingly without any other constraints.
It is noted that a working fluid operating at higher absolute pressures may be preferable
because of improved heat transfer in countercurrent heat exchangers.
[0052] In an alternative embodiment, ammonia and water mixture can be utilized as a thermal
storage medium in the cool storage heat exchanger (having a temperature range of -100°C
to 50°C). Further alternative thermal storage mediums include water without additives
at atmospheric pressure (having a temperature range of 0°C to 100°C), or thermal oil
such as Dowtherm J (having a temperature range of -80°C to 315°C) or molten salt mixture
(having a temperature range up to 566°C). Therefore, it can be seen that the choice
of thermal storage medium is broad and the charging and discharging cycle parameters
can be adapted, dependent upon the thermal storage mediums chosen, for best efficiency
and cost tradeoff. Clearly, where very high temperatures are utilized in the heat
exchange parts of a TEES cycle, then other components in the system may need to be
adapted to operate effectively under high temperature and pressure conditions.
[0053] Although carbon dioxide is the preferred working fluid for the present invention,
the skilled person will be aware that other fluids may be used alternatively. For
example, other gases have properties which enable them to match similar operating
conditions exemplified in Figure 4, but at lower pressures.
1. A thermoelectric energy storage system having a charging cycle (10) for providing
thermal energy to a hot thermal storage arrangement (18, 22, 24), and a discharging
cycle (30) for generating electricity by retrieving the thermal energy from the hot
thermal storage arrangement (18, 22, 24), the thermoelectric energy storage system
comprising;
a working fluid circuit adapted to circulate a gaseous working fluid through the hot
thermal storage arrangement (18, 22, 24) and a cold thermal storage arrangement (14,
26, 28),
the hot thermal storage arrangement comprises a hot storage heat exchanger (18) and
at least two hot storage tanks (22, 24) coupled via the hot storage heat exchanger
(18),
the cold thermal storage arrangement comprises a cold storage heat exchanger (14)
and at least two cold storage tanks (26, 28) coupled via the cold storage heat exchanger
(14), characterized in that;
a further heat exchanger (20) is adapted to further cool the working fluid at the
output of the hot thermal storage arrangement (18, 22, 24) during the charging cycle
(10),
the further heat exchanger (20) is adapted to pre-heat the working fluid at the input
into the hot thermal storage arrangement (18, 22, 24) during a discharging cycle (30),
and
the working fluid is constantly in gas phase during the charging cycle (10) and the
discharging cycle (30).
2. The system according to claim 1, wherein, during the charging cycle (10), the further
heat exchanger (20) comprises;
a first input from the hot thermal storage arrangement (18, 22, 24) connected to a
first output leading to a turbine (12), and
a second input from the cold thermal storage arrangement (14, 26, 28) connected to
a second output leading to a compressor (16).
3. The system according to claim 1, wherein, during the discharging cycle (30), the further
heat exchanger (20) comprises;
a first input from a compressor (32) connected to a first output leading to the hot
thermal storage arrangement (18, 22, 24), and
a second input from a turbine (34) connected to a second output leading to the cold
thermal storage arrangement (14, 26, 28).
4. The system according to any preceding claim, wherein at least one section of a charging
cycle (10) or a discharging cycle (30) runs supercritically.
5. The system according to any preceding claim, wherein the thermal storage medium in
the hot thermal storage arrangement is a molten salt and the thermal storage medium
in the cold thermal storage arrangement is water.
6. The system according to any preceding claim, wherein the working fluid is carbon dioxide.
7. The system according to any preceding claim, wherein an external cooler (36) is positioned
in the discharging cycle system (30) directly after the cold thermal storage arrangement,
and the external cooler (36) is connected to an ambient heat sink.
8. A method for storing and retrieving energy in a thermoelectric energy storage system,
comprising;
circulating a gaseous working fluid through a hot thermal storage arrangement (18,
22, 24) and a cold thermal storage arrangement (14, 26, 28),
charging the system by compressing the working fluid and heating a thermal storage
medium circulating in the hot storage arrangement (18, 22, 24) and by cooling a thermal
storage medium circulating in the cold storage arrangement (14, 26, 28),
discharging the system by heating the working fluid circulating through the hot thermal
storage arrangement (18, 22, 24) and expanding the working fluid through a turbine
(34) and by cooling the working fluid circulating through the cold thermal storage
arrangement (14, 26, 28),
the method characterized by;
cooling further the working fluid output after heat exchange with the hot thermal
storage arrangement (18, 22, 24) during charging,
pre-heating the working fluid input into the hot thermal storage arrangement (18,
22, 24) during discharging, and
maintaining the working fluid constantly in a gas phase during both charging and discharging.
9. The method according to claim 8, wherein the step of cooling further the working fluid
output after heat exchange with the hot thermal storage arrangement during charging,
further comprises;
transferring heat from the working fluid after heat exchange with the hot thermal
storage arrangement (18, 22, 24) to the working fluid output after heat exchange with
the cold thermal storage arrangement (14, 26, 28).
10. The method according to claim 8, wherein the step of pre-heating the working fluid
input into the hot thermal storage arrangement (18, 22, 24) during discharging, further
comprises;
transferring heat from the working fluid exiting the turbine to the working fluid
input into the hot thermal storage arrangement (18, 22, 24).
11. The method according to any of claim 8 to claim 10, wherein at least one section of
a charging cycle or a discharging cycle is performed supercritically.