WO2013064317A1

WO2013064317A1 - Thermoelectric energy storage system with regenerative heat exchange and method for storing thermoelectric energy

Info

Publication number: WO2013064317A1
Application number: PCT/EP2012/069157
Authority: WO
Inventors: Florian Buchter; Jaroslav Hemrle; Mehmet Mercangoez
Original assignee: Abb Research Ltd
Priority date: 2011-10-31
Filing date: 2012-09-28
Publication date: 2013-05-10
Also published as: WO2013064524A1; EP2587005A1

Abstract

A thermoelectric energy storage (TEES) 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 including regenerative heat exchange. The system comprises 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), wherein 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 a cold storage heat exchanger (14). The system also provides a regenerative heat exchanger (20), adapted to further cool the working fluid at the output of the hot thermal storage arrangement during the charging cycle (10), and to pre-heat the working fluid at the input into the hot thermal storage arrangement during a discharging cycle (30). Notably, the working fluid is constantly in gas phase during the charging and discharging cycles.

Description

THERMOELECTRIC ENERGY STORAGE SYSTEM WITH REGENERATIVE HEAT EXCHANGE AND METHOD FOR STORING THERMOELECTRIC ENERGY

FIELD OF THE INVENTION

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

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. 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.

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.

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.

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. 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.

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. 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. The discharging cycle of the present invention may be considered to be the reverse of the above described charging cycle. 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.

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. 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.

There is a need to provide an efficient thermoelectric energy storage having a high round- trip efficiency, whilst minimising the storage footprint in m³ per kWh stored energy. DESCRIPTION OF THE INVENTION

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.

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. 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. 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. In a preferred embodiment of the invention at least one section of a charging cycle or a discharging cycle runs supercritically.

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.

The working fluid of the present invention is preferably carbon dioxide. 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.

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.

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.

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. 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.

Preferably, at least one section of a charging cycle or a discharging cycle is performed supercritically.

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

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.

For consistency, the same reference numerals are used to denote similar elements illustrated throughout the figures. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

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.

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.

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.

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.

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.

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.

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. 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.

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.

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.

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. 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. 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.

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.

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.

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.

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.

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. 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. 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.

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.

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. 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.

Claims

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).

1 1 . 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.