THERMOELECTRIC ENERGY STORAGE SYSTEM AND METHOD FOR STORING THERMOELECTRIC ENERGY

Abstract

 A thermoelectric energy storage system (10) has an intercooler for intercooling a working fluid between two compression stages. The intercooling may be carried out by flashing a portion of the working fluid (taken from the output of an expander (20)) in a flash intercooler (28) and/or by heating a secondary thermal storage (46) with a further heat exchanger (30).

Description

FIELD OF THE INVENTION

[0001] The present invention relates generally to the storage of electric energy. It relates in particular to a system and a method for storing electric energy in the form of thermal energy in a thermal energy storage.

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 EP-A 1577548 the applicant has described the concept of a thermoelectric energy storage (TEES) system. A thermoelectric energy storage converts excess electricity to heat in a charging cycle, stores the heat, and converts the heat back to electricity in a discharging cycle, when necessary. Such an energy storage system may be robust, compact, site independent and may be suited to the storage of electrical energy in large amounts. Thermal energy may be stored in the form of sensible heat via a change in temperature or in the form of latent heat via a change of phase or a combination of both. The storage medium for the sensible heat may be a solid, liquid, or a gas. The storage medium for the latent heat occurs via a change of phase and may involve any of these phases or a combination of them in series or in parallel.

[0004] The round-trip efficiency of an electrical energy storage system may be defined as the percentage of electrical energy that can be discharged from the storage in comparison to the electrical energy used to charge the storage, provided that the state of the energy storage system after discharging returns to its initial condition before charging of the storage. Thus, in order to achieve high roundtrip efficiency, the efficiencies of both modes need to be maximized inasmuch as their mutual dependence allows.

[0005] The roundtrip efficiency of the thermoelectric energy storage system is limited for various reasons rooted in the second law of thermodynamics. The first reason relates to the coefficient of performance of the system. When the system is in the charging mode, its ideal efficiency may be governed by the coefficient of performance (COP) of a heat pump. The COP depends on the temperatures of the cold side (T_c) and the hot side (T_h) as given by

[0006] Thus, it can be seen that the COP of a heat pump declines with increased difference between input and output temperature levels. Secondly, the conversion of heat to mechanical work in a heat engine is limited by the Carnot efficiency. When the system is in the discharging mode, the efficiency (η) is given by

[0007] Thus, it can be seen that efficiency increases when the cold side temperature decreases. 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.

[0008] It is noted that many industrial processes involve provision of thermal energy and storage of the thermal energy. Examples are refrigeration devices, heat pumps, air conditioning and the process industry. In solar thermal power plants, heat is provided, possibly stored, and converted to electrical energy. However, all these applications are distinct from thermoelectric energy storage systems because they are not concerned with heat for the exclusive purpose of storing electricity.

[0009] In an earlier patent application EP-A 2157317 the applicant has described the concept of a transcritical thermoelectric energy storage. In such a system, the working fluid undergoes transcritical cooling during the charging and transcritical heating during the discharging cycle as it exchanges heat with the thermal storage medium.

[0010] US patent number 3,165,905 (Ware) describes a refrigerating machine including an economizer with the aim of improving the efficiency of the refrigerating cycle.

[0011] An article entitled "The commercial feasibility of the use of water vapor as a refrigerant" by Lachner B. F., Nellis G. F., Reindl D. T. (2007) International Journal of Refrigeration 30, 699-708, describes the use of flash intercooling in between compression stages in order to improve the coefficient of performance of refrigeration systems.

[0012] However, in certain cases it would be disadvantageous to apply such techniques for improving the efficiency of refrigeration cycles to a system having both charging and discharging cycles, since in applying such techniques to such a system, an efficiency improvement in one cycles could result in an efficiency reduction in the other cycle.

DESCRIPTION OF THE INVENTION

[0013] There is a need to provide an efficient thermoelectric energy storage having a high round-trip efficiency, whilst minimising the system costs involved.

[0014] This object is achieved by the subject-matter of the independent claims. Further exemplary embodiments are evident from the dependent claims.

[0015] An aspect of the invention relates to 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.

[0016] According to an embodiment of the invention, the thermoelectric energy storage system comprises a working fluid circuit circulating a working fluid; a first compressor, in the charging cycle, compressing the working fluid from a low pressure to an intermediate pressure (such that the temperature of the working fluid is rising), an intercooler, in the charging cycle, cooling the working fluid at the intermediate pressure (for lowering the temperature of the working fluid), a second compressor, in the charging cycle, compressing the working fluid from the intermediate pressure to a high pressure, a first heat exchanger, in the charging cycle, transferring heat from the working fluid at the high pressure to the thermal storage and, in the discharging cycle, transferring heat from the thermal storage to the working fluid at the high pressure.

[0017] The working fluid may be compressed in two stages: from the low pressure to the intermediate pressure in a first stage and from the intermediate pressure to the high pressure in a second stage.

[0018] According to an embodiment of the invention, the intercooler comprises a flash intercooler and/or a second heat exchanger. In other words, the intercooling may be carried out by (a) flashing a portion of the working fluid (taken from the output of a expander) in the flash intercooler and/or by (b) heating a secondary thermal storage with the second heat exchanger. This may have the advantage of (a) reducing the compressor energy of the first stage without compromising the thermal energy delivered to the main thermal storage and/or of (b) carrying out a reheat in the discharging cycle by using the secondary thermal storage to increase the power output.

[0019] This may mean that second heat exchanger, in the charging cycle, transfers heat from the working fluid at the intermediate pressure to a second thermal storage and, in the discharging cycle, transfers heat from the second thermal storage to the working fluid at intermediate pressure.

[0020] If there are multiple compressor stages then for one stage one can use the flash intercooler and for another one can use a thermal storage heat exchanger. However, it is possible to use flash intercoolers exclusively, for example two flash intercoolers. In this case there may not be any reheat stages in the discharging cycle. Further, it is possible to use thermal storage heat exchangers for intercooling exclusively, for example two heat exchangers. In this case there may be more than one reheat stage in the discharging cycle.

[0021] It is noted that the charging cycle of a thermoelectric energy storage system may be referred to as a heat pump cycle and the discharging cycle of a thermoelectric energy storage system may be referred to as a heat engine cycle. In the thermoelectric energy storage concept, heat needs to be transferred from a hot working fluid to a thermal storage medium during the charging cycle and back from the thermal storage medium to the working fluid during the discharging cycle. A heat pump requires work to move thermal energy from a cold source to a warmer heat sink. Since the amount of energy deposited at the hot side, i.e. the thermal storage medium part of a thermoelectric energy storage, is greater than the compression work by an amount equal to the energy taken from the cold side, i.e. 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 thermoelectric energy storage system.

[0022] The charging cycle of a thermoelectric energy storage system may comprise a work recovering expander, an evaporator, a compressor and a heat exchanger, all connected in series by a working fluid circuit. Further, a cold storage tank and a hot storage tank, for example, containing a fluid thermal storage medium may be coupled together via the heat exchanger. Whilst the working fluid passes through the evaporator, it absorbs heat from the ambient or from a thermal bath and evaporates. The discharging cycle of a thermoelectric energy storage system may comprise a pump, a condenser, a turbine and a heat exchanger, all connected in series by a working fluid circuit. Again, a cold storage tank and a hot storage tank, for example, containing a fluid thermal storage medium may be coupled together via the heat exchanger. Whilst the working fluid passes through the condenser, it exchanges heat energy with the ambient or the thermal bath and condenses. The same thermal bath, such as a river, a lake or a water-ice mixture pool, may be used in both the charging and discharging cycles.

[0023] Advantageously, the present invention overcomes the problem of an excessive temperature rise in the working fluid during compression in the charging cycle. This problem occurs where the ratio of the highest operating pressure of a transcritical thermoelectric energy storage system to the evaporator pressure of the charging cycle is relatively great. Specifically, this excessive temperature rise is detrimental to the completion of the compression process in a single stage unless the working fluid is heated to an acceptably high temperature.

[0024] Thus, the skilled person will appreciate that the present invention describes a thermoelectric energy storage system where the charging and discharging cycles are designed to have corresponding compressor intercooling and reheat sections, respectively, with matching heat loads and temperature levels, and where an intercooler may be used for cooling each of the additional compression stages of the charging cycle. Such intercoolers are located at the corresponding compressor discharges and are fed with partially expanded working fluid from the condenser exit, such that the heat of compression is absorbed by the process of vaporizing the liquid part of the working fluid.

[0025] Advantageously, the present invention provides a multi-stage compression system in which the working fluid is cooled close to its saturation temperature as it is output from each intermediate compression stage. The heat released from the working fluid during said cooling is recovered and utilised to improve roundtrip efficiency of the thermoelectric energy storage system.

[0026] A further aspect of the invention relates to a method for storing electrical energy in a charging cycle and retrieving electrical energy in a discharging cycle.

[0027] According to an embodiment of the invention, the charging cycle comprises the steps: compressing the working fluid from a low pressure to an intermediate pressure for storing electrical energy (particularly for converting electrical energy into heat energy); cooling the working fluid at the intermediate pressure; compressing the working fluid from the intermediate pressure to a high pressure for storing electrical energy; transferring heat from the working fluid at the high pressure to the thermal storage.

[0028] According to an embodiment of the invention, the discharging cycle comprises the steps: transferring heat from the thermal storage to the working fluid at the high pressure; expanding the working fluid from the high pressure for generating electrical energy.

[0029] It has 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.

[0030] If technically possible but not explicitly mentioned, also combinations of embodiments of the invention described in the above and in the following may be embodiments of the method and the system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The subject matter of the invention will be explained in more detail in the following text with reference to exemplary embodiments, which are illustrated in the attached drawings.

Figure 1a shows a simplified schematic diagram of a charging cycle of a thermoelectric energy storage system according an embodiment of the invention.

Figure 1b shows a simplified schematic diagram of a discharging cycle of a thermoelectric energy storage system according an embodiment of the invention.

Figure 2a shows an enthalpy-pressure diagram of the heat transfer in the charging cycle of a transcritical thermoelectric energy storage system according to an embodiment of the invention.

Figure 2b shows an enthalpy-pressure diagram of the heat transfer in the discharging cycle of a transcritical thermoelectric energy storage system according to an embodiment of the invention.

Figure 3a shows an enthalpy-pressure diagram of the heat transfer in the charging cycle of a thermoelectric energy storage system according to an embodiment of the invention.

Figure 3b shows an enthalpy-pressure diagram of the heat transfer in the discharging cycle of a thermoelectric energy storage system according to an embodiment of the invention.

[0032] For consistency, in general the same reference numerals are used to denote similar elements illustrated throughout the figures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

[0033] Figures 1a and 1b show a simplified schematic diagram of a thermoelectric energy storage system 10 according to an embodiment of the invention.

[0034] The charging cycle system 12 shown in Figure 1 a comprises a first compression stage with a compressor 14, a second compression stage with a compressor 16 and a third compression stage with a compressor 18. The charging cycle system 12 comprises further a first expansion stage with an expander 20 and a second expansion stage with an expansion valve 22. A working fluid circulates through all components of a working fluid circuit 24 as indicated by the solid line with arrows.

[0035] Further, the charging cycle system 12 comprises a stream splitter 26 between the expander 20 and the expansion valve 22, a flash intercooler 28 between the compressor 14 and the compressor 16 and a heat exchanger 30 between the compressor 16 and the compressor 18.

[0036] At the high pressure side 32, the charging cycle system 12 comprises a heat exchanger 34, and at the low pressure side 36, the charging cycle system 12 comprises a heat exchanger 38.

[0037] In operation, the charging cycle system 12 performs a transcritical cycle and the working fluid flows around the thermoelectric energy storage system 10 in the following manner.

[0038] In the first expansion stage, the working fluid enters the expander 20 where the working fluid is expanded from a high pressure to a lower (intermediate) pressure. On exiting the expander 20, the working fluid stream is split in two streams by the stream splitter 26, with a first portion of the working fluid flowing to the second expansion stage with expansion valve 22 and a second portion passing directly to the flash intercooler 28.

[0039] After the second expansion stage, where the working fluid is expanded by expansion valve 22 from the intermediate pressure to a low pressure, the working fluid passes to the heat exchanger 38 where the working fluid absorbs heat from the ambient or from a cold storage 40 and evaporates. For example, the heat exchanger 38 is a counter flow heat exchanger 38 and a cold storage medium circulates from a first cold storage tank 42 to a second cold storage tank 44 for exchanging heat with the working fluid.

[0040] The vaporised working fluid is circulated to a first compression stage in which surplus electrical energy is utilized to compress and heat the working fluid in a compressor 14 from the low pressure to the intermediate pressure. On exiting the compressor 14, this first portion of working fluid is mixed with the relatively cooler, second portion of working fluid in the flash intercooler 28.

[0041] The mixed working fluids pass to a second compression stage which comprises the compressor 16. In the second compression stage, further surplus electrical energy is utilized to compress the working fluid from the intermediate pressure to a higher second intermediate pressure. The working fluid mass flow through the second compression stage is greater than the working fluid mass flow through the first compression stage.

[0042] Next, the working fluid passes through the heat exchanger 30 where it is cooled as heat energy is transferred from the working fluid to a thermal storage medium from a further heat storage 46. For example, the heat exchanger 30 is a counter flow heat exchanger 30 and the storage medium circulates from a first storage tank 48 to a second storage tank 50 for exchanging heat with the working fluid.

[0043] The working fluid is then directed to a third compression stage where it passes through the compressor 18 before entering the heat exchanger 34. In the third compression stage, again surplus electrical energy is driving the compressor 18 for compressing the working fluid from the second intermediate pressure to the (higher) high pressure.

[0044] Again, in the heat exchanger 34 heat energy is transferred from the working fluid into a thermal storage medium from a hot storage 52. For example, the heat exchanger 34 is a counter flow heat exchanger 34 and the storage medium circulates from a first hot storage tank 54 to a second hot storage tank 56 for exchanging heat with the working fluid.

[0045] Finally, the working fluid is again directed into the first expansion stage.

[0046] In the embodiment of Figure 1a, the flash intercooler 28 is a spray intercooler 28. In alternative embodiments, other types of flash intercoolers 28 may be used.

[0047] Further it should be noted that additional compression and expansion stages may be added. However, It should also be noted that at least one intercooler 28, 30 is required in the charging cycle 12 in order to achieve improved efficiency of the system 10. For example, there may be only two compression stages with a first compressor and a second compressor and only the flash intercooler 28 or the heat exchanger 30 in between the two stages (in the second case only one expansion stage may be needed).

[0048] In a further embodiment, each compression stage may be equipped with a flash intercooler 28, when reheat options are not considered in the discharging cycle. It should be noted that different working fluids may be used for the different cycles, as long as the temperature levels for the heat load of the heat pump, the heat storage and the heat engine are chosen appropriately.

[0049] In an exemplary embodiment, in which the working fluid is carbon dioxide and the thermal storage medium is water, the charging cycle may operate in the temperature range of 5°C and 120 °C. The intercooling occurs at a temperature levels well distributed within this range.

[0050] Summarized, according to an embodiment, the system 10 comprises a first expander 20, in the charging cycle, expanding the working fluid after the first heat exchanger 34 to the intermediate pressure; wherein, in the charging cycle, a first portion of the working fluid at the intermediate pressure is input directly into the flash intercooler 28.

[0051] According to an embodiment, the system 10 comprises a second expander 22, in the charging cycle, expanding a second portion of the working fluid at the intermediate pressure to the low pressure.

[0052] According to an embodiment, the system 10 comprises a third heat exchanger 38, in the charging cycle, transferring heat from a third thermal storage 40 to the working fluid at low pressure and, in the discharging cycle, transferring heat from the working fluid at low pressure to the third thermal storage.

[0053] According to an embodiment, the intercooler comprises a flash intercooler 28 and a third heat exchanger 30, wherein, in the charging cycle, the working fluid between the flash intercooler and the third heat exchanger is compressed from a first intermediate pressure to a second intermediate pressure by a further compressor 16.

[0054] With respect to Fig. 1b, the heat stored in the heat storages 40, 46 and 52 is subsequently utilised in the discharging cycle system 56 shown in Fig. 1 b.

[0055] The working fluid in the discharging cycle system 58 coming from the heat exchanger 38 is pumped from the low pressure to the high pressure by pump 60. After that the working fluid is heated in the heat exchanger 34 and enters a first turbine 62 for converting the heat into mechanical and subsequently into electrical energy. The working fluid is reheated again in heat exchanger 30 and enters a second turbine 64 for generating further electrical energy. In the first turbine 62 the working fluid is expanded from the high pressure to the intermediate pressure and in the second turbine 64 to the low pressure. After that the working fluid is cooled in the heat exchanger 38.

[0056] According to an embodiment, the system 10 comprises a first turbine 62, in the discharging cycle, expanding the working fluid from the high pressure to the intermediate pressure for generating electrical energy and/or a second turbine 64, in the discharging cycle, expanding the working fluid from the intermediate pressure to the low pressure for generating electrical energy.

[0057] According to an embodiment, the system 10 comprises a pump 60, in the discharging cycle, pumping the working fluid from the low pressure to the high pressure during the discharging cycle.

[0058] Figures 2a and 3a show a charging cycle 12a, 12b, and Figures 2b and 3b a discharging cycle 58a, 58b of a transcritical thermoelectric energy storage system 10. The cycles are depicted in pressure-enthalpy diagrams.

[0059] In each of the diagrams, a vapor dome 66 is indicated. The critical point 68 of the working fluid is shown on top of the vapor dome. Left of the vapor dome 66, the working fluid is in liquid phase, right of the vapor dome 66, the working fluid is in gas phase (wet steam phase). Under the vapor dome 66, the working fluid is in a mixed liquid and gas phase. A phase change of the working fluid only occurs, when a state change passes the limiting line of the vapor dome 66. Thus, stated change over the vapor dome 66 and over the critical point 68 do not contain phase changes and may be called transcritical. As may be seen from the diagrams, nearly all state changes of the working fluid in the charging cycle and the discharging cycle are transcritical, and therefore the charging cycle and the discharging cycle are referred to as transcritical.

[0060] Figure 2a illustrates the charging cycle 12 of a storage system 10 which may comprise two heat exchangers 30 for intercooling the working fluid. The charging cycle 12a follows a counter-clockwise direction as indicated by the arrows. The charging cycle 12a starts at point A where the working fluid is first evaporated at a low pressure 70 by utilizing, for example a low grade heat source such as ambient air or by a heat exchanger 38. This transition is indicated in Figure 2a with the line from point A to point B1.

[0061] In the next section of the charging cycle 12a, the resultant vapor is compressed utilizing electrical energy in three stages from point B1 to C1 to a first intermediate pressure 72, from B2 to C2 to a second intermediate pressure 74, and from B3 to C3 to a high pressure 76. Such compression occurring in three stages is a consequence of the thermoelectric energy storage 10 having a compressor train comprising three individual units, for example the compressors 14, 16, 18. In between each of these compression stages the working fluid is cooled from point C1 to B2 and point C2 to B3. For example, the working fluid may be cooled by two heat exchangers 30.

[0062] The hot compressed working fluid exiting the compression train at point C3 is cooled down at constant pressure 76 to point D, for example in a heat exchanger 34. Since the cycle 12a is supercritical between the points C3 and D, no condensation of the working fluid takes place. The heat rejected between point C1 to B2, C2 to B3, and C3 and D is transferred to a thermal storage medium via heat exchangers 30, 34, thereby storing the heat energy. After reaching point D, the cooled working fluid is returned to its initial low pressure state 70 at point A via a thermostatic expansion valve 22 or alternatively with an energy recovering expander.

[0063] Figure 2b illustrates the discharging cycle of a thermoelectric energy storage system 10 with one turbine 62 that follows a clockwise direction as indicated by the arrows. The discharging cycle 58a starts with the compression of the working fluid as it is pumped from point E to point F from low pressure 70 to high pressure 76, for example by pump 60. From point F to point G, the working fluid is in contact with the thermal storage medium in a direct or indirect manner, wherein stored heat is transferred from the thermal storage medium to the working fluid. For example, this may be done with a heat exchanger 34.The working fluid is in a supercritical state between point F and point G, hence no evaporation takes place.

[0064] The subsequent expansion of the working fluid in a turbine 62 from pressure 76 to pressure 70 in order to generate electricity is represented between point G and point H. Finally, the working fluid is condensed to its initial state by exchanging heat, for example with a cooling medium such as ambient air or with a cold storage 40 via a heat exchanger 38. This is represented from point H to point E on Figure 2b.

[0065] When both thermodynamic cycles 12a, 58a shown in Figures 2a and 2b would use the same working fluid, it is noted that the total heat energy generated in the charging cycle 12a is greater than the heat energy requirement of the discharging cycle 58a. Specifically, the total heat energy required for functioning of the discharging cycle 58a, which is equal to the enthalpy difference from point F to point G in Figure 1b, can be provided solely by the heat energy released during the charging cycle between point C3 and point D in Figure 1a.

[0066] Therefore, it would be beneficial to efficiently utilize the excess heat resulting from compressor intercooling. However, this excess heat cannot be used to increase the enthalpy content at point G (which may be envisaged as pushing point G further to the right in the cycle in Figure 1b), because the temperature at which this excess heat is available is lower than the temperature of point G. Thus, according to an embodiment of the invention, a storage system 10 with a charging cycle 12a comprises a discharging cycle, wherein the heat stored during intercooling is used for reheating the working fluid between the expansions in turbines 62, 64.

[0067] Also, the excess heat generated by intercooling cannot be used to increase the power output of the discharging cycle 58 a through increasing the working fluid flow. Thus, according to an embodiment of the invention, a storage system 10 with a discharging cycle 58a comprises a discharging cycle, wherein a flash intercooler 28 is used for cooling the working fluid between two compression stages.

[0068] Figure 3a and Figure 3b depict a charging cycle 12b and a discharging cycle 58b, respectively, on a pressure-enthalpy diagram, which may be performed by an embodiment of the transcritical thermoelectric energy storage system 10 shown in Figures 1 a and 1 b.

[0069] Referring first to Figure 3a, the charging cycle 12b follows a counter-clockwise direction as indicated by the arrows. The charging cycle 12b starts with the expansion of the working fluid which occurs in two stages, between point D and point A1 from pressure 76 to pressure 72 (expander 20), and between point A1 and point A2 from pressure 72 to pressure 70 (expansion valve 22). The working fluid stream is divided at point A1 (stream splitter 26), where a first portion is diverted to point B2 and the remaining portion is expanded further to point A2 (expansion valve 22).

[0070] There is an increase in enthalpy in the remaining portion as it reaches point B1 and in the first compression stage between B1 and C1 from pressure 70 to pressure 72 (compressor 14) and there is an increase in both pressure and enthalpy. The discharge of this first compression stage is cooled by intercooling (intercooler 28). Specifically, point B2 represents the flash intercooler 28 where the hot working fluid from point C1 is mixed with the expanded working fluid from point A1.

[0071] The discharge from the second compression stage, between point B2 and point C2 from pressure 72 to pressure 74, is directed to a heat exchanger 30 where the thermal energy of the working fluid is delivered to a thermal energy storage 46 between points C2 and B3.

[0072] The third compression stage from pressure 74 to pressure 76 occurs between points B3 and C3 (compressor 18).

[0073] Such compression occurring in three stages is a consequence of the thermoelectric energy storage 10 having a compressor train comprising three individual units 14, 16, 18. In between each of these compression stages the working fluid is cooled from point C1 to B2 and point C2 to B3 at constant pressure.

[0074] Similarly, the hot compressed working fluid exiting the compression train at point C3 is cooled down at constant pressure 76 to point D (heat exchanger 34). Since the cycle 12b is supercritical between the points C3 and D, no condensation of the working fluid takes place. The rejected heat energy between points C3 and D is stored in a thermal storage medium (hot storage 52). After reaching point D, the cooled working fluid is returned to its initial low pressure state 70 at point A1 via a work recovering expander 20 / thermostatic expansion valve 22.

[0075] The flash intercooler 28 utilized in the charging cycle 12b may be a direct-contact heat exchanger, where the liquid working fluid from point A1 to be evaporated is injected or sprayed into the compressed working fluid vapour flow at C1. Such a direct-contact heat exchanger comprises a shell filled with a packing of a high specific surface area in order to increase the wetted heat transfer area.

[0076] Figure 3b illustrates the discharging cycle 58b of the thermoelectric energy storage system 10 that follows a clockwise direction as indicated by the arrows. The discharging cycle starts with the compression (pump 60) of the working fluid from low pressure 70 to high pressure 76 and this transition is indicated in Figure 3b with the line from point E to point F.

[0077] From point F to point G1, the working fluid is in contact with the thermal storage medium in a direct or indirect manner, wherein stored heat is transferred from the thermal storage medium to the working fluid at constant pressure (heat exchanger 34). The working fluid is in a supercritical state between point F and point G1, hence no evaporation takes place.

[0078] The subsequent expansion of the working fluid in a turbine 62 in order to generate electricity is represented between point G1 and point H1. Between points H1 and G2 there is a reheat stage at pressure 74, where the reheat energy is provided from the thermal storage 46. Specifically, said thermal storage 46 is coupled to the heat exchanger 30 corresponding to the second intercooling stage in the charging cycle 12b.

[0079] The second expansion of the working fluid from G2 to H2 from pressure 74 to pressure 70 occurs in a second turbine stage (turbine 64). Finally, the working fluid is condensed to its initial state at constant pressure by exchanging heat with a cooling medium such as ambient air or with a heat exchanger 38. This is represented from point H2 to point E on Figure 3b.

[0080] It should be noted that, in an embodiment in which reheat options are not utilized in the discharging cycle, then every compressor stage in the charging cycle can be equipped with a separate flash intercooler.

[0081] In an alternative embodiment, different working fluids may be utilized in the charging and discharging cycles. However, the temperature levels for the charging cycle, the heat storage and the discharging cycle must be adjusted to ensure transfer of heat in the desired direction.

[0082] In a further alternative embodiment, water is used as the working fluid in the charging cycle. Furthermore, another fluid with a high boiling point may be utilised instead of water. In this embodiment, the intercooling heat load is at a suitably high temperature to be stored and used to drive a secondary discharging cycle having a low boiling point working fluid (such as hydrocarbon). In this embodiment, thermal energy stored during intercooling can be efficiently recovered without utilising a flash intercooler.

[0083] The skilled person will be aware that the condenser and the evaporator in the thermoelectric energy storage system may be replaced with a multi-purpose heat exchange device that can assume both roles, since the use of the evaporator in the charging cycle and the use of the condenser in the discharging cycle will be carried out in different periods. Similarly the turbine and the compressor roles can be carried out by the same machinery, referred to herein as a thermodynamic machine, capable of achieving both tasks.

[0084] Further the temperatures, the pressures and the amount of working fluid exiting the stream splitter 26 may be measured and these values may be controlled by valves situated in the working fluid circuit.

[0085] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art and practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or controller or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A thermoelectric energy storage system (10) for storing electrical energy by transferring thermal energy to a thermal storage (52) in a charging cycle, and for generating electricity by retrieving the thermal energy from the thermal storage (52) in a discharging cycle,
the thermoelectric energy storage system (10) comprising:

a working fluid circuit (24) circulating a working fluid;

a first compressor (14), in the charging cycle, compressing the working fluid from a low pressure to an intermediate pressure,

an intercooler (28, 30), in the charging cycle, cooling the working fluid at the intermediate pressure,

a second compressor (18), in the charging cycle, compressing the working fluid from the intermediate pressure to a high pressure,

a first heat exchanger (34), in the charging cycle, transferring heat from the working fluid at the high pressure to the thermal storage (52) and, in the discharging cycle, transferring heat from the thermal storage to the working fluid at the high pressure.

2. The system (10) according to claim 1,
wherein the intercooler comprises a flash intercooler (28).

3. The system (10) according to claim 1 or 2,
wherein the intercooler comprises a second heat exchanger (30),
wherein the second heat exchanger (30), in the charging cycle, transfers heat from the working fluid at the intermediate pressure to a second thermal storage and, in the discharging cycle, transfers heat from the second thermal storage to the working fluid at intermediate pressure.

4. The system (10) according to one of the preceding claims, further comprising:

a first expander (20), in the charging cycle, expanding the working fluid after the first heat exchanger (34) to the intermediate pressure;

wherein, in the charging cycle, a first portion of the working fluid at the intermediate pressure is input into the intercooler.

5. The system (10) according to one of the preceding claims, further comprising:

a second expander (22), in the charging cycle, expanding the working fluid at the intermediate pressure to the low pressure.

6. The system (10) according to one of the preceding claims, further comprising:

a third heat exchanger (38), in the charging cycle, transferring heat from a third thermal storage (40) to the working fluid at low pressure and, in the discharging cycle, transferring heat from the working fluid at low pressure to the third thermal storage.

7. The system (10) according to one of the preceding claims,
wherein the intercooler comprises a flash intercooler (28) and a third heat exchanger (30),
wherein, in the charging cycle, the working fluid between the flash intercooler and the third heat exchanger is compressed from a first intermediate pressure to a second intermediate pressure.

8. The system (10) according to one of the preceding claims, further comprising:

a first turbine (62), in the discharging cycle, expanding the working fluid from the high pressure to the intermediate pressure for generating electrical energy,

a second turbine (64), in the discharging cycle, expanding the working fluid from the intermediate pressure to the low pressure for generating electrical energy.

9. The system (10) according to one of the preceding claims, further comprising:

a pump (60), in the discharging cycle, pumping the working fluid from the low pressure to the high pressure during the discharging cycle.

10. A method for storing electrical energy in a charging cycle and retrieving electrical energy in a discharging cycle,
wherein the charging cycle comprises the steps:

compressing the working fluid from a low pressure to an intermediate pressure for storing electrical energy (particularly for converting electrical energy into heat energy);

cooling the working fluid at the intermediate pressure;

compressing the working fluid from the intermediate pressure to a high pressure for storing electrical energy;

transferring heat from the working fluid at the high pressure to the thermal storage;

wherein the discharging cycle comprises the steps:

transferring heat from the thermal storage to the working fluid at the high pressure;

expanding the working fluid from the high pressure for generating electrical energy.

11. The method of claim 10,
wherein the charging cycle comprises the step:

transferring heat from the working fluid at the intermediate pressure to a second thermal storage;

wherein the discharging cycle comprises the step:

expanding the working fluid from the high pressure to the intermediate pressure for generating electrical energy in a first turbine;

transferring heat from the second thermal storage to the working fluid at intermediate pressure;

expanding the working fluid from the intermediate pressure to the low pressure for generating electrical energy in a second turbine.

12. The method of claim 10 or 11,
wherein the charging cycle comprises the steps:

expanding the working fluid after the heat exchanging at high pressure to the intermediate pressure;

using a first portion of the working fluid at intermediate pressure after the heat exchanging at high pressure for cooling the working fluid before heat exchanging at high pressure.

13. The method of one of claims 10 to 12,
wherein the charging cycle comprises the steps:

expanding the working fluid at the intermediate pressure to the low pressure;

transferring heat from a third thermal storage to the working fluid at low pressure;

wherein the discharging cycle comprises the step:
transferring heat from the working fluid at low pressure to the third thermal storage.

14. The method of one of the steps 10 to 13,
wherein the charging cycle comprises the steps:

compressing the working fluid from a first intermediate pressure to a second intermediate pressure between a flash intercooling with working fluid at the first intermediate pressure and heat exchanging with a second thermal storage at the second intermediate pressure.

15. The method according to one of the claims 10 to 14,
wherein at least one section of the charging cycle and/or the discharging cycle is performed transcritically.

Drawing