Heat recovery

Abstract

A power recovery system for recovering power from a working fluid, comprising a heat exchanger that is configured to receive a first stream of the working fluid, one or more expansion stages for expanding the working fluid to recover power from the working fluid, wherein one or more of the expansion stages is in fluid communication with the heat exchanger, wherein the heat exchanger is configured to transfer heat between the first stream of the working fluid and another stream of the working fluid that is received from one or more of the expansion stages.

Claims

1. A power recovery system for recovering power from a working fluid, comprising: a first heat exchanger that is configured to receive a first stream of the working fluid; and one or more expansion stages for expanding the working fluid to recover power from the working fluid, wherein the one or more expansion stages is in fluid communication with the first heat exchanger, wherein the first heat exchanger is configured to transfer heat between the first stream of the working fluid and a second stream of the working fluid that is received from the one or more expansion stages; wherein the first heat exchanger is configured to transfer heat from the first stream of the working fluid to the second stream of the working fluid that is received from the one or more expansion stages; wherein the heat that is transferred by the first heat exchanger is recovered by cooling the first stream of the working fluid; wherein the first heat exchanger is configured to receive the first stream of the working fluid from a working fluid input; wherein the working fluid input comprises: a source of a liquid, a pump for pumping the liquid to a high pressure, and an evaporator for evaporating the liquid to form a gaseous working fluid; and wherein the source of the liquid comprises a liquid storage tank or a condenser for producing the liquid from a gas.

2. A system according to claim 1, wherein the working fluid is a gaseous working fluid.

3. A system according to claim 1, wherein each of the one or more expansion stages comprises an expander for expanding the working fluid to recover power.

4. A system according to claim 1, wherein the liquid comprises a cryogen, such as liquid air or liquid nitrogen.

5. A system according to claim 1, wherein the first heat exchanger is configured to receive the first stream of the working fluid from another heat exchanger or an expansion stage.

6. A system according to claim 1, further comprising a waste heat recovery apparatus for recovering heat from an external process and using the recovered heat to heat the first stream of the working fluid before the first stream of the working fluid is transported to the first heat exchanger; wherein the waste heat recovery apparatus comprises one or more waste heat exchangers.

7. A system according to claim 6, wherein the waste heat recovery apparatus is for recovering waste heat from an external process, wherein the waste heat recovery apparatus is for recovering waste heat from hot gas in an external process or from an exhaust of an external process.

8. A system according to claim 6, wherein the waste heat recovery apparatus is configured to heat the first stream of the working fluid to a temperature that is higher than an inlet temperature of a first expansion stage.

9. A system according to claim 1, wherein the first heat exchanger is configured to cool the first stream of the working fluid to an inlet temperature of a first expansion stage prior to the first stream of the working fluid being expanded in the first expansion stage.

10. A system according to claim 9, wherein the first expansion stage is configured to return expanded working fluid to the first heat exchanger as the second stream of the working fluid, wherein the first heat exchanger is configured to transfer heat from the first stream of the working fluid to the second stream of the working fluid and output the heated second stream of the working fluid as a third stream of the working fluid; and further comprising a second expansion stage that is configured to receive the third stream of the working fluid and expand the third stream of the working fluid to recover power from the third stream of the working fluid.

11. A power recovery system for recovering power from a working fluid, comprising: a first heat exchanger that is configured to receive a first stream of the working fluid; and one or more expansion stages for expanding the working fluid to recover power from the working fluid, wherein the one or more expansion stages is in fluid communication with the first heat exchanger; wherein the first heat exchanger is configured to transfer heat between the first stream of the working fluid and a second stream of the working fluid that is received from the one or more expansion stages; wherein the first heat exchanger is configured to cool the first stream of the working fluid to an inlet temperature of a first expansion stage prior to the first stream of the working fluid being expanded in the first expansion stage; wherein the first expansion stage is configured to return expanded working fluid to the first heat exchanger as the second stream of the working fluid; wherein the first heat exchanger is configured to transfer heat from the first stream of the working fluid to the second stream of the working fluid and output the heated second stream of the working fluid as a third stream of the working fluid; further comprising a second expansion stage that is configured to receive the third stream of the working fluid and expand the third stream of the working fluid to recover power from the third stream of the working fluid; and further comprising a second heat exchanger that is configured to cool the third stream of the working fluid to an inlet temperature of the second expansion stage prior to the third stream of the working fluid being expanded in the second expansion stage.

12. A system according to claim 11, wherein the second heat exchanger is configured to transfer heat from the third stream of the working fluid to a fourth stream of the working fluid to be transported to the one or more expansion stages.

13. A system according to claim 11, wherein the second expansion stage is configured to return expanded working fluid to the second heat exchanger as a fourth stream of the working fluid.

14. A system according to claim 13, wherein the second heat exchanger is configured to transfer heat from the third stream of the working fluid to the fourth stream of the working fluid and output the heated fourth stream of the working fluid as a fifth stream of the working fluid.

15. A system according to claim 11, wherein at least one heat exchanger and/or at least one expansion stage is configured to return the working fluid to a waste heat recovery apparatus so that the working fluid can be reheated before further expansion and/or heat exchange.

16. A system according to claim 1, comprising at least one valve configured to control a bypass flow of the working fluid to bypass at least one heat exchanger to control a temperature of the first stream of the working fluid exiting the heat exchanger.

17. A system according to claim 1, wherein the working fluid is produced from a cryogen, such as liquid air.

18. A method of using a working fluid within a power recovery system comprising: providing a first heat exchanger within the power recovery system with a first stream of the working fluid; and using the first heat exchanger to transfer heat between the first stream of the working fluid and a second stream of the working fluid that is received from one or more expansion stages; wherein the first heat exchanger is configured to transfer heat from the first stream of the working fluid to the second stream of the working fluid that is received from the one or more expansion stages; wherein the heat that is transferred by the first heat exchanger is recovered by cooling the first stream of the working fluid; wherein the first heat exchanger is configured to receive the first stream of the working fluid from a working fluid input; wherein the working fluid input comprises: a source of a liquid, a pump for pumping the liquid to a high pressure, and an evaporator for evaporating the liquid to form a gaseous working fluid; and wherein the source of the liquid comprises a liquid storage tank or a condenser for producing the liquid from a gas.

19. A method according to claim 18, further comprising: using the first heat exchanger to transfer heat from the first stream of the working fluid to the second stream of the working fluid; and cooling the first stream of the working fluid to recover the heat to transfer to the second stream of the working fluid and expanding the first stream of the working fluid in a first expansion stage to recover power from the first stream of the working fluid.

20. A method according to claim 18, further comprising: returning the expanded first stream of the working fluid to the first heat exchanger as the second stream of the working fluid; and outputting the heated second stream of the working fluid from the first heat exchanger as a third stream of the working fluid.

21. A method according to claim 18, wherein the working fluid is a gaseous working fluid.

22. A method according to claim 18, further comprising using a waste heat recovery apparatus to recover heat from an external process and using the recovered heat to heat the first stream of the working fluid before the first stream of the working fluid is transported to the first heat exchanger, wherein the waste heat recovery apparatus comprises one or more waste heat exchangers.

23. A method according to claim 22, wherein the waste heat recovery apparatus recovers waste heat from an external process, wherein the waste heat recovery apparatus is for recovering waste heat from a hot gas in an external process or from an exhaust of an external process.

24. A method according to claim 22, wherein the waste heat recovery apparatus heats the first stream of the working fluid to a temperature that is higher than an inlet temperature of a first expansion stage.

25. A method according to claim 22, further comprising returning the working fluid from at least one heat exchanger and/or at least one expansion stage to the waste heat recovery apparatus so that the working fluid can be reheated before further expansion and/or heat exchange.

26. A method according to claim 18, wherein the working fluid is produced by evaporating a liquid to form a gas and pumping the gas to a high pressure.

27. A method according to claim 18, wherein the working fluid is produced from a cryogen, such as liquid air.

28. A method according to claim 1, wherein the first heat exchanger is configured to cool the first stream of the working fluid to an inlet temperature of a first expansion stage prior to the first stream of the working fluid being expanded in the first expansion stage; wherein the first expansion stage is configured to return expanded working fluid to the first heat exchanger as the second stream of the working fluid; wherein the first heat exchanger is configured to transfer heat from the first stream of the working fluid to the second stream of the working fluid and output the heated second stream of the working fluid as a third stream of the working fluid; further comprising a second expansion stage that is configured to receive the third stream of the working fluid and expand the third stream of the working fluid to recover power from the third stream of the working fluid; further comprising a second heat exchanger that is configured to cool the third stream of the working fluid to an inlet temperature of the second expansion stage prior to the third stream of the working fluid being expanded in the second expansion stage; wherein the second heat exchanger is configured to transfer heat from the third stream of the working fluid to a fourth stream of the working fluid to be transported to the one or more expansion stages; and wherein the second expansion stage is configured to return expanded working fluid to the second heat exchanger as the fourth stream of the working fluid.

29. A system according to claim 1, wherein a last stream of the working fluid is exhausted to atmosphere after being expanded by a last stage of the one or more expansion stages.

30. A method according to claim 18, further comprising receiving a last stream of the working fluid from a last stage of the one or more expansion stages and exhausting the last stream of the working fluid to atmosphere.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will now be described with reference to the accompanying drawings, in which:

(2) FIG. 1 shows a known power recovery system;

(3) FIG. 2 shows a power recovery system according to a first embodiment of the present invention;

(4) FIG. 3 shows a power recovery system according to a second embodiment of the present invention;

(5) FIG. 4 shows a power recovery system according to a third embodiment of the present invention;

(6) FIG. 5 shows a power recovery system according to a fourth embodiment of the present invention;

(7) FIG. 6 shows a power recovery system according to a fifth embodiment of the present invention;

(8) FIG. 7 shows a power recovery system according to a sixth embodiment of the present invention; and

(9) FIG. 8 shows a power recovery system according to a seventh embodiment of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

(10) In embodiments of the present invention, the power recovery system utilises a working fluid. The working fluid is transported around the power recovery system, such that various streams (e.g. first, second, third, etc.) form a flow path around the power recovery system. It will be understood that any denomination of first, second, third, etc. expansion stages is not necessarily intended to indicate an order in terms of the flow of working fluid. For example, a second expansion stage may be upstream of a first expansion stage.

(11) Whilst cryogenic energy storage systems and methods are mentioned herein, the principles of the invention apply to any any power recovery systems involving multi-stage expansion of a hot gas when the heat is supplied from a heat source which may either be within (thus belong to) or be (external to and) nearby said power recovery systems. The heat supplied by the heat source may comprise one element or a combination of the elements of the following list: waste heat, ambient heat, intentionally-produced heat, heat stored in one or a plurality of thermal stores.

(12) The term intentionally-produced heat refers to any heat produced specifically to heat the working fluid circulating within the power recovery systems prior to each expansion stage. Waste heat may comprise one element or a combination of the elements of the following list: heat produced by power plants (such as nuclear, fossil fuel-based, biofuel-based or solar power plants), heat produced by data centres, heat produced by manufacturing plants (using kilns, ovens or exothermic chemical reactions).

(13) Intentionally-produced heat may comprise one element or a combination of the elements of the following list: concentration solar collector, combustor, load bank.

(14) The heat source may be one thermal store or a plurality of thermal stores, or may comprise at least one thermal store.

(15) The power recovery systems in question may comprise one element or a combination of the elements of the following list: Rankinecycle, Brayton cycle.

(16) In all drawings, the circle labelled with a G represents an electrical generator.

(17) FIG. 2 shows a power recovery system 1010 according to a first embodiment of the present invention. The system is designed to recover power from a working fluid, such as a gaseous working fluid (e.g. air). The system 1010 comprises one or more heat exchangers that are configured to receive a first stream of the working fluid, and transfer heat between the first stream of the working fluid and another stream of the working fluid at another location within the power recovery system. The system also comprises one or more expansion stages for expanding the working fluid to recover power from the working fluid. One or more of the expansion stages is in fluid communication with one or more of the heat exchanger(s).

(18) In particular, the system 1010 comprises first 1601, second 1602 and third 1603 heat exchangers and first 1501, second 1502, third 1503 and fourth 1504 expansion stages. Each expansion stage comprises an expander for expanding working fluid to recover power. Whilst three heat exchangers and four expansion stages are shown in FIG. 2, the skilled person will understand that any suitable number of heat exchangers and expansion stages can be used.

(19) The system 1010 comprises a cryogenic liquid storage tank 1100 for storing a cryogenic liquid, such as liquid air, a pump (e.g. a cryogenic pump) 1200 and an evaporator 1300. The skilled person will understand that any source of liquid, such as a condenser for producing a liquid from a gas, could be used instead of, or in addition to, the tank 1100. Liquid air is drawn from the tank 1100, pumped to a high-pressure (e.g. 140 bar) by the pump 1200, and evaporated in the evaporator 1300 to form a gaseous high-pressure working fluid at approximately ambient temperature (e.g. 15 C.). The cold recovered from the evaporator 1300 may either be ejected to atmosphere or recovered in a cold storage system (not shown) to be used later in a charge phase of a LAES system.

(20) The system 1010 further comprises a waste heat recovery apparatus 1400 for recovering heat from an external process and using the recovered heat to heat a stream of the working fluid before the stream of the working fluid is transported to a heat exchanger. The waste heat recovery apparatus 1400 is typically configured to heat the stream of the working fluid to a temperature that is higher than an inlet temperature of an expansion stage. A heat exchanger can then be used to cool the stream of the working fluid to the inlet temperature of the expansion stage prior to the stream of the working fluid being expanded in the expansion stage.

(21) In the system shown in FIG. 2, the waste heat recovery apparatus 1400 comprises a first 1401 waste heat exchanger that is thermally coupled to a waste heat source, such as an exhaust stack of an Open-Cycle Gas Turbine (OCGTnot shown). Whilst one waste heat exchanger is shown in FIG. 2, the skilled person will understand that any suitable number of waste heat exchangers can be used.

(22) The high-pressure working fluid is conveyed e.g. at 140 bar from the evaporator 1300 to the first waste heat exchanger 1401 where it is heated in heat exchange with the exhaust gases of the OCGT to a high temperature, for example to approximately 450 C.

(23) The first heat exchanger 1601 is configured to receive a first stream 1701 of the working fluid. In the system 1010 shown in FIG. 2, the first heat exchanger 1601 is configured to receive the first stream 1701 of the working fluid from the first waste heat exchanger 1401, and to transfer heat from the first stream 1701 of the working fluid to another stream of the working fluid elsewhere in the power recovery system 1010. The heat that is transferred by the heat exchanger is typically recovered by cooling the first stream 1701 of the working fluid. In alternative embodiments, the first heat exchanger may receive the first stream of the working fluid from another heat exchanger or an expansion stage. The cooled first stream 1701 of the working fluid is then expanded in the first expansion stage 1501 to produce work.

(24) The first heat exchanger 1601 is configured to transfer heat from the first stream 1701 of the working fluid to a second stream 1702 of the working fluid and output the heated second stream 1702 of the working fluid as a third stream 1703 of the working fluid. The system further comprises a second expansion stage 1502 that is configured to receive the third stream 1703 of the working fluid and expand the third stream 1703 of the working fluid to recover power from the third stream 1703 of the working fluid

(25) In an exemplary embodiment, the heated high-pressure working fluid (the first stream 1701) is conveyed from the first waste heat exchanger 1401 to the first heat exchanger 1601 where it is cooled to a suitable input temperature of the first expansion stage 1501 (typically approximately 275 C.) before being expanded in the first expansion stage 1501 to produce work. The first expansion stage 1501 is configured to return the expanded working fluid to the first heat exchanger 1601 as a second stream 1702 of the working fluid. The first heat exchanger 1601 is configured to transfer heat from the first stream 1701 of the working fluid (i.e. the working fluid received from the first waste heat exchanger 1401) to the second stream 1702 of the working fluid (i.e. the working fluid received from the first expansion stage 1501) and output the heated second stream of the working fluid as a third stream 1703 of the working fluid.

(26) In particular, in one embodiment, the second stream 1702 of the working fluid (i.e. exhaust from the first expansion stage 1501) emerges from the first expansion stage 1501 at approximately 160 C. and 45 bar and is heated to approximately 340 C. in the first heat exchanger 1601 in heat exchange with the working fluid received from the first waste heat exchanger 1401.

(27) As described previously, the system 1010 further comprises a second heat exchanger 1602 and a second expansion stage 1502. The second heat exchanger 1602 is configured to cool the third stream 1703 of the working fluid to an inlet temperature of the second expansion stage 1502 prior to the third stream 1703 of the working fluid being expanded in the second expansion stage 1502. In a similar manner to the first expansion stage 1501, the second expansion stage 1502 is configured to return expanded working fluid (i.e. expanded working fluid from the third stream 1703) to the second heat exchanger 1502 as a fourth stream 1704 of working fluid. The second heat exchanger 1502 is then configured to transfer heat from the third stream 1703 of the working fluid to the fourth stream 1704 of the working fluid and output the heated fourth stream 1704 of the working fluid as a fifth stream 1705 of the working fluid.

(28) In particular, in one embodiment, the third stream 1703 of the working fluid is cooled in the second heat exchanger 1602 to 275 C. before being expanded in the second expansion stage 1502. The expanded third stream 1703 of the working fluid emerges at approximately 150 C. and 15 bar as the fourth stream 1704 of the working fluid. The fourth stream 1704 of the working fluid is then reheated in the second heat exchanger 1602 to approximately 220 C. using heat recovered from the third stream 1703 of the working fluid.

(29) This process is then repeated as desired. As described previously, the system 1010 shown in FIG. 2 further comprises a third heat exchanger 1603 and third 1503 and fourth 1504 expansion stages. The fifth stream 1705 of the working fluid that is output by the second heat exchanger 1602 is cooled (e.g. to 200 C.) in the third heat exchanger 1603 and expanded in third expansion stage 1503. Working fluid emerges from the exhaust of the third expansion stage 1503 (e.g. at approximately 75 C. and 4 bar) and is reheated (e.g. to 94 C.) in the third heat exchanger 1603 before being expanded in the fourth expansion stage 1504 and exhausted to atmosphere.

(30) A person skilled in the art will recognise that if the exhaust (i.e. waste heat) of the OCGT is at a high enough temperature, for example 650 C., the first stream 1701 of the working fluid may be heated to a high enough temperature in the first waste heat exchanger 1401 to provide sufficient heat to reheat all stages to 275 C. (i.e. a suitable input temperature of one or more of the expansion stages).

(31) Whilst first 1601, second 1602 and third 1603 heat exchangers have been described, it will be understood that each of these heat exchangers is configured to transfer heat between a first stream of the working fluid and another stream of the working fluid that is received from one or more of the expansion stages. Thus, any of the heat exchangers 1601, 1602 and 1603 can be a heat exchanger (or a first heat exchanger) within the meaning of the present invention.

(32) FIG. 3 shows a system 2010 according to a second embodiment of the present invention. The system 2010 comprises one or more heat exchangers that are configured to receive a first stream of the working fluid, and transfer heat between the first stream of the working fluid and another stream of the working fluid at another location within the power recovery system. The system also comprises one or more expansion stages for expanding the working fluid to recover power from the working fluid. One or more of the expansion stages is in fluid communication with the heat exchanger.

(33) The system 2010 shown in FIG. 3 is like the system 1010 shown in FIG. 2 except that the second heat exchanger 1602 of FIG. 2 is replaced with a second waste heat exchanger 2402, which is situated in the waste heat recovery apparatus 2400.

(34) In particular, the system 2010 comprises first 2601 and second 2602 heat exchangers and first 2501, second 2502, third 2503 and fourth 2504 expansion stages. Each expansion stage comprises an expander for expanding working fluid to recover power. Whilst two heat exchangers and four expansion stages are shown in FIG. 3, the skilled person will understand that any suitable number of heat exchangers and expansion stages can be used.

(35) Like the system 1010 shown in FIG. 2, the system 2010 comprises a cryogenic liquid storage tank 2100 for storing a cryogenic liquid, such as liquid air, a pump (e.g. a cryogenic pump) 2200 and an evaporator 2300. The skilled person will understand that any source of liquid, such as a condenser for producing a liquid from a gas, could be used instead of, or in addition to, the tank 2100. Liquid air is drawn from the tank 2100, pumped to a high-pressure (e.g. 140 bar) by the pump 2200, and evaporated in the evaporator 2300 to form a gaseous high-pressure working fluid at approximately ambient temperature (e.g. 15 C.). The cold recovered from the evaporator 2300 may either be ejected to atmosphere or recovered in a cold storage system (not shown) to be used later in a charge phase of a LAES system.

(36) Like the system 1010 shown in FIG. 2, the system 2010 further comprises a waste heat recovery apparatus 2400 for recovering heat from an external process and using the recovered heat to heat a stream of the working fluid before the stream of the working fluid is transported to a heat exchanger. The waste heat recovery apparatus 2400 is typically configured to heat the stream of the working fluid to a temperature that is higher than an inlet temperature of an expansion stage. A heat exchanger can then be used to cool the stream of the working fluid to the inlet temperature of the expansion stage prior to the stream of the working fluid being expanded in the expansion stage.

(37) In the system shown in FIG. 3, the waste heat recovery apparatus 2400 comprises first 2401 and second 2402 waste heat exchangers that are thermally coupled to a waste heat source, such as an exhaust stack of an Open-Cycle Gas Turbine (OCGTnot shown). Whilst two waste heat exchangers are shown in FIG. 3, the skilled person will understand that any suitable number of waste heat exchangers can be used.

(38) The high-pressure working fluid is conveyed from the evaporator 2300 to the first waste heat exchanger 2401 where it is heated in heat exchange with the exhaust gases of the OCGT to a high temperature, typically approximately 450 C.

(39) The first heat exchanger 2601 is configured to receive a first stream 2701 of the working fluid. In the system 2010 shown in FIG. 3, the first heat exchanger 2601 is configured to receive the first stream 2701 of the working fluid from the first waste heat exchanger 2401, and to transfer heat from the first stream 2701 of the working fluid to another stream of the working fluid elsewhere in the power recovery system 2010. The heat that is transferred by the heat exchanger is recovered by cooling the first stream 2701 of the working fluid. The cooled first stream 2701 of the working fluid is then expanded in the first expansion stage 2501 to produce work.

(40) In alternative embodiments, the first heat exchanger may receive the first stream of the working fluid from another heat exchanger or an expansion stage.

(41) In particular, the first heat exchanger 2601 is configured to transfer heat from the first stream 2701 of the working fluid to a second stream 2702 of the working fluid and output the heated second stream 2702 of the working fluid as a third stream 2703 of the working fluid. The system further comprises a second expansion stage 2502 that is configured to receive the third stream 2703 of the working fluid and expand the third stream 2703 of the working fluid to recover power from the third stream 2703 of the working fluid.

(42) In an exemplary embodiment, the heated high-pressure working fluid (the first stream 2701) is conveyed from the first waste heat exchanger 2401 to the first heat exchanger 2601 where it is cooled to a suitable input temperature of the first expansion stage 2501 (typically approximately 275 C.) before being expanded in the first expansion stage 2501 to produce work. The first expansion stage 2501 is configured to return the expanded working fluid to the first heat exchanger 2601 as a second stream 2702 of the working fluid. The first heat exchanger 2601 is configured to transfer heat from the first stream 2701 of the working fluid (i.e. the working fluid received from the first waste heat exchanger 2401) to the second stream 2702 of the working fluid (i.e. the working fluid received from the first expansion stage 2501) and output the heated second stream of the working fluid as a third stream 2703 of the working fluid.

(43) In particular, in one embodiment, the second stream 2702 of the working fluid (i.e. exhaust from the first expansion stage 2501) emerges from the first expansion stage 2501 at approximately 160 C. and 45 bar and is heated to approximately 340 C. in the first heat exchanger 2601 in heat exchange with the working fluid received from the first waste heat exchanger 2401.

(44) As described previously, the system 2010 further comprises a second expansion stage 2502. The third stream 2703 of the working fluid is expanded in the second expansion stage 2502 to recover power from the third stream 2703 of the working fluid. The second expansion stage 2502 is configured to return expanded working fluid (i.e. expanded working fluid from the third stream 2703) to the second waste heat exchanger 2402 in the waste heat recovery apparatus 2400 as a fourth stream 2704 of the working fluid so that the fourth stream 2704 of the working fluid can be reheated by the second waste heat exchanger 2402 (e.g. to approximately 410 C.). The working fluid that is reheated in the second waste heat exchanger 2402 is then conveyed to the second heat exchanger 2602 where it is cooled (e.g. to 275 C.) before being expanded in the third expansion stage 2503. The exhaust from the third expansion stage 2503 emerges (e.g. at approximately 130 C.) and is heated in the second heat exchanger 2602 (e.g. to 275 C.) before being expanded in the fourth expansion stage 2504 and exhausted to atmosphere.

(45) Whilst first 2601 and second 2602 heat exchangers have been described, it will be understood that both of these heat exchangers are configured to transfer heat between a first stream of the working fluid and another stream of the working fluid that is received from one or more of the expansion stages. Thus, any of the heat exchangers 1601 and 1602 can be a heat exchanger (or a first heat exchanger) within the meaning of the present invention.

(46) The system 2010 provides for increased performance over the system 1010 due to the higher inlet temperatures of the third and fourth expansion stages. However, it is more costly due to the second return trip to the waste heat recovery unit. It is nevertheless less costly than existing systems, such as the system 10 shown in FIG. 1. In other words, the system 2010 provides an advantageous solution of high performance at a low cost.

(47) FIG. 4 shows a system 3010 according to a third embodiment of the present invention. The system 3010 comprises one or more heat exchangers that are configured to receive a first stream of the working fluid, and transfer heat between the first stream of the working fluid and another stream of the working fluid at another location within the power recovery system. The system also comprises one or more expansion stages for expanding the working fluid to recover power from the working fluid. One or more of the expansion stages is in fluid communication with one or more of the heat exchanger(s).

(48) In particular, the system 3010 comprises first 3601, second 3602 and third 3603 heat exchangers and first 3501, second 3502, third 3503 and fourth 3504 expansion stages. Each expansion stage comprises an expander for expanding working fluid to recover power. Whilst three heat exchangers and four expansion stages are shown in FIG. 4, the skilled person will understand that any suitable number of heat exchangers and expansion stages can be used.

(49) Like the systems 1010 and 2010 shown in FIGS. 2 and 3, respectively, the system 3010 comprises a cryogenic liquid storage tank 3100 for storing a cryogenic liquid, such as liquid air, a pump (e.g. a cryogenic pump) 3200 and an evaporator 3300. The skilled person will understand that any source of liquid, such as a condenser for producing a liquid from a gas, could be used instead of, or in addition to, the tank 3100. Liquid air is drawn from the tank 3100, pumped to a high-pressure (e.g. 140 bar) by the pump 3200, and evaporated in the evaporator 3300 to form a gaseous high-pressure working fluid at approximately ambient temperature (e.g. 15 C.). The cold recovered from the evaporator 3300 may either be ejected to atmosphere or recovered in a cold storage system (not shown) to be used later in a charge phase of a LAES system.

(50) Like the systems 1010 and 2010 shown in FIGS. 2 and 3, respectively, the system 3010 further comprises a waste heat recovery apparatus 3400 for recovering heat from an external process and using the recovered heat to heat a stream of the working fluid before the stream of the working fluid is transported to a heat exchanger. The waste heat recovery apparatus 3400 is typically configured to heat the stream of the working fluid to a temperature that is higher than an inlet temperature of an expansion stage. A heat exchanger can then be used to cool the stream of the working fluid to the inlet temperature of the expansion stage prior to the stream of the working fluid being expanded in the expansion stage.

(51) In the system shown in FIG. 4, the waste heat recovery apparatus 3400 comprises first 3401 and second 3402 waste heat exchangers that are thermally coupled to a waste heat source, such as an exhaust stack of an Open-Cycle Gas Turbine (OCGTnot shown). Whilst two waste heat exchangers are shown in FIG. 4, the skilled person will understand that any suitable number of waste heat exchangers can be used.

(52) In the system shown in FIG. 4, each of the first 3601, second 3602 and third 3603 heat exchangers is configured to receive a first stream of the working fluid, and transfer heat between the first stream of the working fluid and a second stream of the working fluid at another location within the power recovery system. Furthermore, the heat that is transferred by the first 3601, second 3602 and third 3603 heat exchangers is typically recovered by cooling the first stream of the working fluid and transferring heat recovered from the cooling to the second stream of the working fluid. Additionally, each of the first 3601, second 3602 and third 3603 heat exchangers is configured to transfer heat from the first stream of the working fluid to the second stream of the working fluid and output the heated second stream of the working fluid as a third stream of the working fluid.

(53) In the arrangement shown in FIG. 4, the first heat exchanger 3601 is configured to receive a first stream 3701a of the working fluid from the first waste heat exchanger 3401, and to transfer heat from this first stream 3701a of the working fluid to a second stream 3702a of the working fluid received from the third heat exchanger 3603 via the first expansion stage 3501. The first heat exchanger 3601 then outputs a third stream 3703a of the working fluid.

(54) Similarly, the second heat exchanger 3602 is configured to receive a first stream 3701b of the working fluid from the first heat exchanger 3601, and to transfer heat from this first stream 3701b of the working fluid to a second stream 3702b of the working fluid received from the second expansion stage 3502. The second heat exchanger 3602 then outputs a third stream 3703b of the working fluid.

(55) Similarly, the third heat exchanger 3603 is configured to receive a first stream 3701c of the working fluid from the second waste heat exchanger 3402, and to transfer heat from this first stream 3701c of the working fluid to a second stream 3702c of the working fluid received from the third expansion stage 3503. The third heat exchanger 3603 then outputs a third stream 3703c of the working fluid.

(56) Thus, each of the first 3601, second 3602 and third 3603 heat exchangers shown in FIG. 4 applies the advantageous principles of the present invention.

(57) In the system shown in FIG. 4, high-pressure working fluid is conveyed from the evaporator 3300 to the first waste heat exchanger 3401 where it is heated in heat exchange with the exhaust gases of the OCGT to a high temperature, typically approximately 450 C.

(58) The heated high-pressure working fluid is then conveyed to the first heat exchanger 3601 as first stream 3701a where it is cooled (e.g. to approximately 220 C.) before being conveyed back to the waste heat recovery apparatus 3400 where it is heated (e.g. to approximately 410 C.) in the second waste heat exchanger 3402. The heated working fluid exiting from the second waste heat exchanger 3402 is conveyed to the third heat exchanger 3603 as first stream 3701c where it is cooled (e.g. to 275 C.) before being expanded in first expansion stage 3501 to produce work. The exhaust from the first expansion stage 3501 emerges (e.g. at approximately 160 C. and 45 bar) as second stream 3702a and is heated (e.g. to approximately 390 C.) in the first heat exchanger 3601 in heat exchange with the first stream 3701a of the working fluid received from the first waste heat exchanger 3401. The first heat exchanger 3601 then outputs third stream 3703a of the working fluid.

(59) The third stream 3703a (or first stream 3701b) of the working fluid is then cooled in the second heat exchanger 3602 (e.g. to 275 C.) before being expanded in the second expansion stage 3502, emerging as second stream 3702b of working fluid, for example at approximately 150 C. and 15 bar. The second stream 3702b of the working fluid is then reheated in the second heat exchanger 3602 (e.g. to approximately 275 C.) using heat from the first stream 3701b of the working fluid before being output as the third stream 3703b and expanded in the third expansion stage 3503. The exhaust from the third expansion stage 3503 emerges (e.g. at approximately 130 C. and 4 bar) as the second stream 3702c of the working fluid and is reheated (e.g. to approximately 275 C.) in the third heat exchanger 3603 using heat recovered from the first stream 3701c of the working fluid before emerging as third stream 3703c, being expanded in fourth expansion stage 3504 and exhausted to atmosphere.

(60) Similarly to system 2010, system 3010 provides for increased performance over system 1010 due to the higher inlet temperatures on the third and fourth expansion stages. System 3010 is more costly then system 1010 due to the second return trip to the waste heat recovery apparatus. System 3010 may nevertheless be less costly than system 2010 as the working fluid is typically conveyed to the waste heat recovery apparatus at approximately 140 bar (i.e. a higher pressure), which requires a smaller pipe diameter. While the pipework must withstand higher pressure, the cost may be less than a lower pressure, larger diameter pipe. In other words, the system 3010 provides an advantageous solution of high performance at a low cost.

(61) FIG. 5 shows a system 4010 according to a fourth embodiment of the present invention. The system comprises first 4601, second 4602 and third 4603 heat exchangers and first 4501, second 4502, third 4503 and fourth 4504 expansion stages. The system 4010 also comprises a cryogenic liquid storage tank 4100 for storing a cryogenic liquid, such as liquid air, a pump (e.g. a cryogenic pump) 4200 and an evaporator 4300. The skilled person will understand that any source of liquid, such as a condenser for producing a liquid from a gas, could be used instead of, or in addition to, the tank 4100. Additionally, the system 4010 comprises a waste heat recovery apparatus 4400 comprising a first 4401 waste heat exchanger that is thermally coupled to a waste heat source, such as an exhaust stack of an Open-Cycle Gas Turbine (OCGTnot shown).

(62) The system 4010 shown in FIG. 5 operates in the same way as the system 1010 shown in FIG. 2, but with the addition of control valves 4801, 4802, 4803 and 4804, which allow the temperatures of the streams emerging from the waste heat exchanger 4401 and the heat exchangers 4601, 4602 and 4603 to be controlled by bypassing the heat exchangers with a portion of the flow on the cold side of the exchanger.

(63) FIG. 6 shows a system 5010 according to a fifth embodiment of the present invention. The system comprises first 5601, second 5602 and third 5603 heat exchangers and first 5501, second 5502, third 5503 and fourth 5504 expansion stages. The system 5010 also comprises a cryogenic liquid storage tank 5100 for storing a cryogenic liquid, such as liquid air, a pump (e.g. a cryogenic pump) 5200 and an evaporator 5300. The skilled person will understand that any source of liquid, such as a condenser for producing a liquid from a gas, could be used instead of, or in addition to, the tank 5100. Additionally, the system 5010 comprises a waste heat recovery apparatus 5400 comprising a first 5401 waste heat exchanger that is thermally coupled to a waste heat source, such as an exhaust stack of an Open-Cycle Gas Turbine (OCGTnot shown).

(64) The system 5010 shown in FIG. 6 operates in the same way as the system 1010 shown in FIG. 2, but differs in the working fluid can be shared between the first 5601, second 5602 and third 5603 heat exchangers (both at an input side 5901 and an output side 5902). Working fluid can also be delivered to, or output by, the first 5601, second 5602 and third 5603 heat exchangers in parallel. If the working fluid exiting 5401 is at a high enough temperature, for example 650 C., it may provide sufficient heat to reheat all stages to a suitable input temperature (e.g. 275 C.).

(65) An advantage of system 5010 is that the heat exchangers can operate with a higher pressure on the hot side, which offers potential savings in the cost of the exchanger due to reduced surface area for heat exchange on the hot side. This may be offset by the requirement for more of the heat exchangers to withstand higher pressure, but the embodiment shown in FIG. 6 is intended to show the flexibility in selecting the cheapest heat exchangers by adapting the process within the limits of the present invention.

(66) FIG. 7 shows a system 6010 according to a sixth embodiment of the present invention. The system comprises first 6601, second 6602 and third 6603 heat exchangers and first 6501, second 6502, third 6503 and fourth 6504 expansion stages. The system 6010 also comprises a cryogenic liquid storage tank 6100 for storing a cryogenic liquid, such as liquid air, a pump (e.g. a cryogenic pump) 6200 and an evaporator 6300. The skilled person will understand that any source of liquid, such as a condenser for producing a liquid from a gas, could be used instead of, or in addition to, the tank 6100. Additionally, the system 6010 comprises a waste heat recovery apparatus 6400 comprising a first 6401 waste heat exchanger that is thermally coupled to a waste heat source, such as an exhaust stack of an Open-Cycle Gas Turbine (OCGTnot shown).

(67) The system 6010 shown in FIG. 7 operates in the same way as the system 1010 shown in FIG. 2, but differs in that the working fluid flows through the first 6601, second 6602 and third 6603 heat exchangers in series. Whilst FIG. 7 shows an embodiment in which the working fluid is transported through the first heat exchanger 6601, second heat exchanger 6602 and third heat exchanger 6603 in that order, it will be understood that, in other embodiments, the working fluid can be transported through the heat exchangers 6601, 6602, 6603 in any order. If the working fluid exiting the first waste heat exchanger 6401 is at a high enough temperature, for example 650 C., it may provide sufficient heat to reheat all stages to 275 C.

(68) An advantage of system 6010 over system 5010 is that it is possible to simplify the pipework required as complex manifold arrangements are not required to divide the stream between multiple heat exchangers.

(69) FIG. 8 shows a system 7010 according to a seventh embodiment of the invention. The system 7010 comprises first 7601, second 7602 and third 7603 heat exchangers and first 7501, second 7502, third 7503 and fourth 7504 expansion stages. The system 7010 also comprises a cryogenic liquid storage tank 7100 for storing a cryogenic liquid, such as liquid air, a pump (e.g. a cryogenic pump) 7200 and an evaporator 7300. The skilled person will understand that any source of liquid, such as a condenser for producing a liquid from a gas, could be used instead of, or in addition to, the tank 7100. Additionally, the system 7010 comprises a waste heat recovery apparatus 7400 comprising a first 7401 waste heat exchanger that is thermally coupled to a waste heat source, such as an exhaust stack of an Open-Cycle Gas Turbine (OCGTnot shown).

(70) The system 7010 combines aspects of systems 1010 and 6010 described above.

(71) The present invention provides the advantage that the working fluid of a power recovery system is used in place of a conventional intermediate heat transfer fluid to transfer heat between streams of the working fluid within the power recovery system. An advantage of this arrangement is that working fluid may be conveyed fewer times to and from a waste heat recovery apparatus. This results in improved performance and, crucially, reduced pipework costs.

(72) The present invention has been described above in exemplary form with reference to the accompanying drawings which represent a single embodiment of the invention. It will be understood that many different embodiments of the invention exist, and that these embodiments all fall within the scope of the invention as defined by the following claims.