HEAT-OF-COMPRESSION RECYCLE SYSTEM, AND SUB-SYSTEMS THEREOF

Abstract

Power recovery sub-systems, cryogenic energy storage systems, and methods of capturing, storing, and re-using thermal energy are disclosed.

Claims

1. A power recovery sub-system for a cryogenic energy storage system, the power recovery sub-system comprising: a first heat source; a first heat exchanger; a second heat exchanger; a first expansion stage; a second expansion stage; a first arrangement of conduits, having an upstream end and a downstream end, and configured to pass a working fluid through the first heat exchanger, the first expansion stage, the second heat exchanger, and the second expansion stage; and a second arrangement of conduits configured to pass a first heat transfer fluid from the first heat source, through the first heat exchanger and the second heat exchanger, wherein the second arrangement of conduits is further configured to pass a first portion of the first heat transfer fluid through the first heat exchanger and pass a second portion of the first heat transfer fluid through the second heat exchanger.

2. The sub-system of claim 1, further comprising: a third heat exchanger; and a third expansion stage; wherein the first arrangement of conduits is further configured to pass the working fluid through the third heat exchanger and the third expansion stage; and wherein the second arrangement of conduits is further configured to pass a third portion of the first heat transfer fluid through the third heat exchanger.

3. The sub-system of claim 2, further comprising; a second heat source; a fourth heat exchanger; a fourth expansion stage; and a third arrangement of conduits configured to pass a second heat transfer fluid from the second heat source, through the fourth heat exchanger, wherein the first arrangement of conduits is further configured to pass the working fluid through the fourth heat exchanger and the fourth expansion stage.

4. The sub-system according to claim 3, further comprising: a fifth heat exchanger; and a fifth expansion stage; wherein the first arrangement of conduits is further configured to pass the working fluid through the fifth heat exchanger and the fifth expansion stage; and wherein the third arrangement of conduits is further configured to pass a first portion of the second heat transfer fluid through the fourth heat exchanger and pass a second portion of the second heat transfer fluid through the fifth heat exchanger.

5. The sub-system of claim 3, wherein the or each heat exchanger through which the third arrangement of conduits passes is positioned along the first arrangement of conduits upstream of the heat exchangers through which the second arrangement of conduits passes.

6. The sub-system of claim 3, wherein the or each heat exchanger through which the third arrangement of conduits passes is positioned along the first arrangement of conduits downstream of the heat exchangers through which the second arrangement of conduits passes.

7. The sub-system according to claim 3, further comprising: a sixth heat exchanger, wherein the first arrangement of conduits is further configured to pass the working fluid through the sixth heat exchanger upstream of both (i) the furthest upstream heat exchanger through which the second arrangement of conduits passes and (ii) the furthest upstream heat exchanger through which the third arrangement of conduits passes, and wherein the first arrangement of conduits is further configured to pass the working fluid output from the furthest downstream expansion stage through the sixth heat exchanger to an exhaust.

8. The sub-system according to claim 1, further comprising: a fourth arrangement of conduits configured to divert a portion of the working fluid from a downstream position in the first arrangement of conduits through an evaporator and a first compressor, and return it to an upstream position in the first arrangement of conduits.

9. The sub-system according to claim 8, wherein the evaporator is positioned along the first arrangement of conduits upstream of the furthest upstream heat exchanger, wherein the downstream position is downstream of the furthest downstream expansion stage; and wherein the upstream position is immediately upstream of the furthest downstream expansion stage.

10. The sub-system according to claim 3, configured such that the second arrangement of conduits passes through the first, second and third heat exchangers and preferably no other heat exchanger and the third arrangement of conduits passes through the fourth heat exchanger and preferably no other heat exchanger, and wherein the heat exchanger through which the third arrangement of conduits passes is upstream of the heat exchangers through which the second arrangement of conduits passes.

11. The sub-system according to claim 4, configured such that the second arrangement of conduits passes through the first, second, and third heat exchangers and preferably no other heat exchanger and the third arrangement of conduits passes through the fourth and fifth heat exchangers and preferably no other heat exchanger, wherein the heat exchangers through which the second arrangement of conduits passes are upstream of the heat exchangers through which the third arrangement of conduits passes.

12. The sub-system according to claim 3, configured such that the second arrangement of conduits passes through the first, second and third heat exchangers and preferably no other heat exchanger and the third arrangement of conduits passes through the fourth heat exchanger and preferably no other heat exchanger, wherein the heat exchangers through which the second arrangement of conduits passes are upstream of the heat exchanger through which the third arrangement of conduits passes.

13. The sub-system of claim 3, wherein the first heat source is a first thermal energy storage device and the second arrangement of conduits is further configured to return the first heat transfer fluid to the first thermal energy storage device after passing it through each heat exchanger through which the second arrangement of conduits is configured to pass, such that the second arrangement of conduits forms a first closed circuit.

14. The sub-system of claim 13, wherein the second heat source is a second thermal energy storage device and the third arrangement of conduits is further configured to return the second heat transfer fluid to the second thermal energy storage device after passing it through each heat exchanger through which the third arrangement of conduits is configured to pass, such that the third arrangement of conduits forms a second closed circuit.

15. The sub-system of claim 14, wherein the first thermal energy storage device is configured to store at least a portion of a heat of compression generated by a recycle air compressor and the second thermal energy storage device is configured to store at least a portion of a heat of compression generated by a main air compressor, optionally wherein the second thermal energy storage device may comprise pipework suitable for transporting molten salts.

16. The sub-system of claim 3, further comprising: a tenth heat exchanger; and an eleventh heat exchanger, wherein: the second heat source is a second thermal energy storage device, the first arrangement of conduits is further configured to pass the working fluid through the tenth heat exchanger immediately upstream of the fourth heat exchanger, and wherein; the third arrangement of conduits is configured to form a first closed loop and a second closed loop, the first closed loop passing through the second thermal energy storage device and the eleventh heat exchanger, and the second closed loop passing through the eleventh heat exchanger and the fourth heat exchanger, optionally wherein a heat transfer fluid in the first closed loop comprises molten salts, further optionally wherein a heat transfer fluid in the second closed loop comprises a thermal oil or a mixture of thermal oils.

17. The sub-system of claim 16, wherein the first thermal energy storage device is configured to store at least a portion of a heat of compression generated by a main air compressor and at least a portion of the heat of compression generated by a recycle air compressor, and the second thermal energy storage device is configured to store and at least a portion of the heat of compression generated by the main air compressor, optionally wherein the second thermal energy storage device may comprise pipework suitable for transporting molten salts.

18. The sub-system of claim 14, wherein the second thermal energy storage device is configured to store thermal energy at a higher temperature than the temperature of the thermal energy stored in the first thermal energy storage device, optionally wherein the second thermal energy storage device is configured to store thermal energy between 150 C. and 550 C., preferably between 200 C. and 400 C., and the first thermal energy storage device is configured to store thermal energy between 150 C. and 350 C.

19. A cryogenic energy storage system, comprising: a power recovery sub-system comprising a plurality of expansion stages configured to receive, via a corresponding plurality of heat exchangers, hot thermal energy from a first thermal energy storage device and a second thermal energy storage device and transfer it to a working fluid passing through the plurality of expansion stages and plurality of heat exchangers, wherein the power recovery sub-system is according to claim 3; and a liquefaction sub-system configured to supply thermal energy to the first and second thermal energy storage devices, and further comprising; a main air compressor; a recycle air compressor; an eighth heat exchanger; a ninth heat exchanger; a fifth arrangement of conduits configured to pass a process stream through the main air compressor, eighth heat exchanger, recycle air compressor, and ninth heat exchanger; a sixth arrangement of conduits forming a third closed circuit and configured to pass a third heat transfer fluid between the second thermal energy storage device and the eighth heat exchanger; and a seventh arrangement of conduits forming a fourth closed circuit and configured to pass a fourth heat transfer fluid between the first thermal energy storage device and the ninth heat exchanger, wherein the eighth heat exchanger is positioned along the fifth arrangement of conduits immediately downstream of the main air compressor and configured to transfer at least a portion of a heat of compression of the process stream from the main air compressor, via the third heat transfer fluid to the second thermal energy storage device, and wherein the ninth heat exchanger is positioned along the fifth arrangement of conduits immediately downstream of the recycle air compressor and configured to transfer at least a portion of the heat of compression of the process stream from the recycle air compressor, via the fourth heat transfer fluid to the first thermal energy storage device.

20. A thermal energy recycle system, comprising: a main air compressor a recycle air compressor; a second thermal energy storage device; a first thermal energy storage device; a working fluid; and a plurality of expansion stages, comprising a first and second subset; wherein the system is configured to capture at least a portion of a heat of compression produced by the main air compressor and store it in the second thermal energy storage device during a liquefaction phase, and to apply the heat of compression stored in the second thermal energy storage device to the working fluid upstream of each of the first subset of expansion stages during a power recovery phase, and wherein the system is further configured to capture at least a portion of the heat of compression produced by the recycle air compressor and store it in the first thermal energy storage device during a liquefaction phase, and to apply the heat of compression stored in the first thermal energy storage device to the working fluid upstream of each of the second subset of expansion stages during a power recovery phase.

21. A method for recycling thermal energy in a cryogenic energy storage system, comprising: providing a liquefaction sub-system comprising: a main air compressor; a recycle air compressor; a second thermal energy storage device; and a first thermal energy storage device; providing a power recovery sub-system comprising: a working fluid; and a plurality of expansion stages, comprising a first and second subset; capturing at least a portion of a heat of compression from the main air compressor and storing it in the second thermal energy storage device; capturing at least a portion of the heat of compression from the recycle air compressor and storing it in the first thermal energy storage device; applying the heat of compression stored in the second thermal energy storage device to the working fluid upstream of each of the first subset of expansion stages; and applying the heat of compression stored in the first thermal energy storage device to the working fluid upstream of each of the second subset of expansion stages.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0184] Embodiments of the present invention will now be described with reference to the figures in which:

[0185] FIG. 1 shows a conventional generating regime of an energy storage device;

[0186] FIG. 2 shows a schematic view of a conventional cryogenic energy storage (CES) system;

[0187] FIG. 3A. shows direct heat exchange between the process fluid of a liquefaction unit and a packed-bed TESD and between the packed-bed TESD and the working fluid of a power recovery unit.

[0188] FIG. 3B. shows indirect heat exchange between the process fluid of a liquefaction unit and a packed-bed TESD and between the packed-bed TESD and the working fluid of a power recovery unit.

[0189] FIG. 3C shows a direct heat exchange between the process fluid of a liquefaction unit and a packed-bed TESD and subsequently an indirect heat exchange between the packed-bed TESD and the working fluid of a power recovery unit.

[0190] FIG. 3D shows an indirect heat exchange between the process fluid of a liquefaction unit and a packed-bed TESD and subsequently a direct heat exchange between the packed-bed TESD and the working fluid of a power recovery unit.

[0191] FIG. 4A shows a standalone CES system according to the invention displaying a main air compressor-related and a recycle air compressor-related TESDs storing at least some heat of compression, another TESD storing at least some of the cold thermal energy embedded in the cryogen and a power island according to the invention that may be as shown in any one of FIGS. 5A to 5F.

[0192] FIG. 4B shows a thermally-integrated CES system according to the invention receiving some waste cold thermal energy from a LNG regasification terminal, that displays a main air compressor-related and recycle air compressor-related TESDs storing at least some heat of compression, another TESD storing at least some cold thermal energy embedded in the cryogen, two LNG-based cooling loops and a power island according to the invention that may be as shown in any one of FIGS. 6A to 6F.

[0193] FIG. 4C shows a thermally-integrated CES system according to the invention receiving an abundant amount of cold thermal energy from a LNG regasification terminal, that displays a main air compressor-related and recycle air compressor-related TESDs storing at least some heat of compression, three LNG-based cooling loops and a power island according to the invention that may be as shown in any one of FIGS. 6A to 6F.

[0194] FIG. 5A-5F depict six alternative embodiments of a power island for a standalone CES system according to the invention.

[0195] FIG. 6A-6F depict six alternative embodiments of a power island for a thermally-integrated system according to the invention.

[0196] FIGS. 7A, 7B and 7C show embodiments of cryogenic energy storage systems similar to the systems shown in FIGS. 4A, 4B, and 4C, respectively.

[0197] FIGS. 8A and 8B represent two further alternative embodiments of the power island (33) for a standalone CES system as shown in FIG. 7A.

[0198] FIGS. 9A and 9B represent a further two alternative arrangements of the power island (330) for a thermally-integrated CES system as shown in FIGS. 7B and 7C, which are embodiments of the present invention.

[0199] FIG. 10 depicts an embodiment of a CES system according to the present invention and shows first (501) and second (502) intermediate closed loops.

DETAILED DESCRIPTION OF THE INVENTION

[0200] A first embodiment of the invention is depicted in FIG. 4A and is directed to a standalone CES system displaying a liquefaction unit (1), a cryogenic tank (2) and a power recovery unit (3), which exhibits a power island (33) that could adopt any of the configurations depicted in FIG. 5A-5F, each of which is also an embodiment of the present invention. This standalone CES system possesses a heat-of-compression recycle device (11, 12, 15, 16, 12A, 16A) and a first separate closed double loop (130) which transfers the cold thermal energy embedded in the cryogen to the process stream of the liquefaction unit. The hot thermal energy provided to the working fluid of the power recovery unit via at least one power recovery heater may stem from the heat-of-compression recycle device through the second (11, 12, 12A) and third (15, 16, 16A) separate closed double loops. Although it is a standalone system, it is possible to have this embodiment using not only the hot thermal energy provided by the heat-of-compression recycle device to heat the working fluid of the power recovery unit via at least one power recovery heater, but also some waste hot thermal energy from at least one system co-located with and external to the CES system producing waste hot thermal energy, such as nuclear power plants, thermal power plants (e.g. open cycle gas turbine gas plants; combined cycle gas turbine plants and conventional steam cycles), data centres, steel works, furnaces used by ceramics, terra cotta, glass-making and cement-making industries.

[0201] The liquefaction unit (1) turns a stream of ambient air (0) into liquid air that is subsequently stored in the cryogen tank (2). The liquefaction unit (1) may comprise at least a main air compressor (10), a first heat-of-compression capturing heat exchanger (11), a main air compressor-related TESD (12) to store the heat of compression stemming from the main air compressor, an Air Purification Unit (APU) (13), a recycle air compressor (14), a second heat-of-compression capturing heat exchanger (15), a recycle air compressor-related TESD (16) to store the heat of compression stemming from the recycle air compressor, a cold box (17), a set of two liquefaction turbo-expanders (100, 101) placed in series, an expansion device (18) (e.g. a Joule-Thomson valve, a wet turbo-expander, etc.), a phase separator (19), a first conduit to convey the process stream of the liquefaction unit from the main air compressor going through the first heat-of-compression capturing heat exchanger, the APU, the recycle air compressor, the second heat-of-compression capturing heat exchanger, the cold box, the expansion device to the phase separator, a second conduit to divert part of the process stream of the liquefaction unit (conveyed by the first conduit) when crossing the cold box, a third conduit to convey the gaseous output stream of the phase separator (121) through the cold box to the recycle air compressor input (the merging occurring downstream of the APU and upstream of the recycle air compressor), a fourth conduit to convey the liquid output stream of the phase separator (122) to the cryogenic tank (2) and a fifth conduit to convey the heat transfer fluid circulating through the first separate closed double loop (130) through the cold box.

[0202] The main air compressor compresses ambient air (i.e. the air present in the atmosphere surrounding the CES system) from the ambient air pressure to a first pressure, which may be between two bar to tens of bar, prior to its purification in the APU, which is placed downstream of the main air compressor. The APU is made up of adsorption vessels able to adsorb hydrocarbons, water and carbon dioxide to obtain cleaned air at its output. Downstream of the APU, the recycle air compressor compresses cleaned air from a pressure slightly below the first pressure (to take into account the pressure drop introduced by the APU) to a second pressure, which equals tens of bars with an upper limit of 200 bar.

[0203] The cleaned air processed by the recycle air compressor encompasses not only the cleaned air output by the APU but also the cleaned air stemming from the gaseous output stream (121) of the phase separator, whose cold thermal energy has been stripped out when going through the cold box (before reaching the recycle air compressor) to be transferred to the process stream of the liquefaction unit conveyed by the first conduit. Consequently, the air mass flow output by the recycle air compressor is greater than that of the main air compressor and affects the amount of generated heat of compression.

[0204] The cleaned air output by the recycle air compressor is conveyed through the cold box to be cooled, then through the expansion device to decrease its pressure to the first pressure, or to a pressure greater than the first pressure and lower than the second pressure, allowing its total or partial liquefaction depending upon the conditions under which the stream output by the recycle air compressor was subjected (i.e. pressure of the stream output by the recycle air compressor relative to the critical pressure of air, amount of cold thermal energy supplied through the cold box, pressure change via expansion through the expansion device, etc.). The gaseous and liquid mixture, output by the expansion device (18), is subsequently conveyed to a phase separator where it separates into a liquid phase and a gaseous phase.

[0205] Part (120) of the process stream of the liquefaction unit (conveyed by the first conduit) is diverted while crossing the cold box via the second conduit which exits the cold box to go through a first liquefaction turbo-expander (100) and re-enters the cold box (via a re-entry point controlled by the amount of cooling embedded in the liquefaction turbo-expander (100) output) to cool the process stream of the liquefaction unit (conveyed by the first conduit) on a given length of the cold box, after which it exits the cold box and is processed by the second liquefaction turbo-expander (101). The gas/liquid mixture output by the second liquefaction turbo-expander (101) is then conveyed to the phase separator.

[0206] The gaseous output stream of the phase separator conveyed by the third conduit comprises the gaseous phase resulting from the expansion of the stream conveyed by the first conduit through the expansion device (18) and the gaseous phase resulting from the stream conveyed by the second conduit that is subjected to two consecutive expansions through the two liquefaction turbo-expanders (100, 101) placed in series and subsequently injected into the phase separator.

[0207] The gaseous output stream (121) of the phase separator goes through the cold box to transfer its cold thermal energy to the process stream of the liquefaction unit conveyed by the first conduit and is subsequently conveyed to the recycle air compressor input (the merging occurring downstream of the APU and upstream of the recycle air compressor).

[0208] The liquid output stream (122) of the phase separator is conveyed by the fourth conduit to the cryogenic tank (2).

[0209] The term separate closed double loop is closely associated with the presence of a TESD: one single loop through which a heat transfer fluid circulates captures the thermal energy from one fluid and the other single loop through which another heat transfer fluid circulates supplies this thermal energy to another fluid. Said single loops could be of simple design i.e. each exhibiting a circulation pump, an arrangement of conduits that goes through the TESD, and a heat transfer fluid. Or said single loops may have the same heat transfer fluid and may share a circulation pump and part of their arrangement of conduits, said part going through the TESD which involves the presence of valves (e.g. three-way valves) as shown in the first separate closed double loop (130) of FIG. 4A.

[0210] The single loops of the first separate closed double loop (130) shares a TESD (131), part of their arrangement of conduits, a heat transfer fluid and a circulation pump (132) to circulate the heat transfer fluid through both single loops. During the power recovery phase, one single loop allows for capturing at least part of the cold thermal energy embedded in the cryogen via the evaporator (32) after being pumped via the cryogen pump (31), and storing it in the TESD (131). During the liquefaction phase, the other single loop allows for providing the cold thermal energy stored in the TESD (131) to the process stream of the liquefaction unit via the fifth conduit.

[0211] The process stream of the liquefaction unit conveyed by the first conduit is cooled down by the streams conveyed by the second conduit, the third conduit and the fifth conduit so as to be partially liquefied after passing through the expansion device (18).

[0212] The cryogen produced by the liquefaction unit during the liquefaction phase (i.e. the liquid output stream (122) of the phase separator) is conveyed to the cryogenic tank (2). During the power recovery phase, some cryogen contained in the cryogenic tank is conveyed to the power recovery unit (3): it is pumped to a high pressure by the cryogenic pump (31), heated in the evaporator (32) and transferred to a power island (33) in which it is superheated via at least one power recovery heater and expanded via at least one expansion stage of at least one turbo-expander. Whatever the amount of turbo-expanders present in the power island is, they are all mechanically coupled to a generator to produce electricity.

[0213] The power island (33) may adopt any of the configurations depicted by FIG. 5A-5F, each of which is also an embodiment of the present invention.

[0214] The turbo-expander of the power recovery unit may preferably display four expansion stages.

[0215] The recycle air compressor-related TESD (16) may supply hot thermal energy to the working fluid of the power recovery prior to each of the first three-expansion stages (61, 62, 63) via power recovery heaters (81, 82, 83) while the main air compressor-related TESD (12) may supply hot thermal energy to the working fluid of the power recovery unit prior to the last fourth stage (64) via power recovery heater (84) (See FIG. 5A). An extra power recovery heater (80) may be placed upstream of the power recovery heater (81) upstream of the first expansion stage (61), wherein the working fluid of the power recovery unit may be heated by the output of the fourth expansion stage (64) prior to be heated further by the power recovery heater (81) placed upstream of the first expansion stage (See FIG. 5B). After having transferred its hot thermal energy to the working fluid of the power recovery unit via the extra power recovery heater (80), the output of the fourth expansion stage may be discharged into the atmosphere or used to regenerate the adsorption vessels of the APU.

[0216] Alternatively, the main air compressor-related TESD (12) may supply hot thermal energy to the working fluid of the power recovery unit prior to the first expansion stage (6100) via a power recovery heater (8100) while the recycle air compressor-related TESD (16) may supply hot thermal energy to the working fluid of the power recovery unit prior to the each of the last three expansion stages (6200, 6300, 6400) via power recovery heaters (8200, 8300, 8400) (See FIG. 5E). An extra power recovery heater (8000) may be placed upstream of the power recovery heater (8100) upstream of the first expansion stage (6100), wherein the working fluid of the power recovery unit may be heated by the output of the fourth expansion stage (6400) prior to be heated further by the power recovery heater (8100) placed upstream of the first expansion stage (6100) (See FIG. 5F). After having transferred its hot thermal energy to the working fluid of the power recovery unit via the extra power recovery heater, the output of the fourth expansion stage may be discharged into the atmosphere or used to regenerate the adsorption vessels of the APU.

[0217] The turbo-expander may display five expansion stages.

[0218] The recycle air compressor-related TESD (16) may supply hot thermal energy to the working fluid of the power recovery unit prior to each of the first three-expansion stages (610, 620, 630) via power recovery heaters (810, 820, 830) while the main air compressor-related TESD (12) may supply hot thermal energy to the working fluid of the power recovery unit prior to the last fourth and fifth expansion stages (640, 650) (See FIG. 5C). An extra power recovery heater (800) may be placed upstream of the power recovery heater (810) upstream of first expansion stage (610) wherein the working fluid of the power recovery unit may be heated by the output of the fifth expansion stage prior to be heated further by the power recovery heater (810) placed upstream of the first expansion stage (610) (See FIG. 5D). After having transferred its hot thermal energy to the working fluid of the power recovery unit via the extra power recovery heater, the output of the fifth expansion stage may be discharged into the atmosphere or used to regenerate the adsorption vessels of the APU.

[0219] The extra power recovery heater (80, 800, 8000) may be placed downstream of the evaporator (32).

[0220] A second embodiment of the invention is depicted in FIG. 4B and is directed to a thermally-integrated system displaying a liquefaction unit (1), a cryogenic tank (2) and a power recovery unit (3) which exhibits a power island (330) that could adopt any of the configurations depicted in FIG. 6A-6F, each of which is also an embodiment of the present invention. This thermally-integrated CES system possesses a heat-of-compression recycle device (11, 12, 15, 16, 12A, 16A) and a first separate closed double loop (130) which transfers the cold thermal energy embedded in the cryogen to the process stream of the liquefaction unit. This CES system receives some waste cold thermal energy from a LNG regasification terminal external to and co-located with said CES system via a first (401) and a second (403) separate closed single loops. The waste cold thermal energy provided by the LNG regasification terminal does not fulfil entirely the needs of the liquefaction unit, which still requires the presence of the first separate closed double loop (130). The reference number 400 appended next to several streams refers to LNG streams.

[0221] The hot thermal energy provided to the working fluid of the power recovery unit via at least one power recovery heater may stem from the heat-of-compression recycle device through the second (11, 12 12A) and the third (15, 16, 16A) separate closed double loops. The hot thermal energy provided to the working fluid of the power recovery unit via at least one power recovery heater may stem from the heat-of-compression recycle device and at least one system co-located with and external to the CES system producing waste hot thermal energy, such as nuclear power plants, thermal power plants (e.g. open cycle gas turbine gas plants; combined cycle gas turbine plants and conventional steam cycles), data centres, steel works, furnaces used by ceramics, terra cotta, glass-making and cement-making industries.

[0222] The liquefaction unit (1) turns a stream of ambient air (0) into liquid air that is subsequently stored in the cryogen tank (2). The liquefaction unit (1) may comprise at least a main air compressor (10), a first heat-of-compression capturing heat exchanger (11), a main air compressor-related TESD (12) to store the heat of compression stemming from the main air compressor (10), an Air Purification Unit (APU) (13), a recycle air compressor (14), a second heat-of-compression capturing heat exchanger (15), a recycle air compressor-related TESD (16) to store the heat of compression stemming from the recycle air compressor, a cold box (17), a liquefaction turbo-expander (102), an expansion device (18) (e.g. a Joule-Thomson valve, a wet turbo-expander, etc.), a phase separator (19), a first conduit to convey the process stream of the liquefaction unit from the main air compressor going through the first heat-of-compression capturing heat exchanger, the APU, the recycle air compressor, the second heat-of-compression capturing heat exchanger, the cold box and the expansion device to the phase separator, a second conduit to divert part of the process stream of the liquefaction unit (conveyed by the first conduit) while crossing the cold box, a third conduit to convey the gaseous output stream (124) of the phase separator to the atmosphere, a fourth conduit to convey the liquid output stream (122) of the phase separator to the cryogenic tank (2), a fifth conduit to convey the heat transfer fluid of the first separate closed double loop (130) through the cold box, a sixth conduit to convey the heat transfer fluid of the first separate closed single loop (401) through the cold box.

[0223] The main air compressor compresses ambient air (i.e. the air present in the atmosphere surrounding the CES system) from the ambient air pressure to a first pressure, which may be between two bar to tens of bar, prior to its purification in the APU, which is placed downstream of the main air compressor. The APU is made up of adsorption vessels able to adsorb hydrocarbons, water and carbon dioxide to obtain cleaned air at its output. Downstream of the APU, the recycle air compressor compresses cleaned air from a pressure slightly below the first pressure (to take into account the pressure drop introduced by the APU) to a second pressure, which equals tens of bars with an upper limit of 200 bar.

[0224] The cleaned air processed by the recycle air compressor encompasses not only the cleaned air output by the APU but also the cleaned air stemming from the gaseous stream (123) output by the liquefaction turbo-expander (102), whose cold thermal energy has been stripped out when going through the cold box (before reaching the recycle air compressor) to be transferred to the process stream of the liquefaction unit conveyed by the first conduit. Consequently, the air mass flow output by the recycle air compressor is greater than that of the main air compressor and affects the amount of generated heat of compression.

[0225] The cleaned air output by the recycle air compressor (conveyed by the first conduit) is conveyed through the cold box (17) to be cooled, then through the expansion device (18) to decrease its pressure to the first pressure, or to a pressure greater than the first pressure and lower than the second pressure, allowing its total liquefaction. The liquid stream output by the expansion device (18) is subsequently conveyed to a phase separator.

[0226] Part (123) of the process stream of the liquefaction unit (conveyed by the first conduit) is diverted while crossing the cold box via the second conduit which exits the cold box to go through the liquefaction turbo-expander (102) to re-enter the cold box via its cold side (i.e. lower side of the cold box) to cool the rest of the process stream of the liquefaction unit conveyed by the first conduit and exits the cold box via its warm side to finally merge with the first conduit downstream of the APU and upstream of the recycle air compressor.

[0227] The third conduit conveying the gaseous output stream (124) of the phase separator allows any gas present after expansion of the process stream of the liquefaction unit through the expansion device (18) to escape from the phase separator to the atmosphere. This situation normally occurs when the CES system is started up as total liquefaction is only achieved when a steady state is established.

[0228] The liquid output stream (122) of the phase separator is conveyed by the fourth conduit to the cryogenic tank (2).

[0229] The single loops of the first separate closed double loop (130) shares a TESD (131), part of their arrangement of conduits, a heat transfer fluid and a circulation pump (132) to circulate the heat transfer fluid through both single loops. During the power recovery phase, one single loop allows for capturing at least part of the cold thermal energy embedded in the cryogen via the evaporator (32) after being pumped via the cryogen pump (31), and storing it in the TESD (131). During the liquefaction phase, the other single loop allows for providing the cold thermal energy stored in the TESD (131) to the process stream of the liquefaction unit via the fifth conduit.

[0230] The first separate closed single loop (401) is a refrigeration loop that increases the grade of the waste cold thermal energy supplied by the LNG stream (400): the heat transfer fluid (circulating through the first separate closed single loop (401)) is compressed then cooled by the LNG stream (400) via heat exchanger (402), then compressed again and cooled again by the LNG stream (400) via heat exchanger 402), then expanded through a turbo-expander and heated while crossing the entire cold box (going from its cold side to its warm side) by the process stream of the liquefaction unit that is then cooled down.

[0231] The process stream of the liquefaction unit conveyed by the first conduit is cooled down by the streams conveyed by the second conduit, the fifth conduit and the sixth conduit so as to be entirely liquefied after passing through the expansion device (18).

[0232] The cryogen produced by the liquefaction unit during the liquefaction phase (i.e. the liquid output stream (122) of the phase separator) is conveyed to the cryogenic tank (2). During the power recovery phase, some cryogen contained in the cryogenic tank is conveyed to the power recovery unit (3): it is pumped to a high pressure by the cryogenic pump (31), heated in the evaporator (32) and transferred to a power island (330) in which it is superheated via at least one power recovery heater and expanded via at least one expansion stage of at least one turbo-expander. Whatever the amount of turbo-expanders present in the power island is, they are all mechanically coupled to a generator to produce electricity. Nevertheless, the power recovery unit of the second embodiment is different to that of the first embodiment with respect to its configuration.

[0233] The power recovery unit (3) is thermally interacting with the second separate closed single loop (403). The second separate closed single loop (403) contains a recirculation pump to circulate the heat transfer fluid which is cooled by the LNG stream (400) via heat exchanger (404) and heated by the stream (35) (diverted from the output of the power island (330)) via evaporator (320). Said diverted stream (35) is thus cooled by the heat transfer fluid of the second separate closed single loop to be subsequently compressed by a power recovery compressor (34) and re-injected into the power island (330), that may adopt any of the configurations depicted in FIG. 6A-6F, each of which is also an embodiment of the present invention.

[0234] The turbo-expander of the power recovery unit may preferably display four expansion stages as depicted in FIGS. 6A, 6B, 6E and 6F.

[0235] The turbo-expander of the power recovery unit may display five expansion stages as depicted in FIGS. 6C and 6D.

[0236] For each of the configurations 6A-6F, all embodiments of the present invention, the stream (35) diverted from the output of the power island (330) is cooled via the evaporator (32) then recompressed by a power recovery compressor (34) and injected back to the power island (330) downstream of the second-to-last expansion stage and upstream of the last power recovery heater and the last expansion stage. The work input of the compressor (34) is reduced by having its input cooled via evaporator (32) by the heat transfer fluid of the second separate closed single loop (403) and the work output of the last expansion stage is increased by augmenting the mass flow rate processed by the last expansion stage. The rest of the output stream of the power island may be exhausted to the atmosphere or may be used to regenerate the adsorption vessels of the APU (13).

[0237] The power island may adopt any one of the configurations depicted in FIG. 6A-6F, each of which is also an embodiment of the present invention. One difference between FIGS. 5A and 6A, FIGS. 5B and 6B, FIGS. 5C and 6C, FIGS. 5D and 6D, FIGS. 5E and 6E, and FIGS. 5F and 6F is the presence of a stream (35) diverted from the output of the power island for FIG. 6A-6F, which is then cooled via evaporator (32) and recompressed by a compressor (34) and is injected back to the power island (330) downstream of the second-to-last expansion stage and upstream of the last power recovery heater (84, 850, 8400) and the last expansion stage (64, 650, 6400). FIGS. 6B, 6D and 6F displays the extra power recovery heater (80, 800, 8000). The extra power recovery heater (80, 800, 8000) may be placed upstream of the power recovery heater (81, 810, 8100) and upstream of the first expansion stage (61, 610, 6100), wherein the working fluid of the power recovery unit may be heated by the output of the last expansion stage (64, 650, 6400) prior to be heated further by the power recovery heater (81, 810, 8100) placed upstream of the first expansion stage.

[0238] The extra power recovery heater (80, 800, 8000) may be placed downstream of the evaporator (32).

[0239] A third embodiment of the invention is depicted in FIG. 4C and is directed to a thermally-integrated system displaying a liquefaction unit (1), a cryogenic tank (2) and a power recovery unit (3) which exhibits a power island (330) that could adopt any of the configurations depicted in FIG. 6A-6F, each of which is also an embodiment of the present invention. This thermally-integrated CES system possesses a heat-of-compression recycle device (11, 12, 15, 16, 12A, 16A). This CES system receives a large amount of waste cold thermal energy from a LNG regasification terminal co-located with and external to said CES system. In other words, the amount of waste cold thermal energy provided by the LNG regasification terminal can fulfil the needs in cold thermal energy of the liquefaction unit in such a manner that there is no need for a first separate closed double loop (130) as displayed in the first and second embodiments. The waste cold thermal energy is transferred from LNG streams (400) to the liquefaction unit via the third (405) and fourth (407) separate closed single loops so as to liquefy entirely the process stream of the liquefaction unit.

[0240] The cold thermal energy embedded in the cryogen is directly used in the power recovery unit to cool down the part of the output stream of the power island (330) that is diverted to be recompressed by compressor (34) and injected back to the power island (330) downstream of the second-to-last expansion stage and upstream of the last power recovery heater (See FIG. 6A-6F: 84, 850, 8400) and the last expansion stage (See FIG. 6A-6F: 64, 650, 6400). The cold thermal energy provided by the second separate closed single loop (403) is used to cool further the output stream (35) in the evaporator (32).

[0241] The hot thermal energy provided to the working fluid of the power recovery unit via at least one power recovery heater may stem from the heat-of-compression recycle device through the second (11, 12, 12A) and third (15, 16, 16A) separate closed double loops. The hot thermal energy provided to the working fluid of the power recovery unit via at least one power recovery heater may stem from the heat-of-compression recycle device and at least one system co-located with and external to the CES system producing waste hot thermal energy, such as nuclear power plants, thermal power plants (e.g. open cycle gas turbine gas plants; combined cycle gas turbine plants and conventional steam cycles), data centres, steel works, furnaces used by ceramics, terra cotta, glass-making and cement-making industries.

[0242] The liquefaction unit (1) turns a stream of ambient air (0) into liquid air that is subsequently stored in the cryogen tank (2). The liquefaction unit (1) may comprise at least a main air compressor (10), a first heat-of-compression capturing heat exchanger (12), a main air compressor-related TESD (12) to store the heat of compression stemming from the main air compressor, an Air Purification Unit (APU) (13), a recycle air compressor (14), a second heat-of-compression capturing heat exchanger (15), a recycle air compressor-related TESD (16) to store the heat of compression stemming from the recycle air compressor, a cold box (17), an expansion device (18) (e.g. a Joule-Thomson valve, a wet turbo-expander, etc.), a phase separator (19), a first conduit to convey the process stream of the liquefaction unit from the main air compressor going through the first heat-of-compression capturing heat exchanger, the APU, the recycle air compressor, the second heat-of-compression capturing heat exchanger, the cold box, the expansion device to the phase separator, a second conduit to convey the gaseous output stream (124) of the phase separator to the atmosphere, a third conduit to convey the liquid output stream (122) of the phase separator to the cryogenic tank (2), a fourth conduit to convey the heat transfer fluid of the third separate closed single loop (405) through the cold box, a fifth conduit to convey the heat transfer fluid of the fourth separate closed single loop (407) through the cold box.

[0243] The main air compressor compresses ambient air (i.e. the air present in the atmosphere surrounding the CES system) from the ambient air pressure to a first pressure, which may be between two bar to tens of bar, prior to its purification in the APU, which is placed downstream of the main air compressor. The APU is made up of adsorption vessels able to adsorb hydrocarbons, water and carbon dioxide to obtain cleaned air at its output. Downstream of the APU, the recycle air compressor compresses cleaned air from a pressure slightly below the first pressure (to take into account the pressure drop introduced by the APU) to a second pressure, which equals tens of bars with an upper limit of 200 bar.

[0244] The cleaned air output by the recycle air compressor (conveyed by the first conduit) is conveyed through the cold box (17) to be cooled, then through the expansion device (18) to decrease its pressure to the first pressure, or to a pressure greater than the first pressure and lower than the second pressure, allowing its total liquefaction. The liquid stream output by the expansion device (18) is subsequently conveyed to a phase separator.

[0245] The second conduit conveying the gaseous output stream (124) of the phase separator allows any gas present after expansion of the process stream of the liquefaction unit through the expansion device (18) to escape from the phase separator to the atmosphere. This situation normally occurs when the CES system is started up as total liquefaction is only achieved when a steady state is established.

[0246] The liquid output stream (122) of the phase separator is conveyed by the third conduit to the cryogenic tank (2).

[0247] A heat transfer fluid is circulated through the third separate closed single loop (405) thanks to a circulation pump, retrieves some cold thermal energy from the LNG stream (400) via a heat exchanger (406) and enters the cold box at a distance a (See cold box of FIG. 4C) from the warm side of the cold box to give up its cold thermal energy to the process stream of the liquefaction unit (conveyed by the first conduit) which goes on the opposite direction. The length of the cold box equals to the sum of the distances a and b.

[0248] In the fourth separate closed single loop (407) which is a refrigeration loop, its heat transfer fluid is compressed by a compressor, then cooled by the LNG stream (400) via a heat exchanger (408), expanded by a turbo-expander, heated in the cold box from its cold side to the distance b (at which it exits the cold box) by the process stream of the liquefaction unit conveyed by the first conduit.

[0249] The process stream of the liquefaction unit conveyed by the first conduit is cooled by the heat transfer fluids of the third (405) and fourth (407) separate closed single loops, conveyed by the second and third conduits, respectively so as to be entirely liquefied after passing the expansion device (18).

[0250] The cryogen produced by the liquefaction unit during the liquefaction phase (i.e. the liquid output stream (122) of the phase separator) is conveyed to the cryogenic tank (2). During the power recovery phase, some cryogen contained in the cryogenic tank is conveyed to the power recovery unit (3): it is pumped to a high pressure by the cryogenic pump (31), heated in the evaporator (32) and transferred to a power island (330) in which it is superheated via at least one power recovery heater and expanded via at least one expansion stage of at least one turbo-expander. Whatever the amount of turbo-expanders present in the power island is, they are all mechanically coupled to a generator to produce electricity. Nevertheless, the power recovery unit of the third embodiment is different to that of the first embodiment with respect to its configuration, but similar to that of the second embodiment.

[0251] The power recovery unit (3) is thermally interacting with the second separate closed single loop (403). The second separate closed single loop (403) contains a recirculation pump to circulate a heat transfer fluid which is cooled by the LNG stream (400) via heat exchanger (404) and heated by the stream (35) diverted from the output of the power island (330) via evaporator (320). Said diverted stream (35) is cooled by the heat transfer fluid of the second separate closed single loop (403) and also by the cold thermal energy embedded in the cryogen, to be subsequently compressed by a power recovery compressor (34) and re-injected into the power island (330), that may adopt any of the configurations depicted in FIG. 6A-6F, each of which is also an embodiment of the present invention.

[0252] The turbo-expander of the power recovery unit may preferably display four expansion stages as depicted in FIGS. 6A, 6B, 6E and 6F.

[0253] The turbo-expander of the power recovery unit may display five expansion stages as depicted in FIGS. 6C and 6D.

[0254] For each of the configurations 6A-6F, all embodiments of the present invention, the stream (35) diverted from the output of the power island (330) is cooled via the evaporator (32) then recompressed by a power recovery compressor (34) and injected back to the power island (330) downstream of the second-to-last expansion stage and upstream of the last power recovery heater and the last expansion stage. The work input of the compressor (34) is reduced by having its input cooled via evaporator (32) by the heat transfer fluid of the second separate closed single loop (403) and the pressurised cryogen, and the work output of the last expansion stage is increased by augmenting the mass flow rate processed by the last expansion stage. The rest of the output stream of the power island may be exhausted to the atmosphere or may be used to regenerate the adsorption vessels of the APU (13).

[0255] The power island may adopt any one of the configurations depicted in FIG. 6A-6F, each of which is also an embodiment of the present invention. One difference between FIGS. 5A and 6A, FIGS. 5B and 6B, FIGS. 5C and 6C, FIGS. 5D and 6D, FIGS. 5E and 6E, and FIGS. 5F and 6F is the presence of a stream (35) diverted from the output of the power island in FIG. 6A-6F, which is then cooled via evaporator (32) and recompressed by a compressor (34) and is injected back to the power island (600) downstream of the second-to-last expansion stage and upstream of the last power recovery heater (84, 850, 8400) and the last expansion stage (64, 650, 6400). FIGS. 6B, 6D and 6F displays the extra power recovery heater (80, 800, 8000). The extra power recovery heater (80, 800, 8000) may be placed upstream of the power recovery heater (81, 810, 8100) and upstream of the first expansion stage (61, 610, 6100), wherein the working fluid of the power recovery unit may be heated by the output of the last expansion stage (64, 650, 6400) prior to be heated further by the power recovery heater (81, 810, 8100) placed upstream of the first expansion stage.

[0256] The extra power recovery heater (80, 800, 8000) may be placed downstream of the evaporator (32).

[0257] What follows is common to the first, second and third embodiments of the present invention depicted in FIG. 4A-4C and also to the other embodiments of the invention depicted in FIG. 5A-5F and FIG. 6A-6F.

[0258] The cold box (17) is an assembly of heat exchangers, pipes and pressure vessels contained inside a metal structure filled with high quality insulation material, such as perlite. The cold box may encompass at least one single multi-pass heat exchanger. The cold box displays a warm side (upper side) and a cold side (lower side).

[0259] The main air compressor (10) may have at least one compression stage, preferably two compression stages, more preferably four compression stages. There may be a heat-of-compression capture heat exchanger downstream of at least one compression stage of the main air compressor, preferably downstream of its last compression stage. The task of a heat-of-compression capture heat exchanger is to capture at least part of the heat of compression generated by a compressor or a set of compression stages or a compression stage. There may be a cooler downstream of at least one compression stage. There may be a cooler downstream of at least one heat-of-compression capture heat exchanger. Typically, coolers (i.e. heat exchangers using air or water) are placed either upstream of the compression stages of a compressor to pre-cool the stream of gas prior to its compression through them (reduction of the compression work) or downstream of the compressor output to cool it down and ease its subsequent liquefaction. Preferably, there may be no cooling/heating during compression and in between the compression stages of the main air compressor i.e. the main air compressor may be adiabatic.

[0260] The recycle air compressor (14) may have preferably one compression stage, or at least one compression stage, or more preferably four compression stages. There may be a heat-of-compression capture heat exchanger downstream of at least one compression stage of the recycle air compressor, preferably downstream of its last compression stage. There may be a cooler downstream of at least one compression stage. There may be a cooler downstream of at least one heat-of-compression capture heat exchanger. Preferably, there may be no cooling/heating during compression and in between the compression stages of the recycle air compressor i.e. the recycle air compressor may be adiabatic.

[0261] With respect to the main air compressor and to the recycle air compressor, the amount of heat-of-compression capture heat exchangers and coolers and their respective locations relatively to the compression stage of the main air compressor and the recycle air compressor depend on the main air compressor- and the recycle air compressor-output temperature targets and on the parasitic losses they introduce (e.g. pressure drop, etc.), which increases the power consumption while affecting the grade of the heat of compression generated by the main air compressor and the recycle air compressor.

[0262] The power recovery unit of the CES system may comprise at least one turbo-expander, preferably one turbo-expander. Each turbo-expander may in turn comprise at least one expansion stage, preferably four or five expansion stages. There may be a power recovery heater upstream of each expansion stage.

[0263] The heat-of-compression recycle device may comprise a second separate closed double loop (11, 12, 12A) and a third separate closed double loop (15, 16, 16A). The advantage of having separate closed double loops (due to the occurrence of indirect heat exchange from a compressor to a TESD and from a TESD to a turbo-expander) lies in the ease of replenishing the heat transfer fluid in case of leakage and to control the pressure of the heat transfer fluid circulating through the separate closed double loops.

[0264] The second separate closed double loop may comprise the main air compressor-related TESD (12). Each single loop of the second separate closed double loop possesses a heat transfer fluid, a circulation pump and an arrangement of conduits that goes through the main air compressor-related TESD. Part of their respective arrangements of conduits that go through the main air compressor-related TESD may be shared, which supposes the presence of three-way valves and of a single heat transfer fluid.

[0265] One single loop of the second separate closed double loop captures at least some of the heat of compression generated by the main air compressor (10) using at least one heat-of-compression capturing heat exchanger (11) placed downstream of the main air compressor, and stores it in the main air compressor-related TESD (12). The other single loop (12A) of the second separate closed double loop provides hot thermal energy to the working fluid of the power recovery unit via at least one power recovery heater (84, 840, 850, 8100), prior to its expansion via at least one of the expansion stage (64, 640, 650, 6100) of the power recovery turbo-expander, as shown on FIG. 5A-5F and FIG. 6A-6F, which are also embodiments of the present invention. Said single loop may comprise at least one valve (e.g. three-way valve).

[0266] The main air compressor-related TESD (12) is thermally coupled to the main air compressor (10) via the heat-of-compression capturing heat exchanger (11).

[0267] The main air compressor-related TESD (12) may be a packed bed TESD, a stationary liquid phase-based TESD or a two-reservoirs TESD or preferably a thermocline TESD.

[0268] If the main air compressor-related TESD (12) is a packed bed TESD, the packed bed matrix may comprise particles randomly stacked on each other made of sensible matter (e.g. pebbles) or made of latent-heat phase change matter, or of combination thereof. If the main air compressor-related TESD is a packed bed TESD, the packed bed matrix may comprise particles non-randomly stacked on each other made of sensible matter (e.g. metal oxide beads) or made of latent-heat phase change matter, or of combination thereof. If the main air compressor-related TESD is a packed bed TESD, the packed bed matrix may comprise fused particles (e.g. ceramics).

[0269] The main air compressor-related TESD may store heat of compression whose temperature is between 200 and 400 C. The heat transfer fluid circulating through the second separate closed double loop may be a gas or a liquid. Said heat transfer fluid may comprise water or a mixture of water and glycol, or thermal oil or a mixture of thermal oils (synthetic oils, natural oils, mineral oils) or molten salts.

[0270] The third separate closed double loop may comprise the recycle air compressor-related TESD (16). Each single loop of the third separate closed double loop possesses a heat transfer fluid, a circulation pump and an arrangement of conduits that goes through the recycle air compressor-related TESD. Part of their respective arrangements of conduits that go through the recycle air compressor-related TESD may be shared, which supposes the presence of three-way valves and of a single heat transfer fluid.

[0271] One single loop of the third separate closed double loop captures at least some of the heat of compression generated by the recycle air compressor (14) using at least one heat-of-compression capturing heat exchanger (15) placed downstream of the recycle air compressor, and stores it in the recycle air compressor-related TESD (16). The other single loop (16A) of the third separate closed double loop provides hot thermal energy to the working fluid of the power recovery unit via at least one power recovery heater (81, 82, 83, 810, 820, 830, 8200, 8300, 8400) prior to its expansion via at least one of the expansion stage (61, 62, 63, 610, 620, 630, 6200, 6300, 6400) of the power recovery turbo-expander, as shown on FIG. 5A-5F and FIG. 6A-6F, which are also embodiments of the present invention. Said single loop may comprise at least one valve (e.g. three-way valve).

[0272] The recycle air compressor-related TESD (16) is thermally coupled to the recycle air compressor (14) via the heat-of-compression capturing heat exchanger (15).

[0273] The recycle air compressor-related TESD may be a packed bed TESD, a stationary liquid phase-based TESD or a two-reservoirs TESD or preferably a thermocline TESD.

[0274] If the recycle air compressor-related TESD (16) is a packed bed TESD, the packed bed matrix may comprise particles randomly stacked on each other made of sensible matter (e.g. pebbles) or made of latent-heat phase change matter, or of combination thereof. If the recycle air compressor-related TESD is a packed bed TESD, the packed bed matrix may comprise particles non-randomly stacked on each other made of sensible matter (e.g. metal oxide beads) or made of latent-heat phase change matter, or of combination thereof. If the recycle air compressor-related TESD is a packed bed TESD, the packed bed matrix may comprise fused particles (e.g. ceramics).

[0275] The recycle air compressor-related TESD (16) may store heat of compression whose temperature is between 150 and 350 C. The heat transfer fluid circulating through the third separate closed double loop may be a gas or a liquid. Said heat transfer fluid may comprise water or a mixture of water and glycol, or thermal oil or a mixture of thermal oils (synthetic oils, natural oils, mineral oils) or molten salts.

[0276] The gas to be liquefied by the liquefaction unit of the CES system may be ambient air, nitrogen gas or any air whose concentrations in oxygen and nitrogen differ to those in ambient air. The cryogen produced by the liquefaction unit, subsequently filling the cryogen tank and processed by the power recovery unit may be liquid air, liquid nitrogen or any liquid air whose concentrations in oxygen and nitrogen differ to those from the ambient air.

[0277] The hot thermal energy provided to the working fluid of the power recovery unit via at least one power recovery heater may stem from the heat-of-compression recycle device through the second (11, 12, 12A) and third (15, 16, 16A) separate closed double loops.

[0278] FIGS. 7A, 7B and 7C show cryogenic energy storage systems similar to the systems shown in FIGS. 4A, 4B, and 4C, respectively. The differences between the systems of FIG. 7 and FIG. 4 are as follows: [0279] The first heat of compression-capturing heat exchanger (11) depicted in FIGS. 4A-4C is split into a third heat-of-compression-capturing heat exchanger (110) and a fourth heat-of-compression-capturing heat exchanger (150). [0280] The third (110) and fourth (150) heat of compression-capturing heat exchangers are both thermally coupled to the main air compressor (10). In other words, the third (110) and fourth (150) heat of compression-capturing heat exchangers each capture heat of compression from the main air compressor (10). [0281] The third heat of compression-capturing heat exchanger (110) is thermally coupled to the second thermal energy storage device (12); [0282] The fourth heat of compression-capturing heat exchanger (150) is thermally coupled to the first thermal energy storage device (16); [0283] The second (11, 12, 12A) and third (15, 16, 16A) separate closed double loops depicted in FIGS. 4A, 4B and 4C are now replaced by the fourth (110, 12, 12A) and the fifth (500; 16, 16A) separate closed double loops depicted in FIGS. 7A, 7B and 7C.

[0284] The fourth separate closed double loop (110, 12, 12A) has two single loops (110, 12) and (12A): a heat transfer fluid circulates through the single loop (110, 12) and may capture a portion of the heat of compression from the main air compressor (10) via the heat of compression-capturing heat exchanger (110). The temperature of the heat of compression stored in the second thermal energy storage device (12) may be between 150 C. and 550 C. The heat of compression is transferred from the heat exchanger (110) by the heat transfer fluid and stored in the second thermal energy storage device (12). A further heat transfer fluid circulating through the single loop (12A) conveys at least some of the stored heat of compression to the power recovery heater (85000) to heat the working fluid of the power recovery unit (3) as shown in FIGS. 8A-8B and 9A-9B.

[0285] The fifth separate closed double loop (500; 16, 16A) has two single loops (500) and (16A): a heat transfer fluid circulates through the single loop (500) and may capture a portion of the heat of compression from the main air compressor (10) via the heat of compression-capturing heat exchanger (150), and a portion of the heat of compression from the recycle air compressor (14) via the second heat of compression-capturing heat exchanger (15). The temperature of the heat of compression stored in the first thermal energy storage device (16) may be between 150 C. and 350 C. The portion of heat of compression from the main air compressor captured by the heat of compression-capturing heat exchanger (150) and the portion of heat of compression from the recycle air compressor captured by the heat of compression-capturing heat exchanger (15) may be at the same temperature and they each are stored in the first thermal energy storage device (16). A further heat transfer fluid circulating through the single loop (16A) conveys at least some of the stored heat of compression to the power recovery heaters (81000; 82000; 83000; 84000) to heat the working fluid of the power recovery unit (3) as shown in FIGS. 8A-8B and 9A-9B. The power recovery heater (85000) is located downstream of the power recovery heater (84000) and upstream of the fourth expansion stage (64000).

[0286] The temperature of the heat of compression stored in the second thermal energy storage device (12) is higher than the temperature of the heat of compression stored in the first thermal energy storage device (16).

[0287] The turbo-expander of the power recovery unit (3) may preferably display four expansion stages.

[0288] FIGS. 8A and 8B represent two further alternatives of the power island (33) for a standalone CES system as shown in FIG. 7A, which are embodiments of the present invention.

[0289] FIGS. 9A and 9B represent a further two alternative arrangements of the power island (330) for a thermally-integrated CES system as shown in FIGS. 7B and 7C, which are embodiments of the present invention.

[0290] FIG. 10 depicts an alternative view of a CES system according to the present invention. FIG. 10 shows an arrangement of a first (501) and a second (502) intermediate closed loop. FIG. 10 provides an alternative view of the embodiments of the present invention as shown in FIGS. 7A-7C, 8A-8B, 9A-9B when the first (501) and the second (502) intermediate closed loops are introduced.

[0291] FIGS. 7 to 10 depict power recovery sub-systems and cryogenic energy storage systems which are embodiments of the claimed invention, and are further embodied by the numbered clauses of the invention.

[0292] The first thermal energy storage device (16) may supply hot thermal energy to the working fluid of the power recovery prior to each of the four expansion stages (61000, 62000, 63000, 64000) via power recovery heaters (81000, 82000, 83000, 84000) while the second thermal energy storage device (12) may supply hot thermal energy to the working fluid of the power recovery unit prior to the last fourth expansion stage (64000) via the power recovery heater (85000) placed downstream of the power recovery heater (84000) and upstream of the last expansion stage (64000) (See FIG. 8A). An extra power recovery heater (80000) may be placed upstream of the power recovery heater (81000) upstream of the first expansion stage (61000), wherein the working fluid of the power recovery unit may be heated by the output of the fourth expansion stage (64000) prior to being heated further by the power recovery heater (81000) placed upstream of the first expansion stage (See FIG. 8B). After having transferred its hot thermal energy to the working fluid of the power recovery unit via the extra power recovery heater (80000), the output of the fourth expansion stage may be discharged into the atmosphere or used to regenerate the adsorption vessels of the APU.

[0293] For each of the configurations 9A-9B, which are embodiments of the present invention, the stream (35) diverted from the output of the power island (330) is cooled via the evaporator (32) then recompressed by a power recovery compressor (34) and injected back to the power island (330) downstream of the penultimate expansion stage and upstream of the power recovery heaters (84000, 85000) and the last expansion stage (64000). The work input of the compressor (34) is reduced by having its input cooled via evaporator (32) by the heat transfer fluid of the second separate closed single loop (403) and the work output of the last expansion stage is increased by augmenting the mass flow rate processed by the last expansion stage. The rest of the output stream of the power island may be exhausted to the atmosphere or used to regenerate the adsorption vessels of the APU (13).

[0294] One difference between FIGS. 8A and 9A and FIGS. 8B and 9B, is the presence of a stream (35). In FIGS. 9A and 9B, at least a portion of the output from the power island (330) downstream of the last expansion stage (64000) is diverted, cooled via evaporator (32), and recompressed by a compressor (34), before being injected back to the power island (330) downstream of the penultimate expansion stage (63000) and upstream of the power recovery heaters (84000, 85000) and the last expansion stage (64000). FIG. 9B displays the extra power recovery heater (80000). The extra power recovery heater (80000) may be placed upstream of the power recovery heater (81000) and upstream of the first expansion stage (61000), wherein the working fluid of the power recovery unit may be heated by the output of the last expansion stage (64000) prior to be heated further by the power recovery heater (81000) placed upstream of the first expansion stage (61000).

[0295] In FIGS. 7A-7C, 8A-8B, 9A-9B and 10, the first thermal energy storage device (16) is thermally coupled to the recycle air compressor (14) via the heat of compression-capturing heat exchanger (15) and to the main air compressor (10) via the heat of compression-capturing heat exchanger (150). The heat transfer fluids circulating through the two single loops of the fifth separate closed double loop (500; 16, 16A) may contain only water or may contain a mixture of water and glycol. The first thermal energy storage device (16) may store heat of compression at a temperature between 150 C. and 350 C.

[0296] In FIGS. 7A-7C, 8A-8B, 9A-9B and 10, the second thermal energy storage device (12) is thermally coupled to the main air compressor (10) via the heat of compression-capturing heat exchanger (110). The heat transfer fluids circulating the two single loops of through the fourth separate closed double loop (110, 12, 12A) may contain molten salts. The second thermal energy storage device (12) may store heat of compression at a temperature between 150 C. and 550 C., preferably between 200 C. and 400 C.

[0297] The advantage of providing both a third (110) and a fourth (150) heat of compression-capturing heat exchanger, as in FIGS. 7A-7C and 10, is to ensure that the process stream remains hot enough while passing through the third (110) heat of compression-capturing heat exchanger so as to avoid the molten salts circulating in the single loop (110, 12) freezing.

[0298] The advantage of providing the two power recovery heaters (84000; 85000) in FIGS. 8A-8B and 9A-9B is to heat the working fluid enough while passing through the power recovery heater (84000) so as to avoid the molten salts circulating through the single loop 12A of the fourth separate closed double loop (110, 12, 12A) freezing while passing through the power recovery heater (85000).

[0299] Molten salts may be advantageously used as a heat transfer fluid in the fourth separate closed double loop. Using molten salts as heat transfer fluids may provide the following advantages: [0300] they have an ultra-low vapour pressure (approximately 0 kPa) i.e. they can be maintained in a liquid state by pressurising them moderately and thus only require the use of a low-pressure vessel (e.g. pressurised to, for example, a few hundred millibars, which is inexpensive) to store them, [0301] they need less energy to be pressurised via a pump than gaseous heat transfer fluids via a compressor; [0302] they have a high density, typically, for example, between 1600 and 2500 kg/m.sup.3; [0303] they are stable at high temperatures; [0304] they are not flammable; [0305] they have a low viscosity at high temperatures; [0306] they have a high heat capacity per unit volume; [0307] they are used in a wide range of applications, from energy storage to nuclear reactors and concentrating solar power (CSP) plants.

[0308] However, conveying molten salts through pipework requires the use of a bespoke and expensive type of pipework which is able to maintain the temperature of the molten salts above the temperature at which they solidify or freeze through the use of heat tracing. Without this type of pipework, the molten salts within the pipework between the main air compressor (10) and the power recovery heater (85000) may freeze, causing operation and maintenance issues in the overall system. In addition, molten salts are corrosive and may damage pipework and expensive mechanical equipment within the CES system.

[0309] An example of the type of pipework required for transporting molten salts is described in U.S. Pat. No. 8,895,901 B2 to BASF. Molten salt pipework differs from traditional pipework, such that conventional pipework may not be suitable for transporting molten salts. For example, molten salt pipework may maintain the salts above freezing point in order to avoid the process of re-melting. In another example, molten salt pipework may contain a circulation pump specifically designed for pumping molten salt.

[0310] The second thermal energy storage device may be specifically configured to store thermal energy, or heat, at a higher grade than that stored by the first thermal energy storage device. This may include being configured to contain the bespoke pipework required for molten salts, detailed above.

[0311] A first intermediate closed loop (501) and a second intermediate closed loop (502) may be incorporated in the present invention as depicted in FIG. 10 to solve the problems with molten salts mentioned above. In other words, the first and second intermediate closed loops reduce the amount of required heat tracing-equipped pipework, thereby decreasing the capital expenditure and keeping apart the pipework conveying the molten salts and the pipework conveying the process stream of the liquefaction unit and the working fluid of the power recovery unit.

[0312] A first intermediate closed loop (501) may be introduced in between the main air compressor (10) and one single loop (110, 12) of the fourth separate closed double loop (110, 12, 12A). In this case, the fourth separate closed double loop (110, 12, 12A) becomes the sixth separate closed double loop (503, 12, 504). The first intermediate closed loop (501) goes through the heat of compression-capturing heat exchanger (110) and an additional heat exchanger (110A) that allows for thermal transfer between the heat of compression capturing heat exchanger (110) and the single loop (503) of the sixth separate closed double loop (503, 12, 504).

[0313] A second intermediate closed loop (502) may be introduced in between the single loop (504) of the fourth separate closed double loop (110, 12, 12A) and the power recovery heater (85000). In this case, the fourth separate closed double loop (110, 12, 12A) becomes the sixth separate closed double loop (503, 12, 504). The second intermediate closed loop (502) goes through the heat exchanger (110B) and the power recovery heater (85000). An additional heat exchanger (110B) allows for thermal transfer between the single loop (504) of the sixth separate closed double loop (503, 12, 504) and the power recovery heater (85000).

[0314] Each of the first (501) and second (502) intermediate closed loops may comprise: [0315] a heat transfer fluid; [0316] a pump (if the heat transfer fluid is a liquid) or a mechanical blower (if the heat transfer fluid is a gas) to circulate the heat transfer fluid through said intermediate closed loops; [0317] a pressurisation unit that accommodates the change in volume occupied by the heat transfer fluid in the intermediate closed loops induced by the thermal variations imposed to said heat transfer fluid.

[0318] The heat transfer fluid in the intermediate closed loops may be a single type of thermal oil or a mixture of thermal oils. Examples of thermal oils that could be used are fluids from the DowTherm range of heat transfer fluids and the SylTherm range of silicone fluids, both manufactured by The Dow Chemical Company. Other suitable fluids may also be used.

[0319] The mechanical blower or the pump is used to offset the pressure drop that affects said heat transfer fluid when circulating through the first (501) and the second (502) intermediate closed loops.

[0320] The first (501) and second (502) intermediate closed loops thus maintain the molten salts above the temperature at which they freeze to avoid their solidification, which would otherwise cause operation and maintenance issues in the pipework between the main air compressor (10) and the power recovery heater (85000), as well as the overall system. Also, the first (501) and second (502) intermediate closed loops keep the molten salts away from the pipework conveying the process stream of the liquefaction unit and the working fluid of the power recovery unit.

[0321] Numbered clauses of the invention: [0322] 1. A power recovery sub-system for a cryogenic energy storage system, the power recovery sub-system comprising: [0323] a first heat source; [0324] a first heat exchanger; [0325] a second heat exchanger; [0326] a first expansion stage; [0327] a second expansion stage; [0328] a first arrangement of conduits, having an upstream end and a downstream end, and configured to pass a working fluid through the first heat exchanger, the first expansion stage, the second heat exchanger, and the second expansion stage; and [0329] a second arrangement of conduits configured to pass a first heat transfer fluid from the first heat source, through the first heat exchanger and the second heat exchanger, [0330] wherein the second arrangement of conduits is further configured to pass a first portion of the first heat transfer fluid through the first heat exchanger and pass a second portion of the first heat transfer fluid through the second heat exchanger. [0331] 2. The sub-system of clause 1, further comprising: [0332] a third heat exchanger; and [0333] a third expansion stage; [0334] wherein the first arrangement of conduits is further configured to pass the working fluid through the third heat exchanger and the third expansion stage; and [0335] wherein the second arrangement of conduits is further configured to pass a third portion of the first heat transfer fluid through the third heat exchanger. [0336] 3. The sub-system of clause 1 or 2, further comprising; [0337] a second heat source; [0338] a fourth heat exchanger; [0339] a fourth expansion stage; and [0340] a third arrangement of conduits configured to pass a second heat transfer fluid from the second heat source, through the fourth heat exchanger, [0341] wherein the first arrangement of conduits is further configured to pass the working fluid through the fourth heat exchanger and the fourth expansion stage. [0342] 4. The sub-system according to clause 3, further comprising: [0343] a fifth heat exchanger; and [0344] a fifth expansion stage; [0345] wherein the first arrangement of conduits is further configured to pass the working fluid through the fifth heat exchanger and the fifth expansion stage; and [0346] wherein the third arrangement of conduits is further configured to pass a first portion of the second heat transfer fluid through the fourth heat exchanger and pass a second portion of the second heat transfer fluid through the fifth heat exchanger. [0347] 5. The sub-system of clause 3 or clause 4, wherein the or each heat exchanger through which the third arrangement of conduits passes is positioned along the first arrangement of conduits upstream of the heat exchangers through which the second arrangement of conduits passes. [0348] 6. The sub-system of clause 3 or clause 4, wherein the or each heat exchanger through which the third arrangement of conduits passes is positioned along the first arrangement of conduits downstream of the heat exchangers through which the second arrangement of conduits passes. [0349] 7. The sub-system according to any preceding clause, further comprising: [0350] a sixth heat exchanger, [0351] wherein the first arrangement of conduits is further configured to pass the working fluid through the sixth heat exchanger upstream of both (i) the furthest upstream heat exchanger through which the second arrangement of conduits passes and (ii) the furthest upstream heat exchanger through which the third arrangement of conduits passes, and [0352] wherein the first arrangement of conduits is further configured to pass the working fluid output from the furthest downstream expansion stage through the sixth heat exchanger to an exhaust. [0353] 8. The sub-system according to any preceding clause, further comprising: [0354] a fourth arrangement of conduits configured to divert a portion of the working fluid from a downstream position in the first arrangement of conduits through an evaporator and a first compressor, and return it to an upstream position in the first arrangement of conduits. [0355] 9. The sub-system according to clause 8, wherein the evaporator is positioned along the first arrangement of conduits upstream of the furthest upstream heat exchanger, wherein the downstream position is downstream of the furthest downstream expansion stage; and wherein the upstream position is immediately upstream of the furthest downstream expansion stage. [0356] 10. The sub-system according to clauses 3 to 9, configured such that the second arrangement of conduits passes through the first, second and third heat exchangers and preferably no other heat exchanger and the third arrangement of conduits passes through the fourth heat exchanger and preferably no other heat exchanger, and wherein the heat exchanger through which the third arrangement of conduits passes is upstream of the heat exchangers through which the second arrangement of conduits passes. [0357] 11. The sub-system according to clauses 4 to 9, configured such that the second arrangement of conduits passes through the first, second, and third heat exchangers and preferably no other heat exchanger and the third arrangement of conduits passes through the fourth and fifth heat exchangers and preferably no other heat exchanger, wherein the heat exchangers through which the second arrangement of conduits passes are upstream of the heat exchangers through which the third arrangement of conduits passes. [0358] 12. The sub-system according to clauses 3 to 9, configured such that the second arrangement of conduits passes through the first, second and third heat exchangers and preferably no other heat exchanger and the third arrangement of conduits passes through the fourth heat exchanger and preferably no other heat exchanger, wherein the heat exchangers through which the second arrangement of conduits passes are upstream of the heat exchanger through which the third arrangement of conduits passes. [0359] 13. The sub-system of any preceding clause, wherein the first heat source is a first thermal energy storage device and the second arrangement of conduits is further configured to return the first heat transfer fluid to the first thermal energy storage device after passing it through each heat exchanger through which the second arrangement of conduits is configured to pass, such that the second arrangement of conduits forms a first closed circuit. [0360] 14. The sub-system of any one of clauses 3 to 13, wherein the second heat source is a second thermal energy storage device and the third arrangement of conduits is further configured to return the second heat transfer fluid to the second thermal energy storage device after passing it through each heat exchanger through which the third arrangement of conduits is configured to pass, such that the third arrangement of conduits forms a second closed circuit. [0361] 15. The sub-system of clause 14, wherein the first thermal energy storage device is configured to store at least a portion of a heat of compression generated by a recycle air compressor and the second thermal energy storage device is configured to store at least a portion of a heat of compression generated by a main air compressor, optionally wherein the second thermal energy storage device may comprise pipework suitable for transporting molten salts. [0362] 16. The sub-system of any one of clauses 3 to 13, further comprising: [0363] a tenth heat exchanger; and [0364] an eleventh heat exchanger, wherein: [0365] the second heat source is a second thermal energy storage device, the first arrangement of conduits is further configured to pass the working fluid through the tenth heat exchanger immediately upstream of the fourth heat exchanger, and wherein; [0366] the third arrangement of conduits is configured to form two closed loops, the first closed loop passing through the second thermal energy storage device and the eleventh heat exchanger, and the second closed loop passing through the eleventh heat exchanger and the fourth heat exchanger, [0367] optionally wherein a heat transfer fluid in the first closed loop comprises molten salts, further optionally wherein a heat transfer fluid in the second closed loop comprises a thermal oil or a mixture of thermal oils. [0368] 17. The sub-system of clause 16, wherein the first thermal energy storage device is configured to store at least a portion of the heat of compression generated by a main air compressor and at least a portion of the heat of compression generated by a recycle air compressor, and the second thermal energy storage device is configured to store and at least a portion of the heat of compression generated by the main air compressor, optionally wherein the second thermal energy storage device may comprise pipework suitable for transporting molten salts. [0369] 18. The sub-system of any one of clauses 14 to 17, wherein the second thermal energy storage device is configured to store thermal energy at a higher temperature than the temperature of the thermal energy stored in the first thermal energy storage device, optionally wherein the second thermal energy storage device is configured to store thermal energy between 150 C. and 550 C., preferably between 200 C. and 400 C., and the first thermal energy storage device is configured to store thermal energy between 150 C. and 350 C. [0370] 19. A cryogenic energy storage system, comprising: [0371] a power recovery sub-system comprising a plurality of expansion stages configured to receive, via a corresponding plurality of heat exchangers, hot thermal energy from a first thermal energy storage device and a second thermal energy storage device and transfer it to a working fluid passing through the plurality of expansion stages and plurality of heat exchangers, preferably wherein the power recovery sub-system is according to any one of clauses 3 to 18; and [0372] a liquefaction sub-system configured to supply thermal energy to the first and second thermal energy storage devices, and further comprising; [0373] a main air compressor; [0374] a recycle air compressor; [0375] an eighth heat exchanger; [0376] a ninth heat exchanger; [0377] a fifth arrangement of conduits configured to pass a process stream through the main air compressor, eighth heat exchanger, recycle air compressor, and ninth heat exchanger; [0378] a sixth arrangement of conduits forming a third closed circuit and configured to pass a third heat transfer fluid between the second thermal energy storage device and the eighth heat exchanger; and [0379] a seventh arrangement of conduits forming a fourth closed circuit and configured to pass a fourth heat transfer fluid between the first thermal energy storage device and the ninth heat exchanger, [0380] wherein the eighth heat exchanger is positioned along the fifth arrangement of conduits immediately downstream of the main air compressor and configured to transfer at least a portion of the heat of compression of the process stream from the main air compressor, via the third heat transfer fluid to the second thermal energy storage device, and [0381] wherein the ninth heat exchanger is positioned along the fifth arrangement of conduits immediately downstream of the recycle air compressor and configured to transfer at least a portion of the heat of compression of the process stream from the recycle air compressor, via the fourth heat transfer fluid to the first thermal energy storage device. [0382] 20. The system according to clause 19, further comprising: [0383] a cold box; [0384] a liquefaction turbo-expander; [0385] an eighth arrangement of conduits, configured to pass at least a portion of the process stream through part of the cold box and the liquefaction turbo-expander before going through the cold box and merging with the fifth arrangement of conduits upstream of the recycle air compressor, such that the mass flow rate of the fluid through the main air compressor is less than the mass flow rate of fluid through the recycle air compressor; [0386] a ninth arrangement of conduits, configured to pass at least a portion of the process stream through the cold box, an expansion device, preferably a Joule-Thompson valve or a wet turbo-expander, to a phase separator, such that the portion of the process stream in the eighth arrangement of conduits transfers cold thermal energy to the portion of the process stream in the ninth arrangement of conduits via the cold box; and [0387] a first cold recycle loop, wherein the first cold recycle loop passes through the cold box and is configured to transfer waste cold thermal energy from a system which is external to, but thermally integrated with, the cryogenic energy storage system, to at least the portion of the process stream in the ninth arrangement of conduits. [0388] 21. The system according to clause 19 or 20, wherein the power recovery sub-system further comprises an evaporator; the system further comprising: [0389] a second cold recycle loop passing through the evaporator and configured to transfer waste cold thermal energy from a system which is external to, but thermally integrated with, the cryogenic energy storage system, to at least a portion of the working fluid, preferably a portion of the working fluid downstream of the plurality of expansion stages and plurality of heat exchangers of the power recovery sub-system. [0390] 22. The cryogenic energy storage system according to any one of clauses 19 to 21, wherein the main air compressor has different input and output pressures to those of the recycle air compressor and the main air compressor and/or the recycle air compressor are adiabatic. [0391] 23. The cryogenic energy storage system according to any one of clauses 19 to 22, further comprising a twelfth heat exchanger, wherein: [0392] the fifth arrangement of conduits is further configured to pass the process stream through the twelfth heat exchanger downstream of the eighth heat exchanger and upstream of the recycle air compressor, [0393] the seventh arrangement of conduits is further configured to pass the fourth heat transfer fluid through the twelfth heat exchanger, and wherein the twelfth heat exchanger is configured to transfer at least a portion of the heat of compression of the process stream from the main air compressor, via the fourth heat transfer fluid, to the first thermal energy storage device. [0394] 24. The cryogenic energy storage system according to any one of clauses 19 to 23, wherein the temperature of the thermal energy received from the second thermal energy storage device is greater than the temperature of the thermal energy received from the first thermal energy storage device, optionally wherein the second thermal energy storage device is configured to store thermal energy between 150 C. and 550 C., preferably between 200 C. and 400 C., and the first thermal energy storage device is configured to store thermal energy between 150 C. and 350 C. [0395] 25. The cryogenic energy storage system according to any one of clauses 19 to 24, further comprising a thirteenth heat exchanger, and wherein: [0396] wherein the sixth arrangement of conduits is configured to form two closed capture loops, the first closed capture loop passing through the eighth heat exchanger and the thirteenth heat exchanger, and the second closed capture loop passing through the thirteenth heat exchanger and the second thermal energy storage device, [0397] optionally wherein a heat transfer fluid in the second closed capture loop may comprise molten salts, further optionally wherein a heat transfer fluid in the first closed capture loop may comprise a thermal oil or a mixture of thermal oils [0398] 26. A thermal energy recycle system, comprising: [0399] a main air compressor; [0400] a recycle air compressor; [0401] a second thermal energy storage device; [0402] a first thermal energy storage device; [0403] a working fluid; and [0404] a plurality of expansion stages, comprising a first and second subsets; [0405] wherein the system is configured to capture at least a portion of the heat of compression produced by the main air compressor and store it in the second thermal energy storage device during a liquefaction phase, and to apply the heat of compression stored in the second thermal energy storage device to the working fluid upstream of each of the first subset of expansion stages during a power recovery phase, and [0406] wherein the system is further configured to capture at least a portion of the heat of compression produced by the recycle air compressor and store it in the first thermal energy storage device during a liquefaction phase, and to apply the heat of compression stored in the first thermal energy storage device to the working fluid upstream of each of the second subset of expansion stages during a power recovery phase. [0407] 27. A method for recycling thermal energy in a cryogenic energy storage system, comprising: [0408] providing a liquefaction sub-system comprising: [0409] a main air compressor; [0410] a recycle air compressor; [0411] a second thermal energy storage device; and [0412] a first thermal energy storage device; [0413] providing a power recovery sub-system comprising: [0414] a working fluid; and [0415] a plurality of expansion stages, comprising a first and second subset; [0416] capturing at least a portion of the heat of compression from the main air compressor and storing it in the second thermal energy storage device; [0417] capturing at least a portion of the heat of compression from the recycle air compressor and storing it in the first thermal energy storage device; [0418] applying the heat of compression stored in the second thermal energy storage device to the working fluid upstream of each of the first subset of expansion stages; and [0419] applying the heat of compression stored in the first thermal energy storage device to the working fluid upstream of each of the second subset of expansion stages. [0420] 28. The system according to clause 26 or the method according to clause 27, further comprising: [0421] a cold box; [0422] a first cold recycle loop, configured to pass through the cold box and to transfer waste cold thermal energy from a system which is external to, but thermally integrated with, the cryogenic energy storage system, to a portion of a process stream which passes through said cold box, an expansion device, preferably a Joule-Thompson valve or a wet turbo-expander, to a phase separator. [0423] 29. The system according to clause 26 or 28, or the method according to clause 27 or 28, further comprising: [0424] a second cold recycle loop, [0425] and wherein the power recovery sub-system further comprises: [0426] an evaporator; and [0427] a compressor; [0428] wherein the second cold recycle loop is configured to pass through the evaporator and to transfer waste cold thermal energy from a system which is external to, but thermally integrated with, the cryogenic energy storage system, to a portion of the working fluid which passes from the output of the power recovery unit, through said evaporator and said compressor, and re-enters the power recovery unit. [0429] 30. The system according to clause 26, 28, or 29, or the method according to clauses 27 to 29 wherein the mass flow rate of the fluid through the main air compressor is less than the mass flow rate of fluid through the recycle air compressor. [0430] 31. The system according to clause 26, 28, 29, or 30, or the method according to clauses 27 to 30, wherein the main air compressor has different input and output pressures to those of the recycle air compressor and the main air compressor and/or the recycle air compressor are adiabatic. [0431] 32. The system according to clause 26, 28, 29, 30 or 31, or the method according to clauses 27 to 31, wherein the external system is a liquid natural gas regasification terminal. [0432] 33. The system according to clause 26, 28, 29, 30, 31, or 32, or the method according to clauses 27 to 32, wherein the second thermal energy storage device is configured to capture, store, and apply heat of compression at a temperature different to, preferably higher than, that of the heat of compression captured, stored, and applied by the first thermal energy storage device, optionally wherein the second thermal energy storage device is configured to store thermal energy between 150 C. and 550 C., preferably between 200 C. and 400 C., and the first thermal energy storage device is configured to store thermal energy between 150 C. and 350 C. [0433] 34. The system according to clauses 26, 28 to 33, wherein the system is further configured to capture and store at least a portion of the heat of compression produced by the main air compressor and store it in the first thermal energy storage device during the liquefaction phase. [0434] 35. The system according to clause 34, wherein the system is configured to apply the heat of compression stored in the first thermal energy storage device to the working fluid via a heat transfer fluid during the power recovery phase. [0435] 36. The system according to clause 34 or 35, wherein the system is further configured to capture and store at least a portion of the heat of compression from the main air compressor in the first thermal energy storage device and at least a portion of the heat of compression from the recycle air compressor in the first thermal energy storage device during the liquefaction phase via a heat transfer fluid. [0436] 37. The system according to any one of clauses 26, 28 to 36, wherein the system further comprises: [0437] a first pair of conduit loops, configured to thermally interact with each other via a first intermediate heat exchanger; and [0438] a second pair of conduit loops, configured to thermally interact with each other via a second intermediate heat exchanger, wherein the system is configured to capture and store in the second thermal energy storage device the at least a portion of the heat of compression from the main air compressor via the first pair of conduit loops, and wherein the system is configured to apply the heat of compression stored in the second thermal energy storage device to the working fluid via the second pair of conduit loops. [0439] 38. The system according to clause 37, wherein the first pair of conduit loops comprises a first conduit loop and a second conduit loop, and wherein the first pair of conduit loops is configured to transfer at least a portion of the heat of compression from the main air compressor to the first intermediate heat exchanger via a first conduit loop, and from the first intermediate heat exchanger to the second thermal energy storage device via a second conduit loop, optionally wherein a heat transfer fluid in the second conduit loop may comprise molten salts. [0440] 39. The system according to clause 37 or 38, wherein the second pair of conduit loops comprises a third conduit loop and a fourth conduit loop, and wherein the second pair of conduit loops is configured to transfer at least a portion of the heat of compression stored in the second thermal energy storage device to the second intermediate heat exchanger via a third conduit loop, and from the second intermediate heat exchanger to the working fluid via a fourth conduit loop, optionally wherein a heat transfer fluid in the third conduit loop may comprise molten salts. [0441] 40. The method according to any one of clauses 27 to 33, further comprising capturing at least a portion of the heat of compression from the main air compressor and storing it in the first thermal energy storage device. [0442] 41. The method according to clause 37, wherein during applying the heat of compression stored in the first thermal energy storage device to the working fluid, the heat of compression is transferred via a heat transfer fluid. [0443] 42. The method according to clause 37 or 38, wherein during capturing at least a portion of the heat of compression from the main air compressor and storing it in the first thermal energy storage device and during capturing at least a portion of the heat of compression from the recycle air compressor and storing it in the first thermal energy storage device, the heat of compression is transferred via a heat transfer fluid. [0444] 43. The method according to any one of clauses 27 to 33, or 40, wherein capturing at least a portion of the heat of compression from the main air compressor and storing it in the second thermal energy storage device comprises: [0445] transferring at least a portion of the heat of compression from the main air compressor to a first intermediate heat exchanger via a first conduit loop, and from the first intermediate heat exchanger to the second thermal energy storage device via a second conduit loop, optionally wherein a heat transfer fluid in the second conduit loop may comprise molten salts. [0446] 44. The method according to any one of clauses 27 to 33, or 40, or 43, wherein applying the heat of compression stored in the second thermal energy storage device to the working fluid comprises: [0447] transferring at least a portion of the heat of compression stored in the second thermal energy storage device to a second intermediate heat exchanger via a third conduit loop, and from the second intermediate heat exchanger to the working fluid via a fourth conduit loop, optionally wherein a heat transfer fluid in the third conduit loop may comprise molten salts.