Systems and methods for controlling pressure in a cryogenic energy storage system

Abstract

A cryogenic energy storage system comprises at least one cryogenic fluid storage tank having an output; a primary conduit through which a stream of cryogenic fluid may flow from the output of the fluid storage tank to an exhaust; a pump within the primary conduit downstream of the output of the tank for pressurising the cryogenic fluid stream; evaporative means within the primary conduit downstream of the pump for vaporising the pressurised cryogenic fluid stream; at least one expansion stage within the primary conduit downstream of the evaporative means for expanding the vaporised cryogenic fluid stream and for extracting work therefrom; a secondary conduit configured to divert at least a portion of the cryogenic fluid stream from the primary conduit and reintroduce it to the fluid storage tank; and pressure control means within the secondary conduit for controlling the flow of the diverted cryogenic fluid stream and thereby controlling the pressure within the tank. The secondary conduit is coupled to the primary conduit downstream of one or more of the at least one expansion stages.

Claims

1. A cryogenic energy storage system, comprising: at least one cryogenic fluid storage tank having an output; a primary conduit through which a stream of cryogenic fluid may flow from the output of the fluid storage tank to an exhaust of the cryogenic energy storage system; a pump within the primary conduit downstream of the output of the tank for pressurising the cryogenic fluid stream to form a pressurised cryogenic fluid stream; evaporative means within the primary conduit downstream of the pump for vaporising the pressurised cryogenic fluid stream to form a gaseous stream of a vaporised cryogenic fluid stream; two or more expansion stages in series within the primary conduit downstream of the evaporative means for expanding the vaporised cryogenic fluid stream and for extracting work therefrom; a heating device between a or each pair of adjacent expansion stages in series and within the primary conduit, wherein the or each pair of adjacent expansion stages comprises an upstream expansion stage and a downstream expansion stage; a secondary conduit configured to divert at least a portion of the cryogenic fluid stream from the primary conduit and reintroduce it to the fluid storage tank; and pressure control means within the secondary conduit for controlling the flow of the diverted cryogenic fluid stream and thereby controlling the pressure within the tank; wherein the secondary conduit is coupled to the primary conduit downstream of one or more of the two or more expansion stages; wherein a connection between the primary and secondary conduits is immediately upstream of a heating device and immediately downstream of the upstream expansion stage in at least one pair of adjacent expansion stages, or a connection between the primary and secondary conduits is immediately downstream of a heating device and immediately upstream of the downstream expansion stage in at least one pair of adjacent expansion stages.

2. The cryogenic energy storage system of claim 1, further comprising: a cold recycle system comprising a cold store for storing cold energy; a liquefier for producing cryogen for storage in the cryogenic fluid storage tank; and pipework coupling the cold store to the evaporative means and to the liquefier for transferring cold energy from the evaporative means to the liquefier via the cold store; and a tertiary conduit configured to divert at least a portion of the cryogenic fluid stream from the primary conduit and introduce it to the cold recycle system, thereby increasing the pressure within the cold recycle system; wherein the tertiary conduit is coupled to the primary conduit downstream of one or more of the at least one expansion stages.

3. The cryogenic energy storage system of claim 1, wherein the evaporative means comprises a heat exchanger, wherein the pressure control means within the secondary conduit comprises a valve, wherein the at least one cryogenic fluid storage tank is a plurality of cryogenic fluid storage tanks, and further comprising a heating device immediately upstream of the first expansion stage and within the primary conduit.

4. The cryogenic energy storage system of claim 1, further comprising an additional connection between the primary and secondary conduits downstream of the downstream expansion stage in the at least one pair of adjacent expansion stages.

5. The cryogenic energy storage system of claim 1, wherein the two or more expansion stages in series comprise first and second expansion stages, the secondary conduit is connected to the primary conduit by first and second branches, and wherein the connection between the first branch and the primary conduit is between the first and second expansion stages, and wherein the connection between the second branch and the primary conduit is downstream of the second expansion stage.

6. The cryogenic energy storage system of claim 1, wherein the two or more expansion stages in series comprises first, second and third expansion stages in series and a connection between the primary conduit and the secondary conduit is between the second and third expansion stages.

7. The cryogenic energy storage system of claim 6, wherein the secondary conduit is connected to the primary conduit by first and second branches, and wherein the connection between the first branch and the primary conduit is between the first and second expansion stages, and wherein the connection between the second branch and the primary conduit is between the second and third expansion stages.

8. The cryogenic energy storage system of claim 7, wherein the first and second branches of the secondary conduit join at a valve configured to selectively connect the first and second branches to the downstream end of the secondary conduit.

9. The cryogenic energy storage system of claim 1, further comprising: an ambient vaporizer coupled to the cryogenic fluid storage tank for controlling the pressure therein; and pressure sensing means configured to sense a pressure within a headspace of the tank and a pressure within the primary conduit at an intersection with the secondary conduit; wherein: the system is configured to cause the ambient vaporizer to control the pressure within the cryogenic fluid storage tank when the pressure within the primary conduit at the intersection with the secondary conduit is insufficient to pressurise the fluid storage tank.

10. The cryogenic energy storage system of claim 9, wherein the two or more expansion stages in series comprises first, second and third expansion stages in series, wherein the secondary conduit is connected to the primary conduit by first and second branches, and wherein the connection between the first branch and the primary conduit is between the first and second expansion stages, and wherein the connection between the second branch and the primary conduit is downstream of the second expansion stage, and wherein said intersection of the primary conduit and secondary conduit is an intersection of the primary conduit and the first branch of the secondary conduit.

11. The cryogenic energy storage system of claim 8, further comprising processing means configured to control operation of the valve; and pressure sensing means configured to sense: a first pressure within the primary conduit at the intersection with the second branch; optionally, a second pressure within the primary conduit at the intersection with the first branch; and, optionally, a third pressure within the headspace of the tank; and wherein the processing means is configured to: cause the valve to connect the downstream end of the secondary conduit to the second branch when the first pressure is determined to be sufficient to pressurise the fluid storage tank; and cause the valve to connect the downstream end of the secondary conduit to the first branch when the first pressure is determined to be insufficient to pressurise the fluid storage tank.

12. The cryogenic energy storage system of claim 1, wherein a connection between the primary and secondary conduits is immediately downstream of a heating device and immediately upstream of the downstream expansion stage in at least one pair of adjacent expansion stages.

13. A method of re-pressurising at least one cryogenic fluid storage tank in a cryogenic energy storage system, comprising: passing a stream of cryogenic fluid through a primary conduit from an output in the cryogenic fluid storage tank; pressurising the stream of cryogenic fluid with a pump within the primary conduit downstream of the output of the tank to form a pressurised cryogenic fluid stream; vaporising the stream of pressurised cryogenic fluid with an evaporative means within the primary conduit downstream of the pump to form a gaseous stream of a vaporised cryogenic fluid stream; expanding and extracting work from the gaseous stream of the vaporised cryogenic fluid with two or more expansion stages in series within the primary conduit downstream of the pump; heating the expanded cryogenic fluid stream with a heating device between a or each pair of adjacent expansion stages in series within the primary conduit, wherein the or each pair of adjacent expansion stages comprises an upstream expansion stage and a downstream expansion stage; and diverting at least a portion of the expanded stream of pressurised cryogenic fluid from the primary conduit through a secondary conduit and reintroducing it into the cryogenic fluid storage tank, thereby controlling the pressure within the tank; wherein said at least a portion of the expanded stream of pressurised cryogenic fluid is diverted from the primary conduit after the stream has been expanded in one or more of the two or more expansion stages and work has been extracted from it; wherein a connection between the primary and secondary conduits is immediately upstream of a heating device and immediately downstream of the upstream expansion stage in at least one pair of adjacent expansion stages; or a connection between the primary and secondary conduits is immediately downstream of a heating device and immediately upstream of the downstream expansion stage in at least one pair of adjacent expansion stages.

14. The cryogenic energy storage system of claim 5, wherein the first and second branches of the secondary conduit join at a valve configured to selectively connect the first and second branches to the downstream end of the secondary conduit.

15. The cryogenic energy storage system of claim 1, wherein the primary conduit is further configured to convey a remaining portion of the stream of cryogenic fluid in gaseous form from the point at which the secondary conduit is coupled to the primary conduit to the exhaust of the cryogenic energy storage system.

16. The method of claim 13, further comprising conveying a remaining portion of the expanded stream of pressurised cryogenic fluid in gaseous form from the point at which the secondary conduit is coupled to the primary conduit to an exhaust of the cryogenic energy storage system.

17. The method of claim 13, wherein substantially all of the cryogenic fluid stream from the output of the fluid storage tank is conveyed in the primary conduit through the pump and the evaporative means and at least a first of the two or two or more expansion stages in series.

18. The cryogenic energy storage system of claim 1, wherein the primary conduit is configured to convey substantially all of the cryogenic fluid stream from the output of the fluid storage tank through the pump and the evaporative means and to a first of the least two or more expansion stages in series.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

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

(2) FIG. 1 is a system diagram of a cryogenic energy storage system according to a first embodiment of the invention;

(3) FIG. 2 is a system diagram of a cryogenic energy storage system according to a second embodiment of the invention;

(4) FIG. 3 is a system diagram of a cryogenic energy storage system according to a third embodiment of the invention;

(5) FIG. 4 is a system diagram of a cryogenic energy storage system according to a fourth embodiment of the invention;

(6) FIG. 5 is a system diagram of a cryogenic energy storage system according to a fifth embodiment of the invention;

(7) FIG. 6 is a system diagram of a cryogenic energy storage system according to a sixth embodiment of the invention;

(8) FIG. 7 is a system diagram of a cryogenic energy storage system according to a seventh embodiment of the invention;

(9) FIG. 8 is a system diagram of a cryogenic energy storage system according to an eighth embodiment of the invention;

(10) FIG. 9 is a system diagram of a cryogenic energy storage system according to a ninth embodiment of the invention; and

(11) FIG. 10 is a system diagram of a cryogenic energy storage system according to a tenth embodiment of the invention; and

(12) FIG. 11 is a system diagram showing possibilities for cryogenic energy storage systems according to further embodiment of the invention.

DETAILED DESCRIPTION

(13) The pressures, temperatures and flow rates used in the following description are intended to illustrate the invention. A person skilled in the art will understand that a wide range of possible values of pressure, temperature and flow rates exist depending on the particular design of the power recovery part of the LAES system.

(14) At supercritical pressures the distinction between liquid and gaseous phases is not definite. Purely for ease of understanding, the fluid state from the outlet of the evaporator will be described herein as being in the gaseous phase.

(15) A first embodiment of the invention is shown in FIG. 1, which illustrates a power recovery unit of a LAES system. According to this embodiment, cryogenic liquid is stored in cryogenic storage tank 100 with a pressure of approximately 8 bar in the headspace of the tank.

(16) During a first power recovery period cryogenic liquid stored in cryogenic storage tank 100 is withdrawn from the bottom of tank 100 at a rate of 100 kg/s and pumped to a pressure of 100 bar in cryogenic pump 200. The resulting high-pressure cryogenic liquid is then substantially vaporised in evaporator 300, emerging as a gaseous stream, at a temperature of approximately 15 degC. Said gaseous stream is then further heated in first heating device 501 to a temperature of 80 degC before being expanded in first expansion stage 401 to a pressure of approximately 32 bar. The gaseous stream is now at a temperature of approximately 0 degC and is reheated in second heating device 502 to 80 degC before entering second expansion stage 402. The gaseous stream emerges at a pressure of approximately 10 bar and a temperature of approximately 0 degC. Downstream of expansion stage 402 (specifically, between the second and third expansion stages), at connection point P, a portion of the gaseous stream is diverted, forming a pressurisation stream.

(17) The remainder of the gaseous stream has a flow rate that is, on average (during the power recovery phase), approximately 98% of the flow rate of the original gaseous stream prior to diversion. This remainder is reheated to 80 degC in third heating device 503 before entering third expansion stage 403 from which emerges at a pressure of approximately 4 bar and a temperature of approximately 0° C. The remainder of the gaseous stream is reheated to 80 degC in fourth heating device 504 before entering fourth expansion stage 404 where it is expanded to approximately ambient pressure before being exhausted to atmosphere. In this case, connection point P is immediately upstream of the third heating device 503 (between the second expansion stage 402 and the third heating device 503).

(18) First, second, third and fourth expansion stages 401, 402, 403 and 404 are mechanically coupled to an electric generator such that the work generated by expansion of the gaseous stream in first, second, third and fourth expansion stages 401, 402, 403 and 404 is converted into electrical energy.

(19) The pressurisation stream has a flow rate that is, on average, approximately 2% of the flow rate of the original gaseous stream prior to diversion. The pressurisation stream is connected to the headspace of cryogenic storage tank 100 via pressure control means 600. Pressure control means 600 is configured to regulate the pressure in the headspace of the cryogenic tank at a constant 8 bar.

(20) During a second power recovery period, in response to a change in electrical load, the output of the system is decreased to approximately 85% of capacity by reducing the discharge pressure of the cryogenic pump to approximately 48 bar (according to techniques known in the art). The rate of outflow of liquid from tank 100 drops to approximately 85 kg/s and the reheat temperatures remain identical. The outlet pressure from expansion second stage 402 is now approximately 8.5 bar.

(21) During this second power recovery period, the rate of outflow from the tank is lower than during the first power recovery period and the required flow of the pressurisation stream is also lower. Since the pressurisation stream is diverted from the gaseous stream, the ratio of the flow rates of the outflow of liquid from the tank and the pressurisation stream are approximately the same during the first and second periods of power recovery.

(22) It will be recognised that during the second power recovery period, the pressure available in the pressurisation stream is approaching the pressure in tank 100. The system is therefore approaching a limit beyond which it would no longer be possible to pressurise tank 100 as the pressure differential would cause vapour to flow in reverse from tank 100 to connection point P downstream of second expansion stage 402. While the addition of non-return valve means would prevent reverse flow, it would not be possible to pressurise tank 100 from the gaseous stream. Advantageously, connection point P is provided at a point in the system where the pressure remains above the minimum required tank pressure over the entire range of output required of the system. This point will depend on various system parameters and may be tailored to suit particular circumstance by a skilled person.

(23) Alternatively, the system may further comprise a small ambient vaporiser coupled to the tank for maintaining the pressure in the headspace of the tank during the LAES storage phase when the power recovery unit is not running. In this case, when the pressure at connection point P drops below the pressure in the tank during the power recovery period, since the outflow from the tank will be lower, it may be practicable to use the small ambient vaporiser to maintain tank headspace pressure for the lower end of the output range. Suitable sensing and control means may be provided to achieve this, as a skilled person would appreciate.

(24) It is known in the art of cryogenic liquid storage that the boil-off rate of a liquefied gas is lower at low pressure. Optionally, during the storage phase, cryogenic storage tank 100 may be held at lower headspace pressure, for example 4 bar, to reduce the quantity of gas lost to boil-off, and, during the power recovery phase, the pressure may be raised using the above described system to the operating pressure (in this case 8 bar). This would have the effect of sub-cooling the fluid by taking it away from the saturation curve, providing greater available NPSH to the cryogenic pump.

(25) A person skilled in the art will recognise that the system may comprise any number of expansion stages and that connection point P may be situated downstream of one or more of the stages, provided that the pressure at point P is greater than or equal to the required pressure in the cryogenic storage tank. In the case where only one expansion stage is provided, connection point P may be situated downstream of the expansion stage; that is, between the expansion stage and the exhaust of the system. However, in that case it would be necessary for the exhaust of the system be at a pressure greater than or equal to the required pressure in the cryogenic storage tank. Preferably, the connection point P is immediately downstream of the expansion stage; that is, without any other components in between. Where there are two or more expansion stages, connection point P may be situated between any two adjacent stages or between the final stage and the exhaust of the system. Specifically, the connection point P may be situated between the first and second expansion stages; or between the second and third expansion stages; and so on. For example, in the embodiment shown in FIG. 1, the pressurisation stream is diverted from the outlet of the second expansion stage 402 but this is simply an exemplary arrangement. The power recovery unit may have at least one and as many as “n” expansion stages, and the pressurisation stream may be diverted from the outlet of any of the said “n” expansion stages, provided that the pressure at the outlet of expansion stage “n” is equal to or higher than the pressure in the cryogenic storage tank 100. FIG. 11 shows a generic representation of embodiments formed by “n” expansion turbines, n being equal or higher than 1, where the stream is diverted from the outlet of turbine “j”, j being equal or higher than 1 and equal or lower than n.

(26) Furthermore, it will be understood that cryogenic storage tank 100 may be formed of a plurality of cryogenic storage tanks with a common connection to cryogenic pump 200 and a common header in fluid communication with the fluid connection.

(27) A second embodiment of the invention is shown in FIG. 2 and is identical to the first embodiment except that connection point P is situated downstream of expansion stage 402 (specifically, between the second and third expansion stages) but downstream (rather than upstream) of third heating device 503 (specifically, between the third heating device 503 and the third expansion stage 403). Compared with the first embodiment, the pressurisation stream is at an elevated temperature of 80 degC.

(28) The warmer pressurisation stream is less dense and occupies more space per unit mass, meaning that the same pressure may be achieved in the tank headspace using a smaller quantity of gas as compared with the first embodiment. A portion of the warm gas will condense at the surface of the liquid in the tank, thus forming a layer of saturated liquid in equilibrium with the vapour phase, which is maintained by thermal stratification and provides a barrier between the vapour in the headspace and the bulk of the liquid.

(29) This method may also provide for faster pressurisation of the tank, which could be useful in cases where the cryogenic liquid is stored in the tank at lower pressure and then its pressure is raised at the start of the power recovery phase. Optionally, the system would operate in the manner of the second embodiment during start-up of the power recovery unit in order to provide faster start, and then operate in the manner of the first embodiment once the pressure had been raised to the required operating pressure for the power recovery phase. This could be achieved by providing two connection points (for instance, one upstream of the heating device 503 and one downstream of the heating device 503), in a similar fashion to embodiments discussed below.

(30) It should be understood that, as with the embodiment of FIG. 1, the embodiment of FIG. 2 is merely exemplary, and the same invention can be implemented with the power recovery unit having at least one and as many as “n” expansion stages, and the pressurisation stream may be diverted from a heating device downstream the outlet of any of the said “n” expansion stages, provided that the pressure at the outlet of expansion stage “n” is equal or higher to the pressure in the cryogenic storage tank 100.

(31) A third embodiment of the invention is shown in FIG. 3 and is identical to the first embodiment except that the fluid connection between the headspace of tank 100 and the gaseous stream is connected at two connection points P and Q rather than one. As shown, connection point Q is between the first expansion stage 401 and the second expansion stage 402; whilst connection point P is between the second expansion stage 402 and the third expansion stage 403. In this case, each connection point is upstream of the heating device that is situated between the same two adjacent stages as the connection point. However, one or more of the connection points may be downstream of the heating device that is situated between the same two adjacent stages as the connection point.

(32) Valve means 601 is provided to alternatively connect either connection point P or connection point Q to the headspace of tank 100 via pressure control means 600. A skilled person would appreciate that where circumstances render it impractical to provide a single pressure control means covering the full range of pressures in the two branches connected at P and Q, two pressure control means may be used—one for each branch.

(33) The advantage of this third embodiment is that if the pressure at point P falls below the pressure in the headspace of tank 100 due to a reduction in the power output of the system, connection point Q, which is at a higher pressure, may be selected instead. Suitable sensing and control means may be provided to achieve this, as a skilled person would appreciate. In circumstances in which the pressure at connection point P is sufficient, however, this connection point may be selected such that further work may be extracted from the gaseous stream before a portion is diverted to the pressurisation stream.

(34) As is common practice in the safe design of all cryogenic energy storage systems, the pressure in the tank of all the above embodiments may be prevented from rising above design value by means of a pressure relief valve (not shown).

(35) A person skilled in the art will understand that the above-described embodiments are purely exemplary arrangements that depict implementations of the invention. The number of expansion stages, the pressures ratios and the temperatures at the inlet of the turbines are design parameters that may vary depending on the particular implementation whilst still falling within the scope of the claims. Moreover, the pressure ratio in each turbine may or may not be the same in all of the stages. Similarly, the inlet temperature at the entrance of each expansion stage may or may not be the same.

(36) A fourth embodiment is shown in FIG. 4. This embodiment is identical to the first embodiment with the exception that an additional fluid connection R is provided downstream of the third expansion stage 403 (specifically, between the third and fourth expansion stages), which provides a pressurisation stream to a cold recycle system 700, comprising cold store 701, cold recovery stream 702 flowing through evaporator 300 and cold supply stream 703 for supplying cold to the liquefier in the LAES system during the LAES charging phase (not shown).

(37) In the exemplary embodiment of FIG. 4, the cold recycle system is maintained at a pressure of 3.5 bar. The fluid connection R is used to maintain the pressure in the cold recycle system. The circulation of gas in the cold recycle system may be ensured by blowers. The flow rate diverted to the cold recycle system is controlled by pressure control means 602 which is configured to open once pressure in the cold recycle system falls below a predetermined threshold, thus allowing the pressure in the cold recycle system to be controlled at the desired level, compensating for the effects of small leaks or thermal contraction as the mean temperature of the fluid in the cold recycle system falls, for instance. Suitable pressure sensing and control means may be provided to achieve this, as a skilled person would appreciate.

(38) In this embodiment, the connection between the conduit carrying the diverted cryogen and the cold recycle system 700 is provided upstream of the blower 801. The portion of cryogen that is diverted at point R is at 0 degC. The gas circulating in cold recycle system 700 emerges from cold store 701 at approximately ambient temperature. It is beneficial to provide the connection upstream of the blower such that the diverted cryogen can provide a slight cooling effect on the gas circulating in the cold recycle system 700, thus reducing the work required to circulate the fluid in blower 801.

(39) The flow required to control the pressure in the cold recycle system depends on the volume of the the cold store, which in turn depends on the energy capacity (MWh) and the operating regime of the LAES system. Compared with utilising the present invention to pressurise a cryogenic storage tank, the gain in useful energy output from the LAES system that results from pressurising the cold recycle system in the manner described above may be small where the cold store is small. This is due to the small flow of the cold recycle pressurisation stream, compared with the higher flow of the cryogenic tank pressurisation stream.

(40) Nevertheless, even marginal gains contribute to the overall round-trip efficiency of the LAES system, and in the case of pressurising the cold recycle system, the gains outweigh the costs of providing the requisite infrastructure of additional pipework and a pressure control system. This is particularly so when pressurisation for the tank is also being provided, but may also be the case in isolation of such a system.

(41) Connection points R and P might be the same connection point along the main fluid stream. In that case, the diverted stream is further split into two separate streams, one of them fluidly connected with the headspace of the cryogenic tank 100 and the other with the cold recycle system 700. The pressure of each stream is accurately controlled by a pressure control means.

(42) A fifth embodiment is shown in FIG. 5. It is identical to the fourth embodiment except that the connection point R is replaced with a connection to the headspace of the cryogenic storage tank and the connection to cold recycle system 700 is provided downstream of the evaporator and upstream of cold store 701. This embodiment is particularly advantageous in cases where cold recycle system 700 operates at the same or slightly lower pressure than the cryogenic storage tank. The cold recycle system 700 is pressurised using gaseous cryogen from the cryogenic tank 100. This embodiment provides for controlling the pressure of cold recycle system 700 during the power recovery phase but also during the storage phase. In the latter case, it may replace gas lost through small leaks in the system. Pressure control means 607 is provided to control the pressure in the cold recycle system.

(43) In this embodiment, the portion of cryogen diverted to cold recycle system 700 leaves the headspace of the cryogenic storage tank at approximately −160 degC. It is therefore beneficial to introduce it to cold recycle system 700 immediately upstream of cold store 701 such that the cold embodied in it is transferred to the thermal storage medium.

(44) A sixth embodiment is shown in FIG. 6. This is identical to the fourth embodiment with the following exceptions. Firstly, the fluid streams diverted from connection points R and P in the sixth embodiment are at the same pressure but have different temperatures. Secondly, the connection point between the conduit carrying the diverted cryogen and the cold recycle system 700 is provided downstream of cold store 701 and also downstream of the blower 801 (whilst remaining upstream of evaporator 300). In this exemplary embodiment the cold recycle system operates at approximately 8.5 bar and connection points P and R are both downstream of the same expansion stage (in this case, the second expansion stage 402—that is, they are both between the second and third expansion stages). However, connection point P is upstream of heating device 503 whereas connection point R is downstream of heating device 503. In this case, both diverted streams have a pressure around 10 bar, but the stream headed to the cryogenic tank headspace is at a temperature of 0° C. whereas the stream directed to the cold recycle system is at 80° C. Topping up the cold recycle system 700 with a higher temperature stream may enhance evaporation.

(45) It should be understood that the described embodiments are just exemplary arrangements of the invention. The same invention may be implemented having one or more fluid connections between the headspace of the cryogenic tank 100 and a point in the main flow stream downstream at least a first expansion stage 401 and/or one or more fluid connections between the main flow stream downstream the evaporator 300 and the cold recycle system 700. In all cases, the condition is that the pressure of the diverted stream or streams is equal or higher to the target pressure.

(46) A seventh embodiment of the invention is shown in FIG. 7. The seventh embodiment is identical to the sixth except that a connection is provided between cold recycle system 700 and the air liquefaction system. Accordingly, FIG. 7 further illustrates the air liquefaction system, wherein, during the liquefaction phase, ambient air is compressed to approximately 8 bar in compressor 901 before being purified of moisture and other impurities in air purification unit 1000. The now clean air joins air vapour returning from liquefier 4000 before being further compressed in compressor 902 to approximately 60 bar before entering liquefier 4000. A portion of the air is liquefied and sent to cryogenic storage tank 100 via pump 201 while a portion returns to the inlet of compressor 902 During the liquefaction stage cold is being delivered from cold store 701 to liquefier 4000 via cold supply stream 703. The cold supply stream 703 enters liquefier 4000 at around minus 160 degC and leaves it at close to ambient temperature. As result, the mean temperature in cold recycle system 700 gradually increases from approximately minus 160 degC towards ambient. As the air in cold recycle system 700 expands, a portion is relieved via connection point Z and introduced into the air liquefaction system, upstream of compressor 902, where the process pressure is approximately 8 bar. Pressure control means 604 is provided so that when the pressure in the cold recycle loop 700 increases above 8.5 bar, air is diverted from the cold recycle system 700 to the inlet of the recirculating air compressor 902. The advantage of this aspect of the current invention is that instead of venting the clean and compressed air, it is fed into the liquefaction cycle, reducing the duty of the main air compressor 901 and the air purification unit 1000.

(47) As a person skilled in the art will know, the main air compressor 901 and the recirculation air compressor 902 are usually composed of various stages in an arrangement known as multistage compression. Thus, the connection point to the recirculation air compressor 902 will preferably be provided at the inlet of the stage whose inlet pressure is the closest, but inferior, to the pressure in the cold recycle system 700.

(48) An eighth embodiment of the invention is shown in FIG. 8. It is identical to the seventh embodiment except that the same principle is applied to the control of the pressure in the headspace of the cryogenic storage tank during the liquefaction phase. Accordingly, a further connection is provided between the headspace of cryogenic storage tank 100 and the inlet of compressor 902. During the liquefaction phase, as cryogenic storage tank 100 is filled, the level of liquid in the tank increases and gas in the tank headspace gets gradually compressed as it has less volume to occupy. To avoid an excessive pressure build-up, said gas in the tank headspace is usually vented to ambient. The embodiment of FIG. 8 provides a means to avoid wasting that portion of clean and compressed gas by providing a fluid connection to the inlet of the recirculation air compressor 902. This way, the round trip efficiency of the system is increased, even if marginally, as the main air compressor and air purification system need to compress and clean a relatively smaller amount of gas. Pressure control means 605 is provided to the control the pressure in the headspace of the tank.

(49) A ninth embodiment of the invention is shown in FIG. 9. It is identical to the eighth embodiment except that the fluid connection from the headspace of the cryogenic storage tank is connected to cold supply stream 703 of cold recycle system 700 instead of the inlet of the recirculation air compressor 902. This allows the cold embodied in the vapour released from the headspace of the cryogenic tank to be utilised for cooling in the air liquefier, before being introduced to the inlet of compressor 902 via the same connection provided for the control of pressure in cold recycle system 700 during the liquefaction phase, as explained above in connection with FIG. 7. Pressure control means 606 is provided to control the flow of displaced gas from the cryogenic storage tank to the cold recycle system, thus controlling the pressure in the headspace of the tank. Pressure control means 606 controls the pressure in the headspace of the cryogenic storage tank to slightly above the pressure in the cold recycle system as controlled by pressure control means 604, such that the flow of gas is always from the cryogenic storage tank to the cold recycle system to the liquefaction system.

(50) A tenth embodiment of the invention is shown in FIG. 10. It is identical to the ninth embodiment except that the fluid connection between the headspace of tank 100 and the gaseous stream is connected at two connection points P and Q rather than one and the fluid connection between the cold recycle system and the gaseous stream is connected at two points R and S, and valve means 601 and 603 are provided for selecting between connection points P and Q, and connection points R and S respectively.

(51) The advantage of this tenth embodiment is that if the pressures at points P or R fall below the pressure in the headspace of tank 100 or the pressure of cold recycle system 700 respectively, due to a reduction in the power output of the system, connection points Q or S respectively, which are at a higher pressure respectively, may be selected instead.

(52) It should be understood that the described embodiments are just exemplary arrangements of the invention. The same invention may be implemented using any combination of connections, including: between the cryogenic tank and the liquefaction system; between the cold recycle system and the liquefaction system; and between the cryogenic tank and the cold recycle system (with or without a subsequent connection between the cold recycle system and the liquefaction system). There may also be provided a connection between the cryogenic tank and the cold recycle system upstream of the cold store; and/or a connection between the cryogenic tank and the cold recycle system downstream of the cold store (again either with or without a subsequent connection between the cold recycle system and the liquefaction system).

(53) Irrespective of such modified embodiments, the invention is defined solely by the appended claims.