Improved re-liquefier system

Abstract

A re-liquefier system for recovering boil-off gas from a cryogenic storage tank, the re-liquefier system having a phase separator vessel connectable to the cryogenic storage tank to receive boil-off gas therefrom and return liquid thereto, and an expansion valve to cause a portion of the gas to condense into liquid. The system also has a cryocooler with a plurality of cold heads, a plurality of heat exchangers; and a compressor connected to drive circulation of gas through the re-liquefier system and provide pressure differential to the expansion valve. The heat exchangers are connected in series, and the cold heads are connected in series.

Description

[0001] IMPROVED RE-LIQUEFIER SYSTEM

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a re-liquefier system. More particularly, but not exclusively, it relates to a hydrogen re-liquefier system for use in recovering boil-off hydrogen gas from cryogenic storage tanks.

[0004] BACKGROUND OF THE INVENTION

[0005] Various substances used in industrial applications are gases at room temperature and have extremely low boiling points, such that in liquid form they must be stored in specially thermally insulated cryogenic tanks. For example, hydrogen has a boiling point of -252.9 °C (20 K) at atmospheric pressure, and liquid hydrogen is used in applications such as liquid rocket fuel and fuel cells. Helium has a boiling point of -268.93 °C (4.22 K) at atmospheric pressure, and liquid helium is used as a refrigerant and for superconducting magnets like those in MRI machines. Storing such substances in liquid form is also more space efficient even when it is the gas form which is ultimately used.

[0006] Due to heat leakage into a cryogenic storage tank, it is typical for some portion of the stored liquid to be lost to boil-off over time, usually vented to atmosphere via a pressure relief valve. Typical losses for hydrogen may be around 1% per day, although this will depend on the insulation and the resulting rate of heat leakage.

[0007] Instead of venting the boil-off gas to atmosphere and having it go to waste, it is possible to re-liquefy the boil-off and return it to the cryogenic tank. However, this requires hardware capable of both reaching sufficiently low temperatures and providing adequate cooling power. The most efficient systems typically used for industrial cryogenic liquefaction are large and expensive, and are impractical for use in merely re-liquefying boil-off from storage tanks. A smaller scale solution is needed which is suitable for use with individual cryogenic storage tanks. However, this has proved difficult to achieve without significant inefficiency and high levels of complexity, hence wasteful venting of boil-off to atmosphere remains dominant. It is desirable to develop re-liquefier systems which are more efficient and simple, such that they are economical to adopt in the prevention of wasting stored cryogenic liquids. It is an object of the present invention to provide a re-liquefier system which overcomes or at least partially ameliorates some of the abovementioned disadvantages or which at least provides the public with a useful choice.

[0008] BRIEF DESCRIPTION OF THE INVENTION

[0009] According to a first aspect the invention broadly comprises a re-liquefier system for recovering boil-off gas from a cryogenic storage tank, the re-liquefier system comprising: a phase separator vessel connectable to the cryogenic storage tank to receive boil- off gas therefrom and return liquid thereto, and comprising a reliquefying outlet and a reliquefying inlet; an expansion valve connected to the re-liquefying inlet to expand gas entering the phase separator vessel therethrough and to thereby cause a portion of the gas to condense into liquid; a cryocooler comprising a cryostat and a plurality of cold heads mounted within the cryostat; a plurality of heat exchangers; and a compressor connected to drive circulation of gas through the re-liquefier system and provide pressure differential to the expansion valve; wherein the heat exchangers are connected in series to heat a first flow of gas coming from the re-liquefying outlet by transferring heat from a second flow of gas returning to the re-liquefying inlet, the compressor is fed by the first flow of gas and returns the second flow of gas, and the cold heads are connected in series to cool the second flow of gas.

[0010] According to another aspect the cold heads and the heat exchangers are connected such that the second flow of gas flows through the heat exchangers and the cold heads in an alternating fashion.

[0011] According to another aspect the plurality of cold heads is three cold heads.

[0012] According to another aspect the plurality of heat exchangers is four heat exchangers.

[0013] According to another aspect the plurality of heat exchangers are mounted within the cryostat of the cryocooler.

[0014] According to another aspect one of the plurality of heat exchangers is mounted outside the cryostat of the cryocooler and a remainder of the plurality of heat exchangers are mounted within the cryostat. According to another aspect the plurality of heat exchangers are mounted within a secondary cryostat connected to the cryostat of the cryocooler.

[0015] According to another aspect the plurality of heat exchangers are counterflow heat exchangers.

[0016] According to another aspect the phase separator vessel and the expansion valve are mounted outside the cryostat of the cryocooler.

[0017] According to another aspect the phase separator vessel and the expansion valve are mounted within the cryostat of the cryocooler.

[0018] According to another aspect the cryostat comprises a gas warming outlet and a gas cooling inlet between which the compressor is connected, outside of the cryostat.

[0019] According to another aspect the re-liquefier system further comprises a supply conduit and a return conduit each connected to the phase separator vessel, the conduits being connectable to the cryogenic storage tank for receiving boil-off gas therefrom and returning liquid thereto, respectively.

[0020] According to another aspect the re-liquefier system further comprises a vapour supply valve in the supply conduit and a liquid return valve in the return conduit.

[0021] According to another aspect one of the plurality of heat exchangers is connected to both the supply conduit and the re-liquefying outlet, and a proportional control valve is connected between that heat exchanger and the re-liquefying outlet.

[0022] According to another aspect the re-liquefier system further comprises an integrated conduit and a valve assembly configured to allow the phase separator vessel to switch between receiving vapour or returning liquid via the integrated conduit.

[0023] According to another aspect the re-liquefier system further comprises a controller operatively connectable to a pressure sensor associated with the cryogenic storage tank, the controller being configured to monitor the pressure in the cryogenic storage tank and switch the re-liquefier system between receiving boil-off gas or returning liquid accordingly.

[0024] According to another aspect the compressor is a multi-stage compressor cooled at each stage.

[0025] According to another aspect the re-liquefier system further comprises pressure buffer tanks connected either side of each stage of the compressor, the buffer tanks being configured to regulate pressure in the re-liquefier system during transient events. According to another aspect the invention broadly comprises a method of operating the re-liquefier system of any one of the preceding claims, the method comprising: connecting the phase separator vessel to the cryogenic storage tank; supplying boil-off gas from the cryogenic storage tank; re-liquefying gas by running the compressor and the cryocooler to circulate gas through the re-liquefier system; returning liquid from the phase separator vessel to the cryogenic storage tank.

[0026] According to another aspect the method further comprises operating the plurality of cold heads at progressively lower temperatures along the second flow.

[0027] According to another aspect the progressively lower temperatures are between 120 K to 50 K.

[0028] According to another aspect the re-liquefier system is operated to intermittently return liquid to the cryogenic storage tank.

[0029] According to another aspect the re-liquefier system is operated to continuously return liquid to the cryogenic storage tank.

[0030] According to another aspect the boil-off gas supplied by the cryogenic storage tank is hydrogen.

[0031] According to another aspect the cryocooler is operated at approximately 30 kW input power.

[0032] According to another aspect the compressor is operated to raise the pressure of gas passing therethrough by between 15 to 25 bar.

[0033] According to another aspect the compressor is operated to raise the pressure of gas passing therethrough by approximately 20 bar.

[0034] According to another aspect the temperature after passing through the expansion valve is maintained at approximately 30 K.

[0035] According to another aspect a gas supply is provided, and the system operates in both a liquefication mode and a re-liquefication mode.

[0036] According to another aspect the invention broadly comprises a method of reliquefying boil-off gas, the method comprising: supplying boil-off gas from a cryogenic storage tank; warming the boil-off gas; compressing the warmed boil-off gas to increase its pressure; cooling the compressed boil-off gas; expanding the cooled boil-off gas to cause a portion of the expanded boil-off gas to condense into liquid; and continuously circulating the boil-off gas through each above step, wherein cooling the compressed boil-off gas comprises successive stages of active cryocooling, and successive stages of heat transfer to the boil-off gas being warmed.

[0037] According to another aspect the successive stages of active cryocooling and heat transfer are alternated.

[0038] According to another aspect the successive stages of active cryocooling are performed at progressively lower temperatures.

[0039] According to another aspect the progressively lower temperatures are between 120 K to 50 K.

[0040] According to another aspect the method comprises comprising intermittently returning liquid to the cryogenic storage tank.

[0041] According to another aspect the method comprises continuously returning liquid to the cryogenic storage tank.

[0042] According to another aspect the boil-off gas is hydrogen.

[0043] According to another aspect compressing the boil-off gas is performed to increase pressure by between 15 to 25 bar.

[0044] According to another aspect compressing the boil-off gas is performed to increase pressure by approximately 20 bar.

[0045] Other aspects of the invention may become apparent from the following description which is given by way of example only and with reference to the accompanying drawings.

[0046] As used herein the term "and / or" means "and" or "or", or both.

[0047] As used herein "(s)" following a noun means the plural and / or singular forms of the noun.

[0048] The term "comprising" as used in this specification and claims means "consisting at least in part of". When interpreting statements in this specification and claims which include that term, the features, prefaced by that term in each statement, all need to be present but other features can also be present. Related terms such as "comprise" and "comprised" are to be interpreted in the same manner. In this specification, where reference has been made to external sources of information, including patent specifications and other documents, this is generally for the purpose of providing a context for discussing the features of the present invention. Unless stated otherwise, reference to such sources of information is not to be construed, in any jurisdiction, as an admission that such sources of information are prior art or form part of the common general knowledge in the art.

[0049] For the purpose of this specification, where method steps are described in sequence, the sequence does not necessarily mean that the steps are to be chronologically ordered in that sequence, unless there is no other logical manner of interpreting the sequence.

[0050] BRIEF DESCRIPTION OF THE DRAWINGS

[0051] The invention will now be described by way of example only and with reference to the drawings in which:

[0052] Figure 1 shows a schematic of a re-liquefier system with a phase separator vessel mounted outside of a cryostat of a cryocooler;

[0053] Figure 2 shows a schematic of the inside of the cryostat of the re-liquefier system of figure 1;

[0054] Figure 3 shows a schematic of a re-liquefier system where the phase separator vessel is mounted inside the cryostat of the cryocooler and a two-stage compressor is used;

[0055] Figure 4 shows a schematic of the phase separator vessel and the cryogenic storage tank connected by an integrated conduit;

[0056] Figure 5 shows a schematic of a re-liquefier system where the components are split between two connected cryostats;

[0057] Figure 6 shows a perspective view of the inside of the cryostat of figure 3; and

[0058] Figure 7 shows a front view of the inside of the cryostat of figure 3.

[0059] DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0060] According to various aspects of the various embodiments of the present invention as illustrated in figures 1 -7, there is provided a re-liquefier system 10 which will now be described. The re-liquefier system 10 is for recovering boil-off gas from a cryogenic storage tank, but it will be understood that the cryogenic storage tank itself may not form part of the invention. The cryogenic storage tank may be of an entirely conventional type, and the reliquefier system 10 is external and connectable to the cryogenic storage tank. The invention is primarily directed towards liquid hydrogen storage, however it may be equally applicable to other cryogenic liquids such as helium for example. Thus, the generic terms "gas" and "liquid" are used throughout the specification, but it will be understood that the invention is intended for use with substances having a boiling point low enough to necessitate cryogenic storage in liquid form.

[0061] As shown in figure 1, the re-liquefier system 10 comprises a phase separator vessel 12 connectable to the cryogenic storage tank. The phase separator vessel 12 receives boil-off gas from the cryogenic storage tank, and returns liquid after the boil-off gas is re-liquefied. The capacity of the phase separator vessel 12 may be approximately 20 L, but could be adapted to the size of the cryogenic storage tank. Depending on the configuration of the re-liquefier system 10 the liquid may be returned intermittently or continuously, and the connection to the cryogenic storage tank may take various forms, as will subsequently be described in more detail. Separate from the connection to the cryogenic storage tank, the re-liquefier system 12 comprises a re-liquefying outlet 14 and a re-liquefying inlet 16 for circulating gas through the re-liquefier system 10 and thereby re-liquefying it.

[0062] The re-liquefier system 10 further comprises an expansion valve 18 connected to the re-liquefying inlet 16. The expansion valve 18 acts as a Joule-Thompson valve, i.e. it facilitates isenthalpic expansion which causes gas passing through to drop in temperature without heat transfer to the environment. The expansion valve 18 may be throttled or porous to cause this effect. Thus, when gas enters the phase separator vessel 12 via the re-liquefying inlet 16 and therefore passes through the expansion valve 18, a portion of the gas flow will condense into liquid. In order for the temperature drop through the expansion valve 18 to cause condensation, the incoming gas must be at a sufficiently low temperature and there must be a sufficient pressure differential across the expansion valve 18. These two parameters are codependent and dependent on the gas to be re-liquefied. For example, a system to re-liquefy hydrogen which has an incoming temperature in the order of 39K and a pressure drop from 24 bar to 7 bar will liquefy nominally 20% of the fluid flow.

[0063] To provide the necessary cooling, the re-liquefier system 10 further comprises a cryocooler 20 comprising a cryostat 22. The cryostat 22 is preferably thermally insulated to maintain a low internal temperature while minimising heat leakage, and fully sealed except for inlets and outlets to facilitate the entry / exit of gas as part of the re-liquefying cycle. The cryocooler 20 may be modular to some extent, with a shell of the cryostat 22 being removably engageable to allow components to be assembled within the envelope of the cryostat 22 before being sealed inside for operation.

[0064] To provide the necessary pressure differential, the re-liquefier system 10 further comprises a compressor 28 connected to drive circulation of gas through the re-liquefier system 10 and raise the pressure. In general, compressors do not operate efficiently at cryogenic temperatures, and in fact ambient temperature may be close to optimal. However, gas exiting from the re-liquefying outlet 16 will be at low temperature and hence requires warming if the compressor is to operate efficiently.

[0065] As shown in figure 2, a plurality of cold heads 24 are mounted within the cryostat 22. The cold heads 24 are part of the cryocooler 20 and may utilise any suitable cooling cycle, for example pulse tube or Stirling. Warming of the gas is provided by a plurality of heat exchangers 26, some or all of which may also be mounted within the cryostat 22 of the cryocooler 20 - in which case the heat exchangers 26 may be external components retrofitted within the cryostat 22 before the shell is engaged as described above. However, some number of the heat exchangers 26 may be mounted outside the cryostat 22 as will subsequently be described.

[0066] The heat exchangers 26 are connected in series to warm a first flow of gas coming from the re-liquefying outlet 14 by transferring heat from a second flow of gas returning to the re-liquefying inlet 16. The compressor 28 is fed by the first flow of gas after being warmed by the heat exchangers 26, such that the compressor 28 can operate at near-ambient temperature and hence high efficiency. The compressor 28 raises the pressure and then returns the second flow of gas to flow through the cold heads 24, which are connected in series to actively cool the flow back down.

[0067] The compressor 28 can be situated outside of the cryostat 22, since it is more efficiently operated at ambient temperature. By using cascaded heat exchangers 26 to warm the first flow of gas before it is fed to the compressor 28, instead of active heating or drawing in heat from the environment, less cooling power is required from the cold heads 24 to remove the added heat from the second flow of gas - this cooling power is instead provided by the first flow of gas itself. Containing the heat exchangers 26 within the cryostat 22 further increases thermal efficiency by reducing the heat drawn in from the environment. These effects, in combination with the use of the compressor 24 and expansion valve 18 to provide the final stage of cooling, enable a more compact cryocooler 20 to be used for the re-liquefying system 10 while still maintaining high capacity.

[0068] The preferred connection arrangement is that in which the second flow of gas flows through the heat exchangers 26 and the cold heads 24 in an alternating fashion, which will in general be most thermally efficient. Use of counterflow heat exchangers will also improve efficiency. The figures depict a cryocooler 20 having three cold heads 24, such that between two to four heat exchangers 26 can be alternated with the cold heads 24. Preferably the number of heat exchangers 26 is at least equal to the number of cold heads 24, to facilitate an alternating path.

[0069] In general, it is preferable to mount as many of the heat exchangers 26 as possible in a thermally insulated environment in order to improve efficiency, for example by mounting them within the cryostat 22. However, figure 1 shows a final heat exchanger 30 being mounted outside of the cryostat 22 yet still in series with the other heat exchangers 26 and continuing the alternating path for the second flow of gas. This arrangement may be beneficial in allowing the final heat exchanger 30 to be mounted adjacent to the phase separator vessel 12 and closer to the expansion valve 18, for example if the phase separator vessel 12 is large and the cryocooler 20 cannot be installed on the phase separator vessel 12 to keep them in close proximity. The re-liquefier system 10 of figures 1 and 2 has the phase separator vessel 12 and the final heat exchanger 30 situated outside of the cryostat 22. Thus, the cryostat 22 comprises a gas warming inlet 32 for receiving the first flow of gas from the re-liquefying outlet 16 and a gas warming outlet 34 for transmitting the first flow of gas to the compressor 28. The heat exchangers 26 are connected in between. The cryostat 22 also comprises a gas cooling inlet 36 for receiving the second flow gas from the compressor 28 and a gas cooling outlet 38 for transmitting the second flow of gas to the re-liquefying inlet 14. The cold heads 24 are connected in between. These are preferably the only points of ingress and egress from the cryostat 22 except for purposes of the cooling cycle of the cold heads 24.

[0070] However, to achieve further improvement in efficiency it may be preferable to mount additional components inside of the cryostat 22, which also changes the nature of the inlets and outlets of the cryostat 22.

[0071] As shown in figure 3, in one embodiment the phase separator vessel 12 is mounted inside the cryostat 22. The fourth heat exchanger 26 is also mounted inside of the cryostat 22 instead of being an externally mounted final heat exchanger 30. Thus, all the key components which handle gas at low temperature are contained within the cryostat 22 and thereby thermally insulated. As a result, the external connections of the cryostat 22 are direct to the cryogenic storage tank rather than to the phase separator vessel 12. Thus, the cryostat 22 comprises a boil-off inlet 56 instead of a gas warming inlet 34, and a liquid outlet 58 instead of a gas cooling outlet 38.

[0072] Figure 3 also shows the compressor 28 configured as a multi-stage compressor instead of a single-stage compressor. Thus, the compressor 28 comprises a first cylinder 39 and a second cylinder 41 connected in series to successively increase the gas pressure, although further cylinders may also be used. After each stage the gas is preferably cooled to remove the heat of compression, which may be achieved by a coolant circuit comprising a plurality of coolant heat exchangers 43.

[0073] The re-liquefier system 10 may experience transient events which cause pressure fluctuations in some parts of the system, for example during liquid return. To compensate, preferably the compressor 28 is provided with buffer tanks to dampen pressure pulses. For the single-stage compressor of figure 1, there may be a low-pressure buffer tank 52 connected on a first side of the compressor 28 and a high-pressure buffer tank 54 connected on the second side of the compressor 28. For the multi-stage compressor of figure 3, there may additionally be an intermediate buffer tank 55 connected between each pair of cylinders, in addition to the low-pressure buffer tank 52 connected before the first cylinder 39 and the high-pressure buffer tank 54 connected after the second / final cylinder 41. The buffer tanks can assist in regulating pressure during such transient events.

[0074] Figures 1 -3 show the phase separator vessel 12 configured for connection to the cryogenic storage tank via two separate conduits, namely a supply conduit 40 for receiving boil-off gas and a return conduit 42 for returning liquid. The conduits may have a vapour supply valve 44 and a liquid return valve 46, respectively.

[0075] As shown in figure 3, the supply conduit 40 may connect to one of the heat exchangers 26 which is also connected to the phase separator vessel 12 via the re-liquefying outlet 16. Thus, boil-off gas received from the cryogenic storage tank may begin circulating through the re-liquefying system 10 without first passing through the phase separator vessel 12.

[0076] A proportional control valve 45 may be connected in between the re-liquefying outlet 16 and the connected heat exchanger 26, facilitating control of the rate at which gas circulates through the re-liquefier system 10 and thus the rate at which liquid is condensed. The proportional control valve 45 also causes the pressure within the phase separator vessel 12 to be slightly higher than that within the cryogenic storage tank during operation of the reliquefier system 10, a pressure difference which may be sufficient to drive liquid return via the return conduit 42. Thus, the proportional control valve 45 may facilitate continuous liquid return during operation. A simple orifice may be a suitable alternative for the proportional control valve 45 to provide the necessary pressure difference, but it will not provide for active control of flow rate. As shown in figure 4, in an alternative embodiment the phase separator vessel 12 may connect to the cryogenic storage tank 47 via an integrated conduit 48. This necessitates a valve assembly 49 configured to allow the phase separator vessel 12 to switch between receiving vapour or returning liquid via the integrated conduit 48. The integrated conduit 48 may project to near the bottom of the phase separator vessel 12 to ensure that liquid can be collected. When vapour is being received from the cryogenic storage tank, it may bubble through the liquid portion in the phase separator vessel 12.

[0077] Whether using a separate supply conduit 40 and return conduit 42, or an integrated conduit 48, each conduit is thermally insulated (preferably with a vacuum jacket) to keep the gas or liquid cool during transfer. However, the integrated conduit 48 may be more thermally efficient given that it will be conveying vapour the majority of the time and liquid only intermittently, with heat leak being less significant into vapour than into liquid.

[0078] Figure 4 also shows a controller 50 connected to the valve system 49 and to a pressure sensor 51 of the cryogenic storage tank 47. The controller 50 may be configured to monitor the pressure in the cryogenic storage tank 47 and switch the re-liquefier system 10 between receiving boil-off gas or returning liquid accordingly, by actuating the valve system 49. The controller 50 may alternatively (or additionally) monitor liquid level within the phase separator vessel 12, and commence liquid return based on this level.

[0079] For embodiments where separate conduits are used, the controller 50 may instead actuate the vapour supply valve 44 and the liquid return valve 46 to achieve the same - however, if the re-liquefier system 10 is configured for continuous liquid return then there may be no need to actuate these valves during normal operation. It will be appreciated that the controller 50 might also be configured to control other aspects of the re-liquefier system 10, for example the cryocooler 20 and the compressor 28, or alternatively multiple controllers may be used.

[0080] I n either configuration, when the re-liquefier system 10 is connected to the cryogenic storage tank to receive boil-off gas and operated by running the compressor 28 and the cryocooler 20, gas is circulated through the system and liquid accumulates in the phase separator vessel 12 at some mass flow rate. The mass flow rate of condensation will be some fraction of the mass flow rate through the expansion valve 18, and may for example be around 10-20% for hydrogen under typical operating conditions.

[0081] For intermittent liquid return, the valving can be configured to allow boil-off gas to freely enter during normal operation, but to prevent liquid from exiting. When pressure in the cryogenic storage tank drops to a certain set point (e.g. 6 bar) and / or the phase separator vessel 12 becomes sufficiently full of liquid, the valving may be actuated to cut off the supply of boil-off gas. Pressure within the phase separator vessel 12 is then increased until it exceeds that within the cryogenic storage tank 47, thereby driving the return of liquid once the valving is actuated to allow this.

[0082] Pressure increase in the phase separator vessel 12 can be provided by controlling the cryocooler 20 and / or the compressor 28 to reduce the flow and / or driving pressure - stopping the compressor 28 may cause pressure across the expansion valve 18 to begin equalising. The high-pressure buffer tank 54 may assist in maintaining this increased pressure for long enough to transfer all of the accumulated liquid, and preferably the pressure is high enough to drive a relatively rapid transfer of liquid to minimise heat leak. Transfers may for example occur somewhere between every 30 minutes to 3 hours depending on the thermal loading on the cryogenic storage tank and the resulting flow rate of boil-off gas.

[0083] For continuous liquid return, the rate of liquid transfer may be matched to the rate of boil-off. Continuous liquid return may be achieved for example by the proportional control valve 45 or orifice arrangement previously described, or alternatively by the provision of one or more additional phase separator vessels 12 such that one vessel transfers liquid while the others accumulate it. The system then switches between the vessels each time one is emptied of liquid.

[0084] It will be appreciated that the ideal operating parameters for the system will depend on various factors including the gas being re-liquefied and hence the pressure in the cryogenic storage tank (generally 6-10 bar for hydrogen), and the specific cryocooler 20 and compressor 28 being used. The cryocooler 20 may for example be unable to provide sufficient cooling power below a certain temperature, and thus the compressor 28 must raise the pressure high enough for the expansion valve 18 to provide the remainder of the cooling.

[0085] Preferably, the cryocooler 20 is rated for at least 20 kW input power and runs at least at this level during operation - for example it may be operated at 30 kW input power. The compressor 28 may be operated to increase pressure by between 15 bar to 25 bar, for example approximately 20 bar. At least for hydrogen, typical pressure of the received boil-off gas may be approximately 7 bar, and there will be pressure drop when flowing through the heat exchangers 26 to be warmed - for example down to approximately 5 bar. The compressor 28 may then increase the pressure to approximately 25 bar, and will again be some losses upon flowing back through the cold heads 24 and heat exchangers 26.

[0086] The cold heads 24 of the cryocooler 20 are preferably operated at successively lower temperatures, facilitating efficient heat transfer via the heat exchangers 26. For hydrogen, the cold heads 24 are preferably operated at between 120 K to 50 K. For example, the first cold head 24 that the second flow passes through may be at approximately 117 K, the second cold head 24 may be at approximately 72 K, and the third cold head 24 may be at approximately 58 K. The last heat exchanger 26 the second flow passes through may further drop the temperature to approximately 39 K. Finally, the expansion valve 18 may achieve the final temperature drop to below 30 K (which will in general be approximately equal to the temperature of the incoming boil-off gas) and in doing so liquefy a portion of the hydrogen.

[0087] As shown in figure 5, in one embodiment the heat exchangers 26 and optionally the phase separator vessel 12 may be mounted within a secondary cryostat 59 which is external to the cryostat 22 of the cryocooler 20. Connection of the cold heads 24 and the heat exchangers 26 in an alternating fashion therefore involves a plurality of inter-cryostat connections 60 between the cryostat 22 and the secondary cryostat 59, specifically six connections in the case of three cold heads 24. The inter-cryostat connections 60 may be collectively vacuum jacketed such that the cryostats share a vacuum space, resulting in little difference thermally compared to mounting within a single cryostat. However, the cryostat 22 of the cryocooler 20 may be a pre-assembled machine as previously described and therefore have limited space available within, restricting the size of the heat exchangers 26 and the phase separator vessel 12 that can be used - in which case as secondary cryostat 59 is beneficial.

[0088] Figures 6 and 7 show one way in which the heat exchangers 26 may fit within the cryostat 22 of the cryocooler 20 - specifically using a vertical stacked configuration. In the configuration shown where three cold heads 24 are provided in a triangular arrangement, the heat exchangers 26 may be centrally located inwardly of the cold heads 24. The phase separator vessel 12 is also shown here contained within the cryostat 22, and it may be located between two of the cold heads 24. Fluid connections between the components are made as necessary.

[0089] It will be appreciated that many other configurations are possible, although in general the location of the cold heads 24 is fixed for a given cryocooler 20. Thus, the placement of the other components may need to accommodate this, unless the cryocooler 20 is custom- built or significantly modified. Use of a large phase separator vessel 12 may be preferable to reduce the frequency of valve operation if the system is configured for intermittent liquid return, such that it is preferable to mount the phase separator vessel 12 outside the cryostat 22.

[0090] As described above, the re-liquefier system 10 recovers boil-off gas from a cryogenic storage tank, which may contain liquid hydrogen, helium or other cryogenic liquids. In these configurations, the supply conduit 40 receives boil-off gas and then returns liquid via return conduit 42.

[0091] In some configurations, the system is a system 10 which converts gas (not boil-off gas, or not only boil-off gas), to a liquid state. It should be appreciated, the system 10 could include the combination of components as described above, to provide the same or similar advantages. However, the source supplied to the system 10 may be a gas e.g. from a Hydrogen gas tank rather than a storage tank with both liquid and boil-off gas, thus making it a liquefier system. The supplied gas has not been liquefied in the system prior to passing through the cryocooler 20.

[0092] In these configurations, the supplied gas (not being a boil-off gas), should have high purity to avoid freezing or corrosion issues and to improve the efficiency and performance of system. Optionally, the system may include a purification stage to remove impurities from the supplied gas. The system may operate as both a liquefier where gas from a gas tank may be supplied, and a re-liquefier where boil-off gas from a liquid is supplied. For example, the system may operate in a liquefication mode initially, and then the system may operate in a re- liquefication mode.

[0093] The system 10 of the present invention provides a number of key benefits over alternative liquefier or re-liquefier systems:

[0094] Firstly, the use of the compressor 28 and the heat exchangers 26 enable the efficient use of a cryocooler 20 that has multiple cold heads 24 but does not necessarily have sufficient cooling power at the temperatures needed to re-liquefy hydrogen. Such cryocoolers 20 are readily available as a unit that is relatively compact and easy to transport, and the compressor 28 (which can be similarly compact) makes up for the shortage of cooling power. Without the improvement in thermal efficiency provided by the present invention when using such a cryocooler, a much larger and more inconvenient system might be needed to re-liquefy boil- off gas at an adequate level of efficiency.

[0095] Secondly, the re-liquefier or liquefier system 10 can be easily retrofit to existing cryogenic storage tanks. In general, all such tanks have an inlet and a vent, allowing connection via separate conduits or an integrated conduit 48. No additional components must be built into a cryogenic storage tank, and the re-liquefier system need not be mounted directly on the cryogenic storage tank.

[0096] Thirdly, the ability to continuously return liquid to the cryogenic storage tank reduces the frequency of valve cycling, and thus extends the lifetime of the valves before maintenance is needed. The alternative configuration of an integrated conduit and intermittent liquid return can simplify connection and provide additional thermal efficiency improvement.

[0097] Where components are referred to as 'connected to' or 'connectable to' other components throughout the specification and in the appended claims, it will be understood that such terminology references fluid connections (or electrical connections, based on context), via appropriate fluid (or electrical) conduits, which are not necessarily direct. There may be intermediate components which form part of any such connection, whether or not such intermediate components are explicitly referenced. To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims.

[0098] This invention may also be said broadly to consist in the parts, elements and features referred to or indicated in the specification of the application, individually or collectively, and any or all combinations of any two or more of said parts, elements or features, and where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

Claims

CLAIMS1. A re-liquefier system for recovering boil-off gas from a cryogenic storage tank, the reliquefier system comprising: a phase separator vessel connectable to the cryogenic storage tank to receive boil-off gas therefrom and return liquid thereto, and comprising a reliquefying outlet and a reliquefying inlet; an expansion valve connected to the re-liquefying inlet to expand gas entering the phase separator vessel therethrough and to thereby cause a portion of the gas to condense into liquid; a cryocooler comprising a cryostat and a plurality of cold heads mounted within the cryostat; a plurality of heat exchangers; and a compressor connected to drive circulation of gas through the re-liquefier system and provide pressure differential to the expansion valve; wherein the heat exchangers are connected in series to heat a first flow of gas coming from the re-liquefying outlet by transferring heat from a second flow of gas returning to the re-liquefying inlet, the compressor is fed by the first flow of gas and returns the second flow of gas, and the cold heads are connected in series to cool the second flow of gas.

2. The re-liquefier system of claim 1, wherein the cold heads and the heat exchangers are connected such that the second flow of gas flows through the heat exchangers and the cold heads in an alternating fashion.

3. The re-liquefier system of claim 1 or 2, wherein the plurality of cold heads is three cold heads.

4. The re-liquefier system of any one of the preceding claims, wherein the plurality of heat exchangers is four heat exchangers.

5. The re-liquefier system of any one of the preceding claims, wherein the plurality of heat exchangers are mounted within the cryostat of the cryocooler.

6. The re-liquefier system of any one claims 1 to 4, wherein one of the plurality of heat exchangers is mounted outside the cryostat of the cryocooler and a remainder of the plurality of heat exchangers are mounted within the cryostat.

7. The re-liquefier system of any one of claims 1 to 4, wherein the plurality of heat exchangers are mounted within a secondary cryostat connected to the cryostat of the cryocooler.

8. The re-liquefier system of any one of the preceding claims, wherein the plurality of heat exchangers are counterflow heat exchangers.

9. The re-liquefier system of any one of the preceding claims, wherein the phase separator vessel and the expansion valve are mounted outside the cryostat of the cryocooler.

10. The re-liquefier system of any one of claims 1 to 8, wherein the phase separator vessel and the expansion valve are mounted within the cryostat of the cryocooler.

11. The re-liquefier system of any one of the preceding claims, wherein the cryostat comprises a gas warming outlet and a gas cooling inlet between which the compressor is connected, outside of the cryostat.

12. The re-liquefier system of any one of the preceding claims, further comprising a supply conduit and a return conduit each connected to the phase separator vessel, the conduits being connectable to the cryogenic storage tank for receiving boil-off gas therefrom and returning liquid thereto, respectively.

13. The re-liquefier system of claim 12, further comprising a vapour supply valve in the supply conduit and a liquid return valve in the return conduit.

14. The re-liquefier system of claim 12 or 13, wherein one of the plurality of heat exchangers is connected to both the supply conduit and the re-liquefying outlet, and a proportional control valve is connected between that heat exchanger and the re-liquefying outlet.

15. The re-liquefier system of any one of claims 1 to 11, further comprising an integrated conduit and a valve assembly configured to allow the phase separator vessel to switch between receiving vapour or returning liquid via the integrated conduit.

16. The re-liquefier system of any one of the preceding claims, further comprising a controller operatively connectable to a pressure sensor associated with the cryogenic storage tank, the controller being configured to monitor the pressure in the cryogenic storage tank and switch the re-liquefier system between receiving boil-off gas or returning liquid accordingly.

17. The re-liquefier system of any one of the preceding claims, wherein the compressor is a multi-stage compressor cooled at each stage.

18. The re-liquefier system of any one of the preceding claims, further comprising pressure buffer tanks connected either side of each stage of the compressor, the buffer tanks being configured to regulate pressure in the re-liquefier system during transient events.

19. A method of operating the re-liquefier system of any one of the preceding claims, the method comprising: connecting the phase separator vessel to the cryogenic storage tank; supplying boil-off gas from the cryogenic storage tank; re-liquefying gas by running the compressor and the cryocooler to circulate gas through the re-liquefier system; returning liquid from the phase separator vessel to the cryogenic storage tank.

20. The method of claim 19, further comprising operating the plurality of cold heads at progressively lower temperatures along the second flow.

21. The method of claim 20, wherein the progressively lower temperatures are between 120 K to 50 K.

22. The method of claim 19, wherein the re-liquefier system is operated to intermittently return liquid to the cryogenic storage tank.

23. The method of claim 19, wherein the re-liquefier system is operated to continuously return liquid to the cryogenic storage tank.

24. The method of any one of claims 19 to 23, wherein the boil-off gas supplied by the cryogenic storage tank is hydrogen.

25. The method of any one of claims 19 to 24 wherein the cryocooler is operated at approximately 30 kW input power.

26. The method of any one of claims 19 to 25, wherein the compressor is operated to raise the pressure of gas passing therethrough by between 15 to 25 bar.

27. The method of claim 26, wherein the compressor is operated to raise the pressure of gas passing therethrough by approximately 20 bar.

28. The method of any one of claims 19 to 27, wherein the temperature after passing through the expansion valve is maintained at approximately 30 K.

29. The method of any one of claims 19 to 28, wherein a gas supply is provided, and the system operates in both a liquefication mode and a re-liquefication mode.

30. A method of re-liquefying boil-off gas, the method comprising: supplying boil-off gas from a cryogenic storage tank; warming the boil-off gas; compressing the warmed boil-off gas to increase its pressure; cooling the compressed boil-off gas; expanding the cooled boil-off gas to cause a portion of the expanded boil-off gas to condense into liquid; and continuously circulating the boil-off gas through each above step, wherein cooling the compressed boil-off gas comprises successive stages of active cryocooling, and successive stages of heat transfer to the boil-off gas being warmed.

31. The method of claim 30, wherein the successive stages of active cryocooling and heat transfer are alternated.

32. The method of claim 30 or 31, wherein the successive stages of active cryocooling are performed at progressively lower temperatures.

33. The method of claim 32, wherein the progressively lower temperatures are between120 K to 50 K.

34. The method of any one of claims 30 to 33, further comprising intermittently returning liquid to the cryogenic storage tank.

35. The method of any one of claims 30 to 34, further comprising continuously returning liquid to the cryogenic storage tank.

36. The method of any one of claims 30 to 35, wherein the boil-off gas is hydrogen.

37. The method of any one of claims 30 to 36, wherein compressing the boil-off gas is performed to increase pressure by between 15 to 25 bar.

38. The method of claim 37, wherein compressing the boil-off gas is performed to increase pressure by approximately 20 bar.

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