WO2022058543A1

WO2022058543A1 - A system for conditioning of lng

Info

Publication number: WO2022058543A1
Application number: PCT/EP2021/075693
Authority: WO
Inventors: Jørn Magnus JONAS
Original assignee: Rondane Teknologi As
Priority date: 2020-09-21
Filing date: 2021-09-17
Publication date: 2022-03-24
Also published as: NO346152B1; NO20201033A1

Abstract

The present invention relates to a system for conditioning of liquid natural gas (LNG) in a LNG storage system, the system comprising at least one tank for storage of LNG, a circuit for LNG comprising a pump and a heat pump cooling system, where the circuit for sub cooled LNG is configured to be used as a cooling circuit for the heat pump cooling circuit, and where the circuit for LNG and the heat pump cooling circuit are connected through a first, second and third heat exchanger, the heat pump cooling circuit comprising a gas expansion device and a compressor.

Description

A system for conditioning of LNG

Technical field

The present invention relates to a plant comprising one or more tanks for storing of liquid natural gas (LNG).

More particularly, the present invention relates to a system and a method for conditioning liquid natural gas (LNG), the system comprising one or more tanks for storing of LNG and a heat pump installation, where LNG cooled by the heat pump installation is used to cool the heat pump installation internally.

However, it should be understood that the system and method according to the invention also could be used for conditioning of other cryogenic liquids, such as liquid hydrogen or the like.

Background

Liquid natural gas (LNG) storage is increasingly used in the marine industry. A major reason for this is that LNG is stored as fuel and, when used as fuel, the LNG from a storage tank is vaporized and heated to gaseous state for combustion in the propulsive engines and/or other consumers on board the vessel. The fuel gas is mostly stored in liquid condition, as LNG, because that requires much less space than in gaseous form.

LNG is stored at a very low temperature, less than -100 °C and down to -163 °C. The colder the LNG is stored, the lower is the pressure in the LNG storage tank. At -163 °C the overpressure in the tank is zero.

A technical problem with LNG storage is the heat transfer from an outside of the tank with stored LNG to an inside of the tank with stored LNG due to the extreme low temperature of LNG. This heating of LNG inside the tank will result in pressure increase in the tank, which in the long term is not feasible.

Therefore, in many cases, the LNG needs to be cooled during the storage.

Cooling apparatuses for LNG storage is a proven technology. Also, cooling apparatuses utilizing the LNG fuel flow as cooling source is known technology.

The most known heat pump cooling apparatuses of these categories are Joule Thomson and Brayton cycles.

A number of gas-powered systems are known in the marine industry. These systems include both LNG cargo ships, where a part of the cargo is consumed as fuel, and other ships where LNG is stored on board in tanks solely for the purpose of being consumed as fuel. Solutions for LNG cargo ships often use atmospheric LNG cargo tanks, but solutions that use pressure tanks have also been used. The use of pressure tanks has been the dominant solution in cargo ships, passenger ships and other vessels, but the use of atmospheric tanks may increase in the future, especially for long-endurance ships.

WO20171 14815 Al relates to a method of fueling a transporter with liquefied fuel gas, the method comprising the steps of: providing a transporter, the transporter comprising a fuel gas storage tank for holding a liquefied fuel gas, a sub-cooler fluidly connected to the fuel gas storage tank, and a consumer; pumping the liquefied fuel gas from the fuel gas storage tank into the sub-cooler to create subcooled liquefied fuel gas; and introducing the subcooled liquefied fuel gas into the fuel gas storage tank. The subcooled liquefied fuel gas may be sprayed into a vapor space of the fuel gas storage tank. The method further comprises: pumping the liquefied fuel gas from the fuel gas storage tank to provide pressurized liquefied fuel gas; vaporizing the pressurized liquefied fuel gas to provide vaporized fuel gas; and providing the vaporized fuel gas to the consumer for propelling the means of transport using the vaporized fuel gas as a fuel.

WO 2011/019284 Al relates to a plant comprising a tank for storing of liquid natural gas (LNG) as marine fuel for use in a propulsion engine or another consumer in a ship, and a heater for vaporizing of LNG discharged from the tank for such use. In order to cool tank filling piping the plant comprises a closed heat exchanging loop containing a non-flammable gas in gaseous form as heat exchanging medium, a compressing device for compressing the gas, a heat exchanger for cooling of the compressed gas by use of LNG discharged from the tank, a pressure reduction device for the gas and one or more heat exchangers for cooling of the tank filling piping. The loop may also be used for cooling of LNG being filled into the tank and for cooling inside the tank.

WO 2019/137359 Al relates to an LNG cold energy utilization system based on an argon cycle and a method thereof. The LNG cold energy utilization system comprises a natural -gas cold-energy recovery system, an argon cycle system, and a cold energy utilization system. The natural-gas cold-energy recovery system comprises an LNG-argon heat exchanger. The argon cycle system comprises a series loop consisting of a circulating-argon expansion valve, a circulating-argon heat exchanger, a circulating-argon compressor, and the LNG-argon heat exchanger. A cooled medium in the cold energy utilization system contacts a heat release pipeline of the circulating-argon heat exchanger, and exchanges heat with argon in a heat regeneration pipeline of the circulating-argon heat exchanger, realizing LNG cold energy exchange. EP 1.990.272 Al relates to a fuel gas supply system of an LNG carrier for supplying fuel gas to a high-pressure gas injection engine of an LNG carrier, wherein LNG is extracted from an LNG storage tank of the LNG carrier, compressed at a high pressure, gasified, and then supplied to the high-pressure gas injection engine.

WO 2019/145643 Al relates to a cryogenic heat pump for a liquefied gas treatment device, where the cryogenic heat pump comprises, in closed circuit, at least one compressor, at least one expansion device, a cold-receiving first circuit extending between said at least one compressor and said at least one expansion device, and a cold-transmitting second circuit extending between said at least one expansion device and said at least one compressor, said closed circuit comprising a cryogenic fluid configured to be in a biphasic state in at least part of this circuit, the said coldtransmitting second circuit being configured to have an outlet temperature below - 40°C.

FR 3.093.785 Al relates to a system for controlling the pressure in a vessel fitted to a ship, the vessel being configured to contain a cargo of gas and the system including: at least one cold production unit comprising at least a first heat exchanger configured to evaporate the gas received by the first heat exchanger in the liquid state and at least one second heat exchanger configured to cool the gas and adjust the pressure in the tank, the first heat exchanger being configured to supply evaporated gas to a gas consuming device, at least one condensing unit of the gas evaporated by the first heat exchanger and which comprises at least one heat exchanger configured to operate a heat exchange between part of the gas evaporated by the first heat exchanger and gas taken in the liquid state in the tank.

There is thus a need for alternatives to today’s LNG fuel systems, or at least supplementary solutions for LNG fuel systems.

The present invention relates generally to a system and a method for conditioning liquid natural gas (LNG) in a tank storage system. The conditioning is more specifically a subcooling of the LNG, which means cooling the LNG below its boiling point. This is also called undercooling.

However, it could also be envisaged that the system and method according to the present invention also could be used for conditioning of other cryogenic liquids, such as liquid hydrogen or the like.

The system and method according to the present invention may, for instance, in one embodiment, be used on board a vessel adapted for storage and transport of liquid natural gas (LNG) for the purpose of utilizing some or all of the LNG to supply fuel to the one or more consumers on board the vessel.

The consumer, for instance one or more gas engines, will then use LNG in the vapor phase as LNG has been evaporated and heated prior to consumption.

The present invention also relates to a heat pump cooling apparatus for conditioning of LNG. The heat pump apparatus is more specifically a cooling process where a circulation medium in gas phase is expanded through one or more devices, for instance one or more turbines, one or more valves or the like and where the circulation medium becomes colder during expansion.

For such a turbine expansion process the work by the circulation medium on the turbine causes temperature reduction on the circulation medium during the expansion. The thermodynamic model for determining the temperature reduction is the isentropic expansion.

For such a valve expansion process the temperature reduction during the pressure reduction is due to a property of the circulation medium, the circulation medium being a gas or gas mixture, often named Joule Thomson coefficient. The thermodynamic model for describing the valve expansion process is isenthalpic expansion, however the specific temperature drop is explained with reference to a Joule Thomson coefficient which is specific for a gas or gas mixture.

The invention is applied to a heat pump cooling apparatus where the heat transfer medium (heat pump circuit) is transferring heat from the LNG content in the tank to the heat pump circuit, the heat pump circuit is at the same time transferring heat to one or more heat pump cooling medium. The heat pump cooling media are advantageously including LNG fuel flow to one or more consumers or end users. The heat pump apparatus is thus subcooling the LNG in the tank.

An object according to the present invention is to provide a system and method for conditioning liquid natural gas (LNG) in a LNG storage system, wherein the conditioned LNG is used for increasing the functionality and in some cases also the efficiency of such a LNG storage system, minimizing and/or reducing one or more of the disadvantages of the prior art, or at least providing a viable alternative.

It is also an object according to the present invention to provide a system and a method for conditioning LNG in fuel systems, where such a system also may be used to supply natural gas to one or more consumers or end users, which minimizes the cost of operating the system. These objects are achieved with a system and a method for conditioning of liquid natural gas in a LNG storage system as defined in the independent claim.

Advantageous embodiments of the present invention are indicated in the dependent claims.

Summary of the invention

The present invention relates to a system for conditioning of liquid natural gas (LNG) in a LNG storage system, where the system comprises at least one tank for storing of LNG, a circuit for LNG comprising a pump, and a heat pump cooling circuit, where a part of a circuit for LNG is configured to be used as a cooling circuit for the heat pump cooling circuit, where the circuit for LNG and the heat pump cooling circuit are connected through a first, second and third heat exchanger, the heat pump cooling circuit further comprising a gas expansion device and a compressor.

The system and method according to the invention could also be used for conditioning of other cryogenic liquids, such as liquid hydrogen or the like.

According to one aspect, the gas expansion device may be a valve, a turbine, or the like.

According to one aspect, the system for conditioning of liquid natural gas (LNG) may be used for conditioning of liquid natural gas in a fuel system and for supplying natural gas (NG) to one or more consumers in the fuel system. The fuel system may be arranged on board a vessel adapted for storage and transport of liquid natural gas (LNG), where some or all of the LNG may be used to supply fuel to one or more consumers on board the vessel.

According to one aspect, the heat pump cooling circuit comprises a circulation or working medium in form of a gas, where the gas, for instance, may be nitrogen, argon, helium, or a mixture of hydrocarbon gases.

In one embodiment of the present invention, the heat pump cooling circuit may comprise an additional heat exchanger.

According to one aspect, the circuit for LNG fuel may comprise a first branch extending through the second heat exchanger, a second branch extending through the first heat exchanger and a third branch extending through the third heat exchanger.

According to the present invention, it is provided a heat exchanging arrangement for selective cooling of the heat pump cooling circuit in which a flow from the sub cooled LNG is used as a cooling source.

The heat pump cooling circuit may be arranged between one or more tanks for storing of LNG and a flow of LNG from the one or more tanks for storing of LNG. The purpose of the heat pump cooling circuit is to sub cool the LNG stored in the one or more tanks for storing of LNG. The subcooling may be done by using a pump to pump LNG out of the one or more tanks for storing of LNG, subcooling the LNG by heat exchange with the heat pump cooling circuit and returning the cooled LNG back to the one or more tanks for storing of LNG as subcooled LNG. As used herein, the term subcooling or undercooling refers to a liquid existing at a temperature below its normal boiling point. The boiling point of a substance is the temperature at which the vapor pressure of a liquid equals the pressure surrounding the liquid and the liquid changes into a vapor.

A part of the LNG that is subcooled by the heat pump cooling circuit is used as a cooling source in the same heat pump cooling circuit.

The LNG flow from the pump may be branched up before the LNG is subcooled by the heat pump cooling circuit. This can be done by operating one or more valves arranged in the branch pipes. The branch pipes are arranged in such a way that in one branch the LNG is subcooled by heat exchanging with the heat pump cooling circuit, the heat pump cooling circuit is then heated up from its coldest condition out of an expansion device.

In a second branch the LNG is further subcooled by heat exchanging with the heat pump cooling circuit, whereby the heat pump cooling circuit then further is heated.

The LNG in the branch that is heat exchanged with the coldest heat pump circulation gas will have the potential for higher subcooling than in the other branch.

The subcooled LNG out of the branch wherein the LNG is heat exchanged with the coldest heat pump cooling circuit, may according to the invention, in another heat exchanger be used for cooling the heat pump cooling circuit before the gas in the heat pump cooling circuit is expanded.

According to one embodiment the subcooled LNG from the branch that is used for cooling the heat pump cooling circuit will thereafter be directed to a fuel pipe, where the subcooled LNG may be further used as cooling source for the heat pump cooling circuit and thereafter be supplied as fuel to the one or more consumers, for instance one or more gas engines.

According to another embodiment, the subcooled LNG from the branch that is used for cooling the heat pump circuit may thereafter be returned to the one or more tanks for storing of LNG.

The flow through the branches will be regulated so that the cooling process is best possible for intended use. For instance, for any embodiment of the invention a certain flow of subcooled LNG through the branch that is cooling the heat pump cooling circuit will cause a lower cooling temperature in the heat pump cooling circuit and thus increase the subcooling of LNG in the cooling process. The remaining sub cooled LNG will be returned to the one or more tanks for storing of LNG.

This arrangement of flow with subcooled LNG can be particularly suitable during a tank filling wherein a coldest possible LNG shall be sprayed inside the tank and into a vapor phase inside the tank for purpose of condensing the vapor. The flow regulation between the branches will depend on many conditions and is not within the description of the invention.

In an embodiment wherein the expansion device is a valve and the cooling process is of Joule Thomson type, a certain flow through the branch that is cooling the heat pump circulation gas before expansion is preferably done all the time during a continuous subcooling process. In the embodiment wherein no LNG is supplied as fuel, but the subcooled LNG is returned to the one or more tanks for storing of LNG the entire flow of subcooled LNG may preferably be used for cooling the heat pump circulation gas before expansion. By cooling the heat pump circulation gas before expansion as described the efficiency of the cooling process will increase because the Joule Thomson factor will increase.

When the sub cooled LNG flow is colder than any other cooling source for the heat pump cooling circuit the sub cooled LNG fuel flow from the branch is used for selectively cooling the heat pump cooling circuit. This is cooling the circulation medium of the heat pump cooling circuit before the circulation medium is expanded. The cooling source used on marine cooling plants is in general water- glycol mixture cooling circuit that is cooled by seawater, which means the temperature is slightly above the seawater temperature.

A heat pump cooling circuit, according to the present invention, will lower its cooling temperature when its temperature before expansion is lowered. To cool with sub cooled LNG before expansion as described above will therefore cause lower cooling temperature in the heat pump cooling circuit.

Lower cooling temperature in the heat pump cooling circuit, on the other hand, causes more sub cooling of LNG in a heat exchanger due to the higher temperature difference between the heat pump cooling circuit and the LNG during heat exchanging.

The present invention is creating an interaction between the LNG sub cooling and the heat pump cooling circuit selective cooling. A further advantage according to the present invention when used in a Joule Thomson (JT) process, is that the cooling process may be described with reference to a JT coefficient for a heat pump circulation gas. The JT coefficient indicates how much the temperature will reduce or increase in a pressure reduction through a valve.

The present invention, when used in a JT cooling process, is concerned with gases that reduce its temperature in a pressure reduction through a valve. For all these gases the JT coefficient will be higher the colder the circulation gas is before expansion or the higher pressure it is before expansion. Thus, there will be a higher temperature reduction over an expansion in a valve when the temperature before expansion is reduced or when the pressure before expansion is increased.

The present invention is concerned with temperature reduction of the heat pump circulation gas before expansion. It is a method to reduce the temperature before expansion. It is thus a method that can increase the JT coefficient for a gas or gas mixture in a JT cooling process. A certain cool down ATi of the heat pump gas by the present invention before valve expansion will cause a higher temperature reduction AT2 over the valve than is the case if not cooled down ATi before the valve.

A common gas used as a working medium for JT process is nitrogen, however other gases and mixture of such gases may be used, for instance such as argon or mixture of hydrocarbon gases.

Other advantages and characteristic features of the present invention will be seen clearly from the following detailed description, the appended figures and the following claims whereas it is to be understood that the figures show principles of different embodiments of the invention and not the processes in detail.

Brief description of the drawings

The present invention will now be described in more detail with reference to the following figures, wherein

Figure 1 shows a first exemplary embodiment of a system for conditioning of liquid natural gas (LNG) according to the present invention,

Figure 2 shows an embodiment of a system for conditioning of liquid natural gas (LNG) according to figure 1, where the system is used for conditioning of liquid natural gas in a fuel system, Figure 3 shows an alternative embodiment of the system for conditioning of liquid natural gas according to figure 2,

Figure 4 shows yet an alternative embodiment of the of the system for conditioning of liquid natural gas according to figure 2, and

Figure 5 shows a temperature - entropy diagram (T-S diagram) for circulation medium nitrogen.

Detailed description

Figure 1 shows in a schematic way how a system S for conditioning of liquid natural gas (LNG) in a LNG storage system according to the present invention can be used to subcool or undercool the liquid natural gas (LNG). The system S for conditioning of LNG is shown with only one tank 1 for storage of LNG for reasons of clarity, but it should be understood that the system S may comprise more than one tank 1 for storage of LNG.

However, it should be understood that the system and method according to the invention also could be used for conditioning of other cryogenic liquids than liquid natural gas (LNG), such as liquid hydrogen or the like.

A first pipe 31 extending out from the tank 1 for storage of liquid natural gas will form an outlet from the tank 1 for storage of liquid natural gas (LNG). A pump 2 is connected to the first pipe 31 in order to pump the liquid natural gas (LNG) through the first pipe 31. The first pipe 31 is branched into a second pipe 32, where the second pipe 32 extends back and into the tank 1 for storage of liquid natural gas (LNG), the second pipe 32 forming an inlet in the tank 1 for storage of liquid natural gas (LNG). Furthermore, the second pipe 32 is branched into a third pipe 33, where the third pipe 33 thereafter is connected to the tank 1 for storage of liquid natural gas (LNG) or to the second pipe 32 again.

When the first, second and third pipe 31, 32, 33 are connected to each other and the tank 1 for storage of liquid natural gas (LNG), the first, second and third pipe 31, 32, 33 will form a “closed” circuit 3 for the liquid natural gas (LNG). Furthermore, the third pipe 33 is arranged to run through corresponding heat exchangers 6a, 4 in a heat pump cooling circuit 5, where liquid natural gas (LNG) flowing through the third pipe 33 will be used as a “cooling circuit” in the heat pump cooling circuit 5.

The second pipe 32 will form a first branch and the third pipe 33 will form a second branch in the system S for conditioning of LNG. The system S for conditioning of liquid natural gas (LNG) comprises also a heat pump cooling circuit 5, where the heat pump cooling circuit 5 is arranged to be a closed circuit. The heat pump cooling circuit 5 comprises a first heat exchanger 4, a second heat exchanger 6a and a third heat exchanger 6b, where the first, second and third heat exchangers 4, 6a, 6b are connected through a pipe 81 in order to provide the closed circuit in the heat pump cooling circuit 5.

A gas expansion device 7 and a compressor 12 are also connected to the pipe 81, where the gas expansion device 7 is arranged between the first and second heat exchangers 4, 6a and the compressor 12 is arranged between the third heat exchanger 6b and the first heat exchanger 4.

An additional heat exchanger 11 is connected to the pipe 81 in such a way that a part of the pipe 81 extending between the third heat exchanger 6b and the compressor 12 will run through the additional heat exchanger 11, and a part of the pipe 81 extending between the compressor 12 and the first heat exchanger 4 will also run through the additional heat exchanger 11. The additional heat exchanger 11 is therefore arranged between the third heat exchanger 6b and the compressor 12 and between the compressor 12 and the first heat exchanger 4.

The closed circuit of the heat pump cooling circuit 5 is filled with a working medium, for instance a gas, where the gas is in a gaseous state or in a two-phase state, i.e. gaseous and liquid state. The gas may, for instance, be nitrogen.

However, it can also be envisaged that other gases may be used, for instance argon if the expansion device 7 is a valve and helium if the expansion device 7 is a turbine. It can also be envisaged that a mixture of gases may be used as the working medium in the heat pump cooling circuit 5, where a mixture of gases may, for instance, comprise hydrocarbon gases.

Furthermore, the first pipe (first branch) 32 of the closed circuit 3 for liquid natural gas (LNG), where subcooled liquid natural gas flows through the first pipe (first branch) 32, is arranged to extend through the third heat exchanger 6b of the heat pump cooling circuit 5, while the third pipe (second branch) 33 of the closed circuit 3 for liquid natural gas (LNG), where subcooled liquid natural gas flows through the third pipe (second branch) 33, is arranged to extend through the second heat exchanger 6a and the first heat exchanger 4 of the heat pump cooling circuit 5.

When the liquid natural gas in the tank 1 for storage of liquid natural gas for some reasons is to be conditioned, the pump is started and an amount of liquid natural gas is pumped from the tank 1 for storage of liquid natural gas and through the first pipe 31. The amount of liquid natural gas will thereafter be divided or separated, such that a part of the liquid natural gas will be transported through the second pipe 32 and the third heat exchanger 6b of the heat pump cooling circuit 5, while a remaining part of the liquid natural gas is transported through the third pipe 33, the second heat exchanger 6a and the first heat exchanger 4 of the heat pump cooling circuit 5 in order to be heat exchanged with the working medium/gas of the heat pump cooling circuit 5, whereafter the subcooled LNG is returned to the tank 1 for storage of liquid natural gas. Simultaneously, the working medium/gas circulating through the heat pump cooling circuit 5 is heated when heat exchanged with the liquid natural gas running through the second and third heat exchanger 6a, 6b and through a cold side of the additional heat exchanger 11, then cooled through the warm side of the additional heat exchanger 11, the first heat exchanger 4, and preferably through cooling water heat exchangers (not shown) after each compression step in the compressor 12.

A number of regulation valves (indicated) are provided in the first, second and third pipes 31, 32, 33 for optimal utilization of the cooling arrangement as described the according to the present invention. The regulation valves are then used to control an amount of liquid natural gas running through the first, second and third pipes 31,

32, 33 in order to route a greater or lesser amount of the LNG through the third heat exchanger 6b and/or first and second heat exchangers 4, 6a of the heat pump cooling circuit 5. A person skilled in the art would know how the regulation valves should be arranged in the closed circuit 3 for LNG and how the regulation should be controlled, whereby this is not described any further herein.

The regulation valves can be controlled to be between a fully closed or fully open position, or positions between a fully closed and fully open position. The regulation valves may then be connected to a common control system (not shown), or the regulation valves may be connected to separate control systems (not shown), each separate control system controlling or regulating one or more regulation valves.

The purpose of the heat pump cooling circuit 5 is to transfer heat from the cold LNG in tank 1 to external cooling media (not shown) that are warmer, where such external cooling medium, on a marine plant, may be one or more water/glycol cooling circuits that is/ are cooled by sea water. To achieve a colder working media/gas in the heat pump cooling circuit 5 than the LNG that is pumped from tank 1, the gas in the heat pump cooling circuit 5 is expanded through the expansion device 7.

If the expansion device 7 is a turbine, the gas in the heat pump cooling circuit 5 is cooled by that the gas is performing work on the turbine by using its internal energy, and thus reducing the temperature in the gas. If the expansion device is a valve, the cooling during the expansion through the valve is depending on a property of the gas in the heat pump cooling circuit 5, i.e. the Joule Thomson coefficient. Nitrogen and argon are gases that will be used during the cooling in a valve expansion under conditions with LNG conditioning. Helium and hydrogen, on the other hand, will be warmer and may not be used for such cooling process. The colder the gas is before expansion and the higher pressure before expansion the better are the cooling properties of the gas during valve expansion.

The compressor 12 is increasing the pressure so that an expansion through the expansion device 7, the expansion device 7 being in form of a turbine or a valve, is possible. The heat pump cooling circuit 5 is a closed circuit. In a closed circuit total energy input equals total heat extraction. Energy inputs to the heat pump cooling circuit 5 are the cooling of LNG through the second and third heat exchanger 6a, 6b, compressor work through the compressor 12 and friction work. Heat extraction is the cooling of the working medium/gas of the heat pump cooling circuit 5, which in figure 1 is done through the first heat exchanger 4. Through the additional heat exchanger 11 is obtained a heat recovery process, that is transferring heat from one place to the other in same circuit, as the compressor 12 will increase the pressure in the working media/gas of the heat pump cooling circuit 5. There may also be other cooling media for the working medium/gas of the heat pump cooling circuit 5, typical cooling water related to the compression process performed in the compressor 12, but this is not shown in the figure 1 or any other figure, as a person skilled in the art would know how to arrange this.

The additional heat exchanger 11 is therefore arranged between the third heat exchanger 6b and the compressor 12 and between the compressor 12 and the first heat exchanger 4, such that the heated gas of the heat pump cooling circuit 5 (after being heat exchanged with the LNG in the third heat exchanger 6b) is run through the additional heat exchanger 11, further through the compressor 12 and thereafter once again run through the additional heat exchanger 11 in order to be heat exchanged with the heated gas running from the third heat exchanger 6b.

It is a benefit to achieve as much heat extraction in the heat pump cooling circuit 5 as possible, so that the possible energy input in terms of cooling capacity is as large as possible. A limitation of heat extraction of the heat pump cooling circuit 5 is the temperature of the cooling sources. The cooling source on marine cooling plants is in general water-glycol that is cooled by sea water.

The higher the pressure increase in the compressor 12 is, the more energy can be extracted by heat exchanging through the water-glycol heat exchangers and through the additional heat exchanger 11, because the working medium/gas of the heat pump cooling circuit 5 becomes warmer during compression. Higher increase of pressure in the compressor 12 causes higher pressure drop through the expansion device 7 and higher temperature reduction in the working medium/gas of the heat pump cooling circuit 5.

The working medium/gas temperature needs to be reduced so much during expansion that the working medium/gas out of expansion is colder than the LNG that is pumped from tank 1 for storage of liquid natural gas (LNG). Then the working medium/gas can be used for subcooling the LNG.

In most cases the pressure increasing in compressor 12 needs to be very high to achieve a temperature of the working medium/gas that is colder than LNG.

A high pressure increasing in the compressor 12 leads to increased energy and operation cost of the process performed by the heat pump cooling circuit 5. Furthermore, the upper pressure from the compressor 12 is technically limited by items like compressor motor size and heat pump cooling circuit strength.

According to the embodiment of the invention there is an arrangement for cooling down of the working medium/gas of the heat pump cooling circuit 5 prior to the expansion device 7. Some of the cold energy that is produced in the heat pump and used for subcooling of LNG is further used for cooling the working medium/gas prior to expansion in the expansion device 7. The subcooled LNG is the coldest cooling medium for the heat pump cooling circuit 5.

The cooling source for the additional heat exchanger 11, on the other hand, is the working medium/gas of the heat pump cooling circuit 5 that already has been heated during subcooling LNG through the second and third heat exchanger 6a, 6b and will have a temperature that is close to the LNG temperature from tank 1 in normal heat exchanger configurations.

The heat exchanger 4, according to the embodiment of the present invention, will have cooling performance that is depending on the degree of LNG subcooling. The more degree of subcooled LNG in the first heat exchanger 4, the more cooling of the working medium/gas of the heat pump cooling circuit 5 will be obtained through the first heat exchanger 4. It is in that way a certain interaction between the degree of subcooling of LNG in second heat exchanger 6a and the cooling of circulation gas in 4 in that increased subcooling of LNG in second heat exchanger 6a will give higher cooling of the working medium/gas in the first heat exchanger 4 because the cooling source in the first heat exchanger 4 is the subcooled LNG that has been cooled in the second heat exchanger 6a. LNG from the tank 1 for storage of LNG running through the third pipe 33 and to the second heat exchanger 6a of the heat pump cooling circuit 5 will firstly be heat exchanged with the working medium/gas circulating through the heat pump cooling circuit 5 and thereby be subcooled, whereafter the subcooled LNG from the second heat exchanger 6a is heat exchanged through the first heat exchanger 4, thereby cooling the working medium/gas of the heat pump cooling circuit 5 to a temperature that is lower than possible with any other external cooling source. When the liquid natural gas (LNG) is heat exchanged through both the second heat exchanger 6a and the first heat exchanger 4, the LNG will be sent back to the second pipe 32 and thereafter be run through the third heat exchanger 6b or alternatively to tank 1 for storage of LNG without further heat exchanging.

The present invention also relates to a method to increase the functionality of a heat pump cooling circuit 5 which is used for subcooling LNG. For instance, in a closed liquid natural gas (LNG) tank containing LNG wherein LNG is to be filled into the tank 1 for storage of LNG. A common method to control the tank pressure during filling is to condense vapor inside the liquid natural gas (LNG) tank by spraying cold LNG into the vapor. Natural gas vapor takes up to 600 times more volume compared to when the same amount of natural gas is in liquid phase. The colder the LNG spray is, or the more subcooled the LNG spray is, the faster will the condensation of the vapor be because vapor condensation is caused by vapor cooling.

Seen from the aspect of tank filling where a coldest possible LNG shall be sprayed into the tank the present invention will differ from prior art in that cooling in the first heat exchanger 4 will give a higher subcooling of the LNG. The function of the present invention during tank filling is not to condition or to sub cool LNG inside the LNG tank, but to increase the condensation rate of the vapor inside the tank 1 for storage of LNG.

Another and wider aspect of the present invention comes up when the expansion device 7 is a valve and the cooling circuit is a Joule Thomson process. A further cooling of the working medium/gas of the heat pump cooling circuit 5 through the first heat exchanger 4 as described in the present invention changes the properties of the working medium/gas. The temperature reduction will be higher over the expansion device 7 the colder the gas is before expansion. This may be visualized in a Temperature-Entropy (T-S) diagram for a gas used in the Joule Thomson cooling process. Such a T-S diagram is a thermodynamic diagram used in thermodynamics to visualize changes to temperature and specific entropy during a thermodynamic process or cycle as the graph of a curve, where an axis of the diagram shows temperature and an axis of the diagram shows entropy for a given gas. An example is in the following given with reference to T-S diagram for a working medium in form of nitrogen as shown in figure 5. The pressure of the working medium into the expansion device 7 is 25 bara, and the outlet pressure of the working medium from the expansion device 7 is 2.5 bara. Isenthalpic lines are shown in the T-S diagram for nitrogen and are the model for valve expansion. Figure 5 reveals:

If the temperature into the valve is - 145 °C, then the temperature out is -176 °C, that is a temperature drop of 31 °C in the expansion device 7. The expansion process is shown as the isenthalpic (dashed) line 1-4 in figure 5,

If the temperature into the valve is - 150 °C, then the temperature out is -184 °C, that is a temperature drop of 34 °C in the expansion device 7. The expansion process is shown as the isenthalpic (continuous) line 2-3 in figure 5.

That is, by cooling 5 °C before the expansion valve 7 according to the embodiment of the present invention, the temperature reduction in the expansion will be 3 °C higher than without the cooling before expansion, i.e. a 8 °C lower temperature out of the expansion valve 7 when applying the method of the present invention.

The cool down of 5 °C before the expansion valve is shown in the isobaric line 1-2 in figure 5. The higher temperature drop during expansion is due to changing properties of the gas caused by the cooling before expansion through the expansion valve 7.

The present invention used in a JT process is a method to increase the efficiency both in terms of achieved cold temperature and in terms of achieved cooling capacity.

The colder temperature out of the expansion device 7 compared to prior art is evident.

When it comes to difference in cooling capacity the energy balance reveals:

The following produced cooling is used for internal cooling in the heat pump cooling circuit 5: Cp x m x 5 °C, where Cp x m is the heat capacity and mass flow of the working medium/gas before the valve expansion, 5 °C is the cool down according to the arrangement of present invention before expansion. After expansion the available cooling capacity has increased with (Cp x m x 8°C) because the same mass flow of heat pump circulation gas is 8°C colder compared to prior art technology.

The net increase in cooling capacity Cp x m x (8-5) °C = Cp x m x 3 °C, will then give the following increase in percentage (Cp x m x 3 °C) /(Cp x m x 31 °C) = 0.097, i.e. 9.7%

From this aspect the present invention improves the efficiency of a Joule Thomson cooling process in addition to also reduce the cooling temperature.

Figures 2 to 4 show in a schematic way different embodiments of how the system for conditioning of liquid natural gas (LNG) according to figure 1 can be used for conditioning of liquid natural gas in a fuel plant on board a vessel (not shown). The principles of functionality and efficiency improvement are similar for all described embodiments of the present invention.

The different embodiments of figures 2 to 4 are showing different arrangement of cooling sources for the closed heat pump circuit 5 which will influence the equipment and power consume for the heat pump cooling apparatus. Furthermore, there are shown different arrangements for distribution of subcooled LNG. In figure 2 and 3 the subcooled LNG from the third heat exchanger 6a is utilized as cooling source in both the first heat exchanger 4 and the second heat exchanger 10 and the sub cooled LNG is as such heated in both the first heat exchanger 4 and the second heat exchanger 10 before processed in the fuel system. The same flow in figure 4 is directly processed in the fuel system after being heated in heat exchanger 4.

A number of regulation valves (indicated) are provided in the first, second and third pipes 31, 32, 33 for optimal utilization of the cooling arrangement as described according to the present invention. The regulation valves are then used to control an amount of liquid natural gas running through the first, second and third pipes 31, 32, 33 in order to route a greater or lesser amount of the LNG through the third heat exchanger 6b and/or the first and second heat exchangers 4, 6a of the heat pump cooling circuit 5. A person skilled in the art would know how the regulation valves should be arranged in the closed circuit 3 for LNG and how the regulation should be controlled, whereby this is not described any further herein.

The regulation valves can be controlled to be between a fully closed or fully open position, or positions between a fully closed and fully open position. The regulation valves may then be connected to a common control system, or the regulation valves may be connected to separate control systems, each control system controlling or regulation one or more regulation valves. The system S for conditioning of LNG in a fuel system, which system also supplies natural gas to one or more end consumers 9, is shown with only one tank 1 for storage of LNG for reasons of clarity, but it should be understood that the system S may comprise more than one tank 1 for storage of LNG.

Figure 2 shows a first embodiment of the system S for conditioning of liquid natural gas (LNG) in a fuel plant on board a vessel (not shown), where the tank 1 for storage of liquid natural gas (LNG) is connected to a pump 2 and an end consumer 9 through a first pipe 31. The first pipe 31 is branched into a second pipe 32, where the second pipe 32 extends back and into the tank 1 for storage of liquid natural gas (LNG). The second pipe 32 is branched into a third pipe 33, where the third pipe 33 extends back and into the first pipe 31, before the first pipe 31 reaches the end consumer 9.

The first, second and third pipe 31, 32, 33 will form a circuit 3 for LNG, where the circuit 3 for LNG fuel will be used as a cooling circuit for a heat pump cooling circuit 5.

The second pipe 32 will form a first branch, the third pipe 33 will form a second branch and the first pipe 31 will form a third branch in the circuit 3 for LNG fuel.

The system S for conditioning of LNG comprises also a heat pump cooling circuit 5, where the heat pump cooling circuit 5 is arranged to be a closed circuit. The heat pump cooling circuit 5 comprises a first heat exchanger 4, a second heat exchanger 6a, a third heat exchanger 6b and a fourth heat exchanger 10, where the first, second, third and fourth heat exchangers 4, 6a, 6b, 10 are connected through a pipe 81 in order to provide the closed circuit in the heat pump cooling circuit 5. A gas expansion device 7 and a compressor 12 are also connected to the pipe 81, where the gas expansion device 7 is arranged between the first and second heat exchangers 4, 6a and the compressor 12 is arranged between the third heat exchanger 6b and the fourth heat exchanger 10.

The closed circuit of the heat pump cooling circuit 5 is filled with a gas, where the gas is in a gaseous state or in a two-phase state, i.e. gaseous and liquid state. The gas may, for instance, be nitrogen.

However, it can also be envisaged that other gases may be used, for instance argon if the expansion device 7 is a valve and helium if the expansion device 7 is a turbine. Furthermore, the first branch 32 of the circuit for sub cooled LNG is arranged to extend through the third heat exchanger 6b of the heat pump cooling circuit 5, the second branch 33 of the circuit for LNG fuel is arranged to extend through the second heat exchanger 6a and first heat exchanger 4 of the heat pump cooling circuit 5, while the third branch 31 of the circuit for LNG fuel is arranged to extend through the fourth heat exchanger 10 of the heat pump cooling circuit 5.

When the liquid natural gas in the tank 1 for storage of liquid natural gas for some reasons is to be conditioned, an amount of liquid natural gas is pumped from the tank 1 for storage of LNG and through the first pipe 31. The amount of liquid natural gas is thereafter divided or separated, such that a part of the liquid natural gas is transported through the second pipe 32 and the third heat exchanger 6b of the heat pump cooling circuit 5, while a remaining part of the liquid natural gas is transported through the third pipe 33, the second heat exchanger 6a and the first heat exchanger 4 of the heat pump cooling circuit 5 in order to be heat exchanged with the working medium/gas of the heat pump cooling circuit 5, whereafter the subcooled liquid natural gas (LNG) is returned to the tank 1 for storage of LNG. Simultaneously, the working medium/gas circulating through the heat pump cooling circuit 5 is heated when heat exchanged with the liquid natural gas running through the second and third heat exchanger 6a, 6b, whereafter the working medium/gas is cooled through the first heat exchanger 4, the gas expansion device 7 and preferably through cooling water heat exchangers (not shown) after each compression step in the compressor 12.

However, a subcooled LNG from the tank 1 for storage of LNG may also be run through the third pipe 33, while simultaneously being run through the second pipe 32, and to the second heat exchanger 6a of the heat pump cooling circuit 5 in order to firstly be heat exchanged with the working medium/gas circulating through the heat pump cooling circuit 5 and thereafter to be heat exchanged through the first heat exchanger 4. When the LNG is heat exchanged through both the second heat exchanger 6a and the first heat exchanger 4, the subcooled LNG will be returned to the first pipe 31 and thereafter through the fourth heat exchanger 10, to be sent to a consumer/end user 9.

Figure 3 shows an alternative embodiment of the system S for conditioning of liquid natural gas in a fuel system according to figure 2, where the system S in this embodiment also is shown with only one tank 1 for storage of LNG for the sake of simplicity. However, it should be understood that the system S may comprise more than one tank 1 for storage of LNG.

The tank 1 for storage of liquid natural gas (LNG) is connected to a pump 2 and an end user 9 through a first pipe 31. The first pipe 31 is branched into a second pipe 32, where the second pipe 32 extends back and into the tank 1 for storage of LNG. The second pipe 32 is branched into a third pipe 33, where the third pipe 33 extends back and into the first pipe 31, before the first pipe 31 reaches the end consumer 9.

The first, second and third pipe 31, 32, 33 will form a closed circuit 3 for LNG fuel, where the closed circuit 3 for LNG fuel will be used as a cooling circuit for a heat pump cooling circuit 5.

The system S for conditioning of LNG comprises also a heat pump cooling circuit 5, where the heat pump cooling circuit 5 is arranged to be a closed circuit. The heat pump cooling circuit 5 comprises a first heat exchanger 4, a second heat exchanger 6a, a third heat exchanger 6b, a fourth heat exchanger 10, where the first, second, third and fourth heat exchangers 4, 6a, 6b, 10 are connected through a pipe 81 in order to provide the closed circuit.

A gas expansion device 7 and a compressor 12 are also connected to the pipe 81, where the gas expansion device 7 is arranged between the first and second heat exchangers 4, 6a and the compressor 12 is arranged between the third heat exchanger 6b and the fourth heat exchanger 10.

An additional heat exchanger 11 is connected to the pipe 81 in such a way that a part of the pipe 81 extending between the third heat exchanger 6b and the compressor 12 will run through the additional heat exchanger 11, and a part of the pipe 81 extending between the compressor 12 and the first heat exchanger 4 also will run through the additional heat exchanger 11. The additional heat exchanger 11 is therefore arranged between the third heat exchanger 6b and the compressor 12 and between the compressor 12 and the fourth heat exchanger 10.

However, it can also be envisaged that other gases may be used, for instance argon if the expansion device 7 is a valve and helium if the expansion device 7 is a turbine.

Furthermore, the first branch 32 of the circuit for sub cooled LNG is arranged to extend through the third heat exchanger 6b of the heat pump cooling circuit 5, the second branch 33 of the circuit for LNG fuel is arranged to extend through the second heat exchanger 6a and the first heat exchanger 4 of the heat pump cooling circuit 5, while the third branch 31 of the circuit for LNG fuel is arranged to extend through the fourth heat exchanger 10 of the heat pump cooling circuit 5.

When the liquid natural gas in the tank 1 for storage of liquid natural gas for some reasons is to be conditioned, the pump is started and an amount of liquid natural gas is pumped from the tank 1 for storage of LNG and through the first pipe 31. The amount of liquid natural gas is thereafter divided or separated, such that a part of the liquid natural gas is transported through the second pipe 32 and the third heat exchanger 6b of the heat pump cooling circuit 5, while a remaining part of the liquid natural gas is transported through the third pipe 33, the second heat exchanger 6a and the first heat exchanger 4 of the heat pump cooling circuit 5 in order to be heat exchanged with the working medium/gas of the heat pump cooling circuit 5, whereafter the subcooled LNG is returned to the tank 1 for storage of LNG. Simultaneously, the working medium/gas circulating through the heat pump cooling circuit 5 is heated when heat exchanged with the liquid natural gas running through the second and third heat exchanger 6a, 6b, whereafter the working medium/gas is cooled through the first heat exchanger 4, the gas expansion device 7 and preferably through cooling water heat exchangers (not shown) after each compression step in the compressor 12.

However, LNG from the tank 1 for storage of LNG may also be run through the third pipe 33 and to the second heat exchanger 6a of the heat pump cooling circuit 5 in order to firstly be heat exchanged with the medium/gas circulating through the heat pump cooling circuit 5 and thereafter to be heat exchanged through the first heat exchanger 4. When the LNG is heat exchanged through both the second heat exchanger and the first heat exchanger 4, the LNG will be returned to the first pipe 31 and thereafter be run through the fourth heat exchanger 10.

Figure 4 shows yet an alternative embodiment of the system S for conditioning of liquid natural gas in a fuel system according to figure 1, where the system S in this embodiment also is shown with only one tank 1 for storage of LNG for the sake of simplicity. However, it should be understood that the system S may comprise more than one tank 1 for storage of LNG.

The tank 1 for storage of liquid natural gas (LNG) is connected to a pump 2 and an end user 9 through a first pipe 31. The first pipe 31 is branched into a second pipe 32, where the second pipe 32 extends back and into the tank 1 for storage of LNG. The second pipe 32 is branched into a third pipe 33, where the third pipe 33 extends back and into the first pipe 31, before the first pipe 31 reaches the end consumer 9. The first, second and third pipe 31, 32, 33 will form a circuit 3 for LNG fuel, where the circuit 3 for LNG fuel will be used as a cooling circuit for a heat pump cooling circuit 5.

The system S for conditioning of LNG comprises also a heat pump cooling circuit 5, where the heat pump cooling circuit 5 is arranged to be a closed circuit. The heat pump cooling circuit 5 comprises a first heat exchanger 4 and a second heat exchanger 6a, where the first and second heat exchangers 4, 6a are connected through a pipe 81 in order to provide the closed circuit.

A gas expansion device 7 and a compressor 12 and a are also connected to the pipe 81, where the gas expansion device 7 is arranged between the first and second heat exchangers 4, 6a and the compressor 12 is arranged between the third heat exchanger 6b and the first heat exchanger 4.

An additional heat exchanger 11 is connected to the pipe 81 in such a way that a part of the pipe 81 extending between the third heat exchanger 6b and the compressor 12 will run through the additional heat exchanger 11, and a part of the pipe 81 extending between the compressor 12 and the first heat exchanger 4 also will run through the additional heat exchanger 11. The additional heat exchanger 11 is therefore arranged between the third heat exchanger 6b and the compressor 12 and between the compressor 12 and the first heat exchanger 4.

Furthermore, the first branch 32 of the circuit for sub cooled LNG is arranged to extend through the third heat exchanger 6b of the heat pump cooling circuit 5 and the second branch 33 of the circuit for LNG fuel is arranged to extend through the second heat exchanger 6a and the first heat exchanger 4 of the heat pump cooling circuit 5.

The third branch 31 of the circuit for LNG fuel will not be in contact with or connected to the heat pump cooling circuit 5. When the liquid natural gas in the tank 1 for storage of liquid natural gas for some reasons is to be conditioned, an amount of liquid natural gas is pumped from the tank 1 for storage of LNG and through the first pipe 31. The amount of liquid natural gas is thereafter divided or separated, such that a part of the liquid natural gas is transported through the second pipe 32 and the third heat exchanger 6b of the heat pump cooling circuit 5, while a remaining part of the liquid natural gas is transported through the third pipe 33, the second heat exchanger 6a and the first heat exchanger 4 of the heat pump cooling circuit 5 in order to be heat exchanged with the working medium/gas of the heat pump cooling circuit 5, whereafter the subcooled LNG is returned to the tank 1 for storage of LNG. Simultaneously, the working medium/gas circulating through the heat pump cooling circuit 5 is heated when heat exchanged with the liquid natural gas running through the second and third heat exchanger 6a, 6b, whereafter the working medium/gas is cooled through the first heat exchanger 4, the gas expansion device 7 and preferably through cooling water heat exchangers (not shown) after each compression step in the compressor 12.

The present invention differs from prior art in the way the cooling temperature in the heat pump circuit is achieved. A part of the cooling capacity that is produced for subcooling of LNG in the heat pump circuit is further used for selective, internal cooling in the heat pumps circuit to further cool the heat pump circuit before expansion in a way that creates an interaction between the heat pump circuit cooling temperature and the coldest cooling source for the heat pump circuit.

Advantageously, the coldest part of the subcooled LNG is used for the selective internal cooling in the heat pump circuit.

The sequence of happenings when introducing the embodiment of the present invention in heat a pump process is in the following explained by an example of a Joule Thomson (JT) cooling process, with reference to any of the Figures 1-4.

The process is sub-cooling LNG in the storage tank as follow:

A stable JT process (5) is sub cooling LNG in the third heat exchanger 6b without the present invention in operation, all sub cooled LNG is returned to the tank With the present invention in operation the heat pump circulation gas in pipe 81 is cooled by the sub cooled LNG from pipe 33 and second heat exchanger 6a before it is expanded in the expansion device 7, where the expansion device 7 in a Joule Thompson cooling process may be a type of an expansion valve that is partly open,

The cold temperature of the working medium/gas out of the expansion valve 7 will be lower than with prior JT technology because the cooling source temperature is reduced by the embodiment of the present invention The cold temperature of the working medium/gas out of the expansion valve 7 will be further lowered than with prior JT technology because the JT coefficient will be higher with the lowered temperature before expansion

Lower cold (cooling) temperature in the heat exchanger 6a will further sub cool the LNG through the second and third heat exchangers 6a, 6b and thereby increase the cooling capacity of the heat pump cooling circuit,

Further sub cooled LNG in the first heat exchanger 4 will further cool the heat pump circuit gas into the expansion valve.

The effect of the present invention can be visualized in a Temperature-Entropy diagram as shown for nitrogen in figure 5. The lines for isenthalpic expansion, which is the thermodynamic model for JT expansion, are steeper between the iso baric lines the colder the gas is before expansion. That is, the temperature reduction is higher between two pressure levels the colder the gas is before expansion. The diagram is showing the effect of the increasing the JT coefficient.

A real expansion process through a valve will be some different from the isenthalpic lines due to friction work through the expansion valve (7), but the effect of the present invention will basically be same.

The function of the present invention may be seen from different aspects:

Any compressor in a heat pump circuit has a pressure ratio limitation. Within a pressure ratio limitation, the feasible cooling temperature with the present invention is lower than with prior art

Because of this, a certain cooling temperature may be achieved with a lower pressure compressor than prior art.

The sub cooled LNG in the heat exchanger 6a and the cooling temperature in the heat pump circuit is interacting when the sub cooled LNG is colder than any other cooling source for the heat pump circuit. Then a part of the cooling capacity produced in the heat pump circuit is used for internal cooling. This causes a reduction of the cooling temperature in the heat pump circuit which again results in higher sub cooling of LNG.

The system and method according to the invention could also be used for conditioning of other cryogenic liquids than liquid natural gas (LNG), such as liquid hydrogen or the like.

The invention has now been explained with reference to non-limiting exemplary embodiments. A person of skill in the art will understand however that a number of variations and modifications can be made to the system for conditioning of LNG as described within the scope of the invention, as defined in the appended claims.

Claims

1. A system (S) for conditioning of liquid natural gas (LNG) in a LNG storage system, the system (S) comprising at least one LNG tank (1), a circuit (3) for LNG comprising a pump (2), and a heat pump cooling circuit (5), the heat pump cooling circuit (5) further comprising a gas expansion device (7) and a compressor (12), characterized in that the circuit (3) for sub cooled LNG is configured to be used as a cooling circuit for the heat pump cooling circuit (5), wherein the circuit (3) for LNG and the heat pump cooling circuit (5) are connected through a first, second and third heat exchanger (4, 6a, 6b, ).

2. The system according to claim 1, characterized in that the gas expansion device (7) is a valve, a turbine, or the like.

3. The system according to claim 1, characterized in that the system further comprises a plurality of regulation valves.

4. The system according to claim 3, characterized in that each regulation valve is controlled between a fully closed or fully open position, or positions between a fully closed and fully open position.

5. The system (S) according to claim 1, characterized in that the heat pump cooling circuit (5) comprises an additional heat exchanger (11).

6. The system (S) according to claim 5, characterized in that the additional heat exchanger (11) is arranged between the third heat exchanger (6b) and the compressor (12) and between the compressor (12) and the first heat exchanger (4).

7. The system (S) according to claim 1, characterized in that the system (S) comprises a fourth heat exchanger (10).

8. The system (S) according to claim 1, characterized in that the system (S) supplying natural gas (NG) to one or more consumers (9) is configured to be used as cooling medium for the heat pump cooling circuit (5).

9. The system (S) according to claim 1 and 2, characterized in that the system (S) for sub cooled LNG is configured to increase the Joule Thomson coefficient of a gas in the heat pump cooling circuit (5).

10. The system (S) according to claim 1, 2 and 8, characterized in that the heat pump cooling circuit (5) comprises a gas expansion cooling.

11. The system (S) according to claim 9, characterized in that the gas is nitrogen. The system (S) according to claims 2-6, characterized in that the circuit (3) for LNG fuel comprises a first branch (32) extending through the second heat exchanger (6a), a second branch (33) extending through the first heat exchanger (4) and a third branch (31) extending through the fourth heat exchanger (10). Use of a system (S) according to any of the preceding claims 1-12 for conditioning of liquid hydrogen.