DK202070207A1

DK202070207A1 - A gaseous fuel supply system for a main internal combustion engine of a marine vessel, a power system for a marine vessel, a marine vessel and a method of sub-cooling liquefied gaseous fuel stored in a gaseous fuel storage tank

Info

Publication number: DK202070207A1
Application number: DKPA202070207A
Authority: DK
Inventors: Nikolaos Kourtidis
Original assignee: Man Energy Solutions Filial Af Man Energy Solutions Se Tyskland
Priority date: 2020-04-03
Filing date: 2020-04-03
Publication date: 2021-12-07
Also published as: DK180711B1

Abstract

A method and system for sub-cooling liquefied gaseous fuel stored in a gaseous fuel storage tank (26) under cryogenic conditions for supply to a main internal combustion engine (100) of a marine vessel (60).

Description

. DK 2020 70207 A1

A GASEOUS FUEL SUPPLY SYSTEM FOR A MAIN INTERNAL COMBUSTION ENGINE OF A MARINE VESSEL, A POWER SYSTEM FOR A MARINE VESSEL, A MARINE VESSEL AND A METHOD OF SUB-COOLING LIQUEFIED GASEOUS FUEL STORED IN A GASEOUS FUEL STORAGE TANK

TECHNICAL FIELD The disclosure relates to a gaseous fuel supply system for providing a supply of pressurized gaseous fuel to a main engine of a marine vessel, to a power system for a marine vessel, a marine vessel and to a method of sub-cooling liquefied gaseous fuel stored in a gaseous fuel storage tank.

BACKGROUND Large two-stroke turbocharged uniflow scavenged internal combustion engines with crossheads are for example used for propulsion of large oceangoing vessels or as prime mover in a power plant. Not only due to sheer size, these two-stroke diesel engines are constructed differently from any other internal combustion engines.

These large two-stroke turbocharged uniflow scavenged internal combustion engines are increasingly being fuelled with gaseous fuel, such as e.g. liquified natural gas (LNG) or liquified petroleum gas (LPG), instead of the conventional liquid fuels such as e.g. marine diesel or heavy fuel oil.

This change towards gaseous fuels has mainly been driven by a desire to reduce emissions and to provide a more environmentally friendly prime mover.

Gaseous fuels such have very low energy density compared to conventional fuels. To serve as a convenient energy source, the density needs to be increased. This is done by cooling

, DK 2020 70207 A1 the gaseous fuel to cryogenic temperatures, creating, in the example of natural gas, liquefied natural gas (LNG). For LNG to be stored in a fuel tank the temperature of gas needs to be below boiling point (for LNG this is typically in the range of -155 to -165°C) depending on the pressure and the actual composition of the gas). A gaseous fuel supply system for such a gaseous operated engine comprises insulated tanks in which the liquefied gas is stored, keeping it in a liquid state for longer periods.

However, heat flux from the surroundings will increase the temperature inside the tank, thus causing the liquified gas to evaporate.

The gas from this process is known as boil-off gas (BOG). The boil-off from the storage tanks causes a substantially steady flow of boil-off gas that needs to be removed from the storage tanks and needs to be handled.

One way of dealing with the boil-off gas is to apply sub- cooling to the liquefied fuel in the storage tanks.

Hereto, a sub- cooling system is provided that sub-cools liquefied gaseous fuel from the storage tanks to a temperature lower than the temperature of the liquefied gaseous fuel in the storage tank and returns it to the storage tanks in a sub- called state.

For example, liquefied natural gas is typically stored at approximately -160%C in the storage tanks and the temperature of the liquified gas is reduced to approximately -165%C by the sub- cooling system.

Thus, the generation of boil off gas is prevented or at least slowed down and the liquefied gas has a certain "buffer" prior to evaporation by natural heat ingress, reducing or even eliminating the need for a boil-off gas compressor.

However, the sub-cooling system

2 DK 2020 70207 A1 uses a substantial amount of power, typically electric power to drive electric motors that drive compressors that are used to compress a cooling medium of the sub-cooling system. Thus, the use of a sub-cooling system increases investment in equipment and increases energy consumption.

SUMMARY It is an object to provide a gaseous fuel supply system, a power supply system, a marine vessel as well as methods that overcome or at least reduce the problems indicated above.

The foregoing and other objects are achieved by the features of the independent claims. Further implementation forms are apparent from the dependent claims, the description, and the figures.

According to a first aspect, there is provided a fuel supply system for supplying pressurized gaseous fuel to a main internal combustion engine of a marine vessel, the fuel supply system comprising: - a liquified gas storage tank for storing liquefied gaseous fuel under cryogenic conditions, - a sub-cooling system for sub-cooling liquified gaseous fuel stored in the liquified gas storage tank, - the sub-cooling system operating with a cooling medium, - a first heat exchanger for exchanging heat between the cooling medium and the liquefied gaseous fuel for sub-cooling the liquefied gaseous fuel, - a feed pump for pumping liquefied gaseous fuel from the liquified gas storage tank through a fuel line to the main internal combustion engine, and

2 DK 2020 70207 A1 - a second heat exchanger for exchanging heat between the pressurized liquefied gaseous fuel in the fuel line and the cooling medium, directly or indirectly, for cooling the cooling medium.

By exploiting the cooling capacity that the flow of liquefied gaseous fuel to the main internal combustion engine provides for covering at least a part of the cooling needs of the sub- cooling system, the energy efficiency of the overall system is significantly increased, i.e. less (electrical power) is required to operate the sub- cooling system. The liquefied gaseous fuel that is delivered to the main internal combustion engine needs to be heated anyway, since it will need to be vaporized before delivery to the main internal combustion engine. Thus, the waste heat of the sub-cooling system can be used to heat the liquefied gaseous fuel on its way to the main internal combustion engine. The cooling capacity of the sub-cooling system is increased by exploiting the cooling capacity of the flow of liquefied gaseous fuel to the main internal combustion engine, or alternatively the size of the sub-cooling system can be reduced whilst the cooling capacity of the sub- cooling system can be maintained to a required level. Both a reduced size sub-cooling system and the improved energy efficiency reduces costs.

In a possible implementation form of the first aspect, the liquefied gases fuel is stored at atmospheric pressure in the liquefied gas storage tank.

In a possible implementation form of the first aspect, the cooling system comprises a cooling circuit and the cooling circuit passes through the first heat exchanger.

5 In a possible implementation form of the first aspect, the cooling circuit comprising at least one compressor and at least one expander.

In a possible implementation form of the first aspect, the at least one compressor is driven by an electric drive motor, and the at least one expander preferably assists the electric drive motor in driving the compressor.

In a possible implementation form of the first aspect, a sub- cooling feed conduit connects an outlet of the feed pump to a first inlet of the first heat exchanger.

In a possible implementation form of the first aspect, a sub- cooling return conduit connects a first outlet of the first heat exchanger to the liquified gas storage tank for returning sub-cooled liquefied gaseous fuel to the liquefied gas storage tank.

In a possible implementation form of the first aspect, the second heat exchanger comprises at least one liquefied gaseous fuel to water based medium heat exchanger and at least one water based medium to cooling medium heat exchanger.

In a possible implementation form of the first aspect, the second heat exchanger comprises a liquefied gaseous fuel to cooling medium heat exchanger.

; DK 2020 70207 A1 In a possible implementation form of the first aspect, the fuel line comprises a high-pressure pump for increasing the pressure of the liquefied gaseous fuel to meet the pressure demands of a high-pressure fuel injection system of the main internal combustion engine.

In a possible implementation form of the first aspect, the fuel line comprises a third exchanger in the fuel line downstream of the second heat exchanger for heat exchanging with a heating medium for vaporizing the liquefied gasses fuel before it is supplied to the main internal combustion engine, the heating medium preferably being cooling water of the main internal combustion engine and/or boil-off gas from the liquefied gas storage tank that has been compressed after removal from the liquefied gas storage tank.

In a possible implementation form of the first aspect, the second heat exchanger is integral with the first heat exchanger and preferably the first and second heat exchanger are one integral unit or one single heat exchanger body.

According to a second aspect there is provided a power system for a marine vessel comprising the fuel supply system according to the first aspect and any implementations thereof, and a main internal combustion engine.

According to a third aspect there is provided a marine vessel comprising a power system according to the second aspect.

; DK 2020 70207 A1 According to a fourth aspect there is provided method of sub- cooling liquefied gaseous fuel stored in a gaseous fuel storage tank under cryogenic conditions for supply to a main internal combustion engine of a marine vessel, the method comprising: - pumping a first stream of liquefied gaseous fuel from the gaseous fuel storage tank to a sub-cooling system for subcooling the first stream of liquefied gaseous fuel in the sub-cooling system, - returning the sub-cooled first stream of liquefied gaseous fuel to the gaseous fuel storage tank, - pumping a second stream of liquefied gaseous fuel from the gaseous fuel storage tank to the main internal combustion engine, - using the second stream of liquefied gaseous fuel for cooling the sub-cooling system.

According to a possible implementation of the fourth aspect the sub-cooling system comprises a cooling circuit that is operated with a cooling medium and comprises heat exchanging the second stream of liquefied gaseous fuel with the cooling medium for cooling the cooling medium.

According to a possible implementation of the first aspect the method comprises vaporizing the liquefied gaseous fuel before suppling the gaseous fuel the main internal combustion engine.

These and other aspects will be apparent from and the embodiments described below.

2 DK 2020 70207 A1

BRIEF DESCRIPTION OF THE DRAWINGS In the following detailed portion of the present disclosure, the aspects, embodiments, and implementations will Lbe explained in more detail with reference to the example embodiments shown in the drawings, in which: Fig. 1 is a diagrammatic representation of an embodiment of a fuel supply system for supplying pressurized gaseous fuel to a main internal combustion engine of a marine vessel, Fig. 2 is a diagrammatic representation of an embodiment of a sub-cooling system of the fuel supply system of Fig. 1, Fig. 3a is a diagrammatic representation of another embodiment of a sub-cooling system of the fuel supply system of Fig. 1, Fig. 3b is a diagrammatic representation of yet another embodiment of a sub-cooling system of the fuel supply system of Fig. 1, and Fig. 4 is a diagrammatic representation of another embodiment of a fuel supply system for supplying pressurized gaseous fuel to a main internal combustion engine of a marine vessel,

DETAILED DESCRIPTION In the following detailed description, a gaseous fuel supply system for supplying gaseous fuel to a main internal combustion engine 100 of a marine vessel 60 will be described. The main internal combustion engine 100 is in an embodiment a large two-stroke low-speed turbocharged internal combustion engine. In an embodiment the marine vessel 60 has further engines (not shown), so called auxiliary engines that are coupled to generators to form generator sets that provide electric power for the marine vessel 60, in particularly when the main engine

2 DK 2020 70207 A1 100 is not operating. The auxiliary engines are in an embodiment also supplied with gaseous fuel from the gaseous fuel supply system.

In an embodiment the main engine 100 has crossheads. In an embodiment the main engine 100 scavenge ports in the lower end of the cylinder liners and a central exhaust valve in the cylinder cover. In an embodiment the main engine is uniflow scavenged.

The main engine has in an embodiment between four and fourteen cylinders in line. The total output of the main engine 100 may, for example, range from 1,000 to 110,000 kW.

The main engine combines in an operational mode with gaseous fuel as the main fuel for both the Diesel cycle and the Otto cycle since it is compression ignited but also compresses an air-fuel mixture from a first amount of pressured gaseous fuel admitted during the compression stroke of a piston. The compressed air-fuel mixture is ignited when a second amount of high-pressure gaseous fuel is injected at or near top dead center (TDC).

In another operation mode, the main engine 100 operates according to the Diesel cycle with no fuel being admitted during the compression stroke and all fuel being injected at high pressure at or near top dead center of the piston concerned, this mode also has gaseous fuel as the main fuel.

In yet another operation mode the main engine 100 operates according to the Otto cycle with all gaseous fuel being mixed i. DK 2020 70207 A1 with at relatively low pressure with the scavenging air and the air-fuel mixture being compressed during the compression stroke, and timed ignition being provided at or near TDC.

By “at or near TDC” is meant a range that comprises injection of gaseous fuel starting earliest at a time when the piston is approximately 15 degrees before TDC and ending at the latest by approximately 40 degrees after TDC.

In all operation modes ignition liquid (e.g. pilot oil) can be provided at or near TDC to ensure reliable ignition of the gaseous fuel.

In an embodiment the main engine 100 is dual fuel engine that has at least one operation mode on gaseous fuel and at least one operation mode on a conventional fuel, such as e.g. marine diesel or heavy fuel oil.

Fig. 1 is a diagrammatic representation of a gaseous fuel supply system for supplying gaseous fuel to the large two- stroke turbocharged internal combustion engine 100 in a marine vessel 60. The gaseous fuel supply system is in an embodiment installed in a liquefied gas tanker, i.e. a marine vessel that transports a large amount of liquefied gaseous fuel, such as e.g. liquefied natural gas (LNG) or liquefied petroleum gas (LPG). In another embodiment, the gaseous fuel supply system is installed in a cargo vessel, such as e.g. a container ship.

The gaseous fuel supply system is configured to supply pressurized gaseous fuel to the main engine 100 of a marine

1 DK 2020 70207 A1 vessel and to other consumers of gaseous fuel of the marine vessel, such as e.g. generator sets 110 for producing heat and electrical power for the marine vessel (generator sets 110 are typically four-stroke internal combustion engines that are significantly smaller than the main engine and drive an electric generator/alternator), in particular when the main engine 100 is stopped (e.g. when the marine vessel is in a harbor for transfer of cargo), or boilers (not shown) that operate on gaseous fuel.

The gaseous fuel supply system is configured to supply pressurized gaseous fuel to the main engine 100. The gaseous fuel can be any suitable liquified gaseous fuel for powering a large two-stroke turbocharged internal combustion engine 100, such as e.g. liquified natural gas (LNG), Lliquified petroleum gas (LPG) Liquid Ethane GAS (LEG) or Liquid Ammonia. The gaseous fuel supply system is in this embodiment configured to supply medium pressure (e.g. between 5 to 40 bar) vaporized gaseous fuel to the main engine 100, i.e. a pressure that is suitable for admission of the gaseous fuel in vaporized form for mixing with the scavenging air and admission during stroke of the piston from BDC to TDC.

The gaseous fuel supply system comprises one or more (insulated) gaseous fuel storage tanks 26 for storing liquefied gaseous fuel under cryogenic conditions and at atmospheric pressure. For LNG this will be a temperature of approximately -155 to -165°C.

i. DK 2020 70207 A1 A sub-cooling system 40 sub-cools the liquified gaseous fuel stored in the liquified gas storage tank 26 for preventing or at least delaying the generation of boil-off gas in the fuel storage tank 26.

A feed pump 30 pumps the cryogenic fuel from the storage tank 26 through a fuel line to the main engine 100 for medium- pressure fuel admission. The fuel line comprises a first feed conduit 31 that transports the liquified gaseous fuel from fuel storage tanks 26 to an inlet of a second heat exchanger 34,35,36,45,46 associated with the sub-cooling system 40 by the action of a feed pump 30. The second heat exchanger 34,35,36,45,46 will be described in greater detail further below. A second feed conduit 32 connects an outlet of the second heat exchanger 34,35,36,45,46 to an inlet of a third heat exchanger 33. The third heat exchanger 33 is configured to heat and vaporize the medium-pressure liquefied gaseous fuel by heat exchanging with a suitable medium, such as e.g. cooling water of the main engine 100 and/or boil-off gas from the liquefied gas storage tank 26 that has been compressed after removal from the liquefied gas storage tank 26. Typically, the gaseous fuel is heated to approximately 45°C at the outlet of the third heat exchanger 33 and no longer liquified.

A third feed conduit 39 connects an outlet of the third heat exchanger 33 to an inlet of the fuel admission system of the main engine 100 for supplying pressurized vaporized gaseous fuel the main engine 100.

12 DK 2020 70207 A1 A sub-cooling feed conduit 47 branches off from the first feed conduit 31 and connects an outlet of the feed pump 30 to the sub-cooling system 40 for supplying liquefied gaseous fuel from the liquified gas storage tank 26 to the sub-cooling system 40 for sub-cooling. A sub-cooling return conduit 48 returns sub-cooled liquefied gaseous fuel from the sub- cooling system 40 back to the liquified gas storage tank 26.

Fig. 2 is a diagrammatic representation of the sub-cooling system used in the fuel supply system of Fig. 1. The sub- cooling system 40 operating with a cooling medium, such as a suitable gas, e.g. a mixture of noble gases in a cooling circuit 41. A number of compressors 43 driven by electric drive motors 42 compress the cooling medium and a number of expanders 44 allow the cooling medium to expand to thereby reduce its temperature. In an embodiment the expanders 44 are used to assist the electric drive motors for driving the compressions 43. At least an intercooler 45 and in an embodiment also an aftercooler 46 cool the compressed cooling medium before the compressed heating medium is supplied to the expanders 44.

The cooling circuit 41 passes through a first heat exchanger

49. The cooling medium leaving the expanders 44 is passed through a first heat exchanger 49 for heat exchanging with liquified gaseous fuel supplied from the liquefied gas storage tank 26 through the subcooling feed conduit 47 and returned after sub-cooling in the first heat exchanger to the liquefied gas storage tank 26 via the sub-cooling return conduit 48.

Thus, the sub-cooling feed conduit 47 connects an outlet of the feed pump 30 to a first inlet of the first heat exchanger

DK 2020 70207 A1 14

49. The sub-cooling return conduit 48 connects a first outlet of the first heat exchanger 49 to the liquified gas storage tank 26 for returning sub-cooled liquefied gaseous fuel to the liquefied gas storage tank 26. In an example with LNG as the fuel, the LNG in the liquefied gas storage tank 26 is sub-cooled from - 160 to -165%C by passing through the first heat exchanger 49. In this embodiment the inter- and aftercooler 45,46 exchange heat with a water based medium, such as e.g. a glycol-water mixture. The glycol-water mixture exchanges heat with liquified gaseous fuel from the fuel line in a first water based medium to gaseous fuel heat exchanger 35 and in a in second water based medium to gaseous fuel heat exchanger 36.

The first feed conduit 31 connects to an inlet of the first- and second water based medium to gaseous fuel heat exchangers 35,36 and the second feed conduit 32 connects to an outlet of the first- and second water based medium to gaseous fuel heat exchangers 35,36. Thus, the liquefied gaseous fuel from the first feed conduit 31 cools the water based medium and the cooled water based medium cools the cooling medium, thereby removing heat from the sub-cooling system 40.

The cooling effect of the liquified gaseous fuel in the fuel line is higher than what can be obtained with cooling the sub-cooling system 40 by heat exchanging with the atmosphere or with sea water, thereby increasing the sub-cooling capacity of the sub-cooling system 40. Further, the liquefied gaseous fuel is heated in the process which reduces the capacity needed in the third heat exchanger 33 for vaporizing the

Ls DK 2020 70207 A1 liquified gaseous fuel before delivery to the main engine

100.

In this embodiment the second heat exchanger for exchanging heat between the pressurized liquefied gaseous fuel in the fuel line and the cooling medium, is formed by the first- and second water based medium to gaseous fuel heat exchangers 35,36 and the inter- and aftercooler 45,46. Thus, in this embodiment the cooling medium is indirectly cooled by the liquified gaseous fuel in the fuel line. The liquefied gaseous fuel is stored in the gaseous fuel storage tank 26 under cryogenic conditions and preferably at atmospheric pressure for supply to the main internal combustion engine 100 of the marine vessel 60. A first stream of liquefied gaseous fuel is pumped from the gaseous fuel storage tank 26 to the sub-cooling system 40 for subcooling the stream of liquefied gaseous fuel in the sub-cooling system

40. After sub-cooling the liquefied gaseous fuel the sub- cooled first stream of liquefied gaseous fuel to the gaseous fuel storage tank 26. A second stream of liquefied gaseous fuel is pumped from the gaseous fuel storage tank 26 to the main internal combustion engine 100 for combustion and the second stream of liquefied gaseous fuel is used for cooling the sub-cooling system 40, i.e. for removing (waste) heat from the sub-cooling system. The cooling medium of the sub- cooling system heat exchanges with the second stream of liquefied gaseous fuel for cooling the cooling medium. Thereby the sub-cooling capacity of the is increased and the (electrical) power consumption of the sub-cooling system 40 is reduced.

Le DK 2020 70207 A1 In an embodiment the gaseous fuel supply stem is part of a power system for the marine vessel 60, the power system also comprising the main engine 100.

Fig. 3a shows another embodiment of the gaseous fuel supply system that is similar to the gaseous fuel supply system according to the embodiment of Figs. 1 and 2. In the embodiment of Fig. 3a, structures and features that are the same or similar to corresponding structures and features previously described or shown herein are denoted by the same reference numeral as previously used for simplicity. In the embodiment of Fig. 3a the second heat exchanger is formed by a liquified gaseous fuel to cooling medium heat changer 34. Both the cooling circuit 41 and the fuel supply line flow tough the liquified gaseous fuel to cooling medium heat changer 34. As shown by the interrupted lines, the cooling circuit 41 may pass twice through the liquified gaseous fuel to cooling medium heat changer 34. In this embodiment, the inter- and aftercooler 45,46 for heat exchanging with a water based medium can be included in addition to the liquified gaseous fuel to cooling medium heat changer 34 for additional cooling of the cooling medium.

Fig. 3b shows another embodiment of the gaseous fuel supply system that is similar to the gaseous fuel supply system according to the embodiment of Figs. 1 and 2. In the embodiment of Fig. 3b, structures and features that are the same or similar to corresponding structures and features i DK 2020 70207 A1 previously described or shown herein are denoted by the same reference numeral as previously used for simplicity.

In the embodiment of Fig. 3b the second heat exchanger is integral with the first heat exchanger 49 and liquified gaseous fuel in the fuel supply line exchanges heat with the cooling medium in the first heat exchanger for cooling the cooling medium. Thus, in this embodiment the first and second heat exchanger are one integral unit.

Fig. 4 shows another embodiment of the gaseous fuel supply system that is similar to the gaseous fuel supply system according to the embodiment of Fig. 1. In the embodiment of Fig. 4, structures and features that are the same or similar to corresponding structures and features previously described or shown herein are denoted by the same reference numeral as previously used for simplicity. In this embodiment the main engine 100 is of the type that has a high-pressure fuel injection system that requires a supply of vaporized gaseous fuel at a pressure in the range of approximately 200 to 350 bar. The embodiment of Fig. 4 differs from the embodiment of Fig. 1 in that a high pressure cryogenic pump 37 is arranged upstream of the third heat exchanger/vaporized 33. The high pressure cryogenic pump 37 increases the pressure of the liquefied gaseous fuel to meet the pressure demands of a high- pressure fuel injection system of the main internal combustion engine 100. In the same way as for the embodiments above, the liquefied fuel is vaporized in the third heat exchanger 33 i.

DK 2020 70207 A1 before delivery to the main engine 100, just at a significantly higher pressure than in the embodiments above.

The various aspects and implementations have been described in conjunction with various embodiments herein.

However, other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed subject-matter, from a study of the drawings, the disclosure, and the appended claims.

In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality.

A single processor, controller or other unit may fulfill the functions of several items recited in the claims.

The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.

The reference signs used in the claims shall not be construed as limiting the scope.

Claims

19 DK 2020 70207 A1 CLAIMS

1. A fuel supply system for supplying pressurized gaseous fuel to a main internal combustion engine (100) of a marine vessel (60), said fuel supply system comprising: - a ligquified gas storage tank (26) for storing liquefied gaseous fuel under cryogenic conditions, - a sub-cooling system (40) for sub-cooling liquified gaseous fuel stored in said liquified gas storage tank (26), - said sub-cooling system (40) operating with a cooling medium, - a first heat exchanger (49) for exchanging heat between said cooling medium and said liquefied gaseous fuel for sub- cooling said liquefied gaseous fuel, - a feed pump (30) for pumping liquefied gaseous fuel from said liquified gas storage tank (26) through a fuel line (31,32,39) to said internal combustion main engine (100), and - a second heat exchanger (34,35,36,45,46,49) for exchanging heat between said liquefied gaseous fuel in said fuel line (31,32,39) and said cooling medium, directly or indirectly, for cooling said cooling medium.

2. The fuel supply system according to claim 1, wherein said cooling system (40) comprises a cooling circuit (41), said cooling circuit (41) passing through said first heat exchanger (49).

3. The fuel supply system according to claim 1 or 2, wherein said cooling circuit (41) comprising at least one compressor (43) and at least one expander (44).

DK 2020 70207 A1 20 4, The fuel supply system according to any one of the preceding claims, wherein said at least one compressor (43) is driven by an electric drive motor (42), and wherein said at least one expander (44) preferably assists said electric drive motor (42) in driving said compressor (43.

5. The fuel supply system according to any one of the preceding claims, wherein a sub-cooling feed conduit (47) connects an outlet of said feed pump (30) to a first inlet of said first heat exchanger (49).

6. The fuel supply system according to any one of the preceding claims, wherein a sub-cooling return conduit (48) connects a first outlet of said first heat exchanger (49) to said liquified gas storage tank (26) for returning sub-cooled liquefied gaseous fuel to said liquefied gas storage tank (26).

7. The fuel supply system according to any one of the preceding claims, wherein said second heat exchanger (34,35,36,45,46) comprises at least one liquefied gaseous fuel to water based medium heat exchanger (35, 36) and at least one water based medium to cooling medium heat exchanger (45, 46).

8. The fuel supply system according to any one of the preceding claims, wherein said second heat exchanger (34,35,36,45,46) comprises a liquefied gaseous fuel to cooling medium heat exchanger (34).

1 DK 2020 70207 A1

9. The fuel supply system according to any one of the preceding claims, wherein said fuel Lline (31, 32, 39) comprises a high-pressure pump (37) for increasing the pressure of said liquefied gaseous fuel to meet the pressure demands of a high-pressure fuel injection system of said main internal combustion engine (100).

10. The fuel supply system according to any one of the preceding claims, wherein said fuel line comprises a third exchanger (33) in said fuel line downstream of said second heat exchanger (34,35,36,45,46) for heat exchanging with a heating medium for vaporizing the liquefied gasses fuel before it is supplied to said internal combustion main engine (100), said heating medium preferably being cooling water of said main internal combustion engine (100) and/or boil-off gas from said liquefied gas storage tank (26) that has been compressed after removal from said liquefied gas storage tank (26).

11. The fuel supply system according to any one of the preceding claims, wherein said second heat exchanger is a part of said first heat exchanger (49).

12. A power system for a marine vessel (60) comprising the fuel supply system according to any one of claims 1 to 11 and said main internal combustion marine engine (100).

13. A marine vessel (60) comprising a power system according to claim 12.

> DK 2020 70207 A1

14. A method of sub-cooling liquefied gaseous fuel stored in a gaseous fuel storage tank (26) under cryogenic conditions for supply to a main internal combustion engine (100) of a marine vessel (60), said method comprising: - pumping a first stream of liquefied gaseous fuel from said gaseous fuel storage tank (26) to a sub-cooling system (40) for subcooling said first stream of liquefied gaseous fuel in said sub-cooling system (40), - returning said sub-cooled first stream of liquefied gaseous fuel to said gaseous fuel storage tank (26), - pumping a second stream of liquefied gaseous fuel from said gaseous fuel storage tank (26) to said main internal combustion engine (100), - using said second stream of liquefied gaseous fuel for cooling said sub-cooling system (40).

15. The method according to claim 14, wherein said sub-cooling system (40) comprises a cooling circuit (41) that is operated with a cooling medium and comprising heat exchanging said second stream of liquefied gaseous fuel with said cooling medium for cooling said cooling medium.

16. The method according to claim 15, comprising vaporizing said liquefied gaseous fuel before suppling said gaseous fuel said main internal combustion engine (100).