BACKGROUND

Marine vessels, in particular container vessels or reefer ships often have a substantial need for cooling cargo in addition to cooling of the traditional crew accommodation, when in port and when sailing. Passenger vessels have a substantial need for cooling in connection with a HVAC system for the passenger and crew accommodation. Hereto, known marine vessels are provided with cooling equipment that includes compressors that are driven by electric motors. Electricity for the cooling system is provided using fuel form of gas or diesel in one or more generator sets, i.e. a four stroke internal combustion engines driving a generator. The generators are connected to the main switchboard that distributes the electrical power to the various electrical consumers. One of the electrical consumers is a compressor station where the electrical energy is converted to mechanical energy and the mechanical energy is converted into cooling energy in a vapor compression cycle. The efficiency of such a cooling system could be improved and since the costs for cooling aboard a marine vessel can be significant, substantial costs can be saved providing cooling energy in a less expensive manner. SUMMARY

On this background, it is a first aspect to provide a more cost efficient system for providing cooling energy to a cooling medium circuit of a marine vessel, the cooling system comprises a liquefied gas storage tank, a liquefied gas vaporizer comprising a first heat exchanger that exchanges heat between the liquefied gas and a second medium in an intermediate flow circuit, a liquefied gas feed line for transporting liquefied gas from the liquefied gas storage tank to the liquefied gas vaporizer, a second heat exchanger configured to exchange heat between the second medium and a cooling medium in the cooling medium circuit, an adsorption or absorption refrigerator operated by burning gas from the liquefied gas vaporizer, and a gas feed line for transporting gas from the liquefied gas vaporizer to the absorption or adsorption refrigerator, the absorption or adsorption refrigerator comprising a third heat exchanger configured to exchange heat with the cooling medium in the cooling medium circuit.

By combining the direct cooling using the cold stored in the liquefied gas that is on board the marine vessel in the liquefied gas storage tank with a gas refrigerator using heat from burning gas from evaporating the liquid natural gas and optional surplus heat from steam or the like, a highly energy effective system for providing cooling energy aboard a marine vessel is obtained. Such a system can be fitted or retrofitted on marine vessels that have main engines and generator sets that are operated conventionally with fuel oil and/or heavy fuel oil. The system can also be fitted or retrofitted on marine vessels that have a main engine operated on liquefied natural gas.

In a first possible implementation form of the first aspect the second cooling medium circuit is in connection with a cold store in the marine vessel.

In a second possible implementation form of the first aspect the second cooling medium circuit is in connection with a HVAC system in the marine vessel.

In a third possible implementation form of the first aspect the system further comprises a first temperature transducer configured to sense a temperature of the cooling medium, wherein the liquefied natural gas feed pump is driven by a variable speed drive that is in receipt of a signal from the first temperature transducer, and wherein the variable speed drive is configured to adjust the speed of the liquefied natural gas feed pump in relation to the signal from the first temperature transducer.

In a fourth possible implementation form of the first aspect the system further comprises a second temperature transducer configured to sense a temperature of the cooling medium, a second medium pump in the intermediate circuit driven by a variable speed drive, the variable speed drive that is in receipt of a signal from the second temperature transducer, and wherein the variable speed drive is configured to adjust the speed of the second medium pump in relation to the signal from the second temperature transducer. In a fifth possible implementation form of the first aspect the system further comprises a variable output liquefied gas feed pump for pumping liquefied gas from the liquefied gas storage tank to the liquefied gas vaporizer via the liquefied gas feed pipe.

According to a second aspect, there is provided a marine vessel with a hull and an internal combustion engine operated on fuel oil or heavy fuel oil, the fuel tank for fuel oil, a plurality of generator sets operated on fuel oil, and the system according to the first aspect and its implementation forms.

In a first implementation of the second aspect the marine vessel further comprises means to feed heat from a source other than the heat from burning said gas to said absorption or adsorption refrigerator for operating said absorption or adsorption refrigerator. These and other aspects of the invention will be apparent from the example embodiments described below.

BRIEF DESCRIPTION OF THE DRAWINGS In the following portion of the present description, the invention will be explained in more detail with reference to the example embodiments shown in the drawings, in which :

Fig. 1 is a sectional view of a marine vessel showing diagrammatically a cooling system according to an example embodiment,

Fig. 2 is a diagrammatic representation view of a cooling system for a marine vessel according to an example embodiment, Fig. 3 is a diagrammatic representation of an absorption refrigerator, and

Fig. 4 is a diagrammatic representation of an adsorption refrigerator .

DETAILED DESCRIPTION

In the following detailed description, the cooling system for a marine vessel will be described by the example embodiments. Fig. 1 diagrammatically shows a marine vessel 1 in a sectional view showing a cooling system. The cooling system can be used for cooling of accommodation on cargo ships. This could be (non- exhaustive list) :

roll-on-roll-off ships (ROROs), container ships, supply- ships, bulk-carries or tankers, etc. The cooling system can also be used for cooling of accommodation on passenger ships, ferries and ROPAX (roll-on-roll-off ships that are suitable to carry a substantial amount of passengers), and for cargo cooling on cargo ships e.g. banana boats and container ships (non-exhaustive list) . In the shown example the process is represented by the box 5. The cooling system is shown with reference to numeral 60, for cooling the crew accommodation 2 with a HVAC system and for cooling the cargo space 3. A liquefied natural gas storage tank 20 shown on the aft of the vessel. An optional steam boiler 80 is also shown. The marine vessel 1 has a hull and is provided with at least one internal combustion engine, such as the main engine for driving the propeller. The main engine can be a four stroke internal combustion engine or in an example embodiment the main engine is a large slow running two- stroke internal combustion engine (not shown) . The internal combustion engines can be operated on fuel oil or heavy fuel oil, and the marine vessel includes at least one fuel tank and at least one generator set operated on fuel oil. Fig. 2 is a diagrammatic representation of a cooling system according to an example embodiment. The cooling system includes an insulated tank 20 for holding liquefied gas, such as liquefied natural gas (LNG) at a low temperature and at a pressure, i.e. below -150°C and above 1 atm. Hereafter, reference is made to liquefied natural gas (LNG) and natural gas, but it is understood that any other gas with similar properties, such as propane or butane could be used instead. The LNG is supplied from the insulated storage tank 20 by a liquefied natural gas feed pump 10 that is driven by variable speed drive (VSD) 11 (alternatively the LNG can be delivered by increasing the pressure in the tank 20 by evaporation of a small amount of gas in the tank 20. An LNG feedline 22 transports the LNG from the LNG storage tank 20 to an LNG vaporizer 30.

The LNG vaporizer 30 includes a first heat exchanger exchanges heat between the LNG supplied from the storage tank 20 and a second cooling medium in an intermediate flow circuit 36. The first heat exchanger transfers heat from the second medium to the LNG so that the natural gas leaving the vaporizer 30 has in an embodiment a temperature of approximately 45°C. The second medium can in embodiment be glycol or a similar medium. The intermediate flow circuit 36 includes a second medium pump 32 that is driven by a variable speed drives (ESD) 37 for circulating the second cooling medium in the intermediate flow circuit 36 in the direction indicated by the arrows. An expansion tank 39 is connected to the intermediate flow circuit 36 for absorbing any fluctuations in the volume of fluid in the intermediate flow circuit.

The intermediate flow circuit 36 also passes the second cooling medium through a second heat exchanger 50.

The cooling circuit 55 of the cooling system 60 of the marine vessel 1 includes a cooling medium circulation pump 46 and is provided with an expansion tank 48 for absorbing any fluctuations in the volume of fluid in the cooling circuit 55. The cooling circuit 55 passes the cooling medium through the second heat exchanger 50 in order to exchange heat with the second medium. In this process heat from the cooling medium is transferred to the second medium. The cooling medium circuit 55 uses a cooling medium, such as water, brine or gas. A gas feed conduit 23 connects an outlet of the LNG vaporizer 30 to the inlet of a "gas" refrigerator in the form of a absorption or adsorption refrigerator 40. The natural gas delivered by the gas feed conduit 23 is burned in the absorption or adsorption refrigerator 40 and the heat created by the burning process is used for the operation of the absorption or adsorption refrigerator 40. Other sources of heat for operating the absorption or adsorption refrigerator 40 can be added to the heat coming from burning the natural gas. Such an additional source of heat can be surplus steam from a steam boiler 80 or surplus (waste) heat from the engine (s) of the marine vessel. The absorption or adsorption refrigerator 40 includes a third heat exchanger that is configured to exchange heat with the cooling medium in the cooling medium circuit 55. In this process heat from the cooling medium is transferred to the gas refrigerator 40.

The cooling medium circuit 55 is provided with a first temperature transducer 42 at a position close to where the cooling medium leaves the gas refrigerator 40 and with a second temperature transducer 44 at a position close to where the cooling medium leaves the second heat exchanger 50. In an embodiment the cooling medium is water or brine and the temperature of the cooling medium leaving the gas refrigerator 40 is approximately 6°C and the temperature of the cooling medium leaving the cooling system 60 is approximately 12°C at full load.

An electronic control unit 70 is connected via signal cables to the first temperature transducer 42, to the second temperature transducer 44 to the variable speed drive 11 and to the variable speed drive 37. The electronic control unit is configured to adapt the speed of the LNG feed pump 10, and/or the speed of the second medium pump 32 in response to the temperature signals from the two pressure transducers 42, 44 and in order to maintain the desired temperature of the cooling medium leaving the gas refrigerator 40. The electronic control unit 70 can also be configured to control the temperature of the second medium between the second heat exchanger 50 and the LNG vaporizer 30.

Thus, the energy in the form of both coldness in the LNG and chemical energy in the LNG is transferred in the form of cooling to a cooling system through an evaporator and a gas refrigerator. The purpose of the absorption or adsorption process other than to translate large amounts of energy bound in the gas, is to prevent methane and other gas emissions to the atmosphere.

As mentioned above, the "gas" refrigerator 40 can be of the adsorption type or of the absorption type. The gas refrigerator 40 will hereafter be described in greater detail .

Fig. 3 is a diagrammatic representation of an absorption type gas driven refrigerator 40. The refrigerator 40 comprises a generator where the heat from burning the natural gas (eventually supplied by other sources such as surplus steam) is applied and evaporates a separated part of the process liquid into vapor. The remainder liquid part flows to the absorber. The vapor enters a condenser, e.g. cooled with sea water, where the vapor separated part from the generator is condensed. The liquid is transported from the condenser to the evaporator by pressure differences. In the evaporator, where an under pressure is established, the external circuit is cooled as the condensed liquid boils to vapor prior to entering the absorber. In the absorber the vapor from the evaporator is absorbed by the remainder liquid part from the generator stage. The liquid is transported from the absorber to the generator by means of a pump. The process liquid is a solution of a refrigerant and an absorber with properties suitable for the thermodynamical process.

Fig. 4 is a diagrammatic representation of an adsorption type gas driven refrigerator. The adsorption type refrigerator uses solid sorption materials. The system comprises of a number of adsorbers, as often 2, filled with a material that adsorbs water, a condenser, and an evaporator. More than one adsorber is needed to maintain continuously cooling. One of the adsorbents, which has adsorbed a refrigerant liquid, is dried out by applying heat and vapor is generated which flows to the condenser, which is e.g. cooled with sea water, where it is condensed while heat is transferred. The heat input stops to this particular adsorbent when the absorber is dried out.

The condensed liquid is continuously transferred to the evaporator where it evaporates at low pressure when in contact with the cooling liquid coil. The dried adsorbent aspires the vapor and accumulates it. Due to the evaporation cooling is generated. Water is expected to be the refrigerant liquid and the solid sorption material to be silica gel or alternative material. The driving force for refrigerant circulation is vapor pressure differentials created by a mechanical or thermal device and gravity.

The invention has 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 invention, 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 or other control unit may fulfil 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.