Carbon dioxide liquefaction device

Abstract

A carbon dioxide liquefaction device (20) comprises: a liquefaction heat exchanger (5) that exchanges heat so that carbon dioxide contained in a mixed gas discharged from a fuel cell (2) is cooled and liquefied by the cold energy of a liquefied gas; a pressure sensor (13) that detects an internal pressure of the liquefaction heat exchanger (5); a liquid level sensor (14) for detecting a liquid amount that is the amount of liquefied carbon dioxide liquefied by the liquefaction heat exchanger (5); and a control device (1) that controls the amount of the liquefied gas fed into the liquefaction heat exchanger (5), on the basis of the internal pressure detected by the pressure sensor (13) and the liquid amount detected by the liquid level sensor (14).

Description

Carbon dioxide liquefaction equipment

[0001] The present invention relates to a carbon dioxide liquefaction device for liquefying carbon dioxide.

[0002] It is generally known that LNG (liquefied natural gas) is used as fuel for fuel cells. When LNG is used as fuel, the fuel cell emits carbon dioxide, which is often released into the atmosphere.

[0003] However, the release of carbon dioxide into the atmosphere is a concern due to its impact on the environment, and there is a demand for liquefying and storing carbon dioxide in order to prevent its release into the atmosphere.

[0004] An object of an embodiment of the present invention is to provide a carbon dioxide liquefaction device that efficiently liquefies carbon dioxide discharged from a fuel cell.

[0005] A carbon dioxide liquefaction device according to an aspect of the present invention comprises a liquefaction heat exchanger that exchanges heat to cool and liquefy carbon dioxide contained in a mixed gas discharged from a fuel cell using the cold energy of a liquefied gas, a pressure sensor that detects the internal pressure of the liquefaction heat exchanger, a liquid quantity sensor that detects the liquid quantity, which is the amount of liquefied carbon dioxide liquefied in the liquefaction heat exchanger, and a control device that controls the amount of liquefied gas sent to the liquefaction heat exchanger based on the internal pressure detected by the pressure sensor and the liquid quantity detected by the liquid quantity sensor.

[0006] Fig. 1 is a configuration diagram showing the configuration of a carbon dioxide liquefaction device according to a first embodiment of the present invention. Fig. 2 is a configuration diagram showing the configuration of a ship according to a second embodiment of the present invention. Fig. 3 is a configuration diagram showing the configuration of a ship according to a third embodiment of the present invention. Fig. 4 is a configuration diagram showing the configuration of a ship according to a fourth embodiment of the present invention. Fig. 5 is a configuration diagram showing the configuration of a ship according to a fifth embodiment of the present invention.

[0007] 1 is a diagram showing the configuration of a carbon dioxide liquefaction device 20 according to a first embodiment of the present invention. Note that the same parts in the drawings are given the same reference numerals, and duplicated explanations will be omitted as appropriate.

[0008] The carbon dioxide liquefaction device 20 is not limited to being installed fixedly on land or sea, but may also be provided on a moving body such as a ship, a vehicle, or an aircraft.

[0009] The carbon dioxide liquefaction device 20 includes a control device 1, a fuel cell 2, a condenser 3, a compressor 4, a liquefaction heat exchanger 5, a liquefied carbon dioxide tank 6, an LNG tank 7, a pump 8, a bypass valve 9, a heater 10, a gas sensor 11, an exhaust valve 12, a pressure sensor 13, and a liquid level sensor 14. As long as the carbon dioxide liquefaction device 20 includes at least the control device 1 and the liquefaction heat exchanger 5 as its constituent devices, the other devices may be provided as external devices of the carbon dioxide liquefaction device 20.

[0010] The mixed gas containing carbon dioxide discharged from the fuel cell 2 passes successively through a condenser 3, a compressor 4, and a liquefaction heat exchanger 5, where the carbon dioxide is extracted and the gas is liquefied. The liquefied carbon dioxide is sent to a liquefied carbon dioxide tank 6. A path (pipe) is provided to sequentially connect the fuel cell 2, the condenser 3, the compressor 4, the liquefaction heat exchanger 5, and the liquefied carbon dioxide tank 6.

[0011] The LNG taken out from the LNG tank 7 is sent as fuel to a supply destination of the LNG via a pump 8, a liquefaction heat exchanger 5, and a heater 10 in that order. A path (pipe) is provided to sequentially connect the LNG tank 7, the pump 8, the liquefaction heat exchanger 5, the heater 10, and the supply destination.

[0012] The control device 1 performs main control for liquefying carbon dioxide in the carbon dioxide liquefaction device 20. The control device 1 may control or monitor any of the components that make up the carbon dioxide liquefaction device 20. The control device 1 may also be used as a control device that controls the power generation of the fuel cell 2. The control device 1 includes a computer for performing various types of arithmetic processing, and may be composed of multiple computers.

[0013] The fuel cell 2 generates electricity by using oxygen in the air and hydrogen contained in natural gas vaporized from LNG supplied as fuel. For example, the fuel cell 2 is a solid oxide fuel cell (SOFC), but other types of fuel cells may be used. Furthermore, although LNG is used as the fuel for the fuel cell 2 in the following description, LPG (liquefied petroleum gas) may also be used. The mixed gas discharged from the fuel cell 2 mainly contains water vapor and carbon dioxide, and may also contain nitrogen.

[0014] In addition, the carbon dioxide liquefaction device 20 may include a device for removing or separating any substances emitted from the fuel cell 2, and such processing may be performed at any point in the process of liquefying carbon dioxide.

[0015] The condenser 3 converts the water vapor contained in the mixed gas discharged from the fuel cell 2 back into water. In this way, the condenser 3 removes the water vapor from the mixed gas and extracts carbon dioxide. The condenser 3 sends the extracted carbon dioxide to the compressor 4. In this way, complete removal of moisture from the mixed gas prevents ice from being formed when cooling to liquefy the carbon dioxide.

[0016] Any configuration may be used as long as it is capable of removing moisture from the mixed gas. For example, in order to completely remove moisture that cannot be removed by the condenser 3, a device for removing moisture such as a dryer or a dehumidifier may be additionally provided, or these devices may be provided instead of the condenser 3. Furthermore, the configuration for removing moisture may be provided anywhere on the path from the gas discharged from the fuel cell 2 to the liquefaction heat exchanger 5.

[0017] The compressor 4 compresses the carbon dioxide sent from the condenser 3. The compressor 4 sends the compressed carbon dioxide to the liquefaction heat exchanger 5.

[0018] The liquefaction heat exchanger 5 uses LNG to be supplied to a destination as fuel from the LNG tank 7 to cool and liquefy the carbon dioxide sent from the compressor 4. This allows heat exchange between the LNG and the carbon dioxide. The liquefaction heat exchanger 5 is provided with a pipe through which the LNG for cooling the carbon dioxide passes. The liquefaction heat exchanger 5 sends the liquefied carbon dioxide to the liquefied carbon dioxide tank 6. The LNG used for cooling in the liquefaction heat exchanger 5 is supplied to the destination as fuel. The destination may be the fuel cell 2 or any other device. The LNG to be liquefied may be cooled in advance by a cooler or the like before being sent to the liquefaction heat exchanger 5.

[0019] The liquefied carbon dioxide tank 6 stores the carbon dioxide liquefied by the liquefaction heat exchanger 5. The liquefied carbon dioxide tank 6 may be installed in a removable manner. This allows the liquefied carbon dioxide tank 6 in which the liquefied carbon dioxide is stored to be transported, making it easier to handle the liquefied carbon dioxide.

[0020] The LNG tank 7 is a tank for storing LNG and is a supply source for supplying LNG. The LNG stored in the LNG tank 7 is used as fuel at a supply destination such as the fuel cell 2, and is also used as a refrigerant for cooling carbon dioxide in the liquefaction heat exchanger 5. The number of LNG tanks provided is not limited to one, and any number of LNG tanks may be provided.

[0021] The pump 8 is a device for pumping LNG from the LNG tank 7 to a supply destination. The number of pumps 8 provided and the location of the pumps 8 may be any number in the path for supplying LNG from the LNG tank 7 to a supply destination.

[0022] The bypass valve 9 is provided in a path for supplying LNG from the LNG tank 7 to a supply destination, and is a device for forming a bypass path for supplying LNG to the supply destination while bypassing the liquefaction heat exchanger 5. By closing the bypass valve 9, the LNG supplied from the LNG tank 7 is supplied to the supply destination via the liquefaction heat exchanger 5. By opening the bypass valve 9, a part or all of the LNG supplied from the LNG tank 7 is supplied to the supply destination without passing through the liquefaction heat exchanger 5. The bypass valve 9 may have a function for adjusting its aperture. This makes it possible to adjust the amount of LNG supplied from the LNG tank 7 to the liquefaction heat exchanger 5. For example, the opening / closing or aperture operation of the bypass valve 9 is controlled by the control device 1. In addition to the bypass valve 9, a valve for adjusting the amount of LNG supplied to the liquefaction heat exchanger 5 may be provided.

[0023] The heater 10 is a device for heating LNG to natural gas at a temperature suitable for the supply destination. When LNG is fed to the heater 10 via the liquefaction heat exchanger 5, the heater 10 further heats the LNG heated in the liquefaction heat exchanger 5. When LNG is fed to the heater 10 without passing through the liquefaction heat exchanger 5, the heater 10 heats LNG supplied directly from the LNG tank 7. Therefore, when LNG is supplied to the heater 10 via the liquefaction heat exchanger 5, the heater 10 can reduce the amount of heating of the LNG by the amount heated in the liquefaction heat exchanger 5.

[0024] The gas sensor 11 is a sensor for determining the amount of nitrogen inside the liquefaction heat exchanger 5. The gas sensor 11 is not limited to a sensor capable of detecting nitrogen, as long as it can determine the amount of nitrogen inside the liquefaction heat exchanger 5. For example, the gas sensor 11 may be a sensor for detecting the concentration of carbon dioxide. In this case, the amount of nitrogen inside the liquefaction heat exchanger 5 is determined based on the concentration remaining after subtracting the amount of carbon dioxide (concentration detected by the gas sensor 11) from the total amount of gas inside the liquefaction heat exchanger 5 (concentration 100%). The detection value detected by the gas sensor 11 is transmitted to the control device 1.

[0025] The exhaust valve 12 is a valve for exhausting nitrogen accumulated inside the liquefaction heat exchanger 5. Nitrogen is not liquefied by cooling by the liquefaction heat exchanger 5, and therefore accumulates inside the liquefaction heat exchanger 5. For example, when the amount of nitrogen determined based on the detection value by the gas sensor 11 exceeds a predetermined threshold, the exhaust valve 12 is opened. The exhaust valve 12 may have a function for adjusting its opening degree. For example, the opening / closing or opening degree of the exhaust valve 12 is controlled by the control device 1. As a result, the nitrogen accumulated inside the liquefaction heat exchanger 5 is released into the atmosphere. The gas released from the exhaust valve 12 may contain carbon dioxide. Furthermore, a device (such as a filter) for suppressing the release of carbon dioxide may be provided.

[0026] The pressure sensor 13 is a sensor for detecting the internal pressure of the liquefaction heat exchanger 5. The pressure sensor 13 transmits the detected pressure value to the control device 1. The pressure value detected by the pressure sensor 13 is used to control the liquefaction process of carbon dioxide.

[0027] The liquid level sensor 14 is a sensor for detecting the liquid level (height of the liquid surface) of the liquefied carbon dioxide accumulated in the liquefaction heat exchanger 5. The liquid level of the liquefied carbon dioxide indicates the amount of liquefied carbon dioxide. The liquid level sensor 14 transmits the detected liquid level to the control device 1. The liquid level detected by the liquid level sensor 14 is used to control the carbon dioxide liquefaction process. Note that the sensor is not limited to the liquid level sensor 14, and any sensor may be used as long as it detects a physical quantity that indicates the amount of liquefied carbon dioxide.

[0028] Next, the control of the carbon dioxide liquefaction process by the control device 1 will be described. The control device 1 controls the carbon dioxide liquefaction process so as to balance the cold energy of LNG (LNG cold energy) and the energy for liquefying carbon dioxide (liquefaction energy). When the balance between the cold energy of LNG and the liquefaction energy for liquefying the fed carbon dioxide is maintained, the pressure value inside the liquefaction heat exchanger 5 and the amount of liquefied carbon dioxide (liquid level) always fall within a certain range. The control device 1 controls the carbon dioxide liquefaction device 20 to maintain this state. Specifically, the control device 1 adjusts the inflow amount of LNG so that the pressure value detected by the pressure sensor 13 falls within a predetermined range and the liquid level detected by the liquid level sensor 14 falls within a predetermined range.

[0029] The control device 1 determines the status of the carbon dioxide liquefaction process based on the internal pressure of the liquefaction heat exchanger 5 detected by the pressure sensor 13 and the liquid level of the liquefied carbon dioxide detected by the liquid level sensor 14.

[0030] If the LNG cold energy is greater than the liquefaction energy estimated from the amount of carbon dioxide fed, the liquefaction process of carbon dioxide may proceed faster than expected, and the carbon dioxide may turn into dry ice. In this case, the liquid level of the liquefied carbon dioxide rises, and the internal pressure of the liquefaction heat exchanger 5 decreases.

[0031] If the LNG cold energy is less than the liquefaction energy estimated from the amount of carbon dioxide fed, the liquefaction process of carbon dioxide will be slower than expected. In this case, the liquid level of the liquefied carbon dioxide will drop, and the internal pressure of the liquefaction heat exchanger 5 will increase.

[0032] When the control device 1 determines that the LNG cold energy is greater than the expected liquefaction energy based on the pressure value detected by the pressure sensor 13 and the liquid level detected by the liquid level sensor 14, it controls to reduce the amount of LNG that flows in. Specifically, the control device 1 opens the bypass valve 9 or controls to increase the opening degree of the bypass valve 9. Note that the control device 1 may control the pump 8 to reduce the amount of LNG taken out from the LNG tank 7, or may control the operation of the fuel cell 2 to increase the amount of carbon dioxide that is output from the fuel cell 2.

[0033] When the control device 1 determines that the LNG cold energy is less than the expected liquefaction energy based on the pressure value detected by the pressure sensor 13 and the liquid level detected by the liquid level sensor 14, it controls to increase the inflow of LNG. Specifically, the control device 1 closes the bypass valve 9 or controls to reduce the opening of the bypass valve 9. Note that the control device 1 may control the pump 8 to increase the amount of LNG taken out from the LNG tank 7, or may control the operation of the fuel cell 2 to reduce the amount of carbon dioxide output from the fuel cell 2.

[0034] In addition to the control performed to balance the LNG cold energy and liquefaction energy described above, the control device 1 constantly controls the discharge of nitrogen to prevent nitrogen from accumulating inside the liquefaction heat exchanger 5. Nitrogen tends to accumulate in the upper part of the liquefaction heat exchanger 5. When the control device 1 determines, based on the detection value by the gas sensor 11, that nitrogen has accumulated inside the liquefaction heat exchanger 5, it opens the discharge valve 12.

[0035] If nitrogen accumulates inside the liquefaction heat exchanger 5, the internal pressure of the liquefaction heat exchanger 5 will increase regardless of the liquid level of the liquefied carbon dioxide. Therefore, the control device 1 may use the detection value by the pressure sensor 13 or the liquid level sensor 14 in addition to the detection value by the gas sensor 11 to determine whether nitrogen has accumulated.

[0036] If the gas sensor 11 is a sensor that detects nitrogen, the control device 1 outputs an open command to the exhaust valve 12 when the value detected by the gas sensor 11 exceeds a predetermined reference value. If the gas sensor 11 is a sensor that detects carbon dioxide, the control device 1 outputs an open command to the exhaust valve 12 when the value detected by the gas sensor 11 falls below a predetermined reference value.

[0037] An optional cooler for cooling carbon dioxide for liquefaction may be provided at least in one of the upstream and downstream stages of the liquefaction heat exchanger 5. The cooling method of the cooler provided in this manner is not limited to using LNG cold energy, and any cooling method may be adopted. For example, such a cooler is used when the liquefaction heat exchanger 5 alone is not sufficient to cool the carbon dioxide.

[0038] According to this embodiment, carbon dioxide can be liquefied by using the liquefied gas (LNG, LPG, or the like) that is the fuel for the fuel cell 2 as cold energy to cool and liquefy the carbon dioxide discharged from the fuel cell 2. Furthermore, by adjusting the amount of LNG used as cold energy in accordance with the amount of carbon dioxide discharged from the fuel cell 2, carbon dioxide can be efficiently liquefied.

[0039] Second Embodiment Fig. 2 is a configuration diagram showing the configuration of a ship 30 according to a second embodiment of the present invention. The ship 30 is equipped with the carbon dioxide liquefaction device 20 according to the first embodiment.

[0040] In this embodiment, some of the components of the carbon dioxide liquefaction device 20 will be omitted from the description, but the ship 30 may include all of the components of the carbon dioxide liquefaction device 20, or some of the components that are not necessarily required for liquefying carbon dioxide may be removed. Further, duplicate descriptions of the components described in the first embodiment will be omitted as appropriate. The arrangement of the components in the ship 30 shown in FIG. 2 is an example, and the components may be arranged in any manner, and the ship 30 may have any configuration or shape.

[0041] The ship 30 is a ship that obtains propulsion power from electric power generated by the fuel cell 2. The ship 30 may be any ship as long as it has facilities for storing LNG. For example, the ship 30 may be an LNG ship intended for transporting LNG, or a ship that uses LNG as fuel. Furthermore, LNG may be used as fuel other than for the fuel cell 2.

[0042] The ship 30 is equipped with a control device 1, a fuel cell 2, a condenser 3, a compressor 4, a liquefaction heat exchanger 5, a liquefied carbon dioxide tank 6, an LNG tank 7, a pump 8, a bypass valve 9, a vaporizer 15, a distribution panel 21, a propulsion motor 22, a diesel generator 23, a compressor 24, and a heater 25.

[0043] Although not shown in Fig. 2, the control device 1 is provided in, for example, a control room inside the accommodation area including the bridge, etc. The control device 1 may be implemented as a part of the function of any device or system in the ship 30.

[0044] The fuel cell 2 generates electricity using as fuel LNG supplied from the LNG tank 7 and boil-off gas (BOG) generated in the LNG tank 7. BOG is gaseous natural gas that is generated when a portion of the LNG stored in the LNG tank 7 is vaporized by external heat. The fuel cell 2 supplies the generated electricity to a propulsion motor 22 via a switchboard 21.

[0045] The LNG tank 7 is a tank for storing LNG on board the ship. The LNG tank 7 may be a fuel tank for storing LNG as fuel, or may be a tank for transporting LNG.

[0046] A pump 8 supplies LNG stored in an LNG tank 7 to the fuel cell 2 via a liquefaction heat exchanger 5 and a vaporizer 15 in that order. A bypass valve 9 is provided in a path between the pump 8 and the vaporizer 15. The LNG passing through the bypass valve 9 bypasses the liquefaction heat exchanger 5 and is sent to the vaporizer 15.

[0047] The vaporizer 15 is a device that forcibly vaporizes LNG to generate natural gas. The natural gas generated by the vaporizer 15 is supplied to the fuel cell 2. The vaporizer 15 may be a device that corresponds to the heater 10 according to the first embodiment.

[0048] The switchboard 21 supplies the electric power generated by the fuel cell 2 and the diesel generator 23 to the propulsion motor 22. The switchboard 21 has a plurality of switches for selecting the electric power supply source and the supply destination, respectively. This allows the switchboard 21 to form a path (wiring) that selects any combination of supply source and supply destination.

[0049] The propulsion motor 22 is an electric motor that serves as a power source that supplies power to drive a propulsion device using electric power supplied from the fuel cell 2 and the diesel generator 23. The propulsion device is a device that generates propulsive power for the vessel 30. The propulsion device may be any device that can generate propulsive power for the vessel 30. For example, the propulsion motor 22 rotates the rotary shaft of the propulsion device.

[0050] The diesel generator 23 is a generator that generates electricity by burning fuel. The diesel generator 23 may be oil-fired or gas-fired. The diesel generator 23 may be supplied with LNG as fuel from the LNG tank 7, or may be supplied with fuel from a fuel tank provided separately from the LNG tank 7. The generator 23 is not limited to a diesel type, and may be any combustion type generator. The electric power generated by the diesel generator 23 is supplied to the propulsion motor 22 via the switchboard 21. For example, the diesel generator 23 is used to supply electric power to the propulsion motor 22 when the ship 30 is navigating within a harbor or during standby.

[0051] The compressor 24 compresses the BOG generated in the LNG tank 7. The compressor 24 sends the compressed BOG to the heater 25.

[0052] The heater 25 supplies the compressed BOG sent from the compressor 24 to the fuel cell 2 as fuel.

[0053] According to this embodiment, a ship 30 can be constructed to which the carbon dioxide liquefaction device 20 according to the first embodiment is applied, and the same effects as those of the first embodiment can be obtained.

[0054] Third Preferred Embodiment FIG. 3 is a configuration diagram showing the configuration of a vessel 30A according to a third preferred embodiment of the present invention.

[0055] The vessel 30A has a configuration in which the propulsion motor 22 in the vessel 30 according to the second embodiment is replaced with a propulsion motor 22A, a main engine 26 and the heater 10 according to the first embodiment are added, and the carburetor 15 is removed. In other respects, the vessel 30A is similar to the vessel 30 according to the second embodiment.

[0056] The main engine 26 is an engine that serves as a power source that combusts LNG supplied from the LNG tank 7 and supplies power to drive the propellers. For example, the main engine 26 rotates the rotary shaft of the propeller. The main engine 26 is, for example, a diesel engine, but is not limited to a diesel engine and may be any combustion engine.

[0057] The heater 10 heats the LNG that is sent to the main engine 26. The heater 10 heats the LNG to a temperature that is suitable for use as fuel for the main engine 26.

[0058] The propulsion motor 22A is an electric motor (power source) that supplements the power supplied to the propulsion unit from the main engine 26. The propulsion motor 22A supplies power to drive the propulsion unit using electric power supplied from the switchboard 21. For example, the propulsion motor 22A supplies power to supplement the rotation of the rotary shaft of the propulsion unit, which rotates using the power supplied from the main engine 26.

[0059] The vessel 30A may be configured to supply power from the switchboard 21 to any equipment on board without providing the propulsion motor 22A.

[0060] According to this embodiment, in the ship 30A having a diesel engine as the main engine 26, the same effects as those of the second embodiment can be obtained.

[0061] Fourth Preferred Embodiment FIG. 4 is a configuration diagram showing the configuration of a vessel 30B according to a fourth preferred embodiment of the present invention.

[0062] The ship 30B has a configuration in which a heat exchanger 27 is added to the ship 30A according to the third embodiment. In addition, a path is additionally formed so that the BOG compressed by the compressor 24 is supplied as fuel to the main engine 26 via the heater 10. In other respects, the ship 30B is similar to the ship 30A according to the third embodiment. Furthermore, like the ship 30A according to the third embodiment, the ship 30B may be configured to supply electric power from the switchboard 21 to any equipment on board the ship without providing a propulsion motor 22A.

[0063] The BOG compressed by the compressor 24 is heated by the heater 10 and supplied to the main engine 26. The main engine 26 is driven by the BOG as fuel in addition to the LNG supplied from the LNG tank 7.

[0064] The heat exchanger 27 acts as a cooler that pre-cools the carbon dioxide before it is cooled (liquefied) by the liquefaction heat exchanger 5. The heat exchanger 27 is provided midway along the path in which the carbon dioxide is sent from the compressor 4 to the liquefaction heat exchanger 5. The heat exchanger 27 is provided with a pipe through which BOG passes to cool the carbon dioxide. A pipe for sending BOG from the LNG tank 7 to the compressor 24 is formed so as to pass through the heat exchanger 27.

[0065] The heat exchanger 27 uses the cold energy of the BOG to cool the carbon dioxide sent from the compressor 4. This allows heat exchange between the BOG and the carbon dioxide. The heat exchanger 27 sends the cooled carbon dioxide to the liquefaction heat exchanger 5. The BOG used for cooling in the heat exchanger 27 is supplied as fuel to the fuel cell 2 via the compressor 24 and heater 25.

[0066] According to this embodiment, by providing the heat exchanger 27 that uses BOG to cool carbon dioxide, it is possible to improve the efficiency of cooling carbon dioxide compared to the third embodiment.

[0067] Furthermore, by using the BOG generated in the LNG tank 7 as fuel for the main engine 26, the fuel cost for the main engine 26 can be reduced.

[0068] Fifth Preferred Embodiment FIG. 5 is a configuration diagram showing the configuration of a vessel 30C according to a fifth preferred embodiment of the present invention.

[0069] The vessel 30C has a configuration in which the path for supplying BOG from the compressor 24 to the main engine 26 is removed from the vessel 30B according to the fourth embodiment, and a vaporizer 15C is added. In other respects, the vessel 30C is similar to the vessel 30A according to the third embodiment. Furthermore, like the vessel 30A according to the third embodiment, the vessel 30C may be configured to supply electric power from the switchboard 21 to any equipment on board the vessel, without providing a propulsion motor 22A.

[0070] The vaporizer 15C has the same configuration as the vaporizer 15 according to the second embodiment, and therefore differences will be mainly described here. The vaporizer 15C is a heat exchanger that uses liquefied carbon dioxide as a heat medium to heat the LNG supplied to the fuel cell 2. This allows heat exchange between the carbon dioxide and the LNG.

[0071] The vaporizer 15C is provided midway along the path along which the LNG used as a refrigerant by the liquefaction heat exchanger 5 or the LNG that has bypassed the liquefaction heat exchanger 5 and passed through the bypass valve 9 is supplied to the fuel cell 2. The vaporizer 15C is provided with a pipe through which carbon dioxide passes to heat the LNG. A pipe for sending the carbon dioxide cooled by the heat exchanger 27 to the liquefaction heat exchanger 5 is formed so as to pass through the vaporizer 15C.

[0072] The vaporizer 15C vaporizes the LNG by heating it using carbon dioxide as a heat medium. The vaporizer 15C supplies natural gas obtained by vaporizing the LNG to the fuel cell 2. The carbon dioxide used as a heat medium in the vaporizer 15C is sent to the liquefaction heat exchanger 5.

[0073] According to this embodiment, the same effects as those of the fourth embodiment can be obtained, and further, by using carbon dioxide before liquefaction to vaporize LNG, which serves as fuel for the fuel cell 2, the energy efficiency of liquefying carbon dioxide and vaporizing LNG can be improved.

[0074] It is to be understood that additional advantages and modifications may readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.

Claims

1. A carbon dioxide liquefaction device comprising: a liquefaction heat exchanger that exchanges heat to liquefy carbon dioxide contained in a mixed gas discharged from a fuel cell by cooling it with the cold heat of a liquefied gas; a pressure sensor that detects the internal pressure of the liquefaction heat exchanger; a liquid volume sensor that detects the liquid volume, which is the amount of liquefied carbon dioxide liquefied in the liquefaction heat exchanger; and a control device that controls the amount of liquefied gas sent to the liquefaction heat exchanger based on the internal pressure detected by the pressure sensor and the liquid volume detected by the liquid volume sensor.

2. A carbon dioxide liquefaction device as described in claim 1, further comprising a valve for supplying the liquefied gas from a supply source of the liquefied gas to a destination without passing through the liquefaction heat exchanger, and the control device operates the valve to control the amount of the liquefied gas sent to the liquefaction heat exchanger.

3. A carbon dioxide liquefaction device as described in claim 1, further comprising: a gas sensor for determining the amount of nitrogen inside the liquefaction heat exchanger; and an exhaust valve for discharging the nitrogen inside the liquefaction heat exchanger, and the control device controls the operation of the exhaust valve so as to discharge the nitrogen inside the liquefaction heat exchanger based on the detection value by the gas sensor.

4. The carbon dioxide liquefaction device according to claim 1, characterized in that the liquefied gas that has undergone the heat exchange in the liquefaction heat exchanger is supplied to a destination as fuel.

5. The carbon dioxide liquefaction device according to claim 4, characterized in that the supply destination includes the fuel cell.

6. The carbon dioxide liquefaction device according to claim 1, further comprising a moisture removal device for removing moisture contained in the mixed gas discharged from the fuel cell in order to extract carbon dioxide.

7. The carbon dioxide liquefaction device according to claim 6, characterized in that the moisture removal device includes a condenser for removing water vapor.

8. A ship equipped with the carbon dioxide liquefaction device described in claim 1, characterized in that the ship comprises: a liquefied gas tank for storing the liquefied gas; a fuel cell for generating electricity using the liquefied gas supplied from the liquefied gas tank as fuel; and a propeller for obtaining propulsion power using the liquefied gas supplied from the liquefied gas tank as fuel.

9. The vessel according to claim 8, characterized in that the propulsion device obtains propulsive force from the electric power generated by the fuel cell.

10. The ship according to claim 8, further comprising a main engine that uses the liquefied gas supplied from the liquefied gas tank as fuel to drive the propeller.

11. A vessel according to claim 8, characterized in that the propeller is provided with a generator for supplying electric power for obtaining a propulsive force.

12. The vessel according to claim 11, characterized in that the propulsion device obtains propulsive force from the electric power generated by the fuel cell and the electric power generated by the generator.

13. A method for liquefying carbon dioxide, comprising: exchanging heat in a liquefaction heat exchanger so as to liquefy carbon dioxide contained in a mixed gas discharged from a fuel cell by cooling it with the cold heat of a liquefied gas; detecting an internal pressure of the liquefaction heat exchanger; detecting a liquid amount which is the amount of liquefied carbon dioxide liquefied in the liquefaction heat exchanger; and controlling the amount of the liquefied gas sent to the liquefaction heat exchanger based on the detected internal pressure and the detected liquid amount.

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