Abstract:
Provided are a cold heat recovery apparatus using an LNG fuel and a liquefied gas carrier, in which costs involved in the installation and operation of a reliquefying facility can be reduced by recovering and utilizing cold heat of the LNG fuel when boil-off gas of liquefied gas as a cargo is reliquefied, and energy saving and reduction of environmental pollutants can be achieved because additional energy is not consumed in a reliquefaction process. A cold heat recovery apparatus for processing boil-off gas generated in a liquefied gas tank by using cold heat of an LNG fuel includes cold heat usage means using cold heat of an LNG supplied from an LNG fuel tank storing an LNG as a fuel to an engine, so as to processing the boil-off gas generated in the liquefied gas tank.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a cold heat recovery apparatus using an LNG fuel and a liquefied gas carrier including the same, and more particularly, to a cold heat recovery apparatus using an LNG fuel and a liquefied gas carrier, in which costs involved in the installation and operation of a reliquefying facility can be reduced by recovering and utilizing cold heat of the LNG fuel when boil-off gas of liquefied gas as a cargo is reliquefied, and energy saving and reduction of environmental pollutants can be achieved because additional energy is not consumed in a reliquefaction process. 
       BACKGROUND ART 
       [0002]    Recently, there has been an increasing demand for reducing SOx, NOx and CO 2  generated during the voyage of vessels. Due to such environmental problems and high oil prices, there have been proposed and developed vessels using clean fuel, LNG, as a propulsion fuel, instead of oil such as heavy oil or diesel oil. However, the vessel using the LNG as the propulsion fuel requires a separate apparatus for enabling the LNG to be used as fuel, which increases the cost of the vessel and blocks commercialization. 
         [0003]    In order to reduce a volume of a fuel tank, the vessel using the LNG as the propulsion fuel stores the fuel as the LNG that is natural gas of a liquefied state. The fuel is evaporated before the LNG is used as the fuel. Cold heat (cold energy) generated at that time is all discharged to the outside. 
         [0004]    On the other hand, in a liquefied gas carrier such as an LPG carrier or a CO 2  carrier, liquefied gas inside a liquefied gas tank is evaporated by continuous heat invasion from the outside during voyage, and thus, an internal pressure is increased. One of methods for controlling this problem is to use a reliquefaction facility that reliquefies the generated boil-off gas. 
         [0005]    However, such a reliquefaction facility has a problem that increases the manufacturing cost of the liquefied gas carrier, makes the operation of the vessel complicated, consumes additional energy for operating the reliquefaction facility, and emits a large amount of environmental pollutants. 
       DISCLOSURE 
     Technical Problem  
       [0006]    The present invention has been made in an effort to solve the above problems, and is directed to provide a cold heat recovery apparatus and a liquefied gas carrier including the same, in which cold heat of an LNG used as fuel is recovered and used to control a pressure of liquefied gas being a cargo, thereby reducing costs involved in the installation and operation of a reliquefaction facility of the liquefied gas carrier and reducing environmental pollutants because additional fuel is not consumed in a reliquefaction process. 
       Technical Solution 
       [0007]    According to one aspect of the present invention, a cold heat recovery apparatus for processing boil-off gas generated in a liquefied gas tank by using cold heat of an LNG fuel includes: cold heat usage means using cold heat of an LNG supplied from an LNG fuel tank storing an LNG as a fuel to an engine, so as to processing the boil-off gas generated in the liquefied gas tank. 
         [0008]    The cold heat usage means may be a pressure control unit configured to control a pressure of the liquefied gas tank by using the cold heat of the LNG discharged from the LNG fuel tank, and the pressure control unit may include an LNG line installed to pass through the liquefied gas tank from the LNG fuel tank and configured to cause heat exchange between the LNG and a fluid stored in the liquefied gas tank. 
         [0009]    The pressure control unit may further include a heater installed in front of the liquefied gas tank in the LNG line. 
         [0010]    The pressure control unit may further include: a first bypass line connected to the LNG line to bypass the liquefied gas tank; a control valve configured to control a flow of the LNG passing through the first bypass line; a pressure detection unit installed in the liquefied gas tank and configured to measure a pressure inside the liquefied gas tank and output a detection signal; and a control unit configured to receive the detection signal output from the pressure detection unit and control the control valve. 
         [0011]    The cold heat recovery apparatus may further include: an evaporator installed in rear of the liquefied gas tank in the LNG line; a temperature detection unit installed in rear of the evaporator in the LNG line and configured to measure a temperature of the fluid passing through the evaporator and output a detection signal; and a control unit configured to receive the detection signal output from the temperature detection unit and control a heat quantity of the evaporator. 
         [0012]    The cold heat usage means may be a pressure control unit configured to control a pressure of the liquefied gas tank by using the cold heat of the LNG discharged from the LNG fuel tank, and the pressure control unit may include: an LNG line configured to transfer the LNG from the LNG fuel tank; a heat exchanger installed in the LNG line; and a coolant circulation line configured to circulate and supply a coolant to the heat exchanger and installed to pass through the liquefied gas tank. 
         [0013]    The pressure control unit may further include: a make-up tank configured to supply the coolant to the LNG line; a pressure detection unit installed in the liquefied gas tank and configured to measure a pressure inside the liquefied gas tank and output a detection signal; and a control unit configured to receive the detection signal output from the pressure detection unit and control the supply of the coolant from the make-up tank to the LNG line. 
         [0014]    The pressure control unit may further include: a coolant pump configured to provide a pumping force for circulating the coolant through the coolant circulation line; a pressure detection unit installed in the liquefied gas tank and configured to measure a pressure inside the liquefied gas tank and output a detection signal; and a control unit configured to receive the detection signal output from the pressure detection unit and control the pumping force of the coolant pump provided with a variable pump. 
         [0015]    The pressure control unit may further include: a second bypass line connected to the LNG line to bypass the heat exchanger; a control valve configured to control a flow of the LNG passing through the second bypass line; a pressure detection unit installed in the liquefied gas tank and configured to measure a pressure inside the liquefied gas tank and output a detection signal; and a control unit configured to receive the detection signal output from the pressure detection unit and control the control valve. 
         [0016]    The cold heat usage means may be a reliquefaction tank configured to reliquefy boil-off gas, which is supplied from the liquefied gas tank storing the liquefied gas through a gas pipeline, by using the cold heat of the LNG discharged from the LNG fuel tank, and return the reliquefied boil-off gas to the liquefied gas tank through a liquid pipeline. 
         [0017]    An LNG line configured to discharge the LNG from the LNG fuel tank may be installed to pass through the reliquefaction tank, such that the boil-off gas is reliquefied by heat exchange with the LNG. 
         [0018]    The cold heat recovery apparatus may further include: a gas manifold configured to supply the reliquefaction tank with boil-off gas supplied from each of a plurality of liquefied gas tanks; and a liquid manifold configured to distribute the reliquefied gas discharged from the reliquefaction tank to each of the liquefied gas tanks and return the reliquefied gas thereto. 
         [0019]    The cold heat usage means may further include a coolant circulation unit configured to transfer the cold heat of the LNG discharged from the LNG fuel tank through the circulating coolant to the reliquefaction tank. 
         [0020]    The coolant circulation unit may include: a heat exchanger installed in the LNG line through which the LNG is discharged from the LNG fuel tank; and a coolant circulation line configured to circulate and supply the coolant to the heat exchanger and installed to pass through the reliquefaction tank. 
         [0021]    The cold heat recovery apparatus may further include: a bypass line connected to the LNG line to bypass the reliquefaction tank; and a control valve configured to control a flow of the LNG through the bypass line. 
         [0022]    The cold heat recovery apparatus may further include: a detection unit configured to measure a temperature or pressure inside the reliquefaction tank and output a detection signal; and a control unit configured to receive the detection signal output from the detection unit and control the control valve. 
         [0023]    The cold heat usage means may be a heat exchanger configured to perform heat exchange between boil-off gas, which is supplied from a liquefied gas tank storing the liquefied gas through a gas pipeline, and the LNG, which is discharged from the LNG fuel tank, and return the boil-off gas to the liquefied gas tank through a liquid pipeline. 
         [0024]    The cold heat recovery apparatus may further include: a heater installed to provide heat to an inside of the heat exchanger; a temperature detection unit installed in the liquid pipeline and configured to measure a temperature of the reliquefied gas and output a detection signal; 
         [0025]    and a control unit configured to receive the detection signal of the temperature detection unit and control the heater. 
         [0026]    According to another aspect of the present invention, a liquefied gas carrier for transporting liquefied gas includes: a storage tank capable of storing the liquefied gas as a cargo; an LNG fuel tank configured to store an LNG as a fuel; and the above-described cold heat recovery apparatus using the LNG fuel. The liquefied gas stored as the cargo may be LPG or CO 2 . 
       Advantageous Effects  
       [0027]    According to the present invention, since the LNG is used as fuel, it is possible to reduce the emission of pollutants into the atmosphere. Furthermore, since the LNG fuel is used to control the pressure of the liquefied gas tank, the cost of the reliquefaction facility installed in the existing liquefied gas carrier can be reduced, the operation thereof can be simplified, and emission of CO 2  generated during the operation of the reliquefaction facility can be reduced. 
         [0028]    In addition, according to the present invention, since the pressure of the liquefied gas tank is controlled using the LNG, the thickness of the liquefied gas tank is not increased, resulting in a reduction in the manufacturing cost of the liquefied gas tank. 
         [0029]    In addition, according to the present invention, since the cold heat of the LNG is used to reduce the temperature or pressure, of the liquefied gas, the cold heat can be recovered. Therefore, energy introduced through the evaporator for the purpose of use as the fuel can be reduced. Since the volume of the evaporator is reduced, the cost and energy consumption can be reduced. 
         [0030]    Furthermore, according to the present invention, since energy necessary for driving the reliquefaction facility is minimized, the operation cost and the installation cost can be reduced. 
         [0031]    Moreover, according to the present invention, the reliquefaction facility is configured in a forcible transmission method, it is possible to remove pipeline plugging caused by the sublimation of the boil-off gas that may be generated within the heat exchanger. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  is a configuration diagram illustrating a cold heat recovery apparatus using an LNG fuel according to a first embodiment of the present invention. 
           [0033]      FIG. 2  is a configuration diagram illustrating a cold heat recovery apparatus using an LNG fuel according to a second embodiment of the present invention. 
           [0034]      FIG. 3  is a configuration diagram illustrating a cold heat recovery apparatus using an LNG fuel according to a third embodiment of the present invention. 
           [0035]      FIG. 4  is a configuration diagram illustrating a cold heat recovery apparatus using an LNG fuel according to a fourth embodiment of the present invention. 
           [0036]      FIG. 5  is a configuration diagram illustrating a cold heat recovery apparatus using an LNG fuel according to a fifth embodiment of the present invention. 
           [0037]      FIG. 6  is a configuration diagram illustrating a cold heat recovery apparatus using an LNG fuel according to a sixth embodiment of the present invention. 
           [0038]      FIG. 7  is a configuration diagram illustrating a cold heat recovery apparatus using an LNG fuel according to a seventh embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0039]    Hereinafter, configurations and operations of preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In addition, the following embodiments can be modified in various forms and are not intended to limit the scope of the present invention. 
         [0040]      FIG. 1  is a configuration diagram illustrating a cold heat recovery apparatus using an LNG fuel according to a first embodiment of the present invention. 
         [0041]    As illustrated in  FIG. 1 , the cold heat recovery apparatus  100  using the LNG fuel according to the first embodiment of the present invention may include an LNG fuel tank  110  configured to store an LNG as a fuel, and a pressure control unit  120  serving as cold heat usage means configured to control a pressure of a liquefied gas tank  10  by using a low-temperature characteristic, that is, cold heat of the LNG discharged from the LNG fuel tank  110 . Herein, a liquefied gas tank  10  may store CO 2  or hydrocarbon liquefied gas such as an LPG, and other various liquefied gases as a cargo. 
         [0042]    The LNG fuel tank  110  is made of a container for withstanding a pressure and low-temperature of the LNG, and stores the LNG used as a fuel of a liquefied gas carrier. 
         [0043]    The pressure control unit  120  may include an LNG line  121  and a first pump  122 . The LNG line  121  is installed to extend from the LNG fuel tank  110  and pass through the liquefied gas tank  10 , such that heat exchange is performed between the LNG and the fluid stored in the liquefied gas tank  10 . The first pump  122  is installed on the LNG line  121  to provide a pumping force for transferring the LNG through the LNG line  121 . 
         [0044]    A coil portion  123  having a coil shape may be formed in a portion of the LNG line  121  which passes through the inside of the liquefied gas tank  10 . Therefore, the coil portion  123  of the LNG line  121  increases a pipeline length or a pipeline area inside the liquefied gas tank  10 , which can increase the efficiency of heat exchange with the fluid stored in the liquefied gas tank  10 . In addition, a rise in the pressure of the liquefied gas tank  10  can be efficiently reduced. 
         [0045]    Furthermore, a temperature of the LNG within the LNG fuel tank  110  is about −163° C., and the temperature of the LNG slightly rises while the LNG passes through the first pump  122 . While the LNG is stored at about −163° C., liquefied gas as a cargo, for example, CO 2 , is stored at −50 to −20° C., and the LPG is stored at about −50° C. Therefore, the temperature of the LNG rises when the LNG passes through the liquefied gas tank  10 , and then, the LNG has a condition suitable for an engine when the LNG passes through an evaporator  130  to be described below. 
         [0046]    The LNG line  121  may pass through a portion where gas is located within the liquefied gas tank  10 . Therefore, a rise in the pressure of the liquefied gas tank  10  is suppressed by reliquefying gases close to the cold LNG line  121  within the liquefied gas tank  10 . 
         [0047]    The LNG line  121  may pass through a liquid portion, that is, a portion where a liquid is located within the liquefied gas tank  10 . Since a temperature of the liquefied gas within the liquefied gas tank  10  is lowered by the cold LNG line  121 , a temperature of a free surface of the liquid is lowered, and thus, a saturation pressure on the free surface is lowered. Therefore, the pressure inside the liquefied gas tank  10  can be controlled. 
         [0048]    The pressure control unit  120  may further include a heater  124  installed in the front of the liquefied gas tank  10  in the LNG line  121 . The heater  124  increases the temperature of the LNG supplied to the liquefied gas tank  10  to some degree, so as to prevent thermal deformation of the liquefied gas tank  10  caused when the temperature of the LNG is too low, or prevent the liquefied gas from condensing around the LNG line  121  and becoming frost. 
         [0049]    The pressure control unit  120  may further include a first bypass line  125  and a control valve  126 . The first bypass line  125  is connected to the LNG line  121  to bypass the liquefied gas tank  10 . The control valve  126  is installed in the LNG line  121  and the first bypass valve  125  to control the flow of the LNG through the liquefied gas tank  10  and the first bypass line  125 . Therefore, the supercooling of the liquefied gas tank  10  is prevented by supplying the liquefied gas tank  10  with the LNG fuel such that only an amount of LNG fuel necessary for controlling the pressure of the liquefied gas tank  10  is used as a coolant. On the other hand, a pair of control valves  126  may be installed in the LNG line  121  and the first bypass line  125  one by one as in the present embodiment, but the present invention is not limited thereto. For example, a three-way valve may be used. 
         [0050]    The pressure control unit  120  may further include a pressure detection unit  127  and a control unit  128 . The pressure detection unit  127  is installed in the liquefied gas tank  10  to measure the pressure inside the liquefied gas tank  10  and output a detection signal. The control unit  128  receives the detection signal output from the pressure detection unit  127  and controls the control valve  126 . The control unit  128  measures the pressure inside the liquefied gas tank  10  through the pressure detection unit  127 . When the measured pressure is increased to above a set pressure, the control unit  128  may control the control valve  126  such that the LNG passes through the liquefied gas tank  10 . When the measured pressure is decreased to the set pressure, the control unit  128  may control the control valve  126  such that the LNG bypasses the liquefied gas tank  10 . 
         [0051]    An evaporator  130  may be installed in the rear of the liquefied gas tank  10  in the LNG line  121 . Therefore, the LNG has a condition suitable for an engine by evaporating the LNG having passed through the liquefied gas tank  10 . 
         [0052]    The cold heat recovery apparatus  100  using the LNG fuel according to the first embodiment of the present invention may further include a temperature detection unit  140  and a control unit  128 . The temperature detection unit  140  is installed in the rear of the evaporator  130  in the LNG line  121  to measure the temperature of the fluid having passed through the evaporator  130  and output a detection signal. The control unit  128  receives the detection signal output from the temperature detection unit  140  and controls a heat quantity of the evaporator  130 . When the temperature of the evaporated LNG measured in the rear of the evaporator  130  by the temperature detection unit  140  is too low to use the LNG as the fuel, the control unit  128  increases the heat quantity of the evaporator  130 . On the contrary, when the temperature of the evaporated LNG is high, the control unit  128  reduces the heat quantity of the evaporator  130 . The control unit  128  can control the heat quantity of the evaporator  130  by controlling a heat supply unit  131  that supplies the evaporator  130  with heat by electricity or through the medium of liquid or gas. 
         [0053]      FIG. 2  is a configuration diagram illustrating a cold heat recovery apparatus using an LNG fuel according to a second embodiment of the present invention. 
         [0054]    As illustrated in  FIG. 2 , like the first embodiment, the cold heat recovery apparatus  200  using the LNG fuel according to the second embodiment of the present invention may include an LNG fuel tank  210  and a pressure control unit  220  serving as cold heat usage means. The following description will focus on differences from the cold heat recovery apparatus  100  using the LNG fuel according to the first embodiment of the present invention. 
         [0055]    The pressure control unit  220  may include an LNG line  221 , a second pump  222 , a heat exchanger  223 , a coolant circulation line  224 , and a coolant pump  225 . The LNG line  221  is configured to transfer the LNG fuel from the LNG fuel tank  10 . The second pump  222  is configured to provide a pumping force for transferring the LNG fuel through the LNG line  221 . The heat exchanger  223  is installed in the LNG line  221 . The coolant circulation line  224  is installed to pass through the liquefied gas tank  10  and configured to circulate the coolant and supply the coolant to the heat exchanger  223 . The coolant pump  225  is configured to provide a pumping force for circulating the coolant through the coolant circulation line  224 . The coolant is cooled by the heat exchange with the LNG passing through the LNG line  221 , and the cooled coolant passes through the liquefied gas tank  10  by way of the coolant circulation line  224 . In this manner, since cold heat is supplied to the liquefied gas tank  10 , the liquefied gas inside the liquefied gas tank is cooled, and the pressure inside the liquefied gas tank  10  is controlled. 
         [0056]    Preferably, the coolant has a freezing point (melting point) lower than the temperature of the LNG so as not to be condensed. Since the temperature of the coolant inside the closed loop, that is, the coolant circulation line  224 , is higher than the temperature of the LNG, a thermal stress of the liquefied gas tank  10  can be reduced. In addition, it is possible to avoid a condensation problem in the vicinity of the pipeline due to the liquefied gas having a higher melting point than the LNG, for example, CO 2  or LPG. 
         [0057]    A coil portion  232  having a coil shape may be formed in a portion of the coolant circulation line  224  which is inserted into the liquefied gas tank  10 . 
         [0058]    The pressure control unit  220  may include a make-up tank  226 , a pressure detection unit  227 , and a control unit  228 . The make-up tank  226  supplies the coolant to the LNG line  221 . The pressure detection unit  227  is installed in the liquefied gas tank  10  to measure the pressure inside the liquefied gas tank  10  and output a detection signal. The control unit  228  receives the detection signal output from the pressure detection unit  227  and controls the supply of the coolant from the make-up tank  226  to the LNG line  221 . The control unit  228  can control an amount of coolant supplied from the make-up tank  226  to the LNG line  221  by a valve or a separate pump. 
         [0059]    When the pressure of the liquefied gas tank  10  measured through the pressure detection unit  227  is increased, the control unit  228  discharges the coolant from the make-up tank  226 , so that a flow rate of the coolant is increased. Thus, more heat can be removed in the heat exchanger  223 , and a large heat quantity can be removed from the liquefied gas tank  10 . 
         [0060]    The pressure control unit  220  may further include a pressure detection unit  227  and a control unit  228 . The pressure detection unit  227  is installed in the liquefied gas tank  10  to measure the pressure inside the liquefied gas tank  10  and output a detection signal. The control unit  228  receives the detection signal output from the pressure detection unit  127  and controls a pumping force of the coolant pump  225  provided with a variable pump. Therefore, the control unit  228  can control a coolant flow speed of the closed loop such that the pressure inside the liquefied gas tank  10  measured by the pressure detection unit  227  has a set pressure. 
         [0061]    The pressure control unit  220  may further include a second bypass line  229 , a control valve  230 , a pressure detection unit  227 , and a control unit  228 . The second bypass line  229  is connected to the LNG line  221  to bypass the heat exchanger  223 . The control valve  230  is installed in the LNG line  221  and the second bypass line  229  to control the flow of the LNG through the heat exchanger  223  and the second bypass line  229 . The pressure detection unit  227  is installed in the liquefied gas tank  10  to measure the pressure inside the liquefied gas tank  10  and output a detection signal. The control unit  228  receives the detection signal output from the pressure detection unit  227  and controls the control valve  230 . When the pressure inside the liquefied gas tank  10  measured by the pressure detection unit  227  is increased, the control unit needs to control the control valve  230  so as to remove a large heat quantity of the liquefied gas by the heat exchanger  223 . Therefore, the control unit  228  reduces the flow rate of the LNG bypassing the heat exchanger  223  by the second bypass line  229 . In the opposite case, the control unit  228  increases the flow rate of the LNG bypassing the heat exchanger  223 . On the other hand, the control valve  230  may be provided with a pair of valves like the present embodiment, but the present invention is not limited thereto. For example, a single three-way valve may be used as the control valve  230 . 
         [0062]    Like the cold heat recovery apparatus  100  using the LNG fuel according to the first embodiment, the cold heat recovery apparatus  200  using the LNG fuel according to the second embodiment of the present invention may include an evaporator  240 , a heat supply unit  241 , and a temperature detection unit  250 . 
         [0063]      FIG. 3  is a configuration diagram illustrating a cold heat recovery apparatus using an LNG fuel according to a third embodiment of the present invention. 
         [0064]    As illustrated in  FIG. 3 , the cold heat recovery apparatus  300  using the LNG fuel according to the third embodiment of the present invention may include a fuel tank  310  and a reliquefaction tank  320  serving as cold heat usage means. 
         [0065]    The LNG fuel tank  310  is a tank for storing an LNG fuel of a liquefied gas carrier and is made of a container for withstanding a pressure and low-temperature of the LNG. 
         [0066]    The reliquefaction tank  320  reliquefies boil-off gas supplied from the liquefied gas tank  10  storing the liquefied gas through a gas pipeline  321  by using cold heat of the LNG discharged from the LNG fuel tank  310 , and supplies the reliquefied boil-off gas through a liquid pipeline  322  to the liquefied gas tank  10 . 
         [0067]    An LNG line  311  configured to transfer the LNG as the fuel discharged from the LNG fuel tank  310  is installed to pass through the reliquefaction tank  320 . The reliquefaction tank  320  reliquefies the boil-off gas by the heat exchange with the LNG. The reliquefaction tank  320  may be installed at a position higher than the liquefied gas tank  10  such that reliquefied gas obtained by the heat exchange between the boil-off gas and the LNG is supplied through the liquid pipeline  322  to the liquefied gas tank  10  by gravity. 
         [0068]    The LNG line  311  provides a passage for supplying the LNG to the engine or the like. In the LNG line  311 , an LNG pump  311   a  is installed for pumping the LNG. A coil portion  311   b  may be provided for increasing a contact area in a portion of the LNG line  311  located within the reliquefaction tank  320 . An evaporator  350  is installed to evaporate the LNG such that the LNG meets a condition as a fuel. 
         [0069]    The cold heat recovery apparatus  300  using the LNG fuel according to the third embodiment of the present invention may further include a gas manifold  330  and a liquid manifold  340 . The gas manifold  330  is configured to supply the reliquefaction tank  320  with boil-off gas supplied from each of the liquefied gas tanks  10 . The liquid manifold  340  is configured to distribute the reliquefied gas discharged from the reliquefaction tank  320  to each of the liquefied gas tanks  10 . 
         [0070]    On the other hand, one reliquefaction tank  320  may be provided like the present embodiment, but the present invention is not limited thereto. For example, a plurality of reliquefaction tanks  320  may be provided. In this case, gas manifolds  330  and liquid manifolds  340  may be installed in the respective reliquefaction tanks  320 , or a gas manifold  330  and a liquid manifold  340  may be integrally installed with respect to all reliquefaction tanks  320 . 
         [0071]    The gas manifold  330  and the liquid manifold  340  are installed in the gas pipeline  321  and the liquid pipeline  322 , respectively. The gas pipeline  321  and the liquid pipeline  322  are branched into a plurality of pipelines such that the branched pipelines are connected to a plurality of liquefied gas tanks  10  on the basis of the gas manifold  330  and the liquid manifold  340 . 
         [0072]    In the cold heat recovery apparatus  300  using the LNG fuel according to the third embodiment of the present invention, when boil-off gas generated from the liquefied gas tank  10  usable as a cargo tank is supplied through the gas pipeline  321  to the reliquefaction tank  320 , the boil-off gas is reliquefied by the heat exchange with the LNG of the LNG line  311  passing through the reliquefaction tank  320 , and the reliquefied gas is resupplied to the liquefied gas tank  10  through the liquid pipeline  322 . 
         [0073]    In addition, it is possible to integrally manage the boil-off gas generated from the plurality of liquefied gas tanks  10  through the gas manifold  330  and the liquid manifold  340 . 
         [0074]      FIG. 4  is a configuration diagram illustrating a cold heat recovery apparatus using an LNG fuel according to a fourth embodiment of the present invention. 
         [0075]    As illustrated in  FIG. 4 , like the cold heat recovery apparatus  300  using the LNG fuel according to the third embodiment, the cold heat recovery apparatus  400  using the LNG fuel according to the fourth embodiment of the present invention includes an LNG fuel tank  410  and a reliquefaction tank  420  serving as cold heat usage means. However, the reliquefaction tank  420  is configured to reliquefy boil-off gas by the heat exchange with a coolant supplied by a coolant circulation unit  430  using the LNG discharged from the LNG fuel tank  410 , instead of the direct heat exchange with the LNG discharged from the LNG fuel tank  410  in order for use as the fuel. 
         [0076]    The coolant circulation unit  430  may include a heat exchanger  431 , a coolant circulation line  432 , and a circulation pump  433 . The heat exchanger  431  is installed in the LNG line  411  through which the LNG is discharged from the LNG fuel tank  410 . The coolant circulation line  432  is installed to circulate and supply the coolant to the heat exchanger  431  and pass through the reliquefaction tank  420 . The circulation pump  433  is configured to provide a pumping force for circulating the coolant through the coolant circulation line  432 . 
         [0077]    A coil portion  432   a  may be provided for increasing a contact area in a portion of the coolant circulation line  432  which is inserted into the reliquefaction tank  420 . 
         [0078]    As described above, in the cold heat recovery apparatus  400  using the LNG fuel according to the fourth embodiment of the present invention, the LNG discharged from the LNG fuel tank  410  through the LNG line  411  exchanges heat with the coolant circulated by the pumping force of the circulation pump  433  in the heat exchanger  431 . The cooled coolant passes through the reliquefaction tank  420 , and the boil-off gas supplied from the liquefied gas tank  10  through the gas pipeline  421  to the reliquefaction tank  421  is reliquefied by the heat exchange with the coolant and is resupplied through a liquid pipeline  422  to the liquefied gas tank  10 . 
         [0079]    Preferably, the coolant has a freezing point (melting point) lower than the temperature of the LNG so as not to be condensed. Since the temperature of the coolant inside the closed loop, that is, the coolant circulation line  224 , is higher than the temperature of the LNG, a thermal stress of the liquefied gas tank  10  can be reduced. In addition, it is possible to avoid a condensation problem in the vicinity of the pipeline due to the liquefied gas having a higher freezing point than the LNG, for example, CO 2  or LPG. 
         [0080]      FIG. 5  is a configuration diagram illustrating a cold heat recovery apparatus using an LNG fuel according to a fifth embodiment of the present invention. 
         [0081]    As illustrated in  FIG. 5 , like the cold heat recovery apparatus  300  using the LNG fuel according to the third embodiment of the present invention, the cold heat recovery apparatus  500  using the LNG fuel according to the fifth embodiment of the present invention includes an LNG fuel tank  510  and a reliquefaction tank  520  serving as cold heat usage means. An LNG line  511  configured to discharge the LNG from the LNG fuel tank  10  is installed to pass through the reliquefaction tank  520 , so that boil-off gas is reliquefied by the heat exchange with the LNG. 
         [0082]    The cold heat recovery apparatus  500  using the LNG fuel according to the fifth embodiment of the present invention may further include a bypass line  531  and a control valve  532 . The bypass line  531  is connected to the LNG line  511  to bypass the reliquefaction tank  520 . The control valve  532  is installed in the LNG line  511  and the bypass line  531  to control the flow of the LNG through the reliquefaction tank  520  and the bypass line  531 . Like the present embodiment, a pair of control valves  532  may be installed in the bypass line  531  and the LNG line  511 , respectively. Alternatively, the control valve  532  may be a single three-way valve installed in a connection portion of the bypass line  531  and the LNG line  511 . 
         [0083]    The cold heat recovery apparatus  500  using the LNG fuel according to the fifth embodiment of the present invention may further include a detection unit  533  and a control unit  534 . The detection unit  533  measures the temperature or pressure inside the reliquefaction tank  520  and outputs a detection signal. The control unit  534  receives the detection signal output from the detection unit  533  and controls the control valve  532 . 
         [0084]    The reliquefaction tank  520  may be forcibly supplied with boil-off gas from the liquefied gas tank  10  by a compressor (not illustrated) installed in a gas pipeline  521 , or may forcibly transfer the reliquefied gas to the liquefied gas tank  10  by a transfer pump (not illustrated) installed in a liquid pipeline  522 . 
         [0085]    The cold heat recovery apparatus  500  using the LNG fuel according to the fifth embodiment of the present invention may further include a control valve  535 , a pressure detection unit  536 , and a control unit  534 . The control valve  535  is installed in the gas pipeline  521 . The pressure detection unit  536  measures the pressure inside the liquefied gas tank  10  and outputs a detection signal. The control unit  534  receives the detection signal of the pressure detection unit  536  and controls the control valve  535 . 
         [0086]    The cold heat recovery apparatus  500  using the LNG fuel according to the fifth embodiment of the present invention may further include a control valve  537 , a water level detection unit  538 , and a control unit  534 . The control valve  537  is installed in the liquid pipeline  522 . The water level detection unit  538  measures a water level of the reliquefaction tank  520  and outputs a detection signal. The control unit  534  receives the detection signal of the water level detection unit  538  and controls the control valve  537 . 
         [0087]    The cold heat recovery apparatus  500  using the LNG fuel according to the fifth embodiment of the present invention may further include a control valve  537 , a pressure detection unit  536 , and a control unit  534 . The control valve  535  is installed in the gas pipeline  522 . The pressure detection unit  536  measures the pressure inside the liquefied gas tank  10  and outputs a detection signal. The control unit  537  receives the detection signal of the pressure detection unit  536  and controls the control valve  535 . 
         [0088]    As described above, in the cold heat recovery apparatus  500  using the LNG fuel according to the fifth embodiment of the present invention, when the detection unit  533  detects that the temperature or pressure of the reliquefaction tank  520  is higher than a set temperature or a set pressure, the control unit  534  controls the control valve  532  such that the flow of the LNG via the LNG line  511  passing through the reliquefaction tank  520  is increased. When the detection unit  533  detects that the temperature or pressure of the reliquefaction tank  520  is lower than the set temperature or the set pressure, the control unit  534  controls the control valve  532  such that the flow of the LNG via the LNG line  511  is reduced and the flow of the LNG passing through the bypass line  531  is increased. 
         [0089]    The reliquefaction tank  520  is configured to be smoothly supplied with boil-off gas from the liquefied gas tank  10  through the compressor, and to stably supply the reliquefied gas to the liquefied gas tank  10  by the transfer pump. When the temperature inside the reliquefaction tank  520  or the temperature of the LNG supplied to the reliquefaction tank  520  falls to below a set temperature, the reliquefaction tank  520  is configured to increase the temperature by causing a heater to provide heat. 
         [0090]    When the pressure detection unit  536  detects that the pressure inside the liquefied gas tank  10  is higher than the set pressure, the control unit  534  may reduce the pressure inside the liquefied gas tank  10  by opening the control valve  535 . When the pressure inside the liquefied gas tank  10  becomes lower than the set pressure, the control unit  534  may increase the pressure inside the liquefied gas tank  10  by closing the control valve  535 . 
         [0091]    In addition, the control unit  534  controls the control valve  537  such that the water level of the reliquefaction tank  520  measured by the water level detection unit  538  is maintained at a set water level. When the pressure inside the liquefied gas tank  10  detected by the pressure detection unit  536  is higher than the set pressure, the control unit  534  reduces the pressure and temperature inside the liquefied gas tank  10  by opening the control valve  537  to cause the cold reliquefied gas to flow into the liquefied gas tank  10 . 
         [0092]      FIG. 6  is a configuration diagram illustrating a cold heat recovery apparatus using an LNG fuel according to a sixth embodiment of the present invention. 
         [0093]    As illustrated in  FIG. 6 , the cold heat recovery apparatus  600  using the LNG fuel according to the sixth embodiment of the present invention may include an LNG fuel tank  610  and a heat exchanger  620  serving as cold heat usage means. The cold heat recovery apparatus  600  using the LNG fuel according to the sixth embodiment differs from the cold heat recovery apparatus according to the third embodiment in that the heat exchanger  620  is used instead of the reliquefaction tank  320 . 
         [0094]    The LNG fuel tank  610  is a tank that stores an LNG fuel of a liquefied gas carrier. The LNG fuel tank  610  is made of a container for withstanding a pressure and low-temperature of the LNG, and supplies the LNG as a fuel through the LNG line  611 . The heat exchanger  620  is installed in the LNG line  611 . An LNG pump  611   a  and an on/off valve  611   b  may be installed in the front of the heat exchanger  620 . The LNG pump  611   a  is configured to provide a pumping force for transferring the LNG. The on/off valve  611   b  is provided with a control valve as an example of opening and closing the supply of the LNG. An evaporator  650  may be installed in the rear of the heat exchanger  620  to evaporate the LNG such that the LNG can be used suitably as a gaseous fuel. 
         [0095]    The heat exchanger  620  reliquefies boil-off gas supplied from the liquefied gas tank  10  storing the liquefied gas through a gas pipeline  621  by the heat exchange with the LNG discharged from the LNG fuel tank  610  by using a low-temperature characteristic of the LNG, and supplies the reliquefied boil-off gas through a liquid pipeline  622  to the liquefied gas tank  10 . In addition, the heat exchanger  620  may be forcibly supplied with the boil-off gas from the liquefied gas tank  10  by a compressor  623  installed in a gas pipeline  621 . Therefore, when the heat exchange occurs in the heat exchanger  620 , the boil-off gas sublimates, and thus, the boil-off gas can be liquefied without being solidified. 
         [0096]    The cold heat recovery apparatus  600  using the LNG fuel according to the sixth embodiment of the present invention may further include a gas manifold  630  and a liquid manifold  640 . The gas manifold  330  is configured to supply the heat exchanger  620  with boil-off gas supplied from each of the liquefied gas tanks  10 . The liquid manifold  640  is configured to distribute the reliquefied gas discharged from the heat exchanger  620  to each of the liquefied gas tanks  10 . 
         [0097]    On the other hand, one heat exchanger  620  may be provided like the present embodiment, but the present invention is not limited thereto. For example, a plurality of heat exchangers  620  may be provided. In this case, gas manifolds  630  and liquid manifolds  640  may be installed in the respective heat exchangers  620 , or a gas manifold  630  and a liquid manifold  640  may be integrally connected to all heat exchangers  620 . 
         [0098]    The gas manifold  630  and the liquid manifold  640  are installed in the gas pipeline  621  and the liquid pipeline  622 , respectively. The gas pipeline  621  and the liquid pipeline  622  are branched into a plurality of pipelines such that the branched pipelines are connected to liquefied gas tanks  10  on the basis of the gas manifold  630  and the liquid manifold  640 . 
         [0099]    In the cold heat recovery apparatus  600  using the LNG fuel according to the sixth embodiment of the present invention, when boil-off gas generated from the liquefied gas tank  10  usable as a cargo tank is supplied through the gas pipeline  321  to the heat exchanger  620  by the compressor  623 , the boil-off gas is reliquefied by the heat exchange with the LNG of the LNG line  611  passing through the heat exchanger  620 , and the reliquefied gas is resupplied to the liquefied gas tank  10  through the liquid pipeline  622  In this case, the heat exchanger  620  is forcibly supplied with the boil-off gas by the compressor  623 . Therefore, when the heat exchange occurs in the heat exchanger  620 , the boil-off gas sublimates, and thus, the boil-off gas can be liquefied without being solidified. 
         [0100]    In addition, it is possible to integrally distribute and manage the boil-off gas generated from the plurality of liquefied gas tanks  10  through the gas manifold  630  and the liquid manifold  640 . 
         [0101]      FIG. 7  is a configuration diagram illustrating a cold heat recovery apparatus using an LNG fuel according to a seventh embodiment of the present invention. 
         [0102]    As illustrated in  FIG. 7 , like the cold heat recovery apparatus  600  using the LNG fuel according to the sixth embodiment of the present invention, the cold heat recovery apparatus  700  using the LNG fuel according to the seventh embodiment of the present invention includes an LNG fuel tank  710  and a heat exchanger  720  serving as cold heat usage means. The heat exchanger  720  reliquefies boil-off gas supplied from the liquefied gas tank  10  through a gas pipeline  721  by the heat exchange with the LNG discharged from the LNG fuel tank  710  storing an LNG fuel of a liquefied gas carrier through an LNG line  711  by a pumping of an LNG pump  711   a,  and supplies the reliquefied boil-off gas through a liquid pipeline  722  to the liquefied gas tank  10 . 
         [0103]    The heat exchanger  720  may be forcibly supplied with the boil-off gas from the liquefied gas tank  10  by a compressor  723  installed in the gas pipeline  721 . Like the sixth embodiment, the heat exchanger  720  can be supplied with the boil-off gas from a plurality of liquefied gas tanks  19  by a gas manifold and a liquid manifold, and can be distributed and supplied to the plurality of liquefied gas tanks  10  after reliquefying the boil-off gas. 
         [0104]    In addition, the cold heat recovery apparatus  700  using the LNG fuel according to the seventh embodiment of the present invention may further include a bypass line  731  and control valves  732  and  733 . The bypass line  731  is connected to the LNG line  711  to bypass the heat exchanger  720 . The control valves  732  and  733  are installed in the LNG line  711  and the bypass line  731  to control the flow of the LNG through the heat exchanger  720  and the bypass line  731 . Like the present embodiment, a pair of control valves  732  and  733  may be installed in the bypass line  731  and the LNG line  711 , respectively. Alternatively, the control valves  732  and  733  may be a single three-way valve installed in a connection portion of the bypass line  731  and the LNG line  711 . 
         [0105]    The cold heat recovery apparatus  700  using the LNG fuel according to the seventh embodiment of the present invention may further include a detection unit  734  and a control unit  735 . The detection unit  734  measures the temperature or pressure inside the heat exchanger  720  and outputs a detection signal. The control unit  735  receives the detection signal output from the detection unit  734  and controls the control valves  732  and  733 . The detection unit  734  may be a temperature sensor configured to measure the temperature inside the heat exchanger  720 , or a pressure sensor configured to measure the pressure inside the heat exchanger  720 . Therefore, when the temperature or pressure inside the heat exchanger  720  measured by the detection unit  734  is higher than a set temperature or a set pressure, the control unit  735  controls the control valves  732  and  733  such that the flow rate of the LNG passing through the heat exchanger  720  is more increased than that passing through the bypass line  731 . On the other hand, when it is detected that the temperature or pressure inside the heat exchanger  720  is lower than the set temperature or the set pressure, the control unit  735  controls the control valves  732  and  733  such that the flow rate of the LNG passing through the bypass line  731  is increased. 
         [0106]    A heater  740  may be installed inside (or outside) the heat exchanger  720  to provide heat. In addition, a temperature detection unit  739  may be installed in the liquid pipeline  722  to measure the temperature of the reliquefied gas and output a detection signal. The control unit  735  receives the detection signal of the temperature detection unit  739  and controls the heater  740 . For example, when the temperature of the reliquefied liquefied gas within the liquid pipeline  722  detected by the temperature detection unit  739  falls to below a preset temperature, the control unit  735  operates the heater  740  to heat the heat exchanger  720  to thereby increase the temperature of the reliquefied liquefied gas. 
         [0107]    On the other hand, the heater  740  may be installed inside or outside the heat exchanger  720 . The heater  740  may be provided with a hot wire or a heating block configured to convert electric energy into thermal energy, or may be provided with a heating pipe configured to circulate and supply a heating medium such as steam or hot water. 
         [0108]    The cold heat recovery apparatus  700  using the LNG fuel according to the seventh embodiment of the present invention may further include a control valve  736 , a pressure detection unit  737 , and a control unit  735 . The control valve  736  is installed in the gas pipeline  721 . The pressure detection unit  735  measures the pressure inside the liquefied gas tank  10  and outputs a detection signal. The control unit  735  receives the detection signal of the pressure detection unit  737  and controls the control valve  536 . Therefore, when the pressure of the liquefied gas tank  10  detected by the pressure detection unit  737  becomes higher than the preset pressure, the control unit  735  opens the control valve  736  to supply the boil-off gas to the heat exchanger  720  so that the pressure inside the liquefied gas tank  10  is reduced. On the other hand, a control valve  738  configured to perform an operation corresponding to the control valve  736  installed in the gas pipeline may be installed in the liquid pipeline  722 . 
         [0109]    As described above, in the cold heat recovery apparatus  700  using the LNG fuel according to the seventh embodiment of the present invention, when the temperature or pressure of the heat exchanger  720  detected by the detection unit  734  is higher than the set temperature or the set pressure, the control unit  735  controls the control valves  722  and  723  such that the flow of the LNG via the LNG line  711  passing through the heat exchanger  720  is increased. When the temperature or pressure of the heat exchanger  720  is lower than the set temperature or the set pressure, the control unit  735  controls the control valves  722  and  723  such that the flow of the LNG via the LNG line  711  is reduced and the flow of the LNG passing through the bypass line  731  is increased. 
         [0110]    When the temperature of the reliquefied liquefied gas within the liquid pipeline  722  detected by the temperature detection unit  739  falls to below the preset temperature, the control unit  735  operates the heater  740  to heat the heat exchanger  720  to thereby increase the temperature of the reliquefied liquefied gas. In addition, when the pressure inside the liquefied gas tank  10  detected by the pressure detection unit  737  becomes higher than the set pressure, the control unit  735  may reduce the pressure inside the liquefied gas tank  10  by opening the control valve  736 . When the pressure inside the liquefied gas tank  10  becomes lower than the set pressure, the control unit  735  may increase the pressure inside the liquefied gas tank  10  by closing the control valve  536 . 
         [0111]    As described above in detail, the liquefied gas carrier using the LNG as the fuel according to the present invention may include the cold heat recovery apparatus using the LNG fuel according to the first to seventh embodiments of the present invention, and may include a storage tank capable of storing liquefied gas such as LPG or CO 2  as a cargo, and a fuel tank capable of storing the LNG as a fuel. 
         [0112]    The present invention is not limited to the above-mentioned embodiments, and it is obvious to those skilled in the art that various modifications or changes can also be made thereto without departing from the scope of the present invention.