Abstract:
A system for maintaining a substantially constant pressure within an ullage space of a cryogenic storage tank is provided. The system includes a compressor configured to receive fuel gas from the cryogenic storage tank, and compress the fuel gas to produce heated fuel gas. The system further includes a heat exchanger in flow communication with the compressor and configured to cool the heated fuel gas to produce cooled fuel gas, and a turbine in flow communication with the heat exchanger and configured to expand the cooled fuel gas to produce a gas and liquid mixture having a predetermined liquid to gas ratio, and discharge the gas and liquid mixture into the cryogenic storage tank.

Description:
BACKGROUND 
       [0001]    The field of the disclosure relates generally to the storage and delivery of cryogenic fuels, and more specifically, to a system for maintaining a constant pressure in a cryogenic fuel tank. 
         [0002]    The cost of petroleum-based liquid fuels has been escalating because of continuing world demand, and demand will likely continue to increase because of substantial economic growth in countries such as China and India. These countries have relatively large populations, spurring rapid expansion of ground and air transportation. 
         [0003]    The increasing cost of petroleum-based products is due, at least in part, to a dependence on importing energy. This has resulted in numerous efforts to develop alternative energy sources to reduce costs and achieve energy independence. For example, recent advances in shale gas fracking have led to large-scale recovery of natural gas from shale rock. This abundance of natural gas has led to a decrease in the cost of natural gas, while the cost of petroleum-based fuels continues to rise. Accordingly, at least some known vehicle fuel systems utilize natural gas instead of petroleum-based fuels. 
         [0004]    For example, liquefied natural gas may be used as a fuel for an aircraft. The liquefied natural gas may be stored in light-weight cryogenic fuel storage tanks onboard the aircraft. However, changes in altitude of the aircraft may result in pressure changes in the fuel storage tanks, causing at least a portion of the total liquefied natural gas to boil off Accordingly, to avoid boil off, it is desirable to maintain a substantially constant pressure within the fuel storage tanks. However, as liquefied natural gas is essentially pure methane, the substantially constant pressure should not be maintained by venting liquefied natural gas into the atmosphere. 
       BRIEF DESCRIPTION 
       [0005]    In one aspect, a system for maintaining a substantially constant pressure within an ullage space of a cryogenic storage tank is provided. The system includes a compressor configured to receive fuel gas from the cryogenic storage tank, and compress the fuel gas to produce heated fuel gas. The system further includes a heat exchanger in flow communication with the compressor and configured to cool the heated fuel gas to produce cooled fuel gas, and a turbine in flow communication with the heat exchanger and configured to expand the cooled fuel gas to produce a gas and liquid mixture having a predetermined liquid to gas ratio, and discharge the gas and liquid mixture into the cryogenic storage tank. 
         [0006]    In another aspect, a cryogenic system is provided. The cryogenic system includes a storage tank and a cryo circuit in flow communication with the storage tank and configured to receive fuel gas from the storage tank, convert the fuel gas into a gas and liquid mixture having a predetermined liquid to gas ratio, and discharge the gas and liquid mixture into the storage tank. 
         [0007]    In yet another aspect, a method for maintaining a substantially constant pressure within an ullage space of a cryogenic storage tank is provided. The method includes channeling fuel gas from the cryogenic storage tank to a cryo circuit, converting, using the cryo circuit, the fuel gas into a gas and liquid mixture having a predetermined liquid to gas ratio, and discharging the gas and liquid mixture into the cryogenic storage tank such that a pressure in the ullage space remains substantially constant. 
         [0008]    The features, functions, and advantages that have been discussed can be achieved independently in various implementations or may be combined in yet other implementations, further details of which can be seen with reference to the following description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic diagram of a system for maintaining a constant pressure in cryogenic fuel tank. 
           [0010]      FIG. 2  is a flowchart of an exemplary method that may be used with the system shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    The systems and methods described herein enable maintaining a substantially constant pressure in an ullage space of a storage tank. Fuel gas from the storage tank is channeled to a cryo circuit that converts the fuel gas into a gas and liquid mixture. The gas and liquid mixture has a predetermined liquid to gas ratio. Further, the gas and liquid mixture is discharged into the storage tank to facilitate maintaining the substantially constant pressure. 
         [0012]    Flight vehicles, such as airplanes, utilize fuel tanks that store and supply cryogenic fuels such as, for example, natural gas, methane, hydrogen, and/or other fuels stored as a cryogenic liquid. To facilitate minimizing payload carrying penalties, cryogenic fuel tanks should be relatively low-weight. Further, cryogenic fuel tanks should have a minimum differential pressure across tank walls. An internal tank pressure is near ambient pressure when the tank is filled on the ground. As the tank climbs to higher altitudes during flight, the ambient pressure outside of the tank drops. If the pressure inside of the fuel tank is allowed to drop, fuel within the tank will boil off, resulting in a loss of fuel. 
         [0013]    Accordingly, to facilitate minimizing fuel loss, ullage space (i.e., the gaseous space in the tank above the liquid fuel) should be maintained at substantially the same pressure as the pressure during the filling of the tank. Further, during extended periods on the ground, heat transfer into the tank may vaporize at least some of the liquid cryogenic fuel, resulting in a pressure build up that may be relieved by venting. However, if the cryogenic fuel is liquefied natural gas that includes primarily methane, venting into the atmosphere may contribute to global warming. 
         [0014]    During flight operations, when fuel is supplied to vehicle engines for power, the tank should be pressurized to prevent ullage space pressure collapse that would generate large compression loads on tank walls. The systems and methods described herein maintain a substantially constant pressure in the fuel tank to facilitate providing pressure relief during long term storage, preventing boil off when climbing to higher altitudes, and maintaining a positive ullage space pressure during fuel outflow. Further, at least for environmental reasons, the systems and methods described herein avoid venting into the atmosphere. 
         [0015]    Specifically, the systems and method described herein provide a regenerative integrated pressurization and vent gas recovery system for cryogenic fuel tanks. The implementations described herein provide particular benefits for light weight cryogenic fuel tanks where a maximum differential pressure between the inside and the outside of the tank should be minimized. The systems and methods described herein also avoid the use of helium for pressurization, as helium is relatively expensive. In general, nitrogen is not used for pressurization, as it has a higher freezing temperature than some cryogenic fuels (e.g., liquid hydrogen) and thus freezes out. Further, nitrogen cannot be used for pressurization of liquid natural gas, as it has a relatively high solubility in liquefied methane and natural gas when processed into a liquid, resulting in essentially pure methane. 
         [0016]    The implementations described herein provide a gas turbine that drives a cryo-cooler using cryogenic vent gas as a fuel to regeneratively maintain a constant ullage pressure. The fuel tank feeds into a cryo-cooler refrigeration cycle using a variable flow fan in a heat exchanger between a compressor and an expansion cooling turbine. The outflow from the turbine is a mixture of liquefied and gaseous natural gas. The systems and methods utilize a partial liquefaction cycle that continuously recycles through the fuel storage tank. A flow cooling turbine that extracts energy requires expansion of a gas through the turbine. 
         [0017]    For liquefaction, at least some known systems use an auxiliary refrigeration cycle device such as a nitrogen expansion cycle to liquefy the vent gas. However, in the systems and methods described herein, partial liquefaction is done in expansion turbine, allowing gaseous flow expansion to extract energy while partially liquefying the turbine flow. Continuous recycling of the vent flow maintains the constant ullage pressure by the combination of liquefaction and pressurization of vent gas. The cooling flow for the liquefaction cryo-cooler is modulated to vary the heat extracted, which in turn varies a liquefaction fraction of the flow. The exemplary system is powered by regenerative combustion that utilizes a portion of the vent gas. 
         [0018]      FIG. 1  is a schematic diagram of an exemplary system  100  for maintaining constant pressure in a thermally insulated fuel tank  101  that stores cryogenic fuel  102 . In the exemplary implementation, cryogenic fuel  102  is liquefied natural gas. Alternatively, cryogenic fuel  102  may be any fuel that enables the system to function as described herein. 
         [0019]    A ullage space  103  within fuel tank  101  above cryogenic fuel  102  is gaseous. During storage of cryogenic fuel  102 , heat transfer from outside fuel tank  101  may cause at least some of cryogenic fuel  102  to vaporize, resulting in a pressure build up. When cryogenic fuel  102  flows out of fuel tank  101  through an outlet  104  (e.g., to power one or more engines), a quantity of cryogenic fuel  102  in fuel tank  101  decreases, resulting in a pressure drop. 
         [0020]    In the exemplary implementation, where cryogenic fuel  102  is liquefied natural gas, excess pressure is not relieved by venting to the atmosphere. This is because liquefied natural gas is essentially pure methane, which may contribute to global warming Further, venting to the atmosphere at a high altitude may result in relatively large fuel boil offs (e.g., 8 to 10% of the total fuel). 
         [0021]    In system  100 , the pressure in ullage space  103  is maintained at substantially the same pressure as a pressure when fuel tank  101  is filled with cryogenic fuel  102 . Otherwise, differential pressures across walls of fuel tank  101  may become high, particularly during take-off and an initial ascent of the vehicle. Vent gas  105  from fuel tank  101  is channeled into a cryo circuit  110 , and more specifically, into a cryo-cooler compressor  118  in cryo circuit  110 . Vent gas  105  is compression heated in cryo-cooler compressor  118  to produce heated vent gas  119  and then channeled to and cooled in a first heat exchanger  120 . 
         [0022]    To cool vent gas  105 , a variable flow fan  117  provides cooled air  121  to first heat exchanger  120 . After being cooled in first heat exchanger  120 , vent gas  105  is expanded and partially sub-cooled into a liquid in an energy extraction turbine  122 . Energy extraction turbine  122  outputs a gas and liquid mixture  123  that is channeled back into fuel tank  101 . Accordingly, cryo-cooler compressor  118 , first heat exchanger  120 , and energy extraction turbine  122  form cryo circuit  110  that converts vent gas  105  from fuel tank  101  into gas and liquid mixture  123  that is returned to fuel tank  101 . 
         [0023]    Gas and liquid mixture  123  has a predetermined ratio of liquid to gas that facilitates maintaining the pressure in ullage space  103 , as described in detail below. 
         [0024]    In the exemplary implementation, a differential pressure sensor  124  monitors the pressure of ullage space  103 . Depending on whether the pressure of ullage space  103  is increasing or decreasing, a signal  125  is transmitted by a controller  126  to variable flow fan  117  to increase or decrease the cooling of vent gas  105 . Controller  126  is communicatively coupled to differential pressure sensor  124 . 
         [0025]    Specifically, if the pressure of ullage space  103  is increasing, controller  126 , via signal  125 , instructs variable flow fan  117  to increase a cooling flow to increase cooling of vent gas  105 , resulting in an increase in the liquid to gas ratio (i.e., an increase in the amount of liquid and a decrease in the amount of gas) in mixture  123 , which in turn decreases the pressure of ullage space  103 . On the other hand, if the pressure of ullage space  103  is decreasing, controller  126 , via signal  125 , instructs variable flow fan  117  to decrease the cooling flow to decrease cooling of vent gas  105 , resulting in a decrease of the liquid to gas ratio (i.e., a decrease in the amount of liquid and an increase in the amount of gas) in mixture  123 , which in turn increases the pressure of ullage space  103 . 
         [0026]    In the exemplary implementation, differential pressure sensor  124  determines that the pressure of ullage space  103  is increasing if the measured pressure is above a predetermined upper threshold, and differential pressure sensor  124  determines that the pressure of ullage space  103  is decreasing if the measured pressure is below a predetermined lower threshold. The predetermined upper and lower thresholds define a pressure range that ullage space  103  operates within. For example, in one implementation, the pressure is maintained at approximately 3.0 pounds per square inch above atmospheric pressure (i.e., approximately 17.7 pounds per square inch), with the lower threshold set at 17.2 pounds per square inch and the upper threshold set at 18.2 pounds per square inch. Alternatively, differential pressure sensor  124  may determine that the pressure of ullage space  103  is increasing or decreasing using any techniques that enable system  100  to function as described herein. 
         [0027]    A gas turbine  204  provides shaft power  206  to a plurality of components in system  100 , as described herein. For example, gas turbine  204  provides shaft power  206  to at least cryo-cooler compressor  118 , variable flow fan  117 , and energy extraction turbine  122  in the exemplary implementation. Gas turbine  204  is fueled by a portion  207  of heated vent gas  119  that is bled off before entering first heat exchanger  120 . Gas turbine  204  includes a compressor  209 , a combustor  210 , a first turbine  211 , and a second turbine  212 . The portion  207  of heated vent gas  119  is supplied to combustor  210 . 
         [0028]    Compressor  209  receives ambient air  218  and heated air  219 . The compressed air is then burned in combustor  210  to produce energy in the form of high temperature combustion products. The high energy burned air is expanded in first turbine  211  to drive compressor  209 . Any excess high energy air is expanded in second turbine  212  to produce shaft power  206 . An output  223  of second turbine  212  has a temperature higher than the atmosphere. Accordingly, output  223  is passed through a second heat exchanger  224  to generate first hot air  228  included in heated air  219 . 
         [0029]    In the exemplary implementation, at least a portion  227  of cooling air  121  from variable flow fan  117  passes through second heat exchanger  224  to generate first hot air  228 . Further, second hot air  226  output from first heat exchanger  120  also contributes to heated air  219 . Accordingly, waste heat from both gas turbine  204  and first heat exchanger  120  is recovered to facilitate improving an efficiency of gas turbine  204  by regeneration. 
         [0030]    System  100  maintains the pressure of ullage space  103  within a relatively narrow pressure range by generating gas and liquid mixture  123  from vent gas  105  using a closed loop. System  100  consumes a relatively small amount of fuel  102  for power, and releases a relatively low amount of carbon dioxide into the atmosphere. Further the regenerative cycle of system  100  has a relatively high efficiency. As described herein, system  100  facilitates maintaining a substantially constant pressure in ullage space  103  of fuel tank  101  during substantial changes in altitude. 
         [0031]      FIG. 2  is a flowchart of an exemplary method  300  for maintaining a substantially constant pressure within a ullage space of a cryogenic storage tank, such as ullage space  103  in fuel tank  101  (both shown in  FIG. 1 ). Fuel gas, such as vent gas  105  (shown in  FIG. 1 ), is channeled  302  from the cryogenic storage tank to a cryo circuit, such as cryo circuit  110  (shown in  FIG. 1 ). The cryo circuit converts  304  the fuel gas into a gas and liquid mixture, such as gas and liquid mixture  123  (shown in  FIG. 1 ). The gas and liquid mixture has a predetermined liquid to gas ratio. The gas and liquid mixture is discharged  306  into the cryogenic storage tank such that a pressure in the ullage space remains substantially constant. 
         [0032]    In the exemplary implementation, the cryo circuit converts  304  the fuel gas by compressing the fuel gas into heated fuel gas in a compressor, such as cryo-cooler compressor  118  (shown in  FIG. 1 ), cooling the heated fuel gas into cooled fuel gas in a heat exchanger, such as first heat exchanger  120  (shown in FIG.  1 ), and expanding the cooled fuel gas in a turbine, such as energy extraction turbine  122  (shown in  FIG. 1 ). In the exemplary implementation, a power turbine, such as gas turbine  204  (shown in  FIG. 1 ), powers at least one of the compressor and the turbine. The power turbine operates using a portion of the heated fuel gas. 
         [0033]    A cooling flow from a fan, such as variable flow fan  117  (shown in  FIG. 1 ), is channeled through the heat exchanger to cool the heated fuel gas. To control the pressure within the ullage space, the pressure can be monitored using a sensor, such as differential pressure sensor  124  (shown in  FIG. 1 ). To maintain the pressure, when the monitored pressure increases, the cooling flow from the fan is increased, and when the monitored pressure decreases, the cooling flow from the fan is decreased. 
         [0034]    The systems and methods described herein facilitate maintaining a constant pressure in a cryogenic storage tank by using closed loop recycling of vent gas using a cryo-cooler and partial liquefaction in a expansion turbine. The systems and methods described herein do not utilize atmospheric venting for pressure relief during storage or to account for pressure drops when discharging fuel out of a tank. Moreover, the systems and methods described herein utilize a regenerative cycle to recover heat from a gas turbine and the cryo-cooler to improve the thermodynamic efficiency of the gas turbine. Shaft power generated by the gas turbine drives one or more components in the system. 
         [0035]    Although specific features of various implementations of the invention may be shown in some drawings and not in others, this is for convenience only. In accordance with the principles of the invention, any feature of a drawing may be referenced and/or claimed in combination with any feature of any other drawing. 
         [0036]    This written description uses examples to disclose various implementations, which include the best mode, to enable any person skilled in the art to practice those implementations, including making and using any devices or systems and performing any incorporated methods. The patentable scope is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.