Abstract:
A system for the cold-weather storage of gaseous fuels includes a gas source having an inlet pressure, a compressor having an inlet and an outlet, the inlet selectively communicating with the gas source and the outlet having a discharge pressure greater than the inlet pressure, a heat exchange apparatus having an inlet and an outlet, the inlet selectively communicating with the compressor so as to receive pressurized gas therefrom, a high-pressure storage tank having an inlet and an outlet, the inlet selectively communicating with the compressor so as to receive pressurized gas therefrom, and a valve assembly for selectively directing the pressurized gas to the heat exchange apparatus and the high-pressure storage tank in dependence upon a temperature within the storage tank.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to fuel storage and distribution and, more particularly, to a system, apparatus and method for the cold-weather storage and distribution of gaseous fuels. 
     BACKGROUND OF THE INVENTION 
     As gasoline prices have soared and concerns over harmful emissions have mounted in recent years, vehicles that run on alternative fuel sources are becoming increasingly important. For example, the use of compressed natural gas (“CNG”) as an alternative fuel for motor vehicles is becoming increasingly popular throughout the world because it is relatively inexpensive, burns cleanly, is relatively abundant and is adaptable to existing technologies. 
     Natural-gas vehicles use the same basic principles as gasoline-powered vehicles. In other words, the fuel (natural gas) is mixed with air in the cylinder of, e.g., a four-stroke engine, and then ignited by a spark plug to move a piston up and down. Although there are some differences between natural gas and gasoline in terms of flammability and ignition temperatures, natural-gas vehicles themselves operate on the same fundamental concepts as gasoline-powered vehicles. Accordingly, existing gasoline-powered vehicles may be converted to run on CNG, thereby easing the transition between gasoline and CNG in markets where gasoline-powered vehicles are dominant. In addition, an increasing number of vehicles worldwide are being originally manufactured to run on CNG. 
     Advantageously, CNG-fueled vehicles have lower maintenance costs when compared with other fuel-powered vehicles. In addition, CNG emits significantly fewer pollutants such as carbon dioxide, hydrocarbons, carbon monoxide, nitrogen oxides, sulfur oxides and particulate matter compared to petrol. 
     Despite the advantages of compressed natural gas as a motive fuel, the use of natural gas vehicles faces several logistical concerns, including fuel storage and infrastructure available for delivery and distribution at fueling stations. Natural gas suitable for vehicle use is customarily stored in small capacity tank, at 3,600 psi at 70° F., and is distributed from storage tanks to an on-vehicle receiving tank by “cascade filling.” Cascade filling is accomplished by starting out with the storage tank at a higher pressure than the receiving tank and then allowing this pressure to force the gas (or liquid) into the receiving tank. In so doing, natural gas is transferred, and the pressure in the storage tank drops to the point where the pressures of the two tanks become equal and nothing more is transferred. 
     The storage and distribution of CNG is severely affected, however, at low temperatures, and particularly when the temperature drops below 40° F. At low temperatures, the pressure in the storage tank drops, thereby resulting in less of a difference in pressure between the receiving tank and the storage tank, ultimately resulting in inefficiencies in gaseous fuel transfer (i.e., less gaseous fuel being transferred to the receiving tank on board the compatible vehicle, and longer filling times). 
     Moreover, the storage of CNG in large capacity tanks at high pressures is also problematic. In particular, storing CNG in tanks at 3,000-3,600 psi requires that the tank&#39;s walls be cast from thick steel or other suitable metal in order to withstand the enormous stresses caused by the compressed gas. As will be readily appreciated, large capacity CNG storage tanks would therefore be undesirably heavy and inefficient and expensive to manufacture and transport. As a result, transportation and storage of CNG is customarily effectuated by using numerous smaller, tube-shaped cylinders, which themselves are extremely heavy. 
     With the forgoing problems and concerns in mind, it is the general object of the present invention to provide a system and method for the cold-weather storage and distribution of gaseous fuels, which utilizes large capacity tanks that are insulative and have a reduced weight. 
     SUMMARY OF THE INVENTION 
     With the forgoing concerns and needs in mind, it is a general object of the present invention to provide a system and method for the cold-weather storage and distribution of gaseous fuels. 
     It is another object of the present invention to provide a system and method for the cold-weather storage and distribution of compressed natural gas. 
     It is another object of the present invention to provide a system and method for the cold-weather storage and distribution of gaseous fuels that compresses the fuels to a predetermined storage pressure. 
     It is another object of the present invention to provide a system and method for the cold-weather storage and distribution of gaseous fuels that maintains the gaseous fuel at a desired storage temperature. 
     It is another object of the present invention to provide a system and method for the cold-weather storage and distribution of gaseous fuels having a tank that has a greater storage capacity and is lighter than existing storage tanks. 
     These and other objectives of the present invention, and their preferred embodiments, shall become clear by consideration of the specification, claims and drawings taken as a whole. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
       The present invention will be better understood from reading the following description of non-limiting embodiments, with reference to the attached drawings, wherein below: 
         FIG. 1  is a schematic view of a system for the cold-weather storage of gaseous fuels in accordance with one embodiment of the present invention. 
         FIG. 2  is a side elevational view of a gaseous fuel storage tank for use with the system of  FIG. 1 . 
         FIG. 3  is a cross-sectional view of the gaseous fuel storage tank for use in connection with the system of  FIG. 1 , taken along line A-A of  FIG. 2 . 
         FIG. 4  is a diagram illustrating the stresses in the walls of the storage tank of  FIG. 2  at an internal pressure of 3,600 psi. 
         FIG. 5  is a diagram illustrating the stresses in the wall of a single-walled storage tank at an internal pressure of 3,600 psi. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An embodiment of the system of the present invention is indicated in general at  10  in  FIG. 1 . As shown therein, the system includes a slow fill compressor  12 , a heat exchange apparatus  14 , a plurality of gaseous fuel storage tanks  16 , a manifold  18  and a plurality of fast fill dispensers  20 . 
     As described in greater detail below, gaseous fuel, e.g., natural gas, is transferred from a low-pressure source to the slow fill compressor  12 . As used herein, “low pressure” is intended to mean the pressure at which the particular gas is originally introduced to the system  10 . In the preferred embodiment, the low-pressure source is a low pressure gas line  22  extending from a gas main, wherein the low pressure is the line pressure of the gas main. Alternatively, however, the low-pressure source may be a low-pressure gas tank  24  that is fluidly connected to the slow fill compressor  12  by a pipeline  26 . In this embodiment, the natural gas may be delivered by a tanker truck, unloaded from the truck via a loading pipeline  28 , and stored in the low-pressure gas tank  24  for use on demand. In any event, the low pressure gas line  22  and/or the low pressure gas tank  24  provide an on-demand supply of gaseous fuel for compression, storage and distribution by the system  10 , as described in detail hereinafter. 
     Returning to  FIG. 1 , the slow fill compressor  12  includes an inlet and an outlet and may be of the type known in the art, but in any event has a relatively low flow rate. The slow fill compressor  12  is in electrical communication with a power supply  30  for powering the compressor  12 . The power supply  30  may be an electrical outlet hooked up to the power grid. In alternative embodiments, the power supply  30  may be a generator, one or more batteries, or an alternative power generation device such as a solar panel or the like, without departing from the broader aspects of the present invention. In operation, the slow fill  12  compressor intakes and compresses the low-pressure gaseous fuel from the low-pressure source  22  or  24 . The compressed gas is then routed through a direct fill line  32  to the storage tanks  16 , from which it can then be dispensed to compatible vehicles through one or more fast fill dispensers  20 . 
     As alluded to above, gaseous fuel storage and distribution and, in particular CNG storage and distribution are greatly affected when temperatures drop below 40° F. It is therefore crucial for efficient storage and distribution that the CNG in the storage tanks is maintained at roughly 70° F. at 3,600 psi, as is standard in the industry. Importantly, the system  10  further includes a means of maintaining the temperature of the gaseous fuel in the storage tanks at a desired level, even when ambient air temperature drops, as discussed below. 
     In cold weather, especially below 40° F., the temperature of the gaseous fuel in the storage tanks begins to drop, as does the pressure within the storage tanks. As gaseous fuel stored in the tanks  16  is distributed to compatible vehicles, the slow fill compressor  12  is actuated to intake and compress source gas to replenish the gaseous fuel and pressure in the tanks  16 . As the low-pressure source gas is compressed by the slow fill compressor  12 , its temperature, as well as pressure, rises. This heated, compressed gas is then routed along the direct fill pipeline  32  to the storage tanks  16  for storage. The warmer compressed gas enters the tanks  16  so as to allow the incoming, warmer compressed gas to mix with the gaseous fuel already present in the tanks  16  so as to raise its temperature to a desired and optimum point, namely, approximately 70° F. 
     In this manner, compression of low-pressure source gas generates heat, which is then transferred to the gaseous fuel inside the storage tanks  16  to maintain the temperature thereof. As will be readily appreciated, fuel distribution to compatible vehicles triggers an almost continuous, slow pumping and compression of source gas, thereby providing the storage tanks  16  with an almost continuous supply of heat. As a result, cost savings can be realized because stand-alone heaters do not need to be utilized to maintain the temperature of the gaseous fuel within the tanks. 
     As further shown in  FIG. 1 , each of the storage tanks  16  includes a temperature sensor  34  connected to a thermostat  36 , each of which are set to maintain a desirable temperature of gaseous fuel inside each tank  16 . When the desired or setpoint temperature is reached within the tanks  16 , the thermostat  36  sends a signal to a solenoid valve  38  which changes the direction of the compressed gas exiting the slow fill compressor  12 . In particular, a solenoid valve  38  adjacent the exit of the slow fill compressor  12  is actuated such that the compressed gas exiting the slow fill compressor  12  is not routed directly into the storage tanks  16  via the direct fill line, but is instead directed along a heat exchange loop  40  having a heat exchange apparatus  14 . The heat exchange apparatus  14  effectively cools the compressed gas, i.e., heat from the gas is transferred to the heat exchange apparatus  14 , before the gas is directed back to the storage tanks  16 . Once cooling is effectuated, the compressed gas exits the heat exchange loop  40  and is fed into to a downstream portion of the direct fill line  32  and, ultimately, into the storage tanks  16 . 
     In the event that the tanks  16  are full, for instance when no dispensing is occurring, no compression is taking place and thus no heat from the compression of source gas is available to maintain the temperature of the gaseous fuel inside the storage tanks  16 . Accordingly, in order to maintain the temperature of the gaseous fuel in cold weather during times of little or no replenishing of the tanks (i.e., when fuel dispensing is low), the storage tanks  16  are additionally provided with an auxiliary electric heater  42  located in the main body of each of the tanks, discussed in more detail below. In the preferred embodiment, the power supply  30  that powers the slow fill compressor  12  also powers each electric heater  42 , although a separate power supply may also be used without departing from the broader aspects of the present invention. 
     Importantly, as discussed above, the temperature sensor  34  positioned within each storage tank  16  monitors a temperature of the gaseous fuel within each tank  16 . As shown in  FIG. 1 , each temperature sensor  34  is connected to a thermostat  36  that is set to maintain a desired temperature within each tank  16 . In the preferred embodiment, the desired temperature is approximately 70° F., although the thermostat  36  can be configured to maintain any desired setpoint temperature. When the heat generated from compression of the low pressure source gas is not is not available to maintain the temperature of the gaseous fuel within the tanks  16 , or when compression generated heat cannot keep up with temperature demand, the temperature sensor  34  will detect declining temperatures or a temperature below the setpoint temperature of the thermostat  36 . In response, the auxiliary heater  42  will be activated by the thermostat  36  to provide auxiliary heat to each fuel tank  16  to maintain or raise the temperature inside each tank  16 . Once the temperature of the gaseous fuel within the storage tanks  16  again reaches the setpoint temperature of the thermostat  36 , the auxiliary electric heater  42  is automatically switched off. 
     Preferably, the electric heater  42  is envisioned as a “blanket” which surrounds at least a portion of the tanks  16 , although other configurations and positioning of the electric heater  42  are also contemplated in the present invention. 
     As further shown in  FIG. 1 , valves  44  control the flow of low pressure gas from the loading truck into the low pressure tank  24 , from the low pressure tank  24  into the slow fill compressor  12 , and from the low pressure gas line  22  into the slow fill compressor  12 . Other valves  46  control the flow of pressurized gas from the heat exchange apparatus  14  into the storage tanks  16 . The output pipeline  48  of each storage tank  16  is also configured with a valve  50  to control the flow of compressed gaseous fuel from the tanks  16  to the manifold  18 . Finally, valves  52  control the flow of gaseous fuel from the manifold  18  to each fuel dispenser  20 . 
     Check valves  54  are positioned downstream from the solenoid valve along the direct fill line  32  and downstream the heat exchange apparatus  14  along the heat exchange loop  40 . The check valves  54  desirably control the direction of flow through the heat exchange loop  40  and the direct fill line  32  toward the storage tanks  16 , and prevent undesirable flow reversals that might otherwise occur due to unexpected pressure changes, leaks, equipment failures, or the like. Check valves  56  are also positioned along the output pipelines to control the direction of flow therethrough and to prevent similar flow reversals. 
     Importantly, the system  10  of the present invention is, broadly speaking, applicable to CNG storage tank assemblies of any size, both small and large capacity. The large capacity tank concept complements this system in the preferred embodiment, but it is not required. 
     In connection with the above, the configuration of the gaseous fuel storage tanks  16  is another important aspect of the present invention. In the preferred embodiment, each tank  16  is a large capacity tank, capable of storing a large quantity of gaseous fuel, in contrast to known small-volume tanks. Where the gaseous fuel is compressed natural gas, stored at approximately 70° F. and 3,600 psi, each tank  16  has a storage capacity large enough fill 500-700 compatible vehicles with CNG. Moreover, each storage tank is specially designed to withstand the pressures of the gaseous fuel inside the tank  16  and to insulate the gaseous fuel inside the tank from outside, ambient air, while having a lower weight profile than has heretofore been known. 
       FIGS. 2 and 3  show the configuration of a large-capacity storage tank  16 . As shown therein, each tank  16  is generally cylindrical in cross-section and includes an inner tank wall  60  and an outer tank wall  62  defining an annular space  64  therebetween, the inner and outer walls  60 , 62  being generally concentric. Within the annular space  64 , the auxiliary electric heater  42  is preferably disposed. The auxiliary electric heater  42  comprises a fiber carbon or metal electric mesh, through which electrical current is provided to produce heat. The mesh auxiliary heater  42  is preferably wrapped around the outer peripheral surface of the inner wall  60  of the tank  16  and preferably extends the length of the inner wall  60 . 
     As further shown therein, a polymer based resin  66  fills the remainder of the annular space  64 . Importantly, this resin  66  functions as an insulation layer to insulate the interior of the tank from the outside, ambient air (and potential low temperature thereof), as well as functioning as a mechanical reinforcement layer that effectively bonds the inner wall  60  to the outer wall  62 , and as a shock absorber for absorbing stress on the walls of the inner wall  60 . In this manner, the inner wall  60  and outer wall  62  are essentially joined together as a single unit. As will be readily appreciated, this increases the ability of the tank  16  to withstand the high pressures of gaseous fuel stored therein, as discussed below. In addition, the use of two walls bonded together with a polymer resin  66  decreases the weight of the tank  16  as compared to a single-walled tank of equal volume. 
     In the preferred embodiment, each wall is manufactured from steel, although other metals or materials known in the art may also be used without departing from the broader aspects of the present invention. Preferably, the walls of each wall  60 , 62  are approximately 1″ thick in embodiments where steel is utilized. In contrast to the present invention, known single-wall storage tanks not having the structure of the tanks  16  shown in  FIGS. 2 and 3  would have to be manufactured with walls that are 3″ thick to safely withstand the pressures, approximately 3,600 psi, inside the tank. As will be readily appreciated, providing a tank with inch-thick walls is advantageous because the tanks can be manufactured by rolling, whereas a tank with 3″ thick walls cannot be rolled using known methods and devices, but instead must be cast and, of course, would exhibit a much higher weight profile. 
     Through testing, it has been shown that the greatest stresses in cylindrical storage tanks oriented in the horizontal direction are concentrated along the top of the tank. Advantageously, as discussed above, the polymer based resin  66  disposed in the annular space  64  functions as a shock absorber to absorb the stresses upon the inner wall  60  of the tank, such that the outer wall  62  is subject to little stress, thereby allowing the walls  60 , 62  to be manufactured from steel or other metals of a lesser thickness. As compared to a single-walled storage tank having the same capacity and suitable to withstand gaseous fuel at a pressure of 3,600 psi at 70° F., the tank  16  of the present invention provides for an approximately 50% reduction in weight. In addition, significant weight savings are also realized in comparison to utilizing a large number of smaller storage tanks to store the same volume of gas, as more tanks equate more weight. 
     Referring now to  FIG. 4 , a finite element analysis evidences the advantages provided by the large capacity, double-walled tank of the present invention. In particular, as shown in  FIG. 3 , at 3,600 psi, the large capacity of the tank  16  of the present invention, having a 40″ diameter inner chamber defined by an inner wall  60  that is 1″ thick, a 44″ diameter outer chamber defined by an outer wall  62  that is 1″ thick, and a 1″ thick resin  66  disposed in the annular space  64  between the walls  60 , 62  results in a maximum von mises stress of 38,454 psi in the top of the inner wall  60 , within material limits (see top half of tank in  FIG. 4 ). In addition, the outer wall (bottom half of tank in  FIG. 4 ) exhibits a stress of 33,966 psi, also within material limits. The weight of the tank having these parameters is approximately 10 tons. 
     In contrast, finite element analysis of a single walled tank having a 44″ diameter and a 1″ thick wall has shown that the tank would yield to internal pressures prior to reaching the optimum internal pressure of 3,600 psi. As shown in  FIG. 5 , the von mises stress is 72,757 psi in the sidewall, well above material limits. Accordingly, in order to withstand pressurization at 3,600 psi, the walls of a single walled tank having a 44″ diameter would need to be 3″ thick, as discussed above, which would translate to a gross tank weight of approximately 15 tons. As will be readily appreciated, in these examples, the double-walled tank  16  of the present invention allows for a weight savings of 5 tons over a single-walled tank. In addition to the weight savings, in contrast to the 3″ thick single-wall tank, the tank  16  of the present invention can be rolled, rather than cast, thereby decreasing manufacturing time and cost. 
     It is therefore another important aspect of the present invention that the gaseous fuel storage tank  16  of the system of the present invention is capable of withstanding much higher pressures than known single-walled tanks of similar wall thickness. As a result, significant savings in weight, materials, cost, and ease of manufacture are realized, as discussed above. In view of the above, the present invention therefore provides a much lighter tank with the added ability to more precisely control the temperature of pressurized gaseous fuel stored within the tank. Indeed, by utilizing the compression of source gas to maintain the temperature within the storage tanks, significantly less energy is expended than would be the case if a stand-alone heater were utilized. Importantly, the temperature sensor and thermostat allow the temperature within the tanks to be more precisely controlled. Moreover, when the tanks are full and no compression is needed to fill the tanks, the temperature sensor and thermostat are arranged so as to control the auxiliary electric heater located in the main body of the tank to further maintain an optimum temperature of the CNG stored therein. 
     As discussed in detail above, the system  10  of the present invention utilizes the heat generated by gaseous compression of the fuel as a way to maintain the proper temperature and pressure regiment within the CNG storage tanks. In addition, the present invention provides a novel construction for large capacity CNG storage tanks that can be manufactured economically and at a much reduced weight profile. It will therefore be readily appreciated that a combination of the system  10  shown in  FIG. 1 , with the large capacity tanks  16  shown in  FIGS. 2 and 3 , results in a compressed gaseous fuel dispensing assembly that is more economical and efficient than has heretofore been known in the art. 
     Although this invention has been shown and described with respect to the detailed embodiments thereof, it will be understood by those of skill in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed in the above detailed description, but that the invention will include all embodiments falling within the scope of this disclosure.