Abstract:
A method of filling a storage vessel with a fuel gas at a fuel dispenser. In various embodiments, the method includes providing a fuel dispenser comprising a housing defining a fluid flow path therein, the fluid flow path operatively connected with a source of the fuel gas. The fuel dispenser includes a control valve disposed along the fluid flow path and a controller in operative electronic communication therewith. The method also includes actuating the control valve to provide a predetermined difference between the pressure of the fuel gas upstream of the control valve and the pressure of the fuel gas downstream of the control valve. The predetermined difference is selected such that the temperature of the fuel gas is reduced to a predetermined temperature after passing through the control valve. The method also includes dispensing the fuel gas into the storage vessel, the fuel gas having first mass flow rate.

Description:
PRIORITY CLAIM 
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 62/074,138, titled “Compressed Gas Filling Method and System,” filed Nov. 3, 2014, which is hereby relied upon and incorporated herein by reference for all purposes. 
     
    
     BACKGROUND 
       [0002]    The present invention relates generally to methods and systems for dispensing fuel gases, such as compressed natural gas (CNG), to a storage vessel on a vehicle. More specifically, embodiments of the present invention relate to systems and methods for ensuring that a fuel gas storage vessel is completely filled during dispensing, regardless of ambient temperature or the initial pressure of the storage vessel. 
         [0003]    Those of skill in the art are familiar with dispensing systems for vehicles that are fueled by gas that is flowed into a cylinder or vessel, such as natural gas vehicles (NGVs). In general, NGV storage vessels are rated to be completely filled with gas under a given pressure at a given temperature (e.g., 3600 psi at 70° F.). The Ideal Gas Law defines a direct relationship between the temperature and pressure of a gas being added into a fixed-volume storage vessel, and thus as the temperature of the gas increases, greater pressures are required to completely fill the vessel. As the gas inside the vessel cools, the pressure inside the vessel decreases. 
         [0004]    Most filling systems are configured to terminate when the dispenser measures a target pressure of the gas in the NGV storage vessel at which it is expected that the vessel should be completely filled. In the past, the target pressure has been based on the pressure at which the storage vessel is rated to be completely filled at the measured ambient temperature. In addition, as a safety measure, the filling system is typically configured to halt dispensing if the pressure in the vessel reaches a predetermined cutoff pressure. This cutoff pressure is typically greater than the target pressure but lower than the maximum pressure the vessel is designed to safely accommodate. 
         [0005]    As is known, prior art dispensing systems for CNG, including those that compensate for differences in ambient temperature, often fail to completely fill, or “charge,” the vessels, especially at higher ambient temperatures. In particular, during a “fast-fill” dispensing process (i.e., one that is typically designed to take fewer than five minutes), it is often the case that a pressure greater than the cutoff pressure is required to completely charge the NGV&#39;s storage vessel. As a result, the dispenser will terminate the filling process when the gas reaches the cutoff pressure, before the vessel is completely charged. 
         [0006]    Those of skill in the art are familiar with a variety of prior art systems which attempt to solve the problem described above. Examples of such attempts include U.S. Pat. Nos. 5,259,424; 5,641,005; 5,653,269; 5,868,176; and 5,881,779, the disclosure of each of which is incorporated by reference herein in its entirety for all purposes. Unfortunately, however, prior art attempts have failed to solve this problem, and they cannot accurately determine when the storage vessel will be completely filled to its rated capacity, yet not overfilled. 
       SUMMARY 
       [0007]    The present invention recognizes and addresses various considerations of prior art constructions and methods. According to one embodiment, the present invention provides a method of filling a storage vessel with a fuel gas at a fuel dispenser. The method comprises the step of providing a fuel dispenser comprising a housing defining a fluid flow path therein, the fluid flow path operatively connected with a source of the fuel gas and configured for fluid communication with the storage vessel. The fuel dispenser also comprises a control valve disposed along the fluid flow path and a controller in operative electronic communication with the control valve. The method also comprises actuating the control valve to provide a predetermined difference between the pressure of the fuel gas upstream of the control valve and the pressure of the fuel gas downstream of the control valve. The predetermined difference is selected such that the temperature of the fuel gas is reduced to a predetermined temperature after passing through the control valve. Finally, the method comprises dispensing the fuel gas into the storage vessel, the fuel gas having first mass flow rate. 
         [0008]    Those skilled in the art will appreciate the scope of the present invention and realize additional aspects thereof after reading the following detailed description of preferred embodiments in association with the accompanying drawing figures. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    A full and enabling disclosure of the present invention, including the best mode thereof directed to one skilled in the art, is set forth in the specification, which makes reference to the appended drawings, in which: 
           [0010]      FIG. 1  is a chart of the pressure required to fill a storage vessel to 25%, 50%, 75%, and 100% as a function of the ambient temperature in a prior art CNG dispensing system. 
           [0011]      FIG. 2  is a chart of the completeness of fill, expressed as a percentage of the compensated target pressure, as a function of the ambient temperature in a prior art CNG dispensing system. 
           [0012]      FIG. 3  is a schematic representation of a CNG dispensing system according to an embodiment of the present invention. 
           [0013]      FIG. 4  is a schematic detail view of the storage system of the CNG dispensing system of  FIG. 3 . 
           [0014]      FIG. 5  is a schematic detail view of a CNG dispenser of the CNG dispensing system of  FIG. 3 . 
           [0015]      FIG. 6  is a flowchart illustrating steps of a method of filling a storage vessel with a fuel gas according to one embodiment of the present invention. 
       
    
    
       [0016]    Repeat use of reference characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention. 
       DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0017]    Reference will now be made in detail to presently preferred embodiments of the invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present invention without departing from the scope or spirit thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the present disclosure including the appended claims and their equivalents. 
         [0018]    Some embodiments of the present invention may be particularly suitable for use in dispensing natural gas, such as with a CNG dispenser in a retail service station environment, and the below discussion will describe some preferred embodiments in that context. However, those of skill in the art will understand that the present invention is not so limited. In fact, it is contemplated that embodiments of the present invention may be used with any compressible fluid medium having a gaseous end-state and with fluid dispensing equipment (such as nozzles and fluid dispensing hoses) associated with each such fluid medium. For example, embodiments of the present invention may also be used in dispensing of liquid natural gas (LNG), hydrogen, butane, and propane, among other fuel gases. Further, embodiments of the present invention may be used with any type of storage vessel and with any type of vehicle. 
         [0019]    As noted above, prior art CNG dispensing systems often fail to completely charge a storage vessel. This underfilling is the result of a number of complex fluid dynamic and thermodynamic processes. Primary among these is that, during fast-filling, the gas in the storage vessel is compressed (or rather, recompressed) at a rapid rate, which in turn causes increases in gas temperature. This contributes to the result that pressures higher than those estimated based on ambient temperature (and, as explained below, in some cases higher than the dispenser cutoff pressure) are required to completely fill the vessel. In addition to the ambient temperature and the heat of compression, other factors can affect the ability of a dispenser to completely fill a vessel, including the initial vessel and supply gas pressures and the supply gas temperature. Further, hydraulic losses in the filling circuit, which may be as much as or more than 200 psi, and other dynamic fill conditions at the dispenser may reduce the maximum obtainable tank pressure. Moreover, if the mass flow rate of the fuel gas becomes too high, backpressures can develop upstream of the fueling nozzle and trigger opening of a safety relief valve. Depending on the ambient temperature, initial pressure in the storage vessel, and various characteristics of the dispensing system, the storage vessels may be left underfilled by as much as 20%. 
         [0020]    In this regard,  FIG. 1  is a chart of the pressure required to fill a storage vessel to 25%, 50%, 75%, and 100% as a function of the ambient temperature. Horizontal lines on the chart (“Fill limits”) represent exemplary pressures corresponding to the maximum capacity of a storage vessel (4500 psig), dispenser cutoff (4300 psig), and the pressure obtainable as a result of hydraulic losses (4100 psig). The line labeled “Fill Settled” on the chart represents the “settled” pressure, or the pressure inside the vessel when the temperature inside the vessel has cooled to the ambient temperature, at a given ambient temperature. As shown, for example, at ambient temperatures above about 40° F., the dispensing system is prevented from applying the pressure required to completely fill the storage vessel. 
         [0021]      FIG. 2  is a chart of the completeness of fill, expressed as a percentage of the compensated target pressure, as a function of the ambient temperature. The chart includes lines representing the completeness of fill for fills beginning with an empty storage vessel, with a quarter filled storage vessel, with a half-filled storage vessel, and with a three-quarters-filled storage vessel. It will be appreciated from  FIG. 2  that a complete charge of a storage vessel is possible at higher ambient temperatures (at least at those below about 90° F.) only where the filling process starts with a storage vessel that is already three-quarters filled. In contrast, where the filling process starts with a storage vessel that is empty, a complete charge is possible only at much lower ambient temperatures (e.g., up to approximately 40° F.). 
         [0022]    Thus, prior art CNG dispensing systems have failed to adequately address the problem of underfilling of vehicle storage vessels. In contrast, embodiments of the present invention provide systems and methods for ensuring that a fuel gas storage vessel is completely filled during dispensing, regardless of ambient temperature or the initial pressure of the storage vessel. As described in more detail below, some embodiments may employ a multi-stage process comprising, in one stage, using a dispenser control valve to modulate the temperature of the dispensed gas and the temperature inside the vessel to compensate for the increase in gas temperature in the storage vessel as a result of compression. 
         [0023]    Turning now to  FIGS. 3-5 , aspects of a CNG dispensing system in accordance with an embodiment of the present invention are described below.  FIG. 3  is a schematic representation of a CNG dispensing system  10 . First, a CNG supply source  12  is in fluid communication with a dryer  14 . As those of skill in the art will appreciate, dryer  14  is preferably operative to remove water vapor from the CNG supply before it is stored or used. In one embodiment, dryer  14  may be analogous to the natural gas dryers offered by ANGI Energy Systems, Inc. of Janesville, Wis., the assignee of the present invention. In other embodiments, however, dryer  14  may be any suitable dryer technology familiar to those of skill in the art, including non-regenerating inlet dryers, regenerating inlet dryers, and twin-tower fully automatic regenerating dryers, among others. 
         [0024]    CNG leaving dryer  14  may flow along a fluid conduit  16  to one or more compressors  18 . In some embodiments, only a single compressor  18  may be provided, but in other embodiments multiple compressors  18  may be provided for additional capacity and/or redundancy, according to the needs of a particular dispensing system. Those of skill in the art can select suitable compressors  18  for use with embodiments of the present invention. In one embodiment, however, compressors  18  may preferably be analogous to the reciprocating compressors offered by ANGI Energy Systems. After compression, the CNG may be delivered to a storage system  22  and/or to one or more CNG dispensers  24 , and ultimately to a storage vessel in a vehicle. 
         [0025]      FIG. 4  is a schematic detail view of storage system  22 . Those of skill in the art are familiar with storage systems for CNG dispensing applications and can select an appropriate storage system for use with embodiments of the present invention. Moreover, storage system  22  may not be provided in all embodiments. In one embodiment, however, storage system  22  may be analogous to the small or large vessel ASME storage systems offered by ANGI Energy Systems. As shown, storage system  22  may be a cascade storage system comprising low-, mid-, and high-bank storage vessels  26 ,  28 , and  30  in operative fluid communication with a priority panel  32  that directs the CNG to storage system  22  or dispensers  24 . A suitable gas management system may be used to prioritize delivery of CNG as is well known. 
         [0026]    In general, priority panel  32  may comprise a valve  34  that is in fluid communication with supply plenum  20  and controlled by a valve controller  36 . A first pressure transducer  38 , which is in electronic communication with valve controller  36 , is operative to measure the pressure upstream of the hose of fuel dispenser  24 . Valve controller  36  may thus cause valve  34  to open and close based on feedback from pressure transducer  38 . More particularly, valve controller  36  and valve  34  may function as a backpressure regulating system to keep a predetermined amount of gas flowing in supply plenum  20 . Thus, valve controller  36  may cause valve  34  to open an amount based on the extent to which the discharge from compressors  18  exceeds the demand from dispensers  24 . When there is no demand from dispensers  24 , valve  34  may be completely opened to fill vessels  26 ,  28 ,  30 . It will be appreciated that, in other embodiments, this function may be performed by a spring-loaded backpressure valve. In the illustrated embodiment, valve  34  may be sized for full turndown and to accommodate one half of the total flow rate from compressors  18  when fully opened. In this regard, valve  34  may preferably be appropriately sized for best back-pressure control and is able to turn down the flow with the most precise control. Pressure transducer  38  may be configured to measure pressures between 0 and 6000 psig. Depending on the demand from dispensers  34  and the stages of dispensing then ongoing, valve controller  36  may target pressures of between 4300 and 5000 psig at pressure transducer  38 . 
         [0027]    A second pressure transducer  40  may be provided as redundant to transducer  38 . In particular, in some embodiments, if readings are different between transducers  38 ,  40 , a control algorithm may assume that one of the transducers is broken and give a fault notification. Pressure transducer  40  may also be used in lead/lag logic for controlling compressors  18 , for example to control when compressors  18  should operate when there is no demand from dispensers  24 . 
         [0028]      FIG. 5  is a schematic detail view of a CNG dispenser  24  of CNG dispensing system  10 . Certain components of dispenser  24 , such as valves, pressure transducers, temperature indicators, mass flow meters, dispensing hoses, and nozzles, may preferably be analogous to corresponding components found in CNG dispensers familiar to those of skill in the art, including those offered by ANGI Energy Systems. Thus, a detailed description of these components is not provided herein, except where the components differ in accordance with embodiments of the present invention. 
         [0029]    Dispenser  24  preferably comprises a housing  42  in which a gas conduit  44  extends. As shown, gas conduit  44  is in fluid communication with supply plenum  20  at one end and a pressure-tight dispensing hose  46  at the other. Gas flowing from supply plenum  20  may enter dispenser  24  via a gas inlet valve  45 . Dispensing hose  46  may terminate in a conventional CNG refueling nozzle, such as an NGV type 1 or type 2 nozzle, which may have a three-way valve  48 . The nozzle may interface with a conventional connector on a vehicle to allow fluid communication between gas conduit  44  and a storage vessel  50  of the vehicle. 
         [0030]    In addition, dispenser  24  may comprise a proportional control valve  52  disposed along gas conduit  44 . Control valve  52 , which may be a rising stem valve in some embodiments, is preferably under the control of a controller  54 . Controller  54  may be a driver or actuator for valve  52 , and controller  54  is preferably in operative electronic communication with (and controlled by) a dispenser control system  55 . Control system  55  may be any suitable electronics with associated memory and software programs running thereon whether referred to as a processor, microprocessor, controller, microcontroller, or the like. In one embodiment, however, control system  55  is preferably comparable to the microprocessor-based control systems for CNG dispensers offered by ANGI Energy Systems, but modified in accordance with the present invention. The memory of control system  55  may be any suitable memory or computer-readable medium as long as it is capable of being accessed by the controller, including random access memory (RAM), read-only memory (ROM), erasable programmable ROM (EPROM), or electrically EPROM (EEPROM), CD-ROM, DVD, or other optical disk storage, solid-state drive (SSD), magnetic disc storage, including floppy or hard drives, any type of suitable non-volatile memories, such as secure digital (SD), flash memory, memory stick, or any other medium that may be used to carry or store computer program code in the form of computer-executable programs, instructions, or data. Control system  55  may also include a portion of memory accessible only to control system  55 . Control system  55  preferably stores computer-executable instructions which may cause control system  55  to carry out the steps of a method of filling a storage vessel with a fuel gas described below. 
         [0031]    In accordance with embodiments of the present invention, controller  54  is preferably operative to actuate valve  52  to modulate the temperature of CNG flowing in gas conduit  44  and to control the pressure of CNG downstream of valve  44 , depending on the mode of operation of dispenser  24 . Controller  54  and control system  55  are preferably in electronic communication with a mass flow meter  56 , which may output to control system  55  information representative of the mass flow rate  58  and the temperature  60  of CNG being dispensed. In some embodiments, flow meter  56  may be a Coriolis mass flow meter, such as the Coriolis flow meters offered by Micro Motion, Inc. of Boulder, Colo. Control system  55  is preferably operative to determine the mass and volume of CNG dispensed based on flow rate information received from flow meter  56 . 
         [0032]    Dispenser  24  may also comprise a pressure transducer  62  in electronic communication with controller  54  and control system  55  that is operative to measure the pressure in conduit  44  and hose  46  downstream of mass flow meter  56 . Further, dispenser  24  may comprise a temperature sensor  64 , also in electronic communication with control system  55 , that is operative to measure the ambient temperature. Pressure transducer  62  and temperature sensor  64  may respectively transmit information representative of their measured pressures and temperatures to control system  55 , which may use this information in control decisions as described in more detail below. As those of skill in the art will appreciate, control system  55  may also be in electronic communication with other fuel dispensing equipment and components, such as solenoid valves associated with low-, mid-, and hi-bank storage vessels  26 ,  28 , and  30 . 
         [0033]    Dispenser  24  may additionally comprise a safety relief valve  66  in fluid communication with conduit  44 . Relief valve  66  is preferably operative to relieve excessive backpressure in conduit  44  and may, for example, be configured to open at pressures at or exceeding 4500 psig in some embodiments. Finally, a fast-acting, positive shutoff valve  68  and a one-way check valve  70  may be disposed along conduit  44  as shown. 
         [0034]    In various embodiments, system  10  described above may be used to dispense CNG to a vehicle and completely fill the vehicle&#39;s storage vessel, regardless of ambient temperature or initial vessel pressure. In general, embodiments of the present invention may comprise a multi-stage dispensing process. In a first stage, the process comprises obtaining information about (or “characterizing”) a vehicle that is to be refueled at a dispenser  24 . This stage may include determining the mass of CNG already in the storage vessel and determining the filling resistance. A second stage of the process comprises prescribing a “fueling profile,” including target values for fueling, and then dispensing CNG in accordance with the fueling profile. Depending on the fueling profile prescribed, this second stage may involve filling the storage vessel with a predetermined amount of CNG that has been cooled in accordance with the fueling profile. In preferred embodiments, a control valve within the dispenser is used to reduce the temperature of the CNG prior to the CNG being introduced into the storage vessel. In a third stage, the process may complete filling of the storage vessel at the highest flow rate possible that does not cause the pressure of the CNG in the fuel dispensing hose or in the storage vessel to exceed a predetermined limit. This stage may involve filling without regard to the temperature of the dispensed CNG. As described below, the process may vary depending on the fueling profile prescribed, and the process may include fewer or more steps in some embodiments. 
         [0035]    In this regard,  FIG. 6  is a flowchart illustrating steps of a method of filling a storage vessel with a fuel gas according to one embodiment of the present invention.  FIG. 6  is explained in detail below as used with system  10  described in  FIGS. 3-5  above, and thus in the embodiment illustrated in  FIG. 6 , the fuel gas is CNG. As explained above, however, those of skill in the art will appreciate that the method may be used with any compressible fluid medium having a gaseous end-state. Additionally, those of skill in the art will appreciate that, in various embodiments, some steps of the method may be executed in a different order than the order described below without sacrificing benefits of the present invention. 
         [0036]    At step  100 , the process starts. At this point, a user may have initiated a fueling transaction. For example, the user may have connected a nozzle of a fuel dispenser  24  with a vehicle storage vessel and started the dispensing process, for example by obtaining authorization for a transaction and pressing a “start” button on the dispenser. As those of skill in the art will appreciate, if dispenser  24  is equipped to dispense fuel to more than one size of storage vessel, control system  55  of dispenser  24  may know the unfilled capacity of the storage vessel based on the particular nozzle the user connects to his or her vehicle. 
         [0037]    At step  102 , control system  55  of dispenser  24  may obtain the ambient temperature and the initial (static) storage vessel pressure (“static pressure”). For example, control system  55  may receive information representative of the ambient temperature from temperature indicator  64 . Further, control system  55  may receive information representative of the pressure at pressure transducer  62 . Assuming that the hose is pressurized from a previous fill and that the hose pressure has equalized with the pressure inside the storage vessel, the pressure at pressure transducer  62  may be assumed to be the initial pressure inside the storage vessel. It will be appreciated that, in some embodiments, it may be necessary to re-pressurize the hose and allow the flow and pressure to settle before determining the initial storage vessel pressure. The values of the ambient temperature and initial storage vessel pressure may be stored in memory accessible to control system  55 . 
         [0038]    Control system  55  may then determine the mass of CNG that is needed to completely fill the vehicle storage vessel (step  104 ). Those of skill in the art are familiar with methods for making this determination, and any such method may be used in embodiments of the present invention. For example, one such method is disclosed in the above-mentioned U.S. Pat. No. 5,653,269. 
         [0039]    In general, control system  55  may determine the needed mass of CNG by first calculating the volume of storage vessel  50 . Control system  55  may open the appropriate valve (e.g., a solenoid valve associated with low bank storage vessel  26  in storage system  22 ) to allow a predetermined mass of CNG to be dispensed into storage vessel  50 . Control system  55  may then close the valve and obtain the pressure measured at pressure transducer  62 , which will have risen from its previous value. The change in pressure between the initial pressure and the pressure measured after a known mass of CNG has been dispensed may then be used to determine the volume of the vehicle tank using the Ideal Gas Law, as is known. 
         [0040]    Next, control system  55  may use the initial pressure of storage vessel  50  and the ambient temperature (both determined above) to calculate the temperature-compensated full tank pressure, for example with reference to supercompressibility data stored in memory, as is also known. In some embodiments, an empirical factor to compensate for compression heating may be multiplied to the full tank pressure value, though this is not required. In any event, the full tank pressure may be calculated in a manner similar to that by which prior art dispensers calculate the cutoff pressure expected to correspond to a completely full storage vessel  50 . Control system  55  may then use the values of the above-calculated volume of vessel  50  and cutoff pressure as inputs into the Ideal Gas Law, solved for mass, to determine the mass of CNG needed to fill vessel  50 . 
         [0041]    At step  106 , control system  55  may determine the filling resistance. The filling resistance is representative of the hydraulic pressure losses in the filling circuit, which as noted above may reduce the pressure obtainable at dispenser  24 . The filling resistance also makes it difficult for dispenser  24  to dynamically sense or estimate the pressure in vessel  50  during the fill cycle. As a result of this resistance, for example, the pressure within the fuel dispensing hose may be dynamically much greater than the actual (dynamic) pressure within vessel  50 . 
         [0042]    To determine the filling resistance in one embodiment, control system  55  may open the appropriate valve (e.g., a solenoid valve associated with low-bank storage vessel  26 ) to allow CNG to be dispensed. When the flow rate of CNG has peaked and is steady, control system  55  may record the maximum flow rate and the pressure at pressure transducer  62  (“dynamic pressure”). Control system  55  may cause the valve to close after recording these measurements. Also, after closing the valve, control system  55  may again sample the pressure at pressure transducer  62  to obtain the pressure in storage vessel  50 . 
         [0043]    For subsonic flow conditions at the nozzle, the filling resistance is calculated as follows: Filling resistance=(dynamic pressure−static pressure)/maximum flowratê2. Therefore, control system  55  may estimate the pressure inside storage vessel  50  throughout the filling process by rearranging the previous equation as follows: static pressure=dynamic pressure−filling resistance*maximum flowratê2. 
         [0044]    In some embodiments, based on the above measurements and calculations, control system  55  may prescribe a fueling profile for the vehicle storage vessel. The fueling profile may indicate, for example, the type of compensation for ambient temperature and/or compression heating to be applied during dispensing. For example, control system  55  may evaluate one or more of the ambient temperature, the initial pressure of storage vessel  50 , the full tank pressure, and/or the filling resistance to determine whether storage vessel  50  can be completely filled without additional compensation for the heat of compression (step  108 ). In other words, depending on some or all of these factors, control system  55  may determine that standard temperature-compensation methods may be adequate to provide a complete fill of storage vessel  50 . This may be the case where, for example, the ambient temperature is below a predetermined temperature and the initial pressure of vessel  50  is above a predetermined pressure. In some embodiments, control system  55  may access a lookup table in memory comprising ambient temperatures, initial pressures, and/or other conditions for which complete filling is possible with only temperature compensation. If it is determined that a complete fill is possible without additional compensation, as shown in  FIG. 6 , the process proceeds to step  114 , described below. 
         [0045]    If, however, control system  55  determines that standard temperature compensation methods are inadequate to provide a complete fill, the fueling profile may indicate that additional compensation is necessary. In this case, system  10  is preferably operative to compensate for compression heating by dispensing a predetermined amount of cooled CNG into vessel  50 . Thus, control system  55  may actuate control valve  52  to cool the CNG to a predetermined temperature (step  110 ) and may cause dispensing of a predetermined amount of CNG at that temperature (step  112 ). As described below, in various embodiments, the predetermined temperature and/or the predetermined mass may be calculated based on previous measurements (e.g., of ambient temperature and initial vessel pressure) and specified in the fueling profile. 
         [0046]    More particularly, based on the Ideal Gas Law, gas moving from a higher pressure to a lower pressure will have a reduction in temperature. Thus, in embodiments of the present invention, cooling of the CNG may be achieved by controller  54  actuating control valve  52  such that a pressure differential is created across control valve  52  that is sufficient to cause the desired cooling. In one exemplary embodiment, the pressure upstream of valve  52  may be 5000 psig, and the valve  52  may be closed to a position that causes the pressure downstream of valve  52  to be 1000 psig. In contrast to the prior art processes, wherein temperatures at the fuel dispenser nozzle could approach or exceed 120° F., during this stage temperatures at the nozzle may be approximately −20° F. in one embodiment. Control system  55  may then allow a continuous stream comprising a predetermined mass of CNG to be dispensed at that temperature, or it may dispense the predetermined mass in several (e.g., 3-7) small bursts. When the CNG enters vessel  50  it may cool further due to further expansion. In some embodiments, one or more intercooling devices may also be provided along conduit  44  and/or supply plenum  20 , though this is not required in all embodiments. 
         [0047]    The dispensed, cooled CNG will reduce the temperature inside vessel  50  during this stage of dispensing, and this reduction in temperature will compensate for increases in temperature of the gas inside vessel  50  due to the heat of compression when the remainder of the CNG is dispensed. As the temperature of the CNG in vessel  50  falls, so will its pressure. Thus, embodiments of the present invention can dispense the entire mass of CNG needed to fill storage vessel  50 , regardless of ambient temperature or initial vessel pressure, without having to fill storage vessel  50  to a pressure higher than dispenser  24 &#39;s cutoff pressure. 
         [0048]    Those of skill in the art will appreciate that the temperature to which the CNG is cooled and the predetermined mass of cooled CNG dispensed may vary in embodiments of the present invention. In some embodiments, the fueling profile may comprise a particular mass of CNG to be dispensed at a preset, cooled temperature. In other embodiments, the fueling profile may also comprise the temperature to which the gas should be cooled. In some embodiments, for example, control system  55  may determine that it is necessary to create only a 500 psig pressure differential across control valve  52 , rather than a 4000 psig differential noted above. The temperature to which the CNG is cooled and the mass dispensed at that temperature may vary depending on the ambient temperature, the initial pressure of vessel  50 , the full tank pressure, and/or the filling resistance. Information output from mass flow meter  56  may be used to verify that the CNG has been cooled to the desired temperature and that the desired mass is dispensed. Those skilled in the art may determine by experimentation the temperatures to which the CNG should be cooled and the mass dispensed at the cooled temperature to provide the desired compensation for compression heating at various ambient temperatures and initial tank pressures. Further, the reduction in temperature achieved by the reduction in pressure may be determined by reference to the supercompressibility tables for the gas involved. 
         [0049]    In compensating for the heat of compression, the predetermined mass of cooled CNG is dispensed at lower flow rates due to the pressure differential across control valve  52 . Once this compensation has been performed, however, in embodiments of the invention the remaining mass of CNG needed to completely fill storage vessel  50  is dispensed at higher flow rates. The temperature of the CNG dispensed at these higher flow rates will likewise be higher, but compression heating caused by dispensing of the remainder of CNG is offset by the lower temperature of the predetermined mass of CNG dispensed in the steps described above. 
         [0050]    As explained previously, if the mass flow rate of the CNG becomes too high, backpressures can develop upstream of the fueling nozzle and cause the dispenser to shutoff prematurely, for example by triggering safety relief valve  66 . Thus, the higher flow rate is preferably selected (and, if necessary, iteratively adjusted) to provide the highest flow rate that does not cause the pressure measured at pressure transducer  62  to exceed a level that would cause dispenser  24  to shut off. 
         [0051]    Accordingly, at step  114 , the process comprises actuating control valve  52  to adjust the allowable mass flow rate of CNG. As explained above, the dynamic pressure measured at transducer  62  depends on the mass flow rate, and thus adjusting the mass flow rate via valve  52  also adjusts the pressure measured at transducer  62 . (Also as noted above, if the additional compensation described above with reference to steps  110  &amp;  112  is not required, the process may skip those steps and proceed with step  114 .) At step  116 , the remaining CNG needed to fill storage vessel  50  is dispensed. 
         [0052]    More specifically, control system  55  may cause valve  52  to open an amount that allows a higher mass flow rate than the mass flow rate at the previous stage of dispensing. In some embodiments, adjustments to the position of valve  52  may be based on preset values stored in memory, and in other embodiments the adjustments may be calculated by control system  55  based on the mass of CNG needed to fill vessel  50 , the initial pressure of vessel  50 , and/or the filling resistance. Control system  55  may receive information regarding the mass flow rate  58  from mass flow meter  56 , as noted above. 
         [0053]    At step  118 , control system  55  may determine whether the pressure measured at transducer  62  exceeds a predetermined limit. If so, the process returns to step  114 , whereby control system  55  may iteratively adjust the position of valve  52  (and thus, the mass flow rate of CNG) based on feedback from pressure transducer  62  to maintain the pressure below the predetermined level. In some embodiments, the predetermined pressure level may also be a preset value stored in memory. In other embodiments, the predetermined pressure level may vary based on the mass of CNG needed to fill vessel  50 , the initial pressure of vessel  50 , the filling resistance, the settings of pressure relief valve  66 , and/or other dispenser pressure limitations. In some embodiments, the predetermined pressure level may be specified in the fueling profile. In one embodiment, controller  52  may monitor the output of pressure transducer  62  to keep the pressure below about 4200-4300 psig, for example adjusting the position of valve  52  to reduce the mass flow rate if the pressure reaches or exceeds that value. 
         [0054]    If the pressure measured at transducer  62  does not exceed the predetermined limit, the process proceeds to step  120 . Here, control system  55  may determine whether vessel  50  has been completely filled. In one embodiment, control system  55  may determine whether vessel  50  has been completely filled based on information from mass flow meter  56  indicative of the mass of CNG that has been dispensed. In another embodiment, control system  55  may determine that vessel  50  has been filled based on information from pressure transducer  62  that the pressure has reached the temperature-compensated cutoff pressure, calculated above. If vessel  50  is not completely filled, the process may return to step  116 . If vessel  50  is completely filled, the process ends (step  122 ). 
         [0055]    It can thus be seen that embodiments of the present invention provide novel systems and methods for filling a storage vessel with natural gas and/or other compressible fluid media. By employing a dispenser control valve to reduce the temperature of fuel gas dispensed into a storage vessel, embodiments of the present invention may compensate for the heat of compression that occurs during fuel gas dispensing processes. Embodiments of the present invention may thus enable complete filling of a vehicle storage vessel for fuel gas without regard to ambient temperature or initial storage vessel pressure or fill level. While one or more preferred embodiments of the invention have been described above, it should be understood that any and all equivalent realizations of the present invention are included within the scope and spirit thereof. The embodiments depicted are presented by way of example only and are not intended as limitations upon the present invention. Thus, it should be understood by those of ordinary skill in this art that the present invention is not limited to these embodiments since modifications can be made. Therefore, it is contemplated that any and all such embodiments are included in the present invention as may fall within the scope and spirit thereof.