Abstract:
A method and system for preventing gas pressure in a pressure vessel from dropping below a minimum allowable pressure. Pressure readings from a pressure sensor downstream of a pressure regulator are monitored by a processor as they vary within a steady fluctuation band under normal regulated pressure conditions. When the pressure regulator reaches a fully open position in low vessel pressure conditions, the processor detects a drop in the pressure reading to a value below the recent fluctuation band, and recognizes that the pressure is dropping below the regulation pressure value. The processor can use this information to shut off flow of gas from the vessel, thus preventing the vessel from dropping below its minimum allowable pressure, regardless of the actual magnitude of the pressure reading from the sensor—which can vary through a wide range due to tolerances.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to a method for preventing gas pressure in a pressure vessel from dropping below a minimum allowable pressure and, more particularly, to a method for preventing pressure in one or more pressure vessels in a hydrogen storage system from dropping below a minimum allowable pressure which monitors the normal tolerance-driven fluctuations in pressure readings at a pressure sensor downstream of a pressure regulator and, if a pressure drop in excess of the normal fluctuations is detected, shuts down the hydrogen storage system to prevent the pressure in the vessels from dropping too low. 
     2. Discussion of the Related Art 
     Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cell systems as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today&#39;s vehicles employing internal combustion engines. 
     Typically hydrogen gas is stored at high pressure in pressure vessels on the vehicle to provide the hydrogen necessary for the fuel cell system. The pressure in the vessels can be 700 bar or more. In one known design, the pressure vessels include an inner plastic liner that provides a gas tight seal for the hydrogen gas, and an outer carbon fiber composite layer that provides the structural integrity of the vessel. A hydrogen storage system typically includes at least one pressure regulator that reduces the pressure of the hydrogen gas from the high pressure of the vessels to a pressure suitable for the fuel cell system. 
     If the pressure within the vessels falls below a certain value, and the vessels are then refilled at a high rate of pressure increase, the inner liner layer may begin to separate from the outer structural layer. This separation could cause inner liner damage and loss of leak-tightness, and thus must be avoided. A current solution to avoid this separation is to maintain a high enough pressure within the vessels to prevent inner liner layer separation. For one exemplary vessel design, a minimum pressure of 20 bar must be maintained in the vessels to prevent the inner liner layer from separating from the outer structural layer. 
     One or more pressure sensors provide a measurement of the hydrogen gas pressure within the vessels and elsewhere in the hydrogen storage system. Because the pressure sensors employed in these types of systems need to provide a pressure measurement over a range of nearly 1000 bar, and they need to be relatively inexpensive, they typically have a tolerance band of about 1.5%, which gives an accuracy of +/−15 bar. Further, considering the measurement requirements of the sensor wiring over the entire temperature range that the vessels may encounter typically provides a measurement accuracy of +/−35 bar, which is added to the 20 bar minimum allowable pressure to provide the desired safety margin. Thus, in typical system designs, hydrogen discharge from the vessels needs to be stopped at a vessel pressure sensor reading of about 55 bar, resulting in about 5% of the hydrogen gas within the vessels not being usable for vehicle operation. 
     A method is needed for reliably protecting the pressure vessels from dropping below the minimum allowable pressure, but still allowing the most possible hydrogen gas to be consumed by the fuel cell. Such a method would allow the vehicle to be driven a greater distance between refueling events, thus improving customer satisfaction, while still protecting the vessels from dropping below the minimum allowable pressure. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a method and system are disclosed for preventing gas pressure in a pressure vessel from dropping below a minimum allowable pressure. Pressure readings from a pressure sensor downstream of a pressure regulator are monitored by a processor as they vary within a steady fluctuation band under normal regulated pressure conditions. When the pressure regulator reaches a fully open position in low vessel pressure conditions, the processor detects a drop in the pressure reading to a value below the recent fluctuation band, and recognizes that the pressure is dropping below the regulation pressure value. The processor can use this information to shut off flow of gas from the vessel, thus preventing the vessel from dropping below its minimum allowable pressure, regardless of the actual magnitude of the pressure reading from the sensor—which can vary through a wide range due to tolerances. 
     Additional features of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a hydrogen storage system for a fuel cell; 
         FIG. 2  is a cross-sectional view of a pressure vessel used for hydrogen gas storage in the hydrogen storage system of  FIG. 1 ; 
         FIG. 3  is a graph of gas pressure at three pressure sensors in the hydrogen storage system of  FIG. 1 ; 
         FIG. 4  is a graph of pressure data from a pressure sensor as recorded in a system processor; and 
         FIG. 5  is a flow chart diagram of a method which can be used to shut down the hydrogen storage system in order to prevent the pressure in the vessels from dropping below a minimum allowable pressure. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The following discussion of the embodiments of the invention directed to a method and system for preventing a pressure vessel from dropping below a minimum allowable pressure is merely exemplary in nature, and is in no way intended to limit the invention or its applications or uses. For example, the disclosed methods and systems have particular application to a hydrogen storage system for a fuel cell vehicle, but may also be applicable to any gas storage or gas handling system. 
       FIG. 1  is a schematic diagram of a hydrogen storage system  10  for a fuel cell  34 . Pressure vessels  12  store hydrogen gas at a high pressure. More or fewer of the pressure vessels  12  could be used than the three shown in the hydrogen storage system  10 . A pressure sensor  14  measures the pressure at a filler line  16  which is used to fill the vessels  12  from an external supply (not shown). Shut-off valves  18  are situated just downstream from each of the pressure vessels  12 . The terms upstream and downstream are used throughout this disclosure with respect to the direction of hydrogen gas flow, with the fuel cell  34  being downstream of the pressure vessels  12 . A pressure sensor  20  measures the hydrogen pressure between the shut-off valves  18  and a pressure regulator  22 . The pressure regulator  22  is used to reduce the pressure of the hydrogen gas from the high pressure of the vessels  12  down to a lower pressure which is near that required by the fuel cell  34 . 
     A pressure sensor  24  measures the hydrogen pressure downstream of the regulator  22  and upstream of a shut-off valve  26 . The shut-off valve  26  can be closed to completely isolate the hydrogen storage system  10  from the fuel cell  34 . A second pressure regulator  28  is used to reduce the hydrogen gas pressure down to the low pressure required by the fuel cell  34 , which may be around 6 bar. Fuel supply line  30  connects the hydrogen storage system  10  to the fuel cell  34 . A controller  32 —in communication with the valves  18 , the sensors  14 ,  20 , and  24 , the regulators  22  and  28 , the valve  26 , and the fuel cell  34 —can be used to monitor conditions in the hydrogen storage system  10  and the fuel cell  34 , and control the shutdown of the hydrogen storage system  10  if necessary to prevent the hydrogen gas pressure in the vessels  12  from dropping too low. For simplicity, the details of the fuel cell  34  are omitted, as are various filters, check valves, relief valves, and other components of the hydrogen storage system  10 . 
     The pressure of the hydrogen gas contained in the pressure vessels  12  can be as much as 700 bar or even higher. As a result, a high-range pressure transducer or sensor, typically with a range of about 900 bar, must be used for at least the pressure sensors  14  and  20 . The same type of sensor may also be used for the pressure sensor  24 . High-range sensors inherently have a high tolerance band around the pressure readings, which can significantly affect the accuracy of readings throughout their range. A typical pressure transducer or pressure sensor, such as the pressure sensor  14  which measures the pressure of the hydrogen gas in the vessels  12 , consists of a membrane and possibly several electronic components configured such that an output voltage signal is produced which is proportional to the pressure differential being experienced by the membrane. Each of the components of the sensor  14  has a base tolerance which can be represented as a plus or minus pressure variance. In addition, each of the components also experiences long-term drift, which further impacts the accuracy of the sensor  14 . When all of the tolerances of the components are added up, a typical high-range pressure transducer or sensor, such as the pressure sensor  14 , can have a pressure reading tolerance of +/−35 bar or higher. 
       FIG. 2  is a cross-sectional view of one of the pressure vessels  12  from the hydrogen storage system  10 . The vessel  12  includes an outer structural layer  40 , typically made of a carbon fiber composite material to provide structural integrity, and an inner liner  42 , typically made of a durable molded plastic, such as a high density polyethylene. An interior volume  44  contains the hydrogen gas. The liner  42  provides an impervious surface for containment of the hydrogen gas, and the outer layer  40  provides the structural integrity for the high pressures of the compressed hydrogen gas. The vessel  12  includes an adaptor (not shown) in an opening extending through the outer structural layer  40  and the inner liner  42  that provides access to the interior volume  44  for filling the vessel  12  and removing gas from the vessel  12  in a manner that is well understood to those skilled in the art. 
     The design of the pressure vessel  12  described above has proven to be reliable and cost effective. However, experience has shown that if the vessel  12  is initially pressurized with hydrogen gas, then the gas pressure is subsequently allowed to drop to a very low value, followed by a rapid re-pressurization, delamination of the inner liner  42  from the outer structural layer  40  can occur. In order to prevent this delamination, the hydrogen storage system  10  must be designed to prevent the pressure of the hydrogen gas in the interior volume  44  from dropping below a minimum allowable pressure value, typically about 20 bar in a common vessel design. 
     The high tolerance band on the pressure readings at the pressure sensor  14 , described above, makes it difficult to accurately determine how much hydrogen actually remains in the vessels  12 . This creates a problem when the pressure is nearing the minimum allowable pressure, as the hydrogen storage system  10  may need to be shut down prematurely in order to protect the vessels  12 . For example, the vessels  12  may have a minimum allowable pressure of 20 bar. If the pressure sensor  14  has a tolerance of +/−35 bar, then the minimum allowable pressure of 20 bar could possibly be reached when the pressure sensor  14  reads 55 bar (20 bar actual pressure plus 35 bar tolerance). Therefore, with a +/−35 bar tolerance around readings at the sensor  14 , the hydrogen storage system  10  would have to be designed to shut down when the pressure reading at the sensor  14  reaches 55 bar in order to protect the vessels  12 . However, due to the uncertainty of the pressure reading at the sensor  14 , the actual pressure remaining in the vessels  12  may be as high as 90 bar (the 55 bar reading plus the 35 bar tolerance) in such a situation. The result of all of this is that the hydrogen storage system  10  and the fuel cell  34  will have to be designed to shut down when, in most cases, a significant usable amount of hydrogen still remains in the vessels  12 . 
     It is possible to monitor other data in the hydrogen storage system  10 , besides the high-tolerance band pressure reading at the sensor  14 , to determine when the pressure in the vessels  12  is nearing the minimum allowable pressure. In particular, the pressure at the sensor  24  downstream of the pressure regulator  22  can be monitored to detect a pressure drop, which indicates that the regulator  22  is fully open. In a typical current design of the hydrogen storage system  10 , the regulator  22  has a regulation pressure slightly higher than the minimum allowable pressure of the vessels  12 . This means that, if the regulator  22  is fully open and the pressure downstream of the regulator  22  is dropping below the regulation pressure, then the pressure in the vessels  12  is getting very close to the minimum allowable pressure and the hydrogen storage system  10  must be shut down soon. 
       FIG. 3  is a graph  100  showing the pressure readings at the pressure sensors  14 ,  20 , and  24  as the pressure in the hydrogen storage system  10  drops and the pressure regulator  22  fully opens. Horizontal axis  102  represents time, while vertical axis  104  represents pressure. Curve  106  is the pressure at the sensor  20 , just upstream of the regulator  22 . Curve  108  is the pressure at the sensor  24 , just downstream of the regulator  22 . Curve  110  is the pressure at the sensor  14 , upstream of the vessels  12 . At the left ends of the curves  106 ,  108 , and  110 , it can be seen that the pressure at the sensors  14  and  20  is dropping, while the pressure at the sensor  24  is holding steady at the regulation pressure of the regulator  22 . Around the middle of the curves, at the time indicated by time mark  112 , the pressure at the sensor  20 , shown by the curve  106 , reaches the regulation pressure value. From this time onward, the regulator  22  is fully open, and the pressure at the sensors  20  and  24  are essentially the same. Meanwhile, the pressure at the sensor  14  is somewhat higher, due to pressure drops in the hydrogen storage system  10  caused by various check valves, pipes, and other components. 
     The regulation pressure value, indicated by pressure mark  114 , is about 29 bar in a typical implementation. The minimum allowable pressure in the vessels  12  is shown by line  116 . As mentioned above, the minimum allowable pressure, indicated by pressure mark  118 , is typically about 20 bar. The tolerance bands on the regulation pressure of the regulator  22  are shown by lines  120  and  122 . It can be seen by the relationship of the curves on the graph  100  that the full opening of the pressure regulator  22  can be used as an indication that the pressure in the vessels  12  is approaching the minimum allowable pressure, and that the hydrogen storage system  10  needs to be shut down soon to prevent further pressure drop. The mechanical tolerance band on the regulation pressure of the regulator  22  is much tighter than the combined mechanical and electrical tolerances of the sensors  14 ,  20 , and  24 —especially when the analog to digital conversion tolerances at the controller  32  are taken into account. Therefore, it is possible to design a system shutdown strategy based on the regulation pressure of the regulator  22  which is essentially immune to the large tolerances of the pressure readings at the sensors  14 ,  20 , and  24 . 
     In order to use the full opening of the regulator  22  as a trigger for shutting down the hydrogen storage system  10  to prevent dropping below the minimum allowable pressure, the inherent behavior of the pressure readings at the sensor  24  must be understood.  FIG. 4  is a graph  130  showing the pressure signal from the sensor  24  as stored by the controller  32 . Just as with the graph  100  of  FIG. 3 , horizontal axis  132  represents time, and vertical axis  134  represents pressure. Pressure trace  136  is the pressure reading at the sensor  24  as stored by the controller  32 . Thus, the graph  130  is essentially a greatly magnified version of the curve  108  on the graph  100 . On the graph  130 , the pressure trace  136  shows a fluctuation around a median value designated by the pressure mark  138 . This fluctuation is an inherent trait of the pressure readings as stored in the controller  32 , and this fact can be used as the basis of the control strategy of the present invention. 
     The controller  32  must monitor data from many different devices, perform numerous real-time calculations, and run many control algorithms simultaneously. Therefore, it is critical that memory space and computing power be allocated judiciously. In a typical implementation, the pressure readings from the sensors  14 ,  20 , and  24  are stored in registers of only 8 bits in size. This means that the 900 bar range of the sensor  24 , for example, must be divided up between 2 8  (the number 2 raised to the power of 8) or 256 increments. 900 bar divided by 256 increments equals 3.5156 bar per increment, which is the pressure reading resolution in the controller  32 . This will be rounded to 3.5 bar/increment in this discussion for brevity. Returning attention to the pressure trace  136  on the graph  130 , the fluctuations above and below the median pressure value represent this phenomenon. That is, pressure mark  140  is 3.5 bar higher than the pressure mark  138 , and pressure mark  142  is 3.5 bar lower than the pressure mark  138 . Because of the tolerances in the sensor  24 , including its mechanical tolerances, analog-to-digital and digital-to-analog conversion tolerances, wiring resistance tolerances, and others, the analog voltage signal received by the controller  32  exhibits a slight variation, even when the regulator  22  is not fully open and the pressure at the sensor  24  is essentially constant. The slight variations in signal voltage from the sensor  24  are amplified by the 8-bit storage register resolution of the controller  32 , resulting in the real-world behavior shown by the pressure trace  136 . 
     While the fluctuation shown by the pressure trace  136  at first seems troublesome, the very predictable nature of the fluctuation can be used as the basis for a control strategy. It has been observed over years of actual usage of the hydrogen storage system  10  that the pressure trace  136  consistently remains within plus or minus one 3.5-bar increment of the median pressure value, as long as the regulator  22  is not fully open. Only when the regulator  22  reaches a fully open position, and the real pressure at the sensor  24  begins to drop below the regulation pressure, does the pressure trace  136  drop below the pressure shown by the pressure mark  142 . On the graph  130 , the regulator  22  reaches a fully open position and the pressure at the sensor  24  begins to drop, at the time designated by time mark  144 . After that time, it can be seen that the pressure trace  136  drops an additional 3.5 bar increment, down to a pressure designated by pressure mark  146 . A few time steps later, after some additional fluctuation, the pressure trace  136  drops to an even lower value. This behavior has been consistently observed in real implementations of the hydrogen storage system  10 , and is a reliable indicator that the regulator  22  is fully open and the pressure at the sensor  24  is dropping. 
     Line  148  on the graph  130  represents the minimum allowable pressure in the vessels  12 , which is typically about 20 bar. It is noteworthy that the minimum allowable pressure is sufficiently below the median pressure value, so that the pressure trace  136  can drop at least one 3.5-bar increment below the median value without crossing below the minimum allowable pressure. Also, it was shown on the graph  100  that, when the pressure readings at the sensors  20  and  24  reach the regulation pressure of the regulator  22 , the pressure at the sensor  14  is still somewhat higher. Thus, the pressure in the vessels  12  will not drop below the minimum allowable pressure, even if the pressure at the sensors  20  and  24  does drop slightly below the minimum allowable pressure. 
     Implementing a control strategy based on the phenomenon described above then becomes straightforward.  FIG. 5  is a flow chart diagram  160  of a method which can be used to shut down the hydrogen storage system  10  in order to prevent the pressure in the vessels  12  from dropping below the minimum allowable pressure. At box  162 , a pressure increment value is defined, based on the range of the sensor  24  and the resolution of the data register in the controller  32 . As described in the examples above, a 900 bar pressure range and an 8-bit data register result in a pressure increment of 3.5 bar in the controller  32 . Thus, the pressure increment value will be known for any implementation of the sensor  24  and the controller  32 . At box  164 , the controller  32  monitors the pressure from the sensor  24  and identifies the fluctuation range within which the pressure is varying. At box  166 , the controller  32  calculates a rolling median pressure value for a certain time window. In one example, the time window is the past 60 seconds; however, longer or shorter windows could be defined as appropriate. The rolling median pressure value can be calculated by simply selecting the median (middle) value of the three different pressure readings which have most recently been recorded, as described previously and shown on the graph  130 . Other methods of calculating the rolling median pressure value could also be used. 
     With the rolling median pressure value established and the pressure increment value known, at box  168  the controller  32  can detect a pressure reading at the sensor  24  which is below the normal fluctuation range. The detection activity at the box  168  can be accomplished in one of two ways. First, the controller  32  can compare each new pressure reading with the previous reading to determine if the value has dropped by more than two pressure increments. A drop of two increments is possible under normal steady pressure conditions, as shown on the graph  130 . But a drop of more than two increments indicates that the pressure at the sensor  24  is actually dropping below the regulation pressure of the regulator  22 . Second, the controller  32  can compare each new pressure reading with the rolling median pressure value. If a pressure reading more than one increment below the rolling median pressure is detected, then the pressure at the sensor  24  is actually dropping below the regulation pressure of the regulator  22 . The two techniques for detecting a pressure below the normal fluctuation range may also be combined in a way that can accommodate a slight upward or downward drift of the fluctuation range during vehicle operation, without unnecessarily shutting down the hydrogen storage system  10 . 
     In the event that a pressure outside the normal fluctuation range is detected at the box  168 , the controller  32  commands a shutdown of the hydrogen storage system  10  at box  170 . The shutdown can most effectively be accomplished by closing the shutoff valves  18 , which are situated just downstream of the vessels  12 . By closing the shutoff valves  18 , the hydrogen gas contained in the pipes and components downstream of the valves  18  can be consumed by the fuel cell  34  before the fuel cell  34  stops producing electricity. In addition, the vehicle batteries will have at least a small amount of electrical charge remaining from fuel cell operation. The residual hydrogen gas and the residual electrical energy in the batteries will provide sufficient driving time for the driver to park the vehicle before it completely stops. 
     The shutdown sequence at the box  170  may also include a small time delay before closing the valves  18 . This is based on the fact that the pressure in the vessels  12 , represented by the reading of the sensor  14 , is known to be higher than the pressure at the regulator  22 , due to pressure drops therebetween. This fact can be used to allow a little additional time for the driver to park the vehicle upon being notified that the fuel cell  34  is shutting down imminently, while still preventing the vessels  12  from dropping below the minimum allowable pressure. The amount of time delay can be determined based on the known pressure drop between the sensors  14  and  20 , the capacity of the vessels  12  and the lines and fittings, and the rate at which hydrogen gas is being consumed. 
     It is emphasized that the specific values described above, including the 900 bar range of the sensor  24 , the 20 bar minimum allowable pressure, and the 8 bit storage register size, are all just examples. Higher or lower values could be used for any of these, but the operating principle of the detection and control strategy would remain the same. 
     In actual implementation, a low fuel warning would be issued to the driver of the vehicle well before the system shutdown procedure described above would have to be executed. The low fuel warning would be triggered by a pressure reading at the sensor  14  crossing below some threshold value, such as 80 bar, and most drivers would refuel their vehicle soon thereafter. As such, it is expected that the enforced system shutdown procedure would rarely have to be executed in real world driving situations. Nonetheless, the enforced system shutdown based on a pressure drop at the sensor  24 , as disclosed herein, can provide an extra measure of protection for the reliability of the vessels  12 . In doing so, it also avoids shutdown of the hydrogen storage system  10  when a significant amount of usable fuel still remains in the vessels  12 , thus allowing the greatest possible driving range of the vehicle between refueling stops. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.