Abstract:
A fuel storage system includes a storage vessel including a dielectric liner, a voltage sensor formed by a pair of plates disposed on opposing surfaces of the liner, and a controller configured to determine a gas pressure in the storage vessel based on voltages measured by the sensor.

Description:
BACKGROUND 
       [0001]    A capacitor is a passive electrical component that can store energy in an electric field between a pair of conductors. The process of storing energy in the capacitor is known as “charging,” and involves electric charges of equal magnitude, but opposite polarity, building up on each conductor. A capacitor&#39;s ability to store charge is measured by its capacitance in units of farads. 
         [0002]    Capacitors are often used in electric and electronic circuits as energy-storage devices. They can also be used to differentiate between high-frequency and low-frequency signals. Practical capacitors have series resistance, internal leakage of charge, series inductance and other non-ideal properties not found in a theoretical, ideal, capacitor. 
         [0003]    Some capacitors include two conductive electrodes, or plates, separated by a dielectric, which prevents charge from moving directly between the plates. Charge may, however, move from one plate to the other through an external circuit, such as a battery connected between terminals of the plates. When any external connection is removed, the charge on the plates persists. The separated charges attract each other, and an electric field is present between the plates. 
         [0004]    An example capacitor may include two wide, flat, parallel plates separated by a thin dielectric layer. Assuming the area of the plates, A, is much greater than their separation, d, the instantaneous electric field between the plates, E(t), is generally the same at any location away from the plate edges. If the instantaneous charge on a plate, −q(t), is spread evenly, then 
         [0000]        E ( t )= q ( t )/ε A    (1) 
         [0000]    where ε is the permittivity of the dielectric. The voltage, v(t), between the plates is given by 
         [0000]        v ( t )=∫ o   d   E ( t ) dz=q ( t ) d/εA    (2) 
         [0000]    where z is a position between the plates. 
         [0005]    A capacitor&#39;s ability to store charge is measured by its capacitance, C, which is the ratio of the amount of charge stored on each plate, q, to the voltage, v: 
         [0000]        C=q/v    (3) 
         [0000]    or, substituting (2) into (3): 
         [0000]        C=εA/d.    (4) 
         [0006]    In SI units, a capacitor has a capacitance of one farad when one coulomb of charge stored on each plate causes a voltage difference of one volt between its plates. Capacitance, however, is usually expressed in microfarads (μF), nanofarads (nF) or picofarads (pF). In general, capacitance is greater in devices with large plate areas, separated by small distances. When a dielectric is present between two charged plates, its molecules become polarized and reduce the internal electric field and hence the voltage. This allows the capacitor to store more charge for a given voltage: a dielectric increases the capacitance of a capacitor by an amount proportional to the dielectric constant of the material. 
       SUMMARY 
       [0007]    A fuel storage system may include a storage vessel including a dielectric liner, a voltage sensor formed by a pair of plates disposed on opposing surfaces of the liner, and a controller configured to determine a gas pressure in the storage vessel based on voltages measured by the sensor. 
         [0008]    A fuel storage system may include a storage vessel including a dielectric liner, a first sensor formed by a first pair of plates disposed on opposing surfaces of the liner, and a second sensor formed by (i) a second pair of plates disposed on opposing surfaces of the liner and (ii) a thermistor disposed within the storage vessel and electrically connected with one of the second pair of plates. The system may also include a controller configured to determine a gas temperature in the storage vessel based on respective voltages across the first and second sensors. 
         [0009]    A method for detecting a gas pressure within a storage vessel having a dielectric liner may include measuring a voltage over time across a sensor formed by a pair of metal plates disposed on opposing surfaces of the liner, and determining a gas pressure within the storage vessel based on the measured voltage across the sensor. 
         [0010]    While example embodiments in accordance with the invention are illustrated and disclosed, such disclosure should not be construed to limit the invention. It is anticipated that various modifications and alternative designs may be made without departing from the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a side view, in cross-section, of an example fuel storage system. 
           [0012]      FIG. 2  is an end view, in cross-section, of another example fuel storage system. 
           [0013]      FIG. 3  is a schematic view of an embodiment of the fuel storage system of  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Referring now to  FIG. 1 , an automotive fuel storage system includes a pressurized tank  10  and a valve  12  threadedly engaged with the tank  10 . The valve  12  provides a passageway  14  for hydrogen gas to be provided to the tank  10 . An O-ring  13  provides a seal between the valve  12  and the tank  10 . 
         [0015]    A moveable element  16 , e.g., plunger, of a solenoid  18  may be positioned by the solenoid  18  to restrict or block the flow of hydrogen gas through the passageway  14 . As illustrated in  FIG. 1 , the moveable element  16  is in the open position, thus allowing hydrogen to flow through the passageway  14 . In the closed position (not shown), the moveable element  16  extends into the passageway  14 . 
         [0016]    The solenoid  18  receives control signals from a vehicle controller (not shown) via a pair of solenoid control wires  20 . The solenoid control wires  20  pass through a pressure seal  22  and terminate at an electrical connector  24 . The electrical connector  24  is attached with a mating electrical connector  26  of a wiring harness  27  electrically connected with the vehicle controller. 
         [0017]    A temperature sensor  28  is disposed within the tank  10  and may be attached to the solenoid  18  via a tie-strap  29 . The sensor  28  provides signals indicative of a temperature of the hydrogen within the tank  10  to the vehicle controller via a pair of sensor wires  30 . The sensor wires  30  also pass through the seal  22  within the valve  12  and terminate at the electrical connector  24 . A pressure sensor (not shown) may be similarly situated. 
         [0018]    Referring now to  FIG. 2 , an embodiment of a fuel storage system  32  includes a storage tank  34 , valve  36 , and sensors  38 ,  40 . The valve  36  is electrically grounded (e.g., grounded to a chassis of a vehicle). The storage tank  34  of  FIG. 2  includes a dielectric liner  42 , e.g., high density polyethylene (HDPE), and a wrap  44 , e.g., carbon fiber. As known to those of ordinary skill, the storage tank  34  may store hydrogen for use with an automotive fuel cell system. The storage system  32  also includes first and second circuits  46 ,  48  and a controller  50  in communication with the circuits  46 ,  48 . In other embodiments, the circuits  46 ,  48  may be integrated with the controller  50 . Other configurations are also possible. The circuits  46 ,  48  and controller  50  will be discussed in more detail below. 
         [0019]    The sensor  38  includes a pair of metal plates  52 ,  54  positioned on opposing sides of the liner  42 . The plate  52  is grounded (e.g., grounded to the chassis of the vehicle) via the valve  36 . The sensor  40  includes a pair of metal plates  56 ,  58  positioned on opposing sides of the liner  42  and a thermistor  60  electrically connected between the valve  36  and plate  56 . The plate  56  is grounded (e.g., grounded to the chassis of the vehicle) via the thermistor  60  and valve  36 . In the embodiment of  FIG. 2 , the metal plates  52 ,  54 ,  56 ,  58  are adhered with the liner  42 . Any suitable attachment method, however, may be used. 
         [0020]    As apparent to those of ordinary skill, the metal plates  52 ,  54  as well as the metal plates  56 ,  58  form capacitors. As explained below, the capacitance of sensor  38  may be used to determine the pressure in the storage tank  34 , and the resistance of the thermistor  60  may be used to determine the temperature in the storage tank  34 . 
         [0021]    Referring now to  FIG. 3 , the first circuit  46  may include a resistor  62  (having a known resistance) and an inverting Schmitt trigger  64 . Of course, any suitable circuit configuration, e.g., Op-amp, voltage comparator, analog digital converter, etc., may be used. As apparent to those of ordinary skill, the sensor  38  and resistor  62  form an RC circuit. 
         [0022]    Initially, the output of the trigger  64  applies a HIGH step response to this RC circuit while the input of the trigger  64  senses the voltage across the sensor  38 . Once the voltage rises above the ‘upper’ threshold of the trigger  64 , the output of the trigger  64  will apply a LOW step response. Once the voltage falls below the ‘lower’ threshold, the output is HIGH again. The periodic signal generated by this circuit is detected by the controller  50  to determine the capacitance, C 38 , of the sensor  38 . 
         [0023]    As known to those of ordinary skill, the step response of the above RC circuit is related to the resistance, R 62 , of the resistor  62  and the capacitance, C 38 , of the sensor  38 . Assuming, for example, a HIGH step response of 5 Volts, the voltage across the sensor  40  over time, t, is give by: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       v 
                       38 
                     
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
                     5 
                     · 
                     
                       ( 
                       
                         1 
                         - 
                         
                            
                           
                             
                               
                                 - 
                                 t 
                               
                               / 
                               
                                 R 
                                 62 
                               
                             
                              
                             
                               C 
                               38 
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0000]    The time constant, τ 1 , derived from this step response is the time for the voltage across the sensor  38  to reach approximately 63% of its final (asymptotic) value: 
         [0000]      τ 1 =R 62 C 38    (6) 
         [0000]    The period detected by the controller  50  is proportional to this time constant, τ 1 . The controller  50  may thus find the capacitance, C 38 , of the sensor  38  as it is the only unknown. (The capacitance will increase as the pressure within the tank  34  increases.) 
         [0024]    From (4), the capacitance, C 38 , of the sensor  38  is related to the area, A 38 , of the plates  52 ,  54  in contact with the liner  42  and the thickness, t 42 , of the liner  42 : 
         [0000]        C   38   =εA   38   /t   42    (7) 
         [0000]    where ε is the permittivity of the liner  42 . The controller  50  may thus find the thickness, t 42 , of the liner  42  between the plates  52 ,  54  as it is the only unknown. 
         [0025]    The controller  50  may then apply known analytical techniques or access a look-up table (generated, for example, via testing or simulation) relating the thickness of the liner  42  to the pressure within the storage tank  34  to find the pressure within the storage tank  34 . 
         [0026]    The second circuit  48  may include a resistor  66  (having a known resistance) and an inverting Schmitt trigger  68 . Of course, any suitable circuit configuration may be used. Similar to the sensor  38 , the sensor  40  and resistor  66  form another RC circuit. 
         [0027]    The periodic signal generated by this circuit may be detected by the controller  50  to determine the resistance, R 60 , of the thermistor  60 . This period, however, is also affected by the capacitance, C 40 , of the sensor  40 . By constructing sensor  38  and sensor  40  such that their capacitance is generally the same (e.g. the plates  52 ,  54  and  56 ,  58  are of approximate equal size), the differences in the step response of the first RC circuit (formed by sensor  38  and resistor  62 ) and the step response of the second RC circuit (formed by the sensor  40  and the resistor  66 ) may thus be used to determine the temperature within the tank  34 . 
         [0028]    As known to those of ordinary skill, the step response of the second RC circuit is related to the resistance, R 66 , of the resistor  66 , the capacitance, C 40 , of the sensor  40 , and the resistance, R 60 , of the thermistor  60  (part of sensor  40 ). Assuming, for example, a HIGH step response of 5 Volts, the voltage across sensor  40  over time, t, is given by: 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       v 
                       40 
                     
                      
                     
                       ( 
                       t 
                       ) 
                     
                   
                   = 
                   
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                               R 
                               66 
                             
                             
                               
                                 R 
                                 66 
                               
                               + 
                               
                                 R 
                                 60 
                               
                             
                           
                            
                           
                              
                             
                               
                                 
                                   - 
                                   t 
                                 
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                                   ( 
                                   
                                     
                                       R 
                                       66 
                                     
                                     + 
                                     
                                       R 
                                       60 
                                     
                                   
                                   ) 
                                 
                               
                                
                               
                                 C 
                                 40 
                               
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
         [0000]    Assuming that R 60 ≦R 66 , the time constant, τ 2 , derived from this step response is the time for the voltage across the sensor  40  to reach approximately 63% of its final (asymptotic) value: 
         [0000]      τ 2 =[1+ ln ( R   66 )− ln ( R   66   +R   60 )]( R   66   +R   60 ) C   40    (9) 
         [0000]    The period detected by the controller  50  is proportional to this time constant, τ 2 . By assuming that the capacitance, C 40 , of the sensor  40  is generally the same as the capacitance, C 38 , of the sensor  38  (that is separately determined), the controller  50  may thus find the resistance, R 60 , of the thermistor  60  as it is the only unknown. The controller  50  may then, for example, access a standard look-up table relating the resistance R 60  of the thermistor  60  to the temperature within the tank  60  to find the temperature within the tank  60 . 
         [0029]    As apparent to those of ordinary skill, sensor wires passing from inside the storage tank  34  to outside the storage tank  34  are not required (in contrast, for example, to the sensor wires  30  associated with the temperature sensor  28  illustrated in  FIG. 1 ). As a result, the fuel storage system  32  may be less likely to leak relative to the fuel storage  10  illustrated in  FIG. 1 . 
         [0030]    While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. The words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.