Fuel storage system and method for detecting a gas pressure therein

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.

BACKGROUND

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's ability to store charge is measured by its capacitance in units of farads.

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.

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.

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
E(t)=q(t)/εA(1)
where ε is the permittivity of the dielectric. The voltage, v(t), between the plates is given by
v(t)=∫odE(t)dz=q(t)d/εA(2)
where z is a position between the plates.

A capacitor'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:
C=q/v(3)
or, substituting (2) into (3):
C=εA/d.(4)

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

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.

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.

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.

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.

DETAILED DESCRIPTION

Referring now toFIG. 1, an automotive fuel storage system includes a pressurized tank10and a valve12threadedly engaged with the tank10. The valve12provides a passageway14for hydrogen gas to be provided to the tank10. An O-ring13provides a seal between the valve12and the tank10.

A moveable element16, e.g., plunger, of a solenoid18may be positioned by the solenoid18to restrict or block the flow of hydrogen gas through the passageway14. As illustrated inFIG. 1, the moveable element16is in the open position, thus allowing hydrogen to flow through the passageway14. In the closed position (not shown), the moveable element16extends into the passageway14.

The solenoid18receives control signals from a vehicle controller (not shown) via a pair of solenoid control wires20. The solenoid control wires20pass through a pressure seal22and terminate at an electrical connector24. The electrical connector24is attached with a mating electrical connector26of a wiring harness27electrically connected with the vehicle controller.

A temperature sensor28is disposed within the tank10and may be attached to the solenoid18via a tie-strap29. The sensor28provides signals indicative of a temperature of the hydrogen within the tank10to the vehicle controller via a pair of sensor wires30. The sensor wires30also pass through the seal22within the valve12and terminate at the electrical connector24. A pressure sensor (not shown) may be similarly situated.

Referring now toFIG. 2, an embodiment of a fuel storage system32includes a storage tank34, valve36, and sensors38,40. The valve36is electrically grounded (e.g., grounded to a chassis of a vehicle). The storage tank34ofFIG. 2includes a dielectric liner42, e.g., high density polyethylene (HDPE), and a wrap44, e.g., carbon fiber. As known to those of ordinary skill, the storage tank34may store hydrogen for use with an automotive fuel cell system. The storage system32also includes first and second circuits46,48and a controller50in communication with the circuits46,48. In other embodiments, the circuits46,48may be integrated with the controller50. Other configurations are also possible. The circuits46,48and controller50will be discussed in more detail below.

The sensor38includes a pair of metal plates52,54positioned on opposing sides of the liner42. The plate52is grounded (e.g., grounded to the chassis of the vehicle) via the valve36. The sensor40includes a pair of metal plates56,58positioned on opposing sides of the liner42and a thermistor60electrically connected between the valve36and plate56. The plate56is grounded (e.g., grounded to the chassis of the vehicle) via the thermistor60and valve36. In the embodiment ofFIG. 2, the metal plates52,54,56,58are adhered with the liner42. Any suitable attachment method, however, may be used.

As apparent to those of ordinary skill, the metal plates52,54as well as the metal plates56,58form capacitors. As explained below, the capacitance of sensor38may be used to determine the pressure in the storage tank34, and the resistance of the thermistor60may be used to determine the temperature in the storage tank34.

Referring now toFIG. 3, the first circuit46may include a resistor62(having a known resistance) and an inverting Schmitt trigger64. 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 sensor38and resistor62form an RC circuit.

Initially, the output of the trigger64applies a HIGH step response to this RC circuit while the input of the trigger64senses the voltage across the sensor38. Once the voltage rises above the ‘upper’ threshold of the trigger64, the output of the trigger64will 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 controller50to determine the capacitance, C38, of the sensor38.

As known to those of ordinary skill, the step response of the above RC circuit is related to the resistance, R62, of the resistor62and the capacitance, C38, of the sensor38. Assuming, for example, a HIGH step response of 5 Volts, the voltage across the sensor40over time, t, is give by:

v38⁡(t)=5·(1-ⅇ-t/R62⁢C38)(5)
The time constant, τ1, derived from this step response is the time for the voltage across the sensor38to reach approximately 63% of its final (asymptotic) value:
τ1=R62C38(6)
The period detected by the controller50is proportional to this time constant, τ1. The controller50may thus find the capacitance, C38, of the sensor38as it is the only unknown. (The capacitance will increase as the pressure within the tank34increases.)

From (4), the capacitance, C38, of the sensor38is related to the area, A38, of the plates52,54in contact with the liner42and the thickness, t42, of the liner42:
C38=εA38/t42(7)
where ε is the permittivity of the liner42. The controller50may thus find the thickness, t42, of the liner42between the plates52,54as it is the only unknown.

The controller50may then apply known analytical techniques or access a look-up table (generated, for example, via testing or simulation) relating the thickness of the liner42to the pressure within the storage tank34to find the pressure within the storage tank34.

The second circuit48may include a resistor66(having a known resistance) and an inverting Schmitt trigger68. Of course, any suitable circuit configuration may be used. Similar to the sensor38, the sensor40and resistor66form another RC circuit.

The periodic signal generated by this circuit may be detected by the controller50to determine the resistance, R60, of the thermistor60. This period, however, is also affected by the capacitance, C40, of the sensor40. By constructing sensor38and sensor40such that their capacitance is generally the same (e.g. the plates52,54and56,58are of approximate equal size), the differences in the step response of the first RC circuit (formed by sensor38and resistor62) and the step response of the second RC circuit (formed by the sensor40and the resistor66) may thus be used to determine the temperature within the tank34.

As known to those of ordinary skill, the step response of the second RC circuit is related to the resistance, R66, of the resistor66, the capacitance, C40, of the sensor40, and the resistance, R60, of the thermistor60(part of sensor40). Assuming, for example, a HIGH step response of 5 Volts, the voltage across sensor40over time, t, is given by:

v40⁡(t)=5·(1-R66R66+R60⁢ⅇ-t/(R66+R60)⁢C40)(8)
Assuming that R60≦R66, the time constant, τ2, derived from this step response is the time for the voltage across the sensor40to reach approximately 63% of its final (asymptotic) value:
τ2=[1+1n(R66)−1n(R66+R60)](R66+R60)C40(9)
The period detected by the controller50is proportional to this time constant, τ2. By assuming that the capacitance, C40, of the sensor40is generally the same as the capacitance, C38, of the sensor38(that is separately determined), the controller50may thus find the resistance, R60, of the thermistor60as it is the only unknown. The controller50may then, for example, access a standard look-up table relating the resistance R60of the thermistor60to the temperature within the tank60to find the temperature within the tank60.

As apparent to those of ordinary skill, sensor wires passing from inside the storage tank34to outside the storage tank34are not required (in contrast, for example, to the sensor wires30associated with the temperature sensor28illustrated inFIG. 1). As a result, the fuel storage system32may be less likely to leak relative to the fuel storage10illustrated inFIG. 1.