Abstract:
A fuel supply system for a fuel cell is described. One embodiment of the fuel supply system includes a fuel supply vessel; a fuel spending line in fluid communication with the fuel supply vessel and the fuel cell; a piezoelectric injector in fluid communication with the fuel spending line; and a pressure sensor connected to the fuel spending line and positioned between the fuel supply vessel and the fuel cell. A method for controlling the pressure to a fuel cell is also described.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates generally to controlling compressed feed gases and more particularly to controlling the pressure at the inlet of a fuel cell or other device utilizing a compressed gas feed. 
         [0002]    Electrochemical conversion cells, commonly referred to as fuel cells, produce electrical energy by processing reactants, for example, through the oxidation and reduction of hydrogen and oxygen. The gases are often introduced into the fuel cells from pressurized storage tanks. 
         [0003]    Mechanical pressure regulators have been used to control the pressure at the inlet to the fuel cell or other device. However, mechanical pressure regulators suffer from a number of problems. The spring force of the mechanical pressure regulator decreases over the life of the regulator, and it can also be affected by temperature. The effective pressure of the regulator is influenced by the flow rate through the regulator. The stiffness of the membrane of the spring force regulator changes over time as the material ages and deteriorates. As result of these effects, the set point of pressure regulator changes and will differ over time compared to the requested value. However, if the pressure regulator changes its set point, this deviation cannot be corrected by the mechanical pressure regulator. In addition, the mechanical parts are subject to the stick and slip effect because at high flow transient the friction is different. Furthermore, leaks in a line cannot be avoided because spring forced pressure regulators do not close very tightly. In addition, there is typically a need for several pressure steps for pressure reduction from a high pressure level (e.g., 700 bar) down to a low pressure level (e.g., 2 bar absolute). To realize small tolerance at the target pressure, several reduction steps have to be installed with mechanical spring forced pressure regulators. 
       SUMMARY OF THE INVENTION 
       [0004]    One aspect of the invention is a fuel supply system for a fuel cell. One embodiment of the fuel supply system includes a fuel supply vessel; a fuel spending line in fluid communication with the fuel supply vessel and the fuel cell; a piezoelectric injector in fluid communication with the fuel spending line; and a pressure sensor connected to the fuel spending line and positioned between the fuel supply vessel and the fuel cell. 
         [0005]    Another aspect of the invention is a method of controlling a pressure to a fuel cell. The method includes providing a fuel supply system comprising: a fuel supply vessel; a fuel spending line in fluid communication with the fuel supply vessel and the fuel cell; a piezoelectric injector in fluid communication with the fuel spending line; and a pressure sensor connected to the fuel spending line and positioned between the fuel supply vessel and the fuel cell; providing a gas flow through the fuel spending line from the fuel supply vessel; measuring a pressure with the pressure sensor; comparing the measured pressure with a reference pressure; and controlling the piezoelectric injector based on a difference between the measured pressure and the reference pressure. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is an illustration of one embodiment of a fuel supply system for a fuel cell. 
           [0007]      FIG. 2  is an illustration of a portion of one embodiment of a fuel supply vessel. 
           [0008]      FIG. 3  is an illustration of one embodiment of a pressure control arrangement for a fuel supply system. 
           [0009]      FIGS. 4A-B  are illustrations of a pressure control arrangement with the pulse frequency controlled and the resulting pressure. 
           [0010]      FIGS. 5A-B  are illustrations of a pressure control arrangement with the flow pulse width controlled and the resulting pressure. 
           [0011]      FIGS. 6A-B  are illustrations of a pressure control arrangement with the pulse frequency and flow pulse width controlled and the resulting pressure. 
           [0012]      FIG. 7  is an illustration of another embodiment of a fuel supply system for a fuel cell. 
           [0013]      FIG. 8  is an illustration of another embodiment of a fuel supply system for a fuel cell 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    A piezoelectric injector is used to control the pressure in front of the fuel cell system inlet. It allows the pressure set point for the feed system to be variable. The use of piezoelectric injectors avoids the deviation which occurs in mechanical pressure regulators over the life of the regulator caused by the springs. It also reduces the number of components in the storage system because multiple pressure regulators are not required. Piezoelectric injectors increase the signal quality of the pressure at the fuel cell system inlet and the accuracy of the pressure control. They are not influenced by the temperature or the flow rate of the gas. A pressure increase between several pressure regulators is not possible because there are no mechanical pressure regulators between the vessel and the fuel cell, and as result, no creeping of gas into the several stages is possible. The pressure at the pressure sensor will not increase because the fuel cell system has a relief valve. Using a piezoelecctric injector permits the use of a pressure sensor having a smaller pressure full scale with a very small tolerance band, which allows better pressure measurement. 
         [0015]    In one embodiment, there can be one vessel with a piezoelectric injector coupled to the fuel cell. Alternatively, more than one fuel supply vessel can be connected to the fuel cell. If more than one vessel is used, each vessel can have its its own built-in piezoelectric injector. This allows the fuel supply vessels to be controlled separately (for example, to ensure a fuel reserve). Alternatively, the output from several fuel supply vessels can be controlled by one piezoelectric injector. The system can include an optional shut off valve between the piezoelectric injector and the pressure sensor to assure that the fuel spending line can be closed completely. There can be an optional filter and/or shield to prevent contamination from entering the piezoelectric injector. 
         [0016]    Current vehicles and systems use several pressure regulators for several pressure stages (step by step reduction). One piezoelectric injector can be used in place of two or more mechanical pressure regulators. The piezoelectric injector provides the ability to control the resulting pressure at the inlet of the fuel cell. 
         [0017]    In the embodiment shown in  FIG. 1 , there is a vessel  10  which contains the compressed gas, such as H 2  or compressed natural gas (CNG). The compressed gas is fed though a fuel spending line  15 , which runs from the vessel  10  to the fuel cell  20 . There is a piezoelectric injector  25  to control the pressure of the gas in the fuel spending line. The piezoelectric injector  25  can be located inside the vessel  10  (as shown) or outside the vessel (see  FIG. 8 , for example). When the piezoelectric injector  25  is inside the vessel  10 , a leakproof seal is not required because any leak would be into the vessel. If the piezoelectric injector  25  is positioned in the fuel spending line  15  between vessel  10  and fuel cell  15 , the function of tightness to the environment would be fulfilled by fuel spending line  15 . 
         [0018]    The compressed gas often contains contamination, which can potentially cause failure due to leaking valves. As shown in  FIG. 2 , particles  60  can be found in many parts of fuel cell systems, including the fueling line and fuel inlet, the vessel, filters, and other parts. Generally, the particles are at the bottom area of the vessel  10 . In order to protect the piezoelectric injector  25  from this contamination, the inlet for the piezoelectric injector  25  is located in the middle of the vessel  10  or higher. 
         [0019]    To improve the protection, an optional filter  55  could be positioned in front of the piezoelectric injector  25  inlet. Because the operation flow out of the vessel is much less than the fueling line flow to fill the vessel, smaller filter mesh sizes could be used for the filter  55  in front of the piezoelectric injector  25  to assure the cleanliness of the gas entering the piezoelectric injector. 
         [0020]    There could also optionally be a shield  65  in front of the filter  55  to provide additional protection from particles in the vessel. The shield can be made out of any material which can block particles from entering the filter  55  and the piezoelectric injector  25  behind it. Suitable materials include, but are not limited to, stainless steel, plastic, or fiber. 
         [0021]    The flow through filter  55  can be only in one direction to provide additional protection, if desired. 
         [0022]    If the piezoelectric injector  25  is inside the vessel  10 , it can be located in the boss  70 , as shown in  FIG. 2 . The fueling inlet (not shown) for the vessel  10  could be integrated into boss  70 , if desired. Care should be taken so that particles brought in with the fueling do not enter the fuel spending line  15 , and more importantly, to the piezoelectric injector  25 . One way to accomplish this would be to separate the fueling inlet from the outlet components. In another embodiment, the piezoelectric injector could be used as the fueling inlet for the vessel. 
         [0023]    The piezoelectric injector  25  creates small pressure pulses behind it. To filter these pulses, a pressure pulse filter is provided. The pressure pulse filter is free space which can filter the pulses. For example, in current vehicles, the fuel spending line  15  is longer than  4  m. This provides a large volume for filtering the pulses, and it could be used as the pressure pulse filter. If the fuel spending line  15  is too short and/or the inner diameter is too small, the resulting volume may not provide sufficient volume to filter the pulses. To assure the filter function, an additional volume could be included, such as container  30 . There can also be a flow restrictor  35 , if desired. Alternatively, the flow restrictor could be the outlet for the container  30 . 
         [0024]    The pressure sensor  40  measures the pressure at the fuel cell  20  inlet. There can be a pressure relief valve  45  which protects the pressure sensor  40  and the fuel cell  20  from high pressure. The pressure relief valve  45  avoids a pressure increase when the fuel cell is not operating. 
         [0025]    There can be an optional shut-off valve  50  between vessel  10  and the piezoelectric injector  25 , if desired. 
         [0026]      FIG. 3  is an illustration of one embodiment of a suitable pressure control circuit  100 . In this embodiment, the control principle is a classic feedback control using the resulting pressure. The piezoelectric injector  25  controls the flow in the fuel spending line  15 . The pressure at the fuel cell  20  is measured by the pressure sensor  40 , and compared to a desired nominal pressure at  105 . Controller  110  controls the piezoelectric injector  25  based on the difference between the requested pressure at the fuel cell inlet and the measured pressure. The controller  110  varies the flow pulse alone, or the frequency, or both, as discussed below. Thus, the needed pressure at the fuel cell inlet is controlled by the actual measured value, not a calculated value. Other control schemes could also be used. 
         [0027]    The injector principle controls the gas flow, and the result of the controlled flow is a pressure. Various approaches to pressure control can be used. For example, the flow pulse width can be fixed and the frequency controlled, as shown in  FIG. 4A . The resulting pressure is shown in  FIG. 4B . Alternatively, the pulse frequency can be fixed, while the flow pulse width is varied, as shown in  FIGS. 5A-B . Another type of control involves varying both the flow pulse width and the frequency, as shown in  FIG. 6A-B . 
         [0028]      FIG. 7  shows an embodiment in which there is more than one fuel supply vessel  10  feeding the fuel cell  20 . As shown, there are three vessels  10 , each with its own piezoelectric injector  25  and filter  55 , which would allow each vessel to be controlled separately. For example, one vessel could be used as a fuel reserve. In this embodiment, the piezoelectric injectors  25  are located in the vessels  10 . The output from the three piezoelectric injectors  25  are combined into the same fuel spending line  15 . There can be an optional shut-off valve  50 , followed by the container  30  and flow restrictor  35  as discussed above. There is a pressure relief valve  45  and pressure sensor  40  before the fuel cell  20 . 
         [0029]      FIG. 8  shows another embodiment in which more than one fuel supply vessel  10  feeds the fuel cell  20 . In this embodiment, the output from three vessels  10  is combined in fuel spending line  15 . When the piezoelectric injector  25  is not in the fuel supply vessel, each vessel  10  has an internal flow limiter (or flow restrictor)  75 . The flow limiter  75  at the outlet of the vessel  10  is a desirable safety feature. These flow limiters  75  limit the gas release in case of rupture of the vessel(s). The piezoelectric valve  25  is tight to ambient itself, or it can be integrated inside the fuel spending line  15  . If the piezoelectric valve is integrated in the fuel spending line  15 , the “seal to ambient” function is shifted to the fuel spending line. The tightness to ambient has to be assured always. There can be an optional filter  55  before the piezoelectric injector  25 , if desired. The optional shut-off valve  50 , container  30 , flow restrictor  35 , pressure relief valve  45  and pressure sensor  40  follow. 
         [0030]    For compressed gas systems in vehicles, the design incorporating the piezoelectric injector could be integrated into smaller space than multiple mechanical pressure regulators. 
         [0031]    It is noted that terms like “preferably,” “commonly,” and “typically” are not utilized herein to limit the scope of the claimed invention or to imply that certain features are critical, essential, or even important to the structure or function of the claimed invention. Rather, these terms are merely intended to highlight alternative or additional features that may or may not be utilized in a particular embodiment of the present invention. 
         [0032]    For the purposes of describing and defining the present invention it is noted that the term “device” is utilized herein to represent a combination of components and individual components, regardless of whether the components are combined with other components. For example, a “device” according to the present invention may comprise an electrochemical conversion assembly or fuel cell, a vehicle incorporating an electrochemical conversion assembly according to the present invention, etc. 
         [0033]    For the purposes of describing and defining the present invention it is noted that the term “substantially” is utilized herein to represent the inherent degree of uncertainty that may be attributed to any quantitative comparison, value, measurement, or other representation. The term “substantially” is also utilized herein to represent the degree by which a quantitative representation may vary from a stated reference without resulting in a change in the basic function of the subject matter at issue. 
         [0034]    Having described the invention in detail and by reference to specific embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims. More specifically, although some aspects of the present invention are identified herein as preferred or particularly advantageous, it is contemplated that the present invention is not necessarily limited to these preferred aspects of the invention.