Abstract:
Described is a system to deliver fluid to a fuel cell. The system includes a source of fluid, a fuel cell and a fluid delivery device in fluid communication therewith. The delivery device includes a pump having a reciprocating piston for drawing fluid from the source, pressurizing the fluid and delivering the fluid to the fuel cell accurately and reproducibly. In a further aspect of the disclosure, there is included a reformer system in fluid communication with an outlet of the pump. The reformer system includes a vaporizer which converts the fluid into a gas thereby generating a back pressure with respect to the pump. The back pressure varies with fluid flow rate and the back pressure also has a random component. The pump accurately and reproducibly delivers the fluid to the vaporizer against such back pressure.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to positive displacement pumps for delivering a fluid in an accurate and reproducible manner. More particularly, the invention relates to a positive displacement pump that delivers fuel and/or water to a fuel cell which utilizes direct oxidation of a hydrogen-containing fuel for the production of electricity. 
     Over 150 years ago a British physicist conceived of the first fuel cell consisting of an electrochemical reaction of hydrogen and oxygen to produce electricity and water. The electrochemical reaction in the reverse direction is the subject of many secondary education science experiments. In these experiments, the student passes an electrical current through a beaker of water to split water molecules thereby producing hydrogen gas and oxygen gas in a two to one molar ratio. A fuel cell reverses this secondary education learning experience. Simply put, combining hydrogen (a fuel) with oxygen (an oxidant) under the proper conditions, yields water and an electrical charge. 
     The basic chemical equation reveals the most attractive feature of electrical power production using fuel cell technology. The only byproducts of fuel cells that use hydrogen as the fuel and oxygen as the oxidant are water and electricity. The environmental advantages of using such a fuel cell to generate electrical power are readily apparent. Furthermore, relative to internal combustion engines, fuel cells produce very little waste heat and do not need to “idle” thereby operating more efficiently than internal combustion engines. 
     Of course, implementation of fuel cell technology in an economical and practical manner has proved difficult. The air around us contains an abundant supply of oxygen for use as the oxidant. Providing suitable hydrogen fuel, however, is recognized as a primary hurdle facing commercial realization of fuel cell technology, especially in vehicles. Storing pure hydrogen gas or liquid on board a vehicle or at vehicle filling stations is unfeasible at this time. 
     A promising solution to the hydrogen storage problem is the use of an organic compound such as methanol (CH 4 O; CH 3 OH) as the fuel. The methanol fuel is then chemically treated or “reformed” to increase the percentage of hydrogen within the fuel before introducing it into the fuel cell. Many organic compounds are suitable as fuels, including many hydrocarbons and other compounds. Although using such fuels increases the environmentally harmful emissions from the fuel cell, these emissions remain an order of magnitude or two below that of internal combustion engines and still at least half that of battery-powered vehicles (when emissions from generation of power to charge the battery are included). 
     One difficulty with using organic compound fuels is delivering or pumping the fuel through the reforming process and then to the fuel cell. When starting with a liquid fuel such as methanol, the reforming process includes vaporizing the fuel and then introducing the fuel to a catalyst to strip out carbon and oxygen molecules. The reforming process requires that the fuel be pumped into a vaporizer at a high pressure that is directly related to the flow rate of fuel through the reformer. Additionally, the reforming process produces random pressure fluctuations on top of the flow rate-related pressure. Furthermore, in order to optimize efficiency of the fuel cell, it is very important to deliver precise quantities and flow rates of the fuel to the fuel cell. Otherwise, the fuel will be wasted because there will be either too much or too little oxidant in the fuel cell with which to react. Finally, a fluid delivery system should have a short response time for changes in fuel flow rate in order to adequately respond to the power demands of a typical vehicle. 
     Water circulation is another difficulty with fuel cells that use organic-compound fuels. In addition to being a byproduct of the electrochemical reaction at the fuel cell, water may be used as an additive to or humidifier of the fuel (see, e.g., U.S. Pat. No. 5,573,866 to Van Dine et al.), as a coolant in the fuel cell (see, e.g., U.S. Pat. No. 5,503,944 to Meyer et al.) and as an humidifier of the oxidant-air supply (see, e.g., U.S. Pat. No. 5,366,818 to Wilkinson et al.). Thus, it is important to provide a fluid delivery system that is compatible with both organic solvents and water. 
     Traditional systems of delivering fuel, such as a fuel injector used with an internal combustion engine, have proved inadequate for use in a fuel cell with a reformer process In general, these types of systems do not respond well or quickly to a varying and random back pressure such as that produced by a reformer in a fuel cell system. In particular, and among other difficulties, there are three basic shortcomings present in fuel injectors and related systems. 
     First, a fuel injection system depends upon maintaining precise pressures and pressure differentials within the system. Thus, a relatively complicated and expensive pressure regulating mechanism (often including a booster pump and return fuel line) is required within the fuel injection system. Second, the fuel injector system cannot provide reproducible and accurate fuel delivery rates against a variable and/or random back pressure. The fuel injector system delivers fuel by opening a fuel injector valve. A typical fuel injector valve operates electro-mechanically and opens upon a signal from a fuel injector controller. Other fuel injector valves respond to changes in fuel line pressure and upon a sharp increase line pressure, the valve opens and delivers fuel to the engine. In either case, however, the pressure regulator is too slow to respond satisfactorily, and in the face of variable and random back pressure (such as that from a vaporizer or reformer), a constant pressure differential across the valve is difficult to maintain. Without a constant pressure differential, the flow rate from the valve will be uneven. Thus, fuel injection systems cannot provide reproducible and accurate fuel delivery rates against a variable and random back pressure. The third difficulty is that fuel injection systems are incompatible with water and will corrode when exposed to water based solutions. 
     In view of the foregoing, an object of the invention is to provide an improved apparatus for delivering fluid to a fuel cell and a reforming process. 
     Another object of the invention is to provide such apparatus to deliver fluid to the fuel cell against a back pressure that is flow-rate dependent. 
     Yet another object of the invention is to provide such apparatus to deliver fluid to the fuel cell against a back pressure that is randomly variable. 
     Yet still another object of the invention is to provide such apparatus to deliver fluid to the fuel cell at an accurate and reproducible flow rate against a variable and fluctuating back pressure. 
     An additional object of the invention is to provide such apparatus to deliver fluid to the fuel cell with a short response time for flow rate changes. 
     It is another object of the invention to provide such apparatus for delivering both an organic based fluid and a water based fluid to the fuel cell. 
     Yet still a further object of the invention is to provide such apparatus as can be implemented inexpensively. 
     SUMMARY OF THE INVENTION 
     The foregoing objects are among those attained by the invention, which provides in one aspect a system to deliver fluid to a fuel cell. The system includes a source of fluid, a fuel cell and a fluid delivery device in fluid communication therewith. The delivery device includes a pump having a reciprocating piston for drawing fluid from the source, pressurizing the fluid and delivering the fluid to the fuel cell. 
     In a further aspect of the invention, the system includes a reformer system in fluid communication with an outlet of the pump. The reformer system includes a vaporizer which converts the fluid into a gas thereby generating a back pressure with respect to the pump. The back pressure varies with fluid flow rate and the back pressure also has a random component. The pump accurately and reproducibly delivers the fluid to the vaporizer against such back pressure. 
     In another aspect of the invention the fluid delivery system includes a controllable piston driver coupled to the piston for discharging fluid from the pump chamber. The fluid delivery system also includes a controller coupled to the piston driver. The controller causes the driver to discharge the fluid from the chamber at a controllable flow rate. The flow rate may depend on a signal which is based upon the amount of electricity desired from the fuel cell. 
     In yet another aspect of the invention, the pump delivers a water based solvent and another pump delivers a fuel such as methanol. The same type of pump may be used to deliver either fluid to the fuel cell. Additional pumps may be added to the system in parallel to prevent pulsation in the fluid flow. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a fluid delivery system for a fuel cell and reformer system in accordance with the present invention. 
     FIG.  2 . is a block diagram of another embodiment of the fluid delivery system of the present invention. 
     FIGS. 3A and 3B are detailed cross-sectional views of the side and top of a pump for use in the fluid delivery system. 
     FIG. 3C is an end view of the pump of FIGS. 3A and 3B. 
     FIG. 4 is an exploded view of the pump of FIGS.  3 A and  3 B. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a block diagram of a fuel cell electrical power system indicated generally by reference numeral  10 . The fuel cell system  10  includes a fluid source  12 , a reformer system  20 , a fuel cell  30  and a fluid delivery system  40 . The fuel cell system  10  is particularly well-suited for use in a vehicle (not shown) because rather than providing a cumbersome and expensive pure hydrogen fuel storage system, the reformer system  20  converts another fuel, such as methanol or natural gas, into a hydrogen-rich fuel on board the vehicle itself Fuels such as methanol are readily stored in tanks similar to currently mass-produced gasoline tanks. Similarly, filling stations could easily be converted to methanol dispensing stations. Thus, the fuel cell system  10  can produce the hydrogen necessary for the electrochemical reaction while the vehicle is being operated. It will be readily recognized, however, that the invention disclosed herein is not limited to use with a vehicle, but may also be used with any electricity-generating fuel cell system. 
     The fluid source  12  provides a fuel  14  to the fluid delivery system  40 . The fluid delivery system  40  includes a positive displacement pump  42 . The pump  42  draws fuel from the source  12 , pressurizes the fuel and discharges it into the reformer system  20 . The reformer system  20  includes a vaporizer  22  and a reformer  24 . The reformer system  20  serves to produce a hydrogen rich gas from the fuel  14 . The hydrogen rich gas  17  is introduced into the fuel cell  30  which combines the hydrogen with an oxidant (preferably oxygen from the air)  18  to generate electricity  19 , water and a relatively small amount of heat. A portion of the electricity  19  produced by the fuel cell may be used to power the fluid delivery system  40 , the reformer system  20  and other components within the fuel cell system. The remainder of the electricity  19  can be used to drive a high-efficiency motor, such as an inductance motor utilizing electromagnets, to power, for example, a vehicle. 
     The fluid source  12  contains fuel  14  which is a fuel suitable for use in a fuel cell electrochemical reaction, such as methanol, natural gas (or methane) or other hydrocarbon*based liquid. The fluid source  12  may instead contain a water based solution in order to cool the fuel cell system or to humidify the fuel  14  or the oxidant  18 . The fluid source  12  includes a tank: or any other storage device capable of holding a water or an organic-compound solution. The fluid source is vented to the atmosphere such that the fuel  14  in the tank is substantially atmospheric. A filter  16  prevents contaminating particles from harming the remainder of the system  10 . 
     The fuel cell  30  may be any fuel cell that oxidizes a hydrogen rich solution and produces electrical energy. Countless fuel cells are known in the art, e.g. U.S. Pat. No. 5,262,249 to Beal et al., and many are suitable for use with this system  10 . Similarly, the reformer system  20  for producing hydrogen rich gas is well known in the art. The byproducts of the reforming process generally include carbon monoxide and carbon dioxide  15 . 
     In order to provide sufficient electrical energy to power a vehicle, the fuel  14  must be provided to the reformer system  20  and then to the fuel cell  30  at a certain flow rate and pressure. To produce sufficient quantities of electricity, the fluid delivery system  40  should provide fuel at flow rates varying from about 0.50 to 850 milliliters (ml) per minute, and more preferably providing a maximum flow rate of about 750 ml/min. Furthermore, for practical application in a vehicle, the fluid delivery system  40  should have a dynamic reaction time of about 100 milliseconds (ms) when transitioning from 10% to 90% of the maximum flow rate. 
     The vaporizer  22  and reformer  24  are such that they produce a back pressure relative to the fluid delivery device  40 . It has been found that a reformer system  20  suitable for generating sufficient hydrogen-rich gas to adequately supply the fuel cell  30  produces as much as 300 psi of back pressure and more generally up to 150 psi of back pressure. The back pressure generated by the reformer system is generally related to the flow rate of the fuel through the vaporizer  22 . 
     In addition to back pressure caused by the flow rate of fuel passing through the reformer system  20 , operational variables within the reformer system cause random back pressure fluctuations as well. These random fluctuations have been found to be between 1 and 10 psi and more generally between 3 and 6 psi. 
     The fluid delivery device  40  includes the pump  42 , a motor  44  and a controller  46 . The pump  42  is a positive displacement pump, preferably including a piston  48 . The piston  48  engages a chamber  46  formed by housing  49 . A spring  50  biases the piston  48  away from a bottom  52  of the chamber. When the piston  48  moves away from the chamber bottom  52 , fuel from the source is drawn into the chamber through inlet  54 . When the piston  48  is driven towards the chamber bottom  52 , the piston forces the fuel through outlet  56 . Check valves  58 ,  60  prevent back flow and are located at the inlet  54  and the outlet  56  of the chamber  46 . 
     To provide the required flow rates and pressure, without unduly wearing pump seals, it has been found that a desirable piston diameter for use with a passenger vehicle is between about 1.0 to 1.5 inches. Other sizes for other applications are readily used. Similarly, a piston stroke length is about 0.25 to 0.50 inches. A typical stroke volume for the pump, which is determined by multiplying the piston area by the stroke length, is approximately 0.20 to 0.90 cubic inches. The maximum speed at which the pump operates is generally about 100 to 130 strokes/min. 
     The motor  44  imparts rotational forces on a cam  60  and cam shaft which drives the piston into the chamber  46 . The motor is a standard DC motor or stepper motor and operates on 12 volt power, and, of course, other power supplies having 24 or 48 volts DC may also be used. The cam  60  may be designed so that at a constant rotational speed, fluid is drawn into the chamber quickly and subsequently pressurized and discharged at a desired rate. Of course, those skilled in the art will recognize that means other than a cam may be used to drive the piston  48 . For example, a linear drive or hydraulic drive could be coupled to the piston. Furthermore, depending on the driver means, the spring  50  may be eliminated from the pump  42 . 
     The controller  46  controls the motor  44  to provide accurate and reproducible pump action. The controller  46  operates based upon predetermined commands or may receive an input signal or signals  51  other devices. In a vehicle, for example, the controller  46  is coupled to a gas pedal to control the desired power level of the vehicle. As in a typical automobile, pressing on the gas pedal provides a signal  51  to the controller to pump more fuel to the fuel cell  30 , which in turn generates more electricity to power the vehicle. 
     As will be recognized by those skilled in the art, the flow rate and pressure of the fluid delivered by pump  42  will have at least some pulsation due to the fluid intake portion of the delivery cycle. Although cam  60 , motor  44  and controller  46  design and operation can minimize the pulsation, some pulsation will remain. Two pumps (or more) operating in parallel and out of phase would eliminate such pulsation. See U.S. Pat. No. 3,917,531 to Magnussen, incorporated herein by reference, for an example of such out of phase pump operation. 
     Turning now to FIG. 2, two fluid delivery devices  40   a ,  40   b  are shown. One delivery device  40   a  delivers fuel  14   a  to the fuel cell  30  and the other delivery device  40   b  provides a water-based solution  14   b  to the system. The reference numerals used in FIG. 1 correspond to those used in FIG.  2  and the remainder of the figures, with an “a” suffix on the numeral indicating that it is part of a fuel channel and a “b” suffix on the numeral indicating that it is pan of a water channel. The fluid delivery device  40   b  operates in substantially the same manner as the fluid delivery device  40  described with respect to FIG.  1 . The water based solution  14   b  pumped by device  40   b  may serve a variety of functions in the fuel cell system. As shown in FIG. 2, the water solution  14   b  is vaporized by vaporizer  22   b  and then directed to the reformer  24 . The vaporized water-based fluid serves to humidify the fuel in the reformer  24 . Other application for the water-based channel include using it to cool the fuel cell (not shown). A return line (not shown) from the fuel cell may be used to recycle the water back to the water source  12   b.    
     The controllers  46   a ,  46   b  are coupled together to coordinate the water flow rate and the fuel flow rate in the fuel cell system  10 . In many instances it is essential to deliver and maintain a predetermined ratio of water and fuel in the fuel cell system because otherwise the relatively sensitive fuel cell  30  may be damaged. Controllers  46   a ,  46   b  are preferably powered by the same power source as the motors  44   a ,  44   b . Of course, the controllers  46   a ,  46   b  need not be two separate components. Similarly, the motors  44   a ,  44   b  and the cams  60   a ,  60   b  may also be arranged as one component. 
     FIGS. 3A and 3B depict a detailed cross-sectional side view and top view of the pump  42 , and FIG. 4 shows an exploded view of the pump  42 . The housing  49  includes liquid head  49   a , piston back-up  49   b , back-up disk  49   c , and spring housing  49   d  secured together with fasteners  51 ,  53 . Liquid head  49   a  contains the chamber  46  with the chamber bottom  52 . The inlet  54  and the outlet  56  from the chamber are located opposite each other across the chamber. 
     The piston  48  engages the chamber  46 , and back-up ring  70   a  and seal  70   b  (see also FIG. 4) prevent fluid leakage from chamber  46 . Seal  70   b  is made of a standard sealant material, preferably an ultra-high molecular weight polyethylene. The seal  70   b  should also be hydrophobic and organic-solvent resistant in order to withstand both a water and fuel environment. The back-up ring  70   a  prevents seal  70   b  from cold flowing as a result of piston movement and friction. A sleeve  68  provides support to the piston  48 . 
     A piston shaft  72  having a shoulder  73  extends from the piston  48  through the housing  49  and through an bearing housing  74 . The bearing housing  74  mounts to spring housing  49   d  with fasteners  76 . The shoulder  73  of the piston shaft  72  engages the spring  50  at one end thereof The other end of spring  50  engages back-up disk  49   c  thereby biasing the piston away from the chamber bottom  52 . The spring  50  must be sufficiently strong to draw fluid into the chamber  46 . 
     A piston cup  78  mounts to the end of the piston shaft  72  with a shaft-stop  80  therebetween. The shaft-stop serves to distribute force from the piston cup  78  to the shaft  72 . The piston cup  78  extends outside the bearing housing  74 . Force applied to the piston cup (by, for example, the cam  60  shown in FIG. 1) causes the piston  48  to pressurize fluid in the chamber  46  and discharge the fluid through outlet  56 . A cap  82  contains a linear bearing  84  within bearing housing  74 . The linear bearing  84  maintains a seal for piston cup  78  and provides a surface against which the piston cup  78  moves. 
     The various parts the pump  42  that encounter the fluid being pumped should be both water and organic-solvent compatible. The pump is primarily made of stainless steel or other corrosion resistant materials. 
     FIG. 3C shows an end view of the pump  42 . Check valves  58 ,  60  mount to the outlet  54  a seal  92  is provided in check valve  58  and inlet  56 , respectively. A standard high-pressure valve mechanism is provided in the outlet check valve  58 . A standard ball and seat assembly  94  provides the checking mechanism in the inlet check valve  60 . 
     It should be understood that the preceding is merely a detailed description of certain preferred embodiments. It therefore should be apparent to those skilled in the art that various modifications and equivalents can be made without departing from the spirit or scope of the invention.