Abstract:
Unused fuel in a fuel cell is recirculated in the anode loop and combined with fresh fuel using an electromagnetically driven pump. The pump includes a magnetic rotor mounted inside a conduit of nonmagnetic material that recirculates the fuel, and a plurality of electric stator coils disposed around the outside of the conduit. The stator coils are powered by either a AC or pulse train signal produced by a controller, and generate synchronous electromagnetic forces that spin rotor to force the gas through the recirculation loop.

Description:
FIELD OF THE INVENTION 
   The present invention broadly relates to managing the flow of gaseous fuels used in fuel cells, and deals more particularly with a method and apparatus for recirculating unused fuel using an electromagnetically driven pump. 
   BACKGROUND OF THE INVENTION 
   Fuel cells are electrochemical energy conversion devices that generate electricity and heat by converting the chemical energy of fuels. A single fuel cell normally consists of an electrolyte sandwiched between two electrodes, a porous anode and a cathode. While a variety of different fuel cell types have been developed, all operate on essentially the same principle. For a PEM fuel cell, hydrogen, or a hydrogen-rich fuel is fed to the anode where a catalyst separates the hydrogen&#39;s negatively charged electrons from positively charged ions (protons). The electrons from the anode side of the cell cannot pass through the membrane to the positively charged cathode; they must travel around it via an electrical circuit to reach the other side of the cell. This movement of electrons is an electrical current which is advantageously used to drive a load, such as an electric motor or other electrical system. Once delivered to the cathode via the electrical circuit, the electrons combine with the protons that have crossed the membrane and the oxygen from the air, resulting in water or hydroxide. For proton exchange membrane (PEM) and phosphoric acid fuel cells, protons move through the electrolyte to the cathode to combine with oxygen and electrons, producing water and heat. In other types of fuel cells such as solid oxide fuel cells (SOFC&#39;s), negative ions travel through the electrolyte to the anode where they combine with the hydrogen or other oxidizable “fuel” 
   In the case of hydrogen fuel cells, hydrogen fuel may be fed to the anode in what is sometimes referred to as the anode loop. The quantity of hydrogen fed to the anode is a function of variety of factors, including the relative purity of hydrogen fuel, low demand and other variable parameters that are unique to each fuel cell application. 
   In order to operate efficiently, the fuel cell must be supplied with more hydrogen fuel then it can actually convert. As a result, extra, unused hydrogen gas is discharged from the fuel cell. In order to increase operating efficiencies, it has been proposed that the unused hydrogen gas be circulated and combined with fresh gas from the hydrogen source before being redelivered to the anode of the fuel cell. Known hydrogen recirculating systems rely on comparatively complicated mechanical components, or electrical control systems that expose sensitive electronic components to potentially harsh environments found in fuel cells. It would therefore be desirable to provide a system for recirculating unused hydrogen fuel that is both simple in construction and well suited to operate within the adverse environment of the fuel cell. The present invention is directed toward satisfying this need. 
   SUMMARY OF THE INVENTION 
   Unused hydrogen gas is returned to the anode of a fuel cell using an electromagnetically driven pump which is particularly simple in design, relies on relatively few components and is easily controllable to achieve precise flow rates in the recirculation loop. Another advantage of the invention is that the recirculation pump can be housed directly inside the recirculation conduitline, thereby saving space and simplifying installation. A further feature of the pump is its ability to be precisely controlled by the fuel cell&#39;s master controller that controls a variety of other functions of the cell. It is also an advantage of the invention that the moving parts of the pump are completely contained within a hydrogen gas environment, while electrical parts are housed outside of the fuel rich environment. 
   According to one aspect of the invention, apparatus is provided for recirculating a gaseous fuel used to power a fuel cell. The apparatus includes an electromagnetically powered pump incorporated in the recirculation conduitline, connecting the anode exhaust with the anode gas fuel inlet. The pump includes a rotor formed of a magnetic material and mounted for a rotation within the conduitline, and an electrical stator circumscribing the exterior wall of the conduit. The stator includes a plurality of stator coils that are magnetically coupled with the rotor. A controller delivers an AC power signal or a pulse train signal to the stator, producing a synchronously varying electromagnetic field that forces the magnetic rotor blades to spin and resulting in the rotor pump fuel through the recirculating conduitline. The stator coils are preferably evenly circumferentially spaced around the outside of the conduit. The rotor is mounted on the inside wall of the conduit using one or more support struts. The rotor blades may either be formed of magnetic material, or include magnetic elements near the outer tips of the blades in order to magnetically interact with the field produced by the stator coils. 
   According to another aspect of the invention, a method is provided for recirculating unused gaseous fuel in the anode loop of the fuel cell, comprising: pumping the unused fuel through a recirculation conduit using an electromagnetically driven pump. The pumping includes powering the stator with an electrical signal having a variable characteristic related to the desired rotation speed of the rotor. The method includes placing a magnetic rotor inside the conduit and placing a plurality of stator elements around the outside of the conduit. 
   These non-limiting features, as well as other advantages of the present invention may be better understood by considering the following details of a description of a preferred embodiment of the present invention. In the course of this description reference will frequently be made to the attached drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a highly simplified block diagram of a fuel cell system having a hydrogen gas recirculation loop employing the recirculation pump forming the preferred embodiment of the invention. 
       FIG. 2  is a combined block diagram and cross sectional few of a portion of the recirculating conduitline, showing the mounting position of the pump; 
       FIG. 3  is a cross sectional view of the conduit shown in  FIG. 2 , depicting the relationship between the conduit, the rotor and the stator coils; and, 
       FIG. 4  is a fragmentary, side elevational view of one of the rotor blades, depicting magnetic blade tips. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring first to  FIG. 1 , a fuel cell stack  10  includes an electrolyte  16  sandwiched between anodes  12  and cathodes  14 . An oxidizing agent such as air is delivered from a supply  20  through a conduitline to a plenum (not shown) which directs the air onto the surface of the cathodes  14  where the oxygen contained in the air is reduced. 
   A gaseous fuel such as hydrogen from a supply  18  thereof is similarly delivered by a conduitline to a plenum (not shown) which directs the hydrogen over the surface of the anodes  12 . The oxygen reduced at the cathodes  14  is converted into negatively charged oxygen ions which travel through the electrolyte  16  to the anodes  12  where they react with the hydrogen fuel gas. The fuel gas is oxidized by the oxygen ions and releases electrons to an external circuit (now shown) where they produce an electromotive force to drive a desired load. The oxygen ions also combine with the hydrogen at the surface of the anodes  12  to produce water which is carried away along with excess oxygen depleted air via a discharged line  32 . Electrons continue flowing to the circuit to the cathode  14  where they reduce oxygen from the air, thus continuing the electricity-generating cycle. A purge block  43  is connected with the discharge line  32  to allow purging of a later described recirculation line  22 . 
   In order to generate a desired level of electricity, a plurality of individual fuel cells are stacked together and connected in series to form the fuel cells stack  10 . The individual fuel cells forming the stack  10  may be any of several configurations, including monolithic, planar or tubular. Regardless of the exact cell geometry, the fuel cells are stacked so as to create a series of gas flow channels therebetween. In one well known arrangement, the cells are arranged to provide so-called cross flow or orthogonal flow, in which the hydrogen fuel gas and the air flow in orthogonal directions to each other, and alternating flow channels between the fuel cells. 
   Excess, unused hydrogen gas fuel exits the plenum (not shown) covering the surface of the anodes  12  and is returned in a recirculating conduitline  22  so is to be combined with fresh hydrogen gas from the hydrogen supply  18 . The mass flow of hydrogen delivered to the anodes  12  is measured using a mass flow sensor  26  which may be a conventional device, or a specially designed sensor. A conventional air flow sensor  47  measures the flow of air from the supply  20  to the fuel cell  10  and provides air flow information to the controller  30 . The excess hydrogen gas is drawn through recirculating conduitline  22  by a hydrogen gas recirculation pump  24  which will be described later in more detail. A master controller  30  receives signals from the mass flow sensor  26  indicative of the mass flow rate of hydrogen to the anodes  12  and sends control signals to the supply  18  and pump  24  so as to maintain a desired flow rate of hydrogen to the anodes  12 . The functions of the controller  30  may be included in and carried out by one or more master controllers which control other operations of the fuel cell stack  10 . 
   Referring now also to  FIG. 2-4  the recirculation pump  24  includes a rotor  34  having a central hub on which there is secured a plurality of circumferentially spaced rotor blades  32 . The rotor blades  32  extend radially so as to span across essentially the full diameter of the conduit  22 , and thus across the full cross sectional flow of hydrogen gas which flows in the direction designated by the arrows  42 . The rotor hub is journal led for rotation on a strut  36  which is secured to the interior sidewall of the conduit  22  and functions to support the entire rotor  34 . Although a single strut is shown in the illustrated embodiment, a plurality of the struts  36  or other similar support structure may be employed depending on the application. Such support structure should be designed so as to create minimum drag on the flow of hydrogen gas through the conduit  22 . 
   The rotor blades  32  are preferably formed of a magnetic material. Alternatively, however, rotor blades  32  may be formed of a non-magnetic material such as plastic, in which case one or more of the blades  32  is provided with an insert  44  of magnetic material (i.e., permanent magnets) near the blade tip. An electrical stator is formed by a plurality of circumferentially spaced electrical stator coils  38  which are disposing around the outer circumferential wall of the conduitline  22  so as to be magnetically coupled with the rotor blades  32 . The conduitline  22  must be formed of a non-magnetic material, so as not to interfere with the magnetic circuit formed between the coils  38  and the rotor blades  32 . 
   From the forgoing, it can be appreciated that the electrical portions of the pump  24  are advantageously disposed entirely outside of the hydrogen gas environment of the conduit  22 , while only the simple mechanical components of the pump are subject to the gas environment. 
   In operation, the controller  30  sends either a pulse train or a sinusoidal (AC) signal to the coils  38 , thereby synchronizely energizing the coils  38  at a frequency determined by that of the applied signal. The energized coils  38  produce a synchronously varying electromagnetic field (and related forces) which attracts the ferromagnetic rotor blades  32 , causing the rotor  34  to spin about the central hub, in the direction of the arrow  40 . The blades  32  are configured to force the flow of hydrogen gas through the conduitline  22 . The exact size shape and number of the rotor blades will depend on the specific application and the desired flow rates. Depending on the flow rates that must be achieved, the rotor  34  is made to spin at relatively high rates of speed due to the fact that hydrogen is a relatively light gas. In any event, the speed of the rotor  32  and thus the gas flow rate, is directly dependent on the frequency of the excitation signal delivered by the controller  30   
   Although a stator comprising 6 poles (coils  38 ) has been disclosed (suitable for being powered as in a three phase power system), a different number of poles may be employed, depending on the application. By varying the current delivered through the stator coils  38 , the induced electromagnetic field induced is superimposed over the existing magnetic field of the blades  32 , generating the force that causes the rotor  34  to spin. The number of the stator coils  38  determines the exact nature of the required excitation signals that must be produced by the controller  30 . The frequency of the excitation signal is directly proportional to the rotational speed of the rotor  34 . The flow rate of recirculated hydrogen gas is a function of the selected frequency of the excitation signal. The excitation signal can be any periodic signal with the desired frequency however sinusoidal or pulse train signals are generally most suitable. 
   The mass flow sensor  26  senses the mass flow of hydrogen gas being delivered to the anodes  12  and delivers a signal to the controller  32  indicative of the mass flow rate. The controller  30  then adjusts the speed of the recirculation pump  24  and/or the speed of a pump (not shown) that controls the supply of fresh hydrogen from the supply  18 , to assure that hydrogen gas is supplied to the anodes  12  at the proper rate. In some cases, a conventional gas analyzer sensor (not shown) may also be used to provide information to the controller which is taken into consideration in adjusting the speed of the pump  24 . It should be noted here that it may be necessary to calibrate the recirculation pump  24  prior to initial use. 
   It is to be understood that the device and method which has been described are merely illustrative of one application of the principles of the invention. Numerous modifications may be made to the device of the method as described without departing from the true spirit and scope of the invention.