Abstract:
Fuel concentrations are determinable in a solid oxide fuel cell through voltage measurement of one or more fuel cell units, which voltage is a function of hydrogen gas present in the fuel feed stream to the one or more fuel cell units. The voltage in the one or more fuel cell units is proportionally related to the fuel concentration in the fuel feed stream to the entire fuel cell. A sensor determines concentrations of the fuel flowing in the fuel cell. The sensor comprises a fuel cell unit, and an indicator electrically coupled to the fuel cell unit, the indicator being capable of displaying a voltage or being adapted to convert a voltage to a fuel concentration display. The voltage measured is correlated to the fuel concentration flowing in the fuel cell.

Description:
TECHNICAL FIELD 
     This disclosure relates to fuel cell systems and specifically to sensing a concentration of fuel within a fuel cell system. 
     BACKGROUND 
     A fuel cell is an energy conversion device that generates electrical energy and thermal energy by electrochemically combining a gaseous fuel and an oxidant gas across an ion conducting electrolyte. Several types of fuel cells currently exist. A characteristic difference between distinct types of fuel cell is the type of material used for the electrolyte. The difference in the materials of the electrolyte employed distinguishes the fuel cells due to the operating temperature ranges of the materials. In one type of fuel cell, the Solid Oxide Fuel Cell (SOFC), the fuel cell is constructed from solid-state materials utilizing an ion-conducting oxide ceramic as the electrolyte. To generate a useful quantity of power, a fuel cell is made up of multiple fuel cell units in a series array, typically stacked together. A single SOFC unit consists of two electrodes, one is an anode and one is a cathode. The anode and the cathode are separated by the solid electrolyte just identified. Fuel for the SOFC is typically gaseous hydrogen and carbon monoxide supplied in from reformats, and the oxidant is commonly an air supply. The fuel cell operates when the oxidant contacts the cathode and the fuel contacts the anode. The electrolyte conducts the oxygen ions between the cathode and the anode maintaining an overall electrical charge balance in the system. Electrons are released from the fuel cell to an external circuit forming a flow of electrons. The flow of electrons released from the fuel cell to the external circuit provides useful electrical power. 
     The production of useful electrical power is the primary function of the SOFC. Optimizing the conversion of fuel in the fuel cell is an endeavor that commands a significant amount of time and effort. As in many other energy conversion devices, the function of converting the fuel into useful energy, (electrical energy, thermal energy), is closely monitored by system operators. Quantifying the concentration of fuel flowing in the fuel cell provides a benefit during the operation of the fuel cell. The performance of the fuel cell is related to, and optimized by knowing the concentration of fuel being supplied to the fuel cell. Understanding the fuel concentration allows operators to understand what quantity of fuel to supply, and what electrical load to apply. Unfortunately, directly measuring the concentration of fuel such as hydrogen in the fuel cell creates many engineering challenges due to the limitations of hydrogen concentration sensors. The limitations of directly measuring hydrogen concentrations with sensors are amplified when applied to the SOFC, because the SOFC operates at high temperatures and uses high concentrations of hydrogen. The limitations are greatest with respect to sensing the concentration of hydrogen and the material compatibility of the sensor. 
     Direct measurement hydrogen concentration sensors are designed for concentrations that are very small compared to the relatively high SOFC hydrogen concentrations that exist during fuel cell operation. As a result, the direct measurement hydrogen concentration sensors are inadequate for use with solid oxide fuel cells. 
     In addition to the forgoing, existing hydrogen concentration sensors that measure hydrogen concentrations directly are not compatible with SOFC operating environments. Typically SOFC&#39;s exhibit high operating temperatures and a harsh environment both of which are detrimental to direct measurement hydrogen concentration sensors. Thus, there is a need in the art for a sensor that is compatible with both the operating environment and the relatively high levels of hydrogen concentration of the SOFC. 
     SUMMARY 
     Fuel concentrations are determinable in a solid oxide fuel cell through voltage measurement of one or more fuel cell units, which voltage is a function of hydrogen gas present in the fuel feed stream to the one or more fuel cell units. The voltage in the one or more fuel cell units is proportionally related to the fuel concentration in the fuel feed stream to the entire fuel cell. A sensor determines concentrations of the fuel flowing in the fuel cell. The sensor comprises a fuel cell unit, and an indicator electrically coupled to the fuel cell unit, the indicator being capable of displaying a voltage or being adapted to convert a voltage to a fuel concentration display. The voltage measured is correlated to the fuel concentration flowing in the fuel cell. The above described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The sensor will now be described, by way of an example, with references to the accompanying drawings, wherein like elements are numbered alike in the several figures: 
     FIG. 1 is a schematic plan view of a fuel cell making up all or a part of a fuel cell; 
     FIG. 2 is a schematic plan view of an exemplary embodiment of a fuel cell unit based sensor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, an exemplary embodiment of a fuel cell unit  10  is shown. In one embodiment, the fuel cell unit  10  is an assembly of an electrolyte  12 , an anode  14  and a cathode  16 , with the electrolyte  12  positioned between the anode  14  and the cathode  16  as illustrated. In a working fuel cell, one or more fuel cell units are employable. Typically more than one unit is employed to increase the total electrical energy output. In such multiple unit fuel cells, fuel cell unit  10  is repeated over and over to provide a serial assay of fuel cell units  10  to produce a desired quantity of electrical energy and thermal energy. 
     An understanding of the components of a solid oxide fuel cell and its operation will be helpful to understand this disclosure. The ceramic electrolyte  12 , in one embodiment, is an yttria-stabilized-zirconia (YSZ). This ceramic electrolyte  12  exhibits good oxygen ionic conductivity and little electrical conductivity at high temperatures (700-1000 degrees centigrade). The electrodes, in one embodiment, are porous, gas-diffusion electrodes. The anode  14  is about 20-40 percent porous and is formed from a metallic nickel and an YSZ skeleton for thermal compatibility with the other components. The cathode  16  is made from strontium-doped lanthanum manganite with about the same porosity as the above embodiment of the anode  14 . In other embodiments the materials may vary. Because the fuel cell is solid state, the thermal expansion coefficients of as many as four different ceramic layers must be well matched in the fuel cell unit  10 . A high operating cell temperature in the SOFC is required to maximize the ionic conductivity of the electrolyte and ensure good electrical conductivity of the electrodes and interconnections. As a result, the critical cell components are made from various ceramics, metal-ceramic composites, and high temperature alloys that are compatible with the operating environment of the SOFC. 
     The fuel cell unit  10  may be configured in a variety of geometries including tubular planar stack and radial planar geometries. The fundamental electrochemical processes of the fuel cell unit  10  remain the same for various cell geometries. In the embodiment shown in FIG. 1, during operation, fuel  18 , (typically reformate containing hydrogen reformed from diesel fuel, gasoline, natural gas, propane, or methanol), flows through channel  22  and oxidant  20 , typically air, flows through channel  24 , respectively. Each electrode, (cathode  16 , anode  14 ), is exposed to the reactant gases  20 ,  18 . The anode  14  is exposed to or contacted with the fuel  18  and the cathode  16  is exposed to or contacted with the oxidant  20 . More specifically, the fuel cell unit  10  operates when the oxidant  20  having oxygen ions  26 , contacts the cathode  16 , where the oxygen ions  26  are adsorbed by the cathode  16 . The oxygen ions  26  diffuse to the cathode-electrolyte interface and are reduced, (gains electrons). The mobile ionic species are negatively charged oxygen ions. Continuing with the fuel cell operation, negative ions (anions)  28  migrate across the electrolyte  12 . The migrating anions  28  carry the negative charge to the electrolyte-anode interface. At the anode  14 , hydrogen  19  is oxidized. Because of hydrogen&#39;s affinity for oxygen, the hydrogen  19  flowing past the anode  14  is adsorbed by the anode  14 , where the hydrogen diffuses through the porous anode  14  to the anode-electrolyte interface, where as mentioned above, the hydrogen  19  is oxidized (loses electrons). The fuel cell unit  10  creates a flow of electrons  30  (electron flow). The flow of electrons  30  is conducted to an electrical load  32  via an electrical circuit (not shown). The electrical circuit maintains the flow of electrons  30  from the anode  14  to the electrical load  32  and continues to the cathode  16 . The electron flow  30  flows from the negative charge at the anode  14  to the positive charge at the cathode  16 . The electrical current (not shown), flows opposite the electron flow  30  from a high electrical potential at the cathode  16  to a low electrical potential at the anode  14 . In addition to electron flow  30 , the fuel cell produces reaction products from both electrodes while in operation. The anode reaction products  34  (product gases and depleted fuel, or combustion products) of the fuel cell unit  10  are typically water, carbon dioxide, hydrogen, carbon monoxide and other products, depending on the fuel  18 . Thermal energy is also a discharged product  34 . Cathode reaction products  36  (excess or depleted oxidant and product gases), typically air and water are also discharged. As stated previously, the fuel cell unit  10 , including the electrolyte  12  disposed between the anode  14  and the cathode  16  produces a limited quantity of electrical energy and thermal energy. Combining an individual fuel cell unit  10  with multiple fuel cell units  10  otherwise known as stacking, increases generating capacity amounting to a quantity of useful electrical and thermal energy. The serial array of individual fuel cell units  10 , creates a complete fuel cell, (sometimes known as a fuel cell stack; not shown). 
     The electrochemical processes that occur in the fuel cell unit  10  can be related to the electrochemical processes that occur in the entire fuel cell. The flow of electrons  30  from the fuel cell unit  10  is related to the sum of all electrons flowing  30  through the entire fuel cell. The electrons flowing  30  through the fuel cell are related to an electrical potential of fuel cell. Electrical potential is measured as voltage. The voltage of the fuel cell is a strong function of the concentration of the hydrogen  19  (fuel  18 ) in the feed stream of fuel of fuel cell. Likewise, the voltage of the fuel cell unit  10  is a strong function of the concentration of the hydrogen  19  (fuel  18 ) in the feed stream of fuel to the fuel cell unit  10 . Stated another way, the concentration of the hydrogen in the fuel cell is related to the flow of electrons  30  and to the electrical load  32 . The operability of the fuel cell is related to the concentration of fuel  18  in the fuel cell. Throughout the operation of the fuel cell, the concentration of fuel  18 , is a parameter that indicates SOFC system performance. In a preferred embodiment, the concentration of hydrogen is a parameter that is used to optimize the performance of the fuel cell. More specifically, the knowledge of the concentration of the hydrogen in the fuel stream being presented to the stack of fuel cell units  10  is a parameter that can be used to optimize the performance of the system. It has been determined by the inventors herein that measured voltage of one or more fuel cell units can be repeatably and reliably correlated to a concentration of reformate flowing in the fuel stream to the fuel cell. The relationship of the flow of electrons  30  to the concentration of hydrogen  19  allows for measurement of the concentration of hydrogen  19  indirectly by measuring the voltage of one or more fuel cell unit(s)  10 . The voltage measurement of even a single fuel cell is correlatable to the reformate concentration in the entire fuel cell. 
     Turning now to FIG. 2, an exemplary embodiment of a fuel cell based fuel concentration sensor  40 , hereinafter, sensor  40 , is shown. The fundamental electrochemical processes of the sensor  40  remains the same as the electrochemical processes of the fuel cell unit  10  regardless of the various cell geometries. FIG. 2, illustrates an arrangement of a preferred embodiment of a sensor  40  that directly measures the voltage of the fuel cell unit  10  and indirectly allows determination of the concentration of hydrogen in the fuel cell. The sensor  40  has the same basic components and materials as the fuel cell unit  10  shown in FIG. 1, with the substitution of the electrical load  32  for an indicator  42 . The indicator  42  measures and indicates the voltage of the sensor  40 . The components of an individual fuel cell unit  10  or, in one embodiment, a portion of the fuel cell unit  10  is utilized as the sensor  40 . The sensor  40  is nestable with the fuel cell. In an embodiment, multiple sensors  40  are disposed or nested within the fuel cell. Sensors  40  can be intermittently disposed throughout the fuel cell stack to provide an array of indications within the cell geometry. In certain fuel cell geometries, the fuel cell units  10  may experience different operating conditions at different locations within the fuel cell, so placement of individual sensors  40  in different locations within the stack is also contemplated. In the preferred embodiment, the sensor  40  is not electrically connected to other fuel cell units  10  in the stack of the fuel cell. The sensor  40  is isolatable from the fuel cell stack. The sensor  40  is not connected to the electrical load  32 . 
     The sensor  40  has the material properties to function in the environment of the fuel cell unit  10 . A hydrogen concentration sensor made from the same materials as the fuel cell components can withstand the SOFC operating environment. In a preferred embodiment, the sensor  40  has the same electrolyte  12  materials, the same anode  14  materials and the same cathode  16  materials as an individual fuel cell unit  10 . The sensor  40  is capable of determining the high concentrations of fuel that are encountered in the fuel cell unit  10 . The capability to determine the relatively high concentrations is due to the proportional relationship of the voltage and the fuel concentration in the fuel cell unit  10 . In a preferred embodiment, the sensor  40  is compatible with the SOFC using hydrogen as a fuel, where the hydrogen has a wide range of concentrations. A hydrogen concentration sensor that is not limited to relatively small hydrogen concentrations can measure hydrogen concentrations within the SOFC. 
     Measuring the voltage with the sensor  40  provides data which is correlatable to the hydrogen concentration in the fuel cell because the voltage of the sensor is proportional to the concentration of hydrogen being presented to the fuel cell. The indicator  42 , in one embodiment, can be used simply to provide the data taken from measuring the voltage. The data can then be used to correlate the voltage to the hydrogen concentration. In another embodiment, the indicator  42  can measure the voltage and correlate the data taken from the measurement into a hydrogen concentration in a display. Measuring the voltage of the sensor  40  enables monitoring hydrogen  19  concentrations or other fuel  18  concentrations in other embodiments. 
     A comparison of the voltage measured in the electrically isolated sensor  40  to the total voltage of the electrically loaded fuel cell units  10  stacked in the fuel cell is also considered in an alternate embodiment. A variety of fuel cell performance characteristics can be assessed, such as contamination within the fuel cell unit  10 , aging, and fuel cell efficiency, by knowing the concentration of hydrogen in the fuel cell. The fuel cell unit  10  fuel flow rates as well as electrical load  32  can be controlled more efficiently as a result of having the capability to detect the voltage of the sensor  40  and correlate a fuel concentration in the fuel cell. It is contemplated that applying varying electrical loads  32  to the sensor  40  and measuring the output impedance of the sensor  40  thus determining a relationship of the concentration of reformate (fuel  18 ) with the output impedance of the sensor  40 . 
     While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the claims.