Current or Fuel Utilization Efficiency

Another important factor that needs to be considered while considering fuel cell efficiency is the fact that excess fuel is generally supplied in order to offset for any unwanted consumptions such as fuel crossover loss through electrolyte, incomplete and undesirable reactions, and leakage loss through cell components and to sustain the electrochemical reaction across the entire active surface area. Any unconsumed fuel will exit the cell as an element exhaust gas mixture. The fuel utilization factor or stoichiometric factor is defined as a measure of the excess fuel supplied as 

Fuel supplied at inlet to the cell Wf Fuel consumed in reaction m 

For a given fuel stoichiometric factor, f, the fuel supply rate at inlet is given as 

The current efficiency, ti is then defined as the ratio of the mass of fuel consumed in the reaction to the mass of fuel supplied to the cell and expressed 

Substituting Equation 4.23b into Equation 4.62, the current or fuel utilization efficiency can be expressed as a function of the stoichiometric factor 

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Physically, a current or fuel utilization efficiency value represents the fraction of the fuel converted into current. The remaining fraction of the fuel leaves the cell without reacting or without being consumed for the production of the current. The excess fuel that exits the cell may be recycled back into the cell or may be burnt to produce heat for other system use. 

Most new gas and oil-fueled furnaces and boilers have similar efficiencies. The range of efficiency has narrowed with the introduction of minimum efficiency standards for new products sold since 1992. New gas and oil heating equipment currently available in the marketplace have /knnual Fuel Utilization Efficiency (AFUE) ratings of at least 78 to 80 percent. /VFUE is a measure of how efficient a furnace operates on an annual basis and takes into account cycling losses of the furnace or boiler. It does not include the... 

The flow of protons and the crossover of alcohol (ROH) from the anode to the cathode are also indicated in Fig. 1.5. Alcohol crossover decreases the current through the cell because part of the fuel permeates through the membrane without reacting in the anode. Thus, a fuel efficiency (or fuel utilization) can be defined as ... 

Fuel costs vaiy widely from one area to another because of the cost of the fuel itself and the cost of transportation. Any meaningful cost comparison between fuels requires current costs based on such factors as the amounts used at a particular geographical location, utilization efficiencies or energy-ratio data for the equipment involved, and the effects of Torm v ue. Although the costs given in Table 27-9 do not apply to specific locations, they give fuel-cost trends. 

The efficiency of the automotive proton exchange membrane (PEM) fuel cell is dependent on many factors, one of which is the humidification of the inlet air. If the inlet air is not sufficiently humid (saturated), then the stack can develop dry spots in the membrane and efficiency and voltage will drop. Therefore, it is necessary to ensure that humid inlet air at the proper elevated temperature is supplied to the stack. Current methods involve utilizing a spray nozzle to atomize water droplets onto a cloth or wire mesh substrate. As the ambient inlet air passes over the cloth it picks up moisture however, the relative humidity drops as the air is heated in the fuel cell. If heat could be supplied to the water efficiently, the system would become independent of the ambient conditions, the inlet air could become more humid at the proper temperatures, and the overall stack could maintain a high level of efficiency. Previous work with power electronic heat sinks and automotive radiators has demonstrated the high efficiency of carbon foam for heat transfer. Utilizing the carbon foam in the PEM fuel cell may reduce the inlet air humidification problems. 

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