Small fuel cells operational control

Small fuel cell systems, especially those operating on hydrogen fuel, are characteristically simpler than larger systems, as discussed earlier. Nevertheless, certain control elements must be imposed to foster stable operation. Representative methodology for these is as follows ... 

The addition of H2O and CO2 to the fuel gas modifies the equilibrium gas composition so that the formation of CH4 is not favored. Carbon deposition can be reduced by increasing the partial pressure of H2O in the gas stream. The measurements (20) on 10 cm x 10 cm cells at 650°C using simulated gasified coal GF-1 (38% H2/56% CO/6% CO2) at 10 atm showed that only a small amount of CH4 is formed. At open circuit, 1.4 vol% CH4 (dry gas basis) was detected, and at fuel utilizations of 50 to 85%, 1.2 to 0.5% CH4 was measured. The experiments with a high CO fuel gas (GF-1) at 10 atmospheres and humidified at 163°C showed no indication of carbon deposition in a subscale MCFC. These studies indicated that CH4 formation and carbon deposition at the anodes in an MCFC operating on coal-derived fuels can be controlled, and under these conditions, the side reactions would have little influence on power plant efficiency. 

For future work, improvements can be made to the controller subroutine such that simulation results for systems that employ small battery arrays can be more easily interpreted. With the current controller subroutine, the smallest practical simulation step size is 0.5 hours. However, with a small battery array, it is possible that the battery SOC can change dramatically within that time. Since the controller currently bases most decisions on the SOC value, fuel cell and electrolyzer activation in such a simulation appear to be unsteady and erratic. Examples of this behavior are seen in Case 1 results where wide variations in battery SOC cause the electrolyzer and fuel cell to switch on/off repeatedly, rather than maintaining steady operation. 

In another study by Wu et al. (2005), the optimal operating conditions based on validated multiresolution fuel cell simulation tool has been developed with four control parameters including cell temperature, cathode stoichiometry, pressure, and humidity. The study shows that different optimal solutions exist for different system assumptions, as well as different current loading levels, classified into small, medium, and large current densities. This design can be readily applied to a larger number of control parameters and further to the fuel cell design optimizations. 

Cathode The carbonate fuel cell cathode material has been lithiated NiO from the beginning of development This component is known to have a small but finite solubility in the electrolyte. The extent of its dissolution is controlled mainly by electrolyte composition, applied gas atmosphere, operation pressure and temperature. Some developers have selected an atmospheric pressure system to assure minimal dissolution and adequate long-term life for the cathode. Long-term field operation has shown no issues relating to particle coarsening, indicating a stable structure (Fig. 9). 

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