The performance and life of a fuel cell is strongly affected by the voltage degradation that results from increase of overpotentials and loss of electrolyte conductivity overtime. The lack of valid field data makes degradation difficult to quantify. The few degradation rates reported in the literature so far vary from 0.3% to 0.6% per 1000 hours of operation for the MCFC, and 0.1% to 0.25% per 1000 hours of operation for the SOFC. We performed a parametric analysis to investigate the impact of fuel cell degradation on the overall hybrid system efficiency. We considered fuel cell power loss due to cumulative degradation varying from 1% up to 20% in two different degradation scenarios for each one of MCFC and SOFC. We considered several ways to recover part or all of the lost system efficiency. An optimization of the fuel cell subsystem design as well as the timing of replacement of stacks or cells that operate poorly, leads to complete efficiency recovery. The use of a stand-by gas turbine is an alternative way to produce the power lost by the fuel cell subsystem. However gas turbines are less efficient than the fuel cells and the recovery of the lost power carries a penalty in system efficiency. Cost considerations have also been taken into account. Stack replacement maintains the system high efficiency but leads to higher system capital cost, because the fuel cell stack is the most expensive component of the system. The use of stand-by gas turbines reduces initial costs but operating costs increase because of the lower system efficiency.

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