Final Activity Report Summary - FCTHERM-CRO (Fuel Cell Stack Thermal Behavior) Fuel cells stacks require active cooling in order to maintain the desired temperature. The most common temperature control strategies for an air-cooled fuel cell are on-off or multi-step flow rate control of the cooling air fan. The objective of this research was to analyze the effects of different cooling strategies on stack temperature, particularly at start-up, shut down and immediately after sudden load changes. A MATLAB model was developed and used to study the steady and transient states of a fuel cell stack. A 1 kW air-cooled fuel cell stack, developed by Proton Energy Systems Inc., was used for this analysis. The analysis included stack mass and heat balance, heat transfer from the stack to cooling air and heat transfer to the surrounding air. Operation of the stack under various scenarios was simulated such as start-up from room temperature, steady state at full and half load and sudden changes of load (from full to half load), under constant or variable flow rate of cooling air. The results of these analyses and simulation show how the stack temperature changes in and between the steady states achieved at different loads and at different cooling air flow rates. In variable power mode operation, the stack temperature is not constant. It is possible to generate conditions under which spikes of liquid water are formed at the cathode outlet during transient power changes although at steady states all water may be in vapour phase. This may have significant effect on the fuel cell stack performance and must be addressed either by the stack design or by the stack temperature control strategy. This study will help in defining a proper temperature control strategy for an air-cooled PEM fuel cell stack.