Results And Discussion

To evaluate the usefulness of the present fuel-cell model, a fuel cell stack with 5 cells has been selected as an example for demonstration. The characteristics of a stack is described by (He and Chen, 1996). The responses of stack current and temperature distributions to +10% step voltage are discussed. In accordance with the time range considered earlier, the simulation time for the stack response is 7200 seconds, with a relatively dense time-step grid in the beginning and succeeded by a coarse time-step grid. The simulation results are therefore selectively illustrated in the interval of 40 seconds for the current density, and at 120,3600 ariif7200 seconds for the temperature.

Figure 2 (a) shows the response of the current density and temperature that takes place in the separator of cell unit 5. It indicates that the overall current density decreases when the stack voltage increases, which satisfies the voltage-current relation. Between 0 and 80 s, the current density profile changes significantly, but not between 80 s and 120 s. Figures 2 (b) illustrates that the overall temperature decreases. The rapid change in current density between time 0 s and 80 s may result from the rapid response of mass storage and chemical reaction. The slow change of current density from time 80 s to 7200 s may result from the slow response of temperature, where temperature response is determined by energy storage.

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