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generation Systems, such as load insertion and load rejection. However, it must be said that a model that reproduces the characteristics of fuel cells with high precision degree as a function of the stack load current, is still a challenge.

The results obtained for fuel cells from different manufacturers agree with the results presented in the current literaturę on fuel cells, for the celi polarization curve, validating so the model presented here as a błock that can be used in the modeling of any complete power generating system. Also, tests at load variation suggest the possibility of use of algorithms such as the Hill Climbing Control (HCC) for load power control. With this model, the HCC and other algorithms can be debugged and tested in laboratory, with neither playing nor expending hydrogen, not putting at risk the celi and load integrities, besides having readily available all models and sizes of cells of whose parameters are available.

The partial and total load insertion and rejection tests demonstrated that the FC output voltage present a component directly related to the load current, known as the ohmic overpotential. This varies instantly with the variation of the current. There are, still, two other components: the activation and the concentration overpotential, that are responsible for the attenuation of the voltage variation as a function of the current variation through the celi. Such dynamie voltage variation has significant reflexes on the supplied power, as it could cause power peaks, as discussed by other authors.

The characteristics outlined in this paper should be taken into account during the design stage of FC energy generation systems. It would permit one to conclude about the needs for other sources of energy necessary to attenuate the effects of the abrupt variations of power, sińce these could damage temporary or permanently the cells.

For injection of energy into the grid and driving larger loads, this power characteristic impose the use of power converters to elevate and to control the voltage across the celi terminals and the power supplied to the load.

Using a simple procedurę and data obtained from the manufacturer and from the literaturę, it is possible to obtain a model for a fuel celi stack, whose resulting characteristic is similar to the real system. For example, the results obtained for the SR-12 PEM Modular Generator, for the Ballard Mark V and for the 500 W BCS stack presented good agreement with their manufacturer’s data. In this way, the model for these stacks could be used to analyze the behavior of actual stacks in FC generation systems.

VIII. ACKNOWLEDGEMENTS

The authors are very thankful to the Coordination for Improvement of Advanced Education Personal (CAPES), National Council of Research and Development (CNPq), to AES-Sul Distribution

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