**6. Conclusions**

**5.3. Resonant circuit operation**

356 New Developments in Renewable Energy

Figure 30 corresponds to step-up and step-down load conditions. From its analysis it follows that the converter reacts to the load variation, varying its frequency of operation. Thus, for a small load level (Imin, Rmax) the frequency is low while for a high load (Imax, Rmin) the frequencyishigh.Indynamicterms itcanbeseenthatthetransitioninthefrequencyofoperation is instantaneous, hence, we conclude that the system has good dynamic characteristics. It can be also observed that in any of the load variations the output voltage Vout remains constant. This analysis validates the objective defined to the controller, that is it ensures a constant output voltage in order to satisfy the requirements imposed by the power system applications.

(a) Step-up load condition.

(b) Step-down load condition.

**Figure 30.** Output voltage and current and resonant circuit operation

The main objective of the chapter is to discuss the design and implementation of a power generation system based on fuel cells. Accordingly, a methodology of designing and imple‐ menting an efficient high power converter system is presented. Moreover the chapter presents also an electrical equivalent model of the PEM fuel cell, which was validated by experimental tests made with the commercial system MARK 1020.

Authors make considerations on the most suitable topologies of converters for this application type, and satisfying several criterions a series-resonant converter topology is selected, whose principle is based on soft-switching methodology. In this context the design and implemen‐ tation of the converter consisting of a input filter followed by the full-bridge inverter and the series resonant circuit on the primary side and a diode rectifier and output filter on the secondary side was based on the exploitation of their benefits as compared to other types of converters, namely: low component stresses, high frequency operation and soft-switching commutation. Converter design was done considering the operational constraints of the system MARK 1020.

A particular attention is done to the controller, which ensures a constant output voltage of the converter, in order to satisfy the requirements of the power system application and simulta‐ neously keeps the PEM operating within its optimum operating point. The control implemen‐ tation was divided into two parts namely: i) the voltage controller, which is responsible for keeping constant the output voltage of the converter even under loading variations and ii) the PEM controller, which is responsible for improving its performance by keeping the PEM fuel cell in its optimal operating point.

Due to significance of the PEM cell behavior the results are firstly presented for the PEM fuel cell model and then for the whole system with load.

The results demonstrate that the converter selected is a good solution to support the approach of improving the efficiency of PEM fuel cells because it allows an appropriated control of the power delivered by the fuel cell as it satisfies the requirements imposed by the load regulation with minimum of losses due to adoption of soft switching commutation.
