Fig.4 Stack Power Build-up


Fig.5 Average Cell Voltage Change •with Change in Load Current

Fig.5 Average Cell Voltage Change •with Change in Load Current

For system designers, it is important to know the equivalent electrical circuit of the power source in advance to execute the design particularly for the dynamic response of the overall system. A simplified equivalent electrical circuit (a first order approximation) of a fuel cell can be expressed as is shown in Fig.6, consisting of an electrolyte resistance [Rr], a charge transfer resistance [Re] and a capacitance [C] representing the electrical double layer capacity.

Fig. 7 Voltage Transient at Abrupt Current Break

Fig. 6 Equivalent Electrical Circuit of a Fuel Cell

Fig. 7 Voltage Transient at Abrupt Current Break

To determine the values of the individual elements, an analysis by means of electrical transient has been applied. An abrupt break of the load current of the fuel cell stack gives a step change in the stack voltage, which is then followed by an exponential decay of the voltage. This phenomenon is seen in Fig.7, from which Rr. Rc and C have been calculated. For example, the time constant of the circuit is given as about 360 ms. Thus, the equivalent electrical circuit of the stack has been confirmed.

3. Power Conversion System

One of the fundamental problems during the vehicle design was the selection of the drive motor. A few kff order fuel cell stack usually generates the stack voltage at around 10-20 V at full load under reasonable design conditions. Low voltage d-c motors are available for small capacity vehicles, however, d-c motor drive is no longer advanced technology in view of cost, maintenance and operability for larger capacity vehicles.

The first prototype EV being built at FUT is small and it is no problem to adopt a d-c motor for drive, but in the standpoint of pursuing an optimum design for FC drive EVs for the next stage, a-c motor drive systems are being studied.

Hock-up tests of an induction motor drive system consisting of a d-c to a-c converter, voltage step-up units and a variable frequency controller are conducted. It is working reasonably well but the sophisticated system resulted in lower efficiency of the power conversion process.

An alternative method is the d-c brushless motor drive concept (by means of synchronous motor control). This technology has recently been widely used and well designed components are available in a certain range of the capacity. In-wheel type of the brushless motor will be tested shortly. Comparison of these different power conversion systems will be discussed at the Seminar.

4. Conclusion

Fundamental characteristics of a PEFC stack have thoroughly been studied first to clarify the operational conditions taking into account the interfacial compatibility with the motor drive system. In conclusion, it has been proved that the PEFC has appropriate characteristics as the power source of EVs. In the meantime, the power conversion and transmission system to drive the vehicle is being studied using several different types so as to assess compatibility. The approach to pursue an optimal design for FC powered EVs will be discussed at the Seminar.

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