9818247183
The stack efficiency is shown in Fig. 9. The behavior is similar to the voltage, sińce these are directly related. The steady-state values for the efficiency are: 56.2% for a current of 50 A and 47.3% for a current of 150 A. It can be noticed that there is a significant reduction in the efficiency for variations of the demanded current, which should be taken into consideration when one evaluates a certain system.
Fig. 9. Stack efficiency for partial load insertion and rejection test
V. MODELING OF A SR-12 MODULAR PEM GENERATOR
The SR-12 Modular PEM Generator, manufactured by the Avista Laboratories, is a modular FC stack, which has some characteristics adequate for use in electrical generation Systems [10]. Recently, the Colorado School of Mines received one SR-12 Modular PEM Generator, with a rated power of 500 W. The data provided by Avista Laboratories for the SR-12 stack was used to match the model proposed in this paper. The procedurę to obtain the parameters for this specific stack is:
1) Initially, it is necessary to obtain the basie information from the manufacturer’s data sheet [10]. In this case, the information provided was: (i) number of cells, n, equals 48; (ii) hydrogen pressure: 1.47628 atm; (iii) oxygen pressure: 0.2095 atm (air at atmospheric pressure); and (iv) normal operation temperaturę: 50°C.
2) The Avista Laboratories gently allowed access to the polarization curve for this generator. From this data, it was possible to obtain: (i) maximum current: 42 A; and (ii) open Circuit voltage: 41.7 V. Avista Laboratories also provided information about the membranę thickness, £ = 25 pm and the membranę active area A = 62.5 cm".
3) Initially, he values for the electrochemical parameters and \\i are considered to be the same as the ones used for the Mark V fuel celi (Table I).
4) The maximum current density can be calculated using Eq. (15), for maximum current and membranę active area, resulting in a maximum current density, Jniax, of 0.672 A/cm2.
5) Now it is necessary to obtain the equivalent current density for the intemal currents/fuel crossover (J„) and the fi-parameter, used in the calculation of the concentration overpotential,
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