Behavior of the FCS During Warm-up

In this section, the experimental results regarding the analysis of the FCS during the warm-up phase are presented. The tests are carried out starting from 18°C, for two different acceleration values (150 and 1,500 W/s). Regarding the stack humidification, the inlet air is saturated at the same temperature adopted for the stack at the beginning of the tests, to avoid membrane de-hydration. 

The individual cell uniformity is monitored by using the coefficient of percentage variation as statistical indicator already introduced in Sect. 6.3. For the analysis reported in this paragraph a value of Cv equals 2.5% is considered acceptable for a regular working of the stack. 

In order to study transient response of the stack during the warm-up phases the following experiments are carried out connecting the FCS to a resistive load electronically controlled. Starting from the temperature of 18°C the stack power is gradually increased up to about 8 kW by using two power accelerations (150 and 1,500 W/s). After each acceleration the system is left in steady state condition until the temperature of about 45°C is reached for the stack. 

In Fig. 7.32, the stack current, voltage, and temperature are shown versus time for the test at 150 W/s, while Fig. 7.33 shows a magnification of the stack power and stoichiometric ratio during the acceleration phase. It can be observed that the stack temperature reaches its final value in about 10 min, while the stack voltage reaches the minimum value of 53 V at the end of acceleration slope (55 s), then increases up to about 60 V at 45°C. The stack current reaches the pick value of 

The warm-up phase is also smdied using a slope of acceleration 10 times higher (1,500 W/s). The results are shown in Figs. 7.36, 7.37, 7.38, and 7.39 evidencing the tendency of the FCS to operate with irregularity only at the end of acceleration phase, in particular the total stack voltage decreases to about 50 V for 

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