Dynamic Performance of the FCS

The dynamic behavior of the FCS is firstly verified starting from the analysis of the energy lost during the start-up phases, evaluating the performance as function of acceleration rates [2]. In particular, warm-up tests are performed starting from two different initial stack temperatures, 15 and 30°C. For each one of these temperatures, two accelerations of 20 and 200 Ws are used up to the stack power of 1200 W. At the end of each acceleration phase, a steady-state operation follows until the stack temperature reaches the value of 45°C. 

Other dynamic tests are affected varying the stack power and evaluating the stack response to hydrogen purge, external humidification, and air management 

The Figs. 6.8 and 6.9 show stack and FCS efficiencies versus time for the tests starting from the temperature of 15°C. In both these two tests, the selected value of final stack temperature of 45°C is reached in about 10 min, while the stack efficiency achieves a value (0.53) a little bit lower than that maximum (0.59) already at the end of the accelerations ramps (1200 W). 

When the stack temperature reaches the value of 45°C, the stack efficiency increases from 0.53 to 0.59, and when power reaches its set maximum value (1200 W) the FCS efficiency increases from 0.45 to 0.50 in both tests. From Figs. 6.8 and 6.9, it is possible to evaluate the energy losses due to the warm-up period of 600 s, they resulted about 5% of the FCS steady-state maximum efficiency. Moreover, the energy losses are not significantly affected by the power acceleration variation from 20 to 200 Ws Start-up operations are also verified at the starting temperature of 30°C and 200 W s (Fig. 6.10), the results evidence 

An important point to consider about the stack management, with reference to an electric power train operating in dynamic conditions, as determined by road requirements, is the regulation of the stack temperature together with the other control parameters of water and reactants to avoid mass transfer limitations and membrane drying out or flooding. Moreover, the interaction between stack and auxiliaries has to be balanced taking into account the optimization of fuel cell system efficiency and reliability (see Sect. 4.6). 

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