A scheme of a fuel feeding sub-sect

A scheme of a fuel feeding sub-section in a direct hydrogen FCS for automotive application is shown in Fig. 4.2. Pressure regulation sub-system design has the objective to regulate inlet pressure to assure the hydrogen flow rate necessary to the chemical reaction. The inlet pressure and fuel purge represent the main parameters to be controlled in both operative modes, however, the role of the purge valve is different for the two possible configurations. In a dead-end configuration and when air is used as oxidant, nitrogen tends to crossover from the cathode flow field through the electrolyte to the anode side. As fuel is consumed in the fuel cell, the concentration of nitrogen in the anode compartment increases, thus accumulating therein and negatively impacting the performance of the fuelcell [5]. Moreover, the humidification control (see Sect. 4.5) could create operativephases characterized by a dangerous stagnation of liquid droplets on surface ofelectrodes in cathode and anode sides, favoring flooding of the compartments andfuel starvation, and then interfering with the access of the hydrogen fuel duringstack power requirement. Then the purge action, draining out the possible excessof nitrogen and water accumulated in anode compartment, can eliminate most ofliquid molecules from the catalyst surface and refresh the feed. As a consequencethe overall strategy for maintaining the well-hydrated cell membranes, assuring areliable and efficient stack operation with minimization of nitrogen crossover andsimultaneously avoiding the flooding phenomena, should involve the periodicacting of the purge valve. This is normally closed but, when necessary, the controlstrategy expects to open it by managing the opening frequency for a specifiedopening time, sufficient to drain the contaminants but not too long for undesiredleaking of useful fuel. In order to evaluate the effect of purge operation on FCSefficiency, it is possible to define a coefficient expressing the ratio of fuelconverted to fuel supplied (gutil in Sect. 6.2), which can reach values higher than90% in optimized realizations [6, 7].