Effects of load variation and purge cycles on the efficiency of Polymer Electrolyte Membrane Fuel Cells for stationary applications

Polymer Electrolyte Membrane Fuel Cells have become rather popular for power generation; Dead-Ended Anode design is currently adopted to limit hydrogen consumption. However, gas and water accumulation at the anode outlet decrease stack performance, and so, purges are carried out to remove them. This work focuses on a Polymer Electrolyte Membrane Fuel Cell system featuring a voltage-drop-based purging strategy; 4 electric-load conditions (0.6-1.8 kW) were imposed to evaluate how purges impact the system performance as the applied load varies. Long-duration experimental tests were conducted at a constant load to reproduce cycles typical of stationary applications; various electric, thermal, and transport parameters were measured, and efficiency was ultimately determined. An analogy between increasing the applied load and increasing the cathode-air humidity level was found in terms of purge-related hydrogen losses and purge time. Stack current intensity is not affected remarkably by purging, whereas stack voltage exhibits higher oscillations at the higher loads and is less stable at the lowest one. A relationship is suggested between voltage, anode stoichiometry, and stack temperature, especially over the initial transient trend prior to reaching an approximately steady condition. Overall stack efficiency decreases as polarization losses increase along with the applied load; fuel efficiency is almost constant, even though slightly bigger at the higher loads, which implies that lower fractions of hydrogen are lost during purges. Net efficiency is relatively flat over the operative range, so this purging strategy tends to counterbalance the effects of polarization losses. However, power used to sustain auxiliaries shows a bigger impact than purge-related energy losses.