Hydrogen preheating through waste heat recovery of an open-cathode PEM fuel cell leading to power output improvement

• A study on the effect of hydrogen preheating using waste heat for low temperature PEM fuel cells.
• Theoretical, experimental and analytical framework was established.
• The maximum electrical power output increases by 8–10% under specific operating conditions.
• Open loop hydrogen supply gives a better performance than closed loop.
• The waste heat utilization is less than 10% due to heat capacity limitations.

The electrochemical reaction kinetics in a Polymer Electrolyte Membrane (PEM) fuel cell is highly influenced by the reactants supply pressures and electrode temperatures. For an open cathode PEM fuel cell stack, the power output is constrained due to the use of air simultaneously as reactant and coolant. Optimal stack operation temperatures are not achieved especially at low to medium power outputs. Based on the ideal gas law, higher reactant temperatures would lead to higher pressures and subsequently improve the reaction kinetics. The hydrogen supply temperature and its pressure can be increased by preheating; thus, slightly offsetting the limitation of low operating stack temperatures. The exit air stream offers an internal source of waste heat for the hydrogen preheating purpose. In this study, a PEM open-cathode fuel cell was used to experimentally evaluate the performance of hydrogen preheating based on two waste heat recovery approaches: (1) open-loop and (2) closed loop hydrogen flow. The stack waste heat was channelled into a heat exchanger to preheat the hydrogen line before it is being supplied (open loop) or resupplied (closed loop) into the stack. At a constant 0.3 bar hydrogen supply pressure, the preheating increases the hydrogen temperature in the range of 2–13 °C which was dependant on the stack power output and cathode air flow rates. The achievable maximum stack power was increased by 8% for the closed loop and 10% for the open loop. Due to the small hydrogen flow rates, the waste heat utilization was only 3–6% from the generated stack heat. The analysis indicates a better potential for open loop hydrogen preheating using fuel cell waste heat for open-cathode stacks.