Working Fluid Optimization for Low-Temperature PEMFC+ORC Systems

PEMFCs are a promising energy source but waste nearly 50% of reactant energy as heat. Rankine cycles can recover this waste heat to increase the overall efficiency of the system. This study develops a mathematical model to optimize energy recovery in a Rankine cycle coupled with a PEMFC, determining the best operating conditions and working fluid. The hypothetical optimum fluid (C6H8N1F5) was not found in the literature. Analysis of suboptimal solutions reveals that efficient fluids have high fluorine content, as observed in many refrigerants, along with high molar mass and high latent heat of vaporization; however, they require vacuum conditions, which increase operational costs. Therefore, a second optimization was conducted within the 1–10 atm range, which resulted in more realistic substances, with methyl vinyl ether emerging as the optimal fluid, yielding a thermal efficiency of 7.36%. Based on both optimizations, a new variable, ϵ, is proposed to correlate the efficiency of the cycle (R2 = 0.967), which could be used as a tool for preliminary screening of working fluids that maximize thermal efficiency.