Analysis of a combined proton exchange membrane fuel cell and organic Rankine cycle system for waste heat recovery

Abstract

ABSTRACT Proton exchange membrane fuel cell (PEMFC) has become a promising alternative power source due to its quiet operation, low emission, and high theoretical energy conversion efficiency. However, a relatively large fraction of the energy (around 40 ~ 50%) is dissipated into the ambient environment by the cooling system, leading to a waste of thermal energy. The present study aims to improve the practical PEMFC performance by harvesting the thermal energy from the cooling system utilizing an organic Rankine cycle (ORC) system. The R134a and R245fa are selected as the working liquids for ORC to harvest energy from the PEMFC. A combined PEMFC and ORC model is established based on the first and second law thermodynamic analysis. It is found that the cycle efficiency (ηorc ) of R134a (about 11.33%) is higher than that of R245fa (about 9.16%). The combined system efficiency (ηsys ) for R134a can be increased by approximately 5.84%, compared with that of the fuel cell stack alone. For R134a, the effect of changes in the operating temperature and current density of the PEMFC stack on its cycle efficiency (ηorc ) is investigated. The results indicate that as the working temperature of the PEMFC stack increases, both the ORC and combined system efficiencies (ηorc and ηsys ) increase, while with the increase of the current density of the PEMFC, the ORC cycle efficiency (ηorc ) remains almost constant, and the combined system efficiency (ηsys ) decreases. The exergy efficiency is found to have the same variation trends with the energy efficiency.