Exergy and exergoeconomic comparison between multiple novel combined systems based on proton exchange membrane fuel cells integrated with organic Rankine cycles, and hydrogen boil-off gas subsystem

Abstract

A comparative study is applied to evaluate four combined power producing systems based on low-temperature and high-temperature proton exchange membrane fuel cells (LT-PEMFC and HT-PEMFC) with equal power generation of 1056 kW. Single-stage organic Rankine cycle (SORC) and cascade two-stage ORC (CTORC) are employed to recover the waste heat from the fuel cells as well as exploiting the hydrogen boil-off gas for generating excess power, and feeding to the fuel cells. The performance of the proposed systems is compared to energy, exergy, and exergoeconomic viewpoints. Also, a parametric analysis is carried out to assess the effects on the thermodynamic and economic performance of the investigated systems by considering main parameters such as current density, fuel cell operating pressure and temperature, turbine inlet pressure, and expander lead. The results obtained from the parametric analysis reveal that with increasing the fuel cell operating temperature the overall exergy efficiencies for the proposed combined systems based on LT-PEMFC and HT-PEMFC decreases and increases, respectively. Also, the net output power and the total product unit cost of the considered systems are optimized at a specific value of BOG expander inlet pressure. The results indicate that albeit the highest values of energy and exergy efficiencies among the considered systems (61.42% and 63.20%) are related to LT-PEMFC/CTORC but the lowest total product unit cost of 33.75 $/GJ is related to HT-PEMFC/SORC.