Effects of water evaporation and condensation on the operating heating value and system efficiency of polymer electrolyte membrane fuel cells: Temperature-dependent phase separation modeling

Abstract

During the operation of polymer electrolyte fuel cells (PEFCs), liquid and gas phase water coexist within the cell domain, and evaporation and condensation result in a heating value that lies between the higher heating values (HHVs) and the lower heating values (LHVs). To elucidate the influence of water phase changes on heat transfer, mass transport, and fuel cell efficiency, a three-dimensional, non-isothermal, multiphase, and multicomponent model was developed. The phase change dynamics of water across the cell domain were quantified to establish the operating heating value (OHV), which provides a more detailed evaluation of fuel cell efficiency and coolant cooling load requirements compared to the conventional lower or higher heating value approaches. A novel method was introduced to validate the model that compares stack cooling loads to the traditional polarization curve analysis, achieving strong alignment with experimental data from the literature. Extensive numerical simulations were performed using a temperature-dependent phase separation model (TDPSM) to determine efficient operating strategies under recent trends favoring higher temperatures and pressurization. Macroscopic performance metrics—including I-V curves, OHV profiles, cell efficiency, heat removal mechanisms, Sankey energy flow diagrams, and parasitic system losses—were systematically analyzed. These evaluations were complemented by localized assessments of current density, relative humidity, liquid saturation, and phase change rates. An increase in temperature from 50 °C to 70 °C facilitated evaporative cooling by 136 %, reducing the cooling load despite a 7.8 % rise in current density. Pressurization, which inversely affected evaporation capacity, achieved maximum systemic net power at 2.0 bar. The TDPSM also highlighted the impact of coolant flow direction on heat and water management, enabling optimized design recommendations based on thermofluidic deviations. This integrated framework addresses the dual challenges of heat and water management, extending the application of theoretical modeling to a pseudo-systemic level.