Achieving holistic sustainability in solar-hydrogen systems: A 6E-based multi-objective optimization of a PV-PEMFC-PEME-ORC integrated framework

The global energy transition presents complex challenges, demanding that the sustainability of integrated renewable systems be assessed beyond conventional thermodynamic and economic (e.g., 3E and 4E) analyses. This study introduces a novel framework for the comprehensive 6E (Energy, Exergy, Exergoeconomic, Exergoenvironmental, Emergoeconomic, and Emergoenvironmental) analysis of a solar-hydrogen system, including a photovoltaic (PV) array, a proton exchange membrane fuel cell (PEMFC), a proton exchange membrane electrolyzer (PEME), and an organic Rankine cycle (ORC). Utilizing a Response Surface Methodology (RSM)-based approach, the study identified an optimized operating point for the system. This point, time: 13.6 h, PEMFC current density: 0.40 A/cm 2 , inlet pressure of ORC turbine: 700 kPa, PEME current density: 7.80 A/cm 2 , represents a quantifiable and carefully balanced compromise between achieving a high total exergy efficiency of 67.18%, a low total cost rate of 2.07 US$/h, and a minimal total environmental impact rate of 0.28 Pts/h. The emergy analysis reveals that hydrogen subsystem operations, rather than solar availability, dominate ecological and economic resource use, challenging conventional perspectives on solar energy sustainability. In addition, achieving the highest efficiency coincides with the lowest hydrogen production rates, directly opposing the mandate for substantial energy storage. This trade-off underscores that optimizing for efficiency in isolation is insufficient for systems designed for energy storage, necessitating a balanced approach that considers both efficiency and production targets.