Towards sustainable power generation with free piston engine generators: comparative analysis of hydrogen and methane fuels

• Integrated combustion kinetics with exergy, cost, and sustainability analysis. • Linked heat recovery ratio to LCOE and exergoeconomic factor improvements. • Found HRR ≥ 50 % enables H 2 FPEG to match CH 4 cost at high efficiency. • Oxidizer composition and operation frequency of 20 Hz and 30 Hz are considered. • 20 Hz-methane: highest sustainability index = 2.2 and exergy efficiency = 50.6 % This study presents a comprehensive thermos-economic assessment of a small-scale Free Piston Engine Generator (FPEG) system operating on hydrogen and methane fuels. A detailed multi-domain simulation framework was developed for chemical kinetics (ANSYS CHEMKIN) and integrated with exergy, cost dynamics, and sustainability modeling using an in-house computational code. The model captures combustion behavior, pressure development, and system performance under varying operational frequencies and air/fuel composition. Key performance indicators, including exergy efficiency, exergoeconomic factor ( f ex ), irreversibility, and levelized cost of electricity (LCOE), were evaluated. Results indicate that hydrogen combustion yields faster pressure rise and higher peak pressures (10.8 % higher than that of methane at 20 Hz) compared to methane, contributing to improved exergy efficiency, and faster combustion kinetics with fewer intermediate species. Sensitivity analysis revealed that for a parity of hydrogen systems with a methane-powered system (i.e., LCOE of 0.65–0.75 $/kWh and f ex ∼ 0.30–0.38), the following condition should be established: η ex ≥ 60 %, heat recovery ratios ≥ 50 %, flow rates ≥ 0.8 kg/h, and H 2 prices ≤ $6–8/kg. The findings underscore the potential of FPEG as a flexible and sustainable solution for decentralized power generation, particularly when paired with heat recuperation and upgraded design strategies.