Thermoeconomic Comparison of Alkaline, Solid Oxide and Anion Exchange Membrane Electrolyzers for Power-to-Gas Applications

This study presents a comparative thermoeconomic analysis of three Power-to-Gas (PtG) systems operating under dynamic conditions, each integrating a different electrolysis technology: solid oxide electrolysis cell (SOEC), alkaline electrolysis cell (AEC), and anion exchange membrane (AEM) electrolysis. Each system converts excess electricity from a photovoltaic (PV) field into hydrogen via electrolysis. The hydrogen is then combined with carbon dioxide – captured from the exhaust gases of a combined heat and power (CHP) unit – within a three-stage catalytic methanation reactor to produce synthetic methane. This system supplies energy to mixed-use facilities. The methanation reactor is modelled as a three-stage fixed-bed catalytic reactor using Ni/Al₂O₃ as the catalyst, cooled by liquid water. Both the electrolyzer and methanator models incorporate temperature-dependent reaction kinetics and simulate realistic transient behaviour. Dynamic simulations were carried out using TRNSYS, while advanced components are modelled in MatLab. Energy results show that the SOEC-based system outperforms the AEC and AEM-based systems in terms of overall conversion efficiency (0.56 vs. 0.48 and 0.49), primary energy savings (46.90% vs. 43.90% and 43.96%), and CO₂ emissions avoided (74.16% vs. 71.77% and 71.84%). To further assess system scalability and investment viability, a multi-objective optimization was carried out on the SOEC configuration. The optimization identified two Pareto-optimal configurations: the first achieves 56% primary energy savings (PES) with a simple payback period (SPB) of 5.41 years, while the second reaches 57% PES with a slightly longer SPB of 5.52 years. In both cases, curtailment of excess electricity was kept below 8%, and electrochemical chain costs remained under 45% of total investment. The results confirm that optimal system sizing – particularly of the electrolyzer and methanation units – is crucial to achieve a cost-effective and energy-efficient PtG deployment.