Economic assessment of a power-to-substitute-natural-gas process including high-temperature steam electrolysis

• A previously modelled Power-to-Gas process including HTSE is economically assessed.
• Performance and ageing of electrolysis cell are experimentally obtained.
• Voltage and steam conversion rate during long-term tests are controlled and constant.
• SNG production cost highly depends on stack performance degradation (around 40%).
• Increasing plant capacity can make SNG production cost decrease by around 30%.

Power-to-Substitute-Natural-Gas (SNG) processes are studied since they can offer solutions for renewable energy storage and transportation. In the present study, an original Power-to-SNG process combining high-temperature steam electrolysis and CO2 methanation is economically assessed. This evaluation is based on experimental data describing the electrolyser performance and its degradation during long-term operation. These data are obtained on a commercial single cell. Tests are originally conducted by imposing voltage and steam conversion rate. The main conclusions of this experimental work are that two fields of current density evolution are evidenced in the tested conditions: a first transient one where the current density absolute value decreases very quickly, followed by a second field where the evolution of current density achieves a steady state. The influence of the operating point seems not significant once this state is achieved.The process assessed here has been developed in a previous work where a precise framework matching with a wide range of applications was defined. The design of the process main units and the calculation of matter and energy fluxes are based on this work. To go further, the economic assessment of this process is proposed. In this study, various scenarios of initial performance of the electrolyser are considered and integrate the performance degradation associated. The SNG production cost is estimated between 211 and 570 €/MWhHHV according to the scenario and the hypotheses considered. The two major items accounting for this high cost are the purchase cost of the electrolyser and its performance degradation. It also evidenced that none of the other items can be neglected. Through a sensitivity analysis, it is shown that an increase of plant annual availability and capacity makes the SNG production cost decrease.