Techno-economic modelling of a Power-to-Gas system based on SOEC electrolysis and CO2 methanation in a RES-based electric grid

Renewable energy sources (RES) are growing rapidly as an alternative to fossil energy sources in order to disjoin the society from carbon sources. Wind and photovoltaic power generation is intermittent due to weather conditions. Therefore, remarkable centrality is given to the storage of the electric excess of production, and Power-to-Gas (P2G) systems seem to be one of the most promising technologies to achieve this purpose. In the case analysed in this research, a high temperature electrolyser based on solid oxide cells technology was considered, coupled to catalytic reactors for the hydrogenation of carbon dioxide into methane. The product is a synthetic natural gas (SNG) which could by directly injected into the natural gas distribution network. This work investigates the coupling between a completely RES-based electric profile in a future scenario and a P2G plant to perform a comprehensive technical, managemental and economic assessment of the system. A P2G model was built on MATLAB environment to examine the P2G plant behaviour when it was coupled with the RES-based intermittent electric profile. Firstly, the P2G plant arrangement has been discussed by varying the P2G plant capacity which has been coupled to the electric profile; the P2G capacity was then selected based on the levelized cost of product (LCOP). Secondly, the electrochemical storage has been introduced to perform a continuing operation also when excess of electricity is not available, thanks to the storage of electric energy. Finally, chemical storage tanks of hydrogen (feed for the methanation) and steam (feed for the electrolyser) were considered to reduce the number of shutdowns of the P2G plant. An assessment of the long-term gas storage in the gas distribution network was additionally performed. An optimal plant capacity with respect to the investigated scenario of local electric residual profile was sized, being equal to 7 MW, with a recovery equal to 70% of the total available electric energy from the grid. Moreover, the P2G system could be economically competitive, because the LCOP varies between 60.2 €/MWh and 224.9 €/MWh (LHV basis), which suits with current levelized natural gas prices. The overall efficiency of the plant reached around 77%, with a plant utilization factor of about 30%. The accumulators and the electrolyser are the most expensive technologies, and the LCOP strongly depends on their size (if the batteries are considered). For this scenario, the SNG yearly productivity reached a value of 18.5 GWh/y (LHV basis): in the hypothesis of using SNG to repower the electric grid (efficiency 45%), such a P2G plant could compensate the 29.6% of the amount of deficit energy, with a consequent reduction in CO2 associated emissions.