Cost optimisation and life cycle analysis of SOEC based Power to Gas systems used for seasonal energy storage in decentral systems

Abstract Chemical energy carriers and storage mechanisms will play a significant role in future energy systems. Apart from stabilising network fluctuations caused by renewable energy supply, chemical energy carriers also serve multiple sectors like electricity generation, chemical industry, transportation and shipping. Power to Gas (PtG) is a method that can be adapted for energy storage using chemical energy carriers produced from reserve electricity. This study contains the evaluation of long term energy storage in a decentral energy hub using a high temperature Power to Gas (PtG) plant. The Power to Gas process in this study uses surplus electricity for high temperature SOEC electrolysis. The resulting H2 undergoes methanation to generate Substitute Natural Gas (SNG) which has the same properties of natural gas and can be distributed using existing infrastructure. Compared to PtG processes using PEM or alkaline electrolysis, better overall process efficiencies up to 85% have been estimated for the high temperature PtG process. A pilot plant with thermally coupled SOEC-Electrolysis and Methanation was constructed as a part of the HELMETH project and is used in this study. Based on the experiments conducted in the pilot plant, the technical feasibility of long term energy storage and transient operations were evaluated. It was observed that short term energy storage with transient plant operation resulted in more operational costs when compared to long term storage with continuous plant operation. Novel methods to minimise the operational costs of the plant were also investigated using a dynamic pricing model and numerical optimisation of PtG plant. The numerical optimisation shows that if the duration of plant operation is adapted to target surplus renewable energy production, the concept could also be economically viable. Further, a life cycle analysis (LCA) of the PtG process was performed to evaluate the global warming potential (GWP) of the PtG plant configured with various input feeds. From the LCA, it was determined that if the input electricity is generated from sources with a global warming potential of less than 150 g CO2-eq/kWh, and carbon dioxide used for methanation is derived from biogenic sources, the PtG plant could act as a carbon sink.