Renewable methanol production and use through reversible solid oxide cells and recycled CO2 hydrogenation

The production of hydrogen from renewable energy and its conversion to other valuable fuels, for example methanol, could help to mitigate the negative effect of CO2 emissions on the environment and compensate for the intermittency of renewable energy sources. This paper is focused on the conceptual design and analysis of an integrated energy system for the production and use of renewable methanol, which is a promising way to recycle CO2 avoiding the release of emissions into the atmosphere. Methanol is obtained through the hydrogenation of captured and recycled CO2, using hydrogen produced from renewable energy in a high temperature reversible solid oxide cell (RSOC). Such a system, connected to a micro-grid, can fulfil the electrical load profile and provide methanol as an easy-to-store fuel and chemical feedstock. Indeed, the RSOC can operate in electrolysis cell (SOEC) mode when an excess of renewable energy is available, producing hydrogen and methanol in the methanol synthesis section (MSS). Methanol is stored in the liquid phase and subsequently converted into electricity, by operating the RSOC in fuel cell mode (SOFC) when there is a lack of renewable energy. Therefore, this system represents an interesting solution to provide ancillary services within a smart grid framework. A thermal energy storage (TES) system of latent heat type based on a suitable phase change material (PCM) was also included to recover and store heat released by the SOFC, to be recycled later in order to satisfy the thermal energy requirements of both SOEC (steam production) and MSS (distillation column reboiler). Comprehensive electrochemical models of RSOC, in both SOEC and SOFC operating modes, were specifically developed. A kinetic model was also developed to simulate the methanol synthesis process. These models were used to predict the performance of the main system components (RSOC in SOEC and SOFC modes, MSS) and the overall system.