Performance evaluation of an integrated energy system for the production and use of renewable methanol via water electrolysis and CO2 hydrogenation

Aiming at the decarbonization of society, power-to-liquids processes can favour the exploitation of the excess of renewable energy, producing methanol or other chemicals (such as dimethyl ether) by reacting electrolytic hydrogen and recycled CO2 (captured from industrial and power plants or directly from air). Such a system could behave as: - an energy storage system, storing excess renewable energy as chemical energy in liquid fuels and converting it into electricity during lack of renewable energy, - a source of fuels and chemicals for a variety of applications in many industrial sectors. This work concerns the conceptual design and performance analysis of a small-scale integrated energy system for the production and use of methanol from renewable hydrogen and captured CO2. The main components of the system are: - a reversible high temperature and high efficiency solid oxide cell (RSOC) that can operate in charge (electrolyser, SOEC) and discharge (fuel cell, SOFC) mode to store and use electricity using methanol as energy storage medium, - a catalytic reactor for methanol synthesis via CO2 hydrogenation. A thermal energy storage (TES) system based on a phase change material (PCM) is also included. To predict performance of the main components and of the overall system, numerical simulation models were developed. Performance and efficiencies of each system component and of the overall system were evaluated through extensive mass and energy balances, considering two different configurations with and without TES integration. Performance indexes were calculated to analyse the goodness of introducing a TES. The global efficiency of the overall system increases from 30% to 35% when heat is recovered between sections via the TES system.