Energy analysis of solar thermochemical fuel production pathway with a focus on waste heat recuperation and vacuum generation

Abstract

The solar thermochemical fuel production pathway as an attractive option for the decarbonization of the transportation sector is investigated. Using ceria as the reactive material and the latest published data on inert gas demand and energy demand for vacuum pumping from the literature, the energy balance of the thermochemical reactor is analyzed for vacuum pumping and inert gas sweeping, and the required process parameters for reaching high efficiencies are discussed. It is found that thermochemical energy conversion efficiencies exceeding 20% can only be reached with a vacuum operated system at reduction temperatures of 1900 K, enhanced pump efficiency by 50%, a concentration ratio of 5000 suns, and an energy recuperation effectiveness from the gases and the solid phase of 70%. We then investigate the whole fuel production pathway from incident sunlight to liquid hydrocarbons by performing an energy analysis of a fuel production plant including waste heat recovery. It is found that the energy losses theoretically can be used to cover the demand for electricity and low-temperature heat, as well as the heating of the reactants to the oxidation temperature, enhancing the pathway efficiency from 5.3% to 8.6%. The heat recovery from the single process steps along the fuel production pathway therefore has a large potential to increase the efficiency of the process, improving the economic and ecological performance significantly. Likewise, waste heat may be used to partially relax the likely stringent operating conditions of highly efficient thermochemical reactors, which could have important implications for reactor design.