Theoretical investigation of the system SnOx/Sn for the thermochemical storage of solar energy

The thermodynamic data of the system SnO2/SnO/Sn in the absence and presence of CH4 and C are calculated as a function of temperature. The direct dissociation of SnOx without any reducing substances needs temperatures T>2000 K at 1 bar. In the presence of CH4 or C, SnOx can be reduced at T<1250 K. The production of H2 from Sn, SnO and H2O is investigated. A real overall solar yield ηreal is defined which compares the output of real fuel cells, fed by solar-produced chemicals, with the total solar input necessary to produce these chemicals. ηreal is then used to find the most promising thermochemical reaction of the system SnO2/SnO/Sn+C/CH4. The optimal reaction is SnO2+2CH4↔Sn+2CO+4H2, proceeding at 980 K (ΔrG=−60 kJ), which is followed by Sn+2H2O↔SnO2+2H2. CO and H2 are then fed to fuel cells producing electricity with ηreal=0.23. The amount of solar upgrading of the fossil fuels CH4 and C is given. A combination of solar reactor, heat recovery device and a following reactor to produce H2 is proposed. The dimension, volume and mass flow of the solar reactor are calculated and the amount of simultaneously produced electricity is given.