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 H-2 from Sn, SnO and H2O is investigated. A real overall solar yield eta(real) 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. eta(real) is then used to find the most promising thermochemical reaction of the system SnO2/SnO/Sn + C/CH4. The optimal reaction is SnO2 + 2CH(4) <----> Sn + 2CO + 4H(2), proceeding at 980 K (Delta(r)G = -60 U), which is followed by Sn + 2H(2)O <----> SnO2 + 2H(2)CO and H-2 are then fed to fuel cells producing electricity with eta(real) = 0.23. The amount of solar upgrading of the fossil fuels CH4 and C is given. A combination of solar reactor, beat recovery device and a following reactor to produce H-2 is proposed. The dimension, volume and mass flow of the solar reactor are calculated and the amount of simultaneously produced electricity is given. (C) 2003 Elsevier Ltd. All rights reserved.