R. Bhosale, D. Dardor, A. Kumar, F. AlMomani, U. Ghosh
Keywords: solar hydrogen, thermochemical, copper oxide - copper sulfate, thermodynamics
Summary:H2 is considered as one of the potential energy carriers due to its capacity of producing 143 MJ/kg of energy (higher in comparison with the oil, gas, and coal individually) and its environmentally friendly nature (burning of H2 produces water with no other polluting gases). Water splitting reaction is considered as one of the most promising ways for the production of H2. However, direct water thermolysis is not favored thermodynamically as it requires process heat at temperatures above 2723 K for obtaining a significant degree of dissociation of water. Furthermore, to avoid the formation of an explosive mixture comprised of H2 and O2, a gas separator unit (for the separation of H2 and O2 at high temperatures) needs to be equipped near to the water dissociation reactor which further enhances the overall cost of the H2 production process. As the direct dissociation of water is not practical due to the high operating temperature and gas separation problem, attempts are currently underway to achieve H2 production via water-splitting reaction at lower operating temperatures and via bypassing the formation of H2 and O2 explosive mixture. Several thermochemical cycles such as iron oxide cycle, zinc/zinc oxide cycle, tin/tin oxide cycle, mixed ferrite cycle, ceria cycle, sulfur-iodine cycle, and hybrid sulfur cycle were investigated towards H2 production via water-splitting reaction. Among these, the sulfur-iodine cycle and its variation the hybrid sulfur cycle are more appealing as the required operating temperatures are lower as compared to other thermochemical cycles. Utilization of metal oxides as the catalytic materials (instead of noble metal catalysts) and converting the sulfur-iodine and hybrid sulfur cycle into a ‘metal oxide – metal sulfate’ cycle operated using concentrated solar energy is one of the alternative to achieve H2 production at lower temperatures. This investigation proposes the utilization of the ‘copper oxide – copper sulfate’ (CO-CS) cycle for the production of solar H2 via thermochemical water splitting reaction. In this study, the thermodynamic feasibility of this cycle was investigated and obtained results are presented in detail. At first, the thermodynamic equilibrium compositions during the solar thermal reduction of CuSO4 under inert atmosphere and oxidation of CuO via water splitting reaction were determined. The variation of the reaction enthalpy, entropy and Gibbs free energy for the thermal reduction and water splitting steps with respect to the operating temperatures were studied. Furthermore, the maximum theoretical solar energy conversion efficiency of the CO-CS cycle is determined by performing the second law thermodynamic analysis over different solar reactor temperatures and with/without considering the heat recuperation.