H2 produced via solar water splitting reaction is considered as one of the most promising options for the future renewable/alternative fuels. The energy density of hydrogen (143 MJ/kg) is relatively higher than that of the fossil fuels such as oil (46.4 MJ/kg), gas (53.6 MJ/kg) and coal (32.5 MJ/kg), which makes it attractive and potential energy carrier. Among the several solar thermochemical cycles investigated previously, the sulfur-iodine cycle (SI Cycle) and its variation the hybrid sulfur cycle are more appealing as the required operating temperatures are lower as compared to other thermochemical cycles. For both cycles, the most energy consuming step is the dissociation of SO3 into SO2 and O2, which is possible only under catalytic conditions. As sulfation poisoning is a major concern related to such reactions, simply the noble metal catalysts were observed to be active towards the endothermic dissociation of SO3. Although, the noble metal catalysts are attractive for such reactions, they are less preferable due to the limited availability and high cost.
In this study, a two-step hybrid copper oxide – copper sulphates (CO-CS) water splitting cycle was thermodynamically investigated towards solar H2 production. It is a two-step process in which the first solar step belongs to the endothermic thermal reduction of CuSO4 into CuO, SO2, and O2. The exothermic step two corresponds to the non-solar oxidation of CuO by SO2 and H2O producing CuSO4 and H2. The equibrium thermodynamic compositions associated with both steps were determined by using HSC Chemistry thermodynamic software and databases. The variation of the reaction enthalpy, entropy and Gibbs free energy for the thermal reduction and water splitting steps with respect to the operating conditions were studied. Furthermore, solar absorption efficiency of the solar reactor, net energy required to operate the CO-CS cycle, solar energy input to the solar reactor, radiation heat losses from the solar reactor, rate of heat rejected to the surrounding from the water splitting reactor, and maximum theoretical solar energy conversion efficiency of the CO-CS cycle was determined by performing the exergy analysis by following the principles of second law of thermodynamic over different solar reactor temperatures and with/without considering the heat recuperation. Also, effect of inert carrier gas on solar to fuel conversion efficiency was examined. Findings of this investigation will be presented in detail.