(33b) Solar Hydrogen Production Via Thermochemical Metal Oxide – Metal Sulfate Water Splitting Cycle

Concentrated solar power is a sustainable source of high temperature process heat, enabling the solar-driven operation of many useful endothermic processes. Of particular interest is the storage of solar energy in chemical bonds via the splitting of water to produce hydrogen. 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. To harvest the solar radiation and to convert it effectively into hydrogen via thermochemical cycles provides a promising path for a future sustainable energy economy. Among the several 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. However, requirement of a noble metal catalyst for the endothermic dissociation of SO₃, sulfation poisoning of the catalyst, and the limited availability and high cost associated with the catalytic material are few of the shortcomings of these thermochemical cycles.

In this study, a two-step metal oxide – metal sulfate (MO-MS) cycle was thermodynamically investigated towards thermochemical storage of concentrated solar radiation via water splitting reaction.  It is a two-step process in which the first solar step belongs to the endothermic thermal reduction of MSO₄ into MO, SO₂, and O₂. The second exothermic step corresponds to the non-solar oxidation of MO by SO₂ and H₂O producing metal sulfate (MSO₄) and H₂. The MO and SO₂ produced in step 2 are recycled back to step 1 and hence can be used in multiple cycles. In this investigation, computational thermodynamic analysis of various MO-MS water splitting cycles was performed. Thermodynamic equilibrium compositions during step 1 (solar thermal reduction of MSO₄ under inert atmosphere) and step 2 (oxidation of MO via water splitting reaction producing hydrogen) 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, solar absorption efficiency of the solar reactor, net energy required to operate the MO-MS water splitting cycle, solar energy input to the solar reactor, rediation 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 MO-MS cycle was determined by performing the solar reactor analysis following the second law thermodynamic procedure over different solar reactor temperatures, inert gas flowrates,  and with/without considering the heat recuperation. Findings of this investigation will be presented in detail.