Hybrid photo-thermal sulfur-ammonia water splitting cycle: Thermodynamic analysis of the thermochemical steps

Solar driven hybrid sulfur-ammonia water splitting cycle (HySA) integrates a solar-photocatalytic hydrogen, H2, production step (H2 sub-cycle) with a high-temperature solar thermochemical oxygen, O2, evolution step (O2 sub-cycle), implementing efficient thermal energy storage as part of the cycle operation. Previous studies of the cycle omitted intermediate products, such as ammonium bisulfate, from the O2 sub-cycle and, thus, neglected their potential impact on the cycle's chemistry. Also, there are discrepancies in reported literature for the thermodynamic properties of ammonium sulfate, (NH4)2SO4, and ammonium bisulfate, NH4HSO4. In this study, thermal analysis experiments were conducted in order to determine the phase transition temperatures and enthalpies, and the heat capacity temperature dependence of the ammonium sulfate, (NH4)2SO4, and ammonium bisulfate, NH4HSO4. Our experimentally determined values for these parameters agree well with the data reported in DIPPR Project 801 database. Moreover, an exploratory thermodynamic analyses was performed using AspenPlus© and FactSage©, that included all potential reaction products, in order to identify critical parameters for an optimum O2 sub-cycle. A methodology is proposed and evaluated to mitigate AspenPlus©'s deficiency to handle solid phase changes. The thermodynamic analyses demonstrate that the NH4HSO4 inclusion in the O2 sub-cycle reduces the overall process energy requirements, and allows its use as an energy storage medium. Finally, we show that the use of molten salts, in combination with their interactions, significantly affects the efficiency and the operating conditions of the process, as well as the state of the mixtures.