394473 Thermochemical Storage of Concentrated Solar Radiation Via Two-Step Iron Oxide – Iron Sulfate Cycle
Solar radiation is an essentially inexhaustible energy source that delivers about 100,000 TW to the earth. To harvest the solar radiation and to convert it effectively into storable and transportable chemicals 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. 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 iron oxide – iron sulfate (IO-IS) cycle was thermodynamically investigated towards thermochemical storage of concentrated solar radiation. 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 exothermic step two 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. The thermodynamic equilibrium compositions during step 1 (solar thermal reduction of FeSO4 under inert atmosphere) and step 2 (oxidation of FeO 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, solar absorption efficiency of the solar reactor, net energy required to operate the IO-IS cycle, solar energy input to the solar reactor, rediation heat losses from the solar reactor, rate of heat rejected to the surrounding from the the water splitting reactor, and maximum theoretical solar energy conversion efficiency of the IO-IS cycle was determined by performing the second law thermodynamic analysis over different solar reactor temperatures and with/without considering the heat recuperation. Findings of this investigation will be presented in detail.
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