Abstract
Thermochemical storage of solar heat involves the heat effects of reversible chemical reactions and can be coupled with a solar thermal power plant for continuous electricity generation. A comprehensive study of chemical materials for thermochemical heat storage based on reversible redox reactions was conducted. The evaluation of the selected systems was based on their suitable transition temperatures, chemical conversion rates, reversibility, energy storage density as well as general criteria such as toxicity and cost, and resulted in the selection of metal oxides (based on Co and Mn), perovskites and carbonates/hydroxides (of Ca, Sr, Ba).For metal oxides processed in air, Co3O4/CoO was cycled as powder bed and porous foam without reactivity loss, whereas Mn2O3/Mn3O4 showed poor reversibility, which can be enhanced via the synthesis of porous microstructure. The potential improvement of their performance was further studied using transition metal addition. The addition of Fe, Cu or Mn to Co3O4/CoO was found to adversely decrease the redox activity and energy storage capacity. In contrast, the reaction rate, oxygen exchange capacity, reversibility and stability of Mn2O3/Mn3O4 were significantly enhanced with added Fe, Co, or Cu amounts above ∼15, 40 and 30 mol% respectively, while the energy storage capacity was improved accordingly.Perovskites represent another attractive class of thermally-stable redox materials (e.g., energy storage density above 200 kJ/kg for Ca0.5Sr0.5MnO3), but the continuous lattice oxygen transfer may be a barrier for the recovery of the absorbed energy, thus requiring pressure-swing operation with inert gas utilization. Finally, carbonates and hydroxides were identified as promising candidates chiefly because they exhibit the highest energy storage capacities among the considered materials, but they require closed-loop operation. Among these systems, both SrCO3/SrO and Sr(OH)2/SrO stabilized with MgO as well as Ca(OH)2/CaO showed remarkable cycling stability and energy storage densities.
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