Oxides and porous structures based on earth-abundant elements for hybrid sensible/thermochemical solar energy storage in air-operated solar thermal power plants

Abstract

The concept of regenerative sensible heat storage in porous solids employed in air-operated solar thermal power plants can be hybridized with thermochemical storage by coating/manufacturing entirely the heat exchange modules with oxides of multivalent metals undergoing reduction/oxidation reactions with significant heat effects. A prerequisite for eventual commercialization of such thermochemical storage concepts is the use of low-cost, environmental-friendly, oxide compositions capable of reversible reduction/oxidation under air with high reaction enthalpies. Equally necessary is the shaping of such oxides into structures operating as integrated reactors/heat exchangers. In this perspective, a specific Mn-based mixed oxide system of composition (0.8)(Mn2O3)*(0.2)(Fe2O3) was investigated. The work involved shaping the powder to porous foams and pellets which were comparatively tested in an infrared furnace, to clarify the effect of high heating/cooling rates on redox performance and structure stability. The redox performance of such Mn-rich systems was found sensitive to exposure at high temperatures. As long as a temperature of ~ 1100oC is not exceeded during redox cycling, both powder and pellets seem to operate reversibly for a high number of cycles. However, the high sintering temperatures (1350oC) required to induce strength to high-porosity structures like foams before their use as thermochemical storage media, had an adverse effect on their redox performance.