(712a) High Temperature Solar Thermochemical Energy Storage Materials

High temperature solar thermochemical energy storage (STCES) has promise to provide a low cost and high temperature storage solution for solar thermal power. However, the current status of STCES is at an early stage of development and substantial research and development is required to realize practical implementation. Two thermochemical energy storage systems with high theoretical energy densities at high temperatures have been investigated. In both the cases, we have been able to develop stable energy storage materials with energy densities in excess of 1200 MJ m^-3 that can operate at temperatures above 1100 â?? in both cases.

The first system pertains to thermochemical energy storage via decomposition and carbonation of strontium carbonate/strontium oxide in the temperature range of 1150-1235°C. Porous structures of pure SrO/SrCO₃ were studied via a fractional factorial experimental study to verify the possibility of synthesizing stable undoped SrO/SrCO₃ structures using the sacrificial pore formation with graphite (SPFG) method. Results of the experimental study indicate that the SPFG method is not suitable for fabricating porous structures of pure SrO or SrCO₃ for high temperature thermochemical energy storage. Cyclic volumetric energy density (including only the chemical reaction contribution) calculated from thermogravimetric measurements decreases substantially to values below 600 MJ m^-3 upon repeated cycling, demonstrating that the SPFG method not viable. Further investigations revealed that doping SrO/SrCO₃ with MgO and BaO improved the cyclic stability significantly. Mixtures of SrO-MgO and SrO-BaO in the molar ratio of 2:3 appear to achieve cyclic stability at 860 ± 5 MJ m^-3 (500 ± 3 kJ kg^-1) and 1408 ± 15 MJ m^-3 (1001 ± 11 kJ kg^-1) respectively over 15 cycles. Pure BaO was also tested for thermochemical energy storage and is found to be more stable with a higher energy density of 1690 ± 24 MJ m^-3 (1070 ± 15 kJ kg^-1).

A second study was performed on transition metal oxide based thermochemical energy storage systems. The purpose of the investigation was to validate the use of magnesium oxide (MgO) as a sintering inhibitor/porous structure stabilizer for high temperature solar thermochemical energy storage (STCES) using reversible redox reactions. Solid solutions of reactive metal oxides in MgO prepared using the solid state reaction method (SSR) were investigated for STCES applications. A new concept of using steam and carbon dioxide to lower the partial pressure of oxygen during thermal reduction was tested and validated. 40 and 33 mole % MnO in MgO, 20 mole % CuO in MgO and 40 mole % Fe₂O₃ in MgO were investigated for high temperature thermochemical energy storage via redox reactions. A composition of 40 mole % MnO in MgO appeared to be the most promising material among those tested here for high temperature thermochemical energy storage. The material can release heat at a temperature greater than 1200°C and has an energy storage capacity of 64212 kJkg^-1 (122236 MJm^-3). The material appeared stable over 20 cycles in a thermogravimetric analyzer and 12 cycles in a tube furnace bench scale reactor and is expected to maintain stability over repeated cycling.