Thermochemical cycle of a mixed metal oxide for augmentation of solar thermal energy storage using solid particles

An exploration was done on the feasibility of storing both sensible and thermochemical energy at high temperatures for concentrated solar power in order to mitigate issues with each type of energy storage alone. Two potential processes were suggested and discussed for use with a solid oxide reaction: an augmented solid particle receiver and a dish system with a gaseous heat transfer fluid and solid blocks of active material.;Thermochemical energy storage using the "hercynite cycle" has been explored using the FACTSage™ Gibbs free energy minimization software, which predicted material compositions and enthalpy changes at conditions of interest. Calculations predict that the hercynite cycle material reduces above 1000°C and <2% O2. The hercynite cycle reduces with a reaction enthalpy of 264.8 kJ/kg 1400°C and <0.5% O2; this is 18.5% of the total sensible energy in the same material from 23°C to 1400°C. The reduction enthalpies were compared to a more limited exergy, which was calculated from 900°C instead of 23°C. The highest fraction of this enthalpy comparison was 66.1% at 950°C at 0% O2, despite the fact that the reduction reaction had less conversion. The thermochemical enthalpy compared favorably to this smaller exergy, indicating that it is useful to match the reaction temperature changes to the temperature range of the process. The isothermal thermochemical enthalpy was predicted to be up to 131.3 kJ/kg, which is 9.2% of the full sensible energy at 1400°C.;Various material formulations were cycled in a TGA/DSC at temperatures between 900°C and 1500°C using argon and air during reduction and oxidation. The observed oxidation enthalpies spanned an order of magnitude, from 10–100 kJ/kg. Isothermal energy storage was demonstrated at 1200°C, resulting in enthalpy values of 32.6 kJ/kg. Mixtures with excess Al2 O3 tended to have lower observed specific heats of reaction due to the additional inert material. The heats of reaction obtained for the oxidation exotherms were lower than equilibrium predictions and it is suggested that side reactions not predicted by well-mixed thermodynamic equilibrium are occurring and contributing to changes the total reaction enthalpy; data from XRD and Raman Spectroscopy indicate that this may be occurring.