Thermochemical energy storage system development utilising limestone

For renewable energy sources to replace fossil fuels, large scale energy storage is required and thermal batteries have been identified as a commercially viable option. In this study, a 3.2 kg prototype (0.82 kWh(th)) of the limestone-based CaCO3-Al2O3 (16.7 wt%) thermochemical energy storage system was investigated near 900 degrees C in three different configurations: (i) CaCO3 was thermally cycled between 850 degrees C during carbonation and 950 degrees C during calcination whilst activated carbon was utilised as a CO2 gas storage material. (ii) The CaCO3 temperature was kept constant at 900 degrees C while utilising the activated carbon gas storage method to drive the thermochemical reaction. (iii) A mechanical gas compressor was used to compress CO2 into volumetric gas bottles to achieve a significant under/overpressure upon calcination/carbonation, i.e. <= 0.8 bar and > 5 bar, respectively, compared to the similar to 1 bar thermodynamic equilibrium pressure at 900 degrees C. Scenarios (i) and (iii) showed a 64% energy capacity retention at the end of the 10th cycle. The decrease in capacity was partly assigned to the formation of mayenite, Ca12Al14O33, and thus the absence of the beneficial properties of the expected Ca5Al6O14 while sintering was also observed. The 316L stainless-steel reactor was investigated in regards to corrosion issues after being under CO2 atmosphere above 850 degrees C for approximately 1400 h, and showed no significant degradation. This study illustrates the potential for industrial scale up of catalysed CaCO3 as a thermal battery and provides a viable alternative to the calcium-looping process.