Many reversible thermal dissociation reactions have been reviewed for storing thermal energy at very high temperatures, such as high temperature heat from power plants. Among them, dissociation of CaCO3 seems very promising as the material can dissociate at 1098 K under atmospheric condition and the reactants involved are free from toxicity. One of the dissociation products, CO2 gas, requires a storage system. Three CO2 storage systems are being considered and the thermal operating efficiencies, COP, of the respective CaO-CO2 energy storage systems are evaluated. It is found that the system with a zeolite adsorbent for CO2 storage becomes quite comparable to other systems when the adsorptivity of the adsorbent increases to higher values. The CO2 adsorption characteristics of zeolite 13X and super activated carbon under a temperature range 303-573 K are studied. Based on the adsorption data, it is found that zeolite 13X adsorbent can be used as a CO2 storage medium in a CaO-CO2 high temperature thermal energy storage system.