Storing thermal energy by thermochemical means seems very attractive since large amounts of energy can be stored per unit mass, and such systems can function as a heat pumps, For storing high temperature heat energy such as concentrated solar energy at 773 K, various candidate chemical reactions have been evaluated in terms of energy storage density, turning temperature, toxicity, corrosiveness, and other factors, The dissociation reaction of CaCO3 is found to be very promising, Three methods for storing the dissociation product CO2 gas; (i) storing as a compressed gas, (ii) letting the CO2 gas react with a metal oxide and storing it in the form of another carbonate, and (iii) adsorbing with an appropriate adsorbent and storing as an adsorbed gas; have been proposed and the respective thermal operating efficiencies at various upgraded temperatures are evaluated. The CaO-CO2 metal oxide system seems very effective for temperature upgrading around 1273 K and the CaO-CO2-compressor system seems suitable for storing and delivering heat energy at the same temperature, Whether the efficiency of the CaO-CO2-Adsorbent system is comparable to one of the other two systems or not greatly depends upon the adsorptivity of the adsorbent.