A chemical heat pump system using BaO/BaCO
3 reversible reaction is proposed for high temperature heat storage and temperature upgrading above 1273 K, because the BaO/BaCO
3 reaction can be controlled under moderate pressure, even above 1273 K. Thermogravimetric studies on carbonation of BaO at 973-1523 K and decarbonation of BaCO
3 at 1073-1523 K were carried out under various CO
2 pressures in order to characterize the heat generating and storing rates of the CHP. In both reactions, reaction rate and conversion increase with an increase in temperature. Compared to carbonation, decarbonation is more strongly influenced by reaction temperature as well as pressure of CO
2. It is also found that the eutectic reaction between BaO and BaCO
3 has a great influence on the reaction characteristics; i.e. for carbonation with pressures around P
e and for decarbonation with all pressures lower than P
e above 1303 K, the reaction rate drops after X = 0.4-0.6 due to diffusion resistance of CO
2 caused by the eutectic reaction. A rate expression similar to that of the CaO/CaCO
3 reaction is found to be applicable for the BaO/BaCO
3 reaction in the initial phase-boundary controlled period. Assuming that conversions of carbonation and decarbonation have to reach over 0.6 within 30 and 60 min, the heat releasing step can be operated at 1273 K or higher above the equilibrium pressure, but the heat storing operation must be carried out above 1273 K, below a quarter of P
e.
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