The relationship among the factors determining the thermal efficiency to convert concentrated solar thermal energy to chemical energy by the O2-releasing reaction of the reactive ceramics in the tow-step water splitting process has been studied for the development of rotary-type solar reactors. The α O2-releasing reaction which has been discovered in the present study for Ni-ferrite (NiFe2O4) has a high chemical reaction rate (k = 0.50 mol sec−1/m2). From calculation with the heat transfer equation in terms of heat transfer through the cavity wall with thckness, d (m), and heat absorption by the endothermic process of the O2-releasing reaction with rate constant, k (mol sec−1/m2), it is cralified that the heat flux of 2000 kW/m2 can be absorbed by the α O2-releasing reaction using the cavity wall with the thickness, d = 0.00036 m and thermal conductivity, λ = 70 WK−1m−1. The heat loss in the temperature swing (ΔT = 300K) for the two step water splitting of O2 releasing step (TH = 1773K) and H2 generation step (TL = 1473K) is only 8.3% for the α O2-releasing reaction. However, the O2 releasing reaction of NiFe2O4 in the β region where the reaction rate constant is around 0.0001 mol sec−1/m2, the heat flux of the concentrated solar energy to be used for conversion to chemical energy becomes very low around 300 kW/m2; the heat loss by the temperature swing in the two step water splitting is 87%. It is concluded that the α O2-releasing reaction can be used for the rotary-type solar reactor to convert the concentrated solar thermal energy to chemical energy with a high efficiency.

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