O₂ Releasing Reactivity of Ceramics for Rotary-Type Solar Reactor With Two-Step H₂O Splitting

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 (T_H = 1773K) and H₂ generation step (T_L = 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.