Thermodynamic analysis of a solar electro-thermochemical cycle coupled with chemical-looping deoxygenation for hydrogen production

The two-step solar thermochemical cycle (TC) for water decomposition based on ceria driven by solar energy is a promising hydrogen production path, which is environmentally friendly. However, the conventional two-step solar thermochemical cycle faces challenges such as high reduction temperature and high energy consumption for deoxygenation, which increase the difficulty of hydrogen production and reduce its efficiency. In this work, a system that couples electro-thermochemical cycle (ETC) and chemical-looping cycle (CLC) is proposed to address the aforementioned challenges, in which electro-thermochemical cycle is carried out in a solid oxide electrolysis cell (SOEC). In the proposed system, an external electric potential is introduced to reduce the reduction temperature during the thermochemical cycle, and the oxygen generated by the reduction step of it is absorbed by the oxidation reaction of the chemical-looping cycle, which is carried out at a lower temperature. The thermodynamic conditions for the reduction of chemical-looping cycle oxygen carriers are studied, and several easily reducible oxygen carriers are selected. The effects of these oxygen carriers on efficiency of the system are discussed, including temperature, applied potential, heat recovery, etc. The results show that when CoO/Co₃O₄ is used as the oxygen carrier in the chemical-looping cycle, the system efficiency can reach a maximum of 29.5 % (T_red,TC = 1300 °C, T_ox,TC = 700 °C, E = 0.25 V, T_red,CLC = 936 °C, T_ox,CLC = 550 °C, r = 2), where the efficiency with heat recovery can reach nearly 40.0 %. In addition, when ceria generates an oxygen vacancy value of 0.2 during the reduction process of the thermochemical cycle, the reduction temperature can be reduced from 1800 °C to 1300 °C. The proposed system addresses the challenges faced by conventional solar thermochemical cycle and provides a reference for efficient hydrogen production.