The two-step solar thermochemical cycle for hydrogen production is one of the promising ways to alleviate current energy and environmental issues, and achieve carbon neutrality prospects in the future. Traditional graphical analysis methods investigate thermochemical cycles using temperature-entropy and temperature-enthalpy diagrams, without demonstrating the irreversibility of energy conversion processes in the looping of oxygen carriers. To solve this problem, a novel graphical exergy analysis method is proposed for two-step solar thermochemical cycles by illustrating material state in the looping of oxygen carriers and the exergy destruction quantitatively. The analysis process for describing the material state and exergy destruction with a diagram is introduced. The proposed graphical exergy analysis method is used to investigate a typical case of a solar ZnO/Zn two-step thermochemical cycle. The results show that the use of the inert sweeping gas during reduction to reduce the partial pressure of oxygen product to obtain a lower reduction temperature causes serious irreversibility. The proposed graphical exergy analysis method provides a useful tool for analyzing solar thermochemical cycles.
Keywords Solar energy, Thermochemical cycles, Exergy destruction, Graphical analysis