A “feasible heating-coupled optimization” method for solar thermochemical cycle system: Taking the solar copper-chlorine cycle system as an example

Solar-driven thermochemical water splitting cycle systems are promising for clean, efficient, and large-scale hydrogen production. Few studies focus on optimizing the heat source input temperature (Thot) and output temperature (Tcold) of the integrated systems. On the one hand, the thermochemical cycles have stepped heat absorption curves with multiple pinch points, bring the constraint of the optimization. On the other hand, coupled with the solar part and power cycle, results in the deviations of the system from individual cycles. Additionally, heat is provided to both the thermochemical cycle and the power cycle, resulting a competitive relationship, and the maximum system efficiency does not always correspond to maximum hydrogen production. Therefore, a “feasible heating-coupled optimization” method is proposed. Based on the feasible heating boundary of an individual thermochemical cycle, the constraint relationship between Thot and Tcold is clarified, and the effect of Thot and Tcold under the coupling of each unit and the optimal heating design is determined by analyzing performance on and deviating from the feasible heating boundary. For the solar copper-chlorine cycle, the optimal Thot and Tcold are 520 °C and 469.83 °C, with energy and exergy efficiencies of 19.75% and 22.27%. With power cycle integration, there is a trade-off between “electricity gain” and “hydrogen loss”. Heating away from the feasible heating boundary increases system efficiency but decreases the hydrogen production. When Thot and Tcold are 520 °C and 290 °C, energy efficiency increases to 28.17%, while hydrogen production decreases to 21.90% of that on the feasible heating boundary.