In-Depth Energy and Irreversibility Analysis in the Solar Driven Two-Step Thermochemical Water Splitting Cycle for Hydrogen Production

Hydrogen production via a two-step thermochemical cycle based on solar energy has attracted increasing attention. However, the severe irreversible loss causes the low efficiency. To make sense of the irreversibility, an in-depth thermodynamic model for the solar driven two-step thermochemical cycles is proposed. Different from previous literatures solely focusing on the energy loss and irreversibility of devices, this work decouples a complex energy conversion process in three sub-processes, i.e., reaction, heat transfer and re-radiation, acquiring the cause of irreversible loss. The results from the case study indicate that the main irreversibility caused by inert sweeping gas for decreasing the reduction reaction temperature dominates the cycle efficiency. Decreasing reduction reaction temperature without severe energy penalty of inert sweeping gas is important to reducing this irreversible loss. A favorable performance is achieved by decreasing re-oxidation rate, increasing hydrolysis conversion rate and achieving a thermochemical cycle with a lower equilibrium temperature of reduction reaction at atmosphere pressure. The research clarifies the essence of process irrrversibility in solar thermichemical cycles, and the findings point out the potential to develop efficient solar driven two-step thermochemical cycles for hydrogen production.

Automated summary

Hydrogen production via a solar driven two-step thermochemical cycle has attracted attention, but the low efficiency is caused by severe irreversible loss. This study proposes a thermodynamic model to understand the irreversibility and identifies the main cause of irreversible loss. It also suggests methods to reduce the irreversible loss and improve efficiency.