Electrochemical-thermochemical complementary hydrogen production system for efficient full-spectrum solar energy storage

Solar hydrogen production, which can store unstable solar energy into clean hydrogen, has garnered widespread attention from researchers. However, there are some shortcomings in the single solar hydrogen production pathway: Photovoltaic-electrolytic green hydrogen production primarily harnesses short-wavelength solar energy, neglecting a significant portion of solar energy in the long-wavelength, while solar thermochemical methane reforming for gray hydrogen production underutilizes high-grade solar energy and emits carbon dioxide. This study presents a hybrid system capable of concurrently producing green and gray hydrogen, effectively harnessing the entire spectrum of solar energy while minimizing carbon emissions. Thermodynamic investigations reveal that the system attains energy efficiency of 47.43 %, exergy efficiency of 41.93 %, and solar-to-hydrogen efficiency of 25.61 % at the direct normal irradiance (DNI) of 1000 W/m². It is intriguing to observe the discrepancy in the proportions of energy input from solar energy and methane, along with their respective impacts on hydrogen production. Although solar energy input constitutes 85.26–63.44 % of the total energy input, its contribution to hydrogen production is 64.94 %–33.71 %. Despite the reduced proportion of solar energy's contribution to hydrogen production compared to its input proportion, it significantly contributes to reducing fossil energy usage and carbon emissions. This system's average carbon dioxide reduction rate stands at around 16.84–13.80 kg/kgH₂. The proposed system offers an efficient approach to full-spectrum solar energy storage and hydrogen production, thus contributing to a cleaner energy future. © 2024 Elsevier Ltd