Spanning solar spectrum: A combined photochemical and thermochemical process for solar energy storage

Storage in the form of chemical energy is crucial for efficient utilisation of solar energy. In recent years, solar photon-induced molecular isomerization energy storage, in which solar energy can be directly converted and stored as chemical energy through internal molecular isomerization reactions, has received increasing interest. It is a closed cycle without any CO2 emission. However, the absorption of reactants cannot cover the full spectrum of solar radiation, and only the ultraviolet and a portion of the visible spectrum of solar energy can be stored. Moreover, some absorbed photons are dissipated as heat, leading to an increase in the reaction temperature and a decrease in the solar photochemical efficiency. To address these problems, a new energy storage system which integrates the photochemical process with thermochemical process has been proposed to convert the full spectrum of solar energy into chemical energy. Concentrated sunlight enters the photochemical device. Norbornadiene derivatives present in the photochemical devices can be isomerized to the related quadricyclanes by absorbing the ultraviolet-visible light photons. Some absorbed photons are simultaneously stored in the chemical bonds of the quadricyclanes. The unabsorbed photons corresponding to the visible-infrared spectrum are transmitted to thermochemical reactors, providing heat for methanol decomposition. This portion of the solar energy is stored in the form of chemical energy of the products (H-2 and CO). The heat dissipated in the photochemical process is transferred to the thermochemical reactors for methanol decomposition, resulting in a higher solar chemical efficiency. Results show that the average solar chemical efficiencies corresponding to the design condition and off-design condition are 75.38% and 49.78%, respectively. The results of comparison of the thermochemical performances verify that the proposed system is superior to both the single photochemical system and the single thermochemical system.