• Energy Research

Abstract 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 (H2 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.

Abstract In most concentrated photovoltaic–thermal systems, solar energy that cannot be used by photovoltaic cells is recycled for the thermal process. The unutilized energy is converted into thermal energy for driving other processes. This increases irreversible losses and restricts the efficient utilization of high energetic photons. To address these challenges, a concentrated photochemical–photovoltaic–thermochemical system is proposed to use the full spectrum of solar energy more efficiently. Photons with photonic energy significantly higher than Eg (the bandgap of photovoltaic cells) are used in the photochemical process, and the below-Eg loss of photovoltaic cells is recycled to provide heat for the thermochemical process. The proposed system realizes the cascade utilization of the full-spectrum solar radiation and can use high-energy photons more efficiently. The irreversible loss from the proposed system is decreased, resulting in higher solar utilization efficiency. The numerical results clearly show that the proposed system outperforms concentrated photovoltaic systems, concentrated thermochemical systems, and concentrated photovoltaic–thermochemical systems. The solar utilization efficiency of high-energy photons (before 600 nm) is increased from 44.01% (of common concentrated photovoltaic–thermal systems) to 80.68%. The total solar utilization efficiency in the proposed system attains 66.95% under the design condition.

Riblet and superhydrophobic surfaces are two typical passive control technologies used to save energy. In this study, three microstructured samples—a micro-riblet surface (RS), a superhydrophobic surface (SHS), and a novel composite surface of micro-riblets with superhydrophobicity (RSHS)—were designed to improve the drag reduction rate of water flows. Aspects of the flow fields of microstructured samples, including the average velocity, turbulence intensity, and coherent structures of water flows, were investigated via particle image velocimetry (PIV) technology. A two-point spatial correlation analysis was used to explore the influence of the microstructured surfaces on coherent structures of water flows. Our results showed that the velocity on microstructured surface samples was higher than that on the smooth surface (SS) samples, and the turbulence intensity of water on the microstructured surface samples decreased compared with that on the SS samples. The coherent structures of the water flow on microstructured samples were restricted by length and structural angles. The drag reduction rates of the SHS, RS, and RSHS samples were −8.37 %, −9.67 %, and −17.39 %, respectively. The novel established RSHS demonstrated a superior drag reduction effect and could improve the drag reduction rate of water flows.

Abstract A solar chemical energy storage system with photochemical process and thermochemical process is proposed to convert full-spectrum solar energy into chemical energy. The ultraviolet and part of visible sunlight are firstly absorbed by norbornadiene derivatives, and the norbornadiene derivatives are converted into the related quadricyclane derivatives. When the quadricyclane derivatives are catalyzed, they are converted back to the norbornadiene derivatives for next cycle. The storage and releasing cycle are eco-friendly without CO2 emission. The rest of solar energy, which cannot be used in the solar photochemical process, are exploited by solar thermochemical process, providing heat for methanol decomposition. It is demonstrated that solar photochemical efficiency increases firstly and then it decreases with the increase of cut-off wavelength, while the solar thermochemical efficiency declines with the cut-off wavelength rising. In the hybrid system, the decrease of the solar thermochemical efficiency is balanced by an increase of the solar photochemical efficiency, and the maximum solar-to-chemical efficiency of the hybrid system reaches 68.7%.