Research progress of different solar thermochemical energy storage systems

In recent years, traditional fossil fuels are constantly depleted, and the world is facing a serious energy crisis. Solar energy is a clean and abundant renewable energy resource which offers an option for solving the serious environmental problem caused by the consumption of the fossil energy. Among different forms of renewable energy, solar energy has also become an essential part of daily human life. Solar thermal power generation technology is regarded as one of the most potential power generation methods in the future. Solar thermal power generation technology is important to alleviate the energy crisis and to protect the ecological environment. Due to the intermittent of the solar energy, a conceivable way is to store it to provide the solar energy continuously. Thermal energy storage system is necessary for steady and continuous solar thermal power generation. In this paper, among the existing three kinds of solar thermal energy storage technology: sensible heat storage, latent heat storage system and thermochemical energy storage system, we analyzed the obvious advantages of thermochemical energy storage- high energy density and large scale storage and convenient remote transportation. The material or matrix chosen to act as solar thermochemical energy storage medium must meet the following criteria: high energy storage density, low charging temperature, higher rate of reaction, appropriate heat and mass transfer properties, easy to handle, low cost and thermal stability. According to these standards, we chose five suitable solar thermochemical energy storage systems, namely, methane reforming system, metal oxide system, metal hydride system, hydroxide system and amino thermochemical system. The latest research progress of the existing five kinds of thermochemical energy storage system were introduced on the reaction mechanism, reaction model and design of reactor by the numerical, experimental and technological study methods. Advantages and disadvantages of each system were analyzed. Based on the problems of reaction system, we put forward the main research directions of each thermochemical energy storage system. For methane reforming system, strengthening the heat transfer process in the reactor and the development of high performance catalysts or oxygen carriers are important. Furthermore, the combination of reforming reaction with methanation reaction to form energy storage cycle also need to develop. The problems of metal oxide system include too high temperature and high thermal hysteresis. Therefore, doped with other metal oxides may be a promising method. To improve the reaction rate of metal hydride system, the method of doping with other metal hydrides can also be carried out. The bottleneck of metal hydride system and the amino thermochemical system is too high hydrogen pressure, therefore the research on reactor structure for high efficient and low cost with large scale storage of hydrogen is necessary. The future research on hydroxide system should focus on the settlement of sintering and corrosion, so the particle design is important. In general, we pointed out that future research will mainly develop in two directions: greater scale and more detailed mechanism, specifically the construction of solar thermal energy storage demonstration system and a deeper and more detailed study of the mechanisms of heat and mass transfer coupling with chemical reaction.