# Mg-Based Hydrogen Absorbing Materials for Thermal Energy Storage--A Review
## 1 Introduction  
Energy safety has faced great challenges due to the massive and long-term depletion of fossil fuels to meet the demands of about 80% of primary energy consumption. At the same time, this utilization of fossil fuels also brings severe air pollution. Under these circumstances, it is urgent to shift the focus on fossil fuels to renewable energy resources [1; 2; 3; 4; 5]. Fortunately, various renewable energies technologies have been developed and used in our daily life, such as hydropower, wind energy, solar energy, geothermal energy, tidal energy, etc. Compared with the solar energy radiated on the earth surface, the amount of all the other renewable sources is less than 1% (Table 1) [6]. The solar energy on the surface of the earth is several thousand times more than that consumed by the human beings every year. For solar energy, the way we currently harness it is usually photovoltaic power generation which mainly makes use of solar panels to capture solar energy. However, this kind of energy is intermittent and highly unpredictable--due to different latitudes, weathers, terrains, etc.--resulting in unstable and discontinuous power source and fluctuation of output power. Moreover, the relative lower conversion efficiency from light to electric energy requires the installation of solar panels in a large scale, which in turn increases the cost. Also, we should take environmental contamination into consideration during the photovoltaic cell manufacturing process.  
Solar thermal energy is the second way we use solar energy and it is a better technique to overcome the drawbacks of photovoltaic power generation. Solar thermal power systems could collect and store heat using the solar radiation as energy input and release heat to produce electricity by generating high-temperature steam and then driving a turbine or a motor engine [1], satisfying the electricity usage requirement at the case of poor sunlight or at right. In addition, this kind of energy conversion is effective in solving the problem of electricity supply fluctuation in photovoltaic power generation and address the mismatch dispatchability between supply and demand of electricity [1]. More importantly, solar thermal power hardly involves any polluting emissions or environmental safety issues which nuclear-based or conventional power generation technologies struggle with.  
Generally, a solar thermal power system is mainly comprised of a solar thermal energy concentrator, TES unit, and power conversion device. The TES system is one of the most crucial component of solar thermal power systems due to its contribution to the effective storage of thermal energy. In recent years, a great many groups have focused their research on TES which has attracted great attention [1; 2; 3; 4; 5; 15; 16; 17; 18]. In the current development of TES, researchers concentrate more attention on the low-temperature (<200 \({}^{\circ}\)C) heat storage. Thus, few high temperature heat storage systems are investigated. However, it is known that the efficiency of engines can be improved by raising the temperature to a relatively high degree according to the Carnot principle. The high temperature heat storage not only improves the efficiency of TES, but also has a wider application than that of low temperature heat storage. Based on the above statements, it could be concluded that it is essential to develop a kind of high temperature TES system. This high temperature TES system not only contributes to better utilization of solar energy but is also beneficial to high temperature industrial waste recycling [19]. In summary, the TES system could help to serve as follows: (1) conserves energy with renewable energy sources such as solar energy; (2) creates emissions reduction of toxic and greenhouse gases and is environmentally friendly; and (3) adjusts electricity mismatch problem and maintains a stable electric power transmission [1]. Besides, the cost and safety should be also taken into account as important or even decisive issues to the wide application of TES.