# Mg-Based Hydrogen Absorbing Materials for Thermal Energy Storage--A Review
## 2 Chemical Fundamentals  
For the TES system, there are mainly three potential technologies used to store thermal energy: Sensible storage, latent storage, and chemical storage. In the sensible storage system, the energy density is equal to the product of temperature change by the specific heat of this material. The thermal energy is stored when the temperature of the material increases and released when its temperature decreases. In our daily life, hot water storage, underground thermal energy storage, and rock filled storage are typical applications of sensible heat storage [15]. When a solid/liquid state material transforms into a liquid/gas state, it would absorb energy to overcome the intermolecular force. Under this circumstance, it would release thermal energy during the process of gas/liquid state transforming into liquid/solid state. The thermal energy related to the phase change of material is named latent heat. In the time of absorbing or releasing heat, the temperature of the system would not change and \begin{table}
\begin{tabular}{c c c} \hline \hline
**Renewable Source** & **Max. Power (TW)** & **Percentage of Total Solar Energy** \\ \hline Total surface solar [7] & 85,000 & 100\% \\ Desert solar [6] & 7,650 & 9\% \\ Ocean thermal [8] & 100 & 0.12\% \\ Wind [9] & 72 & 0.08\% \\ Geothermal [10] & 44 & 0.05\% \\ River hydroelectric [11] & 7 & 0.008\% \\ Biomass [6] & 7 & 0.008\% \\ Open ocean wave [12] & 7 & 0.008\% \\ Tidal wave [13] & 4 & 0.003\% \\ Coastal wave [14] & 3 & 0.003\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Power from different renewable sources in the earth [6]. equal to the melting or boiling point. In total, there are more than 150 phase change materials listed by [PERSON] et al. [16]. Compared with sensible heat, latent heat materials are able to store more than 5-14 times heat due to the much higher phase-transition enthalpy [20]. Nevertheless, both sensible heat and latent heat storage are physical thermal energy storage system, in which thermal energy will fade out as time goes by, and they are not reliable for long-term thermal energy storage. There are many publications studying the TES by sensible and latent heat storage [1; 15; 16; 20; 21]. Chemical storage is the technology of storing heat by utilizing reversible chemical reactions [22]. \[\text{AB+Heat}\left(\Delta\text{H}\right)\rightleftharpoons\text{A+B} \tag{1}\]  
In this reversible chemical reaction, the thermal energy transforms into chemical energy. In the forward reaction, thermal energy serves as the power source to drive the chemical reaction and it is able to store heat by breaking the chemical bonds in the AB molecules, dissociating AB into A and B. Obviously, it is an endothermic process. In the reverse reaction, since the intrinsic property of exothermic reaction, the A and B molecules bind together and produce AB compounds, transforming the chemical energy back to thermal energy and releasing a large amount of heat. During the process of thermal energy storage and release, the most important thing is that the heat is stored in chemical form without any thermal waste and the energy in the chemicals do not fade in a long time. Compared with sensible and latent heat storage (Table 2), chemical heat may show much higher energy storage density due to the strength of the chemical bond being considerably larger than the intermolecular force. In this regard, it needs much more energy to break the chemical bonds than overcome the intermolecular force. However, it is noteworthy that two significant issues must be fulfilled in the chemical heat storage technology-the high reaction \(\Delta\text{H}\) for a certain reversible chemical reaction and the completely reversible chemical reaction in a temperature range of heat source. \[\Delta\text{G}=\Delta\text{H}-\text{T}\Delta\text{S} \tag{2}\]  
\[\text{AB (solid or liquid)+Heat}\left(\Delta\text{H}\right)\rightleftharpoons \text{A (solid or liquid)+B(gas)} \tag{3}\]  
Moreover, we should take the reaction temperature into consideration.