## 2 Heat Storage Classification  
In the literature, _therrochemical heat storage_ is employed for a family of reactions involving both physical and chemical processes. A tentative of classification is given in [19] and is presented in Figure 1. However, in this classification, sorption is used to aggregate different physical phenomena and can lead to misunderstandings. Then, we propose, in this section, to derive a classification of thermochemical heat storage based on physical phenomena typology. We deliberately limit the classification development to heterogeneous Involving components of two or more phases) reactions as homogeneous reactions are seldom used for thermal energy storage. Let us first define sorption: according to [20], sorption is _the process by which a substance (sorbate) is sorbed (adsorbed or absorbed) on or in another substance (sorbent)_. The process can be caused by physical bonding, i.e., physical sorption, or chemical bonding, i.e., chemical sorption. The main difference between physical and chemical sorption lies in the nature of created bonds. Physical sorption is weak, long range bonding mostly Van der Walls interactions and hydrogen bonding. Chemical sorption is strong, short range bonding involving orbital overlap and charge transfer. Another main difference between physical and chemical sorption is that the latter requires activation energy whereas it is not the case for the first process. Sorption can be absorption or adsorption. Definitions of both processes can be found in [20]:  
* Absorption is _the process of one material (absorbate) being retained by another (absorbent); this may be the physical solution of a gas, liquid, or solid in a liquid, attachment of molecules of a gas, vapor, liquid, or dissolved substance to a solid surface by physical forces, etc.._
* Adsorption is _an increase in the concentration of a dissolved substance at the interface of a condensed and a liquid phase due to the operation of surface forces. Adsorption can also occur at the interface of a condensed and a gaseous phase_. While molecule undergoing absorption are taken up through the bulk of the absorbent (for example by diffusion), adsorption is a surface process. It is sometimes difficult to find the difference between adsorption and absorption. Taking for example the dehydration of lithium sulphate monohydrate [21] (i.e., chemical sorption), nucleation is supposed to occur at the surface of the grain (adsorption) and then the growth proceeds towards the center of the grains by diffusion (absorption). Physical sorption can be split into absorption and adsorption (also called physisorption). As with physical sorption, chemical sorption can be split into absorption and adsorption:  
* Chemical absorption or reactive absorption is a chemical reaction between the absorbed and the absorbing substances. Sometimes it combines with physical absorption. This type of absorption depends upon the stoichiometry of the reaction and the concentration of its reactants. * Chemical adsorption is called chemisorption. Chemisorption is _Adsorption which results from chemical bond formation (strong interaction) between the adsorbent and the adsorbate in a monolayer on the surface_[20]. Figure 1: Chemical storage and sorption storage classification—reproduced from [19], copyright permission: Elsevier, 2019. From the previous definition, we propose in Figure 2 a classification of heat storage systems based on the physical/chemical phenomena involved. For heat storage, the split between physical and chemical sorption is important as the heat related to the two phenomena is quite different. For example, heat of adsorption is different for physisorption and chemisorption:  
At constant volume (i.e., \(W=0\)), the heat released or absorbed by the reaction is equal to the change in the internal energy that occurs during the reaction:  
\[\Delta U=Qv \tag{5}\]  
Such configuration is close to **closed chemical sorption heat storage systems**.