## 3 Thermochemical Heat Storage: Description of Materials and Processes
### Thermochemical Processes and Materials  
According to [PERSON] [125], sorption is the phenomenon of fixation or capture of a gas or a vapor by a substance in a condensed state. As shown in Figure 2, sorption processes are classified into absorption and adsorption [140]. According to [PERSON] et al. [141] absorption is defined as \"the process of one material (absorbate) being retained by another (absorbent)\". According to [PERSON] et al. [126], adsorption is defined as \"a phenomenon occurring at the interface between two phases, in which cohesive forces act between the molecules of all substances irrespective of their state of aggregation\". An important difference is that absorption occurs at the sorbent molecular level by altering its composition and morphological structure, adsorption occurs at the surface of the adsorbent [142; 34]. As shown in Figure 2, solid/gas and liquid/gas systems are example of working pairs used for sorption processes. These processes are used to store both low-grade heat (\(<\)100 \({}^{\circ}\)C) and medium-grade heat (100-400 \({}^{\circ}\)C) [143; 144; 145]. High kinetics at low temperatures make the sorption processes particularly  
Figure 2: Thermochemical Heat Storage principles classification. attractive for low-temperature applications such as space heating, domestic hot water preparation or other low-grade and medium-grade heat uses [7; 146; 147; 148; 149; 150; 151; 152]. Usually sorption materials are liquid, solid and composite sorbents [35; 153]. Example of working pairs are:  
* LiBr solution/H\({}_{2}\)O [154; 155];
* LiCl solution/H\({}_{2}\)O [156; 157; 158];
* LiCl/activated alumina [159; 160];
* LiCl/expanded graphite [161];
* LiCl\({}_{2}\) solution/H\({}_{2}\)O [162];
* CaCl\({}_{2}\) solution/H\({}_{2}\)O [163; 164; 165];
* Binary sales [166; 167; 168; 169; 170; 171; 172; 173; 174];
* Zeolite 13X [175; 176; 177; 178; 179; 180; 181; 182], Zeolite 4A [183; 184; 185; 186; 187; 188; 189], Zeolite 5A [190; 191];
* Aluminophosphates (ALPOs) [192] and Silico-aluminophosphates (SAPOs) [193; 194; 195];
* Composite materials made up by the combination of a salt hydrate and an additive with a porous structure and high thermal conductivity (expanded graphite [196; 197], metal foam [198], carbon fiber [199] and activated carbon [199]). (ALPOs) and (SAPOs) are among promising examples of sorption materials, in particular, for low temperature heat storage [200; 201]. Among zeolites, Zeolite 13X is one of the most common thermochemical material in current research due to its hydrothermal and mechanical stability and corrosion behavior [190]. Example of composite materials are CaCl\({}_{2}\)-Silica gel/H\({}_{2}\)O [202], CaCl\({}_{2}\)-FeKIL\({}_{2}\)/H\({}_{2}\)O [203; 204], LiBr-Silica gel/H\({}_{2}\)O [205], MgSO\({}_{4}\)-Zeolite/H\({}_{2}\)O [206; 207], MgSO\({}_{4}\)-MgCl\({}_{2}\)/H\({}_{2}\)O [208; 209]. Chemical reactions are used to store medium (100-400 \({}^{\circ}\)C) and high (\(>\)400 \({}^{\circ}\)C) grade heat [210; 211; 212]. Example of chemical reactions are:  
* dehydration of metal hydroxides [213; 214; 215; 216; 217; 218];
* dehydration of metal hydrides [219; 220; 221; 222; 223; 224];
* dehydration of salt hydrates [225; 226; 227; 228; 229; 230];
* deammoniation of ammonium chlorides [172; 231; 232; 233];
* decarboxylation of metal carbonates [234; 235; 236; 237; 238; 239];
* methane steam reforming [240; 241; 242; 243; 244];
* catalytic dissociation [243; 244; 245];
* metal oxide redox [246; 247; 248]. The interest towards dehydration of metal hydroxides is not recent, e.g., the hydration of MgO has been extensively studied as early as 1960 [249; 250], the dehydration of Ca(OH)\({}_{2}\) has found wide attention as early as 1988. In particular, under support of the National Energy Administration, the American Pacific Northwest National Laboratory started the research on Ca(OH)\({}_{2}\)/CaO as energy storage system [251]. In this context, [PERSON] et al.