## 3 Thermochemical Heat Storage: Description of Materials and Processes
### Thermochemical Processes and Materials
[251] developed an experimental set up to investigate thermal cycling stability of the Ca(OH)\({}_{2}\)/CaO system laying the foundation of applying this system to practical. A similar experimental set up was developed by [PERSON] et al. [252]. Ca(OH)\({}_{2}\)/CaO is among more used systems in chemical processes [253; 254; 255; 256]. This system has numerous advantages, e.g., efficient reaction kinetics [257] and high reaction enthalpy (104.4 KJ/mol) [258]. It is a very suitable material in thermal storage systems [259], in particular for high-temperatures (400-600 \({}^{\circ}\)C) applications [260]. In the context of power-to-heat applications the usage of Ca(OH)\({}_{2}\)/CaO thermochemical systems coupled to heat pumps is arousing great investigation with a particular focus on heat and mass transfer process [261; 262; 263]. Also the interest towards metal hydrides is not recent, these thermochemical storage systems were explored since the mid-1970s [264]. Several applications and different metal hydrides systems were explored for thermochemical heat storage [265; 266; 267; 268; 269]. Among metal hydrides, Mg-based systemsare promising as thermochemical storage materials owing to high reaction enthalpy as shown in the studies of [PERSON] et al. [223] and [PERSON] et al. [53]. Mg-based metal systems show cyclic stability over a temperature range from 250 \({}^{\circ}\)C to 550 \({}^{\circ}\)C in which high thermal energy densities of up to 2257 kJ/kg are reached [130]. The abundance of metal hydrides, low cost, high reaction enthalpy, high storage density are among characteristics attracting extensive investigations [219]. These systems, are suitable for both low and high temperature applications [270]. As an example, [PERSON] et al. [270] investigated the potential of metal hydrides for low temperature applications while [PERSON] et al. [220] investigated their use for high temperatures applications, in particular based on experimental and modelling results they designed and fabricated a prototype to store both hydrogen and heat with solar technologies. In accordance to other studies, they showed that metal hydrides show both good reversibility and cycling stability combined with high enthalpies. A study about the future perspectives of thermochemical storage based on use of metal hydrides for solar technologies have been developed by [PERSON] et al. [271]. High energy density and desorption temperatures make salt hydrates fitting with the use of power-to-heat technologies, waste heat sources, solar thermal collectors, particularly investigated and proposed for seasonal heat storage of solar energy in the built environment [150; 272; 273]. [PERSON] et al. [274] investigated 125 salt hydrates for low temperatures heat storage and found that SrBr\({}_{2}\cdot\)6H\({}_{2}\)O and MgCl\({}_{2}\cdot\)6H\({}_{2}\)O are among the most promising choices for thermochemical storage applications. To investigate the potential energy storage density and the storage efficiency of salt hydrates, a micro-combined heat and power system was developed for the storage of heat generated. They found that for applications requiring lower discharging temperatures like 35 \({}^{\circ}\)C, the expectable efficiency and net energy storage density was low. Their results are in accordance to [275; 276; 277; 278]. Salt hydrates are considered the most suitable materials for residential applications owing to their high energy density (400-870 kWh\(\cdot\)m\({}^{-3}\)) and low turning temperature [280]. Metal carbonates have several advantages, e.g., high energy density, nontoxicity, low costs and widespread availability. All these properties make them suitable for thermochemical storage applications [281; 282; 283; 284]. Among suitable alternatives, the combined use of CaO/CaCO\({}_{3}\) (density 0.49 kWh/kg), proposed by [PERSON] in 1973 [283], is largely investigated. In a recent study [PERSON] et al. [235] used the working pair CaO/CaCO\({}_{3}\) to develop a system referred as Photovoltaic-Calcium looping (PV-CaL) as large scale storage system.