## 4 Thermochemical Storage in Power-to-Heat Applications  
### Thermochemical Storage Energy Systems in Power-to-Heat Applications: Case Studies  
PtH technologies show a mature development with latent and sensible storage while only a limited number of applications with thermochemical storage is available in literature [310; 311; 312; 313; 314; 315; 316; 317]. Existing applications focus on different aspects, hence a net comparison was not possible. Based on the usage of the heat stored, in this work the applications were divided into power-to-heat and power-to-heat-to-power as shown in Figure 4. In the first case, heat stored is used in the form of thermal energy for heating and cooling purposes. In the second case, heat, released during the discharging phase, is used to generate electricity when it is needed. [PERSON] et al. [139] developed a hybrid thermochemical storage device to store the excess of power generation. The system was developed for household applications for low to medium temperature range (50-100 \({}^{\circ}\)C). The scheme of this case study is shown in Figure 5.  
Figure 4: Thermochemical storage and power-to-heat uses. \begin{table}
\begin{tabular}{l l l l} \hline \hline
**Project Name/Institution** & & **Description** & **Storage System** \\ \hline \hline \multirow{6}{*}{STU [4][160] (2016)} & \(\bullet\) & Solid sorption system for space heating and domestic & \\  & heat water. & & LiCl with expanded \\  & \(\bullet\) & T\({}_{\rm c}\) = 40 \({}^{\circ}\)C & graphite \\  & \(\bullet\) & T\({}_{\rm d}\) = 85 \({}^{\circ}\)C & \\  & \(\bullet\) & Storage energy density 0.873 kWh/kg. & \\ \cline{3-4}  & \(\bigcirc\) & Liquid seasonal thermal storage system & \\ \cline{3-4}  & \(\bigcirc\) & T\({}_{\rm c}\) = 22 \({}^{\circ}\)C & NaOH/H\({}_{2}\)O \\  & \(\bigcirc\) & T\({}_{\rm d}\) = 50 \({}^{\circ}\)C & \\ \cline{3-4}  & \(\bullet\) & Solid sorption system for seasonal thermal storage for & \\ Heat STRESS [170] (2019) & \(\bullet\) & T\({}_{\rm c}\) = 40 \({}^{\circ}\)C & CaCl\({}_{2}\)/NH\({}_{3}\) \\  & \(\bullet\) & T\({}_{\rm d}\) = 70 \({}^{\circ}\)C & \\ \cline{3-4}  & \(\bigcirc\) & Hybrid energy storage system to store energy from & \\ \cline{3-4}  & Newcastle [245] (2019) & & wind, solar and/or off-peak electricity simultaneously. & Co\({}_{3}\)O\({}_{4}\)/CoO \\  & \(\bigcirc\) & Reaction take places a T \(>\) 800 \({}^{\circ}\)C & \\ \cline{3-4}  & \(\bullet\) & Pilot prototype integrated with Concentrated Solar & \\ \cline{3-4}  & RESTRUCTURE [247] (2019) & & Power (CSP) for power production & \\ \cline{3-4}  & \(\bullet\) & Reaction take places in the temperature interval & Co\({}_{3}\)O\({}_{4}\)/CoO \\  & (800–1000) \({}^{\circ}\)C & \\ \hline \end{tabular}  
* Salt Water Energy Accumulation and Transformation. 2 Modular Chemical Energy Storage. 3 Combined Development of Compact Thermal Energy Storage Technologies. 4 Institute of Refrigeration and Cryogenics (China)
* Institute fur Solartechnik.  
\end{table} 
Table 3: _Cont._The system is based on the reversible hydration/dehydration of SrBr\({}_{2}\cdot\)6H\({}_{2}\)O and graphite as additive material. The power converted into heat by a heat pump driven by solar and wind energy is carried out to the tank storage where the endothermic dehydration reaction takes place at temperature \(<100\)\({}^{\circ}\)C. From the reaction SrBr\({}_{2}\) (sorbent) and H\({}_{2}\)O (sorbate) are formed (SrBr\({}_{2}\cdot\)6H\({}_{2}\)O\(\leftrightarrow\)SrBr\({}_{2}\)+6H\({}_{2}\)O), the sorbate is condensed for use in the discharging process in the case of closed system or released in the environment in the case of open system. Heat stored is use both heating demand and supply of electricity during the discharging phase.