## 4 Thermochemical Storage in Power-to-Heat Applications
### Thermochemical Storage Energy Systems in Power-to-Heat Applications: Case Studies
The heat of desorption is provided by waste heat or solar collectors at about 50 \({}^{\circ}\)C. The authors demonstrated an overall thermochemical cycle has a COP (1-1.4) higher than a conventional MVC operating without thermochemical storage. [PERSON] et al. [175] developed a hybrid compression thermochemical refrigeration system (HCTSR) to show the potential power flexibility of thermal storage and power-to-heat. Power flexibility is in this specific case defined as the thermal response of TES tanks and related electricity consumption of the heat pump during charging, discharging and store mode. The scheme of this case study is shown in Figure 7.  
HCTRS, consists of an MVC cycle and a thermochemical reactor. The heat pump and an electric heater serve as power-to-heat conversion while the storage tank as the source of flexibility. The thermochemical storage device is a packed bed reactor based on zeolite 13X and water as working pair. During desorption, the electric heater serves as a dehydration source. During adsorption, the heat stored is used for space heating or domestic hot water. The system with thermochemical storage was compared with the one obtained coupling the same MVC to a sensible and latent storage tank. Water and CaCl\({}_{2}\)6H\({}_{2}\)O were used as sensible and latent material respectively. Results show that assuming the same dimensions for the storage tank (a cylindrical vessel of 0.5 m\({}^{3}\)) and a volume flow of heat transfer medium of 1 m\({}^{3}\)/h, the thermochemical system has an energy capacity (0.05 GJ) lower than the other storage systems (0.15 GJ). The available storage capacity (COC) and storage efficiency (\(\eta_{\text{OC}}\)) were used to compare the energy flexibility of the three different thermal storage systems. COC is defined as the amount of energy that is shifted during the optimal control to minimize the electricity consumption costs for operating the heat pump and the electric heater. \(\eta_{\text{OC}}\) indicates the effective use of the heat stored to compensate power-to-heat devices during optimal control. Results show that the thermochemical storage has the lower values for both COC (5.6 kWh) and \(\eta_{\text{OC}}\) (0.96). Figure 7: PtH/TCTES system developed by [PERSON] et al. [175]. The following studies are examples of power-to-heat-to-power applications in which the heat stored is converted into electricity by a power plant when it is needed. [PERSON] et al. [245] proposed a hybrid energy system to store excess energy from renewable sources. The system consists of a compressed air energy storage (CAES) integrated with a thermochemical reactor based on the use of the metal oxide redox pair Co\({}_{3}\)O\({}_{4}\)/CoO as sorption working material. In contrast to a conventional Compressed Air Energy Storage (CAES) [317] in which compressed air is superheated by means the combustion of fossil fuel, in the proposed hybrid system this function is replaced by the sorption reactor. The scheme of this case study is shown in Figure 8.  
The proposed system consists of five compressors powered by electricity to compress air and an electric heater as heat source for the charging phase of the thermochemical storage process. The thermal charging phase takes place, in parallel with the CAES compression phase, with the reduction of Co\({}_{3}\)O\({}_{4}\) into CO and CO\({}_{2}\) (2 Co\({}_{3}\)O\({}_{4}\)\(\leftrightarrow\) 6 CoO + O\({}_{2}\)) carried out at 870 \({}^{\circ}\)C and 0.1 bar. The discharging phase takes place and the energy stored in the compressed air and metal oxide CoO (heat released by the exothermic reaction is transferred to air) is converted back into electricity throw air turbines. A value of 3.9 kWh/m\({}^{3}\) was evaluated for the energy storage density, defined in this case as the total power output per unit volume of the stored air (the same as the volume of the storage cavern). Moreover, it was estimated that 65% of the energy storage density relies on thermochemical part of the system while the remaining 35% is achieved via the CAES.