## 4 Thermochemical Storage in Power-to-Heat Applications
### Thermochemical Storage Energy Systems in Power-to-Heat Applications: Case Studies
The authors demonstrated that, in terms of storage energy density, the hybrid system has a value comparable to a conventional CAES (3-6 kWh/m\({}^{3}\)) operating at the same conditions. Based on a thermodynamic analysis it was estimated an efficiency of 56.4%. In comparison to conventional CAES plants, authors showed that this value is higher than the efficiency of the commercialized Huntfort (42%) and McIntosh (54%) CAES plants. [PERSON] et al. [235] developed a power-to-heat-to-power system based on the calcination/carbonation of calcium carbonate as sorption process and a closed CO\({}_{2}\) Brayton regenerative cycle. The scheme of this case study is shown in Figure 9.  
During the charging phase, the electric power is converted into thermal power by Joule effect to heat up the calciner (Fluidized bed thermochemical reactor). In the reactor the calcination endothermic reaction takes place under atmospheric pressure at 950 \({}^{\circ}\)C, CaO and CO\({}_{2}\) are formed (CaCO\({}_{3}\)\(\leftrightarrow\) CaO + CO\({}_{2}\)). During the discharging phase, that takes place at 75 bar and 25 \({}^{\circ}\)C, power is generated in a CO\({}_{2}\) turbine connected to an asynchronous generator that converts mechanical power into electricity. CaO and CO\({}_{2}\) are carried out in the carbonator reactor where the exothermic carbonation reaction occurs. The presence of a calciner and a carbonator is indicative that in the system charging and discharging  
Figure 8: PH/TCTES system developed by [PERSON] et al. [245]. Figure 6.0 PH/TCTES system developed by [PERSON] et al. [235]. cycles are well differentiated and independent. The system is connected to the grid to export electrical power generated during the discharging phase. The proposed system was simulated under different charging and discharging operations modes to assess its potential as large-scale electric energy storage system estimating a maximum reachable efficiency of 39%. [PERSON] et al. [316] developed a phase change redox (PCR) system to convert electricity surplus into heat and to store it using a CuO/Cu\({}_{2}\)O cycle. The scheme of this case study is shown in Figure 6.1  
When there is a surplus of electricity from grid or solar/wind plants heat provided by Joule heating is used for the charging phase of the sorption process. During this phase, CuO\({}_{2}\) is reduced into CuO and O\({}_{2}\) (2 CuO\({}_{2}\leftrightarrows\) 2 CuO + O\({}_{2}\)). The molten CuO/CuO\({}_{2}\) requires a high temperature of about 1200 \({}^{\circ}\)C during the charging phase. When electricity demand in the grid occurs the discharging phase starts. During this phase, the exothermic reaction takes place and the stored molten CuO/CuO\({}_{2}\) is oxidized and cooled into an oxidation reactor using air. Heated air is used into a Brayton cycle coupled with a bottoming organic Rankine cycle (ORC). Energy storage density and round trip efficiency were the indicators used to assess the energy storage performances. Energy storage density is here defined as the heat stored per mass unit of the raw material CuO while the round trip efficiency is the amount of electricity that can be recovered for a given energy input. The PCR system coupled to the Brayton and Rankine power generation cycles is able to achieve a round trip efficiency of about 50%. Advantages of the proposed PCR system are high-energy storage density, high round trip efficiency, enhancement of CuO/Cu\({}_{2}\)O reversibility, abundant and low-cost raw material and oxygen as a valuable by-product. The main disadvantages and potential limits can be summarized as systems complexity, high-temperature heat source, high operating temperature and high equipment, operation and maintenance costs. [PERSON] et al. [318] proposed an innovative hybrid absorption system based on the thermochemical technology to store electrical energy at large scale.