## 4 Thermochemical Storage in Power-to-Heat Applications
### Thermochemical Storage Energy Systems in Power-to-Heat Applications: Case Studies
Their results showed that an energy storage density of 500 kJ/kg can be achieved at a temperature of 80 \({}^{\circ}\)C, a value of 600 kJ/kg by increasing the temperature to 150 \({}^{\circ}\)C. This study shows for the first time how the composite formulation of SbBr\({}_{2}\) affects the energy density, heat and mass transfer and reaction kinetics. [PERSON] et al. [173] developed a hybrid system for household applications. This integrates a thermochemical system with an air conditioning system driven by grid and photovoltaic electricity. The cooling system is a conventional Mechanical Vapor Compression (MVC) while the storage device is a packed-bed reactor with eight compartments based on the use of BaCl\({}_{2}\)/NH\({}_{3}\) as working pair. The scheme of this case study is shown in Figure 6.  
When there is a surplus of electricity generation and no cooling needs, the extra power is used to run the compressor in order to store energy for later use. By means of a smart controller, during the storage process, the evaporator is disconnected from the circuit and the reactor is connected to the compressor. The desorption heat is provided by a low grade waste heat source at 50 \({}^{\circ}\)C or by an electric heater in direct contact with the thermochemical reactor. BaCl\({}_{2}\) reacts with ammonia (NH\({}_{3}\)) to form BaCl\({}_{2}\)-8 NH\({}_{3}\) with an energy density estimated in an approximate value of 200 kJ per kg of reactor. The coefficient of performance, exergy efficiency and cooling capacity were used as indicators to compare a traditional MVC cycle without thermochemical storage and the hybrid system proposed. As example, the authors showed that the COP of the hybrid system, for a given source temperature, is higher than the one of a conventional one. The hybrid system was compared with alternative energy storage processes. In particular Pb and Li-ion batteries (electrochemical storage), ice and chilled water thermal storage was chosen as alternative devices to thermochemical reactor. Their results showed that the hybrid system proposed has a cooling capacity (60 Wh/L) six times larger than chilled water system but comparable to that one of ice storage systems. MVC systems with electrochemical batteries have the highest cooling capacity, 190 Wh/L for MVC and Pb battery and 420 Wh/L for MVC and Li battery respectively, but much shorter life span than MVC with thermochemical storage. The COP of the hybrid system (4.8) is comparable to Pb batteries (4.2), Li-ion batteries (4.2) and chiller (4.2) systems. Figure 5: PH/TCTES system developed by [PERSON] et al. [139]. Figure 6: PH/TCTES system developed by [PERSON] et al. [173] and by [PERSON] et al. [315]. The hybrid system is an example of compressor-driven method for energy storage and deferred cooling. This application for space cooling is not yet widely explored in literature. [PERSON] et al. [315] analyzed an ammonia-based refrigeration system consisting in the hybridization of compression refrigeration with thermochemical storage. The proposed hybrid system has the typical architecture of a MVC cycle (evaporator, compressor, condenser, reservoir and throttling valve), a grid-connected photovoltaic installation and a thermochemical storage reactor. The scheme of this case study is shown in Figure 6.  
MVC cycle and thermochemical storage system have the same condenser, evaporator and refrigerant fluid (NH\({}_{3}\)). The storage device is a packed-bed reactor based on the use of BaCl\({}_{2}\)/NH\({}_{3}\) as working pair. Both the PV installation and the grid are used to meet the electricity requirements for cold production. When there is a surplus of power generation from RES and no cooling demand, the power in excess is used to store energy in the form of heat driving the desorption phase of the reactor. Thermochemical process enables the storage of energy in the form of chemical potential for a deferred cold production without running the compressor.