## 2 Concept Description  
The studied system is based on a sorption storage technology, which relies on the reversible reaction, associated with a high amount of thermal energy, occurring between a sorbent and a sorbate. Such a process is limited by the slow reaction kinetics, because of the large amount of heat associated as well as the heat and mass transfer diffusion resistance within the material. However, this feature is extremely useful in an industrial type application, since it allows the reaction to be safely controlled and the discharge power to be regulated precisely. In the proposed concept, heat from a renewable energy source (e.g., solar heat) is employed to drive a desorption process, which means that incoming heat is stored in the form of adsorption potential energy. Moreover, the heat is stored without any loss until the refrigerant fluid (adsorbate) is kept separated from the adsorbent enabling the long-term storage of the renewable energy. Generally, there are two system configurations for adsorption TES: closed and open cycle [19]. The studied heat storage system belongs to the closed-cycle sorption storage category. Figure 1 reports the working phases of closed adsorption. During the charging phase, the adsorber, in which the adsorbent material is saturated with adsorbate, is regenerated by exploiting heat, e.g., heat coming from the solar field. The desorbed vapour is then condensed in the condenser, and the heat of condensation, \(Q_{cond}\), is either dissipated in the ambient or delivered to a proper load if the temperature level is adequate. Once the charging process is completed, and the adsorbent material is dry, the connection between the condenser and adsorber is closed. In this condition, the system can keep the stored energy for an indefinite time since the thermal energy is stored as adsorption potential between adsorbate and adsorbent material. To recover the stored thermal energy the connection between the liquid adsorbate reservoir, which in this phase acts as an evaporator, and the adsorber is again opened. During this discharging phase, the adsorbate is evaporated adsorbing heat from the ambient, \(Q_{exp}\), or providing a cooling useful effect to a specific user. Then, the vapour fluxes to the adsorber and, since the adsorption process is exothermic, the heat stored is released to the load. It is self-evident that in this heat storage system, the adsorbate is continuously condensed/evaporated in a closed device without any mass exchange with the ambient; this feature ensures the proper functioning of the adsorption TES system also in dry climates. Figures 2 and 3 depict an example of the proposed concept integrated into an industrial context. It consists of the coupling of a Concentrating Solar Field (CSF) with advanced sorption storage (which uses water as refrigerant). As previously explained, the phase changer acts alternatively as an evaporator or condenser during the different phases. Indeed, during the operation of the system, two distinct phases can be distinguished:  
* Charging phase: the heat provided by the solar field, at the maximum temperature level in the system, feeds the sorption storage system and the high-temperature (HT) user into the industrial process. In the sorption storage system, the adsorbent material  
Figure 1: Working phases of a closed adsorption heat storage cycle: (**a**) charging phase and (**b**) discharging phase [19]. is heated and desorbed until it reaches the maximum temperature. The refrigerant desorbed (water) is condensed in the phase changer and the heat of condensation is released in the ambient through an air/water cooler. * Discharging phase: when the solar heat is not available anymore, the system operates as depicted in Figure 3. The heat absorbed by the HT user cools down the storage. When the reactor is cooled until its pressure drops below the phase changer pressure, the adsorbent material begins adsorbing refrigerant vapour from the phase changer (acting as an evaporator) and releases the stored heat.