## 2 Classification of Thermal Storage Systems  
Storage technologies can be classified with respect to underlining heat storage principle into: sensible, latent and thermochemical [82; 111]. Sensible thermal energy storage (STES) is based on storing thermal energy by cooling or heating of a liquid/solid storage medium. Sensible heat determines a temperature linear change (increase or decrease) in the thermal storage material, without changing its chemical composition or phase. Sensible heat Q\({}_{\text{s}}\) depends on the temperature change and the specific heat capacity of the storage material. The amount of energy stored (J) is as followed (1):  
\[Q_{s}=mc_{p}\Delta T \tag{1}\]  
where:  
* \(m\) is the mass of the storage medium (kg);
* \(c_{p}\) is the heat capacity of the storage medium (J/(kg K));
* \(\Delta T\) is the temperature difference (\({}^{\circ}\)C). It is important for sensible heat storage systems to use a heat storage material having high specific heat, good thermal conductivity, long-term stability under thermal cycling, compatibility with its containment, recyclability, a low CO\({}_{2}\) footprint and low cost [112]. Sensible heat storage is most widely used in building applications [30]. Latent thermal energy storage (LTES) is based on storing heat into a storage medium undergoing a phase transition [113]. Thermal storage materials store their latent heat during phase change from solid to liquid. The latent heat is stored without a temperature change. The amount of energy stored (J) is as followed (2):  
\[Q_{l}=m\Delta t \tag{2}\]  
where:  
* \(\Delta t\) is the melting or phase change enthalpy (J/kg). Micro-encapsulated paraffin based phase change materials PCMs or water-based ice-storage are among methods most suitable can be used [114]. Thermochemical or sorption thermal energy storage (TCTES) recovers the reaction enthalpy involved in a reversible chemical/adsorption reaction [115]. According to [PERSON] et al. [36] the chemical reaction takes place between a sorbent, which is typically a liquid or solid, and a sorbate, which is, e.g., a vapor. During the charging process, a heat source is used to induce an endothermic reaction, the sorbent and sorbate are separated. The chemical/physical energy of the two components can then be stored separately. During the discharging process, an exothermic reaction occurs and heat stored is recovered. Characteristics of Thermal Storage Systems  
The following features can be used to characterize an energy storage system [116; 117; 21]:  
* Storage period defines how long the energy is stored (i.e., hours, days, weeks);* Power defines how fast the energy stored in the system can be charged and discharged. In particular, power capacity (W) is the maximum amount of power that can be delivered by the storage system during discharging while Power density (W/l) is the ratio between the power capacity and the capacity of the energy storage system;
* Energy storage capacity or energy capacity is defined as the amount of energy absorbed in the storage system during the charging process under nominal conditions. The quantity of stored energy in the system after it is charged depends on the storage process, storage medium and size of the system;
* Energy density or volumetric heat capacity is defined as the ratio between the stored energy and the volume of the energy storage system;
* Charge and discharge time defines how much time is needed to charge or discharge the system. The maximum number of charge-discharge cycles in the specified conditions is defined as the cycling capacity or number of cycles;
* Self-discharge is the amount of energy initially stored and dissipated over a specified non-use time;
* Efficiency is the ratio of the energy provided to the user to the energy needed to charge the storage system.