## 1 Introduction
In particular, the aim is to provide a comprehensive review on the state of art of thermochemical thermal energy storage systems (TCTESs) and their applications in PtH technologies, including theoretical, experimental and numerical studies. Recent advancements and their potential perspectives will be discussed. This review is structured as follows. In Section 2 a classification of storage system is reported. Section 3 is a review of the state of art of both sorption and chemical reaction TCES processes and the related main operation processes. Section 4 includes a general description of PtH technologies and an analysis of recent case studies on the application of TCTES systems. Section 5 presents the conclusions of this paper. _Power-to-Heat Technologies: Classification_  
Power-to-heat (PtH) is the classification including all devices that perform the conversion of electricity into heat. It is one of the most relevant flexibility options of the DSM [63; 64]. With the aim to ensure the integration of the renewables, PtH technologies (PtHs) are considered crucial sources of system flexibility [65]. PtHs contribute to both a better utilization of existing assets and use of temporary renewable surplus generation [65]. When there is an excess of generation, electricity is converted into heat, in this way, additional power in the situations of increased load, is provided contributing, in the same time, to peak shaving, load shifting and energy conservation [66]. Turning surplus of electricity into heat, including thermal energy storage, offers a significant additional flexibility with a great potential in stabilizing the power grid [67; 68]. The conversion into thermal energy can be performed through centralized and decentralized options. According to [PERSON] et al. [69] in the centralized option the electricity is converted into heat at a location far from the point of actual heat demand. Bydistrict heating systems (DHS) heat is distributed through pipelines to its use. In contrast, according to [PERSON] et al. [70] in the decentralized approach the conversion is in a point right or very close the location of heat demand. Heat is distributed without districting networks. A schematic example of the power-to-heat concept is shown in Figure 1:  
DHS are considered particularly promising due to several advantages in energy production, distribution and consumption, especially for space heating applications [71; 72]. In particular, the systems using RES have the advantage that renewable technologies can be placed on the energy supplier side in the actual distribution network or be installed on individual buildings [73; 74; 75]. In literature, district heating networks are commonly described as one of the most effective solution towards a low-carbon feature [76; 77; 78; 79; 80; 81]. [PERSON] et al. [12] in a review of nearly four hundred studies on energy flexibility showed that the interaction between the electricity and district heating sectors is a promising option for increasing energy system flexibility. Heat pumps (HPs) and resistive heaters are the main centralized technologies to draw electricity from the grid to generate heat to be connected to the thermal storage [73]. According to [PERSON] et al. [70] in the decentralized approach the conversion occurs at a site very close to the location of heat demand without networks, grids and piping. The decentralized technologies have several advantages in energy production, distribution and consumption, in particular, providing a sustainable, economical and future-proof solution for heating large spaces [82; 83]. A common classification of the decentralized options is among technologies combined with thermal energy, referred as thermal energy storage coupled heating, and technologies without energy storage, referred as direct heating [26]. Heat pumps, resistive boilers, smart electric thermal storage, fans, radiators are examples of the more widely used decentralized power-to-heat technologies [63; 84; 85; 86; 87].