## 1 Introduction  
The use of solar energy for thermal applications can lead to significant reductions in the consumption of fossil fuels for both industrial and domestic use [1, 2]. However, the variability of solar energy production represents a challenging issue [3]. The adoption of thermal storage can help to mitigate the daily solar power fluctuations, allowing to accumulate excessive energy during the peak production for later use [4]. In fact, storage allows to increase the efficiency of the solar systems, ensuring greater exploitation of this resource [5]. For this reason, many experimental and numerical studies were performed in the recent years with the aim of investigating the advantages of different thermal energy storage configurations for solar applications [6, 7, 8, 9, 10]. The three main technologies of thermal energy storage are: sensible, latent, and thermo-chemical heat storage. The sensible heat storage is characterised by the temperature increment of a material compared to a reference temperature. This material can be both solid and liquid and generally, liquid water is used thanks to its low cost [11]. However, due to low energy density, heat losses and low-temperature operative range (50-100 \({}^{\circ}\)C), it is preferable to adopt sensible heat storage for domestic applications and without solar concentration [12]. In latent heat storage, solar heat is accumulated due to the phase change latent heat and generally, solid-liquid transition is adopted. Compared to sensible heat storage, it is possible to obtain higher energy density and lower temperature fluctuations [13; 14]. Nevertheless, external heat losses, slow phase change kinetics and high specific volume variations limit the adoption of this technology [15]. Thermo-chemical heat storage is based on physical or chemical reversible reactions between two materials. This process involves solid-gas, liquid-gas and gas-gas reactions, which are characterised by very slow kinetics. The physical reaction, like absorption and adsorption, requires lower charging temperature compared to chemical ones and consequently, exhibits faster kinetics [16; 17; 18]. Moreover, typical temperature values are in the order of 50-200 \({}^{\circ}\)C, making these technologies a suitable choice for industrial applications. In Reference [19], [PERSON] et al. compared different adsorption heat pumps, underlying advantages and disadvantages of the different technologies. In Reference [20], [PERSON] et al. described the state-of-the-art of the adsorption heat storage, reporting the features for different adsorbent materials. In particular, zeolite and zeo-like materials characteristics are reported, underling the excellent compatibility with water vapour. Furthermore, the high energy density and operative temperature (i.e., more than 150 \({}^{\circ}\)C) promote the adoption of adsorption zeolite thermal storage for industrial applications together with the utilisation of concentrated solar power. Consequently, in the recent years, many studies focusing on vapour -zeolite adsorption thermal storage were performed. In Reference [21], [PERSON] et al. estimated heat losses of an experimental apparatus characterised by a closed unit thermal energy storage filled with 10 kg of zeolite 13X. They found high thermal efficiency (higher than 90%), both for charging and discharging phases. In Reference [22], [PERSON] et al. developed a one-dimensional lumped-parameter model for the simulation of both short- and long-term energy storage of a zeolite adsorption thermal storage. In Reference [23], [PERSON] et al. reported and described different configurations of sorption thermal storage for solar energy, highlighting the capabilities of zeolite as adsorbent material for long-term applications. In Reference [24], [PERSON] et al. carried out an experimental and numerical analysis of a combined hot water and sorption store for solar applications using zeolite as adsorbent material.