## 1 Introduction  
One of the key elements to optimize the use of renewable energies and to improve building performances is the development of thermal energy storage [1; 2; 3; 4; 5]. To this purpose, latent heat storage using phase change materials (PCMs) is known to provide a greater energy density than the sensible heat storage with a smaller temperature difference between the energy charge and the energy discharge. [PERSON] et al. [6] reviewed many of PCM complying with this application as well as the eventual associated technical problems as separation, segregation, corrosion, and materials compatibility. Among inorganic systems such as salts and salt hydrates, organic compounds such as paraffin waves, fatty acids or esters and polymeric materials [3; 4; 5], sugar alcohols (SAs) are promising phase change materials for seasonal energy storage applications at low to medium temperatures (below 100 \({}^{\circ}\)C) and allow avoiding these technical problems. These bio-based glass-forming materials have been up to now widely studied in food engineering and pharmaceutics [7; 8]. Recently, they have been also studied for thermal energy storage applications [9; 10; 11; 12; 13; 14]. The idea consists in storing thermal energy into undercooled sugar alcohols at a temperature inferior to their melting one, as close as possible to the ambient temperature, to reduce thermal losses and to limit the risk of spontaneous energy discharge. When heating is needed, the storage system is discharged by triggering sugar alcohols nucleation, which is the first step of crystallization. In spite of the fact that Xylitol has been identified as one of the most promising candidates among sugar alcohols for applications at temperature below 200 \({}^{\circ}\)C due to its high energy storage potential, few studies only have been conducted on it, as reported in [15; 16]. Its melting point of 95 \({}^{\circ}\)C, its latent heat superior to 263 J\(\cdot\)g\({}^{-1}\) and its total energy density 4-5 times higher than that of water (110-150 kWh\(\cdot\)m\({}^{-3}\) whereas it is approximately 30 kWh\(\cdot\)m\({}^{-3}\) for water on a seasonal basis) are well suitable for their use in cheap solar collectors for instance. However, the strong undercooling behavior of Xylitol as well as its nucleation triggering which is very difficult due to the very high activation energy required for atomic rearrangements at the liquid-solid interface (i.e., the energy barrier to trigger the nucleation) are seen as the major obstacles to their employment as PCM. A decrease in temperature causes indeed a drastic increase in viscosity making the atom diffusion slow down or even stop. This is the reason why [15], among others, stipulated the impossibility to solidify pure Xylitol once at liquid state and thus, stopped their investigation about this material. In the case of undercooled Xylitol, the crystallization step is thus a challenging process since it has a very high viscosity at ambient temperature (see the evolution of its viscosity versus temperature in [17]). Indeed, the transition from liquid to solid comes with a conformation change (from a linear structure in the liquid phase, to a bent one in the solid phase) which increases the required energy to overcome the activation energy barrier. These are the reasons why Xylitol crystal growth rates are low at ambient temperature. The rate values were measured and calculated according to temperature and presented in [18]. The maximum growth rates appear around 50-55\({}^{\circ}\)C and are of the order of magnitude of \(\upmu\)m\(\cdot\)s\({}^{-1}\). In this work, it shown that these characteristics are, on the one part, an asset for a medium/long term storage. Indeed, the probability for spontaneous nucleation is negligible at high undercoolings which leads to a negligible risk of losing the stored energy and limited thermal losses.