## 4 Results
### Cycling Stability
#### 4.3.1 Thermophysical Characterization (Phase Change Temperature and Enthalpy)  
DSC results show that four out of the sixteen evaluated PCMs did not show solidification under described conditions during the first cycle and, as a consequence, they cannot be considered for latent heat TES purposes. These PCMs are maleic acid, DMPA, salicylic acid, and urea. Hence, from a thermophysical characterization point of view, only twelve out of the sixteen materials can be considered for latent heat storage unit purposes. Errors of 10% for melting enthalpy and 5% for melting temperature are stated as acceptable for the DSC measurements and are represented in the graphics with error bars [17].  
Figure 1 shows the evolution of the phase change temperature (melting Figure 1a and solidification Figure 1b) of the twelve PCMs which presented suitable thermal results at the initial stages of the thermophysical characterization after 0, 10 and 100 cycles. It is important to highlight that at the zero cycle, some PCMs showed remarkable differences between melting and solidification temperatures, like benzamide, benzanilide, D-mannitol, and dulcitol. After 10 cycles, benzoic acid did not show phase change under analysis conditions, and the remaining materials presented variations lower than 6.5%., i.e., D-mannitol and hydroquinone, which had a higher variation in the solidification temperature, with values of 6.2% and 6.1%, respectively. After 100 cycles, dulcitol and hydroquinone did not show phase changes under analysis conditions. Moreover, sebacic acid and D-mannitol showed a decrease in their phase change temperatures in comparison to their initial values, especially at the solidification temperature, with values of 14.3% and 19.5%, respectively. Hence, from the D-mannitol and sebacic acid results, it can be seen that the modification in the phase change temperature and latent heat with increasing number of cycles is noteworthy (more than 20% in the case of latent heat). On the other hand, the thermophysical properties of benzamide, HDPE, dimethyl terephthalate, adipic acid, potassium thiocynate, and benzanilide remained almost constant during all cycling tests. However, after 100 cycles D-mannitol, benzamide and benzanilide showed important temperature differences between solidification and melting processes.  
The evolution of the melting and solidification enthalpies after 0, 10 and 100 cycles of the twelve PCMs, which presented suitable results at the initial stages of the thermophysical characterization, is presented in Figure 2a,b, respectively. At the initial stage, a wide range of enthalpies values were observed, from a minimum value of 100-120 kJ/kg for benzoic acid, to values up to 400 kJ/kg for dulcitol. The remaining PCMs showed phase change enthalpies that ranged between 120 and 280 kJ/kg. Results after 100 cycles showed that melting enthalpies of benzamide, HDPE, dimethyl terephthalate, adipic acid, potassium thiocynate, phtalic anhydride and benzanilide presented variations lower than 15%.  
From the phase change temperature and enthalpy point of view, six PCMs, benzamide, HDPE, dimethyl terephthalate, adipic acid, potassium thiocynate, phtalic anhydride and benzanilide, presented values which were suitable for partial load operation conditions in latent heat TES systems. The variations in these parameters observed along thermal cycles might be due to the degradation of the chemical structure of the PCMs with increasing number of cycles [18]. Hence, the material could not form the first crystal during the solidification process of the thermal cycling, and hysteresis phenomena took place. Throughout a considerable number of phase change transition processes, new compounds were formed, which may have different latent heat than that of the fresh PCMs. In order to deeply analyse the described phenomena, infrared (FT-IR) spectroscopy was carried out.