## 3 Methodology
### Cycling Stability  
The cycling stability is studied to detect changes in the thermophysical and chemical properties of the PCM after a certain number of melting and freezing cycles. In this study, the cycling stability testswere divided in two steps. First, the sixteen PCMs were subjected to complete cycling tests during 100 cycles. Once the PCM underwent the required number of cycles, the thermophysical and chemical properties were evaluated. Notice that based on previous experiences, the cycling stability changes normally start during the first thermal cycles due to thermal degradation or after many cycles due to thermal stress. The cycling tests were performed using a Venticell oven (Comfort line) from the MMM Group, Munich, Germany, with the air flap lever closed. Due to the fact that in the present study the authors wanted to simulate real operating conditions, samples of 150 \(\pm\) 20 g of the sixteen PCMs were closed with aluminium foil in a glass recipient. This means that the materials were in contact with air during all the tests. Two separate cycling tests were designed, according to the melting temperature reviewed in the literature, to avoid degradation at higher temperatures and to ensure the entire phase change transition during the cycling procedure. The first cycling test identified the PCMs with phase change temperatures between 120 and 150 \({}^{\circ}\)C: sebacic acid, HDPE, benzoic acid, phtalic anhydride, urea, benzamide, maleic acid, and dimethyl terephthalate. The second cycling test identified the PCMs with phase change temperatures between 150 and 200 \({}^{\circ}\)C: adipic acid, DMPA, D-mannitol, benzanilide, potassium thiocynate, dulcitol, hydroquinone, and salicylic acid. A dynamic cycling method was established for both tests at a constant heating and cooling rate of 3.2 \({}^{\circ}\)C/min. In order to evaluate the cycling stability and quantify the variation of the thermophysical and chemical properties of all the PCMs, three different measurement points were established:  
* Measurement 1: 0 cycle;
* Measurement 2: 10 th cycle;
* Measurement 3: 100 th cycle. The thermophysical properties studied in this paper were the phase change temperature and phase change enthalpy. These properties were evaluated using a DSC 822e from Mettler Toledo, Columbus, OH, USA, under a nitrogen atmosphere, which has an uncertainty of \(\pm\)0.1 \({}^{\circ}\)C for temperatures and \(\pm\)3 kJ/kg for enthalpies. The methodology followed a two-cycle program. A first melting and solidification cycle at 10 \({}^{\circ}\)C/min rate was performed to ensure good contact with the crucible base. The second cycle was performed at 0.5 \({}^{\circ}\)C/min rate, and the mean values of temperature and enthalpy were calculated. Two temperature ranges were evaluated following the same criteria used in the cycling tests, i.e., a temperature program which ranged from 90 to 150 \({}^{\circ}\)C for the PCM with a melting temperature lower than 150 \({}^{\circ}\)C and a temperature program which ranged from 150 to 200 \({}^{\circ}\)C for the remaining PCMs. Three samples of each material which were placed in 40 \(\mu\)L aluminum crucibles (maximum filled with two thirds of their volume, with an average sample mass of 10 mg, to avoid oxidation and taking into account volume expansion), were analysed to ensure repeatability of the tests. The chemical characterization was carried out using a Fourier transform infrared (FT-IR) spectroscopy with attenuated total reflectance (ATR), which analyses the PCM chemical degradation caused by thermal cycling. The advantage of ATR is the possibility of obtaining the spectra directly from the sample, without any specific sample preparation. The partial or total disappearance of the characteristic peaks and/or the appearance of new peaks can indicate that the material is being oxidized or degraded. This analysis was carried out with a Spectrum Two(tm) from Perkin Elmer, Waltham, MA, USA. It allows analysing substances in solid and liquid states.