## 2 Calorimetric Analysis, Aging and Compatibility Studies on Industrial-Grade Fatty Acid Mixture
### Aging of SA-PA Mixture  
The SA-PA mixture's thermal stability was assessed through long-term exposure to temperatures above its melting temperature, at levels that could be encountered in an  
Figure 1: SA–PA mixture thermal analysis: (a) DSC determination of transition temperature and enthalpy (Setaram DSC131, sample mass 32.324 \(\pm\) 0.004 mg), (b) DSC cycling (Perkin-Elmer Pyris Diamond, sample mass 11.080 \(\pm\) 0.004 mg) and (c) TG analysis (Setaram SETSYS).  
industrial context. PCM samples in hermetically sealed flasks were put in three stoves and monitored at the following temperatures: 65 \({}^{\circ}\)C, 75 \({}^{\circ}\)C and 85 \({}^{\circ}\)C. The properties were examined after 7, 14, 21 and 28 days, each measurement being conducted on a dedicated flask to avoid sample pollution. The standard operating conditions involve stability at 10 \({}^{\circ}\)C above the melting temperature, while isolated monitoring accidents could expose the PCM at higher temperatures for shorter periods. Those accidental exposure durations are difficult to predict, but as an order of magnitude, a 20 min exposure per week would lead to approximatively 15 days of high-temperature exposure throughout the 20 years of a system's lifetime. The aging studies were then conducted using such durations.  
As a first step, a visual check of the samples in the oven was carried out to highlight the possible impact of the temperature and duration of the experiment on the PCM. Regardless of the temperature to which the sample was subjected, no degradation of the material could be visually observed.  
The second step consisted of a calorimetric analysis of the sample with Setaram DSC 131 following the protocol detailed in paragraph 2.1. The results, presented in Figure 2, show the evolution of key thermo-physical properties as a function of exposure time at the selected temperatures: 65 \({}^{\circ}\)C (black line with circles), 75 \({}^{\circ}\)C (gray dotted line with squares) and 85 \({}^{\circ}\)C (blue dotted line with triangles). Uncertainty has also been added to the graph.  
A disparity in latent heat values between the three temperatures is visible in Figure 2 after seven days, with 190 J/g for the sample placed at 65 \({}^{\circ}\)C, 182 J/g for the sample placed at 75 \({}^{\circ}\)C and 176 J/g for the sample placed at 85 \({}^{\circ}\)C. However, this difference remains at a value within the error interval throughout its thermal exposure, at all studied temperature levels. On the contrary, regarding the graph presenting the onset temperature as a function of time, differences are more noticeable between the samples. If the melting onset temperatures of the PCM placed at 65 \({}^{\circ}\)C and 75 \({}^{\circ}\)C are constant over time, the samples subjected to a temperature of 85 \({}^{\circ}\)C show a reduction of almost 2 \({}^{\circ}\)C between the reference time (D1), where the PCM is unaged, and the 28 th day (D28).  
Figure 2: SA–PA mixture properties’ evolution over constant temperature exposure: latent heat of fusion (**top**) and onset temperature (**bottom**).  
Thus, the SA-PA mixture has a satisfying thermal stability when used up to 75 \({}^{\circ}\)C, but the operating conditions should avoid prolonged use at 85 \({}^{\circ}\)C and above, in order to maintain the initial melting onset temperature.