# Influence of Water Addition on the Latent Heat Degradation of Sodium Acetate Trihydrate
## 3 Aging Study of Sodium Acetate Trihydrate
### Study of the Raw Material (SAT60%)  
To evaluate the potential of the use of water in order to attenuate the aging of the SAT, the behavior of raw material was first evaluated. Therefore, 35 cycles between 20 and 70 \({}^{\circ}\)C were applied to the sample. These cycles may be summarized in Figure 8. It may be noted that a discontinuity appears between the 5 th and 6 th cycle that corresponds to the change of heating and cooling rate. The 1 st cycle of the series was not considered here because of a different thermal history. Besides, some cycles were applied with a different heating and cooling rate (5 th cycle) whose results were used in another study [19]. This led to a different thermal history  
Figure 8: Temperature evolution along the experiment performed on SAT60%. Figure 7: Schematic diagram of SAT aging throughout melting cycles. for the 6 th cycle. Therefore, 1 st, 5 th, and 6 th were not considered in the analysis and plotted as stars in the following graphs to perceive the influence of different experimental parameters. The latent heat of the remaining cycles could be determined from Equation (2), leading to the monitoring of aging. However, some crystallizations that occurred at a lower temperature would influence the latent heat measurement, such as the 34 th cycle of this experiment, which is plotted in Figure 9.  
During this 34 th crystallization, the heat flux was not stabilized and the core of the material may still be cooling as the temperature started to re-increase. As the material remained hot, the energy needed to bring the material back to 70 \({}^{\circ}\)C for the 35 th melting may be lowered. In parallel, it was assumed in the calculation of the sensible heat subtracted (Equation (2)) that the whole material was heated from 20 to 70 \({}^{\circ}\)C. Therefore, the latent heat, determined from Equation (2) during the 35 th melting, may be underestimated. Even if this is an extreme case, the next melting may be impacted by a delay in the solidification of singular points. Besides, a faster solidification was observed when the supercooling degree was larger [29]. This may induce a larger number of porosities [30] and influence its properties and the progression of the melting front on the next heating step. For these reasons, the melting that follows a low-temperature crystallization has been considered as singular data. Before the determination of latent heat, the first step was to measure the crystallization temperatures, based on the procedure given in Figure 3. The histogram of crystallization temperatures is given in Figure 10 with a step of 1 \({}^{\circ}\)C, where the values for the 1 st, 5 th, and 6 th cycles are excluded. It can be seen that the crystallization temperatures are not homogeneous, but they can be approximated by a normal distribution (in red). The mean value \(T_{C,\ mean}\) is here equal to 44.64 \({}^{\circ}\)C and the standard deviation \(\sigma\) is about 0.97 \({}^{\circ}\)C. The crystallization temperatures out of a range of \(T_{C,mean}\pm 3\sigma\) (\(T_{C}<41.7\,^{\circ}\)C) were considered as singular data. In the present case, about 11 over a total number of 32 solidifications (34%) were concerned, after the exclusion of 1 st, 5 th, and 6 th cycles. Figure 9: Singular crystallization at 20 \({}^{\circ}\)C for the 34 th cycle of the experiment. Cryst: crystallization. Melt. : melting. The second step consisted of the latent heat monitoring for these non-singular points. Its evolution throughout cycles is given in Figure 11a. The singular data (open symbols) were separated from other values of latent heat, whose decrease should only be related to aging. However, a fluctuation in the remaining crystallization temperatures (between 41 and 47 \({}^{\circ}\)C) was still noticed, and the latent heat of the next melting may still be influenced. It was assumed from Figure 7 that the quantity of SAT was decreasing throughout cycles.