# Stability Study of Erythrhitol as Phase Change Material for Medium Temperature Thermal Applications
## 3 Results and Discussion
### Latent Heat Degradation  
As described in Section 2.4, each sample was analyzed in triplicate and the obtained data were averaged. The standard deviation of each sample was estimated from the calculated average and presented as error bars. Figure 3a depicts the results of the latent heat degradation for the short-term experiments at 141 \({}^{\circ}\)C. Pure erythroid under air suffered the highest latent heat loss over time. A latent heat reduction of 22.7% regarding the initial value was registered. Adding antioxidant helped to improve the thermal stability of samples under air. However, due to the fact that enthalpy is an extensive property, its magnitude is proportional to the amount of the substance that reacts. Therefore, the latent heat value is reduced by the percentage of added antioxidant. The enthalpy of fusion of samples with antioxidant will then be lower than the value of the polyol itself, as it is also shown in Figure 3a. Both experiments under argon atmosphere registered lower latent heat reductions in comparison with erythroid under air. Pure erythroid and erythroid-antioxidant lost 2.8% and 2% of their initial enthalpy values by the end of the test, respectively. Registered latent heat values were fitted in a linear regression in order to get the degradation rates for all experiments (Figure 3b). An upward trend in the latent heat degradation rate regarding heating temperature was registered for all experiments, with the highest degradation rates for pure erythroid under air. Samples with antioxidant under air showed lower latent heat degradation rates than pure erythroid samples. Latent heat degradation rates of 0.13 \(\pm\) 0.030 kJ/(kg h), 0.14 \(\pm\) 0.024 kJ/(kg h) and 0.15 \(\pm\) 0.021 kJ/ (kg h) were registered for samples at 121 \({}^{\circ}\)C, 131 \({}^{\circ}\)C and 141 \({}^{\circ}\)C with antioxidant under air,respectively. This represents about 3 to 4 times less than samples without any additive and under air. Thus, it seems that the temperature rise in combination with antioxidant did not influence the degradation rates of samples under air as much as those of the samples of pure erythroid under air. However, the degradation rate during the long-term test starts to increase from 260 h onwards to a value of 0.19 \(\pm\) 0.005 kJ/(kg h), which could be due to the gradual consumption of antioxidant during the experiment of 935 h (Figure 4a). Likewise, Figure 3b shows that lower degradation rates were obtained for both experiments conducted under argon atmosphere, the highest one being 0.06 \(\pm\) 0.02 kJ/(kg h) for tests at 141 \({}^{\circ}\)C. Since the latent heat of fusion recorded from tests under an argon atmosphere decreased less than under air, the slopes of linear fittings were close to zero, resulting in a high standard deviation of the slope. In addition, Figure 4a depicts how the latent heat of fusion of the samples under an argon atmosphere decreased less by the end of the long-term test than those with antioxidant under air, with an average degradation  
Figure 4: (**a**) Evolution of latent heat values over time during long-term experiments at 141 \({}^{\circ}\)C. (**b**) Latent heat degradation rate for long-term experiments at 141 \({}^{\circ}\)C.  
Figure 3: (**a**) Evolution of latent heat values over time during the short-term experiment at 141 \({}^{\circ}\)C. (**b**) Latent heat degradation rate for short-term tests at three different temperatures. rate of only 0.01 \(\pm\) 0.001 kJ/(kg h), as shown in Figure 4b. This indicates that an anoxic environment diminishes the erythroid degradation for exposure times up to 935 h.  
Latent heat decrease of sugar alcohols after heating has previously been reported [30; 31]. It is suggested that the hydrogen bonding structure of erythiritol can be weakened and eventually destroyed by temperature increasing, which in turn may cause a decrease in its latent heat [32; 33].