# Stability Study of Erythrhitol as Phase Change Material for Medium Temperature Thermal Applications
## 4 Conclusions  
Sugar alcohols belong to a promising category of organic phase change materials. Erythiritol is an attractive sugar alcohol due to its thermal energy storage properties. However, earlier studies indicate that erythroid tends to degrade under thermal cycling. Most of the available studies report the structural changes of erythroid during cycling rather than its thermal stability at constant temperature, which would allow a degradation rate to be determined under conditions closer to real operation. In this study, the effect of thermal treatment above the melting point on the erythroid thermal, chemical and physical properties, as well as its degradation kinetics and the likely thermal stability enhancement by adding antioxidant and using it under inert atmosphere, were addressed.  
The latent heat of erythroid under air decreased with the increasing temperature of thermal treatment as its hydrogen bonding structure was likely disrupted at high temperatures. In addition, a decrease in latent heat may also be attributed to incomplete crystallization during cooling from the melt.  
Erythritol under air changed its color and some samples remained liquid after thermal treatment. Sample browning upon heating could be related to the formation of conjugated carbonyl compounds during oxidation/dehydration processes. Pure erythroid under air underwent glass transition as no exothermic peak was observed in its DSC thermogram during cooling. Erythiritol under argon atmosphere, on the other hand, did solidify and its color barely changed.  
Fragmentation products generated during oxidation and carbonyl compounds, among others, generated during dehydration increased the mass loss ratio in samples of pure erythiritol under air. Antioxidant helped to reduce this effect.  
FT-IR spectra of erythroid under air showed the occurrence of carbonyl groups and conjugated carbonyl compounds after thermal treatment. FT-IR spectra of erythroid under argon did not show new absorption bands, which confirmed its chemical stability after being heated above its melting temperature and up to 141 \({}^{\circ}\)C.  
Minimal physical changes regarding color and consistency were observed in pure erythiritol under argon atmosphere after being thermally treated, thus physical stability could be confirmed for up to 935 h of treatment. Finally, the degradation rate of latent heat was reduced from 0.19 kJ/(kg h) to 0.011 kJ/(kg h) when using an inert atmosphere.  
Conceptualization, M.P.A.L.; methodology, M.P.A.L.; investigation, M.P.A.L.; resources, M.P.A.L.; data curation, M.P.A.L.; writing--original draft preparation, M.P.A.L.; writing--review and editing, H.N. and S.G.; visualization, M.P.A.L.; supervision, H.N.; project administ  
Figure 11: FT-IR spectra of the long-term-tests for (**a**) erythiritol with antioxidant under air; (**b**) pure erythroid under argon atmosphere.  
tion, S.G.; funding acquisition, S.G. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the German Federal Ministry of Economics and Energy via several research projects, grant number 0325549A.  
Not applicable.  
The data presented in this study are available on request from the corresponding author. The data are not publicly available due to restrictions given by partners.  
The authors are grateful to the Project Management Jollich for the administrative support.  
The authors declare no conflict of interest.