Feasibility study of D-mannitol as phase change material for thermal storage

The feasibility of D-mannitol as PCM for latent heat storage has been studied by keeping it melted at 180 ºC in air for up to 16 days. During this period of time, down to 80% initial mass was lost and sample appearance changed to a dark-brown-sticky paste. The strong mass decrease implies that not only water but also carbon containing volatile species are produced. These results indicate that D-mannitol undergoes thermal degradation which can be associated to caramelization processes similar to the ones occurring in sugars. Such processes involve complex reactions like dehydrations, condensations and polymerizations, which lead to a great variety of volatile and non-volatile products. Various characterization techniques have been applied in order to study both kinetics and reaction products. From the kinetic point of view thermal degradation of D-mannitol proceeds much faster and at lower temperatures than predicted by the models found in the literature. In relation to the degradation products, the analyses with liquid chromatography indicate the formation of three compounds although their stoichiometry could not be fully elucidated with the characterization techniques applied in this work. However it seems that some of the products are polymeric species that contain carbonyl groups and that are the responsible for the sample browning, the strong absorption in the 200 nm–500 nm range, the two bands at 1730 cm^–1–1645 cm^–1 observed in infrared spectra and also for the occurrence of a glass transition. Therefore this study proves that D-mannitol undergoes strong and fast degradation at temperatures close to its melting point which should be enough reason for avoiding its use as PCM in any latent heat storage application unless it is somehow stabilized