Effect of d-mannitol polymorphism in its thermal energy storage capacity when it is used as PCM

• d-mannitol presents polymorphic transformations which have different melting points.
• d-mannitol polymorphism depends upon the thermal history of the material.
• Polymorphic transformation must be considered when using d-mannitol as PCM.
• The considered working range of d-mannitol as PCM is 135 °C and 175 °C.

The main objective of this paper is to study the possible use of d-mannitol as phase change material (PCM) for thermal energy storage. PCM are materials that have high phase change enthalpy and this thermophysical property gives them the ability to store energy as latent heat. d-mannitol is a material which has different morphological phases (polymorphism); here were studied β-form and δ-form. Different polymorphic forms produce changes on melting point of d-mannitol. For this reason it is necessary to establish a suitable working temperature range for the use of d-mannitol as phase change material. The thermal characterization was performed with DSC analysis using 0.5 K min−1 slow-dynamic method. Polymorphism analysis of d-mannitol was analyzed to associate the thermal behavior obtained by DSC with a specific polymorphic phase. d-mannitol presented three different thermal behaviors: the first one had a melting peak at 167 °C, the second was a double melting peak at 155 °C and 166 °C, and the third a single peak at 155 °C. Due to irregular results, two working range were studied and through the thermal characterization, it was possible to define a working range where d-mannitol could be used as PCM for energy storage: this range is between 135 and 175 °C. Furthermore, it was possible to differentiate two crystalline phases of d-mannitol applying FT-IR analysis and to link them with thermal behavior observed in DSC. The percentage of times each thermal behavior is observed in DSC analysis was calculated. δ-form is obtained 15.8% of analyzed cycles, the β-form appears 44.7% of times, and an intermediate transition between the two phases is found 39.5% of cycles.