Thermal behaviour of d-mannitol when used as PCM: Comparison of results obtained by DSC and in a thermal energy storage unit at pilot plant scale

The use of thermal energy storage (TES) systems for solar heating and cooling applications has received considerable attention in recent decades because it has a high potential in energy savings. Phase change materials (PCMs) can store large amount of energy per mass unit compared with other TES materials. Nevertheless, the selection of the suitable PCM for each application is a key issue in any TES system design. The most important properties to take into account to select a PCM are the melting and solidification temperature, the phase change enthalpy and the stability after several thermal cycles. In this paper, d-mannitol was a candidate material to be tested as PCM in a solar cooling application due to its melting point (167°C) and a relatively high enthalpy (316.0kJ/kg). The experiments performed by DSC have shown that the d-mannitol presents polymorphic structural changes and, therefore, its thermal properties are not always the same. Depending on the polymorphic phase obtained, d-mannitol has different melting temperature. This behaviour was corroborated in a storage tank, where it may be seen that the cooling rate of the d-mannitol is a key parameter in the formation of the different polymorphic phases.