Thermomechanical characterisations of PTFE, PEEK, PEKK as encapsulation materials for medium temperature solar applications

Abstract Macroencapsulation of phase change material (PCM) is one of the enhancement techniques to maximise heat transfer area between PCM and the surrounding heat transfer fluid (HTF). However, the selection of suitable encapsulation material, that can address the volumetric expansion problem and does not react with the core PCM, while being stable at elevated temperature, possesses a challenge. In this work, the thermo-mechanical characterisation of polymeric material is reported, which can be used for medium temperature (200–350 °C) solar applications. Three materials, such as polytetrafluoroethylene (PTFE), polyether ether ketone (PEEK) and polyether ketone ketone (PEKK) are chosen as an encapsulation material for storing a commercially available organic PCM, A164 in the capsule. Capsules of three materials are fabricated and the mechanical properties of the materials are evaluated after every 10 accelerated thermal cycles near the degradation temperature of the materials to understand their behaviour in extreme conditions. Dimensional and weight analyses of the macro encapsulated capsules is also performed to gauge any change in their physical properties. Thermo-mechanical properties show that the polymers chosen can be used as an encapsulation material for solar applications. Dimensional and weight analysis indicate no significant change in the properties of the polymers. High-temperature stability of these polymers even for extended duration indicates a maximum of 3 wt% change only. The compression test on these materials reveals that the PEKK can be used up to 275 °C with Young's modulus decreases to 0.4 GPa. These results would form the ground for their applications as PCM-encapsulating materials targeting medium temperature solar applications.