Utilizing energy storage technologies is beneficial for bridging the gap between supply and demand of energy and for increasing the share of renewable energy in the energy system. Phase change materials (PCMs) offer higher energy density and compact storage design compared to conventional sensible heat storage materials. Over the past years, poly(ethylene glycol) (PEG) has gained attention in the PCM field, and several new composites of PEGs have been developed for thermal energy storage purposes. PCMs are investigated at a given heating/cooling rate to evaluate their phase change temperature and enthalpy. In the case of PEG, some molecular weights show a melting behavior that depends on the thermal history, such as the crystallization conditions. This study investigates the relationship between the molecular weights of PEGs (400−6000 g/mol), cooling/heating rates, and the behavior during phase transitions, to evaluate the performance of PEGs as a PCM under various thermal conditions. Experiments were performed using differential scanning calorimeter and the transient plane source method. All PEG molecular weights were subjected to the same cooling and heating conditions, cooling and heating rate, and number of cycles, to decouple the thermal effects from molecular weight effects. The behavior of phase transition for different thermal conditions was thoroughly analyzed and discussed. It was found that the melting temperature range of PEGs with different molecular weight was between 5.8 and 62 °C (at 5 °C/min). Each PEG showed unique responses to the cooling and heating rates. Generally, the behavior of the crystallization changed most between the thermal cycles, while the melting peak was stable regardless of the molecular weight. Finally, it is recommended that the characterization of PEGs and their composites should be conducted at a heating and cooling rate close to the thermal conditions of the intended thermal energy storage application.

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