Thermal and Chemical Stabilities of Organic Phase Change Materials: Paraffin, Beeswax, and Polyethylene Glycol Using Thermal Cycling Method for Waste Heat Recovery Applications

The growing global energy demand and depletion of fossil fuels have intensified the need for efficient thermal energy storage (TES) systems. Waste heat recovery (WHR) offers a promising solution, particularly through organic phase change materials (PCMs) such as paraffin, beeswax, and polyethylene glycol (PEG 6000), which exhibit high latent heat capacity and favorable phase-transition behavior. A major challenge, however, is ensuring their long-term thermal and chemical stability under repeated cycling. In this study, the durability of three organic PCMs was assessed over 500 heating-cooling cycles to simulate extended WHR operation below 100°C. A custom cycling apparatus integrating a thermoelectric cooling (TEC) module with active water cooling was developed to improve heat dissipation and maintain precise temperature control. Differential scanning calorimetry (DSC) results showed that paraffin experienced the smallest reduction in latent heat, decreasing from 173.7 J/g to 162.75 J/g, while beeswax exhibited a greater decline from 206.1 J/g to 173.3 J/g. Thermogravimetric analysis (TGA) indicated initial decomposition temperatures of 244-358°C, with only slight reductions after cycling: 3.19% for paraffin, 0.7% for beeswax, and 0.5% for PEG 6000, confirming excellent thermal stability. FT-IR spectra showed no chemical degradation. Thermal cycling performance further demonstrated that paraffin achieved the fastest cycle time (257 s; 14 cycles/h), followed by beeswax (315 s; 12 cycles/h) and PEG 6000 (380 s; 10 cycles/h). Overall, the results confirm that these PCMs retain strong thermal and chemical stability, supporting their suitability for long-term WHR-based TES applications below 100°C.