Effects of Chemical Composition and Controlled Cooling on the Phase Transition in Coconut Oil Bio-Phase Change...

Vegetable-oil-based phase change materials (bio-PCMs) are attractive for thermal energy storage due to their renewability, low cost, and high latent heat. However, their adoption is hindered by polymorphism, the ability of triacylglycerol molecules to form multiple crystalline arrangements during solidification. Each polymorph exhibits distinct melting ranges, enthalpies, and metastability, making the prediction of thermophysical properties challenging for the energy-materials community, which often treats PCMs as single-transition systems. This work demonstrates how chemical composition and polymorphic state fundamentally govern the phase transition behavior of coconut oil, using a combination of controlled T-history experiments and composition-based thermophysical modeling. A modified T-history apparatus with precise control over heating and cooling rates was developed, enabling the quantification of subcooling and assessment of how thermal history influences the material's crystallization behavior. After validation with n-eicosane, the system was applied to chemically characterized coconut oil samples, and the resulting thermal responses were interpreted considering thermodynamic predictions for the enthalpies associated with the different polymorphic states. Coconut oil was selected as a model lipid system given the extensive compositional information available in the literature, allowing the mechanistic insights obtained here to be extrapolated to other vegetable oils when chemical characterization is available. By integrating fatty-acid and triacylglycerol profiles with T-history data, we show that (i) coconut oil exhibits subcooling levels that depend on both cooling rate and composition, though remaining limited (≤1 °C); and (ii) controlling crystallization conditions influences the predominance of specific polymorphs, enabling effective tuning of the transition temperature range and latent heat. Coconut oil also displayed stable thermophysical behavior over repeated heating/cooling cycles (<5% deviation), confirming its thermal stability. Overall, this study clarifies a key knowledge gap by demonstrating why vegetable oils cannot be treated as simple single-transition PCMs and how their phase change behavior can be rationally controlled. These insights establish a pathway for engineering reliable lipid-based PCMs through compositional and polymorphic design.