Insights on the degradation of capric acid as phase change material for long-term applications

In this work, a protocol for evaluating the long-term thermal and chemical stability of capric acid is presented. Capric acid is a phase change material (PCM) employed as a latent heat thermal storage material to store/retrieve heat during the phase transition from solid to liquid or vice versa. The nature of the materials utilized for latent heat thermal batteries plays a vital role in their long-term performance for the different applications. Therefore, precisely investigating these material thermal and chemical stability is significant. The protocol consists of two tests (accelerated thermal cycling and thermal stress) in which the performance of the PCM will be investigated. A benchmark experiment for accelerated thermal cycling was performed where the impact of different parameters (heating/cooling rate and sample volume) was investigated for the initial 50 thermal cycles. This was followed by long-term accelerated thermal cycling for a total of 1500 thermal cycles (melting followed by crystallization). A thermal stress where the PCM stays in a liquid state was performed for the capric acid to investigate the impact of the higher temperature on the stability of the PCM. The thermal properties reliability over time was measured with differential scanning calorimetry (DSC), which indicated a variation of 3.2 % in onset phase change equilibrium temperature and 10.45 % in phase change enthalpy after 1500 thermal cycles. Gas chromatography (GC) and nuclear magnetic resonance spectroscopy (NMR) were performed to reveal that by-products are formed during capric acid thermal treatment. These results provide valuable guidance for the research community to design adapted thermal cycling protocols and select appropriate characterization methods, thereby enabling more accurate and reliable investigations of the long-term stability of organic PCMs. • Accelerated thermal cycling and Thermal stress test for capric acid • Investigated the impact of heating/cooling rate and volume of sample • After 1500 thermal cycles the variation in T m(onset) and ∆ fus H was 3.2 % and 10.45 % respectively • By-products formed during the thermal cycling were detected with NMR and GC. • The by-products were identified as Octanoic acid, heptly ester, and Heptadecane.