# Edible Oils as Practical Phase Change Materials for Thermal Energy Storage
## 3 Results and Discussion
### Thermal Properties of Coconut Oil  
The melting/solidification properties of refined (R-CNO) and virgin (V-CNO) coconut oil samples that had undergone up to 200 melt-freeze cycles were determined by DSC. The DSC thermograms after 1, 5, 100, and 200 thermal cycles (Figure 3a,b) showed no significant change in the thermal behavior of either type of coconut oil after the initial melt (Cycle 1) and up to 200 melt-freeze cycles. Note that, when a given sample or an aliquot of a cycled bulk sample of R-CNO and V-CNO was cycled in the DSC (i.e., multiple heating-cooling curves measurements), the thermal behavior of that sample always changed slightly after the first DSC melt (see Figure 3c). The first melting curve for each sample showed a broad endotherm, composed of a major peak and a shoulder peak, extending from \(-7\) to 32 \({}^{\circ}\)C for R-CNO and from 5 to 32 \({}^{\circ}\)C for V-CNO (black curves in Figure 3a,b, respectively), as also observed by others in DSC studies [25; 26]. In the subsequent DSC melting curves of the same sample or aliquot, the two peaks merge into one endothermic peak. Similar results were obtained for samples measured at a slower rate of 2 K/min (Figure S5a,b in Supplementary Materials). Figure 3a,b show that for both types of coconut oil, the endothermic peaks of Cycle 100 and Cycle 200 overlap, but they are slightly narrower than the endothermic peak of Cycle 5. There were no significant shifts in the peak temperature or in the onset temperature after 200 thermal cycles. For the exothermic peaks (cooling curves), the shapes of the peaks were noticeably different for all three cycles shown in Figure 3. However, there were no significant shifts in the onset, offset and peak solidification temperatures after 200 cycles. The differences between the endothermic peaks for different cycles gave rise to a small decrease in \(\Delta_{\text{fus}}H\) and a small shift in \(T_{\text{mpt}}\) for both types of coconut oil. However, over the 200 melt-freeze cycles, these changes remained within the uncertainties of the measurements: for R-CNO, \(\Delta_{\text{fus}}H\) determined from the first cycle was \(127\pm 13\) J g\({}^{-1}\) and the average \(\Delta_{\text{fus}}H\) (determined from 20 heating curves of the same sample) was \(105\pm 11\) J g\({}^{-1}\). After 200 cycles, \(\Delta_{\text{fus}}H\) of R-CNO was 94 \(\pm\) 9 J g\({}^{-1}\). For V-CNO, \(\Delta_{\text{fus}}H\) determined from the first cycle was 110 \(\pm\) 11 J g\({}^{-1}\) and the average (from 20 heating curves) was \(106\pm 11\) J g\({}^{-1}\). After 200 cycles, \(\Delta_{\text{fus}}H\) of V-CNO was 93 \(\pm\) 9 J g\({}^{-1}\). The values agree well with most literature values of \(\Delta_{\text{fus}}H\) determined by DSC in other studies (Table 2). (The exception is the report from reference [25], for which no DSC calibration information was provided. The value from reference [29] also is an outlier but is from \(T\)-history measurements which do not  
Figure 2: DSC thermograms showing heating and cooling curves of vegetable shortening sample measured at 10 K/min for two melt-freeze cycles. give accurate values of \(\Delta_{\text{fus}}H\).) The melting temperatures determined from the DSC measurements at 2 K/min (Figure S5a,b in Supplementary Materials, Cycle 2, peak temperatures) are \(T_{\text{mpt}}=24.4\pm 1.5\,^{\circ}\)C for R-CNO and \(T_{\text{mpt}}=24.0\pm 1.5\,^{\circ}\)C for V-CNO. After 200 melt-freeze cycles, \(T_{\text{mpt}}\) was \(24.0\pm 1.5\,^{\circ}\)C for R-CNO and \(23.9\pm 1.5\,^{\circ}\)C for V-CNO. The peak melting temperatures were slightly different for the first cycle but remain virtually the same for the subsequent cycles. The values of \(\Delta_{\text{fus}}H\) and \(T_{\text{mpt}}\) and their variations as a function of cycle number are shown in Figure 4. (Note that, for the sake of comparison, \(T_{\text{mpt}}\) values in Figure 4 were determined from the cycling thermograms at 10 K/min.)