# Edible Oils as Practical Phase Change Materials for Thermal Energy Storage
## 3 Results and Discussion  
### Thermal Properties of Margarine and Shortening  
The DSC thermograms of margarine samples M1 and M2 measured at a heating (and cooling) rate of 10 K/min are shown in Figure 1. The melting behavior of both margarine samples changed after the first cycle: there was a clear shift in the position and in the amplitude of the main endothermic (melting) peaks and additional endothermic and exothermic peaks appeared in the subsequent melting cycles (Figure 1a,b). There were no significant changes in the solidification curves of different cycles of the two samples, with one major exothermic peak remaining nearly the same for each cooling cycle. However, the cooling exotherms show that margarine supercooled by about 10 K, as determined from the estimated melting and solidification onset temperatures of the first cycle. For margarine samples, we note that it was not possible to accurately determine the melting and solidification temperatures from the DSC measurements performed at the rate of 2 K/min due to the emergence of multiple peaks around and within the main endothermic peak (see Figure S3 in Supplementary Materials). Therefore, we approximated the melting and solidification temperatures from the DSC thermograms measured at 10 K/min. For sample M1 (Parkay@margarine), the peak temperature of melting was \(35.5\pm 1.5\)\({}^{\circ}\)C and the latent heat of fusion was \(\Delta_{\text{fus}}H=8\pm 1\) J g\({}^{-1}\), as determined from the heating curve from cycle 1. Upon cooling, sample M1 solidified with a peak temperature of \(12.5\pm 1.5\)\({}^{\circ}\)C. Sample M2 had a peak melting temperature at \(33.0\pm 1.5\)\({}^{\circ}\)C and a latent heat of fusion of \(\Delta_{\text{fus}}H=7\pm 1\) J g\({}^{-1}\) (determined from first cycle). For both margarine samples, the small latent heats of solidification determined from the exothermic peaks (~4 J g\({}^{-1}\) for M1 and ~2 J g\({}^{-1}\) for M2) indicate that margarine is only partially solidified upon cooling. The appearance of extra peaks, especially after the first melt, likely results from the thermal response of the different oils in the blend and other additives that are present in margarine (see Table 1). This change in the thermal behavior and, most importantly, the small values of \(\Delta_{\text{fus}}H\) for both margarine samples, indicate that margarine is not useful as a PCM for TES applications.  
The DSC thermograms of the vegetable shortening (VS) sample are shown in Figure 2. The melting temperature determined from the peak (Figure S4 in Supplementary Materials, measured at 2 K/min) is \(T_{\text{mpt}}=44.7\pm 1.5\)\({}^{\circ}\)C. Compared with margarine, the vegetable shortening was more stable after the first melt. Figure 2 shows that there is no major shift in the endothermic peak and no additional endothermic peaks appeared in the second heating cycle. The only change is a small exothermic peak that appears just before the melting starts around 25 \({}^{\circ}\)C. The appearance of the exothermic peak upon heating can be explained by a sudden crystallization which is likely dependent on the thermal history of the sample: when the same sample was measured again after being left for one day at room temperature, the exothermic peak was absent from the first heating cycle. The absence of additional  
Figure 1: Differential scanning calorimeter (DSC) thermograms of (**a**) margarine sample M1 (Parkay®) and (**b**) margarine sample M2 (Imperial®). The first and third cycles are shown.  
endothermic peaks is not surprising since, unlike margarine, shortening is mostly composed of a blend of oils without much water or milk products, or other additives found in margarine. However, similar to margarine, VS has only a small latent heat of fusion, \(\Delta_{\text{fus}}H=12\pm 1\) J g\({}^{-1}\), as determined from the average of four melt-freeze cycles measured at 10 K/min (Figure 2). Although VS has better thermal stability than margarine, its small \(\Delta_{\text{fus}}H\) indicates that it is not useful for thermal energy storage.