# Edible Oils as Practical Phase Change Materials for Thermal Energy Storage
## 3 Results and Discussion
### Thermal Properties of Coconut Oil
When the temperature of the sample decreased again to 22.5 \({}^{\circ}\)C, the sample was completely transformed to a soft, white solid (Figure S7 in Supplementary Materials). In contrast to the DSC results for a \(\sim\)10 mg sample size, the cooling curve shows that the bulk sample of R-CNO exhibited little supercooling, around 0.9 K as measured from the exothermic peak of the inset of Figure 6. A similar value for supercooling was obtained when the same sample was cooled by blowing cold (10 \({}^{\circ}\)C) air on the test tube containing the sample. The solidification of coconut oil is strongly dependent on the cooling rate, as evident from the DSC results at 0.5, 2, and 10 K/min (Figure S5d in Supplementary Materials), with slower cooling giving less supercooling. Most importantly, the bench-top experiment results with 15 mL samples showed that the melting and solidification temperature ranges of coconut oil overlap and that, even when the room temperature was not below 21 \({}^{\circ}\)C, liquid coconut oil left to cool down at room temperature solidified after a matter of hours, fully releasing its stored enthalpy of fusion. In addition to the melting temperature and latent heat of fusion, accurate data concerning the heat capacity and thermal conductivity of a PCM are important, especially when performing feasibility studies and numerical simulations for a PCM-based thermal energy storage system. The heat capacity as a function of temperature of R-CNO was measured by DSC between 0 and 50 \({}^{\circ}\)C, for both the solid, \(C_{p,s}(T)\), and liquid, \(C_{p,l}(T)\), phases (Figure 7a). For the solid coconut oil, \(C_{p,s}\) varies between 1.6 and 1.7 J K\({}^{-1}\) g\({}^{-1}\) and in the liquid phase, \(C_{p,l}\) is almost temperature-independent at 2.2 J K\({}^{-1}\) g\({}^{-1}\). The values of the heat capacities for the solid and liquid phases are similar to results for fatty acid PCMs determined with the same technique [44]. We also measured the thermal conductivity of R-CNO as a function of temperature, \(\kappa(T)\), in the temperature range \(-\)10 to 50 \({}^{\circ}\)C. As shown in Figure 7b, the thermal conductivities of both the solid and liquid phases are low, 0.19 \(\pm\) 0.03 W K\({}^{-1}\) m\({}^{-1}\) for solid R-CNO and slightly less, 0.17 \(\pm\) 0.03 W K\({}^{-1}\) m\({}^{-1}\), for the liquid phase. As for other organic PCMs, the thermal conductivity of coconut oil is low and does not have a strong temperature dependence within the range measured here. Figure 6: Temperature versus time curve from 21.5 \({}^{\circ}\)C to 40 \({}^{\circ}\)C collected while heating a 15 mL sample of R-CNO by hot air and then letting the sample cool at room temperature. The cooling started at time \(t\) = 0.13 h. The inset shows a close-up of the solidification region, with a measured supercooling of 0.9 \({}^{\circ}\)C.  
Figure 7: (**a**) Heat capacity at constant pressure as a function of temperature, \(C_{p}(T)\), of R-CNO measured by DSC. Data points in the region of the solid-liquid transition approach infinity and are omitted. (**b**) Thermal conductivity, \(\kappa\), of R-CNO as a function of temperature measured by a Physical Property Measurement System (PPMS). The arrows indicate the DSC peak melting temperature, \(T_{\text{mpt}}\), of R-CNO.