# Edible Oils as Practical Phase Change Materials for Thermal Energy Storage
## 4 Conclusions  
In this work, we investigated the possibility of using edible oils as PCMs for thermal energy storage in residential applications. We measured the thermal properties of margarine and shortening, both composed mostly of a blend of vegetable oils, and the thermal properties of refined and virgin coconut oil. The results showed that, despite their long shelf lives, margarine and shortening are not useful as PCMs due to their small latent heats of fusion, \(\Delta_{\text{fus}}H\), and lack of thermal stability as deduced from the changes in their thermal behaviors after the first melt-freeze cycle. On the other hand, coconut oil has a relatively large \(\Delta_{\text{fus}}H\), \(105\pm 11\) J g\({}^{-1}\), and is thermally stable over at least 200 melt-freeze cycle. The melting temperature of coconut oil determined from the peak melting temperature is \(T_{\text{mpt}}=24.5\pm 1.5\)\({}^{\circ}\)C, which is ideal for thermal buffering of buildings. There was no significant difference between the melting properties of refined (R-CNO) and virgin (V-CNO) coconut oil. Further measurements were performed on R-CNO to determine its heat capacity in the solid phase (\(C_{\text{p,s}}=1.6\pm 0.2\) J K\({}^{-1}\) g\({}^{-1}\)) and in the liquid phase (\(C_{\text{p,l}}=2.2\pm 0.2\) J K\({}^{-1}\) g\({}^{-1}\)), along with its thermal conductivity in both the solid (\(\kappa_{\text{s}}=0.19\pm 0.03\) W K\({}^{-1}\) m\({}^{-1}\) ) and liquid (\(\kappa_{\text{I}}=0.17\pm 0.03\) W K\({}^{-1}\) m\({}^{-1}\) ) phases. The DSC thermograms showed that a few-mg sample of coconut oil supercools by up to \(\sim\)20 K when the melt is cooled at 10 K/min and supercooling decreases as the cooling rate decreases. Following this observation, a bench-top setup was used to measure the temperature versus time curve of a 15 mL previously heated and melted R-CNO sample as it was left to cool at room temperature. The temperature-versus-time curve showed that R-CNO supercools by 0.9 K and solidifies in about 2 h after the cooling starts at a room temperature of 21.5 \({}^{\circ}\)C. These experiments and the DSC results demonstrated that during slow cooling, bulk coconut oil can solidify at a temperature above 20 \({}^{\circ}\)C.  
The measured thermal properties of R-CNO were used to calculate the amount of coconut oil needed to store heat and maintain the temperature inside a residential greenhouse at 20 \({}^{\circ}\)C. Using a simple heat load model, we found that a volume of 630 L of R-CNO is needed to store 71.3 MJ of thermal energy. This is only \(\sim\)1/3 the volume of water required to store the same amount of energy as sensible heat. While coconut oil offers a more compact thermal energy storage solution, which is an important consideration for small spaces like a greenhouse, the current retail price of coconut oil does not make it useful based on cost alone, when compared to water. However, coconut oil PCM is less expensive compared with other energy-dense phase-change materials such as fatty acids and commercially available greenhouse PCM products. Compared to electrical energy cost for a heater heating the greenhouse, coconut oil is more cost effective if it maintains its thermal and chemical stability for more than 2 years. The following are available online at [[http://www.mdpi.com/2076-3417/9/8/1627/s1](http://www.mdpi.com/2076-3417/9/8/1627/s1)]([http://www.mdpi.com/2076-3417/9/8/1627/s1](http://www.mdpi.com/2076-3417/9/8/1627/s1)): Figure S1: Edible oils studied; Figure S2: Bench-top setup for bulk heating/cooling curves; Figure S3: DSC curve for margarine M2 at 2 K/min; Figure S4: DSC curves for vegetable shortening at 2 K/min; Figure S5: DSC curves for coconut oil (refined and virgin) at 0.5 and 2 K/min; Figure S6: Heating curve for melting of refined coconut oil; Figure S7: Photos of refined coconut oil during melting; and Figure S8: Schematic diagram of domestic greenhouse used for calculations. Conceptualization, M.A.W. ; methodology, S.K. and M.A.W. ; software, S.K. ; formal analysis, S.K. and M.A.W.