# Edible Oils as Practical Phase Change Materials for Thermal Energy Storage
## 3 Results and Discussion
### Thermal Energy Storage with Coconut Oil in a Residential Greenhouse
For water, the total mass required is 1700 kg (\(\sim\) 1700 L), almost three times that for R-CNO. Considering the size of the greenhouse, it is not difficult to store the 630 L of R-CNO: if this volume is filled in pipes of 10 cm diameter extending along the length of the greenhouse (3.65 m), in total ca. 25 pipes are needed and all could be stacked on one wall. For this design, it is possible to estimate the total heat transfer from the PCM contained in such pipes to check if all the PCM mass discharges (solidifies) during the night. Considering a heat transfer coefficient \(u\approx\) 5.9 W m\({}^{-2}\) K\({}^{-1}\) (for low air convection) and given the total surface area of the 25 pipes (3.65 m length, 0.1 m diameter, \(A\cong\) 29 m\({}^{2}\)), the heat transferred through the PCM pipes for \(\Delta T=\) 10 K and during 10 h is \(Q_{\rm HT}\cong\) 62 MJ (Equation (1)). This heat transfer value indicates that ca. 90% of the PCM solidifies during the night. The PCM containers and their location in the greenhouse could be designed to optimize the heat transfer and the storage and recovery of thermal energy, but this is not within the scope of the present study. Overall, our calculations show that the incorporation of coconut oil in a residential greenhouse to mitigate excessive heat during the day and warm the air during cold nights is feasible. A large volume, \(\sim\) 600 L, of R-CNO would be needed for a typical (12 ft. \(\times\) 10 ft. \(\times\) 10 ft.) greenhouse, but this volume is approximately three times less than a volume of water that would store the same amount of thermal energy, making CNO much more convenient in terms of space. In terms of cost, we compare using 630 L of R-CNO as a passive heating and cooling system with two alternatives: using fatty acid PCM from a chemical supplier, and operating an electrical heater to heat the greenhouse during cold nights. (We neglect cooling costs for the latter as we assume that cooling can be done through ventilation from openings in the greenhouse.) The retail cost of refined coconut oil is \(\sim\) US$ 6/L (Table 1) but bulk quantities of R-CNO and V-CNO can be had at US$ 2.3/L and US$ 5.8/L, respectively [59] (the commodity cost of coconut oil is even lower, ca. US$ 0.8/L [60]). The total cost for 630 L (at US$ 2.3/L) is \(\sim\) US$ 1,500. An unbranched, saturated fatty acid PCM with \(T_{\rm mpt}\) close to that of R-CNO, 24.5 \({}^{\circ}\)C, does not exist [21; 22] but a binary eutectic mixture composed of 78 mass% decanoic acid (aka capric acid, [CH\({}_{3}\)(CH\({}_{2}\))\({}_{8}\)COOH]) and 22 mass% tetradecanoic acid (aka myristic acid, [CH\({}_{3}\)(CH\({}_{2}\))\({}_{12}\)COOH]) has a peak melting temperature around 24 \({}^{\circ}\)C and a \(\Delta_{\rm fus}H\) of 153 kJ kg\({}^{-1}\)[13; 22]. A total volume of 533 L of this mixture is needed to store 71.3 MJ. If purchased in kg quantities from a chemical supply company (e.g., Sigma-Aldrich), the total cost of this eutectic mixture PCM is \(\sim\) US$ 8,400 (the cost of chemicals needed to prepare 1 kg of the eutectic mixture is US$ 18, the eutectic mixture density is 0.874 kg/L [13]), which is almost six times the cost of coconut oil PCM. To compare with electric energy cost, we assume that heat is used during the night to provide 71.3 MJ, which is equivalent to 20 kWh per day. At a cost rate of US$ 0.1/kWh, the electric energy cost is US$ 60/month. Therefore, it would take 25 months of operation to recoup the cost of coconut oil, in place of electrical heating, which could correspond to a few years duration, given that the greenhouse thermal buffering would likely only be used for a fraction of the year. Note that if coconut oil can be purchased at the commodity price of US$ 0.8/L, then the total recoup time, compared with electrical heating, would be much shorter.