# Edible Oils as Practical Phase Change Materials for Thermal Energy Storage
## 1 Introduction  
Phase change materials (PCMs) provide high thermal energy storage density over a small temperature range, in comparison with sensible heat storage materials, such as water [1; 2; 3]. Among the different classes of PCMs [2], organic non-paraffin PCMs including fatty acids, esters, and alcohols, have many features that make them desirable for integration in latent-heat thermal energy storage (TES) systems operating near the ambient temperature range, i.e., 10-40 \({}^{\circ}\)C. These PCMs are particularly desirable for off-grid and renewable energy applications because the materials can be sustainable, non-toxic to the environment and abundant in nature [4; 5; 6; 7; 8; 9]. They also can be integrated easily into building materials and residential structures to mitigate excessive temperature fluctuations, storing heat as they change phases, e.g., from solid to liquid [10; 11; 12; 13]. The stored heat can be recovered later to warm the space when the temperature decreases below the solidification transition. Long-chain saturated fatty acids are naturally occurring PCMs, mainly extracted from renewable plant and animal sources such as tallow, coconut, palm kernel, and soybean oils [14; 15; 16]. A life-cycle assessment study of dodecanoic acid (aka lauric acid, [CH\({}_{3}\)(CH\({}_{2}\))\({}_{10}\)COOH]) has shown [17] that the embodied energy associated with its production from palm kernel oil can be recouped in a matter of months. Although fatty acid PCMs are non-toxic, food-grade chemicals, their availability is somewhat limited as acquisition of large quantities of fatty acids is through chemical suppliers, which could frustrate individual consumers wishing to build a TES system. Organic PCMs are available commerciallyin various encapsulated forms from many suppliers [18; 19; 20], but they are considerably more expensive than raw materials. Furthermore, a key limitation for pure, long-chain, saturated fatty acids as PCMs is that their transition temperatures are outside the 20-28 \({}^{\circ}\)C range that is typically required to mitigate temperature fluctuations in buildings. Eutectic mixtures composed of two or more fatty acids can achieve a transition in this temperature range, but add additional complications for an individual consumer [13; 21; 22]. Since many edible fats and oils from which fatty acids are derived can be purchased easily in large quantities from stores and wholesalers, they could potentially be used as practical PCMs for some TES applications. Or can they? Potential PCMs include margarines and shortening, which have good shelf life, but contain additives that have an unknown influence on their properties. Edible coconut oil is another possibility, due to its high content of saturated fat, up to 92% [14; 15; 23; 24; 25; 26]. Coconut oil products labeled as virgin coconut oil and refined coconut oil are commonly found in the marketplace, where the difference is in the oil extraction method from the flesh of the coconut fruit [24; 25]. Both have relatively good shelf life: 2 years for refined coconut oil and 5 years or more for virgin coconut oil (virgin coconut oil contains more anti-oxidants than refined coconut oil and some manufacturers state that it has an indefinite shelf life), which can even be much longer if coconut oil is stored in well-sealed opaque containers to reduce photodegradation and oxidation [27; 28]. There are no reports assessing margarine and shortening as potential PCMs in terms of their thermal properties and their thermal stabilities, and only a few studies reporting the thermal properties of coconut oil and its application as a PCM for TES [25; 26; 29; 30; 31; 32; 33; 34; 35]. The latter showed that coconut oil could be useful for TES in buildings, given that its transition temperature is between 22 and 27 \({}^{\circ}\)C [25; 26; 30; 31; 32; 33]. In particular, coconut oil can buffer the temperature of a room in tropical climates [32] and can prevent overheating of passenger vehicles [30].