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Molecules 2020, 25, 5572 18 of 26 be seen in (Table 1), this is comparable to the ∆H of SST. Despite being more established as PCMs, salt hydrates still present several disadvantages such as a high tendency to supercool, and a low long-term stability due to segregation. Both phenomena are still poorly understood and require extensive research to be solved. Additionally, most salt hydrates are either toxic or irritant, therefore can encounter limitations in their applicability [5,6]. Most organic PCMs used consist of paraffins [70,71]. Unlike salt hydrates, paraffins present the advantage of possessing low supercooling and being thermally reliable over consecutive heating-cooling cycles. Still, paraffins are derived from petroleum and are highly flammable. In addition, they present lower ∆H ≤ 200 J/g. Triglycerides are sustainable and bio-based, possess higher ∆H than paraffins, and do not appear to undergo segregation. Therefore, they present considerable advantages compared to commonly- used PCMs. To date, some studies have been conducted on readily available triglyceride-rich substances such as olive oil. The main fatty acid constituents of the triglycerides in olive oil are palmitic, palmitoleic, stearic, oleic, linoleic and linolenic acids, comprising of about 94–96% of the total weight of the triglycerides. However, it must be kept in consideration that the composition of olive oil is strongly dependent on the region of extraction and the growth conditions. Gunasekara et al. [72] measured the thermal properties and observed a consistent melting and freezing for four cycles at the temperatures −4.5 to 10.4 ◦C and −8 to −11.9 ◦C, with the enthalpies 105 and 97 kJ/kg respectively. While the enthalpies are too low to be considered suitable for LHS applications, they are in agreement with what has been reported previously, according to which the enthalpy of triglycerides mixtures is lower than that of simple triglycerides. Therefore, it might be difficult to evaluate the suitability of olive oil as PCM due to its complex composition and diversity. One of the most relevant studies was conducted by Sari et al. [40], where the authors synthesized trimyristin, tripalmitin and tristearin with high purities and characterized their thermal properties. The authors also conducted long-term cycling tests by cycling the samples 1000 times in order to prove their thermal stability and reliability. While they did not attribute the melting points registered to any specific polymorphic phase, the values reported (Table 1) suggest that the authors did not successfully reach the stable β phase for any of the triglycerides analyzed (trimyristin 58.5 ◦C, tripalmitin 66.5 ◦C, tristearin 73.0 ◦C). Instead, they formed the α phase for trimyristin (32.0 ◦C) and the β‘ for both tripalmitin (58.5 ◦C) and tristearin (63.5 ◦C). This is not surprising since they performed the heating-cooling tests with heating rates of 5 K/min, which are too fast to allow for the formation of β crystals. Nevertheless, in every case only one crystalline phase was observed and the differential scanning calorimetry (DSC) analysis results showed that the repeated 1000 thermal cycling did not cause any degradation in the chemical structure of the esters or any significant change in their thermal properties. In addition and as reported by Ravotti et al. [44,50] on fatty esters, close to no supercooling was observed. Saturated triglycerides therefore present attractive enthalpies of fusion and a wide temperature range, with high temperatures achievable in the β phase, which proves to be interesting for a variety of applications. Although Sari et al. [40] proved that the α and β’ phases are stable upon consecutive heating-cooling cycles, achieving the thermodynamically stable β form would ensure even higher reproducibility and energy densities. This would be possible for applications requiring extremely slow heating and cooling rates. Alternatively, passive seeding has proven to be a valuable option according to Suppes et al. [25]. Several ways to perform passive seeding exist. Alfutimie et al. [36] found that the addition of small percentages of monoglycerides favour the crystallization rate and the stabilization of the β phase. Another way to promote the β form is to add longer-chain triglycerides as nucleating agents, similarly to what is done with nucleating agents in salt hydrates to reduce the supercooling [73]. Nevertheless, the control of triglycerides’ polymorphism and the reproducible formation of desired phases over consecutive heating-cooling cycles are still far from being mastered.PDF Image | Triglycerides as Novel Phase-Change Materials
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