Analysis of Bio-Based Fatty Esters PCM’s Thermal Properties and Investigation of Trends in Relation to Chemical Structures

Abstract

As global energy demand increases while primary sources and fossil fuels’ availability decrease, research has shifted its focus to thermal energy storage systems as alternative technologies able to cover for the mismatch between demand and supply. Among the different phase change materials available, esters possess particularly favorable properties with reported high enthalpies of fusion, low corrosivity, low toxicity, low supercooling, thermal and chemical stability as well as biodegradability and being derived from renewable feedstock. Despite such advantages, little to no data on the thermal behavior of esters is available due to low commercial availability. This study constitutes a continuation of previous works from the authors on the investigation of fatty esters as novel phase change materials. Here, methyl, pentyl and decyl esters of arachidic acid, and pentyl esters of myristic, palmitic, stearic and behenic acid are synthesized through Fischer esterification with high purities and their properties are studied. The chemical structures and purities are confirmed through Attenuated Total Reflectance Infrared Spectroscopy, Gas Chromatography coupled with Mass Spectroscopy and Nuclear Magnetic Resonance Spectroscopy, while the determination of the thermal properties is performed through Differential Scanning Calorimetry and Thermogravimetric Analysis. In conclusion, some correlations between the melting temperatures and the chemical structures are discovered, and the fatty esters are assessed based on their suitability as phase change materials for latent heat storage applications.

Investigation of the Thermal Properties of Diesters from Methanol, 1-Pentanol, and 1-Decanol as Sustainable Phase Change Materials

“…All diesters presented were synthesized in house according to the synthesis proposed by Ravotti et al [18,19] for saturated fatty esters. Given that the synthesis procedure had been optimized for fatty esters and not diesters, its success in the case of diesters had to be validated first.…”

“…As a result of the similarity of the spectra of commercial and synthesized DiMeOHOx, and of the absence of peaks denoting the presence of unreacted oxalic acid, the synthesis procedure could be considered successful for the diesters. It is important to consider that because the two carboxylic groups in DiMeOHOx are directly linked to each other with no additional carbons in between, the spectrum of DiMeOHOx appears slightly different than the ones of other esters or diesters [18,19]. Figure 1B shows the main differences between DiMeOHOx and DiMeOHSe for comparison.…”

“…Overall, diesters seem to be marked by lower melting temperatures compared to fatty esters of equal or similar chain length. For example, methyl palmitate has carbon number 17 and an onset melting temperature of approximately 26 • C [19], while DiPeOHSub has carbon number 18 but presents an onset melting point of −20 • C. This means that a single additional carbon in the chemical structure generates a difference in phase change temperature of more than 40 • C. Despite having the same number of 30 carbons in their aliphatic chains, decyl arachidate (DEAR) [19] and DiDeOHSe show onset melting at 41 • C and 31 • C, respectively. Therefore, diesters' melting points seem to be consistently lower than those of comparable fatty esters.…”

“…They observed a high thermal stability with low supercooling rates and enthalpies of fusion of around 150 J/g in a low-mid temperature range between 20 and 32 • C. Sari et al [16] synthesized and studied esters from stearic acid with isopropanol, n-butanol, and glycerol, and obtained similar results. Other studies include those of Stamatiou et al [17], where the authors investigated 11 commercial saturated unbranched esters, and Ravotti et al [18,19], where 15 carboxylic fatty esters were synthesized and their thermal properties were characterized. In general, these authors measured enthalpies of fusion between 150 and 190 J/g in a temperature range of 10-50 • C, which would be suitable for low-mid temperature applications such as space heating.…”

“…In particular, MeOH = methanol, PeOH = 1-Pentanol, DeOH = 1-Decanol, Ox = oxalic acid, Su = succinic acid, Sub = suberic acid, Se = sebacic acid (see chemical structures and IUPAC names in Appendix A Figure A1). The alcohols of provenance have been chosen in order to allow the diesters to be comparable to the fatty esters synthesized by Ravotti et al [18,19]. The dicarboxylic acids of origin were selected based on their natural occurrence with increasing carbon number in order to evaluate the influence of the aliphatic chains' lengths on the thermal properties of the diesters produced.…”