Manufacturing of new ester-based PCMs for building applications

Abstract

Increases in energy prices worldwide have made energy saving essential. One energy saving technique is thermal energy storage using phase change materials (PCMs), which has a variety of applications. One of the common applications of PCMs is in storing and releasing thermal energy in buildings in order to decrease electricity consumption during peak usage. Generally, PCMs can be classified into organic, inorganic and eutectic types. Inorganic PCMs are capable of providing high latent heat but they have some disadvantages such as being subject to corrosion and subcooling. On the other hand, most organic PCMs are non-corrosive, chemically stable, and show little or no subcooling. Among the eutectic PCMs, organic-organic PCMs have a high potential to be used for building applications. Literature shows that a binary mixture of fatty acid esters provides a suitable range of melting temperatures (20-24℃) but their latent heat of fusion and the problem of their needing supercooling need to be improved. This research is mainly focused on improving thermal properties of fatty acid esters by designing new binary mixtures. One of the methods of improving the thermal properties of fatty acid esters is to form a new binary mixture. Since no research has been done about binary mixtures of fatty acid esters and fatty alcohols, this research will shed light on these new binary mixtures by investigating their thermo-physical properties. The fatty acid esters that were used in this study are methyl laurate (MEL), methyl palmitate (MEP) and methyl stearate (MES), while the fatty alcohols are 1-dodecanol (DD) and 1-tetradecanol (TD). Six new binary systems were prepared by melt blending of methyl laurate/1-dodecanol, methyl palmitate/1-dodecanol, methyl laurate/1-tetradecanol, methyl palmitate/1-tetradecanol and methyl stearate/1-tetradecanol at different mole fractions. Each binary mixture was heated above the melting temperature of its components and mixed for 2 hours to achieve a homogenous mixture. The thermal properties of the binary mixtures were tested by using DSC, TGA and thermal cycling analyses. Thermal properties of new eutectic mixtures can be predicted theortically. Using thermodynamic models to predict the melting point of the new eutectic mixtures can provide a quick, accurate, and low-cost approach. Therefore, three thermodynamic models (Ideal model, UNIFAC model and Wilson model) were employed to predict the melting temperatures of each binary system. By comparing the DSC results with the predicted results from thermodynamic models, it was realised that the Wilson model had more accurate prediction for MEL-DD, MES-TD and MEP-TD binary systems, and for MES-DD and MEP-DD binary mixtures, the UNIFAC model predicted the solid-liquid phase diagram more accurately than the Ideal and Wilson models. Experimental and theoretical results proved that the MEL-TD binary system was not a eutectic system, and all binary mixtures of this system showed two endothermic peaks in their DSC curves. A eutectic mixture of MEL-DD, MEP-DD, MES-DD, MEP-TD and MES-TD binary systems occurred at mole fractions of 0.70-0.30, 0.30-0.70, 0.10-0.90, 0.60-.040 and 0.30-0.70, respectively. Phase change transition temperature and latent heat of fusion of eutectic mixtures of MEL-DD, MEP-DD, MES-DD, MEP-TD and MES-TD were 3.94℃/177.44 J/g, 20.34℃/224.84 J/g, 22.46℃/201.80 J/g, 26.93℃/209.47 J/g and 32.05℃/209.38 J/g, respectively. From the six binary systems, four binary mixtures were chosen. These were eutectic mixtures of MEP-DD (PCM18), MES-DD (PCM21), MEP-TD (PCM23) and MES-TD (PCM30). The selected MES-TD binary mixture was not the eutectic mixture of this binary system because MES-TD mixture at mole fraction of 0.40-0.60 provided higher latent heat of fusion compared to eutectic mixture of MES-TD binary system. To investigate the thermal stability of the selected PCMs, a thermal cycling test and TGA were employed. The thermal cycling test showed that melting temperature of PCM18, PCM21, PCM23 and PCM30 (as defined on Table 4.20) were 17.86℃, 20.66℃, 23.22℃ and 30.30℃, respectively in the first cycle. The results illustrated that fluctuation of melting temperatures of selected PCMs were significantly low during 1000 thermal cycles. In addition, the thermal cycling test exhibited that the average degrees of supercooling for PCM18, PCM21, PCM23 and PCM30 were 1.55℃, 1.43℃, 1.86℃ and 1.16℃ , respectively. To observe the effect of long thermal cycling, after 1000 cycles the phase change transition temperature and the latent heat of fusion of selected PCMs were measured. The DSC results proved that new PCMs were resistant against significant change in their phase change transition temperature and latent heat of fusion. TGA was used to examine thermal stability of new ester-based PCMs. Decomposition temperatures of PCM18, PCM21 and PCM23 were 136.83℃ , 136.10℃ and 168.70℃, respectively. In terms of decomposition temperature, the selected PCMs were comparable with RT 21, which is a paraffin-based PCM. However, for building applications PCMs with higher thermal stability are preferable. Therefore, further research is needed to improve the thermal stability of the selected PCMs. For future work, thickening agent can be added to the new PCMs in order to increase the decomposition temperature. However, the agent may affect the phase transition temperature and the latent heat of fusion of the PCMs which requires further thermal analysis to observe effect of the additives. In addition, mass loss of the selected PCMs was measured over a period of nine days. It was discovered that PCM 18, PCM 21, PCM 23 and PCM 30 lost 51.79%, 53.26%, 11.84% and 8.21% of their initial masses, respectively. Encapsulation can be used to prevent the new ester-based PCMs from huge mass loss, which requires further research and investigation. In this study new ester-based PCMs were designed and manufactured from binary mixtures of fatty alcohols and fatty acid esters. The selected binary mixtures as PCMs for building applications were 1-dodecanol/methyl palmitate (PCM18), 1-dodecanol/methyl stearate (PCM21) and 1-tetradecanol/methyl palmitate (PCM23). Thermo-physical properties of the new ester-based PCMs were studied. More research and investigations are required to improve the thermal properties of the PCMs.