Thermal Characteristics of Eutectic Mixture of Capric-Lauric Acids as Phase Change Material (PCM) in Gypsum Board

Transcription

1 Jurnal Teknologi Proses Media Publikasi Karya Ilmiah Teknik Kimia 6(1) Januari 2007: ISSN Thermal Characteristics of Eutectic Mixture of Capric-Lauric Acids as Phase Change Material (PCM) in Gypsum Board Medyan Riza Department of Chemical Engineering, Faculty of Engineering, Syiah Kuala University Abstract Thermal characteristics of some eutectic mixtures of fatty acids as phase change materials (PCM) for passive solar building heating and cooling application have been studied previously. This study looked at the effect of using capric lauric acids eutectic mixture with a composition of 65: 35 w/w % as PCM in gypsum board. Capric lauric acids eutectic mixture has melting point of o C and latent heat of kj kg -1. The melting point is considered suitable to maintain a comfortable temperature in a space having a moderate ambient temperature. Gypsum board samples were immersed for 1 hour in the PCM and the thermal characteristics before and after immersion were analysed. The 12.5 mm thick gypsum board was found to have PCM content of 25.6 % of the PCM and the Differential Scanning Calorimetric (DSC) showed that the melting point and latent heat were 17.9 o C and 33.1 kj kg -1, respectively. Meanwhile for 6 mm thickness the PCM content was 29.2 %, the melting point was 18.1 o C and the latent heat was 36.8 kj kg -1. From the result it can be seen that the thermal characteristic of PCM - gypsum board has the same properties as the PCM. Indeed, the immersion process also did not affect the physical properties of gypsum. Keywords: phase change materials (PCMs), thermal characteristic, fatty acids, eutectic mixtures. Introduction Thermal energy storage in domestic solar space heating and cooling application has been given attention since it can utilise this renewable energy to reduce the greenhouse gas emissions. It also provides a reservoar of energy to adjust the mismatch between peak and off peak time and meet the energy demand at all times. Thermal energy storage is basically classified as latent, sensible and chemical energy storage. The concept of using latent heat storage as energy saving vehicle provides the advantages of storing a large amount of energy in a small mass/volume and the phase transition occurring at nearly constant temperature. In Phase Change Materials (PCMs), latent heat storage stores heat as latent heat of fusion during phase changes that undergo melting and solidification process. To gain a comfortable temperature in a space, the PCM can be employed to absorb heat when melting and release it when solidify. For a material to be used as PCM, some of the following criteria need to be fulfilled: high heat of fusion, high thermal conductivity, high density, high specific heat, congruent melting, small volume changes during phase changes, chemical stability, non corrosive, non poisonous, non flammable and non explosive. It is also noteworthy to take into consideration the availability as well as cost (Beghi, 1982).

2 32 Medyan Riza / Jurnal Teknologi Proses 6(1) Januari 2007: The selection of PCM has recently been directed towards the use of low melting organic materials in an effort to avoid some of the problems inherent in inorganic phase change materials, such as supercooling and segregation. Special attention has been given to fatty acids since they can easily be obtained from renewable sources such as oils/fats (Feldman and Banu, 1996). Fatty acids show solid-liquid transitions within narrow temperature ranges. They possess some superior properties over other PCMs such as melting congruency, good chemical stability, non-toxicity and suitable melting temperature range for solar passive heating and cooling applications. In the liquid phases, these materials have surface tensions in the order of dyne cm -1 and are therefore high enough to be retained in the structure of the host material. These materials possess elevated latent heat of transition and high specific heat (in the range J g -1 o C). They also exhibit only small volume changes during melting or solidification (example: melting dilatation is around ml g -1 ). Because of the protected carboxyl group, fatty acids based PCMs are chemically, heat and colour stable, low corrosion activity and non-toxic (Feldman et al, 1995). PCM can be utilised as a single component or eutectic mixtures (binary mixtures that exhibit fixed melting/solidification points at a certain composition between two single components and act as single component). In passive solar application, three methods are proposed for the incorporation of a thermal storage material within the construction element: encapsulation of the storage material in highdensity polyethylene pellets mixed with a gypsum board material, direct incorporation and simple immersion of conventional board in molten PCM. The latter method contributes to the economic impact of the passive solar wall (Zelba et al 2003). Several works have been carried out in order to investigate the thermal properties of the binary mixtures of fatty acid and its compatibility with the building materials. Study that carried out by Feldman and Banu (1996), investigated thermal characteristics of some PCM and found eutectic mixture of butyl palmitate and butyl stearate (49: 41 % weight) with melting point of 17 o C and latent heat of fusion of 140 kj/kg, to be compatible with gypsum wallboard. Nikolić et al. (2002) analysed thermal properties of methyl stearate, methyl palmitate and their binary mixtures use for solar storage in gypsum wallboard. No obvious changes in the thermal performance of materials were recorded after 50 thermal cycles. Their findings indicate that the latent heat of fusion has contributed to the overall heat storage capacity in a wallboard impregnated with the esters and their mixtures. Besides, Shapiro et al. (1987) has shown several PCMs to be suitable for introduction into gypsum wallboard with possible thermal storage applications for the Florida climate. These materials were mixtures of methyl-esters, methyl palmitate, and methyl stearate and mixtures of short chain acids and capric and lauric acid. Although these materials had relatively high latent heat capacity, the temperature ranges required in achieving the thermal storage did not fall sufficiently within the range of comfort for buildings in hot climates. Rudd (1993) analysed coconut fatty acids as PCM in gypsum and found that for absorption of 25 % weight of acids, the melting point is 24.9 o C and latent heat is kj/kg. Room scale test showed that the PCM wallboard has an average thermal storage capacity of 24.2 kj/kg. This varies by only 8.7 % compared to the latent heat recorded by DSC. In this paper, a simple immersion of gypsum board in eutectic mixture of capric-lauric acids (65 : 35 % weights) as phase change material will be discussed. Gypsum is a non-metallic mineral, found as a rock composed of 70.1 % calcium sulphate and 20.9 % water by weight. In construction industry, it is widely used as ceiling, roofing and wall. It is considered as a good supporting medium for phase change material since approximately 41% of the wallboard volume is air voids (Ruud, 1993).

3 Medyan Riza / Jurnal Teknologi Proses 6(1) Januari 2007: Experimental Method Materials Fatty acids used in this work are capric and lauric acid with the purity of 0.99, supplied by Palm-Oleo Sdn.Bhd, Malaysia. The chemicals were used without any purification.capric acid has melting point of 31 o C, whereas for lauric acid is 44 o C. The eutectic mixture of capriclauric acids in the weight ratio of 65.5:35 was prepared by melting the acids together at temperature 80 o C (Sari et al., 2004). Gypsum boards with thickness of 6 or 12.5 mm are used in this study. Gypsum board was prepared by mixing gypsum with water in the ratio of 1.25:1 and poured into 20 x 20 x 1.25/0.6 cm size mould. After stiffening, the gypsum samples were cut into 6 x 15 x 1.25/0.6 cm in size. Physical properties of gypsum samples before and after immersion in PCM were tested. Method The thermal properties of eutectic mixture of lauric-stearic acids such as melting point, solidification point and latent heat were first characterised. Gypsum board samples (6 x 15 x 1.25/0.6 cm size) were immersed for 1 hour in eutectic mixture of acids at 60 o C. After immersion, the samples were taken out, dried and the thermal characteristics of PCM-gypsum board were analysed. DSC Analysis Differential scanning calorimetric (DSC) Perkin-Elmer Thermal Analysis Seri 7 was used to measure the thermal characteristics of eutectic mixture and PCM-gypsum board. Samples were weighted in a sealed aluminium pan with a mass of mg. The DSC thermal analysis was performed in the temperature range of o C with a heating rate of 5 o C min -1 and under constant stream of nitrogen at atmospheric pressure. The melting temperature of the PCM, T m, corresponds to the onset temperature obtained by drawing a line at the point of maximum slope of the leading edge of the peak. The latent heat, ΔH fus, was calculated as the area under the peak by numerical integration. In addition, melting or solidification peak temperatures are defined as the temperature of the points, which are located furthest from the base line. Result and Discussion The DSC analysis for eutectic mixtures of capric-lauric acids (65 : 35 w/w %) shows a sharp peak with no secondary peak or hump in the temperature range of o C. It can be seen from Figure 1 that melting temperature, latent heat and melting temperatures range of the eutectic mixture are o C, J g -1 and o C, respectively. The melting point of eutectic mixtures is lower than those of the single acids. However, the latent heat is high enough that they can be comparable to other PCMs, such as salt hydrates and polyalcohols, which are between J g -1. Thermal characteristics of solidification process as determined by DSC are shown in Figure 2, which show that the solidification point of o C, latent heat of J g -1 and solidification temperatures range o C, respectively. The temperature transitions between melting and solidification processes are and o C and these are considered close. The solid-liquid phase transition was reversible, as it can be seen that heat absorbed as latent heat of fusion was released as heat of solidification (with a small heat loss). After immersion for 1 hour, the amount of PCM in gypsum board does not exceed 30 %. Nikolic et al. (2003) also reported the same result for some fatty acid esters and their eutectic mixture at the same immersion condition. Table 1 shows the amount of PCM in gypsum board. Figures 3, 4, 5 and 6 illustrate the DSC thermogram for 12.5 mm gypsum board loaded with 25.6 % and 6 mm gypsum board loaded with 29.2 % of acid for heating and cooling, respectively. The findings are summarised in Tables 2 and 3.Thermal characteristics of PCM-gypsum board were found to be very close to those of PCM alone. The melting and freezing curves were sharp with no secondary peak or hump observed.

4 34 Medyan Riza / Jurnal Teknologi Proses 6(1) Januari 2007: Some physical properties of gypsum board were also tested to show the immersion process do not influence the physical characteristics of gypsum. The conductivity value, which is higher after immersion for better absorption of heat, was caused by the replacement of air in gypsum by acid. The increasing in the thermal conductivity was higher in gypsum 6 mm thickness and this could be due to the higher amount of acid absorbed. The impregnated PCM does not reduce gypsum hardness. Table 4 shows some physical properties of gypsum board sample before and after immersion. Conclusion Eutectic mixture of capric-lauric acids (65: 35 w/w %) has melting point of o C, latent heat of J g -1 and close temperature transition. When impregnated in gypsum board, the thermal characteristics of the mixtures are practically unchanged, with sharp peak and no additional peak or hump occurs. Indeed, the immersion process does not affect the physical characteristics of gypsum board. For further study it is recommended an accelerated thermal cycle test to be conducted in order to detect thermal behaviour change (if any) during longterm period of used. Acknowledgement The author would like to thank Advanced Oleochemical Technology Centre (AOTC), Malaysian Palm Oil Board (MPOB) and Palm-Oleo Sdn.Bhd, for their supports of this work. References Beghi G Thermal energy storage, Dordrecht, Hollaand: D. Reidel Publising Company. Feldman D. and Banu D DSC analysis for the evaluation of an energy storing wallboard, Thermochimica Acta, 272, Feldman D., Banu D. and Hawes D Low chain esters of stearic acids as phase change materials for thermal energy storage in building, Solar Energy Materials and Solar Cells, 36, Nikolić R., Marinović-Cincović M., Gadzurić S.and Zsigrai I. J New materials for solar thermal storage-solid/liquid transitions in fatty acid ester, Solar Energy Materials and Solar Cells, 79, Rudd A.F Phase change material wallboard for distributed thermal storage in buildings, ASHRAE Transaction, 99, Part 2, Sari A., Sari H., Onal A Thermal properties and thermal reliability of eutectic mixtures of some fatty acids as latent heat storage materials, Energy Conversion and Management, 45, Shapiro M.M., Feldman D., Hawes D. and Banu D. PCM Thermal storage in drywall using organic phase-change material, Passive Solar Journal, 44, Zelba B., Marin J., Cabeza L. F., Mehling H Review on thermal energy storage with phase change; materials, heat transfer analysis and application, Applied Thermal Engineering, 23,

5 Medyan Riza / Jurnal Teknologi Proses 6(1) Januari 2007: FIGURE 1: DSC thermogram of eutectic mixture of capric-lauric acids (65:35 w/w %) scanned at 5 o C min -1 (Heating). FIGURE 2: DSC thermogram of eutectic mixture of capric-lauric acids (65:35 w/w %) scanned at 5 o C min -1 (Cooling).

6 36 Medyan Riza / Jurnal Teknologi Proses 6(1) Januari 2007: TABLE 1: Percentage of eutectic mixture (PCM) in gypsum board after immersion Sample Replicate 1 (%) Replicate 2 (%) Replicate 3 (%) Average (%) Gypsum 12.5 mm thickness Gypsum 6 mm thickness FIGURE 3: DSC thermogram of PCM-gypsum board 12.5 mm thickness scanned at 5 o C min -1, loading 25.6 % weight (Heating). FIGURE 4: DSC thermogram of PCM gypsum board 12.5 mm thickness scanned at 5 o C min -1, loading 25.6 % weight (Cooling).

7 Medyan Riza / Jurnal Teknologi Proses 6(1) Januari 2007: FIGURE 5: DSC thermogram of PCM-gypsum board 6 mm thickness scanned at 5 o C min -1, loading 29.2 % weight (Heating). FIGURE 6: DSC thermogram of PCM-gypsum board 6 mm thickness scanned at 5 o C min -1, loading 29.2 % weight (Cooling).

8 38 Medyan Riza / Jurnal Teknologi Proses 6(1) Januari 2007: TABLE 2: Thermal characteristics of PCM-gypsum board (heating) Sample Replicates Melting Point, T m, ( o C) Latent Heat ΔH fus (J/g) Melting Temperature Ranges ( o C) Gypsum 12.5 mm thickness Gypsum 6 mm thickness TABLE 3: Thermal characteristics of PCM-gypsum board (cooling) Sample Replicates Solidification Point, T m, ( o C) Latent Heat ΔH fus (J/g) Solidification Temperature Ranges ( o C) Gypsum 12.5 mm thickness Gypsum 6 mm thickness TABLE 4: Physical properties of gypsum board before and after immersion process Sample Conductivity value (W m -1 K -1 ) at 25 o C Density (Kg m -3 ) Shore hardness Before After Before After Before After Gypsum 12.5 mm thickness Gypsum 6 mm thickness