Article history:Received 19 January 2009Accepted 20 February 2009Available online 1 March 2009
Keywords:Stearic acidEsterificationPCMSolar energy materialsThermal properties
This paper deals with the synthesis, characterization, thermal properties and thermal reliability of a series ofstearic acid esters as a novel solid–liquid phase change material (PCM) for thermal energy storage. The estercompounds were synthesized via the reaction of stearic acid with n-butyl alcohol, isopropyl alcohol andglycerol and characterized by Fourier transform infrared spectroscopy (FT-IR) and 1H Nuclear MagneticResonance (1H NMR) techniques. Thermal properties of the esters were measured by differential scanningcalorimeter (DSC) method. DSC results indicated that the melting temperatures and latent heats of thesynthesized PCMs are in the range of 23–63 °C and 121–149 J/g, respectively. The thermal cycling testincluding 1000 cycling was conducted to determine the thermal reliability of the synthesized PCMs. Thethermal conductivities of the PCMs were also increased by adding 5 wt.% EG into the esters. Based on theresults, it is concluded that the synthesized esters as novel PCM have significant energy storage potential dueto their satisfactory thermal properties, good thermal reliability and thermal conductivities.
Solid–liquid phase change materials (PCMs) for thermal energystorage (TES) are attractive materials due to their large latent heat, thecharacteristic of constant temperature in the course of absorbing orreleasing energy. They have been potentially used in many applica-tions such as solar heating systems, cooling or heating in buildings,systems of industrial waste heat recovery, etc [1–3].
In recently years several solid–liquid PCMs were prepared andcharacterized. Alkan et al. synthesized ethylene glycol distearate asnovel PCM using Fisher esterificationmethod and characterized by FT-IR, DSC and TGA analysis [4]. Li and Ding prepared a series ofdistearates and butanediol distearate as a novel solid–liquid PCM andcharacterized by FT-IR, 1H NMR analysis and DSC measurements [5,6].As seen from the literature surveys, there is little study in literature onsynthesis, characterization and determination of thermal properties offatty acid esters as PCMs. Nicolic et al. investigated new materials forsolar thermal storage solid-liquid transitions in fatty acids esters [7].Alkan sulfonated paraffin samples slightly to increase the energystorage efficiency without changing thermophysical properties [8].Sarı prepared eutectic mixtures of fatty acids for decreasing fusiontemperature and increasing LHTES efficiency [9].
Stearic acid is a fatty acid having superior properties such as easyavailability, congruently melting/freezing, good thermal and chemicalstability, non-toxicity and suitable phase change temperature [10,11].
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In spite of these desirable properties of stearic acid, the effluvium,high phase change temperature and poor thermal conductivity limitits applications. To overcome the above mentioned problem someesters of stearic acid have been synthesized as novel solid–liquid PCMsusing esterification between stearic acid and n-butyl alcohol,isopropyl alcohol and glycerol. The esterification reactions werecharacterized by FT-IR and 1H NMR spectroscopy methods. Thermalproperties and thermal reliability of the synthesized PCMs weredetermined using DSC and FT-IR analysis techniques. Moreover, thethermal conductivities of the PCMs were increased by addingexpanded graphite (EG) with high thermal conductivity.
2. Experimental
2.1. Materials
Stearic acid (SA: C17H35COOH) was obtained fromMerck Company.n-butyl alcohol, isopropyl alcohol, glycerol and the other chemicalswere purchased from Aldrich Company. All chemical materials wereused without further purification.
2.2. Synthesis of the stearic acid esters
Stearic acid esters were synthesized according to the Fischer es-terification reaction. The esterification reaction was carried out bytaking the calculated amount of stearic acid, n-butyl alcohol (molarratio: 1:1 for stearic acid:n-butyl alcohol) in toluene and in a reactionsystem equipped with a reflux condenser and thermometer. Water in
Fig. 1. The esterification reaction of the stearic acid.
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the reaction medium was continuously removed. After the reaction,the toluene was casted under vacuum at 50 °C. Residue was solved inchloroform and separated using a separating funnel. Afterwards, theproduct washed four times using hot water (80 °C) and dried usinganhydrous sodium sulfate. The same synthesis process was alsoapplied using isopropyl alcohol and glycerol (molar ratio: 1:1 forstearic acid:isopropyl alcohol and 3:1 for stearic acid:glycerol). Thereaction schemes were shown in Fig. 1.
2.3. Chemical characterization of the stearic acid esters
Fourier transform infrared spectra (FT-IR) were obtained by aJASCO 430 spectrometer in the wavenumber range, 4000–400 cm−1.1H NMR spectra in CDCl3 solutions were recorded using BRUKER400 MHz spectrometer.
2.4. DSC analysis of the stearic acid esters
Thermal properties of the SA and its esters such as phase change(melting and freezing) temperatures and latent heats of melting andfreezing were measured using a differential scanning calorimeter(Perkin Elmer Jade DSC). The analyses were performed at 5 °C/min of
Fig. 2. (a) FT-IR spectra of stearic acid and the synthesized esters (
constant heating rate, in the temperature range, 10–75 °C, under aconstant stream of argon at atmospheric pressure. All DSC measure-ments are repeated three times for each sample and the average valueswere considered. In order to determine the thermal reliability of theesters, thermal cycling tests (1000 melting/freezing processes) wereconducted using the experimental procedure in literature [10,11]. Thethermal properties were measured using DSC analysis after thermalcycling again. The chemical stability of the esters was also investigatedafter thermal cycling test using FT-IR analysis.
On the other hand, the EG was added to the PCMs in the massfraction of 5% to improve thermal conductivity of the synthesizedPCMs. Afterwards, thermal conductivities of the synthesized PCMswere measured using KD2 thermal property analyzer.
3. Result and discussion
3.1. Characterization of the synthesized PCMs
The synthesized PCMs were characterized by using FT-IR and 1HNMR spectroscopymethod. The FT-IR characteristic peaks of SA and itsesters were shown in Fig. 2a. The disappearance of hydroxylabsorption peaks in the range of 3200–3650 cm−1 indicated that all
b) 1H NMR spectra of stearic acid and the synthesized esters.
Fig. 3. DSC thermograms of stearic acid and the synthesized esters (a) before and (b) after thermal cycling.
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hydroxyl groups of alcohols had transformed into ester bonds, and thereactive hydroxyl groups were not observed after the esterification.The 1H NMR characteristic peaks of the esters was also shown inFig. 2b. There were no peaks of hydroxyl groups at about 2.24 ppm forn-butyl alcohol, 2.16 ppm for isopropyl alcohol and 4.48 ppm forglycerol. Moreover, the appearance of peaks related to the protons of –COO–CH2– at δ=4.07 ppm for butyl stearate, –COO–CH– at 5.01 ppmfor isopropyl stearate and –COO–CH2– at 4.45 ppm and –COO–CH– at5.12 ppm for glycerol tristearate indicated that the esterificationreactions were completed. The number of H protons and chemicalshift were in good agreement with butyl stearate, isopropyl stearateand glycerol tri stearate and all results confirm that the esters weresuccessfully synthesized.
3.2. Thermal properties of synthesized PCMs
DSC analyses have been conducted to measure the thermalproperties of the synthesized PCMs. DSC thermograms of the SA andthe esters are shown in Fig. 3a. The DSC data were given in Table 1. Asseen in Table 1, phase change temperature of the SA that expected at68–69 °C couldn't be observed in case of its esters. This verified thatthere was no residual SA in the synthesized PCMs. The endothermicpeak (melting) and exothermic peak (freezing) were clearly observedfor the PCMs from DSC measurements. As also seen in Table 1, themelting and freezing temperatures of the synthesized PCMs are in therange of 23–63 °C and 24–64 °C, respectively. The latent heats ofmelting and freezing of the PCMs are in the range of 121–149 J/g and128–151 J/g, respectively. These properties make them promisingPCMs to storage and release energy for thermal applications. More-over, they had better odor and noncorrosivity property and usagepotential for energy storage at lower working temperature comparedwith SA.
3.3. Thermal reliability of the synthesized PCMs
The DSC curves of synthesized PCMs before and after thermalcycling (melting/freezing) are shown in Fig. 3b. After 1000 cycling,
Table 1The thermal properties of the stearic acid and esters before and after thermal cycling.
Thermal properties
Phase change materials Tm, °C ΔHm, J/g Tf, °C ΔHf, J/g
the melting temperatures of the PCMs changed in range of 0.62–0.77 °C as their freezing temperatures changed in range of 0.12–4.52 °C. Moreover, after the thermal cycling, the latent heat values ofmelting changed by 2.8%, −4.6%, and 2.3%, respectively as the latentheat values of freezing changed by −2.2%, 0.9%, and −2.8% for butylstearate, isopropyl stearate and glycerol tristearate, respectively. Basedon the results, it is noteworthy that the synthesized PCMs have goodthermal reliability in terms of thermal properties after thermalcycling. On the other hand, as seen from FT-IR spectrum in Fig. 4,after 1000 thermal cycling, any degradation do not cause in thechemical structure of the esters. This means that the synthesizedPCMs had good thermal stability during a long utility period.
3.4. Thermal conductivity of the synthesized PCMs
Thermal conductivity of a PCM is an important parameter inenergy storage applications, as well as its transition temperature andlatent heat values. Thermal conductivities of butyl stearate isopropylstearate and glycerol tristearate at the room temperature weremeasured as 0.23, 0.15 and 0.17 Wm−1 K−1, respectively. In order toimprove thermal conductivity of the esters, the EG with high thermalconductivity, 2–90 Wm−1 K−1 [12] was added to the synthesizedPCMs in mass fraction of 5%. The thermal conductivity was measuredto be 0.27 Wm−1 K−1 for the butyl stearate, 0.20 Wm−1 K−1 forisopropyl stearate, and 0.19 Wm−1 K-1 for the glycerol tristearate. The
Fig. 4. FT-IR spectra for the synthesized after thermal cycling.
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increase in thermal conductivity of the esters was in range of 12%–33%, respectively.
4. Conclusion
Novel solid–liquid PCMs were synthesized via the direct esterifica-tion reaction of stearic acid with n-butyl alcohol, isopropyl alcoholand glycerol and characterized by using FT-IR and 1H NMR analysis.DSC thermal analyses showed that the synthesized esters can be usedas novel PCMs because of their suitable phase temperature and con-siderable high latent heat energy storage capacity for thermal energystorage applications.
On the other hand, the of FT-IR and DSC analysis results showedthat the repeated 1000 thermal cycling do not cause any degradationin the chemical structure of the esters and allow any significantchange in thermal properties of the esters. As a result, synthesizedesters as novel solid–liquid PCMs can be considered as promising heatstorage materials because of their good thermal properties, thermalreliability, thermal stability and thermal conductivities.
Acknowledgement
The authors would like to thank Gaziosmanpaşa University ScientificResearch Projects Fund (Project No: 2008/29).
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