The magnitude of the energy recovered from latent heatthermal energy storage (LHTES) system basically depends onthe heat storage capacity of a phase change material (PCM)during solid–liquid phase change at a constant temperature [1].
The selection of suitable heat storage material plays animportant role in terms of thermal efficiency, economic feasi-bility and utility life of LHTES system. Therefore, developingPCMs with respect to the energy storage requirements has beengained significant interest. A large number of inorganic, organiccompounds and their mixtures have been studied as PCM fordifferent LHTES purposes [2–4]. Among the evaluated PCMs,fatty acids are promising ones because of the following advan-tages: suitable phase change temperature, high latent heat ca-pacity, no or less volume change during phase change, and easymanufacturing from common vegetable and animal oils [5–7].
By taking into account of predominant characteristics above,binary mixtures of fatty acids can be tailored as new PCMs, with
almost any suitable phase change temperature for LHTES sys-tems used for heating and cooling purposes [8–10]. The deter-mination of thermal properties and thermal reliability of newdeveloped PCM is essential to predict not only its energystorage life but also LHTES performance in practical applica-tions. There are a few studies on assessment of thermal pro-perties and thermal reliability of different PCMs [6,11].
Previous investigations showed that the capric acid (CA; m.p:32.14 °C) and the palmitic acid (PA, m.p: 59.40 °C) havedesirable thermal and heat transfer characteristics, but theirmelting points are quite high for low temperature LHTES re-quirements [7,10]. The phase change temperature of PA can bemodified to a lower value by addition of CA in eutectic ratio andtherefore, a eutectic CA/PA mixture can be formed as suitablePCM for low temperature LHTES applications.
The objective of this study is to prepare CA/PA eutecticmixture as PCM for low temperature LHTES and determine itsthermal properties and thermal reliability using DSC analysistechnique. The cause of the changes in thermal properties of thePCM with accelerated thermal cycling was also investigatedusing FT-IR spectroscopy technique. Since there is no compre-hensive data on this PCM in literature, the present paper is an
Fig. 1. Melting temperatures of CA/PA mixtures versus composition of the components.
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effort to provide its thermal properties which explore its LHTESpotential.
2. Materials and methods
2.1. Materials
Capric acid (CA, 98% pure) and palmitic acid (PA, 97%pure) were supplied by Aldrich and Fluka Companies. Theseacids were used without further purification.
Fig. 2. DSC curves of the CA/PA eute
2.2. Preparation of CA/PA eutectic mixture
The CA/PA binary mixtures were obtained by mixing thelow-to-high molecular weight component in 10% mass ratiosfrom 0% to 100%. The solid components weighed within±0.1 mg accuracy were mixed in liquid state homogenously andthen cooled to room temperature. The melting temperatures ofthe acid components and their mixtures prepared at differentmass combinations were measured using by DSC thermalanalysis technique.
ctic mixture and its components.
Table 1The changes in thermal properties of CA/PA eutectic mixture with respect tothermal cycling number
CA/PA eutectic mixture
Number of cycling Tm, (°C) ΔHm, (J/g) Tf, (°C) ΔHf, (J/g)
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2.3. DSC thermal analysis
Melting temperature (Tm), freezing temperature (Tf), latentheat of melting (ΔHm), and latent heat of freezing (ΔHf) of CA,PA components and their mixtures with different mass fractionswere measured by a DSC instrument (SETARAM DSC 131).The analyses were performed between the temperatures of 0 and100 °C at 5 °C/min heating rate under a constant stream of argonat flow rate of 60mL/min. All DSCmeasurements were repeatedthree times for each sample. The standard deviation was found tobe ±0.12 °C in phase change temperature and ±1.36 J/g in latentheat value.
2.4. Accelerated thermal cycling test
Accelerated thermal cycling (melting and freezing cycling)test was conducted by using experimental procedure in litera-ture [6,11]. The accelerated thermal cycling test was conducteduntil the number of cycling would be 5000. The changes inthermal properties of the after increased number of thermalcycling (1000, 2000, 3000, 4000, and 5000) were evaluatedusing DSC analysis technique. Moreover, the probable reasonsof the changes in thermal properties of the PCM with thermalcycling test were investigated in KBr disk using a FT-IR spec-trophotometer (Jasco 430).
3. Results and discussion
3.1. Thermal properties of CA/PA eutectic mixture
The variations in melting temperature of CA/PA mixture withcomposition of the components (wt.%) are shown in Fig. 1 . As clearlyseen, the melting temperature of the mixture is lower than those of thecomponents. Both of the components in the mixture melt simulta-neously at a constant temperature point (21.85 °C) as the CA/PA binarysystem has eutectic combination ratio (76.5/23.5 wt.%).
Fig. 3. DSC curves of the CA/PA eute
When compared the eutectic combination ratio and the meltingtemperature with that given in literature, it can be observed that there isgenerally good agreement, but small discrepancies between the results.For instance, the eutectic composition ratio and melting temperature ofCA/PA eutectic mixture were found to be 75.2/24.8 wt.% and 22.1 °C[8]. The differences between the results are most probably to be due totwo factors: the presence of a certain amount of impurities of the singleacid used in the mixture and the experimental error through the DSCanalysis [12,13].
Fig. 2 shows the DSC curves for heating and cooling processes ofCA/PA eutectic mixture and its components.
Thermal properties evaluated from the curves indicate that CA andPAmelt at 32.14 and 59.40 °C, solidify at 32.53 and 58.23, respectivelyas they have latent heat of melting of 156.40 and 218.53 J/g, and latentheat of freezing of 154.24 and 216.48 J/g, respectively. These propertiesmake them potential PCMs to form CA/PA eutectic mixture with largelatent heat and suitable phase change temperature for low temperatureLHTES applications. The prepared CA/PA eutectic mixture melts at21.85 °C and solidifies at 22.15 °C. The phase change temperatures ofCA/PA are in the range of indoor comfortable temperature (16–25 °C)which suggests that this mixture can be used in low temperature LHTESsystems. The latent heats of melting and freezing of CA/PA wasmeasured as 171.22 and 173.16 J/g, respectively. The latent heatcapacities of the eutectic mixture are as high as compared to those ofsome PCMs such as paraffin and poly alcohols [2,8].
ctic mixture after thermal cycling.
Fig. 4. FT-IR spectra of uncycled and cycled CA/PA eutectic mixture.
906 A. Sarı, A. Karaipekli / Materials Letters 62 (2008) 903–906
3.2. Thermal reliability of CA/PA eutectic mixture
Fig. 3 shows the DSC curves of the CA/PA eutectic mixture afterthermal cycling.
The thermal properties obtained from these curves also given inTable 1.
After 1000, 2000, 3000, 4000, and 5000 thermal cycling, the Tm valueof the mixture changed as −0.17, −0.41, 0.51, −0.11, and 0.43 °C, andthe Tf value changed as −0.31, −0.39, 0.86, −0.21, and 0.59 °C, respec-tively. These irregular changes in the Tm and the Tf values with increasingnumber of thermal cycling are in negligible magnitudes for LHTESpurposes. Therefore, the CA/PA eutectic mixture has good thermalreliability in terms of the changes in itsmelting and freezing temperatures.
On the other hand, theΔHm value of themixture changed by −2.8%,−7.1%, 4.0%, −1.2%, and 2.2% and theΔHf value changed by −2.6%,−6.9%, 3.8%, −1.7%, and 1.1% after repeated 1000, 2000, 3000, 4000and 5000 thermal cycling, respectively. It is noteworthy that the changesin the latent heat values of the CA/PA eutectic mixture are in acceptablelevel for a PCM to be used for LHTES applications although it wassubjected to a large number of melting/freezing cycling.
In this study, the probable reasons of the changes in the thermalproperties of the PCM with accelerated thermal cycling were alsoinvestigated. Two factors may be responsible for this case: (i) the
impurities (2–3 wt.%) in the acid components (CA and PA) of thePCM, (ii) chemical degradation or decomposition of the PCM. Thesecond one was investigated using spectroscopy technique (Fig. 4).
As clearly seen from the FT-IR spectra all peaks fit into one anotherat the same frequency band. One may concluded that the PCM does notdegrade or decompose into pure acid components during thermalcycling. Therefore, it is remarkably noted that the change occurred inthermal properties of the PCM with accelerated thermal cycling may bedue to the impurities (2–3 wt.%) in the components of the mixture[6,11].
4. Conclusion
In this study, the CA/PA eutectic mixture was prepared asPCM for low temperature LHTES applications. The DSC ther-mal analysis indicated that the CA/PA eutectic mixture (76.5/23.5 wt.%) are promising PCM for LHTES systems in terms ofphase change temperatures (Tm=21.85 °C; Tf =22.15 °C) andlatent heat values of melting and freezing (ΔHm=171.22 J/g;ΔHf =173.16 J/g). Accelerated thermal cycling test showed thatthe eutectic mixture as a PCM had good long-term thermalreliability. Based on all results, it can be also concluded thatthe mixture has a great potential for low temperature LHTESsystems to be used for low temperature heating and coolingapplications.
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