Thermal properties and thermal reliability of eutectic mixtures of some fatty acids as latent heat storage materials Ahmet Sarı * , Hayati Sarı, Adem € Onal Department of Chemistry, Gaziosmanpas ßa University, Tokat 60240, Turkey Received 16 January 2003; accepted 14 June 2003 Abstract The present study deals with two subjects. The first one is to determine the thermal properties of lauric acid (LA)–stearic acid (SA), myristic acid (MA)–palmitic acid (PA) and palmitic acid (PA)–stearic acid (SA) eutectic mixtures as latent heat storage material. The properties were measured by the differential scanning calorimetry (DSC) analysis technique. The second one is to study the thermal reliability of these materials in view of the change in their melting temperatures and latent heats of fusion with respect to repeated thermal cycles. For this aim, the eutectic mixtures were subjected to 360 repeated melt/freeze cycles, and their thermal properties were measured after 0, 90,180 and 360 thermal cycles by the technique of DSC analysis. The DSC thermal analysis results show that the binary systems of LA–SA in the ratio of 75.5:24.5 wt.%, MA–PA in the ratio of 58:42 wt.% and PA–SA in the ratio of 64.2:35.8 wt.% form eutectic mixtures with melting temperatures of 37.0, 42.60 and 52.30 °C and with latent heats of fusion of 182.7, 169.7 and 181.7 J g 1 , respectively. These thermal properties make them possible for heat storage in passive solar heating applications with respect to climate conditions. The accelerated thermal cycle tests indicate that the changes in the melting temperatures and latent heats of fusion of the studied eutectic mixtures are not regular with increasing number of thermal cycles. However, these materials, latent heat energy storage materials, have good thermal reliability in terms of the change in their thermal properties with respect to thermal cycling for about a one year utility period. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Eutectic mixture of fatty acids; Phase change material; Thermal property; Thermal cycling; Thermal reliability; Technique of DSC thermal analysis Energy Conversion and Management 45 (2004) 365–376 www.elsevier.com/locate/enconman * Corresponding author. Tel.: +90-356-252-1582; fax: +90-356-252-1585. E-mail address: [email protected](A. Sarı). 0196-8904/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0196-8904(03)00154-7
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Energy Conversion and Management 45 (2004) 365–376www.elsevier.com/locate/enconman
Thermal properties and thermal reliability of eutectic mixturesof some fatty acids as latent heat storage materials
Ahmet Sarı *, Hayati Sarı, Adem €OOnal
Department of Chemistry, Gaziosmanpas�a University, Tokat 60240, Turkey
Received 16 January 2003; accepted 14 June 2003
Abstract
The present study deals with two subjects. The first one is to determine the thermal properties of lauric
(SA) eutectic mixtures as latent heat storage material. The properties were measured by the differential
scanning calorimetry (DSC) analysis technique. The second one is to study the thermal reliability of these
materials in view of the change in their melting temperatures and latent heats of fusion with respect to
repeated thermal cycles. For this aim, the eutectic mixtures were subjected to 360 repeated melt/freeze
cycles, and their thermal properties were measured after 0, 90,180 and 360 thermal cycles by the technique
of DSC analysis. The DSC thermal analysis results show that the binary systems of LA–SA in the ratio of75.5:24.5 wt.%, MA–PA in the ratio of 58:42 wt.% and PA–SA in the ratio of 64.2:35.8 wt.% form eutectic
mixtures with melting temperatures of 37.0, 42.60 and 52.30 �C and with latent heats of fusion of 182.7,
169.7 and 181.7 J g�1, respectively. These thermal properties make them possible for heat storage in passive
solar heating applications with respect to climate conditions. The accelerated thermal cycle tests indicate
that the changes in the melting temperatures and latent heats of fusion of the studied eutectic mixtures are
not regular with increasing number of thermal cycles. However, these materials, latent heat energy storage
materials, have good thermal reliability in terms of the change in their thermal properties with respect to
thermal cycling for about a one year utility period.� 2003 Elsevier Ltd. All rights reserved.
366 A. Sarı et al. / Energy Conversion and Management 45 (2004) 365–376
1. Introduction
In recent years, the latent heat thermal energy storage (LHTES) method suitable for solarheating and air conditioning has received considerable attention due to its advantages of storing alarge amount of energy in a small mass/volume and phase transition at constant temperature. Theselection of the heat storage material as phase change material (PCM) in the LHTES methodplays an important role from the points of view of thermal efficiency, economic feasibility andutility life of the system. So, many studies have focused on the development of new PCMs andimprovement of their thermal properties for thermal energy storage (TES) with respect to differentclimate conditions. In this regard, a number of organic, inorganic and eutectic PCMs have beenstudied [1–4]. Among these groups of materials, fatty acids were mostly investigated as PCMs inTES systems because of their suitable phase change temperature, high latent heat density, lowcost, ready availability, non-toxicity, non-flammability, non-subcooling, non-corrosiveness, smallvolume change [5–8] and good thermal reliability after a large number of melt/freeze cycles [9–11].Moreover, a considerable number of binary eutectics of some fatty acids may be tailored withalmost any desired melting point for TES systems with respect to climate requirements. A limitednumber of the eutectic mixtures of fatty acids have been studied so far [5–8,12–14]. Therefore,characterization of the thermal properties of the new eutectics is needed as a database forparametric analysis of the systems.
Besides the measurement of the thermal properties of a new PCM developed for TES appli-cations, the determination of its thermal reliability as a consequence of repeated melting andsolidification processes is essential for assurance of the long term performance of a latent heatstore. Therefore, the accelerated thermal cycle test of a single or eutectic PCM should be con-ducted to study the changes in its latent heat of fusion and melting temperature before using it as alatent heat storage material in an actual TES system.
There are several studies on the thermal stability of different PCMs after many heating–coolingcycles. Porisino [15] studied the thermal reliability of salt hydrate PCMs with melting temperaturesbetween 15 and 32 �C by measuring the latent heat of fusion and melting temperature after sub-jection to repeated cycles. Wada et al. [16] investigated the decreasing heat storage capacity ofCH3COONa Æ 3H2O during thermal cycling and performed calorimetric measurements on threekinds of samples. Sharma et al. [17] conducted 1500 accelerated thermal cycle tests to study thechanges in latent heat of fusion and melting temperature of commercial acetamide, stearic acid andparaffin wax. They concluded that paraffin and acetamide have shown reasonably good thermalstability for melting temperature and variations in latent heat of fusion during the cycle process.Sharma et al. [18] also studied the thermal reliability of urea after 50 repeated thermal cycles andrecommended that urea should not be used as a latent heat storage material. Hasan and Sayigh [9]investigated the thermal properties of some saturated fatty acids using the DSC technique after amiddle term period that includes heating–cooling cycles of 450 times. Zhang et al. [8] studied thesolid–liquid phase transitions in lauric, palmitic and stearic acids and their binary systems. Theyalso investigated the stability of the thermal properties after many times of heating–cooling cycles,such as 30, 50, 80 and 100. Sari [11] conducted 1200 accelerated thermal cycle tests to study thethermal reliability of lauric, myristic, palmitic and stearic acid. In that study, it was reported thatthose materials, as latent heat energy storage materials, have good thermal reliability in view of thechange in their thermal properties with respect to thermal cycling for a long term utility period.
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In the light of the literature survey mentioned above, a comprehensive knowledge of thethermal properties and thermal reliability of a pure or eutectic PCM should be verified by thermalcycling life testing to ensure the longevity of the TES system in which it is used. In this regard, thisstudy deals with two subjects. The first is the measurement of the thermal properties of lauric acid(LA)–stearic acid (SA), myristic acid (MA)–palmitic acid (PA) and palmitic acid (PA)–stearic acid(SA) eutectic mixtures as latent heat storage material. The properties were measured by thetechnique of DSC analysis. The second is determination of the thermal reliability of these ma-terials in terms of the changes in their melting temperatures and latent heats of the fusion withrespect to 360 repeated melt/freeze cycles. Since limited data are available on the thermal pro-perties and the effect of thermal cycling on the melting temperatures and the latent heats of fusionof the above mentioned eutectic mixtures, the present paper is an effort to provide some expandedexperimental results on this subject.
2. Experimental studies
2.1. Determination of the thermal properties
Lauric acid (LA, 98% pure), palmitic acid (PA, 96% pure), stearic acid (SA, 97% pure) andmyristic acid (MA, 97% pure) were used in the preparation of the eutectic mixtures. These acidswere supplied by Aldrich and Fluka Companies. A series of the binary systems of LA–SA, MA–PA and PA–SA in different combination proportions were prepared from the liquid mixtures byslow cooling to room temperature in order to determine their eutectic mixture ratio. A GeneralV4.1C DuPont 2000 DSC instrument was used to measure the melting temperatures and fusionheats of the single acids and their binary systems. Stearic acid of reagent grade (m.p.: 70.47 �C)was used as a reference for the temperature calibration. Samples were measured in a sealedaluminum pan with a mass of 6.5 mg. The DSC thermal analyses were performed in the tem-perature range of 0–80 �C with a heating rate of 5 �C/min and under a constant stream of nitrogenat atmospheric pressure. The melting temperature of the PCM, Tm, corresponds to the on settemperature obtained by drawing a line at the point of maximum slope of the leading edge of theDSC peak and extrapolating the base line on the same side as the leading edge of the peak. Thelatent heat of fusion, DHfus, was calculated as the area under the peak by numerical integration.Besides measuring the thermal properties of the eutectic mixtures, specific heats for their solid andliquid phases were calculated by using the following equation,
ceu ¼X
xici
where ceu is the specific heat for the solid or liquid phase of the eutectic mixture. xi and ci, denotethe mole fraction and specific heat of each of the i components in the mixture, respectively.
2.2. Accelerated thermal cycle test
Accelerated thermal cycle tests have been conducted to study the changes in melting temper-atures ðTmÞ and the latent heats of fusion ðDHfusÞ of the LA–SA, MA–PA and PA–SA eutectic
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mixtures as PCMs after repeated numbers of melt/freeze cycles. The experimental set-up consistsof a thermostatic chamber with a temperature controller. The PCM samples were put in threesealed cylindrical capsules, and then the capsules were set into the thermostatic chamber. Thecapsules, made of pyrex glass with a lid, are airtight but contain a certain amount of air. The innerdiameter and height of the capsules were 40 and 60 mm, respectively. A thermal cycle includes aheating process of the PCM to above its melting temperature and a cooling process of the PCM atroom temperature. For accelerated thermal cycle tests, the temperature of the thermostatic waterbath was maintained at approximately 50, 55 and 65 �C to melt the eutectic mixtures of the LA–SA (m.p.: 37.0 �C), MA–PA (m.p.: 42.60 �C) and PA–SA (m.p: 52.30 �C), respectively. Theamounts of the LA–SA, MA–PA and PA–SA were 120, 135 and 150 g, and the times taken formelting were about 25, 30 and 40 min and for solidifying about 35, 40 and 55 min, respectively.The above procedure was performed consecutively until the number of thermal cycles would be90, 180 and 360. To measure the melting temperatures and the latent heats of fusion of the PCMs,the capsules were opened and about 0.500 g of material withdrawn for DSC analysis after thenumber of melt/freeze test cycles mentioned above. In order to evaluate the changes in the latentheats of fusion and the melting temperatures of the on cycled PCMs, DSC thermal analysis wasperformed on the cycled and uncycled (fresh) PCM samples.
3. Results and discussion
3.1. Thermal properties of the eutectic mixtures
The thermal properties of LA–SA, MA–PA and PA–SA binary systems at different combi-nations were measured by the technique of DSC analysis. The measured thermal data for the LA–SA, MA–PA and PA–SA binary systems are given in Tables 1–3, respectively. The followingresults can be derived from the tables: the melting temperatures of the binary systems are lowerthan those of the single acids, and the temperature ranges are becoming narrow when ap-proaching the eutectic mixture ratio. Both of the acids in the mixture melt simultaneously at aconstant temperature point when the binary system has the eutectic combination ratio. This in-
Table 1
Thermal properties of LA–SA binary systems measured by DSC analysis
LA–SA (wt.%) Tm (�C) DHfus (J g�1)
0.0–100.0 69.1 201.8
72.6–27.4 36.3–41.3 186.2
73.5–26.5 36.5–39.4 185.4
74.5–25.5 36.7–38.6 184.3
75.1–24.9 36.9–37.6 183.4
75.5–24.5 37.0–37.0 182.7
76.5–23.5 37.2–36.4 181.8
80.8–19.2 38.2–33.7 180.5
100.0–0.0 42.4 186.4
Table 2
Thermal properties of MA–PA binary systems measured by DSC analysis
MA–PA (wt.%) Tm (�C) DHfus (J g�1)
0.0–100.0 58.9 189.6
50.0–50.0 39.1–45.4 173.7
55.0–45.0 41.0–43.5 171.5
56.0–44.0 41.5–43.2 171.2
57.0–43.0 42.0–42.9 169.9
58.0–42.0 42.6–42.6 169.7
59.0–41.0 43.1–42.1 169.4
80.0–20.0 48.6–30.3 174.3
100.0–0.0 52.2 182.6
Table 3
Thermal properties of PA–SA binary systems measured by DSC analysis
PA–SA (wt.%) Tm (�C) DHfus (J g�1)
0.0–100.0 69.1 201.8
60.0–40.0 51.2–54.2 183.7
62.0–38.0 51.8–53.5 182.9
63.5–36.5 52.1–52.8 182.3
64.2–35.8 52.3–52.3 181.7
64.9–35.1 52.5–51.7 181.4
65.6–34.4 52.8–51.1 181.1
85.0–15.0 56.7–45.9 183.1
100.0–0.0 58.9 189.6
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variant temperature is the eutectic melting temperature of the binary system. The melting tem-peratures were found to be 37.00 �C for the LA–SA (75.5:24.5 wt.%), 42.60 �C for the MA–PA(58:42 wt.%) and 52.30 �C for the PA–SA (64.2:35.8 wt.%) eutectic mixtures, respectively. TheLA–SA, MA–PA and PA–SA eutectic mixtures have a latent heat of fusion of 182.7, 169.7 and181.7 J g�1, respectively. The latent heats of the eutectic mixtures are so high that they can becomparable to other PCMs, such as salt hydrates and polyalcohols [1–3]. In addition, the specificheats for the solid and the liquid phases were calculated as 1.92 and 2.10 J g�1 �C�1 for the LA–SA, 2.02 and 2.27 J g�1 �C�1 for the MA–PA and 2.21 and 2.45 J g�1 �C�1 for the PA–SA eutecticmixtures, respectively.
When comparing the measured thermal properties with the values reported for the same binarysystems by other researchers in the literature, it can be observed that there is generally goodagreement, but small discrepancies, between the results. For instance, for the LA–SA binarysystem, Zhang et al. [8] established the eutectic mixture ratio as 78.0% of LA, and Kauranen et al.[7] determined the ratio as 75.5% of LA. For the same binary system, Kauranen et al. [7] measuredthe eutectic invariant temperature as 37.0 �C, which is identical with the measured value by us,whereas, it was found to be 39.0 �C by Zhang et al. [8]. Kauranen et al. [7] found the fusion heat ofthe mixture as 171.0 J g�1, which is lower than that measured by us.
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For the binary system of PA–SA, on the other hand, Kauranen et al. [7] found the eutectic ratioas 65.7:34.3 wt.%, Tm as 50.7 �C and DHfus as 179.0 J g�1, whereas Cedeno et al. [6] studied threedifferent ratios of the PA–SA binary system and reported the eutectic ratio as 60:40 wt.% and Tmas 58.0 �C. In that study, the purities of the palmitic and stearic acids were 99.3% and 97%, re-spectively. Moreover, Kauranen et al. [7] found the eutectic ratio of MA–PA to be 51.0:49.0 wt.%,Tm to be 39.8 �C and DHfus to be 174.0 J g�1. The discrepancies in the results are most probablydue to two factors: the amount of impurities of the single acids in the mixture and the heating rateperformed for the DSC measurements [19].
It is noteworthy from the DSC thermal analysis results that the latent heats of fusion and themelting temperatures of the LA–SA, MA–PA and PA–SA eutectic mixtures make them possiblefor heat storage in passive solar space heating applications, such as building and greenhouseheating, with respect to the climate conditions.
3.2. Thermal reliability of the tested eutectic mixtures
The measured melting temperatures and the latent heats of fusion of the LA–SA, MA–PA andPA–SA eutectic mixtures after 0, 90, 180 and 360 accelerated test cycles are given in Tables 4 and5, respectively. The DSC curves (showing variation of heat flow with temperature) after the ze-roth, 90th and 360th test cycles of the PCMs are shown in Figs. 1–9, respectively. From thesetables and figures, it is possible to see the change in the melting temperatures and the latent heatsof fusion of the PCMs at various numbers of thermal cycles. The values in Tables 4 and 5 and theDSC curves in Figs. 1, 4 and 7 for the zeroth cycle were taken as references for the PCMs.
Table 4
Melting temperatures, Tm, (�C) of the tested eutectic PCMs after repeated thermal cycles
No. of test cycles Eutectic PCM
LA–SA MA–PA PA–SA
0 37.00 42.60 52.30
90 36.56 42.04 53.43
180 36.12 42.70 51.88
360 37.36 42.40 53.58
Table 5
Latent heats of fusion, DHfus, (J g�1) of tested eutectic PCMs after repeated thermal cycles
No. of test cycles Eutectic PCM
LA–SA MA–PA PA–SA
0 182.7 169.7 181.7
90 171.4 157.4 185.7
180 149.8 163.8 175.5
360 183.1 174.6 186.3
Fig. 2. DSC curve of the LA–LA eutectic mixture subjected to 90 thermal cycles.
Fig. 1. DSC curve of fresh (uncycled) LA–SA eutectic mixture.
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As one can see in Table 4 and in Figs. 1–9, the melting temperature has a variation of )0.44 �Cafter 90 cycles, )0.88 �C after 180 cycles and 0.36 �C after 360 cycles for the LA–SA eutecticmixture. It has varied by )0.56 �C after 90 cycles, 0.10 �C after 180 cycles and )0.2 �C after 360cycles for the MA–PA eutectic mixture and by 1.13 �C after 90 cycles, )0.42 �C after 180 cyclesand 1.28 �C after 360 cycles for the PA–SA eutectic mixture. These results indicated that thechange in the melting temperatures of the LA–SA, MA–PA and PA–SA eutectic mixtures withincreasing numbers of thermal cycles is not regular. However, no major change in the meltingtemperatures of the PCMs was noticed. Similar results were reported for the LA–PA eutecticmixture after repeated 30, 50, 80 and 100 melt/freeze cycles [8]. Also, it can be noted that thechanges in the melting temperatures of the tested PCMs after approximately a one year utilityperiod (corresponds to about 360 melt/freeze cycles) were in an acceptable level for a PCM, thatwill be used in a latent heat energy storage system.
Fig. 3. DSC curve of the LA–SA eutectic mixture subjected to 360 thermal cycles.
Fig. 4. DSC curve of fresh (uncycled) MA–PA eutectic mixture.
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Table 5 depicts the effect of thermal cycling on the thermal stability of the LA–SA, the MA–PAand PA–SA eutectic mixtures in view of the changes in their latent heat of fusion. It can beobserved from Table 5 and Figs. 1–9 that after 90 cycles, the latent heats of fusion, DHfus, has avariation of )6.2% for the LA–SA, )7.2% for the MA–PA and 2.2% for the PA–SA eutecticmixtures, after 180 cycles, it changed by )18.0% for the LA–SA, )3.5% for the MA–PA and)3.4% for the PA–SA eutectic mixtures and after 360 cycles, by 0.2% for the LA–SA, 2.9% for theMA–PA and 2.5% for the PA–SA eutectic mixtures. The positive and negative results meanthat the latent heat of fusion of the eutectic PCMs has no regular change with increasing num-bers of thermal cycles. Similar results for the LA–PA eutectic mixture were reported by Zhanget al. [8].
Fig. 6. DSC curve of the MA–PA eutectic mixture subjected to 360 thermal cycles.
Fig. 5. DSC curve of the MA–PA eutectic mixture subjected to 90 thermal cycles.
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4. Conclusions
From the DSC thermal analysis, it can be concluded that the LA–SA, MA–PA and PA–SAeutectic mixtures are attractive PCMs for heat storage in passive solar heating systems with re-spect to the climate requirements from the points of view of their melting temperatures of 37, 42.6and 52.3 �C and their latent heats of fusion of 182.7, 169.7 and 181.7 J g�1, respectively.
From accelerated thermal cycle tests, it is observed that the change in melting temperature is inthe range of )0.88–0.36 �C for the LA–SA, )0.56 �C and 0.10 �C for the MA–PA and )0.42 �Cand 1.28 �C for the PA–SA eutectic mixtures during the 360 melt/freeze cycles. The latent heats offusion of the LA–SA, MA–PA and PA–SA eutectic mixtures have changed between )18.0% and0.2%, )7.2% and 2.9% and )3.4% and 2.5%, respectively, during the 0–360 thermal cycles. With
Fig. 8. DSC curve of the PA–SA eutectic mixture subjected to 90 thermal cycles.
Fig. 7. DSC curve of fresh (uncycled) PA–SA eutectic mixture.
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the increasing number of thermal cycles, the changes in the melting temperatures and the latentheats of fusion of these materials are not regular, but are in a reasonable level. However, all theresults are in agreement with the values reported in the literature.
It is recommended that before employing a eutectic PCM, it should be subjected to an accel-erated thermal cycle test since its thermal behavior may change. It can be concluded that theinvestigated eutectic mixtures of fatty acids as latent heat energy storage materials used for passivesolar thermal energy storage applications have shown reasonably good thermal reliability in viewof the changes in latent heat of fusion and melting temperature with respect to thermal cycling forabout a one year utility period. However, effective utilization of those materials depends on thedevelopment of an efficient and economical thermal energy storage system designed according tosuitable climate conditions.
Fig. 9. DSC curve of the PA–SA eutectic mixture subjected to 360 thermal cycles.
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Acknowledgement
The authors would like to thank Gaziosmanpas�a University Research Fund for its financialsupport of this work performed under project with grant no: 2000/12.
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