Thermal reliability test of some fatty acids as PCMs usedfor solar thermal latent heat storage applications

Ahmet Sarı *

Department of Chemistry, Gaziosmanpas�a University, Tokat 60240, Turkey

Received 2 July 2002; accepted 28 October 2002

Abstract

The purpose of this study is to determine the thermal reliability of stearic acid, palmitic acid, myristic

acid and lauric acid as latent heat energy storage materials with respect to various numbers of thermal

cycles. The fatty acids, as phase change materials (PCMs), of industrial grade (purity between 90% and

97%) were subjected to accelerated thermal cycle tests. The differential scanning calorimetry (DSC) analysis

technique was applied to the PCMs after 0, 120, 560, 850 and 1200 melt/freeze cycles in order to measure

the melting temperatures and the latent heats of fusion of the PCMs. The DSC results indicated that the

change in melting temperature for the PCMs was in the range of 0.07–7.87 �C, and the change in latent heat

of fusion was )1.0% to )27.7%, except for stearic acid between 560 and 1200 melt/freeze cycles. However,the decrease in the latent heats of fusion for all the PCMs was not regular with increasing thermal cycles.

The experimental results also show that the investigated fatty acids as latent heat energy storage materials

have a good thermal reliability in view of the latent heat of fusion and melting temperature with respect to

thermal cycling for thermal energy storage applications in the long term.

� 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Thermal reliability; Fatty acid; Accelerated thermal cycle; Melting temperature; Latent heat of fusion; DSC

1. Introduction

Latent heat thermal energy storage in phase change materials (PCMs) is considered as adeveloping energy technology, and there has been increasing interest in using this essentialtechnique for thermal applications such as heating, hot water, air conditioning and so on. Thistype of thermal energy storage offers the advantage of storing a large amount of energy in a small

Energy Conversion and Management 44 (2003) 2277–2287www.elsevier.com/locate/enconman

* Tel.: +90-356-252-1582; fax: +90-356-252-1585.

E-mail address: asari@gop.edu.tr (A. Sarı).

0196-8904/02/$ - see front matter � 2002 Elsevier Science Ltd. All rights reserved.

doi:10.1016/S0196-8904(02)00251-0

mass/volume. In a latent heat storage system, when a PCM is subjected to a phase change process(melting or solidification) at an almost constant temperature, it absorbs or releases a quantity ofheat as much as its latent heat [1–3].An ideal PCM must have the following features: appropriate phase change temperature, high

latent heat, low cost, ready availability, non-toxicity, non-flammability and uniform phase changecharacteristics, such as no subcooling or phase separation. Besides these features, it must have along life with regard to its thermal reliability depending on the number of thermal cycles. So, thechanges of latent heat values and of phase transition temperatures for a PCM after a large numberof melting and solidification processes must be as low as possible. In this sense, an acceleratedthermal cycle test of a PCM should be conducted to study the changes in latent heat of fusion andmelting temperature of a PCM before using it as a latent heat storage material in an actualthermal energy storage system.There are several studies on the thermal stabilities of different PCMs after many heating–

cooling cycles. Wada et al. [4] investigated the decreasing heat storage capacity of CH3COONa�3H2O during thermal cycling and performed calorimetric measurements on three kinds of sam-ples. They studied the effect of the addition of a thickening agent to the sample on the latent heatstorage capacity of the sample, and they reported that the addition of the thickening agentconsiderably improves the latent heat capacity after 500 thermal cycles. Ting et al. [5] conductedaccelerated cycle tests of N2SO4 � nH2O as PCM. They performed 1000 melt/freeze cycles to studythe effect of thermal cycling on the container tube but did not analyze the effect on the thermo-physical properties of the PCM. Porosino [6] tested the thermal performance reliability of somesalt hydrates with melting points between 15 and 32 �C after repeated 5650 thermal cycles bymeasuring the latent heat of fusion and melting temperature. Sharma et al. [7] have conducted1500 accelerated thermal cycle test to study the changes in latent heat of fusion and meltingtemperature of commercial acetamide, stearic acid and paraffin wax. They concluded that paraffinand acetamide have shown reasonably good thermal stability for melting temperature and vari-ations in latent heat of fusion during the cycle process. Abhat and Malatidis [8] determined theheats of fusion for palmitic and lauric acid after a short term thermal period which includes 120thermal cycles. Hasan and Sayigh [9] investigated the thermophysical properties of some saturatedfatty acids using the differential scanning calorimetry (DSC) technique after a middle term periodwhich includes heating–cooling cycles of 450 times. Zhang et al. [10] studied the solid–liquid phasetransitions in lauric, palmitic, stearic acid and their binary systems. They also investigated thestability of thermal properties after many times of heating–cooling cycles, such as 30, 50, 80and 100.In the light of the literature survey mentioned above, a comprehensive knowledge of the

thermal reliability of the PCMs as functions of repeated heating–cooling cycles is essentialfor assurance of the long term performance and economic feasibility of a latent heat storagesystem. In this regard, this study aims at determining the change in the melting temperaturesand the latent heats of fusion of stearic, palmitic, myristic and lauric acids (between 90% and97% purity) after 1200 melt/freeze cycles, assuming that the PCM in a solar system under-goes one melt/freeze cycle per day. Since limited data are available on the effect of thermalcycling on the melting temperature and the latent heat fusion of the fatty acids of industrialgrade, the present paper is an effort to provide some expanded experimental results in thissubject.

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2. Experimental

2.1. Materials

The fatty acids tested as PCMs in this work are stearic acid (P 90% pure; assay: stearic andP 40% palmitinic acid as weight, heavy metal as Pb: 6 0.001%, Ni: 6 0.0001%, s€uulfated ash at600 �C: 6 0.1%), palmitic acid (96% pure), myristic acid (95% pure) and lauric acid (97% pure).The melting temperatures of the PCMs were in the range of 40–70 �C. Stearic acid was obtainedfrom the Merck Company, palmitic acid and myristic acid from the Aldrich Company and lauricacid from the Fluka Company. The melting points, Tm, and the latent heat of fusion, DHf€uus, of thefresh (uncycled) PCMs are given in Table 1. As seen in Table 1, the stearic acid melts at 54.7 �C,which is in agreement with the value (P 54 �C) given by the Merck Company. The main impurityin stearic acid is palmitinic acid, and its melting temperature comes down to 54 �C, dependingupon the ratio of stearic acid to palmitinic acid [7,11].

2.2. Accelerated test process and DSC analysis

To determine the effect of a large number of accelerated thermal cycles on the phase changetemperature and the latent heat of fusion of the fatty acids, four cylindrical capsules were used.The capsules, made of pyrex glass with a lid, are air tight but contain a certain amount of air. Theinner diameter and height of the capsules were 40 and 60 mm, respectively. The sealed capsuleswere filled with the fatty acid samples and then set into a thermostatic chamber equipped with atemperature controller. The PCMs were heated above their melting temperature and then cooledto room temperature by shutting off the heating controller. A thermal cycle was conducted as aheating (melting) and a cooling (solidifying) process. The above procedure was performed con-secutively until the numbers of thermal cycle would be 120, 560, 850 and 1200. In order tomeasure the melting temperatures and estimate the latent heats of fusion of the PCMs, thecapsules were opened and about 0.500 g of material withdrawn for DSC analysis after the numberof melt/freeze test cycles mentioned above.The DSC analysis technique was used to evaluate the melting temperature and the latent heat

storage capacity of the uncycled and cycled PCM. For this aim, a General V4.1C DuPont 2000DSC instrument was used. All DSC samples were encapsulated in hermetically sealed aluminumpans with the mass of 4.700–7.100 mg. Stearic acid of reagent grade (m.p.: 69.3 �C) was used as areference for temperature calibration. The heating rate for all runs was 10 �C min. The meltingtemperature of the PCM corresponds to the on set temperature obtained by drawing a line at the

Table 1

Purity degrees and melting points of the tested PCMs

Fatty acid Chemical formula Purity (%) Tm (by manufacturer) (�C) Tm (by DSC) (�C)Stearic acid CH3(CH2)16COOH P 90 P 54 54.70

Palmitic acid CH3(CH2)14COOH 96 59–61 61.31

Myristic acid CH3(CH2)12COOH 95 51–53 52.99

Lauric acid CH3(CH2)10COOH 97 42–44 42.46

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point of maximum slope of the leading edge of the DSC peak and extrapolating the baseline onthe same side as the leading edge of the peak. The latent heat of fusion, DHf€uus, was calculated asthe area under the peak by numerical integration [12].

3. Experimental results and discussion

For the accelerated thermal cycle tests, the temperature of the thermostatic water bath wasmaintained at approximately 75, 70, 65 and 55 �C to melt the palmitic acid (m.p.: 61.31 �C),stearic acid (m.p.: 54.70 �C), myristic acid (m.p.: 52.99 �C) and lauric acid (m.p.: 42.64 �C),respectively. The amounts of the palmitic acid, stearic acid, myristic acid and lauric acid were 160,155, 162 and 170 g, and the times taken for melting were about 50, 40, 30 and 20 min and forsolidifying about 65, 50, 40 and 30 min, respectively.The melting temperatures of those materials were measured after 0, 120, 560, 850 and 1200

accelerated test cycles and are given in Table 2. The DSC curves (showing variation of heat flowwith temperature) after the zeroth, 560th and 1200th test cycles of the PCMs are shown in Figs.1–12. From these figures, it is possible to see the changes in the melting temperatures and the

Table 2

Melting temperatures, Tm (�C) of the PCMs after repeated thermal cycles

No. of test cycles PCM

Stearic acid Palmitic acid Myristic acid Lauric acid

0 54.70 61.31 52.99 42.64

120 54.62 61.22 52.65 42.57

560 49.66 58.79 50.78 42.38

850 48.02 56.60 46.86 41.90

1200 46.83 55.47 46.21 41.26

Fig. 1. DSC curve of fresh (uncycled) stearic acid.

2280 A. Sarı / Energy Conversion and Management 44 (2003) 2277–2287

latent heats of fusion of the PCMs at various numbers of thermal cycles. The values in Table 2and the DSC curves in Figs. 1, 4, 7 and 10 for the zeroth cycle were taken as reference for thePCMs.As seen in Table 2 and in Figs. 1–12, the change in the melting temperatures, Tm, of the PCMs

with increasing number of thermal cycles is noteworthy. Although the melting temperature wasalmost constant after 120 cycles, it decreased by 5.04 �C after 560 cycles and 7.87 �C after 1200cycles for stearic acid, 2.52 �C after 560 cycles and 5.84 �C after 1200 cycles for palmitic acid, 2.21�C after 560 cycles and 6.78 �C after 1200 cycles for myristic acid and 0.26 �C after 560 cycles

Fig. 2. DSC curve of stearic acid subjected to 560 thermal cycles.

Fig. 3. DSC curve of stearic acid subjected to 1200 thermal cycles.

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and 1.38 �C after 1200 cycles for lauric acid. These results indicate that the melting temperaturesdecreased and, at the same time, expanded in a wide range with increasing number of thermalcycles. However, no major change in the melting temperatures of the PCMs was noticed.Nevertheless, stearic acid melts over a wider temperature range when compared with the otherPCMs after the 560th and 1200th test cycle. This phase change behavior in stearic acid may be dueto its impurities. On the other hand, the decrease in the melting temperature of the lauric acid withpurity of 97% is lower than that in the other PCMs. A similar decrease in the melting point was

Fig. 4. DSC curve of fresh (uncycled) palmitic acid.

Fig. 5. DSC curve of palmitic acid subjected to 560 thermal cycles.

2282 A. Sarı / Energy Conversion and Management 44 (2003) 2277–2287

reported for fatty acids with purity of 95% by Hasan and Sayigh [9] after 450 melt/freeze cycles.Also, it can be noted that the decrease and widening of the melting temperatures of the testedPCMs after approximately a 4 year utility period (corresponds to about 1200 melt/freezecycles) were in an acceptable level for a PCM that will be used in a latent heat energy storagesystem.Table 3 depicts the effect of a large number of thermal cycles on the latent heat capacities of the

PCMs. The following results can be derived from Table 3 and Figs. 1–12. After a short termthermal period, such as 120 test cycles, the changes in the latent heats of fusion, DHf€uus, for stearic,

Fig. 6. DSC curve of palmitic acid subjected to 1200 thermal cycles.

Fig. 7. DSC curve of fresh (uncycled) myristic acid.

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palmitic, myristic and lauric acids were +3.3%, )4.8%, )0.9% and )3.9%, respectively. After 560test cycles, the changes in DHf€uus were )17.3%, )11.4%, )1.3% and )27.7% for stearic, palmitic,myristic and lauric acids, respectively. The changes in DHf€uus of stearic, palmitic, myristic andlauric acids were +2.6%, )14.4%, )20.2% and )13.1%, respectively, after 850 test cycles. Thechanges in DHf€uus of stearic, palmitic, myristic and lauric acids were )1.0%, )12.9%, )12.1% and)11.3%, respectively, after 1200 test cycles. It can be observed from these values that the latentheat of fusion of the PCMs decreased with the increased number of thermal cycles, but the de-

Fig. 8. DSC curve of myristic acid subjected to 560 thermal cycles.

Fig. 9. DSC curve of myristic acid subjected to 1200 thermal cycles.

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crease was irregular. Sharma et al. [7] reported the irregular decrease in the latent heat capacity ofstearic acid with 95% purity. On the other hand, the positive and negative changes in the fusionheat of the stearic acid is probably due to the fact that it is a mixture of stearic and palmitinic acidas mentioned in a similar way in the literature [7].

Fig. 10. DSC curve of fresh (uncycled) lauric acid.

Fig. 11. DSC curve of lauric acid subjected to 560 thermal cycles.

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4. Conclusions

It can be concluded from the experimental results that the change in the melting temperatureswas in the range of 0.26–7.87 �C, and the change in the latent heats of fusion for the PCMs was)1.0% to )27.7%, except for stearic acid between 560 and 1200 melt/freeze cycles. However, thedecrease in the latent heats of fusion for all the PCMs was not regular with increasing number ofthermal cycles. The high level of decrease in the melting temperature and irregular change in thelatent heat of fusion of stearic acid have probably resulted from its impurity (P 40% as weight)of palmitinic acid. However, all the results are in agreement with the values reported in the lite-rature.It is recommended that before employing a PCM of industrial grade, it should be subjected to

an accelerated thermal cycle test, since its thermal behavior may change. It can be concluded thatthe investigated fatty acids as latent heat energy storage materials used for solar thermal energystorage applications have shown reasonably good thermal reliability in view of the changes inlatent heat of fusion and melting temperature with respect to thermal cycling. However, furtherwork, which includes the structure analyses of the fatty acids before and after thermal cycling, is

Table 3

Latent heats of fusion, DHf€uus (kJ kg�1) of PCMs after repeated thermal cycles

No. of test cycles Stearic acid Palmitic acid Myristic acid Lauric acid

0 159.3 197.9 181.0 176.6

120 164.6 188.4 179.4 169.7

560 131.7 175.4 178.6 127.6

850 163.4 169.5 144.5 153.4

1200 157.7 172.4 159.1 156.6

Fig. 12. DSC curve of lauric acid subjected to 1200 thermal cycles.

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needed to study the nature of such a change in their thermal reliability and how to prevent it fromoccurring.

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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