Solar Energy Materials & Solar Cells 108 (2013) 98–104

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Fatty acid ester-based commercial products as potential new phase changematerials (PCMs) for thermal energy storage

Ahmet Alper Aydın n,1

Chemical Engineering Department, Faculty of Chemical and Metallurgical Engineering, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey

a r t i c l e i n f o

Article history:

Received 25 May 2012

Received in revised form

1 August 2012

Accepted 24 August 2012Available online 11 October 2012

Keywords:

Phase change

Heat transfer

Thermal energy storage

Renewable energy

Fatty acid

Ester

48/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.solmat.2012.08.015

þ49 176 76535232, þ90 532 3029298.

ail addresses: aydin@wzw.tum.de, ahmetalpe

sent address: Analytical Research Group,

, Technische Universitat Munchen, Am Coulo

y.

a b s t r a c t

Thermal energy storage applications require readily available commercial PCMs which offer suitable

thermal properties desired for thermal applications. However, commercial PCMs in the market are

restricted with few material groups which include mostly different paraffin waxes and salt hydrates.

Therefore, three commercial products which have been currently used in cosmetics and personal care

industry have been investigated from the perspective of thermal energy storage engineering. The

presented commercial products have the main compositions of different high-chain mono- and diesters

with the trade names of Cutina EGMS, Cutina AGS and Cutina CP. Thermal properties of the commercial

products have been presented with necessary statistical calculations in terms of phase change

temperature, enthalpy, specific heat capacity (Cp) values of liquid and solid states, thermal decom-

position and thermal reliability after 1000 thermal cycles. The DSC analyses indicate that the melting

temperatures are between 44 1C and 60 1C with phase change enthalpies above 180 kJ/kg. According to

the presented results, Cutina EGMS has average whereas Cutina AGS and Cutina CP have fairly well

thermal behaviors and properties for low temperature heat transfer applications. The research

outcomes offer a new potential market to the investigated products.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

Utilization of renewable energy sources and enhanced energyefficiency are two important issues to reduce carbon dioxideemissions and in turn, the greenhouse effect. More efficientutilization of various energy sources can provide cost savings byreducing the loss of energy during production with the help ofenergy storage. In this sense, thermal energy storage plays animportant role when there is a mismatch between demand andsupply of the energy.

In addition to efficient utilization of various energy sources,thermal energy storage can also be used to lower the installedpower and energy consumption of heating and cooling applica-tions. Several researches have been conducted to determine thecontribution of PCMs in reducing the installed heating and cool-ing powers in households [1,2]. The common conclusion of theseresearches is that utilization of PCMs in living places with properengineering and design approaches helps to regulate the interiortemperature fluctuations and to lower the energy consumption.

ll rights reserved.

raydin@gmail.com

Institute of Water Quality

mbwall, D-85748 Garching,

From the perspective of retrieving heat from renewable energysources like solar energy, thermal energy storage systems providethe continuity of the heat transfer independently from weatherconditions and solar radiation intensity. Over the past fewdecades, extensive efforts have been made to apply the latentheat storage method to solar energy systems, where heat is storedduring the day for use at night [3–5]. The collectors and heatexchangers are two main parts of solar energy systems in whichPCMs can be utilized. The collectors with embedded PCM layersstore solar energy first in the thermal energy storage volume andthen it is transferred to the water circulating pipes located insidethe PCM [6,7]. Similarly, heat exchangers containing PCM withmodified designs yield more hot water next day in the morning ascompared to the conventional storage unit [3].

PCMs are functional tools for decreasing the installed powerand increasing the efficiency of retrieving energy from renewablesources. However, the thermal properties of the employed PCMsplay the main role in defining the heat sink capacity and servicelife of applications. Therefore, it is preferred using the one whichis available in the required temperature interval with the highestphase change enthalpy in unit mass and the best thermalreliability.

In researches related to investigating new commercial PCMcandidates, it is important to introduce industrial chemicals withproper thermal and chemical properties because efficiency ofthermal energy storage systems relies on the properties of the

A.A. Aydın / Solar Energy Materials & Solar Cells 108 (2013) 98–104 99

employed material. Therefore, not only high phase changeenthalpy and specific heat values, but also properties like repro-ducible crystallization without degradation, small supercoolingdegree and non-corrosive and stable nature should also be met byPCMs. However, most of the inorganic commercial PCMs in themarket are lacking in offering some of these important properties.In this sense, investigations on commercial products which havereadily been used for different industrial purposes gain moreimportance to offer new bulk materials to the field of energystorage.

Compared to the inorganic PCMs in the market, the organicPCMs generally offer better thermal properties, such as long-lasting thermal reliability. They preserve their thermal propertiesafter several hundreds of phase changes. On the other hand, thesalt hydrates which are the most widely used inorganic commer-cial PCMs tend to dehydrate in the process of thermal cycling andthey seriously suffer from high degree of supercooling due tonucleation problem followed by phase segregation [8,9]. Anadverse effect of supercooling is observed when more than afew degrees of phase change difference cannot be prevented.Phase change differences above 5 1C can entirely disturb the heatextraction from the sink [9].

Among the available PCMs in literature, the high-chain fattyacid esters of higher alcohols have recently attracted attention forlow temperature thermal energy storage applications. They offerphase change enthalpies above 185 kJ/kg and melting temperaturerange between 29 1C and 60 1C. These PCMs exhibit better thermalbehavior and provide more preferable heat storage properties thanmost of the known organic and inorganic PCMs in literature. It hasbeen stated that they are suitable for low temperature thermalapplications with their non-toxic, non-corrosive and stable thermalbehavior [10–12]. In addition to the high-chain fatty acid estersof higher alcohols, high-chain dicarboxylic acid diesters of 1-tetradecanol (myristyl alcohol) [13], fatty acid esters of glycerolwith myristic, palmitic and stearic acids [14], fatty acid esters oferythritol and xylitol with stearic and palmitic acids [15,16] havealso been recently investigated from the thermal energy storagepoint of view in the last 2 years.

In the light of the above mentioned researches, the aim of thispaper is to present the thermal properties and behaviors of threecommercial products of BASF The Chemical Company with highchain fatty acid ester-based compositions and to point out theirsuitability to thermal energy storage applications. These chemi-cals have been readily used in cosmetics and personal careindustry, including body to all skin care, shampoo and foam bathproduct formulations. The outcomes of the research suggest theutilization of these products in the field of thermal energy storageand introduce a new promising market.

2. Experimental

2.1. Materials

The commercial products were kindly supplied by BASF TurkKimya San. ve Tic. Ltd. S-ti. which is located in Dilovasi/Kocaeli-Turkey.

Cutina EGMS, Cutina AGS and Cutina CP are three commercialproducts of the chemical company which have been widely usedin shampoo, foam bath and cosmetic skin care formulations.

2.2. Fourier transform infrared spectroscopy (FT-IR)

FT-IR spectra of the commercial high-chain fatty acid mono-and di-esters were recorded on a Perkin Elmer FT-IR Spectrum

100 spectrometer with universal ATR accessory between 4000and 600 cm�1 wavelengths.

2.3. Gas chromatography–mass spectrometry (GC–MS)

GC–MS analyses were conducted with Agilent 7890A GCsystem and Agilent 5975C inert XL EI/CI MSD with Triple-AxisDetector by use of non-polar capillary GC column of Agilent HP-5MS stationary phase (30 m�0.25 mm, 0.25 mm film). The sam-ple solutions were prepared in tetrahydrofurane and the GCtemperature program was as follows: hold at 120 1C for 2 min,increase at 20 1C/min heating rate to 180 1C with 2 min hold andthen, increase at 10 1C/min heating rate to 310 1C with 12 minhold at 310 1C.

2.4. Differential scanning calorimeter (DSC)

A Perkin Elmer Jade DSC was used for the calorimeter analyses.The measurements were carried out under inert nitrogen atmo-sphere at 20 ml/min flow rate. All DSC thermal analyses wereconducted at 5 1C/min heating and cooling rate for the determi-nation of phase change temperature and enthalpy.

DSC analyses were conducted according to the ASTM standardtest method with designation numbers E 792-06 and E 1269-11,explaining the determination of enthalpies of fusion and freezingand specific heat of liquids and solids, respectively [17,18]. Thetemperature and heat calibration of the instrument was per-formed systematically with zinc and indium references prior tothe analyses on each workday. Sapphire reference was used asinternal standard for specific heat measurements. Every pre-sented DSC data in this paper was calculated according to theresults of at least four individual analyses in order to minimizeuncertainty.

2.5. Thermo-gravimetric analyzer (TGA)

Perkin Elmer STA-6000 was used for the thermo-gravimetricdecomposition with temperature, including the decompositionbehavior, onset and 5% weight loss temperatures of the materials.The analyses were carried out under inert nitrogen atmosphere at20 ml/min flow and 10 1C/min heating rates.

The analyses were conducted according to the general princi-ples given in BS EN ISO 11358:1997 [19]. The weight andtemperature calibrations of the instrument were made by usingthe reference weight and according to the sensor calibration ofthe instrument, respectively. The calibration of the instrumentwas performed systematically prior to the first analysis of eachworkday. Every presented TGA data in this paper was calculatedaccording to the results of at least 2 individual analyses.

2.6. DNA thermal cycler

Bio-Rad MJ Mini DNA thermal cycler was used to provideautomated 1000 thermal heating and cooling cycles in order toobserve the thermal performance of each PCM. Each PCM wascycled in the temperature interval of 25 1C to provide completephase change.

3. Results and discussion

3.1. The commercial products and their chemical compositions

Cutina EGMS, Cutina AGS and Cutina CP are sold as additive inthe market of cosmetics and personal care. Cutina AGS and CutinaEGMS are mainly used as opacifier and pearlizing agents for the

Table 1GC–MS composition list and the approximate relative abundance.

PCMs Retentiontime (min)

Chemicalcompound

Aprx. relativeabundance (%)

Cutina EGMS 13.8 Ethylene glycol monopalmitate 47

15.6 Ethylene glycol monostearate 53

A.A. Aydın / Solar Energy Materials & Solar Cells 108 (2013) 98–104100

preparation of shampoos and foam bath products while Cutina CPis preferred due to its consistency-giving properties in skin careformulations. The approximate sale prices of these additives are2.5 h/kg for Cutina EGMS, 3.6 h/kg for Cutina AGS and 5.8 h/kg forCutina CP in the Turkish market. The flake and granule likeappearances of the products are shown in Fig. 1.

Fatty acids and alcohols show distinctive absorptions atvarious wavelengths. On a FT-IR spectrum, the fatty acids showvery broad trough of bonded oxygen–hydrogen stretching vibra-tions between 2500 and 2700 cm�1, whereas alcohols show asimilar trough at higher wave numbers in the range of 3230–3550 cm�1. The carbonyl stretching vibrations of saturated fattyacids absorb in the range of 1700–1725 cm�1 in general [20].

In Fig. 2, the carbonyl stretching vibrations of acyclic saturatedesters are seen around 1732 cm�1. The absence of any vibrationsin the ranges 2500–2700 cm�1 and 1700–1725 cm�1 on the FT-IRspectra indicate that the products do not contain any excess freefatty acids. However, the bonded oxygen–hydrogen stretchingvibration of the ethylene glycol monostearate is clearly seenaround 3271 cm�1 on the spectrum of Cutina EGMS.

In addition to the FT-IR spectra, further GC–MS analyses wereperformed in order to clarify the impurity content of the com-mercial products. Since impurities in the industrial raw materialsdirectly affect the composition of the end product, it is importantto take into account the influence of product composition onthermal behaviors and properties of the samples. According to theGC–MS chromatograms, the investigated commercial productscontain other chemicals including different mono- and/or diestersas impurities. Among the investigated products, Cutina CP has themost complex composition with octadecyl hexadecanoate andhexadecyl hexadecanoate as main constituents. Cutina EGMS isconsisted of a mixture of partially esterified ethylene glycol,whereas Cutina AGS contains predominantly (Z95%) ethyleneglycol diesters in addition to partially esterified ethylene glycol inlower extent.

The chemical composition list of the products is presented inTable 1 with retention times on GC column and approximate

Fig. 1. Commercial products: (a) Cutina EGMS, (b) Cutina AGS and (c) Cutina CP.

%T

4000.0 3600 3200 2800 2400 2000 180

cm

3271.28

a

b

c

Fig. 2. FT-IR spectra: (a) Cutina AGS, (b

relative abundance. The GC–MS chromatogram of Cutina AGS isgiven as an example in Fig. 3.

3.2. Determination of thermal properties of the commercial products

In industrial production, product purity can be maintained upto a certain degree due to process conditions or impurities whichare originated from industrial raw materials and the thermalbehaviors are directly affected from the end product content.Depending on the content and level of impurity, DSC curves caneither show wider phase change paths or formation of secondarypeaks with distorted curve sharpness in both conditions.

The investigated commercial products have mainly similarchemical structures with the reported high-chain fatty acid estersand diesters of high-chain dicarboxylic acids of higher alcohols inliterature. The reported high-chain esters and diesters have onedistinct phase change curve at a constant temperature with-out any significant super-cooling tendency and the changes inthermal properties are less than 1% after 1000 thermal cycles[10–13]. However, different than the reported thermal behaviorin literature, the DSC analyses of the commercial products showbroader phase change intervals and secondary peak formationsdue to the impurities given in Table 1. The DSC curves of CutinaEGMS and Cutina AGS are presented in Fig. 4.

Secondary peak formations and distorted curve sharpnesscause onset temperature differences during phase change and in

0 1600 1400 1200 1000 800 600-1

1732.62

) Cutina EGMS and (c) Cutina CP.

Cutina AGS 13.5 Ethylene glycol monopalmitate 2

14.4 Ethylene glycol monostearate 3

23.2 Ethylene glycol dipalmitate 33

27.1 Ethylene glycol distearate 62

Cutina CP 16.4 Dodecyl dodecanoate o1

17.9 Dodecyl tetradecanoate o1

20.7 Dodecyl octadecanoate 18

22.4 Hexadecyl hexadecanoate 24

24.7 Octadecyl hexadecanoate 43

27.6 Octadecyl octadecanoate 14

Abu

ndan

ce

1e+07

Time (min)

900000

800000

700000

600000

500000

400000

300000

200000

100000

2 4 6 8 10 12 14 16 18 20 22 24 26 28

14.4

13.5

23.2 27.1

Fig. 3. GC–MS chromatogram of Cutina AGS.

-20

-30

-40

-10

0

10

20

30

40

10 20 30 40 50 60 70 80

Temperature (°C)

Hea

t Flo

w E

ndo

Up

(mW

)

b

b

a

a

Fig. 4. DSC curves: (a) Cutina EGMS and (b) Cutina AGS.

Table 2Phase change temperatures of the commercial products.

PCM Melting temperature (1C) Freezing temperature (1C)

Cutina EGMS 52.88 53.07

Cutina AGS 59.65 59.02

Cutina CP 44.72 48.67

Table 3Phase change enthalpies of the commercial products.

PCM Enthalpy offusion (kJ/kg)

795% Conf.interval

Enthalpy offreezing(kJ/kg)

795%

Conf.interval

Cutina EGMS 183.60 73.13 184.17 73.46

RSD (%) 2.04 2.25

Cutina AGS 213.08 73.05 213.07 73.41

RSD (%) 1.36 1.52

Cutina CP 203.50 73.53 203.04 73.93

RSD (%) 2.08 2.31

A.A. Aydın / Solar Energy Materials & Solar Cells 108 (2013) 98–104 101

the case of the investigated three commercial products, thetemperature differences between the onset values are less than4 1C at 5 1C/min heating and cooling rate which is quite goodcompared to commercial inorganic PCMs in the market. AlthoughCutina CP exhibits the highest difference as a consequence of itshigh impurity level and variety of different fatty acid estercomposition, the onset differences of these products do notinhibit the heat transfer process in thermal applications whereslower heat transfer rate dominates the process. The phasechange onset temperatures are given in Table 2.

In addition to the distribution of the phase change tempera-tures between 44 1C and 60 1C, the differences between productcontents also affect the measured phase change enthalpies. Forexample, the main ethylene glycol diester content in Cutina AGSenhances the phase change enthalpy approximately by 30 kJ/kgand increases the phase change temperature approximately by7 1C with respect to ethylene glycol monoester content of Cutina

EGMS. Based on the given GC–MS composition list in Table 1, it ispossible to claim that diester formation of ethylene glycol is morefavorable than partially esterified ethylene glycol in terms ofphase change enthalpies.

In general, the phase change enthalpy values of all productsare above 180 kJ/kg at defined phase change temperatures. Thephase change enthalpies of the commercial products are tabu-lated in Table 3 with relative standard deviation (RSD) and 95%confidence interval of the measured data.

Besides the determination of the phase change temperaturesand enthalpies of the products, the specific heat values of the liquidand solid states have also been determined in order to providemore thermal data for engineering calculations. Even if specific

A.A. Aydın / Solar Energy Materials & Solar Cells 108 (2013) 98–104102

heat capacity defines the amount of sensible heat absorbed perunit mass, it has been omitted by researchers due to its lowerthermal storage density. In this research, Cutina AGS and Cutina CPhave been analyzed for their specific heat values due to theirhigher latent heat absorption capacity than Cutina EGMS.

The specific heat capacity values have been presented as afunction of temperature for both solid and liquid phases. Thereliability of the presented data is 73% and the equationsgenerated represent the temperature interval where no phasechange occurs between 0 1C and 90 1C. The first order coefficientsof the solid and liquid phases are given in Tables 4 and 5,respectively. Additionally, the average specific heat capacityvalues of Cutina AGS and Cutina CP in liquid and solid stateshave also been presented in Table 6 for the same temperatureinterval.

3.3. Thermal reliability of the commercial products

Thermal reliability is an important parameter to estimate theefficient time span of a designed thermal energy storage system.

Table 4Coefficients of the polynomials in solid state. Cp(kJ/kg 1C)¼AT(1C)þB.

PCM A B R2

Cutina AGS 0.0123 1.6009 0.99

Cutina CP 0.0192 2.1654 0.98

Table 5Coefficients of the polynomials in liquid state. Cp(kJ/kg 1C)¼AT(1C)þB.

PCM A B R2

Cutina AGS 0.0036 1.9857 0.96

Cutina CP 0.0026 2.4805 0.97

Table 6Average specific heat capacity values of solid and liquid states (kJ/kg 1C).

PCM Solid state Liquid state

Cutina AGS 1.79 2.28

Cutina CP 2.30 2.68

-10

-15

-5

0

5

10

15

10 30200

Temper

Hea

t Flo

w E

ndo

Up

(mW

)

Fig. 5. DSC curves: (a) Cutina C

Materials tend to lose their thermal performances after repeatedthermal cycles and the phase change temperatures and enthalpieschange significantly. However, a suitable PCM must have reliablethermal properties with reproducible phase change cycles with-out degradation. Therefore, the changes in thermal propertiesafter 1000 phase change cycles have been presented to define theeffects of thermal cycling on stability. Thereby, the suitability ofthe commercial products to thermal applications is also clarifiedin terms of thermal reliability which is expressed by differenceand percentage change in phase change temperatures andenthalpies according to the DSC results of the aged and originalsamples.

The overlapped DSC curves indicate that the materials do notundergo any decomposition steps and they preserve their thermalbehavior. However, the degree of changes in phase change tem-perature and enthalpy values is higher than that of the reportedhigh-chain fatty acid esters in literature [10–13]. The impuritycontent of the products plays an important role in increasing thedeviations of the thermal values after 1000 thermal cycles. How-ever, the deviations do not cause any new secondary peak forma-tions different than the original products. Thus, it can also be statedthat these materials exhibit also chemical stability during 1000thermal cycles. The compared similar thermal behaviors of agedand original samples of Cutina CP and Cutina AGS are shown withoverlapped DSC curves in Figs. 5 and 6, respectively. The changes inthe thermal properties are presented in Table 7.

3.4. Determination of thermal endurance by thermogravimetric

analysis (TGA)

The thermal endurance level of the products defines theirdurability against temperature increase without any decomposi-tion. In this sense, thermogravimetric data present the highesttemperature limit of an exothermic coacervation process whichthe PCM can withstand.

The investigated products have onset decomposition and 5%weight loss temperatures above 285 1C and 240 1C, respectively.According to the TGA curves of the investigated products given inFig. 7, Cutina AGS and Cutina CP can withstand an exothermiccoacervation process up to 200 1C. The TGA data also show thatCutina EGMS contains moisture at around 2% which is clearlyseen with the initial weight loss at lower temperatures. Thethermal decomposition values of the commercial products aregiven in Table 8.

The diester based product Cutina AGS shows better durabilitythan the monoester based Cutina EGMS with higher onset

40 50 60

ature (°C)

a

a

b

b

P and (b) Cutina CP aged.

Temperature (°C)

-40

-30

-20

-10

0

10

20

30

40

10 20 30 40 50 60 70 80

Hea

t Flo

w E

ndo

Up

(mW

)

a

b

b

a

Fig. 6. DSC curves: (a) Cutina AGS and (b) Cutina AGS aged.

Table 7Differences in the phase change temperatures and enthalpies after 1000 thermal cycles.

PCM Melting temperaturedifference (1C)

Freezing temperaturedifference (1C)

Enthalpy of fusiondifference (kJ/kg)

Enthalpy of freezingdifference (kJ/kg)

Cutina EGMS �1.03 (�1.95%) 1.18 (2.22%) �7.51 (�4.09%) �6.95 (�3.77%)

Cutina AGS 0.20 (0.34%) 0.30 (0.50%) �5.12 (�2.40%) �5.37 (�2.52%)

Cutina CP 0.22 (0.48%) 0.06 (0.12%) �4.77 (�2.34%) �4.45 (�2.19%)

100

90

80

70

60

50

40

30

20

10

050 100 150 200 250 300 350 400 450

Temperature (°C)

Wei

ght %

(%

)

a

b

c

Fig. 7. TGA curves: (a) Cutina AGS, (b) Cutina EGMS and (c) Cutina CP.

Table 8Decomposition temperatures of the PCMs.

PCM Onset decompositiontemperature (1C)

5% Weight losstemperature (1C)

Cutina EGMS 308.84 240.48

Cutina AGS 332.42 283.55

Cutina CP 286.32 251.89

A.A. Aydın / Solar Energy Materials & Solar Cells 108 (2013) 98–104 103

decomposition temperature. In this case, the thermal enduranceis favored (similar to enthalpy values) with diester formation ofethylene glycol.

4. Conclusion

Concerning the energy efficiency issues, ‘‘thermal energystorage’’ plays an important role and today, PCMs are importanttools of energy storage. However, further researches on commer-cial chemicals which are readily available in other markets in bulkamount are needed to increase the variety of PCMs for differentthermal applications.

The introduced three products of the company offer goodthermal properties which are appropriate for thermal applications.The melting temperatures of the introduced materials are between44 1C and 60 1C and the phase change enthalpy values vary between183 kJ/kg and 214 kJ/kg. Besides, these chemicals are non-toxic,non-corrosive and can easily be supplied. However, higher enthalpy

A.A. Aydın / Solar Energy Materials & Solar Cells 108 (2013) 98–104104

values of Cutina AGS and Cutina CP, together with better thermalreliability and higher thermal endurance, are the advantages of thetwo products against Cutina EGMS.

In conclusion, it can be stated that the introduced chemicalsare suitable commercial PCM candidates for low temperaturethermal applications with better thermal properties than most ofthe available PCMs in the market. The presented thermal dataoffer a new market for the introduced commercial products.

Acknowledgments

The commercial products were kindly supplied by BASF TurkKimya San. ve Tic. Ltd. S-ti. (Dilovasi/Kocaeli-Turkey).

The author thanks to the Department of Chemistry, Faculty ofArts and Sciences, Marmara University (Kadikoy/Istanbul-Turkey)for the additional instrumental support.

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