Chinese J. Chem. Eng., 14(2) 270-275 (2006) RESEARCH NOTES

Thermal Energy Storage Characteristics of Myristic and Stearic Acids Eutectic Mixture for Low Temperature Heating Applications*

Ahmet Saria,** and Kamil Kaygusuzb a Department of Chemistry, Gaziosmanpasa University, 60240 Tokat, Turkey

Department of Chemistry, Karadeniz Technical University, 61 100 Trabzon, Turkey

Abstract Stearic acid (67.83'C) and myristic acid (52.32"C) have high melting temperatures that can limit their use as phase change material (PCM) in low temperature solar heating applications such as solar space and greenhouse heating in regard to climatic requirements. However, their melting temperahms can be adjusted to a suitable value by prepaxing a eutectic mixture of the myristic acid (MA) and the stearic acid (SA). In the present study, the thermal analysis based on differential scanning calorimetry @SC) technique shows that the mixture of myristic acid (MA) and stearic acid (SA) in the respective composition (by mass) of 64% and 36% forms a eutectic mixture having melting temperature of 44.13'C and the latent heat of fusion of 182.4J.g-'. The thermal energy storage characteristics of the MA-SA eutectic mixture illled in the annulus of two concentric pipes were also experimentally established. The heat recovery rate and heat chargingldischarging fractions were determined with respect to the change in the mass flow rate and the inlet temperature of heat transfer fluid. Based on the results obtained by DSC analysis and by the heat charg- ingldischarging processes of the PCM, it can be concluded that the MA-SA eutectic mixture is a potential material for low temperature thermal energy storage applications in terms of its thermo-physical and thermal characteristics. Keywords eutectic mixture, myristic and stearic acids, phase change material, thermal energy storage, differential scanning calorimetry

1 INTRODUCTION Thermal energy storage (TES) is becoming of

increasing concern in modem technology because of impending shortages and increasing cost of energy resources. A TES system is expected to provide timely and complete energy management by excessive ther- mal energy at energy-rich periods to utilize it at en- ergy poor periods. Therefore, the development of an effective TES system especially needed for solar heating applications has been popular. There are three basic methods for TES, i.e. thermo-chemical, sensible and latent heat storage. TES in the solifliquid phase change has recently attracted great interest in view of solar space heating and cooling applications due to the following advantages: (1) It involves phase change material (PCM) that provides high energy storage density; (2) The temperature of the PCM remains nearly constant during its phase change; (3) It requires much smaller PCM storage tank than that of the used for a sensible energy storage; (4) It needs much less insulation, thereby maintaining reasonable heat losses.

A large number of PCM including salt hydrates, paraffins, fatty acids and their binary and ternary

mixtures have been explored for passive and active solar TES application^"^^. Most of the PCMs, having been studied so far are in the category of salt hydrates. However, the problems of nucleation, and super cool- ing, corrosion on the PCM container and long-term instability have limited their utilization".*]. Fatty acids have also been identified as PCM potential candidates for solar TES application^[^-^]. Fatty acids have some desirable properties over many other PCMs, such as high energy capacity, congruent melting and freezing behavior, and good chemical thermal ~tability'~.']. The added advantages are that they have no toxicity, no corrosivity, less or no super cooling and small volume change during phase change, and ease of production from vegetable and animal oils"].

The adjustment of PCM melting temperature to the climatic specific requirements can be accomplished by preparation of the fatty acid mixture. In this regard, developing binary and ternary eutectics of fatty acids as new PCMs for heating and cooling TES applications has gained importance in recent

Previous investigations showed that both myristic acid and the stearic acid are promising PCMs for pas-

Received 2005-02-02, accepted 2006-01-10. * Supported by the Research Fund of Gaziosmanpasa University (No.2003/42).

** To whom correspondence should be addressed. E-mail: asari@goP.edu.&

Thermal Energy Storage Characteristics of Myristic and Stearic Acids Eutectic Mixture 271

sive solar domestic water heating and building heating from points of their thermal energy storageheleasing chara~teristics"~"~~ and thermal reliability"41. However, the melting points of stearic acid (SA, 67.83"C) and myristic acid (MA, 52.32"C) are high for some solar energy storage requirements such as solar space and agricultural heating with respect to the specific climatic conditions. The melting points of MA and SA can be tailored to a proper temperature of 44.13"C by intro- ducing the lower melting component (MA) into the higher melting component (SA) in the respective 64% and 36% by mass composition. The eutectic mixture has also a sufficient latent heat value of 184.6J.g-'. These thermo-physical properties of the mixture make it a possible candidate as PCM for solar heating re- quirements that can change according to the climatic conditions of any residential or agricultural region.

This paper aims to investigate the feasibility of employing MA-SA eutectic mixture as PCM in terms of its thermo-physical properties by DSC analysis and thermal characteristics in a vertical cylindrical energy storage medium during its heat charging/discharging processes, experimentally. Thereby, the present paper provides basic thermal parameters that are needed to design latent heat energy storage systems, using the eutectic mixture of MA-SA as PCM.

2 EXPERIMENTAL 2.1 Thermal analysis of MA-SA binary mixtures by DSC

Myristic acid (98% purity) and stearic acid (97% purity) were obtained from Aldrich Company. The binary mixtures were obtained by mixing the low-to-high molecular weight component in 10 mass ratios from 0 to 100% different mass ratios. Addi- tional molar ratios were prepared around the eutectic point to clearly establish the thermal data in this re- gion. The solid samples were weighed to within +O.lmg and were then melted and stirred for about lOmin to ensure the homogeneity of the mixture. Melting temperatures and latent heats of the mixtures were measured by differential scanning calorimeter (DSC, DuPont ZOO0 rn~del)"~]. Each sample was placed in an aluminum pan with a mass of about 4- 8.5mg, and pure indium was used to calibrate the in- strument. The analyses were performed with a heating rate of 5 0 ~ m i n - l at the temperature interval of 0- 100°C and under a constant stream of nitrogen at at- mospheric pressure. Melting point was taken at the

intersection of the extrapolated base line and tangent to the heat flow curve drawn at the inflection point of the appropriate side of the peak. Latent heat was ob- tained by numerical integration of the area between the heat flow curve and the extrapolated base line.

2.2 Experimental set-up The experimental set-up to establish the thermal

characteristics of the MA-SA eutectic mixture is shown in Fig.1. It mainly consists of a vertical con- centric heat storage unit, two constant tempera- ture-water baths, circulation pump, flowmeter and temperature measurement systems containing a data loggerkonverter with PC ADC 16 interface card module and a computer. The outer surface temperature of the PCM container (Tcs) and the insulation surface temperature of the storage unit (Zs) were measured to establish the heat loss during the heat charging and discharging processes. The heat transfer fluid (HTF, water) flows upward through the inner pipe during the experimental runs. The inlet and outlet HTF tempera- tures (Ti and To, respectively) were measured using the two probes. The temperature probes including LM 335H temperature sensor are calibrated according to a temperature immersion probe (Fluka model 80 PK type K). Calibration was performed from the reference temperature curves for both cooling and heating to have an accuracy of f 1%. The test unit was insulated by a 30mm thick glass wool to minimize heat loss.

Temperature probe I

Water bath with temperature controller

Circulation pump

Figure 1 A schematic view of the experimental set-up

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272 Chinese J. Ch. E. (Vol. 14, No.2)

0 -

-0.5

-1.0

M 3 -1.5

-2.0

-2.5

-

P A

2.3 Experimental method Before the experimental runs, the solid MA-SA

eutectic mixture (2.15kg) was filled into the annular space in the heat storage unit. About 10% of the an- nular space was left empty to allow for expansion of the mixture upon melting. A typical heat charging run was started at room temperature. The charging run was completed when all robe temperatures in the PCM were above the upper limit of melting tempera- ture range of the PCM. The heat charging run was repeated at different constant inlet HTF temperature of 58, 60 and 62°C and the mass flow rate of 0.025,0.05 and 0.075kg.s-'. The discharging run was directly initiated by the circulation of the cooling HTF at con- stant temperatures (38 and 39°C) with individual flow rates of 0.016, 0.025 and 0.033kg.s-' during the heat discharging processes. It was terminated as all probe temperatures in the PCM were below the lower limit of its melting temperature range. Temperature meas- urements were recorded by a data loggerkonverter at a time interval of 120s.

44.13'C

-

-

-

-

- 46.96'C

3 RESULTS AND DISCUSSION 3.1 Thermo-physical properties by DSC analysis

In the preliminary experiments, the melting points of the pure compounds and their mixtures at different mass combination measured by an apparatus for melting point determination (melting point deter- mination device, Electrothermal 9100 model). Ana- lytical verification of the thermal analysis of different MA-SA binary systems was confirmed employing the DSC method. The melting point and latent heat of fusion were measured with accuracy of k 0.12"C and 1.2J.g-', respectively.

Figure 2 shows the DSC curve of the MA-SA bi- nary system with eutectic composition. Fig.3 shows the melting temperature-composition diagram for the MA-SA binary systems, which is derived from the DSC data given in the previous study"41. As seen in Fig.3, introducing MA into SA decreases the melting tem- perature of SA. In the same way, adding SA into MA decreases the melting point of MA. However, as the mass percentage of the MA in the mixture reached to 64, both of the components melt simultaneously at a con- stant temperature of 44.13OC, which is eutectic invari- ant temperature for the MA-SA mixture. The eutectic mixture has a latent heat of fusion of (182.4k 1.2)J.g-'.

On the other hand, when comparing thermal data with that of given for the same eutectic mixture in the

A P d , 2oo6

-3.0 t -3.5 L I I

30 40 50 60

temperature, 'C

Figure 2 DSC heating curve of the MA-SA eutectic mixture

[latent heat of fusion, 182.4J.g-';sample, 64% MA-36% SA; size, 6.5ooOmg; method, 2.5'Cmin-' (30'C to 80'C);

comment, heating (at atmospheric Nz)]

0 10 20 30 40 50 60 70 80 90 100 SA concentration, %

MA-SA binary system -0- inircducing MA into SA; -+ intmducing SA into MA

Figure 3 Melting point-combination diagram for the

literature, it is observed a little discrepancy. For in- stance, Kauranen et al."O1 found the eutectic mass ra- tio as 65.7% MA-34.3% SA and melting temperature as 44"C, and latent heat of fusion as 18 1 .OJ.g-'. They used the fatty acids with industrial grade and used a heating rate of lO"C.min-' in their DSC analysis. Therefore, the discrepancy between the results is most probably due to that: the amount of impurity of the single acid in the mixture and heating rate performed to the samples through the DSC analysis as well as the experimental error which may be caused by other op- erational parameters' ' 51.

3.2 Heat recovery rate In the present study, the heat recovery rate for the

eutectic PCM during the discharging process was es- timated using the following equation

Thermal Energy Storage Characteristics of Myristic and Stearic Acids Eutectic Mixture 273

The second term on the right-hand side of Eq.(l) pre- sents the heat loss rate from the energy storage unit to surroundings.

The heat recovery rates obtained for the different experimental conditions during the discharging proc- ess of the MA-SA eutectic mixture are shown in Fig.4. By qualitatively examining the curves, some remark- able considerations are noted. First, after the begin- ning stage of heat release process (approximately in first 2000s), it decreases quickly until the inlet HTF temperature reaches the solidification temperature range. This is due to the sensible heat recovery rate from the liquid PCM. As time passes (approximately from 2000s to 6000s), the heat recovery rate decreases slowly. At this stage, liquid sensible heat release ter- minates and the latent heat of fusion of the eutectic mixture is released during phase transition. After this period (approximately 6000s), the temperature of the solidified PCM decreases and the solid sensible heat release continues. Furthermore, from Fig.4, it is pos- sible to see the effect of the inlet HTF temperature and mass flow rate on the heat recovery rate. The average heat recovery rate for riz =0.033kg.s-' is approxi- mately 1.4 times greater than for m =0.016kg.s-' while the average heat recovery rate for Ti:,=38"C is approximately 1.7 times greater than that of the heat recovery rate for Ti=39"C. These results indicate that the effect of solid conduction thermal resistance on the

o.8 ' 0.6 0.7 h

P g 0.5 e $' 0.4 b 8 0.3 3 n

0.2

0.1

I 1 0 2000 4000 6000 8000 10000

time, s

Figure 4 The effett of inlet HTF temperature and mass flow rate on the heat recovery rate

Ti, "2: -0- 38; -0- 39; -A- 39 rit , kgs-': -0- 0.033; -0- 0.033; -A- 0.016

heat recovery rate is more easily decreased as the inlet HTF temperature is decreased.

The time required for the latent heat release (ap- proximately 4000s), on the other hand, is about two times greater than that required for the sensible heat release (approximately 2000s) from the PCM as seen in Fig.4. This time difference is mainly due to the in- creased thermal resistance between the PCM and the HTF pipe with the increase in the amount of the so- lidified PCM[16-'91.

3.3 Heat fraction Heat fraction during phase change process of the

PCM is generally defined as the charged heat to or discharged heat from the system at a given time di- vided by the total heat charged to or discharged from the system. Heat fraction for the heat charging and discharging processes of MA-SA eutectic mixture was calculated by the following general equation:

t

Heat fraction = 0 (2) Qtot

where Q is maximum heat charging or discharging rate calculated by using Eq.(l). Qtot is total heat charged to or discharged from the system during the completion of phase change period of the eutectic PCM. It was calculated during the charging and dis- charging processes by the following equation:

tl 11 1"

Qtot = jQd t+ jQd t+ - . .+ [Qd? (3)

The effect of inlet HTF temperature and the mass flow rate on the heat charging fraction (HCF) and heat discharging fraction (HDF) for the MA-SA eutectic mixture are shown in Figs.5 and 6, respectively. The time to reach the HCF of 1 is reduced approximately by 1200 s when the inlet temperature of the HTF is increased by 272, as seen in Fig.5. It takes 840s longer to reach the HCF of 1 for the mass flow rate of 0.05kg-s-' than that for the mass flow rate of 0.075kg-s-'. On the other hand, as the inlet tempera- ture is decreased from 39°C to 38"C, the time to reach the HDF of 1 is shortened approximately by 540s. It takes 480s longer to reach the HDF of 1 for the mass flow rate of 0.025kg.s-' than that for the mass flow rate of 0.033kg.s-', as seen in Fig.6.

The following conclusions are made in the light of the results:

0 tl 1.-1

Chinese J. Ch. E. 14(2) 270 (2006)

214 Chinese J. Ch. E. (Vol. 14, No.2)

1 .o

h 0.8

8 0.6 ‘n P ct, 9 0.4

8 ;;j 0.2 8

0 2000 4000 6000 8000 10000 12000 time, s

Figure 5 The effect of inlet HTF temperature and mass flow rate on the heat charging fraction

Ti, ‘C: -0- 60; -A- 58; -0- 58 i , kg&’: -0- 0.050; -A- 0.050; -0- 0.075

1 .o c

8 g 0.8

.cl 2 0.6

fi 0.4 n

;;j 0.2 8

.9

3

0 2000 4000 6000 8000 10000 time, s

Figure 6 The effect of inlet HTF temperature and mass flow rate on the heat discharging fraction Ti, ‘C: -0- 38; -0- 39; -A- 39

m , kges-’: -0- 0.033; -0- 0.033; -A- 0.025

(1) Based on the DSC thermal analysis results, it can be concluded that the binary mixture of the myris- tic and stearic acids with respective combination of 64% and 36% (by mass) forms a eutectic mixture. The MA-SA eutectic mixture has a suitable melting tem- perature of 4413°C and a relatively high latent heat of 182.4J.g-’. These thermal properties make it possible for low temperature solar heating requirements de- pending on the climate condition of any residential or agricultural region.

(2) The heat charging and discharging character- istics of the MA-SA eutectic mixture encapsulated in the annular space between the two vertical concentric pipes assess its potential as a latent heat storage me- dium for solar thermal energy storage.

ACKNOWLEDGEMENTS The authors would like to thank Gaziosmanpqa

University Research Fund for the financial support (No. 2003/42). The authors also thank Profi Dr. Teo- man TinGer and expert Sevim Uluprnar in Middle East Technical University for their help in the DSC analy- sis.

NOMENCLATURE C

k

L m

Q heat flow rate, kW

Qm heat recovery rate, kW

Q, total heat, kJ

rI r2 Ti Ti, To Tsc t time, s dt time step, s

specific heat of HTF, J.g-’.K -’ thermal conductivity of glass wool as insulation material, W.m-’.K-’ length of PCM container, m mass flow rate of HTF, kg.s-’

radius of the PCM container without insulation, m radius of the PCM container with insulation, m inlet temperature of HTF, ‘C surface temperature of insulation, “C outlet temperature of HTF, “C surface temperature of PCM container, ‘C

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Chinese J. Ch. E. 14(2) 270 (2006)