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

Chinese J. Chem. Eng., 14(2) 270—275 (2006) RESEARCH NOTES Thermal Energy Storage Characteristics of Myristic and Stearic Acids Eutectic Mixture for...
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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 b

Department of Chemistry, Gaziosmanpasa University, 60240 Tokat, Turkey Department of Chemistry, Karadeniz Technical University, 61100 Trabzon, Turkey

Abstract Stearic acid (67.83℃) and myristic acid (52.32℃) 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 temperatures can be adjusted to a suitable value by preparing 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 (DSC) 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℃ and the latent heat of fusion of 182.4J·g-1. The thermal energy storage characteristics of the MA-SA eutectic mixture filled in the annulus of two concentric pipes were also experimentally established. The heat recovery rate and heat charging/discharging 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 charging/discharging 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

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INTRODUCTION Thermal energy storage (TES) is becoming of increasing concern in modern 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 thermal energy at energy-rich periods to utilize it at energy 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 solid/liquid 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 applications[1―4]. Most of the PCMs, having been studied so far are in the category of salt hydrates. However, the problems of nucleation, and super cooling, corrosion on the PCM container and long-term instability have limited their utilization[1,2]. Fatty acids have also been identified as PCM potential candidates for solar TES applications[3―5]. Fatty acids have some desirable properties over many other PCMs, such as high energy capacity, congruent melting and freezing behavior, and good chemical thermal stability[6,7]. 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[7]. 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 years[8―11]. 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: [email protected]

Thermal Energy Storage Characteristics of Myristic and Stearic Acids Eutectic Mixture

sive solar domestic water heating and building heating from points of their thermal energy storage/releasing characteristics[12,13] and thermal reliability[14]. However, the melting points of stearic acid (SA, 67.83℃) and myristic acid (MA, 52.32℃) 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 introducing 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 1. These thermo-physical properties of the mixture make it a possible candidate as PCM for solar heating requirements 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. Additional molar ratios were prepared around the eutectic point to clearly establish the thermal data in this region. The solid samples were weighed to within ±0.1mg and were then melted and stirred for about 10min to ensure the homogeneity of the mixture. Melting temperatures and latent heats of the mixtures were measured by differential scanning calorimeter (DSC, DuPont 2000 model)[14]. Each sample was placed in an aluminum pan with a mass of about 4— 8.5mg, and pure indium was used to calibrate the instrument. The analyses were performed with a heating - rate of 5℃·min 1 at the temperature interval of 0— 100℃ and under a constant stream of nitrogen at atmospheric pressure. Melting point was taken at the

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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 obtained 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 concentric heat storage unit, two constant temperature-water baths, circulation pump, flowmeter and temperature measurement systems containing a data logger/converter 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 (Tis) 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 temperatures (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 ±1%. The test unit was insulated by a 30mm thick glass wool to minimize heat loss. Temperature probe HTF pipe

PCM container

HTF flow direction

Flowmeter

PC

Valve

Mixer

Water bath with temperature controller Circulation pump

Figure 1

A schematic view of the experimental set-up Chinese J. Ch. E. 14(2) 270 (2006)

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

2.3

44.13

0 0.5

1

1.0 heat flow, W·g

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 annular 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 temperature range of the PCM. The heat charging run was repeated at different constant inlet HTF temperature of 58, 60 and 62℃ and the mass flow rate of 0.025, 0.05 - and 0.075kg·s 1. The discharging run was directly initiated by the circulation of the cooling HTF at constant temperatures (38 and 39℃) with individual flow - rates of 0.016, 0.025 and 0.033kg·s 1 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 measurements were recorded by a data logger/converter at a time interval of 120s.

1.5 2.0 2.5 46.96 3.0 3.5

30

40

50

60

temperature,

Figure 2

DSC heating curve of the MA-SA eutectic mixture [latent heat of fusion, 182.4J·g-1;sample, 64% MA-36% SA; size, 6.5000mg; method, 2.5℃·min-1 (30℃ to 80℃); comment, heating (at atmospheric N2)] 69

April, 2006

melting point,

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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 determination device, Electrothermal 9100 model). Analytical 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 ±0.12℃ and - 1.2J·g 1, respectively. Figure 2 shows the DSC curve of the MA-SA binary 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[14]. As seen in Fig.3, introducing MA into SA decreases the melting temperature 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 constant temperature of 44.13°C, which is eutectic invariant temperature for the MA-SA mixture. The eutectic - mixture has a latent heat of fusion of (182.4±1.2)J·g 1. On the other hand, when comparing thermal data with that of given for the same eutectic mixture in the

57 51 45 44.13 39 0

10

20

30

40

50

60

70

80

90 100

SA concentration, %

Figure 3

Melting point-combination diagram for the MA-SA binary system —●— introducing MA into SA; —◆— introducing SA into MA

literature, it is observed a little discrepancy. For instance, Kauranen et al.[10] found the eutectic mass ratio as 65.7% MA-34.3% SA and melting temperature - as 44℃, and latent heat of fusion as 181.0J·g 1. They used the fatty acids with industrial grade and used a - heating rate of 10℃·min 1 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 operational parameters[15]. 3.2

Heat recovery rate In the present study, the heat recovery rate for the eutectic PCM during the discharging process was estimated using the following equation

Thermal Energy Storage Characteristics of Myristic and Stearic Acids Eutectic Mixture

Qrec = mc (Ti − To ) −

2πkL (Tcs − Tis ) ln( r2 / r1 )

(1)

The second term on the right-hand side of Eq.(1) presents the heat loss rate from the energy storage unit to surroundings. The heat recovery rates obtained for the different experimental conditions during the discharging process of the MA-SA eutectic mixture are shown in Fig.4. By qualitatively examining the curves, some remarkable considerations are noted. First, after the beginning 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 terminates 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 possible to see the effect of the inlet HTF temperature and mass flow rate on the heat recovery rate. The average - heat recovery rate for m =0.033kg·s 1 is approxi- mately 1.4 times greater than for m =0.016kg·s 1 while the average heat recovery rate for Ti=38℃ is approximately 1.7 times greater than that of the heat recovery rate for Ti=39℃. These results indicate that the effect of solid conduction thermal resistance on the 0.8 0.7

heat recovery rate, kW

0.6 0.5 0.4 0.3 0.2 0.1 0

2000

4000

6000

8000

10000

time, s

Figure 4

The effect of inlet HTF temperature and mass flow rate on the heat recovery rate Ti, ℃: —◇— 38; —○— 39; —△— 39 m , kg·s-1: —◇— 0.033; —○— 0.033; —△— 0.016

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heat recovery rate is more easily decreased as the inlet HTF temperature is decreased. The time required for the latent heat release (approximately 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 increased thermal resistance between the PCM and the HTF pipe with the increase in the amount of the solidified PCM[16―19]. 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 divided 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 =

∫ Q dt 0

(2)

Qtot

where Q is maximum heat charging or discharging rate calculated by using Eq.(1). 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 discharging processes by the following equation:

Qtot

t1

t2

tn

0

t1

tn−1

= Q dt + Q dt + ⋅ ⋅ ⋅ +





∫ Q dt

(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 2℃, as seen in Fig.5. It takes 840s longer to reach the HCF of 1 for the mass flow rate of - 0.05kg·s 1 than that for the mass flow rate of - 0.075kg·s 1. On the other hand, as the inlet temperature is decreased from 39℃ to 38℃, 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 1 than that for the mass flow -1 rate of 0.033kg·s , as seen in Fig.6. The following conclusions are made in the light of the results: Chinese J. Ch. E. 14(2) 270 (2006)

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heat charging fraction (HCF)

1.0

ACKNOWLEDGEMENTS The authors would like to thank Gaziosmanpaşa University Research Fund for the financial support (No. 2003/42). The authors also thank Prof. Dr. Teoman Tinçer and expert Sevim Ulupınar in Middle East Technical University for their help in the DSC analysis.

0.8 0.6 0.4 0.2

0

NOMENCLATURE 2000

4000

6000 8000 time, s

10000 12000

Figure 5

The effect of inlet HTF temperature and mass flow rate on the heat charging fraction Ti, ℃: —□— 60; —△— 58; —○— 58 m , kg·s-1: —□— 0.050; —△— 0.050; —○— 0.075

heat discharging fraction (HDF)

1.0 0.8 0.6 0.4

c k L m Q Q

rec

Q tot

total heat, kJ

r1 r2 Ti Tis To Tsc t dt

radius of the PCM container without insulation, m radius of the PCM container with insulation, m inlet temperature of HTF, ℃ surface temperature of insulation, ℃ outlet temperature of HTF, ℃ surface temperature of PCM container, ℃ time, s time step, s

1 2000

4000

6000 time, s

8000

10000

Figure 6 The effect of inlet HTF temperature and mass flow rate on the heat discharging fraction Ti, ℃: —◇— 38; —○— 39; —△— 39 m , kg·s-1: —◇— 0.033; —○— 0.033; —△— 0.025

(1) Based on the DSC thermal analysis results, it can be concluded that the binary mixture of the myristic 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 temperature of 44.13°C and a relatively high latent heat of - 182.4J·g 1. These thermal properties make it possible for low temperature solar heating requirements depending on the climate condition of any residential or agricultural region. (2) The heat charging and discharging characteristics 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 medium for solar thermal energy storage. April, 2006

heat recovery rate, kW

REFERENCES

0.2

0

specific heat of HTF, J·g-1·K -1 thermal conductivity of glass wool as insulation material, W·m-1·K-1 length of PCM container, m - mass flow rate of HTF, kg·s 1 heat flow rate, kW

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Abhat, A., “Low temperature latent thermal energy storage system: Heat storage materials”, Solar Energy, 30, 313—332(1983). Zalba, B., Marin, J.M., Cabeza, L.F., Mehling, H., “Review on thermal energy storage with phase change: Materials, heat transfer analysis and applications”, Appl. Therm. Eng., 23, 251—283(2003). Sarı, A., “An experimental study on thermal energy characteristics of lauric acid as latent heat storage material during melting process”, Chinese J. Chem. Eng., 11(2), 240—243(2003). Sarı, A., Kaygusuz, K., “First and second law analyses of a closed latent heat thermal energy storage system”, Chinese J. Chem. Eng., 12(2), 290—293(2004). Hasan, A., Sayigh, A.A., “Some fatty acids as phase change thermal energy storage materials”, Renew. Energy, 4(1), 69—76(1994). Sarı, A., “Thermal reliability test of some fatty acids as PCMs used for solar thermal latent heat storage applications”, Energy Conversion Manag., 44, 2277 — 2287(2003). Feldman, D., Shapiro, M.M., Banu, D., Fuks, C.J., “Fatty acids and their mixtures as phase change materials for thermal energy storage”, Solar Energy Mater., 18, 201— 216(1989). Zhang, J.J., Zhang, J.L., He, S.M., Wu, K.Z., Liu, X.D., “Thermal studies on the solid-liquid phase transition in

Thermal Energy Storage Characteristics of Myristic and Stearic Acids Eutectic Mixture

9

10

11

12

13

14

binary systems of fatty acids”, Thermochim. Acta, 369, 157—160(2001). Sarı, A., “Thermal characteristics of a eutectic mixture of myristic and palmitic acids as phase change material for heating applications”, Appl. Therm. Eng., 23, 1005— 1017(2003). Kauranen, P., Peippo, K., Lund, P.D., “An organic PCM storage system with adjustable melting temperature”, Solar Energy, 46(5), 275—278(1991). Dimaano, M.N.R., Watanabe, T., “The capric and lauric acid mixture with chemical addivities as latent heat storage materials for cooling application”, Energy, 27, 869— 888(2002). Sari, A., Kaygusuz, K., “Thermal performance of myristic acid as a phase change material for energy storage application”, Renew. Energy, 12, 303—317(2001). Hasan, A., “Thermal energy storage system with stearic acid as phase change material”, Energy Conversion Manag., 35(10), 843—856(1994). Sari, A., “Eutectic mixtures of some fatty acids for low temperature solar heating applications: Thermal prop-

15

16

17

18

19

275

erties and thermal reliability”, Appl. Therm. Eng., 25, 2100—2107(2005). Bo, H., Gustafsson, M., Setterwall, F., “Tetradecane and hexadecane binary mixtures as phase change materials (PCMs) for cool storage in district cooling systems”, Energy, 24, 1015—1028(1999). El-Dessouky, H.T., Bouhamra, W.S., Ettouney, H., Akbar, M., “Heat transfer in vertically aligned phase change energy storage system”, Trans. ASME J. Solar Energy Eng., 121, 98—109(1999). Choi, J.C., Kim, S.D., “Heat transfer in a latent heat energy storage system using MgCl2·6H2O at the melting point”, Energy, 20(1), 13—25(1995). Choi, J.C., Kim, S.D., Han, G.Y., “Heat transfer characteristics in low-temperature latent heat storage systems using salt-hydrates at heat recovery stage”, Solar Energy Mater. Solar Cells, 40, 71—87(1996). Yanadori, M., Masuda, T., “Heat transferential study on a heat storage container with phase change material”, Solar Energy, 36,169—177(1986).

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