Thermal performance of palmitic acid as a phase change energy storage material

Thermal performance of palmitic acid as a phasechange energy storage material

Ahmet Sari a,*, Kamil Kaygusuz b

a Department of Chemistry, Gaziosmanpas�a University, 60100 Tokat, Turkeyb Department of Chemistry, Karadeniz Technical University, 61080 Trabzon, Turkey

Received 27 November 1999; received in revised form 27 October 2000; accepted 19 March 2001

Abstract

Experimental investigation of palmitic acid as a phase change material (PCM) for energy storage hasbeen conducted in this study. The performance and heat transfer characteristics of a simple tube-in-tubeheat exchanger system were studied, and the obtained results were compared with other studies given in theliterature. The present study included some parameters, such as transition times, temperature range andpropagation of the solid–liquid interface, as well as the heat flow rate characteristics of the employed cy-lindrical tube storage system. The experimental results show that the melting front moves in the radialdirection inward, as well as in the axial direction from the top toward the bottom of the PCM tube. It wasobserved that the convection heat transfer in the liquid phase plays an important role in the meltingprocess. The flow rate and inlet temperature of the heat transfer fluid to the PCM tube in the experimentedrange has an insignificant effect on the phase change processes. On the other hand, the melting and so-lidification times of the PCM can be reduced significantly by placing the tube containing the PCM in ahorizontal position rather than a vertical one. The heat storage capacity of the PCM tube is not as good aswe expected in this study, and the average heat storage efficiency (or heat exchanger effectiveness) is 53.3. Itmeans that 46.7% of the heat actually is lost somewhere. � 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Palmitic acid; Thermal performance; Phase change material; Energy storage

1. Introduction

Thermal energy storage has always been one of the most critical components in residential solarspace heating and cooling applications. Solar radiation is a time dependent energy source with an

Energy Conversion and Management 43 (2002) 863–876www.elsevier.com/locate/enconman

*Corresponding author.

E-mail address: [email protected] (A. Sari).

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

PII: S0196-8904(01)00071-1

intermittent character. The heating demands of a residential house are also time dependent.However, the energy source and the demands of a house, in general, do not match each other,especially in solar heating applications. The peak solar radiation occurs near noon, but the peakheating demand is in the late evening when solar radiation is not available. Thermal energystorage provides a reservoir of energy to adjust this mismatch and to meet the energy needs at alltimes. It is used as a bridge to cross the gap between the energy source, the sun, the applicationand the building. So, thermal energy storage is essential in the solar heating system [1–5].

The storage of thermal energy as the latent heat of a phase change material (PCM) represents agood, attractive option for thermal energy storage. A wide range of PCMs has been investigated,including salt hydrates, paraffin waxes and non-paraffin organic compounds [6–10]. Some fattyacids were investigated as PCMs for thermal energy storage by Hasan [11–13] and by the authors[14–16]. They observed that stearic, palmitic and myristic acids are suitable materials for energystorage in solar heating and cooling applications. They also observed that the melting and so-lidification times are not affected by the flow rate of the heat transfer fluid in the tested laminarrange.

In this study, the thermal performance and storage parameters of palmitic acid as a PCM wereinvestigated experimentally. The phase change temperatures and times of the palmitic acid havebeen determined. The melting and solidification behavior of the PCM in the axial and radial

Nomenclature

Cpw specific heat of water (kJ=kg �C)CpPCM specific heat of PCM (kJ=kg �C)mw mass flow rate of water (kg/min)mPCM weight of PCM (kg)Tiw inlet water temperature (�C)Tow outlet water temperature (�C)Ts;hsc outside surface temperature of heat storage container (�C)Tis outside surface temperature of insulation material (�C)T2 final temperature of PCM (�C)T1 initial temperature of PCM (�C)TPCM temperature of PCM (�C)DHmelt latent heat of melting of PCM (kJ/kg)Q heat value of PCM during charging or discharging period at any time (J/m)QT total heat value of PCM during charging or discharging period at any time (J/m)Qloss total heat loss from complete system to surroundings (J/m)k thermal conductivity (J=m �C)L length of heat storage container (m)r2 outside radius of heat storage container (m)r1 inside radius of heat storage container (m)e heat effectiveness of heat exchanger

864 A. Sari, K. Kaygusuz / Energy Conversion and Management 43 (2002) 863–876

directions and in the vertical and horizontal positions was also studied. In addition, the heatstorage efficiency of the container (heat exchanger tube) was calculated.

2. Experimental set-up and procedure

The experimental set-up is shown in Fig. 1. It consists of a heat storage container, a hightemperature bath HTP, a low temperature bath LTP, circulation pumps, piping systems and 14Pt-100 type thermocouples (PtRh–Pt) and multimeter to obtain measured temperature data. Theheat storage container consists of two concentric cylindrical copper pipes. The inner cylindricaltube is 350 mm long with a 50 mm diameter, while the outer cylindrical tube is 200 mm long with a120 mm diameter. The heat storage container was isolated by 20 mm glass wool. Thermocouplesmade of platinum, rhodium–platinum were located at the positions of 5, 15 and 25 mm in theradial direction and 20, 60, 100 and 140 mm in the longitudinal direction from the bottom of thestorage container (Fig. 2). The temperature values during phase change periods were recorded byusing a digital temperature recorder. The measurement system permits the temperature differenceto be measured at the accuracy of 0.1�C due to the calibration made previously. Two thermo-couples were also placed at the inlet and outlet of the heat transfer tube to measure the water (heattransfer fluid) temperature. A desired inlet temperature of the water for each test was maintainedby means of a chiller unit, and its flow rate was measured by a calibrated rotameter.

The PCM (950 g) is filled into the annulus of the two concentric cylindrical tubes. The ther-mophysical properties of the PCM (palmitic acid obtained from Merck Company) were tested byusing DSC analyses, and the results are shown in Fig. 3. It has 97% purity, 203.4 kJ/kg fusionenthalpy at the interval 30–80�C, 0.113 cm3/g dilation volume, 0.16 W/mK thermal conductivityand 61�C m.p. The solid phase and liquid phase densities are 0.942 (30�C) and 0.862 g/cm3 (70�C),respectively.

Prior to starting the phase change periods (initial condition), the heat storage material washeated so that it was in a completely melted state at 75�C, which is just above the transitiontemperature. After that, the melting period was started by circulating inlet hot water at the upper

Fig. 1. Schematic diagram of the apparatus. 1: Cold-water tank; 2: Hot-water tank; 3: Pump; 4: Flowmeter; 5: Heat

exchanger; 6: Temperature controller; 7: Mixer; 8: Data logger; 9: PCM sample; 10: Heater; T: Thermocouple.

A. Sari, K. Kaygusuz / Energy Conversion and Management 43 (2002) 863–876 865

melting temperatures (76–80.5�C) of the PCM with different flow rates (1.2–6.0 kg/min). Duringthis period, the temperature variations at the distances in the axial and radial directions wererecorded by five minutes time intervals. To calculate the heat losses from the PCM and heatexchanger tube to the ambient, the temperatures of the heat storage container surface and outersurface of the glass wool were also measured. The solidification period was started directly aftercompletion of the melting period. For this process, after determining the initial condition, lowtemperature water(38–47�C) at different flow rates (1.2–6.0 kg/min) was circulated through thesystem, and then, the transition temperature data were recorded at five minutes time intervals. The

Fig. 2. Heat exchanger tube and thermoresistance location; T.R: Thermoresistance.

Fig. 3. DSC analysis data for investigated PCM.


phase transition behavior of the palmitic acid was determined separately in two positions of thePCM container for which one is horizontal and other is vertical.

In this study, there is a lot of data regarding the heat transfer fluid flow rate and the inlet heattransfer fluid temperatures, but only some selected data are given in the figures to show the bestphase transition behavior of the PCM.

3. Results and discussion

3.1. Melting curves

Typical temperature data versus time melting curves, obtained in the radial and axial directions,of the palmitic acid are shown in Figs. 4 and 5. Phase transition times for melting and solidifi-cation can be obtained from the curves of the PCM temperature versus time. As seen from themelting curves, the initial energy transferred to the palmitic acid will raise its temperature from the

Fig. 4. Temperature distribution during melting of palmitic acid in radial direction (axial distance: 60 mm; inlet water

temperature: 78�C; water flow rate: 3 kg/min).

Fig. 5. Temperature distribution during melting of palmitic acid in radial direction (axial distance: 100 mm; inlet water

temperature: 78�C; water flow rate: 3 kg/min).


initial temperature to a higher temperature around the melting point of the PCM. This sensibleheat of the PCM is transferred through the PCM by pure conduction. Because of the large initialtemperature gradient between the water and the PCM, the initial change in temperature is a fastprocess due to the high heat transfer rate to the PCM. After this rapid increase, the temperaturebecomes somewhat constant during the melting period. As it melts, a thin layer of liquid is formedbetween the wall and the solid phase, and convection starts to take place. A similar phenomenonwas reported by Hasan [11,12]. The melting time was estimated from the time elapsed between theonset of transition until the completion of transition, which is the phase change temperaturerange. This time was determined as 55 min. In the case of T9 (Fig. 5, R: 5 mm, A: 100 mm),melting behavior is not obvious from the heating curve. On the contrary, one might think thatmelting is occurring at a high temperature of 70�C, even though the melting in the radial and axialdirections of the PCM may occur approximately in the 59–61�C temperature range. The mainreason for this discussion is that the thermocouple located at the T9 distance in the PCM tube israther closer to the heat flow fluid than the other ones. Based on this reason, one can be moreconfident about melting time and temperature range as measured by the thermocouples located atthe T4, T5 and T7, T8 distances into the PCM tube. This result can be estimated from Figs. 6 and7 and was found to agree with the given value in the literature [11] and the given physicalproperties by the Merck company.

Fig. 6. Temperature profile during melting of palmitic acid in radial direction (inlet water temperature: 78�C; waterflow rate: 1.2 kg/min).

Fig. 7. Temperature profile during melting of palmitic acid in axial direction (inlet water temperature: 78�C; water flowrate: 1.2 kg/min).


During the melting process of the palmitic acid, the heat transfer mechanisms undergone in thePCM are controlled by two different heat transfer rates. One is the absorbed sensible heat duringmelting, and the other is the heat transfer inside the PCM. So, the total heat transfer occurring atany point inside the PCM is equal to the sum of the two heat transfers at this point. If there is heattransfer at that point, the temperature will increase continuously as shown in Figs. 4 and 5. Themelting process takes place from the upper point to the bottom in the axial direction throughthe container, and the melting behavior in the radial direction takes place from the near point ofthe heat transfer fluid to the far point. These results were also observed to be in agreement withgiven values in the literature [11,12].

The effective factor for determining the melting process direction is the free convection heattransfer, which is increased at the upper space on the palmitic acid. Consider Figs. 4 and 5 withthe only difference being the axial distance (60 and 100 mm from the bottom, respectively). In thiscase, one might think that there is heat being transferred from the 78�C inlet water temperatureover the axial distance from 0 to 60 mm for Fig. 4 and from 0 to 100 mm for Fig. 5. Thus, thetemperature of the water is expected to be lower than 78�C at 60 mm and even lower at 100 mm,but it must be recalled that the temperature change of the inlet water is different from the tem-perature change, or distribution, in the PCM at the mentioned axial distances because the meltingfront moves in the axial direction from the top toward the bottom of the cylinder. Hence, themelting front is not cylindrical shape. The melting of the upper part of the PCM before the lowerpart is due to natural convection heat transfer in the PCM. Such convection is higher at the topbecause of the buoyancy effects, as the hot melted material rises to the top. Thus, at the same timeand axial position, the melting time of the upper part of the PCM is smaller than that of the lowerpart (less axial distance). Such axial movement of the melting front is reported by other re-searchers [11,12,17,18].

3.2. Solidification curves

Solidification curves for the palmitic acid in the radial and axial directions are shown in Figs. 8and 9. At the beginning of the solidification period, the PCM releases its sensible heat very

Fig. 8. Temperature distribution during solidification of palmitic acid in radial direction (axial distance: 60 mm; mean

storage temperature: 66.6�C; inlet water temperature: 38�C; water flow rate: 2.5 kg/min).


rapidly, then a longer time is needed to release the latent heat during the phase change. As seenfrom Figs. 8 and 9, the outermost thermocouples do not show a constant temperature duringsolidification because of high heat transfer rate. The heat transfer rate is reduced when the so-lidification front moves inward due to the increased conduction thermal resistance of the solidifiedPCM. On the other hand, in the axial direction, the solidification front moves from the top towardthe bottom of the PCM tube, as in the case of the melting process. Namely, the solidification rateat the upper part of the PCM is higher than that at the bottom of the PCM due to the additionalheat losses at the top of the PCM tube. Such losses are enhanced by the presence of the air gap atthe top. Hence, similar to the melting front, the solidification front is not cylindrical in shape. Thesolidification time was estimated as the elapsed time between the onset of transition and thecompletion of transition, which is the phase change temperature range. This time was determinedas 25 min. It is obvious that the PCM shows no subcooling, as shown in Figs. 8 and 9.

3.3. Effect of the inlet heat transfer fluid temperature

The effect of the inlet heat transfer fluid temperature on both the melting and solidificationperiods of the palmitic acid is shown in Figs. 10 and 11. As shown in these figures, the totalmelting time was decreased about 54% by increasing the inlet water temperature from 76�C to80.5�C. It is also observed that the total solidification time was decreased about 44%, as an av-erage, by decreasing the inlet water temperature from 47�C to 38�C. Thus, the melting front speedincreases substantially with increasing inlet heat transfer fluid temperature. On the other hand, thesolidification front speed increases substantially with decreasing the fluid temperature. The mainreason for these two situations is the high temperature difference between the PCM and the inletheat transfer fluid temperature, and therefore, the heat transfer rate is high between them.

When comparing the effects of changing the inlet temperature on the melting and solidificationprocesses, it can be seen that it has a larger effect on melting than on solidification. In the so-lidification process, increasing the convection heat transfer rate from the water to the PCM tubewall by increasing the driving force (temperature difference) does not translate to a similar in-crease in the heat transfer through the solid phase of the PCM due to its high thermal resistance.

Fig. 9. Temperature distribution during solidification of palmitic acid in radial direction (axial distance: 100 mm; mean

storage temperature: 67�C; inlet water temperature: 38�C; water flow rate: 2.5 kg/min).


Meanwhile, for melting, increasing the convection heat transfer rate to the wall of the PCM tuberesulted in a similar increase of its rate from the wall to the PCM due to the convection heattransfer in the liquid phase of the PCM.

3.4. Flow rate of the heat transfer fluid

The experimental heat transfer fluid flow rates (1.2–6 kg/min) are in the laminar regime, and theconvection heat transfer coefficient between the water and the PCM tube wall is constant in thislaminar range. The effect of this characteristic factor on the melting and solidification periods isshown in Figs. 12 and 13, respectively. As seen from these data, this factor has substantially thesame effect as the fluid temperature change on both phase changes of the palmitic acid. We showthat the melting time is decreased only about 20% by increasing the flow rate approximately sixtimes.

Fig. 11. Temperature distribution during solidification of palmitic acid at same water flow (2.5 kg/min) and various

water inlet temperatures.

Fig. 10. Temperature distribution during melting of palmitic acid at same water flow rate (3 kg/min) and various water

inlet temperatures.


3.5. Vertical and horizontal position of the phase change material tube

The effect of placing the heat exchanger in a horizontal position instead of the vertical one isinvestigated in this part of the work. The position effect of the PCM tube on the melting process isshown in Figs. 4, 5 and 14. On the other hand, the position effect of the tube on the solidificationprocess of the palmitic acid is given in Figs. 8, 9 and 15. According to this data, we obtained thatthe heat transfer rate is increased, and thus, the completion time of the melting and solidificationperiod is decreased at the horizontal position of the heat exchanger tube. We believe that thelarger air space around the PCM in the heat exchanger tube when it is located at the horizontalposition is more useful for getting extra conduction heat transfer than the small air space at thevertical position of the tube filled by PCM. The buoyancy effects (hot fluid rising to the top) havea much larger area to act through, which is now the diameter times the length of PCM tube, whilein the vertical case, it is the cross sectional area of the PCM tube. Bareiss [17] and Hasan [11,12]pointed out that increasing the height of a vertical tube does not increase the convection heat

Fig. 12. Temperature distribution during melting of palmitic acid at same water inlet temperature (78�C) and at various

water flow rates.

Fig. 13. Temperature distribution during solidification of palmitic acid at same water inlet temperature (38�C) and at

various water flow rates.


transfer effects. In addition to the above convection effects, the melting or solidification front has ashorter distance to travel in the upper part of the horizontal tube, thus a faster phase transition isexpected. Similar behavior is found for the solidification process, as shown in Fig. 15.

3.6. Heat flow rate, heat fraction and effectiveness of the heat exchanger

The effect of the convection heat transfer from the water to the PCM tube, as a sensible heat,was investigated. In addition, during the melting period, the stored latent heat of the palmitic acid,the extracted total heat from the system during solidification and the lost heat to the surroundingswere calculated by using Eqs. (1) and (2) [19,20]. The obtained data were plotted versus time. Asseen from Figs. 16 and 17 and Eqs. (1) and (2), the rate of heat transfer is mainly proportional tothe temperature difference between the water bulk temperature and the PCM temperature.

mwCpwðTiw � TowÞ ¼ mPCMCpPCMðT2 � T1Þ þ mPCMDHmelt þ Qloss ð1Þ

Fig. 14. Temperature versus time during melting of palmitic acid in radial direction (heat exchanger tube in horizontal

position; water flow rate: 3 kg/min; inlet water temperature: 81�C).

Fig. 15. Temperature versus time during solidification of palmitic acid in radial direction (heat exchanger tube in

horizontal position; mean storage temperature: 63�C; inlet water temperature: 42�C; water flow rate: 3.0 kg/min).


Qloss ¼2pkLðTs;hsc � TisÞ

lnðr2=r1Þð2Þ

Heat fractions, calculated by using Eq. (3), were plotted versus time during the melting andsolidification periods of the palmitic acid. As seen from Fig. 18, there are two time periods neededto receive all the charged heat from the system during melting of the palmitic acid.

Heat fraction ¼ QQT

ð3Þ

The heat flow rate was computed from the change of temperature of the heat transfer fluid as itflows through the heat exchanger. The overall heat losses from the system are calculated as theproduct of a heat loss coefficient and the temperature difference between the system temperatureand the ambient. The heat gained and lost by the PCM is taken as the sensible heat of the water

Fig. 16. Heat flow rate versus time during melting of palmitic acid (inlet water temperature: 78�C; water flow rate: 1.2

kg/min).

Fig. 17. Heat flow rate versus time during solidification of palmitic acid (inlet water temperature: 42�C; water flow rate:

1.2 kg/min).


minus the heat losses. Considering the above, the effectiveness of the heat exchanger tube wascalculated by Eq. (4) and found to be 53.3%.

e ¼ ðTiw � TowÞðTiw � TPCMÞ

ð4Þ

This means that 46.7% of the heat was actually lost somewhere or to the ambient. Because of theinsufficient insulation of the heat exchanger tube and the construction material of the tube, hashigh conduction heat transfer, and this effectiveness was found to be low.

4. Conclusions

In the case that the latent heat storage material of palmitic acid is put into a vertical cylindricalheat storage container and a vertical single pipe is inserted into the container as the heat ex-changer, the heat transfer rates were measured when the heat storage material is melted andsolidified. The following conclusions are obtained:

1. Heat transfer from the heat exchanger (heat transfer pipe) to the palmitic acid is largely influ-enced by natural convection at the melting layer section in addition to forced conduction andconvection heat transfer.

2. Palmitic acid is a good PCM for energy storage for domestic solar water heating. It has a suit-able melting point of 61�C, 97% purity and a relatively high latent heat of 203.4 kJ/kg. In ad-dition, it does not exhibit any subcooling.

3. A heat exchanger and a storage system which consists of a vertical single pipe as a heat ex-changer inserted into another vertical pipe as a heat storage container can be used for energystorage with reasonable charging and discharging times and heat release rate.

4. In the horizontal position, the melting and solidification behaviors occurred at a better steadystate than in the vertical position.

5. The heat charging and the heat discharging times can be altered by changing the water inlettemperature.

Fig. 18. Heat fractions versus time during melting and solidification of palmitic acid (inlet water temperature: 78�C/38�C; water flow rate: 1.2 kg/min).


Acknowledgements

This study was supported by the Karadeniz Technical University Research Fund and theGaziosmanpas�a University Research Fund.

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Thermal performance of palmitic acid as a phase change energy storage material

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