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Performance assessment of heat storage by phase change materials containing MWCNTs and graphite
Applied Thermal Engineering 50 (2013) 637e644
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Performance assessment of heat storage by phase change materials containing MWCNTs and graphite Tun-Ping Teng a, *, Ching-Min Cheng b, Chin-Pao Cheng b a b
Department of Industrial Education, National Taiwan Normal University, No. 162, Sec. 1, He-ping E. Rd., Da-an District, Taipei 10610, Taiwan, ROC Department of Mechatronic Technology, National Taiwan Normal University, No. 162, Sec. 1, He-ping E. Rd., Da-an District, Taipei 10610, Taiwan, ROC
h i g h l i g h t s < We produce the MPCMs with MWCNTs and graphite by the direct-synthesis method. < To assess the thermal resistance for MPCMs by the charging/discharging experiments. < DSC experiments to assess the phase change properties of MPCM. < The MWCNTs are better than graphite to modify the thermal performance of paraffin. < The highest decreased phase change heat of MPCM with MWCNTs is 3.69%.
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Article history: Received 27 May 2012 Accepted 2 July 2012 Available online 7 July 2012
This study reports the production of modified phase change materials (MPCMs) using the directsynthesis method to mix paraffin with MWCNTs and graphite as the experimental sample. The MWCNTs and graphite were dispersed into three concentrations of 1.0, 2.0, and 3.0 wt.%. This study experimentally investigates the influences of the additive concentrations of the additives in the paraffin on their temperature and phase change heat variations by charging/discharging temperature difference and DSC experiments to evaluate the feasibility for thermal storage. Experimental results demonstrate that adding the MWCNTs was more effective than graphite in modifying the thermal storage performance of paraffin for most of the experimental parameters. Furthermore, adding MWCNTs reduced the melting onset temperature and increased the solidification onset temperature for paraffin. This makes the phase change heat applicable to a wider temperature range, and the highest decreased ratio of phase change heat was only 3.69%, compared with paraffin. This study demonstrates that for enhancing the thermal storage characteristic of PCMs by adding MWCNTs to paraffin to form MPCMs has great potential. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Graphite Multi-walled carbon nanotubes (MWCNTs) Paraffin Phase change materials (PCMs) Thermal storage
1. Introduction The production of greenhouse gases in recent years has been restricted by the implementation of the Kyoto Protocol. The rate of increase in renewable or recovery energy is also a global trend. A survey of all the renewable energy sources reveals that solar energy has the most value in Taiwan, which lies in a subtropical area. However, the utilization of solar energy is restricted by the alternation of day and night and weather conditions. Thus, the development of highly efficient thermal storage materials and devices is a critical issue. Common thermal storage materials can be divided into two categories: single-phase materials and phase change materials (PCMs). PCMs have the advantage of high density of * Corresponding author. Tel.: þ886 2 77343358; fax: þ886 2 23929449. E-mail addresses: [email protected], [email protected] (T.-P. Teng). 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2012.07.002
thermal storage, and can be grouped into three types based on phase change state: solidesolid PCMs, solideliquid PCMs, and liquidegas PCMs. Solideliquid PCMs are best suited to thermal energy storage. Solideliquid PCMs include organic PCMs, inorganic PCMs, and eutectics [1e4]. Most PCMs have the disadvantage of low thermal conductivity, making it difficult for them to overcome the problem of rapid load changes in the charging and discharging process [5]. Many studies have proposed various modified techniques to enhance the thermal conductivities of PCMs, such as inserting fins [6e8], adding metallic or nonmetallic particles with high thermal conductivity [9e11], incorporating porous matrix materials or expanded graphite [12e16], inserting fibrous materials [17e20], and incorporating macro, micro, and nano capsules [21e23]. The methods of enhancing the thermal conductivity of PCMs mentioned above are very helpful, and each has its own advantages.
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Nomenclature
u C D F H PW q R T V
weight fraction, wt.% controller experimental data flow meter heat, kJ/kg paraffin heat flow, W/g ratio temperature, C valve
Adding particles or fibers to PCMs is a convenient and cost-effective approach to enhance their thermal conductivity. However, these additives also create the problem of sedimentation because additives size, dispersion technique, and surface property of materials gradually result in lower thermal performance in the long-term. However, the viscosity of solideliquid PCMs in the liquid state is much higher than water, so the additives with micron-scale may be stably suspended in the PCMs. The development of nanotechnology has led to nanoscale additives, which can more effectively solve the problems of sedimentation and increasing viscosity. Nano-additives have a high specific surface area (SSA) that can significantly enhance the heat transfer performance of PCMs. Elgafy and Lafdi [24] added carbon nanofiber (CNF) to paraffin to enhance the thermal properties of PCMs. Adding CNFs at a concentration of 4 wt.% can increase the thermal conductivity from 0.24 to 0.33 W/m K. Zeng et al. [25] synthesized an organic phase change material (1-tetradecanol, TD)/Ag nanoparticles composite material. They also analyzed the effect of containing Ag nanoparticles on the thermal conductivity and phase change enthalpy of composite PCMs. The thermal conductivity of composite PCMs increases with the loading of Ag nanoparticles. The phase change enthalpy can be correlated linearly with the loading of TD, but the phase change temperature was slightly lower than that of pure TD. Shaikh et al. [26] indicated that paraffin with CNF and carbon nanotubes (CNTs) can enhance latent heat more than pure paraffin. Their study disagrees with the general concept that adding solid additives will reduce the latent heat of PCMs. Wang et al. [27] demonstrated that PCMs with multi-walled carbon nanotubes (MWCNTs) of 2.0 wt.% can increase the thermal conductivity by 35% compared to the original PCMs. Ho and Gao [28] added alumina (Al2O3) nanoparticles to paraffin using an emulsion technique with a non-ionic surfactant as the experimental sample, and dispersed the nanoparticles into different concentrations (5 wt.% and 10 wt.%) to study their thermophysical properties. They reported that the dynamic viscosity and thermal conductivity of paraffin containing 5 wt.% and 10 wt.% of Al2O3 nanoparticles increased by approximately 20e28% and ten to four times compared with the original paraffin, respectively. Cui et al. [29] investigated the enhancement of thermal conductivity for soy wax and paraffin wax containing CNFs and CNTs with concentrations of 1, 2, 5, and 10 wt.%. Their experimental results show that the thermal conductivity of PCMs containing CNFs and CNTs increased more than the original PCMs. Adding the CNFs into PCMs more effectively increased the thermal conductivity than CNTs because the CNFs had better dispersion performance. Teng et al. [30] adopted a two-step method of adding MWCNTs and Al2O3 nanoparticles to paraffin, forming nanocomposite-enhanced phase change materials (NEPCMs). Their experimental results show that adding nano-materials to the paraffin slightly increased the phase
Subscripts a additives d difference H heating m melting mo melting onset mp melting peak o onset p phase change peak pw paraffin s solidification so solidification onset sp solidification peak
change temperature and reduced the thermal resistance. Arasu and Mujumdar [31] investigated thermal storage by melting paraffin containing Al2O3 nanoparticles in a square enclosure followed by numerical investigation. The melting rate decreases with the increase in the volumetric composition of Al2O3 nanoparticles for both horizontal wall and vertical wall heating cases. This phenomenon is mainly the result of Al2O3 nanoparticles in the paraffin, which results in a relative increase of the dynamic viscosity compared to pure paraffin. The increased ratio of dynamic viscosity was higher than the effective thermal conductivity of paraffin with Al2O3 nanoparticles, thus significantly degrading its natural-convection heat transfer efficiency at the concentration of nanoparticles increased in the melted region. The heated temperature of non-concentrating solar collector generally does not exceed 90 C. Thus, paraffin was chosen as the PCM thermal storage material. The direct-synthesis method was employed to produce modified phase change materials (MPCMs) by adding the MWCNTs and graphite to paraffin. Charging/discharging temperature difference and differential scanning calorimeter (DSC) experiments were used to assess the charging/discharging temperature difference, onset temperature, peak temperature, and latent heat of melting/solidification for MPCMs and paraffin at the different temperatures, fixed heating and cooling rate to discuss the feasibility of MPCM in thermal storage system. 2. Experimental setup and procedure 2.1. Preparation of phase change materials Fully refined paraffin (Choneye Pure Chemicals, Taiwan) served as the base materials in this study. The MWCNTs (20e30 nm, Cheap Tubes Inc) and graphite (3.2 um, HOMYTECH, Taiwan) are commercial materials used as additives to modify the paraffin. Fig. 1(a) and (b) shows field-emission scanning electron microscope (FE-SEM, LEO 1530, Zeiss, Germany) photographs of MWCNTs and graphite powder, respectively. The MWCNTs appear aggregated, and the outside diameter approximately met the specifications provided by the manufacturer. The graphite exhibited uneven particle size and aggregated phenomenon, but the overall particle size was generally lower than the manufacturer’s specifications. The MPCMs prepared by the direct-synthesis method were used to disperse the MWCNTs or graphite into three weight fractions (1.0, 2.0, 3.0 wt.%) in the paraffin, forming the experimental samples in this study. The main reason is to select the MWCNTs and graphite that are not easily oxidized and have excellent thermal conductivity. Paraffin was first melted to the liquid state by impermeable heating in water tank at 100 C to successfully disperse the MWCNTs or graphite powder in the liquid paraffin. Then, the liquid paraffin was continuously stirred at 120 C by an
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Fig. 1. FE-SEM image of (a) MWCNT and (b) graphite.
electromagnetic stirrer/hot plate (PC420D, Corning, USA). MWCNTs or graphite were divided into several times to add to the liquid paraffin until reaching the desired concentrations to complete the preliminary adding and dispersing procedures for MPCM. Secondly, the liquid MPCMs was continuously dispersed for 40 min at 120 C by a high-speed homogenizer (T25 digital, IKA, Germany) at 6000 rpm to evenly disperse the MWCNTs or graphite in the liquid paraffin. The liquid MPCMs were then dispersed at 90 C for 1 h using an ultrasonic vibrator (D400H, TOHAMA, Taiwan) to complete the modification procedures of PCMs. Finally, 40 g of paraffin and MPCMs was poured into a polypropylene (P.P.) test tube to complete a unit of sample for charging/discharging temperature experiment. 2.2. Charging/discharging temperature experiment for MPCMs This study investigates the effects of additives for charging/ discharging temperature difference and latent heat based on the phase change of paraffin. Fig. 2 shows the test apparatus used for charging/discharging temperature difference of MPCMs. In the experiments of charging/discharging temperature difference of MPCMs, the sample was placed in a heat exchanger and the heating/cooling temperature was controlled by isothermal units (D620, DENG YNG) at different charging (70, 75 and 80 C) and discharging (25 C) temperatures.
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Two isothermal units were adopted for preheating and precooling using internal circulation (valves 1, 2, 4, 5 were closed, and valves 3 and 6 were opened) until the water temperature remained at a setting temperature of 0.5 C. Valves 4 and 5 were opened and valve 6 was closed to circulate water to maintain the temperature of the experimental sample within 25 0.5 C to ensure the same initial temperature for each experimental sample. Valves 1, 2, and 6 were then opened and valves 3, 4, and 5 were closed to conduct the MPCM charging experiment. After completing the MPCM charging experiment, and valves 3, 4, and 5 were opened and valves 1, 2, and 6 were closed to conduct the MPCM discharging experiment. The volume flow rate of the heating water was monitored by a mechanical flow meter (2.0 L/min, FongJei, Taiwan) at an accuracy of 2.0% for flowing to the heat exchanger at 1.6 L/min to achieve steady heating. The volume flow rate of the cooling water was monitored by a digital flow meter (NF05, Aichi Tokei, Japan) at an accuracy of 0.2% for flowing to the heat exchanger at 1.6 L/min to offer stable capacity of cooling. A dataacquisition unit (MV-200, YOKOGAWA, Japan) with K-type thermocouples was used to measure temperature with a precision of 0.1 C. Finally, the times required for the MPCM charging/discharging temperature difference experiments for were 90 and 80 min, respectively, which are determined by the same experiment with paraffin. The experimental results of charging/discharging temperature difference, similar to the form in Fig. 3, the heating temperature minus the sample temperature or sample temperature minus cooling temperature represents the temperature difference (Td) discussed in this study. The charging/discharging experiments are running 90 min and 80 min, respectively. To calculate the temperature difference between the experimental sample and the working fluid is selected the experimental data of 10 min to average before the end of the experiments. The test was repeated with varying parameters such as weight fraction of additives (u) and heating temperature (TH). The charging and discharging temperature difference represents the average of the last 10 min of each experimental sample of MPCMs, and comparing it with the sample of paraffin at the same experimental temperature revealed the effects of additives on the charging/discharging temperature difference of paraffin. 2.3. DSC experiment By the different temperature of heat sources and practical application to select the PCMs must consider the suitable phase change temperature and phase change heat is a very important step. A differential scanning calorimeter (DSC) is often used to determine the phase change temperature and phase change heat of PCMs [12,20,21,23,29]. A DSC (Q20, TA, USA) with a mechanical cooling system (RCS40, TA, USA) was used to assess different samples of charging and discharging experiments to determine the melting and solidification temperature. The range of experimental temperature was 25e90 C at the fixed heating and cooling rate of 4.0 C/min. The calorimetric precision and temperature accuracy of the DSC were 0.1% and 0.1 C, respectively. The sample was held in an aluminum sample pan (Tzero Pan, No.: T100915) with a lid (Tzero Hermetic Lid, No.: T100624), and the DSC experiment was conducted in a high-purity nitrogen (5N) atmosphere. This experiment controlled the sample’s weight at 5.0 1.0 mg in the sample pan using a precision electronic balance (XS-125A, Precisa, Switzerland) at a precision of 0.1 mg. The thermograms of the DSC charging and discharge experiments were analyzed by computer software (Universal Analysis 2000, TA) at a temperature ranging from 30 to 70 C to calculate the phase change latent heat for all samples. Comparing the experimental results of DSC for MPCMs with the paraffin at the same experimental parameters reveals the
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Fig. 2. The test apparatus used for charging/discharging temperature of thermal storage materials.
effects of additives for the melting and solidification temperature and heat of paraffin. 2.4. Experimental data analysis To easily compare the experimental data after adding the MWCNTs and graphite to paraffin (Da), all data were obtained with the paraffin to form baseline values (Dpw). In other words, experimental data obtained from the MPCM were compared with baseline values. The differences before and after adding the MWCNTs and graphite to paraffin (PW) were presented as proportions (R), calculated as follows:
R ¼ Da =Dpw
(1)
Fig. 3. The schematic diagram of temperature difference in charging and discharging process.
3. Results and discussion Figs. 4 and 5 show the charging and discharging temperature difference ratios of MPCMs. Table 1 shows the testing data of charging and discharging temperature difference for paraffin. Fig. 4 shows that adding MWCNTs to paraffin can significantly reduce the temperature difference between the sample temperature and heating temperature with increasing concentrations of MWCNTs at the same charging time. These results show that adding MWCNTs can effectively reduce the thermal resistance of liquid paraffin. However, adding graphite to paraffin is resulting in a greater temperature difference between the sample temperature and heating temperature with increasing concentration. Adding higher concentrations of graphite to the paraffin did not help reduce the thermal resistance of the liquid paraffin. Graphite has a much higher thermal conductivity than paraffin, but adding a higher
Fig. 4. The charging temperature difference ratio of MPCMs.
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Fig. 6. The DSC experiment of endothermic and exothermic thermograms for paraffin.
Fig. 5. The discharging temperature difference ratio of MPCMs.
concentration of graphite did not help reduce the thermal resistance. This is because of the effect of poor dispersion performance, combination property, and the solideliquid interface layer. The reduction in heat transfer performance caused by the large thermal resistance of solideliquid interface is caused by the poor combination of graphite and liquid paraffin. Fig. 5 shows that adding MWCNTs and graphite to paraffin can significantly reduce the temperature difference between the sample temperature and cooling temperature at the same discharging time. The graphite can reduce the temperature difference ratio, but this does not vary with its concentration. A better combination of the graphite and solid paraffin can cause lower temperature difference and thermal resistance. Fig. 6 presents the DSC thermograms of the paraffin. The phase change latent heat of the calculation ranges from 30 to 70 C. Fig. 6 shows that in the melting and solidification process, the phase change peak temperature (Tp) is 60.74 C and 58.68 C, the onset temperature (To) is 54.00 C and 60.64 C, and the latent heat of phase change is 199.4 kJ/kg and 194.5 kJ/kg, respectively. Because the paraffin is not a pure substance, its range of melting and solidification are more widely compared with pure substance. The DSC experimental results of the paraffin can be as a baseline for comparison with the follow-up experimental results of MPCMs. This comparison reveals the effects of additives on the phase change characteristics of paraffin. Figs. 7 and 8 respectively demonstrate the DSC thermograms for MPCMs containing MWCNTs and graphite at different concentrations. Fig. 7 shows that the MWCNTs can slightly decrease the
melting onset temperature and increase the solidification onset temperature of paraffin. The MWCNTs can also enhance the melting peak temperature and slightly decrease the solidification peak temperature of paraffin. Adding MWCNTs resulted in the greater variations of the composition of the paraffin, causing a greater range in the phase change temperature. Although the lower peak of charging and discharging heat of MPCMs with MWCNTs, but the heat changes are more evenly in the phase change range and the wider range of phase change temperature. This phenomenon is mainly from the MWCNTs with the high thermal conductivity, the excellent combination between MWCNTs and paraffin and dispersion performance of MWCNTs in paraffin lead to the heat changes in the phase change process is relatively uniform. However, the combination and dispersion performance are mainly attributed to the surface properties of the MWCNTs and paraffin. Adding MWCNTs with high thermal conductivity to paraffin promotes the early occurrence of phase change onset temperature through rapid heat transfer. Whether adding the materials with high thermal conductivity affects the phase change onset temperature of PCMs depends on the combination and dispersion performance. Fig. 7 shows that adding MWCNTs allows the early phase change of paraffin. This implies that adding MWCNTs to paraffin can lead to good combination and dispersion performance (Fig. 9). Fig. 8 presents the DSC thermograms of the MPCMs with graphite. This figure shows that MPCMs with graphite exhibit two clear phase change peaks. Adding graphite to paraffin can increase the melting onset temperature and decrease the solidification onset temperature of paraffin. The graphite can decrease the melting and solidification peak temperature of paraffin. In addition, MPCMs with graphite exhibit higher peaks of charging and discharging heat. This phenomenon is the result of a poor combination
Table 1 The temperature difference between paraffin and working fluid at different charging/discharging temperature ranges. Items
Temperature range ( C)
Temperature difference (Td, C)
Charging process (average of the last 10 min, C)
25e70 25e75 25e80
1.69 1.70 1.72
Discharging process (average of the last 10 min, C)
70e25 75e25 80e25
0.61 0.70 0.79 Fig. 7. DSC thermograms for MPCMs containing MWCNTs at different concentrations.
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Fig. 8. DSC thermograms for MPCMs containing graphite at different concentrations.
Fig. 11. The solidification onset temperature ratio of MPCMs.
Fig. 9. The melting onset temperature ratio of MPCMs.
between graphite and paraffin and dispersion performance of graphite in paraffin, which leads to the endothermic and exothermic delay phenomenon. A poor combination between graphite and paraffin causes an increase in porosity, which in turn increases interfacial thermal resistance. The higher interfacial thermal resistance of the phase change material delays the phase change onset temperature and causes the number of phase change. The endothermic and exothermic delay phenomenon also contributes to two phase change peaks. Figs. 9e14 respectively demonstrate the ratio of the melting onset temperature (Tmo), melting peak temperature (Tmp), solidification onset temperature (Tso), solidification peak temperature (Tsp), melting heat (Hm), and solidification heat (Hs) of MPCMs. The actual values of the test can be calculated by the experimental results of paraffin (Fig. 6). These experimental results reveal the difference between MPCMs and paraffin. Comparing the phase change temperatures with the range of main endothermic and
Fig. 10. The melting peak temperature ratio of MPCMs.
Fig. 12. The solidification peak temperature ratio of MPCMs.
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and 14, the MWCNTs and graphite in paraffin resulted in a lower melting heat and adding the graphite reduced the melting heat of the paraffin by approximately 12% at a concentration of 3.0 wt.%. Adding MWCNTs and graphite to paraffin resulted in a lower solidification heat and adding the graphite reduced the solidification heat of the paraffin by approximately 10% at a concentration of 3.0 wt.%. Comparing the experimental results above reveals that adding MWCNTs to paraffin can reduce the melting onset temperature and increase the solidification onset temperature for paraffin. This addition also makes the phase change heat applicable to a wider temperature range. Although adding the graphite to the paraffin can obtain a lower onset temperature in the first peak, the share of the first peak of phase change heat is the low percentage of the heat change heat in the overall phase change process. Adding MWCNTs to paraffin has a minor effect on the phase change heat, as the highest decreased ratio was only 3.69% when added concentration of 3.0 wt.%, which is minor higher than the adding concentration. However, adding graphite had a significant effect on the phase change heat. The highest decreased ratio was up to about 12% when adding a concentration of 3.0 wt.%, which is far higher than the adding concentration. Therefore, adding MWCNTs to improve the thermal storage performance of paraffin has great potential in the future. Fig. 13. The melting heat ratio of MPCMs.
exothermic peaks for comparison, the range of phase change heat selected to cover the scope of the entire endothermic and exothermic peaks was calculated at 30e70 C (Fig. 10). In Figs. 9 and 10, adding MWCNTs to paraffin resulted in a lower melting onset temperature and allowed a melting phase change of paraffin at lower temperatures to increase the application of the temperature range of PCMs. However, the MWCNTs in paraffin resulted in a higher melting peak temperature. In Figs. 11 and 12, the MWCNTs in paraffin resulted in a higher solidification onset temperature and allowed a solidification phase change of paraffin at higher temperatures. This situation makes it possible to use a phase change heat at higher temperatures to enhance the heat release rate of PCMs. Furthermore, the MWCNTs and graphite in paraffin led to a lower solidification peak temperature. In Figs. 13
4. Conclusions This study employs a direct-synthesis method to prepare modified phase change materials (MPCMs) by adding different concentrations (1.0, 2.0, and 3.0 wt.%) of MWCNTs and graphite to paraffin. This study experimentally investigates the influences of the concentrations of the MWCNTs and graphite on their temperature and phase change heat variations at the thermal energy charging and discharging process by charging/discharging temperature difference and DSC experiments. Experimental results demonstrate that MWCNTs were more effective than graphite in modifying the thermal storage performance of paraffin. Adding the MWCNTs reduced the temperature difference between the PCMs and the heating fluids, and achieved better results than graphite for most experimental parameters. Adding MWCNTs also reduced the melting onset temperature and increased the solidification onset temperature for paraffin. This makes the phase change heat applicable to a wider temperature range, and the highest decreased ratio of phase change heat is only 3.69% compared with paraffin. Therefore, adding MWCNTs to improve the thermal storage performance of paraffin has great potential in the future. Acknowledgements The authors would like to thank National Science Council and Bureau of Energy, Ministry of Economic Affairs of the Republic of China, Taiwan for their financially support to this research under Contract Nos.: NSC 99-2221-E-003-008 and 101-D0802, respectively. References
Fig. 14. The solidification heat ratio of MPCMs.
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Applied Thermal Engineering journal homepage: www.elsevier.com/locate/apthermeng
Performance assessment of heat storage by phase change materials containing MWCNTs and graphite Tun-Ping Teng a, *, Ching-Min Cheng b, Chin-Pao Cheng b a b
Department of Industrial Education, National Taiwan Normal University, No. 162, Sec. 1, He-ping E. Rd., Da-an District, Taipei 10610, Taiwan, ROC Department of Mechatronic Technology, National Taiwan Normal University, No. 162, Sec. 1, He-ping E. Rd., Da-an District, Taipei 10610, Taiwan, ROC
h i g h l i g h t s < We produce the MPCMs with MWCNTs and graphite by the direct-synthesis method. < To assess the thermal resistance for MPCMs by the charging/discharging experiments. < DSC experiments to assess the phase change properties of MPCM. < The MWCNTs are better than graphite to modify the thermal performance of paraffin. < The highest decreased phase change heat of MPCM with MWCNTs is 3.69%.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 May 2012 Accepted 2 July 2012 Available online 7 July 2012
This study reports the production of modified phase change materials (MPCMs) using the directsynthesis method to mix paraffin with MWCNTs and graphite as the experimental sample. The MWCNTs and graphite were dispersed into three concentrations of 1.0, 2.0, and 3.0 wt.%. This study experimentally investigates the influences of the additive concentrations of the additives in the paraffin on their temperature and phase change heat variations by charging/discharging temperature difference and DSC experiments to evaluate the feasibility for thermal storage. Experimental results demonstrate that adding the MWCNTs was more effective than graphite in modifying the thermal storage performance of paraffin for most of the experimental parameters. Furthermore, adding MWCNTs reduced the melting onset temperature and increased the solidification onset temperature for paraffin. This makes the phase change heat applicable to a wider temperature range, and the highest decreased ratio of phase change heat was only 3.69%, compared with paraffin. This study demonstrates that for enhancing the thermal storage characteristic of PCMs by adding MWCNTs to paraffin to form MPCMs has great potential. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Graphite Multi-walled carbon nanotubes (MWCNTs) Paraffin Phase change materials (PCMs) Thermal storage
1. Introduction The production of greenhouse gases in recent years has been restricted by the implementation of the Kyoto Protocol. The rate of increase in renewable or recovery energy is also a global trend. A survey of all the renewable energy sources reveals that solar energy has the most value in Taiwan, which lies in a subtropical area. However, the utilization of solar energy is restricted by the alternation of day and night and weather conditions. Thus, the development of highly efficient thermal storage materials and devices is a critical issue. Common thermal storage materials can be divided into two categories: single-phase materials and phase change materials (PCMs). PCMs have the advantage of high density of * Corresponding author. Tel.: þ886 2 77343358; fax: þ886 2 23929449. E-mail addresses: [email protected], [email protected] (T.-P. Teng). 1359-4311/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.applthermaleng.2012.07.002
thermal storage, and can be grouped into three types based on phase change state: solidesolid PCMs, solideliquid PCMs, and liquidegas PCMs. Solideliquid PCMs are best suited to thermal energy storage. Solideliquid PCMs include organic PCMs, inorganic PCMs, and eutectics [1e4]. Most PCMs have the disadvantage of low thermal conductivity, making it difficult for them to overcome the problem of rapid load changes in the charging and discharging process [5]. Many studies have proposed various modified techniques to enhance the thermal conductivities of PCMs, such as inserting fins [6e8], adding metallic or nonmetallic particles with high thermal conductivity [9e11], incorporating porous matrix materials or expanded graphite [12e16], inserting fibrous materials [17e20], and incorporating macro, micro, and nano capsules [21e23]. The methods of enhancing the thermal conductivity of PCMs mentioned above are very helpful, and each has its own advantages.
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Nomenclature
u C D F H PW q R T V
weight fraction, wt.% controller experimental data flow meter heat, kJ/kg paraffin heat flow, W/g ratio temperature, C valve
Adding particles or fibers to PCMs is a convenient and cost-effective approach to enhance their thermal conductivity. However, these additives also create the problem of sedimentation because additives size, dispersion technique, and surface property of materials gradually result in lower thermal performance in the long-term. However, the viscosity of solideliquid PCMs in the liquid state is much higher than water, so the additives with micron-scale may be stably suspended in the PCMs. The development of nanotechnology has led to nanoscale additives, which can more effectively solve the problems of sedimentation and increasing viscosity. Nano-additives have a high specific surface area (SSA) that can significantly enhance the heat transfer performance of PCMs. Elgafy and Lafdi [24] added carbon nanofiber (CNF) to paraffin to enhance the thermal properties of PCMs. Adding CNFs at a concentration of 4 wt.% can increase the thermal conductivity from 0.24 to 0.33 W/m K. Zeng et al. [25] synthesized an organic phase change material (1-tetradecanol, TD)/Ag nanoparticles composite material. They also analyzed the effect of containing Ag nanoparticles on the thermal conductivity and phase change enthalpy of composite PCMs. The thermal conductivity of composite PCMs increases with the loading of Ag nanoparticles. The phase change enthalpy can be correlated linearly with the loading of TD, but the phase change temperature was slightly lower than that of pure TD. Shaikh et al. [26] indicated that paraffin with CNF and carbon nanotubes (CNTs) can enhance latent heat more than pure paraffin. Their study disagrees with the general concept that adding solid additives will reduce the latent heat of PCMs. Wang et al. [27] demonstrated that PCMs with multi-walled carbon nanotubes (MWCNTs) of 2.0 wt.% can increase the thermal conductivity by 35% compared to the original PCMs. Ho and Gao [28] added alumina (Al2O3) nanoparticles to paraffin using an emulsion technique with a non-ionic surfactant as the experimental sample, and dispersed the nanoparticles into different concentrations (5 wt.% and 10 wt.%) to study their thermophysical properties. They reported that the dynamic viscosity and thermal conductivity of paraffin containing 5 wt.% and 10 wt.% of Al2O3 nanoparticles increased by approximately 20e28% and ten to four times compared with the original paraffin, respectively. Cui et al. [29] investigated the enhancement of thermal conductivity for soy wax and paraffin wax containing CNFs and CNTs with concentrations of 1, 2, 5, and 10 wt.%. Their experimental results show that the thermal conductivity of PCMs containing CNFs and CNTs increased more than the original PCMs. Adding the CNFs into PCMs more effectively increased the thermal conductivity than CNTs because the CNFs had better dispersion performance. Teng et al. [30] adopted a two-step method of adding MWCNTs and Al2O3 nanoparticles to paraffin, forming nanocomposite-enhanced phase change materials (NEPCMs). Their experimental results show that adding nano-materials to the paraffin slightly increased the phase
Subscripts a additives d difference H heating m melting mo melting onset mp melting peak o onset p phase change peak pw paraffin s solidification so solidification onset sp solidification peak
change temperature and reduced the thermal resistance. Arasu and Mujumdar [31] investigated thermal storage by melting paraffin containing Al2O3 nanoparticles in a square enclosure followed by numerical investigation. The melting rate decreases with the increase in the volumetric composition of Al2O3 nanoparticles for both horizontal wall and vertical wall heating cases. This phenomenon is mainly the result of Al2O3 nanoparticles in the paraffin, which results in a relative increase of the dynamic viscosity compared to pure paraffin. The increased ratio of dynamic viscosity was higher than the effective thermal conductivity of paraffin with Al2O3 nanoparticles, thus significantly degrading its natural-convection heat transfer efficiency at the concentration of nanoparticles increased in the melted region. The heated temperature of non-concentrating solar collector generally does not exceed 90 C. Thus, paraffin was chosen as the PCM thermal storage material. The direct-synthesis method was employed to produce modified phase change materials (MPCMs) by adding the MWCNTs and graphite to paraffin. Charging/discharging temperature difference and differential scanning calorimeter (DSC) experiments were used to assess the charging/discharging temperature difference, onset temperature, peak temperature, and latent heat of melting/solidification for MPCMs and paraffin at the different temperatures, fixed heating and cooling rate to discuss the feasibility of MPCM in thermal storage system. 2. Experimental setup and procedure 2.1. Preparation of phase change materials Fully refined paraffin (Choneye Pure Chemicals, Taiwan) served as the base materials in this study. The MWCNTs (20e30 nm, Cheap Tubes Inc) and graphite (3.2 um, HOMYTECH, Taiwan) are commercial materials used as additives to modify the paraffin. Fig. 1(a) and (b) shows field-emission scanning electron microscope (FE-SEM, LEO 1530, Zeiss, Germany) photographs of MWCNTs and graphite powder, respectively. The MWCNTs appear aggregated, and the outside diameter approximately met the specifications provided by the manufacturer. The graphite exhibited uneven particle size and aggregated phenomenon, but the overall particle size was generally lower than the manufacturer’s specifications. The MPCMs prepared by the direct-synthesis method were used to disperse the MWCNTs or graphite into three weight fractions (1.0, 2.0, 3.0 wt.%) in the paraffin, forming the experimental samples in this study. The main reason is to select the MWCNTs and graphite that are not easily oxidized and have excellent thermal conductivity. Paraffin was first melted to the liquid state by impermeable heating in water tank at 100 C to successfully disperse the MWCNTs or graphite powder in the liquid paraffin. Then, the liquid paraffin was continuously stirred at 120 C by an
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Fig. 1. FE-SEM image of (a) MWCNT and (b) graphite.
electromagnetic stirrer/hot plate (PC420D, Corning, USA). MWCNTs or graphite were divided into several times to add to the liquid paraffin until reaching the desired concentrations to complete the preliminary adding and dispersing procedures for MPCM. Secondly, the liquid MPCMs was continuously dispersed for 40 min at 120 C by a high-speed homogenizer (T25 digital, IKA, Germany) at 6000 rpm to evenly disperse the MWCNTs or graphite in the liquid paraffin. The liquid MPCMs were then dispersed at 90 C for 1 h using an ultrasonic vibrator (D400H, TOHAMA, Taiwan) to complete the modification procedures of PCMs. Finally, 40 g of paraffin and MPCMs was poured into a polypropylene (P.P.) test tube to complete a unit of sample for charging/discharging temperature experiment. 2.2. Charging/discharging temperature experiment for MPCMs This study investigates the effects of additives for charging/ discharging temperature difference and latent heat based on the phase change of paraffin. Fig. 2 shows the test apparatus used for charging/discharging temperature difference of MPCMs. In the experiments of charging/discharging temperature difference of MPCMs, the sample was placed in a heat exchanger and the heating/cooling temperature was controlled by isothermal units (D620, DENG YNG) at different charging (70, 75 and 80 C) and discharging (25 C) temperatures.
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Two isothermal units were adopted for preheating and precooling using internal circulation (valves 1, 2, 4, 5 were closed, and valves 3 and 6 were opened) until the water temperature remained at a setting temperature of 0.5 C. Valves 4 and 5 were opened and valve 6 was closed to circulate water to maintain the temperature of the experimental sample within 25 0.5 C to ensure the same initial temperature for each experimental sample. Valves 1, 2, and 6 were then opened and valves 3, 4, and 5 were closed to conduct the MPCM charging experiment. After completing the MPCM charging experiment, and valves 3, 4, and 5 were opened and valves 1, 2, and 6 were closed to conduct the MPCM discharging experiment. The volume flow rate of the heating water was monitored by a mechanical flow meter (2.0 L/min, FongJei, Taiwan) at an accuracy of 2.0% for flowing to the heat exchanger at 1.6 L/min to achieve steady heating. The volume flow rate of the cooling water was monitored by a digital flow meter (NF05, Aichi Tokei, Japan) at an accuracy of 0.2% for flowing to the heat exchanger at 1.6 L/min to offer stable capacity of cooling. A dataacquisition unit (MV-200, YOKOGAWA, Japan) with K-type thermocouples was used to measure temperature with a precision of 0.1 C. Finally, the times required for the MPCM charging/discharging temperature difference experiments for were 90 and 80 min, respectively, which are determined by the same experiment with paraffin. The experimental results of charging/discharging temperature difference, similar to the form in Fig. 3, the heating temperature minus the sample temperature or sample temperature minus cooling temperature represents the temperature difference (Td) discussed in this study. The charging/discharging experiments are running 90 min and 80 min, respectively. To calculate the temperature difference between the experimental sample and the working fluid is selected the experimental data of 10 min to average before the end of the experiments. The test was repeated with varying parameters such as weight fraction of additives (u) and heating temperature (TH). The charging and discharging temperature difference represents the average of the last 10 min of each experimental sample of MPCMs, and comparing it with the sample of paraffin at the same experimental temperature revealed the effects of additives on the charging/discharging temperature difference of paraffin. 2.3. DSC experiment By the different temperature of heat sources and practical application to select the PCMs must consider the suitable phase change temperature and phase change heat is a very important step. A differential scanning calorimeter (DSC) is often used to determine the phase change temperature and phase change heat of PCMs [12,20,21,23,29]. A DSC (Q20, TA, USA) with a mechanical cooling system (RCS40, TA, USA) was used to assess different samples of charging and discharging experiments to determine the melting and solidification temperature. The range of experimental temperature was 25e90 C at the fixed heating and cooling rate of 4.0 C/min. The calorimetric precision and temperature accuracy of the DSC were 0.1% and 0.1 C, respectively. The sample was held in an aluminum sample pan (Tzero Pan, No.: T100915) with a lid (Tzero Hermetic Lid, No.: T100624), and the DSC experiment was conducted in a high-purity nitrogen (5N) atmosphere. This experiment controlled the sample’s weight at 5.0 1.0 mg in the sample pan using a precision electronic balance (XS-125A, Precisa, Switzerland) at a precision of 0.1 mg. The thermograms of the DSC charging and discharge experiments were analyzed by computer software (Universal Analysis 2000, TA) at a temperature ranging from 30 to 70 C to calculate the phase change latent heat for all samples. Comparing the experimental results of DSC for MPCMs with the paraffin at the same experimental parameters reveals the
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Fig. 2. The test apparatus used for charging/discharging temperature of thermal storage materials.
effects of additives for the melting and solidification temperature and heat of paraffin. 2.4. Experimental data analysis To easily compare the experimental data after adding the MWCNTs and graphite to paraffin (Da), all data were obtained with the paraffin to form baseline values (Dpw). In other words, experimental data obtained from the MPCM were compared with baseline values. The differences before and after adding the MWCNTs and graphite to paraffin (PW) were presented as proportions (R), calculated as follows:
R ¼ Da =Dpw
(1)
Fig. 3. The schematic diagram of temperature difference in charging and discharging process.
3. Results and discussion Figs. 4 and 5 show the charging and discharging temperature difference ratios of MPCMs. Table 1 shows the testing data of charging and discharging temperature difference for paraffin. Fig. 4 shows that adding MWCNTs to paraffin can significantly reduce the temperature difference between the sample temperature and heating temperature with increasing concentrations of MWCNTs at the same charging time. These results show that adding MWCNTs can effectively reduce the thermal resistance of liquid paraffin. However, adding graphite to paraffin is resulting in a greater temperature difference between the sample temperature and heating temperature with increasing concentration. Adding higher concentrations of graphite to the paraffin did not help reduce the thermal resistance of the liquid paraffin. Graphite has a much higher thermal conductivity than paraffin, but adding a higher
Fig. 4. The charging temperature difference ratio of MPCMs.
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Fig. 6. The DSC experiment of endothermic and exothermic thermograms for paraffin.
Fig. 5. The discharging temperature difference ratio of MPCMs.
concentration of graphite did not help reduce the thermal resistance. This is because of the effect of poor dispersion performance, combination property, and the solideliquid interface layer. The reduction in heat transfer performance caused by the large thermal resistance of solideliquid interface is caused by the poor combination of graphite and liquid paraffin. Fig. 5 shows that adding MWCNTs and graphite to paraffin can significantly reduce the temperature difference between the sample temperature and cooling temperature at the same discharging time. The graphite can reduce the temperature difference ratio, but this does not vary with its concentration. A better combination of the graphite and solid paraffin can cause lower temperature difference and thermal resistance. Fig. 6 presents the DSC thermograms of the paraffin. The phase change latent heat of the calculation ranges from 30 to 70 C. Fig. 6 shows that in the melting and solidification process, the phase change peak temperature (Tp) is 60.74 C and 58.68 C, the onset temperature (To) is 54.00 C and 60.64 C, and the latent heat of phase change is 199.4 kJ/kg and 194.5 kJ/kg, respectively. Because the paraffin is not a pure substance, its range of melting and solidification are more widely compared with pure substance. The DSC experimental results of the paraffin can be as a baseline for comparison with the follow-up experimental results of MPCMs. This comparison reveals the effects of additives on the phase change characteristics of paraffin. Figs. 7 and 8 respectively demonstrate the DSC thermograms for MPCMs containing MWCNTs and graphite at different concentrations. Fig. 7 shows that the MWCNTs can slightly decrease the
melting onset temperature and increase the solidification onset temperature of paraffin. The MWCNTs can also enhance the melting peak temperature and slightly decrease the solidification peak temperature of paraffin. Adding MWCNTs resulted in the greater variations of the composition of the paraffin, causing a greater range in the phase change temperature. Although the lower peak of charging and discharging heat of MPCMs with MWCNTs, but the heat changes are more evenly in the phase change range and the wider range of phase change temperature. This phenomenon is mainly from the MWCNTs with the high thermal conductivity, the excellent combination between MWCNTs and paraffin and dispersion performance of MWCNTs in paraffin lead to the heat changes in the phase change process is relatively uniform. However, the combination and dispersion performance are mainly attributed to the surface properties of the MWCNTs and paraffin. Adding MWCNTs with high thermal conductivity to paraffin promotes the early occurrence of phase change onset temperature through rapid heat transfer. Whether adding the materials with high thermal conductivity affects the phase change onset temperature of PCMs depends on the combination and dispersion performance. Fig. 7 shows that adding MWCNTs allows the early phase change of paraffin. This implies that adding MWCNTs to paraffin can lead to good combination and dispersion performance (Fig. 9). Fig. 8 presents the DSC thermograms of the MPCMs with graphite. This figure shows that MPCMs with graphite exhibit two clear phase change peaks. Adding graphite to paraffin can increase the melting onset temperature and decrease the solidification onset temperature of paraffin. The graphite can decrease the melting and solidification peak temperature of paraffin. In addition, MPCMs with graphite exhibit higher peaks of charging and discharging heat. This phenomenon is the result of a poor combination
Table 1 The temperature difference between paraffin and working fluid at different charging/discharging temperature ranges. Items
Temperature range ( C)
Temperature difference (Td, C)
Charging process (average of the last 10 min, C)
25e70 25e75 25e80
1.69 1.70 1.72
Discharging process (average of the last 10 min, C)
70e25 75e25 80e25
0.61 0.70 0.79 Fig. 7. DSC thermograms for MPCMs containing MWCNTs at different concentrations.
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Fig. 8. DSC thermograms for MPCMs containing graphite at different concentrations.
Fig. 11. The solidification onset temperature ratio of MPCMs.
Fig. 9. The melting onset temperature ratio of MPCMs.
between graphite and paraffin and dispersion performance of graphite in paraffin, which leads to the endothermic and exothermic delay phenomenon. A poor combination between graphite and paraffin causes an increase in porosity, which in turn increases interfacial thermal resistance. The higher interfacial thermal resistance of the phase change material delays the phase change onset temperature and causes the number of phase change. The endothermic and exothermic delay phenomenon also contributes to two phase change peaks. Figs. 9e14 respectively demonstrate the ratio of the melting onset temperature (Tmo), melting peak temperature (Tmp), solidification onset temperature (Tso), solidification peak temperature (Tsp), melting heat (Hm), and solidification heat (Hs) of MPCMs. The actual values of the test can be calculated by the experimental results of paraffin (Fig. 6). These experimental results reveal the difference between MPCMs and paraffin. Comparing the phase change temperatures with the range of main endothermic and
Fig. 10. The melting peak temperature ratio of MPCMs.
Fig. 12. The solidification peak temperature ratio of MPCMs.
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and 14, the MWCNTs and graphite in paraffin resulted in a lower melting heat and adding the graphite reduced the melting heat of the paraffin by approximately 12% at a concentration of 3.0 wt.%. Adding MWCNTs and graphite to paraffin resulted in a lower solidification heat and adding the graphite reduced the solidification heat of the paraffin by approximately 10% at a concentration of 3.0 wt.%. Comparing the experimental results above reveals that adding MWCNTs to paraffin can reduce the melting onset temperature and increase the solidification onset temperature for paraffin. This addition also makes the phase change heat applicable to a wider temperature range. Although adding the graphite to the paraffin can obtain a lower onset temperature in the first peak, the share of the first peak of phase change heat is the low percentage of the heat change heat in the overall phase change process. Adding MWCNTs to paraffin has a minor effect on the phase change heat, as the highest decreased ratio was only 3.69% when added concentration of 3.0 wt.%, which is minor higher than the adding concentration. However, adding graphite had a significant effect on the phase change heat. The highest decreased ratio was up to about 12% when adding a concentration of 3.0 wt.%, which is far higher than the adding concentration. Therefore, adding MWCNTs to improve the thermal storage performance of paraffin has great potential in the future. Fig. 13. The melting heat ratio of MPCMs.
exothermic peaks for comparison, the range of phase change heat selected to cover the scope of the entire endothermic and exothermic peaks was calculated at 30e70 C (Fig. 10). In Figs. 9 and 10, adding MWCNTs to paraffin resulted in a lower melting onset temperature and allowed a melting phase change of paraffin at lower temperatures to increase the application of the temperature range of PCMs. However, the MWCNTs in paraffin resulted in a higher melting peak temperature. In Figs. 11 and 12, the MWCNTs in paraffin resulted in a higher solidification onset temperature and allowed a solidification phase change of paraffin at higher temperatures. This situation makes it possible to use a phase change heat at higher temperatures to enhance the heat release rate of PCMs. Furthermore, the MWCNTs and graphite in paraffin led to a lower solidification peak temperature. In Figs. 13
4. Conclusions This study employs a direct-synthesis method to prepare modified phase change materials (MPCMs) by adding different concentrations (1.0, 2.0, and 3.0 wt.%) of MWCNTs and graphite to paraffin. This study experimentally investigates the influences of the concentrations of the MWCNTs and graphite on their temperature and phase change heat variations at the thermal energy charging and discharging process by charging/discharging temperature difference and DSC experiments. Experimental results demonstrate that MWCNTs were more effective than graphite in modifying the thermal storage performance of paraffin. Adding the MWCNTs reduced the temperature difference between the PCMs and the heating fluids, and achieved better results than graphite for most experimental parameters. Adding MWCNTs also reduced the melting onset temperature and increased the solidification onset temperature for paraffin. This makes the phase change heat applicable to a wider temperature range, and the highest decreased ratio of phase change heat is only 3.69% compared with paraffin. Therefore, adding MWCNTs to improve the thermal storage performance of paraffin has great potential in the future. Acknowledgements The authors would like to thank National Science Council and Bureau of Energy, Ministry of Economic Affairs of the Republic of China, Taiwan for their financially support to this research under Contract Nos.: NSC 99-2221-E-003-008 and 101-D0802, respectively. References
Fig. 14. The solidification heat ratio of MPCMs.
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