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This study focuses on the poor thermal conductivity of paraffins. Nano-alumina / paraffin composites as phase change material were prepared. Paraffin was used as a PCM in the composites, and nano-alumina was added for improving the thermal conductivity. The infrared heat camera was used to observe the temperature distribution in the melting process, with ultrasonic treatment being employed to improve the dispersibility of the nano-alumina in paraffin. The thermal performances of the composites were investigated by a differential scanning calorimetry (DSC). After 50 cycles, the melting and solidifying latent heats of the composites were 168 J/g and 176 J/g. The thermal conductivity of the composites was 1.7 times (0.361 W/mK) higher than that of paraffin (0.21 W/mK). No chemical degradation of the nano-alumina/paraffin composites occurred during thermal cycling.
Keywords: nano-alumina; paraffin; PCM
Latent heat storage is a particularly attractive technology to store excess energy that would otherwise be wasted. Latent heat storage has the capacity to absorb or release the heat at a constant or near constant temperature which correspond to the phase transition temperature of the phase change material . Compared to inorganic PCMs, organic PCMs melt and solidify repeatedly without phase segregation. Some organic PCMs such as alkanes, fatty acids and their mixtures have been studied for latent heat storage application[2-4]. Among the organic PCMs investigated, paraffins have been widely used due to their high latent heat storage capacity and appropriate thermal properties such as little or no supercooling, low vapor pressure and good thermal and chemical stability . However, paraffins have unacceptable low thermal conductivities, leading to slow charging and discharging rates. Hence, heat transfer enhancement for paraffins is required in practical applications, including insertion of a metal matrix into the paraffins[6,7], using paraffins dispersed with high conductivity particles, micro-encapsulation of the paraffins[9,10]. The best enhancement as reported in the literature was that due to Velraj et al. where the effective thermal conductivity calculated employing paraffin with metal rings was ten times (2 W/mK) greater than the thermal conductivity of paraffin (0.2 W/mK).
In this paper, the thermal performance of the nano-alumina/paraffin composites as a PCM is presented. In the composites, paraffin was used as a PCM for latent heat storage. It melts at 55.94ï¿½ï¿½-55.23ï¿½ï¿½ with a latent heat of 203.3 J/g and solidifies at 59.15ï¿½ï¿½-50.66ï¿½ï¿½ with a latent heat of 206.4 J/g. In this research, the nano-alumina was added to enlarge the heat transfer area, thus improving the thermal conductivity of the nano-alumina / paraffin composites. Therefore, the nano-alumina / paraffin composite PCM can be applied in solar heating, building energy conservation and heat recovery systems for latent heat storage.
Paraffin was used as PCM in this study. The paraffin was supplied by Sinopharm Chemical Reagent Company. ï¿½ï¿½-alumina (average particle size is 30nm) was supplied by Shanghai Paddy Field Mstar Technology Co., Ltd.
2.2 Preparation of nano-alumina/paraffin composites
The composite PCMs were prepared by stirring of nano-alumina in liquid paraffin with mass fraction of 0.1%, 0.2%, 0.3%, 0.4%, 0.5%. To establish the relationship between thermal conductivity of the composite PCM and mass fraction of nano-alumina, the composites were heated at 60ï¿½ï¿½, and stirred at the rate of 500 rpm for 60 min by a constant temperature magnetic stirrer. Five kinds of the nano-alumina / paraffin composites were obtained, and were denoted as PCM1, PCM2, PCM3, PCM4, PCM5.
The temperature distribution of nano-alumina/paraffin composites were observed using an E40 infrared camera (FLIR). The thermal properties of the composites were measured using a Q200 differential scanning calorimetry (TA American Instrument Company) at 5ï¿½ï¿½/min under a constant stream of argon at a flow rate of 20 ml/min. The accuracy of enthalpy measurements was ï¿½ï¿½5% and the temperature accuracy was ï¿½ï¿½0.2ï¿½ï¿½. The thermal conductivity coefficients of the composites were determined by a TPS2500s Hotdisk thermal constants analyzer (the Swedish Hotdisk company).
3 Results and discussions
3.1 Heat transfer behavior investigation
The composites were heated in a temperature controlled water bath at 60ï¿½ï¿½. The dynamic temperature profile of their melting were taken every 20 min by infrared camera. The temperature distribution of the pure paraffin and PCM1-PCM5 are shown in Fig. 1(a), (b). Heat conduction from the outer surfaces of beaker to the PCM dominated the early stage of the charging process. Conduction was the dominant heat transfer mode at the beginning of the melting process. As the liquid region increased, the effect of buoyancy appeared. The temperature distribution of PCM5 approached the temperature of water bath quite quickly because natural convection sped up the melting of PCM. Among the samples, PCM5 melted most quickly due to the highest mass fraction of nano-alumina added. The heat flux decreased due to the increasing thermal resistance of the growing layer of molten PCM. Therefore, even very strong natural convection has a negligible effect on the solid-liquid interface position compared to the effect of heat conduction in solid PCM.
Fig. 1 (a). Temperature distribution in melting
Fig. 1 (b). Temperature distribution in solidification
The thermal conductivity of pure paraffin and the nano-alumina/paraffin composite PCM are shown in Table 1. The thermal conductivities of the composite PCMs improve evidently compared to that of pure paraffin. When the thermal conductivity of pure paraffin is 0.21 W/mK, the thermal conductivity of the composite PCM including mass 0.1 wt% nano-alumina was found to be 0.3245 W/mK . The thermal conductivity of the paraffin with 0.5 wt% nano-alumina was 1.7 times (0.361 W/mK) higher than that of paraffin (0.21 W/mK).
Table1 Thermal conductivity data of paraffin, and PCM1-PCM5
Sample paraffin PCM1 PCM2 PCM3 PCM4 PCM5
wt% 0 0.1 0.2 0.3 0.4 0.5
ï¿½ï¿½(W/m?K) 0.21 0.3245 0.3322 0.3426 0.3402 0.361
3.2 Thermal kinetic investigation
Paraffin, and PCM1-PCM5 were sunk to the temperature controlled water bath and heated to melt the composites from 30ï¿½ï¿½ to 80ï¿½ï¿½ in 1000 seconds. After that, keeping 80ï¿½ï¿½ for 5 minutes, the composites were completely melted. Then heating was quitted to solidify the composites, the beakers in the water bath was cooled by natural. A temperature change of the composites in the melting and solidification is shown in Fig.2 (a), (b). The temperature variation in the composites was taken place by two different heat transfer rates. One is absorbed sensible heat during melting and the other was heat transfer inside the PCM. So, total heat transfer occurring at any point inside the PCM was equal to the two heat transfers at this point. The melting and solidification times can be considered as a nearly constant temperature. It was observed that the melting and solidification temperatures occurred at the same temperature range. This temperature range ranged from 54ï¿½ï¿½ to 56ï¿½ï¿½. This result was found to be agreed with given physical properties by the Sinopharm Chemical Reagent Company.
Fig. 2 (a). Melting curves of paraffin, and PCM1-PCM5
Fig. 2 (b). Solidification curves of paraffin, and PCM1-PCM5
Table2 DSC data of paraffin, and PCM1-PCM5
Sample wt% Melting Solidifying
Onset temperatureï¿½ï¿½ Peak temperatureï¿½ï¿½ Latent heat
J/g Onset temperatureï¿½ï¿½ Peak temperatureï¿½ï¿½ Latent heat
paraffin 0.0% 55.94 59.23 203.3 59.15 50.66 206.4
PCM1 0.1% 55.65 58.57 201.1 58.83 50.83 201.3
PCM2 0.2% 55.46 58.33 196.0 58.66 49.94 201.3
PCM3 0.3% 56.01 58.44 199.0 58.63 50.57 197.1
PCM4 0.4% 55.93 58.39 190.4 57.96 49.34 190.2
PCM5 0.5% 55.46 58.35 173.0 59.55 50.79 182.4
Thermal data obtained from the DSC thermograms of the composites were presented in Table 2. The melting temperature of the composites are approximately equal to that of the pure paraffin and the temperature ranges are less than 1ï¿½ï¿½. It is also found from Table 2 that the latent heat storage capacity of the samples was decreased by increasing the nano-alumina ratio in the mixture. The heat of fusion of the composites approached 182.4 J/g when the ratio of nano-alumina approached 0.5 wt%. The thermal analysis results showed that the latent heat of the composites were so high that it can be comparable with other PCMs, such as salt hydrates and polyalcohols. Therefore, the melting temperature and the latent heat capacity of the nano-alumina / paraffin composites made it suitable for heat storage in solar heating, building energy conservation and heat recovery systems with respect to the climate conditions.
3.3 Dispersion stability investigation
The PCM5 was heated in a temperature controlled water bath at 60ï¿½ï¿½. After PCM5 melted completely, ultrasonic vibration was employed with ultrasonic treatment time of 20, 40, 60, 80, 100 min. The variation of the depositional time of nano-alumina with the time of ultrasonic treatment is shown in Fig.3. It is found that the dispersibility of the nano-alumina is related to the time of the ultrasonic treatment. The sedimentation time was the longest when ultrasonic treatment time was set at 60 min. But the nano-alumina with longer time of ultrasonic treatment did not have higher dispersing ability.
Fig. 3. Impact of ultrasonic treatment on dispersion stability
Ultrasonic treatment may cause the flow velocity of the medium to increase: an effect known as acoustic streaming. In the experiment, the application of ultrasonic vibrations to the PCM5 liquid may induce acoustic streaming. The acoustic streaming can enhance both thermal convective streaming and mass transport, where convective transport was superimposed on diffusive transport. However, if the ultrasonic treatment time was too long, Brownian motion of nano-alumina would be accelerated with the increase of the temperature. As a result, particle collision may further aggravate the agglomeration.
3.4 Thermal stability investigation
A repetition test of solidification and fusion was conducted to investigate the thermal stability of the composites versus using time. Fig.4 show the DSC curves for the melting and solidification transitions of nano-alumina/paraffin composite PCM5 after 10, 20, 30, 40, 50 thermal cycles scanned at a rate of 5ï¿½ï¿½/min. Throughout the scan cycles, the solidification transition showed one main transition with a maximum solidification point of 59.55ï¿½ï¿½. The peak temperature decreased slightly with the thermal cycle increased (Fig.5). After 50 cycles the latent heats for the melting and solidification transitions were 168 J/g and 176 J/g. As shown in Fig.6, the latent heat did not change significantly from their initially observed values of 172 J/g and 183 J/g for melting and solidification, respectively. This suggests that no chemical degradation of the nano-alumina/paraffin composites occurred during thermal cycling. The nano-alumina/paraffin composite PCM would likely have good thermal reliability as PCMs for latent heat storage applications.
Fig. 4. DSC thermal cycling thermograms of PCM5
Fig. 5. Impact of thermal cycles on phase transition temperature of PCM5
Fig. 6. Impact of thermal cycles on latent heat of PCM5
Preparation and thermal performance of nano-alumina / paraffin composites as phase change material were presented, where paraffin was used as a PCM with nano-alumina added for improving the thermal conductivity. When the mass fraction of nano-alumina was 0.5%, the nano-alumina/paraffin composite PCM melted at 54.24ï¿½ï¿½ with a latent heat of 168 J/g and solidified at 57.68 J/g with a latent heat of 176 J/g after 50 cycles. The thermal conductivity of the composites was 1.7 times (0.361 W/mK) higher than that of paraffin (0.21 W/mK). No chemical degradation of the nano-alumina/paraffin composites occurred during thermal cycling. The nano-alumina/paraffin composite PCM could be used repeatedly in solar heating, building energy conservation and heat recovery systems for latent heat storage.
This study benefits from Shanghai Municipal Education Commission Innovation Program (Grant No. 12ZZ154) and Postgraduate Innovation Program of Shanghai Maritime University (Grant No.YC2012032). The authors also wish to thank the reviewers for kindly giving revising suggestions.