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expanded graphite composite phase change material
Solar Energy Materials & Solar Cells 155 (2016) 141–146
Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Preparation and thermal energy storage properties of D-Mannitol/ expanded graphite composite phase change material Tao Xu a,c,n, Qinglin Chen a, Gongsheng Huang c, Zhengguo Zhang b, Xuenong Gao b, Shushen Lu a a Guangdong Engineering Technology Research Center for Petrochemical Energy Conservation, School of Chemical Engineering and Technology, Sun Yat-sen University, Guangzhou 510275, China b Key Laboratory of Enhanced Heat Transfer and Energy Conservation (Ministry of Education), School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China c Department of Architecture and Civil Engineering, City University of Hong Kong, Kowloon, Hong Kong
art ic l e i nf o
a b s t r a c t
Article history: Received 5 April 2016 Received in revised form 31 May 2016 Accepted 1 June 2016 Available online 9 June 2016
A D-Mannitol/expanded graphite(EG) composite phase change material(PCM) was prepared for solar thermal energy storage or waste heat recovery applications performed at 180–240 °C. EG was produced by microwave irradiation performed. It was found that the EG prepared exhibited the maximum sorption capacity of 85 wt% for D-Mannitol. Scanning electron microscopy (SEM) images showed that D-Mannitol was uniformly dispersed in the micro holes of EG. Differential scanning calorimeter (DSC) analysis indicated that the phase change temperature of the composite PCM was close to that of D-Mannitol, and its latent heat was equivalent to the calculated value based on the mass fraction of D-Mannitol in the composite. With the assistance of hot disk analyzer, the thermal conductivities of the composite PCM first rise and then drop with increasing of its densities, and similar linear relationships between the maximum thermal conductivity and mass fraction of expanded graphite or between corresponding compress density and mass fraction of expanded graphite were obtained. Thermal energy storage performance of the composite PCM was tested in latent thermal energy storage (LTES) system. The heat transfer in the composite PCM during the thermal energy storage process was enhanced through the thermal conductivity improvement, while the heat storage duration was affected by the mass fraction of EG and the apparent density of the composite PCM. & 2016 Elsevier B.V. All rights reserved.
Keywords: D-mannitol Phase change material Thermal energy storage Thermal conductivity
1. Introduction Latent thermal energy storage (LTES) using phase change material (PCM) as a storage unit is one of the most effective methods to obtain high energy efficiency and energy storage densities, which has already widely been applied in solar energy utilization, cooling of electronic devices, waste heat recovering and other fields. In the past decades, LTES systems have been extensively studies for high performance [1–3]. The PCMs with the properties such as high latent heat, chemical inertness, high thermal conductivity, and commercial availability will be taken as ideal thermal energy storage for desired applications. In areas where solar thermal energy consumption focuses on high temperature, PCMs with high phase change temperature has come a main object of investigations in recent years. In n Corresponding author at : Guangdong Engineering Technology Research Center for Petrochemical Energy Conservation, School of Chemical Engineering and Technology, Sun Yat-sen University, Guangzhou 510275, China. E-mail address: [email protected] (T. Xu).
http://dx.doi.org/10.1016/j.solmat.2016.06.003 0927-0248/& 2016 Elsevier B.V. All rights reserved.
temperature range higher than 100 °C, a great interest is focused on inorganic eutectic salts as PCMs because of their lower investment costs and wide range of melting temperatures to apply different applications [4]. Binary and ternary eutectics are frequently applied for energy storage purpose in temperature interval of 100–300 °C [5–9], but these eutectic salts suffer a comparatively low latent heat (o 120 J/g), which leads to a reduction in energy storage capacity per volume of a LTES system. Besides that, the eutectic salts possess a poor thermal conductivities (o1 W/m K), which will suppress their heat storage rates. Even more importantly, almost all salts have a corrosive action to ordinary metals at high temperatures. Thus, those drawbacks of salts will consequently obstacle their widespread applications. According to Kaizawa et al., Erythritol, Xylitol and D-Mannitol appears to be reliable PCMs for high-temperature LTES applications, due to large latent heat and high melting temperature [10]. Especially, DMannitol is a natural polyol having a high phase change temperature ( 4160 °C) and large latent heat of melting (4 300 J/g) [11]. Its ability to retain heat effectively in phase change state affirms its capability for use as a PCM. Furthermore, D-Mannitol's
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relatively high density of 1.52 g/cm3 allows a large quantity of DMannitol to be packed into a small volume in comparison with paraffin waxes. This, coupled with its high specific heat of melting allows for large amounts of energy to be absorbed for storage[10]. As regards the cost, D-Mannitol is quite cheap. Comprehensive above factors, D-Mannitol aroused our interest to be selected as a promising PCM for practical LTES application performed at 180– 240 °C. However, two problems must be resolved actually. One problem is that the slow rate of heat transfer is generally unacceptably, which is caused by the low thermal conductivities of PCMs [12,13]. A great amount of studies on have been carried out to enhance thermal conductivity of PCM. Techniques as addition of high-thermal-conductivity particles, insertion of fins, and impregnation of PCMs into highly conductive porous structures are attempted to be applied in LTES applications [14–17]. However, the configuration of fin tubes and addition of high-thermal-conductivity materials will bring significant weight and cost increase. Another problem is that it is very difficult to encapsulate PCMs in high-temperature applications (above 100 °C) [18,19]. Liquid PCM are usually easy to leak through crack generated by volumetric expansion upon melting. For this reason, a higher thickness of the capsule layer is required, but this leads to a decrease in the heat storage density of composite PCM. Porous materials with a high thermal conductivity are regarded as candidates for solving these two problems simultaneously. It is considered that porous materials can form a stable shape that avoids the leakage of liquid PCM because of the capillary and surface tension forces of a porous structure. Recently, expanded graphite (EG) has been adopted to enhance the heat transfer and prevent from liquid leakage upon melting in PCMs, due to its desirable properties of better stability, higher thermal conductivity, better compatibility with organic PCMs, and lower density as compared with metallic foams [19–24]. Although D-Mannitol has been mentioned previously and EG is widely used to enhance heat transfer process and encapsulate PCMs, few studies about the utilizations of D-Mannitol in LTES applications and the thermal conductivity improvement of DMannitol with the help of EG has been reported. Hence, the development and study on a new D-Mannitol/EG composite PCM is valuable for the promotion of solar thermal energy utilization and waste heat recovery. In the present work, a composite PCM with DMannitol as PCM and EG as encapsulation heat transfer promoter was prepared. The thermal properties of the composites were studied by measuring the melting temperature, latent heat of phase change and thermal conductivity. Meanwhile, its thermal energy storage performance with EG mass fraction and compressing density in a LTES unit were experimentally tested and compared.
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1- Mixture of D-Mannitol and expanded graphite 2-Conduction oil 3-Stainless steel tank 4-Mixer 5-Seal cover 6 -Glass container 7- Valve 8-Vacuum pump Fig. 1. Schematic diagram of the composite process.
Mannitol into the expanded graphite. Fig. 1 shows a schematic diagram of the composite process. First, solid D-Mannitol and expanded graphite were placed in a glass container with a seal cover. Then, the glass container was put into the stainless steel tank containing conduction oil which was heated to about 220 °C. Next, the vacuum pump started to remove the air from the glass container and the mixer also started to stir the mixture for about 2 h. Finally, the vacuum pump was turned off and the composite PCM was removed from the glass container and then it was allowed to cool down until the PCM was solidified. For each EG composite PCM, based on the sorption curve as described in our earlier investigation [25], the maximum sorption capacity was determined by the experimental method. In this study, the mass percentage of D-Mannitol in the composite PCM was used to express the sorption capacity. The maximum sorption capacity of D-Mannitol can reach 85% for the expanded graphite sample prepared. The composite PCMs with different mass percentage of D-Mannitol were chosen as tested PCM samples. 2.2. Characterizations of PCM
2. Experimental
In this study, a scanning electron microscope (SEM, Quanta, FEI, Hotland) was used to observe the micro structures of the expanded graphite and the composite PCM. The melting and heat storage behaviors of the pure D-Mannitol and composite PCM were studied by a differential scanning calorimeter (DSC, DSCQ20, TA Instruments Inc., USA) under the protection of nitrogen. The data were collected at a scan rate of 5 °C/min from 120 °C to 210 °C. The quantity of each sample in the calorimeter sample cells made of aluminum is in the range from 6 to 10 mg. The latent heat and melting temperature for PCM were obtained by DSC. The thermal conductivities of PCM with a type 8563 probe acting as both heat source and sensor was measured by Hot disk thermal constant analyzer(Hot Disk TPS2500, Hot Disk Inc., Sweden). A transient plane source method, in which two disk-like samples for the test materials with a similar thickness were required to be placed in contact with the probe and heated at constant power for a setting scanning time, was selected for these measurements.
2.1. Preparation of composite PCM
2.3. Testing device of thermal energy storage performance
In the experiments, analytically pure D-Mannitol (C6H14O6) with a melting point of 166–169 °C was selected as the PCM, which was made of the Shanghai Bio Science & Technology CO. Ltd. Expanded graphite was used to serve as a support in the composite process. Expanded graphite was made by the expandable graphite with average particle size of 500 mm and expansion rate of 300 ml/ g. The expandable graphite was supplied by the Qingdao Hengsheng Graphite Co. Ltd. After being dried in a vacuum oven at 120 °C for 2 h, the expandable graphite of about 10 g was put into a microwave oven (800 W, Midea, China) for 20 s every time in order to obtain the expanded graphite. A D-Mannitol/Expanded graphite composite PCM was produced by absorbing the liquid D-
The thermal energy storage characteristics of D-Mannitol and composite PCM in a LTES system were investigated, respectively. The schematic diagram of the experimental system shown in Fig. 2 mainly consisted of an automatic temperature test chamber and a LTES unit. The temperature of test chamber was automatically controlled by a temperature control system with an accuracy of 71 °C. The tested PCM samples which were produced by steel shell mould and tablet press (DY-40, Tianjing Keqi Co. Ltd, China) could be packaged completely by the cylindrical alloy aluminum container with inner diameter of 50mm, outer diameter of 48mm and height of 100 mm. The K-type thermocouple was set in the center of each cylindrical PCM sample to investigate the
T. Xu et al. / Solar Energy Materials & Solar Cells 155 (2016) 141–146
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organic. After D-manitol had been absorbed into the micro holes of the expanded graphite, the microstructure of the composite PCM with 85% of D-mannitol appear to be similar to that of the expanded graphite owing to the surface tension force and the capillary force of the porous expanded graphite, as shown in Fig. 4 (c). This shows that the expanded graphite not only has a good compatibility with D-mannitol, but also can adsorb a large mass percentage of D-mannitol into its micro holes. No liquid D-Mannitol has been observed on the surface of the composite PCM at the solid-liquid phase change process, which is due to that D-Mannitol is hold by the tension force and capillary force of the porous expanded graphite.
1-Test chamber 2-Cylindrical alloy aluminum container 3-PCM sample 4-thermocouple 5-data logger 6-computer 7-temperature control system Fig. 2. Schematic diagram of the experimental system.
temperature variations within a measurement error of 71 °C. A data logger (Agilent34970A, USA) was used to gathering the voltage signal generated by the thermocouple and transform it to digital data. At the start, 60% of the container volume was filled with the PCM at 100 °C, and the remaining volume was left to meet the thermal expanding of PCM samples. The test chamber was kept at constant temperature of 220 °C in the process of heat storage.
3. Results and discussion 3.1. Structure of expanded graphite and composite PCM The configurations of the expanded graphite and the composite PCM are shown in Fig. 3 respectively. The expanded graphite appears fluffy and vermicular (Fig. 3(a)). Unlike the expanded graphite, the composite PCM with 85% of D-mannitol consists of granular particles (Fig. 3(b)). SEM images of the expanded graphite and the composite PCM with 85% of D-mannitol are shown in Fig. 4, respectively. Fig. 4(a) shows that the expanded graphite is made up of typical worm-like particles. To the magnification SEM images of the expanded graphite in Fig. 4(b), the microstructure of the worm-like particle presents an irregular honeycomb network constructed from elementary graphite sheets, which supplies enough crack-like and network-like micro holes for adsorbing
3.2. DSC analysis of D-Mannitol and composite PCM The differential scanning calorimetry (DSC) was selected to study the influence of expanded graphite addition on the thermal properties such as the latent heat capacity and melting point of DMannitol and composite PCM. Fig. 5 shows the DSC curves of pure D-Mannitol and the composite PCM with D-Mannitol mass percentages of 85%, respectively. Obviously, the curve shape of pure DMannitol is basically similar to that of composite PCM, embodying a single symmetrical peak that indicates it was the pure D-Mannitol that acted role of the latent thermal energy storage during the process of liquid-solid phase change. However, it can also be observed that expanded graphite has the impact on the phase change characteristics of D-Mannitol. The melting temperatures slightly shifts from 164.87 °C for pure D-Mannitol to 151.82 °C for composite PCM. This phenomenon could be ascribed to the porous expanded graphite network in the composite PCM that break up the crystal structure of D-Mannitol and weaken the interaction force between molecules. Moreover, the phase change latent heats of pure D-Mannitol and composite PCM are 319.0 J/g and 267.7 J/g according to the DSC tests. Because expanded graphite is no change in phase during the change process of composite PCM, the theoretical calculation value of the phase change latent of composite PCM should be about 271.2 J/g according to the mass fraction of D-Mannitol (85%) in the composite PCM. The experimental phase change latent heat of the composite PCM has an agreement with the calculated values with relative errors less than 2%, which also shows there is no chemical reaction between pure D-Mannitol and expanded graphite in the preparation of the composite PCM.
Fig. 3. Photographs of the expanded graphite (a) and the composite PCM (b).
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Fig. 4. SEM images of the expanded graphite and the composite PCM: (a) Expanded graphite( 100);(b)expanded graphite( 1000);(c)composite PCM( 100); (d)composite PCM( 1000).
3.3. Thermal conductivity measurement
0
Heat flow(w/g)
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Composite PCM 0 151.82 C 267.7 J/g Composite PCM D-Mannitol
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D-Mannitol 0 164.87 C 319.0 J/g
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Temperature( C) Fig. 5. DSC curves of the D-Mannitol and the composite PCM.
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Usually, the composite PCM particles are so loose that their apparent densities are too small, which will have an obvious effect on thermal conductivities of porous materials. The moderate compression density is very important for the performances of heat storage and thermal conductivity. The thermal conductivity variation of the composite PCM (85%) with the different densities is shown in Fig. 6. It shows that the thermal conductivities of the composite PCM remarkably higher than that of the D-Mannitol. This improvement was caused by the addition of expanded graphite which could act as a type of high-thermal-conductive substrate. At the same time, it can be noticed that the thermal conductivities first rises and then drop with increasing of the density of the composite PCM. While the density of the composite PCM is within 1.80 g/cm3, the increasing trend could be attributed to the reduction of micropore volume of the porous expanded graphite and the enlargement of the contact area. In the meantime, the apparent density also can't be beyond 1.80 g/cm3 because DMannitol leakage will appear if the mechanical stress that acts upon the composite PCM exceeded the capillary force of the
Thermal Conductivity (W/m ·K
T. Xu et al. / Solar Energy Materials & Solar Cells 155 (2016) 141–146
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3.4. Thermal energy storage performance
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For investigating the thermal energy storage performance of the composite PCM, the apparent density and expanded graphite mass content were considered as an important factor. Three tested samples (Table 1) were applied into the container. The temperature profiles of the composites with different expanded graphite mass fraction during the heat storage process were shown in Fig. 8. It can be observed that the test point of each sample exhibits a three-step behavior during heating process, which is similar to that occurred in different types of LTES systems with various PCMs [15,26–28]. Firstly, the temperature rises quickly until it reaches the melting temperature, mainly because of heat conduction inside the solid phase dominating the heat transfer. Secondly, the liquid phase of D-Manitol begins to occur, but it is confined to small porous holes of expanded graphite and heat conduction still plays a crucial role during the process of heat transfer. The heat conduction-dominated mechanism have already been studied by the previous works on paraffin/EG composite [15], AC/EG composite [26] and NaNO3/EG composite [27]. During the process of thermal energy storage, the LTES unit could get continuous heat from its encapsulated walls and heat is transferred from its outer cylindrical surface to inner, thus the temperature at test point increases at a different time. As shown in Fig. 8, it takes 3400, 3050 and 2850 s for the samples with EG mass fraction of 5%, 15% and 25% and corresponding compress density of 1.64, 1.80 and 2.03 g/ cm3 to reach the thermal balance, respectively. It is clear that the composite PCM can achieve the heat storage in a shorter time with higher mass fraction of EG. There are two main reasons for this result. For one thing, the thermal energy storage process can be considered as an unsteady heat conduction process and the thermal conductivity of the composite PCM is improved with the assistance of increasing EG mass fraction. For another, phase change latent heat of the composite PCM can reduce because of increasing the EG mass fraction, which obviously leaded to a lower energy storage density.
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Density (g/cm ) Fig. 6. Thermal conductivity variation of the composite PCM with the different densities. 22 2.2
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Maximum thermal conductivity (W/m ·K
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Mass fraction of expanded graphite (%) Fig. 7. Maximum conductivities and corresponding compress densities with different mass fractions of expanded graphite.
porous expanded graphite. For the composite PCM with expanded graphite mass fraction of 15%, the maximum conductivity of 7.32 W/m K corresponds to optimal compress density of 1.80 g/ cm3 and the thermal conductivity of pure D-Mannitol (0.60 W/ m K) can be improved by about 12 times. What is more, the same method could be adopted to obtain the maximum conductivities and corresponding compress densities with different mass fractions of expanded graphite. The result is shown in Fig. 7 that indicates almost linear relationships between the maximum thermal conductivity and mass fraction of expanded graphite or between corresponding compress density and mass fraction of expanded graphite. The relations are expressed as Eq. (1) and Eq. (2):
y1 = 1.55 + 0.01843x
(1)
y2 = − 0.725 + 0.52136x
(2)
4. Conclusions In this study, a new high temperature composite PCM was prepared and investigated for the thermal energy storage application by using D-Manitol with large latent heat of phase change and high-thermal-conductivity EG. The results indicate that the composite PCM can be suggested as a potential alternative for solar energy thermal storage. The conclusions are drawn as follows: (1) The D-Mannitol /EG composite PCMs were prepared by absorbing liquid D-Mannitol into the porous structure of EG. The maximum mass fraction of D-Mannitol absorbed in the composite PCM was as high as 85%. Meanwhile, the presence of EG within the composite had no significant change in the phase change temperature of D-Mannitol, but the latent heat of the composite PCM(267.7 J/g) was equivalent to the value calculated by multiplying the latent heat of D-Mannitol (319.0 J/g) with its mass fraction(85 wt%).
Table 1. Properties of tested samples. Properties
Sample #1
Sample #2
Sample #3
Mass fraction of expanded graphite (%) Maximum conductivity(W/m K) Corresponding compress density (g/cm3)
5 1.83 1.64
15 7.32 1.80
25 11.39 2.03
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Sample #1 Sample #2 Sample #3
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Time (s) Fig. 8. Thermal energy storage performance of the composite PCM with various mass fraction of expanded graphite.
(2) The addition of EG into D-Mannitol plays an important role in a remarkable improvement in the thermal conductivity. According to the measurements, the thermal conductivities of the studied composites first rise and then drop with increasing of the density of the composite PCM. For the composite PCM samples, similar linear relationships between the maximum thermal conductivity and mass fraction of expanded graphite or between corresponding compress density and mass fraction of expanded graphite were obtained. (3) During the thermal energy storage process, the thermal energy storage duration of the composite PCM was affected by its phase change latent heat, apparent density and thermal conductivity. Consequently, the thermal energy storage duration was shorten when the composite with higher EG mass fraction was applied in the LTES unit, whereas it was extended with the apparent density and conductivity increased.
Acknowledgment This work was supported by the Fundamental Research Funds for the Central Universities (16lgpy20), Guangdong-Hongkong Joint Innovation Project (2016A05050302), and National Natural Science Foundation of China (U1507201).
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Contents lists available at ScienceDirect
Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat
Preparation and thermal energy storage properties of D-Mannitol/ expanded graphite composite phase change material Tao Xu a,c,n, Qinglin Chen a, Gongsheng Huang c, Zhengguo Zhang b, Xuenong Gao b, Shushen Lu a a Guangdong Engineering Technology Research Center for Petrochemical Energy Conservation, School of Chemical Engineering and Technology, Sun Yat-sen University, Guangzhou 510275, China b Key Laboratory of Enhanced Heat Transfer and Energy Conservation (Ministry of Education), School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China c Department of Architecture and Civil Engineering, City University of Hong Kong, Kowloon, Hong Kong
art ic l e i nf o
a b s t r a c t
Article history: Received 5 April 2016 Received in revised form 31 May 2016 Accepted 1 June 2016 Available online 9 June 2016
A D-Mannitol/expanded graphite(EG) composite phase change material(PCM) was prepared for solar thermal energy storage or waste heat recovery applications performed at 180–240 °C. EG was produced by microwave irradiation performed. It was found that the EG prepared exhibited the maximum sorption capacity of 85 wt% for D-Mannitol. Scanning electron microscopy (SEM) images showed that D-Mannitol was uniformly dispersed in the micro holes of EG. Differential scanning calorimeter (DSC) analysis indicated that the phase change temperature of the composite PCM was close to that of D-Mannitol, and its latent heat was equivalent to the calculated value based on the mass fraction of D-Mannitol in the composite. With the assistance of hot disk analyzer, the thermal conductivities of the composite PCM first rise and then drop with increasing of its densities, and similar linear relationships between the maximum thermal conductivity and mass fraction of expanded graphite or between corresponding compress density and mass fraction of expanded graphite were obtained. Thermal energy storage performance of the composite PCM was tested in latent thermal energy storage (LTES) system. The heat transfer in the composite PCM during the thermal energy storage process was enhanced through the thermal conductivity improvement, while the heat storage duration was affected by the mass fraction of EG and the apparent density of the composite PCM. & 2016 Elsevier B.V. All rights reserved.
Keywords: D-mannitol Phase change material Thermal energy storage Thermal conductivity
1. Introduction Latent thermal energy storage (LTES) using phase change material (PCM) as a storage unit is one of the most effective methods to obtain high energy efficiency and energy storage densities, which has already widely been applied in solar energy utilization, cooling of electronic devices, waste heat recovering and other fields. In the past decades, LTES systems have been extensively studies for high performance [1–3]. The PCMs with the properties such as high latent heat, chemical inertness, high thermal conductivity, and commercial availability will be taken as ideal thermal energy storage for desired applications. In areas where solar thermal energy consumption focuses on high temperature, PCMs with high phase change temperature has come a main object of investigations in recent years. In n Corresponding author at : Guangdong Engineering Technology Research Center for Petrochemical Energy Conservation, School of Chemical Engineering and Technology, Sun Yat-sen University, Guangzhou 510275, China. E-mail address: [email protected] (T. Xu).
http://dx.doi.org/10.1016/j.solmat.2016.06.003 0927-0248/& 2016 Elsevier B.V. All rights reserved.
temperature range higher than 100 °C, a great interest is focused on inorganic eutectic salts as PCMs because of their lower investment costs and wide range of melting temperatures to apply different applications [4]. Binary and ternary eutectics are frequently applied for energy storage purpose in temperature interval of 100–300 °C [5–9], but these eutectic salts suffer a comparatively low latent heat (o 120 J/g), which leads to a reduction in energy storage capacity per volume of a LTES system. Besides that, the eutectic salts possess a poor thermal conductivities (o1 W/m K), which will suppress their heat storage rates. Even more importantly, almost all salts have a corrosive action to ordinary metals at high temperatures. Thus, those drawbacks of salts will consequently obstacle their widespread applications. According to Kaizawa et al., Erythritol, Xylitol and D-Mannitol appears to be reliable PCMs for high-temperature LTES applications, due to large latent heat and high melting temperature [10]. Especially, DMannitol is a natural polyol having a high phase change temperature ( 4160 °C) and large latent heat of melting (4 300 J/g) [11]. Its ability to retain heat effectively in phase change state affirms its capability for use as a PCM. Furthermore, D-Mannitol's
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relatively high density of 1.52 g/cm3 allows a large quantity of DMannitol to be packed into a small volume in comparison with paraffin waxes. This, coupled with its high specific heat of melting allows for large amounts of energy to be absorbed for storage[10]. As regards the cost, D-Mannitol is quite cheap. Comprehensive above factors, D-Mannitol aroused our interest to be selected as a promising PCM for practical LTES application performed at 180– 240 °C. However, two problems must be resolved actually. One problem is that the slow rate of heat transfer is generally unacceptably, which is caused by the low thermal conductivities of PCMs [12,13]. A great amount of studies on have been carried out to enhance thermal conductivity of PCM. Techniques as addition of high-thermal-conductivity particles, insertion of fins, and impregnation of PCMs into highly conductive porous structures are attempted to be applied in LTES applications [14–17]. However, the configuration of fin tubes and addition of high-thermal-conductivity materials will bring significant weight and cost increase. Another problem is that it is very difficult to encapsulate PCMs in high-temperature applications (above 100 °C) [18,19]. Liquid PCM are usually easy to leak through crack generated by volumetric expansion upon melting. For this reason, a higher thickness of the capsule layer is required, but this leads to a decrease in the heat storage density of composite PCM. Porous materials with a high thermal conductivity are regarded as candidates for solving these two problems simultaneously. It is considered that porous materials can form a stable shape that avoids the leakage of liquid PCM because of the capillary and surface tension forces of a porous structure. Recently, expanded graphite (EG) has been adopted to enhance the heat transfer and prevent from liquid leakage upon melting in PCMs, due to its desirable properties of better stability, higher thermal conductivity, better compatibility with organic PCMs, and lower density as compared with metallic foams [19–24]. Although D-Mannitol has been mentioned previously and EG is widely used to enhance heat transfer process and encapsulate PCMs, few studies about the utilizations of D-Mannitol in LTES applications and the thermal conductivity improvement of DMannitol with the help of EG has been reported. Hence, the development and study on a new D-Mannitol/EG composite PCM is valuable for the promotion of solar thermal energy utilization and waste heat recovery. In the present work, a composite PCM with DMannitol as PCM and EG as encapsulation heat transfer promoter was prepared. The thermal properties of the composites were studied by measuring the melting temperature, latent heat of phase change and thermal conductivity. Meanwhile, its thermal energy storage performance with EG mass fraction and compressing density in a LTES unit were experimentally tested and compared.
4
5
6
1 7 2 3
8
1- Mixture of D-Mannitol and expanded graphite 2-Conduction oil 3-Stainless steel tank 4-Mixer 5-Seal cover 6 -Glass container 7- Valve 8-Vacuum pump Fig. 1. Schematic diagram of the composite process.
Mannitol into the expanded graphite. Fig. 1 shows a schematic diagram of the composite process. First, solid D-Mannitol and expanded graphite were placed in a glass container with a seal cover. Then, the glass container was put into the stainless steel tank containing conduction oil which was heated to about 220 °C. Next, the vacuum pump started to remove the air from the glass container and the mixer also started to stir the mixture for about 2 h. Finally, the vacuum pump was turned off and the composite PCM was removed from the glass container and then it was allowed to cool down until the PCM was solidified. For each EG composite PCM, based on the sorption curve as described in our earlier investigation [25], the maximum sorption capacity was determined by the experimental method. In this study, the mass percentage of D-Mannitol in the composite PCM was used to express the sorption capacity. The maximum sorption capacity of D-Mannitol can reach 85% for the expanded graphite sample prepared. The composite PCMs with different mass percentage of D-Mannitol were chosen as tested PCM samples. 2.2. Characterizations of PCM
2. Experimental
In this study, a scanning electron microscope (SEM, Quanta, FEI, Hotland) was used to observe the micro structures of the expanded graphite and the composite PCM. The melting and heat storage behaviors of the pure D-Mannitol and composite PCM were studied by a differential scanning calorimeter (DSC, DSCQ20, TA Instruments Inc., USA) under the protection of nitrogen. The data were collected at a scan rate of 5 °C/min from 120 °C to 210 °C. The quantity of each sample in the calorimeter sample cells made of aluminum is in the range from 6 to 10 mg. The latent heat and melting temperature for PCM were obtained by DSC. The thermal conductivities of PCM with a type 8563 probe acting as both heat source and sensor was measured by Hot disk thermal constant analyzer(Hot Disk TPS2500, Hot Disk Inc., Sweden). A transient plane source method, in which two disk-like samples for the test materials with a similar thickness were required to be placed in contact with the probe and heated at constant power for a setting scanning time, was selected for these measurements.
2.1. Preparation of composite PCM
2.3. Testing device of thermal energy storage performance
In the experiments, analytically pure D-Mannitol (C6H14O6) with a melting point of 166–169 °C was selected as the PCM, which was made of the Shanghai Bio Science & Technology CO. Ltd. Expanded graphite was used to serve as a support in the composite process. Expanded graphite was made by the expandable graphite with average particle size of 500 mm and expansion rate of 300 ml/ g. The expandable graphite was supplied by the Qingdao Hengsheng Graphite Co. Ltd. After being dried in a vacuum oven at 120 °C for 2 h, the expandable graphite of about 10 g was put into a microwave oven (800 W, Midea, China) for 20 s every time in order to obtain the expanded graphite. A D-Mannitol/Expanded graphite composite PCM was produced by absorbing the liquid D-
The thermal energy storage characteristics of D-Mannitol and composite PCM in a LTES system were investigated, respectively. The schematic diagram of the experimental system shown in Fig. 2 mainly consisted of an automatic temperature test chamber and a LTES unit. The temperature of test chamber was automatically controlled by a temperature control system with an accuracy of 71 °C. The tested PCM samples which were produced by steel shell mould and tablet press (DY-40, Tianjing Keqi Co. Ltd, China) could be packaged completely by the cylindrical alloy aluminum container with inner diameter of 50mm, outer diameter of 48mm and height of 100 mm. The K-type thermocouple was set in the center of each cylindrical PCM sample to investigate the
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organic. After D-manitol had been absorbed into the micro holes of the expanded graphite, the microstructure of the composite PCM with 85% of D-mannitol appear to be similar to that of the expanded graphite owing to the surface tension force and the capillary force of the porous expanded graphite, as shown in Fig. 4 (c). This shows that the expanded graphite not only has a good compatibility with D-mannitol, but also can adsorb a large mass percentage of D-mannitol into its micro holes. No liquid D-Mannitol has been observed on the surface of the composite PCM at the solid-liquid phase change process, which is due to that D-Mannitol is hold by the tension force and capillary force of the porous expanded graphite.
1-Test chamber 2-Cylindrical alloy aluminum container 3-PCM sample 4-thermocouple 5-data logger 6-computer 7-temperature control system Fig. 2. Schematic diagram of the experimental system.
temperature variations within a measurement error of 71 °C. A data logger (Agilent34970A, USA) was used to gathering the voltage signal generated by the thermocouple and transform it to digital data. At the start, 60% of the container volume was filled with the PCM at 100 °C, and the remaining volume was left to meet the thermal expanding of PCM samples. The test chamber was kept at constant temperature of 220 °C in the process of heat storage.
3. Results and discussion 3.1. Structure of expanded graphite and composite PCM The configurations of the expanded graphite and the composite PCM are shown in Fig. 3 respectively. The expanded graphite appears fluffy and vermicular (Fig. 3(a)). Unlike the expanded graphite, the composite PCM with 85% of D-mannitol consists of granular particles (Fig. 3(b)). SEM images of the expanded graphite and the composite PCM with 85% of D-mannitol are shown in Fig. 4, respectively. Fig. 4(a) shows that the expanded graphite is made up of typical worm-like particles. To the magnification SEM images of the expanded graphite in Fig. 4(b), the microstructure of the worm-like particle presents an irregular honeycomb network constructed from elementary graphite sheets, which supplies enough crack-like and network-like micro holes for adsorbing
3.2. DSC analysis of D-Mannitol and composite PCM The differential scanning calorimetry (DSC) was selected to study the influence of expanded graphite addition on the thermal properties such as the latent heat capacity and melting point of DMannitol and composite PCM. Fig. 5 shows the DSC curves of pure D-Mannitol and the composite PCM with D-Mannitol mass percentages of 85%, respectively. Obviously, the curve shape of pure DMannitol is basically similar to that of composite PCM, embodying a single symmetrical peak that indicates it was the pure D-Mannitol that acted role of the latent thermal energy storage during the process of liquid-solid phase change. However, it can also be observed that expanded graphite has the impact on the phase change characteristics of D-Mannitol. The melting temperatures slightly shifts from 164.87 °C for pure D-Mannitol to 151.82 °C for composite PCM. This phenomenon could be ascribed to the porous expanded graphite network in the composite PCM that break up the crystal structure of D-Mannitol and weaken the interaction force between molecules. Moreover, the phase change latent heats of pure D-Mannitol and composite PCM are 319.0 J/g and 267.7 J/g according to the DSC tests. Because expanded graphite is no change in phase during the change process of composite PCM, the theoretical calculation value of the phase change latent of composite PCM should be about 271.2 J/g according to the mass fraction of D-Mannitol (85%) in the composite PCM. The experimental phase change latent heat of the composite PCM has an agreement with the calculated values with relative errors less than 2%, which also shows there is no chemical reaction between pure D-Mannitol and expanded graphite in the preparation of the composite PCM.
Fig. 3. Photographs of the expanded graphite (a) and the composite PCM (b).
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Fig. 4. SEM images of the expanded graphite and the composite PCM: (a) Expanded graphite( 100);(b)expanded graphite( 1000);(c)composite PCM( 100); (d)composite PCM( 1000).
3.3. Thermal conductivity measurement
0
Heat flow(w/g)
-1 -2 -3
Composite PCM 0 151.82 C 267.7 J/g Composite PCM D-Mannitol
-4
D-Mannitol 0 164.87 C 319.0 J/g
-5 -6 120
140
160
180
200
0
Temperature( C) Fig. 5. DSC curves of the D-Mannitol and the composite PCM.
220
Usually, the composite PCM particles are so loose that their apparent densities are too small, which will have an obvious effect on thermal conductivities of porous materials. The moderate compression density is very important for the performances of heat storage and thermal conductivity. The thermal conductivity variation of the composite PCM (85%) with the different densities is shown in Fig. 6. It shows that the thermal conductivities of the composite PCM remarkably higher than that of the D-Mannitol. This improvement was caused by the addition of expanded graphite which could act as a type of high-thermal-conductive substrate. At the same time, it can be noticed that the thermal conductivities first rises and then drop with increasing of the density of the composite PCM. While the density of the composite PCM is within 1.80 g/cm3, the increasing trend could be attributed to the reduction of micropore volume of the porous expanded graphite and the enlargement of the contact area. In the meantime, the apparent density also can't be beyond 1.80 g/cm3 because DMannitol leakage will appear if the mechanical stress that acts upon the composite PCM exceeded the capillary force of the
Thermal Conductivity (W/m ·K
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3.4. Thermal energy storage performance
7
For investigating the thermal energy storage performance of the composite PCM, the apparent density and expanded graphite mass content were considered as an important factor. Three tested samples (Table 1) were applied into the container. The temperature profiles of the composites with different expanded graphite mass fraction during the heat storage process were shown in Fig. 8. It can be observed that the test point of each sample exhibits a three-step behavior during heating process, which is similar to that occurred in different types of LTES systems with various PCMs [15,26–28]. Firstly, the temperature rises quickly until it reaches the melting temperature, mainly because of heat conduction inside the solid phase dominating the heat transfer. Secondly, the liquid phase of D-Manitol begins to occur, but it is confined to small porous holes of expanded graphite and heat conduction still plays a crucial role during the process of heat transfer. The heat conduction-dominated mechanism have already been studied by the previous works on paraffin/EG composite [15], AC/EG composite [26] and NaNO3/EG composite [27]. During the process of thermal energy storage, the LTES unit could get continuous heat from its encapsulated walls and heat is transferred from its outer cylindrical surface to inner, thus the temperature at test point increases at a different time. As shown in Fig. 8, it takes 3400, 3050 and 2850 s for the samples with EG mass fraction of 5%, 15% and 25% and corresponding compress density of 1.64, 1.80 and 2.03 g/ cm3 to reach the thermal balance, respectively. It is clear that the composite PCM can achieve the heat storage in a shorter time with higher mass fraction of EG. There are two main reasons for this result. For one thing, the thermal energy storage process can be considered as an unsteady heat conduction process and the thermal conductivity of the composite PCM is improved with the assistance of increasing EG mass fraction. For another, phase change latent heat of the composite PCM can reduce because of increasing the EG mass fraction, which obviously leaded to a lower energy storage density.
6 5 4 3 2 1 0.6
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Density (g/cm ) Fig. 6. Thermal conductivity variation of the composite PCM with the different densities. 22 2.2
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Corresponding compress density g/cm
Maximum thermal conductivity (W/m ·K
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0
5
10
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20
25
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Mass fraction of expanded graphite (%) Fig. 7. Maximum conductivities and corresponding compress densities with different mass fractions of expanded graphite.
porous expanded graphite. For the composite PCM with expanded graphite mass fraction of 15%, the maximum conductivity of 7.32 W/m K corresponds to optimal compress density of 1.80 g/ cm3 and the thermal conductivity of pure D-Mannitol (0.60 W/ m K) can be improved by about 12 times. What is more, the same method could be adopted to obtain the maximum conductivities and corresponding compress densities with different mass fractions of expanded graphite. The result is shown in Fig. 7 that indicates almost linear relationships between the maximum thermal conductivity and mass fraction of expanded graphite or between corresponding compress density and mass fraction of expanded graphite. The relations are expressed as Eq. (1) and Eq. (2):
y1 = 1.55 + 0.01843x
(1)
y2 = − 0.725 + 0.52136x
(2)
4. Conclusions In this study, a new high temperature composite PCM was prepared and investigated for the thermal energy storage application by using D-Manitol with large latent heat of phase change and high-thermal-conductivity EG. The results indicate that the composite PCM can be suggested as a potential alternative for solar energy thermal storage. The conclusions are drawn as follows: (1) The D-Mannitol /EG composite PCMs were prepared by absorbing liquid D-Mannitol into the porous structure of EG. The maximum mass fraction of D-Mannitol absorbed in the composite PCM was as high as 85%. Meanwhile, the presence of EG within the composite had no significant change in the phase change temperature of D-Mannitol, but the latent heat of the composite PCM(267.7 J/g) was equivalent to the value calculated by multiplying the latent heat of D-Mannitol (319.0 J/g) with its mass fraction(85 wt%).
Table 1. Properties of tested samples. Properties
Sample #1
Sample #2
Sample #3
Mass fraction of expanded graphite (%) Maximum conductivity(W/m K) Corresponding compress density (g/cm3)
5 1.83 1.64
15 7.32 1.80
25 11.39 2.03
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Sample #1 Sample #2 Sample #3
0
Temperature ( C)
200 180 160 140 120 100 0
500
1000
1500
2000
2500
3000
3500
Time (s) Fig. 8. Thermal energy storage performance of the composite PCM with various mass fraction of expanded graphite.
(2) The addition of EG into D-Mannitol plays an important role in a remarkable improvement in the thermal conductivity. According to the measurements, the thermal conductivities of the studied composites first rise and then drop with increasing of the density of the composite PCM. For the composite PCM samples, similar linear relationships between the maximum thermal conductivity and mass fraction of expanded graphite or between corresponding compress density and mass fraction of expanded graphite were obtained. (3) During the thermal energy storage process, the thermal energy storage duration of the composite PCM was affected by its phase change latent heat, apparent density and thermal conductivity. Consequently, the thermal energy storage duration was shorten when the composite with higher EG mass fraction was applied in the LTES unit, whereas it was extended with the apparent density and conductivity increased.
Acknowledgment This work was supported by the Fundamental Research Funds for the Central Universities (16lgpy20), Guangdong-Hongkong Joint Innovation Project (2016A05050302), and National Natural Science Foundation of China (U1507201).
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