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expanded graphite composite phase change material
Renewable Energy 50 (2013) 670e675
Contents lists available at SciVerse ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Thermal energy storage cement mortar containing n-octadecane/expanded graphite composite phase change material Zhengguo Zhang a, *, Guoquan Shi a, Shuping Wang a, Xiaoming Fang a, Xiaohong Liu b a
Key Laboratory of Enhanced Heat Transfer and Energy Conservation, the Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China b Automotive Department, Guangdong Industry Technical College, Guangzhou 510300, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 March 2012 Accepted 8 August 2012 Available online 4 September 2012
Here we demonstrate thermal energy storage cement mortar (TESCM) fabricated by integrating ordinary cement mortar with a composite phase change material (PCM) based on n-octadecane and expanded graphite (EG). The mass percentage of n-octadecane in the composite PCM can reach as high as 90% due to the excellent adsorption ability of EG, which endows the composite PCM with large latent heat. SEM images of the composite PCM show that n-octadecane is adsorbed into the pores of EG and uniformly covers on the nanosheets of EG, which microstructure contributes to preventing leakage of melted n-octadecane after it changes phase from solid state to liquid state. The n-octadecane/EG composite PCM has a good compatibility with ordinary cement mortar, and does not obviously deteriorate the apparent densities of the TESCM samples. Based on the thermal energy storage performance evaluation, it is found that the TESCM containing the n-octadecane/EG composite PCM plays a role in reducing the variation of indoor temperature, which helps to decrease the energy consumption for buildings. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Phase change material Thermal energy storage n-octadecane Expanded graphite Cement mortar
1. Introduction The fast economic development leads to a rapidly increasing energy demand worldwide, which results in several energy concerns about supply difficulties, exhaustion of energy resources, climate changes, and environmental pollution. Being an important part of the whole energy demand, the energy consumption for buildings increases very rapidly in the recent years due to the population growth, the enhancement of building services and thermal comfort levels, and the rise in time for people spending inside buildings [1]. Therefore, it is of great significance to develop novel building materials for improving the energy utilization efficiency and reducing the energy consumption for buildings. A phase change material (PCM) that can absorb or release a large quantity of latent heat when it changes phase from solid state to liquid state or vice versa, has been widely applied into thermal energy storage systems. Incorporating a suitable PCM into the walls, ceiling and floor of a building can function as reducing the room temperature swings in the building, thus leading to an improvement in the human comfort and a reduce in the energy
* Corresponding author. Tel.: þ86 20 8711 2845; fax: þ86 20 8711 3870. E-mail address: [email protected] (Z. Zhang). 0960-1481/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2012.08.024
consumption for the building [2e11]. Since the 1970s, a number of researchers have tried to apply PCMs into buildings to enhance the thermal comfort of lightweight constructions. Three general methods are proposed for the incorporation of organic PCMs into construction elements such as wallboards, gypsum boards, concrete, and so on [12,13]. The first one is the immersion of conventional wallboards into molten PCMs [14e18]. Although the method is simple and low cost, the impregnated wallboards are inflammable owing to leakage of the liquid PCMs to the surfaces of the wallboards, especially after the PCMs experienced several heating-cooling cycles. The second one is the integration of microencapsulated PCMs with ordinary building materials [19e27]. However, the method suffers from the complicated polymerization processes and the high costs related to the microencapsulation of PCMs [23]. Nowadays the commercially available microencapsulated PCMs are rare and have been only developed by few companies such as BASF. The third one is the incorporation of a kind of shape-stabilized PCMs into building materials, which shape-stabilized PCMs can be prepared by blending PCMs with supporting materials [28,29]. The commonly-used supporting materials include high-density polyethylene (HDPE) and styrenee butadieneestyrene (SBS), expanded graphite (EG), and so on. Compared with the polymeric supporting materials, EG has the advantages of high thermal conductivity and good adsorption
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Fig. 1. A sketch of the experimental apparatus for testing the thermal storage performances of the TESCM boards.
ability, thus making it a good matrix for preparing composite PCMs [30]. The previous work reported by our group demonstrated that the EG-based composite PCMs not only exhibited excellent thermal energy storage property along with improved thermal conductivity as compared with the pure organic PCMs, but also have the advantage of preventing leakage of the liquid PCMs [31]. Cement mortar that is produced by mixing up Portland cement, sand, and water has wide applications in buildings such as creating a smooth surface to walls. The integration of PCMs with cement mortar can render it feasible to capture solar energy directly and store significant amounts of thermal energy in the building envelope. N-octadecane is an excellent PCM with the properties of nontoxicity, high latent heat, and a suitable phase transition temperature for the application in buildings. In the current work, we focus on the incorporation of the n-octadecane/EG composite PCM into ordinary cement mortar to produce thermal energy storage cement mortar (TESCM). Firstly, the n-octadecane/EG composite PCM was prepared by adsorbing n-octadecane into the pores of EG. The structure and properties of the composite PCM was characterized by SEM, XRD, and DSC, respectively. Then, the mechanical property and thermal conductivity of the TESCM samples containing different mass percentage of the n-octadecane/EG composite PCM were investigated. Finally, the thermal energy storage performances of the TESCM boards containing different mass percentage of the n-octadecane/EG composite PCM were evaluated. 2. Experimental 2.1. Preparation and characterization of n-octadecane/EG composite PCM N-octadecane (Alfa Aesar) with a melting temperature of 27 C and graphite powder (Qingdao Graphite Co. Ltd, China) with an
average particle size of 500 mm and an expandable rate of 300 ml/g were used as the raw materials. After being dried in a vacuum oven at 70 C for 20 h, the graphite powder was heated using a domestic microwave oven (800 W, Midea, China) for 10 s to make it expanded into EG. The n-octadecane/EG composite PCM was prepared by adsorbing the melted n-octadecane into the pores of the obtained EG. The mass percentage of n-octadecane in the composite PCM was set at 90% based on our previous work [31]. The microstructures of the obtained EG and n-octadecane/EG composite PCM were observed using a scanning electron microscope (SEM, S-3700N, Hitachi, Japan), respectively. The crystalline phases of n-octadecane and the obtained EG and composite n-octadecane/EG PCM were characterized by an X-ray diffractometer (XRD, D8-ADVANCE, Bruker, German), respectively. The copper target (CuKa) was used as the diffraction source. The thermal properties of n-octadecane and the composite n-octadecane/EG PCM were measured by using a differential scanning calorimeter (DSC, DSC2910, TA Instruments, USA) under N2 atmosphere. The measuring temperature ranged from 20 to 70 C. The temperature rise rate was 5 C/min. 2.2. Fabrication and performance test of TESCM 2.2.1. Fabrication Conforming to ISO 679:1989 (Method of testing cements e determination of strength), ordinary cement mortar was prepared by mixing up Slag Portland cement (# 325), standard sand (particle size 2.0 mm), and water, in which the mass ratio of sand to the cement is 3, and that of water to the cement was 0.5. Four TESCM samples were produced by adding the n-octadecane/EG composite PCM into the ingredients of the ordinary cement mortar followed by thoroughly mixing, in which the mass ratio of the composite PCM to the cement was 0.02, 0.05, 0.07, and 0.1, respectively. After
Fig. 2. Photographs of the EG (a) and the n-octadecane/EG composite PCM (b).
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the four TESCM samples were dried to constant weight, the percentages of the n-octadecane/EG composite PCM in the four TESCM samples were calculated to be 0.5, 1.2, 1.7, and 2.5%, respectively, based on the final weight of each TESCM sample and the amount of the composite PCM added into it. 2.2.2. Mechanical property test The mechanical properties of the four TESCM samples were evaluated by measuring their compressive strength and apparent density, respectively. Firstly, the ordinary cement mortar and the four TESCM samples were formed into several cubes using a standard tripartite mold with the dimensions of 70.7 mm 70.7 mm 70.7 mm, respectively. After being kept under a moist atmosphere for about 24 h, all the cubes were maintained under water at 20 1 C for 7 days. Then, the cubes used for measuring the compressive strength were kept under a controlled condition (20 2 C in temperature and 60 3% in humidity) until the tests were performed on a compression-testing machine (5000 A, Jinan Shijin Group Co., Ltd., China). The other cubes used for measuring the apparent density were dried at 100 C until their weight didn’t change. The accurate dimensions of the cubes were measured using a vernier caliper with a precision of 0.02 mm, and their weights were measured using an analytical balance with a precision of 0.001 g, respectively. The apparent densities (r) of the four TESCM cubes were calculated by the following formula:r ¼ m=V, respectively, where m represents the weights of the TESCM cubes (unit: g), V represents the volumes of the TESCM cubes (unit: cm3). 2.3. Thermal conductivity measurement and thermal energy storage performance evaluation The ordinary cement mortar and the four TESCM samples were formed into several boards using a home-made stainless steel mold with the dimensions of 100 mm 100 mm 10 mm, respectively. The thermal conductivities of theses boards were measured at room temperature using a hot disk thermal constant analyzer (Hot Disk TPS2500, Hot Disk AB Company, Sweden), which the probe was put between two boards containing the same percentages of the n-octadecane/EG composite PCM PCM. Fig. 1 illustrates a sketch of the experimental apparatus for testing the thermal storage performance of the TESCM boards. Small test rooms (100 mm 100 mm 100 mm) were set up using 6 pieces of the boards with the dimensions of 100 mm 100 mm 10 mm, in which five boards were made of the ordinary cement mortar, and the four boards with the percentages of the n-octadecane/EG composite PCM of 0, 0.5, 1.2 and 2.5% were used as the top board, respectively. A halogen tungsten lamp (500 W) was placed over the top board at a distance of 35 cm, which was used as used as the light source to simulate the sun. One thermocouple linked to a data acquisition/ switch unit (Agilent 34970A) was placed in the center of the test room for recording the indoor temperatures. When the lamp was switched on, the indoor temperature variation of the test room started to be monitored. After 1.5 h, the lamp was switched off. The monitoring of the indoor temperature variation continued until the test room cooled to room temperature.
Fig. 3. SEM images of EG and the n-octadecane/EG composite PCM. (a) EG (2000), and (b) the n-octadecane/EG composite PCM (2000).
particles (Fig. 2b). Fig. 3 displays SEM images of the EG powder and the n-octadecane/EG composite PCM, respectively. The microstructure of the EG particle exhibits a flattened irregular honeycomb network constructed from elementary graphite nanosheets, which provides abundant crevice-like and net-like pores for adsorbing organic substances (Fig. 3a). The microstructure of the composite PCM is similar to that of the EG sample except that the
3. Results and discussion 3.1. Characterization of n-octadecane/EG composite PCM The appearances of the EG powder and n-octadecane/EG composite PCM are shown in Fig. 2, respectively. The EG powder consists of loose and worm-like particles (Fig. 2a). Different from the EG powder, the composite PCM is composed of granular
Fig. 4. XRD patterns of n-octadecane, EG, and the n-octadecane/EG composite PCM.
Z. Zhang et al. / Renewable Energy 50 (2013) 670e675
Fig. 5. DSC curves of n-octadecane and the n-octadecane/EG composite PCM.
honeycomb network of EG is thoroughly covered with n-octadecane, as shown in Fig. 3b. Fig. 4 shows the XRD patterns of n-octadecane, the EG powder, and the n-octadecane/EG composite PCM, respectively. A strong diffraction peak with a lattice spacing (d) of 0.3416 nm is observed in the XRD pattern of the EG powder, which should be attributed to the feature peak (002) of graphite. The XRD pattern of the composite PCM includes all the diffraction peaks of n-octadecane and the only diffraction peak of EG, although the intensities of the peaks are relatively lower than those of the corresponding peaks in the XRD patterns of EG and n-octadecane, respectively. The results suggest that the composite PCM is just the combination of noctadecane with the EG, and no new substance has been produced. In addition, we found that no liquid n-octadecane was observed on the surface of the composite PCM after the composite PCM powder was heated to 50 C (above the melting temperature of n-octadecane). It is revealed that n-octadecane is held inside the porous EG by means of the capillary force and the tension force [32]. Fig. 5 displays the DSC curves of n-octadecane and the n-octadecane/EG composite PCM, respectively. The DSC curve of the composite PCM is similar to that of n-octadecane, exhibiting one narrow peak that represents the solid - liquid phase change of n-octadecane. The melting and freezing temperature were
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measured to be 26.84 C and 25.38 C for n-octadecane, and 26.37 C and 25.79 C for the composite PCM, respectively. The results indicate that the phase transition temperature of the composite PCM is almost the same as that of n-octadecane. Moreover, the melting and freezing latent heat were measured to be 207.2 kJ/kg and 206.1 kJ/kg for n-octadecane, and 184.8 kJ/kg and 182.1 kJ/kg for the composite PCM, respectively. The measured latent heat values of the composite PCM are almost equivalent to their calculated latent heat values based on the mass fraction of n-octadecane in the composite PCM. The melting point and phase change enthalpy of a commercial microencapsulated PCM (MicronalÒPCM) made from BASF are 26 C and 110 kJ/kg, respectively [24]. The shape-stabilized PCM composed of paraffin and highdensity polyethylene had the phase change temperature of 27.5 C and the phase change latent heat as low as 88.6 kJ/kg due to the maximum content of paraffin in the shape-stabilized PCM of 70% [33]. Obviously, the latent heat value of the n-octadecane/EG composite PCM is much higher than those of the microencapsulated PCM and the shape-stabilized PCM owing to the excellent adsorption capacity of EG. Furthermore, the n-octadecane/EG composite PCM has the advantages of sample preparation process and low cost. The n-octadecane/EG composite PCM was continuously experienced 50 times cooling e heating cyclic test ranging from 10 to 50 C to investigate its stability. Fig. 6 shows the XRD patterns of the composite PCM before and after experiencing the cooling-heating cyclic test. The XRD pattern of the composite PCM after the test is almost the same as that of it before the test, indicating that the noctadecane/EG composite PCM exhibits good structure stability. Fig. 7 displays the DSC curve of the composite PCM after the cooling e heating cyclic test. After experiencing the test, the composite PCM shows the melting and freezing temperature of 26.68 C and 25.73 C and the melting and freezing latent heat of 183.7 and 181.2 J/g, respectively, which are very close to those of the composite PCM before experiencing the test (26.37 C and 25.79 C; 184.8 kJ/kg and 182.1) (Fig. 5). These results reveal that the composite n-octadecane/EG composite PCM possesses excellent thermal property stability.
3.2. Effect of the mass percentage of the composite PCM on the mechanical properties of TESCM Fig. 8 shows the compressive strengths of the cement mortar cubes containing the n-octadecane/EG composite PCM with its
Fig. 6. XRD patterns of the n-octadecane/EG composite PCM before and after 50 times cooling e heating cyclic test.
Fig. 7. DSC curve of the n-octadecane/EG composite PCM after 50 times cooling e heating cyclic test.
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Fig. 8. Compressive strengths of the cement mortar cubes with the n-octadecane/EG composite PCM mass percentages of 0, 0.5, 1.2, 1.7, and 2.5%, respectively.
mass percentage of 0, 0.5, 1.2, 1.7, and 2.5%, respectively. We can see that an increase in the mass percentage of the composite PCM results in a decrease in the compressive strengths of the cubes. When the mass ratio of the composite PCM is increased from 0 to 0.5%, the compressive strength obviously reduced from 23.7 to 16.1 MPa. However, with the further increase of the mass ratio from 0.5 to 2.5%, the compressive strength decreased slowly. Fig. 9 shows the apparent densities of the cement mortar cubes containing the n-octadecane/EG composite PCM with its mass percentage of 0, 0.5, 1.2, 1.7, and 2.5%, respectively. We can see that the apparent density decreases linearly with the increase in the mass percentage of the composite PCM in the cement mortar cubes. However, the apparent density of the cube made of the TESCM containing the composite PCM with the mass percentage of 2.5% is only reduced by 9.5% as compared with that of the cube made of the ordinary cement mortar. It is revealed that the composite PCM does not obviously deteriorate the apparent density of the TESCM cubes. Fig. 10 displays the thermal conductivities of the cement mortar boards containing the n-octadecane/EG composite PCM with its mass percentage of 0, 0.5, 1.2, 1.7, and 2.5%, respectively. It is clearly indicated that the thermal conductivities of the boards decrease linearly with the increase in the mass percentage of the composite PCM in the boards. When the mass percentage of the composite PCM in the TESCM boards was 0.5, 1.2, 1.7, and 2.5%, the thermal
Fig. 9. Apparent densities of the cement mortar cubes with the n-octadecane/EG composite PCM mass percentages of 0, 0.5, 1.2, 1.7, and 2.5%, respectively.
Fig. 10. Thermal conductivities of the cement mortar boards with the n-octadecane/EG composite PCM mass percentages of 0, 0.5, 1.2, 1.7, and 2.5%, respectively.
conductivities of the TESCM boards reduced by 4.7, 9.9, 13.1, and 15.5%, respectively, as compared with that of the board made of the ordinary cement mortar. The decrease in the thermal conductivity is probably due to that the addition of the n-octadecane/EG composite PCM leads to an increase in the porosities of the TESCM boards, which benefits their thermal insulation performance.
3.3. Thermal energy storage performance evaluation Fig. 11 shows the indoor temperature variation curves of the test rooms with the top boards containing the n-octadecane/EG composite PCM with its mass percentage of 0, 0.5, 1.2, and 2.5%, respectively. We can see that the indoor temperature variation curves gradually slow down with the increase in the mass percentage of the composite PCM in the top boards. Consequently, the test rooms with different top boards reach different indoor peak temperatures. Specifically, the indoor peak temperature is 39.0, 37.0, 35.0, and 30.3 C for the TESCM top boards containing the composite PCM with its mass percentage of 0, 0.5, 1.2, and 2.5%, respectively. The indoor peak temperature of the test room with the top board containing 1.2% of the composite PCM is 4 C lower than that of the test room with the ordinary top board. It is revealed that the TESCM boards containing the n-octadecane/EG composite PCM have a function of reducing energy consumption by decreasing the indoor temperature variation,
Fig. 11. Indoor temperature variation curves of the test rooms with the top boards containing different mass percentages of the n-octadecane/EG composite PCM.
Z. Zhang et al. / Renewable Energy 50 (2013) 670e675
and the function is enhanced with the increase in the mass ratio of the composite PCM in the TESCM boards. It can be predicted that these kinds of TESCM containing the n-octadecane/EG composite PCM have promising applications in buildings. 4. Conclusions The n-octadecane/EG composite PCM with the mass fraction of n-octadecane as large as 90% has been prepared, which endows the composite PCM with large latent heat. The phase transition temperature of the composite PCM is similar to that of n-octadecane. The composite PCM is just the combination of n-octadecane with EG. The mechanical property test for the TESCM cubes shows that the n-octadecane/EG composite PCM has a good compatibility with building materials, and does not obviously deteriorate the apparent densities of the cement mortar cubes. The thermal conductivities of the TESCM boards decrease with the increase in the mass percentage of the composite PCM, which benefits their thermal insulation performance. The TESCM boards containing the n-octadecane/EG composite PCM have a function of reducing energy consumption by decreasing the indoor temperature variation, and the function is enhanced with the increase in the mass ratio of the composite PCM. Acknowledgments This work was supported by the Joint Funds of NSFC-Guangdong of China (U0934005), and the Natural Science Foundation of Guangdong Province (S2011010001403). References [1] Luis PL, José O, Christine P. A review on buildings energy consumption information. Energy and Buildings 2008;40(3):394e8. [2] Ibanez M, Lazaro A, Zalba B, Cabeza LF. An approach to the simulation of PCMs in building applications using TRNSYS. Applied Thermal Engineering 2005; 25(11e12):1796e807. [3] Bentz DP, Turpin R. Potential applications of phase change materials in concrete technology. Cement Concrete Composites 2007;29(7):527e32. [4] Pasupathy A, Velraj R, Seeniraj RV. Phase change material-based building architecture for thermal management in residential and commercial establishments. Renewable and Sustainable Energy Reviews 2008;12(1):39e64. [5] Baetens R, Jelle BP, Gustavsen A. Phase change materials for building applications: a state-of-the-art review. Energy and Buildings 2010;42(9):1361e8. [6] Khudhair AM, Farid MM. A review on energy conservation in building applications with thermal storage by latent heat using phase change materials. Energy Conversion and Management 2004;45(2):263e75. [7] Heim D. Isothermal storage of solar energy in building construction. Renewable Energy 2010;35(4):788e96. [8] Sadineni SB, Madala S, Boehm RF. Passive building energy savings: a review of building envelope components. Renewable and Sustainable Energy Reviews 2011;15(8):3617e31. [9] Kuznik F, Virgone J, Johannes K. In-situ study of thermal comfort enhancement in a renovated building equipped with phase change material wallboard. Renewable Energy 2011;36(5):1458e62. [10] Alawadhi EM, Alqallaf HJ. Building roof with conical holes containing PCM to reduce the cooling load: numerical study. Energy Conversion and Management 2011;52(8e9):2958e64.
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[11] Behzadi S, Farid MM. Experimental and numerical investigations on the effect of using phase change materials for energy conservation in residential buildings. HVAC and Research 2011;17(3):366e76. [12] Cabeza LF, Castell A, Barreneche C, De GA, Fernández AI. Materials used as PCM in thermal energy storage in buildings: a review. Renewable and Sustainable Energy Reviews 2011;15(3):1675e95. [13] Kuznik F, David D, Johannes K, Roux JJ. A review on phase change materials integrated in building walls. Renewable and Sustainable Energy Reviews 2011;15(1):379e91. [14] Hawes DW, Feldman D. Absorption of phase change materials in concrete. Solar Energy Materials and Solar Cells 1992;27(2):91e101. [15] Hadjieva M, Stoykov R, Filipova T. Composite salt-hydrate concrete system for building energy storage. Renewable Energy 2000;19(1e2):111e5. [16] Lee T, Hawes DW, Banu D, Feldman D. Control aspects of latent heat storage and recovery in concrete. Solar Energy Materials and Solar Cells 2000;62(3): 217e37. [17] Athienitis AK, Liu C, Hawes D, Banu D, Feldman D. Investigation of the thermal performance of a passive solar test-room with wall latent heat storage. Building and Environment 1997;32(5):405e10. [18] Rudd AF. Phase-change material wallboard for distributed thermal storage in buildings. ASHRAE Transactions 1993;99(2):339e46. [19] Kuznik F, David D, Johannes K, Roux JJ. A review on phase change materials integrated in building walls. Solar Energy Materials and Solar Cells 2011; 15(1):379e91. [20] Cabeza LF, Castellón C, Nogués M, Medrano M, Leppers R, Zubillaga O. Use of microencapsulated PCM in concrete walls for energy savings. Energy and Buildings 2007;39(2):113e9. [21] Schossig P, Henning HM, Gschwander S, Haussmann T. Microencapsulated phase-change materials integrated into construction materials. Solar Energy Materials and Solar Cells 2005;89(2e3):297e306. [22] Hunger M, Entrop AG, Mandilaras I, Brouwers HJH, Founti M. The behavior of self-compacting concrete containing micro-encapsulated phase change materials. Cement Concrete Composites 2009;31(10):731e43. [23] Tyagi VV, Kaushik SC, Tyagi SK, Akiyama T. Development of phase change materials based microencapsulated technology for buildings: a review. Solar Energy Materials and Solar Cells 2011;15(2):1373e91. [24] Arce P, Castellón C, Castell A, Cabeza LF. Use of microencapsulated PCM in buildings and the effect of adding awnings. Energy and Buildings 2012;44(1): 88e93. [25] Wang Y, Xia TD, Feng HX, Zhang H. Stearic acid/polymethylmethacrylate composite as form-stable phase change materials for latent heat thermal energy storage. Renewable Energy 2011;36(6):1814e20. [26] Borreguero AM, Luz SM, Valverde JL, Carmona M, Rodríguez JF. Thermal testing and numerical simulation of gypsum wallboards incorporated with different PCMs content. Applied Energy 2011;88(3):930e7. [27] Castellón C, Medrano M, Roca J, Cabeza LF, Navarro ME, Fernández AI, et al. Effect of microencapsulated phase change material in sandwich panels. Renewable Energy 2010;35(10):2370e4. [28] Zhang YP, Lin KP, Yang R, Di HF, Jiang Y. Preparation, thermal performance and application of shape-stabilized PCM in energy efficient buildings. Energy and Buildings 2006;38(10):1262e9. [29] Sari A. Form-stable paraffin/high density polyethylene composites as solidliquid phase change material for thermal energy storage: preparation and thermal properties. Energy Conversion and Management 2004;45(13e14): 2033e42. [30] Zhang YP, Ding JH, Wang X, Yang R, Lin KP. Influence of additives on thermal conductivity of shape-stabilized phase change material. Solar Energy Materials and Solar Cells 2006;90(11):1692e702. [31] Zhang ZG, Fang XM. Study on paraffin/expanded graphite composite phase change thermal energy storage material. Energy Conversion and Management 2006;47(3):303e10. [32] Sarı A, Karaipekli A. Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material. Applied Thermal Engineering 2007;27(8e9): 1271e7. [33] Yan QY, Li LS, Liang C. Thermal performance of shape-stabilized phase change paraffin wallboard. International Journal of Sustainable Energy 2010;29(4):185e90.
Contents lists available at SciVerse ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Thermal energy storage cement mortar containing n-octadecane/expanded graphite composite phase change material Zhengguo Zhang a, *, Guoquan Shi a, Shuping Wang a, Xiaoming Fang a, Xiaohong Liu b a
Key Laboratory of Enhanced Heat Transfer and Energy Conservation, the Ministry of Education, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, China b Automotive Department, Guangdong Industry Technical College, Guangzhou 510300, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 March 2012 Accepted 8 August 2012 Available online 4 September 2012
Here we demonstrate thermal energy storage cement mortar (TESCM) fabricated by integrating ordinary cement mortar with a composite phase change material (PCM) based on n-octadecane and expanded graphite (EG). The mass percentage of n-octadecane in the composite PCM can reach as high as 90% due to the excellent adsorption ability of EG, which endows the composite PCM with large latent heat. SEM images of the composite PCM show that n-octadecane is adsorbed into the pores of EG and uniformly covers on the nanosheets of EG, which microstructure contributes to preventing leakage of melted n-octadecane after it changes phase from solid state to liquid state. The n-octadecane/EG composite PCM has a good compatibility with ordinary cement mortar, and does not obviously deteriorate the apparent densities of the TESCM samples. Based on the thermal energy storage performance evaluation, it is found that the TESCM containing the n-octadecane/EG composite PCM plays a role in reducing the variation of indoor temperature, which helps to decrease the energy consumption for buildings. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Phase change material Thermal energy storage n-octadecane Expanded graphite Cement mortar
1. Introduction The fast economic development leads to a rapidly increasing energy demand worldwide, which results in several energy concerns about supply difficulties, exhaustion of energy resources, climate changes, and environmental pollution. Being an important part of the whole energy demand, the energy consumption for buildings increases very rapidly in the recent years due to the population growth, the enhancement of building services and thermal comfort levels, and the rise in time for people spending inside buildings [1]. Therefore, it is of great significance to develop novel building materials for improving the energy utilization efficiency and reducing the energy consumption for buildings. A phase change material (PCM) that can absorb or release a large quantity of latent heat when it changes phase from solid state to liquid state or vice versa, has been widely applied into thermal energy storage systems. Incorporating a suitable PCM into the walls, ceiling and floor of a building can function as reducing the room temperature swings in the building, thus leading to an improvement in the human comfort and a reduce in the energy
* Corresponding author. Tel.: þ86 20 8711 2845; fax: þ86 20 8711 3870. E-mail address: [email protected] (Z. Zhang). 0960-1481/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.renene.2012.08.024
consumption for the building [2e11]. Since the 1970s, a number of researchers have tried to apply PCMs into buildings to enhance the thermal comfort of lightweight constructions. Three general methods are proposed for the incorporation of organic PCMs into construction elements such as wallboards, gypsum boards, concrete, and so on [12,13]. The first one is the immersion of conventional wallboards into molten PCMs [14e18]. Although the method is simple and low cost, the impregnated wallboards are inflammable owing to leakage of the liquid PCMs to the surfaces of the wallboards, especially after the PCMs experienced several heating-cooling cycles. The second one is the integration of microencapsulated PCMs with ordinary building materials [19e27]. However, the method suffers from the complicated polymerization processes and the high costs related to the microencapsulation of PCMs [23]. Nowadays the commercially available microencapsulated PCMs are rare and have been only developed by few companies such as BASF. The third one is the incorporation of a kind of shape-stabilized PCMs into building materials, which shape-stabilized PCMs can be prepared by blending PCMs with supporting materials [28,29]. The commonly-used supporting materials include high-density polyethylene (HDPE) and styrenee butadieneestyrene (SBS), expanded graphite (EG), and so on. Compared with the polymeric supporting materials, EG has the advantages of high thermal conductivity and good adsorption
Z. Zhang et al. / Renewable Energy 50 (2013) 670e675
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Fig. 1. A sketch of the experimental apparatus for testing the thermal storage performances of the TESCM boards.
ability, thus making it a good matrix for preparing composite PCMs [30]. The previous work reported by our group demonstrated that the EG-based composite PCMs not only exhibited excellent thermal energy storage property along with improved thermal conductivity as compared with the pure organic PCMs, but also have the advantage of preventing leakage of the liquid PCMs [31]. Cement mortar that is produced by mixing up Portland cement, sand, and water has wide applications in buildings such as creating a smooth surface to walls. The integration of PCMs with cement mortar can render it feasible to capture solar energy directly and store significant amounts of thermal energy in the building envelope. N-octadecane is an excellent PCM with the properties of nontoxicity, high latent heat, and a suitable phase transition temperature for the application in buildings. In the current work, we focus on the incorporation of the n-octadecane/EG composite PCM into ordinary cement mortar to produce thermal energy storage cement mortar (TESCM). Firstly, the n-octadecane/EG composite PCM was prepared by adsorbing n-octadecane into the pores of EG. The structure and properties of the composite PCM was characterized by SEM, XRD, and DSC, respectively. Then, the mechanical property and thermal conductivity of the TESCM samples containing different mass percentage of the n-octadecane/EG composite PCM were investigated. Finally, the thermal energy storage performances of the TESCM boards containing different mass percentage of the n-octadecane/EG composite PCM were evaluated. 2. Experimental 2.1. Preparation and characterization of n-octadecane/EG composite PCM N-octadecane (Alfa Aesar) with a melting temperature of 27 C and graphite powder (Qingdao Graphite Co. Ltd, China) with an
average particle size of 500 mm and an expandable rate of 300 ml/g were used as the raw materials. After being dried in a vacuum oven at 70 C for 20 h, the graphite powder was heated using a domestic microwave oven (800 W, Midea, China) for 10 s to make it expanded into EG. The n-octadecane/EG composite PCM was prepared by adsorbing the melted n-octadecane into the pores of the obtained EG. The mass percentage of n-octadecane in the composite PCM was set at 90% based on our previous work [31]. The microstructures of the obtained EG and n-octadecane/EG composite PCM were observed using a scanning electron microscope (SEM, S-3700N, Hitachi, Japan), respectively. The crystalline phases of n-octadecane and the obtained EG and composite n-octadecane/EG PCM were characterized by an X-ray diffractometer (XRD, D8-ADVANCE, Bruker, German), respectively. The copper target (CuKa) was used as the diffraction source. The thermal properties of n-octadecane and the composite n-octadecane/EG PCM were measured by using a differential scanning calorimeter (DSC, DSC2910, TA Instruments, USA) under N2 atmosphere. The measuring temperature ranged from 20 to 70 C. The temperature rise rate was 5 C/min. 2.2. Fabrication and performance test of TESCM 2.2.1. Fabrication Conforming to ISO 679:1989 (Method of testing cements e determination of strength), ordinary cement mortar was prepared by mixing up Slag Portland cement (# 325), standard sand (particle size 2.0 mm), and water, in which the mass ratio of sand to the cement is 3, and that of water to the cement was 0.5. Four TESCM samples were produced by adding the n-octadecane/EG composite PCM into the ingredients of the ordinary cement mortar followed by thoroughly mixing, in which the mass ratio of the composite PCM to the cement was 0.02, 0.05, 0.07, and 0.1, respectively. After
Fig. 2. Photographs of the EG (a) and the n-octadecane/EG composite PCM (b).
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the four TESCM samples were dried to constant weight, the percentages of the n-octadecane/EG composite PCM in the four TESCM samples were calculated to be 0.5, 1.2, 1.7, and 2.5%, respectively, based on the final weight of each TESCM sample and the amount of the composite PCM added into it. 2.2.2. Mechanical property test The mechanical properties of the four TESCM samples were evaluated by measuring their compressive strength and apparent density, respectively. Firstly, the ordinary cement mortar and the four TESCM samples were formed into several cubes using a standard tripartite mold with the dimensions of 70.7 mm 70.7 mm 70.7 mm, respectively. After being kept under a moist atmosphere for about 24 h, all the cubes were maintained under water at 20 1 C for 7 days. Then, the cubes used for measuring the compressive strength were kept under a controlled condition (20 2 C in temperature and 60 3% in humidity) until the tests were performed on a compression-testing machine (5000 A, Jinan Shijin Group Co., Ltd., China). The other cubes used for measuring the apparent density were dried at 100 C until their weight didn’t change. The accurate dimensions of the cubes were measured using a vernier caliper with a precision of 0.02 mm, and their weights were measured using an analytical balance with a precision of 0.001 g, respectively. The apparent densities (r) of the four TESCM cubes were calculated by the following formula:r ¼ m=V, respectively, where m represents the weights of the TESCM cubes (unit: g), V represents the volumes of the TESCM cubes (unit: cm3). 2.3. Thermal conductivity measurement and thermal energy storage performance evaluation The ordinary cement mortar and the four TESCM samples were formed into several boards using a home-made stainless steel mold with the dimensions of 100 mm 100 mm 10 mm, respectively. The thermal conductivities of theses boards were measured at room temperature using a hot disk thermal constant analyzer (Hot Disk TPS2500, Hot Disk AB Company, Sweden), which the probe was put between two boards containing the same percentages of the n-octadecane/EG composite PCM PCM. Fig. 1 illustrates a sketch of the experimental apparatus for testing the thermal storage performance of the TESCM boards. Small test rooms (100 mm 100 mm 100 mm) were set up using 6 pieces of the boards with the dimensions of 100 mm 100 mm 10 mm, in which five boards were made of the ordinary cement mortar, and the four boards with the percentages of the n-octadecane/EG composite PCM of 0, 0.5, 1.2 and 2.5% were used as the top board, respectively. A halogen tungsten lamp (500 W) was placed over the top board at a distance of 35 cm, which was used as used as the light source to simulate the sun. One thermocouple linked to a data acquisition/ switch unit (Agilent 34970A) was placed in the center of the test room for recording the indoor temperatures. When the lamp was switched on, the indoor temperature variation of the test room started to be monitored. After 1.5 h, the lamp was switched off. The monitoring of the indoor temperature variation continued until the test room cooled to room temperature.
Fig. 3. SEM images of EG and the n-octadecane/EG composite PCM. (a) EG (2000), and (b) the n-octadecane/EG composite PCM (2000).
particles (Fig. 2b). Fig. 3 displays SEM images of the EG powder and the n-octadecane/EG composite PCM, respectively. The microstructure of the EG particle exhibits a flattened irregular honeycomb network constructed from elementary graphite nanosheets, which provides abundant crevice-like and net-like pores for adsorbing organic substances (Fig. 3a). The microstructure of the composite PCM is similar to that of the EG sample except that the
3. Results and discussion 3.1. Characterization of n-octadecane/EG composite PCM The appearances of the EG powder and n-octadecane/EG composite PCM are shown in Fig. 2, respectively. The EG powder consists of loose and worm-like particles (Fig. 2a). Different from the EG powder, the composite PCM is composed of granular
Fig. 4. XRD patterns of n-octadecane, EG, and the n-octadecane/EG composite PCM.
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Fig. 5. DSC curves of n-octadecane and the n-octadecane/EG composite PCM.
honeycomb network of EG is thoroughly covered with n-octadecane, as shown in Fig. 3b. Fig. 4 shows the XRD patterns of n-octadecane, the EG powder, and the n-octadecane/EG composite PCM, respectively. A strong diffraction peak with a lattice spacing (d) of 0.3416 nm is observed in the XRD pattern of the EG powder, which should be attributed to the feature peak (002) of graphite. The XRD pattern of the composite PCM includes all the diffraction peaks of n-octadecane and the only diffraction peak of EG, although the intensities of the peaks are relatively lower than those of the corresponding peaks in the XRD patterns of EG and n-octadecane, respectively. The results suggest that the composite PCM is just the combination of noctadecane with the EG, and no new substance has been produced. In addition, we found that no liquid n-octadecane was observed on the surface of the composite PCM after the composite PCM powder was heated to 50 C (above the melting temperature of n-octadecane). It is revealed that n-octadecane is held inside the porous EG by means of the capillary force and the tension force [32]. Fig. 5 displays the DSC curves of n-octadecane and the n-octadecane/EG composite PCM, respectively. The DSC curve of the composite PCM is similar to that of n-octadecane, exhibiting one narrow peak that represents the solid - liquid phase change of n-octadecane. The melting and freezing temperature were
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measured to be 26.84 C and 25.38 C for n-octadecane, and 26.37 C and 25.79 C for the composite PCM, respectively. The results indicate that the phase transition temperature of the composite PCM is almost the same as that of n-octadecane. Moreover, the melting and freezing latent heat were measured to be 207.2 kJ/kg and 206.1 kJ/kg for n-octadecane, and 184.8 kJ/kg and 182.1 kJ/kg for the composite PCM, respectively. The measured latent heat values of the composite PCM are almost equivalent to their calculated latent heat values based on the mass fraction of n-octadecane in the composite PCM. The melting point and phase change enthalpy of a commercial microencapsulated PCM (MicronalÒPCM) made from BASF are 26 C and 110 kJ/kg, respectively [24]. The shape-stabilized PCM composed of paraffin and highdensity polyethylene had the phase change temperature of 27.5 C and the phase change latent heat as low as 88.6 kJ/kg due to the maximum content of paraffin in the shape-stabilized PCM of 70% [33]. Obviously, the latent heat value of the n-octadecane/EG composite PCM is much higher than those of the microencapsulated PCM and the shape-stabilized PCM owing to the excellent adsorption capacity of EG. Furthermore, the n-octadecane/EG composite PCM has the advantages of sample preparation process and low cost. The n-octadecane/EG composite PCM was continuously experienced 50 times cooling e heating cyclic test ranging from 10 to 50 C to investigate its stability. Fig. 6 shows the XRD patterns of the composite PCM before and after experiencing the cooling-heating cyclic test. The XRD pattern of the composite PCM after the test is almost the same as that of it before the test, indicating that the noctadecane/EG composite PCM exhibits good structure stability. Fig. 7 displays the DSC curve of the composite PCM after the cooling e heating cyclic test. After experiencing the test, the composite PCM shows the melting and freezing temperature of 26.68 C and 25.73 C and the melting and freezing latent heat of 183.7 and 181.2 J/g, respectively, which are very close to those of the composite PCM before experiencing the test (26.37 C and 25.79 C; 184.8 kJ/kg and 182.1) (Fig. 5). These results reveal that the composite n-octadecane/EG composite PCM possesses excellent thermal property stability.
3.2. Effect of the mass percentage of the composite PCM on the mechanical properties of TESCM Fig. 8 shows the compressive strengths of the cement mortar cubes containing the n-octadecane/EG composite PCM with its
Fig. 6. XRD patterns of the n-octadecane/EG composite PCM before and after 50 times cooling e heating cyclic test.
Fig. 7. DSC curve of the n-octadecane/EG composite PCM after 50 times cooling e heating cyclic test.
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Fig. 8. Compressive strengths of the cement mortar cubes with the n-octadecane/EG composite PCM mass percentages of 0, 0.5, 1.2, 1.7, and 2.5%, respectively.
mass percentage of 0, 0.5, 1.2, 1.7, and 2.5%, respectively. We can see that an increase in the mass percentage of the composite PCM results in a decrease in the compressive strengths of the cubes. When the mass ratio of the composite PCM is increased from 0 to 0.5%, the compressive strength obviously reduced from 23.7 to 16.1 MPa. However, with the further increase of the mass ratio from 0.5 to 2.5%, the compressive strength decreased slowly. Fig. 9 shows the apparent densities of the cement mortar cubes containing the n-octadecane/EG composite PCM with its mass percentage of 0, 0.5, 1.2, 1.7, and 2.5%, respectively. We can see that the apparent density decreases linearly with the increase in the mass percentage of the composite PCM in the cement mortar cubes. However, the apparent density of the cube made of the TESCM containing the composite PCM with the mass percentage of 2.5% is only reduced by 9.5% as compared with that of the cube made of the ordinary cement mortar. It is revealed that the composite PCM does not obviously deteriorate the apparent density of the TESCM cubes. Fig. 10 displays the thermal conductivities of the cement mortar boards containing the n-octadecane/EG composite PCM with its mass percentage of 0, 0.5, 1.2, 1.7, and 2.5%, respectively. It is clearly indicated that the thermal conductivities of the boards decrease linearly with the increase in the mass percentage of the composite PCM in the boards. When the mass percentage of the composite PCM in the TESCM boards was 0.5, 1.2, 1.7, and 2.5%, the thermal
Fig. 9. Apparent densities of the cement mortar cubes with the n-octadecane/EG composite PCM mass percentages of 0, 0.5, 1.2, 1.7, and 2.5%, respectively.
Fig. 10. Thermal conductivities of the cement mortar boards with the n-octadecane/EG composite PCM mass percentages of 0, 0.5, 1.2, 1.7, and 2.5%, respectively.
conductivities of the TESCM boards reduced by 4.7, 9.9, 13.1, and 15.5%, respectively, as compared with that of the board made of the ordinary cement mortar. The decrease in the thermal conductivity is probably due to that the addition of the n-octadecane/EG composite PCM leads to an increase in the porosities of the TESCM boards, which benefits their thermal insulation performance.
3.3. Thermal energy storage performance evaluation Fig. 11 shows the indoor temperature variation curves of the test rooms with the top boards containing the n-octadecane/EG composite PCM with its mass percentage of 0, 0.5, 1.2, and 2.5%, respectively. We can see that the indoor temperature variation curves gradually slow down with the increase in the mass percentage of the composite PCM in the top boards. Consequently, the test rooms with different top boards reach different indoor peak temperatures. Specifically, the indoor peak temperature is 39.0, 37.0, 35.0, and 30.3 C for the TESCM top boards containing the composite PCM with its mass percentage of 0, 0.5, 1.2, and 2.5%, respectively. The indoor peak temperature of the test room with the top board containing 1.2% of the composite PCM is 4 C lower than that of the test room with the ordinary top board. It is revealed that the TESCM boards containing the n-octadecane/EG composite PCM have a function of reducing energy consumption by decreasing the indoor temperature variation,
Fig. 11. Indoor temperature variation curves of the test rooms with the top boards containing different mass percentages of the n-octadecane/EG composite PCM.
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and the function is enhanced with the increase in the mass ratio of the composite PCM in the TESCM boards. It can be predicted that these kinds of TESCM containing the n-octadecane/EG composite PCM have promising applications in buildings. 4. Conclusions The n-octadecane/EG composite PCM with the mass fraction of n-octadecane as large as 90% has been prepared, which endows the composite PCM with large latent heat. The phase transition temperature of the composite PCM is similar to that of n-octadecane. The composite PCM is just the combination of n-octadecane with EG. The mechanical property test for the TESCM cubes shows that the n-octadecane/EG composite PCM has a good compatibility with building materials, and does not obviously deteriorate the apparent densities of the cement mortar cubes. The thermal conductivities of the TESCM boards decrease with the increase in the mass percentage of the composite PCM, which benefits their thermal insulation performance. The TESCM boards containing the n-octadecane/EG composite PCM have a function of reducing energy consumption by decreasing the indoor temperature variation, and the function is enhanced with the increase in the mass ratio of the composite PCM. Acknowledgments This work was supported by the Joint Funds of NSFC-Guangdong of China (U0934005), and the Natural Science Foundation of Guangdong Province (S2011010001403). References [1] Luis PL, José O, Christine P. A review on buildings energy consumption information. Energy and Buildings 2008;40(3):394e8. [2] Ibanez M, Lazaro A, Zalba B, Cabeza LF. An approach to the simulation of PCMs in building applications using TRNSYS. Applied Thermal Engineering 2005; 25(11e12):1796e807. [3] Bentz DP, Turpin R. Potential applications of phase change materials in concrete technology. Cement Concrete Composites 2007;29(7):527e32. [4] Pasupathy A, Velraj R, Seeniraj RV. Phase change material-based building architecture for thermal management in residential and commercial establishments. Renewable and Sustainable Energy Reviews 2008;12(1):39e64. [5] Baetens R, Jelle BP, Gustavsen A. Phase change materials for building applications: a state-of-the-art review. Energy and Buildings 2010;42(9):1361e8. [6] Khudhair AM, Farid MM. A review on energy conservation in building applications with thermal storage by latent heat using phase change materials. Energy Conversion and Management 2004;45(2):263e75. [7] Heim D. Isothermal storage of solar energy in building construction. Renewable Energy 2010;35(4):788e96. [8] Sadineni SB, Madala S, Boehm RF. Passive building energy savings: a review of building envelope components. Renewable and Sustainable Energy Reviews 2011;15(8):3617e31. [9] Kuznik F, Virgone J, Johannes K. In-situ study of thermal comfort enhancement in a renovated building equipped with phase change material wallboard. Renewable Energy 2011;36(5):1458e62. [10] Alawadhi EM, Alqallaf HJ. Building roof with conical holes containing PCM to reduce the cooling load: numerical study. Energy Conversion and Management 2011;52(8e9):2958e64.
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