- Email: [email protected]
expanded perlite composite PCM
Energy and Buildings 92 (2015) 155–160
Contents lists available at ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Preparation and properties of gypsum based energy storage materials with capric acid–palmitic acid/expanded perlite composite PCM Jianwu Zhang, Xuemao Guan, Xianxian Song, Huanhuan Hou, Zhengpeng Yang, Jianping Zhu ∗ School of Material Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 23 September 2014 Received in revised form 15 January 2015 Accepted 20 January 2015 Available online 7 February 2015 Keywords: Phase change materials Heat storage Thermal-regulated Heat transfer Mechanical property
a b s t r a c t Phase change materials (PCMs) have been widely applied to develop building materials with high thermal energy storage capacity. In this study, the capric acid–palmitic acid (CA–PA)/expanded perlite (EP) composite PCM was prepared using vacuum impregnation method and the thermal-regulated gypsum boards were fabricated by adding the prepared composite PCM. Scanning electron microscope images revealed that the CA–PA eutectic mixture was uniformly distributed in pores of EP and the CA–PA/EP composite PCM has a little effect on the interface gypsum crystals growth. Differential scanning calorimeter results showed that the melting temperature range of the composite PCM was 24.1–31 ◦ C and the latent heat was 88.39 J g−1 . The composite PCM had a good chemical stability by surveying the chemical characterization of the composite PCM. Furthermore, the heat transfer property, thermal conductivity, bending and compressive strength were also tested. The results indicated that the higher the composite PCM volume content, the smaller the thermal conductivity of the gypsum board and the lower the temperature fluctuation in the cubicle system. The bending strength and compressive strength reduced gradually with an increase of the volume fraction of the composite PCM. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Phase change materials (PCMs) are substance that could absorb or release higher latent heat in the process of phase transition. From 1970s, several researchers have tried to introduce PCMs into buildings to enhance the heat storage capacity of constructions [1,2]. Especially in recent years, in order to enhance the level of comfort of the building, many fossil fuel sources have been consumed and the building energy consumption increases gradually, which has already reached about one-third of the total energy consumption [3–5]. Therefore, more and more researchers have focused on the research that implant PCMs into constructions for reducing the building energy consumption in such a situation [6–9], and PCMs will play an increasingly important role in the field of building energy efficiency in future. Fatty acids are a kind of phase change materials, which have attracted much attention for their various applications in building energy efficiency. As one of the most promising PCMs, fatty acids have many desirable characteristics, such as high energy storage density, good phase change reversibility, freezing and melting
∗ Corresponding author. Tel.: +86 13569172672. E-mail address: [email protected] (J. Zhu). http://dx.doi.org/10.1016/j.enbuild.2015.01.063 0378-7788/© 2015 Elsevier B.V. All rights reserved.
congruently with minimum supercooling and without phase segregation, small volumetric change between solid and liquid phase, chemically stable, non-toxic and non-corrosive, etc. [10,11]. However, the drawback of fatty acids is lower thermal conductivity, which is adverse to widespread application. Researchers usually add substances with high thermal conductivity into fatty acids to improve the heat-conducting property. For example, Fauzi et al. enhanced the conductivity of myristic acid/palmitic acid eutectic phase change material by adding 10 wt.% of sodium laurate [12]. Karaipekli et al. studied the enhancement of thermal conductivity of stearic acid by using expanded graphite (EG) and carbon fiber (CF). The result showed that thermal conductivity of the stearic acid increased as 27.6%, 58.6%, 179.3%, and 279.3%, for addition EG in mass fraction 2%, 4%, 7%, and 10%, respectively, and it increased as 24.1%, 106.9%, 162.1%, and 217.2% for addition of CF in mass fractions 2%, 4%, 7%, and 10%, respectively [13]. Literatures [14–17] also studied the thermal properties and characteristics during phase change of fatty acids as PCMs. Most fatty acids usually have a solid–liquid phase change process when applied to buildings for energy storage. Thus, first fatty acids should be stabled to prevent leakage in the process of phase transition. At present, the commonly methods include adsorption method, polymer blends, microencapsulation, etc. However, adsorption is simple and cheaper for preparation of form-stable
156
J. Zhang et al. / Energy and Buildings 92 (2015) 155–160
Table 1 The bending strength and compressive strength of the thermal-regulated gypsum boards. Volume fraction
0%
10%
20%
30%
40%
Bending strength (dry) (MPa) Compressive strength (dry) (MPa)
2.5 4.7
2.17 4.5
1.94 4
1.4 3.27
1.1 2.8
65 60 55 50 45
PCMs. The principle is that some porous materials could strongly adsorb the PCMs by using their own capillary forces even in the case of liquid. Expanded perlite (EP) is a white granular material that has honeycomb structure internally. The porous structure can prevent the leakage of fatty acids during phase change process due to the effect of capillary and surface tension forces [18–20]. This study fixed capric acid–palmitic acid eutectic in pores of EP by means of vacuum impregnation method (VA method), and then prepared thermal-regulated gypsum board by mixing the prepared CA–PA/EP composite PCM and gypsum. The thermal property and chemical compatibility were investigated. Moreover, the bending strength and compressive strength, the heat transfer property were also discussed (Table 1). 2. Experimental 2.1. Materials and equipments Capric acid (CA, 98.5% pure) and palmitic (PA, 99% pure) were purchased from Sinopharm Chemical Reagent Co, Ltd. Expanded perlite (EP) was obtained from Xinyang perlite Plant, China. The bulk density was 45 kg m−3 . Gypsum was produced by Taishan Construction Material Plant, Shandong, China. The microstructure of the composite PCM was observed with a SEM (JSM-6390LV) at the acceleration voltage of 15 kV under low vacuum. DSC (NETZSCH DSC 204) was used to test the thermal property of the composite PCM. The heating rate was 5.0 ◦ C min−1 . The chemical compatibility of the composite PCM was carried out using a FT-IR spectrophotometer (V70 from Bruker Corporation). Thermal conductivity was determined by thermal conductivity measuring apparatus (DRH-III). PID temperature recorder (SWPASR100) was used to record the temperature variation. The bending strength and compressive strength of the thermal-regulated gypsum boards were tested by electronic universal testing machine (WDW-20). 2.2. Experimental method 2.2.1. Preparation of the thermal-regulated gypsum board The CA–PA/EP composite PCM was prepared by VA method. First, the CA–PA eutectic mixture was produced. The mass ratios of CA and PA in the eutectic mixture were determined as 85 wt.% and 15 wt.%, respectively, according to the CA–PA binary phase diagram shown in Fig. 1, which was measured by cooling curve method. Second, the melted CA–PA eutectic mixture and EP were placed in a bottom flask, and stirred for 30 min. This process ensured EP and CA–PA could be mixed uniformly. Third, the mixture was then placed in a vacuum drying oven about 2 h, the temperature and vacuum degree were set at 60 ◦ C and 0.05 MPa, respectively. To determine the maximum combination ratio of CA–PA eutectic mixture, a series of composite PCMs with mass ratio of 35 wt.%, 45 wt.%, 55 wt.%, 65 wt.%, 75 wt.%, 85 wt.% (CA–PA) were prepared. It was found that the CA–PA eutectic mixture as PCM could be retained as 65 wt.% in EP without the leakage of melted PCM. Therefore, the composite with 65 wt.% of CA–PA was defined as form-stable composite PCM. The thermal-regulated gypsum boards were fabricated with volume fraction of the CA–PA/EP composite PCM at 10%, 20% and 30%.
40 35 30 25 20
15:85
45:55
65:35
85:15
98:2
Fig. 1. Experimental phase diagram of the CA–PA binary system.
The typical procedure is described as follows: first, the gypsum slurry was prepared by blending water and gypsum powder with a mass ratio of 49 wt.%. Second, the composite PCM was added into the mixture and stirred until the homogeneous slurry was obtained. Finally, the resulted slurry was put into molds with the size of 30 cm × 30 cm × 5 cm for the heat transfer property test and 20 cm × 20 cm × 1.5 cm for the thermal conductivity test, respectively. Similarly, the size of 40 mm × 40 mm × 160 mm samples with 10%, 20%, 30% and 40% volume fraction of the composite PCM were prepared for the strength test. The whole samples were dried at room temperature before testing. Fig. 2 showed the distribution of the CA–PA/EP composite PCM in gypsum board. 2.2.2. Heat transfer property of the thermal-regulated gypsum board test The heat transfer property of the thermal-regulated gypsum boards was tested by using a self-made design as shown in Fig. 3. The self-made design mainly included a cubicle system (30 cm × 30 cm × 30 cm) and a heat release source (60 W). Furthermore, the cubicle system was composed of five polystyrene boards and one gypsum board (30 cm × 30 cm × 5 cm). The distance between the heat release source and the gypsum board was 25 cm. The temperature signal of the center location of the gypsum board surface and the cubicle system were recorded by two temperature sensors, which connected with a corresponding temperature recorder. The specific test procedure was described as follows: first, the room temperature was modulated at 20 ◦ C, and then the heat release source of self-made design was opened. The heating time was 2 h to ensure that the composite PCM in gypsum board reached or exceeded the melted temperature. Finally, the heat release source was closed and the cubicle system was automatically
Fig. 2. The section of the thermal-regulated gypsum.
J. Zhang et al. / Energy and Buildings 92 (2015) 155–160
157
Fig. 3. The photo and sketch map of the self-made designed for the heat transfer property of the thermal-regulated gypsum board.
cooled. The temperature measurements were recorded by the temperature recorder. 3. Results and discussion 3.1. Morphology and chemical characterization of the CA–PA/EP The SEM images of expanded perlite and the CA–PA/EP composite PCM were shown in Fig. 4. As can be seen, EP had porous structure with pore diameter of 1 to 50 m. Fig. 4(b) presented that the CA–PA eutectic mixture had been absorbed in the pores and the surface of EP was smooth and round. Fig. 5 was the SEM micrograph of thermal-regulated gypsum board. As seen from Fig. 5(a), the crystal size and structure of the gypsum which was away from the composite PCM had no obvious change, and the dihydrate gypsum crystal was well developed according to the degree of acicular. From Fig. 5(b), the composite PCM had a little effect on the interface gypsum crystals growth and leaded to form an obvious interface between the composite PCM and gypsum. This is due to the strong hydrophobic of composite PCM, causing water shortage near the composite PCM and therefore affecting the hydration process of gypsum near the interface.
The chemical compatibility between the CA–PA and EP was characterized by evaluating the interactions among the components of the composite using FT-IR spectroscopy technique. The FT-IR absorption spectrum of the CA–PA/EP composite PCM was compared to that of CA–PA eutectic mixture. The FT-IR spectrum of the CA–PA eutectic mixture (as shown in Fig. 6) indicated that carboxylic acid formed a dimer structure (C O stretching band at 1200–1300 cm−1 ). The absorption peaks caused by the out-of-plane bending vibration and in-plane swinging vibration of functional group OH were also found at 729 cm−1 and 719.95 cm−1 . The peaks at 2933 cm−1 and 2871 cm−1 represented the asymmetrical vibration and symmetrical vibration of functional group of CH2 . Compared with the FT-IR spectra of CA–PA and EP, it was clearly found that the FT-IR spectrum of CA–PA/EP form-stable PCM contains the characteristic peaks of CA–PA and EP. The infrared spectrum with no significant new peaks indicated that there were no chemical reactions between CA–PA and EP. The insignificant frequency shifts of the composite PCM can be attributed to the interaction between the carboxyl group of the CA–PA and the alkaline region in EP. These attractive forces provided easy confining of the fatty acids molecules in the pores of EP and thus prevented the leakage of the melted CA–PA from the surface of the composite.
Fig. 4. SEM morphology of EP and CA–PA/EP: (a) EP; (b) CA–PA/EP.
Fig. 5. SEM images of thermal-regulated gypsum board: (a) gypsum area; (b) interface between the composite PCM and gypsum.
158
J. Zhang et al. / Energy and Buildings 92 (2015) 155–160
Temperature/ ºC
EP
CA-PA
CA-PA/EP
50 48 46 44 42 40 38 36 34 32 30 28 26 24 22 20 18 -2500
(a) 10% volume fraction 20% volume fraction
30% volume fraction
The pure gypsum board
0
2500 5000 7500 10000 12500 15000 17500 20000 22500 25000
Time/s 3500
3000
2500
2000
1500
1000
500
42
-1
Wavenumber ( cm )
40
Fig. 6. The FT-IR spectra of EP, CA–PA and CA–PA/EP.
(b)
10% volume fraction
38
The pure gypsum board
36
20% volume fraction
3.2. Thermal property of CA–PA and CA–PA/EP composite PCM Fig. 7 showed the DSC thermograms of the CA–PA eutectic mixture and the CA–PA/EP composite PCM. From the DSC curves, the melting temperature range was determined as 23.7–29.6 ◦ C for CA–PA eutectic mixture, and 24.1–31.0 ◦ C for CA–CA/EP composite PCM, respectively. It can be found that the phase change temperature of CA–PA/EP was slightly higher than that of CA–PA eutectic mixture. The reason was that the volume expansion of CA–PA was limited by the porous structure of expanded perlite under the melting phase change process and the increasing pressure. Thus, the phase change temperature of CA–PA eutectic mixture in the composite would increase with the relationship of pressure and temperature [21]. The latent heat of melting was found to be 137.9 J g−1 for CA–PA eutectic mixture, and 88.39 J g−1 for the CA–PA/EP composite PCM. The latent heat value of the composite PCM was slightly lower than the theoretical value, which was calculated by multiplying the mass ratio of CA–PA in the composite. The results showed that the CA–PA/EP composite PCM was ideal for the application in building energy conservation.
0.0
Heat flow(mv/mg)
-0.5
(CA-PA/EP
-1.0
ΔΗm=88.39J/g
Onset=25.7 -1.5
(CA-PA
-2.0
ΔΗm=137.9J/g
Onset=23.7
-2.5 -3.0 -3.5 0
10
20
30
Temperature(
40
50
60
)
Fig. 7. DSC curves of CA–PA eutectic mixture and the CA–PA/EP composite PCM.
Temperature/ºC
34 32 30 28 30% volume fraction
26 24 22 20 18 -2500
0
2500 5000 7500 10000 12500 15000 17500 20000 22500 25000
Time/s Fig. 8. Effect of the composite PCM volume fraction on the heat transfer property of thermal-regulated gypsum board: (a) the surface temperature variation curve, (b) the center of the cubicle system temperature variation curve.
3.3. Heat transfer property test results The heat transfer property of the thermal-regulated gypsum was mainly evaluated by its heat storage capacity which was varied with the volume fraction of the composite PCM in gypsum board. In this study, the heat transfer property of the thermal-regulated gypsum boards were investigated by measuring the temperature variation regularity of thermal-regulated gypsum board surface and the inside center of the cubicle system. The obtained results were shown in Fig. 8. From Fig. 8, the temperature gradient of the thermal-regulated gypsum board surface and the center of the cubicle system were gradually reduced with an increase of the composite PCM volume fraction from 10% to 30% during the heating and cooling. The highest temperature both the thermal-regulated gypsum surface and the center of the cubicle system were 41.9 and 35.2 ◦ C throughout the testing process, respectively, while the volume fraction of the composite PCM was 30%. That was lower than that of those gypsum boards which had low composite PCM volume content. In other words, the higher the composite PCM volume content, the higher the energy storage capacity of the gypsum board and the lower the temperature fluctuation in the cubicle system. However, it was found that the heat insulation property of the thermal-regulated gypsum board was poor than that of the pure gypsum board when the volume fraction of the composite PCM was 10% according to the test curve, and it was the opposite for the thermal-regulated
J. Zhang et al. / Energy and Buildings 92 (2015) 155–160
Thermal conductivity/W B m k
-1 -1
0.40 0.35
0.3796
0.3911
with this conclusion. The composite PCM provides an effect of “hole defect”, equivalently. Therefore, the combination of dihydrate gypsum crystals is weakened, causing a decrease of the strength.
0.4125 0.3577
0.3555
159
0.3163
0.30
4. Conclusions 0.2759
In this paper, a novel energy storage material, CA–PA/EP composite PCM based on gypsum, has been prepared. The obtained conclusions can be described as follows:
0.25 0.20 0.15 0.10 0.05 0.00 Pure gypsum board
10%EP
20%EP
30%EP
10%CA-PA/EP
20%CA-PA/EP
30%CA-PA/EP
Different types of gypsum boards Fig. 9. Thermal conductivity of the PCM gypsum boards.
gypsum board with 20% or 30% volume fraction composite PCM. Therefore, the composite PCM had positive and negative effect on the heat transfer property of the gypsum board. This could be explained from two factors. On the one hand, when the temperature reaches the phase change temperature, the PCM in the gypsum board will absorb the heat, thus, the heat transfer process is prevented to a certain degree. On the other hand, the internal structure defect of the gypsum board increases with the incorporation of the composite PCM. That is to say, the “internal connectivity” of the gypsum board is enhanced. The result is that the convection effect within the gypsum board has been reinforced. The heat would be more easily passed through the gypsum board. Maybe, when the composite PCM volume dosage is 10%, the negative effect plays a leading role, but predominantly positive effect when the composite PCM volume content is high. 3.4. Thermal conductivity test Thermal conductivity of the thermal-regulated gypsum boards was tested. In order to verify the negative effect, the thermal conductivity of expanded perlite (without CA–PA)/gypsum boards were also tested. The result was shown in Fig. 9. The thermal conductivity of gypsum board increased with the augment of EP compared with the pure gypsum board. That may be ascribed to that EP has connected pore structure, the convection effects within the gypsum board was enhanced with the mix of EP. And when the volume of the CA–PA/EP composite PCM was 30%, the thermal conductivity of the gypsum board was 0.2759 W m−1 K−1 , which was lower than that of the pure gypsum. From the overall test results, it was further confirmed that the volume of the composite PCM had positive and negative effect on the heat transfer property of the gypsum board compared with the pure gypsum board. 3.5. Bending strength and compressive strength The bending strength and compressive strength of the sample decreased with an increase of the volume fraction of composite PCM. When the composite PCM volume fraction was 40%, the bending strength and compressive strength of the sample were 1.1 MPa and 2.8 MPa, respectively, which were decreased by 56% and 40.4% in comparison with the pure gypsum sample. Such phenomena can be explained as follows. For one thing, the content of gypsum decreases with the incorporation of composite PCM. Therefore, the “skeleton effect” of gypsum is weakened. For another thing, the composite PCM has low strength, and the crystal size and structure of the gypsum on the interface between the composite PCM and gypsum is affected. The SEM images are in good agreement
(1) CA–PA eutectic mixture can be effectively distributed in the microporous structure of EP, and the composite PCM was chemically stable. (2) The phase change temperature range of the composite PCM was from 24.1 to 31 ◦ C, and the latent heat of it was 88.39 J g−1 , indicating that the composite PCM was suitable for thermal energy storage in building. (3) The higher the composite PCM volume content, the higher the energy storage capacity of the gypsum board and the lower the temperature fluctuation in the cubicle system. Moreover, the volume fraction of the composite PCM had positive and negative effect on the heat transfer property of the gypsum board compared with the pure gypsum board. (4) The hydration process of gypsum near the composite PCM was affected. The bending strength and compressive strength of the thermal-regulated gypsum board decreased with an increase of the volume of the composite PCM. When the volume amount was 40%, compared with the pure gypsum board, the bending strength and compressive strength of the PCM gypsum board were decreased by 56% and 40.4%, respectively. Acknowledgements The authors gratefully acknowledge the financial support for this research from the National Natural Science Foundation of China (51272068), the National Science & Technology Pillar Program during the 12th Five-year Plan Period (no. 2011BAE14B06-06). Thank the Central Laboratory in Henan Polytechnic University for SEM and DSC analyses. References [1] D.A. Neeper, Thermal dynamics of wallboard with latent heat storage, Sol. Energy 68 (5) (2000) 393–403. [2] J.S. Kim, K. Darkwa, Simulation of an integrated PCM-wallboard system, Int. J. Energy Res. 27 (2003) 215–223. [3] European Commission, Directive 2010/31/EU of the European Parliament and of the Council of 19 May 2010 on the energy performance of buildings (recast), Off. J. Eur. Union 18 (2010) 13–35. [4] L. Perez-Lombard, J. Ortiz, C. Pout, A review on buildings energy consumption information, Energy Build. 40 (2008) 394–398. [5] D. Fiaschi, R. Bandinelli, S. Conti, A case study for energy issues of public buildings and utilities in a small municipality: investigation of possible improvements and integration with renewables, Appl. Energy 97 (2012) 101–114. [6] K. Pielichowska, K. Pielichowski, Phase change materials for thermal energy storage, Prog. Mater. Sci. 65 (2014) 67–123. [7] E. Oró, A. de Gracia, A. Castell, M.M. Farid, L.F. Cabeza, Review on phase change materials (PCMs) for cold thermal energy storage applications, Appl. Energy 99 (2012) 513–533. [8] A. Waqas, Z. Ud Din, Phase change material (PCM) storage for free cooling of buildings—a review, Renew. Sustain. Energy Rev. 18 (2013) 607–625. [9] R. Baetens, B.P. Jelle, A. Gustavsen, Phase change materials for building applications: a state-of-the-art review, Energy Build. 42 (2010) 1361–1368. [10] Z. Belen, M.M. Jose, et al., Review on thermal energy storage with phase change materials, heat transfer analysis and applications, Appl. Energy 23 (2003) 251–283. [11] A. Sarı, Thermal reliability test of some fatty acids as PCMs used for solar thermal latent heat storage applications, Energy Convers. Manage. 44 (2003) 2277–2287. [12] H. Fauzi, H.S.C. Metselaar, T.M.I. Mahlia, M. Silakhori, Sodium laurate enhancements the thermal properties and thermal conductivity of eutectic fatty acid as phase change material (PCM), Sol. Energy 102 (2014) 333–337.
160
J. Zhang et al. / Energy and Buildings 92 (2015) 155–160
[13] A. Karaipekli, A. Sarı, K. Kaygusuz, Thermal conductivity improvement of stearic acid using expanded graphite and carbon fiber for energy storage applications, Renew. Energy 32 (2007) 2201–2210. [14] H. Fauzi, H.S.C. Metselaar, T.M.I. Mahlia, M. Silakhori, H. Nur, Phase change material: optimizing the thermal properties and thermal conductivity of myristic acid/palmitic acid eutectic mixture with acid-based surfactants, Appl. Therm. Eng. 60 (2013) 261–265. [15] H. Fauzi, H.S.C. Metselaar, T.M.I. Mahlia, M. Silakhori, Thermo-physical stability of fatty acid eutectic mixtures subjected to accelerated aging for thermal energy storage (TES) application, Appl. Therm. Eng. 66 (2014) 328–334. [16] A. Sarı, A. Bic¸er, Thermal energy storage properties and thermal reliability of some fatty acid esters/building material composites as novel form-stable PCMs, Sol. Energy Mater. Sol. Cells 101 (2012) 114–122.
[17] A. Karaipekli, A. Sarı, Preparation and characterization of fatty acid ester/ building material composites for thermal energy storage in buildings, Energy Build. 43 (2011) 1952–1959. [18] Changmei Jiao, Baohua Ji, Dong Fang, Preparation and properties of lauric acid–stearic acid/expanded perlite composite as phase change materials for thermal energy storage, Mater. Lett. 67 (2012) 352–354. [19] A. Sarı, A. Karaipekli, C. Alka, Preparation, characterization and thermal properties of lauric acid/expanded perlite as novel form-stable composite phase change material, Chem. Eng. J. 155 (2009) 899–904. [20] Ting Wei, Baicun Zheng, Juan Liu, Yanfeng Gao, Weihong Guo, Structures and thermal properties of fatty acid/expanded perlite composites as form-stable phase change materials, Energy Build. 68 (2014) 587–592. [21] H.A.N. De-gang, G.A.O. Zhi-li, G.A.O. Pan-liang, Physical Chemistry [M], Higher Education Press, Beijing, 2001.
Contents lists available at ScienceDirect
Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Preparation and properties of gypsum based energy storage materials with capric acid–palmitic acid/expanded perlite composite PCM Jianwu Zhang, Xuemao Guan, Xianxian Song, Huanhuan Hou, Zhengpeng Yang, Jianping Zhu ∗ School of Material Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 23 September 2014 Received in revised form 15 January 2015 Accepted 20 January 2015 Available online 7 February 2015 Keywords: Phase change materials Heat storage Thermal-regulated Heat transfer Mechanical property
a b s t r a c t Phase change materials (PCMs) have been widely applied to develop building materials with high thermal energy storage capacity. In this study, the capric acid–palmitic acid (CA–PA)/expanded perlite (EP) composite PCM was prepared using vacuum impregnation method and the thermal-regulated gypsum boards were fabricated by adding the prepared composite PCM. Scanning electron microscope images revealed that the CA–PA eutectic mixture was uniformly distributed in pores of EP and the CA–PA/EP composite PCM has a little effect on the interface gypsum crystals growth. Differential scanning calorimeter results showed that the melting temperature range of the composite PCM was 24.1–31 ◦ C and the latent heat was 88.39 J g−1 . The composite PCM had a good chemical stability by surveying the chemical characterization of the composite PCM. Furthermore, the heat transfer property, thermal conductivity, bending and compressive strength were also tested. The results indicated that the higher the composite PCM volume content, the smaller the thermal conductivity of the gypsum board and the lower the temperature fluctuation in the cubicle system. The bending strength and compressive strength reduced gradually with an increase of the volume fraction of the composite PCM. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Phase change materials (PCMs) are substance that could absorb or release higher latent heat in the process of phase transition. From 1970s, several researchers have tried to introduce PCMs into buildings to enhance the heat storage capacity of constructions [1,2]. Especially in recent years, in order to enhance the level of comfort of the building, many fossil fuel sources have been consumed and the building energy consumption increases gradually, which has already reached about one-third of the total energy consumption [3–5]. Therefore, more and more researchers have focused on the research that implant PCMs into constructions for reducing the building energy consumption in such a situation [6–9], and PCMs will play an increasingly important role in the field of building energy efficiency in future. Fatty acids are a kind of phase change materials, which have attracted much attention for their various applications in building energy efficiency. As one of the most promising PCMs, fatty acids have many desirable characteristics, such as high energy storage density, good phase change reversibility, freezing and melting
∗ Corresponding author. Tel.: +86 13569172672. E-mail address: [email protected] (J. Zhu). http://dx.doi.org/10.1016/j.enbuild.2015.01.063 0378-7788/© 2015 Elsevier B.V. All rights reserved.
congruently with minimum supercooling and without phase segregation, small volumetric change between solid and liquid phase, chemically stable, non-toxic and non-corrosive, etc. [10,11]. However, the drawback of fatty acids is lower thermal conductivity, which is adverse to widespread application. Researchers usually add substances with high thermal conductivity into fatty acids to improve the heat-conducting property. For example, Fauzi et al. enhanced the conductivity of myristic acid/palmitic acid eutectic phase change material by adding 10 wt.% of sodium laurate [12]. Karaipekli et al. studied the enhancement of thermal conductivity of stearic acid by using expanded graphite (EG) and carbon fiber (CF). The result showed that thermal conductivity of the stearic acid increased as 27.6%, 58.6%, 179.3%, and 279.3%, for addition EG in mass fraction 2%, 4%, 7%, and 10%, respectively, and it increased as 24.1%, 106.9%, 162.1%, and 217.2% for addition of CF in mass fractions 2%, 4%, 7%, and 10%, respectively [13]. Literatures [14–17] also studied the thermal properties and characteristics during phase change of fatty acids as PCMs. Most fatty acids usually have a solid–liquid phase change process when applied to buildings for energy storage. Thus, first fatty acids should be stabled to prevent leakage in the process of phase transition. At present, the commonly methods include adsorption method, polymer blends, microencapsulation, etc. However, adsorption is simple and cheaper for preparation of form-stable
156
J. Zhang et al. / Energy and Buildings 92 (2015) 155–160
Table 1 The bending strength and compressive strength of the thermal-regulated gypsum boards. Volume fraction
0%
10%
20%
30%
40%
Bending strength (dry) (MPa) Compressive strength (dry) (MPa)
2.5 4.7
2.17 4.5
1.94 4
1.4 3.27
1.1 2.8
65 60 55 50 45
PCMs. The principle is that some porous materials could strongly adsorb the PCMs by using their own capillary forces even in the case of liquid. Expanded perlite (EP) is a white granular material that has honeycomb structure internally. The porous structure can prevent the leakage of fatty acids during phase change process due to the effect of capillary and surface tension forces [18–20]. This study fixed capric acid–palmitic acid eutectic in pores of EP by means of vacuum impregnation method (VA method), and then prepared thermal-regulated gypsum board by mixing the prepared CA–PA/EP composite PCM and gypsum. The thermal property and chemical compatibility were investigated. Moreover, the bending strength and compressive strength, the heat transfer property were also discussed (Table 1). 2. Experimental 2.1. Materials and equipments Capric acid (CA, 98.5% pure) and palmitic (PA, 99% pure) were purchased from Sinopharm Chemical Reagent Co, Ltd. Expanded perlite (EP) was obtained from Xinyang perlite Plant, China. The bulk density was 45 kg m−3 . Gypsum was produced by Taishan Construction Material Plant, Shandong, China. The microstructure of the composite PCM was observed with a SEM (JSM-6390LV) at the acceleration voltage of 15 kV under low vacuum. DSC (NETZSCH DSC 204) was used to test the thermal property of the composite PCM. The heating rate was 5.0 ◦ C min−1 . The chemical compatibility of the composite PCM was carried out using a FT-IR spectrophotometer (V70 from Bruker Corporation). Thermal conductivity was determined by thermal conductivity measuring apparatus (DRH-III). PID temperature recorder (SWPASR100) was used to record the temperature variation. The bending strength and compressive strength of the thermal-regulated gypsum boards were tested by electronic universal testing machine (WDW-20). 2.2. Experimental method 2.2.1. Preparation of the thermal-regulated gypsum board The CA–PA/EP composite PCM was prepared by VA method. First, the CA–PA eutectic mixture was produced. The mass ratios of CA and PA in the eutectic mixture were determined as 85 wt.% and 15 wt.%, respectively, according to the CA–PA binary phase diagram shown in Fig. 1, which was measured by cooling curve method. Second, the melted CA–PA eutectic mixture and EP were placed in a bottom flask, and stirred for 30 min. This process ensured EP and CA–PA could be mixed uniformly. Third, the mixture was then placed in a vacuum drying oven about 2 h, the temperature and vacuum degree were set at 60 ◦ C and 0.05 MPa, respectively. To determine the maximum combination ratio of CA–PA eutectic mixture, a series of composite PCMs with mass ratio of 35 wt.%, 45 wt.%, 55 wt.%, 65 wt.%, 75 wt.%, 85 wt.% (CA–PA) were prepared. It was found that the CA–PA eutectic mixture as PCM could be retained as 65 wt.% in EP without the leakage of melted PCM. Therefore, the composite with 65 wt.% of CA–PA was defined as form-stable composite PCM. The thermal-regulated gypsum boards were fabricated with volume fraction of the CA–PA/EP composite PCM at 10%, 20% and 30%.
40 35 30 25 20
15:85
45:55
65:35
85:15
98:2
Fig. 1. Experimental phase diagram of the CA–PA binary system.
The typical procedure is described as follows: first, the gypsum slurry was prepared by blending water and gypsum powder with a mass ratio of 49 wt.%. Second, the composite PCM was added into the mixture and stirred until the homogeneous slurry was obtained. Finally, the resulted slurry was put into molds with the size of 30 cm × 30 cm × 5 cm for the heat transfer property test and 20 cm × 20 cm × 1.5 cm for the thermal conductivity test, respectively. Similarly, the size of 40 mm × 40 mm × 160 mm samples with 10%, 20%, 30% and 40% volume fraction of the composite PCM were prepared for the strength test. The whole samples were dried at room temperature before testing. Fig. 2 showed the distribution of the CA–PA/EP composite PCM in gypsum board. 2.2.2. Heat transfer property of the thermal-regulated gypsum board test The heat transfer property of the thermal-regulated gypsum boards was tested by using a self-made design as shown in Fig. 3. The self-made design mainly included a cubicle system (30 cm × 30 cm × 30 cm) and a heat release source (60 W). Furthermore, the cubicle system was composed of five polystyrene boards and one gypsum board (30 cm × 30 cm × 5 cm). The distance between the heat release source and the gypsum board was 25 cm. The temperature signal of the center location of the gypsum board surface and the cubicle system were recorded by two temperature sensors, which connected with a corresponding temperature recorder. The specific test procedure was described as follows: first, the room temperature was modulated at 20 ◦ C, and then the heat release source of self-made design was opened. The heating time was 2 h to ensure that the composite PCM in gypsum board reached or exceeded the melted temperature. Finally, the heat release source was closed and the cubicle system was automatically
Fig. 2. The section of the thermal-regulated gypsum.
J. Zhang et al. / Energy and Buildings 92 (2015) 155–160
157
Fig. 3. The photo and sketch map of the self-made designed for the heat transfer property of the thermal-regulated gypsum board.
cooled. The temperature measurements were recorded by the temperature recorder. 3. Results and discussion 3.1. Morphology and chemical characterization of the CA–PA/EP The SEM images of expanded perlite and the CA–PA/EP composite PCM were shown in Fig. 4. As can be seen, EP had porous structure with pore diameter of 1 to 50 m. Fig. 4(b) presented that the CA–PA eutectic mixture had been absorbed in the pores and the surface of EP was smooth and round. Fig. 5 was the SEM micrograph of thermal-regulated gypsum board. As seen from Fig. 5(a), the crystal size and structure of the gypsum which was away from the composite PCM had no obvious change, and the dihydrate gypsum crystal was well developed according to the degree of acicular. From Fig. 5(b), the composite PCM had a little effect on the interface gypsum crystals growth and leaded to form an obvious interface between the composite PCM and gypsum. This is due to the strong hydrophobic of composite PCM, causing water shortage near the composite PCM and therefore affecting the hydration process of gypsum near the interface.
The chemical compatibility between the CA–PA and EP was characterized by evaluating the interactions among the components of the composite using FT-IR spectroscopy technique. The FT-IR absorption spectrum of the CA–PA/EP composite PCM was compared to that of CA–PA eutectic mixture. The FT-IR spectrum of the CA–PA eutectic mixture (as shown in Fig. 6) indicated that carboxylic acid formed a dimer structure (C O stretching band at 1200–1300 cm−1 ). The absorption peaks caused by the out-of-plane bending vibration and in-plane swinging vibration of functional group OH were also found at 729 cm−1 and 719.95 cm−1 . The peaks at 2933 cm−1 and 2871 cm−1 represented the asymmetrical vibration and symmetrical vibration of functional group of CH2 . Compared with the FT-IR spectra of CA–PA and EP, it was clearly found that the FT-IR spectrum of CA–PA/EP form-stable PCM contains the characteristic peaks of CA–PA and EP. The infrared spectrum with no significant new peaks indicated that there were no chemical reactions between CA–PA and EP. The insignificant frequency shifts of the composite PCM can be attributed to the interaction between the carboxyl group of the CA–PA and the alkaline region in EP. These attractive forces provided easy confining of the fatty acids molecules in the pores of EP and thus prevented the leakage of the melted CA–PA from the surface of the composite.
Fig. 4. SEM morphology of EP and CA–PA/EP: (a) EP; (b) CA–PA/EP.
Fig. 5. SEM images of thermal-regulated gypsum board: (a) gypsum area; (b) interface between the composite PCM and gypsum.
158
J. Zhang et al. / Energy and Buildings 92 (2015) 155–160
Temperature/ ºC
EP
CA-PA
CA-PA/EP
50 48 46 44 42 40 38 36 34 32 30 28 26 24 22 20 18 -2500
(a) 10% volume fraction 20% volume fraction
30% volume fraction
The pure gypsum board
0
2500 5000 7500 10000 12500 15000 17500 20000 22500 25000
Time/s 3500
3000
2500
2000
1500
1000
500
42
-1
Wavenumber ( cm )
40
Fig. 6. The FT-IR spectra of EP, CA–PA and CA–PA/EP.
(b)
10% volume fraction
38
The pure gypsum board
36
20% volume fraction
3.2. Thermal property of CA–PA and CA–PA/EP composite PCM Fig. 7 showed the DSC thermograms of the CA–PA eutectic mixture and the CA–PA/EP composite PCM. From the DSC curves, the melting temperature range was determined as 23.7–29.6 ◦ C for CA–PA eutectic mixture, and 24.1–31.0 ◦ C for CA–CA/EP composite PCM, respectively. It can be found that the phase change temperature of CA–PA/EP was slightly higher than that of CA–PA eutectic mixture. The reason was that the volume expansion of CA–PA was limited by the porous structure of expanded perlite under the melting phase change process and the increasing pressure. Thus, the phase change temperature of CA–PA eutectic mixture in the composite would increase with the relationship of pressure and temperature [21]. The latent heat of melting was found to be 137.9 J g−1 for CA–PA eutectic mixture, and 88.39 J g−1 for the CA–PA/EP composite PCM. The latent heat value of the composite PCM was slightly lower than the theoretical value, which was calculated by multiplying the mass ratio of CA–PA in the composite. The results showed that the CA–PA/EP composite PCM was ideal for the application in building energy conservation.
0.0
Heat flow(mv/mg)
-0.5
(CA-PA/EP
-1.0
ΔΗm=88.39J/g
Onset=25.7 -1.5
(CA-PA
-2.0
ΔΗm=137.9J/g
Onset=23.7
-2.5 -3.0 -3.5 0
10
20
30
Temperature(
40
50
60
)
Fig. 7. DSC curves of CA–PA eutectic mixture and the CA–PA/EP composite PCM.
Temperature/ºC
34 32 30 28 30% volume fraction
26 24 22 20 18 -2500
0
2500 5000 7500 10000 12500 15000 17500 20000 22500 25000
Time/s Fig. 8. Effect of the composite PCM volume fraction on the heat transfer property of thermal-regulated gypsum board: (a) the surface temperature variation curve, (b) the center of the cubicle system temperature variation curve.
3.3. Heat transfer property test results The heat transfer property of the thermal-regulated gypsum was mainly evaluated by its heat storage capacity which was varied with the volume fraction of the composite PCM in gypsum board. In this study, the heat transfer property of the thermal-regulated gypsum boards were investigated by measuring the temperature variation regularity of thermal-regulated gypsum board surface and the inside center of the cubicle system. The obtained results were shown in Fig. 8. From Fig. 8, the temperature gradient of the thermal-regulated gypsum board surface and the center of the cubicle system were gradually reduced with an increase of the composite PCM volume fraction from 10% to 30% during the heating and cooling. The highest temperature both the thermal-regulated gypsum surface and the center of the cubicle system were 41.9 and 35.2 ◦ C throughout the testing process, respectively, while the volume fraction of the composite PCM was 30%. That was lower than that of those gypsum boards which had low composite PCM volume content. In other words, the higher the composite PCM volume content, the higher the energy storage capacity of the gypsum board and the lower the temperature fluctuation in the cubicle system. However, it was found that the heat insulation property of the thermal-regulated gypsum board was poor than that of the pure gypsum board when the volume fraction of the composite PCM was 10% according to the test curve, and it was the opposite for the thermal-regulated
J. Zhang et al. / Energy and Buildings 92 (2015) 155–160
Thermal conductivity/W B m k
-1 -1
0.40 0.35
0.3796
0.3911
with this conclusion. The composite PCM provides an effect of “hole defect”, equivalently. Therefore, the combination of dihydrate gypsum crystals is weakened, causing a decrease of the strength.
0.4125 0.3577
0.3555
159
0.3163
0.30
4. Conclusions 0.2759
In this paper, a novel energy storage material, CA–PA/EP composite PCM based on gypsum, has been prepared. The obtained conclusions can be described as follows:
0.25 0.20 0.15 0.10 0.05 0.00 Pure gypsum board
10%EP
20%EP
30%EP
10%CA-PA/EP
20%CA-PA/EP
30%CA-PA/EP
Different types of gypsum boards Fig. 9. Thermal conductivity of the PCM gypsum boards.
gypsum board with 20% or 30% volume fraction composite PCM. Therefore, the composite PCM had positive and negative effect on the heat transfer property of the gypsum board. This could be explained from two factors. On the one hand, when the temperature reaches the phase change temperature, the PCM in the gypsum board will absorb the heat, thus, the heat transfer process is prevented to a certain degree. On the other hand, the internal structure defect of the gypsum board increases with the incorporation of the composite PCM. That is to say, the “internal connectivity” of the gypsum board is enhanced. The result is that the convection effect within the gypsum board has been reinforced. The heat would be more easily passed through the gypsum board. Maybe, when the composite PCM volume dosage is 10%, the negative effect plays a leading role, but predominantly positive effect when the composite PCM volume content is high. 3.4. Thermal conductivity test Thermal conductivity of the thermal-regulated gypsum boards was tested. In order to verify the negative effect, the thermal conductivity of expanded perlite (without CA–PA)/gypsum boards were also tested. The result was shown in Fig. 9. The thermal conductivity of gypsum board increased with the augment of EP compared with the pure gypsum board. That may be ascribed to that EP has connected pore structure, the convection effects within the gypsum board was enhanced with the mix of EP. And when the volume of the CA–PA/EP composite PCM was 30%, the thermal conductivity of the gypsum board was 0.2759 W m−1 K−1 , which was lower than that of the pure gypsum. From the overall test results, it was further confirmed that the volume of the composite PCM had positive and negative effect on the heat transfer property of the gypsum board compared with the pure gypsum board. 3.5. Bending strength and compressive strength The bending strength and compressive strength of the sample decreased with an increase of the volume fraction of composite PCM. When the composite PCM volume fraction was 40%, the bending strength and compressive strength of the sample were 1.1 MPa and 2.8 MPa, respectively, which were decreased by 56% and 40.4% in comparison with the pure gypsum sample. Such phenomena can be explained as follows. For one thing, the content of gypsum decreases with the incorporation of composite PCM. Therefore, the “skeleton effect” of gypsum is weakened. For another thing, the composite PCM has low strength, and the crystal size and structure of the gypsum on the interface between the composite PCM and gypsum is affected. The SEM images are in good agreement
(1) CA–PA eutectic mixture can be effectively distributed in the microporous structure of EP, and the composite PCM was chemically stable. (2) The phase change temperature range of the composite PCM was from 24.1 to 31 ◦ C, and the latent heat of it was 88.39 J g−1 , indicating that the composite PCM was suitable for thermal energy storage in building. (3) The higher the composite PCM volume content, the higher the energy storage capacity of the gypsum board and the lower the temperature fluctuation in the cubicle system. Moreover, the volume fraction of the composite PCM had positive and negative effect on the heat transfer property of the gypsum board compared with the pure gypsum board. (4) The hydration process of gypsum near the composite PCM was affected. The bending strength and compressive strength of the thermal-regulated gypsum board decreased with an increase of the volume of the composite PCM. When the volume amount was 40%, compared with the pure gypsum board, the bending strength and compressive strength of the PCM gypsum board were decreased by 56% and 40.4%, respectively. Acknowledgements The authors gratefully acknowledge the financial support for this research from the National Natural Science Foundation of China (51272068), the National Science & Technology Pillar Program during the 12th Five-year Plan Period (no. 2011BAE14B06-06). Thank the Central Laboratory in Henan Polytechnic University for SEM and DSC analyses. References [1] D.A. Neeper, Thermal dynamics of wallboard with latent heat storage, Sol. Energy 68 (5) (2000) 393–403. [2] J.S. Kim, K. Darkwa, Simulation of an integrated PCM-wallboard system, Int. J. Energy Res. 27 (2003) 215–223. [3] European Commission, Directive 2010/31/EU of the European Parliament and of the Council of 19 May 2010 on the energy performance of buildings (recast), Off. J. Eur. Union 18 (2010) 13–35. [4] L. Perez-Lombard, J. Ortiz, C. Pout, A review on buildings energy consumption information, Energy Build. 40 (2008) 394–398. [5] D. Fiaschi, R. Bandinelli, S. Conti, A case study for energy issues of public buildings and utilities in a small municipality: investigation of possible improvements and integration with renewables, Appl. Energy 97 (2012) 101–114. [6] K. Pielichowska, K. Pielichowski, Phase change materials for thermal energy storage, Prog. Mater. Sci. 65 (2014) 67–123. [7] E. Oró, A. de Gracia, A. Castell, M.M. Farid, L.F. Cabeza, Review on phase change materials (PCMs) for cold thermal energy storage applications, Appl. Energy 99 (2012) 513–533. [8] A. Waqas, Z. Ud Din, Phase change material (PCM) storage for free cooling of buildings—a review, Renew. Sustain. Energy Rev. 18 (2013) 607–625. [9] R. Baetens, B.P. Jelle, A. Gustavsen, Phase change materials for building applications: a state-of-the-art review, Energy Build. 42 (2010) 1361–1368. [10] Z. Belen, M.M. Jose, et al., Review on thermal energy storage with phase change materials, heat transfer analysis and applications, Appl. Energy 23 (2003) 251–283. [11] A. Sarı, Thermal reliability test of some fatty acids as PCMs used for solar thermal latent heat storage applications, Energy Convers. Manage. 44 (2003) 2277–2287. [12] H. Fauzi, H.S.C. Metselaar, T.M.I. Mahlia, M. Silakhori, Sodium laurate enhancements the thermal properties and thermal conductivity of eutectic fatty acid as phase change material (PCM), Sol. Energy 102 (2014) 333–337.
160
J. Zhang et al. / Energy and Buildings 92 (2015) 155–160
[13] A. Karaipekli, A. Sarı, K. Kaygusuz, Thermal conductivity improvement of stearic acid using expanded graphite and carbon fiber for energy storage applications, Renew. Energy 32 (2007) 2201–2210. [14] H. Fauzi, H.S.C. Metselaar, T.M.I. Mahlia, M. Silakhori, H. Nur, Phase change material: optimizing the thermal properties and thermal conductivity of myristic acid/palmitic acid eutectic mixture with acid-based surfactants, Appl. Therm. Eng. 60 (2013) 261–265. [15] H. Fauzi, H.S.C. Metselaar, T.M.I. Mahlia, M. Silakhori, Thermo-physical stability of fatty acid eutectic mixtures subjected to accelerated aging for thermal energy storage (TES) application, Appl. Therm. Eng. 66 (2014) 328–334. [16] A. Sarı, A. Bic¸er, Thermal energy storage properties and thermal reliability of some fatty acid esters/building material composites as novel form-stable PCMs, Sol. Energy Mater. Sol. Cells 101 (2012) 114–122.
[17] A. Karaipekli, A. Sarı, Preparation and characterization of fatty acid ester/ building material composites for thermal energy storage in buildings, Energy Build. 43 (2011) 1952–1959. [18] Changmei Jiao, Baohua Ji, Dong Fang, Preparation and properties of lauric acid–stearic acid/expanded perlite composite as phase change materials for thermal energy storage, Mater. Lett. 67 (2012) 352–354. [19] A. Sarı, A. Karaipekli, C. Alka, Preparation, characterization and thermal properties of lauric acid/expanded perlite as novel form-stable composite phase change material, Chem. Eng. J. 155 (2009) 899–904. [20] Ting Wei, Baicun Zheng, Juan Liu, Yanfeng Gao, Weihong Guo, Structures and thermal properties of fatty acid/expanded perlite composites as form-stable phase change materials, Energy Build. 68 (2014) 587–592. [21] H.A.N. De-gang, G.A.O. Zhi-li, G.A.O. Pan-liang, Physical Chemistry [M], Higher Education Press, Beijing, 2001.