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Fabrication and thermal characterization of kaolin-based composite phase change materials for latent heat storage in buildings
Energy and Buildings 96 (2015) 193–200
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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Fabrication and thermal characterization of kaolin-based composite phase change materials for latent heat storage in buildings Ahmet Sarı ∗ Gaziosmanpas¸a University, Department of Chemistry, 60240 Tokat, Turkey
a r t i c l e
i n f o
Article history: Received 16 January 2015 Received in revised form 10 March 2015 Accepted 11 March 2015 Available online 18 March 2015 Keywords: CPCM Capric acid PEG600 Heptadecane Kaolin Thermal energy storage Building Thermal reliability Thermal durability Thermal conductivity
a b s t r a c t Three kinds of kaolin-based composite phase change materials (Kb-CPCMs) including capric acid (CA), PEG600, and heptadecane (HD) as organic PCMs were fabricated using vacuum impregnation method for latent heat storage (LHS) application in buildings. The surface morphology, compatibility, maximum ratio for impregnated PCM, LHS properties, thermal endurance, thermal conductivity and its effect on the melting times of prepared Kb-CPCMs were investigated by using microscopy, spectroscopy, calorimetry and thermal methods. The seepage test indicated that CA, PEG600 and HD were impregnated maximally into the kaolin as 17.5, 21 and 16.5 wt%, respectively. The fabricated three composites, K/CA, K/PEG600, and K/HD, have a phase change temperature of 30.71, 5.16 and 22.08 ◦ C and a latent heat of 27.23, 32.80 and 34.63 J/g, respectively. The thermal cycling test exposed that the thermal reliability of the Kb-CPCMs slightly changed after repeated 1000 heating-cooling cycling. The heat storage rates of the Kb-CPCMs were increased considerably by adding expanded graphite (EG) in mass faction of 5%. All the prepared Kb-CPCMs have good thermal energy storage (TES) function for heating, ventilating and air conditioning (HVAC) in building envelopes because of their suitable LHS properties, high reusability performance and enhanced thermal conductivity. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The primary energy consumers in the world are building and industrial sectors and about one-third of total energy consumption is originated from these sectors [1,2]. Therefore, enhancing the energy efficiency of buildings diminishes the fossil sourcedenergy consumption and the hazardous gas emissions. In this sense, thermal energy storage (TES) has an important function in the improvement of the energy efficiency of building envelopes and reduces the environmental damages caused by the fossil-based energy use. [3,4]. Amongst various TES techniques, latent heat storage (LHS) using phase change material (PCM) has drawn substantial interest since it makes possible to storage and release high energy at narrow servicing temperature range [5–7]. This method has been extensively used for heating, ventilating, and air conditioning (HVAC) purposes in buildings to achieve thermal comfort and acceptable indoor air quality by flattening the fluctuations at indoor temperature [8,9]. The organic solid–liquid PCMs have high latent heat of fusion and suitable phase change temperature for passive solar heating and cooling applications. They are generally non-corrosive,
∗ Tel.: +90 356 2521616; fax: +90 356 2521585. E-mail address: [email protected] http://dx.doi.org/10.1016/j.enbuild.2015.03.022 0378-7788/© 2015 Elsevier B.V. All rights reserved.
non-toxic, chemically stable and do not show no or a little supercooling (except PCMs in PEG group) behavior [10–14]. However, the low thermal conductivity (0.15–0.20 W/mK) can cause disinhibitory effect on the rate of thermal energy storage and release during the heating and cooling periods of PCMs. To resolve this problem, some high conductive materials such as expanded graphite [15,16] and ˇ-aluminum nitride [17] have been introduced into the composite PCMs. On the other hand, to get rid of the leakage problem encountered during the solid–liquid phase change processes of organic PCMs, many researchers have strived about the encapsulation of PCMs using the different methods such as confining into a polymeric network, grafting with polymeric materials and impregnating with porous building materials. Among these methods, the combination of porous materials with PCM is simple, cost-effective and thus relatively proficient way to fabricate form-stable lightweight constructive materials with high LHS capacity [18–20]. In addition to gypsum [21–25] and cement [26–31], natural clays as porous building materials, such as montmorillenite [32–35], perlite [36–39], attapulgite [40,41], vermiculite [42–44], bentonite [45–47], silica-fume [48,49] and diatomite [50–56] have been evaluated in the incorporation of several organic PCMs. Moreover, these materials have been used to fabricate PCM wallboards, plasters, concrete blocks, hollow bricks and wall covering materials for passive solar buildings [57–63].
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As well as the above-mentioned clay materials, kaolin is one of the most common industrial clays and has a chemical composition of Al2 Si2 O5 (OH)4 . Kaolin is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedral [64]. It is generally used in ceramics, toothpaste, light bulbs, cosmetics, water and wastewater treatment, etc. Especially, metakaolin is a supplementary cementitious material (SCM) acting as pozzolan. Additionally, some advantageous properties of this clay such as low cost (e.g., its wholesale price is about 0.25 $/kg in Turkish markets), large surface area, porous structure, abundantly availability, easy handling, direct usability without extra encapsulation and good compatibility with mortar and concrete make it a potential matrix for the fabrication of building composites with organic PCMs. Therefore, successful integration of kaolin with organic PCMs for the TES purposes in buildings can expand its industrial usage areas. However, the number of the studies about the usability of the kaolin as supporting matrix to encapsulate organic PCMs is limited with only a few [65–68]. So, many works to enlarge the varieties of novel composite PCMs with high LHS efficiency are still needed. On the other hand, capric acid (CA), polyethylene glycol 600 (PEG600) and heptadecane (HD) are fitting organic PCMs for the incorporation with kaolin clay because they have suitable phase change temperature range (10–31 ◦ C) and high latent heat capacity in the range of 136–216 J/g for passive solar TES applications in building envelopes. As far as the authors are aware, there is no investigation reported in the literature on the fabrication, chemical-morphological characterization and determination of LHS properties of newly developed K/CA, K/PEG600, and K/HD composites. The morphological and chemical characterization of fabricated three types of kaolin based-building composite PCMs (Kb-CPCMs) were made using scanning electronic microscope (SEM) and Fourier transformation infrared spectroscope (FTIR) techniques. The LHS properties and thermal reliability of the Kb-CPCMs were analyzed by differential scanning calorimetry (DSC). The thermal degradation temperatures of the Kb-CPCMs were measured using thermogravimetric analysis (TGA) method. Furthermore, the thermal conductivities of the Kb-CPCMs were increased by addition of expanded graphite (EG) and the obtained results were confirmed by comparing the melting times of the samples with and without EG additive. 2. Materials and methods 2.1. Materials Kaolin clay used in this work is originated with of Balıkesir city of Turkey. It was kept at 105 ◦ C in an oven to remove its humidity and
then sieved form 200 mesh. The dried sample is mainly consisted of SiO2 (48 wt%), Al2 O3 (36.6 wt%), K2 O (2 wt%) and other metal oxides [69,70]. The selected organic PCMs, capric acid (CA), PEG600 and heptadecanol (HD) were supplied from Sigma–Aldrich company. 2.2. Methods K/CA, K/PEG600, and K/HD composites were fabricated using vacuum impregnation method and the same vacuum conditions reported in our previous studies [71,72] were used here. By following that procedure, the dried kaolin sample in specified weight amount were incorporated with CA, PEG600 and HD, separately in mass fractions varied from 10% to 30%, respectively. The highest mass portion of the PCM retained by kaolin was determined by performing the leakage test. With this aim, each composite sample was subjected to a heating process above the melting temperature of the confined PCM and then cooling process to room temperature. The test results exposed that the maximum fraction of CA, PEG600 and HD in the composite samples, which did not show any PCM loss was determined as 17.5, 21 and 16.5 wt%, respectively. The photograph images of kaolin clay, the selected organic PCMs and fabricated Kb-CPCMs are shown in Fig. 1. The morphologies of kaolin and the produced three kinds of Kb-CPCMs were investigated by using a SEM instrument (LEO 440 model). The compatibility among the components of the composites was characterized chemically using a FT-IR spectrophotometer (JASCO 430 model). The analysis procedures performed for the SEM and FT-IR analysis were the same as in our previous studies [42,43,46]. The LHS properties of the prepared Kb-CPCMs were measured under atmospheric nitrogen and at a heating rate of 5 ◦ C min−1 using by a Perkin Elmer JADE model DSC instrument. For each sample, the DSC measurement was repeated for three times and the mean deviations in the phase change temperature and latent heat values were calculated as ±0.14 ◦ C and ±1.24 J/g, respectively. Thermal durability of the Kb-CPCMs was determined by comparing the measured thermal degradation temperatures of pure organic PCMs and the fabricated Kb-CPCMs using a TGA analyzer (Perkin–Elmer TGA7 model). The analyses were applied at heating rate of 10 ◦ C min−1 between 50 ◦ C and 500 ◦ C. The thermal reliability of the Kb-CPCMs was evaluated in terms of probable changes to be occurred in their LHS properties after the following heating-cooling cycles repeated for 1000 times. The cycling process was realized by using a thermal cycler instrument (BIOER TC-25/H model). In order to reach a decision about the thermal reliability and chemical stability of Kb-CPCMs, their DSC and FT-IR data obtained after and before the cycling process were compared. Furthermore, the thermal conductivities of the prepared
Fig. 1. Photograph images of pure kaolin, pure CA, pure PEG600, pure HD and the fabricated K/CA, K/PEG600 and K/HD composite PCMs.
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Fig. 2. SEM images of (a) kaolin, (b) K/CA, (c) K/PEG600 and (d) K/HD.
Kb-CPCMs were improved by addition of 5 wt% EG with high thermal conductivity (4.26 W/m K). The measurements were carried out using a thermal property analyzer (Decagon KD2 model). The analyzer was calibrated by using a verification standard (glycerin). For the measurements, a 25 mL-test tube was filled the sample and the needle sensor of the analyzer was placed tightly into the sample. The measurements were repeated three times at room temperature and the results were recorded as average values. The accuracy of the measurements was calculated as ±3%. The enhancement in the thermal conductivity of the Kb-CPCMs was confirmed by comparing the melting times of the composite samples with and without EG additive. The temperature data against time during the melting periods of Kb-CPCMs were recorded by using a data logger (NOVA5000 model).
3. Results and discussion 3.1. Morphological characterization of the fabricated Kb-CPCMs Fig. 2a–d shows the SEM photographs of the kaolin and the prepared K/CA, K/PEG600, and K/HD composites. As can be evidently observed from Fig. 2a, the surface of the kaolin clay is consisted of the particles with indiscriminate shape and different sizes and the layers formed by these particles are dispersed randomly through the surface. The holes and hiatuses among the layers facilitate the sorption of the organic PCM molecules. As also clearly seen from Fig. 2b–d, the micro holes or channels between the particles wholly were occupied with CA, PEG600 and HD as organic PCMs. Moreover, the surface tension and capillary forces caused good physically compatibility between the clay matrix and the organic PCMs [46,51,53,65–68], which was resulted in forming shape-stabilized Kb-CPCMs with excellent resistant against the leakage problem regarding with their PCM content.
3.2. Chemical characterization of the fabricated Kb-CPCMs In order to investigate the chemical compatibility between the kaolin and the organic PCMs, The FT-IR spectra of Kb-CPCMs were taken and compared with that of the pure components. The CH2 characteristic bands were observed in the range of 2921–3000 cm−1 for pure CA [73] and 2865–2908 cm−1 for pure PEG600 [46] and 2862–2956 cm−1 for pure HD [74]. The absorption wavenumber values regarding C O and C O groups of pure CA were reported as1735 and 1100 cm−1 , respectively. Moreover, the O H characteristic band was determined in the range of 3174–3633 cm−1 for PEG600 [46] and 3400–3700 cm−1 for CA [73]. In another study, the FT-IR analysis of the kaolin used here indicated that the absorption band in the range of 3478–3552 cm−1 was attributed to the O H stretching vibration of the silanol (SiOH) groups and the bands at 1070 and 584 cm−1 represent the asymmetric stretching and bending vibrations of Si O Si groups, respectively. On the other hand, Fig. 3 shows the FT-IR spectra of the fabricated Kb-CPCMs containing maximum amount of the organic PCM. Compared these results with that mentioned above, it is possible to observe all characteristic bands of the pure components in the spectra of the Kb-CPCMs. After incorporation of the organic PCMs with the kaolin, the absence of any new peak in the spectrum means that any chemical reaction was not occurred between the PCM molecules and the inorganic components of the kaolin. The symmetrical stretching bands of CH2 groups was observed below 3000 cm−1 in the spectrum of all composites. The absorption bands of C O and C O groups were detected at 1710 and 1108 cm−1 for the K/CA composite, respectively. The O H bands of K/CA and K/PEG600 composites were monitored in the range of 3162–3702 cm−1 and 3080–3716 cm−1 , respectively. Additionally, the asymmetric stretching and bending vibrations of Si O Si groups were detected at 1139 and 536 cm−1 for K/CA, 1103 and 538 cm−1 for K/PEG600, at 1137 and 534 cm−1 for K/HD composite,
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Fig. 3. FT-IR spectra of the fabricated Kb-CPCMs.
respectively. The small shifts occurred in the wavenumber values of the characteristic bands of the composites relatively to their pure components could be due to capillary forces or surface tension forces between the confined PCM molecules and the surface functional groups or pore walls of the kaolin [65–68]. All the FT-IR results also proved the presence of good compatibility between the ingredients of the fabricated Kb-CPCMs. 3.3. LHS properties of the fabricated Kb-CPCMs The LHS properties of a building composite PCM such as melting temperature and latent heat capacity are decisive key parameters in terms of its usage in building envelopes to reach the comfort temperature level. Thus, an organic PCM preferred in the production of the composite PCM should have an appropriate melting temperature and sufficiently high enthalpy for this goal. The solid–liquid phase change temperature and enthalpy of the selected organic PCMs were measured as 31.04 ◦ C and 190.21 J/g for pure CA [73], 10.0 ◦ C and 136.42 J/g for pure PEG600 [46] and 21.36 ◦ C (and also solid–solid phase change at 10.87 ◦ C) and 216.21 J/g for pure HD [74]. As can be seen from these DSC data, the LHS properties of the PCMs are rather convenient materials for the fabrication of Kb-CPCMs, which will be used for low temperature-LHS implementations in buildings. On the other hand, as can be clearly seen from the DSC thermograms in Fig. 4, the three kinds of Kb-CPCMs have steady
Fig. 4. DSC curves of the fabricated Kb-CPCMs.
melting-freezing phase change behavior like pure PEG600, CA and HD. The LHS properties obtained from the DSC heating and cooling peaks were also presented in Table 1. From these data, the fabricated K/CA, K/PEG600 and K/HD composites including maximum amount of PCM melt at 30.71 ◦ C, 5.16 ◦ C and 22.08 ◦ C and storage a latent heat as 27.23, 32.80 and 34.63 J/g, respectively, while they freeze at 28.21 ◦ C, 15.44 ◦ C and 21.53 ◦ C and release a latent heat as −25.51, −29.78 and −32.42 J/g, respectively. The melting and freezing temperatures of the prepared Kb-CPCMs make them hopeful composite PCMs with energy storing function for solar passive heating and cooling intentions in buildings with respect to the different climate conditions. Moreover, after the impregnation process, the change occurred in the melting and freezing temperatures of the Kb-CPCMs compared to the pure PCMs could be because of strong physical attractions between the characteristics silanol groups of the kaolin and the functional groups of the PCMs, which are proved by the FT-IR analysis. The similar interpretations were given for some building composites including different PCMs [51,53,65–68]. Additionally, the theoretical PCM fractions (17, 20 and 16 wt% for K/CA, K/PEG600 and K/HD, respectively) in the composites calculated by dividing their latent heat values to that of pure PCMs are very close to their experimental incorporation fractions (17.5, 21 and 16.5 wt%, respectively) determined after the leakage test. However, the slight difference between the results could be due to the restriction of phase change behavior of the PCMs retained into to the porous network of the kaolin [51]. By comparing the latent heat capacity of the fabricated three kinds of Kb-CPCMs with other type building composite PCMs in literature [22,24,28,35,40,41,45,46,49,51,60–63], it is deduced that the latent heat capacities of the composites were changed according to the incorporation ability that are dependent on the chemical and physical characteristics of building material and of PCM. In other words, the abundance and variety of functional groups interacted with PCM, and porosity of the building material can affect the incorporation ratio of the PCM while the latent heat capacity of confined PCM directly influences the LHS capacity of the prepared composite PCM. Moreover, by taking into consideration the LHS capacity of the fabricated K/CA, K/PEG600 and K/HD composite PCMs, it can be drawn a conclusion that they have reasonable LHS capacity for passive solar HVAC applications in buildings. 3.4. Thermal reliability and chemical stability of the fabricated Kb-CPCMs A PCM to be considered for TES practices in buildings should have a good energy storage life even it subjected to an extended number of thermal cycling. Therefore, prior knowledge of the thermal reliability property of newly developed composite PCM is important in term of its handling in real long-term TES applications. For that purpose, the LHS properties of prepared three kinds of the Kb-CPCMs were measured again by DSC analysis after the thermal cycling test corresponding to 1000 melting-freezing processes. Fig. 5 and Table 2 show the DSC thermograms and data obtained after the cycling test. It is possible to see the same peaks on the thermograms like before the cycling process, meaning that the prepared Kb-CPCMs have stable melting and freezing behavior even after 1000 cycles. The changes in the phase change temperatures of the prepared K/CA, K/PEG600 and K/HD composites were calculated as −0.27 ◦ C, 3.45 ◦ C and −0.14 ◦ C for melting period and −0.5 ◦ C, −1.56 ◦ C and 0.01 ◦ C for freezing period, respectively. Moreover, the decrease occurred in their latent heat values were determined as 9.8%, 11.4% and 12.4% for melting period and 2.4%, 10.2% and 9.6% for freezing period, respectively. These results showed that the fabricated three kinds of Kb-CPCMs had good thermal reliability for practical TES applications because of the low deviation occurred in their LHS properties.
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Table 1 The LHS properties of the fabricated Kb-CPCMs before thermal cycling process. K-BCPCM
Solid–solid phase change temperature (◦ C) during heating period
Melting temperature (◦ C)
Latent heat of melting (J/g)
Solid–solid phase change temperature (◦ C) during cooling period
Freezing temperature (◦ C)
Latent heat of freezing (J/g)
K/CA (17.5 wt%) K/PEG600 (20 wt%) K/HD (16.5 wt%)
– – 11.06
30.71 5.16 22.08
27.23 32.80 34.63
– – 8.02
28.21 15.44 21.53
−25.51 −29.78 −32.42
Latent heat of freezing (J/g)
Table 2 The LHS properties of the fabricated Kb-CPCMs after thermal cycling process. K-BCPCM
Solid–solid phase change temperature (◦ C) during heating period
Melting temperature (◦ C)
Latent heat of melting (J/g)
Solid–solid phase change temperature (◦ C) during cooling period
Freezing temperature (◦ C)
K/CA (17.5 wt%) K/PEG600 (20 wt%) K/HD (16.5 wt%)
– – 10.88
30.44 8.61 21.94
24.56 29.05 30.32
– – 7.13
27.71 13.88 21.54
On the other hand, the chemical stability of PCM after a great number of thermal cycling is one of the key parameters to be considered in its selection for actual TES applications. In this context, to evaluate this property of the Kb-BCPCMs, their FT-IR spectra taken after and before 1000 thermal cycling were compared. As evidently seen from Fig. 6, the spectra of the fabricated Kb-CPCMs had no new band and all of the characteristic bands were observed again without no occurring any change in their shapes and wavenumbers after the cycling test. Additionally, these results are apparent pointer of good chemical stability of the fabricated Kb-CPCMs.
−24.89 −26.73 −29.31
also found to be 200 ◦ C, 420 ◦ C and 450 ◦ C, respectively. On the other hand, the weight loss fractions during the thermal degradation stages are almost same with the incorporation ratios of the CA, PEG600 and HD. The sublimit values of the degradation temperatures of the Kb-CPCMs are fairly over their working temperatures. All results indicated that the fabricated composite PCMs have high resistant against to thermal degradation and thus good thermal stability for solar passive TES goals in buildings. 3.6. Thermal conductivity improvement of the fabricated Kb-CPCMs by EG addition
3.5. Thermal stability of the fabricated Kb-CPCMs Thermal stability is one of essential criteria to be taken into account for choosing PCM. So, a newly fabricated PCM should be thermally durable when heated over its operating temperature. In the present study, this property of the fabricated three kinds of Kb-CPCMs was determined by evaluating the temperatures limits regarding their thermal degradation behavior. In this sense, TG analysis was used to determine the thermal durability of the prepared composite PCMs. The pure CA, PEG600 and HD degraded thermally at the temperature limit range of 90–170 ◦ C [73], 230–425 ◦ C [46] and 140–255 ◦ C [74], respectively. Moreover, as apparently seen from the TG curves shown in Fig. 7, the fabricated Kb-CPCMs have two degradation stages. The temperature sublimit value regarding about 3 wt%-weight loss was carried out at 145 ◦ C for K/CA composite, 300 ◦ C for K/PEG600 composite and 135 ◦ C for K/HD composite. The temperature upper limits of the weight losses corresponds to about 17.2, 20.6 ad 16.3 wt% were
Fig. 5. DSC curves of the fabricated Kb-CPCMs after thermal cycling.
Thermal conductivity of a PCM is one of the parameters which noticeably influence the heat transfer rates during heat charging/discharging periods of the LHS systems. Thus, low thermal
Fig. 6. FT-IR spectra of the fabricated Kb-CPCMs after thermal cycling.
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Fig. 7. TG curves of the fabricated Kb-CPCMs.
conductivity can be considered is as essential disadvantage for a PCM. Expanded graphite (EG) has been preferred as thermal conductivity promoter for PCMs because of its several advantageous properties such as relatively high thermal conductivity, light-weightiness, good physicochemical compatibility, and high resistance ability against oxidation, corrosion and radiation, etc. [46,75,76]. Therefore, in this study, the thermal conductivities of the fabricated Kb-CPCMs were improved with EG additive in mass fraction of 5 wt%. The thermal conductivities of K/CA/EG, K/PEG600/EG and K/HD/EG composite PCMs were measured as 0.23, 0.27 and 0.29 W/m K, respectively, while it was measured to be 0.17, 0.18 and 0.20 W/m K, respectively, for the composite PCMs without EG additive. These results mean that the addition of EG in the amount of 5 wt% caused an increase in the thermal conductivities of the fabricated composite PCMs as much as 35%, 50% and 45%, respectively.
On the other hand, the improvement occurred in their thermal conductivities of the fabricated Kb-CPCMs was also checked by comparing their melting times measured before and after EG adding. Figs. 8–10 show the effect of EG additive on the melting times of the fabricated composite PCMs. From the temperature–time curves, the melting time for each of the KbCPCMs was taken account of time duration to be passed from the same initial temperature to the upper limit of melting temperature. The upper limit value was determined as 34 ◦ C, 17 ◦ C and 24 ◦ C for K/CA, K/PEG600 and K/HD composites, respectively, according to the ending temperature value of the DSC melting peak. From Figs. 8–10, the melting time was established as 78, 84 and 124 s for K/CA/EG, K/PEG600/EG and K/HD/EG composites, respectively, as it was determined to be 98, 100 and 174 s, respectively, for the composite samples without EG additive. These results mean that the melting times of the Kb-CPCMs were reduced as about 20%, 16% and 29%, respectively, due to the accelerating effect of the
Fig. 8. Time–temperature data recorded during the heating period of the fabricated K/CA composite PCM.
Fig. 9. Time–temperature data recorded during the heating period of the fabricated K/PEG600 composite PCM.
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References
Fig. 10. Time–temperature data recorded during the heating period of the fabricated K/HD composite PCM.
improved thermal conductivity on the heat transfer occurred in the composites during their heating periods. 4. Conclusions In the present study, three kinds of kaolin-based composite PCMs, K/CA, K/PEG600 and K/HD, were fabricated as new energy storing/releasing building materials through vacuum incorporation method. The following conclusions can be deduced based on all results: (1) According to the leakage test, the maximum weight fraction of CA, PEG600 and HD PCM into the kaolin was established as 17.5, 21 and 16.5 wt%, respectively. The chemical and morphological results obtained by the SEM and FT-IR analyses proved the existence of good compatibility among the components of the prepared composites. (2) The measured LHS properties of the fabricated Kb-CPCMs using DSC analysis make them promising energy storing/releasing materials for passive solar HVAC applications in building envelopes. (3) The DSC and FT-IR results obtained after the thermal cycling test demonstrated that the fabricated Kb-CPCMs have good thermal reliability and chemical stability. (4) TG analysis confirmed that the fabricated composite PCMs had good thermal stability their high resistant against thermal degradation. (5) The EG addition in the mass fraction of 5 wt% caused an increase as much as 35%, 50% and 45%, respectively, in the thermal conductivities of the fabricated K/CA, K/PEG600 and K/HD composite PCMs, respectively. The improvement in the thermal conductivity of the composite PCMs after the EG addition were proved with the reduction taken place in their melting times. (6) Based on all results, it can also be concluded that the fabricated K/CA, K/PEG600 and K/HD composites can be evaluated as potential composite PCMs to diminish the energy requirement for HVAC intention in buildings by adjusting indoor temperature to the human thermal comfort temperature range. However, advanced investigations are needed still to establish the practice TES performances of the wallboards or plasters fabricated using these Kb-CPCMs. Acknowledgements The author would like to thank Scientific Research Project Committee of Gaziosmanpas¸a University and also thank Cahit Bilgin and Alper Bicer for their helps in some parts of the experiments.
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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Fabrication and thermal characterization of kaolin-based composite phase change materials for latent heat storage in buildings Ahmet Sarı ∗ Gaziosmanpas¸a University, Department of Chemistry, 60240 Tokat, Turkey
a r t i c l e
i n f o
Article history: Received 16 January 2015 Received in revised form 10 March 2015 Accepted 11 March 2015 Available online 18 March 2015 Keywords: CPCM Capric acid PEG600 Heptadecane Kaolin Thermal energy storage Building Thermal reliability Thermal durability Thermal conductivity
a b s t r a c t Three kinds of kaolin-based composite phase change materials (Kb-CPCMs) including capric acid (CA), PEG600, and heptadecane (HD) as organic PCMs were fabricated using vacuum impregnation method for latent heat storage (LHS) application in buildings. The surface morphology, compatibility, maximum ratio for impregnated PCM, LHS properties, thermal endurance, thermal conductivity and its effect on the melting times of prepared Kb-CPCMs were investigated by using microscopy, spectroscopy, calorimetry and thermal methods. The seepage test indicated that CA, PEG600 and HD were impregnated maximally into the kaolin as 17.5, 21 and 16.5 wt%, respectively. The fabricated three composites, K/CA, K/PEG600, and K/HD, have a phase change temperature of 30.71, 5.16 and 22.08 ◦ C and a latent heat of 27.23, 32.80 and 34.63 J/g, respectively. The thermal cycling test exposed that the thermal reliability of the Kb-CPCMs slightly changed after repeated 1000 heating-cooling cycling. The heat storage rates of the Kb-CPCMs were increased considerably by adding expanded graphite (EG) in mass faction of 5%. All the prepared Kb-CPCMs have good thermal energy storage (TES) function for heating, ventilating and air conditioning (HVAC) in building envelopes because of their suitable LHS properties, high reusability performance and enhanced thermal conductivity. © 2015 Elsevier B.V. All rights reserved.
1. Introduction The primary energy consumers in the world are building and industrial sectors and about one-third of total energy consumption is originated from these sectors [1,2]. Therefore, enhancing the energy efficiency of buildings diminishes the fossil sourcedenergy consumption and the hazardous gas emissions. In this sense, thermal energy storage (TES) has an important function in the improvement of the energy efficiency of building envelopes and reduces the environmental damages caused by the fossil-based energy use. [3,4]. Amongst various TES techniques, latent heat storage (LHS) using phase change material (PCM) has drawn substantial interest since it makes possible to storage and release high energy at narrow servicing temperature range [5–7]. This method has been extensively used for heating, ventilating, and air conditioning (HVAC) purposes in buildings to achieve thermal comfort and acceptable indoor air quality by flattening the fluctuations at indoor temperature [8,9]. The organic solid–liquid PCMs have high latent heat of fusion and suitable phase change temperature for passive solar heating and cooling applications. They are generally non-corrosive,
∗ Tel.: +90 356 2521616; fax: +90 356 2521585. E-mail address: [email protected] http://dx.doi.org/10.1016/j.enbuild.2015.03.022 0378-7788/© 2015 Elsevier B.V. All rights reserved.
non-toxic, chemically stable and do not show no or a little supercooling (except PCMs in PEG group) behavior [10–14]. However, the low thermal conductivity (0.15–0.20 W/mK) can cause disinhibitory effect on the rate of thermal energy storage and release during the heating and cooling periods of PCMs. To resolve this problem, some high conductive materials such as expanded graphite [15,16] and ˇ-aluminum nitride [17] have been introduced into the composite PCMs. On the other hand, to get rid of the leakage problem encountered during the solid–liquid phase change processes of organic PCMs, many researchers have strived about the encapsulation of PCMs using the different methods such as confining into a polymeric network, grafting with polymeric materials and impregnating with porous building materials. Among these methods, the combination of porous materials with PCM is simple, cost-effective and thus relatively proficient way to fabricate form-stable lightweight constructive materials with high LHS capacity [18–20]. In addition to gypsum [21–25] and cement [26–31], natural clays as porous building materials, such as montmorillenite [32–35], perlite [36–39], attapulgite [40,41], vermiculite [42–44], bentonite [45–47], silica-fume [48,49] and diatomite [50–56] have been evaluated in the incorporation of several organic PCMs. Moreover, these materials have been used to fabricate PCM wallboards, plasters, concrete blocks, hollow bricks and wall covering materials for passive solar buildings [57–63].
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As well as the above-mentioned clay materials, kaolin is one of the most common industrial clays and has a chemical composition of Al2 Si2 O5 (OH)4 . Kaolin is a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedral [64]. It is generally used in ceramics, toothpaste, light bulbs, cosmetics, water and wastewater treatment, etc. Especially, metakaolin is a supplementary cementitious material (SCM) acting as pozzolan. Additionally, some advantageous properties of this clay such as low cost (e.g., its wholesale price is about 0.25 $/kg in Turkish markets), large surface area, porous structure, abundantly availability, easy handling, direct usability without extra encapsulation and good compatibility with mortar and concrete make it a potential matrix for the fabrication of building composites with organic PCMs. Therefore, successful integration of kaolin with organic PCMs for the TES purposes in buildings can expand its industrial usage areas. However, the number of the studies about the usability of the kaolin as supporting matrix to encapsulate organic PCMs is limited with only a few [65–68]. So, many works to enlarge the varieties of novel composite PCMs with high LHS efficiency are still needed. On the other hand, capric acid (CA), polyethylene glycol 600 (PEG600) and heptadecane (HD) are fitting organic PCMs for the incorporation with kaolin clay because they have suitable phase change temperature range (10–31 ◦ C) and high latent heat capacity in the range of 136–216 J/g for passive solar TES applications in building envelopes. As far as the authors are aware, there is no investigation reported in the literature on the fabrication, chemical-morphological characterization and determination of LHS properties of newly developed K/CA, K/PEG600, and K/HD composites. The morphological and chemical characterization of fabricated three types of kaolin based-building composite PCMs (Kb-CPCMs) were made using scanning electronic microscope (SEM) and Fourier transformation infrared spectroscope (FTIR) techniques. The LHS properties and thermal reliability of the Kb-CPCMs were analyzed by differential scanning calorimetry (DSC). The thermal degradation temperatures of the Kb-CPCMs were measured using thermogravimetric analysis (TGA) method. Furthermore, the thermal conductivities of the Kb-CPCMs were increased by addition of expanded graphite (EG) and the obtained results were confirmed by comparing the melting times of the samples with and without EG additive. 2. Materials and methods 2.1. Materials Kaolin clay used in this work is originated with of Balıkesir city of Turkey. It was kept at 105 ◦ C in an oven to remove its humidity and
then sieved form 200 mesh. The dried sample is mainly consisted of SiO2 (48 wt%), Al2 O3 (36.6 wt%), K2 O (2 wt%) and other metal oxides [69,70]. The selected organic PCMs, capric acid (CA), PEG600 and heptadecanol (HD) were supplied from Sigma–Aldrich company. 2.2. Methods K/CA, K/PEG600, and K/HD composites were fabricated using vacuum impregnation method and the same vacuum conditions reported in our previous studies [71,72] were used here. By following that procedure, the dried kaolin sample in specified weight amount were incorporated with CA, PEG600 and HD, separately in mass fractions varied from 10% to 30%, respectively. The highest mass portion of the PCM retained by kaolin was determined by performing the leakage test. With this aim, each composite sample was subjected to a heating process above the melting temperature of the confined PCM and then cooling process to room temperature. The test results exposed that the maximum fraction of CA, PEG600 and HD in the composite samples, which did not show any PCM loss was determined as 17.5, 21 and 16.5 wt%, respectively. The photograph images of kaolin clay, the selected organic PCMs and fabricated Kb-CPCMs are shown in Fig. 1. The morphologies of kaolin and the produced three kinds of Kb-CPCMs were investigated by using a SEM instrument (LEO 440 model). The compatibility among the components of the composites was characterized chemically using a FT-IR spectrophotometer (JASCO 430 model). The analysis procedures performed for the SEM and FT-IR analysis were the same as in our previous studies [42,43,46]. The LHS properties of the prepared Kb-CPCMs were measured under atmospheric nitrogen and at a heating rate of 5 ◦ C min−1 using by a Perkin Elmer JADE model DSC instrument. For each sample, the DSC measurement was repeated for three times and the mean deviations in the phase change temperature and latent heat values were calculated as ±0.14 ◦ C and ±1.24 J/g, respectively. Thermal durability of the Kb-CPCMs was determined by comparing the measured thermal degradation temperatures of pure organic PCMs and the fabricated Kb-CPCMs using a TGA analyzer (Perkin–Elmer TGA7 model). The analyses were applied at heating rate of 10 ◦ C min−1 between 50 ◦ C and 500 ◦ C. The thermal reliability of the Kb-CPCMs was evaluated in terms of probable changes to be occurred in their LHS properties after the following heating-cooling cycles repeated for 1000 times. The cycling process was realized by using a thermal cycler instrument (BIOER TC-25/H model). In order to reach a decision about the thermal reliability and chemical stability of Kb-CPCMs, their DSC and FT-IR data obtained after and before the cycling process were compared. Furthermore, the thermal conductivities of the prepared
Fig. 1. Photograph images of pure kaolin, pure CA, pure PEG600, pure HD and the fabricated K/CA, K/PEG600 and K/HD composite PCMs.
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Fig. 2. SEM images of (a) kaolin, (b) K/CA, (c) K/PEG600 and (d) K/HD.
Kb-CPCMs were improved by addition of 5 wt% EG with high thermal conductivity (4.26 W/m K). The measurements were carried out using a thermal property analyzer (Decagon KD2 model). The analyzer was calibrated by using a verification standard (glycerin). For the measurements, a 25 mL-test tube was filled the sample and the needle sensor of the analyzer was placed tightly into the sample. The measurements were repeated three times at room temperature and the results were recorded as average values. The accuracy of the measurements was calculated as ±3%. The enhancement in the thermal conductivity of the Kb-CPCMs was confirmed by comparing the melting times of the composite samples with and without EG additive. The temperature data against time during the melting periods of Kb-CPCMs were recorded by using a data logger (NOVA5000 model).
3. Results and discussion 3.1. Morphological characterization of the fabricated Kb-CPCMs Fig. 2a–d shows the SEM photographs of the kaolin and the prepared K/CA, K/PEG600, and K/HD composites. As can be evidently observed from Fig. 2a, the surface of the kaolin clay is consisted of the particles with indiscriminate shape and different sizes and the layers formed by these particles are dispersed randomly through the surface. The holes and hiatuses among the layers facilitate the sorption of the organic PCM molecules. As also clearly seen from Fig. 2b–d, the micro holes or channels between the particles wholly were occupied with CA, PEG600 and HD as organic PCMs. Moreover, the surface tension and capillary forces caused good physically compatibility between the clay matrix and the organic PCMs [46,51,53,65–68], which was resulted in forming shape-stabilized Kb-CPCMs with excellent resistant against the leakage problem regarding with their PCM content.
3.2. Chemical characterization of the fabricated Kb-CPCMs In order to investigate the chemical compatibility between the kaolin and the organic PCMs, The FT-IR spectra of Kb-CPCMs were taken and compared with that of the pure components. The CH2 characteristic bands were observed in the range of 2921–3000 cm−1 for pure CA [73] and 2865–2908 cm−1 for pure PEG600 [46] and 2862–2956 cm−1 for pure HD [74]. The absorption wavenumber values regarding C O and C O groups of pure CA were reported as1735 and 1100 cm−1 , respectively. Moreover, the O H characteristic band was determined in the range of 3174–3633 cm−1 for PEG600 [46] and 3400–3700 cm−1 for CA [73]. In another study, the FT-IR analysis of the kaolin used here indicated that the absorption band in the range of 3478–3552 cm−1 was attributed to the O H stretching vibration of the silanol (SiOH) groups and the bands at 1070 and 584 cm−1 represent the asymmetric stretching and bending vibrations of Si O Si groups, respectively. On the other hand, Fig. 3 shows the FT-IR spectra of the fabricated Kb-CPCMs containing maximum amount of the organic PCM. Compared these results with that mentioned above, it is possible to observe all characteristic bands of the pure components in the spectra of the Kb-CPCMs. After incorporation of the organic PCMs with the kaolin, the absence of any new peak in the spectrum means that any chemical reaction was not occurred between the PCM molecules and the inorganic components of the kaolin. The symmetrical stretching bands of CH2 groups was observed below 3000 cm−1 in the spectrum of all composites. The absorption bands of C O and C O groups were detected at 1710 and 1108 cm−1 for the K/CA composite, respectively. The O H bands of K/CA and K/PEG600 composites were monitored in the range of 3162–3702 cm−1 and 3080–3716 cm−1 , respectively. Additionally, the asymmetric stretching and bending vibrations of Si O Si groups were detected at 1139 and 536 cm−1 for K/CA, 1103 and 538 cm−1 for K/PEG600, at 1137 and 534 cm−1 for K/HD composite,
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Fig. 3. FT-IR spectra of the fabricated Kb-CPCMs.
respectively. The small shifts occurred in the wavenumber values of the characteristic bands of the composites relatively to their pure components could be due to capillary forces or surface tension forces between the confined PCM molecules and the surface functional groups or pore walls of the kaolin [65–68]. All the FT-IR results also proved the presence of good compatibility between the ingredients of the fabricated Kb-CPCMs. 3.3. LHS properties of the fabricated Kb-CPCMs The LHS properties of a building composite PCM such as melting temperature and latent heat capacity are decisive key parameters in terms of its usage in building envelopes to reach the comfort temperature level. Thus, an organic PCM preferred in the production of the composite PCM should have an appropriate melting temperature and sufficiently high enthalpy for this goal. The solid–liquid phase change temperature and enthalpy of the selected organic PCMs were measured as 31.04 ◦ C and 190.21 J/g for pure CA [73], 10.0 ◦ C and 136.42 J/g for pure PEG600 [46] and 21.36 ◦ C (and also solid–solid phase change at 10.87 ◦ C) and 216.21 J/g for pure HD [74]. As can be seen from these DSC data, the LHS properties of the PCMs are rather convenient materials for the fabrication of Kb-CPCMs, which will be used for low temperature-LHS implementations in buildings. On the other hand, as can be clearly seen from the DSC thermograms in Fig. 4, the three kinds of Kb-CPCMs have steady
Fig. 4. DSC curves of the fabricated Kb-CPCMs.
melting-freezing phase change behavior like pure PEG600, CA and HD. The LHS properties obtained from the DSC heating and cooling peaks were also presented in Table 1. From these data, the fabricated K/CA, K/PEG600 and K/HD composites including maximum amount of PCM melt at 30.71 ◦ C, 5.16 ◦ C and 22.08 ◦ C and storage a latent heat as 27.23, 32.80 and 34.63 J/g, respectively, while they freeze at 28.21 ◦ C, 15.44 ◦ C and 21.53 ◦ C and release a latent heat as −25.51, −29.78 and −32.42 J/g, respectively. The melting and freezing temperatures of the prepared Kb-CPCMs make them hopeful composite PCMs with energy storing function for solar passive heating and cooling intentions in buildings with respect to the different climate conditions. Moreover, after the impregnation process, the change occurred in the melting and freezing temperatures of the Kb-CPCMs compared to the pure PCMs could be because of strong physical attractions between the characteristics silanol groups of the kaolin and the functional groups of the PCMs, which are proved by the FT-IR analysis. The similar interpretations were given for some building composites including different PCMs [51,53,65–68]. Additionally, the theoretical PCM fractions (17, 20 and 16 wt% for K/CA, K/PEG600 and K/HD, respectively) in the composites calculated by dividing their latent heat values to that of pure PCMs are very close to their experimental incorporation fractions (17.5, 21 and 16.5 wt%, respectively) determined after the leakage test. However, the slight difference between the results could be due to the restriction of phase change behavior of the PCMs retained into to the porous network of the kaolin [51]. By comparing the latent heat capacity of the fabricated three kinds of Kb-CPCMs with other type building composite PCMs in literature [22,24,28,35,40,41,45,46,49,51,60–63], it is deduced that the latent heat capacities of the composites were changed according to the incorporation ability that are dependent on the chemical and physical characteristics of building material and of PCM. In other words, the abundance and variety of functional groups interacted with PCM, and porosity of the building material can affect the incorporation ratio of the PCM while the latent heat capacity of confined PCM directly influences the LHS capacity of the prepared composite PCM. Moreover, by taking into consideration the LHS capacity of the fabricated K/CA, K/PEG600 and K/HD composite PCMs, it can be drawn a conclusion that they have reasonable LHS capacity for passive solar HVAC applications in buildings. 3.4. Thermal reliability and chemical stability of the fabricated Kb-CPCMs A PCM to be considered for TES practices in buildings should have a good energy storage life even it subjected to an extended number of thermal cycling. Therefore, prior knowledge of the thermal reliability property of newly developed composite PCM is important in term of its handling in real long-term TES applications. For that purpose, the LHS properties of prepared three kinds of the Kb-CPCMs were measured again by DSC analysis after the thermal cycling test corresponding to 1000 melting-freezing processes. Fig. 5 and Table 2 show the DSC thermograms and data obtained after the cycling test. It is possible to see the same peaks on the thermograms like before the cycling process, meaning that the prepared Kb-CPCMs have stable melting and freezing behavior even after 1000 cycles. The changes in the phase change temperatures of the prepared K/CA, K/PEG600 and K/HD composites were calculated as −0.27 ◦ C, 3.45 ◦ C and −0.14 ◦ C for melting period and −0.5 ◦ C, −1.56 ◦ C and 0.01 ◦ C for freezing period, respectively. Moreover, the decrease occurred in their latent heat values were determined as 9.8%, 11.4% and 12.4% for melting period and 2.4%, 10.2% and 9.6% for freezing period, respectively. These results showed that the fabricated three kinds of Kb-CPCMs had good thermal reliability for practical TES applications because of the low deviation occurred in their LHS properties.
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Table 1 The LHS properties of the fabricated Kb-CPCMs before thermal cycling process. K-BCPCM
Solid–solid phase change temperature (◦ C) during heating period
Melting temperature (◦ C)
Latent heat of melting (J/g)
Solid–solid phase change temperature (◦ C) during cooling period
Freezing temperature (◦ C)
Latent heat of freezing (J/g)
K/CA (17.5 wt%) K/PEG600 (20 wt%) K/HD (16.5 wt%)
– – 11.06
30.71 5.16 22.08
27.23 32.80 34.63
– – 8.02
28.21 15.44 21.53
−25.51 −29.78 −32.42
Latent heat of freezing (J/g)
Table 2 The LHS properties of the fabricated Kb-CPCMs after thermal cycling process. K-BCPCM
Solid–solid phase change temperature (◦ C) during heating period
Melting temperature (◦ C)
Latent heat of melting (J/g)
Solid–solid phase change temperature (◦ C) during cooling period
Freezing temperature (◦ C)
K/CA (17.5 wt%) K/PEG600 (20 wt%) K/HD (16.5 wt%)
– – 10.88
30.44 8.61 21.94
24.56 29.05 30.32
– – 7.13
27.71 13.88 21.54
On the other hand, the chemical stability of PCM after a great number of thermal cycling is one of the key parameters to be considered in its selection for actual TES applications. In this context, to evaluate this property of the Kb-BCPCMs, their FT-IR spectra taken after and before 1000 thermal cycling were compared. As evidently seen from Fig. 6, the spectra of the fabricated Kb-CPCMs had no new band and all of the characteristic bands were observed again without no occurring any change in their shapes and wavenumbers after the cycling test. Additionally, these results are apparent pointer of good chemical stability of the fabricated Kb-CPCMs.
−24.89 −26.73 −29.31
also found to be 200 ◦ C, 420 ◦ C and 450 ◦ C, respectively. On the other hand, the weight loss fractions during the thermal degradation stages are almost same with the incorporation ratios of the CA, PEG600 and HD. The sublimit values of the degradation temperatures of the Kb-CPCMs are fairly over their working temperatures. All results indicated that the fabricated composite PCMs have high resistant against to thermal degradation and thus good thermal stability for solar passive TES goals in buildings. 3.6. Thermal conductivity improvement of the fabricated Kb-CPCMs by EG addition
3.5. Thermal stability of the fabricated Kb-CPCMs Thermal stability is one of essential criteria to be taken into account for choosing PCM. So, a newly fabricated PCM should be thermally durable when heated over its operating temperature. In the present study, this property of the fabricated three kinds of Kb-CPCMs was determined by evaluating the temperatures limits regarding their thermal degradation behavior. In this sense, TG analysis was used to determine the thermal durability of the prepared composite PCMs. The pure CA, PEG600 and HD degraded thermally at the temperature limit range of 90–170 ◦ C [73], 230–425 ◦ C [46] and 140–255 ◦ C [74], respectively. Moreover, as apparently seen from the TG curves shown in Fig. 7, the fabricated Kb-CPCMs have two degradation stages. The temperature sublimit value regarding about 3 wt%-weight loss was carried out at 145 ◦ C for K/CA composite, 300 ◦ C for K/PEG600 composite and 135 ◦ C for K/HD composite. The temperature upper limits of the weight losses corresponds to about 17.2, 20.6 ad 16.3 wt% were
Fig. 5. DSC curves of the fabricated Kb-CPCMs after thermal cycling.
Thermal conductivity of a PCM is one of the parameters which noticeably influence the heat transfer rates during heat charging/discharging periods of the LHS systems. Thus, low thermal
Fig. 6. FT-IR spectra of the fabricated Kb-CPCMs after thermal cycling.
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Fig. 7. TG curves of the fabricated Kb-CPCMs.
conductivity can be considered is as essential disadvantage for a PCM. Expanded graphite (EG) has been preferred as thermal conductivity promoter for PCMs because of its several advantageous properties such as relatively high thermal conductivity, light-weightiness, good physicochemical compatibility, and high resistance ability against oxidation, corrosion and radiation, etc. [46,75,76]. Therefore, in this study, the thermal conductivities of the fabricated Kb-CPCMs were improved with EG additive in mass fraction of 5 wt%. The thermal conductivities of K/CA/EG, K/PEG600/EG and K/HD/EG composite PCMs were measured as 0.23, 0.27 and 0.29 W/m K, respectively, while it was measured to be 0.17, 0.18 and 0.20 W/m K, respectively, for the composite PCMs without EG additive. These results mean that the addition of EG in the amount of 5 wt% caused an increase in the thermal conductivities of the fabricated composite PCMs as much as 35%, 50% and 45%, respectively.
On the other hand, the improvement occurred in their thermal conductivities of the fabricated Kb-CPCMs was also checked by comparing their melting times measured before and after EG adding. Figs. 8–10 show the effect of EG additive on the melting times of the fabricated composite PCMs. From the temperature–time curves, the melting time for each of the KbCPCMs was taken account of time duration to be passed from the same initial temperature to the upper limit of melting temperature. The upper limit value was determined as 34 ◦ C, 17 ◦ C and 24 ◦ C for K/CA, K/PEG600 and K/HD composites, respectively, according to the ending temperature value of the DSC melting peak. From Figs. 8–10, the melting time was established as 78, 84 and 124 s for K/CA/EG, K/PEG600/EG and K/HD/EG composites, respectively, as it was determined to be 98, 100 and 174 s, respectively, for the composite samples without EG additive. These results mean that the melting times of the Kb-CPCMs were reduced as about 20%, 16% and 29%, respectively, due to the accelerating effect of the
Fig. 8. Time–temperature data recorded during the heating period of the fabricated K/CA composite PCM.
Fig. 9. Time–temperature data recorded during the heating period of the fabricated K/PEG600 composite PCM.
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References
Fig. 10. Time–temperature data recorded during the heating period of the fabricated K/HD composite PCM.
improved thermal conductivity on the heat transfer occurred in the composites during their heating periods. 4. Conclusions In the present study, three kinds of kaolin-based composite PCMs, K/CA, K/PEG600 and K/HD, were fabricated as new energy storing/releasing building materials through vacuum incorporation method. The following conclusions can be deduced based on all results: (1) According to the leakage test, the maximum weight fraction of CA, PEG600 and HD PCM into the kaolin was established as 17.5, 21 and 16.5 wt%, respectively. The chemical and morphological results obtained by the SEM and FT-IR analyses proved the existence of good compatibility among the components of the prepared composites. (2) The measured LHS properties of the fabricated Kb-CPCMs using DSC analysis make them promising energy storing/releasing materials for passive solar HVAC applications in building envelopes. (3) The DSC and FT-IR results obtained after the thermal cycling test demonstrated that the fabricated Kb-CPCMs have good thermal reliability and chemical stability. (4) TG analysis confirmed that the fabricated composite PCMs had good thermal stability their high resistant against thermal degradation. (5) The EG addition in the mass fraction of 5 wt% caused an increase as much as 35%, 50% and 45%, respectively, in the thermal conductivities of the fabricated K/CA, K/PEG600 and K/HD composite PCMs, respectively. The improvement in the thermal conductivity of the composite PCMs after the EG addition were proved with the reduction taken place in their melting times. (6) Based on all results, it can also be concluded that the fabricated K/CA, K/PEG600 and K/HD composites can be evaluated as potential composite PCMs to diminish the energy requirement for HVAC intention in buildings by adjusting indoor temperature to the human thermal comfort temperature range. However, advanced investigations are needed still to establish the practice TES performances of the wallboards or plasters fabricated using these Kb-CPCMs. Acknowledgements The author would like to thank Scientific Research Project Committee of Gaziosmanpas¸a University and also thank Cahit Bilgin and Alper Bicer for their helps in some parts of the experiments.
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