Energy & Buildings 164 (2018) 166–175
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Diatomite/CNTs/PEG composite PCMs with shape-stabilized and improved thermal conductivity: Preparation and thermal energy storage properties Ahmet Sarı a,b,∗, Alper Bicer c,∗∗, F.A. Al-Sulaiman b, Ali Karaipekli d, V.V. Tyagi e a
Department of Metallurgical and Material Engineering, Karadeniz Technical University, 61080 Trabzon, Turkey Center of Research Excellence in Renewable Energy (CORERE), Research Institute, King Fahd University of Petroleum & Minerals (KFUPM) 31261 Kingdom of Saudi Arabia c Department of Chemistry, Gaziosmanpas¸ a University, 60240 Tokat, Turkey d Department of Chemistry, Karatekin University, 18100 Çankırı, Turkey e Department of Energy Management, Shri Mata Vaishno Devi University, Katra 182320, J&K, India b
a r t i c l e
i n f o
Article history: Received 9 July 2017 Revised 6 December 2017 Accepted 9 January 2018
Keywords: Shape-stabilized composite PCM Diatomite PEG CNTs Thermal energy storage Thermal conductivity
a b s t r a c t Polyethylene glycol (PEG) is one the most promising organic phase change materials (PCMs) due to good latent heat thermal energy storage (LHTES) characteristics. However, leakage issue in melting state and low thermal conductivity restrict its further real applications. In order to eliminate these disadvantages as well as increasing its incorporation ratio, polyethylene glycol (PEG600) was impregnated with raw diatomite (RD)/carbon nanotubes(CNTs) pre-composites. Without exhibiting melt leakage, PEG was successfully conﬁned as 42.8, 44.5 and 51.7 wt% in the novel shape-stabilized composite phase change materials (S-SCPCMs) including 0.57, 1.70 and 2.50 wt% CNTs while it was absorbed by RD as 41.0 wt%. The chemical and morphological characterizations of the produced S-SCPCMs were made by FT-IR and SEM techniques. The DSC analysis showed that the S-SCPCMs had melting temperatures in the range of about 7–8°C and latent heat capacity between 53.8 and 62.9 J/g. Moreover, compared to the RD/PEG composite, the thermal conductivities of RD/CNTs/PEG composites were enhanced between 73% and 93% as well as the latent heat capacity of them was increased in the range of 5–31%. The melting times and total heating times of the S-SCPCMs were drastically shortened depending on the improvement in thermal conductivity of them. Thermal cycling test and TGA results demonstrated that the S-SCPCMs had commendable long-term chemical stability, LHTES reliability and thermal durability. Consequently, the loading of CNTs to RD/PEG composite provided beneﬁcial outcomes such as increasing impregnation ratio with no liquid leakage, reducing heat storing/releasing periods depending on enhanced thermal conductivity, without damaging chemical stability and thermal durability. In view of these advantageous properties, the S-SCPCMs can be incorporated with ordinary structural elements to generate various building sections with solar energy harvesting/releasing capability. Such combinations can be also evaluated for passive solar cooling purposes in radiant ﬂoor heating systems, insulation and ceiling panels or walls depending on the climatic circumstances. © 2018 Elsevier B.V. All rights reserved.
1. Introduction The energy consumption of buildings has been increasing considerably depending on the population growth and requirements of indoor thermal comfort . Thermal energy can be stored or released for heating and cooling of a medium or used at a later time
Corresponding author. Corresponding author. E-mail addresses: ahmet.[email protected]
(A. Sarı), [email protected]
(A. Bicer). ∗∗
https://doi.org/10.1016/j.enbuild.2018.01.009 0378-7788/© 2018 Elsevier B.V. All rights reserved.
for power generation and solar heating, ventilating and air conditioning (HVAC) purposes in buildings and industrial processes. Thermal energy storage (TES) is one of the most operational means for passive solar building conception, especially in terms of increasing energy eﬃciency of HVAC systems . Compared to sensible and thermo-chemical TES techniques, latent heat thermal energy storage (LHTES) by means of phase change material (PCM) has been more frequently used in recent years because of its facility to storage/release high amount of latent heat form at an almost constant temperature [3–5]. The practicality of LHTES systems has been tested extensively for passive solar HVAC targets [6–11].
A. Sarı et al. / Energy & Buildings 164 (2018) 166–175 Table 1 Some physicochemical properties of RD, PEG, acetone and CNTs.
Nomenclature LHTES TES HVAC FT-IR SEM TGA DSC RD CNTs MWCNTs SWCNTs Al2 O3 CMPs Cu GO GNP PEG Silicon dioxide EG PCM S-SCPCM S-SCPCM-1 S-SCPCM-2 S-SCPCM-3 S-SCPCM-4
latent heat thermal energy storage; thermal energy storage; heating, ventilating and air conditioning; fourier Transform Infrared; scanning Electron Microscopy; thermogravimetry Analysis; differential Scanning Calorimetry; raw diatomite; carbon nanotubes; multi-walled carbon nanotubes; single-walled carbon nanotubes; Aluminum oxide; carbon microspheres; copper; graphene oxide; graphene nano pellet; polyethylene glycol; SiO2 expanded graphite; phase change material; shape-stabilized composite PCM; PEG: 41.0 wt%; CNTs: 0.00 wt%; PEG: 42.8 wt%; CNTs: 0.57 wt%; PEG: 44.5 wt%; CNTs: 1.70 wt% PEG: 50.7 wt%; CNTs: 2.50 wt%
Polyethylene glycols (PEGs) as promising organic PCMs are placed in wide phase change temperature range of −55 °C-100 °C and have considerable high latent heat of fusion [12–15]. However, low thermal conductivity and packing requirement due to leakage problem during melting phase change are main disadvantages for this PCM group. These drawbacks have lessening effect on the HVAC performance of TES systems, which resulted in restriction of their usage potential. Several studies have been aimed to solve the low thermal conductivity problem of PEGs or PEG-containing composites using different additives such as β -aluminum nitride , expanded graphite(EG) [17–19], situ Cu doping , aluminum oxide nano particles , carbon microspheres (CMPs) , graphitic carbon nitride  and hybrid graphene nano . On the other hands, carbon nanotubes (CNTs) have ultrahigh thermal conductivity (20 0 0–60 0 0 W/m.K), low density, high aspect ratios and extraordinary mechanical properties [25,26]. However, at current conditions, CNTs with high purity (>96%) are still too expensive compared to industrial-grade (>90% purity). CNTs are still one of the most inﬂuential doping agents for improvement of thermal conductivity of composite PCMs, although their usage as ﬁller markedly increases the cost of composite product. In this regard, the inﬂuences of the additive concentrations of CNTs on the TES properties of different kinds of PCMs have long been investigated in detail [27–30]. However, the number of the studies about PEG including-composite PCMs is still limited by a few. The MWCNTs/PEG/SiO2 has excellent form-stable character and good phase change performance . The thermal conductivity of formstable PEG/diatomite composite was enhanced considerably using single-walled carbon nanotubes (SWCNs) . On the other hand, the encapsulation requirement of PEGs can be solved by impregnation of them within lightweight supporting matrixes as shape-stabilized composite PCM (S-SCPCM) [33–35]. Such type a composite keeps solid form even the conﬁned PEG turns to liquid. In this sense, raw diatomite (RD) can be evaluated as proper supporting material for the imprisonment of the PEGs. It is formed from the siliceous fossilized skeletons
Raw diatomite (RD) SiO2 (wt%) Al2 O3 (wt%) Fe2 O3 (wt%) CaO (wt%) Humidity Density (g/cm3 )
92.0 4.2 1.5 0.6 1–5 0.225
Polyethylene glycol (PEG) MW (g/mol): Viscosity (mPa.s) Density (g/cm3 )
570–630 150–190 1.12
Carbon nanotubes (CNTs) Type Dimensions Assay (carbon %) Density (g/cm3 )
MWCNTs OD(nm):6–9; L(μm):5 ˃95 2.1
Acetone Boiling point (°C) Flash point (°C) Density (g/cm3 )
56.2 <−20 0.79
of diatoms and generally used as a building material, heat, cold, and sound insulator, ﬁller absorbent, abrasive, and ingredient in medicines [36,37]. Moreover, some advantageous properties of RD like relatively high porosity and speciﬁc surface area, good adsorption capability, low density, and cost-effectiveness make it a practicable applicant to create S-SCPCMs for TES objectives in buildings [38–41]. By considering the suitability of phase change temperature in terms of thermo-regulation of building envelopes, the PEG600 (PEG) was selected as PCM to build up novel kinds of SSCPCMs. In order to simultaneously improve thermal conductivity, eradicate the leakage issue and increase absorption fraction of PEG, the RD/CNTs pre-composites were prepared and used for the impregnation of the PEG. As understood from the literature survey, an extensive investigation on the development and TES properties of RD/CNTs/PEG composites as S-SCPCMs with enhanced thermal conductivity has not been carried out so far. The chemical and morphological analyses of the created S-SCPCMs were conducted by FTIR and SEM techniques. The effects of CNTs additive on the LHTES characteristics, cycling reliability, chemical stability and thermal durability of the S-SCPCMs were studied. Furthermore, the inﬂuence of improved thermal conductivity with CNTs addition on the melting times and total heating times of the S-SCPCMs was evaluated. 2. Materials and methods 2.1. Materials RD preferred as porous building material in this study was supplied from the company of BEG-TUG Industrial Minerals & Mines. It was dried before use at 105 °C for 24 h to remove its humidity. The PEG used as PCM, acetone used as dispersing solvent and CNTs used as thermal conductivity enhancer were supplied from Sigma Aldrich Company. Some physicochemical properties of all reagents were given in Table 1. 2.2. Preparation method The composite PCMs were prepared at three steps as shown in Fig. 1:(i), RD/CNTs pre-composites were prepared by dispersing
A. Sarı et al. / Energy & Buildings 164 (2018) 166–175
Fig. 1. The experimental procedures used for preparation of pre-composites and S-SCPCMs and performing leakage test.
CNTs in 100 mL of acetone solution. The weight fractions of CNTs within the RD were taken as 1.0, 3.0 and 5.0 wt%. The mixture was homogenized using a magnetic stirrer at 500 rpm for 15 min. Then, the acetone in the pre-composites was entirely evaporated at 40 °C for 48 h. (ii) In the second step, the RD/PEG and RD/CNTs/PEG composites were produced using vacuum impregnation method [42– 45]. For this process, the RD or RD/CNTs pre-composite at speciﬁed amount was put inside a ﬂask and vacuum pump was worked during 90 min at 65 kPa to facilitate the impregnation process. After that, the melted PEG at speciﬁed amount was transferred gradually to the ﬂask by means of a funnel. The obtained RD/PEG or RD/CNTs/PEG composites were cooled to 5 °C for the solidiﬁcation of PEG into the composites. (iii) At the third step, the impregnation process was repeated for different mass fractions of PEG as 30, 40, 50 and 60 wt% to reach the composite PCM with shapestabilized. Each of the composites was subjected to the leakage test. For this test, the composite sample was put on a ﬁlter paper and then heated at 20 °C for 60 min using a heating plate. If any melt leakage was not seen for tried composite, the impregna-
tion process was repeated for another composite including more mass fraction of PEG. The composite with no seepage despite the highest amount of PEG was characterized as shape-stabilized composite PCM (S-SCPCM). Additionally, in order to make sure no mass loss from the S-SCPCM during the leakage test, they were separately weighted before and after the test and the results were compared. The calculated mass amounts and fractions of the components in the fabricated S-SCPCM-1, S-SCPCM-2, S-SCPCM-3 and S-SCPCM-4 are given in Table 2. 2.3. Characterization techniques Microstructures of RD, CNTs, RD/CNTs (5 wt%) and S-SCPCM-4 were investigated by using SEM instrument with LEO 440 model (Japan). The instrument was operated at an accelerated voltage of 20 kV. The samples were coated with gold before the analysis. The chemical characterization of PEG, RD, CNTs, RD/CNTs and S-SCPCMs with/without CNTs were made by using the FT-IR technique (JASCO 430 model FT-IR instrument, USA) at wavenumber range of 400–
A. Sarı et al. / Energy & Buildings 164 (2018) 166–175
Table 2 Weight amounts and percentages of the components in the pre-composites and S-SCPCMs. S-SCPCM
Diatomite amount (g)
CNTs amount (g)
CNTs ratio in pre-composite (wt%)
PEG amount (g)
PEG ratio in composite (wt%)
CNTs ratio in composite (wt%)
S-SCPCM-1 S-SCPCM-2 S-SCPCM-3 S-SCPCM-4
10.0 9.9 9.7 9.5
0.00 0.1 0.3 0.5
0.0 1.0 3.0 5.0
6.95 7.48 8.02 10.28
41.0 42.8 44.5 50.7
0.0 0.57 1.70 2.50
Fig. 2. The experimental set-up designed for the measurement of temperature vs time during the heating periods of the S-SCPCMs.
Fig. 3. The leakage test results obtained at 20 °C depending on the PEG ratio in the S-SCPCM with CNTs additive.
40 0 0 cm−1 . Before the analysis, the samples were prepared in pelletized disc form with KBr under high pressure. The LHTES properties such as melting and freezing temperatures and the corresponding latent heat values of PEG and SSCPCMs with/without CNTs were determined by DSC instrument (Perkin Elmer-JADE model, USA). The measurements were carried out at a heating and cooling rate of 5 °C/min under nitrogen gas atmosphere. The melting and freezing temperatures were taken as onset-temperature point of the peaks. The mean deviation values for three repetitive measurements were calculated as ±0.13 °C phase change temperature and ±1.75 J/g latent heat capacity. The thermal stabilities of pure PEG, RD and S- SCPCMs with/without CNTs were estimated by TGA method (TGA7, PerkinElmer) at a ramping rate of 10 °C/min under nitrogen gas atmosphere. The thermal conductivities of RD, PEG and S-SCPCMs with/without CNTs were measured by using a thermal property analyzer (Decagon KD2 model, USA). The sample (about 50 g) was placed into 25 mL-glass tube and the probe was settled tightly into the sample. The measurements were repeated triplicate at room temperature and the average values were presented here. Thermal cycling test was performed to have a judgment about the chemical stability and LHTES reliability of the S-SCPCMs using a thermal cycler (BIOER TC-25/H model, China). A thermal cycling
was consisted with a heating of the composite sample at 20 °C for 5 min and cooling of the composite at 5 °C for the same duration. After 500 cycling treatments, the DSC analyses for S-SCPCMs were repeated under the same analysis conditions applied for the uncycled composite samples. The possible changes to be occurred in the LHTES properties after the cycling test were determined by DSC analysis. To have information about the chemical stability of S-SCPCMs, the FT-IR spectrum bands taken before and after the cycling test were compared. The effect of enhanced thermal conductivity on melting times of S-SCPCMs with CNTs was investigated using the experimental set-up shown in Fig. 2. The heating process was carried out by circulating water at 30 °C around sample. During this operation, the temperature data were collected using a data logger (NOVA50 0 0 model, USA). 3. Results and discussion 3.1. Microstructure investigation of S-SCPCMs The photographs of the S-SCPCMs with CNTs were taken after the leakage test and demonstrated in Fig. 3. After this test and weighing processes, the weight fraction of PEG within the S-SCPCM-2, S-SCPCM-3 and S-SCPCM-4 including 0.57, 1.70 and
A. Sarı et al. / Energy & Buildings 164 (2018) 166–175
Fig. 5. FT-IR spectrum of RD, pure PEG, CNTs and the S-SCPCMs with/without CNTs.
Fig. 4. SEM (c) S-SCPCM-4.
2.5 wt% CNTs was determined as 42.8, 44.5 and 50.7 wt%, respectively These results indicated that the CNTs additive contributed to the holding fraction of PEG. Especially in case of CNTs loading (2.5 wt%), the mass fraction of absorbed PEG was increased as about 10 wt% relative to S-SCPCM-1 without CNTs. These results mean that the network of RD/CNTs pre-composite could permit
higher mechanical stability against leakage action of PEG retained at more amount during the heating treatments. Fig. 4(a) shows the characteristic surface morphology of the CNTs. As obviously observed, they are coiled together before the dispersion of the CNTs jointed with RD in acetone solution. The SEM photograph of the prepared RD/CNTs (5 wt%) pre-composite was shown in Fig. 4(b) because the RD/CNTs binary composite was used in this work as supporting material instead of RD used alone in our previous study . As clearly seen, even although the agglomeration of CNTs into RD was signiﬁcantly prevented by helping of acetone and mechanical blending, inhomogeneous distribution of CNTs at different regions inside the RD can be still perceived. As also detected from Fig. 4(c), the PEG was hold homogenously into the network structure of the RD/CNTs pre-composite. In spite of the partial melting of PEG caused by electron beam during the SEM analysis, any liquid phase was not viewed in the micrograph. This can be explained the fact that RD/CNTs matrix presents strong capillary and tension forces to keep PEG in solid state and results in shape-stabilized composite structure. This structural stability makes the S-SCPCMs feasible for incorporation with conventional construction elements with free of PCM leakage during to be directly applied to building structural elements without fear of PCM leakage during TES applications in buildings. Additionally, they can be applied in areas with arbitrary size or to be easily used as a retroﬁt to existing buildings already in place.
A. Sarı et al. / Energy & Buildings 164 (2018) 166–175
Fig. 6. DSC curves of pure PEG and the S-SCPCMs with/without CNTs.
3.2. Chemical compatibility between components of S-SCPCMs The chemical compatibility among the components of the SSCPCMs was identiﬁed by using FT-IR spectroscopy. As shown in Fig. 5, the FT-IR spectrum of the RD, PEG, CNTs and the prepared S-SCPCMs were taken and compared each other. In the spectrum of pure PEG, the major absorption bands were observed at 3200– 360 0 cm−1 , 280 0–30 0 cm−1 and 985 cm−1 , which are attributed to the stretching vibration of alkyl (-CH3 and CH2 ), hydroxyl (-OH) and carbonyl single bond (-C–O), respectively. As also seen from the spectrum of the RD, it has large bands at the range of 3400– 360 0 cm−1 and 120 0–90 0 cm−1 due to the stretching vibration of the silanol (Si-OH) and siloxane (-Si-O-Si-) groups, respectively while the small bands at 1635 cm−1 and 748 cm−1 represent the bending vibrations of -OH and Si-O groups. Moreover, in the spectrum of the CNTs, the big peak between 3300 and 3600 cm−1 is regarded with stretching vibration bands of -OH group as the small peak at 1685 cm−1 is assigned by stretching vibration of -C = Cgroups. On the other hand, it can be detected overall characteristic bands in spectrum of the S-SCPCMs, which belong to the pure components. In none of spectrum, any new absorption band resulted from chemical bonding of the constituents was not occurred after the incorporation of PEG. However, little changes in the wavenumbers of some bands could be noticed from the dashed lines because of the weak physical interactions between the functional surface groups of RD, CNTs and PEG. These interactions can be attributed to the hydrogen bridge bonds besides the capillary and surface tension forces, which prevent the leaching of the PEG from the S-SCPCMs. Consequently, all results depicted the good chemical compatibility among the ingredients of the S-SCPCMs. 3.3. Effect of CNTs addition on the LHTES properties Fig. 6 (a–d) shows the DSC thermogram of pure PEG and the S-SCPCMs for heating and cooling periods. Their LHTES properties obtained by evaluation of the thermograms were also given in Table 3. As seen from DSC curve in Fig. 6(a), pure PEG melts and solidiﬁes at 9.85 °C and 6.42 °C and the corresponding latent heat capacity of 136.42 and −134.86 J/g. These LHTES properties
make it promising PCM for low temperature-LHTES applications in buildings. As also seen from Fig. 6(b), the RD/PEG(41.0wt%) called as S-SCPCM-1 shows a melting and solidiﬁcation phase change at 7.86 °C and 6.89 °C, which corresponds to the latent heat storage capacity of 51.43 and −53.21 J/g, respectively. On the other hand, as observed from the thermograms in Figs. 6(c–d), the most of the S-SCPCMs exhibited regular endothermic and exothermic peaks like pure PEG although some of them had minor shoulders. From the on-set points of the curves, the melting and freezing temperatures were found as 7.31 °C and 8.23 °C for S-SCPCM-2, 6.92 °C and 8.08 °C for S-SCPCM-3 and 7.21 °C and 8.83 °C for S-SCPCM-4. On the other hand, the latent heats of melting and freezing of the composite PCMs were determined as 53.79 J/g and −57.87 J/g for S-SCPCM-2 and 55.89 J/g and −59.83 J/g for S-SCPCM-3 and 62.86 J/g and −69.45 J/g for S-SCPCM-4. The melting latent heats of these composite PCMs were in agreement with the theoretically calculated values, 54.44, 56.97 and 63.30 J/g, respectively by multiplying the impregnation ratio (wt%) of PEG with its latent heat value. These results also indicated that contrary to expectation, the latent heat capacities of the S-SCPCMs were increased by about 5, 9 and 22% regarding their melting and 9, 12 and 31% concerning with their freezing periods, respectively in case of 1, 3 and 5 wt% CNTs loading to the RD. These increments were due to the contribution of the CNTs to absorption capacity of RD. When compared pure PEG, it can be noted that these composites melt at lower temperatures and solidiﬁes at higher temperatures. Even so, the phase change temperatures and latent heat capacities of the produced S-SCPCMs can be appropriate for solar cooling applications in smart building and greenhouse structures to reduce the temperature peaks and temperature swing depending on the climatic circumstances. On the other hand, in Table 4 the latent heat capacities of the S-SCPCMs were compared with those of diatomite-based and other kinds of clay based-composite PCMs in literature [33,39,40,42, 46–48]. As evidently seen, the latent heat capacity are become different depending on the mass fraction and latent heat capacity of incorporated CM, absorbing ability of building material and also method used in the fabrication of the composites. It can be also concluded form this comparison that especially S-SCPCM-4 has
A. Sarı et al. / Energy & Buildings 164 (2018) 166–175 Table 3 The measured LHTES properties of the S-SCPCMs. S-SCPCM
Melting temperature (°C)
Latent heat of melting (J/g)
Freezing temperature (°C)
Latent heat of freezing (J/g)
S-SCPCM-1 S-SCPCM-2 S-SCPCM-3 S-SCPCM-4
7.86 ± 0.13 7.31 ± 0.11 6.92 ± 0.12 7.21 ± 0.09
51.43 ± 1.21 53.79 ± 1.07 55.89 ± 1.64 62.86 ± 1.75
6.89 ± 0.09 8.23 ± 0.11 8.08 ± 0.08 8.83 ± 0.10
−53.21 ± 1.70 −57.87 ± 1.23 −59.83 ± 1.47 −69.45 ± 1.58
Table 4 Comparison of latent heat capacities of the S-SCPCMs with those of different kinds of clay based-composite PCMs given in literature. S-SCPCMs
Impregnation ratio (wt%)
Latent heat of melting
PEG/gypsum PEG/natural clay Decanoic–dodecanoic acid/diatomite Stearic acid/diatomite Capric-myristic acid/VMT Paraﬃn/montmorillonite Stearic-capric acid /attapulgite Polynary fatty acid/sludge ceramsite S-SCPCM-1 S-SCPCM-2 S-SCPCM-3 S-SCPCM-4
18.0 22.0 34.0 40.0 20.0 58.8 50.0 46.0 41.0 43.0 45.0 50.0
24.18 28.79 66.8 57.1 27.0 79.3 72.6 47.1 51.43 53.79 55.89 62.86
vacuum impregnation vacuum impregnation direct blending direct blending vacuum impregnation direct blending vacuum impregnation vacuum impregnation vacuum impregnation vacuum impregnation vacuum impregnation vacuum impregnation
        This work This work This work This work
Table 5 The measured LHTES properties of the S-SCPCMs after thermal cycling. S-SCPCM
Latent heat of melting (J/g)
Freezing temperature (°C)
Latent heat of freezing (J/g)
S-SCPCM-1 S-SCPCM-2 S-SCPCM-3 S-SCPCM-4
7.26 ± 0.11 7.41 ± 0.12 6.82 ± 0.11 7.31 ± 0.10
48.23 ± 1.21 48.19 ± 1.07 52.33 ± 1.64 59.25 ± 1.75
6.42 ± 0.09 8.03 ± 0.11 8.28 ± 0.08 8.33 ± 0.10
−51.21 ± 1.45 −54.87 ± 1.14 −56.83 ± 1.21 −65.45 ± 1.15
relatively higher latent heat capacity than most of the composite PCMs. 3.4. Effect of CNTs addition on LHTES reliability and chemical stability of S-SCPCMs On the other hand, the LHTES reliability is one of the imperative properties, which should be determined for a newly formed composite PCM. This property demonstrates probable changes in its LHTES properties after long-term heating/cooling cycling. Table 5 shows the LHTES properties of the S-SCPCMs subjected to 500 cycles. In view of these data, it can be noted that the phase change temperatures and the corresponding latent heat values of the SSCPCMs with/without CNTs were slightly changed after the cycling operation. This ﬁnding signiﬁed that the S-SCPCMs can be reused many times for low-temperature passive solar cooling applications in buildings without substantial degradation in their LHTES properties. The cycling chemical stability is another feature to be taken into account in the selection of a composite PCM for a real application. This parameter deals with the probable change in its chemical structure after a long-term thermal cycling treatment. In this context, the chemical structures of the developed S-SCPCMs were investigated by FT-IR analysis after the cycling test. As observed from the dot lines shown in Fig. 7, a new band was not detected and all characteristic bands mentioned in Section 3.2 preserved their own wavelength positions at 0th cycle because any chemical attraction was not taken place between the components of the S-SCPCMs. 3.5. Effect of CNTs addition on the thermal stability of S-SCPCMs Thermal stability for an S-SCPCM is one of decisive restrictions which are paid attention to prefer it as HTES material. Thus, a
freshly built up composite PCM should be durable against to adequately high temperatures. The present work is also aimed to examine the effect of CNTs on the thermal degradation temperatures of PEG, RD and the S-SCPCMs and the TGA ﬁndings were shown in Fig. 8. As shown from the curves, the PEG completely lost its entire weight until 390 °C while the RD showed only 5.0 wt%-weight loss at about 600 °C. The ﬁrst steps of the S-SCPCMs with/without CNTs concerning the evaporation of their PEG constituent were taken place occurred at 20 0–40 0 °C while the second steps regarding the thermal degradation of RD ingredient of them would be possibly carried above 800 °C. The weight loss during the ﬁrst step of S-SCPCM-1, S-SCPCM-2, S-SCPCM-3 and S-SCPCM-4 was determined as 40, 42, 44 and 51%, respectively, which are in well agreement with their PEG impregnation ratios. Additionally, the limit temperatures of the ﬁrst degradation step (about 200 °C) for all S-SCPCMs with/without CNTs were much higher than their phase change temperatures. This indicated that the developed S-SCPCMs had good thermal stability due to their high thermal durability.
3.6. Effect of CNTs addition on the improvement in the thermal conductivity of S-SCPCMs Thermal conductivity is one of the critical properties of a PCM or composite PCM. With this sense, the thermal conductivity increment of PCM leads to the improvement of its heat storage and release performances. Fig. 9 shows the measured thermal conductivity values of PEG, RD and the prepared S-SCPCMs with/without CNTs. The thermal conductivity was measured as 0.08 W/m.K for RD and 0.22 W/m.K for PEG while it was measured as 0.15 W/m.K for S-SCPCM-1, 0.26 W/m.K for S-SCPCM-2, 0.27 W/m.K for S-SCPCM-3 and 0.29 W/m.K for S-SCPCM-4. Compared to that of S-SCPCM-1 without CNTs it can concluded that thermal conductivity values of S-SCPCMs with CNTs were improved
A. Sarı et al. / Energy & Buildings 164 (2018) 166–175
Fig. 8. TGA curves of RD, PEG and the S-SCPCMs with/without CNTs.
Fig. 9. The measured thermal conductivity values of RD, PEG and S-SCPCMs.
Fig. 7. Comparison of FT-IR spectra of the S-SCPCMs with/without CNTs before and after thermal cycling.
3.7. Effect of improved thermal conductivity on melting and total heating times of S-SCPCMs
as 73, 80 and 93%, respectively. Moreover, although mass fraction of CNTs was increased from 0.57 to 2.5 wt%, the observed enhancement in thermal conductivity of three S-SCPCMs was not in considerable magnitude. These results might be due to the fact the agglomeration diﬃculty of CNTs was not completely eliminated. These discussions were also given in the revised text. On the other hand, Table 6 exhibits the inﬂuence of different type of additives on the thermal conductivity of PEGs and PEG-containing composites. As evidently seen from this table, the improvement (%) is changed considerably depending on the thermal conductivity of building material, thermal enhancer and their mass fractions in the composite. However, in this work the improvement in thermal conductivity of RD/PEG600 composite PCM after adding especially 2.5 wt% CNTs was in considerable high rank compared to those of PEG10 0 0/silica/β AlN(5 wt%) , PEG10 0 0/diatomite/EG(3–10 wt%) , PEG40 0 0/diatomite/EG(wt%) , PEG60 0 0/SiO2 /Cu(2.1 wt%) , PEG60 0 0/SiO2 /Al2 O3 (3.3 wt%) , PEG80 0 0/ CMPs(10 wt%)  and PEG/SiO2 /MWCNTs(3 wt%) .
Fig. 10(a-d) presents the effect of improved thermal conductivity on melting and total heating times of the S-SCPCMs. As shown from the heating curves, the composite PCMs including different amount of CNTs additive have similar temperature plateaus corresponding to the solid-liquid phase change of their PEG component. For comparison, the melting time was taken as passing time from the same initial temperature to the temperature value at which the curves showed break point. It was determined as about 580, 375, 240 and 200 s for the S-SCPCM-1, SCPCM-2 SCPCM-3 and SCPCM-4, respectively. Moreover, the total heating time needed for a 25 °Cincrease in temperature of the S-SCPCMs was determined as about 620, 490, 405 and 390 s, respectively. Consequently, the improvements in thermal conductivity of the S-SCPCMs by loading CNTs caused signiﬁcantly reductions in their melting times or total heating periods depending on the heat transfer enhancement within the composites. Moreover, when the fabricated S-SCPCMs with enhanced thermal conductivity can be adapted to be an ordinary building material with relatively lower thermal conductivity and the ﬁnal combination can be used as an outer plasterboard or external wall over normal wall of a building envelope. In this case,
A. Sarı et al. / Energy & Buildings 164 (2018) 166–175 Table 6 The inﬂuence of different type of additives on the thermal conductivity of PEGs and PEG-containing composites. PEG or PEG-containing composite
Added amount (wt%)
Increase in thermal conductivity (%)
PEG10 0 0/silica PEG10 0 0 PEG10 0 0/diatomite PEG40 0 0/diatomite PEG60 0 0/SiO2 PEG60 0 0/SiO2 PEG80 0 0 PEG10.0 0 0 PEG10.0 0 0 PEG/SiO2 PEG60 0 0/diatomite S-SCPCM-2 S-SCPCM-3 S-SCPCM-4
5–30 10 3–10 10 2.1 3.3 10.0 2.0/4.0 4.0 3.0 2.0 0.57 1.7 2.5
30.2 −156.6 344 28–103 97.2 38.1 12.8 65.1 490.0 450.0 26.7 260.0 73.0 80.0 93.0
           This work This work This work
EG EG EG Cu Al2 O3 CMPs GO/GNP GNP MWCNTs SWCNTs CNTs CNTs CNTs
Fig. 10. Temperature histories regarding the heat storage periods of the S-SCPCMs with/without CNTs.
the interior temperature of the envelope will not exceed during the daytime because the melting of outer layer with high thermal conductivity absorbs a large quantity of heat during the phase change of S-SCPCMs. On the other hand, as the external temperature is decreased, the S-SCPCM within outer layer is solidiﬁed by releasing stored energy to the inside of the envelope. The inside layer with lower thermal conductivity will not favor energy release to the interior. This results in thermo-regulation of inside air and therefore helps to reduce the overall energy consumption of the structure. 4. Conclusions In this study, PEG was incorporated with RD/CNTs binary precomposite to abolish its leakage problem during phase change, to amplify LHTES capacity depending on the increased impregnation ratio and also to reduce heat charging/discharging times with respect to improved thermal conductivity. The following conclusions can be drawn from the experimental ﬁndings:
(3) The FT-IR spectrum taken after cycling test proven that the S-SCPCMs had outstanding chemical stability and LHTES reliability even though they were exposed to long-term thermal cycling operation. The TGA results demonstrated that the CNTs they showed good thermal durability. (4) Compared with RD/PEG composite, the thermal conductivity of the S-SCPMs was improved by 73, 80 and 93%, respectively as well as the latent heat capacity of them was increased in the range of 5–31%. (5) In comparison with RD/PEG composite PCM, the CNTs additive in pre-composites allow noticeable advantageous to the S-SCPCMs such as increased absorption ratio with ability to preserve shape during melting, enhanced thermal conductivity and shortening heat storing/releasing times, as well as no damaging in chemical stability and thermal durability. By taking into account of these properties, the S-SCPCMs can be evaluated for the production of various building sections such as radiant ﬂoor heating systems, insulation and ceiling panels or walls for TES purposes in renewable energy effective-building design depending on the climatic circumstances. However, further studies should be made to establish the large-scale thermal performances of the prepared under the real climatic provisions. (6) Before the preparation of such type S-SCPCMs, the purity grade and amount of CNTs additive should be carefully selected by considering their currently high cost to be used in the practical engineering. Acknowledgments The authors would like to thank the Commission of scientiﬁc research projects (Project number: 2016/75) of Gaziosmanpas¸ a University, Karadeniz Technical University (Project number: FBA-20176863) and also King Fahd University of Petroleum and Minerals due to their facilities. References
(1) The continuation of good physicochemical compatibility between RD/CNTs pre-composite and PEG was conﬁrmed by FT-IR and SEM ﬁndings. The leakage test revealed that the skeleton of the fabricated RD/CNTs pre-composites allowed the absorption of PEG up to about 51.0 wt% by maintaining its structural stability. Such capability of the produced S-SCPCMs can present potential choice for buildings as part of overall energy eﬃciency improvement. (2) The DSC results indicated that the prepared novel S-SCPCMs had melting temperatures in the range of about 7–8 °C and latent heat capacities between 51.4 and 62.9 J/g.
 F. Souayfane, F. Fardoun, P-H. Biwole, Phase change materials (PCM) for cooling applications in buildings: a review, Energy Build. 129 (2016) 396–431.  L. Per´ ez-Lombard, J.F. Coronel, I.R. Maestre, A review of HVAC systems requirements in building energy regulations, Energy Build. 43 (2011) 255–268.  R.K Sharma, P. Ganesan, V.V Tyagi, H.S.C Metselaar, S.C Sandaran, Developments in organic solid–liquid phase change materials, and their applications in thermal energy storage, Energy Convers. Manag. 95 (2015) 193–228.  R. Parameshwaran, S. Kalaiselvam, Energy conservative air conditioning system using silver nano-based PCM thermal storage for modern buildings, Energy Build. 69 (2014) 202–212.  V.V Tyagi, D. Buddhi, R. Kothari, S. Tyagi, Phase change material (PCM) based thermal management system for cool energy storage application in building: an experimental study, Energy Build. 51 (2012) 248–254.
A. Sarı et al. / Energy & Buildings 164 (2018) 166–175  F. Berroug, E.K. Lakhal, M. El Omari, M. Faraji, H. El Qarnia, Thermal performance of a greehouse with a phase change material north wall, Energy Build. 43 (2011) 3027–3035.  J. Shao, J. Darkwa, G. Kokogiannakis, Review of phase change emulsions (PCMEs) and their applications in HVAC systems, Energy Build. 94 (2015) 200–207.  Y. Konuklu, M. Ostry, H.O. Paksoy, P. Charvat, Review on using microencapsulated phase change materials (PCM) in building applications, Energy Build. 106 (2015) 134–155.  N. Soares, J.J. Costa, A.R. Gaspar, P. Santos, Review of passive PCM latent heat thermal energy storage systems towards buildings’ energy eﬃciency, Energy Build. 59 (2013) 82–103.  Z. Khan, Z. Khan, A. Ghafoor, A review of performance enhancement of PCM based latent heat storage system within the context of materials, thermal stability and compatibility, Energy Convers. Manag. 115 (2016) 132–158.  Y. Lv, Y. Zou, L. Yang, Feasibility study for thermal protection by microencapsulated phase change micro/nano particles during cryosurgery, Chem. Eng. Sci. 66 (2011) 3941–3953.  L.-S. Tang, J. Yang, R.-Y. Bao, Z.-Y. Liu, B.-H. Xie, M.-B. Yang, W. Yang, Polyethylene glycol/graphene oxide aerogel shape-stabilized phase change materials for photo-to-thermal energy conversion and storage via tuning the oxidation degree of graphene oxide, Energy Convers. Manage 146 (2017) 253–264.  K. Pielichowski, K. Flejtuch, Differential scanning calorimetry studies on poly(ethylene glycol) with different molecular weights for thermal energy storage materials, Polym Adv Technol 13 (2002) 690–696.  S. Mu, J. Guo, Y. Yu, Q. An, Sen Zhang, D. Wang, S. Chen, X. Huang, S. Li, Synthesis and thermal properties of cross-linked poly(acrylonitrile-co-itaconate)/polyethylene glycol as novel form-stable change material, Energy Convers. Manag. 110 (2016) 176–183.  K. Pielichowska, J. Bieda, P. Szatkowski, Polyurethane/graphite nano-platelet composites for thermal energy storage, Renew. Energy 91 (2016) 456–465.  W. Wang, X. Yang, Y. Fang, J. Ding, J. Yan, Enhanced thermal conductivity and thermal performance of form-stable composite phase change materials by using β -Aluminum nitride, Appl. Energy 86 (2009) 1196–1200.  W. Wang, X. Yang, Y. Fang, J. Ding, J. Yan, Preparation and thermal properties of polyethyleneglycol/expanded graphite blends for energy storage, Appl. Energy 86 (2009) 1479–1483.  S. Karaman, A. Karaipekli, A. Sarı, A. Biçer, Polyethyleneglycol (PEG)/diatomite composite as a novel form-stable phase change material for thermal energy storage, Sol. Energy Mater. Sol. Cells 95 (2011) 1647–1653.  T. Qian, J. Li, H. Ma, J. Yang, Adjustable thermal property of polyethyleneglycol/diatomite shape-stabilized composite phase change material, Polym. Compos. 37 (2016) 854–860.  B. Tang, M. Qiu, S. Zhang, Thermal conductivity enhancement of PEG/SiO2 composite PCM by in situ Cu doping, Sol. Energy Mater. Sol. Cells 105 (2012) 242–248.  B. Tang, C. Wu, M. Qiu, X. Zhang, S. Zhang, PEG/SiO2 -Al2 O3 hybrid form-stable phase change materials with enhanced thermal conductivity, Mater. Chem. Phys. 144 (2014) 162–167.  Q. Sun, Y. Yuan, H. Zhang, X. Cao, L. Sun, Thermal properties of polyethylene glycol/carbon microsphere composite as a novel phase change material, J. Therm. Anal. Calorim. (2018) (article in press), doi:10.1007/ s10973- 017- 6535- 6.  L. Feng, P. Song, S. Yan, H. Wang, J. Wang, The shape-stabilized phase change materials composed of polyethylene glycol and graphitic carbon nitride matrices, Thermochim. Acta 612 (2015) 19–24.  G-Q. Qi, J. Yang, R-Y. Bao, Z.-Y. Liu, W. Yang, B.-H. Xie, M.-B. Yang, Enhanced comprehensive performance of polyethylene glycol based phase change material with hybrid graphene nanomaterials for thermal energy storage, Carbon 88 (2015) 196–205.  S.M. Sohel Murshed, C.A. Nieto de Castro, Superior thermal features of carbon nanotubes-based nanoﬂuids-A review, Renew. Sustain. Energy Rev. 37 (2014) 155–167.  Y. Zhao, S. Thapa, L. Weiss, Y. Lvov, Phase change heat insulation based on wax-clay nanotube composites, Adv. Eng. Mater. 16 (2014) 1391–1399.  T. Xian Li, J-H. Lee, R.Z Wang, K.Y Tae, Enhancement of heat transfer for thermal energy storage application, using stearic acid nanocomposite with multi-walled carbon nanotubes, Energy 55 (2013) 752–761.
 H. Babaei, P. Keblinski, J.M Khodadadi, Thermal conductivity enhancement of paraﬃns by increasing the alignment of molecules through adding CNT/graphene, Int. J. Heat Mass Transf. 58 (2013) 209–216.  S. Yu, S-G. Jeong, O. Chung, S. Kim, Bio-based PCM/carbon nanomaterials composites with enhanced thermal conductivity, Sol. Energy Mater. Sol. Cells 120 (2014) 549–554.  N. Zhang, Y. Yuan, Y. Yuan, X. Cao, X. Yang, Effect of carbon nanotubes on the thermal behavior of palmitic–stearic acid eutectic mixtures as phase change materials for energy storage, Sol. Energy 110 (2014) 64–70.  B. Tang, Y. Wang, M. Qiu, S. Zhang, A full-band sunlight-driven carbon nanotube/PEG/SiO2 composites for solar energy storage, Sol. Energy Mater. Sol. Cells 123 (2014) 7–12.  T. Qian, J. Li, W. Feng, H. Nian, Enhanced thermal conductivity of form-stable phase change composite with single-walled carbon nanotubes for thermal energy storage, Sci. Rep. 7 (2017) 44710.  A. Sarı, Composites of polyethyleneglycol (PEG600) with gypsum and natural clay as new kinds of building PCMs for low temperature-thermal energy storage, Energy Build. 69 (2014) 184–192.  L. Hui, G.Y. Fang, Experimental investigation on the characteristics of polyethyleneglycol/cement composites as thermal energy storage materials, Chem. Eng. Technol. 33 (2010) 1650–1654.  A.R Sakulich, D.P Bentz, Incorporation of phase change materials in cementitious systems via ﬁne lightweight aggregate, Constr. Build Mater. 35 (2012) 483–490.  B. Xu, Z. Li, Paraﬃn/diatomite/multi-wall carbon nanotubes composite phase change material tailor-made for thermal energy storage cement-based composites, Energy 72 (2014) 371–380.  S.G. Jeong, J. Jeon, O. Chung, S. Kim, S. Kim, Evaluation of PCM/diatomite composites using exfoliated graphite nanoplatelets (xGnP) to improve thermal properties, J. Therm. Anal. Calorim. 114 (2013) 689–698.  Z. Sun, Y. Zhang, S. Zheng, Y. Park, R.L. Frost, Preparation and thermal energy storage properties of paraﬃn/calcined diatomite composites as form-stable phase change materials, Thermochim. Acta 558 (2013) 16–21.  M. Li, H. Kao, Z. Wu, J. Tan, Study on preparation and thermal property of binary fatty acid and the binary fatty acids/diatomite composite phase change materials, Appl. Energy 88 (2011) 1606–1612.  X. Fu, Z. Liu, B. Wu, J. Wang, J. Lei, Preparation and thermal properties of stearic acid/diatomite composites as form-stable phase change materials for thermal energy storage via direct impregnation method, J. Therm. Anal. Calorim. 123 (2015) 1173–1181.  Y. Qin, X. Yu, G.H. Leng, L. Zhang, Y.L. Ding, Effect of diatomite content on diatomite matrix based composite phase change thermal storage material, Mater. Res. Innov. 18 (2014) 453–456.  A. Karaipekli, A. Sarı, Capric–myristic acid/vermiculite composite as form-stable phase change material for thermal energy storage, Sol. Energy 83 (2009) 323–332.  A. Sarı, A. Karaipekli, Preparation, thermal properties and thermal reliability of capric acid/expanded perlite composite for thermal energy storage, Mater. Chem. Phys. 109 (2008) 459–464.  S. Liu, H. Yang, Stearic acid hybridizing coal-series pumice composite phase change material for thermal energy storage, Appl. Clay Sci. 101 (2014) 277–281.  S.A. Memon, T.Y. Lo, X. Shi, S. Barbhuiya, H. Cui, Preparation, characterization and thermal properties of lauryl alcohol/kaolin as novel form-stable composite phase change material for thermal energy storage in buildings, Appl. Therm. Eng. 59 (2013) 336–347.  X. Fang, Z. Zhang, A novel montmorillonite-based composite phase change material and its applications in thermal storage building materials, Energy Build. 38 (2006) 377–380.  S. Song, L. Dong, S. Chen, H. Xie, C. Xiong, Stearic-capric acid eutectic/activated-attapulgiate composite as form-stable phase change material for thermal energy storage, Energy Convers. Manag. 81 (2014) 306–311.  H. He, P. Zhao, Q. Yue, B. Gao, D. Yue, Q. Li, A novel polynary fatty acid/sludge ceramsite composite phase change materials and its applications in building energy conservation, Renew. Energy 76 (2015) 45–52.