Thermal energy storage characteristics of bentonite-based composite PCMs with enhanced thermal conductivity as novel thermal storage building materials

Thermal energy storage characteristics of bentonite-based composite PCMs with enhanced thermal conductivity as novel thermal storage building materials

Energy Conversion and Management 117 (2016) 132–141 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www...

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Energy Conversion and Management 117 (2016) 132–141

Contents lists available at ScienceDirect

Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman

Thermal energy storage characteristics of bentonite-based composite PCMs with enhanced thermal conductivity as novel thermal storage building materials Ahmet Sarı ⇑ Karadeniz Technical University, Department of Metallurgical and Material Engineering, 61080 Trabzon, Turkey King Fahd University of Petroleum and Minerals, Centers of Research Excellence, Renewable Energy Research Institute, Dhahran, 31261, Saudi Arabia

a r t i c l e

i n f o

Article history: Received 30 November 2015 Accepted 27 February 2016

Keywords: Composite PCM Bentonite Capric acid PEG600 Dodecanol Heptadecane Latent heat Thermal energy storage Thermal conductivity

a b s t r a c t In this work, for latent heat thermal energy storage (LHTES) applications in buildings, bentonite-based form-stable composite phase change materials (Bb-FSPCMs) were produced by impregnation of capric acid (CA), polyethylene glycol (PEG600), dodecanol (DD) and heptadecane (HD) into bentonite clay. The morphological characterization results obtained by scanning electron microscopy (SEM) showed that the bentonite acted as good structural barrier for the organic PCMs homogenously dispersed onto its surface and interlayers. The chemical investigations made by using fourier transform infrared (FT-IR) technique revealed that the attractions between the components of the composites was physical in nature and thus the PCMs were hold by capillary forces. The results of differential scanning calorimetry (DSC) analysis indicated that the prepared Bb-FSPCMs composites including 40 wt% CA, 43 wt% PEG600, 32 wt% DD and 18 wt% HD, respectively had suitable phase change temperature of 4–30 °C and good latent heat capacity between 38 and 74 J/g, respectively for solar space heating and cooling applications of buildings envelopes depending on climatic conditions. The results of thermogravimetric (TG) analysis demonstrated that all of the fabricated Bb-FSPCMs had good thermal resistance. The Bb-FSPCMs maintained their LHTES properties even after 1000 heating–cooling cycling. Furthermore, the total heating times of the prepared Bb-FSPCMs were reduced noticeably due to their enhanced thermal conductivity by addition of expanded graphite (EG) in the mass fraction of 5 wt%. Ó 2016 Elsevier Ltd. All rights reserved.

1. Introduction Thermal energy storage (TES) has been one of attracting techniques because of reduction of the fossil fuels by raising energy demand and increasing the deficit between energy demand and supply. In this regard, the storage of excess thermal energy in a suitable form has been considered as key solution to close gap between energy demand and supply [1,2]. The interest in TES by using phase change materials (PCMs) has been growth in especially research basis for the last 20 years since PCMs can store or release large amounts of latent heat via phase change in a constant or narrow temperature range [3–6]. The method in this manner is called as latent heat thermal energy storage (LHTES) and taken into account as prudential way in solar heating/cooling systems for domestic [7,8] and greenhouse applications [9,10], thermal man⇑ Address: Karadeniz Technical University, Department of Metallurgical and Material Engineering, 61080 Trabzon, Turkey. Tel.: +90 4623773000; fax: +90 325 3257405. E-mail addresses: [email protected], [email protected], asari061@ hotmail.com http://dx.doi.org/10.1016/j.enconman.2016.02.078 0196-8904/Ó 2016 Elsevier Ltd. All rights reserved.

agement of electronic equipment [11,12], food containers [13], textile products [14] and medical [15]. Moreover, LHTES technique has key action to improve the energy efficiency of building envelopes, to reduce the fossil sourced-energy consumption and thus to prevent hazardous gas emissions [16,17]. This method is becoming attractive in recent years because energy consumption in building and industrial sectors has about one third share in total energy consumption in the world [18]. With this purpose, in order to accomplish thermal comfort and decreasing the fluctuations at indoor temperature, several studies focused determination of LHTES performance of PCM/construction material composites prepared in shape-stabilized form have been carried out in recent years [19–25]. Also, these type form-stable building composite PCMs (FS-BCPCMs) have been used in fabrication of different types of as concrete blocks, hollow bricks and wallboard plasters with LHTES ability [26–30]. Different organic PCMs such as paraffins, fatty acids, fatty alcohols, PEGs, and their eutectic mixtures have been generally preferred for LHTES for heating and cooling applications because of some advantageous properties: high latent heat capacity,

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appropriate phase change temperature, non-corrosivity, nontoxicity, good thermal/chemically stability, low vapor pressure and little supercooling [31–33]. However, the low thermal conductivity (generally in the range of 0.15 and 0.20 W/mK) is one of the most drawbacks of them. By introducing high thermal conductive materials [34,35], this disadvantage could be eliminated. Moreover, the other problem is that they need storage container to dispose the outflow problem occurred in phase change stage. The usage of such a type container opens the way of extra thermal resistance and cost increase. Therefore, the preparation of form-stable or shape stabilized PCMs in macro size by holding them into a polymeric matrix or porous building material can take away the necessity of extra container usage. The integration of PCM with porous and lightweight materials is simple, cost-effective, environment friendly and does not need any solvent [36,37]. The incorporation of different organic PCMs with porous building clays such as perlite [38– 40], diatomite [41–45], vermiculite [46,47] and kaolin [48,49]. Bentonite is consisted basically with clay minerals of the smectite (montmorillonite) group and one of the most common industrial clays. It is generally used in drilling mud, binder, absorbent in cement, and ceramic bodies [36,50]. Additionally, some advantageous properties of bentonite such as low cost, excellent absorption capacity, and direct usability with cement, mortar and concrete due to its good compatibility and chemical inertness make it a prospective matrix for the production of FS-BCPCMs. On the other hand, the abundance of bentonite clay make it low cost (about 0.15 $/kg in Turkish markets). In addition to all beneficial properties above-mentioned, the incorporation feasibility of bentonite with different types of organic PCMs make it strong candidate for fabrication of bentonite or organic modified bentonite-based form-stable composite PCMs (Bb-FSPCMs) for the TES purposes in buildings. Li et al. [51] prepared a novel mineral-based composite PCMs was via microwave-acid treatment of the graphite (G) and bentonite (B) mixture. The SA/GBm composite showed an enhanced thermal storage capacity, latent heats for melting and freezing (84.64 and 84.14 J/g) compared with those of SA/B sample (48.43 and 47.13 J/g, respectively). The thermal conductivity of the SA/B composite was increased as 31% by addition of graphite. Fang and Zhang [52] developed montmorillonite/ paraffin (RT20) composite PCM and investigated its compatibility with gypsum powders and the composite gypsum boards. The composite PCM had good performance stability and exhibited higher heat transfer rate owing to the combination with montmorillonite. Chen et al. [53] investigated comparatively the morphology, composition, crystalline properties, phase transition properties and heat storage/release performances of myristic acid (MA)/bentonite and MA/Eudragit L100 as novel form-stable PCMs. The results show that the maximum MA content of in the composites was 50 wt% and 70 wt%, respectively. Li et al. [54] carried out an experimental investigation on the preparation and thermal performance of paraffin/bentonite composite prepared by a solution intercalation process. The results showed that the composite PCM containing 44.4% paraffin had a melting and freezing points of 41.7 °C and 43.4 °C, and the latent heat capacity of 39.84 J/g. In addition, the heat transfer rate of the composite PCM was enhanced due to bentonite. Moreover, Kao et al. [55] produced EG/paraffin/organic montmorillonite (OMMT) composite PCM by using melt intercalation method and characterized it structurally and thermally. The results showed that EG and OMMT had good sorption ability and the melting point EG/paraffin/OMMT was decreased slightly with an addition of paraffin. It was also reported that the heating time of the composite is decreased to one-sixth of that of paraffin by addition of EG and OMMT depending on the increase in the heat transfer efficiency. It can be seen from the summarized literature, the researchers prepared and characterized bentonite-based composite including

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only one kind of organic PCM. Thus, from such type studies, it is difficult to expose comparatively the adsorption ability and compatibility of bentonite against different types of organic PCMs such as paraffin, fatty acid, fatty alcohol and organic polymer. In addition, in most of the studies mentioned above, intercalated bentonite was used as supporting matrix instead of its natural state. Moreover, especially the number of the studies about the enhancement of the thermal conductivity of the selected PCM is restricted by only a few. When taking into account these cases in the literature, it can be considered that a comprehensive study on the development, characterization of LHTES properties and improvement of heat storage performance depending on the increased thermal conductivity of natural bentonite-based composites with different type organic PCMs are still needed. In this regard, this study provides a deep insight into developing novel four kinds of bentonite based-form-stable PCM (Bb-FSPCMs) with enhanced thermal conductivity and investigation of LHTES characteristics comprehensively. The phase change properties of prepared Bb-FSPCMs were varied and controlled by changing the type of organic PCMs, capric acid (CA), polyethylene glycol-600 (PEG600), dodecanol (DD) and heptadecane (HD), categorized in the class of fatty acid, organic polymer, fatty alcohol and paraffin, respectively. In the selection of these PCMs, especially phase change temperature (about in the range of 10–31 °C) was considered as key criteria, which is suitable for passive solar heating, cooling and air conditioning temperature range in buildings. Moreover, as far as known from the literature, no comprehensive study on the physicochemical–mor phological description, determination of LHTES properties, and enhancement of thermal conductivity and also comparatively investigation of the effect of the improved thermal conductivity on the heat storage performance of novel four types of BbFSPCMs was carried out. These materials can be used in the fabrication of building envelops, thermal insulation coatings of interior and exterior wall, plasterboards, building brick and polyurethane foam to or adjusting indoor temperature to the comfortable range by means of solar passive LHTES. The prepared Bb-FSPCMs, bentonite/CA, bentonite/PEG600, bentonite/DD and bentonite/HD were characterized structurally and morphologically by using FTIR and SEM techniques. The LHTES properties of the Bb-FSPCMs before and after thermal cycling process and thermal durability temperatures were measured by using DSC and TGA methods. The thermal conductivity of the obtained Bb-FSPCMs was enhanced with EG addition and the effect of this additive on their energy storage times was also evaluated.

2. Experimental 2.1. Materials and instruments Bentonite used as supporting matrix is Turkish origin (in Resßadiye region of Tokat, Turkey). After humidity evaporation process at 105 °C, it was sieved from 200-mesh. The dried bentonite is mainly composed of 61.82 wt% SiO2, 17.3 wt% Al2O3, 4.5 wt% CaO, 3 wt% Fe2O3, 2.7 wt% Na2O, 2.1 wt% MgO and other metal oxides [36]. The selected PCMs, CA, PEG600, DD and HD were obtained from Sigma–Aldrich company (Germany). The average size, powder density, assay and thermal conductivity of the expanded graphite (EG) are 200 lm, 0.1 g/cm3, P99.9 and 4.26 W/m K, respectively. It was purchased from Sigma–Aldrich company (Germany) and then expanded (expansion rate: 250 ml/g) through thermal shock at 950 °C in muffle furnace. The morphological investigations were carried out by using a SEM instrument with LEO 440 model (Japan) as the chemical characterization was made by using a FT-IR spectrophotometer with JASCO 430 model (USA). The LHTES properties and thermal

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degradation temperatures were measured by a DSC instrument with Perkin Elmer JADE model (USA) and a TG instrument with Perkin–Elmer TGA7 (USA) model, respectively. Thermal conductivity values were measured by using a thermal property analyzer with Decagon KD2 model (USA). The temperature data were recorded against time during by using a data logger with NOVA5000 model (USA). 2.2. Methods The four types of Bb-FSPCMs, Bentonite/CA, Bentonite/PEG600, Bentonite/DD and Bentonite/HD composites were produced using vacuum impregnation method. The same experimental stages and vacuum procedure presented in literature [36,37,46] were conducted to fabricate the Bb-FSPCMs. During impregnation runs, the amount of the selected organic PCM was changed from 10 to 50 wt%. The maximum holding amount of bentonite for each of PCM was controlled simultaneously with leakage test. In this test, the prepared bentonite/PCM composite was placed in oven above the melting temperature point of PCM for 30 min so as to watch the PCM leakage in the melted state. Based on the leakage tests applied for each combination, the maximum mass fractions of CA, PEG600, DD and HD into the bentonite were determined as 40, 43, 32 and 18 wt%, respectively. These results also mean that as long as the mass fractions of the organic PCMs into the bentonite was below these values, any leakage was not observed on the surface of the form-stable BCPCMs although heated over their melting temperatures. The same procedures for both SEM and FT-IR analysis with literature [24,33] were applied to the composites. During the DSC analysis, the heating and cooling rates were applied at 5 °C/min under nitrogen atmosphere. The measurements were replicated three times to achieve high accuracy. The mean deviation value was calculated as ±0.12 °C for phase change temperature and ±1.21 J/g for latent heat value. TG analyses were conducted at heating rate of 10 °C min 1 in the range of 50–700 °C. In order to evaluate the LHTES dependability properties of the produced Bb-FSPCMs, each of them was subjected to thermal cycling process by using a thermal cycler with BIOER TC-25/H model. The temperature values and duration time was programmed by using software of the thermal cycler for accelerated heating and cooling cycles. During an each heating cycle, the temperature of the cycler was adjusted to 10 °C above the melting temperature of the composite PCM and then heating process was maintained at this temperature for 5 min. The temperature value of the cycler was simultaneously adjusted to 10 °C below the solidification temperature of the composite PCM and then the cooling cycle was carried out for 5 min. This process was consecutively repeated until the cycling number was reached 1000. In addition, the thermal conductivities of the prepared Bb-FSPCMs were enhanced by using EG additive due to the advantageous properties such as low density, considerable thermal conductivity physical/chemical compatibility, chemical inertness, non-corrosively, and homogenously dispersion ability. Moreover, in the selection of its mass fraction of 5 wt%, two factors was taken into account: (i) reach a considerable increase by using minimum amount, (ii) the negative effect on the latent heat capacity and the phase change temperature of the composite PCM in case of its excess usage. The EG was directly added to the composite PCM in glass tube and then homogenously dispersed by using ultrasonic centrifuge. Thermal conductivity of the selected organic PCMs, bentonite and prepared Bb-FSPCM composites were measured at 25 °C values. Prior to measurement, thermal conductivity analyzer was calibrated with a standard (glycerol). The sample was filled into a 25 mL-test tube and the analyzer probe was placed tightly into the sample. The measurements were replicated three

times and the average data were presented with mean discrepancy as ±0.01 W/m K. Additionally, the effect of this addition on the reduction of heating times of the prepared Bb-FSPCMs was investigated by using the experimental set-up shown in Fig. 1. The heating process was carried out by circulating water into the glass test tube including 50 g composite sample until the PCM hold into bentonite was completely melted. For the heating period, the temperature of the circulating water was adjusted between 20 and 50 °C for Bentonite/CA and Bentonite/CA/EG, 20 and 30 °C for Bentonite/ PEG600 and Bentonite/PEG600/EG, 0 and 50 °C for Bentonite/DD and Bentonite/DD/EG and 10 and 30 °C for Bentonite/HD and Bentonite/HD/EG composite sample. The temperature data were recorded against time by using a data logger. 3. Results and discussion 3.1. Microstructure characterization The photographs of unloaded-bentonite and the produced four types of Bb-FSPCMs are presented in Fig. 2. The morphological investigations of bentonite before and after impregnation of PCM were also made by using a SEM instrument with LEO 440 model. Before and after impregnation processes, the microstructure of bentonite was characterized morphologically by SEM analysis technique and the obtained micrographs with same magnification were shown in Fig. 3. As can be seen from micrograph of bentonite before impregnation process, it is consisted with haphazard shaped-coarse particles dispersed arbitrarily through the surface. The most of them are unconnected each other and thus the spaces among the particle layers enable the holding the molecules of organic PCMs and thus preventing their exudation from the surface of matrix. As also observed from Fig. 3, after impregnation process, the spaces between the micro particles of bentonite completely were occupied separately with CA, PEG600, DD and HD used as organic PCMs. Moreover, the surface tension and capillary forces between the PCM molecules and inter layers of bentonite played important role in increasing the compatibility of the components and making whole matrix structurally durable against the exudation problem of PCMs above their melting temperatures [53–55]. 3.2. Chemical characterization In this study, FTIR spectroscopy analysis was used for chemical characterization of the pure components and the prepared BbFSPCMs. As known from the literature, the stretching vibration bands of ACH2 groups are recorded in the close ranges of about

Fig. 1. Experimental-set-up designed for the investigation of the effect of enhanced thermal conductivity on the heating time of the produced BB-FSPCMs.

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Fig. 2. Photograph images of bentonite, CA, PEG600, DD, HD and the produced bentonite/CA, Bentonite/PEG600 Bentonite/DD and Bentonite/HD form-stable composite PCMs.

Fig. 3. SEM images of Bentonite, Bentonite/CA, Bentonite/PEG600, Bentonite/HD and Bentonite/DD.

2800–2970 cm 1 for pure PEG600 [36], pure HD [56], pure CA [57] and for pure DD [20]. In the same studies, the place of OAH stretching band was reported as 3400–3700 cm 1 for CA, 3174–

3633 cm 1 for PEG600 and 3100–3600 cm 1 for pure DD and also the wavenumber values regarded with the stretching and antisymmetric stretching vibrations of C@O and ACAOAC groups of

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pure CA are reported as 1735 cm 1 and 1100 cm 1, respectively. Moreover, as seen from the spectrum of bentonite (Fig. 4), the main peaks shown between 3235 cm 1 and 3617 cm 1 and around 1637 cm 1 are attributed to the stretching vibration and bending vibration regarding with water in the structures of bentonite. It has also other characteristic bands such as Si-antisymmetric vibration and bending vibrations at 1039 cm 1 and 460 cm 1 and 1045 cm 1 and 463 cm 1, respectively. Fig. 4 demonstrates the FT-IR spectrum of four types of BbFSPCMs taken at the wavenumber range of 400–4000 cm 1. When compared the characteristic bands of the Bb-FSPCMs with those of their components given above, it can be easily detected all of the bands in each of the spectrum obtained for the Bb-FSPCMs. After impregnation process, the absorption peaks regarded with the main characteristic bands of the Bb-FSPCMs showed little shifts. For instance; when compared the main characteristic bands of the Bentonite/CA with those of pure PA, it can be observed that the stretching band of AC@O and ACAO groups of the composite shifted to 1742 cm 1 and 1105 cm 1, respectively. Moreover, the OAH stretching band of the composite shifted to the range of 3420–3730 cm 1. It can be considered that the little changes occurred in the wavenumber of the characteristic bands could be due to some physical attractions. These can be described as capillary forces between the walls of the pores of the bentonite and the molecules of the organic PCMs or weak intermolecular attractions between the SiAOH, AlAOH groups and oxide contents of the bentonite and the functional groups (e.g.: AOH and ACOO) of the organic PCMs. Additionally, the absence of additional new absorption peak in the spectrum of the composites confirmed the chemical inertness property of bentonite against the selected organic PCMs. 3.3. LHTES properties Melting temperature and latent heat capacity are crucial LHTES parameters of building composite PCM like pure PCM. Therefore organic PCM to be integrated with building material should have proper phase transition temperature and adequately high phase transition enthalpy. Fig. 5 shows the DSC thermograms of the preferred organic PCMs including heating and cooling cycles. The LHTES properties of these PCMs are also summarized in Table 1.

Fig. 4. FT-IR spectra of the produced Bb-FSPCMs.

As obviously seen from the endothermic and exothermic curves, the HD has two DSC peaks during both heating and cooling periods, smaller one (10.87 °C for heating and 9.86 °C for cooling period) of which corresponds to solid–solid phase transition and the bigger (23.03 °C for heating and 22.36 °C for cooling period) is described as solid–liquid phase change. CA, PEG600 and DD released only one endothermic peak at 31.5, 9.93 and 23.54 °C, respectively and one exothermic peak at 27.16, 6.42 and 19.89 °C, respectively. Moreover, the selected PCMs have a phase transition enthalpy for melting and solidification in the range of about 136–216 J/g and about in the range of about ( 136)–( 231) J/g, respectively. As obviously seen from Fig. 6, the DSC curves of Bb-FSPCMs indicate similar reversible phase change manner with their pure PCM constituents. The LHTES properties derived from the DSC curves were also given in Table 2. From these data, the produced Bb-FSPCMs, Bentonite/CA, Bentonite/PEG600, Bentonite/DD and Bentonite/HD composites have melting temperature of 30.07, 4.03, 22.61 and 22.07 °C, respectively and have a solidification temperature of 26.16, 10.31, 21.11 and 21.43 °C, respectively. These values are suitable for heating, ventilating and air conditioning (HVAC) applications in buildings depending on the climatic conditions. On the other hand, when compared the phase change temperatures of the organic PCMs with that of the composites, it can be observed little amount of decreases, which were due to the weak physical interactions which can be described as capillary or electrostatic intermolecular between the walls of pores of bentonite and the PCM molecules between the components of the composites. In similar trend, Radhakrishnan and Gubbins [58] reported that the probable physical interactions among the components of the composite play important role in decreasing or increasing of the phase change temperature in porous media. On the other hand, the produced Bb-FSPCMs have a melting transition enthalpy as 74.08, 56.72, 67.55 and 38.42 J/g, respectively and a solidification transition enthalpy as 71.13, 55.12, 62.32 and 38.63 J/g, respectively. These properties also make them promising candidate composites for solar passive LHTES and HVAC in buildings. Additionally, by dividing the measured enthalpy value for the melting transition to the corresponding value of pure PCM the theoretical holding ratio of PCM by bentonite was calculated as 38.9, 41.6, 31.4 and 17.8 wt% for Bentonite/CA, Bentonite/PEG600, Bentonite/DD and Bentonite/HD composites, respectively. Evidently seen, these values are close to real impregnation mass fractions 40, 43, 32 and 18 wt%, respectively. However, lower real mass fraction could be owing to the non-free of phase transition of the PCMs hold between inner layers of bentonite.

Fig. 5. DSC curves of the pure CA, PEG600, DD and HD used as energy storing materials in the production of Bb-FSPCMs.

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A. Sarı / Energy Conversion and Management 117 (2016) 132–141 Table 1 The LHTES properties of the selected organic PCMs used to produce Bb-FSPCMs.

CA PEG 600 DD HD

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)

– – – 10.87

31.15 9.93 23.54 23.03

190.21 136.42 215.13 216.21

– – – 9.86

27.16 6.42 19.89 22.36

Latent heat of freezing (J/g) 181.01 136.33 204.17 230.61

Fig. 6. DSC curves of the produced Bb-FSPCMs.

Fig. 7. DSC curves of the produced Bb-FSPCMs after thermal cycling.

On the other hand, the produced four kinds of Bb-FSPCMs have high enough phase transition enthalpy of melting to be compared with those of different form-stable composite PCMs reported in literature [22–27,36–49,51–55]. However, by taking into consideration this comparison, it can be inferred that the phase change-heat capacity of the composites could be varied with respect to the enthalpy value and impregnation percentage of pure constituent.

decrease in melting temperature of Bentonite/CA, Bentonite/ PEG600, Bentonite/DD and Bentonite/HD composites as 0.76, 0.03, 0.44 and 0.06 °C while it was determined as 4.27, 1.99, 0.19 and 0.02 °C in their solidification temperature, respectively. Moreover, after the thermal cycling, the enthalpy values of the BbFSPCMs were reduced as little as 6.5%, 9.32%, 12.3% and 8.6% for melting period and 5.32%, 10.1%, 9.5% and 12.8% for freezing period. However, the produced four kinds of Bb-FSPCMs had good LHTES dependability for solar passive TES applications in buildings. On the other hand, another key criterion to be taken into account prior to use of a newly prepared composite PCM is chemical stability after thermal cycling treatment. Thus, in this study, the chemical stability of the produced Kb-BCPCMs was characterized by using FT-IR analysis. In Fig. 8, the FT-IR spectrums obtained before and after 1000 thermal cycling were given together for comparison. As clearly perceived from all spectrums, the profile and wavenumbers of the characteristic absorption bands of the produced Bb-FSPCMs were kept without change after the cycling test. These results mean that the produced Bb-FSPCMs have good chemical stability.

3.4. LHTES dependability and chemical stability A form-stable composite PCM newly prepared for LHTES practices in buildings should have good LHTES dependability although it is treated with a large number of consecutive heating/cooling cycling. Therefore, the achievement of preceding information about the LHTES dependability of the composite PCM is important from its usage perspective in actual TES systems. With this regard, LHTES dependability of fabricated four kinds of the Bb-FSPCMs were determined by measuring their LHTES properties after treatment of heating/cooling cycles repeated for 1000 times. Fig. 7 shows the DSC curves of the Bb-FSPCMs treated with cycling process and Table 3 summarizes the LHTES data deduced from these curves. The absence of additional new peak or having maintained the presence of current peaks after thermal cycling process indicated that the produced Bb-FSPCMs had good phase change dependability. However, it is possible to notice little amount of

3.5. Thermal resistance Thermal resistance property is one of crucial factors to be considered in the election of composite PCMs for TES practices.

Table 2 The LHTES properties of the produced Bb-FSPCMs before thermal cycling process. Bb-FSPCM

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)

Bentonite/CA Bentonite/PEG600 Bentonite/DD Bentonite/HD

– – – 10.93

30.07 4.03 22.61 22.07

74.08 56.72 67.55 38.42

– – – 7.50

26.16 10.31 21.11 21.45

Latent heat of freezing (J/g) 71.13 55.12 62.32 38.63

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Table 3 The LHTES properties of the produced Bb-FSPCMs after thermal cycling process. Bb-FSPCM

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)

Bentonite/CA Bentonite/PEG600 Bentonite/DD Bentonite/HD

– – – 10.79

29.31 4.00 22.17 22.01

69.30 51.43 59.21 35.13

– – – 7.13

21.89 8.32 20.92 21.43

Latent heat of freezing (J/g) 67.34 49.56 56.37 33.67

Fig. 8. FT-IR spectra of the produced Bb-FSPCMs after thermal cycling.

Therefore, it is requested from a freshly produced PCM to show good thermal resistance until sufficiently high temperature value. In this regard, the selected organic PCMs and the produced four kinds of Bb-FSPCMs were characterized in temperature range of 50–500 °C by using TG analysis method and the obtained results are shown in Figs. 9 and 10. As clearly seen form the curves in Fig. 8, about 3%-part of total weight of pure CA, HD and PEG600 loses by evaporation at 90, 140 and 230 °C, respectively while about of 25%-part of weight of DD loss at 50 °C. This is due to start-

ing of degradation process immediately after its phase change of melting (about 22 °C according to DSC analysis). The weight loss actions of CA, PEG600, DD and HD and were ended at about 220, 425, 255 and 265 °C, respectively. On the other hand, as obviously seen from the curves in Fig. 10, CA, PEG600, DD and HD were evaporated at about 190, 395, 200 and 215 °C, respectively. The percentages of weight loss corresponded to these temperatures were determined as 42, 44, 34 and 19 wt%, respectively, which are rather close the real mass fractions to be found after leakage test and the theoretical ones calculated based on DSC analysis results. Furthermore, DD maintained

Fig. 9. TG curves of pure CA, PEG600, DD and HD.

Fig. 10. TG curves of the produced Bb-FSPCMs.

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in the composite structure without evaporation up to 50 °C. This means that in case of composite DD has better thermal durability compared to its pure state because of good structural resistance provided by bentonite as supporting matrix. On the other hand, the degradation temperatures of the PCMs hold between inner layers of the bentonite clay are extremely over their phase change temperatures defined as working temperatures. Thus, it can be concluded that the produced Bb-FSPCMs have high thermal resistant and thus good thermal stability. 3.6. Thermal conductivity enhancement

Fig. 11. The measured thermal conductivity values of CA, PEG600, DD and HD, Bentonite and the prepared Bb-FSPCMs with and without EG additive.

Thermal conductivity of a PCM is one of the factors which remarkably affect the heat charging/discharging transfer rates to/ from LHTES systems. More time is needed for storing and releasing the heat energy of PCM due to its low thermal conductivity. This means the working of TES unit for longer period during the completion of storage/release processes and also leads to more energy consumption and thereby, cost increase. Also, this drawback restricts the effectiveness of the LHTES systems in large-scale TES applications [59,60]. In this regard, firstly, thermal conductivity

Table 4 The LHTES properties of the produced Bb-FSPCMs after EG (5 wt%) addition. Bb-FSPCM/5 wt% EG

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)

Bentonite/CA Bentonite/PEG600 Bentonite/DD Bentonite/HD

– – – 10.81

29.23 3.97 22.16 22.09

68.23 50.12 57.84 34.05

– – – 7.25

21.50 8.13 21.05 21.53

Fig. 12. Temperature history vs time obtained during the heating period of the produced Bb-FSPCMs.

Latent heat of freezing (J/g) 66.13 47.96 55.45 32.43

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of the pure constituents, CA, PEG600, DD and HD and bentonite were measured as 0.18, 0.15, 0.19, 0.17, and 0.82 W/m K, respectively (Fig. 11). Afterwards, EG (thermal conductivity value: 4.26 W/m K) was added to each of the four kinds of Bb-FSPCMs in mass fraction of 5 wt% and the thermal conductivities of Bentonite/CA/EG, Bentonite/PEG600/EG, Bentonite/DD/EG and Bentonite/HD/EG composites were measured as 0.77, 0.69 and 0.70 and 0.56 W/m K, respectively. When compared to these values with the measured data, 0.43, 0.39, 0.48, and 0.35 W/m K, respectively for Bb-FSPCMs without EG, noteworthy increases in thermal conductivities of Bb-FSPCMs was determined by approximately 65%, 63%, 39% and 47%, respectively for the composite PCMs with EG additive. Additionally, after EG addition, the LHTES properties of the produced Bb-FSPCMs were measured by DSC analysis and the results were summarized in Table 4. As seen from these data, the melting temperatures of Bentonite/CA/EG, Bentonite/PEG600/EG, Bentonite/ DD/EG and Bentonite/HD/EG composites were changed as little as 0.08, 0.03, 0.01 and 0.08 °C while the change occurred in their solidification temperatures were determined as 2.61, 0.19, 0.13 and 0.10 °C, respectively. These irregular changes might be due to the change occurred in the molecular randomness during their phase changes into the pores of EG as well as the inter layers of bentonite. Moreover, the latent heats of melting of the composites were reduced as small as about 1.5%, 2.5%, 2.3% and 3.1% and while their latent heats of solidification were decreased 1.7%, 3.2%, 1.6% and 3.7%, respectively. These reductions in latent heat storage capacities of the composite PCMs were due to the decreases occurred in the mass fractions of the PCMs in the bentonite after the EG addition. However, it can be concluded that the effects of EG addition on all LHTES properties of the produced Bb-FSPCMs were in negligible level.

enhanced Bb-FSPCMs. The produced Bentonite/CA, Bentonite/ PEG600, Bentonite/DD and Bentonite/HD have good energy storing/releasing functions as novel composite construction materials. According to the leakage test results, the maximum absorption ratio for CA, PEG600, DD and HD was found to be 40, 43, 32 and 18 wt%, respectively. The results of chemical and morphological characterization confirmed the presence of good physical compatibility with non-occurring any chemical reaction between the components of the composites. The DSC measurements indicated that the produced Bb-FSPCMs had suitable phase change temperature of 4–30 °C and good latent heat capacity between 38 and 74 J/g, respectively for solar space heating and cooling applications of buildings envelopes depending on climatic conditions. Thermal cycling test showed that the produced Bb-FSPCMs have good LHTES dependability and chemical stability. TG analysis exhibited that the produced Bb-FSPCMs had adequate thermal resistance. Furthermore, after 5 wt% EG addition, the thermal conductivities of Bentonite/CA, Bentonite/PEG600, Bentonite/DD and Bentonite/ HD were increased by approximately 65%, 63%, 39% and 47%, respectively. The enhancement to be occurred in the thermal conductivity values of the composites was also tested by measuring the amounts of reduction taken place in their total heating times. By taking account of all results, it can be also deduced that the produced Bentonite/CA, Bentonite/PEG600, Bentonite/DD and Bentonite/HD composites can be considered as promising construction materials for building envelopes, thermal insulation/ coatings of interior/exterior wall, plasterboards, building bricks, polyurethane foam and etc. These composites with energy harvesting/releasing property have great usage potential for adjusting indoor temperature to the comfortable range by means of solar passive LHTES. However, advanced-level studies are required still to expose their LHTES performances in practice scale.

3.7. Effect thermal conductivity enhancement on the total heating time

Acknowledgements

The increase to be occurred in the thermal conductivities of the produced Bb-FSPCMs was also evaluated by comparing their heating times of the composite PCMs with and without EG additive. Fig. 12 shows the effect of enhanced thermal conductivity on the total heating times of the four kinds of composite PCMs. In the plots of temperature vs time, the total heating time was determined as passing time period from same initial temperature to same final temperature value which is over the melting temperature of the PCM absorbed into the composite. The temperature differences (DT) between the sublimit and upper limit values during the completion of heating period of the composite PCMs were also shown on Fig. 11 as 12.8, 10, 8.2 and 19.8 °C for Bentonite/CA, Bentonite/PEG600, Bentonite/DD and Bentonite/HD composites and also EG included composites, respectively. Thus, the heating time was considered as passing period for increasing the temperature of the composite sample as much as amount of DT. After EG addition to the composites, the amount reduction occurred in their total heating times corresponded to the DT values are measured as 15, 17, 22 and 12 s, respectively for the composites including CA, PEG600, DD and HD. These results also mean that compared to those of the others, the total heating times of the Bb-FSPCMs with EG additive were diminished as about 16%, 19%, 26%, and 11%, respectively because of the increasing effect of the enhanced thermal conductivity on the heat transfer rate to be occurred during their heating periods of the EG added-composites.

The author would like to thank Cahit Bilgin for their helps in some parts of the experiments and also thank Altınay Boyraz due to SEM analysis.

4. Conclusions This study is focused on preparation, characterization and investigation of LHTES properties of four kinds of thermal

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