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micronized silica composite for latent heat thermal energy storage
Energy and Buildings 70 (2014) 180–185
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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Thermal performance of organic PCMs/micronized silica composite for latent heat thermal energy storage Okyoung Chung, Su-Gwang Jeong, Seulgi Yu, Sumin Kim ∗ Building Environment & Materials Lab, School of Architecture, Soongsil University, Seoul 156-743, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 April 2013 Received in revised form 27 October 2013 Accepted 10 November 2013 Keywords: BioPCM Octadecane Micronized silica Heat storage Incorporation Phase change material
a b s t r a c t The aim of this study is to prepare a form-stable composite phase change material (PCM) for latent heat thermal energy storage (LHTES) in buildings. Octadecane as paraffinic PCM and BioPCM as nonparaffinic PCM were used in the experiment as kinds of organic PCM by vacuum impregnation. It is loaded with porous of micronized silica (MS). The composite PCMs were characterized using scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) analysis techniques and determination of thermal properties and thermal reliability of the composite PCMs were analyzed using differential scanning calorimetry (DSC) and thermo gravimetric analyses (TGA) analysis techniques. SEM results showed that ocatadecane and BioPCM confined into porous of MS. FT-IR analysis indicated that the composite formation of porous MS and octadecane and BioPCM were physical. 241.8 J/g of latent heat of composites, 26.9 ◦ C of melting point, and 120.0 J/g of it, 22.1 ◦ C, was respectively measured by DSC analysis. The values for latent heat capacity reduction ratio of BioPCM as non-paraffinic PCM and octadecane as paraffinic PCM were nearly 29% and 56%. Furthermore, TGA analysis result showed that composite PCM had good thermal durability in the working temperature range. As a result, this composite PCM could be considered to have good potential for thermal energy storage because of its good thermal energy storage properties, thermal and chemical reliability. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Recently, given the predictable rise in the cost of fossil fuels and the desire to reduce carbon emissions, many countries have enforced policies to improve energy efficiency, while reducing fossil fuel usage. Thermal energy storage (TES) allows melting and freezing to be stored, which can then be used later. Thermal energy can be stored as latent heat when a substance changes from one phase to another, by either melting or freezing. The latent heat thermal energy storage (LHTES) method, in which energy is stored in the form of latent heat in phase change materials (PCMs), is one of the most efficient ways of storing thermal energy [1,2]. PCMs can play an important role in an energy storage device by utilizing its high storage density and latent heat property [3]. The optimization of integrating PCMs within passive LHTES systems and the optimal integration of these systems within the building provides a much higher heat storage density, with a smaller temperature difference between storage and release of heat [4]. It can be used to shift the cooling or heating load from the peak period, to the off-peak period [5].
∗ Corresponding author. Tel.: +82 2 820 0665; fax: +82 2 816 3354. E-mail address: [email protected] (S. Kim). 0378-7788/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.enbuild.2013.11.055
PCMs have many fields of application. Some of the potential applications of PCM have been investigated by Rodriguez-Ubinas et al. [6], Zhang and Ma [7], and Mónica Delgado et al. [8]. PCM is divided into three kinds, which are organic PCM, inorganic PCM and eutectic PCM. These are used as LHTES, which can be classified into two major categories, of organic and inorganic compounds. Inorganic PCMs include salt hydrates, salts, metals and alloys, whereas organic PCMs are comprised of hexadecane, octadecane, paraffin and fatty acids/esters, etc. A great variety of inorganic and organic PCMs and their mixtures have been investigated as latent heat storage materials. Organic PCMs would be useful for application in various fields, because PCM has a large latent heat and low cost, and is stable, non-toxic and not corrosive [9–12]. Also PCMs are classified into paraffinic PCMs or non-paraffinic PCMs, such as fatty acids/esters [13,14]. Paraffin has been widely used for latent heat thermal energy storage applications due to their large latent heat and proper thermal characteristics [15]. But, composite PCM manufacturing existing paper, composite paraffinic PCM was degrade latent heat storage. Paraffinic PCM and non-paraffinic PCM by heat storage reducing were compared. A octadecane as paraffinic PCM, and BioPCM as non-paraffinic PCM, were used in the experiment as kinds of organic PCM. BioPCM raw materials were made of vegetable oil, such as highly refined soybean and palm oils. Fatty acid PCM can also be manufactured,
O. Chung et al. / Energy and Buildings 70 (2014) 180–185
such that the melting point can be made from −22.7 ◦ C to 78.33 ◦ C (−73 ◦ F to +173 ◦ F), and this facilitates its use in various climatic zones [15,16]. Organic materials are further described as paraffins or non-paraffins. The octadecane as paraffinic PCM was chosen, in order to compare the BioPCM, because the octadecane and BioPCM have a very similar melting temperature range. But, they have different latent heat storage. Latent heat storage of octadecane and BioPCM were 120 J/g and 241.8 J/g. The two types of composite PCM are loaded with micronized silica. Fine powder processing of silica gel is micronized silica (MS). MS, a powder with particles size of 3–3.5 m, is fine noncrystalline silica. We chose micronized silica model SS-SIL 230, because of its pH 7 value, when applied to a cement as alkaline, in order to minimize the impact of the model that has been selected. MS amorphous porous silica gel as a raw material is a porous material. Considering all those mentioned about, MS is one of the most suitable candidates as building material to prepare form-stable composite PCM for LHTES in buildings. In this study, two types of octadecane as paraffinic PCM and BioPCM as non-paraffinic PCM by organic PCM, by impregnated with MS were prepared using a vacuum impregnating process. Hence, the MS/organic PCM blends were prepared as a novel form-stable composite PCM by the method of immersion. This paper analyzed characteristics of form-stable composites PCM, determined by SEM, DSC, TGA and FTIR. The morphologies and compatibilities of the composite PCM were characterized by SEM and FTIR analysis technique. The composite’s thermal properties and thermal reliabilities were determined by DSC analysis. Analysis on thermal durability of the thermal enhanced octadecane and BioPCM were carried out using TGA. This study also deals with improvement of the form-stable composite PCM using micronized silica. Investigations of their morphology and chemical compatibility, as well as their thermal properties, were carried out. The results can show better development on adsorption matrix in producing PCM applied in buildings [16]. Impregnation of octadecane as paraffinic PCM and BioPCM as non-paraffinic PCM by organic PCM were compared with impregnated latent heat loading MS.
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Fig. 1. Vacuum impregnation set-up.
2.2. Preparation The octadecane and BioPCM were prepared using the vacuum impregnation method [19,20]. Fig. 1 shows vacuum impregnation set-up. The MS was dried at 105 ◦ C before the composite PCMs were prepared. Because of water in the pores of MS is removed. Then, 50 g of MS was added in the impregnation set-up. Next, the valve between the flask and container with 100 g of liquid BioPCM or octadecane was opened to allow them to slowly pour into the flask to cover the MS [20]. The vacuum process was continued for 90 min, and then air was allowed to enter the flask again to force the liquid of each two types of PCM to penetrate into the porous structure of MS. After the penetrating process, excess PCM remained in the flask, and needed to be removed through filtering process. The two types of composite PCM in a colloidal shape were filtered by 1 m filter paper, until a granular sample was apparent, which was dried under vacuum at 80 ◦ C for 24 h. After, composite PCMs were cooled at room temperature. 2.3. Characterization techniques
2. Experimental 2.1. Materials This paper used two types of organic PCMs with different melting points and octadecane and BioPCM were selected as organic PCM. The melting enthalpy of paraffinic PCM shows range 100–200 J/g. Also, the melting enthalpy of fatty acid PCM as non-paraffinic PCM shows range 20–160 J/g [17]. The latent heat capacity of reference octadecane as paraffinic PCM shows 241.97 J/g [11]. Also, the latent heat capacity of reference BioPCM as nonparaffinic PCM shows 149.2 J/g [18]. Because of this high latent heat of organic PCM, two kinds of experiments were selected. The BioPCMTM was obtained from Korea C&S Corporation in South Korea. BioPCMTM were manufactured in PHASE CHANGE ENERGY SOLUTION. The octadecane was purchased from the Sigma–Aldrich Company. The MS was supplied from S-CHEMTHECH Corporation in South Korea. The MS has 300 ml/100 g oil absorption, 3.5 m particle size, surface area of 310 m2 /g, and pore volume of 1.6 ml/g. The detailed properties of MS are shown in Table 1.
The microstructures of the octadecane, BioPCM and composite PCMs were studied by using JEOL scanning electron microscopy (SEM) at room temperature. A JEOL SEM with accelerating voltage of 13 kV and working distance of 10 mm was used to collect the SEM images. All samples were gold coated by sputtering to produce conductive coatings onto the samples [10]. Fourier transformed infrared (FTIR, 300E Jasco) spectra was used to confirm the change of chemical groups of composites at room temperature, by analayzing FT-IR spectra of the octadecane, BioPCM and composite PCMs [21]. FTIR analysis was used to study the possible chemical and physical interactions between the two type of composite PCMs and micronized silica. Thermal properties of BioPCM, octadecane and composite PCMs, such as the melting and crystallizing points and phase change enthalpies of shape stabilized composite PCMs, were measured using differential scanning calorimetry (DSC: Q 1000), in the range of 0–80 ◦ C at a melting and freezing rate of 10 ◦ C/min, in an nitrogen atmosphere with the flow rate of 60 ml/min. the latent heat was calculated by intergration of the DSC peak. Thermal durability of the thermal stability composite PCMs were carried
Table 1 Properties of micronized silica. Description
Chemical composition
Particle size (m)
Moisture (%)
Oil Abs. (ml/100 g)
pH
Surface area (m2 /g)
Pore volume (ml/g)
Micronized silica
SiO2
3.5
≤4.0
300
7.0
310
1.6
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Fig. 2. SEM images of (a) BioPCM (b) micronized silica (c) BioPCM/MS, and (d) octadecane/MS.
out using thermo gravimetric analysis (TGA: TA Instruments, TGA Q5000) on around 2–4 mg samples, in the range 0–600 ◦ C at a heating rate of 10 ◦ C/min, in a nitrogen atmosphere (99.5% nitrogen, 0.5% oxygen content) to prevent unwanted oxidation [21,22]. Fig. 1 shows vacuum impregnation set-up. 3. Results and discussion 3.1. Microstructure of the composite PCMs The microstructure of the BioPCM, MS, octadecane/MS and BioPCM/MS was investigated by SEM analysis, and the images are shown in Fig. 2. As shown in Fig. 2(a), BioPCM has a smooth surface with various sizes of particle without sharpness or edge. A lot of research has been conducted for the organic PCM. For example, SEM images of organic PCM were investigated by [23–26]. Fig. 2(b) shows that micronized silica is an amorphous. It shows individual silica, of bulk <10 nm and 3.5 m of average particle size (Table 1). Fig. 2(c) shows the microstructure of octadecane/MS. Fig. 2(d) shows the microstructure of BioPCM/MS. Fig. 2(c) and (d) have a net-like surface. 3.2. FTIR analysis of base materials and composite PCMs The FT-IR spectrum provides useful information about chemical compatibility among the components of the composites. Fig. 3 shows the FTIR spectrum of base materials and composite PCMs. This experiment was carried out to determine chemical properties of the organic composite PCMs. BioPCM, a proprietary configuration of PCM materials, is derived from agricultural and livestock bi-products. It consists of under-used raw material, such as soybean oils, coconut oils and palm oils. Table 2 shows the main component of fatty acids, which contain BioPCM in common vegetable oils [26–28]. In FTIR spectra of the silica, the peaks usually appear between 1200 and 1000 cm−1 . The most intense band at wave number of 1073 cm−1 is due to asymmetric stretching of the Si O Si bonding. Palmitic, Oleic and Linoleic are organic compounds found
Table 2 The main component of fatty acids in palm oil, soybean oil and coconut oil. Fatty acid
Palm oil
Soybean oil
Coconut oil
Myristic (C14:0) Palmitic (C16:0) Stearic (C18:0) Oleic (C18:1) Linoleic (C18:2) Linolenic (C18:3) Lauric acid (C 12:0)
1% 43.5% 5% 36.6% 9.1% 0.5% –
– 11% 4% 24% 54% 7% –
18% 9% 3% 6% 2% – 47%
Heat storage properties of BioPCM, octadecane, BioPCM/MS and octadecane/MS.
in many common vegetable oils. Palmitic acid is the most common fatty acid found in plants, comprising 43% (by wt) in palm oil, 11% in soybean oil and 9% in coconut oil. Soybean oils, coconut oils and palm oil, etc. are composed of fatty acids, such as ␣-linolenic acid, ␣-linolenic acid is an organic compound found. Its molecular formula is CH3 (CH2 )14 CO2 H [18,29]. Oleic acid is a fatty acid that occurs naturally in vegetable fats and oils, comprising 36% (by wt) of palm oil, 24% of soybean oil and 6% of coconut oil. Its molecular formula is C18 H34 O2 [30]. Linoleic acid is abundant in many vegetable oils, comprising 54% (by wt) of soybean oil, 10% of palm oil and 2% coconut oil [31]. Lauric acid is the main component of coconut oil. Its molecular formula is C12 H24 O2 . In Fig. 2(a), the peak of BioPCM shows the experimental results of the CH2 symmetric stretching band for palmitic acid at 2842 cm−1 , increased with increasing molarity of palmitic acid [32]. Also it shows the experimental results of the oleic acid spectrum, which is carbonyl absorbance at 1739 cm−1 , a position that indicates that the molecules are present in dimers that are held together by hydrogen bonding [33]. In the following graphs, BioPCM shows absorption peaks of from 2971 to 2822 cm−1 , 1754–1686 cm−1 , 1250–1013 cm−1 and 721–716 cm−1 . Frequency in the range of 2923 and 2910 is caused by asymmetrical and symmetrical stretching vibration of the methylene ( CH2 ) group. Frequency in the range of 1119 cm−1 and 1096 cm−1 is caused by stretching vibration of the C O ester group. Frequency in the range of 1030 cm−1 is caused by C O stretching. Frequency
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Fig. 4. DSC curves of (a) BioPCM and BioPCM/MS, and (b) octadecane and octadecane/MS. Fig. 3. FTIR spectra of (a) Micronized silica, BioPCM/MS, BioPCM, and (b) octadecane/MS, octadecane.
in the range of 721 cm−1 is caused by overlapping of the methylene ( CH2 ) rocking vibration, and the out of plane vibration of cis-disubstituted olefins. In the following graphs of BioPCM/MS, we determined these to be almost the same, with absorption peaks of BioPCM and MS [33]. Octadecane FTIR absorption spectra have peak wave numbers of 2915, 2847, 1470 and 715 cm−1 , caused by stretching vibration of functional groups of CH2 and CH3 . As a result, Fig. 3 two type of composite PCMs peak have not disappeared PCMs peak and MS peak from the FTIR graphs. FTIR peaks of two type of PCM have not changed during the impregnation process. This means that the characteristics of PCMs were not changed. Therefore, this means that the octadecane and BioPCM molecules were held easily into the pore of micronized silica by these physical interactions and thus leakage of the melted PCM from the porous was prevented [2].
3.3. Thermal properties analysis DSC analysis was conducted, to investigate the thermal properties, such as melting or freezing temperature, and thermal stability composite. The DSC curve of BioPCM and BioPCM/MS in Fig. 4 and octadecane and octadecane/MS in Fig. 4 were taken as references to octadecane and octadecane/MS, respectively. The results obtained by DSC measurements, such as phase transition temperature (Tt ), phase change temperature (Tm ), latent heat and incorporated rate, are listed in Table 3. In DSC thermograms, the melting and freezing temperatures range were determined as 22.14 ◦ C and 23.26 ◦ C for BioPCM, 22.18 ◦ C and 23.16 ◦ C for BioPCM/MS, 26.93 ◦ C and 27.59 ◦ C for octadecane, and 21.18 ◦ C and 26.16 ◦ C for octadecane/MS, respectively. Although there are the small decreases in phase change temperature of composite PCMs, the melting range and freezing range of composites are very close to pure PCMs.
Table 3 Heat storage properties of BioPCM, octadecane, BioPCM/MS and octadecane/MS. PCM samples
BioPCM Octadecane BioPCM/micronized silica Octadecane/micronized silica
Melting range (◦ C)
22.14 26.93 22.18 21.18
Freezing range (◦ C)
23.26 27.59 23.16 26.16
Boiling temperature (◦ C)
418 317 – –
Latent heat (J/g)
Incorporated rate (%)
Solid–liquid melting
Liquid–solid freezing
120.0 241.8 86.07 108.1
118.5 216.4 87.11 111.5
– – 71.17 44.7
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Table 4 Thermal resistance analysis of pure PCMs and composite PCMs. PCM samples
First peak of derivative weight (◦ C)
Second peak of derivative weight (◦ C)
Peak temperature difference (◦ C)
Oxidation rate (%)
BioPCM BioPCM/micronized silica Octadecane Octadecane/micronized silica
228.07 211.48 209.32 201.22
290.23 278.15 – –
62.16 66.67 – –
– 90.65 – 79.14
The phase transition of BioPCM happened between latent heat capacities of 120.0 J/g and 118.5 J/g, during heating and freezing. Also, in the case of BioPCM/MS composite from the results profiles, its latent heat capacity is 89.07 J/g and 87.11 J/g, during heating and freezing. The latent heat capacity of pure octadecane shows 241.8 J/g and 216.4 J/g, during heating and freezing. Also, in the case of the octadecane/MS composite from results profiles, its latent heat capacity is 108.1 J/g and 111.5 J/g, during heating and freezing. The values for latent heat capacity of the BioPCM/MS and octadecane/MS were nearly 71% and 44% of the BioPCM and octadecane. These DSC results are in agreement with those reported by Karaipekli [34] and Zhang et al. [35]. In their studies, they found that the interactions between the components
of the composite play an important role in deciding the shift direction of the melting/freezing point in porous media. For a weakly attractive interaction between fluid and pore surface, a depressed phase change temperature will occur. On the other hand, latent heat values obtained from DSC analyses were suitable for latent heat storage purposes in buildings. Also Karaipekli [34] explain decrease of latent heats of melting point and freezing point. These results were probably caused by interactions such as capillary phenomenon between the PCM and the inner surface of pore of MS. As shown in Fig. 4(a) and (b), BioPCM as non-paraffinic PCM, octadecane as paraffinic PCM are latent heats of melting point and freezing range are much higher. However, in Fig. 4(a) and (b), the paraffinic type octadecane/MS composite showed greater reduction ratio in latent heats (i.e., melting and freezing heats) in comparison with the non-paraffinic type bio-based/MS composite. 3.4. Thermogravimetric analyses The thermogravimetric analysis (TGA) curves under air atmosphere are shown in Fig. 4. List of Table 4 shows the TGA of base PCMs and composite PCMs. In the graph, BioPCM has two curves of thermal oxidation degradation. As shown in the derivative weight curve of BioPCM, the first peak occurred at 228.07 ◦ C, and the second peak occurred at 290.23 ◦ C. The derivative weight of BioPCM/MS shows 221.48 ◦ C at the first peak, and 278.15 ◦ C at the second peak. Peak temperature differences between the first peak and the second peak show 62.16 ◦ C and 66.67 ◦ C, respectively, for the BioPCM and shape stabilized BioPCM. Also, Fig. 5 shows the derivative weight curve of octadecane, where the peak occurred at 209.32 ◦ C. The derivative weight of octadecane/MS shows a 201.22 ◦ C peak. In these graphs, we found the derivative weight peak of shape stabilized composite PCMs are higher than that of pure PCM. This means that shape stabilized composite PCMs have higher flame retardant properties compared to BioPCM. Both octadecane/MS and BioPCM oxidation rates represented 79.14% and 90.65%, respectively. BioPCM impregnation rate of MS was higher than the octadecane of MS. Also TGA analysis showed shape stabilized composite left a more plentiful combustion residue, because of loading contents of MS. This result also showed plenty of BioPCM incorporated into the structure of MS. As a result, MS led to a form stability of the thermal resistance of the composites. 4. Conclusion
Fig. 5. Thermo gravimetric analysis of pure PCMs and composite PCMs: (a) the weight of pure PCMs and composite PCMs, and (b) the derivative weight of pure PCMs and composite PCMs.
Recently, many experiments have been carried out on the latent heat thermal energy storage method. We applied two type of PCM to reduce the energy consumption in building for the reduction of energy saving to latent heat thermal energy storage. In this experiment, we studied the shape stabilized BioPCM and octadecane by impregnation with MS. The characteristics of the composites were determined by using SEM, DSC, FTIR and TGA. SEM images showed the construction of composite PCMs. Conclusion of the experiment can be summarized as follows. SEM analysis showed that the octadecane, micronized silica and two type of composite PCM showed morphology and particle size. FTIR analysis showed peaks of two type of composite PCM have not changed during the impregnation process. DSC analysis showed that the latent heat
O. Chung et al. / Energy and Buildings 70 (2014) 180–185
capacity of octadecane/MS PCM was nearly 44.7% of the octadecane and the latent heat capacity of BioPCM/MS PCM was nearly 71.17% of the BioPCM. The latent heat capacity of two type of composite PCM was decreased. But, in comparison with octadecane/MS, BioPCM/MS are latent heats of reduction ratio are much lower. The TGA result shows plenty of BioPCM and octadecane incorporated into the structure of MS. According to the TGA curves, all composite PCMs show thermal stability at room temperature. Consequently, It was confirmed that PCM’s reduction rate heat storage as of non-paraffinic is lower than PCM’s rate of heat storage as paraffinic. These results were TGA analysis. It showed that BioPCM’s impregnation rate as non-paraffinic of MS was higher than the octadecane’s impregnation rate as paraffinic of MS. Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MEST) (No. 2013-030588). References [1] A. Abhat, Low temperature latent heat thermal energy storage: heat storage materials, Solar Energy 30 (1983) 313–332. [2] X. Zhang, P. Deng, R. Feng, J. Song, Novel gelatinous shape-stabilized phase change materials with high heat storage density, Solar Energy Materials and Solar Cells 95 (2011) 1213–1218. [3] A. Sharma, V.V. Tyagi, C.R. Chen, D. Buddhi, Review on thermal energy storage with phase change materials and applications, Renewable and Sustainable Energy Reviews 13 (2) (2009) 318–345 (Review Article). [4] M.M. Farid, A.M. Khudhair, S.A.K. Razack, S. Al-Hallaj, A review on phase change energy storage: materials and applications, Energy Conversion and Management 45 (9) (2004) 1597–1615 (Review Article). [5] X. Jin, X. Zhang, Thermal analysis of a double layer phase change material floor, Applied Thermal Engineering 31 (10) (2011) 1576–1581 (Original Research Article). [6] E. Rodriguez-Ubinas, L. Ruiz-Valero, S. Vega, J. Neila, Applications of phase change material in highly energy-efficient houses, Energy and Buildings 50 (2012) 49–62 (Original Research Article). [7] P. Zhang, Z.W. Ma, An overview of fundamental studies and applications of phase change material slurries to secondary loop refrigeration and air conditioning systems, Renewable and Sustainable Energy Reviews 16 (7) (2012) 5021–5058. [8] M. Delgado, A. Lázaro, J. Mazo, B. Zalba, Review on phase change material emulsions and microencapsulated phase change material slurries: materials, heat transfer studies and applications, Renewable and Sustainable Energy Reviews 16 (1) (2012) 253–273. [9] M. Kenisarin, K. Mahkamov, Solar energy storage using phase change materials, Renewable and Sustainable Energy 11 (2007) 1913–1965. [10] S. Kim, L.T. Drzal, High latent heat storage and high thermal conductive phase change materials using exfoliated graphite nanoplatelets, Solar Energy Materials and Solar Cells 93 (2009) 136–142. [11] J. Jeon, S.-G. Jeong, J.-H. Lee, J. Seo, S. Kim, High thermal performance composite PCMs loading xGnP for application to building using radiant floor heating system, Solar Energy Materials and Solar Cells 101 (2012) 51– 56. [12] P. Phelan, T. Lee, A. Reddy, Application of Phase Change Material in Buildings: Field Data vs. EnergyPlus Simulation by Karthik Muruganantham, 2013.
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Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild
Thermal performance of organic PCMs/micronized silica composite for latent heat thermal energy storage Okyoung Chung, Su-Gwang Jeong, Seulgi Yu, Sumin Kim ∗ Building Environment & Materials Lab, School of Architecture, Soongsil University, Seoul 156-743, Republic of Korea
a r t i c l e
i n f o
Article history: Received 2 April 2013 Received in revised form 27 October 2013 Accepted 10 November 2013 Keywords: BioPCM Octadecane Micronized silica Heat storage Incorporation Phase change material
a b s t r a c t The aim of this study is to prepare a form-stable composite phase change material (PCM) for latent heat thermal energy storage (LHTES) in buildings. Octadecane as paraffinic PCM and BioPCM as nonparaffinic PCM were used in the experiment as kinds of organic PCM by vacuum impregnation. It is loaded with porous of micronized silica (MS). The composite PCMs were characterized using scanning electron microscopy (SEM) and Fourier transform infrared (FT-IR) analysis techniques and determination of thermal properties and thermal reliability of the composite PCMs were analyzed using differential scanning calorimetry (DSC) and thermo gravimetric analyses (TGA) analysis techniques. SEM results showed that ocatadecane and BioPCM confined into porous of MS. FT-IR analysis indicated that the composite formation of porous MS and octadecane and BioPCM were physical. 241.8 J/g of latent heat of composites, 26.9 ◦ C of melting point, and 120.0 J/g of it, 22.1 ◦ C, was respectively measured by DSC analysis. The values for latent heat capacity reduction ratio of BioPCM as non-paraffinic PCM and octadecane as paraffinic PCM were nearly 29% and 56%. Furthermore, TGA analysis result showed that composite PCM had good thermal durability in the working temperature range. As a result, this composite PCM could be considered to have good potential for thermal energy storage because of its good thermal energy storage properties, thermal and chemical reliability. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Recently, given the predictable rise in the cost of fossil fuels and the desire to reduce carbon emissions, many countries have enforced policies to improve energy efficiency, while reducing fossil fuel usage. Thermal energy storage (TES) allows melting and freezing to be stored, which can then be used later. Thermal energy can be stored as latent heat when a substance changes from one phase to another, by either melting or freezing. The latent heat thermal energy storage (LHTES) method, in which energy is stored in the form of latent heat in phase change materials (PCMs), is one of the most efficient ways of storing thermal energy [1,2]. PCMs can play an important role in an energy storage device by utilizing its high storage density and latent heat property [3]. The optimization of integrating PCMs within passive LHTES systems and the optimal integration of these systems within the building provides a much higher heat storage density, with a smaller temperature difference between storage and release of heat [4]. It can be used to shift the cooling or heating load from the peak period, to the off-peak period [5].
∗ Corresponding author. Tel.: +82 2 820 0665; fax: +82 2 816 3354. E-mail address: [email protected] (S. Kim). 0378-7788/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.enbuild.2013.11.055
PCMs have many fields of application. Some of the potential applications of PCM have been investigated by Rodriguez-Ubinas et al. [6], Zhang and Ma [7], and Mónica Delgado et al. [8]. PCM is divided into three kinds, which are organic PCM, inorganic PCM and eutectic PCM. These are used as LHTES, which can be classified into two major categories, of organic and inorganic compounds. Inorganic PCMs include salt hydrates, salts, metals and alloys, whereas organic PCMs are comprised of hexadecane, octadecane, paraffin and fatty acids/esters, etc. A great variety of inorganic and organic PCMs and their mixtures have been investigated as latent heat storage materials. Organic PCMs would be useful for application in various fields, because PCM has a large latent heat and low cost, and is stable, non-toxic and not corrosive [9–12]. Also PCMs are classified into paraffinic PCMs or non-paraffinic PCMs, such as fatty acids/esters [13,14]. Paraffin has been widely used for latent heat thermal energy storage applications due to their large latent heat and proper thermal characteristics [15]. But, composite PCM manufacturing existing paper, composite paraffinic PCM was degrade latent heat storage. Paraffinic PCM and non-paraffinic PCM by heat storage reducing were compared. A octadecane as paraffinic PCM, and BioPCM as non-paraffinic PCM, were used in the experiment as kinds of organic PCM. BioPCM raw materials were made of vegetable oil, such as highly refined soybean and palm oils. Fatty acid PCM can also be manufactured,
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such that the melting point can be made from −22.7 ◦ C to 78.33 ◦ C (−73 ◦ F to +173 ◦ F), and this facilitates its use in various climatic zones [15,16]. Organic materials are further described as paraffins or non-paraffins. The octadecane as paraffinic PCM was chosen, in order to compare the BioPCM, because the octadecane and BioPCM have a very similar melting temperature range. But, they have different latent heat storage. Latent heat storage of octadecane and BioPCM were 120 J/g and 241.8 J/g. The two types of composite PCM are loaded with micronized silica. Fine powder processing of silica gel is micronized silica (MS). MS, a powder with particles size of 3–3.5 m, is fine noncrystalline silica. We chose micronized silica model SS-SIL 230, because of its pH 7 value, when applied to a cement as alkaline, in order to minimize the impact of the model that has been selected. MS amorphous porous silica gel as a raw material is a porous material. Considering all those mentioned about, MS is one of the most suitable candidates as building material to prepare form-stable composite PCM for LHTES in buildings. In this study, two types of octadecane as paraffinic PCM and BioPCM as non-paraffinic PCM by organic PCM, by impregnated with MS were prepared using a vacuum impregnating process. Hence, the MS/organic PCM blends were prepared as a novel form-stable composite PCM by the method of immersion. This paper analyzed characteristics of form-stable composites PCM, determined by SEM, DSC, TGA and FTIR. The morphologies and compatibilities of the composite PCM were characterized by SEM and FTIR analysis technique. The composite’s thermal properties and thermal reliabilities were determined by DSC analysis. Analysis on thermal durability of the thermal enhanced octadecane and BioPCM were carried out using TGA. This study also deals with improvement of the form-stable composite PCM using micronized silica. Investigations of their morphology and chemical compatibility, as well as their thermal properties, were carried out. The results can show better development on adsorption matrix in producing PCM applied in buildings [16]. Impregnation of octadecane as paraffinic PCM and BioPCM as non-paraffinic PCM by organic PCM were compared with impregnated latent heat loading MS.
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Fig. 1. Vacuum impregnation set-up.
2.2. Preparation The octadecane and BioPCM were prepared using the vacuum impregnation method [19,20]. Fig. 1 shows vacuum impregnation set-up. The MS was dried at 105 ◦ C before the composite PCMs were prepared. Because of water in the pores of MS is removed. Then, 50 g of MS was added in the impregnation set-up. Next, the valve between the flask and container with 100 g of liquid BioPCM or octadecane was opened to allow them to slowly pour into the flask to cover the MS [20]. The vacuum process was continued for 90 min, and then air was allowed to enter the flask again to force the liquid of each two types of PCM to penetrate into the porous structure of MS. After the penetrating process, excess PCM remained in the flask, and needed to be removed through filtering process. The two types of composite PCM in a colloidal shape were filtered by 1 m filter paper, until a granular sample was apparent, which was dried under vacuum at 80 ◦ C for 24 h. After, composite PCMs were cooled at room temperature. 2.3. Characterization techniques
2. Experimental 2.1. Materials This paper used two types of organic PCMs with different melting points and octadecane and BioPCM were selected as organic PCM. The melting enthalpy of paraffinic PCM shows range 100–200 J/g. Also, the melting enthalpy of fatty acid PCM as non-paraffinic PCM shows range 20–160 J/g [17]. The latent heat capacity of reference octadecane as paraffinic PCM shows 241.97 J/g [11]. Also, the latent heat capacity of reference BioPCM as nonparaffinic PCM shows 149.2 J/g [18]. Because of this high latent heat of organic PCM, two kinds of experiments were selected. The BioPCMTM was obtained from Korea C&S Corporation in South Korea. BioPCMTM were manufactured in PHASE CHANGE ENERGY SOLUTION. The octadecane was purchased from the Sigma–Aldrich Company. The MS was supplied from S-CHEMTHECH Corporation in South Korea. The MS has 300 ml/100 g oil absorption, 3.5 m particle size, surface area of 310 m2 /g, and pore volume of 1.6 ml/g. The detailed properties of MS are shown in Table 1.
The microstructures of the octadecane, BioPCM and composite PCMs were studied by using JEOL scanning electron microscopy (SEM) at room temperature. A JEOL SEM with accelerating voltage of 13 kV and working distance of 10 mm was used to collect the SEM images. All samples were gold coated by sputtering to produce conductive coatings onto the samples [10]. Fourier transformed infrared (FTIR, 300E Jasco) spectra was used to confirm the change of chemical groups of composites at room temperature, by analayzing FT-IR spectra of the octadecane, BioPCM and composite PCMs [21]. FTIR analysis was used to study the possible chemical and physical interactions between the two type of composite PCMs and micronized silica. Thermal properties of BioPCM, octadecane and composite PCMs, such as the melting and crystallizing points and phase change enthalpies of shape stabilized composite PCMs, were measured using differential scanning calorimetry (DSC: Q 1000), in the range of 0–80 ◦ C at a melting and freezing rate of 10 ◦ C/min, in an nitrogen atmosphere with the flow rate of 60 ml/min. the latent heat was calculated by intergration of the DSC peak. Thermal durability of the thermal stability composite PCMs were carried
Table 1 Properties of micronized silica. Description
Chemical composition
Particle size (m)
Moisture (%)
Oil Abs. (ml/100 g)
pH
Surface area (m2 /g)
Pore volume (ml/g)
Micronized silica
SiO2
3.5
≤4.0
300
7.0
310
1.6
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Fig. 2. SEM images of (a) BioPCM (b) micronized silica (c) BioPCM/MS, and (d) octadecane/MS.
out using thermo gravimetric analysis (TGA: TA Instruments, TGA Q5000) on around 2–4 mg samples, in the range 0–600 ◦ C at a heating rate of 10 ◦ C/min, in a nitrogen atmosphere (99.5% nitrogen, 0.5% oxygen content) to prevent unwanted oxidation [21,22]. Fig. 1 shows vacuum impregnation set-up. 3. Results and discussion 3.1. Microstructure of the composite PCMs The microstructure of the BioPCM, MS, octadecane/MS and BioPCM/MS was investigated by SEM analysis, and the images are shown in Fig. 2. As shown in Fig. 2(a), BioPCM has a smooth surface with various sizes of particle without sharpness or edge. A lot of research has been conducted for the organic PCM. For example, SEM images of organic PCM were investigated by [23–26]. Fig. 2(b) shows that micronized silica is an amorphous. It shows individual silica, of bulk <10 nm and 3.5 m of average particle size (Table 1). Fig. 2(c) shows the microstructure of octadecane/MS. Fig. 2(d) shows the microstructure of BioPCM/MS. Fig. 2(c) and (d) have a net-like surface. 3.2. FTIR analysis of base materials and composite PCMs The FT-IR spectrum provides useful information about chemical compatibility among the components of the composites. Fig. 3 shows the FTIR spectrum of base materials and composite PCMs. This experiment was carried out to determine chemical properties of the organic composite PCMs. BioPCM, a proprietary configuration of PCM materials, is derived from agricultural and livestock bi-products. It consists of under-used raw material, such as soybean oils, coconut oils and palm oils. Table 2 shows the main component of fatty acids, which contain BioPCM in common vegetable oils [26–28]. In FTIR spectra of the silica, the peaks usually appear between 1200 and 1000 cm−1 . The most intense band at wave number of 1073 cm−1 is due to asymmetric stretching of the Si O Si bonding. Palmitic, Oleic and Linoleic are organic compounds found
Table 2 The main component of fatty acids in palm oil, soybean oil and coconut oil. Fatty acid
Palm oil
Soybean oil
Coconut oil
Myristic (C14:0) Palmitic (C16:0) Stearic (C18:0) Oleic (C18:1) Linoleic (C18:2) Linolenic (C18:3) Lauric acid (C 12:0)
1% 43.5% 5% 36.6% 9.1% 0.5% –
– 11% 4% 24% 54% 7% –
18% 9% 3% 6% 2% – 47%
Heat storage properties of BioPCM, octadecane, BioPCM/MS and octadecane/MS.
in many common vegetable oils. Palmitic acid is the most common fatty acid found in plants, comprising 43% (by wt) in palm oil, 11% in soybean oil and 9% in coconut oil. Soybean oils, coconut oils and palm oil, etc. are composed of fatty acids, such as ␣-linolenic acid, ␣-linolenic acid is an organic compound found. Its molecular formula is CH3 (CH2 )14 CO2 H [18,29]. Oleic acid is a fatty acid that occurs naturally in vegetable fats and oils, comprising 36% (by wt) of palm oil, 24% of soybean oil and 6% of coconut oil. Its molecular formula is C18 H34 O2 [30]. Linoleic acid is abundant in many vegetable oils, comprising 54% (by wt) of soybean oil, 10% of palm oil and 2% coconut oil [31]. Lauric acid is the main component of coconut oil. Its molecular formula is C12 H24 O2 . In Fig. 2(a), the peak of BioPCM shows the experimental results of the CH2 symmetric stretching band for palmitic acid at 2842 cm−1 , increased with increasing molarity of palmitic acid [32]. Also it shows the experimental results of the oleic acid spectrum, which is carbonyl absorbance at 1739 cm−1 , a position that indicates that the molecules are present in dimers that are held together by hydrogen bonding [33]. In the following graphs, BioPCM shows absorption peaks of from 2971 to 2822 cm−1 , 1754–1686 cm−1 , 1250–1013 cm−1 and 721–716 cm−1 . Frequency in the range of 2923 and 2910 is caused by asymmetrical and symmetrical stretching vibration of the methylene ( CH2 ) group. Frequency in the range of 1119 cm−1 and 1096 cm−1 is caused by stretching vibration of the C O ester group. Frequency in the range of 1030 cm−1 is caused by C O stretching. Frequency
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Fig. 4. DSC curves of (a) BioPCM and BioPCM/MS, and (b) octadecane and octadecane/MS. Fig. 3. FTIR spectra of (a) Micronized silica, BioPCM/MS, BioPCM, and (b) octadecane/MS, octadecane.
in the range of 721 cm−1 is caused by overlapping of the methylene ( CH2 ) rocking vibration, and the out of plane vibration of cis-disubstituted olefins. In the following graphs of BioPCM/MS, we determined these to be almost the same, with absorption peaks of BioPCM and MS [33]. Octadecane FTIR absorption spectra have peak wave numbers of 2915, 2847, 1470 and 715 cm−1 , caused by stretching vibration of functional groups of CH2 and CH3 . As a result, Fig. 3 two type of composite PCMs peak have not disappeared PCMs peak and MS peak from the FTIR graphs. FTIR peaks of two type of PCM have not changed during the impregnation process. This means that the characteristics of PCMs were not changed. Therefore, this means that the octadecane and BioPCM molecules were held easily into the pore of micronized silica by these physical interactions and thus leakage of the melted PCM from the porous was prevented [2].
3.3. Thermal properties analysis DSC analysis was conducted, to investigate the thermal properties, such as melting or freezing temperature, and thermal stability composite. The DSC curve of BioPCM and BioPCM/MS in Fig. 4 and octadecane and octadecane/MS in Fig. 4 were taken as references to octadecane and octadecane/MS, respectively. The results obtained by DSC measurements, such as phase transition temperature (Tt ), phase change temperature (Tm ), latent heat and incorporated rate, are listed in Table 3. In DSC thermograms, the melting and freezing temperatures range were determined as 22.14 ◦ C and 23.26 ◦ C for BioPCM, 22.18 ◦ C and 23.16 ◦ C for BioPCM/MS, 26.93 ◦ C and 27.59 ◦ C for octadecane, and 21.18 ◦ C and 26.16 ◦ C for octadecane/MS, respectively. Although there are the small decreases in phase change temperature of composite PCMs, the melting range and freezing range of composites are very close to pure PCMs.
Table 3 Heat storage properties of BioPCM, octadecane, BioPCM/MS and octadecane/MS. PCM samples
BioPCM Octadecane BioPCM/micronized silica Octadecane/micronized silica
Melting range (◦ C)
22.14 26.93 22.18 21.18
Freezing range (◦ C)
23.26 27.59 23.16 26.16
Boiling temperature (◦ C)
418 317 – –
Latent heat (J/g)
Incorporated rate (%)
Solid–liquid melting
Liquid–solid freezing
120.0 241.8 86.07 108.1
118.5 216.4 87.11 111.5
– – 71.17 44.7
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Table 4 Thermal resistance analysis of pure PCMs and composite PCMs. PCM samples
First peak of derivative weight (◦ C)
Second peak of derivative weight (◦ C)
Peak temperature difference (◦ C)
Oxidation rate (%)
BioPCM BioPCM/micronized silica Octadecane Octadecane/micronized silica
228.07 211.48 209.32 201.22
290.23 278.15 – –
62.16 66.67 – –
– 90.65 – 79.14
The phase transition of BioPCM happened between latent heat capacities of 120.0 J/g and 118.5 J/g, during heating and freezing. Also, in the case of BioPCM/MS composite from the results profiles, its latent heat capacity is 89.07 J/g and 87.11 J/g, during heating and freezing. The latent heat capacity of pure octadecane shows 241.8 J/g and 216.4 J/g, during heating and freezing. Also, in the case of the octadecane/MS composite from results profiles, its latent heat capacity is 108.1 J/g and 111.5 J/g, during heating and freezing. The values for latent heat capacity of the BioPCM/MS and octadecane/MS were nearly 71% and 44% of the BioPCM and octadecane. These DSC results are in agreement with those reported by Karaipekli [34] and Zhang et al. [35]. In their studies, they found that the interactions between the components
of the composite play an important role in deciding the shift direction of the melting/freezing point in porous media. For a weakly attractive interaction between fluid and pore surface, a depressed phase change temperature will occur. On the other hand, latent heat values obtained from DSC analyses were suitable for latent heat storage purposes in buildings. Also Karaipekli [34] explain decrease of latent heats of melting point and freezing point. These results were probably caused by interactions such as capillary phenomenon between the PCM and the inner surface of pore of MS. As shown in Fig. 4(a) and (b), BioPCM as non-paraffinic PCM, octadecane as paraffinic PCM are latent heats of melting point and freezing range are much higher. However, in Fig. 4(a) and (b), the paraffinic type octadecane/MS composite showed greater reduction ratio in latent heats (i.e., melting and freezing heats) in comparison with the non-paraffinic type bio-based/MS composite. 3.4. Thermogravimetric analyses The thermogravimetric analysis (TGA) curves under air atmosphere are shown in Fig. 4. List of Table 4 shows the TGA of base PCMs and composite PCMs. In the graph, BioPCM has two curves of thermal oxidation degradation. As shown in the derivative weight curve of BioPCM, the first peak occurred at 228.07 ◦ C, and the second peak occurred at 290.23 ◦ C. The derivative weight of BioPCM/MS shows 221.48 ◦ C at the first peak, and 278.15 ◦ C at the second peak. Peak temperature differences between the first peak and the second peak show 62.16 ◦ C and 66.67 ◦ C, respectively, for the BioPCM and shape stabilized BioPCM. Also, Fig. 5 shows the derivative weight curve of octadecane, where the peak occurred at 209.32 ◦ C. The derivative weight of octadecane/MS shows a 201.22 ◦ C peak. In these graphs, we found the derivative weight peak of shape stabilized composite PCMs are higher than that of pure PCM. This means that shape stabilized composite PCMs have higher flame retardant properties compared to BioPCM. Both octadecane/MS and BioPCM oxidation rates represented 79.14% and 90.65%, respectively. BioPCM impregnation rate of MS was higher than the octadecane of MS. Also TGA analysis showed shape stabilized composite left a more plentiful combustion residue, because of loading contents of MS. This result also showed plenty of BioPCM incorporated into the structure of MS. As a result, MS led to a form stability of the thermal resistance of the composites. 4. Conclusion
Fig. 5. Thermo gravimetric analysis of pure PCMs and composite PCMs: (a) the weight of pure PCMs and composite PCMs, and (b) the derivative weight of pure PCMs and composite PCMs.
Recently, many experiments have been carried out on the latent heat thermal energy storage method. We applied two type of PCM to reduce the energy consumption in building for the reduction of energy saving to latent heat thermal energy storage. In this experiment, we studied the shape stabilized BioPCM and octadecane by impregnation with MS. The characteristics of the composites were determined by using SEM, DSC, FTIR and TGA. SEM images showed the construction of composite PCMs. Conclusion of the experiment can be summarized as follows. SEM analysis showed that the octadecane, micronized silica and two type of composite PCM showed morphology and particle size. FTIR analysis showed peaks of two type of composite PCM have not changed during the impregnation process. DSC analysis showed that the latent heat
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capacity of octadecane/MS PCM was nearly 44.7% of the octadecane and the latent heat capacity of BioPCM/MS PCM was nearly 71.17% of the BioPCM. The latent heat capacity of two type of composite PCM was decreased. But, in comparison with octadecane/MS, BioPCM/MS are latent heats of reduction ratio are much lower. The TGA result shows plenty of BioPCM and octadecane incorporated into the structure of MS. According to the TGA curves, all composite PCMs show thermal stability at room temperature. Consequently, It was confirmed that PCM’s reduction rate heat storage as of non-paraffinic is lower than PCM’s rate of heat storage as paraffinic. These results were TGA analysis. It showed that BioPCM’s impregnation rate as non-paraffinic of MS was higher than the octadecane’s impregnation rate as paraffinic of MS. Acknowledgment This work was supported by the National Research Foundation of Korea (NRF) Grant funded by the Korea government (MEST) (No. 2013-030588). References [1] A. Abhat, Low temperature latent heat thermal energy storage: heat storage materials, Solar Energy 30 (1983) 313–332. [2] X. Zhang, P. Deng, R. Feng, J. Song, Novel gelatinous shape-stabilized phase change materials with high heat storage density, Solar Energy Materials and Solar Cells 95 (2011) 1213–1218. [3] A. Sharma, V.V. Tyagi, C.R. Chen, D. Buddhi, Review on thermal energy storage with phase change materials and applications, Renewable and Sustainable Energy Reviews 13 (2) (2009) 318–345 (Review Article). [4] M.M. Farid, A.M. Khudhair, S.A.K. Razack, S. Al-Hallaj, A review on phase change energy storage: materials and applications, Energy Conversion and Management 45 (9) (2004) 1597–1615 (Review Article). [5] X. Jin, X. Zhang, Thermal analysis of a double layer phase change material floor, Applied Thermal Engineering 31 (10) (2011) 1576–1581 (Original Research Article). [6] E. Rodriguez-Ubinas, L. Ruiz-Valero, S. Vega, J. Neila, Applications of phase change material in highly energy-efficient houses, Energy and Buildings 50 (2012) 49–62 (Original Research Article). [7] P. Zhang, Z.W. Ma, An overview of fundamental studies and applications of phase change material slurries to secondary loop refrigeration and air conditioning systems, Renewable and Sustainable Energy Reviews 16 (7) (2012) 5021–5058. [8] M. Delgado, A. Lázaro, J. Mazo, B. Zalba, Review on phase change material emulsions and microencapsulated phase change material slurries: materials, heat transfer studies and applications, Renewable and Sustainable Energy Reviews 16 (1) (2012) 253–273. [9] M. Kenisarin, K. Mahkamov, Solar energy storage using phase change materials, Renewable and Sustainable Energy 11 (2007) 1913–1965. [10] S. Kim, L.T. Drzal, High latent heat storage and high thermal conductive phase change materials using exfoliated graphite nanoplatelets, Solar Energy Materials and Solar Cells 93 (2009) 136–142. [11] J. Jeon, S.-G. Jeong, J.-H. Lee, J. Seo, S. Kim, High thermal performance composite PCMs loading xGnP for application to building using radiant floor heating system, Solar Energy Materials and Solar Cells 101 (2012) 51– 56. [12] P. Phelan, T. Lee, A. Reddy, Application of Phase Change Material in Buildings: Field Data vs. EnergyPlus Simulation by Karthik Muruganantham, 2013.
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