Encapsulation techniques for organic phase change materials as thermal energy storage medium: A review

Encapsulation techniques for organic phase change materials as thermal energy storage medium: A review

Solar Energy Materials & Solar Cells 143 (2015) 78–98 Contents lists available at ScienceDirect Solar Energy Materials & Solar Cells journal homepag...

3MB Sizes 50 Downloads 305 Views

Solar Energy Materials & Solar Cells 143 (2015) 78–98

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells journal homepage: www.elsevier.com/locate/solmat

Review

Encapsulation techniques for organic phase change materials as thermal energy storage medium: A review Tumirah Khadiran a,b, Mohd Zobir Hussein a,n, Zulkarnain Zainal a, Rafeadah Rusli b a Material Synthesis and Characterization Laboratory (MSCL), Institute of Advanced Technology (ITMA), Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia b Forest Product Division, Forest Research Institute Malaysia (FRIM), 52109 Kepong, Selangor, Malaysia

art ic l e i nf o

a b s t r a c t

Article history: Received 2 April 2015 Received in revised form 5 June 2015 Accepted 18 June 2015

Thermal energy storage based on organic phase change materials (OPCMs) has attracted much attention to various applications for their excellence properties. However, OPCMs suffers from liquid leakage problem and low thermal conductivity, which limit their application as TES material. Encapsulation of OPCMs using organic or inorganic supporting materials is an effective way to overcome the leakage problem and enhancing their thermal conductivity property. In addition, the capsules could prevent possible interaction between OPCMs with the environment. There are many technologies described the encapsulation of OPCMs which depending on the type of supporting material and chemical properties of the OPCMs used. However, no complete overview of the techniques for encapsulation of OPCMs is available in the open literature. In this paper, we reviewed the techniques used for encapsulation of OPCMs and the method used to characterize the physico-chemical and thermal properties of encapsulated OPCMs. We believed that this review could provide useful information on the various encapsulation methods of OPCMs. & 2015 Elsevier B.V. All rights reserved.

Keywords: Organic phase change materials Encapsulation methods Organic supporting materials Inorganic supporting materials Physico-chemical characterization

Contents 1. 2.

3.

4. 5.

n

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 Micro- or nano-encapsulation methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79 2.1. Physico-mechanical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 2.2. Chemical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 2.2.1. Parametric study on the formation of micro- or nano-encapsulated OPCMs using chemical method . . . . . . . . . . . . . . . . . . . . . 80 2.3. Physico-chemical methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 Shape-stabilized methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 3.1. Supporting material based on polymers. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 3.1.1. Polymethylmethacrylate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84 3.1.2. High and low density polyethylene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 3.1.3. Biodegradable polymer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 3.1.4. Styrene maleic anhydrate copolymer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 3.1.5. Other polymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 3.2. Supporting material-based on inorganic frameworks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 3.2.1. Carbon-based materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 3.2.2. Inorganic porous building material. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 3.2.3. Inorganic framework based on silica . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 3.2.4. Others inorganic supporting materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 3.3. Advantages and disadvantages of using polymer and inorganic materials as a supporting material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 Intercalation method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Method for characterization of encapsulated OPCMs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

Corresponding author. Tel.: þ 60 3 89468092; fax: þ60 3 89467006. E-mail address: [email protected] (M.Z. Hussein).

http://dx.doi.org/10.1016/j.solmat.2015.06.039 0927-0248/& 2015 Elsevier B.V. All rights reserved.

T. Khadiran et al. / Solar Energy Materials & Solar Cells 143 (2015) 78–98

79

5.1. Chemical properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2. Physical properties. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3. Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.4. Thermal reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Thermal energy storage (TES) based on organic phase change materials (OPCMs) is an advanced material. They are widely developed for various applications especially for thermal comfort building, solar heating system, thermal protection, air-conditioning, transportation, thermal regulated textiles, electronic devices, etc. OPCMs are more preferred to be used for preparation of TES medium due to their narrow operating temperature range and high energy storage density during their melting processes [1–3]. OPCMs can be selected from paraffins (n-alkanes), fatty acids, polyethylene glycol and sugar alcohol [4,5]. OPCMs can also exist on the form of eutectics. Comprehensive reviews of OPCMs classification, types and their application are discussed by Khudhair et al. [6], Zalba et al. [4], Sharma et al. [7] and Farid et al. [8]. OPCMs offer a lot of advantages such as good thermal stability, freeze without much supercooling, ability to melt congruently, self-nucleating properties, no segregation, non-reactive and nontoxic [4,9]. Among OPCMs, paraffins and fatty acids are considered as the most promising OPCMs [3,10,11]. Unfortunately, OPCMs suffered with low thermal conductivity that causes supercooling and leakage problem during it melting state [12,13], thus limit their application as energy storage material. Direct utilization of fatty acids for TES has another difficulty which is bad odor due to sublimation during heating and poor thermal reliability [14]. Employing OPCMs in traditional manner has several weakness, such as the necessity of using special latent heat devices or heat exchange surface which subsequently could increase the associated cost and thermal resistant between the OPCMs and the environment [15]. Comprehensive investigations have been done using various supporting materials to solve the OPCMs problems. The supporting materials which widely used can be grouped into two; based on polymers and based on inorganic framework. The purpose of using supporting materials is to develop new OPCMs which have good thermal conductivity and form-stable (no more liquid leakage). Many encapsulation methods have been extensively developed previously to prepare new OPCMs such as micro- and nano-

93 93 93 94 95 95 95

encapsulation technology, shape-stabilized composite technique, and intercalation method. In addition, to overcome thermal conductivity and liquid leakage problem, there are many advantages to make OPCMs into form-stable OPCMs such as cost-effectiveness because no further encapsulation is needed and readiness for applications with tunable dimension. Fig. 1 shows the type of supporting materials which can be used to encapsulate OPCMs using various methods. Every encapsulation technique has advantages and disadvantages. The selection of supporting materials for specific OPCMs and specific applications is the key factor for the preparation of high-quality TES materials based on latent heat OPCMs. Nevertheless, all the information regarding the encapsulation OPCMs are not well integrated in a single source. This encourages us to compile and review the information on the existing technology which is used to encapsulate OPCMs. In this paper, a review of the state of art in developing TES material based on OPCMs using various encapsulation techniques will be discussed. The thermal properties and their limitation as energy storage medium are also presented.

2. Micro- or nano-encapsulation methods Micro-encapsulation is the process of coating individual particles or droplets with a film to produce capsules at micrometer to milliliter sized ( o1000 mm), known as a microcapsule [9], microparticles or microspheres. Capsules at nanometer in size (o1000 nm) are known as nanocapsules, nanoparticles or nanosphere. Micro- or nano-encapsulated OPCMs composed of two main parts; OPCMs as a core while polymer or inorganic material as a shell (Fig. 2). The shell will act as a container to protect OPCMs from external environment [16]. Micro- or nano-capsules could exit in several shapes such as spherical, tubular, and oval or can be

Shell Core

Fig. 1. The encapsulation methods and supporting materials that can be used to encapsulate OPCMs.

Fig. 2. Structure of core–shell of micro- or nano-encapsulated OPCMs (a) which has similar structure to a mangoesteen (b).

80

T. Khadiran et al. / Solar Energy Materials & Solar Cells 143 (2015) 78–98

Table 1 List of the physico-mechanical, chemical, and physico-chemical methods used for micro- or nano-encapsulation of OPCMs. Methods

Techniques

Capsule formed

Physico-mechanical

 Spray drying

 Microcapsules

Chemical

 Interfacial

 Microcapsules

polymerization

 Suspension

 Microcapsules

polymerization

 Emulsion polymerization

 Microcapsules

 Miniemulsion

 Nanocapsules

polymerization Physico-chemical

 Coacervation

 Microcapsules

 Sol–gel

 Micro- and nanocapsules

made in an irregular shape. This method is one of the good techniques for preservation of sensitive substances and it is an excellent method for production of materials with new valuable properties. OPCMs with a melting temperature ranging from  10 to 80 °C can be turned into micro- or nanocapsules [9]. The micro- or nanoencapsulation of OPCMs can be prepared using either physicomechanical, chemical or physico-chemical methods depending on the physico-chemical properties of the OPCMs, the shell materials to be used and the preferred size to be obtained. Table 1 list down the most often used micro- and nano-encapsulation methods for micro- or nano-encapsulation of OPCMs. 2.1. Physico-mechanical methods Spray drying technique involve the atomization of a homogeneous sample in liquid form such as in solution, suspension or paste in a drying chamber where the solvent of the samples is evaporated to obtained solid particles [17]. This technique is reported suitable for encapsulation of heat-sensitive material [18,19] therefore, it is widely used in food and pharmaceutical industries [18,20–22]. However, the application of this technique to encapsulate OPCMs is still rare. This could be due to the price of the spray drying system is relatively expensive. Spray drying method was reported could produce a desired particle size distribution depend on the atomizer design [23]. Furthermore, this technique can be easily scaled-up and controlled [17]. This method was also reported could reduce the cost of chemicals and waste stream treatment. Borreguero et al. [24] used this technique to micro-encapsulated RubithermsRT27 (n-alkanes) using low density polyethylene (LDPE) and ethylvinylacetate copolymer (EVA) as the shell materials. They added in 2% carbon nanofibres (CNFs) during fabrication processes to increase the thermal properties of the micro-encapsulated RubithermsRT27. They found that spray drying technique can be used to encapsulate the RubithermsRT27 with latent heat of fusion of 98.1 J g  1 and encapsulation efficiency of 49.32 wt%. Their finding also shows that the developed micro-encapsulated RubithermsRT27 has good thermal stability. It can stand more than 3000 thermal cycling test which indicating that the n-alkanes within the synthesized microcapsules melts and solidifies in a reversible way and can stand about 30 years of continues

operation time. In addition, the developed microcapsule was homogeneous in size. 2.2. Chemical methods Chemical technique is widely used for micro- or nano-encapsulation of OPCMs. This technique is more preferred due to easily set-up in the laboratory compared with physico-mechanical method such as spray-drying. The most important chemical technique used for micro-encapsulation of OPCMs is the method based on in-situ polymerization, in which it consists of interfacial, emulsion and suspension polymerization. The chemical technique used for nano-encapsulation of OPCMs is based on miniemulsion in-situ polymerization such as ultrasonic-assisted miniemulsion polymerization, miniemulsion polymerization, and direct miniemulsion and so on. Miniemulsion in-situ polymerization method is a convenient one-step encapsulation technique for production of nanocapsules [25]. Tables 2 and 3 summarized the particle size, temperature of melting, latent heat of fusion, encapsulation efficiency and particle size distribution of micro- and nanocapsules OPCMs obtained using different types of polymerization methods based on publications for the year of 2000 to 2015. The details about the polymerization methods used to micro- or nano-encapsulation of OPCMs are also shown in Tables 2 and 3. The parameters involve during the formation of micro- or nano-encapsulated OPCMs were also discussed in this paper. Fig. 3 shows the morphology of microencapsulated OPCMs obtained by previous researchers [26]. 2.2.1. Parametric study on the formation of micro- or nano-encapsulated OPCMs using chemical method The morphology, physico-chemical and thermal properties of micro- or nano-encapsulated OPCMs are depending on various parameters which have to be set during the fabrication processes, such as stirring rate, content of emulsifier, content and type of initiator, core/shell weight ratio, shell/initiator weight ratio, polymerization temperature, polymerization time, etc. All of these parameters are very important to be controlled during the fabrication works to obtain micro- or nano-encapsulated OPCMs with desired properties to suit the intended applications. For example, particle size is one of the most important properties that have to be taken into account especially for the application of OPCMs in the construction and textiles industries. 2.2.1.1. Effects of stirring rates on diameters, morphology and phase change properties. According to Zhang et al. [27], the stirring rate had a significant effect on the morphology and particle size distribution of the resulting capsules. The preparation of micro- and nano-encapsulation of n-octadecane with melamine-formaldehyde shell using in-situ polymerization method showed that no microcapsules were formed when the stirring rate was below 3000 rpm because the n-octadecane was not emulsified. The n-octadecane was emulsified at stirring rate of 3000 rpm by continuous stirring the mixture for approximately 240 min. The diameter of the microcapsules decreased with the increased of stirring time and stirring rate. The surface of the capsules becomes smoother with the increased of stirring rate. They also studied the effect of stirring rate on phase change properties. The DSC results show that the onset temperature and the latent heat of fusion remain unchanged even the diameter of the capsules vary from 9.2 to 0.9 mm. This indicates that the stirring rate has no effect on the crystal content of n-octadecane in the capsule. However, they found that the crystallization behavior of n-octadecane in the capsule changes with increase of the stirring rate. TGA result shows that the thermal stability of the capsule was increased with the increasing of the stirring rates. This is due to

Table 2 Properties of micro-encapsulated OPCMs prepared using different chemical method. OPCMs

Shell

Particle size (mm)

Encapsulation efficiency (%) Temperature of melting (Peak) (°C)

Latent heat of melting (J g  1)

Reference

In-situ In-situ In-situ In-situ

n-octadecane n-octadecane n-eicosane n-tetradecane

Melamine-formaldehyde Melamine-formaldehyde Melamine-formaldehyde Poly(styrene), poly(methyl methacrylate), poly(ethyl methacrylate) Melamine resin Melamine-formaldehyde Urea-folmaldehyde

0.9–9.2 – 0.1–10 5–40

70–71 – – 5–6 to 40, 5–6 to 30

30.5 – 36.9 2.06–5.97

170 160–170 134.3 66.26–80.62

[27] [28] [29] [30]

5–20 2.2 1–500

– 59 –

– 40.6 22.0, 35.6, no record for n-eicosane

150–210 144 185.3, 233.8, 155.0

[31] [32] [33]

Melamine-formaldehyde Poly(styrene) Poly(methyl methacrylate) Poly (methyl methacrylate) Poly(styrene) Poly(methyl methacrylate) Poly(methyl methacrylate) Polyurethane Methyl methacrylate/methyl acrylate/methacrylic acid copolymer, Poly(styrene)

3.6 1–20 0.1–35 0.01–100 0.01–115 mm 1.3 mm 0.14–0.4 5–10 244.3

– 63.3 60.3 50.2–65.4 64.4 – 38 93.4–94.9 93.5

– 136.89 139.20 171.14–265.60 156.39 116.25 81.5 110.0–117.5 94.8

[34] [35] [36] [37] [38] [39] [40] [41] [42]

212.54– 441.03

10.92–39.10 (depending on the type of OPCMs)



12.01–119.80 (depending on the type of OPCMs

[43]

Poly(styrene) Poly(styrene) Poly(styrene) Poly(styrene) Styrene-divinybenzene copolymer Poly(butyl methacrylate), Poly (butyl acrylate) n-octadecyl methacrylatemethacrylic acid co-polymer

– 4.80 3.83, 3.97 4.0–53.2 3–4

75.6 – 43.6, 35.1 49.0–67.9 –

– – – 31.56 –

153.5 102.42 58.6, 79.0 75.7–135.3 –

[44] [45] [46] [47] [48]

2–75

47.7–55.6

29.1–31.6

96–112

[49]

1.60–1.68

12–21

26.5–29

974–93

[50]

polymerization polymerization polymerization polymerization

In-situ polymerization In-situ polymerization In-situ polymerization

n-hexadecane, n-octadecane n-octadecane n-hexadecane, n-octadecane, n-eicosane

In-situ polymerization Emulsion polymerization Emulsion polymerization Emulsion polymerization Emulsion polymerization Emulsion polymerization Emulsion polymerization Interfacial polymerization Suspension-like homopolymerization

n-docosane n-heptadecane n-nonadecane Paraffin eutectics Eutectic n-tetracosane/n-octadecane Eutectic capric acid-stearic acid n-heptadecane n-octadecane Paraffin wax

Suspension polymerization

Suspension like polymerization Suspension like polymerization Suspension like polymerization Suspension-like polymerization Suspension-like polymerization

Paraffin wax, n-tetradecane, PEG 800, PEG 1000, Rubitherm 27, Rubitherm 20, n-nonadecane Paraffin wax Paraffin wax Petrepar n-C13, Petrepar n-C14 RubithermsRT31 n-octadecane

Suspension like-polymerization

n-octadecane

Suspension like-polymerization

n-octadecane

21.48 31.23 19.24–35.80 25.96 21.37 20.3 – –

T. Khadiran et al. / Solar Energy Materials & Solar Cells 143 (2015) 78–98

Chemical method

81

82

Table 3 Properties of nano-encapsulated OPCMs prepared using different chemical methods. OPCMs

Shell

Particle size (nm)

Encapsulation efficiency (%)

Ultrasonic-assisted miniemulsion in-situ polymerization In-situ polymerization In-situ polymerization

n-octadecane

Polystyrene

100 –123



n-tetradecane n-octadecane

Urea-formaldehyde Resorcinol-modified melaminefromaldehyde Poly(alkyl methacrylate) Polystyrene and polymethylmethacrylate

100 20

61.8 92

50–140 o 100

Direct miniemulsion polymerization Interfacial redox initiation miniemulsion polymerization Miniemulsion polymerization Miniemulsion polymerization Direct miniemulsion method Ultrasonic-assistant miniemulsion in-situ polymerization Miniemulsion polymerization One step miniemulsion in-situ polymerization Ultraviolet photoinitiated emulsion polymerization Time-saving ultrasonically initiated miniemulsion polymerization Emulsion co-polymerization

n-hexadecane n-octadecane Paraffin n-dodecanol n-octadecane

Temperature of melting (Peak) (°C)

Latent heat of melting (J g  1)

Reference (s)

124.4

[51]

9.01 26.91

134.16 146.25

[52] [53]

– 95

– –

– 114

[54] [55]

100 150 140 and 119

48.6 82.2 89.5

26.2 18.2 32.2 and 31.9

107.1 98.8 198.5 and 208.7

[56] [57] [58]

n-tetradecane

Polystyrene Poly (methylmethacrylate) Poly (ethyl methacrylate, Poly (methyl methacrylate) Poly(styrene)

132

89

4.04

98.71

[59]

RT80 n-octadecane

Styrene-butyl acrylate Styrene-methyl methacrylate co-polymer

52–112 60–130

80 45.1

80.9 29.5

10–20 107.9

[60] [61]

Eicosanoic, stearic acid eutectic n-dotriacontane

Poly(methyl methacrylate)

46

68.8

56.9

126.4

[62]

Poly(styrene)

168.2

61.23

70.9

174.8

[63]

Poly(styrene-co-ethyl acrylate)

166, 265

32.12, 47.79

42.39, 64.57

49.03, 97.93

[64]

n-octadecane, palmitic acid

T. Khadiran et al. / Solar Energy Materials & Solar Cells 143 (2015) 78–98

Chemical method

T. Khadiran et al. / Solar Energy Materials & Solar Cells 143 (2015) 78–98

Fig. 3. SEM micrograph of n-octacosane microcapsules [26].

the emulsion droplet that formed evenly at high stirring rate, thus formation of uniform particle size. Sanchez et al. [44] provided a similar report, that stirring rate strongly affects the particle size of the capsules since it modified the oil phase dispersion. The particle size of the capsule will be smaller if the energy delivered by the stirrer is greater. The stirring rate affects both the formation of droplet and the aggregation through collisions of neighboring droplet. The collisions between the droplets can be minimized by increasing the stirring rate, thus encourage the formations of smaller particle size. Sanchez et al. [44] have also found that the viscosity of the liquid medium in the suspension plays a vital role in determining the capsule size. Suspension with high viscosity need high stirring rate to reduce the size of the droplets. 2.2.1.2. Effect of the shell/core mass ratio on diameter, morphology and phase change properties. Sanchez et al. [44] extensively studied the influence of paraffin wax to styrene. Paraffin wax was used as OPCMs and styrene was used as a shell to encapsulate the paraffin wax. The result shows that the polymer was not successfully encapsulated in the paraffin wax/styrene when mass ratio is 2. This could be due a shortage of polymer that could not completely cover the amount of paraffin. A similar finding was reported by Tumirah et al. [61]. The shell/ core mass ratio also effects the particle size distribution of the capsules. Finding from both authors shows that the average sizes of the capsule increased with the increased of shell/core mass ratio. This could be due to the increased of the viscosity of the mixture and keeping the stirring rate at a constant level, the ability to form the desired droplet will be decreased. It is also believed that by increasing the polymer amount while keeping the surfactants at a constant level, the contribution of the surfactant per molecule will decreased. These causes the size of the stabilized droplets is not consistence anymore. Finding from both authors also agreed that as the shell/core mass ratio was increased, the amount of OPCMs encapsulated was also increased passing through a maximum and then decreased again. Fuensanta et al. [60] studied the influence of styrene to RT80 on thermal properties of the obtained nanoparticles. They found that the melting and freezing behaviors of nanocapsules could be due to the probability of crystal growth in the nanoparticles, which is strongly limited by the dimension of the capsules. 2.2.1.3. Effect of the emulsifier on the encapsulation process, diameter, morphology and thermal properties. Different type of emulsifier has different amphipathic character (ability to lower down the interfacial surface tension) [65]. Emulsifier will form a film around the droplet (in this case OPCMs), and it prevents coalescence and

83

agglomeration [66,67]. Therefore, the existence of emulsifier in the suspension could affect the morphology, the particle size and the amount of OPCMs that was encapsulated into the polymer shell. Sanchez-Silva et al. [47] extensively studied the influence of emulsifier on the encapsulation process. The results indicate that the type of emulsifier used is one of the crucial factors in controlling the particle size in the polymerization process. The emulsifier content also plays an important role during the fabrication of encapsulated OPCMs. Zhang et al. [27] found that the particle size distribution of the capsules become narrow as the emulsifier content was increased. As the emulsifier content was increased, the emulsion droplet is formed evenly and the distribution becomes narrow. Therefore, the capsules are formed with more even shell thickness. These will improve the thermal stability. Zhang et al. [27] reported that the thermal stability of the capsule increase gently with the increased of the emulsion content. However, Fuensanta et al. [60] found that there is a maximum limit of emulsion, which a higher amount of emulsion does not lead to lower diameter of the capsules. Fuensanta et al. [60] reported that the reduction in the nanoparticle diameter with increasing the amount of SDS is higher for lower amount of RT80, because it acts as the hydrophobic material, which suppresses the Ostwald ripening of the droplets. 2.2.1.4. Effect of the initiator type and polymerization temperature. The formation of encapsulated OPCMs can be correlated with the locus of nucleation during the polymerization processes. Therefore, the type of initiator is believes to be one of the important factors that affect the particle morphology and size. Generally, initiator can be classified into two, namely water-soluble initiator and oil-soluble initiator. The type of initiator selected is depending on the method applied for encapsulation process of OPCMs. Li et al. [68] reported that the oil-soluble initiator seemed to be more suitable for the encapsulation of n-octadecane compared with that of water-soluble initiator by mean of suspension-like polymerization method. The water-soluble initiator that they used was ammonium persulfate (APS), while oil-soluble initiator used was 2,2-Azobisisobutyronitrile (AIBN). Li et al. [68] also studied the effect of polymerization temperature against the morphology and structure of encapsulated n-octadecane. They have synthesized the encapsulated n-octadecane using different polymerization temperatures, 70 °C, 75 °C, 80 °C and 85 °C. The results show that no microcapsule was observed when the polymerization temperature was 70 °C. This might be due to the low degree of initiation of AIBN at 70 °C. Further increased of polymerization temperature resulting in the initiation activity of AIBN was increased, and the morphology and structure of microcapsules become better. As a conclusion, various parameters should be considered during fabrication of micro- or nano-encapsulated OPMs using chemical method. The changes of polymer as a shell might influence other parameters. Similarly, different polymerization method needs different parameter optimization. Therefore, extensive parametric study should be done to optimize the condition so that the encapsulated OPCMs obtained have the desired physico-chemical properties. 2.3. Physico-chemical methods Coacervation or gelation method can be classified into two; simple coacervation and complex coacervation methods. Simple coacervation method refers to the interaction of a dissolved polymer with a low-molecular substance. On the other hand, complex coacervation method occurs through the interaction of two polymers with opposite charges. Hawlader et al. [69–72] used complex coaservation technique to encapsulated paraffin wax

84

T. Khadiran et al. / Solar Energy Materials & Solar Cells 143 (2015) 78–98

using gelatin/gum acacia as a shell. Another study was carried out by Bayés-García et al. [71] using complex coacervation method. They successfully encapsulated Rubitherms RT 27 using sterilized gelatin/acacia gum and agar–agar/acacia gum. Ö zonur et al. [72] applied simple and complex coacervation techniques to encapsulate natural coco fatty acid using gelatin/gum acacia. Sol–gel technique is another physico-chemical method that can be used to encapsulate OPCMs. The sol–gel technique can be defined as the polycondensation reactions of a molecular precursor in a liquid phase to form a colloid solution (sol) which is subsequently converted to an oxide network (gel). The encapsulation of OPCMs based on paraffin using this technique was carried out by Wang et al. [73] and Zhang et al. [74]. Chen et al. [75] successfully encapsulated OPCMs based on fatty acid using sol–gel method. The result shows that the melting and freezing temperatures and latent heat of melting and freezing of the microcapsules were determined as 53.5 °C, 52.6 °C and 171.0 kJ/kg and 162.0 kJ/kg, respectively. Latibari et al. [76] encapsulated palmitic acid into nano-sized of SiO2 using sol–gel method. They used palmitic acid as OPCM (core) and SiO2 as shell materials. They found that the size of the capsule can be tailor-made by changing the value of pH in the range of 11–12. The encapsulation ratio of palmitic acid was increased from 83.25 to 89.55% by increasing the pH value in the range of 11–12. Latibari et al. [77] also used titanium dioxide (TiO2) as a shell to nanoencapsulate stearic acid (SA). The nanoencapsulated SA was synthesized using sol–gel method. The diameters of nanocapsules are between 583.4 and 946.4 nm with encapsulation ratio between 30.36% and 64.76%. The increasing mass ratio of SA/TiO2would enhanced phase change properties and improved encapsulation efficiency, however, decrease the thermal conductivity property of the nanocapsules. Chai et al. [78] used TiO2 to microencapsulated n-eicosane using sol–gel method. The perfect spherical shape, smooth and compact of the capsule with a uniform particle size of 1.5–2 mm was obtained.

3. Shape-stabilized methods Encapsulation technology using polymers into micro- or nanosized involve high preparation cost. In addition, most of the developed encapsulation methods especially micro-sized were not be able to improve the thermal conductivity of OPCMs. Therefore, the increasing attention was given to the development of shapestabilized PCMs or form-stable composite PCM [72,79–81] as another new TES material. Shape-stabilized phase change materials (SPCMs) can be defined as a new material which has the ability to keep the shape of OPCMs as in a solid state even when the temperature of OPCMs is over the melting point (melting state) of OPCMs [82]. SPCMs can be prepared using supporting materials, consisting of organic materials based on polymers or

OPCMs

inorganic frameworks

Fig. 4. Description of encapsulated OPCMs into the porous network of an inorganic framework.

inorganic porous materials. This technology also could solve the problems associated with mico- or nano-encapsulation technology, such as the effect of different parameters and type of polymerization methods [83–85]. For supporting organic material based on polymers, OPCMs are encapsulated into the polymeric structure, while supporting material based on inorganic porous materials, OPCMs is encapsulated into the porous structure [11]. Fig. 4 illustrated the structure of OPCMs encapsulated into the porous structure of inorganic material. The detail about the techniques to prepare shape-stabilized OPMCs and types of organic materials based polymers and inorganic porous materials that could be used as supporting materials are described in the following sub-topics. 3.1. Supporting material based on polymers The shape-stabilized OPCMs prepared using polymer as supporting materials need simple preparation step, which would effectively reduce the preparation cost. Different polymers can be used as supporting material for the preparation of shape-stabilized OPCMs. The polymers which are usually used as supporting materials are low density polyethylene [86], styrene maleic anhydride copolymer (SMA) [45], polymethylmethacrylate [87,88], polyurethane [89], polypyrrole [90], biodegradable polymer such as cellulose, chitosan, agarose [91], high density polyethylene (HDPE) [92], polyvinyl alcohol [93], etc. However, the type of polymers chosen as a supporting material depends on the type of OPCMs. Different OPCMs should have different congruent supporting material. For example, OPCMs based on paraffins, the supporting material based on polymers to be used should have a similar skeleton such as high density polyethylene (HDPE), polypropylene (PP) and styrene–butadiene–styrene (SBS) [94–96]. If the OPCMs based on fatty acid are to be used, the supporting materials should have unsaturated chain. The unsaturated chain of the polymer will react with the unsaturated carboxylic acid of fatty acid (C ¼ C), thus the crosslinking can occur. Therefore, the selection of supporting materials based on polymers should be right at the first place. 3.1.1. Polymethylmethacrylate Polymethyl methacrylate (PMMA) is widely used as supporting material for preparation of shape-stabilized OPCMs. PMMA is often used as supporting material due to its moderate properties, easy handling and processing, non-toxic, high impact strength and are cost effective. Wang and Meng [87] studied the eutectic of fatty acid; capric acid (CA)–lauric acid (LA), capric acid (CA)–myristic acid (MA), capric acid (CA)–stearic acid (SA) and lauric acid (LA)–myristic acid (MA) blends with PMMA. They entrapped the CA–LA, CA–MA, CA–SA and LA–MA into PMMA framework through self-polymerization method. The CA–LA, CA–MA, CA–SA and LA–MA eutectics were acting as a OPCMs and PMMA serving as the supporting materials. The method that they used for the preparation of shape-stabilized CA–LA/PMMA, CA–MA/PMMA, CA–SA/PMMA and LA–MA/ PMMA is simple. They just mixed MMA monomer with the eutectic fatty acids using AIBN as catalyst. The maximum percentage of eutectic fatty acids without leakage was found to be 70 wt%. They found that the shape-stabilized eutectic fatty acid/ PMMA are reliable in term of thermal and chemical and have a great thermal energy storage potential in building energy conservation and solar energy application. Alkan and Sari. [88] studied shape-stabilized fatty acid/PMMA such as SA/PMMA, PA/PMMA, MA/PMMA and LA/PMMA. The composites were prepared using a solution casting method with different mass fraction of fatty acids to determine the maximum blending ratio with no leakage above the melting temperature of the fatty acids.

T. Khadiran et al. / Solar Energy Materials & Solar Cells 143 (2015) 78–98

Previous study shows that PMMA-based matrixes can be used as supporting material such as Eudragit S (methyl methacrylate/ methacrylic acid copolymer) [97] and Eudragit E (methyl methacrylate/butyl methacrylate/dimethylaminoethyl methacrylate copolymer) [98]. Sari et al. [97] studied fatty acids (SA, PA and MA) blended with Eudragit S to form shape-stabilized OPCMs. The maximum mass percentage of fatty acids in the composite without seepage during the molten state was found to be 70 wt%. A similar finding was reported by Kaygusuz et al. [99]. Polyethylene glycol (PEG) was also used as OPCMs blends with PMMA [100] and PMMA-based matrixes [98]. Alkan et al. [101] carried out further study by blending different molecular weight of PEG with polyacrylic acid and poly (ethylene-co-acrylic acid) as shape-stabilized OPCMs. The composites have been prepared by solution blending method. They found that the formation of complexes is the results of physical cross-linking due to hydrogen bonding between PEG and carboxyl group in the polymer.

85

Recently, researchers looking for shape-stabilized OPCMs with enhanced thermal conductivity. Zhang et al. [100] used aluminum nitride to prepare the OPCMs composite. They used PEG as a OPCMs and PMMA as supporting material. They found that for a PEG mass fraction less than 70%, the composite remained in solid form even above the melting point of PEG. Thermal analysis showed that the prepared composite possessed desirable latent heat capacities and thermal stability. The addition of aluminum nitride in the composite effectively enhanced the heat transfer properties of the OPCMs. Zhang et al. [2] used graphite nanoplatelets as conductive fillers to improve the thermal and electrical conductivity of shape-stabilized PEG/PMMA. They found that, by using the method of in-situ polymerization upon ultrasonic irradiation, the graphite nanoplatelets were uniformly dispersed and embedded inside the network structure of PMMA, which were endowed with benefit of forming conducting network in polymer matrix, thus increase heat transfer speed of PEG as TES medium.

Fig. 5. SEM micrographs of composite shape-stabilized PEG/PMMA with various mass fractions of graphite nanoplatelets: (a) 0, (b) 1%, (c) 2%, (d) 4%, (e) 6% and (f) 8% (The arrows in the figure denoted the graphite nanoplatelets dispersed in the composites) [2].

86

T. Khadiran et al. / Solar Energy Materials & Solar Cells 143 (2015) 78–98

Fig. 5 shows the SEM micrographs of shape-stabilized PEG/PMMA with different mass fractions of graphite nanoplatelets. 3.1.2. High and low density polyethylene High density polyethylene (HDPE) has a high strength to density ratio, which can stand higher temperature (  120 °C) and could be recycled. It also has branching, which gives it stronger intermolecular forces and tensile strength, thus suitable as a framework for preparation of shape-stabilized OPCMs. Inaba and Tu (a) [80] stabilized paraffin with 26% HDPE by weight, however the heating curves and structural properties were not provided. Krupa et al. [86] stabilized high density paraffin and low density paraffin using low density polyethylene (LDPE). They were found that the high density paraffin and LDPE were miscible, while low density paraffin and LDPE were immiscible. Ehid and Fleischer, [102] stabilized paraffin using 25–30% HDPE. Based on the observation during the mixing process it is clear from observation that the two materials are miscible and mixed readily. The latent heat testing of the samples showed that the increasing in HDPE concentration (more than 30%) lead to reductions in the latent heat and melting temperature of the samples. This is believed to be due to displaced paraffin and some level of plasticizing of the HDPE matrix within the paraffin. In recent study done by Chen and Wolcott [103], stabilized paraffin with three different types of polyethylene which are HDPE, LDPE and linear low density polyethylene (LLDPE) using a parallel co-rotating twin screw extruder. The study was focusing on the morphology and chloroform extraction of the composites. The leakage test shows that paraffin leakage from the paraffin/HDPE is considerably slower than paraffin/LDPE and paraffin LLDPE. To solve the low thermal conductivity problem and flammability of paraffin waxes, some additives such as expanded graphite [EG] and flame retardant compounds were added in during the preparation of shape-stabilized paraffin. Sari [104] studied the effect of EG on the thermal conductivities of paraffin/HDPE composites. The thermal conductivity of paraffin/HDPE were increased by about 14% and 24% for the two types of shape-stabilized paraffin/HDPE with paraffin of melting temperature 42–44 °C and 56–58 °C by adding 3 wt% of EG to the samples, respectively. Cheng et al. [12] studied the effect of graphite powder (GP) and EG in improving the thermal conductivity of shape-stabilized paraffin/HDPE. They found that the thermal conductivity of shape-stabilized paraffin/HDPE with EG was far greater than that of the shape-stabilized paraffin/HDPE with GP. Cai et al. [92] prepared shape-stabilized paraffin/HDPE using twin-screw extruder technique. They added in EG and ammonium polyphosphate (APP) into the composite to improve the thermal conductivity and fire resistance properties, respectively. They found that the loadings of the EG and APP improved the thermal stability, improved flammability and increased self-extinguishing properties of the composite. A similar study was carried out by Zhang et al. [74]. They used EG and intumescent flame retardant (IFR) to improve the thermal conductivity and flame retardant properties. They found that EG improved the flame retardant properties of IFR, because EG could form the first char layer at the beginning of heating process, then an interaction between EG and intumescent char layer formed by paraffin/HDPE/IFR occurred. This phenomenon could increase the stability and strength of the char layer, which finally improve the efficiency of the intumescent char shield. Other study using LDPE as supporting material was carried out by Cheng et al. [105] and Trigui et al. [106]. Both authors stabilized paraffin using LDPE. However, Cheng et al. [105] added 40 wt% GP into the composite sample to improve thermal conductivity of the composite. They also successfully prepared a shape-stabilized paraffin/LDPE with temperature of the material is about 25 °C which has not been obtained before. They believe that

the materials may be used for the thermal control of the electronic devices or other room temperature thermal control applications. Almaadeed et al. [107] studied the effect of EG on the phase change properties of HDPE/paraffin composite. They used up to 15 wt% EG. The results show that the composites possessed good thermal, mechanical and chemical properties. No leakage was observed during material processing or characterization works and no significant changes in tensile strength were detected. The addition of EG has increased the Young’s modulus of the nanocomposites. Other important findings were the EG was well dispersed in the composites and has not affect on the freezing and melting properties of paraffin in the HDPE matrix. The addition of EG was also improved the thermal conductivity and increased the thermal stability of the nanocomposite. They reported that EG could improve the thermal stability of the nanocomposite by reducing chain mobility of the HDPE, thus exhibiting degradation. The significant finding of this research was the addition of a small quantity of the EG has reduced the freezing time, which therefore increased the latent heat storage. 3.1.3. Biodegradable polymer The natural polymers are promising be used as supporting material for OPCMs based fatty acid and PEG. Pielichowska and Pielichowski [108,109] used gelatinized potato starch and cellulose derivatives to stabilized PEG. In both works, they found that the enthalpy values of the composites were lower than theoretically calculated values, due to strong intermolecular interaction (H-bonding interactions) between the PEG and the supporting materials. Şentűrk et al. [91] stabilized PEG using cellulose, agarose and chitosan by solution casting method. The shape-stabilized of 60/40 wt% PEG/cellulose, 70/30 wt% PEG/agarose, and 80/20 wt% PEG/chitosan were found to contain the maximum PEG without leakage during melting phase of PEG. However, due to strong H-bonding interaction between PEG and natural polymers, the phase change enthalpy and melting and freezing temperatures of PEG were observed in wide ranges. Cellulose composite can be also used as supporting material to stabilized fatty acid. Cao et al. [110] stabilized fatty acid eutectics using carboxyl methyl cellulose-1 fibers by absorbing method. The result indicates that the fatty acid eutectics are well adsorbed into the porous structure of the carboxyl methyl cellulose-1. Based on 100 melting/freezing cycles, it proved that the composites have good thermal reliability. Chen et al. [111] have synthesized a shape-stabilized PEG/ polymeric composite using glucose as the molecular skeleton and 4,4-diphenylmethane diisocyanate (MDI) as the crosslinking agent. The preparation of the composite involved four main steps. They found that the maximum PEG content in the composite without melt was 70 wt%. 3.1.4. Styrene maleic anhydrate copolymer Sari et al. [112] encapsulated fatty acid; palmitic acid (PA), lauric acid (LA), stearic acid (SA) and myristic acid (MA) in the styrene maleic anhydrate copolymer (SMA) polymer structure. They prepared shape-stabilized fatty acids/SMA composites with encapsulation ratio of fatty acids were up to 85 wt%, without leakage during melting state of fatty acids. The FTIR results show that fatty acids were physically and chemically compatible with the SMA. DSC test results show that there have small changes in melting and freezing temperatures of shape-stabilized SA/SMA, PA/SMA, MA/SMA and LA/SMA were 0.48,  0.41,  0.69 and  0.66 °C and 0.32,  0.61,  0.42 and  0.42 °C respectively compared to pure SA, PA, MA and LA. The difference in melting and freezing temperatures between the composites and pure fatty acids may be due to the mutual action of functional groups between fatty acids and SMA.

T. Khadiran et al. / Solar Energy Materials & Solar Cells 143 (2015) 78–98

3.1.5. Other polymers Olefin block copolymer is the recent polymer technology, used as supporting material for the preparation of shape-stabilized OPCMs. Olefin block copolymer is a resin which could withstand high temperature. Zhang et al. [113] used olefin block polymer to stabilized n-hexadecane by simple swelling preparation method as latent heat storage material. The result shows that the composite not only can be used as TES material but also has shape memory properties. Shape memory can be defined as the property that materials can be deformed and subsequently return to the temporary shape, which would remain stable unless an appropriate external stimulus triggers the materials to recover permanent shape [113]. The authors believe that the shape-stabilized OPCMs with shape memory properties will extend the OPCMs application as TES materials. Other polymer used as supporting material is polyaniline [114,115]. Zheng et al. [115] stabilized palmitic acid (PA) using polyaniline. They dispersed 7.87 wt% exfoliated graphite nanoplatelets (xGnP) into the composite to enhance the thermal conductivity of the composite. The thermal conductivity of the PA/polyaniline/xGnP composite was 237.5% higher than that of PA/polyaniline composite with latent heat of melting is 157.7 J/g. Wang et al. [116] investigated the thermal properties and electrical conductivity of stearic acid/polyaniline composite. The composite was prepared by entrapping of stearic acid (SA) into polyaniline using self-assembly method. The result show that both SA and polyaniline have good compatibility and have no chemical reaction with the maximum mass fraction of stearic acid loaded in SA/polyaniline composite was 62.1 wt% without seepage of melted SA. The TGA result shows that the prepared SA/polyaniline composite has high thermal durability in working temperature range of the SA. The electrical conductivity, good thermal reliability and high heat storage capacity facilitated this composite to be as a good candidate for thermal energy storage. Polypyrrole has also been reported can be used as supporting material. Silakhori et al. [90] studied the physico-chemical and thermal properties of the shape-stabilized palmitic acid/polypyrrole composite. The composite was prepared using in-situ polymerization method. The highest loading of palmitic acid in mass ratio is 79.9 wt% with desirable latent heat of 166.3 J/g. The composite reported had a superior thermal reliability in terms of thermal properties and chemical structure after 2500 thermal cycling test. Chen et al. [89] studied paraffin/polyurethane composites as TES materials. They used three different typse of paraffins which are n-octadecane, n-eicosane and paraffin wax as OPCMs. The n-octadecane/polyurethane, n-eicosane/polyurethane and paraffin wax/polyurethane were synthesized via bulk polymerization. In order to study the encapsulation capability, the composites were prepared at different mass fractions of OPCMs (10, 20, 25 and 30 wt%. The results indicated that the maximum encapsulation ratio of OPCMs was around 25 wt%. The latent heat of n-eicosane/ polyurethane composite with 25 wt% n-eicosane was as high as 141.2 J/g. However, TGA result shows that the composites degraded at considerably high temperatures. A comprehensive review of polyurethane as supporting material for preparation of shapestabilized OPCM was discussed by Yang et al. [117]. 3.2. Supporting material-based on inorganic frameworks In the most of the experimental and numerical investigations of OPCMs in the inorganic porous media, it was found that the phase change temperature is suppressed, but in some cases it is increased. Radhakrishnan and Gubbins [118] and Radhakrishnan et al. [119] explained that the interaction between the fluid (for

87

TES study it referring to the OPCMs) and pore surface plays an important role in deciding the shift direction of the freezing/ melting point in porous media. If a strong attractive interaction occurs between the fluid and pore surface, an elevated phase change temperature will occur. However, if the interactions between fluid and pore surface is repulsive type and weakly attractive type, a depressed phase change temperature will appear. OPCMs based paraffin waxes, the groups of molecule consist of –CH2 and –CH3, and OPCMs based fatty acid has functional groups of –COOH, while PEG has a –OH. The functional groups of –COOH and –OH help establish a strong attractive interaction between fatty acids or PEG with the inner surface of the porous materials. This strong attractive interaction leads to elevation of the melting/ freezing temperature of the fatty acids or PEG. For paraffin, it is believed that no strong attractive force exists between paraffin and the inner surface of the porous materials thus leads to a depression of the phase change temperature of paraffin in the porous materials. 3.2.1. Carbon-based materials In recent years, a lot of shape-stabilized OPCMs have been reported, prepared by infiltrating OPCMs into the porous structure of carbon-based materials such as expended graphite (EG) [120,121], graphene oxide (GO) [122,123], carbon nanotube (CNT) [124,125], graphite nanofibers [126], graphite nanoplates [127], graphene nanoplates [128,129] and activated carbon (AC) [130]. The resulting carbon based OPCMs were reported to have high thermal conductivity [131]. This properties could be used to overcome the low thermal conductivity problem of OPCMs especially paraffins (0.21–0.24 W m  1 K  1) and PEG. The low thermal conductivity is the major drawback decreasing the rates of heat stored and released during melting and freezing processes which limit their application [132]. In addition, these drawbacks reduce the rate of heat storage and releasing heat during the melting and freezing cycles thus restrict the wide application, respectively. This carbon based material holds the OPCMs form leakage by capillary force and hydrogen bonding. Inorganic framework carbon based materials have high surface area, porous structure, low density, chemical stability, excellent thermal stability, good thermal conductivity and wide availability. Therefore, carbon based materials are highly attractive to be used as a supporting materials. The large surface area and low density of porous materials is expected will enhance the shape stabilization capability which finally will minimize the loss of energy storage capacity [133]. Nevertheless, the phase change behavior of OPCMs in pores of carbon based materials is complicated. This may be due to the pore size distribution, geometrical shape, network inner-connection and the functional groups on the internal surface [134]. The influence of the pore structures is difficult to study because the same material with different pore structures is usually not easy to obtain. Chapotard and Tondeur [135] have shown how the mean pore size of the support could be critical in term of melting and freezing temperature and latent heat storage capacity of the composite. However, carbon based material is flexible material because it can be used to encapsulate paraffin waxes, fatty acids or PEG. 3.2.1.1. Expanded graphite. Py et al. [131] studied paraffin wax/ expanded graphite (EG composite). The finding shows that the composite was maintained in the solid form during melting state of paraffin. This is due to paraffin that was maintained by capillary forces of the porous graphite matrix. The thermal conductivity of the composite was found to be similar to that of the pure EG (4–70 W  1 K  1). Therefore, it overcomes the low thermal conductivity of paraffin, 0.24 W m  1 K  1.

88

T. Khadiran et al. / Solar Energy Materials & Solar Cells 143 (2015) 78–98

Sari and Karaipekli [136] prepared shape-stabilized palmitic acid/EG composite. The maximum amount of palmitic acid retained in EG was found to be 80 wt% without the leakage of PA in melting point of PA. The temperatures of melting and freezing and latent heats of melting and freezing were measured as 60.88 and 60.81 °C and 148.36 and 149.66 J/g, respectively. The thermal conductivity of the palmitic acid/EG composite (0.60 W/mK) was found to be 2.5 times higher than that of pure palmitic acid (0.17 W/mK). Zhang et al. [120] infiltrated n-octadecane into the pores of EG. The SEM micrograph of the composite OPCMs shows that n-octadecane was adsorbed into the pores of EG and uniformly covers on the nanosheets of EG. They found that no liquid n-octadecane was observed on the surface of the composite OPCM after the composite OPCM was heated above the melting temperature of n-octadecane (50 °C). It is revealed that the n-octadecane inside the porous EG was held by mean of the capillary and the tension force. SEM micrographs of their EG and n-octadecane/EG composite are as given in Fig. 6. Wang et al. [121] stabilized sebacic acid into the pores of EG for medium-temperature solar heat storage application. The result shows that the optimal mass percentage of sebacic acid in the sebacic/EG composite was to be around 85%, with melting temperature of 128 °C and latent heat of fusion of 187 J/g. The sebacic acid exhibits a uniform distribution into the pores of EG. They found that the combination of sebacic acid and EG effectively prevents the supercooling in sebacic acid and improve thermal reliability, stability and thermal conductivity. In addition, the sebacic acid/EG composite can easily be form into various shapes by dry pressing, with little loss in thermal properties but a remarkable increase in thermal conductivity. Compared with the supporting materials-based polymer, EG has the advantages of good adsorption ability and high thermal conductivity, thus making it a good matrix for preparing shape-stabilized OPCMs [137]. Zhang et al. [138] developed RT100/EG composite for mediumtemperature latent heat storage applications. RT100 is a mixture of paraffinic hydrocarbons with a melting point of 100 °C. The result shows that the composite containing 80 wt% RT100 was free from seepage even after dry pressing processes. 3.2.1.2. Graphene and graphene oxide. Li et al. [123] stabilized stearic acid (SA) in the interlayer spaces of the multilayer graphene oxide (GO). The SA was hold in the interlayer spaces by the capillary action and interfacial interaction. They prepared the multilayer GO from graphite powder by a high-yield inexpensive chemical oxidation–exfoliation technique. No leakage above the melting point of SA was observed. The GO improved thermal stability of the composites due to the carbonaceous layers creates

a physical protective barrier on the surface of the composites. However, the confinement effects of the nanoscaled spaces of GO resulting in the decreasing of melting and freezing point of SA in the composite. Similar study was carried out by Mehrali et al. [122]. However, they used paraffin as OPCMs. The GO was made by modified Brodie’s method. They used vacuum impregnation method to prepare the shape-stabilized paraffin/GO composite. The results shows that paraffin was bounded into the pores of GO and no chemical reaction between paraffin and GO was observed. The temperature of melting and freezing and latent heats of melting and freezing of the composite were 53.57 °C and 44.59 °C and 63.76 kJ/kg and 64.89 kJ/kg, respectively. The results also show that the thermal conductivity of the composite was highly improved from 0.305 to 0.985 W m  1 K  1 and has good thermal reliability and chemical stability. Qi et al. [139] developed shapestabilized PEG/GO composite by physical blending and impregnation method. They synthesized GO from natural flake graphite by modified Hummer’s method. The maximum weight percentage of PES in the composite was high as 96% without any leakage during melting state of PEG. Dao and Jeong [140] have used latex technology for the preparation of stearic acid (SA)/graphene composite. The method that they developed can be used to prepare SA/graphene composite microcapsules with a core–shell structure, having a good thermal stability and improvement in thermal conductivity property. 3.2.1.3. Other graphene-based material. Liang et al. [141] developed graphene-based material (graphene–nikel foam) for porous supporting material to prepare form-stable PCM composite. The porous material was prepared by coating graphene nanosheets onto nickel foam followed by surface modification with polydimethylsiloxane. The hydrophobic and superhydrophilic properties of the graphene–nickel porous material was reported can improve the PCM (carboxylic acid) absorption capability. 3.2.1.4. Graphite nanoplates and graphene nanoplatelets. Mehrali et al. [128] applied graphene nanoplatelets (GNPs) into palmitic acid to provide shape stability and enhanced thermal conductivity of the palmitic acid. They prepared palmitic acid/GNPs composite with GNPs for three different specific surface areas of 300, 500 and 750 m2/g. The highest mass percentage of palmitic acid was 91.94 wt% absorbed by GNPs with a specific surface area of 750 m2/g with latent heat of fusion was 188.98 kJ/kg and thermal conductivity of 2.11 W/mK, which is about 8 times of palmitic acid (0.29 W/mK). Ĩnce et al. [142] developed shape-stabilized OPCM composite by infiltrated myristic acid into the layers of graphite nanoplates. FTIR results shows that there is no significant shift in the

Fig. 6. SEM micrographs of (a) EG and (b) n-octadecane/EG composite [120].

T. Khadiran et al. / Solar Energy Materials & Solar Cells 143 (2015) 78–98

absorption bands of myristic acid/graphite nanoplates composite in comparison with that of myristic acid, which shows that relatively weak chemical interaction between graphite nanoplates and myristric acid. Thermal conductivity of myristic acid increased up to 38% when using 2% graphite nanoplates.

change properties of shape-stabilized OPCMs/AC can be easily tailor-made depending on the characteristics of the ACs. Different pore structure of AC will give different effects on the phase change properties of shape-stabilized OPCMs AC [133]. Hussein et al. [148] studied the effect of different porous structure of AC against it ability to infiltrate the n-octadecane. They have used commercial AC (CAC), AC prepared using physical activation method (PSAC-P) and H3PO4 chemical activation method (PSAC-C) as an inorganic frameworks. n-Octadecane was used as OPCMs. The physico-thermal properties of the obtained n-octadecane/AC nanocomposites were characterized using surface area analyzer and DSC. Fig. 7a–c shows the nitrogen (N2) adsorption–desorption isotherms of ACs, n-octadecane/AC nanocomposites and BJH desorption of n-octadecane/AC nanocomposites. The extrapolation of isotherm knee for all the AC samples clearly shows the microporous-rich nature of all the ACs in the order of PSAC-C 4 CAC 4PSAC-P. The study on the ability of n-octadecane infiltrates into the porous structure of ACs was characterized using similar technique. The N2 adsorption–desorption isotherm and pore size distribution of n-octadecane/PSAC-P nanocomposite, n-octadecane/PSAC-C nanocomposite and n-octadecane/CAC nanocomposite are shown

0.7

(cm 3g-1) at STP

0.8 PSAC-P PSAC-C CAC

0.6 0.5 0.4

200 160 120

Volume adsorbed

Differential Pore Volume (cc/g/A)

3.2.1.5. Activated carbon. Other technique to stabilized OPCMs is by using activated carbon (AC) as inorganic frameworks. This material is relatively cheaper and easy to prepare compared to expanded graphite and GO. Tumirah et al. [143,144] prepared AC from peat soil using physical and phosphoric acid (H3PO4) and zinc chloride (ZnCl2) chemical activation method. The obtained AC was used as inorganic framework to stabilized n-octadecane. They found that the AC prepared using H3PO4 chemical activation method has high specific surface area with mesopores structure. The shape-stabilized n-octadecane/AC shows it is chemically and physically stable even after 1000 thermal cycling test. AC can be produce form a variety of carbonaceous materials such as agricultural waste [145,146] and industrial waste [147]. However, the specific characteristics of AC such as specific surface area, pore structure, and chemical polarity depend on the carbon precursor and activation method [146]. Therefore, the phase

89

0.3 0.2 0.1 0.0 -0.1 10

100

80

0

1000

D if f e r e n t ia l P o r e V o lu m e ( c c /A /g )

0.0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/Po)

Pore Width (Å)

0.03

n-octadecane/PSAC-P n-octadecane/CAC

40

n-octadecane/PSAC-P n-octadecane/PSAC-C n-octaecane/CAC

0.02

0.01

0.00 100 Pore Width (Å)

1000

Fig. 7. Nitrogen (N2) adsorption–desorption isotherms of ACs (a), n-octadecane/AC nanocomposites (b) and BJH desorption of n-octadecane/AC nanocomposites (c) [148].

90

T. Khadiran et al. / Solar Energy Materials & Solar Cells 143 (2015) 78–98

-1

Latent heat of fusion (Jg )

106 44 104 102

43

100 42

98 96

Encapsulation efficiency (%)

45

108

41 800

1000

1200

1400

1600

1800 2

2000

-1

BET spesific surface area (m g ) Fig. 8. BET specific surface area (m2 g  1) versus encapsulation efficiency (%) and latent heat of fusion (J g  1) [148].

in Fig. 7(b). As shown in Fig. 7(b), the N2 adsorption–desorption isotherm of all the nanocomposites has low adsorption affinity. The adsorption affinity decreased in the order of n-octadecane/ PSAC-P4n-octadecane/CAC 4 n-octadecane/PSAC-P. This agrees nicely with the microporous-rich nature of the ACs. As expected, this is due to the microporous nature of the ACs that was occupied by n-octadecane. They were also found that that PSAC-P has less ability to encapsulate n-octadecane compared to PSAC-C and CAC (Fig. 7(c)). The mesopores are still available even after it was infiltrated with n-octadecane. It may be due to the pores of PSAC-P has less capillary force and hydrogen bonding, thus lead to lower content of n-octadecane in the n-octadecane/PSAC-P nanocomposite. This presumably, due to the surface chemistry of the PSAC-P which contains less surface functional groups. The result also shows that the latent heat of fusion of the nanocomposite PCMs and encapsulation efficiency was increased as the BET specific surface area of the AC materials increased (Fig. 8). However, they conclude that the decrease of the latent heat of the nanocomposite PCMs cannot be attributed to the BET specific surface area of the ACs only. The interactions between n-octadecane and AC materials could be leads to the loss of the latent heat. The strong interaction could hindered the n-octadecane from crystallizing, thus reduce the latent heat value of the nanocomposite PCMs. Feng et al. [149] stabilized PEG using commercial AC by a blending and impregnation method. Based on the results obtained, they concluded that the phase change properties of the shapestabilized PEG/AC composite are influenced by the adsorption of PEG into the porous structure of AC and also the interferences of AC which acting as an impurity during freezing process of PEG. Chen et al. [130] studied the shape-stabilized lauric acid (LA)/AC composite as a phase change material. The commercial AC was used as inorganic supporting material and LA was used as OPCM. The LA was well adsorbed into the porous network of the AC with the maximum mass percentage of the LA in the composites was 33.3%. The melting and freezing temperatures and latent heat of melting and freezing were recorded to be 44.07 °C and 42.83 °C and 65.14 kJ/kg and 62.96 kJ/kg. The AC was reported could improve thermal stability of the composites as to the carbonaceous layers create a physical protective barrier on the surface of the composites. Based on the findings from the previous works, it can be concluded that the method of infiltration of PCM into porous material, is more effective in enhancing the thermal conductivity properties

of OPCMs. The findings from previous works also shows that the heat can easily be transported along the structures of porous materials thus can rapidly reach the inner part of the OPCMs, which finally ensures that every portion of the PCM can be uniformly heated or cooled [1]. 3.2.2. Inorganic porous building material Zhang et al. [134] carried out an experimental study on the phase change behavior of capric acid and paraffin in porous building materials. Three kinds of porous building materials; expanded clay (EC), expanded perlite (EP) and expanded fly ash (EF) granules were used to stabilized capric acid and paraffin. In general, the shape-stabilized capric acid/porous building material has remarkable effect in increasing the melting temperature due to strong interaction with functional group of –COOH in capric acid. But for paraffin, no elevating or depression of the melting temperatures was found. This is because the paraffin has inactive functional groups. Chung et al. [150] also used EP for the function of stabilized paraffin. They compared the chemical and thermal performance of the paraffin/EP composite with paraffin/vermiculite composite. The DSC results shows that both composites have large latent heat capacity and original phase change temperatures, due to the large surface area of EP and vermiculite and good dispersion of the paraffin in the pores of EP and vermiculite. Karaipekli and Sari [151] prepared shape-stabilized OPCMs for latent heat thermal energy storage in buildings using eutectic mixture of capric acid (CA) and myristic acid (MA) infiltrated into the pores of EP. They found that the maximum absorption of CA–MA into EP was found to be 55 wt% without seepage. The composite was reported has a good thermal reliability in term of changes in thermal properties after 5000 thermal cycling. They used 10 wt% EG to the composite to improve the thermal conductivity properties of the resulting composite. The melting and freezing temperatures and latent heat of composite were determined to be 21.70, 20.70 °C, and 85.40, 89.74 J/g, respectively. Sari and Karaipekli [13] studied the shape-stabilized capric acid (CA) using EP. They applied the similar method as Karaipekli and Sari [151]. The DSC results shows that the melting and freezing temperatures and latent heat of composite were determined as 31.80 and 31.61 °C, and 98.12 and 90.06 J/g, respectively. Other studied related with utilization of EP as supporting material was also carried out by Sari et al. [152]. They stabilized lauric acid using EP. They found that the lauric acid could be retained by 60 wt% into EP without melted lauric acid seepage from the composite. The melting and freezing temperatures of the lauric acid/EP composite were measured as 44.13 °C and 40.97 °C, while the latent heat of melting and freezing of the lauric acid/EP composite were measured as 93.36 J/g and 94.87 J/g. They also found that the thermal conductivity of the lauric acid/composite was increased approximately as 86% by adding 10 wt% expanded graphite. Karaipekli and Sari [153] stabilized CA–MA using vermiculite, a porous, light, cheap, non-toxic and abundant available natural mineral. In building, vermiculite is used as lightweight aggregate for plaster, fire stop mortar, concrete component and component of interior fill for wallboards. In addition, vermiculite had a good chemical compatibility with OPCMs. Nevertheless, it has low thermal conductivity, 0.065 W/mK. However, the author reported that the thermal conductivity of the CA–MA/vermiculite was increased by about 85% by introducing 2% EG. But the CA–MA eutectic mixture reported could be retained by 20 wt% in to the pores of the vermiculite without melted CA–MA seepage from the composite. It may be due to the porosity properties of the vermiculite is not really good as compared with EP and other porous building materials. Nomura et al. [154] impregnated erythritol into the porous network of EP, diatom earth (DE) and gamma-alumina (GA) using

T. Khadiran et al. / Solar Energy Materials & Solar Cells 143 (2015) 78–98

vacuum impregnation method. Erythritol is a sugar alcohol or polyol, was used as OPCMs. Li et al. [155] also used diatomite as a supporting material for stabilized binary fatty acid. Chen et al. [156] studied bentonite as supporting material for preparation of myristic acid (MA)/bentonite. Bentonite is an aluminum phyllosilicate, which mainly used for drilling mud, binder, and adsorbent and as a groundwater barrier. Bentonite has a nanolayer structure, high thermal conductivity and low cost, thus suitable to be used as supporting material of OPCMs. This encourages Chan et al. [156] to stabilized myristic acid using bentonite and study the physico-chemical and thermal properties. The results show that the maximum MA content in the composite without melt MA leakage is 50 wt%. 3.2.3. Inorganic framework based on silica The activated-attapulgite is one of the promising inorganic supporting materials. Attapulgite also called as palygorskite or crystalline hydrate magnesium aluminum silicate [157,158] has received extensive attention in medical industries as antidiarrheal by adsorbing the bacteria or germ that may be causing the diarrhea, disposal of sewage, decolorization of cooking oil, catalysis [159,160], etc. This material has good advantages as solid support if compared with bentonite, zeolites, diatomaceous earth and expanded graphite [161]. Attapulgite has a specific surface area of about 300–600 m2/g. Due to high surface area, it was used to stabilized stearic-capric acid eutectic as shape-stabilized phase change material [162]. The maximum mass fraction of stearic– capric acid loaded in a attapulgite was 50 wt% without melted SA– CA seepage form the composite, with melting and freezing temperatures and latent heat of melting and freezing are measured to be 21.8 and 20.3 °C and 72.6 and 71.9 J/g, respectively. Qian et al. [163] used calcium silicate as a supporting material to develop shape-stabilized PEG/calcium silicate composite. The composites were prepared using a facile blending and impregnated method. The crystallinity of PEG in the composites was increased with the increased of the PEG content. Silicon dioxide (SiO2) was also reported could be used as inorganic supporting material. SiO2 can be considered as a good inorganic supporting material candidate because of its non-toxic, excellent thermal stability, good mechanical properties, desirable thermal conductivity, high melting point and specific surface area and multiple porous materials that is fire resistant [164–166]. SiO2 was used to stabilize PEG using a simple technique Wang et al. [167]. SiO2 and PEG was dissolved in water, and then mixed at room temperature at different weight ratios. The composite was obtained after oven dried at 100 °C for 24 h. Cu was used in the PEG/SiO2 composite to enhance the thermal conductivity via in situ chemical reduction of CuSO4 through ultrasound-assisted sol–gel method [168]. The latent heat of the composite was about 110 J/g, with thermal conductivity was enhanced by 38.1% compared to pure PEG. Another study using PEG as OPCMs was carried out for preparation of PEG/SiO2 composite with various PEG mass fractions via gelatinization method [169]. The gelatinization process was carried out by adjusting temperature instead of adding coagulant. The result shows that no chemical reaction between SiO2 and PEG was observed. The latent heat of the composites varies form 63.4 J/g to 128.4 J/g, depending on the PEG mass fraction in the composites, which was proportional to PEG content. Li et al. [169] used 2.7 wt% graphite to enhance the thermal conductivity of the composite to 0.558 W/mK. He et al. [164] studied the effect of preparation method of shape-stabilized PEG/SiO2 composites. They used two different sol–gel-assisted methods which are calcium chloride (CaCl2)-assisted and temperature-assisted. They found that the prepared composite remained in solid state without leakage above the melting point of PEG when the PEG weight percentage was 80%. Nevertheless, the DSC results indicated that the latent heat of

91

composite prepared using temperature-assisted method was higher than that of CaCl2-assisted method. This is because there is a chemical action between PEG and CaCl2. They concluded that the temperature-assisted method was a promising method to prepare PEG/SiO2 composite. Qian et al [170] developed method for the preparation of shape-stabilized PEG/SiO2 composite using hazardous waste oil shale ash. They used oil shale ash as a precursor for SiO2 production. The method that they developed has a lot of advantages especially in reducing the waste for energy application. Fang et al. [171] studied n-hexadecane/SiO2 composite as TES materials. They prepared n-hexadecane/SiO2 using sol–gel method. n-Hexadecane was used as OPCMs and SiO2 was used as inorganic supporting material. EG was added into the composite to improve flame retardant property of the composites. The DSC results indicated that the melting and freezing latent heat of the composite are 147.58 and 145.10 kJ/kg, respectively. The maximum n-hexadecane content in the composite was 73.3% without seepage of the melted n-hexadecane. He et al. [172] studied the phase change characteristics and thermal performance of shape-stabilized n-alkanes/silica composite. The composites were synthesized in a sol–gel process using sodium silicate precursor. The finding on thermal performance demonstrated that the n-alkanes/silica composites achieved a high thermal conductivity, low supercooling and good thermal reliability as a result of using silica as supporting material. Similar to inorganic framework carbon based material, the porous structure of material-based silica could be critical which will affect the latent heat storage capacity [173]. A study done by Goitandia et al. [161] shows that the mesoporous silica can reduce the risk of PCM leakage as compared with the conventional porous materials such as bentonite, zeolites, diatomaceous earth and expended graphite. They concluded that mesoporous silica has advantages to be used as a support material for the preparation of shape-stabilized phase change composite by means of vacuum infiltration technique. 3.2.4. Others inorganic supporting materials Tang et al. [174] prepared stearic acid (SA)/titanium dioxide (TiO2) composites with different mass ratios as shape-stabilized OPCMs for building TES. The maximum SA can retain in the composite without leakage was 33% mass ratio. The melting and freezing temperatures and latent heat of the composites were recorded as 53.84 °C and 53.31 °C and 47.82 kJ/kg and 46.60 kJ/kg, respectively. The advantages of TiO2 as inorganic supporting materials are its non-inflammability, nontoxicity and good thermal stability thus can be used as TES materials for building applications. Kaolinite is another promising inorganic supporting material for stabilized OPCMs. Kaolinite is a mineral (Al2O3  2SiO2  2H2O) with crystal structure formed by superposition of silicon tetrahedral sheets and aluminum octahedral sheets [175]. Moreover, kaolinite has inherent advantages such as low cost, flame retardant properties and porous lamella structure [176]. Song et al. [162] prepared lauric acid/kaolinite by absorbing lauric acid into the pores of modified kaolinite. They modified the kaolinite by exploring the inner layers to increase the adsorption capacity. The maximum mass ratio of lauric acid adsorbed into modified kaolinite without leakage is 48 wt% with the melting temperature and latent heat of fusion of 43.7 °C and 72.5 J/g, respectively. Nevertheless, kaolinite has low thermal conductivity. The stabilized OPCMs using kaolinite will decrease the thermal conductivity value as reported by Song et al. [162]. The performances of energy storage and release energy of TES material based OPCMs are substantially dependent on their thermal conductivity. Therefore, thermal conductivity of TES material based-latent heat OPCMs is one of the important parameter in TES applications. Fang and Zhang [177] developed RT20/montmorillonite (MMT) composite as TES for building applications. They used RT20

92

T. Khadiran et al. / Solar Energy Materials & Solar Cells 143 (2015) 78–98

Table 4 Thermal properties of the OPCMs composite prepared using supporting material based on inorganic materials. Shape-stabilized OPCMs

Melting point (°C)

Freezing point (°C)

Mass fractiona (wt%)

Latent heat (J/g)

Reference

Eutectic capric–palmitic acid/attapulgite Eutectic stearic–capric acid/activated-attapulgite Eutectic capric–lauric acid/diatomite Capric acid/halloysite Stearic acid/halloysite Eutectic capric–myristic acid/vermiculite Palmitic acid/expanded graphite Stearic acid/expanded graphite Paraffin/graphite Paraffin/graphene oxide n-Octadecane/expanded graphite Stearic acid/graphene RT100/EG Capric acid/kaolin; PEG600/kaolin; heptadecane/kaolin

21.71 21.8 16.74 29.34 53.5 19.8 60.88 54.28 – 53.57 26.37 72.1 84.74 30.71; 5.16; 22.08

– 20.3 – 25.28 49.21 17.1 60.81 53.12 – 44.59 25.79 64.9 105.90 –

35 50 57 60 60 20 80 83 65–95 48.8 90 – 80 17.5; 21; 16.5

48.2 72.6 66.81 75.52 93.9 27.0 149.66 155.70 – 63.76 184.8 192.8 177.3 27.23; 32.80; 34.63

[181] [182] [183] [184] [14] [153] [136] [185] [131] [122] [120] [140] [138] [186]

a

Maximum mass fraction of OPCMs in composites without leakage even above melting point of OPCMs.

Table 5 Advantages and disadvantages of OPCMs stabilized by polymer as a supporting material. Advantages

 No seepage of OPCMs during melting states

Disadvantages

 Some polymer may release poisonous gas such as formaldehyde which cause health and environmental problem

 No extra storage container is

Table 6 Advantages and disadvantages of OPCMs stabilized by inorganic framework as a supporting material. Advantages

 No seepage of OPCMs during melting state

 No extra container is needed

Disadvantages

 Latent heat value of composite OPCMs is relatively low depending on the porous structure of the supporting material.

needed

 The interaction between OPCMs

 Thermal resistance by capsule

and environment are prohibited

shell is eliminated

 The interactions between OPCMs and environment are prohibited

 Latent heat value of composite OPCMs is relatively low depending on the ability of polymer to hold the OPCMs.

 Easily prepared with desired dimensions

 High thermal conductivity especially OPCMs stabilized by inorganic framework-based carbon materials

 Easily incorporated into building materials

 Feasible for some heating application in buildings such as under floor space heating application

(saturated hydrocarbons with melting temperature of 23.24 °C and latent heat of fusion of 136.3 J/g) as a OPCMs and organic-modified monmorillonite as supporting material. The composite was prepared by blending OPCM with organic-modified monmorillonite. Further study was carried out by incorporation of the composite with gypsum powders to prepare gypsum composite boards. The results show that the composite able to cut down energy consumption by decreasing the frequency of internal air temperature swings. The latest study using MMT as an inorganic supporting material for the preparation of shape-stabilized PCM was carried out by Jeong et al. [178]. They used exfoliated graphite nanoplatelets (xGnP) to improve thermal conductivity of the PCM composites. The FTIR results show that no chemical interaction between PCM (paraffin) and XGnP/MMT mixture. Khedache et al. [179] used red brick as a supporting material for the preparation shape-stabilized paraffin/red brick composite. Red brick is a clay-based material, with rich surface area, 500 m2/kg with particle size distribution varies from 0.7 to 260 mm. The maximum paraffin retained in the pores of red brick without leakage was found to be 40 wt%. The melting and freezing temperatures and latent heats of the composite were measured to be 52.73 °C and 55.80 °C and 41.31 J/g and 42.39 J/g, respectively. They also used 10% expanded graphite to improve the thermal conductivity property of the composite. Based on the findings,

they concluded that the composite with paraffin/red brick (40:60 wt%) has considerable latent heat energy storage potential due to its good chemical and thermal reliability, thermal conductivity and thermal properties. He at al. [180] used sludge ceramsite as supporting material for preparation of shape-stabilized fatty acid/sludge ceramsite composite for building energy conservation application. The result shows that the fatty acid can be retained in the pores of sludge ceramsite of about 46 wt% without leakage. Table 4 summarizes the thermal properties of the OPCMs composite prepared using inorganic supporting material from the literature. Nevertheless, the study on the effect of surface area of inorganic supporting materials on the encapsulation efficiency is still lacking. Surface area and pore size of the inorganic supporting materials plays an important role in preparing the shape-stabilized OPCM. In addition, the surface area of the composite could be used to determine whether OPCMs is infiltrated into the pores of inorganic supporting material or just spread on the surface of the inorganic supporting material. 3.3. Advantages and disadvantages of using polymer and inorganic materials as a supporting material Shape-stabilized OPCMs which are prepared by encapsulation of OPCMs into a polymeric structure or pores of inorganic material have several advantages [98,131]. The advantage and disadvantages

T. Khadiran et al. / Solar Energy Materials & Solar Cells 143 (2015) 78–98

of using polymer as supporting material for preparation of shapestabilized OPCMs are listed in Table 5. Table 6 shows the advantages and disadvantages of using inorganic material as a framework.

4. Intercalation method Intercalation method is one of the promising technologies that can be used for preparation of shapes-stabilized OPCMs. In general, intercalation can be defined as the insertion of a molecules/ ions (in this case referring to OPCMs) into inorganic compound with layered structure. Fig. 9 illustrated the intercalation of OPCMs into the sandwich structure of layered inorganic material. OPCMs were reported can be stabilized by intercalating into the layered structure of clay (monmorillonite) and graphite. Monmorillonite (MMT) and graphite has a kind of sandwiched structure with the basal spacing is around a few nanometers. The interlayer of both materials consists of removable ion or neutral molecule which can be tailor-made. Therefore OPCMs can be easily intercalated into the interlayer of both materials to form shape-stabilized OPCMs. The advantages of this technique are; it can improve the thermal conductivity and flame retardancy of the composite OPCMs. The detailed about the previous works of intercalating study were reviewed by Li and Wu [85].

5. Method for characterization of encapsulated OPCMs Successful utilization of the TES material based on latent heat OPCMs depends considerably on the physico-chemical and thermal properties of the OPCMs itself. Encapsulation of OPCMs may change the physico-chemical and thermal properties of OPCMs. The right method for characterization purpose is really important to validate that the developed encapsulated OPCMs is successful and fulfill the industrial needs. There are a lot of analytical techniques available that can be used to characterize the properties of encapsulated OPCMs such as Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), surface area analyzer, dynamic light scattering (DLS), and so on. The types of methods chosen are depending on the properties that we wanted to study. Fig. 10 shows the common method for characterization of encapsulated OPCMs. 5.1. Chemical properties Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) are used to characterize the chemical properties of encapsulated OPCMs. FTIR results will show whether any chemical interaction taken place between OPCMs and the supporting materials. The structure changes in encapsulated OPCMs are judged by the changes of FTIR characteristics. XRD analysis is very important especially for encapsulated OPCMs prepared using intercalation technique. XRD analysis could directly prove weather OPCMs have been introduced into the

OPCMs

93

interlayer of layered material or not [85]. XRD could also be used to show the crystallization properties of the encapsulated OPCMs. Fan et al. [187] characterized the crystallographic system of the encapsulated OPCMs using XRD. They conclude that peaks at (011), (012), (101) and (102) are belonging to OPCMs (n-octadecane). Similarly, Fang et al. [188] studied the crystalline structure of encapsulated OPCMs using XRD. They suggested that n-octadecane had been encapsulated by the shell of polystyrene. Wei e al. [32] used XRD to explain the crystallization and amorphous properties of encapsulated OPCM under different environment temperatures. They observed that obvious diffraction peaks located at 19°, 23° and 25° (2θ) when the sample was measured at 20 °C. This is due to the crystallization state of OPCMs (n-octadecane) occurred at that temperature. On the other hand, none of crystallization diffraction peaks was observed when the sample was measured at 35 °C. Fang et al. [185] also studied the crystallization of encapsulated OPCM using XRD technique. They found that the XRD peaks at 21.5° and 23.9° are due to SA, while XRD peak at 26.5° is due to EG regular crystallization. The intensity of the XRD peak of EG in the composites is lower than that pure EG, which indicates that the crystallite size of the EG becomes smaller in the composites due to the restriction of crystal of the SA. XRD analysis can also be used to prove whether OPCMs have been introduced into the interlayer of layered materials or not based on the interlayer distance value (basal spacing) of layered materials before and after intercalation. The interlayer distance can be calculated using formula as below:-

D = Kλ /β cos θ where D is gain size of sample, K is Debye–Sherrer constant (0.89), λ is X-ray wavelength (0.15406 nm), β is peak width of half maximum peak while θ is diffraction angle. The interlayer distance is increased when OPCMs are introduced into the interlayer of layered materials. The detailed about the utilization of XRD to characterize the intercalated OPCMs were reviewed by Li and Wu [85]. Song et al. [162] also used a similar technique to prove that lauric acid is infiltrated into the pores of kaolinite. 5.2. Physical properties Surface area analyzer is another relevant technique to determine whether OPCMs was actually infiltrated into the pores of inorganic framework or not. Therefore it is suitable to characterize the encapsulated OPCMs using inorganic materials such as AC, expanded graphite, etc. This techniques applied adsorption and desorption of nitrogen gas into the sample and studies the changes in their isotherms. The type of isotherms, the changes and the decrease of volume adsorbed and desorbed of nitrogen gas during analysis determine that the pore of inorganic framework was successfully filled up by the PCM material. The increased of pore diameter of composite PCM showing that the micropores and may be some of the mesopores of inorganic framework, was filled up by OPCMs material. This technique is more relevant to prove that OPCMs was actually filled the pores of inorganic framework, since the inorganic frameworks (carbonbased material and building-based material) are porous material. Tumirah et al. [144] used this technique to prove that the OPCM (n-octadecane) was infiltrated into the pores of activated carbon. 5.3. Thermal properties

Layered inorganic nanomaterial

Fig. 9. Intercalation of OPCM into layered inorganic nanomaterials.

One of the indicators reflecting the heat storage properties is the melting and freezing temperatures and latent heat capacity value. These properties can be measured using differential scanning calorimetry (DSC). Rudd [189] carried out large scale and

94

T. Khadiran et al. / Solar Energy Materials & Solar Cells 143 (2015) 78–98

Fig. 10. List of common methods used for characterization of encapsulated OPCMs. Note: PSD ¼particle size distribution; DLS¼Dynamic light scattering.

small scale test of phase change materials (PCMs) to validate a DSC test as standard for the measurements of PCM thermal performance. The small scale test consists of test which carried out using DSC. The large scale tests consist of two side-by side test room heated with an electric resistance heater and equipped with air conditioning. The results show that both techniques vary with only 8.7% where the DSC-test underestimates the results obtained via large-scale test. This indicates that DSC can adequately predict the thermal performance of PCM. This method is popular for determining the latent heat and the specific heat [190]. DSC records the change of heat flux with time. The latent heat capacity of the sample is directly proportional to the peak area under the curve. However, the sample mass and heating rate during analysis is also important, because different sample mass and heating rate will give different temperature scanning responses [121,191]. 5.4. Thermal reliability The thermal reliability of the encapsulated OPCMs can be established by measuring the thermo-physical properties of the encapsulated OPCMs after a number of repeated thermal cycles. Thermal reliability consists of comprehensive study about thermal and chemical stability of the encapsulated OPCMs as functions of number of repeated thermal cycles. A comprehensive knowledge of thermal reliability of the encapsulated OPCMs is essential to ensure the long-term performance of the encapsulated OPCMs as TES materials and it economic feasibility. The economic feasibility of employing encapsulated OPCMs as a latent heat storage material depends on the life service time of the material, i.e. there should not be major changes in the melting temperature and latent heat of fusion with time due to the melt/freeze cycles of the storage material [192]. The repeated thermal cycles or thermal cycling test can be conducted either using hot plate method or climatic chamber method. The hot plate method was reported by Sharma et al. [193] and Sharma et al. [192] to determine the thermal reliability of commercial-grade stearic acid, acetamide and paraffin wax. Sharma et al. [193] conducted about 300 repeated melt/freeze cycles, while Sharma et al. [192] conducted for 1500 repeated melt/freeze cycle. The experimental set-up consists of electric hot plate with temperature controller, steel container with outer diameter and height of the container was 12.3 and 5.5 cm, while inner diameter and height were 9.3 and 4.0 cm, respectively. The PCMs were heated above their melting temperature in the steel container and then cool at room temperature. About 1 g of PCM was taken after a selected melt/freeze cycle. The samples after thermal cycles were characterized using DSC to study the thermal stability properties. Shilei et al. [194] and Sari and Karaipekli [195]

also used hot plate method to performed thermal cycling test. Tumirah et al. [61] used hot plate method equipped with a K-type thermocouple. A K-type thermocouple was placed contacting with the sample during heating and cooling process. About 3 g of the sample was loaded into the glass vials (outer diameter and height was 2.5 cm and 4.5 cm, respectively). The sample was then heated above the melting temperature of the sample and cooled below freezing temperature of sample using an ice-bath. Karaipekli and Sari [153] performed thermal cycling test using climatic chamber method. They carried out the test consecutively up to 3000 thermal cycling using thermal cycle (BIOER TC-25/H model). However, they did not mention the parameters and procedures to setup the instrument before or during testing. Borreguero et al. [24] explained in detail how to set up climatic chamber for thermal cycling test. They performed the test using a climatic chamber (Heraeus Vötsch HC4015) at a constant relative humidity of 65%, equipped with a K-type thermocouple in the middle. About 5 g PCM sample was put in a glass vials. The glass vials containing sample contacting with K-type thermocouple were placed in the chamber. The accuracy of temperature measurements of chamber was 0.1 °C. The sample were subjected to 3000 thermal cycles by heating and cooling above and under the sample melting temperature. Shukla et al. [196] carried out the thermal cycling test using an oven. They used oven with glass widow in the door to see the physical changes of PCM. The 200 g PCM was kept in a stainless steel container with an internal diameter of 9 cm and height of 4 cm. Calibrated T-type thermocouple wires (range  10 to 200 °C) were used to measure the temperatures. The melting process was induced by keeping the container containing PCM in an oven. The oven temperature must be set above the melting point of PCM. The melting process was concluded when the entire PCM turned to liquid. The freezing process was started immediately after taking out the sample from the oven and allowed to cool to room temperature. Tyagi and Buddhi [197] carried out thermal cycle test using a differential scanning calorimetry (DSC) with specific specifications; temperature range (  180 to 725 °C), temperature accuracy (0.1 °C), temperature precision (0.05 °C) and sensitivity (0.2 mW). The thermal cycling test was carried out until 1000 cycles. Silakhori et al. [198] developed their own thermal cycling test facility. They used a strip heater (24VDC), deep cooler (12VDC), PCM storage box (12 mm  6 mm  35 mm), K-Type thermocouple, temperature controller (ACS-13A-R/M-Shinko), communication converter (IF-400-Shinko) and a PC for data acquisition system to set up the thermal cycling test system. The system was then used by other researches to perform thermal cycling test [76,122].

T. Khadiran et al. / Solar Energy Materials & Solar Cells 143 (2015) 78–98

6. Conclusion Encapsulation technology was extensively developed by various approaches to prepare encapsulated OPCMs as a new kind of TES medium with desired properties. The encapsulation technology can be divided into three major categories, namely micro- or nano-encapsulation, shape-stabilized and intercalation method. The selection of encapsulation techniques is highly depending on the specification of encapsulated OPCMs that are required by the industrial needs. The physico-chemical properties of the encapsulated OPCMs can be easily tailor-made using the existence encapsulation technology available. The techniques that were used to measure the life service time of the encapsulated OPCMs are also important to determine the resulting properties particularly their services time of the required TES.

Acknowledgments This work was supported by the Ministry of Higher Education of Malaysia (MOHE), under Grant no. FRGS/1/11/SG/UPM/01/2, and JPA scholarship for Doctoral Program for TK is gratefully acknowledged.

References [1] L. Xia, P. Zhang, R.Z. Wang, Preparation and thermal characterization of expanded graphite/paraffin composite phase change material, Carbon 48 (2010) 2538–2548. [2] L. Zhang, J. Zhu, W. Zhou, J. Wang, Y. Wang, Thermal and electrical conductivity enhancement of graphite nanoplatelets on form-stable polyethylene glycol/polymethyl methacrylate composite phase change materials, Energy 39 (2012) 294–302. [3] S. Mondal, Phase change materials for smart textiles – an overview, Appl. Therm. Eng. 28 (2008) 1536–1550. [4] B. Zalba, J. Marin, L. Cabeza, H. Mehling, Review on thermal energy storage with phase change: materials, heat transfer analysis and applications, Appl. Therm. Eng. 23 (2003) 251–285. [5] T. Makuta, K. Kadoya, H. Izumi, M. Miyatake, Synthesis of cyanoacrylatecovered xylitol microcapsules for thermal storage, Chem. Eng. J. 273 (2015) 192–196. [6] A.M. Khudhair, M.M. Farid, A review on energy conservation in building applications with thermal storage by latent heat using phase change materials, Energy Convers. Manag. 45 (2004) 263–275. [7] A. Sharma, V.V. Tyagi, C.R. Chen, D. Buddhi, Review on thermal energy storage with phase change materials and applications, Renew. Sustain. Energy Rev. 13 (2009) 318–345. [8] M.M. Farid, A.M. Khudhair, S.A.K. Razack, Al-Hallaj, A review on phase change energy storage: materials and applications, Energy Convers. Manag. 45 (2004) 1597–1615. [9] V.V. Tyagi, S.C. Kaushik, S.K. Tyagi, T. Akiyama, Development of phase change materials based microencapsulated technology for buildings: a review, Renew. Sustain. Energy Rev. 15 (2011) 1373–1391. [10] A. Sari, Thermal reliability test of some fatty acids as PCMs used for solar thermal latent heat storage applications, Energy Convers. Manag. 44 (2003) 2277–2287. [11] Y. Yuan, N. Zhang, X. Cao, Y. He, Fatty acids as phase change materials: a review, Renew. Sustain. Energy Rev. 29 (2014) 482–498. [12] W.-l Cheng, R.-m Zhang, K. Xie, N. Liu, J. Wang, Heat conduction enhanced shape-stabilized paraffin/HDPE composite PCMs by graphite addition: preparation and thermal properties, Sol. Energy Mater. Sol. Cells 94 (2010) 1636–1642. [13] A. Sari, 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. [14] D. Mei, B. Zhang, R. Liu, Y. Zhang, J. Liu, Preparation of capric acid/halloysite nanotube composite as form-stable phase change material for thermal energy storage, Sol. Energy Mater. Sol. Cells 95 (2011) 2772–2777. [15] Y. Hong, G. Xin-shi, Preparation of polyethylene–paraffin compound as a form-stable solid–liquid phase change material, Sol. Energy Mater. Sol. Cells 64 (2000) 37–44. [16] S.K. Ghosh, Functional coatings and microencapsulation: a general perspective, in: S.K. Ghosh (Ed.), Functional Coatings: by Polymer Microencapsulation, Wiley-VCH, Zelzate, Belgium, 2006, http://dx.doi.org/10.1002/ 3527608478.ch.1.

95

[17] C.M.V. Land, Industrial Drying Equipment: Selection and Application, Marcel Dekker, New York, 1991. [18] A. Billon, B. Bataille, G. Gassanas, M. Jacob, Development of spray-dried acetaminophen microparticles using experimental designs, Int. J. Pharm. 203 (2000) 159–168. [19] V. Fernandez-Perez, J. Tapiador, A. Martin, M.D. Luque de Casto, Optimization of the drying step for preparation a new commercial powdered soup, Innov. Food Sci. Emerg. Technol. 5 (2004) 361–368. [20] A. Gharsallaoui, G. Roudaut, O. Chambin, A. Voilley, R. Saurel, Application of spray-drying in microencapsulation of food ingredients: an overview, Food Res. Int. 40 (2007) 1107–1121. [21] Q. Shen, A.Y. Quek, Microencapsulation of astaxanthin with blends of milk protein and fiber by spray drying, J. Food Eng. 123 (2014) 165–171. [22] V.M. Silva, G.S. Vieira, M.D. Hubinger, Influence of different combinations of wall materials and homogenization pressure on the microencapsulation of green coffee oil by spray drying, Food Res. Int. (2014) 132–143. [23] J.M. Obon, M.R. Castellar, M. Alacid, J.A. Fernandez-Lopez, Production of the red-purple food colorant from Opuntia stricta fruits by spray drying application in food model system, J. Food Eng. 90 (2009) 471–479. [24] A.M. Borreguero, J.L. Valverde, J.F. Rodriguez, A.H. Barber, J.J. Cubillo, M. Carmona, Synthesis and characterization of microcapsules containing RubithermsRT27 obtained by spray drying, Chem. Eng. J. 166 (2011) 384–390. [25] Y.W. Luo, X.D. Zhao, Nanoencapsulation of a hydrophobic compound by a microemulsion polymerization processes, J. Polym. Sci. 42 (2004) 2145–2154. [26] A. Sari, C. Alkan, A. Karaipekli, O. Uzun, Microencapsulated n-octacosane as phase change material for thermal energy storage, Sol. Energy 83 (2009) 1757–1763. [27] X.X. Zhang, Y.F. Fan, X.M. Tao, K.L. Yick, Fabrication and properties of microcapsules and nanocapsules containing n-octadecane, Mater. Chem. Phys. 88 (2004) 300–307. [28] Y.F. Fan, X.X. Zhang, X.C. Wang, J. Li, Q.B. Zhu, Super-cooling prevention of microencapsulated phase change material, Thermochim. Acta 413 (2004) 1–6. [29] Y. Shin, D. Yoo, K. Son, Development of thermoregulating textile materials with microencapsulated phase change materials (PCM). II. Preparation and application of PCM microcapsules, Thermoregul. Text. Mater. (2005) 2005–2010, http://dx.doi.org/10.1002/app.21438. [30] R. Yang, H. Xu, Y. Zhang, Preparation, physical property and thermal physical property of phase change microcapsule slurry and phase change emulsion, Sol. Energy Mater. Sol. Cells 80 (2003) 405–416. [31] S.H. Lee, S.J. Yoon, Y.G. Kim, Y.C. Choi, J.H. Kim, J.G. Lee, Development of building materials by using micro-encapsulated phase change material, Korean J. Chem. Eng. 24 (2) (2007) 332–335. [32] L. Wei, Z. Xing-Xiang, W. Xue-Chen, N. Jian-Jin, Preparation and characterization of microencapsulated phase change material with low remnant formaldehyde content, Mater. Chem. Phys. 106 (2007) 437–442. [33] N. Sarier, E. Onder, The manufacture of microencapsulated phase change materials suitable for the design of thermally enhanced fabrics, Thermochim. Acta 452 (2007) 149–160. [34] D. Fu, Y. Su, B. Xie, H. Zhu, G. Liu, D. Wang, Phase change materials of n-alkane-containing microcapsules: observation of coexistence of ordered and rotator phases, Phys. Chem. Chem. Phys. 13 (2011) 2021–2026. [35] A. Sari, C. Alkan, K.D. Derya, A. Bicer, Micro/nano-encapsulated n-heptadecane with olystyrene shell for latent heat thermal energy storage, Sol. Energy Mater. Sol. Cells 126 (2014) 42–50. [36] A. Sari, C. Alkan, A. Bicer, A. Altuntas, C. Bilgin, Micro/nanoencapsulated n-nonadecane with poly(methyl methacrylate) shell for thermal energy storage, Energy Convers. Manag. 86 (2014) 614–621. [37] A. Sari, C. Alkam, C. Bilgin, Micro/nanoencapsulation of some paraffin eutectics mixtures with poly(methyl methacrylate) shell: preparation, characterization and latent heat thermal energy storage properties, Appl. Energy 136 (2014) 217–227. [38] A. Sari, C. Alkan, K.D. Derya, Ç. Kizil, Micro/nano encapsulated n-tetracosane and n-octadecane eutectic mixture with polystyrene shell for low-temperature latent heat thermal energy storage applications, Sol. Energy 115 (2015) 195–203. [39] A. Sari, C. Alkan, A.N. Özcan, Synthesis and characterization of micro/nano capsules of PMMA/capric-stearic acid eutectic mixture for low temperaturethermal energy storage in buildings, Energy Build. 90 (2015) 106–113. [40] A. Sari, C. Alkan, A. Karaipekli, Preparation, characterization and thermal properties of PMMA/n-heptadecane microcapsules as novel solid–liquid microPCM for thermal energy storage, Appl. Energy 87 (2010) 1529–1534. [41] J.-F. Su, L.-X. Wang, R. Ren, Synthesis of polyurethane microPCMs containing n-octadecane by interfacial polycondensation: influence of styrene–maleic anhydride as a surfactant, Colloids Surf. A: Physicochem. Eng. Asp. 299 (2007) 268–275. [42] L. Sánchez-Silva, J. Tsavalas, D. Sundberg, P. Sánchez, J.F. Rodriguez, Synthesis and characterization of paraffin wax microcapsules with acrylic-based polymer shell, Ind. Eng. Chem. Res. 49 (2010) 11204–12211. [43] L. Sanchez, P. Sanchez, A. de Lucaz, Microencapsulation of PCMs with a polystyrene shell, Coilloid Polym. Sci. 285 (2007) 1377–1385. [44] L. Sánchez, P. Sánchez, M. Carmona, A. de Lucas, J.F. Rodríguez, Influence of operation conditions on the microencapsulation of PCMs by means of suspension-like polymerization, Colloid Polym. Sci. 286 (2008) 1019–1027.

96

T. Khadiran et al. / Solar Energy Materials & Solar Cells 143 (2015) 78–98

[45] L. Sanchez, E. Lacasa, M. Carmona, J.F. Rodriguez, P. Sanchez, Applying and experimental design to improve the characteristics of microcapsules containing phase change materials for fabric uses, Ind. Eng. Chem. Res. 47 (2008) 9783–9790. [46] A.M. Borreguero, M.V. Sanchez, M.L. Sanchez-Silva, M.S. Carmona, J.F. Rodriguez. Development of microcapsules containing phase change materials for refrigeration. IIR Proceedings Series Refrigeration Science and Technology, 5, 2010, pp. 29–36. [47] L. Sanchez-Silva, J.F. Rodriguez, P. Sanchez, Influence of different suspension stabilizers on the preparation of Rubitherm RT31microcapsules, Colloid Surf. A: Physicochem. Eng. Asp. 390 (2011) 62–66. [48] M. You, X. Wang, X. Zhang, L. Zhang, J. Wang, Microencapsulated n-octadecane with styrene-divinybenzene co-polymer shells, J. Polym. Res. 18 (2011) 49–58. [49] X. Qiu, G. Song, X. Chu, X. Li, G. Tang, Preparation, thermal properties and thermal reliabilities of microencapsulated n-octadecane with acrylic-based polymer shells for thermal energy storage, Thermochim. Acta 551 (2013) 136–144. [50] X. Tang, W. Li, X. Zhang, H. Shi, fabrication and characterization of microencapsulated phase change material with low supercooling for thermal energy storage, Energy 68 (2014) 160–166. [51] Y. Fang, S. Kuang, X. Gao, Z. Zhang, Preparation and characterization of novel nanoencapsulated phase change materials, Energy Convers. Manag. 49 (2008) 3704–3707. [52] G. Fang, H. Li, F. Yang, X. Liu, S. Wu, Preparation and characterization of nanoencapsulated n-tetradecane as phase change material for thermal energy storage, Chem. Eng. J. 153 (2009) 217–221. [53] H. Zhang, X. Wang, Fabrication and performances of microencapsulated phase change materials on n-octadecane core and resorcinol-modified melamine-formaldehyde shell, Colloids Surf. A: Physicochem. Eng. Asp. 332 (2009) 129–138. [54] J.K. Black, L.E. Tracy, C.P. Roche, P.J. Henry, J.B. Pesavento, T. Adalsteinsson, Phase transition of hexadecane in poly(alkyl methacrylate) core–shell microcapsules, J. Phys. Chem. B 114 (2010) 4130–4137. [55] H.J. Kwon, Preparation of n-octadecane nanocapsules by using interfacial redox initiation in miniemulsion polymerization, Macromol. Res. 18 (9) (2010) 923–926. [56] W. Wu, H. Bostanci, L.C. Chow, S.J. Ding, Y. Hong, M. Su, J.P. Kizito, L. Gschwender, C.E. Snyder, Jet impingement and spray cooling using slurry of nanoencapsulated phase change materials, Int. J. Heat Mass Transf. 54 (2011) 2715–2723. [57] Z.-H. Chen, F. Yu, X.-R. Zeng, Z.-G. Zhang, Preparation, characterization and thermal properties of nanocapsules containing phase change material n-dodecanol by miniemulsion polymerization with polymerizable emulsifier, Appl. Energy 91 (2012) 7–12. [58] G.H. Zhang, S.A.F. Bon, C.Y. Zhao, Synthesis, characterization and thermal properties of novel nanoencapsulated phase change materials for thermal energy storage, Sol. Energy (2012) 1149–1154. [59] Y. Fang, H. Yu, W. Wan, X. Gao, Z. Zhang, Preparation and thermal performance of polystyrene/n-tetradecane composite nanoencapsulated cold energy storage phase change materials, Energy Convers. Manag. 76 (2013) 430–436. [60] M. Fuensanta, U. Paiphansiri, M.D. Romero-Sanchez, C. Guillem, A.M. LopezBuendia, K. Landfester, Thermal properties of a novel nanoencapsulated phase change material for thermal energy storage, Thermochim. Acta (2013) 95–101. [61] K. Tumirah, M.Z. Hussein, Z. Zulkarnain, R. Rafeadah, Nano-encapsulated organic phase change material based on copolymer nanocomposite for thermal energy storage, Energy 66 (2014) 881–890. [62] Y. Wang, Y. Zhang, T. Xia, W. Zhao, W.H. Yang, Effects of fabricated technology on particle size distribution and thermal properties of stearic-eicosanoic acid/polymethylmethacrylate nanocapsules, Sol. Energy Mater. Sol. Cells (2014) 481–490. [63] Y. Fang, X. Liu, X. Liang, H. Liu, X. Gao, Z. Zhang, Ultrasonic synthesis and characterization of polystyrene/n-dotriacontane composite nanoencapsulated phase change material for thermal energy storage, Appl. Energy 132 (2014) 551–556. [64] J. Giro-Paloma, Y. Konuklu, A.I. Fernandez, Preparation and exhaustive characterization of paraffin or palmitic acid microcapsules as novel phase change material, Sol. Energy 112 (2015) 300–309. [65] P.J. Dowding, B. Vincent, Suspension polymerization to form polymer beads, Colloid Surf. A 161 (2000) 259–269. [66] E. Vivalso-Lima, P.E. Wood, A.E. Hamielec, An update review on suspension polymerization, Ind. Eng. Chem. Res. 36 (1997) 939–965. [67] D. Wolters, W. Meyer-Zaika, F. Bandermann, Suspension polymerization of styrene with Pickering emulsifiers, Macromol. Mater. Eng. 286 (2001) 94–106. [68] W. Li, G. Song, G. Tang, X. Chu, S. Ma, C. Liu, Morphology, structure and thermal stability of microencapsulated phase change material with copolymer shell, Energy 36 (2011) 785–791. [69] M.N.A. Hawlander, M.S. Uddin, H.J. Zhu, Preparation and evaluation of a novel solar storage material: Microencapsulation paraffin, Inst. J. Sol. Energy 9 (2000) 227–238. [70] M.N.A. Hawlander, M.A. Uddin, M.M. Khin, Microencapsulated PCM thermal energy storage system, Appl. Energy 74 (2003) 195–202.

[71] L. Bayés-García, L. Ventolá, R. Cordobilla, R. Benages, T. Calvet, M.A. CuevasDiarte, Phase change materials (PCM) microcapsules with different shell compositions: preparation, characterization and thermal stability, Sol. Energy Mater. Sol. Cells 94 (2010) 1235–1240. [72] Y. Ö zonur, M. Mazman, H.O. Paksoy, H. Evliya, Micro encapsulation of coco fatty acid mixture for thermal energy storage with phase change material, Int. J. Energy Res. 30 (2006) 741–749. [73] L.-Y. Wang, P.-S. Tsai, Y.-M. Yang, Preparation of silica microspheres encapsulating phase-change material by sol–gel method in O/W emulsion, J. Microencapsul. 23 (2006) 3–14. [74] H. Zhang, X. Wang, D. Wu, Silica encapsulation of n-octadecane via sol–gel process: a novel microencapsulated phase change material with enhanced thermal conductivity and performance, J. Colloid Interface Sci. 343 (2010) 246–255. [75] Z. Chen, L. Cao, F. Shan, G. Fang, Preparation and characterization of microencapsulated stearic acid as composite thermal energy storage material in buildings, Energy Build. 62 (2013) 469–474. [76] S.T. Latibari, M. Mehrali, M. Mehrali, T.M. Indra Mahlia, H.S. Cornelis Metselaar, Synthesis, characterization and thermal properties of nanoencapsulated phase change material via sol–gel methos, Energy 61 (2013) 664–672. [77] S.T. Latibari, M. Mehrali, M. Mehrali, M.A. Amalina, T.M.I. Mahlia, A.R. Akhiani, Metselaar HSC, Facile synthesis and thermal performances of stearic acid/titania core/shell nanocapsules by sol–gel method, Energy (2015) 1–10. [78] L. Chai, X. Wang, D. Wu, Development of bifunctional microencapsulated phase change materials with crystalline titanium dioxide shell for latentheat storage and photocatalytic effectiveness, Appl. Energy 138 (2015) 661–674. [79] A. Sari, M. Akcay, M. Soylak, A. Onal, Polymer-stearic acid blends as formstable phase change material for thermal energy storage, J. Sci. Ind. Res. 64 (12) (2005) 991–996. [80] H. Inaba, J. Tu, Form-stable paraffin/high density polyethylene composites as solid–liquid phase change material for thermal energy storage: preparation and thermal properties, Energy Convers. Manag. 45 (1997) 2033–2042. [81] A. Sari, K. Kaygusuz, Poly (vinyl alcohol)/fatty acid blends for thermal energy storage, Energy Sources Part A 29 (2007) 873–883. [82] H. Inaba, P. Tu, Evaluation of thermo physical characteristics on shape-stabilized paraffin as a solid–liquid phase change material, Heat Mass Transf. 32 (1997) 307–312. [83] K. Kaygusuz, A. Sari, High density polyethylene/paraffin composites as formstable phase change materials for thermal energy storage, Energy Sources Part A 29 (2007) 261–270. [84] M.M. Kenisarin, K.M. Kenisarin, Form-stable phase change materials for thermal energy storage, Renew. Sustain. Energy Rev. 16 (2012) 1999–2040. [85] M. Li, Z. Wu, A review of intercalation composite phase change material: preparation, structure and properties, Renew. Sustain. Energy Rev. 16 (2012) 2094–2101. [86] I. Krupa, G. Mikova, A.S. Luyt, Phase change materials based on low-density polyethylene/paraffin wax blends, Eur. Polym. J. 43 (2007) 4695–4705. [87] L. Wang, D. Meng, Fatty acid eutectic/polymethyl methacrylate composite as form-stable phase change material for thermal energy storage, Appl. Energy 87 (2010) 2660–2665. [88] C. Alkan, A. Sari, Fatty acid/poly(ethyl methacrylate) (PMMA) blends as formstable phase change materials for latent heat thermal energy storage, Sol. Energy 82 (2008) 118–124. [89] K. Chen, X. Yu, C. Tian, J. Wang, Preparation and characterization of formstable paraffin/polyurethane composites as phase change materials for thermal energy storage, Energy Convers. Manag. 77 (2014) 13–21. [90] M. Silakhori, H.S.C. Metselaar, T.M.I. Mahlia, H. Fauzi, S. Baradaran, M.S. Naghavi, Palmitic acid/polypyrrole composites as form-stable phase change materials for thermal energy storage, Energy Convers. Manag. 80 (2014) 491–497. [91] S.B. Şentűrk, D. Kahraman, C. Alkan, I. Gökçe, Biodegradable PEG/cellulose, PEG/agarose and PEG/chitosan blends as shape stabilized phase change materials for latent heat energy storage, Carbohydr. Polym. 84 (2011) 141–144. [92] Y. Cai, Q. Wei, F. Huang, S. Lin, F. Chen, W. Gao, thermal stability, latent heat and flame retardant properties of the thermal energy storage phase change materials based on paraffin/high density polyethylene composites, Renew. Energy (2009) 2117–2123. [93] Z. Li, W. He, J. Xu, M. Jiang, Preparation and characterization of in situ grafted/crosslinked polyethylene glycol/polyvinyl alcohol composite thermal regulating fiber, Sol. Energy Mater. Sol. Cells 140 (2015) 193–201. [94] H. Ye, X. Ge, Preparation of polyethylene-paraffin compound as a form-stable solid–liquid phase change material, Sol. Energy Mater. Sol. Cells 64 (2000) 37–44. [95] I. Krupa, G. Mikova, A.S. Luyt, Polypropylene as a potential matrix for the creation of shape-stabilized phase change materials, Eur. Polym. J. 43 (2007) 895–907. [96] M. Xiao, B. Feng, K.C. Gong, Preparation and performance of shape-stabilized phase change thermal storage materials with high thermal conductivity, Energy Convers. Manag. 43 (2002) 103–108. [97] A. Sari, C. Alkan, U. Kolemen, O. Uzun, Eudragit S (methyl methacrylate methacrylic acid copolymer)/fatty acid blends as form-stable phase change material for latent heat thermal energy storage, J. Appl. Polym. Sci. 101 (2006) 1402–1406.

T. Khadiran et al. / Solar Energy Materials & Solar Cells 143 (2015) 78–98

[98] C. Alkan, A. Sari, O. Uzun, Poly(ethyleneglycole)/acrylic polymer blends for latent heat thermal energy storage, AlChe J. 52 (2006) 3310–3314. [99] K. Kaygusuz, C. Alkan, A. Sari, O. Uzun, Encapsulated fatty acids in an acrylic resin as shape-stabilized phase change materials for latent heat thermal energy storage, Energy Sources Part A 30 (2008) 1050–1059. [100] L. Zhang, J. Zhu, W. Zhou, J. Wang, Y. Wang, Characterization of polymethyl methacrylate/polyethylene glycol/aluminum nitride composites as formstable phase change material prepared by in-situ polymerization method, Thermochim. Acta 254 (2011) 128–134. [101] C. Alkan, E. Gűnther, S. Hiebler, M. Himpel, Complexing blends of polyacrylic acid-polyethylene glycol and poly(ethylene-co-acrylic acid)-polyethylene glycol as shape stabilized phase change materials, Energy Convers. Manag. 64 (2012) 364–370. [102] R. Ehid, A.S. Fleischer, Development and characterization of paraffin-based shape-stabilized energy storage materials, Energy Convers. Manag. 53 (2012) 84–91. [103] F. Chen, M. Wolcott, Polyethylene/paraffin binary composites for phase change material energy storage in building: a morphology, thermal properties, and paraffin leakage study, Sol. Energy Mater. Sol. Cells 137 (2015) 79–85. [104] A. Sari, Form-stable paraffin/high density polyethylebe composites as solid– liquid phase change material for thermal energy storage: preparation and thermal properties, Energy Convers. Manag. 45 (2004) 2033–2042. [105] W.-L. Cheng, W.-F. Wu, J.-J. Song, Y. Liu, S. Yuan, N. Liu, A new kind of shapestabilized PCMs with positive temperature coefficient (TC) effect, Energy Convers. Manag. 79 (2014) 470–476. [106] A. Trigui, M. Karkri, I. Krupa, Thermal conductivity and latent heat thermal energy storage properties of LDPE/wax as shape-stabilized composite phase change material, Energy Convers. Manag. 77 (2014) 586–596. [107] M.A. Almaadeed, S. Labidi, I. Krupa, M. Karkri, Effect of expanded graphite on the phase change materials of high density polyethylene/wax blends, Thermochim. Acta 600 (2015) 35–44. [108] K. Pielichowska, K. Pielichowski, Novel biodegradable shape-stabilized phase change materials: blends of poly (ethylene oxide) and gelatinized potato starch, J. Appl. Polym. Sci. 116 (2010) 1725–1731. [109] K. Pielichowska, K. Pielichowski, Biodegradable PEO/cellulose-based solidsolid phase change materials. Polym. Adv. Technol. 22 (12) (2011) 1633–1641. http://dx.doi.org/10.1002/pat.1645. [110] L. Cao, Y. Tang, G. Fang, Preparation and properties of shape-stabilized phase change materials based on fatty acid eutectics and cellulose composites for thermal energy storage, Energy 80 (2015) 98–103. [111] C. Chen, W. Liu, Z. Wang, K. Peng, W. Pan, Q. Xie, Novel from stable phase change materials based on the composites of polyethylene glycol/polymeric solid–solid phase change material, Sol. Energy Mater. Sol. Cells 134 (2015) 80–88. [112] A. Sari, C. Alkan, A. Karaipekli, A. Onal, Preparation, characterization and thermal properties of styrene anhydrate copolymer (SMA)/fatty acids composites as form stable phase change materials, Energy Convers. Manag. 49 (2008) 373–380. [113] Q. Zhang, K. Cui, J. Feng, J. Fan, L. Li, L. Wu, Q. Huang, Investigation on the recovery performance of olefin block copolymer/hexadecane form stable phase change materials with shape memory properties, Sol. Energy Mater. Sol. Cells 132 (2015) 632–639. [114] J.-L. Zheng, F.-R. Zhu, S.-B. Yu, Z.-L. Xiao, W.-P. Yan, S.-H. Zheng, L. Zhang, L.-X. Sun, Z. Cao, Myristic acid/polyaniline composites as form stable phase change materials for thermal energy storage, Sol. Energy Mater. Sol. Cells 114 (2013) 136–140. [115] J.-L. Zheng, S.-H. Zheng, S.-B. Yu, F.-R. Zhu, J. Gan, L. Zhu, Z.-L. Xiao, X.-Y. Zhu, Z. Zhu, L.-X. Sun, Z. Cao, Preparation and thermal properties of palmitic acid/ polyaniline/exfoliated graphite nanoplatelets form-stable phase change materials, Appl. Energy 115 (2014) 603–609. [116] Y. Wang, H. Ji, H. Shi, T. Zhang, T. Xia, Fabrication and characterization of stearic acid/polyaniline composite with electrical conductivity as phase change materials for thermal energy storage, Energy Convers. Manag. 98 (2015) 322–333. [117] C. Yang, L. Fisher, S. Maranda, J. Worlitschek, Rigit polyurethane forms incorporated with phase change material. A state-of-the art review and future research pathways, Energy Build. 87 (2015) 25–36. [118] R. Radhakrishnan, K.E. Gubbins, Free energy studies of freezing in slit pores: an order-parameter approach using Monte Carlo simulation, Mol. Phys. 96 (1999) 1249–1267. [119] R. Radhakrishnan, K.E. Gubbins, K. Watanabe, K. Kaneko, Freezing of simple fluids in microporous activated carbon fibers: comparison of simulation and experiment, J. Chem. Phys. 111 (1999) 9058–9067. [120] Z. Zhang, G. Shi, S. Wang, X. Fang, X. Liu, Thermal energy storage cement mortar containing n-octadecane/expanded graphite composite phase change material, Renew. Energy 50 (2013) 670–675. [121] S. Wang, P. Qin, X. Fang, Z. Zhang, S. Wang, A novel sebacic acid/expanded graphite composite phase change material for solar thermal medium-temperature applications, Sol. Energy 99 (2014) 283–290. [122] M. Mehrali, S.T. Latibari, M. Mehrali, H.S.C. Metselaar, M. Silakhori, Shapestabilized phase change materials with high thermal conductivity based on paraffin/graphene oxide composite, Energy Convers. Manag. 67 (2013) 275–282.

97

[123] B. Li, T. Liu, L. Hu, Y. Wang, S. Nie, Facile preparation and adjustable thermal property of stearic acid-graphene oxide composite as shape-stabilized phase change material, Chem. Eng. J. 215-216 (2013) 819–826. [124] J. Wang, H. Xie, Z. Xin, Thermal properties of paraffin based composites containing multi-walled carbon nanotubes, Thermochim. Acta 488 (2009) 39–42. [125] B. Li, S. Nie, Y. Hao, T. Liu, J. Zhu, S. Yan, Stearic-acid/carbon-nanotube composites with tailored shape-stabilized phase transitions and light conversion for thermal energy storage, Energy Convers. Manag. 98 (2015) 314–321. [126] R. Ehid, R.D. Weinstein, A.S. Fleischer, The shape stabilization of paraffin phase change material to reduce graphite nanofiber settling during the phase change process, Energy Convers. Manag. 57 (2012) 60–67. [127] L.-W. Fan, X. Fang, X. Wang, Y. Zeng, Y.-Q. Xiao, Z.-T. Yu, et al., Effects of various carbon nanofillers on the thermal conductivity and energy storage properties of paraffin-based nanocomposites phase change materials, Appl. Energy 110 (2013) 163–172. [128] M. Mehrali, S.T. Latibari, M. Mehrali, T.M. Indra Mahlia, H.S. Cornelis Metselaar, M.S. Naghavi, et al., Preparation and characterization of palmitic acid/ graphene nanoplatelets composite with remarkable thermal conductivity as a novel shape-stabilized phase change material, Appl. Therm. Eng. 61 (2) (2013) 633–640. [129] X. Fang, L.-W. Fan, Q. Ding, X. Wang, X.-L. Yao, J.-F. Hou, et al., Increased thermal conductivity of eicosane-based composite phase change materials in the presence of graphene nanoplatelets, Energy Fuels 27 (7) (2013) 4041–4047. [130] Z. Chen, F. Shan, L. Cao, G. Fang, Synthesis and thermal properties of shapestabilized lauric acid/activated carbon composites as phase change materials for thermal energy storage, Sol. Energy Mater. Sol. Cells 102 (2012) 131–136. [131] X. Py, R. Olives, S. Mauran, Paraffin/porous-graphite-matrix composite as a high and constant power thermal storage material, Int. J. Heat Mass Transf. 44 (2001) 2727–2737. [132] A. Sari, A. Karaipekli, Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expended graphite composite as phase change material, Appl. Therm. Eng. 27 (2007) 1271–1277. [133] C. Wang, L. Feng, W. Li, J. Zhen, W. Tian, X. Li, Shape-stabilized phase change materials based on polyethylene glycol/porous carbon composite: the influence of the pore structure of the carbon materials, Sol. Energy Mater. Sol. Cells 105 (2012) 21–26. [134] D. Zhang, S. Tian, D. Xiao, Experimental study on the phase change behaviour of phase change material confined in pores, Sol. Energy 81 (2007) 653–660. [135] C. Chapotard, D. Tondeur, Dynamics of latent heat storage in fixed beds, a non-linear equilibrium model, the analogy with chromatography, Chem. Eng. Commun. 24 (1983) 183–204. [136] A. Sari, A. Karaipekli, Preparation, thermal properties and thermal reliability of palmitic acid/expanded graphite composite as form-stable PCM for thermal energy storage, Sol. Energy Mater. Sol. Cells 93 (2009) 571–576. [137] Y.P. Zhang, J.H. Ding, X. Wang, R. Yang, K.P. Lin, Influence of additives on thermal conductivity of shape-stabilized phase change material, Sol. Energy Mater. Sol. Cells 90 (11) (2006) 1692–1702. [138] Q. Zhang, H. Wang, Z. Ling, X. Fang, Z. Zhang, RT100/expand graphite composite phase change material with excellent structure stability, photo-thermal performance and good thermal reliability, Sol. Energy Mater. Sol. Cells 140 (2015) 158–166. [139] G.-Q. Qi, C.-L. Liang, R.-Y. Bao, Z.-Y. Liu, W. Yang, B.-H. Xie, M.-B. Yang, Polyethylene glycol based shape-stabilized phase change material for thermal energy storage with ultra-low content of graphene oxide, Sol. Energy Mater. Sol. Cells 123 (2014) 171–177. [140] T.D. Dao, H.M. Jeong, Novel stearic acid/graphene core–shell composite microcapsule as a phase change material exhibiting high shape stability and performance, Sol. Energy Mater. Sol. Cells 137 (2015) 227–234. [141] W. Liang, G. Zhang, H. Sun, P. Chen, Z. Zhu, A. Li, Graphene–nikel/n-carboxylic acids composites as form-stable phase change materials for thermal energy storage, Sol. Energy Mater. Sol. Cells 132 (2015) 425–430. [142] S. Ĩnce, Y. Seki, M.A. Ezan, A. Turgur, A. Erek, Thermal properties of myristic acid/graphite nanoplates composite phase change materials, Renew. Energy 75 (2015) 243–248. [143] K. Tumirah, M.Z. Hussein, Z. Zulkarnain, R. Rafeadah, Textural and chemical properties of activated carbon prepared from tropical peat soil by chemical activation method, Bioresources 10 (1) (2015) 986–1007. [144] K. Tumirah, M.Z. Hussein, Z. Zulkarnain, R. Rafeadah, Activated carbon derived from peat soil as a framework for the preparation of shape-stabilized phase change material, Energy 82 (2015) 468–478. [145] Y.J. Tham, A.L. Pauziah, A.M. Abdullah, A. Sharmala-Devi, Y.H. Taufiq-Yap, Performance of toluene removal by activated carbon derived from durian shell, Bioresour. Technol. 102 (2011) 724–728. [146] P. González-Garía, T.A. Centeno, E. Uroness-Garrote, D. Avila-Brande, S. OteroDíaz, Microtsructure and surface properties of lignocellulosic-based activated carbons, Appl. Surf. Sci. 265 (2013) 731–737. [147] J. Kong, O. Yue, L. Huang, Y. Gao, Y. Sun, B. Gao, O. Li, Y. Wang, Preparation, characterization and evaluation of adsorptive proeprties of lather waste based activated carbon via physical and chemical activation, Chem. Eng. J. 221 (2013) 62–71. [148] M.Z. Hussein, K. Tumirah, Z. Zulkarnain, R. Rafeadah, Properties of n-octadecane-encapsulated activated carbon nanocomposites for energy storage medium: the effect of surface area and pore structure, Aust. J. Basic Appl. Sci. 9 (8) (2015) 82–88.

98

T. Khadiran et al. / Solar Energy Materials & Solar Cells 143 (2015) 78–98

[149] L. Feng, J. Zheng, H. Yang, Y. Guo, W. Li, X. Li, Preparation and characterization of polyetylene glycol/active carbon composites as shape-stabilized phase change materials, Sol. Energy Mater. Sol. Cells 95 (2011) 644–650. [150] O. Chung, S.-G. Jeong, S. Kim, Preparation of energy efficient paraffinic PCMs/ expanded vermiculite and perlite composites for energy saving in buildings, Sol. Energy Mater. Sol. Cells 137 (2015) 107–112. [151] A. Karaipekli, A. Sari, Capric–myristic acid/expanded perlite composite as form-stable phase change material for latent heat thermal energy storage, Renew. Energy 33 (2008) 2599–2605. [152] A. Sari, A. Karaipekli, C. Alkan, Preparation, characterization and thermal properties of lauric acid/expanded perlite as novel form-stable composite phase change material, Chem. Eng. J. 155 (2009) 899–904. [153] A. Karaipekli, A. Sari, Capric–myristic acid/vermiculite composite as formstable phase change material for thermal energy storage, Sol. Energy 83 (2009) 323–332. [154] T. Nomura, N. Okinaka, T. Akiyama, Impregnation of porous material with phase change material for thermal energy storage, Mater. Chem. Phys. 115 (2009) 846–850. [155] M. Li, H. Kao, Z. Wu, J. Tan, Study on preparation and thermal property of binary fatty acids/diatomite composite phase change materials, Appl. Energy 88 (5) (2011) 1606–1612. [156] C. Chen, X. Liu, W. Liu, M. Ma, A comparison study of myristic acid/bentonite and myristic acid/Eudragit L100 form stable phase change materials for thermal energy storage, Sol. Energy Mater. Sol. Cells 127 (2014) 14–20. [157] W.F. Bradly, The structural scheme of attapulgite, Am. Mineral. 25 (1940) 405–410. [158] S. Andrejkoviĉová, A. Velosa, A. Gameiro, E. Ferraz, F. Rocha, Palygorskite as an admixture to lime-metakaoline mortars for restoration purpose, Appl. Clay Sci. 83-84 (2013) 368–374. [159] Y.F. Liu, J.H. Huang, X.G. Wang, Adsorption isotherms for bleaching soybean oil with activated attapulgite, J. Am. Oil Chem. Soc. 85 (2008) 979–984. [160] X. Li, C. Ni, C. Yao, C. Chen, Development of attapulgite/Ce1  xZrxO2 nanocomposite as catalyst for the degradation of methylene blue, Appl. Catal. B: Environ. 117 (2012) 118–124. [161] A.M. Goitandia, G. Beobide, E. Aranzabe, A. Aranzabe, Development of content-stable phase change composites by infiltration into inorganic porous supports, Sol. Energy Mater. Sol. Cells 134 (2015) 318–328. [162] S. Song, L. Dong, S. Chen, H. Xie, C. Xiong, Stearic–capric acid eutectic/activated-attapulgite composite as form stable phase change material for thermal energy storage, Energy Convers. Manag. 81 (2014) 306–311. [163] T. Qian, J. Li, X. Min, Y. Deng, W. Guan, H. Ma, Polyethylene glycol/mesoporous calcium silicate shape-stabilized composite phase change material: preparation, characterization, and adjustable thermal property, Energy 82 (2015) 333–340. [164] L. He, J. Li, C. Zhou, H. Zhu, X. Cao, B. Tang, Phase change characteristics of shape-stabilized PEG/SiO2 composites using calcium chloride-assisted and temperature-assisted sol gel methods, Sol. Energy 103 (2014) 448–455. [165] Y. Wang, T.D. Xia, H. Zheng, H.X. Feng, Stearic acid/silica fume composite as form-stable phase change material for thermal energy storage, Energy Build. 43 (2011) 2365–2370. [166] O. Chung, S.G. Jeong, S. Yu, S. Kim, Thermal performance of organic PCMs/ micronized silica composite for latent heat thermal energy storage, Energy Build. 70 (2014) 180–185. [167] W. Wang, X. Yang, Y. Fang, J. Ding, Preparation and performance of formstable polyethylene glycol/silicon dioxide composites as solid–liquid phase change materials, Appl. Energy 86 (2009) 170–174. [168] 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. [169] J. Li, L. He, T. Liu, X. Cao, H. Zhu, Preparation and characterization of PEG/SiO2 composites as shape-stabilized phase change materials for thermal energy storage, Sol. Energy Mater. Sol. Cells 118 (2013) 48–53. [170] T. Qian, J. Li, H. Ma, J. Yang, The preparation of a green shape-stabilized composite phase change material of polyethylene glycol/SiO2 with enhanced thermal performance based on oil shale ash via temperature assisted sol–gel method, Sol. Energy Mater. Sol. Cells 132 (2015) 29–39. [171] G. Fang, Z. Chen, H. Li, Synthesis and properties of microencapsulated paraffin composites with SiO2 shell as thermal energy storage materials, Chem. Eng. J. 163 (2010) 154–159. [172] F. He, X. Wang, D. Wu, Phase-change characteristics and thermal performance of form-stable n-alkanes/silica composite phase change materials fabricated by sodium silicate precursor, Renew. Energy 74 (2015) 689–698. [173] H.F. Erk, M.P. Dudukovic, Phase-change heat regenerations: modeling and experimental studies, AIChE J. 42 (1996) 791–808.

[174] F. Tang, L. Cao, G. Fang, Preparation and thermal properties of strearic acid/ titanium dioxide composites as shape-stabilized phase change materials for building thermal energy storage, Energy Build. 80 (2014) 352–357. [175] B.K. Nandi, A. Goswarni, M.K. Purkait, Adsorption characteristics of brilliant green dye on kaolinite, J. Hazard. Mater. 161 (2009) 387–395. [176] H. Liu, L.J. Dong, H.A. Xie, C.X. Xiong, Novel-modified kaolinite for enhancing the mechanical and thermal properties of poly (vinyl chloride), Polym. Eng. Sci. 52 (2012) 2071–2077. [177] X. Fang, Z. Zhang, A novel monmorillonite-based composite phase change material and its applications in thermal storage building materials, Energy Build. 38 (2006) 377–380. [178] S.-W. Jeong, S.J. Chang, S. We, S. Kim, Energy efficient thermal storage montmorillonite with phase change material containing exfoliated graphite nanoplatelets, Sol. Energy Mater. Sol. Cells 139 (2015) 65–70. [179] S. Khedache, S. Makhlouf, D. Djefel, G. Lefebvre, L. Royon, Preparation and thermal characterization of composite “paraffin/Red Brick” as a novel fromstable of phase change material for thermal energy storage, Int. J. Hydrog. Energy (2015) 1–6. [180] 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. [181] M. Li, Z. Wu, H. Kao, Study on preparation, structure and thermal energy storage property of capric–palmitic acid/attapulgite composite phase change materials, Appl. Energy 88 (2011) 3125–3132. [182] S. Song, L. Dong, S. Zhang, Q. Li, Y. Guo, S. Deng, S. Si, C. Xiong, Lauric acid/ intercalated kaolinite as form-stable phase change material for thermal energy storage, Energy 76 (2014) 385–389. [183] M. Li, Z. Wu, H. Kao, Study on preparation and thermal properties of binary fatty acid/diatomite shape-stabilized phase change materials, Sol. Energy Mater. Sol. Cells 95 (2011) 2412–2416. [184] D.D. Mei, B. Zhang, R.C. Liu, H.Q. Zhang, J.D. Liu, Preparation of stearic acid/ halloysite nanotube composites as form-stable PCM for thermal energy storage, Int. J. Energy Res. 35 (2011) 828–834. [185] G. Fang, H. Li, Z. Chen, X. Liu, Preparation and characterization of stearic acid/ expanded graphite composites as thermal energy storage materials, Energy 35 (2010) 4622–4626. [186] A. Sari, Fabrication and thermal characterization of kaolin-based composite phase change materials for latent heat storage in buildings, Energy Build. 96 (2015) 193–200. [187] Y.F. Fan, X.X. Zhang, X.C. Wang, J. Li, Q.B. Zhu, Super-cooling prevention of microencapsulated phase change material, Thermochim. Acta 413 (2004) 1–6. [188] Y. Fang, S. Kuang, X. Gao, Z. Zhang, Preparation and characterization of novel nanoencapsulated phase change materials, Energy Convers. Manag. 49 (2008) 3704–3707. [189] A.F. Rudd, Phase-change material wallboard for distribution thermal energy in building, ASHRAE Trans. Res. 99 (2) (1993) 339–346. [190] S.D. Kim, Heat transfer characteristics in latent heat storage systems using salt hydrates at heat recovery stage, Sol. Energy Mater. Sol. Cells 40 (1996) 71–87. [191] S. Drissi, A. Eddhahak, S. Caré, J. Neji, Thermal analysis by DSC of phase change materials, study of the damage effect, J. Build. Eng. 1 (2015) 13–19. [192] A. Sharma, S.D. Sharma, D. Buddhi, Accelerated thermal cycles test of acetamide, stearic acid and paraffin wax for solar thermal latent heat storage application, Energy Convers. Manag. 43 (2002) 1923–1930. [193] A. Sharma, S.D. Sharma, D. Buddhi, R.L. Sawhney, Thermal cycle test of urea for latent heat storage application, Int. J. Energy Res. 25 (2001) 465–468. [194] L. Shilei, Z. Neng, F. Guohui, Eitectic mixtures of capric acid and lauric acid applied in building wallboards for heat energy storage, Energy Build. 38 (2006) 208–711. [195] A. Sari, 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. [196] A. Shukla, D. Buddhi, R.L. Sawhney, Thermal cycling test of few selected inorganic and organic phase change materials, Renew. Energy 33 (2008) 2606–2614. [197] V.V. Tyagi, D. Buddhi, Thermal cycle testing of calcium chloride hexahydrate as a possible PCM for latent heat storage, Sol. Energy Mater. Sol. Cells 92 (2008) 891–899. [198] M. Silakhori, M.S. Naghavi, H.S. Cornelis Metselaar, T.M. Indra Mahlia, H. Fauzi, M. Mehrali, Accelerated thermal cycling test of microencapsulated paraffin wax/polyaniline made by simple preparation method for solar thermal energy storage, Materials 6 (2013) 1608–1620.