Microencapsulation of phase change materials (PCMs) for thermal energy storage systems

Microencapsulation of phase change materials (PCMs) for thermal energy storage systems

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R. Al Shannaq, M. M. Farid University of Auckland, New Zealand

10.1 Introduction Encapsulation is a process of engulfing the materials of solids or droplets of liquids or gases in a compatible thin solid wall. The material inside the capsules is referred to as the core, internal phase, or fill, whereas the wall is sometimes called a shell, coating, or membrane. Normally, encapsulation materials are classified as nanocapsules, microcapsules and macrocapsules based on the capsule size diameter. The diameter of microcapsules varies from 1 mm to 1 mm, while capsules smaller than 1 mm are classified as nanocapsules and those larger than 1 mm as macrocapsules. The encapsulation of materials has evolved from examples in nature; the simplest example of a macrocapsule is a bird egg, and a cell along with its content is a microcapsule. The applications of microencapsulation are numerous, but began with carbonless copy papers, where the top sheet of the carbonless paper is coated with dye or ink microcapsules and the bottom layer is coated with reactive clay. As time passed, encapsulation technology has emerged and developed in many fields including the pharmaceutical industry (Pekarek et al., 1994), food (Champagne and Fustier, 2007), cosmetics (Miyazawa et  al., 2000), textile industries (Nelson, 2002) and recently encapsulation of PCMs for thermal energy storage applications (Hawlader et  al., 2000).

10.1.1 Morphology of the capsules The morphology of the capsules depends on the core materials and the deposition process of the shell. Figure 10.1 shows the morphology of three possible types of capsules with their nomenclature. The classical core/shell model of a microcapsule is given in Figure 10.1(a). The capsule in Figure 10.1(b) differs slightly from the previous example in that the core is now divided into many separate regions. This may be called a multi-nuclear microcapsule. Figure 10.1(c) shows a micromatrix particle, where the core material is evenly distributed through the particle and there is no surrounding shell coating.

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Morphology of microcapsules

Mononuclear

Polynuclear

Matrix Polymer and core solution

(a)

(b)

(c)

Figure 10.1 Schematic diagram of several different types of morphology of microcapsules.

10.2 Microencapsulation of phase change materials (PCMs) Phase change materials (PCMs) are organic or inorganic compounds, which melt and solidify with a melting range suitable for the specific application. They have the ability to absorb and release large amounts of heat during phase transition. However, to effectively use these materials, they need to be contained in inert and highly durable capsules. Recently, microencapsulation has been shown to provide an effective encapsulation of PCM, through an increasing heat transfer area, reducing PCM reactivity towards the outside environment and preventing PCM from leaking when it is in liquid state. Microencapsulation has already been proven as a successful technology in commercial applications such as in the pharmaceutical and agrochemical industries and recently in the textile industry and in thermal energy storage applications. In general, microencapsulation may be categorized into three groupings, namely chemical, physico-chemical and physico-mechanical processes (Table 10.1) (Ghosh, 2006). In this chapter, we will discuss the most important methods of encapsulation applicable to PCMs.

10.2.1 Interfacial polymerization (polycondensation) This method is characterized by wall formation via rapid polymerization of monomers at the surface of the droplets of dispersed core material. Droplets are first formed by emulsifying an organic phase consisting of core materials and oil-soluble reactive monomer, which is usually isocyanate or acid chloride, in an aqueous phase. By adding water-soluble reactive monomer, rapid reaction takes place between the two monomers at the interface of the droplets to form a polymer shell, as shown in Figure 10.2. Interfacial polymerization or polycondensation is a popular method, which has been

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Methods used for microencapsulation (adapted from Ghosh, 2006)

Table 10.1

Chemical processes

Physical processes Physico-chemical

Physico-mechanical

∑ Suspension, dispersion and emulsion polymerization

∑ Coacervation

∑ Spray-drying

∑ Polycondensation

∑ Layer-by-Iayer (L-B-L) assembly

∑ Multiple nozzle spraying

∑ Sol-gel encapsulation

∑ Fluid-bed coating

∑ Supercritical CO2-assisted ∑ Centrifugal techniques microencapsulation ∑ Vacuum encapsulation ∑ Electrostatic encapsulation

Polyurea shell (H2N )n Core material (oil)

–(NCO)n H 2O +

emulsion stabilizer

Figure 10.2 Schematic formation of the microencapsulation of PCM by interfacial polymerization (source: Smith, 2009).

used for encapsulation of a wide range of core materials, including oils (Alexandridou and Kiparissides, 1994), liquid crystal (Hsu, 2000), pigment (Mahabadi and Tan, 1996), proteins (Yeo et al., 2001), peptides (Pitaksuteepong et al., 2002), and recently PCMs (Cho et  al., 2002). The preparation of polyurea microcapsules containing octadecane as PCM by interfacial polymerization was reported by Cho et al. (2002). Toluene-2,4-diisocyanate (TDI) and diethylenetriamine (DETA) were used as reactive monomers in disperse phase and aqueous phase, respectively. Multiple reactions occur during the formation of the polyurea shell. Polyurea microcapsules were formed not only by the reaction of toluene-2,4-diisocyanate with diethylenetriamine, but also by the reaction of toluene-2,4-diisocyanate with hydrolyzed toluene-2,4-diisocyanate at the interface and the excess of toluene-2,4-diisocyanate with hydroxyl group of the non-ionic surfactant (NP-10) (Figure 10.3). The resulting polyurea microcapsules had

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OCN CH3

NCO + NH2CH2CH2NHCH2CH2NH2 OCN NHCONHCH2CH2NHCH2CH2NH2

CH3 OCN CH3

OCN NCO + H2O

CH3

NHCOOH

NCO

OCN CH3

OCN

NH2 + OCN

CH3

NCO

OCN CH3

OCN CH3

NHCONH

NCO

+

HO ( CH2CH2O )10

C

OCN CH3

CH2CH2

C

CH3

CH3

CH3

CH3 NHCOO ( CH2CH2O)10

CH3

CH3

CH3 CH3

CH3

NH2 + CO2

CH3

C CH3

CH2CH2

C

CH3

CH3

Figure 10.3 Wall formation reaction of polyurea microcapsules: (a) reaction between TDI and DETA, (b) reaction between TDI and hydrolyzed TDI, and (c) reaction between TDI and NP-10 (source: Cho et al., 2002).

a small size range of 0.1–1 mm. The same method and materials were also employed by Su et al. (2007), along with the addition of styrene-maleic anhydride copolymer as a dispersant. Capsules ranged from 1–20 mm in diameter. Different types of diamine water-soluble reactive monomers were investigated for preparing wall shell PCM microcapsules using interfacial polymerization. Long et al. (2004) used ethylene diamine, 1,6-hexane diamine and their mixture as water-soluble reactive monomers and toluene-2,4-diisocyanate as oil-soluble monomer. The reported results show that the encapsulation efficiency and thermal energy storage capacity of the PCM microcapsules reach highest when ethylene diamine was employed. A few years later, Zhang and Wang (2009a) prepared PCM microcapsules based on n-octadecane as core material and polyurea as shell polymers using toluene2,4-diisocyanate as oil-soluble monomer and various amines containing different soft segments in the molecular chain (ethylene diamine, diethylene triamine and amine-terminated polyoxypropylene (Jeffamine T403)) as water-soluble monomers by polycondensation. The morphological investigation (Figure 10.4) shows that the

Microencapsulation of phase change materials (PCMs) for thermal energy storage systems (a)

(b)

(c)

(d)

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Figure 10.4 SEM images of the microcapsules synthesized by using different amine monomers: (a) EDA, (b) DETA, (c) Jeffamine, and (d) cracked microcapsule particle (source: Zhang and Wang, 2009a).

microcapsules synthesized using Jeffamine as the amine monomer have a smoother and more compact surface than those using ethylene diamine (EDA) and diethylene triamine (DETA). Furthermore, large mean particle size, higher encapsulation efficiency and better anti-osmosis were achieved using Jeffamine. Lone et al. (2013) reported an easy and effective approach for fabricating highly monodisperse PCM polyurea microcapsules using a tubular microfluidic technique. At the tip of the needle, spherical monodisperse droplets formed, stabilized under the action of emulsifier in the continuous phase, and then detached by a shear force caused by the cross-flowing continuous phase to produce an oil-in-water (O/W) emulsion. The O/W droplets were then partially solidified by polycondensation along the tube length and finally received in a collecting reservoir to perform the remainder of the polycondensation reaction (Figure 10.5). The resulting microcapsules were highly monodisperse and the particle size distribution ranged from 35 to 500 mm. The size and morphology of the PCM microcapsules were controlled by changing the flow rates of the two immiscible fluids. In addition, adding conductor filler (Fe3O4 NPs) into the organic phase depressed the supercooling of PCM microcapsules. The microcapsules based on a polyurea shell have gathered much concern as an effective method for preparation of PCM microcapsules, as the polymerization reaction happens quickly and the PCM microcapsules produced are relatively small and uniform in size. However, the brittleness of the microcapsule shells limited the practical applications of PCM microcapsules using this technique. In an attempt to improve the polymer shell strength, a series of paraffin double-shell microcapsules

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Continuous phase (TEPA, PVA, SDS, Water)

Discontinuous phase (PCM, IPDI, DBTDL)

Flow direction

n-octadecane

Polyurea shell

Continuous phase

Discontinuous phase

Figure 10.5 (a) Schematic diagram of the fabrication of monodisperse PCM polyurea microcapsules in a tubular microfluidic device; (b) photograph of O/W PCM droplets produced at the tubular junction (source: Lone et al., 2013).

with relatively low shell permeability were prepared using interfacial polymerization. The inner shell is formed through the reaction between polypropylene glycols and toluene-2,4-diisocyanate and the outer through the reaction between toluene-2,4diisocyanate and amines (water-soluble reactive monomers) added in the aqueous phase (Xing et al., 2006). The same method and materials were also employed for the preparation of double-shell microcapsules by Lu et  al. (2011), along with the addition of styrene-maleic anhydride copolymer as a dispersant and butyl stearate as a PCM. The reported results show that the encapsulation efficiency has a maximum value of 95% at core to monomer ratio of 2, and the surface morphology of the microcapsules were smooth and compact with size range from 1 to 5 mm in diameter. Furthermore, the latent heat was 85 J/g with reasonable PCM weight content of 70 wt%. The stability of the double-shell microcapsule against anti-ethanol wash and anti-heat are obviously improved compared to those of single-shell microcapsule (Liang et al., 2009). Furthermore, novel PCM microcapsules with styrene-divinylbenzene copolymer as inner shell and polyurethane as outer shell were investigated, where styrene and divinylbenzene were employed both as co-solvent and shell-forming monomers (Li et al., 2012).

10.2.2 In-situ polymerization In-situ polymerization is similar to interfacial polymerization, except there are no reactive monomers in the organic phase, and all polymerization takes place in the continuous phase rather than in the interface of the droplets as in interfacial polymerization. The most common example of this method is the condensation polymerization of urea or melamine with formaldehyde to form cross-linked ureaformaldehyde or melamine-formaldehyde capsule shells. In this method, droplets are first formed by dispersing core material (PCMs) into an aqueous phase containing a small fraction of emulsifier, followed by addition of proper monomers or prepolymers of urea with formaldehyde or melamine with formaldehyde. After the pH of the system is lowered, the polycondensation reaction starts, yielding cross-linked urea-formaldehyde or melamine-formaldehyde resins. When the resin reaches a high

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molecular weight, it becomes insoluble in the aqueous phase, precipitates out and deposits at the oil–water interface of the droplets. The resin then hardens, forming the shell of the microcapsules, as shown in Figure 10.6 (Zhang and Wang, 2009b). A number of articles have been published over the last few years on microencapsulation of PCMs using in-situ polymerization. Yamagishi et al. (1996) prepared melamineformaldehyde microcapsules containing low melting temperature PCMs for use in cold energy transportation media by in-situ polymerization. A few years later, Choi et al. (2001) used the same method and materials along with the addition of styrenemaleic anhydride monomethyl maleate copolymer as emulsifier. The experimental results show that the mixer speed in the emulsifying step is one of the most important factors affecting the size and size uniformity distribution of the PCM capsules. By increasing the mixer speed, the size of the microcapsules is decreased and the uniformity size distribution is improved. The optimum emulsifying condition was found at 8000 rpm with a capsule diameter of 4.2 mm. A series of PCM microcapsule articles have been published by a group of Chinese researchers based at Tianjin Polytechnic University using in-situ polymerization (Zhang NH2 N H 2N

N(CH2OH)2

NHCH2OH O

N NH2

N Melamine

H

C

NaOH

n

H

N

HOH2CHN

N

N N

(HOH2C)2N

NHCH2OH

Formaldehyde

N(CH2OH)2

M-F prepolymer

N

N

N N

OH

HO

N

N

CH2

CH2

N

N

N N

N

N NH

NH

H 2C

CH2 pH = 3~5

O

O H 2C

CH2

HO

OH

NH

N N

N N

N

CH2 Resorcinol-modified M-F copolymer shell Hydrophilic group Hydrophobic chain

Core n-Octadecane droplet

M-F prepolymer

Core

Emulsifier Micelle

Microcapsule

Figure 10.6 Schematic formation of the n-octadecane microcapsules by in-situ polymerization (source: Zhang and Wang, 2009b).

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et al., 2004b, 2004c, 2005; Fan et al., 2005). The effects of stirring rate, contents of emulsifier and contents of cyclohexane on diameters, morphology surface, phase change properties and thermal stabilities of the capsules were studied (Zhang et al., 2004b). The diameters had less effect on the melting behavior of microcapsules; however, they had significant effects on the crystallization behavior. As the diameters of the microcapsules decreased, two crystallization peaks were observed from the DSC cooling curves (Figure 10.7). The ratio of the integrated area of the peak at the lower temperature increased with the decrease in diameters of microcapsules. The thermal stability of micro- or nano-capsule rises with the increase in stirring rates and emulsifying content. Thermal stability of PCM microcapsules is crucial for practical applications. So Zhang et al. (2004c) investigated the effect of microcapsule diameter and ureamelamine-formaldehyde mole ratio on the thermal stability of PCM microcapsules. According to their results, the highest thermal stability temperature of the PCM microcapsules was found when the diameter of the PCM microcapsules was in the range 0.4–5.6 mm and the urea-melamine-formaldehyde mole ratio was 0.2:0.8:3. Furthermore, Fan et al. (2005) and Zhang et al. (2005) reported an improvement in the thermal stability of n-octadecane microcapsules by feeding an appropriate content of cyclohexane into oil phase followed by heat treatment at a suitable condition. Microcapsules with 30–40 wt% of cyclohexane in the oil phase have a highest thermal resistant temperature of 270°C, and weight loss less than 1.2% after immersion in petroleum ether for 16 hours. By heat treatment of the PCM microcapsules (100°C), the cyclohexane was removed and approximately 5–28 wt% of expansion space was formed inside the microcapsules. The existence of expansion space in the microcapsule allows the n-octadecane to expand freely when the temperature goes up without exerting stresses on the shell. Therefore, the thermal stability of n-octadecane microcapsules was enhanced. Furthermore, Song et  al. (2007) compared the surface appearance and thermal stability of the PCM microcapsules incorporated with silver nanoparticles (NCPCMMs)

Heat flow (W/g)

Endo

Alpha Beta

Exo

0

20 30 Temperature (°C)

4000 rpm 6000 rpm 8000 rpm 9000 rpm 40

Figure 10.7 DSC cooling curves of PCM microcapsules synthesized with various stirring rates (different mean diameter size of PCM microcapsules) (source: Zhang et al., 2004b).

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and without (PCMMs). According to their results, the surface appearance of the NCPCMMs is coarser than PCMMs (Figure 10.8). In addition, the thermal stability of the NCPCMMs is higher than that of PCMMs and the percentage weight loss at 130°C is less (Figure 10.9). This could be attributed to the nanocomposite structure of the microcapsules, in which metal silver nanoparticles were distributed on the surface to increase wall toughness and strength. As mentioned above, urea-formaldehyde or melamine-formaldehyde resins are usually used as shell materials for microencapsulation of PCM using in-situ polymerization. However, formaldehyde-based microcapsules require significant preparation and strict safety precautions due to the fact that formaldehyde is known to be toxic. In addition, some of the residues of these formaldehyde resins in shells can cause environmental and health problems. Therefore, Japan and European countries have produced a list of the maximum allowed residue limits of formaldehyde in textiles and similar products (Table 10.2). Due to the aforementioned problems associated with formaldehyde, PCM microcapsules with low remnant formaldehyde content have been investigated by several researchers. Li et  al. (2007b) produced n-octadecane microcapsules with low remnant formaldehyde content through intensifying the system with melamine by adding formaldehyde once and melamine three times. Sumiga et al. (2011) fabricated melamine-formaldehyde PCM microcapsules with ammonia as a scavenger for residual formaldehyde reduction. Furthermore, Su et al. (2011a, 2011b, 2011c) used a novel methanol-modified melamine-formaldehyde pre-polymer as a shell material for fabrication of low remnant formaldehyde content PCM microcapsules. Results show that this can reduce the free formaldehyde in shell material by increasing the crosslinking structure, enhance the resistance deformation of melamine-formaldehyde shell and achieve reasonable encapsulation efficiency of 85.4% with PCM content of 71 wt%.

10.2.3 Suspension polymerization Suspension polymerization is one of the encapsulation processes under the category of chemical methods. This method is used for production of many common commercial (a)

(b)

Figure 10.8 SEM photos of (a) conventional PCM microcapsules (PCMMs); (b) silvernanoparticles PCM microcapsules (NCPCMMs) (source: Song et al., 2007).

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NCPCMM

80

PCMM mass (%)

Mass (%)

70 60 50 40 30 20 10 0 150

200

250

300 350 Temperature (°C)

400

450

(a)

Different samples 30.00

Conventional MicroPCMs Nano Ag composite MicroPCMs

Weight loss (%)

25.00

20.00

15.00

10.00

10 minutes 30 minutes 50 minutes 20 minutes 40 minutes Curing duration (b)

Figure 10.9 (a) Thermal gravimetric analysis of silver-nanoparticle PCM microcapsules (NCPCMMs) and conventional PCM microcapsules (PCMMs); (b) comparison of the weight loss percentage under 130°C at different time intervals for NCPCMMs and PCMMs (source: Song et al., 2007).

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Examples of maximum allowed residue limits of formaldehyde in textiles and similar products (adapted from Sumiga et al., 2011)

Table 10.2

Infant garments Garments that (ppm) contact skin (ppm)

Other garments or fabrics (ppm)

Japan Law No. 112

20

75

European Union eco-label

30

75

300

Oeko-tex standard 100

20

75

300

EU restrictions on the use of dangerous chemicals

30

100

300

DIN CERTCO certification scheme for textile products

20

75

300

EU eco-label for footwear

textile 75 leather 150

EU eco-label for bed mattresses

mattress 30

EU eco-label for furniture

leather 150

ppm = parts per million, mg/kg, ng/g.

resins, such as polyvinyl chloride (PVC), polystyrene and poly (methyl methacrylate). Over time, suspension polymerization has been used comprehensively for synthesis of functional microspheres (Yeum and Deng, 2005) and recently for fabrication of PCM microcapsules (Ma et al., 2003). In his PhD thesis, Smith (2009) described the formation mechanism of PCM microcapsules using free radical suspension polymerization within a single droplet. The organic phase, which consists of PCM, water-insoluble monomers and free radical oil-soluble initiator, are dispersed in the aqueous phase as droplets by high shear homogenization along with the use of small amounts of suspending agents. When the temperature reaches the decomposition temperature of the free radical initiator, the reaction starts to take place inside the droplets and the generated polymer precipitates out of the PCM–monomer mixture to form polymer particles. These particles continue to grow in number and size as polymerization continues, and are deposited at the oil/water interface by the action of hydrophobicity to form the capsule shell (Figure 10.10). Ma et al. (2003) first prepared PCM microcapsules using free radical suspension polymerization, in which hexadecane was used as PCM and poly(styrene-co-N,N-dimethylaminoethyl methacrylate) as polymer shell. The morphological investigations show that hexadecane was encapsulated completely when the conversion was high, irrespective of whether a hydrophilic monomer (N,N-dimethylaminoethyl methacrylate) was incorporated into the polymer. Selecting proper shell materials for engulfing PCM is crucial for manufacturing

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PCM/monomer droplet

Polymerization Precipitation of polymer

Polymerization/diffusion Polymer deposits at the interface

Conversion Æ 100% Microcapsule shell is formed

Figure 10.10 Schematic diagram showing mechanism of capsule formation using suspension polymerization (source: Smith, 2009).

robust PCM microcapsules. Intensive work has been carried out to optimize the shell materials through changing the type of shell materials (inorganic, mono-polymer and co-polymer) or adding various crosslinking agents. Sánchez et  al. (2007) prepared polystyrene microcapsules containing different kinds of PCMs (paraffin wax, tetradecane, Rubitherm RT27, Rubitherm RT21, nonadecane and polyethylene glycol) using suspension polymerization. This method allows encapsulating non-polar PCMs, while it was not possible to encapsulate polar PCMs (polyethylene glycol). The microcapsule PCM content depends on the type of PCM used, with a possibility to obtain microcapsules with 50 wt.% PCM. The same method of encapsulation has been used along with the use of polydivinylbenzene as shell material instead of polystyrene (Chaiyasat et al., 2009, 2011; Supatimusro et al., 2012). Polymethyl methacrylate microcapsules containing PCMs have been produced by the Turkish research group based at Gaziosmanpasa University (Sarı et al., 2009a, 2010; Alkan et  al., 2009, 2011). Furthermore, Ma et  al. (2010) and Wang et  al. (2012b) used polymethyl methacrylate as a polymer shell for preparation of PCM microcapsules based on ultraviolet (UV) irradiation-initiated rather than thermal initiation. As mentioned above, the morphology of the capsules depends on the core materials and the deposition process of the shell. In the case of suspension polymerization, the polarity and interfacial tension of the polymer formed within the PCM droplets are crucial for forming core/shell morphology. Sánchez et  al. (2007) revealed a composite salami-like morphology by encasing large spheres within the large capsule

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(800 mm) (Figure 10.11(a)). This phenomenon happened due to the lack of strong driving force for phase separation of polystyrene formed within the paraffin wax droplet, where the values of polarity and interfacial tension of polystyrene are quite similar to paraffin wax, and thus core/shell morphology was not thermodynamically favored during the polymerization process. In an attempt to increase the polarity gap between the polymer formed and the PCM, copolymerization and tert-polymerization of acrylic monomers and styrene as shell materials have been used. In the case of polystyrene-methyl methacrylate PCM microcapsules, the internal structure of the large PCM microcapsules shows similar internal structure (salami-like structures) as in the case of polystyrene microcapsules, as seen in the cross-section environmental scanning electron microscope (ESEM) micrograph (Figure 10.11(b)) (Sánchez-Silva et al., 2010a). However, the average energy storage capacity of the microcapsules obtained using this copolymer was higher than that obtained using polystyrene as shell material. Thus, the higher reactivity and polarity of methyl methacrylate compared to styrene favor paraffin microencapsulation. Three morphological features were observed in the case of methyl methacrylate-based capsules: the internal microstructure seems to be a uniform mixture of paraffin and polymethyl methacylate polymer; the average capsule size is small; and the outer shell is bumpy and irregular (Figures 10.11(c) and (d)) (Sánchez-Silva et al., 2010b). From a thermodynamic point of view the morphology of polymethyl methacrylate microcapsules should resemble a core/shell morphology (fully phase separated), but Figure 10.11(d) shows this was not the case (mixture of paraffin and polymethyl methacrylate polymer). This could be due to the high polymerization rate of methyl methacrylate monomer resulting in rapid polymeric chain growth to high molecular weight before those chains have a chance to separate and diffuse from the paraffin. Furthermore, the polymerization reaction temperature should be higher than the glassy temperature of the polymer chains, but this was also not the case. In addition, pomegranate-like internal morphology was observed when terpolymerization of methyl methacrylate, methyl acrylate and methacrylic acid (MMAco-MA-MAA) was used, as the acrylic polymer appears to have formed seed-like spheres of about 3 mm in diameter inside the large capsule with thin and continuous shell, while the exterior shell of the large capsule was wrinkled (Figure 10.11(e)) (Sánchez-Silva et al., 2010b) Thus, the incorporation of methyl acrylate and methyl methacrylate reduced the wet glass temperature (Tg) of the acrylic polymer, which enhanced the chain diffusion of the polymer. However, the molecular weight of the chains also plays a role. Interestingly, no marked difference in molecular weight or distribution was observed. So, minor enhancement of phase separation was observed, but not sufficient to produce core/shell morphology. In contrast, the differential scanning calorimetry (DSC) result shows that the microcapsules’ heat capacity based on the acrylic monomers was twice that of styrene-based encapsulation. Furthermore, a series of PCM microcapsule articles have been published by a group of Chinese researchers from Tsinghua University using suspension polymerization (Qiu et al., 2012a 2012b, 2013a, 2013b). Qiu et al. (2012b) investigated the effect of copolymerization of acrylic monomers with different side chain length on the thermal

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(d)

(c)

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Figure 10.11 ESEM micrographs of: (a) cross-section of PS/paraffin microcapsules; (b) cross-section of a large microcapsule of poly(styrene-methyl methacylate)/paraffin microcapsules; (c) general view of PMMA microcapsules; (d) cross-section and higher magnifications of inner structure of PMMA microcapsules; (e) cross-section of poly(MMA-co-MA-co-MAA) microcapsules (source: Sánchez-Silva et al., 2010b).

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stability and PCM payload content in the microcapsules. The findings show that the payload content of n-octadecane in the microcapsules is inversely proportional to the length side chain of the acrylic monomers, and reached a maximum value up to 77 wt.% in the case of poly(methyl methacrylate-co-butyl methacrylate) copolymer shell. On the other hand, poly(methyl methacrylate-co-stearyl methacrylate) copolymer shell displays the highest thermal resistance up to a temperature of 255°C.

10.2.4 Coacervation-phase separation method Coacervation-phase separation is one of the common methods of PCM encapsulation under the physico-chemical category, which involves two oppositely charged polyelectrolytes (polycation and polyanion) in an aqueous solution. The polycation is usually gelatin while the polyanion is gum arabic (acacia). Small droplets of oil phase (PCMs) are dispersed in the aqueous phase containing polycation as an emulsion. The polyanion solution was then added proportionately with moderate stirring to the formed emulsion. Upon lowering the pH system using acid, phase separation is induced, and polymer rich phase (coacervate) is formed and deposits on the oil interface droplets to form a gelatinous shell upon cooling. The shell can then be crosslinked using glutaraldehyde, which hardens and prevents the gelatin from melting during heating. A typical encapsulation process based on coacervationphase separation is shown in Figure 10.12 (Smith, 2009). In the early 2000s, Hawlader and his group (Hawlader et  al., 2000) prepared 50–100 mm gelatinous microcapsules containing paraffin wax through complex coacervation. Gelatin, gum arabic and formaldehyde were used as polycation, polyanion and crosslinker, respectively. Hawlader et  al. (2003) used the same materials and method to optimize the operating conditions of the fabricated PCM microcapsules. The results show that, when the emulsification rate of 1000 rpm for 10 minutes was used and 6–8 ml of formaldehyde was added, the PCM mass content inside the microcapsules reached maximum. Following Hawlader et al.’s publication, Bayés-García et al. (2010) successfully prepared commercial Rubitherm RT27 microcapsules with two different coacervate compositions (sterilized gelatin/arabic gum and agar-agar/arabic gum) using complex coacervation. In both cases the encapsulation mass ratio shows similar value of 48 wt.%, while the average particle diameter was 12 mm and 4.3 mm in the case of sterilized gelatin/arabic gum and agar-agar/arabic gum system, respectively. In addition, both systems degraded at the same temperature, but in the case of the agaragar/arabic gum system, the PCM microcapsules degraded in a more gradual way.

10.2.5 Spray-drying and other methods of PCM microencapsulation As mentioned in Table 10.1, spray-drying is classified in the mechanical process category and basically is suitable for heat-sensitive materials. Spray-drying has been

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Core material (water immiscible) 40–60°C

Gelatin solution 40–60°C

Emulsify Polyanion Diffusion water 40–60°C Adjust pH to 4.0–4.6 (coacervate forms)

Slowly cool (shell forms and gels)

Crosslink with glutaraldehyde

Isolate capsules

Figure 10.12 Schematic diagram of typical encapsulation process based on complex coacervation (source: Smith, 2009).

applied widely for microencapsulation of food (Shu et  al., 2006), pharmaceutical ingredients (Wan et  al., 1992) and, more recently, PCMs (Hawlader et  al., 2003). Synthesis polymer microcapsules containing PCMs by the physical technique of spray-drying were first reported by Hawlader et al. (2003). Gelatin-acacia was used as polymer shell and paraffin wax as PCM. The microcapsules have high core loading up to 80% with a homogeneous particle size within 0.1–5 mm and microencapsulation efficiencies between 60 and 92%, depending on the core-to-coating ratio. The same method was also employed by Borreguero et al. (2011), along with the used of low density polyethylene (LDPE) and ethylvinylacetate copolymer (EVA) as shell materials with and without carbon nanofibers (CNFs). The operating conditions and formulation

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used to obtain the PCM microcapsules by the above method were reported in the European patent EP2119498 (A1) (Gravalos et al., 2009), wherein a homogeneous liquid solution (feed stream), which consists of PCM and dissolved polymer using a proper solvent, was atomized by means of a carrier gas stream (compressed N2). Following atomization, the solvent was evaporated and the particles were dried by an additional nitrogen stream (drying N2) in the drying chamber, and then the final product was recovered in the collector (Figure 10.13). The characteristics of RT27 microcapsules depended on the location in which they were collected from spray dryer. The thermal energy storage capacity of the RT27 microcapsules is 98.1 J/g and it was similar to those produced by suspension polymerization using polystyrene as shell material (Sánchez et  al., 2007), while it seemed to be more thermally stable than those formed from PS after 3000 thermal cycles as shown in Figure 10.14. Furthermore, adding 2 wt.% of CNFs to the chemical recipe maintained the thermal energy storage capacity of the PCM microcapsules, and improved the thermal conductivity but increased the force required to produce the same microcapsule deformation. Inlet temperature Feed pump Compressed N2 sensor Feed stream Drying N2 Heating system

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Figure 10.13 Schematic representation of the spray-drying equipment used for fabricated PCM microcapsules (source: Borreguero et al., 2011).

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Figure 10.14 SEM micrographs of the microcapsules containing Rubitherm®RT27 after 3000 cycles of thermal stability using two different shells: (a) polystyrene with magnification ¥700 and (b) LDPE–EVA with magnification ¥6000, operating in both cases at 30 kV (source: Borreguero et al., 2011).

Other methods for microencapsulation of PCMs have been reported in the literature. Zhang et al. (2012c) fabricated polycarbonate microcapsules containing stearic acid as PCM by solution casting. Yang et al. (2009) synthesized n-tetradecane microcapsules with different shell materials by phase separation. Chang et al. (2009) used polymethyl methacrylate network-silica hybrid as polymer shell via the solgel process. The same method was also utilized by Fang et al. (2010a) using SiO2 instead of polymethyl methacrylate network-silica hybrid. Fortuniak et  al. (2013) prepared novel n-eicosane microcapsules coated with polysiloxane based on the co-emulsification method.

10.3 Shape-stabilized PCMs As pointed out previously, encapsulation of PCM has been approved to be an effective method for engulfing PCM and preventing them leaking out when melted. However, due to the high cost of production of PCM microcapsules, a new kind of compound, known as shape-stabilized PCM, has been developed. Recently, shape-stabilized PCMs have gained more attention due to their attractive advantages, such as direct use without additional encapsulation, cost-effectiveness and easy preparation with desirable dimensions. Normally, a form-stable PCM is composed of PCMs and supporting material. The PCM acts as a thermal energy storage material while the supporting material maintains the solid shape of the form-stable PCM when the temperature is higher than the melting point of the PCM. The PCMs are mainly organic compounds, such as paraffin (Zhang et al., 2006b), fatty acids (Yan, 2011), fatty alcohol (Memon et  al., 2013) and polyethylene glycol (Alkan et  al., 2006),

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while the supporting materials include polymers (Hong and Xin-shi, 2000) and inorganic compounds (Zhang, 2004). The solid–liquid PCMs are encapsulated in the voids of the polymers by means of melt blending (Krupa et al., 2007), twin-screw extruder technique (Cai et  al., 2008, 2009), casting molding (Fang et  al., 2010b), electrospinning (Chen et al., 2007) or in-situ polymerization (Zhang et al., 2011), or they are grafted to the chains of polymers (Su and Liu, 2006), to obtain formstable PCMs. Another perspective class of form-stable materials are the materials on the basis of porous compound (inorganic compounds) such as diatomite (Xu and Li, 2013), halloysite nanotube (Mei et al., 2011), carbon nanotube materials (Meng et  al., 2013), expanded graphite (Sarı and Karaipekli, 2007) and expanded perlite (Karaipekli and Sarı, 2008).

10.3.1 Polymer-based shape-stabilized PCMs In the early 1980s, a kind of novel PCM compound, so-called shape-stabilized PCM based on the polymer composite, was developed by Feldman et  al. (1985), where lauric acid (LA), stearic acid (SA) and their mixture were used as PCM and polymeric matrices of poly(vinyl chloride) (PVC), poly(vinyl acetate) (PVAc), poly(vinyl alcohol) (PVA), vinyl acetate-vinyl chloride copolymer (VAc-co-VC), and high density polyethylene (HDPE) were used as supporting materials. The fabricated shape-stabilized PCMs kept their shape and dimensions up to 37–43oC, depending on the composition, without losing any fatty acid and those made from high density polyethylene showed no deformation up to 51°C. Moreover, the mechanical properties of shape-stabilized PCMs were enhanced by adding bleached cellulosic fibers (reinforcing agent) to the system. The same supporting materials (high density polyethylene) was utilized by Inaba and Tu (1997) along with use of a mixture of paraffin (Tm = 54°C) and high viscosity resin (ethylene-a olein) as PCMs instead of fatty acids. The idea behind adding a small amount of low crystallite and high viscosity resin is to reduce the oozing rate of paraffin, which results from repetition of the solidification and melting processes. The test results show that the shapestabilized PCM is composed of 74 wt.% paraffin, and the transition temperature (melting-solidification) coincides with that of the dispersed paraffin (pure paraffin) within a standard deviation of 2%. Furthermore, Sarı and his group investigated different kinds of polymeric supporting materials such as high density polyethylene (Sarı, 2004), poly(vinyl alcohol) (Sari et al., 2005), poly(vinyl chloride) (Sarı and Kaygusuz, 2006), styrene maleic anhydride copolymer (Sarı et al., 2008), polymethyl methacrylate (Alkan and Sari, 2008) and poly(ethylene-co-acrylic acid) (Alkan et al., 2012). In addition, Wang and Meng (2010) used a method of self-polymerization to synthesize fatty acid eutectic mixture/polymethyl methacrylate form-stable PCMs. In order to assess the quality of the prepared PCM composite, two polarized optical microscope (POM) images were taken before and after heating the PCM composite sample above the melting point of the PCM using an infrared lamp (Figures 10.15(b) and (d)). The PCM composite keeps its shape without leaking on the surface when

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Figure 10.15 PCM composite samples: (a) and (b) OM and POM images at room temperature; (c) and (d) OM and POM images at 80°C (source: Wang and Meng, 2010).

the photos have been taken below and above the melting point of the PCM (Figures 10.15(a) and (c)). However, the particles of solid fatty acid eutectic mixture disappear and the sample becomes less transparent with little softening, but without liquid drop or seepage of fatty acid on the surface, when the PCM composite sample is heated above the melting point of the PCM (80°C) (Figures 10.15(c) and (d)).

10.3.2 Electrospun form-stable PCM materials Polymer nanofibers have outstanding features compared with any other known form of polymer materials. Therefore, a number of processing techniques such as drawing (Ondarcuhu and Joachim, 1998), phase separation (Ma and Zhang, 1999), self-assembly (Liu et  al., 1999) and electrospinning (Wang et  al., 2009) have been used to prepare polymer nanofibers. Ultra-fine polymer fibers prepared by electrospinning have gained more attention because of their unique properties such as light weight, small diameter, controllable and multi-scaled porous structure, high surface-to-volume ratio, flexibility in surface functionalities and superior mechanical performance (e.g., stiffness and tensile strength). Basically, electrospinning consists of three components: high voltage supplier, capillary tube with a pipette or needle of small diameter, and a metal collecting screen (Figure 10.16). When a sufficiently high voltage is applied to a liquid droplet, the body of the liquid becomes charged,

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Figure 10.16 Schematic diagram of electrospinning setup for fabricated polymer nanofibers (source: Huang et al., 2003).

and electrostatic repulsion counteracts the surface tension and the droplet is stretched; at a critical point a stream of liquid erupts from the surface. This point of eruption is known as the Taylor cone. If the molecular cohesion of the liquid is sufficiently high, stream breakup does not occur and a charged liquid jet is formed. As the jet dries in flight, the mode of current flow changes from ohmic to convective as the charge migrates to the surface of the fiber. The jet is then elongated by a whipping process caused by electrostatic repulsion initiated at small bends in the fiber, until it is finally deposited in the grounded collector. The elongation and thinning of the fiber, which results from bending instability, leads to the formation of uniform fibers with ultrafine diameters (Huang et al., 2003). Recently, ultrafine fibers of PCM/polymer composites have been developed as a novel shape-stabilized PCM via electrospinning (McCann et al., 2006). A group of researchers from the University of Washington have developed a method based on melt coaxial electrospinning for fabricating phase change nanofibers consisting of long-chain hydrocarbon cores and titanium dioxide–polyvinylpyrrolidone (TiO2-PVP) composite sheaths (McCann et al., 2006). This method combines melt electrospinning with a coaxial spinneret and allows for nonpolar solids such as paraffin to be electrospun and encapsulated in one step. Figure 10.17 shows a schematic representation of the melt coaxial electrospinning setup, in which coaxial spinneret is constructed by inserting a polymer-coated silica capillary into a plastic syringe and then concentrically into a metallic needle. This silica capillary is connected to a glass syringe that is placed in an insulated heating mantle. The temperature of the glass syringe is controlled by a temperature controller. With this spinneret, materials that are solid at room temperature can be injected concentrically into a spinning jet together with a carrier solution. During the spinning process, cooling of the jet due to solvent evaporation causes the inner liquid to quickly solidify, leading to its encapsulation in the nanofibers. Like conventional coaxial electrospinning, the inner

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material (in both melt and solid states) must be insoluble in the solvent used for the outer jet in order to obtain fibers with a core-sheath structure. Thermally stable octadecane/titanium dioxide–polyvinylpyrrolidone nanofiber composites were obtained with PCM loading up to 45 wt.% and 150 nm in diameter. Figure 10.18 shows scanning electron microscope (SEM) images and the corresponding transmission electron microscopy (TEM) images of the prepared nanofiber PCM composite with two different PCM mass content and core material injection feeding rates before and after being soaked in hexane. The TEM images reveal that octadecane broke up into spherical droplets in the interior of each nanofiber. The presence of the droplets and segments inside the nanofibers indicates that varicose breakup of the octadecane occurred inside the spinning jet. The viscosity of the octadecane was too low to avoid varicose breakup, thus leading to the sprayed and/or segmented morphology. At low feeding rates of octadecane (0.2 ml/L), varicose breakup led to the dispersion of small droplets of octadecane throughout the nanofiber matrix (Figure 10.18(b)). These droplets are stretched along the long axis of the fiber, as is consistent with electrohydrodynamic stretching of a non-conducting fluid, whereas large droplets of octadecane in the fibers were observed at high injection feeding rates (0.3 ml/L) (Figure 10.18(d)). Following the publication of McCann’s study, a group of Chinese researchers led by Chen investigated the effect of toluene-2,4-diisocyanate as crosslinking agent on the thermal stability, water resistant and thermal properties of the ultrafine

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Figure 10.18 SEM and TEM images of TiO2-PVP/octadecane nanofibers electrospun with a melt coaxial spinneret: (a) SEM image of the nanofibers with 7 wt.% octadecane and 0.2 and 0.7 ml/L feed rate of PCM and sheath solution, respectively; (b) corresponding TEM images of the nanofibers after soaking in hexane for 24 h; (c) SEM image of the nanofibers with 45 wt.% octadecane and 0.3 and 0.7 ml/L feed rate of PCM and sheath solution, respectively; (d) corresponding TEM images of the nanofibers after soaking in hexane for 24 h (source: McCann et al., 2006).

fibers of polyethylene glycol/cellulose acetate composite (Chen et al., 2007, 2009a, 2011). The water resistance and thermal stability of the crosslinked electrospun polyethylene glycol/cellulose acetate composite fibers were enhanced when the toluene-2,4-diisocyanate was incorporated. However, polyethylene glycol (PCM) payload inside the ultrafine fibers was decreased. Fatty acids have been used extensively as PCMs, because of their outstanding features such as ready availability with a wide range of melting temperatures, high latent heat of fusion, and they are non-toxic and non-corrosive. Therefore, fatty acids/ ultrafine fibers based on the composites of polyethylene terephthalate (PET) (Chen et al., 2009b; Cai et al., 2012b), polyethylene terephthalate/silconoxide (PET/SiO2) (Cai et al., 2011), polyamide 6 (PA6) (Cai et al., 2012a) and polyacrylonitrile (PAN) (Cai et al., 2013b) were prepared successfully via electrospinning as form-stable PCMs. In the case of lauric acid/polyethylene terephthalate (LA/PET) nanofiber composite, the morphology and thermal properties changed dramatically by adjusting the LA/ PET mass ratio. The LA/PET composite with the low mass ratio of PCM maintained cylindrical shape with smooth surface, while the quality of LA/PET composite fibers became markedly inferior and the fibers became non-uniform along the fiber axis when the mass ratio of LA/PET 1.0 (Figure 10.19(d)). In addition, there was a direct

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proportion between the average diameter of the fibers and LA/PET mass ratio, except when the mass ratio reached 1.5 (Figure 10.19(e)) (Chen et al., 2008). This result could be attributed to the variations of the solution properties such as increase of viscosity and decrease of conductivity of the mixed solution by further addition of the PCM into the polymer solution as shown in Figure 10.20 (Chen et al., 2011).

10.3.3 Expanded materials-based intercalation composite PCM 10.3.3.1 Graphite and activated carbon-based intercalation composite PCM Graphite is a soft, black form of carbon and has a layered, planar structure. In each layer, the carbon atoms are arranged in a  honeycomb lattice  with separation of 0.142 nm, and the distance between planes is 0.335 nm. The chemical structure of graphite appears in two different forms, alpha (hexagonal) and beta (rhombohedral). The hexagonal graphite may be either flat or buckled. The alpha form can be converted to the beta form through mechanical treatment and the beta form reverts to the alpha form when it is heated above 1300°C. Small distance between planes and neither hydrophilic nor hydrophobic surface layers of graphite make it difficult to intercalate PCM into the graphite layers. This obstacle could be overcome by producing expanded graphite (EG) through adding graphite flakes into a mixture of acid (sulphuric acid, nitric acid or acetic acid) and oxidants (H 2O2, HNO3 or KMnO4) and stirring for 50 minutes at 45°C followed by washing, drying and heating for 10–30 seconds at 900–1000°C (Figure 10.21) (Li et al., 2006, 2007a). Expanded graphite is widely used in many different applications. For example, it is used for hydrogen storage (Lueking et al., 2005) and treatment of textile wastewater (Kong et al., 2009). Due to the supreme thermal conductivity and large porosity of expanded graphite, it is considered as a promising supporting material or additive 1800

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for intercalation PCM. Zhang and Fang (2006) prepared paraffin/expanded graphite composite PCM by absorbing liquid paraffin into the pores of expanded graphite, using the capillary force and the surface tension force of the porous expanded graphite. The inherent worm-like structure of the expanded graphite keeps its shape even when the PCM is added (Figure 10.22). Furthermore, the paraffin/expanded graphite composite PCM exhibited a uniform distribution of the absorbed paraffin. Moreover, Mills et al. (2006) and Sarı and Karaipekli (2007, 2009) used expanded graphite matrix as supporting material for the preparation of shape-stabilized composite PCM. Mills’ study shows that the thermal conductivity of paraffin wax increased by two orders of magnitude when the paraffin was impregnated inside the porous graphite matrix. The same result was demonstrated by Sarı and his group, in which the thermal conductivity of form-stable palmitic acid/expanded graphite composite (0.60 W/mK) increased by 2.5 times higher than that of pure PA (0.17 W/mK). As mentioned above, EG has been approved to be an effective material for improving the thermal conductivity of PCM composites. However, due to the poor mechanical strength of the PCM/expanded graphite composite, another option has been made by adding conductive filler (expanded graphite and aluminium nitride) to

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Figure 10.22 SEM micrographs of (a) expanded graphite; (b) paraffin/expanded graphite composite PCM (source: Zhang and Fang, 2006).

the PCM polymeric composite. Xiao et al. (2001) prepared PCM composite based on paraffin, styrene–butadiene–styrene (SBS) triblock copolymer and exfoliated graphite (EG) as conductive filler. Zhang et al. (2006a) investigated the influence of adding nine different kinds of conductive filler on the thermal conductivity of shapestabilized PCM. Furthermore, a group of researchers based at the Wuhan University of Technology successfully prepared a new type of form-stable PCM (Zhang et al., 2011, 2012b), where, polyethylene glycol (PCM) and aluminium nitride or graphite nanoplatelets (thermal conductivity promoter) were encapsulated and embedded inside the three-dimensional network structure of polymethyl methacrylate matrix using in-situ polymerization. When the mass fraction of polyethylene glycol was below 70%, the prepared PCM composite remained solid without leakage above the melting point of the polyethylene glycol. Thermal analysis showed that the PCM composite possessed desirable payload mass content up to 68 wt%. Moreover, aluminium nitride (AIN) additive was able to effectively enhance the heat transfer property of organic PCM and volume resistivity of the composite (5.92 ¥ 10–10 W.cm) when the mass ratio of AlN was 30%. Furthermore, Cai et al. (2013a) studied the influence of expanded graphite on the structure morphology and thermal performance of the composite PCMs consisting of fatty acid eutectics mixture and electrospun PA6 nanofibrous mats. The results show that the absorption capacity of fatty acid eutectics within nanofibrous mats was increased by adding expanded graphite, so the interfaces between fatty acid eutectics mixture and PA6 nanofibrous mats become more illegible (Figure 10.23). The enthalpies of the composite PCMs and thermal energy storage/retrieval rates were increased when expanded graphite was used without appreciable changes in the phase transition temperatures. In addition, SEM images showed that composite PCMs had little or no variations in shape and surface morphology after heating/ cooling processes. Activated carbon (AC), also called activated charcoal, is a form of carbon with small and low-volume pores that increase the surface area, which is beneficial for different applications such as hydrogen storage (Hu et  al., 2007), catalyst field (Fukuyama et al., 2004), separation of mixtures (Naono et al., 1996) and recently

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intercalation PCMs for thermal energy storage applications (Feng et  al., 2011b). Feng et al. (2011a) prepared shape-stabilized PCMs by blending and impregnation methods. PEG was used as a PCM and different mesoporous materials (active carbon and silica molecular sieves) were used as supporting materials. Among the various composites, polyethylene glycol/activated carbon composite with 80 wt.% of polyethylene glycol had the largest latent heat, a relatively low melting point, the least supercooling and higher heat storage efficiency. However, some leakage of the PCM was observed when the mass content of polyethylene glycol in the composites was higher than 80 wt.%. Furthermore, Wang et  al. (2012a) extensively investigated the influence of the pore structure of the carbon materials (activated carbon, expanded graphite and mesoporous carbon (CMK-5)) on the phase change behavior of polyethylene glycol in the composites. The porous carbon materials can effectively stabilize the melted polyethylene glycol through both the capillary force of the pores and the hydrogen bonding, which results from the surface functional groups. Expanded graphite with macroporous structures maintained high fraction of crystallinity of polyethylene glycol up to 90%, offering superior performance over the other two mesoporous carbon materials. Taking the pore diameter and the pore geometry into account, the crystallinity of polyethylene glycol in shape-stabilized PCMs was increased in the order of macropores (expanded graphite) > mesopores (CMK-5) > micropores (activated carbon) (Figure 10.24).

10.3.3.2 Clay-based intercalation composite PCM Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Clay is considered as one of the oldest building materials on Earth, among other ancient, naturally occurring geological materials such as stone and organic materials like wood. Between onehalf and two-thirds of the world’s population, in traditional societies as well as developed countries, still live or work in a building made with clay as an essential part of its load-bearing structure. So, more attention has been devoted recently to preparing clay-based intercalation composite PCM and mixing it to be part of the building envelope for improving the energy efficiency and indoor thermal comfort of buildings. Hence, activated montmorillonite (Wang et al., 2012c), attapulgite and bentonite (Li et al., 2011a, 2011b), diatomite (Jeong et al., 2013; Sun et al., 2013; Xu and Li, 2013), granulated blast furnace slag (Memon et  al., 2013), halloysite nanotube (Mei et al., 2011; Zhang et al., 2012a) and expanded perlite (Karaipekli and Sarı, 2008) are considered promising inorganic materials for intercalated PCMs. A group of researchers from Tongji University successfully prepared a new type of form-stable PCM based on the different kinds of inorganic porous materials such as expanded shale aggregate and two kinds of expanded clay aggregates by means of vacuum impregnation (Figure 10.25) (Zhang et al., 2004a). The results show that the PCM can penetrate into pore space with a diameter of 1–2 mm and occupy up to 75% of the total pore space of the porous materials. Moreover, the geometrical

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Figure 10.25 Schematic drawing of the vacuum impregnation setup (source: Zhang et  al., 2004a).

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features of the porous structure of the aggregates had a significant effect on their absorbing ability of the PCM, where aggregates with large pore connectivity factor and a transport tunnel in their boundary part can absorb more PCM. Furthermore, the same method (vacuum impregnation) was used by Sarı and his group for developing a set of form-stable PCM on the basis of expanded perlite (Karaipekli and Sarı, 2008; Sarı et  al., 2009b), vermiculite (Karaipekli and Sarı, 2009) and diatomite (Karaman et al., 2011).

References Alexandridou, S. and Kiparissides, C. 1994. Production of oil-containing polyterephthalamide microcapsules by interfacial polymerization: an experimental investigation of the effect of process variables on the microcapsule size distribution. Journal of Microencapsulation, 11, 603–613. Alkan, C. and Sarı, A. 2008. Fatty acid/poly(methyl methacrylate) (PMMA) blends as formstable phase change materials for latent heat thermal energy storage. Solar Energy, 82, 118–124. Alkan, C., Sarı, A. and Uzun, O. 2006. Poly(ethylene glycol)/acrylic polymer blends for latent heat thermal energy storage. AIChE Journal, 52, 3310–3314. Alkan, C., Sarı, A., Karaipekli, A. and Uzun, O. 2009. Preparation, characterization, and thermal properties of microencapsulated phase change material for thermal energy storage. Solar Energy Materials and Solar Cells, 93, 143–147. Alkan, C., Sarı, A. and Karaipekli, A. 2011. Preparation, thermal properties and thermal reliability of microencapsulated n-eicosane as novel phase change material for thermal energy storage. Energy Conversion and Management, 52, 687–692. Alkan, C., Günther, E., Hiebler, S. and Himpel, M. 2012. Complexing blends of polyacrylic acid-polyethylene glycol and poly(ethylene-co-acrylic acid)-polyethylene glycol as shape stabilized phase change materials. Energy Conversion and Management, 64, 364–370. Bayés-García, L., Ventolà, L., Cordobilla, R., Benages, R., Calvet, T. and Cuevas-Diarte, M. A. 2010. Phase change materials (PCM) microcapsules with different shell compositions: preparation, characterization and thermal stability. Solar Energy Materials and Solar Cells, 94, 1235–1240. Borreguero, A. M., Valverde, J. L., Rodríguez, J. F., Barber, A. H., Cubillo, J. J. and Carmona, M. 2011. Synthesis and characterization of microcapsules containing Rubitherm ®RT27 obtained by spray drying. Chemical Engineering Journal, 166, 384–390. Cai, Y., Wei, Q., Huang, F. and Gao, W. 2008. Preparation and properties studies of halogenfree flame retardant form-stable phase change materials based on paraffin/high density polyethylene composites. Applied Energy, 85, 765–775. Cai, Y., Wei, Q., Huang, F., Lin, S., Chen, F. and Gao, W. 2009. Thermal stability, latent heat and flame retardant properties of the thermal energy storage phase change materials based on paraffin/high density polyethylene composites. Renewable Energy, 34, 2117–2123. Cai, Y., Ke, H., Dong, J., Wei, Q., Lin, J., Zhao, Y., Song, L., Hu, Y., Huang, F., Gao, W. and Fong, H. 2011. Effects of nano-SiO2 on morphology, thermal energy storage, thermal stability, and combustion properties of electrospun lauric acid/PET ultrafine composite fibers as form-stable phase change materials. Applied Energy, 88, 2106–2112. Cai, Y., Gao, C., Xu, X., Fu, Z., Fei, X., Zhao, Y., Chen, Q., Liu, X., Wei, Q., He, G. and

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