Synthesis, characterization and applications of microencapsulated phase change materials in thermal energy storage: A review

Energy and Buildings 144 (2017) 276–294

Contents lists available at ScienceDirect

Energy and Buildings journal homepage: www.elsevier.com/locate/enbuild

Synthesis, characterization and applications of microencapsulated phase change materials in thermal energy storage: A review Guruprasad Alva, Yaxue Lin, Lingkun Liu, Guiyin Fang ∗ School of Physics, Nanjing University, Nanjing 210093, China

a r t i c l e

i n f o

Article history: Received 26 September 2016 Received in revised form 24 March 2017 Accepted 26 March 2017 Available online 29 March 2017 Keywords: Microencapsulation Phase change materials (PCM) Microencapsulated phase change materials (MPCM) Thermal properties Thermal energy storage (TES)

a b s t r a c t In recent years microencapsulation of phase change materials has become popular in thermal energy storage field. Commercially produced microencapsulated phase change material (MPCM) is also available in market today. Microencapsulation enhances thermal and mechanical properties of phase change materials used in thermal energy storage. Microencapsulation can be achieved through different techniques and using different shell materials. As the microencapsulation of PCM is gaining increased attention, more and more research works on MPCM are getting published. This review attempts to summarize the available research information on synthesis, characterization, properties and applications of microencapsulated phase change materials for thermal energy storage. The synthesis methods of microencapsulated phase change materials, such as physical synthesis methods like spray drying, physical chemical synthesis methods like complex coacervation and sol–gel process, and chemical synthesis methods like suspension polymerization, emulsion polymerization, interfacial polymerization, in-situ polymerization and condensation polymerization, are presented. The properties of microencapsulated phase change materials like physical properties, chemical properties and thermal properties are analyzed. The applications of microencapsulated phase change materials in buildings, textiles, MPCM slurry and composite foams are also expounded. © 2017 Elsevier B.V. All rights reserved.

Contents 1. 2.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 The synthesis of microencapsulated phase change materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 2.1. Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 2.1.1. Core . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 2.1.2. Shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 2.1.3. Emulsifier . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 2.1.4. Initiator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 2.1.5. Cross-linking agent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 2.1.6. Nucleating agent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 279 2.1.7. Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 2.2. Physical synthesis methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 2.2.1. Spray drying . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 2.3. Physical chemical synthesis methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 2.3.1. Complex coacervation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283 2.3.2. Sol–gel process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284 2.4. Chemical synthesis methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284 2.4.1. Suspension polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284

∗ Corresponding author. E-mail address: [email protected] (G. Fang). http://dx.doi.org/10.1016/j.enbuild.2017.03.063 0378-7788/© 2017 Elsevier B.V. All rights reserved.

G. Alva et al. / Energy and Buildings 144 (2017) 276–294

3.

4.

5.

277

2.4.2. Emulsion polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284 2.4.3. Interfacial polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284 2.4.4. In-situ polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 2.4.5. Condensation polymerization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286 The characterization of microencapsulated phase change materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286 3.1. Physical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286 3.1.1. Encapsulation efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 286 3.1.2. Microcapsule size distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287 3.1.3. Encapsulation ratio . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287 3.1.4. Shell characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287 3.1.5. Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287 3.2. Chemical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288 3.2.1. Fourier transformation infrared spectroscopy (FT-IR) analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 288 3.2.2. X-ray diffraction (XRD). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .288 3.3. Thermal properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 3.3.1. Differential scanning calorimetry (DSC) analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 3.3.2. Thermogravimetric analysis (TGA) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 290 3.3.3. Flammability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 3.3.4. Thermal reliability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 3.3.5. Thermal conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 Applications of microencapsulated phase change materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 4.1. Buildings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 4.2. Textiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292 4.3. MPCM slurry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292 4.4. Composite foams . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293 Conclusions and outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 293

1. Introduction Phase change materials (PCM) are considered as the best choice for thermal energy storage. They have a high thermal energy storage density due to their high latent heat of fusion. They can store and release thermal energy at an almost constant temperature, near the operating temperature range of many commercial applications like heating and cooling of buildings, thermal comfort textile and solar energy systems. However they have a limitation in low thermal conductivity. Moreover as they melt, the liquid phase creates problems like leakage. MPCM will have a central PCM core around which a coating is grown of organic or inorganic shell materials. The different forms of core-shell structure are shown in Fig. 1. Microcapsules can vary from few microns to millimeter in size. Microencapsulation helps to overcome low thermal conductivity by increasing the surface-to-volume ratio for the PCM. This provides a large heat exchange surface area for each PCM core, thus increasing the heat transfer. It solves leakage issue, by containing the melted PCM inside micron sized capsule made up of material with a higher melting point than PCM. Currently very few review articles on microencapsulation of PCM are available. Most of the previous review articles on microencapsulation are related to its applications in other areas like pharmaceuticals, food, agriculture etc. On microencapsulation of PCM, Zhao and Zhang [1] reviewed their synthesis techniques and applications. Their focus was mainly on chemical techniques. Jamekhorshid et al. [2] did a comprehensive review of microencapsulation synthesis techniques. Paloma et al. [3] reviewed the synthesis techniques, characterization and applications of MPCM. This review attempts to summarize the recent developments in synthesis technologies, characterization and applications of MPCM. 2. The synthesis of microencapsulated phase change materials Microencapsulation technique is popular in various industries. In pharmaceuticals industry it is used for coating drugs. Microen-

Fig. 1. Morphology of microcapsules [2].

capsulated drugs act as oral and transdermal drug delivery vehicles. They provide an extended drug effect at a relatively low dose due to controlled and prolonged release [4]. In food industry it is used for coating probiotic bacteria. Microencapsulation protects the probiotic bacteria from damage due to external factors like heat, pH changes etc during storage and handling and from pH changes and

278

G. Alva et al. / Energy and Buildings 144 (2017) 276–294

bile in human gastrointestinal track until it reaches its destination intestine alive [5]. In agriculture it is used for coating pesticides. Microencapsulation is the technique for the controlled release of pesticide. This reduces the environmental losses of pesticide until it reaches its target [6]. Microencapsulation of PCM for thermal energy storage is relatively a recent phenomenon. There are many different microencapsulation techniques and many different coreshell materials available for PCM. Broadly these techniques can be classified under three categories as physical, physical-chemical and chemical methods. Shell materials can be both organic and inorganic. Below are the details of most common materials and synthesis techniques used for producing MPCM.

2.1. Materials Microencapsulation of PCM requires materials of below listed types.

2.1.1. Core PCM is the core material in MPCM. PCM used in MPCM are mostly organic PCM. Organic PCM have a suitable melting point near the thermal comfort range of humans which is around 20 ◦ C. They are chemically inert and show no phase segregation. They are non toxic and harmless to environment. They do not promote corrosion. They do not have subcooling issue. Organic PCM include different classes like paraffin (n-alkane), fatty acids, alcohols, easters and polyethylene glycol (PEG) etc. Paraffin class materials of organic PCM are the most popular choice as core materials. Paraffin material n-octadecane has suitable melting temperature of 28.4 ◦ C. It is non-polar and insoluble in water. There for it can easily form emulsion. On the other hand polyethylene glycol (PEG) is difficult to be encapsulated. It cannot form emulsion in water because it is soluble in water. Similarly inorganic salts also are rarely encapsulated due to their solubility in water. They are corrosive and have supercooling issue. However there are reports of inorganic hydrated salts also being encapsulated [7]. The PCM that have been reported to be microencapsulated are listed in Table 1.

2.1.2. Shell Shell materials form the capsules that contain the PCM. There are both organic and inorganic shell materials. There are also hybrid shell materials made of organic-inorganic combination. Majority of the shells are organic and prepared through chemical method like polymerization. Monomers are used for building shell through polymerization process. Monomers are repeating blocks of a long polymer chain. Polymer chains can also be formed by more than one monomer. Sometimes the shell material could be a co-polymer which is a combination of more than one polymer [8]. Microcapsule shell can also be built through physical methods like spray drying or complex-coacervation. A shell material should not undergo chemical reaction with the PCM core and should possess good chemical stability. The shell material should have thermal stability at high temperatures. Its surface morphology must be smooth and it should have minimum porosity and prevent any leakage of PCM at temperatures above the melting point of PCM. Table 2 gives a list of commonly used shell materials and synthesis techniques associated. The shell gives the mechanical strength and shape stability to MPCM. Thicker shells have better mechanical strength. It is desirable to have shell material with high thermal conductivity. The shell material should withstand the repeated mechanical stress cycles and thermal cycles for at least 1000 cycles and more. The properties of the shell depends on the type of polymer chosen as shell material and also conditions during the synthesis process.

Fig. 2. Elementary reactions in conventional free radical polymerization [10].

2.1.3. Emulsifier Emulsion is a system of two immiscible liquids where one liquid is dispersed as droplets in the other liquid. Basic microencapsulation technique involves preparation of oil in water (O/W) emulsion with PCM as the oil phase. PCM is dispersed as micron sized oil droplets in the continuous aqueous solution using mechanical stirrers or ultrasound techniques. However emulsions are unstable due to the natural tendency (surface tension) of liquid-liquid system to separate and minimize interfacial area and interfacial energy. Typical emulsifiers are a class of materials called amphiphiles (surfactants, synthetic polymers, polysaccharides etc) that have molecular structure with one end soluble in oil phase and other end soluble in water phase. There for they form a thin film at the interface between the oil droplets and aqueous solution. They reduce the surface tension effect by preventing the direct contact between two different oil droplets. This gives kinetic stability to the emulsion. Sometimes emulsifiers are also called as stabilizers. Surfactants are used for emulsification in almost all chemical synthesis techniques of MPCM. The emulsion stability through the use of mixed surfactants sodium dodecyl sulfate (SDS) and poly vinyl alcohol (PVA) was studied by Al-Shannaq et al. [9] and compared to a system where only a single surfactant (PVA) was used. The use of mixed surfactants induced long-term emulsion stability. They reported surface morphology and the particle size of the microcapsules are related directly to the emulsion stability, where the percentage of surface buckles and dimples on the surface of the microcapsules are reduced significantly in the case of mixed surfactant. Table 2 gives a list of commonly used emulsifier materials along with related shell materials and synthesis techniques. 2.1.4. Initiator Initiators are used in radical polymerization techniques like suspension polymerization and emulsion polymerization. Different stages of a radical polymerization process are shown in Fig. 2. In radical polymerization an initiator decomposes in to two free radicals under the influence of an external trigger like heating, light

G. Alva et al. / Energy and Buildings 144 (2017) 276–294

279

Table 1 Core materials. Type

Class

PCM

Melting point ◦ C

Latent heat of fusion H kJ/kg

Nucleating agents

Remarks

References

Organic

Paraffin

n-Octadecane

28.4

234

Most popular PCM choice

[14,17,21,28]

Organic

Paraffin

Rubitherm-RT21

21

135

1-octadecanol, Sodium Chloride, paraffin 1-Octadecanol, Rubitherm-RT58

Organic Organic Organic Organic Organic Organic Organic Organic Organic Organic Organic

Paraffin Paraffin Paraffin Paraffin Paraffin Paraffin Fatty acid Fatty acid Fatty acid Fatty acid Fatty acid

64 20 32 6 22.6 36.6 64 31 43 53 26.04

189.24 185 194.3 202 216.6 214.2 208.5 190.21 200.18 201.65 176.68

Organic Organic

Ester Alcohol

Paraffin wax n-Hexadecane n-Nonadecane n-Tetradecane n-Heptadecane n-eicosane Palmitic acid Capric acid Lauric acid Myristic acid Capric acid(86%) + stearic acid(14%) Butyl stearate Xylitol

23 93

140 248.7

or ionizing radiation. Free radical initiates the polymerization process by attacking the monomer molecule forming a bond with it and a reactive site on the monomer molecule. Second monomer molecule reacts with the reactive site of first monomer forming a polymer chain and regenerating a fresh reactive site on the chain. Polymerization proceeds (on average adding a monomer unit every millisecond) exclusively by a reaction between the reactive site on the growing polymer chain and fresh monomer molecules, with the regeneration of the fresh reactive site at the end of each growth step. After about a thousand additions, the growing chains undergo either chain termination or chain transfer reactions. In the termination case, a dead polymer chain (i.e. chain without reactive site) is formed and the reactive site is irreversibly lost. In chain transfer case, a dead polymer chain is produced but the reactive site is transferred to another species that can start a new chain [10]. Table 2 gives a list of commonly used initiator materials along with related shell materials and synthesis techniques.

2.1.5. Cross-linking agent Cross-linking is a process where a polymer chain on the shell material is linked to another polymer chain through a chemical bond. Cross-linking provides better mechanical strength to the shell materials. Al-Shannaq et al. [9] reported adding a small amount of cross-linker to the system improves the surface morphology of the microcapsules to be smooth and compact and produces microcapsules with a much higher PCM content. Hawlader et al. [11] reported an example of cross-linking in synthesis using complex-coacervation technique of Gum-Arabica/Gelatin capsules. Authors describe that, carboxylate functional group on gelatin molecule with a formaldehyde molecule, activates carboxylic group to nucleophillic attack. A reaction of such groups on other gelatin molecules with the first activated carboxylate cross-links the gelatin. This hardening reaction is catalyzed by OH (alkaline condition). The change in pH value results in ionization and deionization of the functional groups [4]. For example carboxylic acid group (COOH) are not ionized at low pH of less than 5.5, whereas charged COO groups repulse each other at high pH of 5.5–7.4 [4]. Increasing the amount of cross-linking agent leads to an increase in the encapsulation efficiency. However beyond an optimum 8 ml of cross-linking agent, further increase did not result in much improvement in encapsulation efficiency [11]. Alay et al. [12] reported that cross-linker type used for shell polymerization had an impact on thermal properties, particle size and structure

[13]

Eutectic mixture

[11,22] [12,17,29,70] [17] [19,70] [23,63,70] [68] [31] [24] [24] [24] [26]

[67] [27]

Fig. 3. DSC curves of non-encapsulated and encapsulated PCM [13].

of microcapsules. In the same study they also used polyurethane as a cross-linker to fix the microcapsules to cotton and polyester in thermo-regulated fabrics. Table 2 gives a list of commonly used cross-linking agents along with related shell materials and synthesis techniques. 2.1.6. Nucleating agent Normally inorganic PCM have subcooling problem. Organic PCM do not have serious supercooling issue. However, when organic PCM are encapsulated in microcapsules, they tend to supercool severely, most likely due to the absence of nuclei in such small space. Al-Shannaq et al. [13] reported that due to supercooling, the onset crystallization temperature of microencapsulated PCM in differential scanning calorimetry (DSC) graph could be 10 ◦ C lower than that of non-encapsulated PCM as shown in Fig. 3. This can seriously impact the efficiency of thermal energy storage. Nucleating agents are added to decrease the super cooling problem in MPCM. Al-Shannaq et al. [13] verified using nucleating agents supercooling issue can be eliminated. However the addition of nucleating agents introduces new problems like decrease in latent heat of fusion of core PCM, appearance of buckles and dimples on the shell surface spoiling the smoothness of surface morphology of the shell. Use of 1-octadecanol as nucleating agent for RT21 increased the shell permeability resulting in an increased mass loss. However use of RT58 as nucleating agent for RT21 not only prevented supercool-

280

Table 2 Shell materials. Shell materials

Type

Preparation methods

PCM

Monomers/polymers

Arabic-gum/gelatin

Organic

Complex-coacervation

Paraffin wax

arabic-gum (Polysaccharide), gelatin powder(Protein)

n-Hexadecane

arabic-gum (Polysaccharide), gelatin powder(Protein)

Emulsifier (Surfactant)

Initiators

Dodecyl sulfate sodium salt (SDS) (C12 H25 OSO2 ONa)

Cross linking agents

Remarks

References

Gluteraldehyde, formaldehyde

Emulsified at 10,000 rpm Peak melting temperature: 62.5 ◦ C Latent heat of fusion: 239.78 kJ/kg Potassium peroxodisulfate (K2 O8 S2 ), Sodium thiosulfate (Na2 S2 O3 ), Sodium carbonate (Na2 CO3 ) Are used as electrolytes Latent heat of fusion: 144.7 kJ/kg Latent heat of fusion: 165.8 kJ/kg Latent heat of fusion: 57.5 kJ/kg Latent heat of fusion: 216.44 kJ/kg

[11]

Peak melting temperature: 28.40 ◦ C Latent heat of fusion: 98.1 kJ/kg Dichloromethane(DCM) used as polymer solvent Latent heat of fusion:142.3 kJ/kg Latent heat of fusion: 107.1 kJ/kg

[16]

Formaldehyde (H2 CO)

n-Octadecane n-Nonadecane Spray drying

Paraffin wax

Organic

Spray drying

Rubitherm-RT27

Acrylonitrile–styrene copolymer (AS)

Organic

Phase separation

n-tetradecane

Polyvinyl alcohol (PVA)

ABS(copolymer)

Acrylonitrile–styrene– butadiene copolymer (ABS) Polycarbonate (PC) Polyurea

AS (copolymer)

PC(copolymer) Organic

Interfacial polymerization

n-octadecane

Butyl stearate

Tolylene 2,4-diisocyanate (TDI), ethylene diamine (EDA)

Tolylene 2,4-diisocyanate (TDI), diethylene triamine (DETA) Tolylene 2,4-diisocyanate (TDI), Amine-terminated polyoxypropylene (Jeffamine) Toluene-2,4-diisocyanate (TDI) Ethylene diamine (EDA)

Latent heat of fusion:49.5 kJ/kg Ammonium chloride used as nucleating agent for polymerization. It turns the solution slightly acidic. Peak melting temperature: 28.16 ◦ C Latent heat of fusion: 158.7 kJ/kg Peak melting temperature: 27.73 ◦ C Latent heat of fusion: 165.1 kJ/kg Peak melting temperature: 27.04 ◦ C Latent heat of fusion: 188.9 kJ/kg

Sodium salt of styrene–maleic anhydride copolymer (SMA)

OP-10 (Non-ionic surfactant)

Polyurethane

Organic

Interfacial polymerization

Xylitol

Diphenyl methylene diisocyanate (MDI), Xylitol (Polyol)

Polystyrene

Organic

Emulsion polymerization

n-Heptadecane

Styrene

® Span 85 (sorbitan trioleate)-Non-ionic surfactant, poly(ethylene glycol)dioleate Triton X-100

Capric acid

Styrene

Triton X-100

Ferrous sulfate heptahydrate Ammonium persulphate

Divinylbenzene

Ferrous sulphate heptahydrate, Ammonium persulfate

Divinylbenzene

[11]

[19]

[66]

Cyclohexane is used as a reagent. Peak melting temperature: 33.8 ◦ C Latent heat of fusion:85.92 kJ/kg

[67]

Xylitol is used as both core PCM and polyol type monomer. Latent heat of fusion:196.3 kJ/kg Peak melting temperature: 21.48 ◦ C Latent heat of fusion: 136.89 kJ/kg Latent heat of fusion:86.45 kJ/kg

[27]

[23]

[24]

G. Alva et al. / Energy and Buildings 144 (2017) 276–294

Low density polyethylene(LDPE)/ethylene vinyl acetate(EVA)

arabic-gum (Polysaccharide), gelatin powder(Protein) LDPE (polymer), EVA(polymer)

[17]

Lauric acid Myristic acid Melamine–formaldehyde

Polyurea-formaldehyde

OrganicIn-situ

In-Situ

Melamine, Formaldehyde

Styrene-maleic anhydride copolymer (SMA)

n-eicosane

Melamine, Formaldehyde

Sodium dodecylbenzenesulfonate (SDBS)

n-octadecane

Melamine, Formaldehyde

Sodium salt of styrene-maleic anhydride copolymer (SMA)

1,3-Benzenediol (resorcinol)

n-hexadecane

Urea (NH2)2C O, Formaldehyde (H2CO),

Sodium salt of styrene-maleic anhydride copolymer (SMA) Poly(vinyl alcohol) (PVA), Triton X100

Resorcinol (1,3 dihydroxy benzene)

Paraffin Rubitherm-RT21

Methyl methacrylate (MMA)

Benzoyl peroxide

Disodium hydrogen phosphate hepta-hydrate (Na2 HPO4 ·7H2 O)

Methyl methacrylate (MMA), ethyl acrylate (EA)

Polyvinyl alcohol (PVA)-Nonionic, Sodium dodecyl sulfate(SDS)- ionic sorbitan trioleate (Span-80)

n-Heptadecane

Methylmethacrylate (MMA)

Triton X-100

Ferrous sulphate (FeSO4.7H2O) Sodium thio-sulphate (Na2 S2 O7 ) Tert-butyl hydroperoxide

n-octadecane

PMMA (poly-methyl methacrylate)

OrganicSuspension polymerization

Emulsion polymerization

n-Octacosane

Dibenzoyl peroxide

n-Hexadecane

Silica (SiO2 )

Inorganic Sol-gel

Penta erythritol tetra acrylate (PETRA)

allyl methyl acrylate

ethylene glycol dimethacrylate

Capric acid, Stearic acid (Eutectic Mixture) n-Octadecane

Methylmethacrylate (MMA) Sodium silicate

Paraffin

Tetraethoxysilane (TEOS) (Si(OC2 H5 )4 )

Triton-X100 Poly( ethylene oxide-b-propylene oxide-b-ethylene oxide) tri-block copolymer (EO27-PO61-EO27) Sodium dodecyl sulfate (SDS)

Ammonium persulphate, Ferrous sulphate

Ethylene glycol dimethacrylate

[14]

[68]

[28]

[29]

[13]

[7]

[63]

G. Alva et al. / Energy and Buildings 144 (2017) 276–294

n-Octadecane

[25]

[12]

[26] [21]

[22]

281

Latent heat of fusion:87.21 kJ/kg Latent heat of fusion: 98.26 kJ/kg Triethanolamine used for ph control. Latent heat of fusion: 160 kJ/kg Sodium hydroxide and acetic acid used for pH control Latent heat of fusion:162.4 kJ/kg Acetic acid (acidic pH) and triethanolamine (basic pH) is used for maintaining basic pH. Ammonium chloride used as nucleating agent for polymerization. Peak melting temperature: 26.91 ◦ C Latent heat of fusion: 146.5 kJ/kg Ammonium chloride is the polymirization reaction initiator. Acetic acid (CH3 COOH) for adjusting pH. Peak melting temperature: 22 ◦ C Latent heat of fusion: 115 kJ/kg Peak melting temperature: 33.4 ◦ C Latent heat of fusion: 153 kJ/kg Peak melting temperature: 22.28 ◦ C Latent heat of fusion: 153 kJ/kg Acetone, toluene, carbon tetrachloride and chloroform were used as organic solvent. Hydroquinone mono-methyl ether is used as inhibiter Peak melting temperature: 51 ◦ C Latent heat of fusion: 150 kJ/kg Latent heat of fusion: 81.5 kJ/kg Peak melting temperature: 50.6 ◦ C Latent heat of fusion: 86.4 kJ/kg Peak melting temperature: 17.34 ◦ C Latent heat of fusion: 145.61 kJ/kg Latent heat of fusion:116.25 kJ/kg Hydrochloric acid (pH control) Peak melting temperature: 27.96 ◦ C Latent heat of fusion: 87.4 kJ/kg Anhydrous ethanol and distilled water used as solvent. Hydrochloric acid used as the activator. Peak melting temperature: 58.37 ◦ C Encapsulation ratio: 87 Latent heat of fusion:165.6 kJ/kg

282

Table 2 (Continued) Shell materials

Titania shells(TiO2 )

Inorganic

Organic

Preparation methods

PCM

Monomers/polymers

Emulsifier (Surfactant)

Tris (hydroxymethyl)methyl aminomethane

Tetraethoxysilane (TEOS), (3Aminopropyl)triethoxysilane (APTS)

Sodium dodecyl sulfate (SDS), SPAN 80

Stearic acid

Tetraethoxysilane (TEOS)

Sodium dodecyl sulfate (SDS)

Sol-gel

Palmitic acid

Tetra-n-butyltitanate

Sodium dodecyl sulfate (SDS)

Spray drying

n-Octadecane

Titanium tetra-isopropoxide (TTIP)

Suspension polymerization

n-Octadecane

n-butyl methacrylate (BMA) n-butyl acrylate(BA)

Initiators

Cross linking agents

styrene-maleic anhydride copolymer (SMA)

2,2-Azobisisobutyronitrile (AIBN)

Pentaerythritol triacrylate (PETA)

Triton X-100

Ammonium peroxodisulfate Ferrous sulphate (FeSO4 .7H2 O) Sodium thiosulfate (Na2 O3 S2 )

Ethylene glycol dimethacrylate

Sodium dodecyl benzene sulfonate (SDBS)

Ammonium persulfate

Sodium salt of styrene-maleic anhydride copolymer (SMA)

Benzoyl peroxide (BPO)

n-butyl methacrylate(BMA) methacrylic acid (MAA) n-butyl methacrylate(BMA) acrylic acid (AA)

Poly(styrene-coethylacrylate) (Co-polymer)

Organic

Emulsion polymerization

Tetradecane

n-butyl methacrylate(BMA) n-butyl acrylate(BA) methacrylic acid (MAA) Styrene, Ethyl Acrylate (EA)(C5 H8 O2 )

Pentadecane

Hexadecane

Heptadecane

PMMA-co-SiO2 (Co-polymer)

Hybrid

Interfacial/sol-gel/selfassembly

paraffin

n-Octadecyl methacrylate (ODMA), methacrylic acid (MAA) (Co-polymer)

Organic

Suspension Polymerization

n-octadecane

Methyl methacrylate (MMA), Tetraethoxysilane (TEOS) n-octadecyl methacrylate (ODMA), Methacrylic acid (MAA)

Divinylbenzene (DVB) Allyl methacrylate (AMA)

Remarks

References

Cyclohexane used as solvent. Tris is a solid-solid phase change material. Peak melting temperature: 130 ◦ C Latent heat of fusion:146 kJ/kg Hydrochloric acid used for pH control Peak melting temperature: 53.5 ◦ C Latent heat of fusion: 171.0 kJ/kg Anhydrous ethanol and distilled water used as solvent. Hydrochloric acid used for pH control. Peak melting temperature: 61.7 ◦ C Latent heat of fusion: 63.3 kJ/kg Peak melting temperature:28.7 ◦ C Latent heat of fusion: 97.0 kJ/kg Peak melting temperature: 30.9 ◦ C Latent heat of fusion: 116.4 kJ/kg Peak melting temperature: 32.8 ◦ C Latent heat of fusion: 144.3 kJ/kg Peak melting temperature: 27.6 ◦ C Latent heat of fusion: 141.5 kJ/kg Peak melting temperature: 30.9 ◦ C Latent heat of fusion: 136.3 kJ/kg Peak melting temperature: 7.97 ◦ C Latent heat of fusion: 182.68 kJ/kg Peak melting temperature: 11.60 ◦ C Latent heat of fusion: 121.83 kJ/kg Peak melting temperature: 20.38 ◦ C Latent heat of fusion: 196.08 kJ/kg Peak melting temperature: 24.04 ◦ C Latent heat of fusion: 140.51 kJ/kg Peak melting temperature: 26.8 ◦ C Latent heat of fusion: 71 kJ/kg A novel co-polymer that decreases degree of super cooling. Peak melting temperature: 21.1 ◦ C Latent heat of fusion: 91 kJ/kg

[32]

[65]

[31]

[15]

[69]

[70]

[64]

[8]

G. Alva et al. / Energy and Buildings 144 (2017) 276–294

n-Butylmethacrylate-copolymers (Co-polymer)

Type

G. Alva et al. / Energy and Buildings 144 (2017) 276–294

283

ing but also decreased the mass loss. Fan et al. [14] experimented with three different nucleating agents sodium chloride, ammonium chloride, 1-octadecanol and paraffin for n-octadecane PCM. They found paraffin to be most suited nucleating agent with no impact on surface morphology. Table 1 gives a list of commonly used nucleating agents along with the related PCM. 2.1.7. Others Other than the above listed materials additional materials can be used in the synthesis process. For example solutions of NaOH, hydrochloric acid, triethanolamine and acetic acid are used for controlling the pH during polymerization. Sometimes after the microcapsule shells have polymerized, during the post-processing step inhibiters like hydroquinone monomethyl ether are added to prevent further polymerization reaction [7]. Monomers as supplied by company may contain inhibitors which may inhibit shell polymerization. In such cases monomers are pre-washed in solutions like NaOH and deionized water to remove inhibitors and neutralize the monomer before microencapsulation process. In emulsions, water is the most used solvent. However other solvents like acetone, toluene, carbon tetrachloride and chloroform are also in use. 2.2. Physical synthesis methods In a physical process the ingredients that make up the shell do not undergo any chemical reaction. They retain their original chemical composition. The shells are formed by physical processes like binding due to drying (dehydration), adhesion etc. In food and pharmaceutical industries many purely mechanical techniques like pan coating, air suspension coating, spray drying etc are in use for microencapsulation, but only spray drying technique is most used for microencapsulating PCM. 2.2.1. Spray drying In this synthesis technique, first an emulsion of PCM in a solution containing the shell material like gelatin is prepared with mechanical stirring at a high rpm like 10,000. A binder like gumarabica will be added in drops while a suitable temperature and pH will be maintained [11]. This emulsion will then be fed to an atomizing nozzle at a particular feed rate. At the atomizer a high pressure carrier gas (nitrogen or air), sprays the feed solution as small droplets into a drying chamber. At the drying chamber, sprayed droplets comes into contact with the hot drying gas (nitrogen or air), and evaporation takes place in the droplet’s surface until the moisture content becomes too low to diffuse through the dried droplet surface. Thus a shell is formed around the core producing MPCM. Finally, the recovery of dried MPCM is carried out either in the cyclone, filter bag or electrostatic precipitator. Fig. 4 shows a sample spray drying equipment. Desired particle size distribution can be achieved through suitable atomizer design and the process can be easily controlled and scaled-up. Hawaldar et al. [11] microencapsulated paraffin-wax with gelatin and gumarabica as shell materials using spray-techniquedrying technique. They reported producing microcapsules with spherical shape and uniform size distribution. The surface morphology of the shells was smooth with no sharp edges or dents. Microcapsules had a high heat storage capacity up to 216 kJ/kg. The microencapsulation efficiency decreased with increasing core-to-coating ratio, where as heat storage capacity increased with increasing core-to-coating ratio. Fei et al. [15] used spray-drying technique for synthesizing n-octadecane microcapsules with inorganic titania shell material. They reported synthesizing microcapsules with 0.1–5.0 micrometer size range. Borreguero et al. [16] microencapsulated commercial ® wax, paraffin Rubitherm RT27, using low density polyethylene (LDPE) and ethylvinylacetate copolymer (EVA) as shell materials through spray drying technique. They reported an average parti-

Fig. 4. Spray drying equipment [15].

cle size of 3.9 micrometers, high encapsulation efficiency (63 wt%), high heat storage capacity and good thermal stability over 3000 cycles. However they also reported the problem of agglomeration of microcapsules in the drying chamber. Sometimes, the microcapsules have a complex morphology, such as poly-nuclear or matrix type. Use of large amounts of core material can lead to uncoated particles. 2.3. Physical chemical synthesis methods Physical chemical processes are hybrid process that use combination of physical and chemical methods. Physical processes like phase separation, heating, cooling etc are combined with chemical processes like hydrolysis, cross-linking, condensation etc to achieve microencapsulation. For microencapsulation of PCM most commonly used physical chemical techniques are complex coacervation and sol–gel. 2.3.1. Complex coacervation This method is also called phase separation method. This method is typically used for organic shell material combinations like gelatin/gum-arabic, agar-agar/gum-arabic, chitosan/gumarabic, chitosan/silk-fibroin etc. In this method the shell polymer material is dissolved in a solvent to form solution. Then an emulsion is prepared with PCM material as the oil phase dispersed in the above solution. The dissolved shell is then made to precipitate on the PCM droplet’s surface by evaporating the solvent and then forming a shell around the PCM. Alternately the same can be achieved by coalescing (due to electrostatic attraction) of two different organic polymers around the PCM droplet’s surface and then dropping the temperature and pH of the emulsion to initiate phase-separation. This results in two different phases, one around the PCM droplet surface which is rich in colloids concentration and other in the aqueous phase poor in colloids concentration. The colloid-rich phase coalesces to form a coacervate layer forming a shell around PCM. Gum arabic is a complex and variable mixture of polysaccharides and glycol-proteins [17]. It is negatively charged above pH of 2.2. Gelatin is a heterogeneous mixture of single or multi-stranded polypeptides, each containing between 300 and 4000 amino acids [17]. The final step is hardening and filtering of microcapsules. The hardening step is carried out using the cross-linking agent like formaldehyde or gluteraldehyde solution. Filtered microcapsules will be washed in deionized water or ethanol and dried in a vacuum oven. Santos et al. [18] microencapsulated xylitol using gum-arabica and gelatin as shell materials. They verified that during complex-coacervation, the carboxyl groups of polysaccharides interact with the amino

284

G. Alva et al. / Energy and Buildings 144 (2017) 276–294

groups of proteins to form a complex containing an amide bond. Fourier transformation infrared spectroscopy (FT-IR) analysis verified the formation of amides in the microcapsules, confirming the occurrence of complex-coacervation. The peak that appeared at 3313 cm−1 in the gelatin spectrum is characteristic of amine groups (positively charged), and the peak that appeared with low intensity at 2939 cm−1 in the gum-arabic spectrum is characteristic of carboxylic groups (negatively charged). The binding of positive and negative charges (i.e., amino and carboxyl groups) is expected to promote the process of coacervation and the formation of amides. They observed a peak at approximately 1651 cm−1 in the spectrum, which indicated the presence of an amide, confirming the formation of this complex. Hawalder et al. [11] microencapsulated paraffin-wax with gelatin and gum-arabica as shell materials using complex-coarcervation technique. They reported producing microcapsules with spherical shape and uniform size distribution. The surface morphology of the shells was smooth with no sharp edges or dents. Microcapsules had a high heat storage capacity up to 239.78 kJ/kg. The influence of emulsifying time and cross-linking agent concentration on encapsulation efficiency were also reported. They found 8 ml of cross-linking agent and 10 min emulsifying time to be optimal with maximum encapsulation efficiency. Yang et al. [19] carried out successful microencapsulation of n-tetradecane with different shell materials by phase separation method. They used acrylonitrile-styrenecopolymer (AS), acrylonitrile–styrene–butadiene copolymer (ABS) and polycarbonate (PC) as shell materials. Among them, AS was found to be the most suitable one, with highest encapsulation efficiency (84.5%) and greatest heating enthalpy (142.3 kJ/kg). Garcia et al. [20] observed growth of microorganisms after one month on MPCM prepared with food grade gelatin, using complex-coacervation technique. The microorganism growth was avoided by substituted to sterilized gelatin instead. 2.3.2. Sol–gel process In this technique, first a sol solution is prepared which when added to PCM emulsion, produces a gel of discrete microcapsules containing PCM droplets inside. This method is typically used for inorganic shell materials like silica and titanium oxide. The sol solution is prepared through hydrolysis of precursor compound like tetraethoxysilane (TEOS or Tetraethylorthosilicate), sodium silicate, methyl triethoxysilane etc. Low pH (2–3) condition is maintained in sol solution to promote hydrolysis reaction. After that the silicate sol solution is added to a PCM O/W emulsion drop by drop while emulsion is kept stirring to prevent formation of any continuous gel and allowing the formation of only discrete silica gel walls around PCM droplets. The silica shell will be formed on the surface of the PCM droplets by the condensation polymerization of the silica solid particles. High pH (9–10) condition is maintained in the emulsion to promote condensation reaction. Fig. 5 shows a sample sol–gel process. Organic polymers are usually toxic and flammable, and compared to inorganic polymers, they have poor heat transfer performance and thermal stability. Due to these disadvantages of organic polymers, recently inorganic polymer materials such as silica, titanium dioxides etc have got more attention as shell materials. He et al. [21] microencapsulated n-octadecane through sol–gel synthesis using sodium silicate as a silica precursor. They researched on the effect of pH value on the porosity of the silica shell and found the best anti-osmosis property at pH 2.95–3.05 due to the formation of compact silica wall. Fang et al. [22] microencapsulated paraffin with SiO2 shell using sol–gel method.

when a trigger like heating or pH change is initiated. This type of polymerization technique is called radical polymerization. There are 2 types of radical polymerization namely suspension polymerization and emulsion polymerization. Microencapsulation of PCM has also been achieved through other commonly used chemical synthesis techniques like interfacial polymerization, in-situ polymerization and condensation polymerization. Sometimes a combination of multiple chemical techniques is used to achieve microencapsulation. More details on each of the synthesis techniques are provided below. 2.4.1. Suspension polymerization This method is preferred when both monomers and initiators are either insoluble or poorly soluble in aqueous phase solvent. Water is the most used liquid as a solvent. There for in this method during emulsification, PCM, monomers and initiators form the oil phase and are suspended in the continuous aqueous phase as discrete droplets. Emulsification can be aided by adding surfactants (emulsifiers) and continuous mechanical stirring. This is a radical polymerization process. Initiators trigger the polymerization when they release free radicals in to the system after a trigger like heat is supplied. Typical initiators used in suspension polymerization are benzoyl peroxide, 2,2-Azobisisobutyronitrile (AIBN) etc which are poorly soluble in water. This method is typically used for organic shell material like polymethylmethacrylate (PMMA). Huang et al. [7] microencapsulated inorganic PCM Na2 HPO4 ·7H2 O which is a hydrated salt using PMMA polymer shell using suspension polymerization technique. They reported that use of inhibiters at the post-processing phase gives a smooth, spherical and compact shell surface, while without inhibiters the shell surface was non-spherical with deformed lump shape. They also reported improvement in supercooling problem of the inorganic PCM after microencapsulation. Tang et al. [8] microencapsulated n-octadecane with n-octadecyl methacrylate (ODMA) –methacrylic acid (MAA) co-polymer (PODMAA). They reported that this novel co-polymer reduces the supercooling issue in microencapsulation. The onset crystallizing temperatures of microcapsules were only 4 ◦ C below that of n-octadecane. Many small alkyl nano-domains are formed when PODMAA crystallizes through n-octadecyl side chain packing. During the cooling process, these small alkyl nanodomains located on the inner wall of microcapsule act as crystal nuclei and subsequently induce the heterogeneous nucleation of n-octadecane. This decreases the degree of supercooling of noctadecane cores. 2.4.2. Emulsion polymerization This method is preferred when the monomers are either insoluble or poorly soluble in aqueous phase solvent but initiator is soluble in aqueous phase solvent. Water is the most used liquid as a solvent. There for in this method during emulsification, only the PCM and monomers form the oil phase, dispersed in the continuous aqueous phase as discrete droplets. Initiators will be dissolved in the aqueous phase. Emulsification can be aided by adding surfactants (emulsifiers) and continuous mechanical stirring. This is a radical polymerization process. Initiators trigger the polymerization when they release free radicals in to the system after a trigger like heat is supplied. Typical initiators used in suspension polymerization are ferrous sulphate, ammonium persulphate, sodium thiosulphate etc which are easily soluble in water. This method is typically used for organic shell materials like PMMA and polystyrene. Many researchers [12,23–26] have successfully done microencapsulation using this technique.

2.4. Chemical synthesis methods These methods are purely chemical in nature. Here the polymerization can be started by free radicals that attack the monomers

2.4.3. Interfacial polymerization This method is preferred if we have two (or more) monomers associated with the final shell polymer and also if one monomer is

G. Alva et al. / Energy and Buildings 144 (2017) 276–294

285

Fig. 5. Sol–gel method [21].

Fig. 6. Types of polymerization (a) interfacial, (b) suspension and (c) emulsion [2].

hydrophobic and the other is hydrophilic. Water is the most used liquid as a solvent. During emulsification, PCM and hydrophobic monomers form the oil phase. Hydrophilic monomers are dissolved in the aqueous phase. Hence we have two reactive monomers separated in two immiscible mediums which react only at the interface between two solutions creating a thin film at the surface. But as the polymerization continues, the polymer film becomes a barrier to slow the polymerization process. Thus the shell thickness produced in this type could be very thin. This method is typically used for organic shell materials like polyurea and polyurethane. Polyurea is formed from a reaction of an isocyanate with a multifunctional amine. Polyurethane is formed from a reaction of an isocyanate with a polyol. In this synthesis technique, first the isocyanate will be prepared as oil phase organic solvent with PCM. Then this oil phase is poured in to water to prepare an O/W emulsion. After stirring for few minutes, the water soluble monomer, which is diluted in distilled water before, will be added to the emulsion system slowly and the mixture is heated. Then the interfacial polymerization reaction takes place between isocyanate and water soluble monomer at the oil–water interface. The reaction may last for about 2–3 h. The resultant microcapsules will be filtered, washed and dried. Salaün et al. [27] microencapsulated xylitol (sugar alcohol PCM core) in polyurethane shell using interfacial polymerization technique. Also they used W/O type emulsion instead of the usual O/W type emulsion. Fig. 6 shows the difference between interfacial, suspension and emulsion polymerization.

2.4.4. In-situ polymerization In this synthesis technique, instead of monomers, chemical compounds that produce precursors to polymers are used. Before the polymerization step, the precursors (pre-polymers like oligomers) are prepared in-situ through hydrolysis of the parent chemical compound. Later this precursor polymerizes to form a shell around the PCM droplet. The precursors will be part of aqueous phase and will not be present in the oil phase. This method is typically used for organic shell materials like melamineformaldehyde and polyurea-formaldehyde. It should be noted that formaldehyde is toxic and creates environmental and health problems. In a typical synthesis process melamine-formaldehyde pre-polymer is prepared in distilled water with a mixture of melamine and formaldehyde with pH maintained at around 9. Parallely an emulsion of PCM is prepared in a separate beaker. The emulsion is homogenized for about an hour at high revolutions per minute (rpm). The PCM emulsion will be gradually added to the pre-polymer solution and pH of the solution will be maintained at around 4 and continuously stirred for around 2 h. Once the microcapsules are formed pH is adjusted back to around 9 which terminate the polymerization reaction. Microcapsules are filtered, washed with distilled water and dried. Zhang et al. [28] compared three different emulsifiers, sodium salt of styrene-maleic anhydride copolymer (SMA/anionic), sodium dodecyl sulfate (SDS/anionic) and polyvinyl alcohol (PVA/nonionic) during in-situ polymerization of melamine-formaldehyde. They

286

G. Alva et al. / Energy and Buildings 144 (2017) 276–294

Fig. 7. Synthesis of MPCM with n-octadecane core and melamine–formaldehyde shell through in-situ polymerization [28].

found SMA producing smooth shell morphology with regular spherical shape of microcapsules and no visible disfigurement on shell surface. Using SDS resulted in irregular shapes of microcapsules with a thicker shell. Using PVA resulted in irregular shapes of microcapsule and also problem of agglomeration of microcapsules was noticed. Sarier et al. [29] microencapsulated n-hexadecane or n-octadecane core materials with urea-formaldehyde polymer shell using in-situ polymerization technique. They were successful in depositing silver nanoparticles on the polymer shell using adhesive forces which exist between silver particles and polymer shells containing amide and carbonyl functional groups via metaldipole interaction. They concluded that the depositing silver nano particles improve the thermal conductivity. They also reported agglomeration of microcapsules due to increased cohesive forces between capsule shells incorporated with silver nanoparticles. Fig. 7 shows the in-situ polymerization of melamine-formaldehyde shell around n-octadecane core.

Fig. 8. Condensation polymerization [22].

3. The characterization of microencapsulated phase change materials 3.1. Physical properties 2.4.5. Condensation polymerization In condensation polymerization, the polymerization occurs with the release of simple molecules like water and methanol as by product. The monomers or precursors used in this type of polymerization reaction normally have functional groups like OH (alcohol), NH2 (amino), COOH (carboxylic group) etc. In microencapsulation of PCM condensation polymerization is a part of other synthetic techniques like sol–gel, in-situ and interfacial polymerization. Fig. 8 shows an example condensation process.

3.1.1. Encapsulation efficiency Microencapsulation efficiency is the ratio of mass of microcapsules with fully formed shells in the sample to total mass of microcapsules in the sample. It measures the yield of the microencapsulation synthesis process. Hawlader et al. [11] measured encapsulation efficiency using the following method. One gram of dried microcapsules (test sample) is washed with 25 ml of toluene. Unencapsulated PCM or partially encapsulated PCM will be dissolved in toluene. The encapsulation efficiency is calculated

G. Alva et al. / Energy and Buildings 144 (2017) 276–294

287

• Diameter of the microcapsules. • Thickness of the shell wall (shown in Fig. 10(d)). • If clustering of microcapsules exists.

Fig. 9. PSD diagram [26].

as the ratio of the weight of undissolved capsules to the original weight of sample. (%) =

Weight of undissolved microPCM × 100 Total weight of dry sample microPCM

(1)

Salaün et al. [27] used a different approach. Encapsulation efficiency was calculated as the ratio of weight of xylitol filled particles (MPCM) obtained after correction of xylitol content determined by DSC to total weight of used monomer and xylitol. In this case xylitol is used both as shell monomer and PCM core. It is expressed as follows: mmicroparticles − mxylitol  (%) = (2) × 100 mmonomers 3.1.2. Microcapsule size distribution The particle size distribution (PSD) of the microcapsules is measured using a laser diffraction analyzer. A small sample of micro-capsules is suspended in deionized water bath and this suspension is passed through laser beam which gets scattered. Size is estimated based on the angle of deflection. Each sample is analyzed for multiple times and the average is taken. Fig. 9 shows a sample particle size distribution graph. 3.1.3. Encapsulation ratio This is the ratio of core PCM to shell material. The latent heat capacity of MPCM is dependent on the mass fraction of its PCM core to total mass. Latent heat is obtained from differential scanning calorimeter (DSC) curve. Encapsulation ratio is calculated using below formula, where Hmicro-PCM (kJ/kg) is the latent heat of the microcapsule containing PCM and HPCM (kJ/kg) is the latent heat of pure PCM. R=

Hmicro−PCM × 100% HPCM

(3)

3.1.4. Shell characterization 3.1.4.1. Surface morphology. Particle size and morphology of the microcapsule shell surface can be observed using scanning electron microscope (SEM) or optical microscope (OM) or transmission electron microscopy (TEM). Dried MPCM powder is used as sample for observing the morphology. OM is the simplest to use with least sample preparation requirement. TEM is used for microcapsules smaller than 1 ␮m size. Fig. 10 shows sample images from SEM and TEM showing shell surface morphology of the microcapsules. We can observe the condition of microcapsules as listed below: • If shell surface is smooth and compact with no dents, deformities, pores and ruptures (shown in Fig. 10(c)). • If the shape of MPCMs is spherical and regular (if all capsules are consistently spherical) (shown in Fig. 10(e)).

3.1.4.2. Leakiness. Loose and porous shell structure makes the shell wall fragile and results in leaking of core material. If the shell wall is very thin, the shell can be easily broken upon slight external agitation causing PCM core leakage. However thick shell wall means reduction in encapsulation ratio(R) and there for an optimum shell thickness should exist. Leakage can be tested using centrifugal shear force test. In this test, sample microcapsules are put in a test tube with water. Test tubes are then centrifuged at room temperature at 400 rpm for 4 h. Then using an optical microscope the any breakage and leakage of microcapsules is inspected. Al-Shannaq et al. [13] tested the permeability of PCM through the shell of microcapsules via mass loss analyses. This was performed by placing a known mass of dried microcapsules into an aluminum pan and putting it into a drying oven at 50 ◦ C. The mass of the microcapsules was periodically measured over 25 days. Also, the mass loss of bulk PCM was measured under the same conditions for comparison. The percent mass loss was calculated by the following equation. % mass loss =

Initial mass − Final mass × 100 Initial mass

(4)

3.1.4.3. Shell thickness. Shell thickness is measured using SEM/OM/TEM. Shell thickness provides both mechanical strength and thermal stability. Tang et al. [8] investigated the effect of core/shell mass ratio on thermal stability. They reported by increasing the mass ratio of monomers to PCM, shell thickness can be increased. Increased shell thickness results in higher thermal stability. They used the temperature for occurrence of weigh loss of 5% (Td5% ) for measuring thermal stability. They reported thermal stability of MPCM as a function of the shell thickness. 3.1.4.4. Shell strength. Mechanical strength and shape stability of the microcapsule is provided by the shell material. Stronger the shell, more rigid will be the microcapsule. During repeated charging–discharging cycles, MPCM slurries are circulated through heat exchanger ducts. They are subjected to mechanical stresses leading to abrasion and rupture of shell wall and subsequent loss of effectiveness. There for shell strength is very important. Borreguero et al. [16] used atomic force microscope (AFM) to measure the mechanical strength of microcapsules. Individual microcapsules were fixed to a substrate surface using commercial adhesive (Poxipol). AFM was used to image the sample and locate the microcapsules using semi-contact imaging. AFM probe was brought into contact with an isolated microcapsule on the substrate surface. Zpiezo cantilever extension of the AFM pushes the probe into the microcapsule. It resulted in bending of the AFM cantilever attached to the AFM probe. The recorded cantilever bending was used for calculating the force and plotting a force-displacement graph. Fig. 11 shows the force-displacement curve of atomic force microscope. The total deformation behavior of an individual microcapsule was recorded continually with increasing applied force to give resultant force-displacement curves. This procedure was done five times to check the reproducibility of the experiments, selecting microcapsules of the same particle size. 3.1.5. Density MPCM when dried takes the form of a powder. In case of bulk solids we measure the bulk density. But for powders it is measured as apparent density which is a ratio of dry specimen mass to the apparent volume. The apparent volume also includes the air gap between the individual microcapsules in the powder. Krupa et al.

288

G. Alva et al. / Energy and Buildings 144 (2017) 276–294

Fig. 10. SEM (a,c,e) and TEM(b,d,f) images [15].

3.2. Chemical properties 3.2.1. Fourier transformation infrared spectroscopy (FT-IR) analysis Chemical composition of PCM and MPCM is analyzed using fourier transform infrared spectroscopy (FT-IR) method. Fig. 12 shows a sample FT-IR spectra graph. Table 3 provides hints to identify the appropriate functional group corresponding to each absorption peak. Based on this it is verified that no chemical reaction has taken place between PCM and shell materials. It is also verified, if the polymerization of the shell material has completed successfully and MPCM contains both PCM and shell materials.

Fig. 11. Atomic force microscope force–displacement curve [16].

[30] measured density of the microcapsules using a pycnometer according to ASTM D153 standards.

3.2.2. X-ray diffraction (XRD) Chemical composition can also be identified by using the characteristic diffraction peaks of PCM and shell materials. XRD is typically used when the shell material is inorganic. Fei et al. [15] analyzed pure PCM (n-octadecane), pure shell materials (titania crystals) and micro encapsulated PCM samples separately with X-ray powder diffraction. Using this method first they were able to know that in MPCM synthesized with spray-dry method, titania in the shell wall was amorphous. The MPCM were given a hydrothermal post-

Table 3 FT–IR Spectrum Functional Groups. Wave length from

Wave length to

Material

Strong

820 2800

894 3100

PCM PCM

Rocking

Weak

717

728

PCM

[17,27,29,66] [8,15,17,18,19,21,23,24,26, 27,29,31,32,63,64,65,66] [8,17,19,21,25,28,31,66]

C H

Deformation

Medium

1295

1488

PCM

[15,19,21,23,25,27,28,29,31,32,63,64,68]

Si Si Si Si C

Bending Bending Bending Bending Stretching

Strong

1083 766 420 952 1600

1156 798 465 960 2000

Silica Silica Silica Silica Gum arabica, PMMA, polyurea, polyurethane

[22,32,64,65] [22,32] [21,22,65] [21,22] [8,17,19,24,26,27,29,31,65,66]

Stretching

2400

3400

Gum arabica

[18]

1530

1580

Palmitic acid

[31] [8,15,19,24–26,27,29,32,64,68]

Bond

Vibration type

Intensity

alkane Alkane (CH3 ,CH2 ,CH) Alkane (CH3 and CH2 ) Alkane (CH3 and CH2 ) Silica Silica Silica Silica Carboxylic group

C H C H

Bending Stretching

C H

O O O OH O

Usually appears after polymerization. Indicates presence of amides

References

Carboxylic group Carboxylate group Ester bond

C O

Stretching

C O

Stretching

Strong

950

1381

Amine

N H

Stretching

Weak

3100

3500

Amine

N H

Bending

Medium

1500

1655

Amine

C N

Stretching

Medium

1000

1250

Benzene ring

C C

Stretching

1462

1522

CN

Stretching

2237

2260

420 2200

2280

Strong Medium

3000 1290

3700 1700

PMMA, n-octadecyl methacrylate (ODMA), Titaniumtetra-isopropoxide, Tris, formaldehyde Gelatin, polyurea, polyurethane, melamine Gelatin, polyurea, polyurethane Polyurethane, melamine Polysterene,Polyurea, Resorcinol Acrylonitrile–styrenecopolymer TiO2 Polyurea, polyurethane Water Water, stearic acid

Weak

650

1150

Water

[8,31,65]

Sharp

1500 800

1600 820

Melamine Melamine

[28,68] [28,68]

Nitrite TiO2 Isocyanate

OH

Remarks

Sharp

Ti O N C O

Hydroxyl Hydroxyl

OH OH

Hydroxyl

OH

Triazine ring Triazine ring

C N C N

Stretching In-plane Bending Out of plane bending Stretching Out of plane deformation

[18,27,29,66–68]

[17,18,27,29,66] [27–29,68] Vanishes after polymerization

G. Alva et al. / Energy and Buildings 144 (2017) 276–294

Functional group

[23,24,28] [19]

Vanishes after polymerization

[15] [27,66,67] [17,18,22,24,27,28,29,31,32] [15,22,27,31,65]

289

290

G. Alva et al. / Energy and Buildings 144 (2017) 276–294

Fig. 12. FT–IR spectra (a) shell material, (b) PCM and (c) MPCM [23].

process. After that the XRD pattern verified the formation of titania crystallites. Fang et al. [22] used XRD to verify the successful encapsulation of PCM. The diffraction pattern of a mixture is a simple sum of the scattering from each component phase. They found the microencapsulated PCM’s diffraction pattern as superimposition of pure PCM XRD pattern over the nearly flat XRD pattern of amorphous SiO2 . Fan et al. [14] used XRD to study the effect of nucleating agents on crystallographic system on n-octadecane. In this case the shell material melamine-formaldehyde was amorphous. Melamine formaldehyde does not interfere, as it has a flat XRD pattern with no characteristic peaks of its own. The nucleating agents did not affect the crystallographic system of n-octadecane. Cao et al. [31] by analyzing XRD pattern verified that the crystal structure of the PCM remained unchanged after encapsulation in the TiO2 shell. Thus it is concluded that there was no chemical reaction with the PCM during microencapsulation. Wu et al. [32] used XRD to study the re-crystallization of Tris(hydroxymethyl)methyl aminomethane (a solid–solid PCM) during the period of thermal cycling. They found an increase in the intensity of the diffraction peak which indicated increase in the crystal size with number of thermal cycles. Fig. 13 shows a sample XRD spectra graph.

3.3. Thermal properties 3.3.1. Differential scanning calorimetry (DSC) analysis DSC analysis is used for measuring thermal properties of pure PCM and MPCM. DSC curves show the phase change behaviors during heating and cooling periods as shown in Fig. 3. The figure shows two sample DSC curves for one each for complete cycle of heating and cooling of a pure PCM and MPCM. The DSC curve shows two different phase changes (smaller solid–solid peak and larger solid–liquid peak). Supercooling of MPCM is easily observed using DSC curve by comparing the crystallization temperatures of pure

Fig. 13. XRD patterns of the (a) SiO2 , (b) paraffin and (c) MEPCM [22].

PCM with that of MPCM. The key thermal properties that the DSC curves provide are below: • • • • • •

Tmo : The onset temperature on DSC heating run. Tmp : The peak temperature on DSC heating run. Tco : The onset temperature on DSC cooling run. Tcp : The peak temperature on DSC cooling run. Hm : Enthalpy on DSC heating run (latent heat of melting). Hc : Enthalpy on DSC cooling run (latent heat of crystallization).

3.3.2. Thermogravimetric analysis (TGA) TGA thermographs are used for analyzing thermal stability of MPCM. In this test the MPCM sample is subjected to an increasing temperature and at every step the weight loss of the sample is measured. Fig. 14 shows the TGA curves for PCM, shell material and MPCM. It can be seen that start point of the degradation for both PCM and MPCM is almost same, indicating that weight loss is mainly due to evaporation of PCM and shell material is stable at that

G. Alva et al. / Energy and Buildings 144 (2017) 276–294

291

The thermal conductivity of the MPCM slurry is calculated as below using the Maxwell’s relation [38] for static dilute solution. kb =

Fig. 14. TGA curve [23].

point. This temperature where degradation of MPCM starts should be well above the operating temperature of the thermal energy storage system. 3.3.3. Flammability As organic PCM are moderately flammable, strict building safety laws restrict their use in buildings. The fire safety aspects of a material include its tendency to ignite, heat release rate (HRR) during burning and toxicity of the smoke due to release of harmful chemicals. The flash points of most organic PCM are just above 100 ◦ C where as flash points of polymers such as polystyrene are above 300 ◦ C. There for microencapsulation of PCM will improve its fire safety aspect. For polymers there are different testing methods currently in use for measuring flammability like cone calorimeter, limiting oxygen index (LOI), vertical/horizontal flammability testing. For form stable PCM, flammability has been tested using cone calorimeter [33] and using LOI [34]. For textile fibers microscale combustion calorimeter has been used to study their heat release rate [35]. However currently there is not much research data available on fire safety aspect of MPCM. 3.3.4. Thermal reliability Thermal reliability is analyzed by subjecting the MPCM to repeated very large number of thermal cycles. After repeated melting and freezing cycles if the thermal properties and chemical composition do not change much, then MPCM is considered as reliable. This test may involve subjecting MPCM to up to 5000 thermal cycles and then using DSC, FT-IR, XRD and TGA to check the changes in thermal properties and chemical composition. 3.3.5. Thermal conductivity MPCM as produced exists in powdered form. However during its usage, it is in composite form with other materials like gypsum, water etc. Currently there is not much information available on the measurement of thermal conductivity of MPCM powders. There are few studies on the thermal conductivity of MPCM slurries and theoretical estimation of single MPCM particle. Zhang and Zhao [36] used hot disk instrument to measure thermal conductivity of MPCM slurry. The thermal conductivity of the single microcapsule was estimated using the theoretical formula [36–38] using composite sphere approach as below: dp − dc 1 1 = + kp dp kc dc kw dp dc

(5)

where kc is the thermal conductivity of the MPCM core material, kw is the thermal conductivity of the MPCM shell material, dc is the diameter of the MPCM core material, dp is the diameter of the single MPCM particle, kp is the thermal conductivity of the single MPCM particle.

2kf + kp + 2cv (kp − kf ) 2kf + kp − cv (kp − kf )

· kf

(6)

where kb is the bulk thermal conductivity of the MPCM slurry, kf is the thermal conductivity of the working fluid and cv is the volumetric concentration of the MPCM in the slurry. Zhang and Zhao [36] reported that the thermal conductivity and specific heat decreased with the increase of MPCM concentration due to the lower thermal conductivities and specific heat of the MPCM particles compared to the base fluid, water. Chen et al. [38] also had similar results while studying the thermal conductivity of MPCM slurry. Maxwell’s relation is also used by Lecompte et al. [39] for theoretical estimation of thermal conductivity of concrete and mortar containing MPCM. They also compared the theoretical value against the experimental value measured using hot plate experimental setup. They reported that maxwell model correctly predicts the effective thermal conductivities of the MPCM mixtures. The effective thermal conductivities decreased with the increase of MPCM volume fraction. These results were in agreement with the theoretical predictions and were due to the low thermal conductivity of microencapsulated paraffin. Inorganic shell materials have relatively better thermal conductivity than organic shell materials. Wu et al. [32] attempted microencapsulating Tris(hydroxymethyl)methyl aminomethane (a solid–solid phase change material) with silica. They reported an improvement in thermal conductivity of Tris from 0.211 W/(m.K) to 0.478 W/(m.K). 4. Applications of microencapsulated phase change materials MPCM has applications in passive systems like buildings, thermal comfort textiles, composite foams etc. It also has applications in active systems like heat exchangers, thermal control systems and thermal storage systems. 4.1. Buildings According to IEA estimates in year 2010 the building sector (residential and commercial) used approximately 115EJ of energy globally, accounting for 32% of global energy demand [40]. Out of that space heating and cooling constituted 32–33% of the total energy used in buildings. There for any improvement in efficiency of this sector has a big impact on the global energy demand and can reduce the environmental pollution. One way to improve the efficiency of space heating and cooling in buildings is by incorporating the concept of thermal mass into building designs, which provides inertia against temperature fluctuations. Phase change materials with their high thermal energy storage density near the thermal comfort temperature range are ideal for increasing the thermal inertia for the same mass of buildings. MPCM are embedded into flooring, drywalls, concrete, ceilings, panels, gypsum boards, insulation panels, wallboards etc. Although PCM without microencapsulation (i.e. direct impregnation) is a low cost option but long term stability of the unprotected PCM and negative impact of the PCM on building materials is a concern. For example in concretes with presence of Ca(OH)2, high alkalinity impacts the stability of PCM like fatty esters and fatty alcohols. Inorganic PCM are corrosive and therefore they negatively impact the building materials. Organic materials are flammable and may release toxic fumes during combustion. Moreover the porous nature of building materials like mortar which contain the PCM leads to leakage issues when the temperature is above the melting point of PCM. Direct impregnation of PCM in concrete may also weaken the mechanical strength.

292

G. Alva et al. / Energy and Buildings 144 (2017) 276–294

Attempts to use macro-encapsulated PCM were not much successful due to the poor conductivity of PCM. When it was time to regain the heat from the liquid phase, the PCM solidified around the edges and prevented effective heat transfer. Solution to counter these issues is use of MPCM which is ignition resistant and leak proof. Microencapsulation of PCM increases the compatibility of PCM with the building materials, increases its long term stability and also with small dimensions mitigates thermal conductivity problem. Cabeza et al. [41] tested the inclusion of commercial Micronal PCM (from BASF) in the concrete. The concrete mixture with 5% PCM in weight was tested for mechanical strength and thermal behavior. They concluded that mechanical strength was satisfactory for structural purposes and that there was no negative impact of MPCM on concrete even after six months of observation. They reported that in comparison with conventional concrete without phase change materials, concrete walls with embedded MPCM improved thermal inertia and resulted in lower temperatures in the room. Lecompte et al. [39] studied the thermal and mechanical characteristics of hardened concrete and mortar mixes containing MPCM and compared with classical civil engineering models. The MPCM behave as voids decreasing the mechanical strength of the concrete and mortar mixes. They reported the preparation of concrete mix with MPCM requires additional water up to 10% of MPCM mass. Great care must be taken during the concrete mix preparation to avoid the breakage of microcapsules. They concluded that by maintaining the MPCM volume fraction below 14.8% in mortar, and 8.6% in concrete and by increasing the cement content in the mixture it is possible to achieve mechanical strength of the same order as normal cement-based materials with no MPCM. Cao et al. [42] studied the thermal performance of Portland cement concrete and geopolymer concrete enhanced with MPCM. They concluded that addition of MPCM results in significant loss of concrete compressive strength. For Portland cement concrete addition of 3.2 wt% MPCM resulted in reduction of compressive strength by 42% and for geopolymer concrete addition of 2.7 wt% MPCM resulted in reduction of compressive strength by 51%. They concluded that at these low weight ratios of MPCM, although the loss of concrete compressive strength was significant it still satisfied the European regulations. Energy consumption for heating and cooling to maintain the indoor temperature reduced by 11% for Portland cement concrete with 3.2 wt% MPCM and by 15% for geopolymer concrete with 2.7 wt% MPCM. Authors also noted that further increasing the MPCM concentrations causes a too low workability of the concretes to produce usable samples. Yeliz et al. [43] in their review on use of MPCM in building applications have listed a few research studies on mechanical and thermal properties of cement mortars, concretes and gypsum plasters containing MPCM. Simen et al. [44] in their review on use of PCM in building applications provided a comprehensive list of commercial products and manufacturers for both macro and microencapsulated PCM. Use of PCM in bricks [45] and tiles [46] for passive cooling has also been reported.

4.2. Textiles PCM capsules have application in thermo-regulating textiles. It is used for protection from extreme cold weathers in outdoor wear such as parkas, vests, snowsuits, trousers, ski wear, hunting clothing, ear warmers, boots, gloves and in-house cloths such as blankets, duvets, mattresses and pillowcases [47]. Textiles containing PCM also help to combat overheating. There are two ways to embed MPCM into fabric. PCM microcapsules were coated on the surface of fabric as shown in Fig. 15, and were directly embedded within fiber as shown in Fig. 16.

Fig. 15. PCM microcapsules coated on the surface of fabric [47].

Fig. 16. PCM microcapsules embedded within fiber [47].

4.3. MPCM slurry Active thermal energy systems like heat exchangers, thermal control systems and thermal storage systems use heat transfer fluid (HTF). By enhancing the HTF such as water to a two phase fluid by emulsifying it with PCM, the heat carrying capacity of the HTF can be increased. Advantages from this technique are many: • • • •

Better heat transfer rate. Heat transfer at nearly a constant temperature. Reduced mass flow for the same heat transfer rate. Possibility of usage as both thermal energy transport and storage medium.

Although suspended PCM droplets may impact the fluidity of HTF and increase pumping power requirement it will be within acceptable limits. However PCM slurries there are few issues to be resolved. The suspended PCM droplets could solidify on ducts of heat exchangers and cause clogging. Stability of the PCM emulsion is not good above the melting temperature of PCM and gradually smaller PCM droplets can coalesce to form completely separated layers of PCM and HTF. There for instead of direct PCM slurry with HTF, MPCM slurry with HTF is used which has all the benefits of PCM suspension and it has no coalescing problem as the shell material prevents the contact between PCM droplets. MPCM slurry has potential applications as a HTF for microchannel heat exchangers, solar energy collectors, thermal power plants etc [48,49]. A review on characterization of MPCM slurry was done by Chen et al. [50]. There are many experimental studies done on the thermal and rheological properties of MPCM slurry [51–55]. It is found that wall temperatures of the heat exchanger pipes decrease with the use of MPCM slurry [38]. There are also theoretical studies done to analyze the influential parameters that affect the heat transfer char-

G. Alva et al. / Energy and Buildings 144 (2017) 276–294

acteristics [56–60]. Studies have shown that Stefan number and volumetric concentration of the MPCM in the slurry are the most influential parameters on the laminar flow heat transfer rate [56]. Numerical study predicted the Nusselt number for the MPCM slurry to be 1.5–4 times higher than that of single phase fluid [56]. Goel et al. [61] experimentally analyzed different influencing parameters like Stefan Number, volumetric concentration, microcapsule to duct diameter ratio, Reynolds number and the homogeneity of suspension etc.

293

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant no. 51676095, 51376087) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. The authors also wish to thank the reviewers and editor for kindly giving revising suggestions.

References

4.4. Composite foams Applying MPCM in polyurethane foam can improve its heatinsulating ability. Polyurethane foams find application in areas like automotive interiors, medical products etc. You et al. [62] fabricated polyurethane composite foam containing n-octadecane/melamineformaldehyde MPCM. However there are few challenges involved in fabrication of foams with MPCM as fillers. As the MPCM may contain traces of formaldehyde, emulsifiers etc which are left behind during synthesis process, these impurities can cause defoaming effect. Heat treatment of MPCM eliminates water and formaldehyde. Heat treatment also helps in cross-linking the shell polymer. Test results showed that higher the MPCM content in the composite foam, the higher is the heat storage ability of the foam. However for MPCM content beyond 12.59 wt%, the quality of foam was affected [62].

5. Conclusions and outlook MPCM has applications in buildings, textiles, thermal energy storage, heat transfer fluids and heat insulating foams. MPCM has a huge potential for energy savings in buildings. MPCM has been embedded in to building materials in many innovative ways for passive thermal energy storage. Experimental studies have been conducted on embedding MPCM into cement mortar, concrete, gypsum plaster, bricks, floor tiles, walls, ceiling, roof, windows and shutters, glasses, insulation panels etc. There is scope for further research into performance of MPCM in various building materials and innovative applications of MPCM in building materials. MPCM slurry is not only a good heat transfer fluid but also a good heat storage medium. Therefore, MPCM slurry is the best suited option for active thermal energy storage applications. Currently a wide range of synthesis options are available for producing MPCM. Organic PCM normally do not have supercooling issue, but when microencapsulated show supercooling which is undesirable. Recently MPCM synthesis with inorganic shell materials, and numerical and experimental study on performance of MPCM slurry in circular tube, microchannel heat sink etc is gaining good attention of researchers. Microencapsulation of inorganic PCM has not got much attention so far. non-volatile organic solvents can be tried for emulsifying inorganic PCM in place of the usual water solvent. Microencapsulation of organic eutectic mixtures also has not got much attention so far. Currently MPCM is mainly used in systems operating at low temperatures in the human thermal comfort range. Although shell materials have thermal stability at a much higher temperature than PCM, MPCM thermal stability is close to thermal stability of PCM. Porosity of the shell adversely affects the performance of MPCM at high temperature. Fire safety of MPCM is an important aspect for its building application, where there is scope for further research as currently there is not much information available.

[1] C.Y. Zhao, G.H. Zhang, Review on microencapsulated phase change materials (MEPCMs): fabrication, characterization and applications, Renew. Sustain. Energy Rev. 15 (2011) 3813–3832. [2] A. Jamekhorshid, S.M. Sadrameli, M. Farid, A review of microencapsulation methods of phase change materials (PCMs) as a thermal energy storage (TES) medium, Renew. Sustain. Energy Rev. 31 (2014) 531–542. [3] J.G. Paloma, M. Martinez, L.F. Cabeza, A.I. Fernandez, Types, methods, techniques, and applications for microencapsulated phase change materials (MPCM): a review, Renew. Sustain. Energy Rev. 53 (2016) 1059–1075. [4] P.L. Lam, R. Gambari, Advanced progress of microencapsulation technologies: in vivo and in vitro models for studying oral and transdermal drug deliveries, J. Controlled Release 178 (2014) 25–45. [5] A. De Prisco, G. Mauriello, Probiotication of foods: a focus on microencapsulation tool, Trends Food Sci. Technol. 48 (2016) 27–39. [6] J. Garrido, F. Cagide, M. Melle-Franco, F. Borges, E.M. Garrido, Microencapsulation of herbicide MCPA with native–cyclodextrin and its methyl and hydroxypropyl derivatives: an experimental and theoretical investigation, J. Mol. Struct. 1061 (2014) 76–81. [7] J. Huang, T.Y. Wang, P.P. Zhu, J.B. Xiao, Preparation, characterization, and thermal properties of the microencapsulation of a hydrated salt as phase change energy storage materials, Thermochim. Acta 557 (2013) 1–6. [8] X.F. Tang, W. Li, X.X. Zhang, H.F. Shi, Fabrication and characterization of microencapsulated phase change material with low supercooling for thermal energy storage, Energy 68 (2014) 160–166. [9] R. Al-Shannaq, M. Farid, S. Al-Muhtaseb, J. Kurdi, Emulsion stability and cross–linking of PMMA microcapsules containing phase change materials, Solar Energy Mater. Solar Cells 132 (2015) 311–318. [10] F. di Lena, K. Matyjaszewski, Transition metal catalysts for controlled radical polymerization, Prog. Polym. Sci. 35 (2010) 959–1021. [11] M.N.A. Hawlader, M.S. Uddin, M.M. Khin, Microencapsulated PCM thermal–energy storage system, Appl. Energy 74 (2003) 195–202. [12] S. Alay, C. Alkan, F. Göde, Synthesis and characterization of poly(methyl methacrylate)/n?hexadecane microcapsules using different cross–linkers and their application to some fabrics, Thermochim. Acta 518 (2011) 1–8. [13] R. Al-Shannaq, J. Kurdi, S. Al-Muhtaseb, M. Dickinson, M. Farid, Supercooling elimination of phase change materials (PCMs) microcapsules, Energy 87 (2015) 654–662. [14] 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. [15] B. Fei, H.F. Lu, K.H. Qi, H.F. Shi, T.X. Liu, X.Z. Li, J.H. Xin, Multi–functional microcapsules produced by aerosol reaction, J. Aerosol Sci. 39 (2008) 1089–1098. [16] A.M. Borreguero, J.L. Valverde, J.F. Rodríguez, A.H. Barber, J.J. Cubillo, M. Carmona, Synthesis and characterization of microcapsules containing ® Rubitherm RT27 obtained by spray drying, Chem. Eng. J. 166 (2011) 384–390. [17] E. Onder, N. Sarier, E. Cimen, Encapsulation of phase change materials by complex coacervation to improve thermal performances of woven fabrics, Thermochim. Acta 467 (2008) 63–72. [18] M.G. Santos, F.T. Bozza, M. Thomazini, C.S. Favaro-Trindade, Microencapsulation of xylitol by double emulsion followed by complex coacervation, Food Chem. 171 (2015) 32–39. [19] R. Yang, Y. Zhang, X. Wang, Y.P. Zhang, Q.W. Zhang, Preparation of n-tetradecane–containing microcapsules with different shell materials by phase separation method, Solar Energy Mater. Solar Cells 93 (2009) 1817–1822. [20] L.B. Garcia, L. Ventola, R. Cordobilla, R. Benages, T. Calvet, M.A. Cuevas-Diarte, Phase Change Materials (PCM) microcapsules with different shell compositions: preparation characterization and thermal stability, Solar Energy Mater. Solar Cells 94 (2010) 1235–1240. [21] F. He, X.D. Wang, D.Z. Wu, New approach for sol-gel synthesis of microencapsulated n-octadecane phase change material with silica wall using sodium silicate precursor, Energy 67 (2014) 223–233. [22] G.Y. 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. [23] A. Sari, C. Alkan, D.K. Doguscu, A. Bicer, Micro/nano-encapsulated n-heptadecane with polystyrene shell for latent heat thermal energy storage, Solar Energy Mater. Solar Cells 126 (2014) 42–50. [24] A. Sari, C. Alkan, A. Altintas, Preparation, characterization and latent heat thermal energy storage properties of micro-nanoencapsulated fatty acids by polystyrene shell, Appl. Therm. Eng. 73 (2014) 1160–1168.

294

G. Alva et al. / Energy and Buildings 144 (2017) 276–294

[25] 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. [26] A. Sari, C. Alkan, A.N. Özcan, Synthesis and characterization of micro/nano capsules of PMMA/capric–stearic acid eutectic mixture for low temperature thermal energy storage in buildings, Energy Build. 90 (2015) 106–113. [27] F. Salaün, G. Bedek, E. Devaux, D. Dupont, L. Gengembre, Microencapsulation of a cooling agent by interfacial polymerization: influence of the parameters of encapsulation on poly(urethane–urea) microparticles characteristics, J. Membr. Sci. 370 (2011) 23–33. [28] H.Z. Zhang, X.D. Wang, Fabrication and performances of microencapsulated phase change materials based on n-octadecane core and resorcinol–modified melamine–formaldehyde shell, Colloids Surf. A: Physicochem. Eng. Aspects 332 (2009) 129–138. [29] N. Sarier, E. Onder, G. Ukuser, Silver incorporated microencapsulation of n-?hexadecane and n-octadecane appropriate for dynamic thermal management in textiles, Thermochim. Acta 613 (2015) 17–27. ˇ [30] I. Krupa, Z. Nógellová, Z. Spitalsky, I. Janigová, B. Boh, B. Sumiga, A. Kleinová, M. Karkri, M.A. AlMaadeed, Phase change materials based on high–density polyethylene filled with microencapsulated paraffin wax, Energy Convers. Manage. 87 (2014) 400–409. [31] L. Cao, F. Tang, G.Y. Fang, Preparation and characteristics of microencapsulated palmitic acid with TiO2 shell as shape?stabilized thermal energy storage materials, Solar Energy Mater. Solar Cells 123 (2014) 183–188. [32] C.B. Wu, G. Wu, X. Yang, Y.J. Liu, T. Liang, W.F. Fu, M. Wang, H.Z. Chen, Preparation of microencapsulated medium temperature phase change material of Tris(hydroxymethyl)methyl aminomethane@SiO2 with excellent cycling performance, Appl. Energy 154 (2015) 361–368. [33] Y.B. Cai, Y. Hu, L. Song, Q.H. Kong, R. Yang, Y.P. Zhang, Z.Y. Chen, W.C. Fan, Preparation and flammability of high density polyethylene/paraffin/organophilic montmorillonite hybrids as a form stable phase change material, Energy Convers. Manage. 48 (2007) 462–469. [34] G.L. Song, S.D. Ma, G.Y. Tang, Z.S. Yin, X.W. Wang, Preparation and characterization of flame retardant form-stable phase change materials composed by EPDM, paraffin and nano magnesium hydroxide, Energy 35 (2010) 2179–2183. [35] C.Q. Yang, Q.L. He, R.E. Lyon, Y. Hu, Investigation of the flammability of different textile fabrics using micro-scale combustion calorimetry, Polym. Degrad. Stab. 95 (2010) 108–115. [36] G.H. Zhang, C.Y. Zhao, Thermal and rheological properties of microencapsulated phase change materials, Renew. Energy 36 (2011) 2959–2966. [37] M.I. Hasan, Numerical investigation of counter flow microchannel heat exchanger with MEPCM suspension, Appl. Therm. Eng. 31 (2011) 1068–1075. [38] B.J. Chen, X. Wang, R.L. Zeng, Y.P. Zhang, X.C. Wang, J.L. Niu, Y. Li, H.F. Di, An experimental study of convective heat transfer with microencapsulated phase change material suspension: laminar flow in a circular tube under constant heat flux, Exp. Therm. Fluid Sci. 32 (2008) 1638–1646. [39] T. Lecompte, P. Le Bideau, P. Glouannec, D. Nortershauser, S. Le Masson, Mechanical and thermo–physical behavior of concretes and mortars containing phase change material, Energy Build. 94 (2015) 52–60. [40] D. Ürge-Vorsatz, L.F. Cabeza, S. Serrano, C. Barreneche, K. Petrichenko, Heating and cooling energy trends and drivers in buildings, Renew. Sustain. Energy Rev. 41 (2015) 85–98. [41] L.F. Cabeza, C. Castellon, M. Nogues, M. Medrano, R. Leppers, O. Zubillaga, Use of microencapsulated PCM in concrete walls for energy savings, Energy Build. 39 (2007) 113–119. [42] V.D. Cao, S. Pilehvar, C. Salas-Bringas, A.M. Szczotok, J.F. Rodriguez, M. Carmona, N. Al-Manasir, A.L. Kjøniksen, Microencapsulated phase change materials for enhancing the thermal performance of Portland cement concrete and geopolymer concrete for passive building applications, Energy Convers. Manage. 133 (2017) 56–66. [43] Y. Konuklu, M. Ostry, H.O. Paksoy, P. Charvat, Review on using microencapsulated phase change materials (PCM) in building applications, Energy Build. 106 (2015) 134–155. [44] S.E. Kalnæs, B.P. Jelle, Phase change materials and products for building applications: a state-of-the-art review and future research opportunities, Energy Build. 94 (2015) 150–176. [45] A. Castell, I. Martorell, M. Medrano, Experimental study of using PCM in brick constructive solutions for passive cooling, Energy Build. 42 (4) (2010) 534–540. [46] I. Ceron, J. Neila, M. Khayet, Experimental tile with phase change materials (PCM) for building use, Energy Build. 43 (8) (2011) 1869–1874. [47] G. Nelson, Application of microencapsulation in textiles, Int. J. Pharm. 242 (2002) 55–62.

[48] Y. Wang, Z.Q. Chen, X. Ling, An experimental study of the latent functionally thermal fluid with micro–encapsulated phase change material particles flowing in microchannels, Appl. Therm. Eng. 105 (2016) 209–216. [49] B. Xu, P.W. Li, C. Chan, Application of phase change materials for thermal energy storage in concentrated solar thermal power plants: a review to recent developments, Appl. Energy 160 (2015) 286–307. [50] L. Chen, T. Wang, Y. Zhao, X.R. Zhang, Characterization of thermal and hydrodynamic properties for microencapsulated phase change slurry (MPCS), Energy Convers. Manage. 79 (2014) 317–333. [51] S. Zhang, J.L. Niu, Experimental investigation of effects of supercooling on microencapsulated phase-change material (MPCM) slurry thermal storage capacities, Solar Energy Mater. Solar Cells 94 (2010) 1038–1048. [52] S.H. Song, W.D. Shen, J.L. Wang, S.C. Wang, J.F. Xu, Experimental study on laminar convective heat transfer of microencapsulated phase change material slurry using liquid metal with low melting point as carrying fluid, Int. J. Heat Mass Transfer 73 (2014) 21–28. [53] M. Kong, J.L. Alvarado, W. Terrell, C. Thies, Performance characteristics of microencapsulated phase change material slurry in a helically coiled tube, Int. J. Heat Mass Transfer 101 (2016) 901–914. [54] Y.L. Zhang, S.F. Wang, Z.H. Rao, J.F. Xie, Experiment on heat storage characteristic of microencapsulated phase change material slurry, Solar Energy Mater. Solar Cells 95 (2011) 2726–2733. [55] Z.Z. Qiu, X.L. Ma, X.D. Zhao, P. Li, S. Ali, Experimental investigation of the energy performance of a novel micro–encapsulated phase change material (MPCM) slurry based PV/T system, Appl. Energy 165 (2016) 260–271. [56] P. Charunyakorn, S. Sengupta, S.K. Roy, Forced convection heat transfer in microencapsulated phase change material slurries: flow in circular ducts, Int. J. s Transfer 34 (1991) 819–833. [57] X.X. Hu, Y.P. Zhang, Novel insight and numerical analysis of convective heat transfer enhancement with microencapsulated phase change material slurries: laminar flow in a circular tube with constant heat flux, Int. J. Heat Mass Transfer 45 (2002) 3163–3172. [58] R. Sabbah, M.M. Farid, S. Al-Hallaj, Micro–channel heat sink with slurry of water with micro?encapsulated phase change material: 3D–numerical study, Appl. Therm. Eng. 29 (2008) 445–454. [59] R.L. Zeng, X. Wang, B.J. Chen, Y.P. Zhang, J.L. Niu, X.C. Wang, H.F. Di, Heat transfer characteristics of microencapsulated phase change material slurry in laminar flow under constant heat flux, Appl. Energy 86 (2009) 2661–2670. [60] G. Serale, S. Baronetto, F. Goia, M. Perino, Characterization and energy performance of a slurry PCM?based solar thermal collector: a numerical analysis, Energy Procedia 48 (2014) 223–232. [61] M. Goel, S.K. Roy, S. Senguptas, Laminar forced convection heat transfer in microcapsulated phase change material suspensions, Int. J. Heat Mass Transfer 37 (1994) 593–604. [62] M. You, X.X. Zhang, W. Li, X.C. Wang, Effects of microPCMs on the fabrication of microPCMs/polyurethane composite foams, Thermochim. Acta 472 (2008) 20–24. [63] 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. [64] J. Shi, X.L. Wu, X.Z. Fu, R. Sun, Synthesis and thermal properties of a novel nanoencapsulated phase change material with PMMA and SiO2 as hybrid shell materials, Thermochim. Acta 617 (2015) 90–94. [65] Z. Chen, L. Cao, F. Shan, G.Y. Fang, Preparation and characteristics of microencapsulated stearic acid as composite thermal energy storage material in buildings, Energy Build. 62 (2013) 469–474. [66] H.Z. Zhang, X.D. Wang, Synthesis and properties of microencapsulated n-octadecane with polyurea shells containing different soft segments for heat energy storage and thermal regulation, Solar Energy Mater. Solar Cells 93 (2009) 1366–1376. [67] L. Chen, L.L. Xu, H.B. Shang, Z.B. Zhang, Microencapsulation of butyl stearate as a phase change material by interfacial polycondensation in a polyurea system, Energy Convers. Manage. 50 (2009) 723–729. [68] F. Mohaddes, S. Islam, R. Shanks, M. Fergusson, L. Wang, R. Padhye, Modification and evaluation of thermal properties of melamine–formaldehyde/n-eicosane microcapsules for thermo–regulation applications, Appl. Therm. Eng. 71 (2014) 11–15. [69] X.L. Qiu, L.X. Lu, J. Wang, G.Y. Tang, G.L. Song, Preparation and characterization of microencapsulated n-octadecane as phase change material with different n-butyl methacrylate–based copolymer shells, Solar Energy Mater. Solar Cells 128 (2014) 102–111. [70] Y. Konuklu, H.O. Paksoy, M. Unal, Nanoencapsulation of n-alkanes with poly(styrene–co-ethylacrylate) shells for thermal energy storage, Appl. Energy 150 (2015) 335–340.