Design and Fabrication of Microencapsulated Phase Change Materials for Energy/Thermal Energy Storage and Other Versatile Applications

Abstract

Microencapsulated phase change materials have been considered as potential candidates to overcome the global energy shortage, as these materials can provide a viable method for storing thermal energy and offering consistent energy management by controllable heat release in desirable environments. Microencapsulation technology offers a method for overcoming the trouble associated with the handling of solid–liquid phase change materials (PCMs) via encapsulating PCMs with thin or tiny shells which are known as ‘microcapsules’. Microcapsule shells not only keep PCMs isolated from the surrounding materials but also provide a stable structure and sufficient surface for PCMs to enhance heat transfer. Thus microencapsulation technology received remarkable attention from fundamental studies to industrial growth in recent years. In order to provide a reliable source of information on recent progress and development in microencapsulated PCMs, this chapter emphases on methods and techniques for the encapsulation of PCMs with a diversity of shell materials from traditional organic polymers to novel inorganic materials to pursue high encapsulation efficiency, excellent thermal energy-storage performance and long-term operation durability. The chapter also highlights the design of bi- and multi-functional PCM-based microcapsules by fabricating various functional shells in a multilayered structure to meet the growing demand for versatile applications.

1. Introduction

The rapid global economic growth and population explosion resulted in increased consumption of nonrenewable energy resources such as coal, petroleum and natural gas which not only reduces these fossil fuels sources but also leads to major global environmental issues like CO₂ emission, and global warming and air pollution. If the world energy requirements totally depend on fossil fuel which is continuously exhausting will results in energy crisis in the near future. To minimize the reliance on fossil fuels for energy production, the development of renewable energy resources and the enrichment of energy efficiency have been deliberated as the alternative strategy that could be adopted [1, 2]. The scientific development of thermal energy storage by utilizing phase change materials (PCMs) to store latent heat has been considered as a worthy solution for reducing the worldwide energy scarcity as these materials provide viable ways of keeping thermal energy and offering reliable energy management by controllable heat release in suitable environments [3]. PCMs are a class of heat storage materials, able to absorb and release sufficient amounts of latent-heat energy at a constant temperature when a state change occurs from a solid form to the liquid one and vice-versa. In addition to higher thermal energy-storage density compared to conventional heat-storage materials, PCMs can bridge the gap between energy availability and energy use to reduce energy waste [4].

The application of PCMs as a means of thermal-energy storage has been practiced since 1970s, and PCMs have been developed and designed to fulfill the desired requirements. Nowadays, PCMs have not been only applied in renewable energy effective utilization such as solar thermal energy and low-temperature waste heat utilization but also used for thermal regulation and thermal management in the fields of photovoltaic-thermoelectric systems, temperature-sensitive electronic parts or devices requiring cool or thermal protection, biological products or pharmaceutical needing cool storage, smart fibers and textiles with a thermoregulatory function, telecom shelters in tropical regions, thermal buffering of Li-ion batteries, energy-saving buildings, thermal comfort in vehicles, etc. [5].

Though PCMs due to their desirable properties is widely used in both domestic and industrial areas in recent years, their phase transition brings some difficulties during their application for thermal energy storage and management. After fusion PCMs are converted to low viscous liquids which can then easily diffuse or flow over other materials and thus cause difficulty in handling the process in the liquid state [6]. Other problems associated with the commonly used PCMs include the need of using special latent heat devices, the hysteresis of thermal response due to low thermal conductivity and supercooling, the poor heat transfer during the charging and recovery processes, absorb moisture from the atmosphere or lose water through evaporation, the leakage and loss of PCMs, etc. [7]. Due to these problems, pristine PCMs are generally not recommended for thermal energy storage applications. To avoid the problem, microencapsulation technology was introduced which involves the packing/encapsulation of PCMs into tiny closed ampules that not only protect the liquid PCMs from the interference and interaction of the surrounding materials but also give them a stable form in the liquid state. The product obtained as a result of this packing technology was named microcapsule. The microcapsules which pack the PCM core individually with a firm shell can, therefore, handle even liquids as a solid material [8]. Additionally, the development of a microcapsule shell provides a large heat transfer surface to the encapsulated PCMs and hence considerably increases the heat transfer and thermal response [9]. Thus, microencapsulation of PCMs has been accepted as a more consistent technology for liquid PCMs compare to form stable composite PCMs. Microencapsulation technology of solid–liquid PCMs has received great attention for over 20 years, and several studies can be found in the literature on this topic [10]. Usually, the microencapsulated PCMs could be prepared by making a polymeric shell via coacervation, in-situ polymerization, interfacial polymerization and suspension polymerization techniques, for which the commonly used shell materials include polyureas, poly (methyl methacrylate), melamine-formaldehyde resins, polystyrene, urea-formaldehyde resins, and bio-based polymers such as Arabic gum, agar and gelatin. Moreover, many inorganic materials such as titanium dioxide (TiO₂), silica (SiO₂), calcium carbonate and aluminum oxide have been reported in the recent literature that could also be used as shell materials for encapsulating PCMs [11, 12, 13]. These inorganic shells have shown much higher mechanical strength and rigidity than the polymeric ones and can form a much more secure barrier around PCMs to protect them from damaging interaction with the environment.

Currently, the researchers are interested in the design and development of multifunctional microencapsulated PCMs. One of the potential approaches to achieve the additional functionality involve the use of inorganic functional shell assembly on the microencapsulated PCM core. In this way, not only the additional functions for microencapsulated PCMs along with the wall materials is achieved but also allows the establishment of signal or multilayered shells with various designed functions. Pointing at the high-tech designs and versatile applications of microencapsulated PCMs for thermal energy storage and thermal management, this chapter provides a reliable source of information on recent progress and development in microencapsulation technology for solid–liquid PCMs and especially introduces the diverse designs for PCMs-based microcapsules with various special functions. Moreover, a thorough analysis of the trend in the development and applications of microencapsulated PCMs is also presented. The chapter also highlights the design of bi- and multi-functional PCM-based microcapsules by fabricating various functional shells in a multilayered structure to offer a great potential to meet the growing demand for versatile applications.

2. Phase change materials (PCMs)

PCMs due to their higher latent heat values can store and release a large amount of heat energy during melting and solidifying processes [14]. These materials have been thought to act as a storage medium with numerous applications such as cooling of food products, buildings, textiles, solar systems, spacecraft thermal systems and waste heat recovery systems [15]. On the basis of phase conversion PCMs are categorized into solid–liquid, solid–solid, solid–gas and liquid–gas [1]. Among these categories, solid–liquid PCMs due to their high density, favorable phase equilibrium, minor volume changes and low vapor pressure at the operating temperature during phase transition are more suitable for thermal energy storage systems. Furthermore, solid–liquid PCMs show little or no subcooling during freezing, melting/freezing at the same temperature and phase separation, and sufficient crystallization rate.

PCMs possess high chemical stability, nontoxic, nonexplosive and noncorrosive nature, do not undergo degradation after long-term thermal cycles, and have good chemical properties capable of completing reversible freezing/melting cycle [6]. Solid–liquid PCMs can be divided into three major types: (a) organic PCMs (b) inorganic PCMs and (c) eutectic PCMs [16]. Organic PCMs include paraffin and nonparaffin (alcohols, fatty acids and glycols) materials [2]. Inorganic PCMs generally include salt hydrates, metallic compounds and metal alloys with the advantages of a broader range of transition temperature, high thermal conductivity and high latent heat storage capacity, low cost and nonflammable nature. In contrast, lack of thermal stability, phase segregation, supercooling, corrosion and decomposition, are problems that dominate their benefits [17]. Eutectic PCMs are the combination of two or more low melting components, each of which freezes and melts congruently to make a mixture of the components’ crystals upon crystallization [6]. Eutectic PCMs can be prepared for a specific application by mixing organic–organic, inorganic–inorganic, or a combination of the two PCMs at a given ratio. These PCMs have high thermal conductivity and density without segregation and supercooling, while their specific heat capacity and latent heat are much lower than those of paraffin/salt hydrates [18].

3. Microencapsulation shell materials

In recent years, microencapsulation of PCMs has been widely used to avoid the leakage and reaction of the PCMs with the surrounding environment during the solid–liquid phase transition.

Microencapsulation of PCMs can also be responsible for relatively constant volume, high thermal cycling stability and large heat transfer area for PCM-based thermal storage [19]. Shell/wall materials play a vital role in controlling various physical properties like morphology, mechanical and thermal properties of the produced microcapsules [7]. On the basis chemical nature shell material can be divided into three groups: (a) organic, (b) inorganic and (c) organic–inorganic hybrid materials [20].

3.1 Organic shells

Organic shell materials include synthetic and natural polymeric materials, which have excellent structural flexibility, good sealing properties and high resistance to the volume change associated with repeated phase transformations of PCMs [21]. The organic shell materials most frequently used consist of urea-formaldehyde (UF) resin [22], melamine-formaldehyde resin [23], and acrylic resin [24]. Many workers around the world used MF resin as the wall material due to its good chemical compatibility, low cost and thermal stability [25]. Mohaddes et al. effectively utilized MF as the shell material for encapsulation of n-eicosane for application to textiles [26]. Fabrics doped with this type of microcapsules have higher thermoregulation capacity and low thermal delay efficiency. Among the group of acrylic resins, the copolymers of methacrylate have significant thermal stability, chemical resistance, nontoxic nature and easy preparation. Alkan et al. has shown that n-eicosane microencapsulation with polymethylmethacrylate (PMMA) shell had good thermal stability [27]. Ma et al. successfully encapsulated binary core materials, butyl stearate and paraffin using poly(methylmethacrylate-co-divinylbenzene) (P(MMA-co-DVB)) copolymer as the shell material [28]. The microcapsules so obtained possess a uniform size of 5–10 μm with a uniform spherical shape and dense surface. Moreover, the phase transition temperature of these microcapsules can be adjusted by adjusting the butyl stearate to paraffin ratio. Wang et al. studied the effect of GO on the thermal properties of capric acid@UF microcapsules by adding various contents of graphene oxide (GO) [29]. It was found that the microcapsules with 0.6% GO had the highest enthalpy of 109.60 J/g and encapsulation ratio of 60.7%. The microcapsules with GO presented smoother surfaces and good thermal conductivity.

3.2 Inorganic shells

Microcapsules prepared by using organic polymeric shell materials are usually not suitable for application in some situations due to the low thermal conductivity, flammable nature and poor mechanical strength of the organic shell materials [30]. In recent years, inorganic shells due to their good thermal conductivity, high rigidity and high mechanical strength, have been progressively employed as an alternative shell material for microcapsule preparation [21]. The commonly used inorganic shell materials include Silica (SiO₂) [31], zinc oxide (ZnO) [32], titanium dioxide (TiO₂) [33] and calcium carbonate (CaCO₃) [34].

Silica because of its fire resistance nature, high thermal conductivity and ease of preparation are one the most commonly used shell materials for encapsulation of fatty acids [35], paraffin waxes [34] and inorganic hydrated salts [36]. Liang et al. prepared nanocapsules by encapsulating n-octadecane core material using silica as the shell material via interfacial hydrolysis and polycondensation of tetraethoxysilane (TEOS) in miniemulsion [37]. The thermal conductivity of nanocapsules so obtained was observed to be higher than 0.4 Wm⁻¹ K⁻¹ with melting enthalpy and encapsulation ratio of 109.5 J/g and 51.5%, respectively. The enthalpy of the nanocapsules was not altered and no leakage was observed after 500 thermal cycles. However, the hydrolysis and polycondensation of TEOS, used as a silica precursor, could cause a reduction in the compactness of the silica shell and have a relatively weak mechanical strength. CaCO₃ shells have higher rigidity and better compactness compared to silica. Yu et al. employed CaCO₃ as shells material for encapsulating n-octadecane through the self-assembly method [12]. The microcapsules obtained were of spherical morphology with a uniform diameter (5 μm) and had good thermal stability, thermal conductivity, anti-osmosis properties and serving durability.

Metal oxides, like ZnO and TiO₂, owing to their multifunctional properties including photochemical, catalytic and antibacterial characteristics are frequently used as shell materials to obtain PCM microcapsules with some remarkable characteristics. Li et al. synthesized multifunctional microcapsules with latent heat storage and photocatalytic and antibacterial properties by using ZnO as the shell material and n-eicosane as the core material [38]. Similarly, Liu et al. utilized TiO₂ as shells material for encapsulating n-eicosane through interfacial polycondensation followed by ZnO impregnation [39]. The prepared microcapsules have both thermal storage and photocatalytic capacities with a melting temperature of 41.76°C and latent heat of 188.27 J/g.

3.3 Organic: inorganic hybrid shells

Organic–inorganic hybrid shells materials are used to overcome the shortcomings related to the individual organic or inorganic materials for encapsulating PCMs. In hybrid shells, organic materials offer structural flexibility while inorganic materials can improve thermal conductivity, thermal stability and mechanical rigidity [21]. Polymers (such as PMMA and PMF) based shells doped with SiO₂ or TiO₂ are extensively used to encapsulate PCMs [40]. Wang et al. prepared n-octadecane microcapsules using PMMA-silica hybrid shells via photocurable Pickering emulsion polymerization with good morphology and particles size of 5–15 μm [41]. The highest encapsulation efficiency (62.55%) was achieved with the weight ratio of MMA to n-octadecane of 1:1. Zhao et al. successfully synthesized bifunctional microcapsules by using PMMA doped with TiO₂ as the hybrid shell and n-octadecane as the core material [42]. TiO₂ was observed to improve microcapsules’ thermal conductivity but reduce encapsulation efficiency and enthalpy. The initial degradation temperature of microcapsules with 6% TiO₂ reached 228.4°C, confirming good thermal stability of the microcapsules. Wang et al. prepared multifunctional microcapsules with regular-spherical morphology by using poly(melamine-formaldehyde)/silicon carbide (PMF/SiC) hybrid shells and n-octadecane as cores material [43]. The thermal conductivity of microcapsules with 7% SiC had improved by 60.34% compared to those microcapsules with no SiC, which is also accompanied by a significant increase in heat transfer rate.

4. Technologies for microencapsulation of PCMs

Microencapsulation techniques are of several types which are broadly classified into three major categories on the basis of fabrication mechanism: (1) physical methods (2) chemical methods and (3) physico-chemical methods. All these techniques involve the formation of a solid shell/coat around small liquid or solid particles of 1–100 μm diameter to accomplish the desired properties such as, protection competency, time-dependent release of material, provision of the substance to the particular target, minimize interaction with the environment, corrosion prevention, steadiness of function and to facilitate the use of toxic materials. The microencapsulation of PCMs is a special packaging methodology in which solid–liquid PCMs can be enclosed in some wall materials by using physical or chemical process to make small particles termed ‘microcapsules’ [44]. The PCM in a microcapsule is named as the core material while the outer shell which encloses the PCM from the surrounding environment is called the wall material. Microencapsulation as an emerging technology, commonly applied in many fields like thermal energy storage, medicine, food preservation, catalysis, dyes, textile, cosmetics, self-healing, coatings, engineering and defense [45]. A detailed classification of the microencapsulation methods is listed in Figure 1.

Figure 1.

Major physical and chemical microencapsulation methods for solid–liquid PCMs.

4.1 Physical methods

Physical methods involve involves physical processes, like drying, dehydration and adhesion in the formation of microcapsule shells. The most frequently used physical methods for PCMs encapsulation are spray-drying and solvent evaporation. The spray-drying process can be accomplished in the following steps: (1) preparation of oil–water emulsion comprising PCMs and shell materials, (2) spraying of the oil–water emulsion in a drying chamber via an atomizer, (3) drying of the sprayed droplets by using a stream of drying gas at a particular temperature, and (4) separating the solid particles by cyclone and filter [46]. Borreguero et al. employed a spray drying method for microencapsulation of paraffin Rubitherm®RT27 core using polyethylene EVA shell with and without carbon nanofibers (CNFs) [47]. The CNFs addition improved the thermal conductivity and mechanical strength of microcapsules, and the heat storage capability was retained. Also, the DSC analysis shown that even after the 3000- thermal charge/discharge cycles the microcapsules still had good thermal stability. Hawlader et al. synthesized spherical shape and uniform size microcapsule with paraffin core and gelatin and Arabic gum using spray-drying method [48]. The microcapsules prepared at the core-to-shell ratio of 2:1 have heat storage and release capacity reached 216.44 J/g and 221.217 J/g, respectively.

The solvent evaporation method includes: (1) preparation of polymer solution by dissolving shell materials in a volatile solvent; (2) addition of PCMs to the polymer solution to form O/W emulsion; (3) developing shells on the droplets by evaporating the solvent; (4) filtration and drying to obtaining the microcapsules. Lin et al. encapsulated myristic acid (MA) with ethyl cellulose (EC) using the solvent evaporation method [49]. The melting and solidifying temperatures were observed to be 53.32°C and 44.44°C, while the melting and solidifying enthalpies were found 122.61 J/g and 104.24 J/g, respectively. Wang et al. applied the solvent evaporation method to synthesize high-performance microcapsules by using sodium phosphate dodecahydrate (DSP) as the core and poly(methyl methacrylate) (PMMA) as the shell [50]. The optimal preparation temperature for the microencapsulation process was 80–90°C, reaction time 240 min, and stirring rate 900 rpm. The microcapsules obtained had an energy storage capacity of 142.9 J/g at the endothermic peak temperature of 51.5°C.

4.2 Chemical methods

In chemical methods, microencapsulation is done by the polymerization or condensation of monomers, oligomers, or prepolymers as raw materials to form shells at an oil–water interface. The chemical methods mostly involve in-situ polymerization, suspension polymerization, interfacial polymerization, and emulsion polymerization. The schematic diagrams of these four polymerization methods are shown in Figure 2. In situ polymerization method (Figure 2(a)), involves the formation of a shell on the surface of the droplet by polymerization of the prepolymers which can be accomplished in the following steps [52]: (1) preparation of the O/W emulsion by adding PCMs to surfactant aqueous solution; (2) preparation of a prepolymer solution; (3) addition of the prepolymer solution to the O/W emulsion, followed by adjusting the appropriate reaction conditions; and (4) microcapsule synthesis. Konuklu et al. successfully utilized in situ polymerization method for microencapsulation of decanoic acid using poly(urea-formaldehyde) (PUF), poly(melamine-formaldehyde) (PMF), and poly(melamine urea-formaldehyde) (PMUF) [53]. The microcapsules obtained by coating of PUF displayed higher heat storage capacity but weaker mechanical strength and lower heat resistance, while the microcapsules coated with PMF shells had higher thermal stability but lower thermal energy storage capacity. However, the PMUF-encapsulated microcapsules possessed seamless thermal stability and no leakage was found at 95°C. Zhang et al. utilized in situ polycondensation method for synthesizing dual-functional microcapsules containing n-eicosane cores and ZrO₂ shells [54]. The microcapsules synthesized have a spherical shape with a size of 1.5–2 μm have good thermal energy storage and possessed better thermal stability, and thermal properties almost unchanged after 100 thermal cycles. Su et al. used methanol-modified melamine-formaldehyde (MMF) prepolymer as shell material for microencapsulation of dodecanol and paraffin via in situ polymerization [55]. They observed that the average diameter of dodecanol-based microcapsules sharply decreased and encapsulation efficiency increased with increasing stirring rates. The maximum encapsulation efficiency was found to be 97.4%.

Figure 2.

Schematic diagrams of chemical methods for PCMs microencapsulation: (a) in situ polymerization, (b) interfacial polymerization, (c) suspension polymerization, and (d) emulsion polymerization [51].

Interfacial polymerization is used in the preparation of organic shell materials such as polyurea and polyurethane. In this method, two reactive monomers are separately dissolved in the oil phase and the aqueous phase, then in the presence of an initiator polymerization occurs at the oil–water interface as shown in Figure 2(b). This method includes the following steps: (1) preparation of an O/W emulsion having hydrophobic monomer and PCMs; (2) addition of the hydrophilic monomer under proper conditions to initiate polymerization; (3) filtration, washing, and drying to get microcapsules. Ma et al. successfully used the interfacial polymerization method for microencapsulation of binary core materials like butyl stearate (BS) and paraffin with polyurea/polyurethane as the shell material [56]. The microcapsules phase change temperature was adjusted by changing the ratio of the two core materials. The microcapsules obtained possessed high thermal stability. Lu et al. encapsulated the butyl stearate core with a polyurethane-based cross-linked network shell via interfacial polymerization [57].

In the suspension polymerization method, the dispersed droplets of PCMs, monomers and initiators are suspended in a continuous aqueous phase by using surfactants and mechanical stirring. The oil-soluble initiator free radicals are then released into the emulsion system to initiate polymerization of the monomers at a suitable temperature and stirring rate [46], as presented in Figure 2(c). Wang et al. successfully employed the suspension polymerization method to encapsulate n-octadecane with thermochromic pigment/PMMA shells at five different pigment/MMA ratios varying as 0, 1.4, 4.3, 7.1, and 14.3 wt.% [58]. It was observed that the microcapsules without pigment achieved the highest melting and crystallization enthalpies of 149.16 J/g and 152.55 J/g, respectively. Tang et al. prepared spherical shape microcapsules with an average diameter of about 1.60 μm using n-octadecane core material and n-octadecyl methacrylate (ODMA)-methacrylic acid (MAA) copolymer as shell material via the suspension polymerization method [59]. The microcapsules attained the highest phase change enthalpy of 93 J/g at monomers to the n-octadecane ratio of 2:1. Sanchez-Silva et al. microencapsulated Rubitherm®RT31 with polystyrene via suspension polymerization by using different suspension stabilizers [60]. The DSC investigations have shown that when PVP and gum Arabic were used as suspension stabilizers the microcapsules obtained presented the lowest thermal storage capacity of 75.7 J/g and highest of 135.3 J/g.

In emulsion polymerization (Figure 2(d)), first, the PCMs and monomers dispersed phase is suspended in a continuous phase in the presence of surfactants at constant stirring, followed by the addition of water-solution initiators to start the polymerization process [61]. This method is used to prepare microcapsule shells by polymerizing organic materials like PMMA and polystyrene. Şahan et al. encapsulated stearic acid (SA) with poly(methyl methacrylate) (PMMA) and four other PMMA-hybrid shell materials via emulsion polymerization technique [62]. The average diameter of microcapsules so obtained was found to be 110–360 μm, the thickness of 17–60 μm, heat storage capacity below 80 J/g and degradation temperature above 290°C. Sarı et al. successfully utilized the emulsion polymerization technique to microencapsulate paraffin eutectic mixtures (PEM) containing four different contents with PMMA shells [63]. The microcapsules obtained were spherical with a particle size of 1.16–6.42 μm, heat storage capacity of 169 J/g and melting temperature in the range of 20–36°C.

4.3 Physico-chemical methods

In the physical–chemical method, microencapsulation is accomplished by combining the physical processes like phase separation, heating and cooling, with chemical processes, like hydrolysis, cross-linking and condensation. Normally, the coacervation and sol–gel methods are the most frequently employed methods. The coacervation method is of two types, one is single coacervation which requires only one type of shell material and the other is complex coacervation which requires two kinds of opposite-charged shell materials for microcapsules preparation. The microcapsules synthesized by the complex coacervation method usually have a more uniform size, better morphology and stability.

The complex coacervation processes involve the following key steps: (1) formation of emulsion by dispersing PCMs in polymer aqueous solution; (2) addition of a second aqueous polymer solution with opposite charges and deposition of shell material on droplet surface by electrostatic attraction and (3) Getting of microcapsules by cross-linking, desolation or thermal treatment. Hawlader et al. encapsulated paraffin cores with gelatin and acacia by using a complex coacervation process [48]. The melting and solidifying enthalpies of microcapsules obtained reached 239.78 J/g and 234.05 J/g, respectively, when the amount of cross-linking agent was 6–8 ml, homogenizing time was 10 min, and the ratio of core to the shell was 2:1. Onder et al. employed complex coacervation to microencapsulate n-hexadecane, n-octadecane and n-nonadecane core materials with natural and biodegradable polymers, like gum Arabic-gelatin mixture [64]. The microcapsules having n-hexadecane and n-octadecane cores showed good enthalpies of 144.7 J/g and 165.8 J/g, respectively, were obtained at the dispersed content of 80% in the emulsion and the microcapsules containing n-nonadecane prepared at the dispersed content of 60% in the emulsion presented enthalpy value of only 57.5 J/g.

The sol–gel method is a cheap and mild process for synthesizing PCMs microcapsules by inorganic shells, such as SiO₂ and TiO₂ shells. The major steps involved in the preparation of microcapsule by the sol–gel method are as follows: (1) preparation of colloidal solution by uniformly dispersing the reactive materials like PCMs, precursor, solvent and emulsifier in a continuous phase via hydrolysis reaction; (2) formation of a three-dimensional network structured gel system through condensation polymerization of monomers and (3) drying, sintering and curing processes to obtain microcapsules [65]. Cao et al. used the sol–gel process to microencapsulate paraffin core with TiO₂ shells. They found that the sample with a microencapsulation ratio of 85.5% had melting and solidifying latent heat of 161.1 kJ/kg (at the melting temperature of 58.8°C) and 144.6 kJ/kg (at the solidifying temperature of 56.5°C), respectively [66]. Latibari et al. successfully employed the sol–gel method to synthesize nanocapsules containing palmitic acid (PA) core with SiO₂ shell by controlling solution pH [67]. The nanocapsule obtained presented an average particle size of 183.7, 466.4 and 722.5 nm, at pH 11, 11.5 and 12, respectively, and the corresponding melting latent heats values of 168.16, 172.16 and 180.91 kJ/kg, respectively.

5. Design of microencapsulated PCMs for versatile application

5.1 PCMs microencapsulation with function inorganic shells

Microencapsulation with conventional polymeric, inorganic or composite shells can provide only protection for the PCM core, but at the same time, these inert wall materials cause a reduction in their latent heat-storage capacities which make them unsuitable for thermal energy storage and thermal management systems. In view of that various inorganic materials have a feature of functional diversity, it will be possible to synthesize bi-function PCMs-based microcapsules by encapsulating the PCM core with a functional inorganic shell. This idea was first used by Fei et al. [68] and successfully synthesized a novel multi-functional microcapsules based on an anatase TiO₂ shell and n-octadecane/titania aerosol core via the hydrothermal method. The microcapsules obtained presented multi-functional properties with photocatalytic activity and UV-blocking effectiveness as well as a thermal energy-storage function. Chai et al. [69] introduced a new synthetic strategy by fabricating a well-defined core-shell structured PCM microcapsule based on a functional TiO₂ shell. The crystallization of amorphous TiO₂ was initiated by adding fluorine ions when the in-situ polycondensation of titanic precursors was performed in a nonaqueous O/W emulsion-templating system. The microcapsules so prepared have excellent thermal energy-storage capacity and show photocatalytic and antibacterial functions. Liu et al. [70] introduced a new technology by modifying the brookite TiO₂ shell of the n-eicosane core with graphene nanosheets. It was observed that graphene promotes the charge transfer and separation ability of microcapsule which leads to a significant increase in its photocatalytic activity. Liu et al. [39] also explored that modification of TiO₂ shell with ZnO boosts the latent heat-storage capacity and photocatalytic activity of the resultant microcapsules. A study on the utilization of microcapsules doped with ZnO presented good thermal regulation and thermal management properties when incorporated into the gypsum-matrix composites. These explorers make the modified microcapsules good candidates for direct solar energy utilization. Additionally, Liu et al. [71] introduced a morphology-controlled synthetic technology to fabricate PCM-based microcapsules with crystalline TiO₂ shells by using different structure-directing agents and effectively obtained the microcapsules in the tubular, octahedral and spherical shapes. They also studied the influence of structural morphology on the thermal energy-storage capacity of these microcapsules and observed the highest latent heat-storage efficiency with microcapsules of spherical morphology while the tubular ones displayed the fastest heat response rate. Li et al. [38] successfully encapsulated n-eicosane with ZnO shell via in-situ precipitation reaction of Zn(CH₃COO)₂·2H₂O and NaOH in an emulsion templating system. The microcapsule prepared exhibited good thermal energy-storage capability and high working reliability as well as high photocatalytic activities and antimicrobial effectiveness against Staphylococcus aureus. These microcapsules, therefore, have gained potential applications in medical care and surgical treatment. Gao et al. [72] designed multi-functional microcapsules by a microencapsulating n-eicosane core with a Cu₂O shell, through emulsion templated in-situ precipitation and reduction. The microcapsules obtained exhibited multifunctional properties of effective photothermal conversion, high latent-heat storage/release efficiency for solar photocatalysis and solar thermal energy storage, as well as demonstrated sensitivity to some toxic organics gases due to a p-type semiconductive feature of Cu₂O shell.

5.2 Advanced design of microencapsulated PCMs for versatile applications

In recent years due to the fast development in microencapsulation technology, a large number of innovative designs have been introduced for fabricating bi- or multi-functional PCMs-based microcapsules. Jiang et al. [73] designed magnetic PCM-based microcapsules as an applied energy microsystem for bio-applications as thermoregulatory enzyme carriers. They synthesized the magnetic microcapsules by encapsulating n-eicosane with a TiO₂/Fe₃O₄ hybrid shell by Pickering emulsion-templated interfacial polycondensation and then Candida rugosa lipase (CRL) was immobilized onto the microcapsules obtained by covalent bonds through a series of complicated surface modification and immobilization reactions. The microcapsules obtained were observed to have higher thermal stability, longer storage stability, higher biocatalytic activity and better reusability compared to traditional inert enzyme carriers. Likewise, Li et al. [74] also developed thermoregulatory enzyme carriers based on the magnetic microcapsules containing n-docosane core and SiO₂/Fe₃O₄ hybrid shell with α-amylase immobilized onto the microcapsule and examined the effect of ambient temperature on their biocatalytic activity. They found that the biocatalytic activity was increased considerably for the immobilized α-amylase on the developed enzyme carriers due to the thermoregulation microenvironment around the microcapsules. These innovative designs provide a novel approach for the preparation and applications of microencapsulated PCMs in areas of bioengineering and biotechnological.

Choi et al. [75] designed a novel, temperature-sensitive drug release system based on PCMs. They first prepared the gelatin nanoparticles containing fluorescein isothiocyanate-dextran as a drug via emulsification technique, and then 1-tetradecanol was used to synthesize the PCM-matrix microbeads containing these gelatin nanoparticles by using a simple fluidic device based on an O/W emulsion. Moreover, Wang et al. [58] designed and synthesized thermochromic microencapsulated PCM by encapsulating n-octadecane with PMMA shell with simultaneous dispersion of thermochromic pigments in core and shell by suspension-like polymerization. The microcapsules obtained showed a visible color change with change in temperature, confirmed the occurrence of thermal energy storage or release at the specific temperature.

Geng et al. [76] designed a three-component core consisting of 1-tetradecanol as a PCM, leuco dye and phenolic color developer as an electron donor and fabricated reversible thermochromic microcapsules for application in thermal protective clothing. They encapsulated the three-component core with a poly (methylated melamine-formaldehyde) (PMMF) shell via emulsion-templated copolymerization. The as synthesized microcapsules exhibited thermochromic reversibility with good energy storage/release capability and have a great potential for applications in thermal protective clothing of firefighters as well as intelligent textiles or fabrics, food and medicine package and so on. Wu et al. [77] synthesized reversible thermochromic microcapsules by encapsulating 1-hexadecanol with modified gelatin and gum Arabic via a complex coacervation process. The wall materials of this microcapsule system were fused with 2-phenylamino-3-methyl-6-di-n-butylamino-fluoran as a color former and 2,2-bis(4-hydroxyphenyl) propane as a color developer. The microcapsule prepared acts as an indicator for the states of energy saturation and consumption through color changes. In addition, Zhang et al. [78] introduced polysaccharide-assisted microencapsulation as an innovative methodology for encapsulation of volatile PCMs with a fluorescent retention indicator to determine the retention of microencapsulated volatile PCM in diverse working environments. They microencapsulated heptane core with polymeric shell by one-step in-situ polymerization path using Nile red as a fluorescent indicator which was incorporated into the heptane core during the synthetic process, and therefore the fluorophores in Nile red could give a clear indication for the core and shell structures of microcapsules.

6. Applications of microencapsulated PCMs

Microencapsulated PCMs due to their unique properties such as solid-to-liquid phase transition, chemical and thermal stability and higher amount of energetic changes, has received special attention for their applications in in our ordinary daily life and various industries. In recent years, PCMs have been designed and fabricated to meet the requirements around the world. The potential applications of PCM microcapsules are shown in Figure 3, and discussed as bellows:

Figure 3.

Potential applications of PCMs microcapsules.

6.1 Application in fibers and textiles

In textile industries, microencapsulated PCMs are embedded within the fibers or coated onto the surface of fabrics which are used in the preparations of outdoor dress such as snowsuits, trousers, gloves, ear warmers and boots etc. The microencapsulated PCMs enhance the thermal storage capacities of the fibers/fabrics (2.5–4.5 times) and thus protect from extremely cold weather [46]. Microencapsulation is a promising technology for applications in the textile industry such as agriculture textiles, medical textiles automotive textiles and sportswear/protective clothing. Scacchetti et al. explored the thermal and antimicrobial properties of cotton with silver zeolites functionalized via a chitosanzeolite composite and microcapsules of PCMs [79]. They suggested the use of chitosan zeolite for the production of textiles for superior antibacterial and thermoregulating properties. Microencapsulated PCMs increase the flame-retardant property thermal and comfort of the textiles, as these PCM microcapsules were scattered homogeneously onto textile substrates and were durable with repeated washings [80].

6.2 Application in slurry

PCM microcapsules with high latent heat are used in the slurry industry as an enhanced heat transfer fluid (HTF) and a thermal storage medium (TSM). Song et al. considered laminar heat transfer of PCM microcapsules slurry and proved that the heat transfer coefficient improved with increasing Reynolds number and volume concentration of microcapsules [81]. Roberts et al. compared the heat transfer capability of metal-coated and nonmetal-coated PCM microcapsules slurry and noticed an additional 10% increment in heat transfer coefficient and PCM microcapsules inducing pressure drop in slurry [82]. Zhang and Niu reported higher thermal storage capacity for PCM microcapsules slurry storage devices and stratified water storage tanks [83]. Xu et al. prepared PCM microcapsules with Cu-Cu₂O/CNTs shell and their dispersed slurry for direct absorption solar collectors [84]. They reported that the PCMs@Cu-Cu₂O/CNTs microcapsule slurry had high heat storage competency and outstanding photothermal conversion performance which made it as one of the most potential HTFs for direct absorption solar collector.

6.3 Application in energy-saving building

Another amazing application of PCM microcapsules is their utilization in building materials to overcome overheating problems in summer and provided a new effective solution for thermal management and energy saving in buildings. The PCM microcapsules in construction materials boost the thermal and acoustic insulation of walls. Usually, the PCM microcapsules are embedded into concrete mixtures, cement mortar, gypsum plaster, wallboards, sandwich, slabs, panels and to fulfill the energy demand of the building for heating, cooling, lighting, air conditioning, ventilation and domestic hot water systems [85]. Many researchers around the world worked on the application of PCMs microcapsules in the building industry. Cabeza et al. reported an innovative concrete material with high thermal properties by mixing it with PCM microcapsules [86]. It was found that the concrete wall with PCM microcapsules increase its overall mechanical resistance and stiffness and causes even temperature fluctuations and thermal inertia, making it to be a promising technology to save energy for buildings [87]. Su et al. studied nano-silicon dioxide hydrosol as the surfactant for the preparation of PCM microcapsules for thermal energy storage in buildings [88]. Essid et al. investigated the compressive strength and hygric properties of microencapsulated PCMs concretes [89]. They reported that the use of concrete containing PCM microcapsules as structural material is sufficiently safe, though its compressive strength is lower and porosity is higher than the pure concrete. Schossig et al. [90] directly integrated formaldehyde-free microencapsulated paraffins in building materials and studied their effect for application in conventional construction materials. They observed that the utilization of these PCMs microcapsule could help to keep the indoor temperature up to 4°C lower than typical conditions and could reduce the number of hours that the indoor temperature was greater than 28°C.

6.4 Application in foams

The application of microencapsulated PCMs in foams can enhance their thermal insulating efficiency. Borreguero et al. reported that the thermal energy storage capacity of rigid polyurethane foams was improved when it was embedded with PCM microcapsules investigated rigid polyurethane foams containing and indicated that improved [91]. Li et al. introduced a new approach to enhancing the latent heat energy storage ability by embedding PCM microcapsules in metal foam [92]. They observed that compared to the surface temperature of virgin PCM modules, the surface temperature for the PCM microcapsule/foam and PCM/foam composite modules was reduced from about 90 to 55 and 45°C, respectively. PCM microcapsule/foam composites solved the problem of low thermal conductivity and leakage. Bonadies et al. synthesized poly(vinyl alcohol)- (PVA-) based foams containing PCM microcapsules and investigated their thermal storage and dimensional stability [93]. They observed that the formation of crystalline domain and amount of water uptake was influenced by microcapsules which in turn affected the number of intra- and intermolecular hydrogen bonds as many PVA –OH groups interact with microcapsule shells.

6.5 Others applications

There are many other potential applications of PCM microcapsules. These include biomedical applications, solar-to-thermal energy storage and electrical-to-thermal energy storage [65]. Zang et al. prepared multifunctional microencapsulated PCMs that can be used for sterilization [94]. They reported that these microcapsules have high antibacterial activity against Escherichia coli, S. aureus, and Bacillus subtilis, and the antibacterial efficiency of 2-hour contacting PCM microcapsules was inhibited up to 64.6%, 99.1%, and 95.9%, respectively. Zhang et al. also studied solar-driven PCM microcapsules with efficient Ti₄O₇ nanoconverter for latent heat storage [95]. The solar absorption capacity of the novel PCM microcapsules was found to be 88.28%, and the photothermal storage efficiency of the PCMs@SiO₂/Ti₄O₇ microcapsules was 85.36% compared with 24.14% for pure PCMs. Zheng et al. proposed a joule heating system to reduce the convective heat transferring from the electrothermal system of the surrounding by inserting the highly conductive and stable PCMs microcapsules [96]. They showed that the working temperature could be improved by 30% with the loading of 5% PCMs microcapsules even at lower voltage and ambient temperature.

7. Conclusion

Microencapsulation is a promising technology to fabricate PCM microcapsules for thermal energy storage and other versatile applications. Microencapsulation technology not only overcome the problem of leakage, volatilization and handling the difficulty of liquids PCMs but also improve the heat transferring ability of PCMs and thus makes them a favorable means for many broad range of applications. The thermal, physical, chemical and mechanical properties of PCM microcapsules are highly dependent on the type of the core materials, shell materials and synthesis processes. Encapsulation with inorganic shells can provide more advantages for microencapsulated PCMs and therefore will gain much more attention from fundamental research to commercial development in the future. The designing parameters, such as weight ratio of raw materials for core and shell, the selection of dispersion medium, reaction temperature, time, agitation speed, particles size and its distribution and other additives should be carefully addressed to obtain PCMs microcapsules with well-defined core-shell structured and good thermal energy-storage capability. Though PCM microcapsules are considered smart thermal energy storage materials, still much more new materials and synthetic techniques are to be explored to offer numerous possibilities for the design and fabrication of innovative bi- and multi-functional PCMs microcapsules with better properties and functions than the traditional ones.