XIAO Anna, YUAN Qingchun



Phase change microcapsules in thermal Energy applications: A critical review

1,2

（1GSK, Global Manufacturing and Supply, Ware SG12 0DP, United Kingdom;2Aston Material Research Institute, Aston University, Aston Triangle, Birmingham B4 7ET, United Kingdom）

Phase change microcapsules can carry large amounts of heat and be dispersed into other mediums either as a solid composite or as slurry fluids without changes to their appearance or fluidity. These two standout features make phase change microcapsules ideal for use in thermal energy applications to enhance the efficiency of energy utilisation. This review paper includes methods used for the encapsulation of phase change materials, especially the method suitable for large scale productions, the trends of phase change microcapsule development and their use in thermal energy applications in static and dynamic conditions. The effect of phase change microcapsules on convective heat transfer through addition to thermal fluids as slurries is critically reviewed. The review highlighted that so far the phase change microcapsules used mainly have polymeric shells, which has very low thermal conductivities. Their enhancement in convective heat transfer was demonstrated in locations where the phase change material experiences phase change. The phase change results in the slurries having higher apparent local specific heat capacities and thus higher local heat transfer coefficients. Out of the phase change region, no enhancement is observed from the solid microcapsule particles due to the low specific heat capacity and thermal conductivity of the phase change microcapsules compared to that of water, which is normally used as slurry media in the test. To further the research in this area, phase change microcapsules with higher specific heat capacity, higher thermal conductivity and better shape stability need to be applied.

phase change microcapsule; complex microencapsulation; slurry; phase change patterns; convective heat transfer enhancement

1 Introduction

In recent years, significant interest has developed for materials that release or absorb large amounts of latent heat during phase change between the three states. In process engineering, the heat energy released from steam is utilised for process optimizations and, can also be converted into mechanical force to generate ower through steam turbines. Typically phase changes from vapour to liquid like steam involve dramatic pressure and volume changes. Such systems are not ideal for applications in construction or textiles for temperature management, nor in recovering and storing waste heat from process engineering or solar thermal energy where the amount of heat is large but heat intensity and density are not very high. Liquid fluids are preferably used in process engineering, where a faster rate of heat transfer is desired to limit the amount of thermal fluids, and at the same time improve efficiency of energy utilisation such as in secondary refrigeration and air conditioning [1]. To recover and store low and intermediate ranks of heat, thermal fluids with a high specific heat content and thermal conductivity are required to lower the pumping rate and reduce storage tank sizes compared to single-phase fluids of water, for example, for the same energy content. Water with its high specific heat capacity[~4.18 kJ/(kg·K)], good availability and low cost has been favoured in many popular applications so far. However, its heat carrying capacity at low pressures (less than 105 kJ/kg) is limited by its boiling point when the temperature change is ~25 ℃ per heat exchange pass.

Compared to the phase change of vaporisation and condensation, changes between melting and crystallisation is applied in microcapsule slurries to improve the heat capacity of thermal fluids, and maintain good flowability when the phase change materials crystallise. In the last 30 years, extensive research into phase change microcapsule slurries have been carried out to investigate ways of enhancing heat transfer. Microencapsulation of phase change materials, their formulation into a liquid and enhancement of the heat transfer are the major topics for the development of phase change microcapsule slurries as thermal fluids. This review paper covers the development so far in microencapsulation of phase change materials, and includes critical analysis of the threshold limitations in applications of phase change microcapsule, under static condition for temperature management and in slurries as thermal fluids and energy storage media. The critical review of research findings will allow a step change in the forthcoming development of phase change materials for thermal energy applications.

2 Phase change materials and their microencapsulation

2.1 Phase change materials

Phase change materials (PCMs) are widely studied and used as energy storage materials due to its ability to absorb and release large amounts of latent heat when changing phase from solid to liquid, or liquid to solid. Generally, PCMs can be categorised as organic and inorganic materials, where organic materials are further described as paraffin and non-paraffin (fatty acids) and inorganic materials are made up of salt hydrates, eutectics and metallics as shown in Fig.1, and have been thoroughly reviewed in literatures [2-3]. The strengths and drawbacks for organic and inorganic PCMs are summarised in Table 1.

Table 1 Comparisons of organic and inorganic phase change materials

2.2 Microencapsulation of phase change materials

Microencapsulation of phase change materials require the breakage of a PCM from bulk phase down to particles in the micrometres range, and then covered with a layer of solid material to create individual solid microcapsules. Achieving the desired and uniform sized microcapsules with uniform thickness of the solid shell layer are challenges faced in microencapsulation engineering. These are addressed by physico-chemical sciences and engineering. To eneable their utilization in thermal energy systems, the shell ideally needs to be produced as form- or shape-stable to keep their shape while undergoing phase changes from solid to liquid. It also minimises or prevents the loss of PCM from the microencapsulation structure for a long period of time. The following are some key requirements for phase change microcapsules to be successful in thermal energy applications:① Mechanically robust in structure; ② Desired sizes and uniformity in size; ③ High encapsulation ratio for good loading of phase change materials. The encapsulation ratio can be found with, ΔMEPCMthe melting latent heat of the microcapsules and ΔPCMthe melting latent heat of the PCM material.

④ Compatibility with the dispersion medium; ⑤ Desired thermal conductivity of the materials;⑥ Low cost for commercial production

2.2.1 Structure of phase change microcapsules

Individual microcapsules of a phase change material are covered by a layer of polymer or inorganic materials. Spheres are the most popular shape with varing internal structures as shown in Fig.2. Spherical microcapsules can have structures such as mononuclear, also called core/shell structure, polynuclear or matrix. Mononuclear microcapsules are composed of a shell of uniform thickness from a single or several distinct layers of shell, polynuclear (or multi-core) structures have multiple core particles within the shell and matrix capsules have the phase change material embedded in the shell material.

The different microcapsule structures can be designed and obtained by formulation to use different physico-chemical properties of the PCM and the shell material through different microencapsulation techniques.

Among these structures, the mononuclear structure can give the highest encapsulation ratio of phase change material; the thinner the shell, the higher the encapsulation ratio. However, when the shell is very thin, mechanical strength and shape-stability may be compromised especially when polymer materials are used. The microcapsules in polynuclear and matrix structures can be more advantageous in mechanical strength and, size and shape stability, but their encapsulation ratio is lower.

2.2.2 Encapsulation methods of phase change materials

Characteristically microencapsulated phase change materials are in the range of 1—100 μm. One can manufacture microcapsules of phase change material by coating particles in a solid state or droplets in a liquid state. To coat individual PCM micro- particles with a continuous shell, the physical and chemical properties of the shell material is important for successful microencapsulation, especially to ensure suitability for thermal energy applications later on. Technologies such as pan-coating and air suspension coating are established encapsulation methods preferred for use in the pharmaceutical, food, ceramic and paint industries for its operational low cost. Both technologies coat the solid core particles with a melted shell material, hence the melting temperature of the shell material is lower than that of the core material. This makes methods like pan-coating and air suspension unsuitable for encapsulating phase change materials as the melting temperature of the shell need to be higher than the core to contain the PCM during its melting/ crystallisation cycles without leaking. The following are common microencapsulation techniques developed for phase change materials, which take liquid droplets as templates. ① spray drying; ② coacervation; ③ in-situ polymerization (suspension and interfacial polymerization); ④ sol-gel; ⑤ Pickering emulsion.

Using these methods, the phase change material is firstly formulated as a dispersed phase to be added to a continuous phase, which is an immiscible liquid. The dispersed PCM droplets are then encapsulated by the shell material, which are either long chain polymers dissolved in the continuous phase (spray drying and coacervation), or monomers as precursor of polymers to be polymerised in-situ, or solid particles to be arranged on the surface of the phase change material droplets (sol-gel and Pickering emulsion).

2.2.2.1 Spray drying

Spray drying is a physical technique used commercially to produce phase change microcapsules for its low operational costs [4-5]. The process feed is made up of the phase change material emulsion which contains the shell material dissolved in the continuous phase. The emulsion forms complex droplets when sprayed into a heated drying chamber. Solvent in the droplet vaporises in the hot chamber, leaving the shell material on the external surface of the droplets, as schematically summarised in Fig.3. A spray nozzle called an atomizer is used to spray the emulsion into the hot gas phase. The hot gas flows into the chamber at a temperature sufficient for complete evaporation of the solvent. Consequently, the emulsion droplets decrease in size and a solid shell forms until the solvent is completely evaporated. The solid microcapsules are collected from a cyclone or filters downstream.

Using spray drying, the microcapsule size can be controlled by the size and structure of the nozzles in the atomizer, controlling the spray and gas flow velocity and the temperature inside the drying chamber. The shell thickness is sensibly controlled by the emulsion concentration and the concentration of polymers in the continuous phase. Due to the possibility that the emulsion droplet can contain more than one droplet of phase change material, the microcapsules can form polynuclear structures. The solvent evaporation is normally fast so it is difficult to avoid droplet agglomeration or coalescence to form uniform particle sizes and structures. This technique normally results in a wide size distribution of microcapsules. The encapsulation of® RT27 paraffin wax with polyethylene-ethylene-vinyl alcohol as the shell material yielded an encapsulation ratio of 63% reported by Borreguero et al [4].

HAWLADER et al [5] utilised the spray drying technology in the microencapsulation of paraffin wax with gelatin and acacia. They investigated the effect of varying the core to shell ratio on the energy storage and release capacity. The paraffin wax microcapsules produced were spherical particles with smooth surface and fairly uniform sizes. With a core to shell ratio of 2∶1, the paraffin wax microcapsules showed high energy storage and release of 216.44 kJ/kg and 221.52 kJ/kg, respectively.

By spray drying, it is possible to include other solid particles in the microcapsule, for example, carbon nanofibers have been successfully included to enhance the thermal conductivity of the shell [4]. Thermal analysis showed the presence in the phase change microcapsules maintained the thermal energy storage capacity, enhanced its thermal conductivity as well as improving the stiffness of the microcapsules. Life cycle tests showed the encapsulated paraffin wax melting and solidifying reversibly over 3000 times, suggesting a possible continuous usage lifecycle of 30 years, making it a good candidate for applications in thermal energy storage.

2.2.2.2 Coacervation

Coacervation, also called phase separation, is well studied for applications in the food and drug industries with the purpose of increasing shelf-life, allowing alternative food processing, masking the taste or controlling their release [6]. The coacervaton method usually utilises polyelectrolites as shell materials. The microencapsulation is achieved by dissolving the shell material in the continuous phase of the phase change material emulsion. After the core material is well dispersed by emulsification, coacervation is induced by the addition of opposite charged electrolytes and to change the solubility of the dissolved polymer chains in the solvent. The polymer chains being salted out adsorb onto the phase change material droplet for the encapsulation to conclude. Afterwards, the shell can be enhanced and hardened for strength and resistance to solvents. Fig.4 shows a typical encapsulation path.

Like the spray drying technique, an oil-in-water (o/w) emulsion is prepared as the initial step for coacervation. The phase change material (in oil form) is mixed with an aqueous gelatin solution, for example. After emulsification, a second aqueous solution of gum acacia is added while the pH of the solution is adjusted to be in the range of 4—4.6 with salts, dilution, or temperature changes [5,7]. This enables the adsorption of a shell around the phase change material droplets. Finally, the microcapsules are formed from crosslinking, dessolvation or thermal treatment [8-9]. Formaldehyde or glutaraldehyde are applicable cross-linking agents to react with active hydrogen groups in the polymer [11-12]. MALEKIPIRBAZARI et al [7] prepared microcapsules of paraffin with gelatin/gum acacia through the coacervation method. The microcapsules had average diameters of 300 µm with an energy storage capacity of 116 kJ/kg. HAWLADER et al [5] compared microencapsulated paraffin wax with gelatin and acacia obtained from coacervation with the spray drying method. For core to shell ratio of 2∶1, microcapsules obtained from coacervation have energy storage and release capacities of 239.78 J/g and 234.05 J/g, respectively, which are higher than that from spray drying.

BAYES-GARCIA et al [10] microencapsulated commercial phase change material® RT 27 (paraffin wax) with shell materials Sterilized Gelatine/Arabic Gum and Agar-Agar/Arabic Gum. The encapsulation ratios were 48%—49%. OZONUR et al [11] encapsulated natural coco fatty acid mixtures using gelatin–gum Arabic by the coacervation method. The authors noted the importance of emulsification on the success of microencapsulation, and gelatin-gum Arabic was found to be the best shell material for the coco fatty acid mixture. The microcapsule retained its physical shape after 50 melting and solidifying cycles showing its suitability for use in a thermal energy storage system within a temperature range of 22—34 ℃.

Encapsulation by spray drying and coacervation use dissolvable polymers, and very often water soluble polymers as the shell material. The shell formed may swell or dissolve when subjected to moisture or water due to the polarity of the chemical structure of the polymer chain. In-situ polymerisation can be used to generate non-dissolvable polymers as shell materials.

2.2.2.3 In-situ polymerization

In-situ polymerization is one of the most widely investigated chemical techniques for microencapsulation of PCMs. The method is initiated by dispersing monomers, the precursor of polymer, in the phase change material emulsion. When the monomer is in the dispersed phase, suspension polymerisation follow to form matrix structured microcapsules, or core/shell structured microcapsules if phase separation is successfully achieved as shown in Fig.5. For condensation polymerisation, two or more reactants are involved. One reactant is arranged in the continuous or dispersed phase, and the reactants will meet and react at the droplet surfaces. Following this, interfacial polymerisation will occur.

Different polymer systems have been used in the encapsulation of phase change materials, as reviewed by JAMEKHORSHID et al [12]. Poly(methyl methacrylate), poly(melamiformaldehyde), poly urea, poly(urea formaldehyde) and polystyrene[13] are among the most popular studied. Poly(methyl methacrylate) and polystyrene are polymerised from monomers that have carbon-carbon double bonds via radical polymerisation. Monomers in phase change material droplets are commonly used to encapsulate the phase change material by suspension polymerisation. Poly(melamiformaldehyde), poly urea and poly(urea formaldehyde) are made from two or more different reactants by polycondensation. Interfacial polymerisation is normally involved for encapsulation. Recent development has moved a step forward from using single or simple co-polymers to using polymer hybrids or more complex chemical structures for the encapsulation, in order to achieve better control over the encapsulation ratio, leakage, mechanical strength, size and more importantly, enhancement of thermal conductivity [7,14-19].

QIU et al [17]polymerised ethyl methacrylate withacrylic acid and styrene in the presence of trimethylolpropane-triacrylate as a crosslinking agent to encapsulate paraffin. The prepared microcapsules exhibited higher phase change enthalpies of melting (117.8 kJ/kg) and crystallisation (115.3 kJ/kg) compared with the microcapsule without polystyrene. TG analysis demonstrated that the thermal resistance of the phase change microcapsules produced were 10℃ higher than that of the raw paraffin. The loss of phase change material content was~6% after 1000 heating and cooling cycles.

KONOKLU et al [20] successfully microencapsulated caprylic acid (octanoic acid) using urea-formaldehyde as shell material and achieved an encapsulation ratio of 59.3%. Urea-formaldehyde shell was also cross linked with poly(methyl methacrylate) to form double layer encapsulated n-tetradecane microcapsules [18]. The hybrid shells were cross-linked by n-methylol acrylamide or hydroxymethyl. The microcapsules have an enthalpy of 175.5 kJ/kg which is higher than the single-layer shell samples of poly(methyl methacrylate) or urea-formaldehyde, and enhanced leakage prevention. Thermal tests of 100 heating and cooling cycles showed good chemical stability and thermal reliability of the hybrid shell.

Double shell microcapsules of n-octadecane were prepared from ethylenediamine, 2,4-toluene diisocyanate, melamine and formaldehyde by interfacial polymerisation [19]. With optimised compositions, the outer shell surface was smooth and displayed high thermal resistance up to 250 ℃ and energy storage/release capacity about 180 kJ/kg.

More interestingly, small solid particles such as silicon nitrile [21-22], graphite [23], expanded graphite [24], graphite oxide [25] and silicon carbide [26] have been incorporated into the polymer shell via Pickering emulsion. SUN et al [21] prepared paraffin wax-based phase change microcapsules with silicon nitride nanoparticles embedded in the shell. The oil phase of melted paraffin and monomers were emulsified in an aqueous suspension of nano-Si3N4. The phase change material induced polymerisation and phase separation with the nano-Si3N4stabilised droplets to form phase change microcapsules embedded with nano-Si3N4in the polymer shell. The oil phase of melted paraffin and monomers were emulsified in an aqueous suspension of nano-Si3N4. The composite PCMs formed have well-defined spherical structures and significantly enhanced thermal conductivity due to the presence of nano-Si3N4. YANG et al [22] successfully supplemented polymethylmethacrylate silicon nitride particles by suspension polymerisation. The silicon nitride particle surface was modified and well bonded within the polymer shell. The microcapsules showed regular spherical shapes with well-defined core and shell structure. The addition of silicon nitride enhanced the thermal conductivity by 56.8%.

The composite phase change microcapsules formed have well-defined spherical structures and significantly enhanced thermal conductivity due to the presence of nano-Si3N4. YANG et al [22] successfully supplemented polymethylmethacrylate silicon nitride particles by suspension polymerisation. The silicon nitride particle surface was modified and well bonded within the polymer shell. The microcapsules showed regular spherical shapes with well-defined core and shell structure. The addition of silicon nitride enhanced the thermal conductivity by 56.8%.

Graphite and its derivatives are well known for their high thermal conductivity. Their nanoparticles were used to modify phase change microcapsules for better thermal conductivity [23]. The graphite nanoparticles were embedded in the prepolymer of melamine-formaldehyde to form a shell around the paraffin core after polymerisation. DSC tests showed the melting temperature and latent heat were 50.5 ℃ and 90.8 kJ/kg respectively; in which the mass ratio of paraffin in the microcapsule was calculated to be 51.1%. The phase change microcapsules were dispersed into ionic liquids forming a novel latent functional thermal fluid. The fluid’s thermal storage capacity was two times greater than that of the pure ionic liquid.

ZHANG et al [25] prepared microencapsulated n-hexadecane with a polystyrene/graphene oxide composite shell by using Pickering emulsion as the template. Graphene oxide was modified using [2-(methacryloxy) ethyl]-trimethylammonium chloride or decyltrimethyl ammonium chloride to stabilize oil-in-water Pickering emulsions. Thermal analysis showed an enhancement in the evaporation activation energy for n-hexadecane from 32.2 kJ/mol to 60.9 kJ/mol due to restrictions of the polymer/graphene oxide composite shell.

WANG et al [24] developed a new type of phase change composites with a double-layer network to enhance thermal conductivity and thermal stability. Different mass fractions of expanded graphite were added in the microencapsulated phase change materials. The phase change composite with 20% (weight fraction) expanded graphite showed distinct carbon network structure and had up to 7.5 times increase in the thermal conductivity comparing to that of the pure paraffin. The samples showed negligible change in thermal properties after 100 times thermal cycling and 7days serving durability tests. The enhancement on thermal properties of the phase change composites is a promising route to achieve high energy storage efficiency targets for different thermal applications.

More interestingly, ZHANG et al [27] have synthesized a novel type of multifunctional microcapsules based on n-eicosane core and a silver/silica double-layered shell through interfacial polycondensation followed by silver reduction. The resultant microcapsules showed the regular spheres with a well-defined core-shell structure and a silver shell. The microcapsules had a perfect silver shell when the reaction time for reduction and deposition of silver ions was set to 20 h. The microcapsules contained about 67% (weight fraction) of n-eicosane core. The microcapsules had high latent heat storage and release efficiencies, and displayed good thermal regulating capability. Most of all, the microcapsules achieved a high electrical conductivity of 130 Ω·m.

2.2.2.4 Sol-gel

Microencapsulation through sol-gel is a physico-chemical method. It involves the formation of a colloidal solution (sol) through polycondensation of a molecular precursor in a liquid phase which transforms into an oxide network (gel). The gel self-assembles onto the surfaces of PCM droplets [13,29]. Organo-metal compounds of such as silicon and titanium are normally used as the precursors for the formation of silica and titanuium oxide [13, 28-29]. Such phase change microcapsules can have better shape stability than polymeric ones.

JIN et al[28] prepared n-octodecane/silica microcapsules by the-hydrolysis of methyl trimethoxysilane and 3-aminopropyl trimethoxysilane. The core of n-octodecane was covered by a monolayer of silica nanoparticles as shell, giving an encapsulation ratio of 65%. An optimum encapsulation ratio of 87.5% was achieved by FANG et al with paraffin as the core [30]. Methyl triethoxysilane and tetraethyl orthosilicate were also successfully used in the encapsulation of octadecane [31] and montmorillonite/ stearic acid emulsion [32], respectively. The former showed a melting temperature of 28.32 ℃ with a latent heat of 227.66 kJ/kg, and solidifying temperature at 26.22 ℃ with a latent heat of 226.26 kJ/kg. The microcapsules obtained showed good thermal stability and performed well in preventing leakage of the octadecane core.

Microencapsulation of phase change materials using titanium dioxide (TiO2) nanoparticles is of great interest for applications in solar energy recovery. Microencapsulation of palmitic acid with TiO2as shell material showed low encapsulation ratios [29], CAO et al. reported an encapsulation ratio of 30.4% with the use of TNBT and anhydrous ethanol as the precursor solution. The encapsulated particles were in the size range of 200—400 nm. Analysis of the TGA and DTG graphs showed the TiO2shell improved thermal stability of the composite phase change material. Low encapsulation ratio of 22.5% was obtained using TiO2as shell to encapsulate n-octadecane [33]. The microcapsules were on average 2—5 µm in diameter with high thermal stability and good thermal reliabilities.

2.2.2.5 Development trend of phase change microcapsules

Phase change microcapsules have been developed for the feasibility of enhancing the PCM’s heat storage capacity for better temperature management, or for more efficient heat transportation and storage. The heat storage capacity is directly determined by the amount of phase change materials in the microcapsule. Whereas the heat transfer rate and efficiency are related to the phase change microcapsule thermal properties , its usage and engineering conditions. The low thermal conductivity and shape stability of organic polymer materials are often the critical constraints that hinder the performance of phase change microcapsules. Recent trends in phase change microcapsule development has addressed some critical constraints by incorporating high thermal conductive materials such as metal oxides, carbon material to create hybrids with the organic polymers. Inorganic materials are also used widely to encapsulate phase change materials. GONDORA et al [34] used Rice-husk-char, a by-product in biofuel production of carbon-based particles in the encapsulation of hexadecanePickering emulsions. Polymers, poly(diallyldimethyl-ammonium chloride) and poly(sodium styrene sulfonate) were used to fixate the rice-husk-char nanoparticles on the emulsion droplets through layer-by-layer assembly to enhance the structural stability of the microcapsules. DAO et al [35] functionalized graphene sheets with poly(vinyl alcohol) and dispersed them in water. The aqueous graphene suspension was used to stabilise stearic acid droplets using the Pickering emulsion method. Properties of the graphene shell ensured it effectively serves as a protective layer for the SA core to improve the thermal stability of the composite. WANG et al [36] microencapsulated phase change materials with binary cores and a calcium carbonate shell via a self-assembly method. The thermal conductivity of the microcapsules was significantly enhanced due to the highly conductive calcium carbonate shell. The phase change temperature of the microcapsules could be adjusted from 25℃ to 50 ℃ by changing the ratio of binary cores.

Novel sodium nitrate microcapsules [37] were also synthesised for high-temperature thermal energy storage. Perhydropolysilazane, a ceramic precursor resin with outstanding high-temperature resistance, was used as the shell material to prepare novel sodium nitrate microcapsules (MCP-NaNO3) for high- temperature thermal energy storage. The MCP-NaNO3had a thin shell, and the weight percentage of NaNO3in the MCP-NaNO3was about 85%. The MCP-NaNO3presented a melting point of 306 ℃ and latent heat of 159.2 kJ/kg. The melting point of MCP-NaNO3had almost no change, and the supercooling of MCP-NaNO3increased slightly by 2.7 ℃ compared with that of NaNO3. The thermal decomposition temperature of NaNO3in the MCP-NaNO3was enhanced significantly to 647.6 ℃. The PHPS shell was sufficient in ensuring the structural stability of MCP-NaNO3after being heated at 350 ℃. The MCP-NaNO3developed in this study has great promise in future energy and chemical processes.

The cost associated with using phase change microcapsules is a significant challenge to overcome in order to improve their market status. The production cost of phase change microcapsules is composed of materials and production. Material selection can contribute significantly to the lowering of the total cost, especially when more complex structure is designed for enhanced performance in applications. Recent attempts such as encapsulating PCMs using rice-husk-char from biofuel production represents new development aiming to lower the cost and obtain better thermal conductivity and compatibility to inorganic building materials [34]. Spray drying and polymerisation are technologies being developed for large scale production. The latter has been used in the commercial production such as Micronal® phase change microcapsules by BASF. Thorough and careful process and detailed designs for engineering is crucial for the control over product quality and processing costs.

3 Phase change microcapsules applications under static conditions

Typical applications of phase change materials under static conditions are for smart temperature management in building and textile materials. Paraffin wax with desired melting temperatures is popularly used as phase change materials in these applications. Applications in building materials are the most attractive and best developed area, where phase change microcapsules as additives are added into conventional building materials. The relevant research and development has been reviewed in [38]. Incorporating phase change microcapsules into interior building walls can lower heat charge and discharge frequency for better energy utilization efficiency, if the melting temperature is close to that of comfortable living temperatures.A well known-development is Micronal® phase change microcapsules. Their use in a stagnant building space has proved benefits of this application[39-41]. CASTELLON et al built small test houses using concrete incorporated with Micronal® phase change microcapsules. The test showed a 4℃ difference with addition of the phase change microcapsules (melting point of 26℃ and a phase change enthalpy of 110 kJ/kg) comparing to its counterpart without the incorporation [41]. A two-hour delay of the peak temperature was also recorded.

Textile materials are fibre based, hence suitable for fabric production, paraffin wax such as-hexadecan,-octadecane,-nonadecane have been used as PCMs for their desired melting points. Polyurea and polyurethane are commonly used shell materials. Incorporation of phase change microcapsules changes the energy absorption capacity dramatically. The enhancement can be 2.5—4.5 times of that of the reference fabric at given temperature intervals [42-43].

4 Phase change microcapsule slurries

The use of solid particles to enhance the heat transfer of working fluids has been a long-standing research topic since the 1950s [44]. Existence of solid particles in fluids has shown significant enhancement in thermal conductivity due to microconvection of the particles in the suspension [45]. Early and more recent experimental results have demonstrated that the enhancement depends on the composition, concentration and size of particles, flow pattern and kinematic viscosity of the fluids, thermal diffusivity of the suspending liquid and geometry of pipes [46-47]. The inclusion of phase change material microcapsules introduces solid particles in the fluid as well as a large amount of latent heat that accompany the phase change. The phase change interferes with temperatures of both the slurry and the heat transfer at the surface. Significant changes in the microcapsule volume and viscosity of the slurry is normally expected as well. These result in continuously changing flow patterns, which is difficult to measure accurately. This situation increases the complexity and difficulty of the topic and has continuously attracted more research for better knowledge and designs. In published works, different algorithms have been used to approximate the local temperature, fluidic and thermal properties of the slurry in order to examine the fluid structures and the inclusion effect of phase change material microcapsules on their heat transfer performance.

Parallel and counter current flows are two ideal flow patterns used in heat exchanger designs for forced convective heat transfer. The heat transfer in parallel flow is close to constant wall temperature conditions, while the heat transfer in counter current flow is close to the constant heat flux condition in the precondition that the heat capacities of the two fluids do not differ greatly [48]. The heat capacity here is the product of the fluid mass flowrate and specific heat content. The flow and heat transfer characteristics of phase change microcapsule slurries have been examined under constant temperature and constant heat flux provided by electrical heating.

4.1 The hydrodynamics of phase change microcapsule slurry

The addition of phase change microcapsules in water cause significant changes in the hydrodynamics of the slurry fluid, but it does not affect the steady state flow when no heating is applied. Fig.6 shows the experimental results by YAMAGISHI et al [49]. using aqueous slurries containing microencapsulated phase-change materials with diameter of 2—10 µm in a circular tube. The inclusion of the microcapsules resulted in an increase in pressure drop and decrease of Reynolds number with the microcapsule fraction. When the microcapsule fraction is high, i.e. 0.3 in volume fraction, the pressure drop showed an irregular pattern due to the significant increase of viscosity, which keeps the flow in the laminar and transient region as the flow velocity increases. The friction force in the laminar to turbulent region follows the same trend in relation to the Reynolds number.

When heat is provided, the phase change material inside the microcapsule will gradually melt resulting in continuous expansion. The expansion increases the volume fraction of the particulate material in theslurry, so that the local pressure drop decrease significantly at the point where the phase change microcapsule melted [51-52].

（a）

（b）

Fig.6 (a) The variation of pressure drop with average flow velocity and (b) the relationship between the friction force and Reynolds number of pure water and phase change microcapsule (2—10 µm) slurries at 298K[49]

4.2 Temperature profile

Temperature is one of the most sensible parameters in examining heat transfer efficiency. Under ideal conditions, phase change microcapsules are solid from inside out when the bulk stream temperature is lower than the melting point. As the temperature rises to the melting point, the phase change material starts to melt, and the temperature will be maintained at the melting point. Following which the bulk temperature will increase again. In this case, a temperature profile shown in Fig.7 can be seen, and the heat transfer can be classified as three regions by the state which the phase change material in the microcapsule is in solid (region I), solid and liquid mixture during melting (region II) and liquid (region III).

A parametric analysis by ZHAO et al[48] demonstrated the melting region of the phase change materials in slurry at constant temperature condition. Under laminar flow conditions (=200—600), the influence of the parameter is in the order of Reynolds number, the bulk Stefan number and the concentration of phase change microcapsules, Fig.8. The length of the melting region in the duct increases proportionally with Reynolds number in the examined laminar flow region, the concentration of microcapsules and inversely proportional with the bulk Stefan number.

The bulk Stefan number is the ratio of sensible heat content to the melting heat of the bulk fluid.

（a）

（b）

（c）

Fig.8 Influence of (a) bulk Reynolds number, (b) bulk Stephan number and (c) volumetric particle concentration on phase-change region for the microencapsulated suspension[48]

whereis the specific heat. It is the specific heat of solid phase in the freezing process, and the specific heat of liquid phase in the melting process. ∆is the temperature difference between phases, andis the latent heat of melting.

The three-region model was confirmed by experimental results by YAMAGISHI et al [49]. The length of the melting region in the duct is determined by the heat transfer rate. The PCM starts to melt when the temperature reaches to its melting temperature, and become fully melt when the heat transferredreaches the heat required to melt all the phase change material if supercoiling is negligible. When a larger heat flux is used, the initial melting point of a slurry appears earlier with a shorter melting region length compared to that at a lower heat flux, shown in Fig.9(a). However, when both the heat flux and the phase change microcapsule concentration is doubled, the melting position was delayed and melting region length dramatically shortened, as in Fig.9(b). More interestingly, the bulk temperature rises significantly higher than when the phase change material is fully melted, Fig.9(c). More heat is carried by sensible heat, resulting in significantly higher temperature than predicted based on complete phase change. These phenomena reflect not only the heat transfer capacity/efficiency across the heat transfer surface, but also suggest that melting and heat transfer rates from the bulk fluid into the microcapsules can be a limiting factor preventing the full capacity utilization of phase change materials in the slurry.

When constant heat fluxes are used in the test, the heat transfer enhancement of phase change material slurries are reflected as temperature reduction of the bulk fluid or heat transfer wall due to the latent heat storage. The temperature reduction of the fluid comparing to that of water is related to the heat flux and the latent heat capacity of fluids, which is determined by either PCM concentration in the slurry or by the flow rate of the slurry [50], as shown in Fig.10(a) and Fig.10(b) for the local temperature or the overall mean temperature reduction. The wall temperature reduction in the melting region tends to be constant.

By numerical modelling and experimental work, CHARUNYAKORN [51] and GOEL et al [52] examined the relationship of wall temperature reduction in laminar flow along the heat transfer duct with properties of the phase change microcapsule slurry. Studies predict the use of phase change material suspensions can reduce rise in wall temperature by up to 50% as compared to a single phase fluid for the same, and heat fluxes could be approximately 2—4 times higher than single phase flow based on available analytical and experimental results of the fluid properties and energy conservation[51,53] The enhanced heat flux is expected to be reflected in enhanced heat transfer coefficients when the temperature difference is not altered dramatically.

4.3 Heat transfer coefficient

The heat transfer coefficients reported are mainly measured in constant flux tests. They are calculated as local heat transfer coefficients,h, through dividing the heat flux applied,q, by local temperature differences,.

Different methods from theoretical calculation based on energy balance to experimental calibration have been used to obtain more accurate temperatures and heat fluxes. CHOI et al [54] studied the heat transfer of phase change-material emulsions in a long horizontal circular duct in the turbulent flow region with constant heat flux. The local convective heat transfer coefficient varied significantly when the particles melted, hence applying the LMTD method to analyse the heat transfer was difficult. Local heat transfer coefficients of phase change microcapsule slurries have been successfully obtained [49-50,55-56]. The heat-transfer performance is determined by the flow region of the slurry and closely related to the change of flow structure. In the laminar and transient flow region, the local heat transfer coefficient is generally higher than that of pure water. In the case of turbulent flow, the heat transfer performance of the slurry was significantly influenced by melting of the phase change materials and turbulence of the slurry. The local heat transfer coefficient increased in the melting region relative to that in the non-melting regions, as represented in Fig.11, but generally lower than that of pure water at the same flow velocity.

DELGADO et al’s recent work showed approximately 25% enhancement of the local convective heat transfer coefficients in the melting region of laminar flows compared to that of water [50]. Outside of the melting region, little enhancement showed in the region I. A declining trend appeared at the end of the region II, where the PCM in the microcapsule is close to melting fully. This might be explained by an apparent local specific heat content that was enhanced by the melting of phase change materials in the slurry.

The heat transfer efficiency of phase change microcapsule slurries largely follows that of their counterpart of pure water, which is closely determined by the flow pattern and thermal properties of the liquid. The heat transfer is much more efficient in turbulent flow than in laminar flow. Enhancement to the heat transfer of phase change materials are observed during melting. This is closely related to the bulk Stefan number and the fraction of microcapsules or phase change materials, the latter also determines the heat storage capacity of the fluids. Limited experimental results showed that the inclusion of phase change microcapsules resulted in little enhancement in region I (solid) and region III (liquid) [49-50,55-56]. Considering the effect of viscosity change due to particle inclusion, the heat transfer is not as good as that of pure water. This phenomenon does not comply with the research in the inclusion of other types of solid particles, which are usually more thermally conductive materials such as metal oxides [57] and carbon nanotubes [47]. However, the studies of phase change microcapsules in slurries for convective heat transfer are dominated by less conductive organic materials. Table 2 lists the phase change microcapsules used in the convective heat transfer reviewed here.

Table 2 Phase change material microcapsules used in the investigation of thermal fluids

Different phase change materials have been investigated, but all of them are organic oils such as paraffin or fatty acids. The shell materials are organic polymers, mainly formaldehyde resins. Such phase change microcapsules have low thermal conductivity and shape stability. Their inclusion results in the lowering of thermal conductivity of pure water and a continual increase of the viscosity when heat is transferred to the slurry, deteriorating the thermal property and flow pattern in order to achieve more efficient heat transfer. Modelling temperature changes inside the phase change material microcapsules during melting by a moving-boundary model showed that the melting time can be minimized through reducing the capsule diameter and increasing the thermal conductivity of the PCM [7]. Insufficient considerations for the thermal properties of phase change microcapsules may be responsible for the limited enhancement in convective heat transfer.

5 Summary remarks

The distinctive benefits of phase change microcapsules have inspired researchers to design different micro-structures for desired properties. The primary properties that researchers seek to achieve in their development to meet specific applications have been high heat content, desired shell chemistry for tight encapsulation, good mechanical strength and long-term stability in chemistry and shape. Building upon this, high thermal conductivity, high specific heat capacity and low expansion ratio are pursued to enhance heat transfer and minimise the undesired impact. The progress towards this direction has seen the impressive development of phase change microcapsules to be utilized in thermal engineering for improved energy efficiency such as those produced commercially for use in construction and fabrics.

In comparison, development in heat transfer and heat storage for a convective application environment has been relatively limited. In terms of heat storage enhancement, the maximum is the same as in static conditions which is determined by the amount of phase change microcapsules and, the latent and sensible heat the microcapsule can carry in the application conditions. However, enhancement for heat transfer is conditional. In laminar flows, the convective heat transfer coefficient improved by ~20% in the melting region of the phase change material in comparison to that of water. When the flow moves to the turbulent flow region, the convective heat transfer coefficient was lower than that of water, although some improvement can be seen in the melting region of the phase change material. Little enhancement showed from the solid particle feature of phase change microcapsules in published results, which is in contradiction to that discussed in an earlier section in this review [44-45]. Two factors may be responsible for the result: ① The specific heat capacity of the phase change microcapsules used (organic oil and polymers) were lower than that of water; and ② the flow pattern has been drastically changed due to the significant expansion of the organic oil and polymer microcapsules melting. Therefore, to further the research in this area, phase change microcapsules with higher specific heat capacity, higher thermal conductivity and better shape stability are required. Recent development has seen successful formulations of more thermally conductive phase change microcapsules by using inorganic materials. Investigations into heat transfer in convective conditions using these more promising phase change microcapsules are expected in the coming years.

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2017-05-15.

YUAN Qingchun, lecturer in chemical engineering, research in the development of advanced materials for energy and environment applications. E-mail: q.yuan@aston.ac.uk.

10.12028/j.issn.2095-4239.2017.0076

TK 02

A

2095-4239（2017）04-607-16

The first author: XIAO Anna (1991—), female, project engineer, interested in new technologies and materials, sustainable energy and applications. E-mail: anna.x.xiao@gsk.com;