Micro-Encapsulated Phase Change Materials : A Review of Encapsulation , Safety and Thermal Characteristics

Phase change materials (PCMs) have been identified as potential candidates for building energy optimization by increasing the thermal mass of buildings. The increased thermal mass results in a drop in the cooling/heating loads, thus decreasing the energy demand in buildings. However, direct incorporation of PCMs into building elements undermines their structural performance, thereby posing a challenge for building integrity. In order to retain/improve building structural performance, as well as improving energy performance, micro-encapsulated PCMs are integrated into building materials. The integration of microencapsulation PCMs into building materials solves the PCM leakage problem and assures a good bond with building materials to achieve better structural performance. The aim of this article is to identify the optimum micro-encapsulation methods and materials for improving the energy, structural and safety performance of buildings. The article reviews the characteristics of micro-encapsulated PCMs relevant to building integration, focusing on safety rating, structural implications, and energy performance. The article uncovers the optimum combinations of the shell (encapsulant) and core (PCM) materials along with encapsulation methods by evaluating their merits and demerits.


Introduction
Phase change materials (PCMs) are considered to be potential energy saving materials since, due to latent heat absorption, they can substantially increase the thermal mass of buildings compared to conventional building materials presented in Figure 1 [1].

Introduction
Phase change materials (PCMs) are considered to be potential energy saving materials since, due to latent heat absorption, they can substantially increase the thermal mass of buildings compared to conventional building materials presented in Figure 1 [1].Phase change materials have been extensively reviewed in terms of their classifications, thermo-physical properties, applications and impact on the energy performance of buildings [2][3][4][5][6].The previous reviews have mainly identified three types of PCMs for incorporation into buildings to reduce their energy consumption, namely paraffin waxes, salt hydrates and fatty acids, summarized in Figure 2. Phase change materials have been extensively reviewed in terms of their classifications, thermophysical properties, applications and impact on the energy performance of buildings [2][3][4][5][6].The previous reviews have mainly identified three types of PCMs for incorporation into buildings to reduce their energy consumption, namely paraffin waxes, salt hydrates and fatty acids, summarized in Figure 2. As the concept has developed, the more recent reviews have focused on the improvement in PCM properties, different ways of integration into buildings and PCM behaviors in a built environment.Liu et al. reviewed the thermal conductivity enhancement to increase the charging/discharging rate of the PCM used in building application to increase energy efficiency and achieve better indoor thermal comfort.This review summarized the carbon based and metal based conductivity enhancers.Carbon based additives include carbon fibers, Nano-tubes, carbon foam, graphite, graphite powder, graphene.Metal based additives include particles and or containers of aluminum, copper, nickel and gold particles and metal salts.This review also compiled the models to calculate thermal conductivity [7].J. Pereira da Cunha and P.C. Eames reviewed microencapsulated PCMs with a melting point range of 0 °C to 250 °C including organic PCMs, salt hydrates and eutectics as heat transfer fluids in heat exchangers.This review summarized the applications of encapsulated PCMs in active heating/cooling systems as a means to increase heat transfer rates [8].M. Jurkowska and I. Szczygieł reviewed PCMs as heat transfer enhancers prepared as slurries and suspensions.They particularly focused on the stability, durability and hydrodynamic properties of such PCMs [9].Akeiber et al. summarized different types of phase change materials for building applications with the melting points within the thermal comfort zone.They reviewed thoroughly the integration of PCMs into the building components i.e., walls, roofs, windows and floors [10].A. Kylili and P.A. Fokaides reviewed life cycle assessment of PCMs for building integration.Their work includes sustainability and environmental impact of these materials as compared to the conventional buildings [11].Zhang et al. reviewed PCMs for thermal energy storage and thermal management applications mainly focusing on modeling of the effective thermal conductivities and methodologies to enhance heat transfer rates [12].Silva et al. reviewed the application of PCMs in the windows and glazing of the buildings as an approach for building integration [13].The previous reviews do not identify a suitable microencapsulation process for a particular PCM type, the effect of PCM toxicities on the built environment and the limitations of the PCM applicability into buildings considering the structural component.Conclusively, a comprehensive review is desired to identify the optimal microencapsulation process for a particular PCM.For example, a metallic shell would be less suited to encapsulate salt hydrates due to salt's affinity to corrode metals.On the contrary, a polymeric shell material is better suited for salt hydrates.On the other hand, the paraffin waxes, being non-corrosive, can be encapsulated equally in the metallic and polymeric shell.There is also a lack of safety related issues of the PCMs (such as toxicity), as an important design parameter when it comes to PCM integration into buildings, as dwellings would be exposed to PCMs over a As the concept has developed, the more recent reviews have focused on the improvement in PCM properties, different ways of integration into buildings and PCM behaviors in a built environment.Liu et al. reviewed the thermal conductivity enhancement to increase the charging/discharging rate of the PCM used in building application to increase energy efficiency and achieve better indoor thermal comfort.This review summarized the carbon based and metal based conductivity enhancers.Carbon based additives include carbon fibers, Nano-tubes, carbon foam, graphite, graphite powder, graphene.Metal based additives include particles and or containers of aluminum, copper, nickel and gold particles and metal salts.This review also compiled the models to calculate thermal conductivity [7].J. Pereira da Cunha and P.C. Eames reviewed microencapsulated PCMs with a melting point range of 0 • C to 250 • C including organic PCMs, salt hydrates and eutectics as heat transfer fluids in heat exchangers.This review summarized the applications of encapsulated PCMs in active heating/cooling systems as a means to increase heat transfer rates [8].M. Jurkowska and I. Szczygieł reviewed PCMs as heat transfer enhancers prepared as slurries and suspensions.They particularly focused on the stability, durability and hydrodynamic properties of such PCMs [9].Akeiber et al. summarized different types of phase change materials for building applications with the melting points within the thermal comfort zone.They reviewed thoroughly the integration of PCMs into the building components i.e., walls, roofs, windows and floors [10].A. Kylili and P.A. Fokaides reviewed life cycle assessment of PCMs for building integration.Their work includes sustainability and environmental impact of these materials as compared to the conventional buildings [11].Zhang et al. reviewed PCMs for thermal energy storage and thermal management applications mainly focusing on modeling of the effective thermal conductivities and methodologies to enhance heat transfer rates [12].Silva et al. reviewed the application of PCMs in the windows and glazing of the buildings as an approach for building integration [13].The previous reviews do not identify a suitable microencapsulation process for a particular PCM type, the effect of PCM toxicities on the built environment and the limitations of the PCM applicability into buildings considering the structural component.Conclusively, a comprehensive review is desired to identify the optimal microencapsulation process for a particular PCM.For example, a metallic shell would be less suited to encapsulate salt hydrates due to salt's affinity to corrode metals.On the contrary, a polymeric shell material is better suited for salt hydrates.On the other hand, the paraffin waxes, being non-corrosive, can be encapsulated equally in the metallic and polymeric shell.There is also a lack of safety related issues of the PCMs (such as toxicity), as an important design parameter when it comes to PCM integration into buildings, as dwellings would be exposed to PCMs over a longer period of time and must be assured not to contract toxic elements during material preparation, construction and post-construction occupancy.
This current review is on the relevant and relatively less explored issues related to safety via toxicity, chemical interaction related properties (such as corrosion), suitability of the particular microencapsulation process for the specific PCMs and their structural and thermal performances.The review would evaluate PCMs' integration into buildings from a scientist's, designer's, construction manager's and policy maker's point of view, in view of their pre and post integration implication on buildings.The PCMs are critically reviewed for their suitability in building integration starting from PCM properties.A comprehensive description and detailed comparison of the encapsulation processes that would eventually affect their building integration in terms of structural performance of buildings is provided.The review links suitable types of shell material and encapsulation processes for certain core materials.The performance of PCMs, specific to PCM types integrated into building components, is summarized.
For predominantly hotter climates, PCMs can be selected with the melting point below the upper range of comfort zone so that it can melt during daytime and absorb excess thermal energy to restrict indoor temperature close to comfort.Similarly, in predominantly colder climates, the PCM melting point should be above the lower limit of the comfort zone to store and supply heat for space heating near comfort [14].In the case of improper selection of a PCM melting point, with regard to the ambient temperature at nighttime, PCMs may not regenerate to solid, and absorb heat during the next day, thereby undermining their effectiveness [15,16].
Initially, PCMs were integrated into buildings by impregnation [17][18][19], immersion [20] or direct incorporation [21] of PCM into the porous aggregate of the concrete mix.The PCMs filled in porous aggregate can enhance concrete density and heat storage capacity resulting in enhanced thermal inertia [19].However, all these methods suffered through leakage of PCM with the passage of time [22].The leaked PCM would react with the building material leading to corrosion, along with affecting strength, thereby reducing building life [23].An approach to solve the aforementioned issue is to contain PCMs in a suitable shell material leading to the development of microencapsulated PCMs [24].
Encapsulation is a technique to cage the required material inside another shell material to achieve desired characteristics of preservation, time-dependent release of material, delivery of the substance to the specific target, reduction of reactions to the environment, prevention of corrosion, the stability of function and to facilitate the use of toxic materials [25,26].The caging shell covers and protects the core material from the external environment to facilitate the desired application.Microencapsulation technology is applied in major fields such as medicine, food preservation, thermal energy storage, cosmetics, textile, and defense [27][28][29][30][31].To benefit from this technique, technology was further improved to encapsulate such materials that are small in diameter, in the range of micrometers, thereby called microencapsulation [32].The characteristics desired or achieved through microencapsulation for building applications are summarized in Table 1.In the sections below, the characteristics of each class of core materials (PCMs) are discussed with suitable shell materials depending on different applications.

Optimum Core (PCM) and Shell (Encapsulant) Materials
The choice of a core material (PCM) depends on the application, with the heat of fusion, density, thermal conductivity, heat capacity, toxicity, corrosion and fire resistance being potentially important.Since different PCMs exhibit different toxicological and interaction properties, it is extremely important to evaluate such properties in order to identify suitable shell materials (encapsulant).The Sections 2.1-2.4 briefly review the PCM properties which impact on the choice of optimal encapsulant material.Section 2.5 reviews commercially available PCMs and their encapsulant materials.

Paraffin Waxes
Paraffin waxes are by-products of petroleum and are abundantly available at quite a low cost of $4/kg [39].Paraffin waxes have favorable characteristics for thermal energy storage applications such as higher latent heat of fusion, up to 259 kJ/kg [40], a lower thermal conductivity of 0.2-0.4W/m•K and reasonable solid density of 0.93 kg/L [41,42].Paraffin exhibits no super-cooling, no incongruent melting and has the ability to self-nucleate [43] which improves their solidification behavior.Superior solidification properties make them the favorable candidate for building applications where complete solidification at nighttime, through efficient heat removal, guarantees maximum heat absorption on the proceeding day [44].Paraffin remains structurally stable without derating their thermal energy storage capacity over several thousand cycles [45].Paraffins are available in a wide range of the melting points, from −5 • C to 76 • C [3], which renders them suitable for a variety of applications related to the passive and active cooling of buildings [46].
Paraffins have been proved effective in heat storage in traditional building skin, resulting in a decrease in space cooling load in warmer climates and heating load in colder climates [47].Advanced building skins composed of building integrated photovoltaic (BIPV) systems have also employed paraffin to control PV temperature and reduce heat gain in buildings [48].The thermal energy stored in paraffin behind the PV can be utilized in various temperature ranges by an appropriate selection of a paraffin melting point [49,50].
Lower thermal conductivities of paraffin compared to conventional building materials such as concrete (1.8 W/m•K), ordinary clay bricks (1.4 W/m•K) and mortar (1.73 W/m•K) [42] imparts insulating effect on buildings when integrated into building materials [51].The insulating effect further increases when the low conductivity core (paraffin) is encapsulated in an even lower conductivity shell, such as polymers (poly methyl methacrylate) with thermal conductivity 0.17 W/m•K to 0.25 W/m•K [42].However, in colder climates, a higher thermal conductivity shell or core materials (paraffin with Nano-graphite-NG) are desired.The addition of 10% NG by weight is reported to increase the thermal conductivity of paraffin from 0.2 to 0.94 W/m•K [52].The enhanced thermal conductivity increases the transmission of stored heat in PCMs to indoors at nighttime, thereby reducing space heating demand [53].Paraffins show better contact to the surface of containment, showing a lower subtended angle with the surface, resulting in better heat transfer to/from the shell material [54].
The superscripts H, is hazardous in a particular form of transport, while NH is non-hazardous in any form of transport [55].The downside of paraffin is the itching with polymers if exposed to air at elevated temperatures for prolonged durations [43].Additionally, paraffins exhibit a lower flash point in the range of 108 • C to 170 • C [56] which makes their use in building vulnerable to fire.Therefore, a fire resistant shell material is crucially important to enable them to withstand the fire [57].

Organic Non-Paraffins (Fatty Acids)
Non-paraffin organic PCMs also have characteristics favorable for building applications including higher heat of fusion, up to 259 kJ/kg [3]; availability in a wide range of melting points, from 7.8 • C to 127.2 • C [3]; lower thermal conductivity, from 0.14 to 0.17 W/m•K [5]; and no super-cooling.In addition, they show solid-solid phase transition as which makes them unique compared to paraffin.The fatty acids and their appropriate encapsulation materials are summarized in Table 3.

NA
In addition to corrosion, some fatty acids are toxic or release toxic fumes at elevated temperatures [80], therefore, cannot be directly incorporated into the buildings without encapsulation [81] as listed in Table 4 [82].Some PCMs show phase transitions beyond human comfort so they cannot be integrated into building materials.Instead, such PCMs can be included in the building skin via an active system such as solar collector, concentrator photovoltaic and high-temperature heat exchanger to deliver building energy demand [83].

Metals Stearic Acid Palmitic Acid Myristic Acid Lauric Acid
Steel C 20 [82] Resistant Slightly corroded Slightly corroded Resistant Cu [82] Slightly corroded Slightly corroded Slightly corroded Slightly corroded Table 5 highlights that fatty acids are toxic, however, they are still being used in micro-encapsulations without regard to their toxicity.Since most of the building codes may prohibit the use of such toxic materials, it is crucially important to be aware of their environmental and code compliance requirements before investigating their use in buildings.Highly toxic fumes on heating Acute rating from U.S. EPA product label Formic acid [84] Acute toxic PAN pesticide database Methyl palmitate [84] Acute toxic PAN pesticide database Acetic acid [84] Acute toxic PAN pesticide database Capric acid [84] Acute toxic PAN pesticide database D-Lactic acid [85] Slightly toxic Medical hospital Caprylone [86] Varying level of toxicity Journal paper Oxalate [87] Toxic Examiner of alternative medicine Cyanamide [88] Toxic Product description Hypophosphoric acid [89] Acute toxic Chemical manufacturer-product description

Salt Hydrates
Salt hydrates have desirable characteristics of a higher density of 1640 kg/m 3 [90]; negligible (less than 1%) volume changes during phase change [91,92]; cheaper availability with an average cost of $0.13-0.20/kgfor calcium chloride [93,94]; higher heat of fusion of up to 296 kJ/kg [95]; higher thermal conductivity of 0.6 W/m•K [96]; and favorable transition temperatures in the range of 25 • C-34 • C [97].The salt hydrates are abundantly available [98] in large quantities as by-products of industrial processes [95,99] and as waste co-products in many chemical processes including the production of soda ash [100].
The main problems with salt hydrates are their lower thermal stability [101,102], chemical instability [95], corrosion to metals [102,103], incongruent melting [101] and sub-cooling below the solidification temperature [103].The incongruence can be solved by mechanical stirring [104], using excess water to prevent forming a supersaturated solution from melted crystals [105] and modification of their chemical composition [106,107].Mechanical stability can be improved by PCM encapsulation to reduce separation [108].Sub-cooling can be suppressed by the addition of thickening agents [109] and a nucleating agent such as borax [110].However, high-density borax may settle down when added into PCM which may require further investigation to solve this problem [111,112].Thermo-physical properties of salt hydrates with different shell materials investigated in literature are summarized in Table 6.

Low Melting Point Metals and Alloys
Low melting point metals and alloys (liquid at temperatures near comfort zone) are being used as PCMs with favorable characteristics such as a higher thermal conductivity of 29.4 W/m•K, a higher density of 5907 kg/m 3 , higher thermal cycling stability for millions of cycles and a moderate heat storage capacity of up to 80 kJ/kg [36].Heat storage capacity per volume of these materials is much higher as compared to the heat storage capacity per mass because of their huge density as compared to conventional PCMs [117].These PCMs are currently studied for the cooling of electronic equipment [118,119], high power LEDs [120], heat exchangers [120] and for exergy from low-grade heat energy to produce electricity [121,122].Thermo-physical properties of phase change metals and alloys are presented in Table 7.

Commercial PCMs
Table 8 contains commercial PCMs targeted for building applications with the melting point within thermal comfort.For most of these materials, shell or coating is not revealed by the manufacturers.The available characteristics are provided to help select a specific material for the certain application.
The overall merits and demerits of different types of PCMs are summarized in Table 9.
Low thermal conductivity to render insulating effect in building envelopes [116] No super-cooling [3] Solid-solid phase transition exists [79] Higher heat of fusion up to 296 kJ/kg [3] Negligible volume change during phase change [138] Higher density (1640 kg/m 3 for T m = 26.8Non-compatible with plastic container [3] Some PCM are toxic [116] Mostly flammable [116] Most of them have low flash point and can be vulnerable to fire [116] Impurities greatly affect melting point [139] Produce harmful fumes [116] Unable to sustain high temperatures, oxidizing agents and flames [3] Some PCMs show corrosion after thermal cycling as shown in Table 4 Most of them show incongruent melting and lack of easy reversibility [2] Super cooling [140] Potentially corrosive to metals [138] Higher thermal conductivity in hot climates [112] Dehydration [141] Slightly toxic [3] Thermal conductivity (0.4-0.7 W/m•K) [

Methodologies of Microencapsulation of PCMs
There are several types of microencapsulation techniques, based on the underlying phenomena, which can be classified into three major categories: chemical processes, physico-chemical processes and mechanical processes.These methods are very common in medicine, food preservation, textile, cosmetic, agriculture, biotechnology, sensor industries, defense and engineering fields [143].

Chemical Processes
In chemical processes, a microcapsule wall is formed around the core material through chemical reactions.The processes consist of interfacial polymerization and in-situ polymerization, as explained in the following section.

Interfacial Polymerization
In interfacial polymerization, the solvent is selected to be non-reactive to the core material.The first monomer generates first polymers, which must be insoluble in the core material.Hence, the solution of the core material is prepared with this selected solvent.This solution is then spread all over the non-reactive medium (immiscible to this solution) and prepared as homogeneous droplets.A second monomer is added to this colloid with continuous stirring, which generates the first polymer.Before the generation of the first polymer, the second polymer wall is generated on the droplet.Continuous mild stirring causes the generation of first polymer wall inside the second polymer wall and, in doing this, a core material is micro-encapsulated inside a double-walled shell.This method of microencapsulation requires the first monomer to be composed of a minimum of two organic compounds (for instance two isocyanate groups and polypropylene glycol) [144].Butyl stearate, as a core with the shell material of polyurea [145]; octadecane as a core with the shell material of polyurea [146]; di-ammonium hydrogen phosphate, as core with the shell material of polyurethane-urea [147]; xylitol, as a core with the shell material of polyuria-urethane [148]; butyl stearate, as a core with the shell material of polyurea/polyurethane [149]; N-octadecane, as a core with the shell material of polyurethane [150]; and N-octadecane, as a core with the shell material of polyurea [151], were successfully microencapsulated through this process.This process can yield microcapsules with a size range of 0.5-1000 µm [152] and its parameters are easy to control.The maximum temperature encountered is below 80 C which is considered much less hazardous than other methods [148].However, the issue with this process is that bio-materials cannot be encapsulated using these materials [153].The schematic diagram of the interfacial polymerization process for microencapsulation is shown in Figure 3, representing the various stages involved in the process [148].

Chemical Processes
In chemical processes, a microcapsule wall is formed around the core material through chemical reactions.The processes consist of interfacial polymerization and in-situ polymerization, as explained in the following section.

Interfacial Polymerization
In interfacial polymerization, the solvent is selected to be non-reactive to the core material.The first monomer generates first polymers, which must be insoluble in the core material.Hence, the solution of the core material is prepared with this selected solvent.This solution is then spread all over the non-reactive medium (immiscible to this solution) and prepared as homogeneous droplets.A second monomer is added to this colloid with continuous stirring, which generates the first polymer.Before the generation of the first polymer, the second polymer wall is generated on the droplet.Continuous mild stirring causes the generation of first polymer wall inside the second polymer wall and, in doing this, a core material is micro-encapsulated inside a double-walled shell.This method of microencapsulation requires the first monomer to be composed of a minimum of two organic compounds (for instance two isocyanate groups and polypropylene glycol) [144].Butyl stearate, as a core with the shell material of polyurea [145]; octadecane as a core with the shell material of polyurea [146]; di-ammonium hydrogen phosphate, as core with the shell material of polyurethane-urea [147]; xylitol, as a core with the shell material of polyuria-urethane [148]; butyl stearate, as a core with the shell material of polyurea/polyurethane [149]; N-octadecane, as a core with the shell material of polyurethane [150]; and N-octadecane, as a core with the shell material of polyurea [151], were successfully microencapsulated through this process.This process can yield microcapsules with a size range of 0.5-1000 µm [152] and its parameters are easy to control.The maximum temperature encountered is below 80 C which is considered much less hazardous than other methods [148].However, the issue with this process is that bio-materials cannot be encapsulated using these materials [153].The schematic diagram of the interfacial polymerization process for microencapsulation is shown in Figure 3

In-Situ Polymerization
In situ polymerization occurs in two stages.In the first stage, an aqueous polymer is added to the reactor along with distilled water at a certain temperature, followed by the addition of a cross-linking agent to the reactor.The mixture is stirred at high rotation (600 rpm) for 10 min followed by the addition of nucleating agents, resulting in a colloidal dispersion of the mixture.The pH of this mixture is adjusted to 3.5-4.0with continuous stirring, followed by the addition of a calculated amount of coating to the dispersion.The reaction is carried out along with stirring, while gradually heating the dispersion up to 50 • C for a certain time.The suspension of formed capsules is cooled down, washed repeatedly, and eventually dried under controlled conditions.This method enables one to adjust the size of capsules, the size distribution and encapsulation ratio [154,155].Paraffin wax, as a core material with urea formaldehyde as the shell material [59]; palmitic acid, as a core with aluminium oxyhydroxide (AlOOH) as the shell material [34]; hexadecane or octadecane, as a core with melamine-resin as the shell material [156]; higher hydrocarbons, as a core with amino-aldehyde resins as the shell material [157]; and N-hexadecane as a core with melamine-formaldehyde as the shell material, were encapsulated using this methodology.Through this process, capsules of a size range of 0.05-1100 µm [158][159][160] can be produced and it is capable of encapsulating many oil phase organic compounds [158].Figure 4 represents the microencapsulation of essence oil by the process of in-situ polymerization [161].

In-Situ Polymerization
In situ polymerization occurs in two stages.In the first stage, an aqueous polymer is added to the reactor along with distilled water at a certain temperature, followed by the addition of a crosslinking agent to the reactor.The mixture is stirred at high rotation (600 rpm) for 10 min followed by the addition of nucleating agents, resulting in a colloidal dispersion of the mixture.The pH of this mixture is adjusted to 3.5-4.0with continuous stirring, followed by the addition of a calculated amount of coating to the dispersion.The reaction is carried out along with stirring, while gradually heating the dispersion up to 50 °C for a certain time.The suspension of formed capsules is cooled down, washed repeatedly, and eventually dried under controlled conditions.This method enables one to adjust the size of capsules, the size distribution and encapsulation ratio [154,155].Paraffin wax, as a core material with urea formaldehyde as the shell material [59]; palmitic acid, as a core with aluminium oxyhydroxide (AlOOH) as the shell material [34]; hexadecane or octadecane, as a core with melamine-resin as the shell material [156]; higher hydrocarbons, as a core with amino-aldehyde resins as the shell material [157]; and N-hexadecane as a core with melamine-formaldehyde as the shell material, were encapsulated using this methodology.Through this process, capsules of a size range of 0.05-1100 µm [158][159][160] can be produced and it is capable of encapsulating many oil phase organic compounds [158].Figure 4 represents the microencapsulation of essence oil by the process of in-situ polymerization [161].

Physico-Chemical Processes
These processes involve physical chemistry i.e., gelatin or coacervation yielding, forming stable and regular microspheres or microcapsules, which include coacervation and phase separation, solgel process, supercritical CO2 assisted microencapsulation and solvent evaporation.Details of these processes are given in the following section.

Coacervation and Phase Separation
Fully solvated macromolecules are partially de-solvated in the macromolecular aggregation process.The difference between simple coacervation and complex coacervation is the method of phase separation.In simple coacervation, phase separation is induced by adding alcohol or salt, changing temperature or altering pH, while in complex coacervation, an oppositely charged polymer is added to the polymer solution to form the coacervate phase by a cation-anion interaction [162].Microencapsulation efficiency, via the coacervation method, is inversely proportional to the "core-to-

Physico-Chemical Processes
These processes involve physical chemistry i.e., gelatin or coacervation yielding, forming stable and regular microspheres or microcapsules, which include coacervation and phase separation, sol-gel process, supercritical CO 2 assisted microencapsulation and solvent evaporation.Details of these processes are given in the following section.

Coacervation and Phase Separation
Fully solvated macromolecules are partially de-solvated in the macromolecular aggregation process.The difference between simple coacervation and complex coacervation is the method of phase separation.In simple coacervation, phase separation is induced by adding alcohol or salt, changing temperature or altering pH, while in complex coacervation, an oppositely charged polymer is added to the polymer solution to form the coacervate phase by a cation-anion interaction [162].Microencapsulation efficiency, via the coacervation method, is inversely proportional to the "core-to-coating ratio".It is obvious that smaller amounts of shell material may lead to an incomplete coating of PCM, which decreases its encapsulation efficiency, but increasing the proportion of shell material reduces the heat storage capacity of the microcapsules.The amount of cross-linking agent has a major role on encapsulation efficiency as it causes hardening of the shell due to the reaction of carboxylate with formaldehyde.Increasing the amount of agent increases its efficiency, but beyond a certain limit, efficiency decreases due to abundant amounts of the cross-linking agent that leads to inefficiency of the mechanism of the reaction.Similar is the case of emulsifying time where encapsulation efficiency increases with an increase in time, as it gives generation to droplets.However, if this homogenizing time is stretched beyond a certain limit of 10 min, then the phenomenon of coalesce occurs which reverses the solid into a liquid phase again [25].Ammonium phosphate, as a core material, was encapsulated in different polymers by this method for fire retardation [163].This process gives efficient control of the particle size [164]; simple coacervative is insensitive to water soluble additives and capable of the wide pH range in the system [165].However, it is difficult to scale-up and it causes an agglomeration of Nano-particles [164].Figure 5 shows the schematics of the equipment used for coacervation and phase separation process [166].
Sustainability 2016, 8, 1046 12 of 30 coating ratio".It is obvious that smaller amounts of shell material may lead to an incomplete coating of PCM, which decreases its encapsulation efficiency, but increasing the proportion of shell material reduces the heat storage capacity of the microcapsules.The amount of cross-linking agent has a major role on encapsulation efficiency as it causes hardening of the shell due to the reaction of carboxylate with formaldehyde.Increasing the amount of agent increases its efficiency, but beyond a certain limit, efficiency decreases due to abundant amounts of the cross-linking agent that leads to inefficiency of the mechanism of the reaction.Similar is the case of emulsifying time where encapsulation efficiency increases with an increase in time, as it gives generation to droplets.However, if this homogenizing time is stretched beyond a certain limit of 10 min, then the phenomenon of coalesce occurs which reverses the solid into a liquid phase again [25].Ammonium phosphate, as a core material, was encapsulated in different polymers by this method for fire retardation [163].This process gives efficient control of the particle size [164]; simple coacervative is insensitive to water soluble additives and capable of the wide pH range in the system [165].However, it is difficult to scale-up and it causes an agglomeration of Nano-particles [164].Figure 5 shows the schematics of the equipment used for coacervation and phase separation process [166].This method was used to achieve stain free incorporation of PCM into encapsulated woven fabric to improve its thermal performance, however, minor leakage in two out of six samples was also reported [167].The impact of time, the cross-linking agent, and mix ratio on the microencapsulation efficiency of the PCM, encapsulated by coacervation and the phase separation method, is investigated and these factors were in agreement with the above statements [25].Nhexadecane, N-octadecane and N-nonadecane as a core material and gelatin and Arabic gum as the shell material [167]; paraffin oil as a core material with gelatin and Arabic gum as the shell material [168]; and N-octadecane as a core material with melamine-formaldehyde and styrene-maleic anhydride as the shell materials [169], used this method for microencapsulation.

Sol-Gel Encapsulation
In this method, an organic core material is dispersed in an aqueous solution to form droplets with the help of surfactants to obtain oil-in-water micro-emulsion.The sol-gel route starts with a colloidal suspension, often more accurately a solution.In silica-based preparations, the commonly used tetraethoxysilane (TEOS) as a silica origin is dissolved in ethanol or a similar alcohol, hydrolyzed to silicic acid, and eventually condenses to form a silica gel network.Gelation of the sol is obtained through low-temperature hydrolysis and condensation reactions.The key to this method is the self-assembly of the silica precursor on the organic droplets under elaborate conditions, This method was used to achieve stain free incorporation of PCM into encapsulated woven fabric to improve its thermal performance, however, minor leakage in two out of six samples was also reported [167].The impact of time, the cross-linking agent, and mix ratio on the microencapsulation efficiency of the PCM, encapsulated by coacervation and the phase separation method, is investigated and these factors were in agreement with the above statements [25].N-hexadecane, N-octadecane and N-nonadecane as a core material and gelatin and Arabic gum as the shell material [167]; paraffin oil as a core material with gelatin and Arabic gum as the shell material [168]; and N-octadecane as a core material with melamine-formaldehyde and styrene-maleic anhydride as the shell materials [169], used this method for microencapsulation.

Sol-Gel Encapsulation
In this method, an organic core material is dispersed in an aqueous solution to form droplets with the help of surfactants to obtain oil-in-water micro-emulsion.The sol-gel route starts with a colloidal suspension, often more accurately a solution.In silica-based preparations, the commonly used tetraethoxysilane (TEOS) as a silica origin is dissolved in ethanol or a similar alcohol, hydrolyzed to silicic acid, and eventually condenses to form a silica gel network.Gelation of the sol is obtained through low-temperature hydrolysis and condensation reactions.The key to this method is the self-assembly of the silica precursor on the organic droplets under elaborate conditions, required for a reasonably rapid hydrolysis reaction to eventually form the silica shell [71].TiO 2 as shell material is used to encapsulate paraffin by the sol-gel method and obtained good thermal stability and encapsulation efficiency [170].Silica microsphere (hollow spheres) [171], monodisperse microspheres and gigantic hollow structures from silica shell material [172], and monodisperse microspheres with silica material [173] were prepared using this method.In this process, an inorganic shell with high thermal conductivity is formed around the capsule which is the disadvantage for building applications [164].A schematic diagram of the encapsulation of N-octadecane core material with a silica shell is shown in Figure 6.
Sustainability 2016, 8, 1046 13 of 30 required for a reasonably rapid hydrolysis reaction to eventually form the silica shell [71].TiO2 as shell material is used to encapsulate paraffin by the sol-gel method and obtained good thermal stability and encapsulation efficiency [170].Silica microsphere (hollow spheres) [171], monodisperse microspheres and gigantic hollow structures from silica shell material [172], and monodisperse microspheres with silica material [173] were prepared using this method.In this process, an inorganic shell with high thermal conductivity is formed around the capsule which is the disadvantage for building applications [164].A schematic diagram of the encapsulation of N-octadecane core material with a silica shell is shown in Figure 6.

Supercritical CO2 Assisted Microencapsulation
This process is further divided into static and dynamic.The static version involves the premixing of supercritical CO2 and a solution containing a solute of interest at a pressure higher than the critical pressure of CO2.After equilibrium is established, the mixture in a high-pressure chamber is allowed to expand to atmospheric pressure through a flow restrictor (or a capillary tube), by expansion into a drying chamber.The dynamic version involves continuous intimate mixing of a solution containing a solute of interest and supercritical CO2 or near-critical CO2.The two fluid streams become intimately mixed in a low, dead volume, subsequently expanded through a flow restrictor to atmospheric pressure, where the micro-droplets are rapidly dried [174].The schematics of this process are shown in Figure 7. Generally, this technology is used for the encapsulation of biomaterials and medicines.Piroxicam (drug) as a core material with the poly lactic acid and poly lactic-co-glycolic acid as the shell material [175]; and proteins and bovine serum albumin as a core material with liposome as the shell material [176] were microencapsulated through this process.The literature on microencapsulating starch using this method is compiled in [177].The advantages of this process are low critical temperature value, nontoxic, nonflammable, cost effective and readily available [178] but it is still in the research phase [153].

Supercritical CO 2 Assisted Microencapsulation
This process is further divided into static and dynamic.The static version involves the pre-mixing of supercritical CO 2 and a solution containing a solute of interest at a pressure higher than the critical pressure of CO 2 .After equilibrium is established, the mixture in a high-pressure chamber is allowed to expand to atmospheric pressure through a flow restrictor (or a capillary tube), by expansion into a drying chamber.The dynamic version involves continuous intimate mixing of a solution containing a solute of interest and supercritical CO 2 or near-critical CO 2 .The two fluid streams become intimately mixed in a low, dead volume, subsequently expanded through a flow restrictor to atmospheric pressure, where the micro-droplets are rapidly dried [174].The schematics of this process are shown in Figure 7. Generally, this technology is used for the encapsulation of biomaterials and medicines.Piroxicam (drug) as a core material with the poly lactic acid and poly lactic-co-glycolic acid as the shell material [175]; and proteins and bovine serum albumin as a core material with liposome as the shell material [176] were microencapsulated through this process.The literature on microencapsulating starch using this method is compiled in [177].The advantages of this process are low critical temperature value, nontoxic, nonflammable, cost effective and readily available [178] but it is still in the research phase [153].
of this process are shown in Figure 7. Generally, this technology is used for the encapsulation of biomaterials and medicines.Piroxicam (drug) as a core material with the poly lactic acid and poly lactic-co-glycolic acid as the shell material [175]; and proteins and bovine serum albumin as a core material with liposome as the shell material [176] were microencapsulated through this process.The literature on microencapsulating starch using this method is compiled in [177].The advantages of this process are low critical temperature value, nontoxic, nonflammable, cost effective and readily available [178] but it is still in the research phase [153].

Solvent Evaporation
Oil in water emulsion is produced by the agitation of two immiscible liquids in the solvent evaporation encapsulation method.Microcapsule shell material (polymer) is dissolved in a volatile solvent, immiscible with the emulsion, followed by the dissolution or dispersion of core material in the coating polymer solution.The core-coating material mixture is dispersed with agitation in the emulsion to obtain appropriately sized microcapsules.Agitation of the system is continued until the solvent partition and water contents are removed by evaporation.Several methods can be used to achieve dispersion of the oil phase in the continuous phase summarized in [180]; a schematic representation of this process is represented in Figure 8 [181].Generally, this encapsulation method is adapted in the pharmaceutical industry.Bovine serum albumin as a core with poly(DL-lactide-co-glycolide) as the shell material [182]; proteins and peptides as a core with poly(lactic acid) and poly(lactic-co-glycolic acid) as the shell material [181]; verapamil and propranolol as a core with Eudragit RS and Eudragit RL polymers as the shell material [183], were encapsulated using this process.This process is economical but restricted to lab scale production [164] and constrained to the pharmaceutical industry only [181][182][183].

Solvent Evaporation
Oil in water emulsion is produced by the agitation of two immiscible liquids in the solvent evaporation encapsulation method.Microcapsule shell material (polymer) is dissolved in a volatile solvent, immiscible with the emulsion, followed by the dissolution or dispersion of core material in the coating polymer solution.The core-coating material mixture is dispersed with agitation in the emulsion to obtain appropriately sized microcapsules.Agitation of the system is continued until the solvent partition and water contents are removed by evaporation.Several methods can be used to achieve dispersion of the oil phase in the continuous phase summarized in [180]; a schematic representation of this process is represented in Figure 8 [181].Generally, this encapsulation method is adapted in the pharmaceutical industry.Bovine serum albumin as a core with poly(DL-lactide-coglycolide) as the shell material [182]; proteins and peptides as a core with poly(lactic acid) and poly(lactic-co-glycolic acid) as the shell material [181]; verapamil and propranolol as a core with Eudragit RS and Eudragit RL polymers as the shell material [183], were encapsulated using this process.This process is economical but restricted to lab scale production [164] and constrained to the pharmaceutical industry only [181][182][183].

Mechanical Processes
In these processes, the coating is carried out around core material through some mechanical process.These processes are spray drying and congealing, fluid bed coating, multi-orifice centrifugal process, air suspension coating and pan coating as explained below.

Spray Drying and Congealing
Spray drying is a physical method used for microencapsulation of PCMs.A chamber is used for spraying in which some inert gas (air or nitrogen gas) is kept.The inside temperature is adjusted so that it can cause evaporation of the solvent and can formulate encapsulated PCM in solid form.Thermoplastics are used here as the shell material.A solution is prepared containing a core material, encapsulating agent and a medium that can accommodate the homogenous mixture of these components in the liquid phase.An atomizer is used to spray small proportions of emulsion into the chamber with set conditions of temperature and inert gas.That gas will evaporate the solvent and condense to form solid capsules.In the following step, these solid capsules are separated by using a filter or cyclones.The solvent can be condensed by some means to reuse it for the next encapsulation [184].Paraffin as a core material and gelatin and acacia as the shell material [25]; paraffin RT27 as a core material with polyethylene/ethylene vinyl acetate as the shell material [185], were used for

Mechanical Processes
In these processes, the coating is carried out around core material through some mechanical process.These processes are spray drying and congealing, fluid bed coating, multi-orifice centrifugal process, air suspension coating and pan coating as explained below.

Spray Drying and Congealing
Spray drying is a physical method used for microencapsulation of PCMs.A chamber is used for spraying in which some inert gas (air or nitrogen gas) is kept.The inside temperature is adjusted so that it can cause evaporation of the solvent and can formulate encapsulated PCM in solid form.Thermoplastics are used here as the shell material.A solution is prepared containing a core material, encapsulating agent and a medium that can accommodate the homogenous mixture of these components in the liquid phase.An atomizer is used to spray small proportions of emulsion into the chamber with set conditions of temperature and inert gas.That gas will evaporate the solvent and condense to form solid capsules.In the following step, these solid capsules are separated by using a filter or cyclones.The solvent can be condensed by some means to reuse it for the next encapsulation [184].Paraffin as a core material and gelatin and acacia as the shell material [25]; paraffin RT27 as a core material with polyethylene/ethylene vinyl acetate as the shell material [185], were used for microencapsulation through this process.This method is reviewed to produce Nano-particles electronic and optoelectronic, mechanical, chemical, cosmetic, medical, drug, and food technologies [186].This process is cost economic and commercial [152], equipment and knowledge are widely available [153,164], it is versatile for many materials [153,164] and easy to scale-up [153].Its disadvantages are high process temperature, the of particles, and incomplete coating [164].Figure 9 shows the schematic diagram of the spray drying process.
Sustainability 2016, 8, 1046 15 of 30 microencapsulation through this process.This method is reviewed to produce Nano-particles electronic and optoelectronic, mechanical, chemical, cosmetic, medical, drug, and food technologies [186].This process is cost economic and commercial [152], equipment and knowledge are widely available [153,164], it is versatile for many materials [153,164] and easy to scale-up [153].Its disadvantages are high process temperature, the agglomeration of particles, and incomplete coating [164].Figure 9 shows the schematic diagram of the spray drying process.

Fluid Bed Coating
This method of encapsulation is basically a coating process in which droplets of coating material solution are atomized through a nozzle into a hot gas, fluidized particle bed.After the atomized droplets of the coating material come into contact with core particles, the solvent has to evaporate quickly.An excess of droplets in the bed would induce agglomeration, therefore, appropriate droplet atomizing and drying conditions are maintained to eliminate agglomeration of solid particles.Usually, this is formed for fine particles (Ø <70 µm) due to their strong cohesive forces.So this method cannot be used for fine particles using the conventional fluid bed process [162].Bischofite as a core material and acrylic polymer as the shell material [188], and foods as the core material [189] were encapsulated through this process.Literature related to fluid bed coating to encapsulate food items is reviewed in [190].Top-spray coater and bottom-spray coater, being most popular in the fluid bed coating process, are shown in Figure 10.

Fluid Bed Coating
This method of encapsulation is basically a coating process in which droplets of coating material solution are atomized through a nozzle into a hot gas, fluidized particle bed.After the atomized droplets of the coating material come into contact with core particles, the solvent has to evaporate quickly.An excess of droplets in the bed would induce agglomeration, therefore, appropriate droplet atomizing and drying conditions are maintained to eliminate agglomeration of solid particles.Usually, this is formed for fine particles (Ø <70 µm) due to their strong cohesive forces.So this method cannot be used for fine particles using the conventional fluid bed process [162].Bischofite as a core material and acrylic polymer as the shell material [188], and foods as the core material [189] were encapsulated through this process.Literature related to fluid bed coating to encapsulate food items is reviewed in [190].Top-spray coater and bottom-spray coater, being most popular in the fluid bed coating process, are shown in Figure 10.

Multi Orifice Centrifugal Process
In this process, core material particles are accelerated through an encapsulant membrane with the help of centrifugal force yielding microcapsules.This process is capable of producing microcapsules with solid and liquid core materials, different shell materials and of different sizes.These outputs depend on the parameters of the rotational speed of the cylinder, the flow rate of the core and shell material, concentration, viscosity and surface tension of the core material.The production rate of the process is 50 to 75 pounds per hour [180].The production rate is comparable or even higher than that of regular spray drying or spray cooling processes and the processing cost is also very similar.It involves a high process temperature and creates clogging problems [191].It is suitable for bio-encapsulation [164].
quickly.An excess of droplets in the bed would induce agglomeration, therefore, appropriate droplet atomizing and drying conditions are maintained to eliminate agglomeration of solid particles.Usually, this is formed for fine particles (Ø <70 µm) due to their strong cohesive forces.So this method cannot be used for fine particles using the conventional fluid bed process [162].Bischofite as a core material and acrylic polymer as the shell material [188], and foods as the core material [189] were encapsulated through this process.Literature related to fluid bed coating to encapsulate food items is reviewed in [190].Top-spray coater and bottom-spray coater, being most popular in the fluid bed coating process, are shown in Figure 10.

Air Suspension Coating
In air suspension coating, the core material is dispersed in the upward air stream that is supporting the suspended solid particles.In this upward air chamber, the shell material is sprayed onto the suspended particles and the air is circulated again in order to achieve the required thickness of the coating, along with drying the capsules [180].This complex process, with almost 20 parameters, is generally applied in the pharmaceutical industry [192].Micro-processes (drying, droplet impact and spreading, and stickiness) are identified [193], which play a vital role in the efficiency of this method.This process is cost economic and gives higher production volume [164].The necessity of a high skill level, being unsuitable to encapsulate PCM [194], applicable only to solid cores [195] and substantially non-uniform capsules [196] are shortcomings of this process.Figure 11 represents the schematic diagram of this process.In this process, core material particles are accelerated through an encapsulant membrane with the help of centrifugal force yielding microcapsules.This process is capable of producing microcapsules with solid and liquid core materials, different shell materials and of different sizes.These outputs depend on the parameters of the rotational speed of the cylinder, the flow rate of the core and shell material, concentration, viscosity and surface tension of the core material.The production rate of the process is 50 to 75 pounds per hour [180].The production rate is comparable or even higher than that of regular spray drying or spray cooling processes and the processing cost is also very similar.It involves a high process temperature and creates clogging problems [191].It is suitable for bio-encapsulation [164].

Air Suspension Coating
In air suspension coating, the core material is dispersed in the upward air stream that is supporting the suspended solid particles.In this upward air chamber, the shell material is sprayed onto the suspended particles and the air is circulated again in order to achieve the required thickness of the coating, along with drying the capsules [180].This complex process, with almost 20 parameters, is generally applied in the pharmaceutical industry [192].Micro-processes (drying, droplet impact and spreading, and stickiness) are identified [193], which play a vital role in the efficiency of this method.This process is cost economic and gives higher production volume [164].The necessity of a high skill level, being unsuitable to encapsulate PCM [194], applicable only to solid cores [195] and substantially non-uniform capsules [196] are shortcomings of this process.Figure 11 represents the schematic diagram of this process.

Pan Coating
Pan coating is used for capsules of greater size (Ø >600 µm), and is often used for the encapsulation of controlled release beads and pills in the pharmaceutical industry [194].Shell material in the form of a solution or atomized spray is coated over the solid core material.Later, warm air or a drying oven is used to remove the coating solvent [180].Using this method, the mixing of tablets is investigated by altering parameters of rotation speed and rotation cycles and the results are validated by using the discrete element model [198], and another study evaluated that pan coating can be applied uniformly to the capsules [199].This process is used in the pharmaceutical industry for pills [194] and in the food industry for candies [200].It is time-consuming [195], not suitable to encapsulate PCM [194] and yields substantially non-uniform coating [196].
All the processes are reviewed from the perspectives of manufacturing parameters, economics, and suitability for different categories of core and shell materials and their comparison is drawn in Table 10.

Pan Coating
Pan coating is used for capsules of greater size (Ø >600 µm), and is often used for the encapsulation of controlled release beads and pills in the pharmaceutical industry [194].Shell material in the form of a solution or atomized spray is coated over the solid core material.Later, warm air or a drying oven is used to remove the coating solvent [180].Using this method, the mixing of tablets is investigated by altering parameters of rotation speed and rotation cycles and the results are validated by using the discrete element model [198], and another study evaluated that pan coating can be applied uniformly to the capsules [199].This process is used in the pharmaceutical industry for pills [194] and in the food industry for candies [200].It is time-consuming [195], not suitable to encapsulate PCM [194] and yields substantially non-uniform coating [196].

Modelling Studies
Most of the numerical models focus on PCM wall optimization that includes PCM wall thickness, placement of a PCM layer inside the wall, phase transition temperature and heat storage capacity of the materials.Heat conductivity of the material and wall thickness play vital roles in the completion of the charging-discharging cycle over a 24 h period.Optimal thickness will depend on the thermal conductivity of the material.These models ignore the effect of convective heat transfer within the melted PCM.In the following section, numerical models are surveyed with regard to PCM in building applications.
Effective thermal conductivity of mono-disperse and poly-disperse encapsulated particles, randomly or uniformly distributed inside a matrix, was predicted.It is reported that effective thermal conductivity has no effect on capsule size and spatial distribution, however; volume fraction of core and shell and thermal conductivities of the core, shell, and matrix do influence the effective thermal conductivity [203].Darkwa presented that the heat transfer rate depends on the spatial distribution of the PCM particles inside building components and this rate is more in triangular particles as compared to square ones [204].Zhou et al. adopted a one-dimensional enthalpy model, using a fully implicit finite difference scheme to demonstrate thermal performance of PCM in wallboards and reported that above critical values of latent heat of fusion and thickness of PCM layers inside wallboard have very little impact on damping and delay of temperature peaks [205].
A uni-directional heat transfer model was developed using a finite volume scheme to predict the performance of roof comprising of PCM, concrete and steel, considering concrete slab and roof top, independent of temperature.It showed that the daytime inner surface temperature for PCM is low as compared to a non-PCM ceiling due to higher thermal inertia, while it was opposite in the nighttime due to the heat energy stored in the PCM [206].An inverse method was applied to solve numerical modeling and assumed the conductivity of all materials involved to be constant to validate the experimental results.The results concluded that a 5 mm of PCM layer board had twice the thermal inertia of non-PCM wallboard and performed equivalently to an 8 cm thick concrete wall [207].A finite element algorithm validated the prototype experimental results that an air layer between PCM and metal in the wallboard can enhance thermal mass sensibly, with ease of installation [208].
The incorporation of PCM filled cylindrical holes, created inside bricks, was numerically studied for cooling potential in hot climates through the finite element method.The two-dimensional heat flow was analyzed, neglecting thermal expansion of brick containing PCM and natural convection of PCM in the liquid phase [209].It was reported that the effect of natural convection in liquid regime is significant where Raleigh Number (R a ) is greater than 5 × 10 6 while insignificant when R a is below 1 × 10 6 [210].It is proved numerically that laminated PCM wallboards are better than randomly distributed PCM wallboards in terms of enhanced thermal performance and rapid heat transfer rate [211].A validated numerical model predicted the performance of a form-stable PCM, thereby proposing optimum transition temperatures, thermal conductivity, and acceptable mechanical properties [212].The inclusion of PCM (paraffin) as an ingredient to self-compacting concrete was modeled and its effect on hydration temperature of the concrete was studied [213].It was observed that the inclusion of PCM substantially lowered the hydration temperature and thermal conductivity, however, the compressive strength was reported to decrease by up to 71% with the inclusion of PCM, by a 5% weight ratio within 28 days of exposure [213].A heat transfer model predicted that laminated wallboards exhibit a higher heat storage capacity compared to randomly distributed wallboards [214].It was predicted that PCM performance is strongly dependent on the thickness of the walls, phase change temperature, and total latent heat of the PCM [215], crucial to enhance thermal inertia of the buildings [216].Phase changing component materials are better than ordinary masonry wall for energy efficiency and thermal comfort [217], especially those with sharp melting points [218].

Experimental Studies
Drissi et al. carried out the experimental characterization using differential scanning calorimetry (DSC) to study the damage in the microencapsulated PCM (powder Micronal ® DS 5038X from BASF encapsulated in polymethyl methacrylate shell) and reported that a damaged PCM can lose 28% of latent heat compared to non-damaged PCM [219].Stearic acid (SA) is used as a PCM and its composite, with titanium dioxide (TiO 2 ), is prepared [76].No chemical reaction occurred during its formation.It is non-flammable, non-toxic and thermally stable but its thermal storage capacity is low i.e., 46.6 kJ/kg.Table 11 contains the summary of different types of PCMs integrated into building components and their thermal performance is tabulated.Bio-based PCM is microencapsulated with fiber insulations for thermal regulations.Kosny et al. worked on fiber insulations containing microencapsulated PCM, numerically and experimentally.They have justified the use of a blend in terms of peak load shifting and energy optimization.The PCM achieved a time shift of three hours, with a 20%-35% peak hour load reduction being achieved [220].
In different studies, PCM-cellulose blends are investigated for the thermal insulation of buildings to keep room temperature independent of outside temperature fluctuations.It is obvious that the addition of PCM can insulate a building more efficiently.This study focused on the optimal quantity of PCM inside insulator fibers with constant thermal conductivity.The PCM, blended with fiber, increased thermal inertia of the building while increasing the PCM melting time to 15 h [62,221].
Pomianowski et al. introduced the concept of PCM and thermally activated building system (TABS) jointly for cooling buildings.In this study, they carried out simulations and experimental work to validate numerical predictions for PCM impact on the thermal performance of the building.They reported major discrepancies between theoretical and experimental results and cooling performance of TABS reduced when PCM was added to it [222].Navarro et al. also experimented on the inclusion of macro encapsulated PCM inside TABS and tried to cover the heating and cooling of a building during winter and summer seasons.Simulations were conducted for hot, mild, and cold weather conditions.This system was capable of providing 66% of the cooling demand in hot weather and eliminated the mechanical cooling system in mild weather.The energy saving was 45% in mild winter and 21% in severe winter [223].

Problems of Microencapsulation
Microencapsulated N-octadecane PCM in a sodium silicate shell was prepared by the sol-gel method.Spherical microcapsules with a specified chemical composition were successfully fabricated.After integration to concrete blocks, PCM release was reported, reaching up to 40% in 30 days of exposure, which undermined the reliability of the method over long-time building integration.Additionally, it was observed that using a larger shell to core ratio, to ensure effective encapsulation, resulted in a 59% loss of energy storage capacity of PCM [72].The method was improved with slight modifications, resulting in better microencapsulation efficiency (86.5%) and a better encapsulation ratio (86%), however, thermal instability was observed at elevated temperatures (releasing 20% PCM at 250 • C reaching up to 100% at 500 • C).This sharp weight loss with an increase in temperature raises the safety concerns in case the building catches fire [71].
In another study, N-octadecane PCM was encapsulated in calcium carbonate, achieving up to 40% microencapsulation efficiency with up to 40% microencapsulation ratio.However, up to a 23% loss of PCM was reported in 30 days of exposure in a temperature range of 160 • C to 250 • C [58].Wolcott blended paraffin with polymers and investigated their behavior for thermal energy storage.Although this blend did not considerably affect the crystallinity and melting temperature of PCM, however up to 10% of paraffin PCM leaked within 40 h of exposure at 60 • C [70].Caprylic acid as PCMs was encapsulated in different shell materials by opting for simple and complex coacervation techniques, concluding that emulsion temperature and stirring time were major factors affecting micro-encapsulation efficiency [78].
Salt hydrate was microencapsulated in the polymer shell by the suspension copolymerizationsolvent volatile method.An improved thermal cycling through modification in its chemical composition was reported.However, incongruent melting was observed due to a change in the number of water molecules with the fluctuation of temperature yielding different melting points.The incongruence can be attributed to dehydration of the hydrated salts (observed by weight loss) with an increase in temperature [115].In order to achieve integrity in salt hydrates, salts were micro-encapsulated by the solution impregnation method in expanded graphite.The micro-encapsulation resulted in a six-fold increase in encapsulation efficiency and a thermal stability of up to 100 cycles [114].
A eutectic mixture of fatty acids was tested for passive cooling of buildings which shows structural integrity of up to 120 • C, however, the addition of only 2% PCM in concrete reduced its compressive strength by 30% [35].Nano-graphite (NG) was added to paraffin wax to produce a composite material and its thermal conductivity was investigated.Paraffin was immersed in expanded graphite and was studied numerically and experimentally for melting and solidification behavior of the composite.It was reported that thermal conductivity enhanced greatly with the formation of flake structures.However, the leakage of PCM was observed after exposure, thereby asking for an improvement in PCM micro-encapsulation to avoid leakage [68].

Conclusions and Recommendation
In the article, the research on microencapsulated PCMs for thermal energy storage in buildings is reviewed, focusing on thermal, structural and environmental aspects.It is observed that paraffins are preferred due to self-nucleating properties, the absence of sub-cooling and congruent melting, higher latent heat of fusion and the availability in a wide range of melting points suitable for many applications.Its lower thermal conductivity can be exploited for building applications, creating an insulating effect.Most of the microencapsulation techniques yield fine capsules with good microencapsulation efficiency.When integrating PCM into a concrete, temperature rise, due to hydration, causes PCM leakage which hinders the movement of water that ultimately reduces the compressive strength of concrete.
Core to coating ratio is a major optimization parameter that affects microencapsulation efficiency.Increasing the amount of coating material increases microencapsulation efficiency but, in turn, reduces heat storage capacity.On the other hand, decreasing the amount of coating results in reduced strength of micro-encapsulated PCMs.
Furthermore, the presence of PCM Microcapsules affects the mechanical strength of the building structure.Increasing the amount of PCM in construction materials can increase thermal inertia of the building however, simultaneously, it reduces its compressive strength considerably.Encapsulant materials that offer a strong and durable shell around PCM and provide good bonding with the construction material, i.e., concrete, need to be investigated.In the context of encapsulant materials, metal oxides are studied to improve mechanical strength, however, they need further attention for improvement in terms of environmental corrosion.Low melting point metals have a huge latent heat of fusion and better thermal stability for millions of charging-discharging cycles and can be potential PCMs for building applications.
The thermal models predicting PCM behavior in building components need to be improved to address natural convection within PCM, sub-cooling, super cooling and dehydration in certain materials.In addition, the application of microencapsulated PCMs is not widely tested in various climatic conditions, with various heat loads, to determine their long-term structural and thermal stability, which is a potential area of future research.

Figure 1 .
Figure 1.Comparison of the maximum energy storage capacity of 10 mm thickness of different building materials operating between 18 °C and 26 °C for 24 h [1].

Figure 1 .
Figure 1.Comparison of the maximum energy storage capacity of 10 mm thickness of different building materials operating between 18 • C and 26 • C for 24 h [1].

Figure 2 .
Figure 2. Classification of phase change materials (PCMs) conventionally applied for latent heat storage in buildings [3].

Figure 2 .
Figure 2. Classification of phase change materials (PCMs) conventionally applied for latent heat storage in buildings [3].
, representing the various stages involved in the process [148].

Figure 3 .
Figure 3. Schematic representation of the various stages of the microencapsulation process by the interfacial polycondensation method [148].

Figure 3 .
Figure 3. Schematic representation of the various stages of the microencapsulation process by the interfacial polycondensation method [148].

Figure 4 .
Figure 4. Schematics of the fabrication process of microcapsules containing essence oil by in situ polymerization [161].

Figure 4 .
Figure 4. Schematics of the fabrication process of microcapsules containing essence oil by in situ polymerization [161].

Figure 5 .
Figure 5. Illustration of experimental equipment for the generation of liquid drops using (a) dispersion cell with the simple paddle and (b) continuous pulsed phase flow [166].

Figure 5 .
Figure 5. Illustration of experimental equipment for the generation of liquid drops using (a) dispersion cell with the simple paddle and (b) continuous pulsed phase flow [166].

Figure 10 .
Figure 10.Schematic diagrams of two types of the most commonly used fluid bed coaters [187].

Figure 10 .
Figure 10.Schematic diagrams of two types of the most commonly used fluid bed coaters [187].

Figure 10 .
Figure 10.Schematic diagrams of two types of the most commonly used fluid bed coaters [187].

Table 1 .
Desirable characteristics for microencapsulation, shell material, and PCMs.

Table 2 .
Thermo-physical properties of research-based paraffin and paraffin composites with shell materials and encapsulation methods.

Table 3 .
Thermo-physical properties of organics fatty acids mixtures with suitable shell materials and encapsulation method.

Table 4 .
Corrosion effects observed with metals by metallographic examination after 910 thermal cycles.

Table 5 .
Toxicological level and rating of fatty acids.

Table 6 .
Thermo-physical properties of salt hydrates with a suitable shell material and encapsulation method.

Table 7 .
Thermo-physical properties of low melting point metals.

Table 8 .
Thermo-physical properties of commercially available micro-encapsulated PCMs.

Table 9 .
Merits and demerits of different PCMs.

Table 10 .
Comparison of different microencapsulation techniques.

Table 11 .
Summary of the PCM performance in the buildings.