# Short recent summary review on evolving phase change material encapsulation techniques for building applications

Khaireldin Faraj, Mahmoud Khaled, Jalal Faraj, Farouk Hachem, Khaled Chahine, Cathy Castelain

► To cite this version:

Khaireldin Faraj, Mahmoud Khaled, Jalal Faraj, Farouk Hachem, Khaled Chahine, et al. Short recent summary review on evolving phase change material encapsulation techniques for building applications. Energy Reports, 2022, 8, pp.1245-1260. 10.1016/j.egyr.2022.07.135. hal-03758507v2

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Submitted on 24 Jan 2025

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TMREES22-Fr, EURACA, 09 to 11 May 2022, Metz-Grand Est, France

# Short recent summary review on evolving phase change material encapsulation techniques for building applications

Khaireldin Farajª, Mahmoud Khaledb, c, *, Jalal Farajb, d, Farouk Hachemª, Khaled Chahinee, Cathy Castelainf

Received 29 June 2022; accepted 21 July 2022

Available online 9 August 2022

## Abstract

As the world deviates towards sustainability in buildings due to massive increase in energy demand, the effective integration of Phase Change Material (PCM) in buildings gained substantial attention. The capability of PCM in enhancing the thermal performance of buildings relies heavily on the encapsulation technique used as it is in direct link with enhancement techniques required for upgrading thermo-physical, chemical and environmental PCM properties. The current study reviews shortly recent literature involving PCM integration in buildings and highlights different encapsulation techniques used for their proper active and passive incorporation. It also summarizes recent methodologies of properties enhancements prior to encapsulation. Preliminary results reflect the importance of proper encapsulation using the five distinct techniques: direct mixing, imbibing, shape-stabilization, macro-encapsulation and micro-encapsulation. Commercialization of macro-encapsulated PCMs is dominant over other techniques, where micro/nano-encapsulation technique is still limited and requires further study being the most promising.

© 2022 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Peer-review under responsibility of the scientific committee of the TMREES22-Fr, EURACA, 2022.

**Keywords:** Phase change materials; Macro-encapsulation; Micro-encapsulation; Shape-stabilization; Direct impregnation; Imbibing

## 1. Introduction

World's energy demand is continuously increasing at rapid strides as a consequence for the exponentially increasing population. The building sector in developing countries, solely, accounts for 15%–25% of local annual overall energy consumption. These percentages rise to reach 30%–40% in developed regions [1]. Consequently,

<footnote>ª Energy and Thermo-Fluid Group, Lebanese International University, LIU, PO Box 146404, Bekaa, Lebanon</footnote>

<footnote>ᵇEnergy and Thermo-Fluid Group, The International University of Beirut BIU, PO Box 146404, Beirut, Lebanon</footnote>

<footnote>ᶜUniversity Paris Diderot, Sorbonne Paris Cite, Interdisciplinary Energy Research Institute (PIERI), Paris, France</footnote>

<footnote>d Lebanese University, Faculty of Technology, Saida, Lebanon</footnote>

<footnote>e College of Engineering and Technology, American University of the Middle East, Kuwait</footnote>

<footnote>f Laboratory of Thermal Energy of Nantes, LTEN, Polytech' Nantes, University of Nantes, Nantes, France</footnote>

<footnote>* Corresponding author at: Energy and Thermo-Fluid Group, The International University of Beirut BIU, PO Box 146404, Beirut, Lebanon. E-mail address: mahmoud.khaled@liu.edu.lb (M. Khaled).</footnote>
**Nomenclature**

<table><tr><td>PCM</td><td>Phase change material</td></tr><tr><td>TES</td><td>Thermal energy storage</td></tr><tr><td>EG</td><td>Expanded graphite</td></tr><tr><td>NZEB</td><td>Net zero energy building</td></tr><tr><td>PEB</td><td>Positive energy building</td></tr><tr><td>SHS</td><td>Sensible heat storage</td></tr><tr><td>LHS</td><td>Latent heat storage</td></tr><tr><td>PW</td><td>Paraffin wax</td></tr><tr><td>SBS</td><td>Styrene-butadiene-styrene</td></tr><tr><td>HDPE</td><td>High density polyethylene</td></tr></table>

**Units and symbols**

<table><tr><td>Temperature</td><td>T (°C)</td></tr><tr><td>Enthalpy</td><td>h (kJ/kg)</td></tr><tr><td>Conductivity</td><td>k (W/m K)</td></tr><tr><td>Specific Heat</td><td>Cp (kJ/kg K)</td></tr></table>

greenhouse gases emissions increased worldwide, where residential and commercial buildings hold the responsibility of 40% of the total emissions [2]. Moreover, environmental pollution, ecological impairment and climate change of which is the global warming, are all respective results for producing and utilizing fossil energy due to the use of harmful and toxic ingredients in energy production and processing [3]. For that, researchers in the engineering scientific community are seeking alternative solutions such as the integration of renewable and sustainable energy sources: wind, solar, hydrogen, geothermal, biomass, hydro, wave, tidal and other renewable energy sources. One of the most suitable methods, used to ameliorate building thermal performance and thus reduce energy consumption, is the integration of thermal energy storage (TES) in buildings. TES is divided into two main categories: sensible heat storage (SHS) and latent heat storage (LHS). The incorporation of phase change materials (PCM) as a LHS gained much attention during the recent two decades, and is still evolving as it undergoes the transformation phase from conceptualization to commercialization. The importance of PCM depends on the fact that PCM possess high energy storage capacity (latent heat of fusion) at a nearly constant temperature during the processes of charging and discharging [4–7]. In fact, PCMs are capable of enhancing the building thermal inertia by 5 to 14 times the capability of conventional SHS materials [8]. This reflects the important role PCM has on improving building energy efficiency and controlling thermal comfort [9], and emphasizes on the role of PCM in upgrading conventional buildings into net zero energy buildings (NZEB) and further to positive energy buildings (PEB) [10].

One major parameter, that is marked as a priority upon studying any system encountering the use of PCM, is the adequate method or technique of PCM incorporation. PCM incorporation techniques are mainly five [11]: (1) direct mixing, (2) imbibing or immersion, (3) shape-stabilization, (4) macro-encapsulation and (5) micro-encapsulation. The current study is a short summary review that focuses on reviewing recent research covering the methods of PCM incorporation in building applications, and gives a comprehensive discussion that ends with perspectives and recommendations for guiding future studies in this field. The structure of the current review article reflects its scope and is summarized as follows: Section 2 highlights the importance of the subject of TES and PCM with their classifications, and builds a background for the criteria of PCM selection. Then, enhancement methods used for improving the thermophysical properties of PCM are summarized in Section 3. After that, Section 4 constitutes the main context of the article as it covers the recent work done on studying the evolving PCM incorporation techniques. Finally, proper conclusions and future perspectives are summarized in Section 5.

## 2. Building latent thermal energy storage

The study of building construction material including physical, thermal, chemical and environmental aspects is long road in material science that was initiated centuries ago. Introducing the concept of thermal energy storage
(TES) compensated the reduction in thermal inertia as a consequence of the transformation from small heavy-weight into high-rise light-weight buildings. This section introduces the concepts of thermal energy storage and phase change materials, and summarizes the selection criteria and building applications for the latter.

## 2.1. Thermal energy storage

As mentioned previously, thermal energy storage takes one of two forms: sensible and latent. For SHS the material stores/releases heat energy as the storage medium temperature increases/decreases with no change of phase (physical state). The stored/released heat is, in this case, function of the temperature change (K), material’s specific heat (J/kg K) and mass (kg), and is calculated in Joules, as follows:

$$Q = mC_p\Delta T \qquad (1)$$

On the other hand, LHS material is capable of storing/releasing heat energy involving a change of the physical state of the material. This phenomenon occurs at the material melting/freezing temperature. The stored/released heat in this case is function of the material latent heat (J/kg) and mass (kg). It is calculated in Joules, as follows:

$$Q = m\Delta H \qquad (2)$$

The classification of energy storage showing the location of PCM in the families of TES is summarized in Fig. 1. It is clear from the figure that the LHS material used in buildings is mostly solid–liquid PCM that has three main categories: organic, in-organic and eutectic.

<figure><img src="image_2.png" /><figcaption>Fig. 1. Classification of TES and PCM [11].</figcaption></figure>

## 2.2. Phase change materials

The importance of PCM is reflected by its capability to store a larger amount of energy in a smaller volume compared to SHS materials. PCM is capable of decreasing the energy consumption, decreasing the fluctuations in indoor temperature, shifting the heating/cooling load from high-peak to low-peak periods in countries that have variable tariff policies between day and night, and achieving thermal comfort for the residents. Fig. 2 shows the variation of the indoor temperature as a function of time for two buildings: the first containing PCM and the second deprived from PCM. It is depicted that the PCM is capable of shifting the load by decreasing the thermal frequency, and obtaining thermal comfort by decreasing the temperature fluctuations amplitude.
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Reduction in temperature fluctuations and load shifting by means of PCM leading to thermal comfort.</figcaption></figure>

## 2.3. PCM selection criteria

It is crucial to properly select the most relevant PCM for a specified building application. The criteria followed for selection is based on choosing, from available PCM candidates, the PCM capable of achieving highest thermal performance. The properties of the PCM that make it suitable for a specified application are summarized in Fig. 3.

<figure><img src="image_4.png" /><figcaption>**Fig. 3.** PCM relevant properties for proper selection [11].</figcaption></figure>

## 2.4. PCM building applications

The use of PCM in buildings covers several applications that are categorized as passive and active systems. The main difference between the two systems is the TES driving force. When the storage medium temperature variation is the only influencer for the PCM to store/release heat, the system is said to be passive. However, when an external driving force provided by a mechanical equipment is utilized to facilitate the charging/discharging of the PCM, then
the system is known as active system [12]. Another differentiation explanation is relating each system to the type of
heat transfer affecting its performance, where active systems are accompanied by forced convection whereas passive
systems are accompanied by natural convection (free convection). Passive applications cover the integration of PCM
in building envelope: walls, roofs and ceilings, floors, doors, windows, shatters and blinds, furniture and textiles;
also, passive systems such as PCM in Trombe walls (TW), solar air heaters (SAH) and solar façades are applicable.
For Active applications, PCM is installed in HVAC systems, active TW, ventilated solar façades, active SAH, solar
water heaters (SWH), underfloor radiant heating systems, geothermal systems and evaporative and radiative cooling,
solar absorption chillers and solar chimneys. Plenty of recent research: original and review articles, covered the use
of PCM in buildings. It was shown that the combination of PCM applications as passive–passive, active–passive or
passive–active improves the thermal performance of the building and boosts the transformation towards NZEB [7].

**3. Thermophysical properties enhancement methods**

PCM generally encounters several limitations that affects its thermal performance. These major limitations are
present during fusion and crystallization of PCM, and are as follows: (1) In-congruent melting accompanying phase
separation, (2) low thermal conductivity, (3) sub-cooling, and (4) infrequent long-term stability.

3.1. *Phase separation associated with incongruent melting*

Phase separation is a phenomenon that occurs when cyclic precipitation, containing new solid and liquid phase
with dissimilar compositions, is obtained from the PCM's original composition. These precipitations may lead to
degradation in the latent heat storage capacity of the PCM as the melting and solidifying temperatures of the new
compositions shifts outside the application temperature range [13].

3.2. *Low thermal conductivity*

The majority of PCMs lack thermal conductivity which in turns hinders the heat transfer between the PCM and its surroundings, and within the PCM itself. The concept behind limitations in heat transfer capability is reflected by the fact that: upon solidification, the PCM portion adjacent to the very extreme surface in direct contact with the varying temperature zone crystallize first as it losses heat by convection. After that, the obtained solid layer acts as a conduction thermal barrier due to its low thermal conductivity, that hinders the heat transfer from the remaining quantity of liquid PCM [14]. For that, researchers developed enhancement methods and techniques to compensate the lack of PCM thermal conductivity. Accordingly, Ji et al. [15] performed the method of vacuum impregnation to prepare n-Octadecane PCM composites containing nano porous carbon supportive material. Significant enhancements in thermal energy storage performance were monitored. Yang et al. [16] produced a novel composite PCM containing 1-tetradecanol and carbonized wood. The thermal conductivity of the prepared composite PCM was enhanced by 114% (reaching 0.669 W/m K.) in correspondence to the pure PCM. Righetti et al. [17] introduced novel 3D periodic structured macro-containers, made from aluminum, containing paraffin wax PCM. Significant enhancements in storing and releasing latent heat were monitored reflecting the effectiveness of the method on macro-encapsulation techniques. Unique macro-encapsulation containers constituting baffles and copper wires at its mid-horizontal plane were proposed by Faraj et al. [18] for the purpose of enhancing coconut oil PCM thermal conductivity. Also, a review article was published by Lin et al. [19] covering recent studies related to thermal conductivity enhancements of PCM. It was revealed that thermal conductivity additives such as carbon and relevant encapsulation of PCM are methods that improves the PCM's thermal performance. Methods of heat transfer enhancements are summarized in Fig. 4.

3.3. *Sub-cooling*

The thermal performance of PCMs is also affected by phenomenon of supercooling or subcooling. Materials that exhibit such phenomenon are known by their slow nucleation rate (also known as rate of growth). The PCM in this case crystallize at a lower temperature than the melting temperature [14]. A major disadvantage of sub-cooling is reflected by its prevention of latent heat storage full utilization since no phase change occurs by the PCM to regain a complete solid state when cooling down below fusion point [20]; As a consequence, a reduction in the
<figure><img src="image_5.png" /><figcaption>Fig. 4. Summary of thermal conductivity enhancement methods for PCM [19].</figcaption></figure>

efficiency is triggered for the cooling systems incorporated with PCM as freezing starts at lower temperatures [21]. Efficient solutions are developed by the scientific community to overcome sub-cooling: (1) Addition of relevant nucleating agents to PCM, (2) applying cold finger method, (3) roughening the encapsulating container surface, and (4) increasing the size of the microencapsulation containers [14]. For instance, Zhang et al. [22] proved that the sub-cooling degree of Mg(NO3)2·6H2O shape-stabilized composite PCM was significantly decreased from 29.2 °C to 1.9 °C by the addition of graphitic carbon nitride (g-C3N4).

### 3.4. Deficient long-term stability

Another major threat to the performance of PCM systems is having a deficient long-term stability. Such a problem related to thermo-physical and chemical properties accompanied by the problem of corrosion among the encapsulating container and the PCM, bounds the spread of LHS systems [23]. Before the initiation of thermophysical and chemical PCM properties degradation, the PCM must sustain a large amount of fusion and crystallization cycles, typically in a margin of 1000 to 2000 cycles. Also, the PCM and the container have to possess high level of compatibility [14]. One major method to overcome degradation is micro-encapsulation that isolates the PCM from the surrounding resulting in material protection from environment [24].

## 4. Evolving PCM incorporation techniques

Thermal performance of PCM depends highly on the proper incorporation technique used. There are 5 main techniques/methods used for PCM incorporation (summarized in Fig. 5) and are as follows: (1) direct impregnation (direct mixing), (2) immersion (imbibing), (3) shape-stabilization, (4) macro-encapsulation, and (5) micro-encapsulation.

### 4.1. Direct incorporation (direct mixing)

Direct incorporation (impregnation) or direct mixing technique utilization is limited to passive applications of the building envelope. It is considered the easiest and most economical method of PCM incorporation. Typically, in this technique, PCM is mixed with the construction materials directly upon manufacturing to form a composite construction element [2]. However, two main disadvantages arise: (1) PCM leakage and (2) interaction with the
<figure><img src="image_6.png" /><figcaption>Fig. 5. Evolving PCM incorporation techniques/methods.</figcaption></figure>

porous container. In their review, Kuznik et al. [25] listed the materials compatible for direct mixing with PCM that are: cement, wood, compound, mortar, vermiculite and concrete. Direct mixing may contribute to fire hazards if not treated properly. Gnanachelvam et al. [26] reported that PCM-plasterboard greatly affected the fire resistance poet to the first fire side plasterboard layer fall-off. For that, Bake et al. stated that the incorporation of <30% concentration of mPCM secures the fire resistance feature of the product.

Plenty of experimental studies used the direct mixing method for PCM incorporation. Lee et al. [27] performed an experimental study on the thermal performance of walls incorporated with PCM-enhanced cellulose insulation in the cavities. The authors used paraffin wax that was melted at a water bath then sprayed evenly in the cellulose to form a composite insulation wallboard that was installed in the cavities of the prototype walls at a ratio of 30% PCM to 70% cellulose. This decreased the latent heat capacity from 147 to 60.45 kJ/kg. Preliminary results showed that: The results showed that individual walls possessed a daily average peak heat flux reduction of 25.4%, and a 20.1% reduction in hourly average peak heat for all the walls combined. Another study by Tian et al. [28] investigated the performance of a wall coated with a plastering mortar mixed with 50% ceramsite PCM compared to a reference wall coated with pure mortar. Results revealed that 55% reduction in temperature fluctuations were achieved on the inner surface of the wall reflecting the effectiveness of PCM-enhanced mortar in attaining indoor thermal comfort during summer. Kontoleon et al. [29] performed a study to investigate the thermophysical and mechanical behavior of a reinforced concrete wall coated with PCM-enhanced plastering mortar compared a wall coated with pure mortar. The preparation procedure of the enhanced mortar utilized a combined incorporation methodology: (1) micro-encapsulation of paraffin PCM by polymethyl methacrylate membrane (PMMA), and (2) direct mixing of the obtained capsules with the mixture of cement CEM II32.5 and natural sand. The authors prepared two samples of PCM-enhanced mortar with two different PCM portions: 10% and 20% of total weight in addition to the control conventional sample. It was shown that as the portion of PCM increased, heat flux penetration through the wall decreased and the time lag between indoor and ambient peak temperatures increased, resulting in indoor thermal comfort enhancement. However, the increase in the PCM ingredients decreased the bending and compressive strengths.

## 4.2. Imbibing or immersion

Immersion is the technique used for thermal inertia enhancement of already manufactured construction materials. The method requires that the construction material is dipped in a liquid PCM bath by which absorption of the PCM by the capillaries of the construction product takes place. Unfortunately, one main drawback is the interference between the PCM and the hydrating products of the construction element resulting in negative effects on both durability and mechanical properties [30]. Immersion being one of the preliminary methods of PCM incorporation is
followed by plenty of researchers throughout their studies. Rozanna et al. [31] investigated the thermal performance of a gypsum wallboard enhanced with eutectic PCM: lauric-stearic acid. The wallboard was incorporated with PCM by imbibing as it was dipped for 60 min in the liquid PCM bath. Results showed that the PCM thermal properties were similar to the obtained PCM-gypsum boards’ properties. Recently, Boussaba et al. [32] prepared a composite PCM board by mixing natural clay and graphite with cellulose fibers and then dipping the product in a bio-based PCM liquid, namely coconut fat, water bath to be absorbed through its capillaries. It was shown that with increasing the graphite amount to a ratio of 3:7, the thermal conductivity increased to reach 1.06 W/m K in sample 4; also, the DSC results showed that the thermal characteristics remained conserved, however, the latent heat of the samples decreased compared to the pure PCM.

### 4.3. Shape-stabilization

The third method for incorporating PCM into building applications is known as shape-stabilization. It is mainly recommended for organic PCMs as a solution for two problems: poor thermal conductivity and leakage during phase change. Shape-stabilization technique includes the addition of two types of supporting materials to the PCM. These materials are: porous material for PCM leakage prevention and high conductive nano-particles for thermal characteristics enhancements. The formed product is called shape-stabilized composite PCM (ss-CPCM) [33]. The effectiveness of proper utilization of porous materials is reflected through enhancements at the levels of mechanical strength, chemical stability, thermal conductivity and fire retardation.

Plenty of articles by several research groups were published in the domain of PCM in buildings by using the shape-stabilization technique. Huang et al. [34] reviewed recent research on ss-PCM and summarized the porous materials used for shape-stabilization. The study divided the recent applications into three groups: metal-foams, porous carbon, porous oxide, and other porous supports based ss-PCM. Fig. 6 shows the various porous supports and their characteristics. Another categorization was developed by Zhang et al. [35] dividing the porous materials into 4 groups: Microporous (metal and graphite foams), Mesoporous (mesoporous silica and carbon nano-tubes (CNTs)), Microporous (metal organic frameworks (MOF) and porous coordination polymer (PCP)), and Hierarchical porous (hierarchical porous carbon (HPC) and hierarchical porous carbon (HPC)). Other porous materials are polymers such as poly styrene-butadiene-styrene (SBS), epoxy-based organo-gels, cetyl-alcohol high density polyethylene, polystyrene-graft-PEG polymer, 1-octadecanethiol/epoxy resin, and polyurethane/hexadecanol, ... etc [34].

<figure><img src="image_7.png" /><figcaption>Fig. 6. Summary of porous materials for supporting ss-PCM and their characteristics [34].</figcaption></figure>

Liu et al. [36] prepared a highly effective thermal energy storage ss-PCM by fabricating hyper cross-linked polymers (HCPs). The preparation procedure utilized an $FeCl_3$ catalyst for the cross-linking chemical reaction. The produced ss-PCM endured promising thermal conductivity with an enhancement that reached 17% compared to pure PCM. This percentage increased to 55% by converting the cross-linking catalyst to $Fe_3O_4$. A study by Yang et al. [37] investigated the thermal performance of a novel ceramsite-based ss-CPCM for energy storage in buildings. It was shown that the prepared ss-CPCM possessed efficient thermal characteristics (melting point of
21.3 °C and latent heat of 205.9 kJ/kg) for an optimal composition of 82 wt% lauryl alcohol, 18 wt% stearic acid and 0.5 wt% Al₂O₃ nanoparticles. The ss-CPCM is then absorbed into ceramsite and encapsulated by dry cement powder mixed with styrene-acrylic emulsion for shape-stability, which in turns, reduced the latent heat to 133.4 kJ/kg. Fang et al. [38] fabricated a ss-PCM from calcium chloride hexahydrate PCM and urea, with strontium chloride hexahydrate as a nucleating agent and hydrophilic fumed silica as a carrier. The DSC results showed that the obtained ss-CPCM acquired favorable thermo-physical properties as having a high latent heat 114.3 kJ/kg and suitable fusion temperature of 21.03 °C. A low degree of supercooling with high levels of cyclic reliability and thermal stability were monitored. Using a novel cross-linked polymer swelling method, Yang et al. [39] prepared a flexible ss-PCM. It was shown that the electronic device temperature and indoor temperature of buildings can be reduced by 5.5 °C and 9.3 °C, respectively, by means of ss-CPCMs. As study by Rahmalina et al. [40] yielded a ss-CPCM with 160.4 kJ/kg latent heat capacity and 66.8 °C melting temperature reflecting the beneficial role such a PCM can play in thermal regulation.

### 4.4. Macro-encapsulation

Macro-encapsulation is the technique used to encapsulate a PCM in a shell material with dimensions larger than 5 mm. The PCM is then in the core of the macro-capsule which in turns protect the PCM from the surrounding environment against any intervention from hazardous threats, such as fire, and increases its stability and durability. Also, encapsulation of the PCM increase the surface area and thermal conductivity depending on the geometry and material used in the macro-capsule [33]. The geometry of the macro-containers can take any form depending on the application. Four main forms are used: quadratic prisms (rectangular and square), cylindrical, spherical and concentric tube annulus.

Most of the researchers use the macro-encapsulation as it is more flexible than the other techniques. In a previous study [41], coconut oil PCM was macro-encapsulated in an aluminum container for incorporation with electrical underfloor radiant heating system in a small scale-prototype. Results showed that the PCM was able to shift the load from high peak to low peak period resulting in economic savings. Another study was prepared by enhancing the encapsulation through the addition of copper wires and steel baffles in rectangular steel containers [42]. The used system in this study was a hydronic heating system for larger prototypes. Fig. 7 shows the macro-containers used to be filled with coconut oil and paraffin wax PCMs. The results of the study indicated the enhancement in thermal comfort and energy savings are favorable through the help of the macro-encapsulated PCMs.

<figure><img src="image_8.png" /><figcaption>Fig. 7. Enhanced macro-encapsulation containers with baffles and copper wires [42].</figcaption></figure>

A study by Hamidi et al. [44] shows that the macro-encapsulation of PCM having a melting temperature of 26 °C in hollow brick walls can increase the energy savings up to 56% in the Mediterranean climates. Yan et al. [43] designed a novel self-activated PCM wall equipped with gravity heat-pipes linked to a pipe-encapsulated PCM system, operating with nocturnal radiative cooling. The PCM is macro-encapsulated in the annulus of the concentric pipes. Fig. 8 shows the schematic diagram of the operating system and the incorporation of PCM. Results revealed that the integration of the novel self-activated PCM wall possess a high level of application potential for energy savings in buildings. A new promising method of combining shape-stabilization and macro-encapsulation techniques yields higher thermal performance, stability and durability. Zhang et al. [45] prepared a ss-CPCM by infiltrating
<figure><img src="image_9.png" /><figcaption>**Fig. 8.** Novel self-activated PCM wall mechanism [43].</figcaption></figure>

Mg(NO₃)₂·6H₂O into graphitic carbon nitride (g-C₃N₄) and then macro-encapsulating the formed composite ss-PCM with epoxy resin adhesive and silicon sealants. Results showed that the problem of hydrated salts have been solved with the combined techniques as the weight and the latent heat of fusion were approximately conserved after 100 cycles ensuring the reliability, stability and durability of the PCM. Fig. 9 shows the combined techniques used for encapsulating the salt-hydrate PCM. He et al. [46] performed a parametric study on the effectiveness of a novel RC-PCM-wall. The investigation was carried out on three identical test prototypes: prototype A utilizing conventional brick walls, prototype B containing PCM, and prototype C equipped with the novel RC-PCM-wall. The novelty was reflected by combining the effect of micro-channel heat pipes (MCHPs) to compensate the insufficient solidification of PCM during the evening and that of a highly emissive radiative cooling plate. It was shown that the cooling loads from the southern wall of prototype C were reduced by 47.9% and 23.8% from south wall, compared to A and B. that. Meng et al. [47] studied the effect of integrating two types of macro-encapsulated PCMs in the building envelope where: SP29 was placed in the floor, ceiling and west wall, while RT18 was placed in the north and east walls. Results revealed that the temperature fluctuations were reduced significantly ensuring thermal comfort during summer and winter. Rathore and Shukla [48] impregnated a room envelope with pipe macro-encapsulated PCMs to test the enhancement in thermal performance during hot summer seasons. Results showed that the thermal amplitude was reduced by 40.67% to 59.79% compared to the room without PCM.

<figure><img src="image_10.png" /><figcaption>**Fig. 9.** Graphical representation of the multi-stage encapsulation process for Mg(NO₃)₂·6H₂O [45].</figcaption></figure>
## 4.5. Micro-encapsulation

Micro-encapsulation is referred to the process of encapsulating a PCM in the core surrounded by polymeric material forming a shell at the micro-scale level. Basically, two main methods are followed to obtain a microencapsulated inorganic PCMs: Physical and Chemical [49]. Physical methods are mainly three: fluidized bed, complex coacervation and spray drying. Chemical methods are namely 4: in situ polymerization, interfacial polymerization, suspension-like polymerization and sol-gel. Cardenas-Ramirez et al. [24] summarized the various methods of microencapsulation for organic and in-organic PCMs adding to the new classification: physio-chemical category. According to Alehosseini and Jafari [50], research interests are directed towards nano-encapsulation with a particle size of 1 to 1000 nm. This is due to the problem of increasing viscosity of the end fluid in microencapsulated PCMs and the ability to be disrupted by pumping. A review on nano-encapsulation was also done by Yang et al. [51]. The structure of the micro-capsule and nano-capsule is in one of the forms present in Fig. 10. It is depicted in the figure that in all classifications, the microencapsulated PCM constitutes a shell and a core. Environmentally friendliness is reflected by the shell layer that forms a shield against any adverse environmental interactions such as fire hazards [52].

<figure><img src="image_11.png" /><figcaption>Fig. 10. Classification of capsule types based on core and shell natures [50].</figcaption></figure>

The effectiveness of microencapsulation in thermal performance enhancements of buildings for cooling and heating applications is reflected in plenty of studies by several researchers. A study by Hunger et al. [53] revealed that the mixing of 5% microencapsulated Micronal DS 5008X PCM with fresh concrete can result increasing the heat capacity and lowering the thermal conductivity of concrete wall leading to up to 12% energy savings. However, the loss in strength accompanied by a microstructural analysis ensured that a part of the micro-capsules were destroyed and PCMs were released within the surrounding concrete matrix. Bake et al. [54] developed a gypsum plasterboard imbedded with microencapsulated Micronal DS 5040X PCM and proved that the PCM-gypsum plasterboard can store energy at a rate of 0.4 W/min higher than the conventional gypsum wallboard. Microencapsulated PCM can be integrated in plastering mortars. Illampas et al. [55] studied the effect of adding paraffin-based microencapsulated PCM into polymer modified cement mortar. It was shown that the mechanical properties experienced a significant drop however thermal properties were substantially enhanced resulting in a built ability to reduce and shift temperature peaks. The use of micro-encapsulated bio-based PCM in micro concrete composite was studied by Parameshwaran et al. [52] and results showed that the addition of 0.075 wt% mPCM to the micro-concrete composite yielded adequate compressive strength of 38 MPa, thus sustaining its structural integrity while enhancements in the thermal characteristics prevail. Other studies incorporating mPCM were delivered by Praveen et al. [56] showing the effect of Graphene nano particles (GnP), Romero Rodriguez et al. [57] and Zhu et al. [58] (incorporated with novel heat storage composite coatings).

The evaluation of the data reviewed from recent articles requires the summarization of the PCMs used in the studies with their detailed types, ingredients, thermophysical properties, and incorporation techniques which are shown in Table 1. Also, a summary diagram is constructed including all the incorporation techniques and their corresponding methods of preparation (refer to Fig. 11).
<figure><img src="image_12.png" /><figcaption>Fig. 11. Preparation methods for the different PCM incorporation techniques [24,33].</figcaption></figure>

<table><caption>Table 1. Thermophysical properties of PCMs used in recent studies and the corresponding incorporation technique.</caption><thead><tr><th>Incorporation technique</th><th>Ingredients relevant to incorporation method (material matrix)</th><th>PCM</th><th>Detailed technique</th><th>Melting temperature T<sub>m</sub> (°C)</th><th>Latent heat, L<sub>H</sub> (kJ/kg)<sup>a</sup></th><th>Thermal conductivity, K (W/m K)<sup>a</sup></th><th>Environmental Friendliness</th><th>Ref.</th></tr></thead><tbody><tr><td>Direct mixing</td><td>Cellulose insulation</td><td>Paraffin wax</td><td>30% liquid paraffin was sprayed in 70% cellulose</td><td>28</td><td>60.45</td><td>0.2</td><td>N.A.</td><td>[27]</td></tr><tr><td>Direct mixing</td><td>Plastering mortar</td><td>Paraffin</td><td>50% paraffin mixed with ceramics, cement and sand</td><td>52.6</td><td>205.7</td><td>0.739</td><td>N.A.</td><td>[28]</td></tr><tr><td>Combined: direct mixing and micro-encapsulation</td><td>Plastering mortar</td><td>Paraffin</td><td>Paraffin + PMMA then, CPCM + cement and sand (10/90 and 20/80)</td><td>23</td><td>96 (CPCM)</td><td>0.193 (CPCM)</td><td>N.A.</td><td>[29]</td></tr><tr><td>Immersion (imbibing)</td><td>Natural clay, graphite & cellulose fibers</td><td>Coconut fat</td><td>30 min immersion</td><td>23.7</td><td>47.29–60.7</td><td>0.53–1.06</td><td>N.A.</td><td>[32]</td></tr><tr><td>Immersion (imbibing)</td><td>Gypsum wallboard</td><td>Lauric-stearic acid eutectic</td><td>60 min immersion</td><td>34</td><td>50.28</td><td>–</td><td>N.A.</td><td>[59]</td></tr><tr><td>Macro-encapsulation</td><td>Steel plates with baffles and copper tubes</td><td>Bio-based coconut oil</td><td>Rectangular prisms</td><td>25</td><td>103</td><td>0.321–0.5</td><td>Coconut oil and macro-container are fire resistant</td><td>[42]</td></tr><tr><td>Macro-encapsulation</td><td>Steel plates with baffles and copper tubes</td><td>Paraffin wax</td><td>Rectangular prisms</td><td>58–60</td><td>189</td><td>0.21</td><td>Paraffin wax and macro-container are fire resistant</td><td>[42]</td></tr><tr><td>Macro-encapsulation</td><td>Hollow bricks</td><td>Paraffin RT26</td><td>Rectangular prisms</td><td>26</td><td>180</td><td>0.2</td><td>N.A.</td><td>[44]</td></tr><tr><td>Macro-encapsulation</td><td>Expanded Graphite, EG</td><td>Paraffin RT28</td><td>Concentric-pipe annulus</td><td>26.5–28.5</td><td>160.2</td><td>9.8 (i) 9.5 (s)</td><td>Metallic pipes are fire resistant</td><td>[43]</td></tr><tr><td>Macro-encapsulation</td><td>–</td><td>RT28HC</td><td>Rectangular prisms</td><td>28</td><td>250</td><td>0.2</td><td>Aluminum container is fire resistant</td><td>[46]</td></tr><tr><td>Macro-encapsulation</td><td>–</td><td>In-organic SP29</td><td>Rectangular prisms</td><td>28–30</td><td>190</td><td>0.6</td><td>Aluminum panels are fire resistant</td><td>[47]</td></tr><tr><td>Macro-encapsulation</td><td>–</td><td>Paraffin RT18</td><td>Rectangular prisms</td><td>17–19</td><td>225</td><td>0.2</td><td>Aluminum panels are fire resistant</td><td>[47]</td></tr><tr><td>Macro-encapsulation</td><td>–</td><td>SavE<sup>®</sup> OM-37 inorganic</td><td>Cylindrical containers</td><td>36–40</td><td>218</td><td>0.13 (1) 0.16 (s)</td><td>Aluminum pipes are fire resistant</td><td>[48]</td></tr><tr><td>Macro-encapsulation</td><td>–</td><td>Bio-PCM_ENRG Blanket TM</td><td>Rectangular prisms</td><td>25</td><td>165–200</td><td>0.128</td><td>N.A.</td><td>[60]</td></tr><tr><td>Macro-encapsulation</td><td>–</td><td>Paraffin-based DuPont TM Energain<sup>®</sup></td><td>Rectangular prisms</td><td>20–35</td><td>72.4</td><td>0.18</td><td>Aluminum sheets are fire resistant</td><td>[61]</td></tr><tr><td>Combined: Micro-encapsulation +direct mixing</td><td>Polymethyl methacrylate (micro) then mixed with concrete</td><td>Micronal DS 5008X</td><td>5 wt% PCM 95 wt% concrete</td><td>23</td><td>–</td><td>2.1</td><td>N.A.</td><td>[53]</td></tr><tr><td>Micro-encapsulation</td><td>Polymethyl methacrylate (micro) then mixed with gypsum</td><td>Micronal DS 5040X</td><td>35% Gypsum 15% PCM and 50% H<sub>2</sub>O</td><td>22–23</td><td>>95 (for mPCM)</td><td>0.139</td><td>Product is fire resistant (for mPCM concentrations < 30%)</td><td>[54]</td></tr><tr><td>Combined: Micro-encapsulation+direct mixing</td><td>–</td><td>Paraffin Nextek 37D</td><td>20 wt% PCM + 80 wt% mortar</td><td>37</td><td>>190 (for mPCM)</td><td>–</td><td>N.A.</td><td>[55]</td></tr></tbody></table>

(continued on next page)
<table><caption>**Table 1 (continued).**</caption><thead><tr><th>Incorporation technique</th><th>Ingredients relevant to incorporation method (material matrix)</th><th>PCM</th><th>Detailed technique</th><th>Melting temperature T<sub>m</sub> (°C)</th><th>Latent heat, L<sub>H</sub> (kJ/kg)<sup>a</sup></th><th>Thermal conductivity, K (W/m K)<sup>a</sup></th><th>Environmental Friendliness</th><th>Ref.</th></tr></thead><tbody><tr><td>Micro-encapsulation</td><td>CaCO<sub>3</sub>/Fe<sub>3</sub>O<sub>4</sub></td><td>N-docosane</td><td>–</td><td>–</td><td>140</td><td>0.798</td><td>Environmentally friendly</td><td>[62]</td></tr><tr><td>Micro-encapsulation</td><td>Urea, Tween80 and Tween20 surfactants and sodium hydroxide</td><td>Lauric acid bio-PCM</td><td>–</td><td>42.52</td><td>47.3–54.3</td><td>0.1309</td><td>Environmentally friendly</td><td>[52]</td></tr><tr><td>Micro-encapsulation</td><td>Graphene nanoparticles (GnP) and polyurethane</td><td>Paraffin</td><td>3 wt% GnP</td><td>62.71</td><td>110.71</td><td>0.379</td><td>N.A.</td><td>[56]</td></tr><tr><td>Micro-encapsulation</td><td>Melamine-formaldehyde</td><td>Paraffin wax</td><td>PCM: 30% by volume of cement paste</td><td>–</td><td>131.27 (for mPCM)</td><td>–</td><td>N.A.</td><td>[57]</td></tr><tr><td>Micro-encapsulation</td><td>polyurea</td><td>n-octadecane</td><td>–</td><td>27.36</td><td>151.89 (for mPCM)</td><td>–</td><td>N.A.</td><td>[58]</td></tr><tr><td>Micro-encapsulation</td><td>–</td><td>Organic-based (Micronal DS 5001)</td><td>commercial</td><td>26</td><td>110</td><td>–</td><td>N.A.</td><td>[63]</td></tr><tr><td>Combined: shape-stabilization and macro-encapsulation</td><td>graphitic carbon nitride (micro) Epoxy resin and silicon sealant (macro)</td><td>Mg(NO<sub>3</sub>)<sub>2</sub> · 6H<sub>2</sub>O</td><td>–</td><td>87</td><td>112.3</td><td>–</td><td>Silicon sealant resists fire up to 450 °C</td><td>[22]</td></tr><tr><td>Shape-stabilization</td><td>HDPE and SBS</td><td>Paraffin-based</td><td>Paraffin + HDPE (80/20) then, CPCM + SBS (80/20)</td><td>66.8</td><td>160.4</td><td>0.268 (solid) –0.196 (liquid)</td><td>N.A.</td><td>[40]</td></tr><tr><td>Shape-stabilization</td><td>Al<sub>2</sub>O<sub>3</sub> nanoparticles and ceramsite</td><td>LA-SA eutectic mixture</td><td>LA-SA (82/18) + 0.5 wt% Al<sub>2</sub>O<sub>3</sub> then, absorbed into ceramsite</td><td>22.5</td><td>133.4</td><td>–</td><td>N.A.</td><td>[37]</td></tr><tr><td>Shape-stabilization</td><td>CO(NH<sub>2</sub>)<sub>2</sub> and SiO<sub>2</sub></td><td>CaCl<sub>2</sub> · 6H<sub>2</sub>O salt-hydrate PCM</td><td>6% CO(NH<sub>2</sub>)<sub>2</sub> and 20% SiO<sub>2</sub></td><td>21.03</td><td>114.3</td><td>–</td><td>N.A.</td><td>[38]</td></tr><tr><td>Shape-stabilization</td><td>Porous Al<sub>2</sub>O<sub>3</sub></td><td>Paraffin wax</td><td>50% Al<sub>2</sub>O<sub>3</sub>/PW</td><td>56.3</td><td>101.5</td><td>1.27</td><td>N.A.</td><td>[64]</td></tr></tbody></table>

## 4.6. Discussion

The proper utilization of the incorporation technique adequate for a specified building application results in maximal thermal performance enhancement. The priority in choosing the technique depends on aligning the technique characteristics with each application compatibility and PCM choice. For instance, direct mixing of some kinds of PCMs with construction materials can hinder its properties; also, the corrosion behavior of certain PCMs with metallic containers or metallic heat transfer enhancers (fillers) is a limitation that must be considered [65]. According to the reviewed literature and by referring to Table 1 it is shown that the most used PCMs are paraffin based organic PCMs and the most used incorporation technique is macro-encapsulation. The range of the melting temperature convenient to building applications is basically near the thermal comfort temperature that is desired. PCMs with high melting temperatures are typically used in building for active heating applications.

Incorporation of PCM using a combination of two techniques is promising. PCM Leakage, stability and thermal conductivity are enhanced with combined incorporation techniques. All methods other than immersion are capable of enhancing the thermal conductivity each through a suitable method. For instance, the addition of fillers in direct mixing technique and the integration of structural high conductive baffles in macro-containers are efficient. Besides, the incorporation of GnP is favorable in microencapsulation and shape-stabilization. The incorporation of PCMs in construction materials enhances the thermal performance but reduces the mechanical strength mostly.

Thus, the best incorporation technique to be used, within any building application, is the technique that acquires the highest level of effectiveness in terms of thermal, thermophysical, chemical and mechanical behaviors.

## 5. Conclusions and recommendations

The current review article summarized the recent studies on the evolving techniques/ methods for PCM incorporation in buildings. Five main incorporation techniques are studied: direct mixing, imbibing, shape-stabilization, macro-encapsulation and micro-encapsulation. The study produced graphical summaries for the PCM characterization, classification and operation. Enhancement methods for PCM limitations at the level of thermophysical and chemical properties were also reviewed. Based on the current short review, the following conclusions are drawn out:

* Proper incorporation of PCM ensures its maximal benefit at the level of thermal energy storage, energy saving, load shifting and thermal comfort achievement; all towards the transformation into NZEB.

* The combination of incorporation techniques yields novel integration methods in building applications and improves the PCMs efficiency.

<footnote><sup>a</sup>The italic values in the columns of properties refer to the pure PCM properties.</footnote>
* Macro-encapsulation of PCMs preserves the latent heat capacity of PCM but requires a space for the container and needs proper treatment in installation to ensure complete thermal contact with the partition.

* Enhancement methods of PCM thermophysical properties can be directly linked with the incorporation technique used.

* Commercialization of macro-encapsulated PCMs is dominant over other incorporated PCMs and the most problematic technique having the largest number of limitations is imbibing.

As an attempt to direct upcoming research on the track, the following perspectives are recommended:

* More research must be performed in the domain of micro/nano-encapsulation as it interferes in combined incorporation techniques and is most promising in terms of enhancing building thermal inertia.

* Commercialization of macro- and microencapsulated PCMs must be standardized and expanded.

* Studies on macro-encapsulation of shape-stabilized PCMs using sealants is to be explored.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Data availability

The authors are unable or have chosen not to specify which data has been used.

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