Advances in phase change building materials: An overview

4.2.1 Organic PCMs

There are two types of organic PCMs: (1) paraffins which are comprised of saturated linear and branched hydrocarbon chains, and (2) non-paraffins (e.g. fatty acids and fatty acid esters, unmodified/modified polymers, and polyalcohol).

4.2.1.1 Paraffin-based PCMs

Organic paraffins or paraffin waxes are derivatives of fossils and are usually found in natural gas, petroleum, or can be produced synthetically from coal. They are comprised of straight n-alkanes, CH₃–(CH₂)–CH₃ chain, and other hydrocarbon (CH) molecules. Their chemical composition can be determined using the general formula C _n H_2n+1, where n represents the number of carbon atoms in the compound, whereby alkane mixtures preferred for most applications exist as wax-like solids at room temperature which fall in the range of 20 ≤ n ≤ 40. Most paraffins show a volume change by ∼10–15% while having good thermal reliability which is attributed to their low specific gravity [59]. Paraffins are soft waxes, therefore will not generate large expansion forces. The latent heat of paraffins depends upon the crystallisation of the alkyl chain in the saturated hydrocarbon chain (e.g. the synthesis process of the methyl CH₃-chain) since this process is responsible for releasing energy in the form of latent heat. Hence, the crystallisation of CH₃ chain releases a significant amount of latent heat. The melting temperature of paraffins and paraffin waxes are a function of the number of carbon atoms in the chain according to Figure 4 [24,60]. Paraffins are hydrophobic, and therefore, will not dissolve in water or chemical solvents; thus, this makes them ideal candidates as PCMs in the encapsulation process [61,62]. Exposing them to elevated temperatures causes evaporation of short chain hydrocarbon molecules as result of paraffin bond breakage. Paraffins and waxes with melting temperature in the range of −12 to 71℃, corresponding to a latent heat of storage in the range of 128–198 kJ/kg [63], have been studied for commercialisation purposes in several studies. Paraffins show no phase segregation even after being subjected to numerous phase transition cycles; however, paraffin blends/waxes have commonly been used over pure paraffins due to their lower costs, and because their phase transition temperatures can be tailored easily for the intended application by mixing alkanes of different chain lengths [64]. Table 5 summarises the thermal properties of the selected paraffins and their waxes [65].

Figure 4

The relationship between the number of carbon atoms in the chain and thermal properties of paraffins.

Table 5

Phase transition temperature and the corresponding latent heat capacity of paraffin and their waxes

PCM name	Phase transition temperature (°C)	Latent heat (J/g)
Hexadecane	18–20	224
Cetane	18	231
Heptadecane	23	182
n-Eicosane	37	250
Paraffin ZDJN-28	26–28	231
Paraffin	44–46	259
Paraffin wax P56–58	42–72	250
Paraffin wax	18	237
Paraffin wax	44	190
Paraffin RT21	25	133

4.2.1.3 Poly(ethylene glycols) (PEGs) and their derivatives as PCMs

PEGs, also known as poly(oxyethylene), are a class of polymeric organic solid–liquid PCMs made up of linear dimethyl ether chains with hydroxyl (OH) ending groups. The general formula of PEGs is H–(O–CH₂–CH₂) _n –OH where n represents the number of carbon atoms in the chain. They are polar substances due to the presence of two terminal OH groups [73] in their chain and are comprised of a highly defined macromolecule [74] with good characteristics, such as non-toxicity, good biodegradability, and hydrophilicity. The width to height ratio (e.g. aspect ratio) of PEG particles are dictated by the molecular weight of PEG [75]. Moreover, this environmentally friendly polymer has proven to be suitable in several fields including cosmetics, food, pharmaceutical, and biomedical. Currently, they are reported as the most versatile environmentally friendly polymeric PCM due to its high phase change enthalpy, controllable and suitable phase change temperature i.e. phase change temperature is in the range of −55 to 100℃ [76], and broad range of molecular weight, along with desirable characteristics including high latent heat capacity usually in the range of 150–180 J/g [77], congruent melting, non-toxicity, no super-cooling behaviour, low vapour pressure in melted state, high resistance to corrosion, chemically inert, small volume changes during phase change, high thermal and chemical stability even after long-term utility duration, and competitive pricing [78]. Also, they require no encapsulation but can be stabilised through both physical and chemical shape stabilisation approaches material [79]. Owing to their excellent desirable characteristics, they have been investigated in various TES applications, which range from building envelopes to fabrics, foams, and fibres. Melting temperatures of PEGs vary according to its molecular weight (Figure 5) [59], which enables their utilisation in various applications including industrial heat utilisation and electronic device management and protection. In addition, they can also be used in active and passive heating or cooling systems of buildings [80,81]. Adjusting the melting temperature and heat absorption capacity should be done by mixing PEGs with different molecular weights to enhance its feasibility.

Figure 5

The relationship between the molecular weight of PEG and its thermal properties.

4.2.2 Inorganic PCMs

Inorganic PCMs include salt hydrates, alloy of inorganic anhydrous salts with water, and metals and alloys, e.g. copper (Cu), zinc (Zn), aluminium (Al), magnesium (Mg), silicon (Si), and tin (Sn). They consist of several remarkable characteristics compared to organic PCMs, such as high latent heat capacity, high thermal conductivity, non-toxicity, non-flammability, and relatively low costs. Therefore, the is a high potential in several applications including building, concentrated solar plants (CSPs), and thermal management of lithium-ion battery.

4.2.2.1 Salt hydrate PCMs

Salt hydrates are ionic compounds usually composed of an alloy of inorganic anhydrous salts and few water molecules. They are characterised by the general formula M·nH₂O, where n represents the number of water molecules present in the compound. Common salt hydrates investigated in literature include fluoride, chloride, and carbonate salts because they have good potential as high temperature PCMs. During the phase transition process, they experience phase transformations involving hydration and dehydration processes resembling the typical melting and crystallisation processes of other solid–liquid PCMs. Benefits of salt hydrates include high energy storage density (∼350 MJ/m³) and they are cheaper than paraffins, paraffin waxes [54,82], and metal PCMs; however, their major drawbacks such as incongruent melting and supercooling (Figure 6) tends to reduce its effectiveness as PCM. Thus, its disadvantages are reported to overshadow their advantages which makes them unsuitable for building applications [83]. Most of the salt hydrates available are observed to undergo changes in the stoichiometric composition during the heating and cooling processes [84]. Previous studies have reported that microencapsulating salt hydrate is difficult; however, Huang et al. (2013) [85] demonstrated the successful microencapsulation of disodium hydrogen phosphate heptahydrate (Na₂HPO₄·7H₂O) by employing the suspension copolymerisation-solvent volatile method using modified PMMA as shell material. They concluded that microcapsules of smooth and compact surface with average diameter ranging from 650 to 740 µm was obtained. Table 6 [86] presents the thermal properties of salt hydrate PCMs with operating temperatures matching building applications.

Figure 6

The latent heat and supercooling temperature of the selected salt hydrate PCMs.

Table 6

Thermal properties of salt hydrate PCMs suitable for building applications

Salt hydrate PCM	Melting temperature (℃)	Latent heat (J/g)
KF·4H2O	19	231–330
K2HPO4·4H2O	19	231
FeBr3·6H2O	21	105
Mn(NO3)2·6H2O	26	126–148
CaCl2·6H2O	29	170–192
Na2CO3·10H2O	32–34	267
LiBr·2H2O	30	296

4.2.3 Eutectic mixtures

Eutectic mixtures of PCMs are comprised of a combination of two or more chemical compounds of single chemical composition and solidify at a lower melting temperature than any other composition obtained from the same component. Therefore, they tend to melt and freeze congruently to produce a mixture of component crystals during the crystallisation process. A blend of crystals is formed during the freezing process. Eutectic mixtures may be composed of either inorganic with inorganic, organic with inorganic, and organic with organic. Table 7 shows the melting point and latent heat of different combinations of eutectic mixtures synthesised in the past with phase transition temperatures matching the operating temperature of building applications [63]. Due to the existence of large number of individual salt species, and large number of mixtures, the combination space of eutectic mixtures is enormous. Main advantage is that their melting points can be easily adjusted by combining different weight percentages of components, which means for every temperature range several potential eutectic PCM mixtures can be obtained, which encourages optimal selection for the intended application. For instance, tetradecane with melting temperature in the range of 1.5–5.6℃ can be obtained by combining octadecane, docosane, and hexadecane [94]. Moreover, eutectic mixtures have other desirable characteristics such as high thermal conductivity, density, and experiences no segregation and supercooling effect; however, their specific heat capacity is smaller than the ones of salt hydrates and paraffin, which limits their potential applications significantly.

5.2.1 Physical encapsulation methods

Here spray drying and solvent evaporation approaches will only be discussed since the remaining physical encapsulation approaches have not been used for the encapsulation of PCMs.

5.2.1.1 Spray drying

Spray-drying, pan-coating, air-suspension, and centrifugal are the physical encapsulation techniques available for the encapsulation of solid–liquid PCM cores. Spray drying was the first and one of the oldest physical methods developed in the 1930s, and since then it has attracted much attention [111], because the technology is simple, convenient, reproducible, and scalable [112,113,114]. Moreover, it is comparable with the “one stage drying operation” [112] and are comprised of simple steps of high feasibility. First, the chosen PCM gets dispersed in an aqueous coating solution to encourage the production of MPCM of desirable particle size under the spray drying process. Second, the atomisation of the liquid PCM results in producing droplets which are then passed through the drying chamber. Evaporation of water results in producing dried isolated droplets called MPCMs [112,113,115]. Figure 10 illustrates schematically the spray drying process [116].

Figure 10

Schematic illustration of the spray drying technique [116].

Hawlader et al. [96] encapsulated paraffin wax within gelatine/acacia shell material and observed a reduction in microencapsulation efficiency with the increase in the core/shell ratio and concluded that enough shell material was required to successfully encapsulate the PCM within he chosen polymeric film. Incorporation of cross-linking agent in the range of 6–8 mL enhanced the microencapsulation efficiency significantly. Borreguero et al. [117] encapsulated paraffin with/without carbon nanofibres within a low-density polyethylene ethylvinylacetate shell material. Experimental results demonstrated that particles of better size and shape were obtained from the collection vessel; thus, microcapsules collected from the inside of the drying vessel were bound together, which is attributed to the melting of polymer. Fei et al. [118] encapsulated n-octadecane (C18) (e.g. paraffin) within a titania shell material via a rapid aerosol process and concluded that microcapsules of two structures, namely dense and hollow, with diameters in the range of 0.1–5.0 microns were produced successfully. Overall, the spray drying method is well-established approach in the food industry; however, the approach does not produce a good yield of encapsulated particles which is attributed to the agglomeration of the particles due to the use of very high temperatures during the process. Moreover, the spray drying equipment is bulky and expensive.

5.2.2 Physio-chemical encapsulation methods

Pan-coating, air-suspension coating, and ionic gelation are all physio-chemical approaches used commonly in the pharmaceutical, food, and cosmetic industries for the encapsulation of the chosen core; and therefore, will not be discussed here since they have not been used for the encapsulation of PCMs. In the pan-coating method small, coated capsules are prepared by mixing the coating material with the solid particles followed by melting of coating material and then cooling to encourage solidification [105].

5.2.2.1 Coacervation

Coacervation technique used for the encapsulation of PCMs can be simple or complex. The formation mechanism of MPCMs is the same with the only exceptions being that both techniques use a different polymer system and approach for phase separation. The simple coacervation process is comprised of simple steps as illustrated in Figure 11(a). First, the core substance gets dispersed in the shell polymer solution; and for the separation of coacervate from the solution, a desolvation agent gets added during this process. As a result, the core substance gets covered via the coacervate droplets and coalescence of coacervate results in the formation of a continuous shell around the core which leads to the formation of encapsulated capsules in the form of particles. The steps of the complex coacervation technique are illustrated in Figure 11(b). In the first step, an emulsion gets prepared by dispersing the core material into the aqueous polymer solution. Then, a second polymer solution gets added followed by the addition of relevant salt or dilution of medium to encourage the deposition of shell materials onto the core particles. Finally, crosslinking agents, desolvation, or thermal treatments are used for the stabilisation of microcapsules produced. Overall, this approach of encapsulation is cheap and has a good control over the size of encapsulated particles produced, but the major drawbacks include the use of large amount of solvent during synthesis, agglomeration of particles, and difficulty in scaling-up the production [124].

Figure 11

Schematic illustration of (a) simple and (b) complex coacervation techniques [116].

5.2.2.2 Sol–gel method

In the term solution-gelation, sol refers to the stable dispersion of particles in solvent and gel refers to the three-dimensional network of covalent bonds and intermolecular forces comprised of aggregates of sol particles. The steps involved in this approach of encapsulation is illustrated schematically in Figure 12. In the first step, a solution (e.g. sol) is formed by mixing the chosen precursor (e.g. metal alkoxide) with the chosen solvent, catalyst, and complexing agent, which initiates the chemical hydrolysis and condensation reaction, and this results in producing a stable dispersion of polymers or colloidal particles in the solvent which are added to the PCM-in-water emulsion to form a gel consisting of discrete microcapsules with PCM droplets entrapped inside them. This approach allows the utilising of inorganic shell materials with high thermal conductivity, and they are suitable to produce nano sized PCM particles using silicon dioxide as shell [125].

Figure 12

Sol–gel method of encapsulation [116].

5.2.3 Chemical encapsulation methods

The properties of the MPCMs prepared via different chemical approaches are presented in Table 9.

5.5.1 Supporting material for the shape stabilisation of solid–liquid PCM

The two major drawbacks of solid–liquid PCMs are their poor shape stability which causes leakage problems during phase change, and their inherently low thermal conductivity; however, they can be overcome by selecting a suitable supporting material. In cases where the leakage problem needs to be overcome, a porous material will be selected as the supporting material but if composite PCM is expected to have improved thermal conductivity, the porous material will be combined with the chosen nanomaterial since such materials have very high thermal conductivity. It is important to ensure that nanomaterial is not incorporated in excess since high contents of nanomaterial will reduce the latent heat of resulting composite PCM because the supporting material makes no contribution to latent heat. Vast number of papers have mainly utilised porous materials as supporting material due to the desirable characteristics they exhibit which includes increased surface area, high thermal conductivity, and enhanced chemical compatibility. The interaction existed between PCM and porous materials which arises due to Van der Waals, hydrogen bonds, surface tension, and capillary force tend to prevent leakage of PCM, thus, enabling PCM to retain its solid-state during phase change. Moreover, the choice of supporting material used for the stabilisation has great impact on the final properties of the prepared composite PCM. The different types of supporting materials used in previous literature to stabilise the chosen PCM are presented in Table 11 with a summary of their characteristics [29,189].

5.5.2 PEG composite through physical blending

Physical blending also known as polymer blending is a contemporary method for the development of new polymeric materials and is an ideal technique for the synthesis of materials with desired properties. It consists of three simple steps as illustrated schematically in Figure 20 and they include the dissolution of the chosen PCM and supporting material in chosen solvent at 70℃ followed by pouring solution onto a glass dish and then allowing solvent to evaporate at room temperature which results in the production of the desired composite PCM. Hence, using this method, it is possible to prepare final products of superior properties than the individual constituent polymers (Table 12). The final properties of synthesised polymers are influenced mainly by the miscibility of the composite blend [182]. Wang et al. [81] attempted PEG–EG composite PCM of mass ratio in the range of 50–90 wt%, and Sundararajan et al. [183] prepared PEG-cellulose acetate composites via a microwave-assisted blending approach and reported that optimum PEG content to form film composites was 60 wt%. Li et al. [195] developed a novel graphene oxide (GO)-PEG composite, as photo-thermal conversion material, with latent heat capacity higher than 180 J/g and a phase transition temperature of 50.5℃, via an ultrasonic assisted physical blending approach. Composites of high latent heat capacity was obtained since no changes occurred to the crystalline properties and chemical structure of PEG. A novel PEG/poly(l-lactic acid) (PLLA) composite PCMs were prepared and results indicated that the PLLA content caused no significant impact on the crystalline properties of PEG [175], which was attributed to the partial miscibility of PEG and PLLA but increasing PLLA content above 30 wt% reduced the crystallinity of PEG in the composite PCM from 80 to 70%, and this was observed to decrease the latent heat capacity insignificantly. Moreover, it was concluded that PEG/PPLA blends with PLLA content less than 35 wt% were suitable for TES purposes since they had latent heat capacity greater than 100 J/g and good thermal reliability even after being subjected to 100 thermal cycles.

Figure 20

Schematic representation of physical vacuum impregnation approach for the synthesis of solid–solid PCMs.

5.5.3 Thermal properties of composite PCMs prepared via physical shape stabilisation approaches

Vacuum impregnation is the most used physical approach for the stabilisation of PCMs because it is simple, yet highly effective [200,201,202,203]. Vacuum conditions are usually used to enhance the impregnation efficiency by forcing the liquid PCM into the pore system of the supporting material, and the conventional set-up used in previous literature is shown in Figure 20.

Since physical shape stabilisation processes is very simple, easy and a highly feasible approach along with a flexible design which means any PCM and supporting material can be selected and used. Therefore, previous studies have prepared numerous composite PCMs using variety of PCMs and supporting material; thus, unlimited PCM/supporting material combinations have been prepared and investigated in literature as shown in Table 13. However, there is a lack of understanding regarding the changes caused to the thermal properties of PCM, namely its phase transition temperature and latent heat after the physical shape stabilisation process, which are discussed in the following paragraphs by comparing the design parameters of different studies; thereby, determining major factors influencing the thermal properties of PCMs.

In general, after the shape stabilisation through either physical, chemical, or a combination of both, the thermal properties determined using differential scanning calorimetry (DSC) shows that the phase transition temperatures (e.g. melting and crystallisation temperatures) and the corresponding latent heat of PCM gets reduced but the main reasons for this is yet to be established since results produced in literature varies a lot. This section focuses on identifying design parameters that alternates the phase transition temperature and latent heat since thermal performance of final composite PCM prepared depends entirely upon this. Also, knowing the key parameters that decline the intrinsic thermal properties of PCMs means more attention can be paid during synthesis process, and it would be easier to propose formulation procedures that produce composite PCMs with optimum thermal performance. Moreover, it reveals the ways of tailoring phase transition temperature to match the operating temperature of building application (e.g. 18–30°C) because PCMs with high phase transition temperatures are reported to have higher latent heat capacities. Nomura et al. [203] prepared erythritol/expanded perlite (EP) composite PCM with latent heat of 332.2 J/g which was lower than pure erythritol which had a latent heat of 354.7 J/g, and this phenomenon was predicted to occur due to the existence of an unusual interaction between the PCM and the nano-sized pores of EP. While Lee et al. [204] prepared erythritol/EG composite PCMs via a simple melting method with latent heats of 317.5, 322.1, 327.7, 334.3, 38.5, and 351.2 J/g, respectively. The difference in latent heat of the composite PCMs was because the interlayer distance of EG varied and the change in latent heat of pure erythritol (380.5 J/g) was reported to be attributed to the presence of EG in the composite but according to literature other factors (e.g. temperature) may have affected the latent heat but this was not addressed in this study. Jeong et al. [205] prepared n-hexadecane/diatomite composite PCMs with melting temperature and latent heat of 23.68°C and 120.1 J/g using pure hexadecane with melting temperature and latent heat of 20.84 and 254.7 J/g, respectively. The cause for the change in melting temperature and latent heat of melting and freezing was not reported. The reduction in the crystallisation temperature from 16.78 to 13.17°C (e.g. 21.51% decrease) was attributed to the poor intermolecular forces existing between n-hexadecane molecule and pore walls of diatomite. Li et al. [206] prepared paraffin/diatomite composite PCMs via a simple mixing/surface absorption approach and observed that the phase transition temperature of paraffin reduced insignificantly from 26.1 to 25.2°C (e.g. 3.45% decrease) which was due to the interaction generated between paraffin and diatomite, and due to the confinement of paraffin within the pores of diatomite but reason for the drastic reduction in the latent heat capacity of pure paraffin from 133.7 to 45.96 J/g in the composite PCM was not evaluated and reported in the study. This may be attributed to the PCM content present in the composite PCM. Hence, composite PCM with 47.9 and 34.4% of PCM had latent heats of 63.98 and 45.96 J/g which suggests that higher PCM loading results in producing composite PCMs of better thermal performance.

Karaman et al. [207] prepared PEG/diatomite composite PCMs via the vacuum impregnation and observed changes in the melting temperature of pure PEG from 33.32 to 27.7°C (e.g. 16.87% decrease) and reported that the insignificant changes in the phase transition temperature may be attributed to the physical interaction existing between PEG and diatomite. DSC results showed changes in the latent heat of PEG in composite PCM containing optimum content of PEG (e.g. 50 wt%) from 143.16 to 87.09 J/g in the composite; however, this was not addressed in the study but could have been explained by studying and evaluating changes in the crystalline properties of PEG because PEG is a polymeric PCM. The leakage problem of PEG was resolved, and this was attributed to the prevention caused by physical interactions such as capillary and surface tension forces, resulting from physical interactions existing between the functional groups of both PEG and diatomite. Results from thermal cycling test indicated that the composite PCM exhibited good thermal reliability and chemical stability, even after being exposed to 1,000 repeated heating and cooling cycles. Zhang and Fang [62] prepared paraffin/EG composite PCMs via the melting approach with melting temperature and latent heat of 48.93°C and 161.45 J/g. The melting temperature and latent heat of pure paraffin was 57.99°C and 188.69 J/g, respectively. Results showed that the physical stabilisation of paraffin had affected its thermal properties, but the authors concluded that significant changes did not occur because no chemical reaction took place between paraffin and EG.

Sari and Karaipekli [208] prepared paraffin/EG composite PCMs and investigated the influence of EG content on thermal properties of paraffin. Mass fraction of 2, 4, 7, and 10% of EG led to the production of composite PCMs with melting temperatures of 41.10, 41.00, 40.70, and 40.20°C and corresponding latent heats of 192.6, 188.0, 181.9, and 178.3 J/g, respectively. Results demonstrated that increasing the amount of supporting material tends to impact the latent heat of chosen PCM negatively; thus, the thermal performance of composite PCM becomes poor when EG content is high. The melting temperature and latent heat of composite PCM was lower than pure paraffin which had a melting temperature and latent heat of 41.6°C and 194.6 J/g, respectively, but the cause for the change in melting temperature and latent heat of paraffin in the composite PCM was not discussed anywhere in the study. Mehrali et al. [209] prepared paraffin/graphene oxide (GO) composite PCMs via vacuum impregnation using paraffin with melting temperature and latent heat of 53.46°C and 130.92 J/g, respectively. The composite PCM containing the highest mass percentage of paraffin (e.g. 48.3%) had melting and crystallisation temperatures of 53.57 and 42.25°C, respectively; thus, no significant changes had occurred in the melting temperature, but the crystallisation temperature increased; however, the reason for the change was not reported. The latent heat of composite PCM seemed to have a positive correlation with the paraffin content (in wt%) because composite PCMs with 44.3, 47.39, 47.80, and 48.30 wt% had latent heats of 59.12, 62.53, 63.11, and 63.77 J/g, but the enhancement in latent heat was not significant. Pure paraffin had a latent heat of melting of 131.92 J/g so the percentage difference between latent heat of pure and composite PCM containing optimum paraffin content was 51.7% which is very high, therefore, indicating that thermal properties of paraffin was affected greatly by elevated temperature exposure during the vacuum impregnation process. Sari [210] prepared PEG600/gypsum and PEG600/natural clay composite PCMs with maximum absorption ratio of 18 and 22 wt% via the vacuum impregnation approach and observed changes in the phase transition and latent heat of pure PEG600 after impregnation, which was due to the confinement of PEG600 into the pores of gypsum or natural clay by capillary and surface tension forces which resulted in restricting the mobility of PEG chain during phase change. The cubicles of PEG600/gypsum and PEG600/natural clay composites, constructed at small scale, exhibited good thermal properties such as thermal reliability and chemical stability, even after being subjected to a lengthy thermal cycling test. Li et al. [189] prepared paraffin/TiO₂ foam composite PCMs of three-dimensional interpenetrating structures [189]. The composite PCMs had enhanced melting and freezing temperatures with a difference of no more than 3°C which was reported to be attributed to the surface tension forces and the capillary effect present between paraffin and the TiO₂ foam. Reduction in latent heat of melting and freezing from 184.3 and 182.67 J/g to 97.27 and 99.05 J/g suggested that the thermal properties of paraffin had changed after vacuum impregnation although no chemical reactions occurred between paraffin and foam. Hence, calculated percentage difference of 47 and 46% suggests that major changes in phase change properties had occurred after shape stabilisation which was due to the presence of solid content in the composite; thus, the porosity and pore size of the supporting material was reported as the two influential factors likely to impact the thermal properties of paraffin compared to the surface tension forces and capillary forces. The composite PCMs were observed to undergo thermal degradation when exposed to an elevated temperature of 150°C and thus, its onset degradation temperature was higher than that of paraffin which suggests that the composite PCMs had enhanced thermal stability below 200°C. Zhang et al. [211] prepared paraffin/silica composite PCMs containing 30, 50, 55, and 60 wt% of paraffin via the blending/impregnation approach. The melting temperature and latent heat of composite PCM with optimum paraffin content was 25°C and 95 J/g, respectively. The latent heat of paraffin was observed to have reduced from 168 to 95 J/g which was attributed to the accommodation of paraffin in the pores of silica. Kim et al. [212] prepared paraffin/light weight sand composite PCMs through a simple blending/impregnation approach with melting temperature in the range of 19.67–25.36°C and latent heat in the range of 19.01–19.11 J/g, respectively. Incorporating 50 and 100% of the prepared composites in cement mortar resulted in enhancing its latent heat by 6 J/g. Sarı et al. [76] impregnated PEG within a binary composite consisting of raw diatomite (RD)/carbon nanotubes (CNTs) to prepare composite PCMs of enhanced thermal conductivity, and with latent heat capacity and melting temperature in the range of 51.4–62.9 J/g and 7–8℃. Leakage test results showed that the skeleton of RD/CNTs pre-composite prepared could absorb an optimum PEG content of ∼51.0 wt% without sacrificing its structural stability which indicates the feasibility of incorporating them successfully in conventional building elements without fearing PCM leakage. They also showed good thermal reliability and chemical stability even after the long-term cycling operation, making them suitable for low-temperature passive solar cooling applications in buildings. Presence of CNTs enhanced the thermal durability of the composites significantly. Yang et al. [213] prepared the PEG/EG composite, by filling the pores of EG with liquid PEG under vacuum by air pressure and pore syphoning. Cheng et al. [214] prepared PEG10000/CNTs/PVC/C (CPC) composite PCMs via a blending/impregnation approach and reported that 8% increase in latent heat of melting of the composites was attributed to the hydrogen bonds present between the PCM and the supporting material.

Liu et al. [215] prepared novel PCM/carbon composite PCMs using carbon foam derived from insulation materials polyisocyanurate foam. Pure PCM was reported to have a degree of supercooling of around 4.10°C while the composite PCMs only experienced a degree of supercooling in the range of 1.90–2.50°C which was attributed to the enhancement of thermal conductivity of PCM in the composite due to the presence of carbon foam. With the increase in the PCM content, the melting temperature shifted, and the degree of supercooling was observed to increase which was suggested to be caused by the physical interaction existing between PCM molecules, and the three-dimensional foam structure. Also, the latent heat of melting and freezing was observed to be impacted by the PCM content; thus, increasing PCM loading from 76 to 90.8 wt% resulted in increasing the latent heat of melting from 80.5 to 105.2 J/g which indicated that the thermal properties of the prepared composite PCMs were influenced greatly by the total pore volume of the carbon foam.

5.5.4 Thermal properties of composite PCMs prepared via chemical shape stabilisation approaches

Chemical processes of enhancing the shape stability of solid–liquid PCMs involve combining the chosen PCM with the supporting material with the help of a suitable crosslinking agent. It is important that both the PCM and the supporting material consists of suitable functional groups which enables the formation of chemical bonds called covalent bonds between them. Preparing chemically bonded composite PCMs via chemical approaches like chemical crosslinking is preferred over non-covalently bonded composite PCMs prepared via physical approaches since the latter exhibit drawbacks including poor mechanical performance and solvent resistance [216]. Also, if supporting material gets damaged by external forces or solvent, the PCM will suffer from leakage in addition to phase segregation due to poor compatibility between the PCM and the supporting material. Whereas covalently bonded composite PCMs acquire better mechanical performance and solvent resistance along with enhanced anti-leakage properties. In general, covalently bonded composite PCMs are prepared by reacting the chosen polymeric PCM like PEG with the chosen supporting material since PEG are susceptible to chemical modification due to the presence of plentiful of OH groups, while paraffins are saturated hydrocarbons which means they hardly react due to their inert nature [217,218]. Li et al. [181] prepared PEG10000/4,4′-diphenylmethane diisocyanate (MDI)/pentaerythritol (PE) composite PCM via chemical crosslinking. The latent heat of pure PEG10000 was reported to reduce from 189.45 to 152.97 J/g after chemical crosslinking which suggests possible weakening of the intrinsic endothermic and exothermic capacity of PEG due to the reduction in total number of crystallisable segments available attributed to the presence of the rigid benzene ring in the crosslinked structure. In addition, the crystalline regions had become smaller which was reflected by a decrease in phase transition temperature from 59.43 to 58.68°C. Chen et al. [219] prepared PEG10000/CA composite PCMs via electrospinning/crosslinking reaction with melting temperature and latent heat of 52.14°C and 36.72 J/g. The latent heat of PEG10000 was observed to have dropped by more than 50% in the composite which was attributed to the restriction of PEG phase due to the presence of interchain crosslinks within the molecules which caused confinement of PEG on the surface of the CA fibres during the phase transition process. Subjecting the crosslinked composite PCMs to 100 repeated heating and cooling cycles resulted in changing the melting temperature and latent heat from 52.14 to 52.12°C and 36.79 to 36.72 J/g, respectively. Thus, insignificant changes were caused to its thermal properties after thermal cycling exposure since the percentage difference was 0.038 and 0.19% which indicates that crosslinked composite PCMs have enhanced thermal stability which was accompanied by a reduction in the latent heat compared to the non-covalently bonded composite PCMs. Jiang et al. [199] prepared a series of PEG/cellulose acetate composite PCMs via crosslinking and discussed in their study the phenomenon behind the reduction in latent heat of pure PEG after crosslinking. First, the crosslinked composite PCM consisted of cellulose acetate impurity which resulted in destructing the perfection of the crystallisation course. Second, end OH groups of PEG were attached onto the backbone of cellulose acetate which meant PEG was no more able to form crystals freely due to the confinement of its crystalline segments. Also, it was evaluated that if the composite contains low PEG content, the latent heat of resulting composite PCM will be almost zero since at extremely low concentrations PEG is unable to aggregate which is necessary for it to form crystals. Xi et al. [220] prepared novel polymeric solid–solid PCMs and reported that the latent heat of prepared composites was lower than the pure PCM due to the incorporation of support materials which makes no contribution to latent heat. Chen et al. [219] prepared PEG/cellulose acetate composite PCMs with latent heat capacity of 36.79 J/g via the chemical crosslinking approach for TES applications [219]. A novel PEG/MDI/ (PE) tertiary cross-linking copolymer achieved a solid–solid phase change with phase transition temperature of 58.68℃ and enthalpy of 152.97 kJ/kg via a two-step condensation reaction at 90℃ for 24 h [181]. It is apparent that the solid–solid composite PCMs had enhanced thermal resistance; therefore, it was able to retain its solid state even when exposed to elevated temperature conditions. Chen et al. [179] prepared PEG10000/poly(glycidyl methacrylate) (PGMA) via chemical crosslinking. The latent heat of melting and freezing of pure PEG reduced after chemical crosslinking from 166.7 and 161.8 J/g to 69.8 and 73.2 J/g, and its melting temperature and freezing temperature reduced from 63.4 and 39.20°C to 55.9 and 31.1°C. This was attributed to the confinement of PEG molecular chain by PGMA skeleton which resulted in suppressing and partially restricting the orientation of the PEG molecules due to the steric effect which caused reduction in the crystallinity. Overall, the number of crystallisable segments and crystalline regions had reduced after crosslinking. This had impacted the intrinsic thermal performance of PEG negatively. Chen et al. [64] prepared paraffin/polyurethane (PU) composite PCMs containing PEG as soft segment via the bulk polymerisation approach. The phase transition temperature of the composite PCMs were observed to increase with the increase in the PEG content which was attributed to the attainment of a more ordered polymer which led to the enhancement of the crystalline properties of the resulting composite PCM. Three different paraffins, namely n-octadecane, n-eicosane, and paraffin were used to prepare the composite since this was predicted to enhance the latent heat and broaden the phase transition temperature of the resulting composite. As a result, two dips were observed in the heating cycle at 36.93 and 53.96 which indicates that no chemical bonds had formed in the composite PCM, instead weak intermolecular forces existed between the PCM and the supporting material. In the composite, insignificant changes were caused to the melting temperature of pure paraffin in the composite. However, increasing paraffin content reduced the latent heat and the melting temperature of the composite PCM because the lubrication of paraffin enhanced the mobility of the soft PEG chain. The synthesised composite PCMs were susceptible to thermal degradation when exposed to an elevated temperature of 300°C because this caused severe thermal destruction to the PEG chain present in the composite. Polarised optical microscopy (POM) analysis confirmed changes in crystallinity of PEG in the composite PCM. Hence, the spherulite size were observed to reduce with increasing 2,4-toluene diisocyanate (TDI) and 1,4-butanediol BDO (e.g. hard segment); therefore, the characteristic spherulite shape was not evident because the hard segment had reduced the degree of crystallisation significantly which is the main reason why the latent heat of the composite PCMs had changed after synthesis. Yanshan et al. [147] prepared a series of PEG/melamine/formaldehyde composite PCMs using PEG with different molecular weights (e.g. 1,000, 2,000, 4,000, 6,000, and 10,000) via condensation/crosslinking reaction. The latent heat of pure PEG was observed to have reduced in the composite PCMs mainly due to two reasons: (1) in the covalently crosslinked network structure, fewer crystallisable segments were available due to the fixation of nitrogen atoms in the end groups of PEG caused by the steric hindrance effect which resulted in reducing the crystallinity of PEG chains; and (2) reduced mobility of PEG chain and its fixed end groups meant that its crystal lattice was unable to arrange regularly to form perfect crystals. The melting temperature of pure PEG10000 and its composite was 61.46 and 54.84°C, respectively. Reduction in melting temperature was associated with the formation of covalent linkages near the end groups of PEG, e.g. bonds between PEG and nitrogen atoms from amino in melamine, which resulted in reducing the number of crystallisable segments of PEG available near the end groups of PEG.

PEGs are semicrystalline polymers which means they are comprised of both crystalline and amorphous phases. The degree of crystallinity also known as the crystallinity index (CI) can be calculated using the XRD data obtained from XRD analysis using equation (5), and it is conventionally used in previous literature to evaluate changes caused to the crystalline phases of PEG since this is known to affect its thermal properties like the latent heat capacity and the phase transition temperature. In addition, changes in the XRD patterns will indicate changes caused to the crystal morphology of PEG which impacts the thermal properties of PEG. Therefore, the current section will focus on studying the changes caused to the crystalline properties of PEG in the different composite PCMs prepared in the previous literature since this is influenced by various factors including reaction condition, temperature, time, chemical composition of the reaction mixture, type of crosslinking agent, solvent, etc.

Li and Ding [181] reported that the formation of the crosslinked network did not affect the crystal type since the XRD pattern of PEG remained almost unchanged after the chemical reaction; however, changes in the intensity of the crystalline peaks indicated that the size of the crystals of PEG had reduced which was attributed to the restricted mobility of PEG during the crystallisation process in the crosslinked network formed. Thus, the CI value of pristine PEG10000 was observed to have decreased insignificantly from 85.05 to 81.76% (e.g. 3.87% decrease) which suggested that the crystalline regions of PEG10000 was preserved and remained intact after chemical reaction. This is in good agreement with the DSC results which showed that the latent heat of melting of pristine PEG10000 had decreased from 189.45 to 152.97 kJ/kg (e.g. 19.26% decrease) due to the insignificant decrease in the number of crystalline phases. The POM images were in good agreement with the XRD results since it showed that the shape of PEG crystal did not change but the size of the crystals had reduced. Overall, PEG10000-based composite PCMs were observed to have good latent heat capacity because the chemical reaction was observed to cause no major changes to its crystallinity; however, the prepared composite PCMs were reported to have a melting temperature of 58.68°C which is outside the operating temperature range of building applications.

Li et al. [197] prepared a series of PEG-microcrystalline cellulose based composite PCMs using PEG of three different molecular weights, namely 1,100, 2,000, and 5,000 g/mol via the chemical stabilisation approach using MDI as crosslinking agent. The latent heat of melting of the prepared composite PCMs were reported to lie in the range of 46–153.6 J/g and was observed to depend mainly upon the molecular weight of PEG and the PEG content in wt% because composite PCM prepared using PEG5000 containing an optimum PEG content of 90.1 wt% was reported to have an optimum latent heat of melting of 153.6 J/g and a melting temperature of 58.9°C. Based on the experimental result, conclusion was made that increasing the PEG content (e.g. the working material) resulted in enhancing the latent heat capacity but this is not always true since PEG is a semicrystalline polymer; therefore, increasing PEG content does not only increase the crystalline phases but will be accompanied by an increase in the number of amorphous phases. For PEG-cellulose composites containing 57.6, 70.5, and 77.4 wt% of PEG1100, the latent heat of melting was determined to be 46.0, 60.5, and 73.3 J/g which was much lower than the one of pristine PEG1100 which had an enthalpy of 140.3 J/g. Hence, crosslinking reaction had caused enthalpy of PEG1100 to decrease significantly which was attributed to the restricted mobility of the PEG chain due to the formation of the highly crosslinked structure. In addition, the incorporation of the amorphous cellulose resulted in destructing the perfection of the crystal lattice of PEG which resulted in decreasing the latent heat enthalpy of PEG drastically. The phase transition temperatures of pristine PEG experienced negligible changes since after chemical crosslinking, it was observed to reduce from 27.0 to 25.7, 25.1, and 26.0°C which was attributed to the reduced lamellae thickness of the crystals that led to the formation of low melting point crystals. Overall, the study demonstrated the possibility of preparing PEG–cellulose composite PCMs with latent heat in the range of 79.9–94.7 J/g and the phase transition temperature in the range of 26.0–27.0°C which is within the operating temperature of building applications. However, the main drawback of current study was the use of MDI crosslinking agent since it is toxic even though it enhances the thermal resistance of the resulting composite PCMs due to the introduction of the phenyl groups in the crosslinked structure [185].

Yanshan et al. [147] reported that the degree of crystallinity of PEG6000-based composites was lower than pristine PEG6000 which indicated that the crystallite size became larger after reaction. In addition, small amount of supporting material in the composite was reported to cause no significant changes in the crystallinity of PEG. Changes in crystallinity was confirmed with the POM images obtained. The characteristic cross petal shaped spherulites of PEG at room temperature was less apparent in the composite which confirms the reduction in PEG crystallinity in the composite PCMs. Sundararajan et al. [184] prepared PEG8000/hyperbranched composite PCMs via crosslinking and reported that reduction in latent heat was attributed to the interference of hard polyurethane segment with PEG during the crystallisation process. Also, formation of a highly branched structure was another cause for making PEG less crystallisable. However, increasing PEG content seemed to have a positive effect on its thermal properties; thus, with the increase in the PEG content, less hard segment and increased number of crystallisable segments were present in the composite which resulted in enhancing the crystallinity of PEG. POM images confirm that the crystallite size of PEG had reduced after chemical crosslinking reaction which indicates that crystalline properties of PEG get changed after chemical reaction which may be due to elevated temperature exposure during reaction. Sari et al. [221] synthesised novel polystyrene-co-maleic anhydride (SMA)-graft-PEG copolymers as solid–solid PCMs (S-SPCMs) for solar passive TES applications. The phase transition temperature was in the range of 39–45℃, with latent heat capacity in the range of 107–155 J/g. Hence, the phase transition enthalpies and temperature were influenced greatly by the amount of PEG side chains bonded to the backbone. The resulting copolymers were reported to exhibit good thermal reliability and chemical stability even after 5,000 heating–cooling cycles because negligible changes in their LHTES properties was observed and no changes in their chemical structures occurred. Zhou et al. [222] prepared a novel PEG/cellulose nanocrystal (CNC) PCM via a UV-induced thiol-ene click chemistry and solvent exchange. Results showed that the loading capacity of PEG could be as high as 97% with latent heat capacity of 150 J/g. The phase transition of pristine PEG was observed to reduce from 40.1 to 33.5℃ which was attributed to high specific area of CNC network, and the presence of strong intermolecular hydrogen bonds between PEG and CNC. Zhou et al. [223] prepared PCM/PVA composite PCMs and reported that changes in latent heat of PCM was attributed to the incorporation of supporting material and crosslinking agents since both make no contribution to latent heat. Also, increasing GA content (e.g. crosslinker content) was reported to cause no changes to the phase transition and latent heat of the composite PCMs, and subjecting the crosslinked composites to 100 repeated heating and cooling cycles only reduced the latent heat by 2 and 1 J/g. Yin et al. [224] prepared PEG6000/Poly(glycerol-itaconic acid) (PGI) composite PCMs with optimum PCM content of 72.67% and corresponding latent heat of 86.93 J/g. The crystallinity of the prepared composites was observed to be lower than the one of pure PEG6000 which was attributed to the incorporation of PGI which acts as an impurity to impact the crystallinity of PEG. Also, hydrogen bonds between PEG and PGI were also reported to influence the crystallinity of PEG. Increasing PCM content in the composites from 62.76% to 68.70% to 72.67% resulted in increasing the crystallinity from 50.15% to 60.78% to 60.95% which indicates that the mobility of the PEG chain appears to improve with the increase in the PCM content; thus, composite PCM containing optimum PCM content of 72.67% had the highest latent heat of melting of 86.93 J/g which suggests that composite PCMs with high crystallinity have higher number of crystallisable segments.

Kuru and Aksoy [225] prepared PEG-cellulose composite PCMs using PEG1000 and PEG2000 as PCMs, cotton waste as supporting material and GA as the crosslinking agent. The latent heat capacity and melting temperature of the prepared composite PCMs were in the range of 8.1–33.8 J/g and 17.3–51.2°C. The results demonstrated that using PEG with low molecular weights resulted in producing composite PCMs of poor thermal properties. Also, it had an operating temperature outside one of building applications. Qin et al. [226] prepared PEG based composite PCMs via the chemical process called free radical polymerisation and reported that the crystalline properties of pristine PEG experienced negligible changes after the chemical reaction because the two characteristic peak of pristine PEG4000 appearing at 2θ = 19.4 and 23.5° attributed to the lattice planes of (1 2 0) and (1 3 2) of the triclinic crystal remained intact. The XRD pattern of both PEG and the prepared composite PCMs were observed to be similar but in the XRD pattern of the prepared composite PCMs, a wide broad hump was observed at 2θ = 21.0° which confirmed the existence of amorphous structures in the crosslinked composite PCMs. The calculated CI values indicated that the formation of a crosslinked structure resulted in decreasing the crystallinity of PEG which was attributed to the increased number of amorphous phases in the PEG based composite PCMs. Overall, XRD results suggested that the crystal structure of PEG became less perfect after the chemical reaction which resulted in impacting its latent heat of melting and crystallisation negatively. Table 14 shows the relationship between design parameters and the thermal properties of prepared composite PCMs and outlines the characteristic properties and performance of PEG/cellulosic material based composite PCMs prepared in previous literature.

5.6.1 Shape stabilised composite PCMs in buildings

Major advantage of PCMs is that they can be incorporated virtually into all constituent building elements such as wall, floor, ceiling, and roof due to its easy installation process for the enhancement of the thermal performance and energy efficiency of buildings [63]. Different approaches have been used to combine PCM with the building envelope: direct incorporation, immersing incorporation, shape-stabilisation method, and encapsulation. In general, lightweight buildings are known to suffer from indoor temperature fluctuations which is attributed to their low thermal mass. Hence, literature reports that the indoor temperature of lightweight buildings (LWB) are usually 10°C higher than the outdoor temperature which results in decreasing the indoor thermal comfort level while increasing the energy consumption of air-conditioning [244]. Kuznik and Virgone [245] installed shape stabilised in LWB and the results showed that PCM was capable of effectively controlling indoor temperature fluctuations and increase during summer; thus, the optimum indoor temperature was decreased by 8.5°C. Kim et al. [246] demonstrated the possibility of stabilising the indoor room temperature and reducing the heating load in winter by installing a paraffin-polypropylene/elastomer composite PCM sheets of melting temperature in the range of 19.0–26.0°C and latent heat of 62.24 J/g on the floors, walls, and ceilings of a building. The power consumption of a hut without and with composite PCM sheets were 15.30 and 12.48 MJ. Thus, installation of PCM had resulted in reducing the power consumption by 18.40% and increasing the PCM installation area was observed to reduce the power consumption further. Findings suggested that the effectiveness of PCMs in buildings depended mainly upon the PCM installation area, PCM content, and position of PCM. Yao et al. synthesised a novel composite PCM wallboard (PCMW) by a horizontal vacuum absorption rotate roller technique, in which paraffin and EP was used as the PCM and supporting material. They embedded the composite PCMs in the walls, and roofs of the test room of dimensions 1.7 m × 1.7 m × 2.2 m. PCMW can be used in building envelopes since they had sufficient latent heat capacities: latent heat of melting and freezing was 67.13 and 69.06 J/g, respectively. The latent heat values of PCMW were greater than the criterion (50 J/g), therefore fabricated composite PCMs are feasible to be utilised in buildings. The incremental cost of PCMW can be fully recovered in 5.84 years. The potential composites can be used to enhance the thermal comfort as well as the energy efficiency of buildings. Dodecanol is one of the commonly used fatty acid as PCM for the preparation of composite PCMs using different building materials. In 2013, Memon et al. [247] prepared Dodecanol/cement composite PCMs with a very low latent heat capacity of 18.39 J/g with a melting temperature of 21.06°C. Therefore, majority of current studies have worked on optimising the latent heat capacity of the resulting PCBMs. Concrete and cement are the most used building material; therefore, incorporating PCM in concrete and cement can reduce energy consumption of buildings greatly. One of the commonly employed methods of increasing the energy performance of the building envelope includes employing innovative super-insulating or functional mortars which are prepared by combing PCMs in cement mortars. Li et al. [248] incorporated cement mortar with EG and PCM with melting temperature and latent heat capacity of 28.8°C and 209.3 kJ/kg and reported that the resulting composite PCMs had a melting temperature and latent heat capacity of 28.55°C and 183 kJ/kg. PCM incorporation had reduced the compressive strength of the cement mortar greatly; thus, results indicated that the optimum PCM content was less than 20% to produce functional cement mortars of adequate compressive strength of 5 MPa.

5.6.2 MPCMs in buildings

Main obstacle for long-term use of PCM in building application is that they are susceptible to leakage. Also, direct incorporation of PCM with building materials will influence their intrinsic properties by interacting with its building structure. Integrating MPCMs into conventional building materials can overcome this problem [249]. MPCMs since its introduction has commonly been applied in building materials for the preparation of PCBMs. The following paragraph describes the thermal performance of different PCBMs prepared in the past since the impacts caused by PCM to different building materials is known to vary; however, literature is yet to compile relevant information regarding this. Thereby, better understanding the compatibility between PCMs and the different building materials can be determined.

In general, incorporation of encapsulated PCMs in building material is considered crucial to avoid affecting the function and the intrinsic properties of the building material [231]. Gypsum plaster and boards are one of the most popular and conventionally used material in building construction of internal and external walls and ceilings because they are available at relatively low costs, have good fire-resistance and aesthetics, and are non-toxic. Therefore, gypsum board and/or plaster containing MPCMs have been used increasingly in surrounding wall construction of buildings for the optimisation of building energy performance. Schossig et al. [250] conducted both simulation and experiments for a duration of 5 years to investigate the feasibility of applying MPCMs in building materials. Cabeza et al. [251] conducted experiments to compare the performance of two small house sized cubicles: one with integrated MPCMs; and the other without MPCMs, for a duration of 6 months. They used PCM with a melting point of 26℃ and a phase change enthalpy of 110 kJ/kg. Incorporating 5% of PCM in concrete resulted in producing composite PCM with compressive strength and tensile strength of 25 and 6 MPa. They reported that the results demonstrated a real opportunity in energy savings for buildings because energy storage in concrete walls integrated with MPCMs had enhanced its thermal inertia and caused reduction in the inner temperatures. Lee et al. [252] prepared microcapsules with latent heat capacities of 210 J/g (23℃) and 200 J/g (24℃) and integrated these microcapsules into gypsum wallboards and olefin film for building materials. The thermal conductivity of gypsum board without PCM (0.144 W/m K) increased significantly. Gypsum board with PCM had a thermal conductivity in the range of 0.128–0.163 W/m K. Also, with the increase in the thickness of the olefin film, the thermal storage capacity inside the chamber increased. For practical applications, the thickness of PCM plate must not exceed 20 mm to cause significant reduction in the indoor fluctuation temperature [253]. The amount of PCM incorporated in the gypsum board dictates its thermal performance. Oliver [254] found that increasing the PCM content from 37.5 to 44.5% resulted in improving the thermal performance of the gypsum-based composite by 20% and concluded that 45 wt% of PCM incorporation was capable of absorbing five times the energy that traditional plasterboards could absorb. Barzin et al. [255] investigated the thermal performance of PCM impregnated gypsum board and reported that an energy saving of 93% was achieved which corresponded to a cost saving of 92% provided that PCM charging was facilitated by a combination of night ventilation and free cooling method since improper control strategy of PCM led to an increase in the amount of air-conditioning energy required. Borreguero et al. [256] studied the thermal performance of three gypsum boards one without PCM and two containing 4.7 and 7.5 wt% of Rubitherm^® (RT27) MPCMs at an optimal core/coating mass ratio of 1.50. Results indicated that gypsum board incorporated with 7.5 wt% of MPCM could enhance the thermal comfort of the indoor conditions by either increasing or decreasing the room temperature by 1.3°C. Borreguero et al. [257] discovered that incorporating 15% of MPCM in the gypsum board could save 4.5 kW h of energy per operating cycle in a standard room covered with 1 m³ of gypsum board; however, presence of MPCM in the pores of gypsum resulted in reducing its compressive strength which was attributed to the decreased bulk density of gypsum due to the modification of its porosity. Impregnating fresh concrete/or cement mortars with PCM is a traditional approach used to effectively enhance its energy efficiency. Moreover, it is reported to have an adverse effect on the mechanical strength of concrete. MPCMs are added into concrete and cement mortars to avoid PCM bleeding during the melting process. Cabeza et al. [251] reported that MPCM had real opportunity in energy savings for buildings since incorporation of MPCM in concrete wall caused significant enhancement of thermal inertia which subsequently resulted in decreasing the indoor temperature because for the enhancement of the thermal performance of buildings, the thermal conductivity of concrete needs to reduce and the TES capacity of concrete needs to increase. Hunger et al. [104] reported that an inclusion of 4–5% of PCM with a phase transition temperature in the range 24–36°C optimised the thermal mass of concrete to 6,800 J/K which corresponded to an energy saving of up to 12% which was accompanied by a significant compressive strength loss; and 3% incorporation of PCM resulted in producing PCM-concrete with a compressive strength of 35 N/mm² which was reported as being acceptable for constructional purposes. Entrop et al. [258] investigated the effects of applying MPCM with a melting point of 23°C and latent heat capacity of 110 J/g. Results showed that concrete floors containing 5% of PCM was able to reduce the additional need of heating in houses. Also, concrete floors with a thickness of 50 mm were observed to have a large latent heat capacity but its charging period was too long. Overall, the main problem of incorporating PCMs encapsulated within an organic polymeric film via the microencapsulation approach is that it reduces the compressive strength of concrete drastically. Increasing the PCM content in cement mortar results in decreasing the thermal conductivity and the compressive strength which is attributed to the increased porosity of cement mortar after MPCM incorporation [259].

However, this problem was demonstrated to be overcome through the incorporation of macro-encapsulated PCMs since these techniques causes no intrinsic changes to the concrete because the PCM is stored in containers of desired shape. Metals like steel is conventionally used as the container for this purpose since they can overcome both the leakage problem and the inherently low thermal conductivity of PCMs. Dong et al. [260] encapsulated an optimum PCMs mass of 80.3% within hollow steel ball (HSB) containers with an outer diameter of 22 mm. To investigate the influence of PCM-HSB on the thermal performance and compressive strength of concrete, 25, 50, 75, and 100% of coarse aggregates in concrete was replaced with PCM-HSB, and results indicated that the compressive strength of concrete decreased with the increase in the PCM-HSB content. Cui et al. [261] reported that exceeding a replacement level of aggregates by 75% of PCM-HSB will cause severe reduction in the compressive strength of concrete (e.g. 55% reduction). Shi et al. [262] investigated the effects of using straight and hooked steel fibres on the compressive strength of concrete because steel fibres are reported to enhance the mechanical and thermal properties of concrete which is attributed to its high bending resistance, tensile strength, and thermal conductivity. Thus, incorporating steel fibres in concrete containing PCM-HSB as aggregate is known to improve the compressive strength concrete. Therefore, Cui et al. [263] used steel fibre reinforced PCM-HSB concrete for the enhancement of energy piles performance. Results showed that incorporating 0.70% of steel fibre in PCM-HSB concrete resulted in enhancing the compressive strength by 63% and the heat capacity of concrete was also observed to have improved. Kuznik and Virgone [245] investigated experimentally, under controlled thermal and radiative effects, the thermal performance of a PCM composite wall containing 60% of microencapsulated paraffin within the copolymer in a full-scale test room. The tests were carried out in three different external climates: summer, winter, and mid-season, to evaluate the potential of PCM wallboards. Tested PCM could maintain the room air temperature within the comfort zone by reducing the maximum air temperature of the room to 4.2℃. Enhancement of thermal comfort is very critical in cases where the wall surface temperature is lower for PCMWs compared to the regular ones. Also, no thermal stratification was observed in the room containing PCM wallboard which indicated that natural convection existed with PCMWs.