A Comprehensive Review on Phase Change Materials and Applications in Buildings and Components

Energy consumption of the building sector is over 30% of the world´s final energy use with a high tendency of increase in the upcoming years because of the continuous growth of the world population and upgrading housing conditions. The thermal losses and solar heat gains in buildings depend on the design, orientation, material used for construction, wall areas, glazed areas, and roof area. Improvements of the thermal performance of buildings and increase energy efficiency require improving the thermal design of buildings, innovation of construction materials, and the introduction of new and proven concepts for windows, walls, roofs, and floors. The processes of heat and mass transfer between the building and external environment occur through the building envelop. Hence, the thermal resistance of the walls can be increased by incorporating insulating material, new construction bricks with low effective thermal conductivity, green walls, and/or the application of reflective coatings on the outer surface of the envelop [1–5].

Thick walls offer more resistance for heat transfer and hence decrease the indoor temperature, decrease the indoor temperature swings, and enhance the time delay factor. Phase change materials are investigated as substitutes for thick walls and thermal insulation since phase change material (PCM) has high heat capacity and changes phase isothermally. PCM can provide a more adequate solution if inserted in the wall. During the day it absorbs heat and changes phase and during the night releases heat to the external ambient maintaining the indoor environment at nearly constant temperature [6].

The inclusion of PCMs in roofs, external walls, floors, windows, sunshades, mortars, and finishing plasters in residential buildings showed significant improvements in the thermal performance of these elements. The inclusion of PCM in the construction materials such as mortars, concrete, and similar products needs great attention because of the possible degradation of the mechanical strength of these products along with possible leakage and fire hazards. The integration of PCMs in building elements can be divided into three types: direct incorporation and immersion, macro-encapsulation, and micro-encapsulation. The encapsulation geometries and their use depend on the element where they are to be incorporated. Encapsulation material serves as a barrier between PCM and the surrounding environment and provides long-term durability and structural requirements. Micro-encapsulation of PCMs provides faster charging and discharging. PCMs incorporated in construction materials modify the thermal and physical properties, increase the thermal resistance, and enhance the thermal performance of bricks concrete, mortars, and components as well as plasterboards [7–9]. Two recent reviews [10,11] present new fields of applications of PCM for ice and thermal control of equipments and PV panels, in the automotive, food, and textile industries.

PCM technology applied in the building industry is a real fact in recent commercial and residential constructions where energy management is a solid concern such as residential buildings and low energy consuming residences. As will be seen in some applications there is a need for further development and research work while in others the technology is already in the production process.

This review covers PCMs and their applications in different areas of the building sector such as modeling of processes and components with PCM, development of materials like mortars, concrete, bricks, panels, windows, and wallboards. The review starts with a brief survey of PCMs including classification, thermal and physical properties, and products suitable for the construction industry available in the market. Materials, elements, and components treated in the review are phase change materials, PCM mortars and ricks, PCM concrete and brick walls, PCM wallboards, PCM roofs, PCM floors, PCM Trombe walls, PCM windows, and PCM facades.

In the present review, the authors tried to comment on the reviewed material in an individual form to make the analysis clearer and more objective. To avoid repeated descriptions and comments as well as to make the text less tedious to read, we used tables and included articles collectively (lumped) cited to give some more details and fair recognition. The reviewed articles cover mainly the last 12 years but some relatively old articles were also included because of their relevance, contribution, and importance.

The authors hope that this review can be of help to practicing engineers, architects, researchers, students, and whoever wants to know about PCM and their continuously growing potential in the building sector and their possible energy and environmental impacts.

Energy storage is turning to be a key component for efficient use of energy, a vital member of any integrated and conservation system. There are different means for energy storage depending on the form of energy such as mechanical, thermal, and so on. For the present study, our interest is in thermal energy and hence other forms of energy storage will not be discussed.

Thermal energy can be stored basically in two forms, as sensible heat in fluids and solids by rising of the temperature of the solid or fluid storage mass as in the case of domestic solar heating of water. Also, thermal energy can be stored in the form of latent heat which is utilized during the phase change process. In the charging process, heat is used to melt the solid into liquid isothermally. When the stored energy is needed, heat is retrieved from the liquid mass and solidification occurs. Latent heat storage has some advantages over sensible heat storage including nearly constant temperature during the phase change process, higher heat capacity, smaller volume and lower cost, along with other favorable thermal properties. Because of these favorable properties latent heat materials are more preferred than sensible heat storage materials. In recent years, phase change materials have been widely used in many fields such as thermal storage, thermal shield, enhancement of thermal mass, control of thermal processes, and many other applications as in the building industry which is the main focus of the present work.

The incorporation of adequate PCM in the construction materials, building components, and envelops can produce significant reduction in energy consumption and emissions and makes the buildings more sustainable. In new constructions, there is no problem of including new materials, components, etc. New buildings are only a small part of the already existent residential and commercial buildings. A significant impact can be achieved if building retrofitting and adaptation of existent buildings are financially facilitated and could incorporate these new concepts and PCM-based elements in the building adaptation process by installing PCM wallboards, shutters, windows, etc.

In most of the thermal applications of PCM, the transformation from solid to liquid and vice versa is of primary interest due to some inherent advantages while changes from liquid to gas and the opposite are not applicable because of some technical limitations. There is a big variety of phase change materials of different melt heat and melt temperature. Numerous classification methods exist but the most common is the subdivision into organic, inorganic, and eutectic as in Fig. 1 [2,3,12–14].

Organic PCMs cover a wide range of materials but the most known in storage applications are pure n-alkanes, esters, and fatty acids. The high heat capacity, adequate range of phase change temperature, chemical stability, being noncorrosiveness, nonsubcooling, and inactiveness are the advantages of these PCMs for energy storage and building applications. Organic PCMs are known to be poor conductors of heat, with high changes in volume during phase change and they have the defect of high flammability. Fatty acids can be useful for cooling applications because these acids have favorable properties such as high heat of fusion, low super-cooling, no segregation, and various ranges of phase change temperatures, but they are expensive in comparison to paraffin [2,14].

Paraffin organic PCMs: The increase of the chain length of paraffin increases both the latent heat which is usually in the range from 128 kJ/kg to 198 kJ/kg and phase change temperature range from −12 °C to 71 °C. The thermal conductivity of paraffin varies from 0.21 to 0.24 W/m K. These low values limit its widespread application.

Nonparaffin organic PCMs: Nonparaffin PCMs include alcohols, esters, glycols, and fatty acids and have different physical and chemical characteristics. Fatty acids have many suitable characteristics for building and other applications but are more expensive and slightly corrosive [2,14].

Advantages of inorganic PCMs include high latent heat, low cost, and flammability but they suffer from decomposition, super-cooling, phase segregation, and are considered thermally unstable and corrosive. Salt hydrates as part of the inorganic PCMs are more used in storage applications because of their high volumetric storage capacity of about 350 MJ/m³, high thermal conductivity of about 0.5 W/m K and generally have a lower price in comparison with organic PCMs. Inorganic materials cover a wide temperature range but have the disadvantage of compatibility with metals [2,14].

Eutectic PCMs are formed by combining different phase change materials. During the solidification process, they produce a mixture of crystals with unlikely separation of components since they change phase without segregation. In the fusion process, all elements are converted to the liquid state simultaneously. Some of these eutectic PCMs can be used for passive cooling systems of buildings [2,14].

Phase change temperature of eutectic water–salt solutions is below 0 °C and can be decreased further by adding salt to the mixture. Since these solutions are composed of at least water and salt phase separation may occur. To avoid phase separation, increase storage capacity and increase cycling stability it is recommended to use eutectic compositions. The thermal conductivity and subcooling of eutectic water–salt solutions are similar to those of water and exhibit volume change of 5–10% during phase change, are chemically very stable and safe but corrosive to metals. Besides, they are cheap, often costs less than 1 €/kg [14]. Table 1 shows some eutectic water–salt solutions used as PCM.

Salt hydrates are composed of salt and water, have their melting temperature in the range of 5 °C and 130 °C, and have phase change temperature higher than that of water and thermal conductivity close to that of water. Salt hydrates suffer from both phase separation and subcooling and have a volume change during phase change of about 10%. In general, they are chemically stable but incompatible with metals. The price is about 1–3 €/kg. Table 2 shows some examples of salt hydrates that are used for commercial PCM [14].

High melting temperature salts usually have high latent heat since the latent heat increases by the increase of the phase change temperature. The salt is composed of two components hence phase separation is a real concern. Salts usually are thermally conductive and have marginal subcooling and variation in volume change is about 10%. They are in general chemically stable and can be corrosive to metals. The price varies according to the type of salt. Table 3 shows some salts used as high-temperature PCM.

To obtain phase change materials with different characteristics mixtures of inorganic materials can be prepared experimentally and their properties can be measured to ensure achieving the required parameters. NaCl and KCl when mixed with CaCl₂ ⋅ 6H₂O can improve only the melting performance, while mixing Mg(NO₃)₂ ⋅ 6H₂O and MgCl₂ ⋅ 6H₂O reduces only the melting temperature. This mixing process is usually conducted experimentally and may require extensive experimental efforts. Table 4 shows some inorganic mixtures used as PCMs.

Organic PCMs are adequate for the temperature range up to 200 °C but are not stable at higher temperatures and have a density of less than 103 kg/m³. Hence, except for sugar alcohols, organic PCMs have less latent heat of fusion in comparison with inorganic materials. Fatty acids have latent heat similar to that of paraffin, are stable upon cycling, have no phase separation or subcooling and low thermal conductivity. Tables 5 and 6 show some of the thermal properties of paraffin and fatty acids, respectively.

Polyethylen glycol, PEGs for buildings applications: Polyethylene glycol (PEG), as well as paraffin and fatty acid, are organic PCMs. PEG is a polyether compound with numerous uses, including biochemistry, medicine, biology, and commercial uses. PEG has high latent heat at phase change temperatures that can be adjusted. It has poor thermal conductivity and low heat transfer behavior. These two parameters can be enhanced by using metallic foams, fins, metallic meshes, dispersed metallic particles, and more recently by adding nanoparticles to form a nanofluid or a nanocomposite. Table 7 shows the thermal properties of some polyethylene glycols which can be used as PCM.

PCMs incorporated in buildings materials and components should have the required economic and thermo-physical properties which are hardly all met by a PCM. Usually, phase change temperature, melting enthalpy, thermal conductivity, volume change during phase change, and densities of the PCM liquid and solid phases are the key factors. In a practical application, a technical compromise is usually made to include the other factors so that one can achieve the most adequate choice that satisfies nearly most of the required features.

PCMs selected for incorporation into a building must have a melting temperature matching the operational temperature and high latent heat capacity and high specific heat to use less PCM and small container. The heat transfer to and from the phase change materials during the charging and discharging processes requires that the PCM has a high thermal conductivity which could also be enhanced by metallic additives, fins, porous foams, or both depending on the application. Low vapor pressure and small variation in volume during the phase change process are required design parameters. The thermal properties of some PCMs are listed in Tables 8–10. A general comparison between the different types of PCM is shown in Fig. 2.

The main methods for the insertion of PCMs in the construction materials and buildings elements are direct incorporation and encapsulation. The direct incorporation of PCM into the building materials and structures such as concrete and slabs is not well accepted in the building industry for practical difficulties such as leakage, diffusion of PCM into the material and thermal and mechanical gradual degradation of the element. Innovation of new PCMs, extensive experimental research and developments are required to solve these problems associated with the direct incorporation of PCM in the construction materials.

The encapsulation of PCM has several advantages one of which is the increase of the surface area which increases the effective heat exchange rate and hence reduces the full charging and discharging times. Also encapsulation can be regarded as a mechanical shield protecting the PCM from aggressive environmental factors which can degrade its thermal performance and deteriorate its composition. Encapsulation of PCM is classified as macro (diameters of 1 mm and more), micro (from 1 µm to 1 mm), and nano (less than 1 µm).

Macro-encapsulation refers to PCMs encapsulated in containers such as cylinders, spherical shells, and panels made in dimensions according to the applications. Figure 3 shows macro-encapsulation in spherical shells and cylinders.

Because of the low thermal conductivity of some PCMs, macro-encapsulated PCMs tend to solidify or melt at the edges and near the contact surface. This impairs the heat transfer process causing an increase in the charging and discharging times. Another relevant issue is the size of the macro-capsules which requires special attention to avoid possible destruction or perforation of the containers. If these aspects are considered and rigorously observed macro-encapsulated PCM can be adequately incorporated with other construction materials permitting the use of the current construction methods [13,14].

Micro-encapsulation presents significant advantages such as the increase of the heat transfer area which enhances heat transfer and reduces heat charging and discharging times. It also avoids leakage during the phase change process, reduces PCM reactivity with surroundings, and controls the volume changes during phase change. The general form of micro-encapsulated PCM can be either a regular or an irregular shape. Micro-encapsulated PCMs are available as a powder or dispersed in a liquid. This enables adding the micro-encapsulated PCM directly to concrete, mortars, and gypsum without the risk of leakage. Although micro-encapsulation permits the integration of PCM in the building materials, it reduces the effective storage capacity and may affect the structural strength of the material.

PCMs have demonstrated their abilities to enhance the thermal performance of components and other materials to which they are added, increase the thermal mass of buildings envelops, walls, roofs, etc., reduce indoor temperature fluctuations and increase the time lag factor of internal ambient. The conversion of the concepts and materials based on organic and nonorganic PCM to real products and elements for the building industry was relatively quick and efficient. Commercial PCMs in macro-encapsulation form are available in spherical capsules, cylinders, and flat containers, while micro-encapsulated PCMs are usually in liquid form or dry powder to be mixed with other materials such as gypsum, mortars, plasters, and concrete. The commercial products are usually certified against leakage, degradation, and fire hazards. Tables containing description of PCM commercial products, manufacturers, and other relevant data can be found in Refs. [12–16].

Building envelops, floors, roofs, windows, and facades are weak barriers against heat loss and gains from the external environment. The high thermal resistance and high thermal capacity of construction materials are important parameters that help to reduce consumed energy and improve thermal comfort. A recent comprehensive review presented various studies on evaluating micro-encapsulated PCM capacity to improve the thermal properties of construction materials like mortar, bricks, cement, plasterboard, and gypsum [17]. Other studies were destined to the development of new materials, increasing the mechanical and thermal efficiency of building materials, and reducing the temperature variations. Relevant technical details on manufacturing processes, thermal and mechanical tests as well as tests of performance on PCM mortars and concrete can be found in Ref. [18].

PCM can be incorporated into the construction materials by two methods direct and indirect. The indirect methods are widely investigated and they showed significant potential for application in buildings. The direct method although easy to be applied suffers serious operational and performance problems which limited its use. Figure 4 shows construction bricks with macro-encapsulated PCM. Drissi et al. [19] reviewed PCM micro-encapsulation and composites in cement-based materials and highlighted gaps for future studies. In another study, Rao et al. [20] reviewed the literature involving PCM mortars and their thermal and mechanical properties. Song et al. [16] reviewed building energy performance improvement using phase change materials. Akeiber et al. [13] and Kalnæs and Jelle [12] presented a review on the applications of PCMs for passive thermal control and presented a detailed list of PCMs appropriate for passive comfort in buildings.

Some interesting reviews were directed to evaluate numerical methods used for the simulation of phase change materials within building elements. Such numerical techniques include the enthalpy-based methods, CFD methods, immobilization techniques, and others [5,6].

Some investigations were directed to modeling the complex structure of mortar and bricks with PCM impregnated or incorporated in micro-capsules as in Mankel et al. [21] who investigated the modeling of cement-based PCM mortars by using the enthalpy-based approach and an apparent calorific capacity method. In another study, Younsi and Naji [22] addressed the thermal performance enhancement of PCM brick walls by using a numerical approach based on one-dimensional transient model. The predictions were validated with experimental results. Using simulations, Sharma and Rai [23] assessed the potential of PCM-envelops for reducing the cooling requirements of the residential sector and studied the influence of PCM design parameters such as the layer thickness and its location and the PCM physical parameters. Gao et al. [24] analyzed the thermal behavior of hollow bricks filled with PCM and observed an attenuation rate from 13.07% to 0.92–1.93% as well as an increase in the delay time from 3.83 h to 8.83–9.83 h. Lucas and Aguiar [25] studied experimentally the use of cement, lime, and gypsum as possible binders for PCM mortars. The laboratory tests showed that the addition of PCM is viable and the experiments demonstrated that the inclusion of PCM in mortars helps to absorb heat, lowers the energy demands of buildings, reduces the peak temperature, and increases the time delay.

Other investigations were directed to experimental treatment to identify adequate binders, loss of mechanical properties or thermal properties, leakage of PCM and possible chemical reactions with binders. The experimental investigations were done to assess the effects of binders and the determination of cement mortars of adequate thermal and mechanical characteristics. The results proved that nonencapsulated PCM mortars are adequate for application [26,27].

The problems associated with encapsulation and impregnation of PCM in the mortar, bricks, and concrete are of extreme importance since they affect both the thermal and mechanical properties. Figueiredo et al. [28] and Haurie et al. [29] investigated the PCM concrete and analyzed its thermal and mechanical properties, effects of the PCM fractions in mortars and their impact on the comfort and energy savings of the building. Cunha et al. [30] investigated the PCM fresh and hardened mortars and commented on the workability, microstructure, mechanical and thermal properties, and adhesion. Cui et al. [31] prepared a graphite-modified PCM and incorporated into the cement mortar. The results revealed the reduction of temperature swings and the internal ambient temperature.

The inclusion of phase change materials in plastering mortars is shown to be effective in reducing energy consumption in buildings. Kheradmand et al. [32] and Lucas et al. [33] proposed the demonstration of the effectiveness of the inclusion of PCM in construction mortars, compared their performance with and without PCM, and addressed the effect of micro- and nanomaterials in the mortars matrix. Xu et al. [34] developed a cement-PCM composite, while Lecompte et al. [35] studied the thermal and mechanical characteristics of PCM concretes and mortars and their possible adequacy for use in the construction of buildings.

The thermal performance of a building is highly dependent on the PCM and where it is inserted in the construction elements such as envelops and roofs. Izquierdo-Barrientos et al. [36] investigated numerically the external PCM walls and reported that PCM reduced the wall heat gain and increased the maximum temperature delay. Lee et al. [37] and Vaz Sá et al. [38] investigated numerically and experimentally PCM plastering mortars and thin layers of PCM on increasing the time delay factor and reducing the heat gain. Vicente and Silva [39] conducted an experimental study of brick masonry walls with PCM macro-capsules. They evaluated the influence of the PCM on the attenuation of temperature fluctuations and time constant. Silva et al. [5] investigated the addition of macro-encapsulated paraffin in brick wall and the results revealed that PCM reduces the internal temperature and increases the time delay. In other two studies, Zhang et al. [40] assessed numerically and Castell et al. [4] investigated the performance of PCM brick walls and found that the insertion of PCM increased the capacity and improved the thermal performance. Alawadhi [41] investigated numerically the effects of holes filled with PCM in normal construction bricks on their thermal performance and reported a reduction of heat gain of about 17.55%. Ming and Ming [42] inserted PCM in insulating bricks and found a temperature reduction of 4.9 °C in comparison with bricks without PCM.

As can be seen from the literature review intensive studies were dedicated to find solutions for incorporating PCM in mortars without reducing their mechanical properties which is essential as a construction element. It seems that only indirect incorporation shows well-accepted potential for practical use. Bricks with indirect insertion using micro-encapsulation appear to be acceptable. Direct insertion is still under development and several problems still exist as limited amounts of incorporated PCM, leakage, reduction of mechanical properties, and variation of binder's properties with cycling and temperature variation are still to be solved.

The use of PCM with the construction materials proved to be adequate to enhance the storage capacity of building envelops and improves the energy efficiency. Arıcı et al. [43] and Al-Absi et al. [44] in an attempt to optimize the use and application of PCM in walls conducted investigations to assess the influence of the PCM and its properties, its location and thickness on the thermal characteristics of the wall. Lakhdari and Chikh [45] modeled the problem of phase change in a wall composed of a mixture of mortar and micro-capsules of binary PCM and found that the increase of the PCM fraction (<20%) in the wall panel decreases in the effective thermal conductivity and increases the stored energy. Cui et al. [15], Bland et al. [46], and Akeiber et al. [13] presented reviews on phase change material application in building and discussed types of PCM, thermal–physical properties of PCM. The position of the PCM layer within the concrete wall produces significant effects on the thermal performance and time delay factor. A concrete wall with PCM located at different positions is shown in Fig. 5. Mavrigiannaki and Ampatzi [48] and Cuia et al. [49] reviewed the potential of insertion of phase change materials in building elements. The reviews indicated the necessity of more research, development, and tests on buildings in real use for more credibility. Memon [50] reviewed the different modes of incorporating PCM into constructive elements. Macro-encapsulation of PCM in concrete walls was investigated numerically to assess its impact on thermal comfort and it was found that the numerical models are efficient and adequate for correct predictions [47,51]. Another study focused on applying PCM on the external envelop surface indicated that the fusion temperature is an important parameter [52].

Akeiber et al. [53] reviewed the development of PCM models in buildings and concluded that thermal benefits of the inclusion of PCM in building materials and elements are dependent on the PCMs and their thermal, physical, and chemical characteristics, the construction element and its operational conditions. Pomianowski et al. [54] focused their review on PCM and the corresponding technologies adapted to facilitate using PCM and PCM-based products in the building sector. In another interesting review, Soares et al. [7] examined previous studies related to the use of PCM for thermal comfort, efficient energy management, and improvement in building performance. Figure 6 shows a micro-encapsulated PCM within the internal plaster of a lightweight wall.

Incorporation of PCM in concrete showed some good benefits such as high storage capacity and thermal insulation effects but deteriorated the mechanical resistance properties of concrete. The reduction of the mechanical resistance properties can be alleviated by choosing adequate PCM and an adequate method of incorporation in the concrete. Types of PCMs adequate for building applications, methods of integration of PCM in the concrete and possible impacts on the mechanical and thermal properties in the fresh and hardened states were reviewed in Ref. [55].

Incorporation of PCM in concrete was shown to be useful in increasing the heat capacity of the concrete but degraded the mechanical properties of the concrete. This degradation of the mechanical properties can be alleviated by using adequate inclusion methods for manufacturing PCM concrete. Ling and Poon [55] presented a review on PCM, methods of inclusion in concrete and the effects of the PCM properties of concrete at the fresh and hardened stages. Entrop et al. [56] presented a study utilizing PCM in concrete floor. The floor temperature was reduced by 2% while the minimum temperature was increased by 3%. Cabeza et al. [57] investigated a high quality PCM concrete to achieve high energy savings in buildings and found substantial improvements in the storage capacity and the internal temperature of the building. Evola et al. [58] presented a technique to assess the positive impacts of PCMs on the thermal comfort. Ismail and Castro [59] presented the results of a numerical and experimental study on PCM in walls and roofs. They compared the numerical predictions with their experimental results and reported good agreement, reduction of indoor temperature and heat gains.

The use of PCM in building external walls increases the thermal mass and thermal resistance and reduces temperature fluctuations and consumed energy. There are many ways to incorporate PCM in the construction materials and elements adequate choice is important to ensure efficiency, safety, and durability. Numerous numerical models to handle problems of PCM walls, predictions of performance, internal temperature attenuation, and increase of temperature delay factor are available in the literature. Also, massive experimental results on laboratory scale are available in comparison to model predictions. All these are apparently not enough to accelerate the entry of these concepts in the building sector which we believe is waiting for tests and results in real buildings under real working conditions to prove reliability and ensure safety and durability. To achieve these targets there is a need for development and research focused on these aspects to illustrate the validity and viability of these concepts.

The incorporation of phase change materials in building materials and construction elements proved to be an efficient means to reduce energy demands and maintain thermal comfort. PCM wallboards are interesting elements because they can be inserted in new and also in existing buildings. Numerical modeling and simulations were used as investigation tools in walls, two- and three-dimensional modeling of wall panels, and building envelops and wallboards because of cost, availability of experimental installations and involved time. Phase change materials can also be used as insulation layers in buildings for energy saving. The results of investigations showed that utilizing PCMs integrated insulation layers could reduce the heat loads. Also, the application of PCM wallboards on the exterior side can decrease the solar heat gain and maintain thermal comfort [60–62].

A fair number of numerical studies were dedicated to numerical simulation of PCM walls and PCM wallboards to assess the influence of phase change materials on controlling the building solar heat, minimizing indoor temperature oscillations the internal temperature oscillations and maintain thermal comfort. Within this context, Li et al. [63], Cascone et al. [64], and Saffari et al. [65] applied multi-objective optimization analysis and conducted numerical simulations on PCM external walls and indicated significant improvements in thermal efficiency of the building and reduction of energy demands besides the reduction and attenuation of temperature oscillations. Wang et al. [66] simulated a PCM wall with the objective of decreasing the building heat load and found a reduction of the heat flow by 34.9%. In another work, Sharifi et al. [67] incorporated PCM in gypsum boards, evaluated the thermal improvements, and reported significant effects such as reducing temperature oscillations and delaying maximum temperature occurrence.

Other relevant numerical and simulation studies to evaluate the insulation effects of lightweight PCM insulation layers can be found in Refs. [68,69]. They reported a reduction of 15% in heat transfer rate with a delay of about 2 h. In another study, Zhou et al. [70] investigated numerically the interior and exterior PCM walls including the PCM thermal characteristics and validated the model by experiments. Lightweight buildings usually have low thermal mass and hence suffer from overheating in summer. The thermal mass can be increased by incorporating PCM wallboards in the opaque wall and in the ceiling to increase the effective thermal mass, reduce temperature swings, and improve indoor temperature [71]. Kong et al. [72] developed two new PCM systems and presented the corresponding mathematical model and the numerical solution and validated the numerical model and predictions. Table 11 shows some references focused on investigating the effects of wallboards on energy loss and attenuation of indoor temperature oscillations.

A comprehensive review was presented by Whiffen and Riffat [73] on PCM technology. Paraffin is most used in these applications irrespective of its low thermal conductivity and flammability risks. The low thermal conductivity was improved by incorporating of conductivity enhancers, while flammability risks were minimized by doping with fire retardants.

PCM incorporation into building walls can create problems such as leakage of liquid PCM, risk of fire and deterioration of thermal and physical properties. The incorporation of PCM via micro- or macro-encapsulation can prevent most of the mentioned problems. It was concluded that the location of PCMs within the wall plays an important role in reducing heat losses and undesirable solar gains [74]. It was found that the phase change layer should be 20% distant from the surface, reduction of maximum heat gain by 41%, and a time delay of 2 h. Zwanzig et al. [75] reported based on the simulation results that there is an optimal position of PCM within building envelop and PCM wallboards can reduce energy consumption. Hasse et al. [76] conducted a numerical study on honeycomb panels for short-term heat storage and the numerical simulation agreed well with the experiments. Numerical and simulation models developed by Borreguero et al. [77] and Chen et al. [78] indicated that higher fractions of PCM increase the thermal capacity of the wallboard and reduce wall surface temperature oscillations. Koo et al. [79] investigated numerically the effects of the thermal characteristics of PCMs and the wallboard thickness and found that the melting temperature must be close to the internal ambient temperature. Zhou et al. [80,81] conducted numerical simulations on PCM wallboard and compared with traditional building materials and showed that the decrement factor is strongly affected by the internal convection coefficient. Mathieu-Potvin and Gosselin [82] investigated numerically the thermal protection of an external PCM wall. The model allowed assessing the influence of both the position and melting temperature of the PCM layer.

Table 11 highlights some details in the cited references on PCM wallboards.

PCMs incorporated or encapsulated into wallboard or concrete mixtures enhance the heat capacity of the element and improve the thermal performance and thermal comfort.

PCM incorporation into building walls is a serious design challenge where the integration method and the location of PCMs within the walls need to be determined beforehand. This requires more research and perhaps creation of new dimensionless parameters to permit some independence of the specific application.

The literature review showed many investigations focused on formulating numerical models to represent PCM wallboards along with simulations and optimization studies to meliorate its thermal mass and insulation characteristics. In a way, the building market is still waiting for full-scale and long-duration testing of buildings and components in real working conditions to accelerate and intensify the incorporation of these recent technologies in the building industry.

The roof is one of the buildinǵs weak barriers against heat flow to the interior of the building since it is subject to direct solar radiation and continuous climatic variation. There are various means to improve roof thermal resistance including shading, reflecting solar radiation, or incorporating PCM in the roof. The effectiveness of PCM roofing depends on the thermal and physical characteristics of the PCM especially phase temperature range and latent heat, method of application, position with the roof slab, thickness of the PCM layer, and climatic conditions. Because of the difficulties in conducting experiments including cost and time, most of the work done was basically numerical simulations using in-house built codes or available software. Bhamare et al. [83] conducted a numerical study on a PCM roof to assess the effects of phase change material on reducing the internal heat gain. The numerical predictions showed that the PCM roof maintained a nearly constant internal temperature and reduced the internal heat gain. Yu et al. [84] investigated a building with a PCM roof and reported that the decrement factor was reduced by over 85% while the temperature of the ceiling surface decreased by 3.7 °C. In another study, Liu et al. [85] optimized a PCM panel at the micro-level and macro-level simultaneously. The results showed a good performance for heat storage and release processes. Velasco-Carrasco et al. [86] experimentally investigated a new PCM panel to use for ceiling, and significant enhancements in the performance as well as energy control were observed. Figure 7 shows one possible PCM roof and how the PCM is incorporated into the roof system.

The incorporation of PCM in the construction material and building elements is a proven means for temperature control and energy saving. Al-Yasiri and Szabo [88] focused their study on investigating different PCMs, and specification of PCM properties for use in PCM walls and roofs. Wu et al. [89] prepared a novel form-stable and thermally flexible composite PCM (paraffin), with expanded graphite as the additive for thermal conductivity enhancement. The results indicated that the PCM composite is practical for application in buildings.

Li et al. [90] conducted a numerical study to assess the effects of phase change material on the thermal behavior of walls. The findings showed that the effectiveness of the layer of phase change material depends on its location within the wall and that the thermal properties have a strong effect on the heat gain of the wall.

Generally, the roof is subjected to dynamic conditions of solar radiation and radiation reflections from other surfaces besides conduction and convection heat gains from neighboring components and structures. These varying factors are not correctly taken into consideration when evaluating the roof thermal performance. Perhaps if they can be grouped into smaller number of influencing dimensionless factors they can be easily introduced in performance codes making the evaluation task less tedious [91].

Li Dong and collaborators [92,93] studied experimentally the effect of PCM incorporated into a glazed roof system. The results indicated that heat gain was significantly reduced with possible energy economy of about 47.5%. Liu et al. [94] reviewed studies on phase change encapsulation for building applications and included the thermal and physical PCM properties, location of the PCM layer, heat reduction, and PCM improvement. Kharbouch et al. [95] conducted a numerical investigation on PCM integrated in a building envelop and roof. The results showed significant thermal improvements. Hasan et al. [96] conducted an experimental study for using PCM as thermal insulation material by incorporating it with layers of the walls and the ceiling. Results obtained from the experimental study showed a reduction in indoor temperature and the reduction in cooling load.

A fair amount of numerical studies were directed to improve the roof thermal performance and reduce heat gain through the roof structure. Qin et al. [97] theoretically modeled the daily accumulative heat gain from building roof and correlated the penetrating heat with the roof thickness and solar radiation. They found that to decrease the heat penetrating into the building through the roof, the roof thickness can be increased or thermal insulation can be employed. Reddy et al. [98] investigated the thermal performance of PCM roofs including the number and thickness of PCM layers and the thermal properties of the PCM. The results of their assessment showed that a single layer of PCM decreased the heat gain by 17–26% while two layers achieved a reduction of 25–36% in comparison with the case without PCM. Liu et al. [99] investigated the case of a PCM-glazed roof to verify the effects of PCM thickness on heat gain and found that opaqueness increases the time delay factor and decreases solar heat gain.

Roofs received a good share of attention because they are weak barriers for penetration of solar heat. Several techniques were tested to verify their effectiveness in attenuating solar heat gain and maintaining nearly constant indoor temperature independent of the hour of the day. Green and PCM roofs are among these techniques [87,97,100].

Residential roofs usually receive a large amount of incident solar energy. If this energy is left accumulating under the attics it will eventually penetrate to the internal ambient and increase the cooling load. Different means to cope with this problem were presented and discussed in Ref. [101].

The thermal characteristics of concrete roofs can be improved by integrating PCM in layer form or by inserting PCM in cylindrical holes in the roof structure. In both cases, the reported results indicated a significant reduction of the penetrating heat rate [102,103].

Incorporation of PCM to the roof or in the form of internal finishing gypsum panels showed to be effective in reducing the heat gain, reducing temperature oscillations as well as indoor peak temperature [104,105]. Table 12 highlights some details of the cited references in the text.

From the reviewed articles, the building sector contributes much to energy consumption and also to total emissions. To make the building sector more sustainable in the near future, it is necessary to make major changes in the conceptions, construction materials, and general architectural design of buildings and principally reduction in costs and increase in safety. New technologies incorporating PCM for building materials such as concrete, bricks, windows, walls, finishing panels, and wallboards as well as external walls and facades are crucial to improve thermal performance, reduce heat gain or loss, and reduce temperature swings within the buildinǵs internal ambient.

Thermally, the roof is one of the weak parts of a building envelop and is always subject to variable solar and climatic conditions. There are several passive measures available that can help to enhance thermal performance of roofs such as shading and PCM integrated roofs among others.

The roof, specifically PCM roof, performance is sensitive to solar radiation and other climatic parameters which make estimation of the PCM roof performance a difficult task. Hence, some research has to be done to establish dimensionless groups which can represent the different parameters and facilitate the computational task.

There are several research gaps that should be addressed to make the design task for the architect less tedious and more precise. These fronts include full-scale testing, long-duration tests, choice of PCM, and determination of optimal locations prior to PCM incorporation.

Heating floor is essential for cold countries to help achieving thermal comfort. Normally, electric heating and hot water heating are mostly used for this purpose by using heating coils and coiled water tubes. Because of the actual restrictions on admissible emissions, increased prices and shortage of conventional fuels, other changes and improvements were investigated to substitute electricity for heating floors and water with alternative sources. One promising solution is the PCM floor which is a combination of PCM placed in the holes of a concrete block as shown in Fig. 8. During the day, the slab receives incident solar radiation which is stored in the concrete slab as sensible heat and in the enclosed PCM as latent heat. At nighttime, the hot concrete slab and hot PCM radiate their heat to the internal ambient making the indoor temperature more comfortable. Another version is the use of PCM with cheap off-peak electric power. The simulations of PCM floor electric heating system showed results close to experimental results [107]. Figure 9 shows PCM incorporated in flat containers where it can be placed under the floor to store and release latent heat.

Lightweight structures are widely used in recently constructed buildings and the incorporation of phase change materials in the building and components can help to enhance the thermal performance but the risk of leakage of liquid PCM and its low thermal conductivity can limit its use. Gandhi et al. [108] reviewed the potential shape-stabilized PCMs to avoid possible leakage of PCM and found a significant reduction in temperature oscillations and heat gains. Cunha et al. [109] investigated the PCM characteristics and their possible use in envelop and its components as well as floors and glazed areas.

Park and Kim [110] and Drissi et al. [19] conducted investigations on PCM floor heating systems and the possible improvements of the floor heat capacity and its thermal performance. Devaux and Farid [111] demonstrated the possible thermal gains if PCM is integrated into walls, ceilings, and under-floor heating systems. The simulations results were validated with the experimental data. Mavrigiannaki and Ampatzi [48] concluded that PCM incorporation in building elements enhances the thermal mass and reduces the thermal load and indoor temperature oscillations.

Lecompte et al. [35] investigated the inclusion of micro-encapsulated phase change materials in concretes and mortars and they compared the results with conventional materials. They concluded that PCMs included in a mineral matrix improve the wall thermal performance. Barzin et al. [112] conducted experiments on PCM wallboards and PCM floors and reported relevant reductions in consumed energy. Cheng et al. [113] utilized a shape-stabilized PCM in a floor heating system to improve the thermal characteristics and enhance the performance of the system. Zhou and He [114] conducted an experimental study to investigate incorporating PCM in radiant floor heating system and found that sand helped keeping uniform temperature distribution inside the room.

Karim et al. [106] proposed the improvement of the thermal performance of hollow concrete floor panels by the insertion of a phase change material in the enclosures of the floor panel to increase the thermal inertia. The PCM was a polymer composite of 85% of paraffin with a melting temperature of about 27 °C. Mazo et al. [115] presented the results of a study on PCM radiant floor systems working with low-temperature sources and observed that it can be beneficial to use available residual heat and hence improve the energy efficiency of the system. Additionally, the incorporation of PCM provides an extra energy storage capacity.

Entrop et al. [56] investigated PCM concrete floor heated by incident solar energy. They reported that the thermal performance of the floor was improved as demonstrated by the reduction of maximum temperature and increase of the minimum temperature of the floor. In another study, Cerón et al. [116] proposed PCM tiles on the floor subjected to solar radiation. The proposed design with the PCM tiles is restricted to the floor area receiving solar radiation. Song et al. [117] and Lim et al. [118] proposed a cooling system based on radiant floor. To evaluate the behavior of the radiant floor cooling coupled with dehumidified ventilation, they used both physical experiments and TRNSYS simulation and found that the proposed system prevented condensation and maintained thermal comfort.

Table 13 highlights some details of the cited references on PCM floors.

Nearly 50% of newly constructed houses in Europe use radiant heating–cooling techniques where energy supplied by hot water comes from district heating systems, solar energy heated water, and residual energy whenever available. Hence, these systems can produce substantial energy savings for heating. The incorporation of PCM in floors and floor tiles is a fact although additional work is needed to avoid risks of leakage and possible fire hazards. PCM has a poor thermal conductivity and this impairs its thermal performance. The literature shows enormous efforts have been made to improve the thermal conductivity of PCM.

Heating, ventilating, and air-conditioning consume a big part of the total primary energy of buildings with severe impacts on operational cost, energy requirement, and emissions. Trombe walls have potential for addressing the environmental and energy requirements in residential and commercial buildings. Recent configurations of Trombe walls can provide adequate solutions for passive thermal comfort, energy demand, and emissions reductions in buildings, making them more sustainable. Figure 10 shows a Trombe wall with a PCM concrete storage wall for heating indoor ambient.

The combination of Trombe wall and solar photovoltaic panels is a viable combination to generate electricity and supply heat simultaneously. Ahmed et al. [120] reviewed available literature on PV-Trombe wall systems via influencing parameters and how they affect the performance of PV-Trombe walls. Sergei et al. [121] investigated the potential of Trombe walls for cold climates and indicated possible directions for future studies.

The thermal performance of Trombe walls is well recognized and the thermal properties of PCM are also known. The thermal effects on the building’s internal temperature, comfort, and energy reductions of combining the two elements are not very clear and also not very well investigated yet. Omara and Abuelnuor [122] presented a review addressing the possible comfort and energy gains as well as the increase of the internal heat capacity and consequently they observed a significant reduction of the temperature oscillations within the indoor ambient. Mohamad et al. [123] proposed a novel design of a Trombe wall incorporating hot storage tank and water supply for heating and ventilation as well as cooling in summer. In another study, Zhang and Shu [124] elaborated a method to estimate the thermo-economic performance as well as the environmental impact of Trombe walls with and without ventilation. They found that the solar radiation parameters affect significantly the performance of Trombe walls. Dong et al. [125] improved experimentally the Trombe wall design and its heating performance and commented the impacts of the improvements on the thermal performance.

Buildings are considered as substantial contributors to global energy consumption. Free cooling has recently gained much attention to replace totally or partially conventional cooling and heating systems as indicated by Zeinelabdein et al. [126]. They reviewed the free cooling technologies focusing on PCM incorporation in the system. The review paper by Ranđelović et al. [127] discussed the construction details of the Trombe wall to reduce both the thermal oscillations and energy demands. Effects of climatic factors, the materials’ heat capacity, the thickness and color of the thermal mass, thermal insulation, glazing and other relevant parameters were considered. Based on the outcomes of the study, they elaborated some relevant conclusions and general guidelines to help Trombe wall designers in their projects.

Ventilation, heating, and air-conditioning are responsible for a big share of consumed energy in a building. In this scenario, Trombe wall appears as a potential candidate to address the buildings’ environmental and energy issues. Hu et al. [128] reviewed Trombe walls divided into heating- and cooling-based types and established design parameters of the Trombe wall. In another study, Hu et al. [129] designed, constructed, and tested a novel PV blind-integrated Trombe wall module. The results showed better thermal performance with a rate of more than 14.5%.

Several numerical and experimental studies were conducted on Trombe walls to assess their thermal performance, investigate, and analyze the heat transfer processes and how they affect the temperatures on the back and absorber sides, and the resulting heat gain and delay time of the system. Also, in some other investigations, the nano-Al₂O₃ was mixed with paraffin to improve the thermal conductivity of the nano-PCM and its thermal parameters [119,130–135].

The thermal properties of walls, envelops, windows, and roofs are determinant factors for the efficient thermal performance and sustainability of the buildings. Passive thermal walls are receiving much attention from researchers. The main targets include enhancing the thermal performance, improving the thermal qualities of materials, developing efficient simulation methods to predict the performance and evaluate the effects of the geometrical and operational parameters as well as developing testing stands for small-scale prototypes. Various configurations of thermal walls such as Trombe walls with and without PCM, ventilated concrete walls, double skin walls with and without thermal fillings, green walls, PCM shutters, PCM building blocks, PCM floor heating, ceiling boards with and without PCM, and other novel concepts for optimizing energy efficiency in building envelops were explored. Hence, many reviews were presented over the years to cover the continuous progress in the area such as Refs. [136–140].

Koyunbaba et al. [141] compared the numerical predictions with experimental results from BIPV Trombe wall and reported that the electric and thermal efficiencies were about 4.52% and 27.2%, respectively. In another study, Koyunbaba and Yilmaz [142] compared the performance of three types of glass fitted to Trombe wall façade and reported close agreement with the experimental measurements.

Table 14 highlights some details of the cited references on Trombe walls and applications.

The thermal performance of Trombe wall depends basically on its mass since it stores sensible heat. More mass means more capacity to store solar energy and less temperature fluctuations but it may increase the building's dead load and cause structural problems. Phase change materials can provide appropriate solution for the massive mass problem.

Many variations of the Trombe wall were introduced over the years to reduce its weight, improve its thermal performance, extend its range of application, and integrate it with other systems to generate electricity such as PV and PCM. These variations were investigated to extend its use to lightweight buildings. Irrespective of the volume of research reported in the literature there are still gaps, technical and thermal problems and the future trends and studies of the Trombe wall are needed to be addressed.

Some studies indicated the viability of coupling PCM with the Trombe wall to achieve good thermal performance with reduced weight. Life cycle analysis can be a useful tool to evaluate the environmental impacts while handling the solar energy gains. The internal humidity is another factor which should be controlled to avoid impairing indoor thermal comfort. These problems need to be addressed carefully to give the required insurance for architects and designers.

Windows are essential elements in the building allowing visual contact between the occupants and the outdoor environment, enabling natural illumination and ventilation. Unfortunately, they are thermally weak barriers against heat penetration or escape from the building and can upset thermal comfort of the occupants. Mitigation of emissions and the use of renewable energy sources gained globally more attention in promoting, developing, and urging for innovative solutions to cope with the global problems. These energy and emission issues have similar effects on the tendencies in the building sector. Hence, much attention was dedicated to developing windows that allows natural illumination, less heat loss/gain while maintaining adequate aesthetic appearance. This resulted in many window proposals some of which were successfully implemented while some others are still under investigation and development [143,144]. These innovative technologies combine several methods, such as the use of low conductivity gases, low emissivity films, and filling air layers with aerogel [145]. Gao et al. [146] tested a double glass window with aerogel and compared the thermal performance results with those of the reference case and concluded that aerogel reduced energy consumption by 21% with a payback time of about 4.4 years. In addition, there are some advanced glazing technologies available on the market or in development; some examples are vacuum glazing, intelligent or dynamic active (electrochromic, gasochromic, thermotropic), photovoltaic glazing [147,148]. Sun et al. [149] reviewed key types of transparent insulation materials and their thermal and optical behaviors as well as the benefits of their application to buildings. Lago et al. [150] and Ismail et al. [151] conducted numerical investigations on a doubled-glazed window with solar reflective film and compared the predictions with available experimental results and own experiments. The results indicated that the window spacing must be more than 0.025 m and that solar heat gain was reduced by 64.7% due to the solar film.

Li et al. [152] assessed the effects of PCM incorporated in windows on thermal performance and energy reduction. Good agreement with experiments was found. They reported a reduction in both indoor temperature and solar thermal gain. Ismail and Henriquez [153] conducted a study on PCM windows and showed that incorporating PCM in the double glass window can decrease the heat gains and reduce the indoor temperature. In another study [154], they determined experimentally the optical and thermal coefficients of composite glass systems and showed their effects on reducing the heat gains and indoor temperature. Later, they developed a simplified thermal model for a window under forced ventilation conditions [155]. The results showed the favorable effects on decreasing the heat load and indoor temperature.

Other configurations of double glass windows are reported in the literature such as PCM window [156], window with natural induced airflow [157], window filled with absorbing gases [158,159], and window with water flow [160].

Figure 11 shows an illustration of a PCM-filled window (left) and a commercialized PCM window where the PCM is in the liquid state (right) [12].

Figure 12 shows a double glass window with induced air flow heated by the incident solar radiation. The airflow induced in the channel absorbs part of the heat transmitted through the first glass sheets, heats the flowing air mass, reduces the temperature of the second glass, and consequently transmits less heat to the indoor ambient. Heated air can be expelled to the outdoor during hot days or directed to the indoor ambient when heating is needed [150].

Numerous studies are reported concerning the potential of using PCM in windows, curtains, and shutters to reduce solar heat gains and improve thermal comfort while keeping natural daylight and high indoor quality [161].

The PCM incorporated in glazed windows can reduce the consumed energy in the building. Experimental thermal and optical investigations on PCM windows were conducted to characterize the PCM performance and optical benefits which can impact its use in windows and facades. Also, a great deal of studies was dedicated to the development of numerical and thermal models for PCM windows [99,162]. Liu et al. [163] conducted a numerical modeling study on PCM window with different PCMs of different thicknesses and reported a decrease in the total heat gain by about 109.1%.

Goia and collaborators published a series of articles on PCM-filled windows covering different aspects of this type of window including the optical properties characterization necessary to design an efficient window system. They also proposed a simple prototype of a PCM glazing system, analyzed and compared its energy performance with that of a conventional window system. The experimental results indicated that PCM-glazed window stores incident solar heat, smoothens and delays peak values of the heat flux [164–167].

Vigna et al. [168] presented a review to identify PCM transparent/translucent components of building envelop showing the positive, negative, and possible limitations of these technologies. PCM transparent envelops included simple double glass windows, windows with more cavities incorporating aerogel, solar reflectors, and PCM curtains. Dong Li and collaborators [169] developed a model to evaluate the performance of nano-PCM window units. The influence of the type of nanoparticles and their quantity and size on the window thermal behavior were evaluated numerically and compared with the results of pure PCM. They reported significant improvements in the optical and thermal performance. In another work [170], they numerically evaluated the performance of a doubled-glazed PCM window using different PCMs. They reported the strong effect of the optical properties on the performance of the double-glazed window. Uribe et al. [171] developed a model of a double-glazed PCM window to be integrated with the software energy plus for general evaluation of windows.

In an innovative study, Giovannini et al. [172] investigated the effects of double-glazed PCM window on the visual comfort of occupants. As a result, they managed to define and assess the impact of PCM incorporated in windows and facades on luminosity and visual comfort.

Because of the thermal capacity of PCM windows, when it is incorporated into double-glazed windows it attenuates transmitted solar radiation and reduces the thermal load of the environment. Kolácek et al. [173] conducted an experimental/numerical investigation on a window with incorporated PCM and reported that the investigated window system reduced the indoor temperature and enhanced the effective thermal mass of the building. In another study, a solar shading device to control solar heat gain and daylighting as well as the thermal mass of the transparent envelop was developed by Bianco et al. [174]. They reported a reduction of 40% of the cooling load and increased the delay time by three hours.

In other studies, the impacts of PCM properties on the thermal performance of PCM double-glazed windows were addressed numerically and experimentally with good impact on temperature lag time and decrement factor [175–177].

Current PCM technologies and applications in the building sector more specifically in transparent envelops and elements such as windows, shutters, and other shading devices were reviewed by many researchers along the time to cover the increased number of studies directed to enhance the thermal performance, visual and thermal comforts as well as reducing energy demands and reduction of emissions as in Silva et al. [178], Kasaeian et al. [179], Fokaides et al. [180], and Madessa et al. [181]. Table 15 explores the contents of cited articles on PCM windows and comments in more detail the principal findings.

A manufacturer of PCM windows and some products are given in Table 16. Other manufacturers can be found in Ref. [12].

Windows are essential building elements that allow visual contact between internal and external ambient as well as imposing a pleasant aesthetic building appearance. Because of their small thermal mass and physical properties they are considered as the weak barrier against heat loss and heat gains of the building. The review showed a significant amount of research and development work dedicated to resolve the window thermal problem without affecting too much its major function of visible contact and its pleasant aesthetic visual appearance. The review showed the big advancement in vacuum glazing, intelligent or dynamic active (electrochromic, gas chromic, thermo tropic) and photovoltaic glazing. Other achievements were found in double glass windows with natural and forced ventilation, with water flow, reflective solar films, and windows with sealed absorbing gases, windows with PCM or aerogel. The review also showed the results of investigations of the optical and thermal performance of PCM-glazed windows and indicated future development trends, priorities, and challenges. Table 16 presents a manufacturer of PCM windows and some of the available products. It is hoped that this part of the review can be of big help for developing engineers and beginners on research on PCM widows.

Highly glazed facades and envelops in commercial and multifloor residential buildings are increasing due to the recent architectural tendency of using highly glazed buildings because of their possible fashionable appearance. The use of glazed ventilated facades has rapidly increased causing severe energy and comfort impacts demanding adequate solutions. Li et al. [182] presented an excellent review of investigations on the optical and thermal properties and performance parameters of PCM-glazed elements. They also indicated possible challenges and future tendencies and developments.

New approaches and methods to assess the suitability of PCM for thermal comfort applications in buildings and its potential to alleviate energy demands and emissions by developing building envelops with PCM and smart glazing were reviewed as well as the thermal and physical properties of materials and methods of preparation and application in buildings [183,184]. There are some worries about the impact of new glazed elements with PCM, other materials, and high technology coatings satisfying the lighting design recommendations especially in offices and classrooms [185]. They conducted investigations to evaluate possible impacts of circadian lighting design recommendations and indicated the need for further investigations to understand better the relation between light and human physiology.

Some review studies [45,168,186–188] investigated the available technologies to enhance the thermal performance of existing nonresidential buildings, the possible integration of PCM in opaque ventilated façades and in transparent/translucent building envelop components as well as the integration of PCM technology with different cooling techniques such as free and evaporative cooling. Some of the findings of these reviews indicated the advantages and disadvantages of PCM incorporation in transparent envelop components, opportunities for new developments and future research trends and building applications. Other studies [12,189,190] assessed possible applications and integration of PCM technologies with free cooling and heating of buildings using passive and active methods. Besides, Barbosa and Ip [191] conducted a literature review about double-skin facade technologies for application in naturally ventilated buildings. They highlighted the potential of these technologies and their impacts on improving the indoor thermal conditions. Garcia et al. [192] reviewed models used for facades simulations and highlighted their benefits and limitations. Soares et al. [7] reviewed PCM passive construction solutions and their potential for alleviating heating and cooling demands and to increasing indoor thermal comfort, while Cabeza et al. [193,194] reviewed the use of PCM in buildings materials and indicated problems associated with their use and suggested possible solutions.

Some interesting investigations treated the thermal performance and potential of double skin facades with PCM to handle the thermal comfort in the interior of a building as in Garcia et al. [193], Diarce et al. [195], Mei et al. [196], Seferis et al. [197], Corgnati et al. [198], and Weilander et al. [199].

Table 17 highlights references on ventilated and nonventilated facades to help understanding other additional details.

Highly glazed facades and envelops in commercial and multifloor residential buildings have a tendency to increase due to the architectural tendency of using highly glazed buildings for a fashionable and pleasant appearance. However, this causes severe energy and comfort problems. Satisfying the lighting design recommendations especially in offices and classrooms is a recent preoccupying issue from the health and comfort viewpoints. From the technological viewpoint, there are few real operational data, performance tests in big installations and long-duration tests. The present review results indicated the advantages and disadvantages of PCM transparent elements such as transparent and semi-transparent facades, development opportunities, and trends in future research and building applications.

This review on the PCM utilization in the building sector reveals the potential of PCM insertion in construction materials, envelops, facades, walls, roofs, floors, and components such as windows and shading equipment to reduce the thermal effects, energy consumption, and the building sector emissions share. The review showed an immense amount of numerical and experimental results as well as modeling and simulations on small-scale and experimental installations. The main findings of the present review can be useful for providing information on potential improvements and future research and development trends in the field of energy storage materials and possible new applications in the building sector.

PCM in mortars and bricks: The results of PCM applications in external and internal finishing mortars indicate that insertion of PCM reduces the mechanical strength of the mortar and a safe limiting fraction is about 29%. Long duration and cycling tests are needed to ensure durability. No results are available on energy savings due to the application of PCM mortars while risks of leakage and inflammability exist. Irrespective of the extensive research work on binders and additives to reduce risks of fire and leakage, long duration and cycling tests are essential to certify the effectiveness of using these mortars.

With reference to bricks, the review results indicate that using micro-encapsulation reduces mechanical qualities of the bricks, leakage is imminent and more work is necessary to introduce effectively bricks of moderate cost, long working life, and proven efficiency. Macro-encapsulation of PCM in bricks with voids seems to be more acceptable with fewer risks of leakage but needs additional research and development, real-scale performance tests, cycling tests, and reduction of costs.

PCM in concrete, brick walls, and wallboards:

The insertion of PCM into external walls and building envelops proved to be an efficient measure to increase the thermal inertia of the walls and improve energy consumption as well as temperature swings. The reviewed works and developments indicate the necessity of more development and investigations on real buildings in real operation to gain confidence and validate the results of current investigations as well as further work on encapsulation techniques, stabilization, and fire hazards. PCM wallboards have been widely studied but the reviewed literature indicates the necessity of additional research and development to improve the thermal performance and fire risks along with full-scale and long-duration tests.

PCM roofs: Roofs are normally subject to varying solar radiation and climatic conditions which make the prediction of their thermal performance a really difficult and tedious job. To avoid excessive heat gains (or loss) from the building they need to be protected against these external thermally disturbing sources. Available passive measures that may help to thermally protect the building include shading, reflective paints, green roofs, and PCM integrated roofs. The thermal performance of PCM roofs was evaluated both numerically and experimentally in laboratory and small-scale outdoor testing equipment. The results showed the good potential of PCM roofs to reduce energy gain (or loss) and reduce emissions. Nevertheless, there is a great need to improve the modeling techniques and to include real conditions to obtain better long-duration simulations. Several research gaps which should be addressed to make the design task for the architect less tedious and more efficient. These aspects include full-scale tests, long-duration tests, choice of PCM, encapsulation method, and determination of optimal location of the PCM layer.

PCM floors: Heat loss through floors is considerable because of the surface area. Floor heating is important especially in cold countries for thermal comfort. A radiant PCM floor is a real fact and many conventional and/or renewable-based installations employ PCM to increase the thermal mass and reduce temperature swings which cause undesirable discomfort. More investigations and product developments are required to provide the market with a ready for-use product.

Trombe wall: Classical Trombe walls are passive and massive solar heating systems and because of their weight they are not recommended for multistory and lightweight buildings due to possible structural problems. The incorporation of PCM in Trombe walls can solve the weight problem and allow PCM application in these new areas. The application of PCM in Trombe walls is relatively incipient and needs further research. Numerical modeling and long-period simulations are essential for building designers to gain confidence and experience.

PCM windows: Windows are essential elements for a building since they allow visual contact of the occupants of the building with the external ambient and give the building a pleasant aesthetic appearance. Many numerical and experimental studies on small-scale and laboratory models reported the use of PCM in double glass windows and the effectiveness of these windows in reducing heat gains, however, a reduced natural visibility was also reported. There is a need for more research to solve the problem of visibility. Window incorporation with other elements such as aerogel and reflective films may enhance their thermal qualities and make them more acceptable. The review demonstrates the technical challenges, future developments, and priorities in investigations on PCM-glazed windows, the necessity of dedicated research programs, testing, simulations, and full-scale testing to provide acceptable products.

PCM facades: Highly glazed facades and envelops in commercial and multifloor residential buildings are the current architecture tendencies. The review indicates that there is much to learn about the relation between the light and human physiology. From the technological viewpoint, there are few real operational data, performance tests in big installation, and long duration tests. The review demonstrated the advantages and disadvantages of PCM transparent elements such as transparent and semi-transparent facades, development and trends in future research and building applications.

The first author wishes to thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for the PQ Research Grant (Grant No. 304372/2016-1).

There are no conflicts of interest. This article does not include research in which human participants were involved. Informed consent not applicable. This article does not include any research in which animal participants were involved.

The datasets generated and supporting the findings of this article are obtainable from the corresponding author upon reasonable request.