The Strategy of Using PCMs in Building Sector Applications

With the recent large increase in the building sector's energy use to provide thermal comfort to consumers, the restrictions of climate change, and the scarcity of energy supplies, there has been a need to develop ways to reduce energy consumption. The use of phase change materials (PCMs) as a thermal energy storage (TES) system has attracted a lot of attention as a technique to improve thermal performance, conserve energy, and improve occupant comfort. When storing energy via phase change, the key advantage in terms of building materials is the higher energy density, which indicates that more energy can be stored in a constant volume. This study focuses on the vital role of PCMs in building applications by presenting the different classifications of PCMs as well as their integration techniques, systems, and benefits for building applications. Different case studies for full scale buildings are presented. A design strategy is concluded for integrating PCMs in buildings.


Figure 1. Classification of Thermal Energy Storage
Systems. Source: Authors based on [16]. One of the most efficient ways to store thermal energy is by latent heat storage via phase change materials (PCMs). When compared with sensible heat storage, PCM has a larger heat storage capacity and more isothermal behavior during charging and discharging [17]. Moreover, in thermochemical process at ambient temperature, the two components are kept separate. As a result, this method of TES is more suited to long-term storage, such as seasonal storage.
The utilization of PCMs is based on a basic idea. The material turns from solid to liquid as the temperature rises. The PCM absorbs heat since the process is endothermic. Similarly, when the temperature drops, the material transitions from a liquid to a solid state. The PCM desorbs heat since the process is exothermic [18].
This study addresses the idea of using PCMs in building applications, their classifications, properties, integration techniques, benefits, systems, and full-scale case studies. Finally, summarizing a design strategy for integrating PCMs in building applications.

PCM CLASSIFICATIONS AND PROPERTIES
Based on the chemical composition, three primary classifications of PCMs can be incorporated into building materials [1,19]. Incorporating organic, inorganic, and eutectics types of PCMs involves four techniques like direct, immersion, encapsulation, and shape stabilization [20]. Some properties of PCMs like thermodynamic, chemical, kinetic, and economic are desired to be employed in the applications in buildings [21]. Moreover, various physical phenomena of PCMs like melting temperature, supercooling effect, etc. are recommended to be evaluated when considering the effectiveness of applications in buildings (see Table 1).

BENEFITS OF USING PCMS IN BUILDING APPLICATIONS
The usage of PCMs in buildings provides improved indoor thermal comfort for residents by minimizing interior temperature swings and decreasing total energy consumption due to load reduction. TES systems provide the advantage of controlling energy through storage as well as lowering the consumption of fossil fuels, which are the primary source of CO2. Furthermore, the construction sector relates to the urban heat island (UHI) phenomenon, which causes higher surface and air temperatures in metropolitan areas. The purpose of installing PCMs in a building is to lower and manage the power demand for heating and cooling by practically reducing the maximum thermal load on the building [25,26].

Enhancing Thermal Comfort
PCMs have the potential for being employed in current construction materials to help regulate interior temperature fluctuations and improve thermal comfort [25]. For instance, temperature variations are minimized when PCMs are installed. The emphasis should be on choosing a PCM that melts/freezes at the correct temperature so that temperatures remain constant around the comfort temperature. This improves the interior environment in two ways. First, the temperature is more stable throughout the day, thereby decreasing sensations of thermal discomfort caused by temperature fluctuations. Second, the maximum temperature is lowered, and it will not reach a level that causes greater thermal discomfort. Another advantage of PCMs is that they may result in a more consistent temperature between surfaces and air temperatures, decreasing thermal discomfort caused by radiative heat [27] (see Figure 2).
Moreover, temperature stability and control for temporary constructions can be provided by PCMs, which is an excellent approach in crisis situations such as the Covid-19 pandemic [28]. Inside-building air quality has a significant impact on human comfort, and this is a major post-pandemic concern [30]. The humidity level is known to impact indoor air quality, specifically virus survival and dissemination, as well as other important qualities for residents, such as sleep quality and probable eye and airway discomfort [31,32].
Phase change humidity control material (PCHCM) is a novel type of composite that combines high-performance PCM microcapsules with hygroscopic materials to manage humidity [33]. By absorbing and releasing heat and moisture, the PCHCM composite can control the hygrothermal environment indoors [34]. Hygroscopic materials can absorb and desorb moisture and behave as sponges. When the interior air humidity rises, these porous materials may absorb water vapor from the air, slowing the rate of increase in the indoor humidity. When the interior air humidity drops, it releases the water vapor to maintain the indoor humidity [35].

Energy Saving
The rapid expansion of the construction sector is estimated to contribute to an increase in energy consumption. Heating and cooling accounts for over a quarter of all energy consumed in buildings worldwide. Therefore, one of the most difficult contemporary issues in sustainable development is energy conservation. The heat transmission of the building envelope results in significant heat loss, and in response, researchers are focusing on energy saving enclosure structure technologies [36,37]. By using PCMs, excess heat and cold can be stored in buildings by enhancing their thermal inertia, the immediate cooling and heating loads encountered in these structures can be reduced, and, as a result, the energy consumption of mechanical heating and cooling systems can be lowered [38]. According to studies, PCMs may store 5-14 times more heat per unit volume than sensible heat storage materials [39].
Active TES techniques have a large initial investment. Because of this, in order to maximize the energy savings during operation, an appropriate control strategy must be used. By using such automation, it is possible to cut operational expenses and energy consumption while also contributing to climate and environmental preservation without losing comfort [39,40].

Peak Load Shifting
The demand for electricity changes through the day and night, depending on industrial, commercial, and residential activity. This change results in a varied pricing scheme between peak and off-peak periods, which are typically from midnight to early hours of the morning [41]. The highest demand over a billing cycle is known as peak demand or peak load, and it varies depending on the building type. Peak loads during the day strain the electrical grid and necessitate the sizing of heating, ventilation, and air conditioning systems to accommodate higher heating or cooling loads. This requires constructing additional power generation facilities. The peak load can be distributed during the day, minimizing the highest peaks, by moving the peak load away from peak hours of electrical demand using PCMs [27]. Figure 3 explains how using PCMs can minimize and shift the peak due to temperature reduction. This shift in power demand from peak to off-peak hours will result in considerable financial savings. The development of an energy storage system might be one solution to the problem of power supply and demand: surplus energy will be stored in energy storage devices until needed. [41].

Mitigating the UHI Effect
Owing to heat storage and release in buildings, increased concrete coverings, population growth, and heat sources created in urban areas, UHI phenomena occurs, resulting in a greater temperature in core sections of cities than in suburban regions. Traditional methods to minimizing UHI include increasing the reflectivity of buildings and roadways, as well as adding additional green areas, wind paths, and water space. [43].
Building roofs can be enclosed with PCM, which could help with thermal balancing. These materials receive solar and infrared light and release some of the accumulated thermal energy into the atmosphere via convective and radiative processes. This is accomplished through PCM impacting the surface temperature rather than the roof's thermal resistance [44] (see Figure 4).
Moreover, the UHI effect and thermal distresses in concrete pavements are caused by higher pavement temperatures. One of the potential solutions to lower this temperature is to use PCM [45].

4.PCM SYSTEMS
PCMs can be integrated with different systems in a passive or active way. Figure 5 shows different PCM systems.

PCM-Building Envelope
Generally, the building envelope consists of materials such as bricks, cement, and concrete that store thermal energy in a sensible form. To absorb, store, and release heat energy in the structures, all these materials employ sensible heat storage strategies [47].
The PCM is generally installed in a passive or active design in the building envelope. Heat transmission during melting and solidification occurs spontaneously in passive applications; however, the active approach requires active methods, such as fans and pumps, to improve the heat transfer rate during phase transition or generate additional heat, such as solar collectors. The passive pattern has been tested in various climates and has been shown to significantly reduce building energy use during the day [48][49][50].
Because of their reasonable efficiency, affordability, and ease of use, PCM-building envelopes remain of interest among available PCM systems [51]. Figure 6 shows different building components for envelope system.

PCM-Night Ventilation
To fully utilize PCM's capacity, it must be fully charged and discharged at each cycle. This means that the PCM should solidify overnight in order to absorb heat the next day. PCM is usually installed on the inner surface of the building envelope to regulate the temperature of the indoor environment. As a result, it may not fully solidify at night, and then there will be no significant energy savings. For this, night ventilation is a useful technique that can be used in conjunction with PCM-enhanced structures to charge the PCM at night (see Figure 7) [67].

PCM-Free Cooling
Free cooling technology requires a storage unit that stores the thermal energy either by varying the storage medium's internal energy (sensible heat storage), varying the storage material phase (latent heat storage), or both [68]. The main advantages are cooling with greenhouse gas reduction and the excellent maintenance of indoor air quality inside the building. Since the difference in temperature between day indoors and night outdoors is small, the best storage option is PCM [69] (see Figure 8).

PCM-Air Conditioning
PCM in an air conditioning (AC) system might considerably reduce cooling load, allowing for the use of AC with reduced power sizes [72]. PCMs are usually integrated with AC as flat plates, double tube storage, or spherical capsule units [73] (see Figure 9). Latent heat TES might be employed in a chilled water circuit, ventilation system, or the thermal power generation of desiccant cooling and absorption systems in an air conditioning system [74].

PCM-Solar Cooling
The main drawback of solar energy is the mismatch between energy demand and supply since it is time dependent. As a result, when energy is accessible but not allocated to any operation, TES may become an important issue [75,76]. PCMs are commonly utilized to store thermal energy in renewable energy technologies, particularly solar systems, until it is needed. For example, Sudhakar, et al. [77] evaluated the performance of solar photovoltaic panel with PCM integrated natural water circulation cooling method. Results showed an improvement in the power output performance (see Figure 10).

PCM-Active Heating
Renewable energies, such as solar, ambient air, and geothermal energy, have the drawback of fluctuating outputs, which means they may not be able to meet energy demands while buildings are in use. One major technique for addressing this problem is to employ TES devices (e.g., PCM), which can store renewable energy when it is abundant and release it when it is in short supply to meet building energy demands [78,79] (see Figure 11).  There was a 50% reduction in HVAC consumption in the unit with PCM. [89]

5.FULL SCALE PCM-BUILDING CASE STUDIES
The majority of studies on integrating PCMs into buildings are numerical with the use of different simulation tools like EnergyPlus, TRNSYS, ANSYS Fluent, and COMSOL Multiphysics, or by creating experimental prototypes. However, some case studies for full scale buildings with PCMs are shown in Table 2 to prove the influence of PCMs. It's worth noting that all cases include a PCM-building envelope system with different components.

DISCUSSION
The case studies' outcomes indicate that using PCMs in buildings have a positive impact on energy usage for cooling and heating while also enhancing interior temperatures.
Among organic, inorganic, and eutectic PCMs, organic PCMs are the most commonly used (both paraffin and non-paraffin), according to case studies of building field testing. Taking into account climate circumstances, PCM melting temperature ranges from 21°C to 28°C. The encapsulation method, which includes both micro and macro encapsulation, is the most frequently used of the integration techniques. The outcomes indicate that PCMs can increase thermal comfort while lowering energy consumption. Based on the analyses of various cases, a design strategy for integrating PCMs in building applications is summarized in Figure 12 There are five main parts to this strategy:  First, begin with the PCM type, which falls into one of the following three categories: organic, inorganic, and eutectics, with organic PCM being the most common.  Second, select the melting temperature of PCM, which is primarily determined by two factors: climate and the building system selected.  Third, implement integration techniques, which are mainly divided into four categories, with encapsulation techniques being preferred to overcome leakage issues and prevent reactions to the outside environment.  Fourth, choose a building system, which includes a range of alternatives, with the building envelope system being most prevalent.  Finally, conduct a verification with experimental prototypes or simulation tools such as EnergyPlus, ANSYS Fluent, TRNSYS, and COMSOL Multiphysics. This step is crucial to ensure the correct choice of PCM properties, melting temperature, and thicknesses because each case study has its own unique set of conditions.

CONCLUSION
This study presents the vital role of PCMs in building applications to improve thermal comfort, energy saving, and other benefits. The following points can be concluded:  It is possible to integrate PCMs into passive and active systems in buildings, giving preference to the building envelope system.  PCM use in different applications showed great potential in terms of improving thermal comfort either in cooling or heating, energy savings, peak load shifting, and mitigating the UHI effect with various integration techniques and building systems, making it an effective and innovative material to be used in buildings. o More research is needed to improve different incorporation techniques in order to avoid any possible leakage.
o Further investigation of cases of demolition and maintenance of buildings containing PCMs in order to ensure optimal handling and to overcome the possibility of leakage or other risks.

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

Declaration of Funding
No funding.