A review on phase change material (PCM) for sustainable passive cooling in building envelopes
Hussein Akeiber
a,n, Payam Nejat
b,nn, Muhd Zaimi Abd. Majid
c, Mazlan A. Wahid
a, Fatemeh Jomehzadeh
b,d, Iman Zeynali Famileh
d,e, John Kaiser Calautit
f,
Ben Richard Hughes
f, Sheikh Ahmad Zaki
gaFaculty of Mechanical Engineering, Universiti Teknologi Malaysia, UTM, Skudai, Johor, Malaysia
bFaculty of Civil Engineering, Universiti Teknologi Malaysia, UTM, Skudai, Johor, Malaysia
cConstruction Research Center, Institute of Smart Infrastructure and Innovative Construction, Universiti Teknologi Malaysia, Skudai, Johor, Malaysia
dAdvanced Built and Environment Research (ABER) Center, Tehran, Iran
eCenter of Excellence on Modeling and Control Systems, (CEMCS) & Department of Mechanical Engineering, Faculty of Engineering, Ferdowsi University of Mashhad, Iran
fDepartment of Mechanical Engineering, University of Sheffield, Sheffield S10 2TN, UK
gMalaysia–Japan International Institute of Technology, Universiti Teknologi Malaysia, Kuala Lumpur, Malaysia
a r t i c l e i n f o
Article history:
Received 13 September 2015 Received in revised form 4 February 2016 Accepted 8 March 2016 Available online 24 March 2016 Keywords:
Phase change material (PCM) Thermal energy storage Passive cooling Nano material Building Envelope
a b s t r a c t
The most significant threat that mankind faces in the 21th century is global warming. Buildings, which account for 40% of global energy consumption and greenhouse gas emissions, play a pivotal role in global warming. Estimates show that their destructive impact will grow by 1.8% per year through 2050, which indicates that future consumption and emissions will be worse than today. Therefore, the impact of cooling systems cannot be ignored, as they, along with ventilation and heating systems, account for 60%
of the energy consumed in buildings. Passive cooling techniques are a promising alternative to con- ventional cooling systems. Of the various passive cooling strategies, thermal energy storage by means of latent heat is an efficient way to increase the thermal inertia of building envelopes, which would reduce temperaturefluctuations, leading to the improved thermal comfort of occupants. Phase change materials (PCMs) with high density for thermal energy storage can be efficiently employed to this purpose. This paper reviews recent studies of the application of PCMs for passive cooling in buildings. From the lit- erature, a comprehensive list of different organic, inorganic and eutectic PCMs appropriate for passive cooling in buildings are reviewed. Full-scale testing and numerical modeling were found to be the most popular investigative methods used for experimental and theoretical analysis of PCMs. The combination of these two methods can provide a detailed and valid technique for PCM investigations. Finally, incorporating PCMs into building walls with macro encapsulation was also a dominant interest in pre- vious studies.
& 2016 Elsevier Ltd. All rights reserved.
Contents
1. Introduction . . . 1471
2. Methods of thermal energy storage . . . 1472
3. Phase change material classifications . . . 1472
3.1. Organic phase change materials . . . 1472
3.1.1. Paraffin organic PCMs . . . 1473
3.1.2. Non-paraffin organic PCMs . . . 1473
3.2. Inorganic phase change materials. . . 1474
3.3. Eutectics . . . 1474 Contents lists available atScienceDirect
journal homepage:www.elsevier.com/locate/rser
Renewable and Sustainable Energy Reviews
http://dx.doi.org/10.1016/j.rser.2016.03.036 1364-0321/& 2016 Elsevier Ltd. All rights reserved.
nCorresponding author. Tel.:+601111261344.
nnCorresponding author. Tel.:+60108931457.
E-mail addresses:husseinutm@gmail.com(H. Akeiber),payam.nejaat@gmail.com(P. Nejat).
4. Selection criteria for PCMs for passive cooling of buildings . . . 1474
5. PCM cooling operation . . . 1475
5.1. Operation (charging) with HVAC systems . . . 1475
5.2. Operation with night cooling . . . 1476
6. Different methods of PCM encapsulation . . . 1476
7. Review of previous studies of PCM integration with building envelopes . . . 1476
7.1. Application of PCM in the walls . . . 1476
7.2. Application of PCM in roofs . . . 1485
7.3. Application of PCM in windows . . . 1487
7.4. Application of PCM infloors . . . 1489
8. Summary and discussion. . . 1491
9. Future potential studies . . . 1494
10. Conclusions . . . 1494
Acknowledgments . . . 1495
References . . . 1495
1. Introduction
Currently, global warming is a serious threat to our planet and will result in a wide variety of harsh environmental impacts[1].
Greenhouse gas (GHG) emissions (especially CO2 emissions) are the underlying cause of this warming. Unfortunately, buildings (either commercial or residential) play a pivotal role [2] in increasing fossil fuel consumption. Buildings are responsible for 40% of the total world energy consumption and CO2 emissions [3,4].
The demand of the building sector has experienced an upward trend of 1.8% annually for the last forty years[5], and, if it cannot be controlled, it will exceed 180 exajoules by the middle of this century[6]. Heating, ventilating and air conditioning (HVAC) sys- tems account for 60% of the total energy consumed in buildings [7,8]. Therefore, passive cooling systems are a primary backbone of sustainable building concepts because they encompass the miti- gation of energy consumption and GHG simultaneously.
Cooling strategies can be classified into three major groups, including active, passive and hybrid. Active strategies cover all conventional HVACs (e.g., AHU and chillers). By contrast, passive cooling is attributed to the utilization of energy available from the natural environment rather than the consumption of conventional energy resources[9].
AsFig. 1demonstrates, passive cooling can be implemented in buildings by means of heat prevention reduction (decreased heat
absorption), thermal moderation (modifying heat gains) and heat dissipation (removal of internal heat). In Fig. 1, thermal energy storage with phase change materials (PCM) is a heat modulation technique, one of the passive cooling methods.
In recent years, the application of PCM has grown incremen- tally in different industries, such as the space industry[10], elec- tronic industry [11], solar cooling and solar power plants [12], solar dryers in agricultural industry[13], photovoltaic electricity systems [14], preservation of food and pharmaceutical products [15], waste heat recovery systems [16] and domestic hot water [17]. Apart from the preceding utilizations, PCMs and improve energy performance and thermal comfort in buildings [18].
Therefore, PCM applications could be a powerful tool in designing net zero energy buildings[19].
In recent years, the building envelope has been used as an insolation application in various ranges of outdoor conditions.
Therefore, it could be applied to thermal energy management and the control of indoor conditions. This means that the building envelope could have a considerable influence on the energy demand of HVAC systems. Therefore, an increase in the thermal mass of the building envelope is one of the most important techniques for energy management. With this technique, the insulating envelope absorbs the heat and releases it when the outdoor temperature drops, leading to a shift and decrease in the peak indoor loads. Phase change materials (PCMs), as innovative thermal storage systems, can absorbs heat energy from inside the
Fig. 1. Different types of cooling techniques.
building and release it to the outdoors[20,21]. As highlighted in a recent study[22], PCMs could save HVAC system energy usage in the range of 10–30% in different weather conditions in the US.
For example, an experimental study estimated that in summer, incorporating PCMs in assembly walls could save up to 30% of the energy cost[23]. In a similar study conducted in Germany, it was reported that applying micro-capsules of PCM can help keep the indoor temperature up to 4°C lower than typical conditions and could decrease the number of hours that the indoor temperature is greater than 28°C[24].
Even though numerous papers (Fig. 2) related to PCMs in dif- ferent fields have been published in recent years, only a few review papers have addressed the application of PCMs in buildings for cooling. Accordingly, the objective of this study is to explore the application of PCMs for passive cooling in recent publications with a focus on building envelopes.
2. Methods of thermal energy storage
Thermal energy storage (TES) systems store energy in thermal form for utilization at a later time. Broadly, TES systems involve three main steps: thermal charging, thermal storing and thermal discharging[26]. Materials can reserve heat in three primary ways, including sensible heat, latent heat and chemical reactions (Fig. 3) [27,28].
In sensible heat storage (SHS), heat can be stored in a tem- perature increase in the material. The specific heat of the medium, temperature variation and the quantity of the material are the principal factors that determine the amount of heat storage
capacity of the SHS, as shown in Eq.(1):
Q¼Z T2 T1
mCpdT¼ mCpðT2T1Þ ð1Þ
In contrast, in latent heat storage (LHS), thermal energy is reserved (or released) when the phase of the material changes from one state to another (e.g., solid to liquid). The amount of stored energy can be determined by:
Q¼Z Tm
Ti
mCpdTþmamΔhmþZT1 Tm
mCpdT ð2Þ
Q¼ m C spðTmTiÞþamΔhmþCLpðTfTmÞ
ð3Þ The materials used for latent heat energy storage (also called the phase change materials) have the characteristic of absorbing or releasing thermal energy via temperature variations in controlled conditions[30,31].
Singular storage capacity with small temperature intervals[32]
and generally negligible volume changes are the key advantages of PCMs, which allow them to be successfully implemented in buildings for thermal management [33]. For instance, some investigations have found that PCMs integrated into buildings can mitigate energy by 10% to 87% for cooling purposes [34]. As a result, PCMs have been recognized as one of the most progressive materials to enhance energy efficiency and sustainability in buildings, especially for heating and cooling[10,35,36]. In contrast to customary construction materials (e.g., concrete), PCMs can store thermal energy in both sensible heat as well as latent heat [37,38]. For example, a wall with 25 mm thickness incorporated with PCM is able to reserve the same amount of thermal energy as a concrete wall with 420 mm thickness[18].
3. Phase change material classifications
Changing of material phase can be classified into four states:
solid–solid, solid–liquid, gas–solid and gas–liquid. For practical purposes, only the solid–liquid variety can be used for building cooling or heating because the other varieties have technical limitations[39,40]. There is a wide variety of PCMs on the market with different melting point ranges. The most common classifi- cations of PCMs are organic, inorganic and eutectic [41,42], as presented in Fig. 4 and discussed in detail in the following sections.
3.1. Organic phase change materials
Even though organic PCMs cover a wide range, pure n-alkanes, fatty acids and esters are the most well-known for latent heat storage[44]. Organic PCMs have drawn attention because of their additional latent heat capacity, appropriate phase-transition tem- perature and stable physical and chemical characteristics. Pure Fig. 2. Papers published related to PCMs from 2000 to 2014 in the Web of Science
database[25].
Fig. 3. A comparison between (a) sensible heat and (b) latent heat storage and (c) chemical storage[29].
organic PCMs demonstrate some shortcomings that limit their usage in practice, including low thermal conductivity (usually less than 0.2 W/m2K for organic PCMs [45]), high volume variation and liquid seepage during state changes[46]. Organic PCMs are classified as paraffin or non-paraffin. Organic PCMs usually do not display corrosive characteristics and have congruent melting. The melting point and heat of fusion of some organic PCMs suitable for building applications are presented inTable 1.
3.1.1. Paraffin organic PCMs
Paraffin is composed of a straight chain alkane mixture (CH3– (CH2)–CH3). The process of CH3chain making can release a large amount of thermal energy. For paraffin, increasing the chain length leads to an increase in the latent heat of fusion and melting
point. The range of a paraffin's melting point is from 12 °C to 71°C, which can store 128 kJ/Kg to 198 kJ/Kg of heat[47]. Paraffin materials are one of the most popular PCMs used because they are non-corrosive and non-sub-cooling. Sub-cooling refers to a situa- tion where the temperature of a PCM needs to be sufficiently beneath the melting point before it solidifies. In addition, paraffin materials are also considered to be safe, reliable, cheap and to have a high latent heat [48], all of which are important factors for applications in buildings. In addition, from a chemical point of view, paraffin materials are stable and inert at temperatures lower than 500°C and their volumes do not change considerably when changing phase. Paraffin materials and their properties are listed inTable 1 [49]. Apart from their appropriate characteristics such as congruent melting, they have some detrimental properties, such as being relatively flammable and not being compatible with plastics. However, the low thermal conductivity of paraffin is the main factor that limits its widespread application (0.21– 0.24 W m1K1)[50,51].
3.1.2. Non-paraffin organic PCMs
Unlike paraffin PCMs, which have similar characteristics, non- paraffin PCMs have a wide range of varieties with different prop- erties. Therefore, they are considered to be the most suitable category for thermal storage applications [52]. Alcohols, esters, glycols and fatty acids are the most well-known non-paraffin PCMs[44].
Fig. 4. Different types of PCMs[43].
Table 1
Latent heat and melting points of some organic PCMs (including paraffin and fatty acids) suitable for cooling in buildings[52–57].
No Material Melting point (°C) Latent heat (kJ/kg)
1 Glycerin 17.9 198.7
2 Paraffin C16 18.2 238
3 Butyl stearate 19 140
4 Propyl palmitate 19 186
5 Butyl stearate 19 140
6 Propyl palmitate 19 186
7 Emerest 2325 20 134
8 Emerest 2326 20 139
9 Lithium chloride ethanolate 21 188
10 Dimethyl sabacate 21 135
11 Paraffin C17 21.7 213
12 RT20 22 172
13 Polyglycol E600 22 127.2
14 D-Lattic acid 26 184
15 MICRONAL26 26 110
16 MICRONAL 5001 26 110
17 1-dodecanol 26 200
18 Octadecyl thioglyate 26 90
19 n-Octadecane 27 243.5
20 Paraffin C18 28 244
21 Methyl palmitate 29 215
22 Acid Methyl pentacosane 29 197
23 Methyl palmitate 29 205
24 Capric acid 29.62 139.8
25 ERMEST2325 17–20 138
26 Heptadecane 20.8–21.7 172
27 Polyethylene glycol 600 20–25 146
28 Paraffin C13–C24 22–24 189
29 RT27 26–28 179
30 Vinyl stearate 27–29 122
Table 2
Latent heat and melting points of some salt hydrates[52,54–56,62].
No Material Melting point (°C) Latent heat (kJ/kg)
1 KF 4H2O 18.5 231
2 K2HPO4 4H2O 18.5 231
3 FeBr3 6H2O 21 105
4 Mn(NO3)2 6H2O 25.5 148
5 LiBO2 8H2O 25.7 289
7 FeBr3 6H2O 27 105
8 CaCl2 6H2O 29 191
10 CaCl2 12H2O 29.8 174
12 LiNO3 2H2O 30 296
13 LiNO3 3H2O 30 189
Table 3
Latent heat and melting points of a selection of eutectic PCMs[52,54–56,62].
No Material Melting point
(°C)
Latent heat (kJ/kg)
1 Capricþlauric acid 21 143
2 Capricþmyrstic 21.4 152
3 Capricþpalmitate 22.1 153
4 Methyl stearateþcetyl stearate 22.2 180
5 Capric acidþmyristic acid 22.6 154.8
6 Methyl stearateþmethyl palmitate 23.9 220
7 C14H28O2þC10H20O2 24 147.7
8 Na2S4þMgSO4þH2O 24 n.a.
9 C14H28O2þC10H20O2 24 147.7
10 Tetradodecanolþlauric acid 24.5 90
11 Capric acidþstearic acid 24.7 178.6
12 CaCl2þMgCl2 6H2O 25 95
13 CaCl2 6H2OþNucleatþMgCl2 6H2O 25 127
14 Capricþstearate 26.8 160
15 CH3CONH2þNH2CONH2 27 163
16 Methyl stearateþcetyl palmitate 28.2 189
17 Triethylolethaneþurea 29.8 218
18 Ca(NO3) 4H2OþMg(NO3)3 6H2O 30 136 19 CH3COONa 3H2OþNH2CONH2 30 200.5
20 CaCl2þNaClþKClþH2O 26–28 188
One of drawbacks of this PCM type is its high flammability, which restricts them from being exposed to high temperature, flames and oxidizing substances. In this sub-category, fatty acids seem most likely to be employed practically for building cooling applications. Because it displays many suitable properties, such as high latent heat, low super cooling, no phase segregation and different melting temperature, it may be suitable for different climates and conditions [58]. Furthermore, this type exhibits reproducible freezing and melting properties with no super cool- ing during freezing[52]. However, compared to paraffin, they are more expensive. In addition, they can be corrosive.
3.2. Inorganic phase change materials
Comparing to organic PCMs, inorganic PCMs have higher heat of fusion per unit mass with lower cost andflammability (usually).
However, they do suffer from super cooling phase segregation, lack of thermal stability, corrosion and decomposition, which overshadows their advantages[59–61]. This category includes salt hydrates, salt solutions and metals[62], however, salt hydrates are the most well-known variety and numerous studies have used them for thermal storage applications. Table 2 summarizes the latent heat and melting points of several inorganic PCMs. The attractiveness of salt hydrates for heat storage purposes in build- ings is due to their considerable volumetric storage density (350 MJ/m3), high thermal conductivity (0.5 W/m K [63]) and low price compared to organic PCMs[56].
3.3. Eutectics
Eutectic PCMs consist of a combination of at least two other PCMs. During the freezing process they form a blend crystal[64].
This mixture can consist of inorganic with inorganic, organic with inorganic and organic with organic (Table 3)[60]. The separation of the components is very unlikely because they mostly change phase without segregation (due to freezing to an intimate crystal mixture) and during the melting process all components change to liquid simultaneously. Some of the common eutectic PCMs that can be employed for passive cooling in buildings are shown inTable 3.
4. Selection criteria for PCMs for passive cooling of buildings
PCMs can reduce the energy needs of cooling systems and indoor temperaturefluctuations, however, for PCMs to be effec- tively implemented for passive cooling in the building envelope, several selection criteria must be considered. From a physical point of view, the melting point of the PCM should be in the range of 10°C to 30 °C to provide thermal comfort for occupants. This temperature should be selected with respect to average day and night temperatures and other climatic conditions of the building site[65,66].
Thermodynamically, the PCM should have high latent heat per volume unit, which is an important factor in building applications because it means that with lower volume, the PCM can absorb/
release higher amounts of energy leading to a lighter building envelope[67]. Moreover, it should also have a large specific heat capacity (Cp)[68].
Another significant thermodynamic factor is its heat transfer ability (conductivity). Higher conductivity results in faster thermal responses. Even though the thermodynamic properties are the main selection criteria for the use of a PCM, other important properties are related to its chemical aspects, including chemical stability, low volume expansion and low/no super-cooling during freezing; it is also important for PCMs to be non-toxic, non-cor- rosive, nonflammable and non-explosive[66,67,69].
Furthermore, a PCM is suitable for applications if it is stable after a number of repeated melting/freezing cycles, that is, if it has a proper cycling stability. This is also called long-term stability [62]. Afinal qualification, which overshadows all other aspects, is economics. A PCM needs to have a reasonable price and avail- ability on the market[52,70,71](Fig. 5).
Low thermal conductivity is considered to be the main problem for most PCMs and can be a serious challenge for the application of PCMs as passive cooling systems. There are three main techniques to improve thermal conductivity:
Thefirst technique is to make a composite of PCM with porous metal foam or expanded graphite matrices[72]. The second technique is to add metallic spheres, screens, fins and wools to develop a new thermal conductivity enhanced material[73,74]. The final method is the application of nano-materials. Nano- particles are capable of enhancing micro-convection; therefore, the application of nano-particles can improve the heat transferFig. 5. Selection criteria based on PCM characteristics[35,52,56,71].
significantly. Nano-materials that can be used for this purpose include carbon in various forms (carbon nanotubes), stable metals (e.g., gold and copper), oxide ceramics (e.g., Al2O3and CuO), metal oxide (e.g., silica, alumina and zirconia), oxide ceramics (e.g., Al2O3 and CuO), metal nitrides (e.g., AIN and SiN) and metal carbides (e.g., SiC)[75–77].
In summary, the physical requirements for a PCM are to have a suitable phase change temperature, a completely reversible freeze/melt cycle, a large change in enthalpy (ΔH), a large spe- cific heat capacity (Cp), a large thermal conductivity (k) and little sub-cooling. The chemical requirements are small volume pres- sure, low vapor pressure, good compatibility with other mate- rials, chemical stability, physical stability and non-toxicity. The economic requirements are low price and being recyclable and abundant.
5. PCM cooling operation
The thermal mass concept helps PCM manage the thermal conditions inside a building. Thermal mass refers to a typical building structure that adds inertia to the temperature variation.
The building structure itself constitutes a source of thermal energy storage and plays a key role in buffering heat to mitigate external heatflows and reduce indoor temperature swings. Therefore, the structure acts as a heat sink during warm periods[56]. When PCM is added to the building structure, it increases the thermal mass and prevents the heat from reaching the occupied space[10,20].
The energy absorbed by the latent heat is much greater than the sensible heat; for instance, the heat capacity of wall board with 30% PCM can befive times greater than a conventional one of the same size[78]. The thermo-physical properties of ordinary build- ing envelope materials are compared to a selection of commercial PCMs in Fig. 6. It can be observed that conventional building materials have much lower heat capacity than PCM in general, which makes them unsuitable for thermal storage purposes due to their low heat conductivity.
An obvious merit of increasing the thermal mass can be observed in light-weight building structures that suffer from low thermal inertia, which promotes considerable temperaturefluc- tuations in the hot season due to overheating by solar radiation.
Moreover, it can be useful in the winter when PCMs can act as insulators, decreasing the heating load. As a result, PCMs can prevent overheating during the daytime on hot days and
potentially reduce the heating needs during the nighttime in the cold season[81].
In addition to its considerable potential for energy conserva- tion, PCMs can enhance the overall thermal comfort in the occu- pied space of a building by maintaining a steady temperature near the comfort zone for longer periods without depending on HVAC systems. Accordingly, attention should be drawn towards opting for an appropriate melting point of PCM to stabilize temperature near the comfort zone. This can improve the indoor environment in two ways. First by eliminating temperature fluctuations and providing stable thermal conditions, and second, by modifying the peak temperature during the daytime. Likewise, PCM can allow for an approximately uniform temperature between the air and sur- face, therefore diminishing the discomfort due to radiated heat [81].
In spite of having several different varieties, PCMs follow the same procedure for passive cooling, which consists of two stages:
The charging stage or PCM solidification: In this step the heat transfers from the PCM to the ambient air, which has lower temperature. As a consequence, the PCM changes from liquid to solid at a certain constant temperature. This process stops when the temperature gradient (between the PCM and the indoor air) is insufficient to drive the transition. The discharging stage or PCM liquefying: During the daytime when the indoor temperature rises above the comfort zone, the PCM absorbs the heat and cools the air by converting from a solid to a liquid at a constant temperature[54].Two strategies can be used to derive the force needed to change the phase of the PCM: a HVAC system or night cooling, both of which are discussed below.
5.1. Operation (charging) with HVAC systems
With this technique, the HVAC system reduces the indoor air temperature. While the HVAC is in operation, the PCM is fully charged. Later, when the indoor temperature increases, the air transfers its thermal energy to the PCM to be cooled (discharging stage). Through this process, a promising solution can be found for the mismatch correction between the supply and demand of electricity because PCM provides the opportunity to shift the energy demand from high cost tariff periods to off-peak times [82,83]. This is known as "peak shaving" or "peak shifting". While this strategy can be utilized for both cooling and heating, it is
Fig. 6. Comparing the heat storage capacity of a selection of commercial PCMs with typical construction materials[79,80].
Fig. 7. Illustration of potential peak load shifting and energy saving due to the application of PCMs[81].
better known for cooling purposes. By employing this strategy, HVAC systems can fully operate during off-peak periods to meet the cooling needs as well as charge the PCM. That is, the PCM can provide the total cooling load during peak hours while the HVAC system is completely out of operation. From an economic per- spective, buildings can benefit from lower electricity tariffs (lower operation fees) during off-peak periods[84,85].Fig. 7depicts how the peak may be reduced and shifted by the use of PCMs.
5.2. Operation with night cooling
As illustrated in Fig. 8, with the night cooling strategy, the charging procedure is handled during a time when there is a temperature gradient between the ambient air temperature and the interior space of a building. In the daytime, the discharging process occurs to keep the indoor environment in the comfort zone[51,54].
6. Different methods of PCM encapsulation
Despite the possibility of directly incorporating PCM into building structures or materials (e.g., concrete), which is known as shape-stabilized PCM[86], the encapsulation of PCM is preferred.
This can be explained by the high potential of leaking during changing of the PCM phase to liquid. In addition, encapsulation can prevent the low viscous liquids from diffusing throughout the material. Consequently, direct incorporation and immersion of PCM with other construction materials is not regarded as a prac- tical technique. The issues mentioned highlight the necessity of encapsulation for efficient PCM implementation in building envelopes[81].
Furthermore, by enlarging the surface area, encapsulation can increase the thermal conductivity, which means increasing the heat transfer between the PCM and surrounding environment.
Similarly, with encapsulation, PCM can be isolated from harmful environmental factors. Other key functions of encapsulation include compatibility between PCM and surrounding materials, decline in corrosion and managing volume variation during state changes[87]. The classification of encapsulation is based on the size which encompasses the PCM: macro (with diameters of 1 mm and more), micro (from 1μm to 1 mm) and nano (less than 1 μm) [88].
Macro encapsulation has more applications because it can be integrated more conveniently with other building materials and consequently makes it possible to construct a building in a way similar to the current construction method[81]. For instance, PCM impregnated in concrete can both escalate the specific heat capacity and diminish the density, which has a positive influence on the structure of the building[29]. Low thermal conductivity is the main drawback of macro encapsulation; consequently, it needs
to be simultaneously corrosion resistant, reliable and thermally stable as well as effectively transfer heat[89].
Micro encapsulation may be the best solution in terms of increasing the heat transfer rate, preventing leakage of the melted PCM during the phase change process in latent heat thermal energy storage (LHTES) systems, controlling the alterations in the volume as phase change occurs and decreasing the PCM reactivity with the external environment [90]. In the micro encapsulation procedure, individual particles or droplets are coated by a con- tinuous film to form a capsule with the size of a micrometer, known as a microcapsule. Two main parts can be observed in microencapsulation: the PCM, which is the core, and the shell, which can be a polymer or an organic substance. The shape of microencapsulation is not limited, it can be either a regular (e.g., tubular, oval or spherical) or an irregular shape[90].
7. Review of previous studies of PCM integration with building envelopes
The envelope of a building can be defined as what separates the indoor from the outdoor environments. It is an important element that affects the quality and controls the indoor conditions irre- spective of the transient outdoor conditions. Various components, such as walls, fenestration, roofs and foundations, compose the envelope of a building[20]. PCM can be incorporated into virtually all elements of the building envelope. Nevertheless, the most common PCM integration in the envelope are in the walls,floors, ceilings, roofs and windows due to easy installation and more effective heat transfer[9]. Therefore, the order of this section is based on the position of the PCM incorporated into the building, including the walls, roofs, windows andfloors. In the following section, recent studies (during the last two years) concerned with the incorporation of PCM into the building envelope for passive cooling are reviewed.
7.1. Application of PCM in the walls
Memon et al.[91]conducted thermal performance tests (small- scale experiments) of a lightweight aggregate concrete (LWAC) containing macro encapsulated paraffin–lightweight aggregate (LWA) for wall application, as shown inFig. 9(left). Their study also assessed the economic and environmental aspects of the proposed PCM application for a residential building in Hong Kong.
The results of the indoor test revealed that the macro encapsulated Paraffin–LWA panel was able to decrease the interior temperature at the room center and at the internal surface of the panel by 4.7°C and 7.5 °C, respectively (Fig. 9a and b). Furthermore, 2.9°C of indoor temperature reduction was observed in the Paraffin–
LWA room model during the test. The findings of the environ- mental evaluation found an annual reduction of 465 kg CO2-eq.
The recovery period of the LC–100% PCM–LWA was 29 years, while Fig. 8. The operation of a PCM with nighttime cooling[54].
Fig. 9. Schematics of an indoor test (left), the internal temperature of LWAC with and without Paraffin–LWA (a) at the room center and (b) at the internal surface of the panel[91].
Fig. 10. Structure of the building envelope for (a, b) the PCMOW and for (c, d) the PCMIW[92].
the average life span of a residential building in Hong Kong is 60 years. Therefore, from an economic point of view, the application of macro encapsulated Paraffin–LWA in LWAC building walls was proven to be feasible.
Kong et al.[92]investigated the thermal performance of two new PCM systems incorporated into building envelopes using full- scale experiments and numerical methods. In the experimental study, panels containing Capric acid (PCMOW) as well as Capric acid and 1-dodecanol (PCMIW) were installed on the outside and inside surfaces of the walls and roofs, as shown inFig. 10. In the numerical study, which was conducted using computationalfluid dynamics (CFD) software, the internal temperature development, temperature variation and thermal energy saving were analyzed in two ventilation modes including free cooling and opening the window and door at night. The results showed that the thermal performance of PCMIW was better than PCMOW, particularly for the condition of opening the window and door at night (Fig. 11);
however, an interior retrofit in current buildings was required for the PCMIW application. Nevertheless, the PCM panel application in both walls and roof was found to be effective.
Ye et al. [93] experimentally and numerically evaluated the performance of shaped-stabilized PCM using a software simula- tion. As shown inFig. 12, they used two similar rooms (experiment unit and control room) to analyze the PCMs effectiveness in reducing the indoor temperaturefluctuations. Based on the defi- nitions of the Energy saving index (ESI) and the energy saving equivalent, a comparison of PCM and Expanded polystyrene (EPS),
as an insulation material, was made. To comprehensively evaluate the PCM's application performance, three different cities with different climates were chosen for the simulations. The compar- ison of the ESIs of the PCM placed on the exterior wall's inner surface, the north wall's inner surface and thefloor's upper surface is shown inFig. 13for the summer and an entire year. As shown in Fig. 13, in the summer, the ESIs of the PCM placed on the exterior walls were lower than other two placements (less than0.002).
Even though using PCM could improve more than 0.03 of the ESIs in Beijing and Shanghai for an entire year, the energy saving in Guangzhou was not sensible and the ESI parameter for the exterior Fig. 11. Temperature variations of the wall and roof for PCMOW, PCMIW and reference rooms for the condition of free cooling (left) and for the condition of opening the window and door at night (right)[92].
Fig. 12. Schematic diagram of the experiment units and the control room[93].
Fig. 13. Comparison of Energy Saving Index (ESI) of a room with PCM and EPS in the summer (upper panel), and over an entire year (lower panel)[93].
wall placement was negative. The study concluded that internal thermal comfort could be improved by PCM application in a building.
Biswas et al.[94]conducted a full-scale experimental test of a nano-PCM-enhanced interior wallboard in a natural exposure test facility in the hot and humid climate of the USA. The numerical modeling was developed with COMSOL Multiphysics to assess the effect of nano-PCM wallboards on heating and cooling loads.
Fig. 14depicts the test wall of the NET facility with PCM-enhanced wallboards. Cooling electricity use per month for the south wall at three couples of temperature set points is shown inFig. 15. The results indicate that at a 22°C room temperature set point, the highest heat gain reduction per year was observed for the south- oriented wall. Higher load factors were achieved by the nano-PCM wallboard, which would potentially result in lower capacity and costs for electricity providers and power plants.
In a recent study conducted by Barreneche et al. [95], the thermal and acoustic performance of a shape-stabilized PCM layer for an intermediate wall of a building was experimentally inves- tigated in Spain. The new shape stabilized PCM was composed of a polymeric matrix, 12% paraffin PCM and electric-arc furnace dust (EAFD) (a waste from the steel recycling process). The thermal study involved in-situ measurements of ambient temperature, humidity and wall temperatures for two identical cubicles with and without PCM dense sheet (Fig. 16). The results of the thermal experiment demonstrated the potential of the PCM dense sheet to reduce the interior temperature up to 3°C (Fig. 17). The acoustic insulation capability of the PCM cubicle was found to be more than the reference cubicle (up to 4 dB) due to the EAFD content.
Biswas and Abhari[96]evaluated the thermal operation of an external building wall incorporating a low-cost organic PCM using experimental and numerical approaches.Fig. 18depicts the details of the test wall with PCM-high-density polyethylene (HDPE) pel- lets distributed in cellulose insulation to fill in wall cavities.
Fig. 14. Test wall with the PCM-improved wallboards (upper panel), the location of the test wall on the southeast façade of the building (lower panel)[94].
Fig. 15. Cooling electricity use per month for the south wall heat gain at different cooling set points[94].
Fig. 16. Two identical cubicles used for thermal performance measurements[95].
Numerical analysis using COMSOL Multiphysics was conducted to study the effect of the PCM–HDPE pellets on the cooling load and electricity usage. For the simulation work, a wall with cellulose- only insulation, as well as walls with two different configurations of PCM–cellulose (adding PCM to the inner half of the wall cavity and the entire cavity) were modeled. According to Table 4, the annual heat gains of the wall with the PCM–cellulose were lower than the wall with the regular cellulose insulation, particularly for the south wall, which had a 10.6% difference. Furthermore, similar cooling electricity consumption (approximately 1500–2500 Wh/
m2 per year) was achieved for the two PCM configurations, as shown inFig. 19. The study concluded that the thermal perfor- mance of the wall with PCM in the cavity's inner half was better than that with PCM in the entire cavity.
Ascione et al. [97] studied the effect of using PCM on the exterior building envelope during the cooling season in five Mediterranean cities. Based on a “one-dimensional conduction finite difference” method, as a heat balanced algorithm, the building behavior and a comparison of the phase change tem- perature, the thickness of the PCM wallboard and the location of the PCM layer were analyzed. The results revealed that at fixed phase change enthalpy, the phase change temperature was the most effective parameter.Fig. 20illustrates the effect of different PCM wall configurations with varying PCM thicknesses on the energy demand in a semi-arid climate (Ankara) and in a mild Mediterranean climate (Marseille) during the same month (July).
FromFig. 20(upper panel), energy saving rates ranging from 1.4%
(0.7%) for the internal face of South and East facades (P_Sew) to 3.0% (1.9%) for Internal face of all vertical facades (P_Aw) can be
observed when the melting temperature is 27°C. The comfort hours increased from 32.9% to 51.0% and from 11.2% to 21.9% for the occupied hours in Marseille and Athens, respectively. Accord- ing toTable 5, in all the investigated PCM wall configurations, the efficiency of the PCM was more effective in a semi-arid climate compared to hot or subtropical Mediterranean climates. From an energy saving point of view, it was concluded that cooling energy saving was strongly dependent on the PCM melting temperature and the cooling season and that using a thicker PCM layer improved the efficiency under the investigated conditions.
Sayyar et al.[98]used experimental and numerical approaches to evaluate the thermal performance of a nano-PCM integrated with gypsum wallboards. The nano-PCM consisted of a fatty acid- based PCM and graphite joined nano sheets. The performance of the nano-PCM walls (test cell) and commercial drywall panels (control cell) in an experimental chamber was compared, as shown in Fig. 21. The results demonstrated the potential of the nano-PCM wall panels to maintain the internal temperature
Fig. 17. Temperature variations inside the PCM cubicle and the reference cubicle and the directly measured temperature on the dense sheets[95].
Fig. 18. Details of the test wall with the PCM–HDPE pellets[96].
Table 4
Annual wall heat gains at different orientations with cellulose and PCM–cellulose insulation[96].
Wall orientation Heat gain (Wh/m2) % Difference
Cellulose PCM–cellulose
East 10,451 10,307 1.4
West 10,143 9754 3.8
North 5761 5769 0.1
South 11,289 10,088 10.6
Fig. 19. Annual cooling electricity consumption for two PCM configurations and the wall without PCM[96].
variations from 18.5°C to 26.5 °C, while the range was 13–32 °C for the control cell. Further, the peak load shifting of the test cell with nano-PCMs was 8 hours, which was significantly greater than that of control cell. The new nano-PCMs also improved indoor thermal comfort and reduced energy demand up to 79% (Table 6).
Shi et al.[99]conducted an experimental study on the perfor- mance of concrete walls integrated with macro encapsulated PCM in the subtropical climate of Hong Kong. The study included small- scale measurements of internal air temperatures and humidity levels at the center of the test rooms to highlight the influence of three positions of PCM (paraffin) in concrete walls (externally and internally bonded with concrete walls, as well as laminated within the concrete wall) as shown inFig. 22. The obtained results indi- cate that the air temperature and humility levels inside the room models were adjusted by the macro encapsulated PCM; never- theless, its position in the concrete walls played a significant role in its efficiency. A thermal performance comparison of three dif- ferent cases demonstrated that the best indoor temperature con- trol was achieved by the model with PCM laminated within the concrete walls with a 4°C reduction in the maximum temperature.
In addition, the case with internally bonded PCM was able to decrease the indoor level of relative humidity by 16% (Fig. 23). The authors concluded that the models incorporating macro encap- sulated PCM can provide healthy and comfortable internal environments.
Zhu et al.[100]carried out a numerical study to evaluate the energy and thermal performance of a double shape-stabilized PCM (SSPCM) wallboard in an air-conditioned office building in Wuhan Fig. 20. Effect of different configurations of PCM on cooling energy demand in
Ankara (upper panel) and Marseille (lower panel)[97].
Table 5
Effect of varying configurations of PCM wallboard on cooling energy savings and its rate for all climates[97].
P_SEw P_Aw P_AwR
ΔEP cool (kW h)
ξpcm (%) ΔEP cool
(kW h)
ξpcm (%) ΔEP cool
(kW h)
ξpcm (%)
Ankara 139 1.1 290.6 2.4 159 1.3
Athens 132 0.6 272.7 1.3 253.2 1.2
Marseille 173.6 1 304 1.8 258.1 1.5
Naples 6.1 0.03 142.6 0.8 96.6 0.5
Seville 33.6 0.2 160 0.8 145.1 0.70%
*Internal face of all vertical facades (‘‘P_Aw’’), only internal face of South and East facades (‘‘P_SEw’’), internal face of all facades and 1 cm of PCM plaster on the roof (‘‘P_AwR’’).
Fig. 21. (a) Control cell and (b) nano-PCM sandwich cell in (c) an environmental chamber[98].
City, China. The SSPCM was composed of paraffin (80%), high- density polyethylene (15%) and expanded graphite (5%). As illu- strated inFig. 24, the new double SSPCM wallboard consisted of three layers including an internal SSPCM layer (active in cold seasons), an external SSPCM layer (active in hot seasons) and a conventional concrete layer. The work also assessed the effect of the thickness and melting temperature of double SSPCM wall- boards on their operation. The achieved results demonstrated that for internal and external SSPCM wallboard, the optimum thick- nesses was between 30 mm and 60 mm. Comparing the right and left panels ofFig. 25, we see that the external SSPCM layer reduced the temperature of the wallboard outer surface (point 1), and therefore the gain through the wallboard was restricted. Conse- quently, a reduction of up to 3.8% in peak cooling load during the hot season was achieved. A high reduction of the inner surface temperature of the wallboard was prevented by the internal SSPCM wallboard; therefore, it was able to decrease the internal temperaturefluctuations in heating conditions. The annual energy savings in the hot and cold seasons were 3.4–3.9% and 14.8–18.8%, respectively.
Lee et al.[101] experimentally evaluated the thermal perfor- mance of thin sealed polymer punched PCM walls based on heat flux reduction and heat time delay. They used “PCM thermal shield” (PCMTS) on the south and west walls of two identical test houses and investigated the effect of PCMTS locations atfive dif- ferent levels (Fig. 26). Summaries of the results for the south and west PCMTS walls are listed inTables 7and8. Performance com- parisons of these two walls demonstrated that the optimum location of PCMTS was different for the south and west walls. For the south walls, location 3 had the best performance (51.3%), whereas location 2 was the best position of the PCMTS in the west walls with a performance of 29.7%. The maximum peak heatflux time delay occurred at location 1 (6.3 h) and location 2 (2.3 h) for the south and west walls, respectively.
Evola et al. [102] used Energy Plus software to analyze the effect of a ventilated cavity on the thermal performance of microencapsulated PCM wallboards in an office building (Fig. 27).
To enhance summer thermal comfort, a passive ventilated cavity between the PCM wallboards and the partition wall was con- sidered, as shown in Fig. 27, to improve the PCM solidification process and increase the storage efficiency of the PCM. This study developed a mathematical model to determine the air tempera- ture distribution in the ventilated cavity. Simulated and calculated results were in a good agreement, with a deviation of less than 7%, as shown in Fig. 28. Furthermore, Fig. 29 illustrates that the operative temperature in the PCM room with the cavity was less than the other two cases, particularly at the peak points, up to 0.4°C compared to the room without PCM.Table 9demonstrates the effect of PCM and the ventilated cavity on the internal thermal comfort. The highest value of frequency of thermal comfort (FTC), and a reduction of 16.2% in the intensity of thermal discomfort (ITD), was observed for the case with both the PCM and the cavity.
This occurred because the stored heat in the PCM during the warm
daytime hours is efficiently transferred to the air flowing into the cavity at night, instead of being ejected into indoor air. This phe- nomenon improves the values of the convective heat transfer coefficient occurring on the surface of the PCM wallboards by the cavity side (between 3.1 W/m2K and 6.5 W/m2K), which was much more than those occurring on the room side (approximately 1.5–2 W/m2K).
Jin et al.[103]conducted an experimental study to compare the thermal performance of a building wall with different PCM layer locations.Fig. 30depicts the control wall as well as the six loca- tions of the PCM layer based on its distance from a gypsum wallboard. A performance comparison of the results of varying PCM locations demonstrated that case 1/5 L was the optimal
Table 6
Energy demands for nano-PCM and control walls[98].
Energy demand (J)
Cooling Heating Total
Nano-PCM wall 544 332 877
Control wall 2224 1967 4191 Fig. 22. Three room models with different locations of PCM (a) externally bonded, (b) laminated within and (c) internally bonded[99].
location with an approximate 41% reduction in the average peak heatflux compared to the control wall. Moreover, the study con- cluded that the thermal operation enhancement of the wall was considerably affected by the PCM state (fully or partially melted).
Borderon at al. [104] analyzed four ventilation modes of a double stream ventilation system in a retrofitted house (Fig. 31) in different cities and climates in France. They used PCM layers for cooling forced airflow in a channel. The developed models were used to conduct simulations of day and night periods. The results revealed that for most of the PCM system configurations, the solidification of the PCM was partially achieved at night, due to the non-optimal performance of the system. Therefore, airflow control was needed to improve the solidification and performance of the PCM. The number of hours with overheating problems was reduced to 8% and 2.6% in Lyon and Trappes, respectively, in the
summer period. In all the tested configurations, thermal comfort could not be 100% guaranteed for the four climates, however, the best configuration of the PCM/air system had promising results.
Sage-Lauck and Sailor[105]investigated the performance of a commercial PCM (BioPCM25TM) in a passive house in Portland, USA, to reduce the number of overheating hours and to enhance the internal comfort in an experimental and simulation (Energy Plus) study. The house included two units; the walls and ceiling of the second floor of the west unit were integrated with PCM (behind the gypsum layer) and the east unit served as a control. To validate the simulation results, in-situ measurements of indoor air temperature and surface temperature in both units were used. The results indicated that the overheated hours per year were decreased from approximately 400 to 200 with the PCM applica- tion, and therefore the occupant's thermal comfort was improved.
It was also established that a further temperature reduction in the overheated zone (up to 60%) could be achieved by altering the PCM location from behind the drywall to the inner surface of the wall.
De Gracia et al.[106]investigated the energy performance of a novel ventilated double skin façade (VDSF) incorporated with PCM under varying climatic conditions and its suitability for cooling purposes. The experimental analysis was conducted for a Medi- terranean climate (Spain) (Fig. 32left); however, to optimize the system's operational schedule and to maximize its energy benefits, a numerical model was also developed for several locations representative of varying suitable climates (based on the Köppen– Geiger climate map). Fig. 32 (right) shows a schematic of the system installed in the south wall with different operational modes. Thefindings indicated that free cooling, cold storage and solar radiation prevention were the potential benefits of the VDSF with PCM.Fig. 33illustrates the potential of the system to provide a free cooling supply during the summer; the highest value (more than 200 MJ/day) is found for Quito while some cities such as Kuala Lumpur, Singapore and New Delhi have no potential for free cooling. By enhancing the thermal resistance of the system's outer skin, the free cooling supply was increased by over 80% in regions with high vertical solar radiation (Stockholm, Quito, Berlin, Johannesburg and Montreal). Even though the potential of the cold storage sequence for providing a cooling supply was less than 12 MJ/day, it only occurred during 3 or 4 h of the peak cooling demand period of the building. The study also revealed that the system was able to provide net energy savings of more than 1.2 MJ in all the modeled cities under warm temperature climatic con- ditions (with the exception of Auckland).
Diarce et al.[107]used CFD in ANSYS Fluent to develop a model for an innovative ventilated façade integrated with PCM. Fig. 34 Fig. 23. The indoor relative humidityfluctuations for the internally bonded case
[99].
Fig. 24. Schematic of the double SSPCM wallboard[100].
Fig. 25. Layer temperatures of the SSPCM room (left) and the reference room (right) in cooling conditions[100].
Fig. 26. Southeast view of the test houses (left) with the different locations of the PCMTS in the wall[101].
Table 7
Thermal results of the PCMTS at varying locations in the south wall[101].
PCM shield location 1 2 3 4 5 Avg.
Avg. peak heatflux reduction (%) 10.5 24.7 51.3 37.4 23.5 29.5
Avg. peak heatflux time delay (h) 6.3 5 5 2.3 3 –
Avg. daytime heat transfer reduction (%) 23.5 35.7 47.9 39.9 27.3 34.9
Avg. daily heat transfer reduction (%) 11.1 5.9 27.1 15.6 14.7 14.9
Avg. temperature range of the PCM melting (°C) 21.74–25.86 23.12–27.08 24.95–30.24 24.84–37.20 25.26–41.11 – Avg. percent the total available heat storage capacity used by the PCM during phase tran-
sition (%)
8.1 11.3 44.5 90.6 89.5 –
Avg. heatflux reduction when the heat fluxes of the control wall were at their peaks (%) 28.9 34.2 57.4 49.4 25.9 39.1
Table 8
Thermal results of the PCMTS at varying locations in the west wall[101].
PCM shield location 1 2 3 4 5 Avg
Avg. peak heatflux reduction (%) 5.9 29.7 21.8 1.2 7.2 12.7
Avg. heatflux reduction when the heat fluxes of the control wall were at their peaks (%) 26.1 37.3 36.3 2.5 14.2 23.3
Avg. peak heatflux time delay (h) 2 2.3 1 0.7 1 –
Avg. daytime heat transfer reduction (%) 32.5 32.4 35.1 17.2 22.6 28
Avg. daily heat transfer reduction (%) 2.6 27.9 6.0 14.2 3.6 3.8
Avg. temperature range of the PCM melting process (°C) 21.56–26.46 23.54–26.98 24.82–36.33 21.87–47.87 22.60–52.26 – Avg. percent the total available heat storage capacity used by the PCM during phase
transition (%)
10.9 10.6 90.8 95.7 94.8 –
Fig. 27. Position and performance of the passive ventilated cavity[102].
displays the details of the system and a 3D model of the ventilated façade system with PCM in the external layer. Full-scale experi- mental measurements in the PASLINK test facility were also con- ducted to validate the numerical results. To model the PCM, two methods were used. Thefirst employed the melting and solidifi- cation model available in ANSYS Fluent; the second treated the PCM as a solid material with a variable Cp (specific heat). The results indicated that the second approach was more suitable than the melting and solidification model. Note that when the thermal hysteresis phenomena or convective effects were insignificant, this PCM treatment was valid.
Zhou et al.[108]assessed the influence of the thermal prop- erties of interior and exterior PCM wallboards (Fig. 35) on its thermal performance using a numerical method. The study found that the melting temperature of an exterior PCM wallboard was strongly affected by the outside environment due to PCM exterior to the insulation material of the wallboard. However, without using the insulation material, the melting temperature range was 2–4 K for both the interior and exterior PCM wallboards. The effect of the thermal conductivity of the PCM wallboard on the diurnal
energy storage at different melting temperatures for both posi- tions is illustrated inFig. 36. For a high thermal conductivity value (greater than 0.4 W/m K), the melting temperature had little influence on the diurnal energy storage; however, this effect was more significant when the thermal conductivity was 0.1 W/m K.
That is, the diurnal energy storage can reach an enhancement of 20% for lower thermal conductivity (0.1 W/m K) compared to higher values (0.4 W/m K).
Kheradmand et al. [109] experimentally and numerically investigated the thermal performance of plastering mortars con- taining hybrid PCM blends for façade walls. A comparison of the total required cooling temperature showed that for the tested summer days, a hybrid blend of embedded PCMs (HPCMM) had the best potential to save energy and improve thermal comfort compared to regular mortars.
Lie et al.[21]numerically studied the energy performance of a building envelope integrated with PCMs for cooling load reduction in the tropical climate of Singapore. A simplified cubic model with a dimension of 3 3 2.8 m3was used to conduct the benchmark study to reveal the efficacy of incorporating PCMs into building envelopes for cooling load reduction. All parts of the building were constructed with concrete with a thickness of 150 mm. The PCM was located on the external shells of all the vertical walls in the model with a thickness of 10 mm. The results proved that the PCM was able to decrease the heat gains through the building envel- opes in the range of 21–32% when the phase change temperature of the PCM was appropriately chosen. Moreover, better perfor- mance was achieved when the PCM was used on the exterior surfaces of the walls. A comparison of PCMs with different thick- nesses demonstrated that the smallest thickness of PCM layer had the higher efficiency and cost benefits.
7.2. Application of PCM in roofs
Li et al.[110]conducted a numerical investigation to compare the thermal performance of different PCM roofs for a dwelling in China, as shown inFig. 37(right). In the study, the effect of varying PCM transition temperatures (30°C, 34 °C and 38 °C), PCM layer thicknesses (40-100 mm) and roof slopes (0.05–0.5%) were assessed using the ANSYS Fluent software. The simulation results showed that the delay time of the peak temperatures of the base layer in the PCM roofs were 3 h more than the control roof.
Fig. 38a also shows that the roof with different PCM transition temperatures (30°C, 34 °C and 38 °C) demonstrated 10 °C, 13 °C and 12°C temperature delays, respectively, compared to the con- trol roof. The heat flux and average temperature of the upper surface, as well as the utilization rate of the PCM, were all sig- nificantly affected by the PCM layer thickness (Fig. 38b and c). The influence of roof slope on the thermal performance of the PCM roofs was small compared to the liquid fraction of the PCM layer.
Tokuç et al. [111]used experimental and CFD techniques to evaluate the thermal performance of PCM incorporated into a building'sflat roof in Istanbul, Turkey. ANSYS Fluent software was Fig. 28. Verification of the Energy Plus model by comparison with the mathema-
tical model[102].
Fig. 29. The operative temperature trend in the room model[102].
Table 9
Thermal comfort enhancement with the ventilated cavity[102].
Condition Intensity of thermal discomfort (ITD) (°C h)
Frequency of thermal comfort (FTC) (%)
Mean operative temperature (°C)
Average peak temperature (°C)
No PCM 535 16.5 30.6 32.1
PCM without cavity 448 23.5 30.3 31.8
PCM with cavity 373 31.6 29.9 31.5
Variation due to the PCM 16.2% 7.00% 0.3 °C 0.3 °C
Further variation due to the cavity
16.7% 8.10% 0.4 °C 0.3 °C
Fig. 30. Schematic of wall construction (a) control wall (No PCMTS), (b) location 0/5 L (c) location 1/5 L (d) location 2/5 L (e) location 3/5 L (f) location 4/5 L (g) location 5/5 L Layers include: 1: gypsum wallboard, 2: insulating layer, 3: oriented strand board (OSB), 4: PCM thermal shield (PCMTS) and 5: heat source[103].
Fig. 31. Diagram of different ventilation modes of a building a with double stream ventilation system with PCM for cooling[104].
