# Phase change materials integrated into building walls: An updated review

B. Lamrani, K. Johannes, Frédéric Kuznik

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B. Lamrani, K. Johannes, Frédéric Kuznik. Phase change materials integrated into building walls: An updated review. Renewable and Sustainable Energy Reviews, 2021, 140, pp.110751. 10.1016/j.rser.2021.110751. hal-04918199

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# Phase change materials integrated into building walls: An updated review

Lamrani, B.¹, Johannes, K.¹, Kuznik, F.¹,*

¹ Univ Lyon, CNRS, INSA-Lyon, Université Claude Bernard Lyon 1, CETHIL, UMR5008, F-69621, Villeurbanne, France

* Corresponding author. E-mail address: frederic.kuznik@insa-lyon.fr

## Abstract

This paper is an updated, but totally new, version of “A review on phase change materials (PCMs) integrated in building walls”, an article published in 2011 in Renewable and Sustainable Energy Reviews. Both numerical and experimental studies on building walls containing PCMs during the last ten years (2011-2020) are reviewed. The paper also summarizes the main PCMs used in this application and the recent progress in the integration techniques of PCMs in building construction elements. Most of the proposed studies (about 66 %) are focused on PCM in walls. In addition, more than half of these studies are numerical where several assumptions are made such as the neglected of both the occupant presence in the building and its behaviour. Based on this review, it is concluded that the used PCMs in building walls showed good potential for reducing both indoor air temperature fluctuations and energy consumption. However, further numerical and experimental studies are needed that take other aspects into consideration, such as the real use of buildings with the occupant behaviour, the economic viability and the environmental impact. This review paper will help scientific researchers and engineers to update the integration techniques of PCM in building walls and to define potential future research works.

## Highlights

* An updated review on PCMs integrated into building walls is presented.

* Both experimental and numerical studies over the last ten years are reviewed.

* Further studies that take into consideration the real use of buildings are needed.

* The economic viability and the environmental impact are not studied sufficiently.

**Keywords:** PCM; building construction elements; latent heat storage.

**Word Count:** 7215
## Contents

<table><tr><td>1. Introduction</td><td>2</td></tr><tr><td>2. Analysis of published papers and sources</td><td>3</td></tr><tr><td>2.1. Source of published papers</td><td>3</td></tr><tr><td>2.2. Category of published papers</td><td>5</td></tr><tr><td>3. PCM selection for PCMIBW and classification</td><td>5</td></tr><tr><td>3.1. PCM selection</td><td>6</td></tr><tr><td>3.2. PCM classifications</td><td>6</td></tr><tr><td>3.2.1. Organic PCMs</td><td>6</td></tr><tr><td>3.2.2. Inorganic PCMs</td><td>7</td></tr><tr><td>3.2.3. Eutectic PCMs</td><td>8</td></tr><tr><td>4. Techniques for integrating PCMs into building walls</td><td>11</td></tr><tr><td>4.1. Direct incorporation</td><td>11</td></tr><tr><td>4.2. Impregnation</td><td>11</td></tr><tr><td>4.3. Encapsulation</td><td>12</td></tr><tr><td>4.3.1. Microencapsulation</td><td>12</td></tr><tr><td>4.3.2. Macroencapsulation</td><td>16</td></tr><tr><td>4.4. Shape-stabilized PCMs</td><td>18</td></tr><tr><td>5. Numerical studies</td><td>21</td></tr><tr><td>6. Experimental studies</td><td>23</td></tr><tr><td>7. Conclusion and recommendation</td><td>28</td></tr><tr><td>References</td><td>28</td></tr></table>

## 1. Introduction

Over the past twenty years, phase change materials (PCMs) have been used widely in buildings due to their ability to improve the thermal inertia of the building envelope and to reduce energy demand. In fact, the building sector, which includes tertiary and housing building, is considered energy intensive and is responsible for about 40% of the total greenhouse gas emissions [1]. To reduce this energy consumption, which is mainly used for heating/air conditioning, implanting energy efficiency measures and improving the building envelope energy performance are essential.

Using passive thermal energy storage (TES) in the building envelop presents an attractive solution for improving the building envelope's energy efficiency and reducing both energy consumption and carbon dioxide emissions [2]. Generally, passive TES are classified into two types namely sensible and latent heat storage. In the first type, thermal energy is stored/released through increasing/decreasing the medium storage temperature without
changing its physical state. For centuries, sensible heat storage has been used in the building
sector, and it requires a large volume of storage material compared to latent heat storage. In
the latter, the heat is stored through changing the physical state of the storage medium
(PCMs) with the temperature variation. Integrating PCM into building walls (PCMIBW) is
an efficient way not only to achieve heat storage but also for temperature control [3].
Therefore, the integration of PCMIBW has received increasing attention in the literature over
the last ten years, and several numerical and experimental studies have been presented in the
literature.

In the present paper, these published papers are reviewed and a comprehensive discussion of
the recent progress of this application is presented. The structure of this review can be
summarized as follows: An analysis of published papers and their sources over the past ten
years is presented in **section 2**. The main criteria of PCM selection for PCMIBW and their
classification are detailed in **Section 3**. The recent progress on the modes of incorporating
PCMs into building envelopes is given in **section 4**. In **sections 5 and 6**, special interest is
given to numerical and experimental studies on PCM integration in walls/wallboards. Based
on the present literature review, which reviews the literature of the past 10 years, conclusions
and recommendations for future works are highlighted in **section 7**.

## 2. Analysis of published papers and sources

2.1. Source of published papers

The papers published since 1980, which include research articles, conference papers, review
papers, conference reviews and book chapters, are presented in **Fig. 1**. The total number of
published papers from 1980 until 2019 is approximately 820 papers. It should be noted that
the number of papers published in 2020 is still increasing; therefore, published papers during
this year are excluded from this curve. It can be clearly seen that the integration of PCM into
building elements and envelopes has been attracting growing interest during this period, and
approximately 80% of these studies were published over the last ten years. In fact, during the
last ten years, several new technologies have been developed and used for integration PCM
into building envelopes, namely, new PCM incorporation modes, new PCM heat transfer
enhancement techniques and new energy standards and agendas for sustainable development.

To analyse the sources of the articles, the distribution of the published papers per journal
during the last ten years are presented in **Fig. 2**. It is worth noting that, in this figure, only the
journals with ten or more publications are illustrated. It can be observed that most of the
published papers come from the same publisher, and the highest number of publications (80
articles) comes from the Energy and Building Journal (approximately 11.5% of the total
published papers from 2011 to 2020).
<figure><img src="image_1.png" /><figcaption>**Fig. 1.** Yearly variation in the number of published papers from 1980 to 2019 (source: Scopus).</figcaption></figure>

<figure><img src="image_2.png" /><figcaption>**Fig. 2.** Published papers per journal during the last ten years.</figcaption></figure>

The distribution of the number of published papers per country is given in **Fig. 3**. In this analysis, only the countries with more than twenty papers published over the last ten years are presented. It can be seen that approximately 47% of these papers are published mainly by three countries, which are China (29%), France (9.3%) and the USA (8.7%).
<figure><img src="image_3.png" /><figcaption>Fig. 3. Distribution of published papers per country over the last ten years.</figcaption></figure>

## 2.2. Category of published papers

Since 2011, review papers on PCM integration into building envelopes, which include, walls, glazed areas, ceilings and floors represent approximately 3.7% of the total published papers (25 review articles [2–26]). Based on the first review paper focusing on this application in 2011 [2], the number of review papers has increased by a factor of 2.66.

As the present review is focused on PCM integrated into building walls/wallboards and through analysing only research papers, which represent approximately 66.2% of the total amount of published papers, the following categories of publications are identified:

* Approximately 30% of these research papers, including experimental studies and numerical studies, are dedicated to the investigation of PCMs into building walls.

* Experimental studies on the integration of PCMs in building walls/wallboards represent 35% of these papers.

* Numerical studies on the integration of PCMs in building walls/wallboards represent 52.5% of these papers.

* Only 12.5% of these papers are dedicated to both numerical and experimental investigations.

It is worth noting that all of these selected research articles are reviewed in the present review paper.

# 3. PCM selection for PCMIBW and classification

In this section, the main selection criteria for PCMs for integration into building walls/wallboards and their classifications are presented.
### 3.1. PCM selection

As it well known, PCMs are integrated mainly into the building envelope to increase their heat storage capacity and to reduce both the fluctuations in indoor temperature and heating/cooling energy demand. However, the selection of an adequate PCM is an important step, and it depends on several criteria. These criteria can be described as the follows [8,23]:

* **Thermodynamic properties:** One of the main criteria for PCM selection is its phase change temperature. This physical parameter must be chosen carefully and it should be within the thermal comfort temperature range (from 15 °C to 30 °C). This temperature can be selected by taking into consideration the average temperature during both daytime and night time and other weather conditions [8,27]. In addition to this criteria, the selected PCM should be characterized by a high-energy storage density. In other words, the latent heat per volume unit should be high to absorb/release more thermal energy during the charging and discharging processes. Two other thermodynamic factors should also be considered during the PCM selection, which are the specific heat capacity and the thermal conductivity. The latter is considered as the main disadvantage of PCMs and several techniques for heat transfer enhancement are presented in the literature, such as using porous foams [28], adding metallic spheres or nano-particles [29].

* **Chemical properties:** These criteria are related mainly to the chemical stability of the PCM during the melting/solidification cycles and to the complete reversibility of the melting/solidification cycles. It should be noted that the selected PCM should be non-flammable, non-corrosive, non-toxic and non-explosive.

* **Kinetic and economic properties:** The selected PCM should have low volume expansion during the phase change process and should be without or with only a small amount of super-cooling. From an economic point of view, the PCM should be available in a large scale with a reasonable price.

### 3.2. PCM classifications

Generally, PCMs can be classified based on to their physical state variation. For building applications and especially their integration into walls and wallboards, only solid-liquid PCMs are used and are available with a wide range of phase change temperatures on the market [30]. These PCMs are classified according to Abhat [31] into the following three categories (Fig. 4): organic, inorganic and eutectic.

#### 3.2.1. Organic PCMs

This type of PCM is the most used for TES due to their suitable melting/solidification temperature, high storage density, stable chemical and physical characteristics and to their abundance [32,33]. The thermal properties of some integrated PCM-walls/wallboards are summarized in Table 1, and their main advantages/disadvantages are given as the following:
<figure><img src="image_4.png" /><figcaption>Fig. 4. Solid-liquid PCM classifications [14].</figcaption></figure>

The advantages of organic solid-liquid PCMs (paraffin and non-paraffin) used in building walls/wallboards are as follows:

* Available in a wide range of temperatures and particularly in the comfort temperature range (18 °C – 26 °C);

* High latent heat of fusion;

* Long-term stability;

* Compatible with most building materials;

* Phase transition with a small volume change; and

* Recyclable.

The main disadvantages of organic solid-liquid PCMs (paraffin and non-paraffin) are as follows:

* Thermal conductivity of about 0.2 W/m K;

* Incompatible with some container materials, e.g., plastic containers; and

* Flammable.

### 3.2.2. Inorganic PCMs

The main type of inorganic PCM integrated into building walls over the last ten years is salt hydrate. In **Table 2**, we present the thermophysical properties of integrated inorganic PCM-walls, and their advantages are as follows [34,35]:

* High storage density compared to organic PCMs;
* High thermal conductivity (almost double than of organic PCMs);

* Available at low cost;

* Compatible with plastic containers; and

* Non-flammable.

The disadvantages of this type of PCM are as follows:

* Corrosive;

* Phase transition with a high volume change; and

* Segregation and supercoiling.

### 3.2.3. Eutectic PCMs

As defined before, eutectic PCMs are produced through combining at least two other PCMs. Compared to organic and inorganic PCMs, this type of PCMs is not widely used in building wall applications, and there is limited available data on their properties. Some of the common eutectic PCMs with melting/solidification temperatures between 20 °C and 30 °C are given in **Table 3**.
<table><caption>**Table 1.** Organic PCMs for PCMIBW (n.a.: not available)</caption><thead><tr><th>Reference</th><th>T<sub>f</sub>(°C)</th><th>L<sub>s</sub> (kJ/kg)</th><th>C<sub>ps</sub> (kJ/kgK)</th><th>C<sub>pl</sub><br>(kJ/kgK)</th><th>ρ<sub>s</sub> (kg/m<sup>3</sup>)</th><th>ρ<sub>l</sub> (kg/m<sup>3</sup>)</th><th>k<sub>s</sub> (W/mK)</th><th>k<sub>l</sub> (W/mK)</th></tr></thead><tbody><tr><td>[36]</td><td>23</td><td>200</td><td>2.1</td><td>n.a.</td><td>860</td><td>n.a.</td><td>0.2</td><td>0.2</td></tr><tr><td>[37]</td><td>27-29</td><td>245</td><td>2.0</td><td>n.a.</td><td>880</td><td>770</td><td>0.2</td><td>n.a.</td></tr><tr><td>[38]</td><td>25.8</td><td>28.7</td><td>1.2</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>0.134</td><td>n.a.</td></tr><tr><td>[39]</td><td>18.2-26.2</td><td>177</td><td>n.a.</td><td>n.a.</td><td>784</td><td>n.a.</td><td>2.2</td><td>n.a.</td></tr><tr><td>[40]</td><td>25.1-28</td><td>119.8</td><td>2.85</td><td>2.59</td><td>881.25</td><td>931.36</td><td>0.12</td><td>0.17</td></tr><tr><td>[41]</td><td>27</td><td>172.42</td><td>1.42</td><td>2.22</td><td>1530</td><td>1710</td><td>1.05</td><td>0.58</td></tr><tr><td>[42]</td><td>25</td><td>170</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td></tr><tr><td>[42]</td><td>21.4</td><td>162</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td></tr><tr><td>[43]</td><td>28</td><td>250</td><td>2</td><td>n.a.</td><td>880</td><td>770</td><td>0.2</td><td>n.a.</td></tr><tr><td>[44]</td><td>17-19</td><td>225</td><td>2</td><td>n.a.</td><td>880</td><td>770</td><td>0.2</td><td>n.a.</td></tr><tr><td>[45]</td><td>23-27</td><td>139</td><td>0.9424</td><td>1.0771</td><td>n.a.</td><td>n.a.</td><td>0.72</td><td>0.9</td></tr><tr><td>[45]</td><td>23-27</td><td>139</td><td>1.0136</td><td>1.2157</td><td>1590</td><td>n.a.</td><td>0.63</td><td>0.8</td></tr><tr><td>[45]</td><td>23-27</td><td>139</td><td>1.0848</td><td>1.3542</td><td>n.a.</td><td>n.a.</td><td>0.58</td><td>0.67</td></tr><tr><td>[46]</td><td>26</td><td>160</td><td>2</td><td>n.a.</td><td>850</td><td>n.a.</td><td>0.5</td><td>n.a.</td></tr><tr><td>[47]</td><td>27.9-30.2</td><td>142.7</td><td>3.01</td><td>3.39</td><td>752</td><td>815</td><td>0.1</td><td>0.2</td></tr><tr><td>[47]</td><td>26.1-26.5</td><td>126.90</td><td>5.47</td><td>3.01</td><td>754</td><td>817</td><td>0.125</td><td>0.21</td></tr><tr><td>[48]</td><td>28</td><td>151</td><td>2.11</td><td>n.a.</td><td>880</td><td>n.a.</td><td>n.a.</td><td>n.a.</td></tr><tr><td>[48]</td><td>6</td><td>160</td><td>2.08</td><td>n.a.</td><td>880</td><td>n.a.</td><td>n.a.</td><td>n.a.</td></tr><tr><td>[49]</td><td>28-33</td><td>63</td><td>1.5</td><td>n.a.</td><td>750</td><td>n.a.</td><td>0.15</td><td>n.a.</td></tr><tr><td>[50]</td><td>25-27.5</td><td>33.5</td><td>1.29</td><td>1.46</td><td>829</td><td>790</td><td>n.a.</td><td>0.17</td></tr><tr><td>[51]</td><td>27.4</td><td>243.5</td><td>2.15</td><td>2.18</td><td>778</td><td>775</td><td>0.358</td><td>n.a.</td></tr><tr><td>[52]</td><td>22.5-27.5</td><td>153</td><td>2.5</td><td>n.a.</td><td>774</td><td>814</td><td>0.15</td><td>n.a.</td></tr></tbody></table>
<table><caption>**Table 2.** Inorganic PCMs for PCMIBW (n.a.: not available)</caption><thead><tr><th>Reference</th><th>T<sub>f</sub>(°C)</th><th>L<sub>s</sub> (kJ/kg)</th><th>C<sub>ps</sub> (kJ/kgK)</th><th>C<sub>pl</sub> (kJ/kgK)</th><th>ρ<sub>s</sub> (kg/m<sup>3</sup>)</th><th>ρ<sub>l</sub> (kg/m<sup>3</sup>)</th><th>k<sub>s</sub> (W/mK)</th><th>k<sub>l</sub> (W/mK)</th></tr></thead><tbody><tr><td>[42]</td><td>27.5</td><td>194</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td></tr><tr><td>[44]</td><td>28-30</td><td>190</td><td>2</td><td>n.a.</td><td>1530</td><td>1520</td><td>0.6</td><td>n.a.</td></tr><tr><td>[53]</td><td>25-27</td><td>122.3</td><td>1.34</td><td>2.31</td><td>1100</td><td>n.a.</td><td>1.08</td><td>0.54</td></tr><tr><td>[54]</td><td>22-26</td><td>176</td><td>2.53</td><td>2.53</td><td>1380</td><td>n.a.</td><td>0.5</td><td>0.4</td></tr><tr><td>[55]</td><td>21.6-27.1</td><td>118.7</td><td>2.516</td><td>n.a.</td><td>875</td><td>n.a.</td><td>3</td><td>n.a.</td></tr><tr><td>[52]</td><td>28.9</td><td>190</td><td>3.020</td><td>n.a.</td><td>1700</td><td>n.a.</td><td>1</td><td>n.a.</td></tr><tr><td>[52]</td><td>18.9</td><td>160</td><td>1.9</td><td>n.a.</td><td>1520</td><td>n.a.</td><td>0.43</td><td>n.a.</td></tr><tr><td>[52]</td><td>18-22</td><td>185</td><td>3.4</td><td>n.a.</td><td>1400</td><td>n.a.</td><td>0.82</td><td>n.a.</td></tr><tr><td>[56]</td><td>22.6-30.7</td><td>122.3</td><td>1.34</td><td>2.31</td><td>1100</td><td>n.a.</td><td>1.08</td><td>0.54</td></tr><tr><td>[57]</td><td>18-22</td><td>150</td><td>2</td><td>2</td><td>1500</td><td>1400</td><td>0.5</td><td>0.5</td></tr></tbody></table>

<table><caption>**Table 3.** Eutectic PCMs in the literature.</caption><thead><tr><th>Reference</th><th>T<sub>f</sub>(°C)</th><th>L<sub>s</sub> (kJ/kg)</th></tr></thead><tbody><tr><td>[58]</td><td>22</td><td>130.2</td></tr><tr><td>[58]</td><td>14.2</td><td>98.21</td></tr><tr><td>[2]</td><td>21</td><td>155</td></tr><tr><td>[15]</td><td>24</td><td>147.7</td></tr><tr><td>[2]</td><td>24.7</td><td>179</td></tr><tr><td>[12,15]</td><td>25.5-27</td><td>203.8</td></tr><tr><td>[12,15]</td><td>25.8-26</td><td>173.93</td></tr><tr><td>[12,15]</td><td>30</td><td>80.9</td></tr><tr><td>[12,15]</td><td>30</td><td>136</td></tr></tbody></table>
## 4. Techniques for integrating PCMs into building walls

The integration of PCMs into building walls, which have storage densities approximately 5 to 10 times greater than that of walls, will ensure human thermal comfort by reducing both indoor temperature fluctuations and peaks (Fig. 5). This integration can be achieved by using the direct incorporation technique, impregnation, encapsulations and stabilization of PCMs.

The characterization of these integrated PCMs into building elements is necessary for both numerical and experimental studies, as the data provided by the manufacturer are not always correct. These measurement techniques include DSC (Differential Scanning Calorimetry), the guarded hot-plate setup and the T-history method. To avoid redundancy, more details about the measurement of PCM thermal properties are extensively discussed in our previous paper and readers are referred to article [2].

<figure><img src="image_5.png" /><figcaption>Fig. 5 Effect of PCM integration into building walls on indoor temperature [22,34]</figcaption></figure>

### 4.1. Direct incorporation

This technique is considered as the easiest and the most economical way to incorporate PCMs into building walls. The PCM is generally mixed directly with the construction elements during the manufacturing process [59,60]. As reported by several authors, such as Kuznik et al. [2] and Memon SA [12], this incorporation mode, for example into cement, should not (a) affect the product durability, (b) affect the mechanical properties of the mixed product, or (c) affect the hydration process.

### 4.2. Impregnation

The impregnation process, which can be done through immersion or imbibing, consists of dipping construction materials in a PCM liquid [61]. This procedure is more expensive than the traditional direct incorporation mode, and the construction materials most often used for building walls are gypsum [48,62] or gypsum-cement [63]. The produced wallboard can contain up to 30% PCM by weight. An example of PCM incorporation into gypsum board using the immersion process is presented in Fig. 6. In this example, the impregnation rate of the PCM (PT 20) is approximately 22 wt. %, and it is done through the immersion of several 0.6 × 0.6 m pieces of gypsum boards into a molten PCM. The produced PCM-impregnated
gypsum board was integrated into passive solar building walls in New Zealand and obtained
results that showed significant energy savings.

<figure><img src="image_6.png" /><figcaption>**Fig. 6** Direct immersion of a PCM into Gypsum board [62].</figcaption></figure>

4.3. Encapsulation

Unlike the two previous incorporation techniques, in this method, the PCM is first encapsulated in micro or macro-encapsulation and then is integrated into construction elements. According to Konuklu et al. [10] and Rathore et al. [64], the encapsulation should meet several criteria, such as thermal stability and durability, PCM protection and enhancement of the heat transfer rate. In the following section, studies on PCMIBW using PCM encapsulation over the last ten years are reviewed and analysed.

4.3.1. Microencapsulation

In this incorporation mode, the PCM is enclosed in a small capsule (from 1 µm to 1000 µm) made from natural and synthetic polymers (Fig. 7). In the literature, two main microencapsulation methods have been reported, which are classified as physical and chemical methods [16]. In the first technique, the PCM can be microencapsulated through spry drying, pan coating, centrifugal extrusion, pan coating or air-suspension coating. In the second method, the PCM is microencapsulation through matrix, interfacial or in situ polymerization. Organic PCMs are the most suitable PCM type for this technique and, in particular, paraffin and fatty acid PCMs. In the direct incorporation technique, the PCM is leaked during the phase change process while using microcapsules prior to the mixing process with the construction elements, which on one hand, successfully overcomes this problem and, on the other hand, improves the heat transfer processes through increasing the heat transfer area, reducing the PCM volume change and improving the thermal reliability [12,21,65].
<figure><img src="image_7.png" /><figcaption>**Fig. 7 Microencapsulated PCM.**</figcaption></figure>

Over the last ten years, this technique of incorporating PCMs into building materials, such as concrete and gypsum, has received increasing attention. Some of the prominent studies on the characterization and the thermal performance evaluation of the PCMs microencapsulated into these construction materials are discussed in the following section.

To characterize an improved gypsum wallboard using microencapsulated PCMs, Hang Yu et al. [66] designed an experimental apparatus and compared three different measurement methods. The used PCM is an organic paraffin encapsulated in a PMMA shell, which is incorporated into the gypsum mortar (Fig. 8). The transition temperature of the used PCM is approximately 31 °C, and the spherical microcapsules range in size from 0-100 µm. The thermal characteristics of the produced wallboard with PCM were determined, which include the effective thermal conductivity, heat capacity and curves of enthalpy/temperature. They have shown that adding microencapsulated PCM into gypsum wallboard leads to a reduction in the thermal conductivity compared to a pure gypsum wallboard. However, the apparent heat capacity is increased by approximately 2.71 times. They have concluded that both dynamic and analogous step methods are suitable to determine enthalpy-temperature curves compared to the theoretical calculation from DSC data.
<figure><img src="image_8.png" /><figcaption>**Fig. 8** (a) Microencapsulated PCM; (b) After hydration of the Gypsum; (c) Gypsum-Microencapsulated PCM; (d) Picture of the Microencapsulated PCM-wallboard [66].</figcaption></figure>

Marchi et al. [67] studied two microencapsulated PCMs (paraffin in PMMA and paraffin in silicon dioxide) to prepare a composite panel for a building application. The selected PCMs were embedded into three different matrices, i.e., acrylic, epoxy and polyurethane. To select the PCM-based panel with the best performances, their porosities were determined experimentally. The thermal characteristics of these PCMs and the produced PCM-based panels were determined using DSC. They found that the microencapsulated paraffin-silicon dioxide embedded in the polyurethane matrix is more suitable compared to the other microencapsulated PCM-panels for improving the building's energy efficiency.

Errebai et al. [68] studied numerically and experimentally a novel technical solution for enhancing the heat transfer ability of a microencapsulated PCM-plasterboard. In order to improve the heat transfer between the air and panel (absorbed/released) during the ventilation process, the PCM panel is perforated with small holes. The used PCM is a commercial paraffin encapsulated in a polymethyl methacrylate shell with a size of approximately 5 µm and a melting temperature equal to 26 °C (Fig. 9). The authors also reported that increasing the amount of PCM in the plaster leads to an improvement in the heat capacity, and the most suitable PCM percent, taking into consideration both the heat transfer improvement and mechanical characteristics of the panel, is approximately 30%. Based on the obtained results, using the perforated microencapsulated-PCM plasterboard leads to a significant increase in the absorbed/released heat compared to an ordinary microencapsulated-PCM plasterboard.
This enhancement technique showed promising results; however, an application on a local scale is necessary.

<figure><img src="image_9.png" /><figcaption>**Fig. 9** Microencapsulated PCM powder (left) and perforated microencapsulated-PCM plasterboard with 36 holes (right) [68].</figcaption></figure>

Recently, Frazzica et al. [69] proposed a hybrid microencapsulated PCM - cement mortar for increasing the thermal comfort in buildings during both summer and winter seasons. The two PCMs used are Micronal 5040X and Micronal 5038X encapsulated in PMMA (200-800 µm) with melting temperatures of 23 °C and 26 °C, respectively. First, the thermal characteristics of the proposed hybrid microencapsulated PCM-mortar are determined using a DSC apparatus. Then, the composite mortar-PCMs thermal performances are compared with that of an ordinary mortar, and the obtained results show that the presence of the PCM leads to a decrease in indoor temperature fluctuations and to a shift in the daily maximal temperature.

**Table 4** summarizes the studies on microencapsulated-PCMs for building construction elements.

<figure><img src="image_10.png" /><figcaption>**Fig. 10** SEM of mortar filled with 10% of Micronal 5040X: a) 500 × of the 1040 sample; b) 25 k× of the 1040 sample.</figcaption></figure>
<table><caption>**Table 4.** Studies on microencapsulated PCMs for PCMIBW.</caption><thead><tr><td>PCM</td><td>Shell material</td><td>Capsule size (μm)</td><td>T<sub>f</sub> (°C)</td><td>Reference</td></tr></thead><tbody><tr><td>Paraffin</td><td>Polymethylmethacrylate (PMMA)</td><td>10-100 (Fig. 8)</td><td>31</td><td>[66]</td></tr><tr><td>Paraffin</td><td>PMMA</td><td>50-300</td><td>23</td><td>[67]</td></tr><tr><td>Paraffin</td><td>Silicon dioxide</td><td>300-4750</td><td>28</td><td>[67]</td></tr><tr><td>Micronal DS</td><td>PMMA</td><td>5</td><td>26</td><td>[68,70]</td></tr><tr><td>Micronal 5040X</td><td>PMMA</td><td>200-800</td><td>23</td><td>[69]</td></tr><tr><td>Micronal 5038X</td><td>PMMA</td><td>200-800</td><td>26</td><td>[69]</td></tr><tr><td>Organic MPCM</td><td>Organic polymer</td><td>10-50</td><td>12</td><td>[65,71]</td></tr><tr><td>Organic MPCM</td><td>Organic polymer</td><td>10-50</td><td>18</td><td>[65,71]</td></tr><tr><td>Organic MPCM</td><td>Organic polymer</td><td>10-50</td><td>29</td><td>[65,71]</td></tr><tr><td>Paraffin</td><td>Copolymer</td><td>5</td><td>17.8-22.3</td><td>[27,72-77]</td></tr><tr><td>Micronal- BASF</td><td>Plastic shell</td><td>2-20</td><td>23</td><td>[78]</td></tr><tr><td>Micronal- BASF</td><td>Plastic shell</td><td>2-20</td><td>26</td><td>[79,80]</td></tr><tr><td>Paraffin</td><td>Copolymer</td><td>n.a.</td><td>20.6</td><td>[81]</td></tr><tr><td>Paraffin</td><td>Polymer shell</td><td>17-20</td><td>37</td><td>[82]</td></tr><tr><td>Paraffin</td><td>Melamine-formaldehyde</td><td>15-20</td><td>24</td><td>[83]</td></tr></tbody></table>

### 4.3.2. Macroencapsulation

PCM macroencapsulation is defined as the process of packing PCMs into containers shell with sizes greater than 5 mm. Several forms of containers can be used, such as spheres, tubes, panels and pouches (Fig. 11). The main advantages of this encapsulation technique can be summarized as follows:

* Improving the heat transfer rate;

* Increasing the amount of used PCM;

* Reducing the PCM volume change;

* Avoiding the leakage problem.

However, the disadvantages of macroencapsulation include the following:

* Corrosion;

* Should be protected in the case of drilling holes in building walls;

* Low thermal conductivity; and

* Requires more work during the integration into the building envelope.

A summary of the studies on the integration of macroencapsulated PCMs into building walls is given in Table 5.
<figure><img src="image_11.png" /><figcaption>Fig. 11 PCM macroencapsulation forms [24,42,84,85].</figcaption></figure>
<table><caption>**Table 5.** Various studies on macroencapsulated PCMs for PCMIBW.</caption><thead><tr><td>PCM</td><td>Macroencapsulation Shape</td><td>T<sub>f</sub>(°C)</td><td>References</td></tr></thead><tbody><tr><td>Mixture of hydrated salts</td><td>Rectangular polyolefin envelope</td><td>27</td><td>[85]</td></tr><tr><td>CaCl<sub>2</sub>·6H<sub>2</sub>O</td><td>Rectangular PVC container</td><td>25-27</td><td>[53]</td></tr><tr><td>Paraffin</td><td>Aluminium panel</td><td>40-44</td><td>[86]</td></tr><tr><td>Hydrated salt</td><td>Polyethylene flat bubbles</td><td>24-34</td><td>[84]</td></tr><tr><td>CaCl<sub>2</sub>·6H<sub>2</sub>O</td><td>Rectangular PVC container</td><td>22.6-30.7</td><td>[56]</td></tr><tr><td>n-octadecane</td><td>Plastic pouches</td><td>26-28</td><td>[87]</td></tr><tr><td>n-octadecane</td><td>Copper pipes</td><td>27.5-38</td><td>[88]</td></tr><tr><td>Salt hydrate SP-22</td><td>CSM panel</td><td>18-22</td><td>[57]</td></tr><tr><td>Paraffin</td><td>Stainless steel box</td><td>20.78-25.09</td><td>[89]</td></tr><tr><td>SP-25 A8</td><td>CSM panel</td><td>23-29</td><td>[80]</td></tr><tr><td>PEG 600</td><td>Sheet form</td><td>17-22</td><td>[90]</td></tr><tr><td>PCM-27</td><td>Plexiglas parallelepiped-shaped</td><td>27</td><td>[41]</td></tr><tr><td>Fatty acid</td><td>Glass bricks</td><td>21</td><td>[42]</td></tr><tr><td>Paraffin</td><td>Glass bricks</td><td>25</td><td>[42]</td></tr><tr><td>Salt hydrate</td><td>Glass bricks</td><td>27.5</td><td>[42]</td></tr><tr><td>SP 29</td><td>Aluminium panels</td><td>28-30</td><td>[44]</td></tr><tr><td>RT 18</td><td>Aluminium panels</td><td>17-19</td><td>[44]</td></tr><tr><td>CaCl<sub>2</sub>·6H<sub>2</sub>O</td><td>Stainless steel container</td><td>22-26</td><td>[54]</td></tr><tr><td>Bio-PCM</td><td>Rectangular panel</td><td>27</td><td>[91]</td></tr></tbody></table>

### 4.4. Shape-stabilized PCMs

In this technique, the PCM in liquid state and its supporting material are mixed at high temperature then cooled to form the PCM into a shape-stabilized form (SSPCM). For PCM integration into a building envelope, high-density polyethylene (HDPE), expanded perlite (EP) and styrene butadiene styrene (SBS) are commonly used as supporting materials for SSPCMs, which are characterized by a high stabilization index, a large apparent heat capacity and an appropriate thermal conductivity. In addition, through using these supporting materials, up to 80% of the PCM's mass proportion can be used.

Some of research studies on SSPCMs integrated into building envelopes are presented and discussed in the following section.

Yao et al. [92] investigated a wallboard with shape-stabilized PCM (SSPCM-wallboard) in which an organic PCM (paraffin) and EP supporting material are mixed using a horizontal vacuum absorption rotating roller (Fig. 12). The melting and freezing temperatures of the used PCM are approximately 27.6 °C and 23.5 °C, respectively. The produced SSPCM-wallboard is integrated into a building in north zone of China and the obtained results showed that the indoor maximal temperature could be reduced by approximately 2.53 °C. The same supporting material is used by the same authors [93] and is mixed with a liquid paraffin with a melting temperature of about 25.22 °C, and the SSPCM-wallboard is incorporated into both walls and ceiling of a room. They have concluded that the SSPCM-wallboard had a compact and firm microstructure with a satisfactory thermal performance.
<figure><img src="image_12.png" /><figcaption>**Fig. 12** Horizontal vacuum absorption rotate roller [92,93]</figcaption></figure>

Through using HDPE as the supporting material, Zhu et al.[94] studied a double layer SSPCM-wallboard in which paraffin is used with a mass percentage of 80% (Fig. 13). Two PCMs with different melting temperatures are used and are placed in the interior and the exterior faces of the wallboard. The developed double layer SSPCM-wallboard is tested in a reduced-scale room and is compared with a reference room without PCM under the weather conditions of Wuhan city. It has been found that using SSPCM is advantageous and is able to ensure thermal comfort during both summer and winter periods.

<figure><img src="image_13.png" /><figcaption>**Fig. 13.** Double layer SSPCM-wallboard [94].</figcaption></figure>

As the thermal conductivity of the SSPCM is not very large, researchers add materials with high thermal conductivity to the SSPCM to overcome this problem. Recently, Yang et al. [95] added alumina (Al₂O₃) nanoparticles to a binary PCM (lauryl alcohol + stearic acid) to improve its thermal conductivity. The obtained mixture is absorbed into ceramsite as the supporting material and encapsulated by cement powder and a styrene-acrylic emulsion (Fig. 14). To characterize the produced ceramsite-SSPCM/cement, a DSC apparatus is used and the obtained results showed that the phase transition temperature is approximately 22.5 °C and the heat of fusion is equal to 133.4 kJ/kg. To integrate the ceramsite-SSPCM/cement into the building envelope, a concrete block and panels based ceramsite-SSPCM/cement are fabricated and tested. From this study, the authors have concluded that the proposed ceramsite-
SSPCM/cement is economical to prepare and suitable for regulating the indoor temperature and reducing both the heating and cooling energy consumption.

<figure><img src="image_14.png" /><figcaption>**Fig. 14** Ceramsite-SSPCM/cement: (a) ceramsite; (b) ceramsite with PCM/ Al₂O₃ absorbed; (c) after encapsulation by a styrene-acrylic emulsion; (d) after encapsulation by cement powder [95].</figcaption></figure>

A summary of the various studies on SSPCMs integrated into building envelopes is given in **Table 6**. It can be seen that in the majority of the studies, paraffin is used as the PCM and high-density polyethylene is the common supporting material.

<table><caption>**Table 6.** Various studies on SSPCMs for PCMIBW.</caption><thead><tr><th>PCM</th><th>Supporting material</th><th>T<sub>f</sub>(°C)</th><th>References</th></tr></thead><tbody><tr><td>Paraffin</td><td>Expanded perlite</td><td>23.5-27.6</td><td>[92]</td></tr><tr><td>Paraffin</td><td>Expanded perlite</td><td>25.22</td><td>[93]</td></tr><tr><td>Paraffin</td><td>High-density polyethylene</td><td>26.35-36.25</td><td>[94]</td></tr><tr><td>Paraffin</td><td>High-density polyethylene</td><td>17.03-26.97</td><td>[94]</td></tr><tr><td>Lauryl alcohol + Stearic acid</td><td>Ceramsite</td><td>22.5</td><td>[95]</td></tr><tr><td>Paraffin</td><td>High-density polyethylene</td><td>15.80-22.28</td><td>[96]</td></tr><tr><td>n-octadecane</td><td>Exfoliated graphite nanoplatelets</td><td>28</td><td>[97]</td></tr><tr><td>Paraffin</td><td>Expanded perlite</td><td>24.92-29.05</td><td>[98]</td></tr><tr><td>Hexadecane + octadecane</td><td>Polypropylene</td><td>19-26</td><td>[99]</td></tr><tr><td>Paraffin</td><td>High-density polyethylene</td><td>18-25</td><td>[100]</td></tr><tr><td>CaCl<sub>2</sub>·6H<sub>2</sub>O</td><td>High density polyethylene</td><td>24-29</td><td>[101]</td></tr><tr><td>Paraffin</td><td>Expanded perlite</td><td>24.88</td><td>[102]</td></tr><tr><td>Paraffin</td><td>Carbon based material</td><td>20-30</td><td>[103]</td></tr><tr><td>Paraffin</td><td>High density polyethylene</td><td>27.5-28.5</td><td>[104]</td></tr><tr><td>Paraffin</td><td>Polypropylene</td><td>19-26</td><td>[105]</td></tr><tr><td>Capric acid and palmitic acid</td><td>Exfoliated graphite nanoplatelets</td><td>18.2-26.2</td><td>[39]</td></tr></tbody></table>
<table><tr><td>Paraffin</td><td>Graphite foam</td><td>38.7</td><td>[106]</td></tr><tr><td>Paraffin</td><td>High density polyethylene</td><td>27-29</td><td>[37]</td></tr></table>

## 5. Numerical studies

Numerical modelling and simulation of PCMIWB are advantageous for system design and optimization and have received increasing attention during the past ten years. In this section, numerical studies on PCMIWB are reviewed and some of the prominent numerical studies are presented and discussed here.

To model the phase change process in PCMIWB, two methods were extensively used, which are the effective heat capacity and the enthalpy methods [107,108]. In most of the presented numerical studies on PCMIWB, commercial software, such as EnergyPlus [91,105,109–122], TRNSYS [40,42,44,123–129], MATLAB and Simulink [45,48,76,77,130–135], COMSOL [69,136–138] and ANSYS Fluent [38,47,55,58,139–141], is used to solve the energy governing equations and to carry out numerical simulations under variable weather conditions.

It is worth noting that most of these numerical studies deal with the effect of the integration of PCMs into building walls, its position in the walls, its thermophysical properties, such as the melting temperature and thermal conductivity, and the effect of its thickness. As an example of these studies, Li et al. [52] have numerically studied the thermal performance of a PCMIWB under the weather conditions of Isfahan city (Iran) with the use of thirteen different PCMs. Three different wall designs were studied (Fig. 15), and the effect of PCM position, PCM thickness and PCM melting temperature were investigated. Generally, the PCM selection is based first on the melting temperature. However and based on their numerical results, they have shown that the PCM selection must be based first on the PCM’s thermal conductivity and then on the PCM’s melting temperature. This choice is justified by the ability of PCM with low thermal conductive to reduce more the heat transfer to the indoor space and therefore reduce the energy consumption. In addition and compared with other thermophysical properties, the thermal conductivity must have more priority according to the same authors. In the case of two PCMs with similar thermal conductivities, the PCM melting temperature must be closer to the indoor temperature. For the PCM position effect, they recommended integrating the PCM closer to the exterior for better performance in terms of decreasing the heat transfer in the studied city.

Another numerical study where a PCMIWB is investigated and the influences of some of the design and operation parameters on the PCMIWB performance are investigated is presented by Arıcı et al.[142]. In this study, the unidirectional conduction transfer equation is used to model the heat transfer in both the PCM and its wall components. An optimization study of the PCM position, PCM thickness and its melting temperature is carried out under the weather conditions of three cities in Turkey. They have shown that the thickness and the location of the PCM in the wall depends on both the weather conditions and the building site. For example, in heating conditions, they recommended placing the PCM close to the exterior. However, an interior location is suitable during cooling conditions.

Wang et al.[143] presented a parametric analysis of PCMIWB for reducing heating loads under the weather conditions of Shanghai city. A two-dimensional heat equation is used to model the heat transfer in the wall, which includes the PCM layer and concrete. They have
also shown that integrating a PCM into the wall significantly reduces the wall's surface temperature and the heat transfer rate during the summer period. During this season, the optimal RT-42 PCM thickness is approximately 20 mm, and its suitable location is the exterior side.

To find both the optimal phase transition temperature and the optimal latent heat of a PCM integrated into the interior sides of walls, a simple analytical model is developed by Jiang et al.[144]. The proposed simplified model depends mainly on the lower limit of the comfort temperature range, the ventilation rate and envelope characteristics and the outdoor temperature. From their numerical results, they concluded that the optimal PCM melting temperatures in different climatic regions in China are approximately similar and are higher by approximately 1.1-3.3 °C than the lower limit of the comfort temperature range.

Based on the analysis of the published numerical studies on PCMIBW, we concluded that most of these studies are focused only on one common objective, which is the optimization of PCMs that includes the PCM location, the PCM melting temperature, the PCM thickness, etc. However, other aspects should be taken into consideration and could be carried out successfully using numerical tools, such as the occupant behaviour (real use of building), the economic viability and the environmental impact.
<figure><img src="image_15.png" /><figcaption>**Fig. 15.** Studied wall configurations from Ref [52].</figcaption></figure>

**6. Experimental studies**

Durant the last ten years, many experimental studies have been performed to investigate the thermal performances of building walls integrated with PCMs. Kong et al [93] developed a shape stabilized composite PCM wallboard (CPCMW) using paraffin and expanded perlite, which were then fixed on the internal faces of the walls and ceiling of a test room (Fig. 16). To demonstrate the effect of the integration of CPCMW on the building thermal performance, the indoor temperature variations in two rooms (with and without CPCMW) are compared during summer conditions in Tianjin city. The measured results have shown that the integration of a CPCMW leads to a passive adjustment of the indoor temperature and to an increase in the envelope's thermal mass.
<figure><img src="image_16.png" /><figcaption>**Fig. 16.** Composite PCM wallboard and the test room [93].</figcaption></figure>

Kong et al. [98] recently proposed a novel design of a hybrid PCM wall for the heating supply during winter conditions. The proposed hybrid PCM wallboard is composed mainly of a coupled solar thermal system shape stabilized PCM wallboard (Fig. 17). Two experiment cells were fabricated (1.7 m × 1.7 m × 2 m) to test the hybrid PCM wall (with and without hybrid system). To investigate the performance of this hybrid PCM wall, three experimental tests were carried out and indoor air temperature and daily energy consumption are measured and compared. The authors have shown that the integration of the proposed system in building walls is suitable to ensure thermal comfort and to reduce the daily energy consumption by approximately 44% compared to the ordinary building without the PCM. This system showed promising results; however, a techno-economic assessment is necessary.

<figure><img src="image_17.png" /><figcaption>**Fig. 17.** Hybrid PCM wall [98].</figcaption></figure>

Mourid et al. [145] analysed the feasibility of the integration of PCM panels (Energain wallboard) into building walls during summer conditions in a Moroccan city. Two identical test cells were fabricated (3 m × 3 m × 3 m) using alveolar brick walls, and the PCM panel is placed on the inner surface of one of these rooms. The selected PCM is a microencapsulated paraffin wax from DuPont™ with a phase change temperature between 20 °C and 35 °C. The evolution of the indoor temperature and the inner surfaces' temperatures in both rooms are recorded and compared. The obtained results show that the thermal mass of the room with the
PCM is strongly increased compared to the reference room, and the maximal recorded difference between the air temperatures is approximately 6 °C. In terms of the heating energy savings, using the PCM panel leads to a 20% reduction in the energy consumption. These results indicate that using PCM panels in building walls under Moroccan climatic conditions (Casablanca city) is a suitable application and can contribute successfully to the reduction of energy demand and CO2 emissions.

Berardi et al.[146] studied the effect of integrating a composite PCM wallboard on the thermal performance of a test cell. They proposed a composite PCM wallboard composed mainly of two types of commercial PCMs with different melting temperatures (Fig. 18). The selected PCMs are BioPCM, with phase transition temperature equal to 25 °C, and Energain PCM, with 21.7 °C as melting temperature. The composite PCM is embedded in the walls of one test cell between the gypsum board and insulation, and the test is conducted from July to October under the weather conditions of Toronto city. The influence of the cell orientation, shading and night cooling were analysed and the indoor air temperature and surface walls' temperatures were recorded and analysed. They found that using the proposed composite PCM leads to a reduction in the temperature peak up to 6 °C. The results also showed that night ventilation is beneficial for the complete solidification of the PCM and leads to a fast discharging process. They concluded that using a composite PCM with different melting temperatures is suitable and presents an advantageous approach for PCM applications in the building envelope.

<figure><img src="image_18.png" /><figcaption>Fig. 18 Composite PCM wall and test cells [146].</figcaption></figure>

Sun et al.[88] proposed a novel design of a wallboard enhanced with a pipe encapsulated PCM (Fig. 19). A laboratory dynamic wall simulator is fabricated to reproduce real conditions in residential buildings. The selected PCMs are the organic paraffin type (RT 27 and RT 28) and are encapsulated in pipes with two different sizes. Typical summer conditions are reproduced and the indoor temperature, inner surface wall temperature and liquid fraction are analysed. The authors have recommended that for a real application and to avoid incomplete solidification/melting processes, the PCM must be encapsulated in small pipes. It worth nothing that this study is also carried out only under laboratory test conditions without taken into consideration the real use of building.
<figure><img src="image_19.png" /><figcaption>**Fig. 19** wallboard enhanced with pipe encapsulated PCM [88].</figcaption></figure>

Lee et al. [59] experimentally investigated two identical test cells with and without using a PCM in the walls (Fig. 20). The PCM is incorporated in the walls through mixing it with the thermal insulation (cellulose), and the thermal performances of these cells are evaluated under the weather conditions of the central part of the US. The used PCM is paraffin based with a melting temperature between 26-28 °C. In both cells, the indoor air temperature and wall surfaces' temperatures are recorded and analysed. The obtained results are analysed in terms of the wall heat flux and have shown that using the PCM-enhanced cellulose in the walls leads to reducing the daily average peak heat flux by approximately 25.4%.

<figure><img src="image_20.png" /><figcaption>**Fig. 20** Test cells with and without PCM-cellulose [59].</figcaption></figure>

Recently, Cabeza et al. [147] studied the long-term thermal performance of a test cell integrated with a PCM in walls after 10 years its fabrication. The thermal performance of this cell is compared with that of a reference test cell without PCM during summer conditions. The obtained results are compared with those recorded during the summer period of 2005, and the same thermal response is observed. From this study, it is confirmed that the PCM integrated into building walls preserves the same thermal response without degradation and successfully contributes to the reduction of the indoor air fluctuation even 10 years after its incorporation.

In Table 7, a summary of experimental studies on PCMIBW is presented, and it can be seen clearly that most of them are carried out in outdoor conditions. The commonly used PCM in these studies is paraffin, with a phase transition temperature between 20 and 30 °C. It should
be noted that none of these studies take into consideration internal loads and the existence of
the occupant or his behaviour (real use of building).
<table><caption>Table 7. Various experimental studies on PCMIWB.</caption><thead><tr><td>PCM</td><td>Container</td><td>Cell size</td><td>Number of cells</td><td>Conditions</td><td>References</td></tr></thead><tbody><tr><td>Paraffin: P56-P58</td><td>Brick wall cavity</td><td>0.508 m × 0.251 m × 0.152 m</td><td>2</td><td>Laboratory</td><td>[148]</td></tr><tr><td>Paraffin</td><td>Shape stabilized</td><td>1 m × 1 m × 1 m</td><td>2</td><td>Outdoor</td><td>[102]</td></tr><tr><td>Paraffin</td><td>Shape stabilized</td><td>1.7 m × 1.7 m × 2.1 m</td><td>2</td><td>Outdoor</td><td>[92,93]</td></tr><tr><td>Paraffin</td><td>Shape stabilized</td><td>1.2 m × 1.2 m × 1.2 m</td><td>2</td><td>Outdoor</td><td>[94]</td></tr><tr><td>CaCl<sub>2</sub>·6H<sub>2</sub>O</td><td>Shape stabilized</td><td>2.96 m × 2.41 m × 2.6 m</td><td>1</td><td>Outdoor</td><td>[101]</td></tr><tr><td>Paraffin</td><td>Shape stabilized</td><td>1.7 m × 3.8 m × 3.8 m</td><td>3</td><td>Outdoor</td><td>[99]</td></tr><tr><td>Paraffin</td><td>Shape stabilized</td><td>1.7 m × 1.7 m × 2 m</td><td>2</td><td>Outdoor</td><td>[98]</td></tr><tr><td>Paraffin</td><td>Shape stabilized</td><td>3.25 m × 3.86 m × 2.91 m</td><td>1</td><td>Outdoor</td><td>[96]</td></tr><tr><td>Paraffin</td><td>Energain panel</td><td>5.2 m × 3.55 m × 2.7 m</td><td>2</td><td>Outdoor</td><td>[34]</td></tr><tr><td>Organic PCMs</td><td>Bio-PCM + Energain panel</td><td>0.7 m × 0.6 m × 0.4 m</td><td>2</td><td>Outdoor</td><td>[146]</td></tr><tr><td>Paraffin</td><td>Energain panel</td><td>3 m × 3 m × 3 m</td><td>2</td><td>Outdoor</td><td>[145]</td></tr><tr><td>Paraffin</td><td>Energain panel</td><td>3 m × 1.5 m × 2.65 m</td><td>1</td><td>Laboratory</td><td>[73,149]</td></tr><tr><td>Organic PCMs</td><td>Gypsum</td><td>0.42 m × 0.36 m × 0.36 m</td><td>3</td><td>Outdoor</td><td>[65]</td></tr><tr><td>Paraffin</td><td>Energain panel</td><td>2.62 m × 2.62 m × 2.64 m</td><td>2</td><td>Outdoor</td><td>[72]</td></tr><tr><td>Micronal</td><td>Gypsum</td><td>9.9 m × 12.33 m × 8.47 m</td><td>1</td><td>Outdoor</td><td>[78]</td></tr><tr><td>Micronal</td><td>Concrete</td><td>2.64 m × 2.64 m × 2.52 m</td><td>2</td><td>Outdoor</td><td>[79]</td></tr><tr><td>Paraffin</td><td>Gypsum</td><td>2.64 m × 2.6 m × 2.6 m</td><td>2</td><td>Outdoor</td><td>[80,150]</td></tr><tr><td>Paraffin</td><td>Concrete</td><td>0.45 m × 0.45 m × 0.45 m</td><td>1</td><td>Laboratory</td><td>[83]</td></tr><tr><td>Hydrated salts</td><td>Polyolefin envelope</td><td>1 m × 0.73 m × 0.6 m</td><td>1</td><td>Outdoor</td><td>[85]</td></tr><tr><td>CaCl<sub>2</sub>·6H<sub>2</sub>O</td><td>PVC container</td><td>1.2 m × 0.66 m × 0.8 m</td><td>2</td><td>Laboratory</td><td>[53,56]</td></tr><tr><td>Paraffin</td><td>Aluminium panel</td><td>3 m × 2.5 m × 2 m</td><td>2</td><td>Outdoor</td><td>[86]</td></tr><tr><td>Hydrated salt</td><td>Polyethylene flat bubbles</td><td>1.19 m × 1.19 m × 1.19 m</td><td>1</td><td>Laboratory</td><td>[84]</td></tr><tr><td>Paraffin</td><td>Plastic pouches</td><td>1.19 m × 1.19 m × 1.19 m</td><td>1</td><td>Laboratory</td><td>[87]</td></tr><tr><td>Paraffin</td><td>Pipes</td><td>1.2 m × 1.2 m × 1.2 m</td><td>1</td><td>Laboratory</td><td>[88]</td></tr><tr><td>Salt hydrate SP-22</td><td>CSM panel</td><td>2.4 m × 2.4 m × 5.1 m</td><td>2</td><td>Outdoor</td><td>[57]</td></tr><tr><td>Paraffin</td><td>Stainless steel box</td><td>0.545 m × 0.545 m × 0.56 m</td><td>3</td><td>Outdoor</td><td>[89]</td></tr><tr><td>PEG 600</td><td>sheet form-concrete</td><td>1 m × 0.5 m × 2.2 m</td><td>2</td><td>Laboratory</td><td>[90]</td></tr><tr><td>PT 20</td><td>Gypsum</td><td>2.4 m × 2.4 m × 2.4 m</td><td>2</td><td>Outdoor</td><td>[62]</td></tr><tr><td>Paraffin</td><td>Cellulose insulation</td><td>1.83 m × 1.83 m × 1.52 m</td><td>2</td><td>Outdoor</td><td>[59]</td></tr><tr><td>Paraffin</td><td>Shape stabilized</td><td>1.8 m × 1.2 m × 1 m</td><td>1</td><td>Laboratory</td><td>[60]</td></tr></tbody></table>
## 7. Conclusion and recommendation

In this paper, an update of the first comprehensive review on PCM integration into building walls is presented. Both numerical and experimental studies on PCMIBW over the last ten years (2011-2020) are reviewed. It has been shown that the use of PCMs in buildings has received increasing attention and most of the proposed studies (about 66 %) are focused on PCM in walls. More than half of these studies are numerical where several assumptions are made such as the neglected of the occupant presence in the building and its real behaviour. The experimental studies are generally lower than the numerical one and they represent only about 35 % of all published works.

Based on this ten years review, all the used PCMs in building walls, in particular those with a melting temperature near to the indoor temperature, showed good potential for reducing both indoor air temperature fluctuations and energy consumption. However, more attention should be given to the thermal conductivity enhancement techniques for PCMIWB. For both numerical and experimental studies, other aspects should be taken into consideration in future works, which can be summarized in the following three points:

* Take into consideration the real use of the building i.e. the presence of occupants in the building and their real behaviour with better accuracy.

* Consider the economic viability of each proposed solution for improving the human comfort and reducing the energy consumption.

* Include both the environmental impact of using PCM in building walls and the safety for human health.

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