3.2.2.1 Organic

Organic PCMs are divided into paraffins and non-paraffins. In general, organic PCMs do not suffer from phase segregation and crystallize with little or no supercooling (Rathod and Banerjee, 2013).

Paraffins are available in a large temperature range (Fig. 3.2), opening them up for use in various other areas besides building related applications. The latent heat is mass based, they show no signs of phase separation after repeated cycling through solid–liquid transitions, and have a low vapor pressure (Alkan, 2006). However, paraffins used as PCMs have some drawbacks. They have low thermal conductivity (around 0.2 W/(mK)), they are not compatible with plastic containers, and they are moderately flammable (Sharma et al., 2009).

Non-paraffins used as PCMs include fatty acids and their fatty acid esters and alcohols, glycols, etc. Fatty acids have received the most attention for use as PCMs in buildings. An extensive review on fatty acids used for PCM purposes has been written by Yuan et al. (2014). In this review, fatty acids and their esters and alcohols were reviewed for their potential as PCMs. The most interesting fatty acids for PCM purposes include lauric acid, myristic acid, palmitic acid, and stearic acid. As with paraffins, these also suffer from low thermal conductivity, ranging from 0.15 to 0.17 W/(mK).

Overall, organic PCMs have many qualities that make them suited for building applications. However, the fact that many organic PCMs are considered flammable is a crucial drawback that impacts the safety aspect of organic PCMs considerably when aimed at building applications.

3.2.2.2 Inorganic

Inorganic PCMs of interest consist of hydrated salts and metallics. For building applications, however, metallics are not within the desired temperature range, and in addition, they have severe weight penalties, making them unsuitable. Hence, they will not be further addressed in this review.

Hydrated salts consist of an alloy of inorganic salts and water and enable a cost-effective PCM due to easy availability and low cost. The phase change transformation involves hydration or dehydration of the salts in a process that resembles typical melting and freezing. The salt hydrate may either melt to a salt hydrate containing less water or to an anhydrous form where salt and water are completely separated (Sharma et al., 2009).

Desirable properties of salt hydrates used as PCMs include high latent heat of fusion per unit volume, higher thermal conductivity than organic PCMs and small volume change and easy availability at a lower cost.

Hydrated salts suited for commercial use suffer from incongruent melting. This is a typical problem for hydrated salts as the melting process in many cases leads to the salt releasing water and turning into a salt in its anhydrous from, or a different salt. The density of the anhydrous salt may be higher, causing it to sink to the bottom of the container. When the temperature reaches the freezing point, the salt will be stacked at the bottom and some salt will be unable to reabsorb the water. Hence, the total volume of salt that can undergo phase change has decreased, and the effectiveness of the PCM is reduced. Another issue for hydrated salts is supercooling, which occurs because of poor nucleating properties in many salt hydrates. When supercooling occurs, the liquid phase of a PCM does not freeze (solidify) at the intended temperature, but reaches temperatures lower than the freezing point before actually solidifying. This is a critical flaw as it alters the temperature of the phase change.

3.2.2.3 Eutectic Mixtures

An eutectic is a melting composition of minimum two or more components, each of which melts and freezes congruently. During the crystallization phase, a mixture of the components is formed, hence acting as a single component. The components freeze to an intimate mixture of crystals and melt simultaneously without separation (Lane, 1989). Eutectics can be mixtures of organic and/or inorganic compounds. Hence, eutectics can be made as either organic–organic, inorganic–inorganic or organic–inorganic mixtures (Baetens et al., 2010b). This gives room for a wide variety of combinations that can be tailored for specific applications.

Of organic eutectic mixtures, the most commonly tested consist of fatty acids. Some organic eutectics that have been studied include capric acid/myristic acid (Karaipekli and Sari, 2008), lauric acid/stearic acid, myristic acid/palmitic acid and palmitic acid/stearic acid (Sari et al., 2004), and capric acid/lauric acid (Shilei et al., 2006b). The most common inorganic eutectics that have been investigated consist of different salt hydrates.

The benefits of eutectic mixtures are their ability to obtain more desired properties such as a specific melting point or a higher heat storage capacity per unit volume.

Though it has been given significant interest by researchers, the use of eutectic PCMs for use in (LHTS) systems is not as established as pure compound PCMs. Hence, thermophysical properties of eutectics is still a field for further investigations as many combinations have yet to be tested and proved.

3.2.2.4 Comparison Summary

The advantages and drawbacks for organic, inorganic, and eutectic PCMs are compared and summarized in Table 3.1.

3.2.3.1 Microencapsulation

Microencapsulation of PCMs involves packing the PCM materials in capsules that range from less than 1 µm and up to around 300 µm (Hawlader et al., 2003). The end product consists of the outer shell and the PCM fill in the center. The process can either be performed physically through, e.g., spray-drying (Hawlader et al., 2003) or a coating process (Kaygusuz et al., 2008), or chemically through, e.g., coacervation (Hawlader et al., 2003; Saihi et al., 2006a) or interfacial polymerization (Cho et al., 2002; Liang et al., 2009; Saihi et al., 2006b).

Morphologies of different microencapsulated PCMs are illustrated in Fig. 3.3, which depicts mononuclear, polynuclear, matrix, and multiwall or multifilm microcapsules (Jurkowska and Szczygieł, 2016). The shell materials commonly used for microencapsulation consist of organic polymers or silica. Recently, materials such as SiO₂, AlOOH, and TiO₂ have been studied (Cao et al., 2015).

The advantages of microencapsulation include an increased heat exchange surface giving an increased heat transfer, reduced reactivity with surrounding materials, and the fact that PCMs’ volume can expand without affecting the structure around it. Due to the microencapsulated PCMs’ small size they can be produced in either powder form or dispersed in a liquid (Fig. 3.4) and they do not need additional protection from destruction (Tyagi et al., 2011). This makes it possible to add the PCM directly to materials such as concrete and gypsum without the risk of leakage. However, considerations have to be made as adding microcapsules may affect the structural strength of a material. See also the studies by Zhang et al. (2012) and Zhao and Zhang (2011).

Microencapsulation has shown to be the most promising method of encapsulation for integration of PCMs in building materials. However, microencapsulation leads to a lower latent heat storage capacity per unit volume and unit weight than the pure PCM due to the addition of the encapsulating material.

3.2.3.2 Macroencapsulation

Macroencapsulation refers to PCMs encapsulated in any type of container such as tubes, spheres, or panels, which can be incorporated into building materials or serve as heat exchangers by themselves (Fig. 3.5). The size of these containers is usually larger than 1 cm (Cabeza et al., 2011). Because many PCMs have low thermal conductivity, a disadvantage of macroencapsulation is the tendency to solidify/melt at the edges leading to a slower energy uptake and release, which may prevent the system from discharging completely overnight. The size of the macrocapsules imply that they have to be protected against destruction or perforation and may also make them more difficult to integrate into the building structure, which may make these systems more expensive (Schossig et al., 2005).

3.4.1.1 Free Cooling

Free cooling systems with PCMs work by storing outdoor coolness (e.g., during the night) and release the coolness indoors during the day. The PCM can then be used during the day to absorb the heat from, e.g., passing air in a ventilation system or water in a pipe system, and stored as latent heat, to cool the building in the day when temperatures are higher and the need for cooling arises. These systems work as long as the ambient temperature allows the PCM to freeze and melt over the day, i.e., the ambient temperature must be above the phase change temperature during the day and below it during the night (Fig. 3.6) (Zalba et al., 2004).

Mosaffa et al. (2013) described a free cooling system using PCM slabs separated by air gaps for air to flow through (Fig. 3.6). The model developed from this can be used to evaluate performances of latent heat storage systems for free cooling air-conditioning systems.

3.4.1.2 Peak Load Shifting

Peak loads that hit during the day put pressure on the electrical grid and also lead to the need for heating, ventilation, and air conditioning (HVAC) systems being dimensioned for higher heating or cooling loads. Ultimately, this could lead to a need for more power generation facilities being built. By shifting the peak load away from the peak hours of electrical demand using PCMs, the peak load may be divided throughout the day, reducing the highest peaks (Halford and Boehm, 2007). Fig. 3.7 illustrates how the peak may be both reduced and shifted by the use of PCMs.

Sun et al. (2013) reviewed strategies involving PCMs for peak load shifting and control that have been tested so far. From the studies reviewed, peak cooling load reductions ranging from 10% to 57% with no or simple control strategies were found. The greatest reductions were found in cases where the PCM was compared against an insulated lightweight construction, while the lowest reduction was found when comparing against structures containing more mass, e.g., concrete. However, the cost saving potential of these systems could be further improved if more sophisticated load shifting control strategies were developed.

3.4.1.3 Active Building Systems

The storage capability of PCMs can be integrated into systems such as, e.g., solar heat pump systems, heat recovery systems and floor heating systems. Such systems can be combined to attain a peak load reduction as described in the previous chapter. However, if they are made even more effective they can achieve further savings through reduced electrical demand for HVAC systems. As an example, see the study by Fiorentini et al. (2015).

An example of a radiant floor incorporating PCMs in an active system has been described by Ansuini et al. (2011). The system investigated consists of a lightweight piped radiant floor system with an integrated PCM layer aimed at buffering internal gains during the summer season without affecting the winter warming capacity.

3.4.1.4 Passive Building Systems

Passive building systems and their use have attained the most interest so far. For passive applications, PCMs are integrated into the building envelopes to increase the thermal mass. This is especially beneficial in lightweight constructions, which suffer from low thermal inertia. A known issue for these buildings is large temperature fluctuation in the summer due to excessive overheating caused by a lack of thermal mass. This is especially the case in cold climates, where buildings have been built according to passive house standards, often involving large amounts of thermal insulation to reduce heating loads in the winter.

The materials incorporating PCMs will melt during the daytime and solidify during nighttime. This will help prevent rooms from overheating during the daytime in warm months and may also reduce the need for heating during the nighttime in the winter.

An issue that has been brought up is the importance of getting passive PCM systems to completely discharge during the nighttime in warm periods. If the PCM is not able to completely solidify, the effectiveness of the system may be considerably reduced. This point makes PCMs more effective in climates with large daily variation in temperatures. For areas where the discharge does not happen naturally, cool air has to be supplied during the nighttime to reset the PCMs completely.

3.4.1.5 Thermal Comfort Control

Though PCMs show potential for energy savings, another important factor to highlight is the benefits PCMs may have towards increasing the overall indoor thermal comfort. Lan et al. (2011) showed a correlation between workers’ performance and productivity compared to the sensation of thermal comfort due to shifting temperatures. Seppänen and Fisk (2006) showed that elevated air temperatures had a negative effect on performance and productivity. When temperatures increased up to 20°C there was an increase in working performance. However, when temperatures increased above 23°C there was a decrease in productivity. Maintaining a steady temperature around the comfort zone for longer periods without relying on HVAC systems may be possible with PCMs.

With PCMs installed, temperature fluctuations are reduced. The focus should be placed on selecting a PCM within the desired melting/freezing point so temperatures stay stable around the comfort temperature. This will benefit the indoor climate in two ways. First, the temperature will be held more stable, reducing the feelings of thermal discomfort due to temperature fluctuations throughout the day. Second, the peak temperature will be reduced and should not reach a temperature that leads to increased thermal discomfort. Another possible benefit of PCMs may be that they lead to a more uniform temperature between surfaces and air temperature, reducing thermal discomfort from radiative heat.

3.4.3.1 Walls

The most common solution for implementing PCMs into buildings so far is by installing PCM enhanced wallboards towards the interior side of the building envelope. For lightweight structures, which have low thermal inertia, PCMs can provide a significant increase in thermal storage capacity. When facing the interior rooms of a building or being used in partition walls, wallboards will be able to absorb and release heat throughout the room for large parts of the day. Several investigations, both experimental and practical, have been conducted to see how this affects overall indoor climate and energy usage.

Experimental hot box tests have shown that PCM wallboards give an obvious reduction in temperature fluctuations, as demonstrated in Fig. 3.8 (Cao et al., 2010). The dynamics of external wall structures with PCMs investigating the possibility of substituting thick and heavy thermal mass external walls used in high latitude countries by thin and light thermal mass ones were studied by Chwieduk (2013).

A full-scale investigation was performed on a lightweight building’s inner partition walls. In this study, several positive effects could be seen when a room with PCM wallboards was compared to one without PCMs added. Air temperature fluctuations were reduced, the overheating effect was lower, and the PCM wall was shown to release energy when temperatures fell. The tests also showed reduced fluctuations of the surface temperature of the wall, i.e., lower surface temperatures during the day and higher surface temperatures during the night. Hence, the thermal comfort by radiative effects could be increased during daytime (Kuznik et al., 2008).

Kuznik and Virgone (2009) performed an experimental investigation in a full-scale test room for a typical day in winter, summer, and midseason with PCM enhanced wallboards. The PCM wallboards were created by adding 60% microencapsulated paraffins in a flexible sheet of 5 mm thickness. The PCMs were shown to reduce overheating effects, reduce the surface temperature on the walls, and enhance the natural convection mixing of the air.

Evola et al. (2013) performed a simulated case study of an office building refurbished with PCM enhanced wallboards during summer conditions. The wallboards were made of an aluminum honeycomb matrix and filled with 60% microencapsulated PCMs with paraffin as the core material. This simulation showed that even if the PCMs are frequently activated, on average they only utilized 45% of their total latent heat storage potential, meaning that the entirety of the PCM will not melt or solidify within each cycle. The utilization of total latent heat storage potential is affected by factors such as, e.g., convective heat transfer across the wall surface, whether the placement is in an area of a room that receives low amounts of direct solar radiation, and climate conditions such as cloudy weather or extreme temperatures that are outside the expected normal temperatures. The study also discussed the importance of evaluating the PCM over a longer period of time, such as a few months rather than a few days, to be able to better evaluate the activation rate and the utilization rate of the total latent heat storage potential of a PCM under a given climate.

Diaconu (2011) studied the influence of the occupancy pattern and ventilation that may be affected by PCM enhanced envelopes. Through numerical simulations the potential for thermal energy savings for heating was investigated. It was clear that occupancy patterns and ventilation must be considered when attempting to select the optimal PCM melting temperature.

Ascione et al. (2014) investigated the possibility of refurbishing a building with PCM plaster on the inner side of the exterior building envelope and the effect it would have on energy savings and indoor comfort in the cooling season. The results were simulated while varying the phase change temperature, thickness of the wallboard and the location of the PCM layer. Phase change temperatures ranging from 26°C to 29°C were tested. With a melting temperature of 29°C the highest energy saving potential and increase in comfort hours were seen. However, the achievable benefit for energy savings in climates simulating Seville and Naples were no more than 3%, while Marseille and Athens received a benefit of 4.1% and 3.5%, respectively. The highest energy saving effect was reached in Ankara, with energy savings of 7.2%. The comfort hours during the occupied hours increased by 15.5% (Seville), 22.9% (Naples), 19.8% (Marseille), 15.8% (Athens), and 20.6% (Ankara).

The experiments conducted by Ascione et al. (2014) also highlight another important factor for PCMs. They found that during the summer, the period with temperatures lower than the phase change temperature, i.e., the period where the PCM solidifies, would not always be sufficient for the PCM to fully solidify. The same would also occur during the winter, when the heat available during the day would not be enough to completely melt the PCM. This indicates that the optimal phase change temperature is seasonal, and complete discharge, or complete absorbance of heat, is difficult to obtain for a single PCM.

Shilei et al. (2006a) tested the impact of PCM incorporated gypsum boards in winter climate conditions in the northeast of China. The experiment was performed on two similar rooms, one with PCMs and one without PCMs. The PCM room showed that it could reduce the heat transfer to outdoor air and improve the thermal comfort. Furthermore, the possibility to reduce the scale of the heating equipment was shown.

A renovation project in the south of Lyon using PCM enhanced wallboards was monitored over a 1-year period by Kuznik et al. (2011b). The room was compared to a room in the same building that was renovated without PCMs. The PCMs showed increased thermal comfort of the occupants due to air temperature and radiative effects. However, the PCMs seemed to be unable to utilize their latent heat storage capacity over several periods over the year due to excessive temperatures and the lack of a complete discharge overnight.

Becker (2014) investigated how thermal and energy performance of a building would be affected by placing PCMs on the inner surface of the walls during summer conditions. By using EnergyPlus and a simplified model, the thickness of the PCMs needed to store heat gains from solar radiation and occupancy patterns occurring during the day, and ventilation needed by night to discharge the system, were studied for lightweight buildings, semilightweight buildings, and heavyweight buildings in a Mediterranean climate. In heavyweight constructions, PCMs could improve thermal conditions, but had a marginal effect on energy savings. For lightweight dwellings and offices and semilightweight schools the analyses showed positive effects on both thermal and energy performance. The greatest effect was seen in lightweight office buildings, where energy savings up to 57% were found. However, this study also pointed out the importance of occupancy patterns and the focus on including night ventilation needed for discharging when studying PCMs. It is still important to get full-scale validation in occupied buildings under normal working and climate conditions.

3.4.3.2 Floors

Areas that are in direct contact with solar radiation hold large potential for storage of thermal heat energy. Floor solutions incorporating PCMs in areas of a building where the sun shines for large parts of the day may benefit from incorporating PCMs. Fig. 3.9 shows flat profiles filled with PCMs that were used in the floor in North House, a competitor in the US Department of Energy’s solar decathlon.

Xu et al. (2005) performed a simulation of the thermal performance of PCMs used in a passive floor system during the winter season. The performance on the systems was influenced by the choice of covering material, the air gap between the PCM and covering material, and the thickness of the PCM. For the simulations performed, the thickness should not exceed 20 mm as this would not increase the influence of the thermal storage significantly.

3.4.3.3 Roofs

Implementing PCMs into roof systems does not seem to have received much attention. Only a few studies on the possible effects of PCMs in passive roof systems have been found. The thought is that PCMs placed on the roof will be able to absorb the incoming solar energy and the thermal energy from the surroundings to reduce temperature fluctuations on the inside.

Pasupathy and Velraj (2008) studied the effects of a double layer of PCM for year round thermal management in Chennai, India. An experiment was performed with a PCM roof panel compared to a reference room without the PCM panel. The PCM used was an inorganic eutectic of hydrated salts. The experiment showed that the PCM panel on the roof narrowed the indoor air temperature swings, and that such a system could perform during all seasons when the top panel had a melting temperature 6–7°C higher than the ambient temperature in the early morning during the peak summer month, and the bottom panel had a melting temperature near the suggested indoor temperature.

Kosny et al. (2012b) set up a naturally ventilated roof with a photovoltaic (PV) module and PCMs to work as a heat sink (Fig. 3.10). The goal was that the PCM would absorb heat during the day in winter and release it in the night to reduce heating loads. In the summer the PCM would absorb heat to reduce the cooling loads in the attic beneath. A full scale experiment was performed over a whole year from November 2009 until October 2010 in Oak Ridge, Tennessee, USA. The data from the tests were compared with a conventional asphalt shingle roof. The PV-PCM attic showed a 30% reduction in heating loads during the winter and a 55% reduction in cooling loads. Furthermore, a 90% reduction in peak daytime roof heat fluxes was observed.

3.4.3.4 Windows and Shutters

Windows represent a part of the building that is considered to lead to a higher energy consumption. In warm climates dominated by cooling loads, excessive solar heat gains lead to an increased need for mechanical cooling. In cold climates, much of the energy escapes through glazed facades, leading to a need for mechanical heating (Ismail et al., 2008). Several new advanced window technologies such as electrochromic windows, low-emissivity glazing, evacuated glazing, self-cleaning glazing, building integrated photovoltaics (BIPV) as solar glazing, etc., have been explored to counter these issues (Baetens et al., 2010d; Jelle, 1993, 2013a, 2013b, 2015; Jelle and Hagen, 1993, 1999a; Jelle and Breivik, 2012a,b; Jelle et al., 1998, 2007, 2012a,b, 2015b; Midtdal and Jelle, 2013). However, glazed facades still suffer from low thermal inertia, and have no way of storing excess heat. Transparent PCMs for use in windows represent an opportunity that has been explored for this purpose.

The first issue to overcome is the desire to have sufficiently transparent windows. Fig. 3.11 shows a commercialized glazing filled with a PCM, also depicting the visibility in its liquid state. As of today, only translucent PCMs have been used for PCM windows; though they enable relatively high amounts of visible light to pass through, they do not offer the same visibility as regular windows.

The solar transmittance of a commercial grade PCM was tested by Jain and Sharma (2009). For a pure PCM with a thickness of 4–30 mm the solar transmittance was found to be 90.7–80.3%, respectively. Due to the fact that PCMs have low thermal conductivity, they concluded that PCMs may be interesting as a transparent thermally insulating medium.

Goia et al. (2013) compared a prototype PCM glazing system with a traditional double glazed insulating unit with regard to the effect on thermal comfort in the winter, summer, and midseason. The two glazing systems were measured over a 6-month experimental campaign, and the data was used to numerically evaluate an indoor office room. The thermal conditions were considerably improved for most parts of the year with the PCM prototype compared to the regular double glazing. However, on cloudy days, the two systems showed similar properties on thermal comfort. The study also highlighted the importance of selecting the correct melting temperature for the PCM as this could affect the system negatively if not optimized.

A similar study was performed by Weinläder et al. (2005), comparing a double glazing unit combined with PCM compared to a regular double glazing unit. The test specimens were studied experimentally in an outdoor test facility and the data gained from the experiment was used for further simulations. The light transmittance from this solution was found to be 0.4, giving them the ability to be used as daylighting elements. Compared to regular double glazing they gave a more equalized energy balance, achieving moderate heat gains with very low heat losses. During the winter season, PCM windows improved the thermal comfort considerably and in the summer they shifted heat gains to later in the evening. However, the PCM windows cannot be used where visual contact to the environment is desired.

Goia et al. (2014) performed a full-scale test on a PCM glazing prototype. The test was performed on a south facing wall during summer, midseason, and winter days in a subcontinental climate and compared to a conventional double glazing for reference. The experiment showed that PCM glazing can reduce the energy gain in the summer by more than 50%. In the winter, heat loss reduction during the day was observed; however, this technology might not be effective if the purpose is to achieve solar heat gains. The experiment also pointed out the importance of removing the stored heat during the night via, e.g., night cooling, and selecting the correct phase change temperature, if the system is to function optimally.

Grynning et al. (2013a) performed measurements on a state-of-the-art commercial glazing including a PCM in a large scale climate simulator. The tested glazing was a 4-layer glazing with a prismatic glass in the outer pane and a PCM fill in the cavity between the inner panes. The study showed that characterization of static components (e.g., U-value, solar heat gain coefficient, thermal transmittance) is insufficient for describing the performance of PCMs due to their dynamic nature.

A zero energy office building using translucent PCMs in the window construction has been built in Kempen, Switzerland (Fig. 3.12). In this project, each second window panel has been equipped with PCM windows. The aim for the windows is to effectively store solar energy during the warmer parts of the day, and release thermal energy throughout the colder periods of the day to reduce the total energy required for space heating.

Alawadhi (2012) investigated the possibility to implement PCMs in window shutters, where the goal was to see if the solar heat could be absorbed before it reached the indoor space. When compared to foam shutters, the shutters containing a PCM could lead to a reduction in heat gain through the windows by as much as 23.29%.

3.4.3.5 Concrete

Adding PCMs directly into concrete has shown some promising results through lower thermal conductivity and an increase in thermal mass at specific temperatures. However, PCM concrete has shown some undesirable properties such as lower strength, uncertain long-term stability, and lower fire resistance (Ling and Poon, 2013). Several studies have been conducted on PCM-concrete and have shown positive effects through reduced indoor temperatures in warm climates (Cabeza et al., 2007).

Combining concrete structures with PCMs has been tried in several different ways. One studied solution is to drill holes in the concrete, which may then be filled with a PCM (Alqallaf and Alawadhi, 2013).

Royon et al. (2013) tested the possibility of filling the already hollow areas in a hollow concrete floor with PCMs. The concrete was filled with a paraffin PCM with a melting temperature of 27.5°C. This test showed that the temperature on the other side of the hollow concrete was lower during summer conditions. Hence, such floors can be used as a passive thermal conditioner during the summer. However, more tests are needed with real life climate conditions to validate the effects.

3.4.3.6 Thermal Insulation Materials

Incorporation of PCMs into fibrous thermal insulation materials has received considerable attention. Kosny et al. (2012a) performed an experimental and numerical analysis of a wood-frame wall containing PCM enhanced fiber insulation. The wall assembly had an R-value of 4.14 (m² K)/W (U-value of 0.241 W/(m² K)). For fiber insulation filled with 30 wt% PCM in summer conditions, results showed a reduced peak hour heat gain of 23–37% in Marseille and 21–25% in Cairo and Warsaw.

3.4.3.7 Furniture and Indoor Appliances

A point that has not been investigated for this study, but should be mentioned, is the possibility of using PCMs in furniture and other indoor appliances. The benefit of PCMs is as mentioned their ability to store heat in periods where there is a surplus, and release the heat when there is a deficit. It would be interesting to study how incorporation of PCMs into other components in a building besides the structural components could benefit energy savings and thermal comfort. PCMs have already been widely studied for textile applications (Sarier and Onder, 2012), showing that there is a possibility of adding PCMs to various forms of materials.

3.5.1.1 Increasing Thermal Storage Capacity

A desirable trait for PCMs is a high thermal storage per unit volume and unit weight. Hence, there is always a focus on finding new materials and solutions that may increase the thermal storage capacity for a given volume or weight of a PCM. However, as mentioned earlier, this should not come at the cost of using potentially environmentally harmful materials and chemicals.

Microencapsulation reduces the risk of PCMs leaking from the material in their liquid state. Unfortunately, the encapsulation leads to a lower latent heat storage capacity. The method is a promising way for integrating PCMs with building materials, so finding new methods or materials to encapsulate with, which will give a better thermal storage capacity, is desirable. Possibilities to achieve this may include developing a thinner shell for the capsules, which will increase the weight percent of PCMs in the final products.

Research with miscellaneous microcapsules will be continued, and which morphologies and types of microencapsulated PCMs, as depicted in Fig. 3.13, and even other configurations not hitherto known or explored, will come out as the better ones, remain to be seen. The exploration of these comprises many parameters including, e.g., material types, dimensions, and interfaces. Microencapsulated PCMs may also be incorporated in various materials; one example with gypsum boards is shown in Fig. 3.14.

3.5.1.2 Enhancing Heat Transfer

A problem that has been addressed throughout the literature is the low thermal conductivity for many promising PCMs (around 0.15–0.2 W/(mK) for organic PCMs and around 0.5 W/(mK) for inorganic salts). Low thermal conductivity reduces the rate of heat absorption or heat release throughout the PCM, i.e., reducing the effectiveness at which it can store and release thermal energy. This may lead to a system that does not fully utilize the full latent heat storage of PCM materials. In the study conducted on wallboards under summer conditions by Evola et al. (2013) they found that the wallboards tested only took advantage of about 45% of the possible latent heat storage. Note that this is affected by several other factors than thermal conductivity alone, such as local climate conditions and the way the system is installed.

Fan and Khodadadi (2011) reviewed methods that have been used to enhance the thermal conductivity for the last few decades. Recently, graphite based PCM systems and metal foams have been getting increased attention.

One solution that has been investigated includes adding of a material with a high thermal conductivity to the PCM. A set of materials that has been investigated for this purpose is various carbon-based nanofillers (Babaei et al., 2013; Ji et al., 2012; Yu et al., 2014). Though the results vary, the increase in thermal conductivity has increased between 65% and 336% in these studies.

Another solution is to add the PCM to a material with a porous structure and a high thermal conductivity (Fig. 3.15). Investigations on graphite foam composites (Sedeh and Khodadadi, 2013; Zhong et al., 2010; Song et al., 2012) and metal foams (Jiang et al., 2012; Zhao et al., 2010) have been conducted. Graphite foam composites have shown potential for creating a structure with high thermal conductivity (ranging from 230–570 times higher than the original PCM).

Note that adding a new material to the PCM and creating a composite material reduce the total latent heat storage ability per unit weight due to the adding of new particles. The energy uptake and release of the system may improve, but as of yet, this comes at the cost of less thermal heat storage per unit weight.

3.5.2.1 Nanoencapsulated Phase Change Materials

Though it has not yet been used in commercial products, nanoencapsulation of PCMs may have potential to improve PCM materials. The idea is similar to that of microencapsulation, but with even smaller capsules ranging in the size of nanometers. A smaller size of the capsules could be interesting due to a higher heat transfer area, leading to a higher heat transfer speed. Several studies have been performed and given functional nanocapsules with PCMs.

Chen et al. (2012) prepared nanocapsules containing n-dodecanol by miniemulsion polymerization. These capsules reached a latent heat capacity of 98.8 J/g.

Rao et al. (2013) performed a simulation on nanocapsules by using n-octadecane as the core material and SiO₂ as the shell material. This study pointed out that the possibilities of creating excessive thick or thin shells might be problematic for the current nanocapsules.

Latibari et al. (2013) created nanocapsules containing palmitic acid as the core material and SiO₂ as the shell material by a sol-gel method. The latent heat was measured by a differential scanning calorimetry (DSC) analysis and was found to be 180.91 kJ/kg when melting and 181.22 kJ/kg when freezing.

Nanoencapsulation is still a novel technology. However, it would be interesting to see how the nanocapsules perform in comparison to microcapsules when integrated into building materials, with regards to both energy usage and structural stability in materials such as concrete. An issue that has not been discussed in the studies found is how nanocapsules will compare with respect to overall costs.

3.5.2.2 Adjustable Phase Change Temperature

Through most of the studies investigated in this review it is clear that the phase change temperature is one of the most important criteria for optimizing a PCM system. It is also clear that the optimal phase change temperature for summer conditions will not be optimal for winter conditions and vice versa. Some systems have been described where PCMs of different phase change temperatures are used to partly counter this issue. These systems work by adding different layers of PCMs with different phase change temperatures that can work optimally at different temperatures (Jin and Zhang, 2011; Pasupathy and Velraj, 2008). However, if this could be taken a step further, and a system with an adjustable phase change temperature could be created, the efficiency could be significantly improved. This could either be performed through a controllable or adaptive system and would enable a dynamic change of the phase change temperature in response to user preferences, different climates, and different seasons.

3.5.3.1 Developing a Standard Test Scheme

With more and more commercial PCM products reaching the market, properties relevant for their use in construction have to be identified and tested. Mehling et al. (2006) showed that PCMs cannot be measured accurately enough by using standards that have been developed for other materials. These tests do not fully map all relevant effects around the phase change process due to PCMs’ low thermal conductivity and large melting enthalpy. This leads to an error in the stored heat as a function of temperature and may give errors that cannot be tolerated when evaluating the application of PCMs.

It has been pointed out that there is a lack of common test methods to compare and model the results of various PCMs (Agyenim et al., 2010; Shrestha et al., 2011). This makes it difficult to assess the suitability for each individual application. Accurate knowledge of a PCM’s enthalpy as a function of temperature or storage capacity at each temperature is important for design purposes (Lázaro et al., 2006).

Kousksou et al. (2014) reviewed the applications and challenges for energy storage systems, including PCMs. From this study it was found that there were still discrepancies between the literature and actual measurements of thermophysical properties of technical grade PCMs, further proving that a uniform test standard is needed.

It is important to note that the values measured for pure PCM compounds will not be representative when they are integrated into buildings. Encapsulation materials and combination with building materials will alter the response of the PCMs. It is therefore of importance that finished products aimed for the use in buildings are tested properly. Having a standard test scheme will also be beneficial for future work within field testing of PCMs as all values can be compared with respect to the same conditions.

There are still many issues regarding correct modeling of PCM behavior. Some of these issues include:

Subcooling, which is a common problem for PCMs based on salt hydrates, is still an effect that cannot be treated analytically or numerically. If salt hydrate PCMs become widespread, this is an effect that can have a major effect on a system’s performance, even at just 1 or 2°C of subcooling (Mehling et al., 2007).

The most common test methods used for evaluating the performance of PCMs to date are mentioned in the following sections. Further research into these methods may give a method that can be used as a common standard for testing PCMs in the future.

3.5.3.2 Differential Scanning Calorimetry

Currently, differential scanning calorimetry (DSC) is the most used method to determine a PCM’s thermophysical properties (Barreneche et al., 2013). DSC can only be used for testing small samples of PCMs. Through DSC it is possible to obtain melting temperatures and the heat of fusion. For this purpose, DSC is the most used method. However, for small samples, the degree of supercooling may be increased while the degree of phase segregation may be decreased, giving different results from actual use of the bulk materials used in practical systems (Zhang and Jiang, 1999). DSC can only be used on relatively uniform test specimens, which is not very realistic when testing building envelope products (Kosny et al., 2012a).

3.5.3.3 T-History

The T-history method allows for testing of the melting temperature, degree of supercooling, heat of fusion, specific heat, and thermal conductivity of several PCMs simultaneously (Zhang and Jiang, 1999).

Solé et al. (2013) wrote a review on the T-history method as it has been used to test PCMs up until recently. The aim of their study was to increase the consensus around the use of this method as it has several advantages when measuring PCMs. The T-history method is efficient at determining fusion enthalpy, specific heat, and thermal conductivity for large PCM samples. Nevertheless, there is still no commercial T-history equipment available yet.

3.5.3.4 Dynamic Heat Flow Apparatus

The dynamic heat flow apparatus method is based on the traditional heat flow meter apparatus method, which is used to determine steady-state heat transfer properties, thermal conductivity, and thermal resistance of flat slab specimens. The traditional heat flow apparatus method is already used in accordance with standards such as EN 12667, ISO 8301, and ASTM C518 to determine these properties for traditional insulation materials. When performing a dynamic heat flow test, the plates on each side of the specimen to be tested are held at the same temperature, and both plates are changed to a different temperature, whereas different temperatures are used in normal heat flow tests. By using the dynamic method, results were found to be more accurate when testing PCMs. Dynamic properties such as heat capacity profiles, peaks of melting and solidification cycles, and amount of subcooling were found to be relatively similar to those measured by DSC (Shukla et al., 2012).

3.5.3.5 Dynamic Hot Box

The dynamic hot box method can be used to simulate changes in temperature on the climate side of a test specimen. From this, the dynamic thermal characteristics of a building component can be found. When testing PCMs it is important to get as correct a picture as possible of their dynamic properties. With the dynamic hot box, the temperature is held constant until a steady-state is reached, then the temperature on one side is changed and results are measured until a new steady-state has been reached.

3.5.3.6 Dynamic Guarded Hot Plate

A study performed by Pomianowski et al. (2014) investigated the possibility of using a dynamic guarded hot plate apparatus to determine the specific heat capacity as a function of the temperature for PCM-incorporated concrete. The study describes the experimental setup and proposes various methods to calculate the specific heat capacity of PCM concrete. The advantage of this method is the possibility to attain a very small heating rate, which can imitate the temperature increase in real building constructions. That is, it can simulate realistic thermal conditions of the PCM in actual use. Though the study focuses on PCM concrete, it is pointed out that this experimental setup could also be used on various other PCM composites.

3.5.3.7 M-Value

PCMs are added to structures and lead to an increase in thermal mass. As PCMs have a specific temperature range, they have also been referred to as “smart thermal mass”. However, the phase change temperature of a PCM is not always absolute. It may happen over a small temperature interval and differ between melting and freezing (Fig. 3.16). This is a cause for confusion as there is no standard that specifies in which part of the phase change process this value should be stated. A new energy performance label for PCMs has been mentioned, i.e., the M-value, which should express the phase transition related enthalpy change of the PCM within a standardized range so that all values given for a PCM can be evaluated on an equal basis.

3.5.3.8 Environmental Impact Assessments

New materials that may be suitable for use as PCMs are continuously being investigated. As mentioned earlier, it is critical that PCMs for use in buildings are not hazardous or poisonous in any way. This also includes during production and handling after a PCM’s useful lifetime. It is important that possible new materials are evaluated with regard to waste from the production and necessary treatment when they are to be disposed. As there may be many different chemicals in use in PCMs, environmental assessments could be conducted to avoid unknown pollutants of any kind.

3.5.3.9 Expected Lifetime Predicament of Phase Change Materials

As a thawing/freezing cycle may happen thousands of times, some PCMs may be prone to deterioration over time. Gradual breakdown of the material will end up reducing the amount of latent heat storage per phase transition and may also affect the phase change temperature, and hence reduce the overall energy performance. Such deterioration is particularly common in inorganic PCMs, and to some extent in less pure organic PCMs. It is important that future possible PCMs are tested with regard to their aging over time. As of 2016 there is no standard method to do this. An aim for the future should be to develop a standard testing scheme that all commercial grade PCMs will have to follow when stating the lifetime of their products.

3.5.3.10 Quantifying the Effect of Phase Change Materials in Real-Life Buildings

Though the theory behind PCMs shows that they have the ability to reduce temperature fluctuations in buildings, there has been no study to prove an increase in overall thermal comfort or productivity. A way of measuring the effect and number of full cycles in different environments is needed.

To fully utilize the effect of PCMs, an overview of how they perform in different climates is needed. Ideally, a PCM should undergo a full phase change cycle once a day. However, the lack of guidelines for selecting PCMs for various climates is a source for uncertainty when used for building applications. It would be preferable if a detailed overview of the effect that could be expected for various PCMs in various climates was developed. It has been pointed out that it is difficult to select a PCM that functions optimally for every season and location.

From all the experiments and simulations reviewed in this study, one of the most important factors for implementing PCM enhanced materials is found to be selecting the correct phase change temperature. Other crucial aspects are the difficulty of selecting the correct PCM for specific climates and that the effect of PCMs will vary with varying climate conditions over the year.

The selection of a PCM based on a specific phase change temperature in one climate region will not be appropriate for another. Important factors that govern the selection of a PCM include phase change temperature, local climate, type of PCM and design and orientation of the building (Pasupathy and Velraj, 2008). Several other authors have also highlighted the need for guidelines to select PCMs for specific and different climates (Kosny et al., 2013; Ascione et al., 2014).

From the various manufacturers that have been investigated in this study a large amount of reference projects have been found. However, to the authors’ knowledge, there have been no studies to evaluate and quantify the effect of PCMs in these projects. Although every project has been deemed to increase energy efficiency and thermal comfort, monitoring projects from real life buildings should be conducted and evaluated so the beneficial effects of PCMs can be documented and shared.

3.5.3.11 Investigating Payback Times for Various Systems

As PCMs offer a decrease in overall energy usage, in many cases through minimal interference with the original structure, it is highly relevant to know the expected payback time of the initial investment. To the authors’ knowledge, little research has been carried out on this topic. This may be due to several factors, the most important one being that there is still a lack of knowledge on how PCMs actually perform in real life constructions in all sorts of various climates.

There is still much uncertainty regarding the overall effect of PCMs. Showing the long-term economical benefits of selecting PCMs may help to increase interest among customers to use PCMs. Should the results of such analyses show unfavorable payback times, it may also help to drive the research and development forward towards more cost-efficient solutions.

3.5.3.12 Development of Advanced Building Envelopes

PCMs for building applications may be viewed as part of the development of new materials and solutions for the building envelope in energy- efficient buildings. A seamless and robust integration of these materials and technologies with the PCM parts is important in order to ensure energy-efficient and durable solutions during the whole anticipated lifetime. Although not thermal insulation materials in themselves, PCMs have already been mentioned as part of the thermal building envelope (Kalnæs and Jelle, 2015). State-of-the-art thermal insulation material solutions include vacuum insulation panels (VIP) (Baetens et al., 2010a; Brunner et al., 2014; Grynning et al., 2011; Haavi et al., 2012; Jelle et al., 2013a; Johansson et al., 2014; Kalnæs and Jelle, 2014; Sveipe et al., 2011; Tenpierik, 2009; Wegger et al., 2011), gas-filled panels (GFP) (Baetens et al., 2010c), and aerogels (Baetens et al., 2011; Jelle et al., 2015c; Koebel et al., 2012; Levy and Zayat, 2015). Furthermore, investigations with superinsulation materials (SIM), e.g., nanoinsulation materials (NIM), are ongoing (Baetens et al., 2010a; Jelle, 2011a; Jelle et al., 2009, 2010, 2011a), including experimental attempts with hollow silica nanospheres (HSNS) (Gao et al., 2012, 2013, 2014a, 2015; Grandcolas et al., 2013; Jelle et al., 2011a, 2013b, 2014a, 2015a; Sandberg et al., 2013). Life-cycle assessments of these HSNS have also been performed (Gao et al., 2013, 2014a; Schlanbusch et al., 2014a,b). Moreover, experimental investigations with aerogel–concrete mixtures have also been conducted (Gao et al., 2014b; Ng et al., 2015a,b, 2016).

In connection to windows and fenestration in general, there is a lot of development in several fields (Jelle et al., 2012a), including, e.g., smart and electrochromic windows (Granqvist, 1995, 2005, 2012; Jelle, 1993, 2013a, 2015; Jelle and Hagen, 1993, 1998, 1999a,b; Jelle et al., 1992a,b, 1993a,b,c, 1998, 2007; Lampert, 1998, 2004; Mortimer et al., 2015; Ribeiro and Mortimer, 2015), window pane spacer materials (Van Den Bergh et al., 2013; Asphaug et al., 2016), low-emissivity coatings (Jelle et al., 2015b), window frame technologies (Gustavsen et al., 2008, 2011), self-cleaning surfaces (Midtdal and Jelle, 2013; Jelle, 2013b), solar cell glazing products (Jelle, 2016; Jelle and Breivik, 2012a,b), various fenestration aspects (Jelle et al., 2012a), energy optimization (Grynning et al., 2013b), translucent aerogel window studies (Gao et al., 2014c, 2016; Ihara et al., 2015a,b,c), and new lightweight aerogel window pane glass materials (Gao et al., 2014d).

Building integrated photovoltaics (BIPV) will probably become more and more important in the years to come, thus providing energy-efficient buildings with an electricity power generating source (Breivik et al., 2013; Jelle, 2016; Jelle and Breivik, 2012a,b; Jelle et al., 2012b), where nanotechnology plays a crucial role in the development of new solar cell materials (Jelle, 2016; Jelle et al., 2012b), also including the challenge of removing or avoiding snow and ice formation on the solar cell surfaces, e.g., by the formation and application of superhydrophobic water-repelling and icephobic anti-icing surfaces (Dash et al., 2012; Eberle et al., 2014; Jelle, 2013b, 2016; Kreder et al., 2016; Schutzius et al., 2015; Xiao and Chaudhuri, 2012; Zhang and Lv, 2015).

To ensure energy-efficient and durable buildings all kinds of different aspects have to be considered, where satisfactory resistance towards weather exposure is one important aspect, e.g., from shielding of fresh air ventilation intakes (Jelle and Noreng, 2012) to wind-driven raintightness of BIPV (Breivik et al., 2013; Fasana and Nelva, 2013). Contrary to the heavy climate exposure that walls and roofs experience, the climate exposure for the ground may seem much less harsh; nevertheless, the ground materials and solutions must prevent, e.g., penetration of air (leakage), moisture, and radon (Jelle et al., 2011b; Jelle, 2012a; Pacheco-Torgal, 2012), thus also comprising important health issues (fungi growth and radioactive radiation exposure due to moisture damages and radon ingress, respectively). Air, moisture, and radon barriers towards the ground may be combined into one barrier depending on the placement location of the barrier, and may also incorporate a low-emissivity surface coating (Jelle et al., 2015b) to decrease the heat loss towards the ground.

Miscellaneous advantages and drawbacks of applying nanotechnology in the construction sector are reviewed by Pacheco-Torgal and Jalali (2011). Certainly, as one in the coming years will experience many new and innovative materials and solutions based on nanotechnology, one also has to address any toxicity issues to avoid any health and safety hazards. That also goes for the development of new PCMs and other technologies to be used in the building envelope, e.g., the ones mentioned in the above. Several of these new materials will probably be developed by research groups within the European Commission Horizon 2020 program, where the utilization of nanotechnology also will play an important role (Pacheco-Torgal, 2014).

In general, it is of major importance to investigate the durability of building materials and components, also newly developed ones, e.g., by carrying out accelerated climate aging in the laboratory (Jelle, 2011b, 2012b, 2013c; Jelle and Nilsen, 2011; Jelle et al., 2012c). Thus, performing a robustness assessment of these materials and components may also be found to be beneficial (Jelle et al., 2014b). That is, a durability and robustness evaluation of the new PCMs should be carried out. Applying all these different technologies with their specific properties fulfilling various functions, also including, e.g., PCMs, one may start denoting these building envelopes as multifunctional building envelopes.

Several of the materials to be utilized in advanced and multifunctional building envelopes may be denoted as dynamic materials and further categorized as either adaptive or controllable materials. Photochromic and thermochromic materials that change their solar radiation transmittance according to the ambient solar radiation exposure and temperature, respectively, are examples of adaptive materials. Electrochromic materials that change their solar radiation transmittance according to an applied voltage represent an example of controllable materials, i.e., properties controlled by human beings. PCMs are categorized as adaptive materials. However, if the phase change temperature could be made adjustable either by controllable or adaptive means, especially by the former, one may start talking about semicontrollable PCMs, thus extending the usefulness through increased flexibility and increasing the energy efficiency of the PCM system.