Phase-changing material (PCM) is a substance with a high heat of fusion, which melt and solidify at certain temperature, is capable of storing and releasing large amount of energy. Heat is absorbed or release when material changes from solid to liquid and vice versa. PCM is one of the possible solutions available for reducing the energy consumption of buildings. The possibility of incorporation of PCM in building materials has attracted lot of interest worldwide due to global warming and ability of PCM to reduce energy consumption of building. This paper reviews application of PCMs in building materials like walls, Floors, Roof, windows and concrete.
Phase Change Material (PCM) is a substance with a high heat of fusion which, melts and solidifies at certain temperatures. PCM is capable of storing or releasing large amounts of energy. Phase change materials are latent heat storage substance, in which energy is stored or released in the process of changing phase i.e. either by solid to liquid or liquid to solid. When phase change materials attain the temperature at which phase change occur, they absorb large amount of energy and phase change material solidifies. It releases the stored latent heat when the ambient temperature around phase change material drops. Schematic representation of phase change process is shown in figure 1 (1).
1.1 Properties of phase change material
PCM should release and absorb large amounts of energy when freezing and melting; this requires the PCM to have a large latent heat of fusion and to be as dense as possible.
PCM should have a fixed and clearly determined phase change temperature (freeze/melt point); The PCM needs to freeze and melt cleanly over as small a temperature range as possible. Water is a best example in this respect. It freezes and melts at exactly 0°C (32°F). However many PCMs freeze or melt over a range of several degrees, and will often have a melting point that is slightly higher or lower than the freezing point. This phenomenon is known as hysteresis.
Avoid excessive super cooling; super cooling is observed with many eutectic solutions and salt hydrates. The PCM in its liquid state can be cooled below its freezing point whilst remaining a liquid.
It should remain stable and unchanged over many freeze/melt cycles; PCMs should have operational life time of many cycle because they will undergo thousand time of freeze/melt cycles. It is very important that the PCM is not prone to chemical or physical degradation over time which will affect the energy storage capability of the PCM.
PCMs will have applications where they will come in contact with people, for example in food cooling or heating applications, or in building temperature maintenance. For this reason it should be safe and non-hazardous.
It should be economical; PCMs can range in price from very cheap (e.g. water) to very expensive (e.g. pure linear hydrocarbons). If cost outweighs the benefits obtained using the PCM, its use will be very limited.
Classification of Phase change materials (PCMs) commonly used in concrete can be generally divided into three types: inorganic, organic & eutectic mixtures. From a concrete design point of view, it is important to identify what kind of PCM are suitable for use in concrete, because different kinds of PCM have different chemical natures and melting/transition temperatures. The general characteristics of inorganic and organic PCM are discussed in the following sections.
1.2 Classification of Phase Changing Materials(PCM)
Hydrated salts (MnH2O) are a most common type of inorganic PCM. Inorganic PCM have high volumetric heat storage capacity and thermal conductivity. They find application in some types of building materials. They add advantages of low cost, easy availability and non-flammable nature. Unsuitable characteristics of inorganic PCM such as very high volume change and super cooling during solid-liquid transition, have led to it not being considered as an appropriate material to be incorporated into concrete. Super cooling is a problematic issue of inorganic PCMs because the liquid state can be cooled to below its freezing point whilst remaining a liquid; which makes the associated phase change ineffective. Another concern of the inorganic PCMs is their degradation and inoperative characteristic after repeated phase change cycles (2).
Paraffin and non-paraffin are the two types of Organic PCMs. Most of the organic PCMs are chemically stable, safe and non-reactive. They also have an ability to melt without segregation. Organic PCM have self-nucleating properties that are compatible with traditional construction materials without posing any significant problems of super cooling. Paraffin wax (PAR) is a hydrocarbon that has the chemical structure CnH2n+2. Commercial PAR generally has a melting point ranging from 20ºC up to 70ºC, depending on the number of carbon (C) atoms. The more C atoms present in the chain, the higher the melting point of the PAR. Extensive use of organic PAR with a phase change temperature of 26 °C in concrete has been successfully demonstrated. PAR is regarded as one of the most popular PCMs used in concrete, because it is inactive in an alkaline medium, chemically stable and inexpensive. Main disadvantage of PAR is its flammable nature and low thermal conductivity in solid state. The majority of organic non-paraffin PCMs are acids (CH3 (CH2)2nCOOH). The melting point of paraffin PCM is similar to that of non-paraffin and they have excellent melting and freezing properties. Non paraffin is much more expensive (about three times) than paraffin PCM. Various types of non-paraffin PCM have been studied in concrete application. They include butyl stearate (BS), 1-Dodecanol (DD), Polyethylene glycol (PEG), 1-Tetradecanol (TD) and Dimethyl sulfoxide. Out of above non-paraffin PCMs, BS is found to be the most appropriate material because of its relatively low cost, suitable melting point at temperature comfortable to human, high latent heat storage, low volume change during phase change transition, and inflammable and stable nature (2).
Eutectic is a minimum melting composition of two or more components, each of which melts and freezes correctly. During the crystallization phase, a mixture of the components is formed, thereby acting as a single component. The components freeze to an intimate mixture of crystals and melt simultaneously with-out separation. Eutectic mixtures can be organic and/or inorganic compounds. Thus, eutectics can be made as organicorganic, inorganicinorganic or organicinorganic mixtures. This gives a wide range of possibility of combinations that can be made for specific applications of eutectic mixtures. Fatty acid is most commonly used mixture. Capric acid/myristic acid, lauric acid/stearic acid, myristic acid/palmitic acid and palmitic acid/stearic acid and capric acid/lauric acid are the various types of organic eutectic. The most common inorganic eutectics that have been investigated consist of different salt hydrates (3).
1.3 Means of PCM incorporation in concrete
This technique involves immersion of porous concrete products in a container which is filled with the liquid PCM. Absorption capacity of concrete, temperature and type of PCM are the three main factors which determine time required (immersion time) for the liquid PCM to be fully soaked into the porous concrete. This process of immersion normally takes several hours. Hawes and Feldman investigated the effect of different types of porous concrete on the absorptivity of PCM in concrete. Their results showed that at 80°C±5, the immersion time must be adequate to allow the liquid PCM to soak into the voids of the concrete blocks. Two types of concrete products, autoclaved concrete blocks and regular concrete blocks, were compared. Autoclaved concrete blocks is a better choice for immersion due to its higher porosity and higher degree of absorption compared to the regular concrete blocks. The time required for the autoclaved concrete blocks to be fully soaked with BS and PAR varied from 40 min to 1 hr. while regular concrete blocks with PAR required about 6 hrs. Immersion process is inversely proportion to temperature at PCM is immersed (2).
The impregnation technique involves three simple steps. First, air and water are evacuated from the porous or lightweight aggregates with a vacuum pump. Then, the porous aggregates are soaked in the liquid PCM within a controlled environment (under vacuum). Finally, the pre-soaked PCM porous aggregate functioning as a “carrier for the PCM” is mixed into the concrete. Three types of porous aggregates were examined: expanded clay aggregate (C1), normal clay aggregate (C2) and expanded shale aggregate (S) as the “carrier” for butyl stearate (BS) PCM. When compared to the absorption capacity for water, the vacuum impregnation method was much more effective than the simple immersion technique. The difference was more pronounced in the clay aggregate than in the shale aggregate. A rough estimation of PCM-absorption capacity for C1, C2 and S were 0.876, 0.176 and 0.081 ml/g of the porous aggregate, respectively. This indicates that PCM can occupy up to 75% of the total pore space of the C1 porous aggregate. Furthermore, soaking porous aggregates in PCM enhanced the heat transfer between the PCM in the porous aggregate and the bulk concrete (2).
Direct mixing technique
PCM can also be incorporated using direct mixing technique. It involves encapsulation of chemically and physically stable form of PCM which is added into concrete constituents during the mixing process. This is essential to retain the PCM in its pure form and ensure no interference with the concrete constituents. Encapsulation of PCM can also prevent leaking of PCM while in liquid phase. The most common processes used to encapsulate organic PCM are interfacial polymerization, emulsion polymerization, in situ polymerization as well as spray drying. During direct mixing, it is possible that capsules breaks. To avoid this, membrane reinforcement products such as Zeolite or Zeocarbon (mostly derived from charcoal and volcanic rock) can be used for surface reinforcement to withstand high friction or impact (2).
- PCMS APPLICATIONS IN BUILDINGS
2.1 PCM Filled Glass Windows
PCM can be used in preparation of thermally effective window. Ismail et.al  proposed a different concept for this application as shown in figure 3. From the figure, it is clear that window consists of double sheets with a gap in between them. This gap is filled by air. The sides and bottom of the window are sealed with help of sealer. It contains two holes at bottom, which are connected by plastic tube to a pump and PCM tank. The pump is connected in turn to the tank containing the PCM which is in liquid phase. Temperature sensor controls the pump operation. When the temperature difference reaches a pre-set value, the pump is operated and the liquid PCM is pumped out of the tank to fill the gap between the glass panes. Because of the lower temperature at the outer surface, the PCM starts to freeze, forming a solid layer that increases in thickness with time and hence prevents the temperature of the internal ambient from decreasing. This process continues until the PCM changes to solid phase. A well designed window system will ensure that the external temperature will start to increase before the complete solidification of the enclosed PCM .
2.2 PCM Assisted Sun-Shading
The PCM utilized in PCM assisted sun-shading system is hydrated salt CaCl2·6H2O. This system is very suitable in hot summer climate, especially for those areas where there is significant temperature variation during daytime and nighttime. Figure 3 represent a conventional and PCM sun-shading system.
During the daytime, with high temperatures (compared to the thermal comfort value), the face of the inner blind integrated with PCM is rotated towards the solar radiation so that excess solar energy is stored in PCM and thus control the temperature fluctuations inside the room. During the night time, with relatively low temperatures, the face of the inner blind integrated with PCM is rotated to be exposed towards the room so that the stored energy is released back to the room, thus avoiding excessive reduction of the room temperature below the thermal comfort value(5).
2.3 PCM Assisted Under-Floor Electric Heating System
PCM Assisted Under-Floor Electric Heating System is a system designed to investigate the thermal performance of the floor tested in Tsinghua University, Beijing, China. The experimental house was equipped with Thermal Energy Storage system having Phase Change Material. The compositions of under-floor heating system having different components is shown in figure 4, which included polystyrene insulation, electric heaters, PCM, some wooden supporters, air layer and wood floor. This system can store heat generated by electric heater operated at cheaper nighttime electricity rate and discharge the heat stored during daytime (7).
2.4 PCM Integrated Roof
A roof-integrated solar air heating/storage system uses existing iron roof sheets as a solar collector for heating air. A PCM thermal storage unit is used to store the heat during the day and this stored heat can be supplied at night or when there is no sunshine. The system operates in three modes. During times of sunshine and when heating is required, air is passed through the collector and subsequently into the home. When heating is not required, air is pumped into the thermal storage facility, melting the PCM, charging it for future use. When sunshine is not available, room air is passed through the storage facility, heated and then forced into the house. When the storage facility is frozen, an auxiliary gas heater is used to heat the home. Adequate amounts of fresh air are introduced when the solar heating system is delivering heat into the home as shown figure 5 (8).
2.5 PCM Assisted Ceiling
University of Nottingham (2002) was first to introduce PCM assisted ceiling. They replaced full air conditioning system by the new system which is called a night time cooling system. The proposed module (figure 6) has a ceiling-mounted fan to throw air over the exposed ends of heat pipes. The other end of the heat pipes is in a PCM storage module. During the day, the PCM acts as heat absorber, thereby the warm air generated in the room is cooled by the PCM. During the night, the fan is reversed and the shutters are opened so that heat will be extracted from PCM when cool air from outside passes over the heat pipes containing PCM. The cycle is then repeated next day (6).
2.6 Thermal energy storage in concrete
PCM incorporated in concrete can be used as an energy storing system. During the daytime, PCM absorbs surplus heat and melts, on the other hand in cooler nights, PCM becomes solid and the heat is released back into the environment as shown in figure 7.
Hunges et al. performed the experiment using 1%, 3% and 5% of PCM in Self Compacting Concrete (SCC). He found that the specific heat capacity of the samples increased by up to 1.7, 3.0 and 3.5 times with the increase in PCM levels and the melting temperature of all the samples with PCM ranged from 23°C to 26°C(2)
2.7 Office with PCM incorporated wall
Evaluation of ambient temperature change in summer and winter
Lotfi Derradji et al performed experiments to compare the thermal behavior of a conventional office walls against the walls incorporating a phase change material (PCM) in winter and summer.
He found that the use of phase change materials in the concrete ceiling and the hollow brick walls increased the ambient temperature in office by 4 °C in the winter period and decreased the indoor air temperature by 7 °C in the summer(9).
Energy requirements for heating and cooling
Comparative study of air conditioning of offices having walls with and without PCM shows that office with PCM containing walls consume less energy for heating and cooling. During winter period in January, the highest energy consumption was 33kWh for office without PCM as compared to 18kWh for office with PCM.
Similarly, during summer i.e. in the month of august, highest energy consumption for cooling in office without PCM is slightly higher than office with PCM (9).
Incorporating phase change materials (PCM) into building materials enable a more dynamic use of energy. Due to the storage capabilities of PCMs, excess heat can be stored during warm periods and released during cold periods. It may also work the other way around, storing cold energy and using it for free cooling systems in warm periods. The benefits of using PCMs in buildings mainly revolve around a decrease in energy usage along with a peak load shifting. This review paper is focused on the current thermal energy storage technology available with PCMs and its different applications. These technologies are very beneficial for the energy conservation and find several applications like PCM Filled Glass Windows, Thermal energy storage in concrete, PCM Assisted Ceiling, PCM Integrated Roof, PCM Assisted Under-Floor Electric Heating System, PCM Assisted Sun-Shading etc. Lotfi Derradji et. al even showed that the use of phase change materials in the interior plaster of building brings up to 25% energy savings for heating and cooling.
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