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tiles to the solid concrete ground floor that supply thermal mass. The structural thermal performance of buildings with thermal mass walls would dominate, and PCM needs to be applied only in a thin interior facing layer to achieve the thermal benefits

Richardson and Woods (2008) analytically show the relationship between PCM performance and amount of PCM or thickness of PCM product. They held the temperature at a constant comfort level thereby establishing a useful basis for

comparing the performance of thermal mass with different amounts of PCMs in walls.

The study shows that as the amount of PCM increases, PCM performance improves. To be effective the surface of the PCM must be sufficiently exposed to allow heat transfer.

The larger the amount of these materials that are exposed to the internal environment in a building, the greater the benefits in thermal mass. However, there exists a limit beyond which the structural integrity of the wall fails. PCM thickness that achieves only partial melting at the interior side is more effective at anchoring surface temperatures to the building thermal capacity rather than the interior air temperature of the building. The study concludes that thickness for both PCM should be sufficiently large, such that heat movement cannot reach the interior side of the wall. PCM latent heat and thickness

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should be as high as structural integrity and cost permit; until it reaches a critical point where as more of the latent heat cannot be accessed.

In another study, Xu et al.(2005) using analytic calculations validated, with an experimental cabin the PCM built in Tsinghua University of Beijing concludes that the thickness of PCM used for floor temperature regulations should not be larger than 20mm . The heat of fusion and the thermal conductivity of PCM should be larger than 120 kJ kg and 0.5 W/m K, respectively. The values for heat of fusion and conductivity are in agreement with the work of Zhang et.al. (2006).

Zalba et al. (2004) design, construct and run an experimental air-conditioning system to study PCMs and report that PCM of 25 mm thickness is suitable with solidification average time of 6.5 hours, and melting average time of 9 hours. An increase of thickness means an increase in the duration of the solidification process, but the effect of the temperature is higher when the thickness of the PCM is also higher. This means that in climatic areas with a night temperature of about 16 °C an increase in thickness would be more efficient, while in areas with an average night temperature of 18 °C, an increase in the thickness of the PCM could endanger the viability of the system. In the melting process, an increase of PCM thickness is more critical at low temperatures, and an increase in temperature is more critical with higher thickness of the PCM. This means, that with applications where the inlet temperature of the air is about 28 °C, an increase in PCM thickness could delay substantially the melting process, while in situations where the inlet temperature is about 30 °C, the increase in time of the melting process would be not so important.

It is apparent from the works by Xu et al.(2005) and Zalba et al. (2004) that the optimal thickness of any PCM wall depends variables such as the properties of the PCM, the type of construction, application and the climate.

2.3.2.4 Conductivity

Generally, PCMs have low levels of thermal conductivity (Atul Sharma, 2009). Due to their low heat conductivity, PCM in thick layers within the building fabrics may not melt or solidify completely by diurnal temperature variations alone. Thus, there can be sufficient energy stored but insufficient capacity to dispose of this energy quickly enough (Belen Zalba, 2003). To counteract this negative feature, the systems are enhanced with mostly metallic fins, matrices, tubes, encapsulation, carbon brushes, graphite flakes; and placed in a combination of tube and shell or rectangular

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containment systems (Agyenim et al., 2011). However, in some cases of thermal protection or insulation, it is appropriate to have low conductivity values.

On-going research is being conducted to evaluate the performance of copper foam enhanced PCM building fabric of a model of terrace houses in hot and humid Malaysia in a climatic laboratory chamber (Isa et al., 2010). The integration of copper foam with microencapsulated PCM is hypothesized to cause an increase in heat transfer that will increase heat storage capacity and reduce internal temperature fluctuations compared with other materials. This is because the diurnal temperatures in such climates have little variation and is assumed that night temperatures will not be low enough to solidify the PCMs passively. The distribution of the small PCM microcapsules in a wall offers a larger heat exchange surface where the heat transfer rate to store and release heat is raised significantly. PCM in copper coating is utilized in this study (see Figure 2-6) because the PCM melts faster than with coatings such as acrylic and aluminium.

Figure 2-6 Walling system with copper foam integrated PCM panelling and ventilation holes (Isa et al., 2010)

A building model measuring 3.0 m in depth, width, and height was built and the panel installed directly onto the interior surface of the brick wall, in a climate chamber. All the materials used are the exact same materials of a typical terrace house. One side of the wall was installed with copper foam integrated panels and the other three walls with insulation material to imitate connected terrace houses; and the façades installed with insulation materials will be considered as internal partitions of the house as shown in Figure 2-7.

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Two ventilation strategies are explored and shown in Figure 2-8. The ventilation systems are:

 To circulate air, warmed and raised to the top part of the interior into the air gap between the copper foam integrated PCM panels and the wallboard using a ventilation fan, to reduce the air temperature. This cooler air will be ventilated back to the internal space through openings at the bottom of the internal wallboard

Figure 2-7 Walling system - wallboard finishing (Isa et al., 2010)

 To use night cooling, by circulating air into the ventilation openings at the bottom of the external wall. The air then moves into the ventilation holes of the copper foam integrated PCM panel and out through the ventilation holes and openings located in the top part of the external wall. Heat stored during the day will be discharged back into the environment that will reduce the temperature of the copper foam integrated PCM panels rapidly, and the PCM will reverse the phase transition from liquid back to a solid ready to store heat the next day The preliminary results showed:

 Smaller PCM encapsulates, leading to larger surface areas, in the wall perform better than that of bigger ones

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 Force ventilation is required in tropical climates due to a low diurnal variance in temperature

 A panel-type installation is better than direct integration of PCM into the building structure

Figure 2-8 Forced ventilation for PCM layer (Isa et al., 2010)

It should be noted that this study is on-going so measured or predicted results are yet to be published.

Zhou et al.(2007), also examine the effects on room air temperature of conductivity, and results show that conductivity has an effect on indoor temperature only in the

solidification process. Above 0.5Wm/^oC, the conductivity has no obvious influence for both melting and solidification processes. Therefore, in hypothetical studies, the use of 0.5Wm/^oC conductivity is adequate for successful PCM performance. This value is agreed on by Xu et al. (2005).

2.3.2.5 Other thermo-physical variables

Combinations of other thermo-physical properties have been extensively reviewed in literature. They include:

 Enthalpy/temperature relationship

 Latent heat

 Position

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 Transition time

 Density

The thermo-physical properties of PCM are significantly correlated to one another. For instance the performance of a certain thickness of PCM product is affected by its latent heat capacity, conductivity and climate. A large surface area increases the melting and solidification rate of PCM panels. The temperature amplitude decreases with the increase of heat transfer coefficient and surface area of the PCM panels. Human thermal comfort decreases with increase in temperature amplitude.(Zhou et al., 2008; Xiao et al., 2009; Isa et al., 2010).

The thermo-physical properties of PCM are therefore complex variables that need to be optimized depending on other variables. The need for a critical examination of the effect of the thermo-physical properties of the PCM based on application is apparent for its successful performance.

2.3.3 Chemical classification of PCM

Chemically, PCM are classified as organic, inorganic and a combination of the two, known as eutectic compounds (Farid et al., 2004). Figure 2-9 shows the different types of organic, inorganic and eutectic PCMs. The properties of inorganic and organic PCMs are shown in Table 2-5.

Figure 2-9 Chemical classification of PCM (Pasupathy et al., 2008a)

Although PCMs are made of different chemical content, two studies (Ahmet, 2005 and Diaconu and Cruceru, 2010) found in literature indicate the possibility of combining

PCM

Organic

Paraffin