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Article Excerpt
Abstract: Phase change materials (PCMs) can store much larger amounts of thermal energy per unit mass than conventional building materials and can be used to add thermal stability to lightweight construction without adding physical mass. This paper reviews how PCMs could be incorporated in building materials, particularly in passive applications. A simulation study using IES Virtual Environment package 'Apache' was carried out on PCM impregnated plasterboard, investigating various fusion temperatures of the PCM during night, day, and week-long test durations in hot weather conditions. Different ventilation rates and alternative conductivity values of the gypsum in the plasterboard were tested. It was shown that use of PCMs has significant advantages for both commercial and residential building applications, provided sufficient night ventilation is allowed.
Keywords: Building components, Building materials, Fusion temperature, Lightweight construction, PCM impregnated plasterboard, Phase change materials (PCM), Plasterboard, Simulation, Thermal comfort, Thermal energy, Thermal insulation, Thermal simulation, Ventilation, Virtual environments
Introduction
The UK government is actively encouraging higher speeds and standards of construction using off-site construction techniques (Prescott, 2005). In addition, the Part L Building Regulation requirements on energy efficiency and carbon, demand increasingly stringent energy performance for new construction (Building Regulations, 2006). These factors have led to a significant shift away from conventional brick and block building techniques towards lightweight timber and steel frame construction, because it is a competitive solution for new pre-fabricated building elements and tends to be more cost effective to achieve compliance with thermal performance demands.
However, a move to lightweight construction raises concerns over internal comfort conditions due to lack of thermal storage properties, resulting in rapid swings of internal temperature. This concern is exacerbated by projected climate change because of global warming. Recent studies have indicated that hotter, drier summers are likely. Without optimised design of new buildings, many of the benefits accruing from reduced heating consumption may be lost as air conditioning units are fitted to reduce internal temperatures. There is a tension between the drive to construct more efficient structures with minimum environmental impact and the tendency to add more mass (in practice concrete) for thermal storage.
Phase change materials (PCMs) can store much larger amounts of thermal energy per unit mass than conventional building materials by storing energy as latent rather than sensible heat. Compared to the amount of mass required for energy storage in a block or brick building, the amount of PCMs needed is minimal. Similarly, the energy needed to produce the PCMs would only be a fraction of the energy needed to produce blocks, bricks or concrete with the same heat storage capacity (Anderson & Shires, 2002).
PCMs can be used for cooling a building in three conventional ways:
* Passive cooling: Cooling through the direct heat exchange of indoor air with PCMs incorporated into the existing building materials such as plasterboards, floorboards and furniture
* Assisted passive cooling: Passive cooling with an active component (for example, a fan) that accelerates heat exchange by increasing the air movement across the surface of the PCM
* Active cooling: Using electricity or absorption cooling to reduce the temperature and/or change the phase of the PCM
As active cooling, and to a lesser extent supportive passive cooling, require the use of additional energy (refrigeration and fans) it is likely that the simplest, most cost-effective and environmentally sound means of using PCM is in a purely passive way. Although more research is needed to investigate if this is in fact so, the main focus of this paper is on the use of PCMs for passive cooling.
PCMs in Building Applications
PCM can be impregnated into building materials such as plasterboard, either directly or as impregnated pellets. Various materials have been investigated (see Table 1). Paraffin wax, because of its cheapness and ready availability, combined with its flexibly adjustable phase change temperature is seen as a particularly promising material for use in building components (Amar, Kudhair & Farid, 2004; Chen, Nelson & Polanski, 1982; Demirbas, 2006).
Another method of incorporating PCM into conventional construction components is micro-encapsulation (Hawlander, Uddin & Khin, 2002; Hawlander, Uddin & Zhu, 2002; Schossig, Henning, Gschwander & Haussmann, 2005). Schossig P., Henning, H., Gschwander, S., & Haussmann, T. (2005) describe simulation and testing of PCM plasterboard-lined rooms, and finds notable decreases in peak temperature in both when compared with conventionally finished rooms. However, it is noted that the main difficulty is night cooling, and mechanical ventilation is used at four air changes per hour to cool the PCM. Micro-encapsulated PCM has the appearance of beads or powder, depending upon size. The PCM is contained within small polymer spheres (for example 'Thermocules', 10 micron diameter acrylic spheres which contain paraffin wax), increasing the area to volume ratio available for heat transfer in PCM applications. This effectively overcomes the problems experienced by most encapsulation systems caused by reduced heat flux through the solid-liquid interface as it moves away from the heat transfer surface. In addition, a wall thickness of approximately 1 micron reduces thermal resistance of the encapsulant to negligible proportions. This technique has been a significant breakthrough, as it enables easy integration of organic PCM into building components (for example, plaster, plasterboard and concrete) whilst avoiding odour and handling problems.
Micro-encapsulated PCM, already used in some specialized clothing such as ski wear and in some electronics cooling applications, can be incorporated into a variety of building products, including wallboards and insulation foams. At an experimental stage, conventional, resistive insulation has been converted to capacitive or dynamic insulation by addition of PCM, thus adding thermal storage and consequent time lags where previously none was possible. Experiments have been carried out with perlite-based loft insulation, in which perlite is impregnated with hydrated calcium chloride and contained between layers of conventional insulation (Petrie, Childs, Christian & Childs, 1997). Ceiling tiles is another possible PCM application that has been tested using monitored test cells (University of Brighton Thermal Storage Research Group, 2006). The same group has also undertaken work...
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