Design for energy efficiency in construction materials is gaining popularity as an increasing number of companies seek LEED certification and participate in energy efficiecy programs such as the Department of Energy’s Net-Zero Energy Commercial Building Initiative.
One area in which significant energy efficiency improvements are possible is in window and daylighting technology. Heat loss through standard windows accounts for about 10% to 25% of a typical home heating/cooling bill. Standard techniques to reduce the energy impact of windows include the use of drapes to prevent heat loss. However, this method has a negative impact on the use of windows for daylighting purposes, increasing the use of artificial lighting within the building. A design in which the heat loss from the window can be minimized while the daylighting benefits are not impacted is desirable.
In this project, the use of a paraffin-based phase change material (PCM) within a polyester based panel is explored. This design will allow the PCM to absorb heat through the latent heat of fusion as it melts. The PCM is either encapsulated within a plastic shell so that the liquid phase is contained or contained within an aluminum honeycomb matrix. The melt temperature of the PCM is selected such that it melts during the daylight hours when the window panes are exposed to high solar heat fluxes. At night when the solar flux is significantly reduced the PCM sheds heat back into the building or the environment as it cools.
This design thus provides a dual benefit by lowering the cooling load during the day and the heating load at night. This experiment analyzes the thermal conductivity of various mixtures of thermoplastic polyester and PCMs. It was determined that the addition of PCM slightly increases the thermal conductivity of panels when the PCM is solid. Once the PCM has melted, the panel conductivity is lowered.