As a common modern architectural design, glazed fenestration is widely adopted in residential and non-residential buildings. Windows are the source of heat either gain or loss that call for proper design. Glazing type and window areas are the subjects to be analyzed and evaluated to balance light gain and heat gain requirement from daylight. Due to highly developed technologies, there are a number of high-performance glazing types such as double glazing, low-e coasting, and selective glazing available in markets to fulfill the requirements of customers . Despite pleasant outside views, the necessity to minimize radiant heat gain results in use of severe externally shaded windows or highly reflective glazing with severe internal shading. With severely shaded windows, the utilization of natural light in building is minimal even though ambient illuminance levels are very high.
Well planned external shading is the most effective method of reducing solar heat gain. In buildings where solar load affects the cooling requirement, shading may reduce a building’s total cooling load by as much as half. Exterior shading requires more thought and innovation than most energy conservation techniques because of many choices offered, but no well established strategies for using them. There are some types of external shading devices that can be commonly met: projecting horizontal shelves, overhangs, louvers (multiple slats), awnings, external blinds. Internal multiple slats is a versatile shading device.
In modern offices, the need for efficient automation systems is evident, espescially in fenestration systems with large heights. A theoretical and experimental study for automated blind system has been reported. But, this automated glazing system is not common in residential building. Usually, in residential buildings, the internal multiple slats system is static. Unfortunately, continuous human control of this movable shade is however not tenable . Therefore, for static system, it needs a small calculation to find the best angle for the slats preventing building from radiation heat gain. In this study, the author will try to presenting a study on heat transfer theory of double glazing window with internal multiple slats. A discussion about the performance of the use of double glazing window with internal multiple slats is also given in this paper.
One of the purpose of Double Glazed Window with internal multiple slats as a shading device is to reduce heat gain to the building. Some studies both experimental and numerical studies had been done to assess the performance of this shading device. The performance of this shading device can be seen from its U Values, and also effects to heating and cooling load. U Value is a value that determine the value of thermal transmissivity (W/m2K).
In the case of a window without internal multiple slats, the U-Value concept to calculate heat losses is adequate. However, in the case of a double-glazed window with internal multiple slats, use of the U Value can lead to large errors because the U-Value varies with slat angle, slat colour and material, and the position of the sun.
Avedissian (2007) reported that Huang in University of Westerloo had made an experimental study to assess U Values. The Experimental data can be seen in Figure 1. His experiment used isothermal vertical surfaces at various pane spacings and louver angles. The results showed better window performance when the internal multiple slats were tilted from their fully open position and also when the low-e coating was used.
From Figure 1, we can see that the presence of internal multiple slats has a strong effect on the U Values except when the blind is close to fully open.
Figure 1 Experimental data of U Values on double glazed window with internal multiple slats
In other study, Garnet et al. reported that in the horizontal position, the effect of internal multiple slats is to reduce the thermal resistance of the window by almost 10%. As the internal multiple slats are closed, the performance of the window steadily improves .