Solar Shading devices
Sunlight admitted into a building impacts on the building energy consumption in different ways in different seasons. In summer, excessive solar heat gain results in greater energy consumption due to the increased cooling load requirement; in winter, sunlight reaching the south-facing facade can provide passive solar heating; in all seasons of the year the sun improves daylight quality. Well-designed shading devices can significantly reduce building peak cooling load and corresponding energy consumption and enhance daylight utilization in buildings. Shading devices can also avoid glare by reducing contrast ratios of building interior.
Different shading strategies
Shading of external windows can be provided by natural landscaping such as trees and hills, or by building elements such as overhangs, awnings, fins and trellises. Some shading devices can also act as light reflectors, which called light shelves, to reflect sunlight into the deep building interior. Installing fixed shading devices can be an efficient way to provide thermal and visual comfort to building occupants. The design of fixed shading devices will depend on the daily and yearly variation of solar position. For example, the use of overhangs is most effective on the south-facing window in the summer when sun angles are high. In the winter, overhangs allow the low winter sun to enter south-facing windows. However, the same horizontal device is ineffective at blocking low morning and afternoon sun from entering east- and west-facing window respectively in the winter.
There are a number of external shading strategies for buildings. The figure below illustrates some examples of shading strategies. The design of the shading strategy will depend on the size and orientation of the window openings. Shading devices can also affect the building appearance. Although the design of external shading devices involves a number of factors, the following recommendations are generally applied to all designs:
||Use fixed overhangs on south-facing glass to control direct beam solar radiation. Diffuse (indirect) radiation should be controlled by other measures such as low-e glass.
||Limit the area of east or west glass. Vertical or egg-crate fixed shading can be considered if the shading projections are fairly deep or close together; however these may limit views. North-facing glass receives little direct solar gain for Hong Kong altitude, usually no shading is required this exposure.
||Interior shading devices such as Venetian blinds or vertical louvers do not reduce cooling load since the solar gain has already been admitted into the indoors. However these interior devices do offer glare control.
||The durability of shading devices should be considered. Operable shading devices usually require more maintenance and repair.
External shading devices are particularly effective in conjunction with clear glass facades. However, high-performance glazings such as low-e glass are now available that have very low shading coefficient (SC), which indicates the amount of solar heat gain that is admitted into a building relative to a single-glazed reference glass. These new glass products reduce the need for external shading devices. However, well-designed fixed shading devices are the most effective installation for minimizing direct solar heat gain.
||Public Health Laboratory Centre, Shek Kip Mei
Horizontal overhangs are installed on the external facades of the building to reduce solar heat gain as shown in the photo.
External overhangs at Public Health Laboratory Centre
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||Hong Kong Wetland Park
The perimeter of the visiting center is built with wooden vertical screens with horizontal louvers to block sunlight.
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