Users have a range of different needs from shading devices, such as thermal comfort, low costs, high reliability, aesthetic requirements, compliance with technical conditions
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(mounting dimensions, dimensions when the blind is fully retracted, etc.) and defence against fire, weather and theft. These requirements cover different aspects: some are important for the choice of a shading device; others are significant for the way the device is used, but thermal and visual comfort are the critical parameters that affect how occupants will manage the device (Kuhn, Bühler, & Platzer, 2000).
Currently there are no established criteria for the evaluation of shading systems, partly because there are such a wide variety of devices on offer. However, a range of methods with various approaches are used to estimate the effectiveness of these non-structural devices, based on calculation of solar gain to the space, or measurement of interior light levels (Stack, Goulding, & Lewis, 1999).
Performance evaluation
Two methods are regularly used for evaluating the effectiveness of various shading devices. The first one is calculating the shading coefficient, which is the relation of the amount of solar energy passing through a protected opening to the amount of energy which would pass through the opening if it was unprotected (Goulding, Owen, Steemers, & Directora, 1992). Pure glasses have a shading coefficient of 1 and while opaque insulated wall has a shading coefficient of zero (Baker & Steemers, 2002). To calculate the shading efficiency of various materials, Button (1993) recommend the following procedures:
1. For fixed shading devices, take into account the standard daily solar diffusion. 2. For blinds, the standard for all orientation where the slats are to cut out solar
radiation.
3. For glass, take into account the total of the short wave and long wave shading coefficients, considered for radiation at standard incidence. For any other incidence angle of radiation, the shading coefficient is compared with that for clear glass in the similar condition table 2.4, which marks in deriving shading coefficients that are approximately at all occurrence angles of solar radiation. It should be noted that there is a large deviation in the overall transmission with different incidence angles and the orientation as well as the time of the year; therefore, the shading coefficient can be a deceptive guide for judgment.
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Table 2.4 Solar gain factors for various shading elements (strictly for UK only, but
approximately correct world-wide).
Shading
Element Solar gain factor Glazing Type
Position Type Single Double
Internal
Dark green open weave plastic blind 0.62 0.56
White venetian blind 0.46 0.46
White cotton curtain 0.41 0.40
Cream Holland linen blind 0.30 0.33
Mid pane White venetian blind - 0.28
External
Dark green open weave plastic blind 0.22 0.17
Canvas roller blind 0.14 0.11
White louvered sun breaker, blades at 45o 0.14 0.11 (Goulding, Owen, Steemers, & Directora, 1992)
Based on the evaluation of the shading efficient, it has been concluded that exterior shading devices are 35% extra efficient than interior ones (Olgyay, 1963).
The second technique used to calculate the efficiency of a shading device (particularly its thermal result) is to compare the inside air temperatures with the device to those obtained with the similar windows un-shaded (Santamouris & Asimakopolous, 1996).
Comparative evaluation
Variable shading is more efficient than fixed shading devices for the reason that they can admit all of the wanted solar radiation as it is the situation in winter (Santamouris, et al., 2001).
A number of shading devices can have double roles (McNicholl & Lewis, 1994): 1. Insulation blinds or louvres help in reducing heat loss when closed at night. 2. Treated glass and prismatic strategy offer shading and forwarding of light. Internal shading devices tend to be cheaper and more easily modifiable than external devices but they are not so efficient at reducing heat gains, as the sunlight heats up the shades and the air in the region of them. Interior shading installed within a double or triple glazed window (with ventilation of the void to the outside) combines the advantages of both kinds, because it permits heat gains to be dissipated to the exterior at the same time as shading the window (O'Cofaigh, Owen, & Fitzgerald, 1999).
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1. Orientation evaluation
South facades:
It is possible that a mixture of horizontal and vertical shading devices might be more efficient if the vertical devices are inclined at 45o to the south (Givoni, 1976). If the designers decide to use horizontal or vertical devices in the orientation, they must be useful, but at the same time, they should not limit the vision or reduce the solar gains during winter season. Horizontal shading is more effective than vertical shading for south-east and south-west orientations (Santamouris & Asimakopolous, 1996).
East and west facades:
Low angle direct sunlight, which is generally common inward on east and west facing elevations, presents problems in shading. Overhangs are not efficient in this case, as the use of fixed vertical fins and rotating ones eliminates a considerable amount of daylight towards the inside of building and blocks the outlook. Internal blinds are able to be left open intermittently and rose according to the sun’s angle; however this will increase the heat gain, particularly on west facing elevations. Adaptable external devices are the most effective devices to avoid this drawback even though they are expensive and can result in issues with maintenance and stability (McNicholl & Lewis, 1994).
2. Visual characteristics of shading devices
The visual properties of shading devices were classified by Littlefair, 1999 into two main groups, as follows:
1 Providing a view out
The majority of shading devices affect the outward view, and some sort of compromise is required. This will depend on the principle of the device:
A Controlling overheating
If shading is used just for controlling the overheating, there are a number of devices which protect the outward view: among them are overhangs, light shelves, window film and tinted glazing. Furthermore, coloured glazing and window film must not be dark, or the view will be perceived to be gloomy.
B Controlling glare
The major source of glare is the sun itself. Less often, glare might arise from a vivid patch of sky, or by reflection from a building opposite. The best solution to manage glare and also permit a recognizable view out of the window is usually adaptable opaque shading that occupants can control. Venetian and roller blinds which fall from
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the top of a window are extremely efficient in controlling high angle summer sun and at the same time allow a view from the lower half of the window.
2 Providing privacy
During the day time, privacy can be offered by transparent shading devices such as net curtains and reflective glazing. Through the night, adjustable opaque shading is required to give privacy. Opaque devices such as thick curtains offer the most excellent screening.
A Loss of Daylight
All shading devices will prevent a part of daylight from reaching the room. In order to make the most of daylight while providing shading, the designer can select one of the following:
1. Adjustable shading that can be withdrawn in cloudy days.
2. Devices that can redirect received sun, such as mirrored louvres. These devices cut off sunlight that would usually cause discomfort to people close to the front of the room.
3. Spectrally selective devices similar to thermally reflective glasses, which permit additional daylight through rather than other parts of the solar spectrum.
The percentage of daylight entering is based on the visual permeability of the shading device. Glass manufacturers usually quote the direct standard transmittance for light passing through the window at right angles. However, the general day-lighting performance is better measured by the diffuse transmittance which includes light approaching in from all angles. The diffuse transmittance is frequently lower than the direct usual value. Clear single glazing has a direct standard visible transmittance of 0.9, but its diffuse value is approximately 0.8.
3. Ventilation characteristics of shading devices
Alternative shading devices have various effects on natural ventilation Figure 2.30 (Baker & Steemers, 2002):
1. Retraction types guarantee unobstructed observation and ventilation when retracted. However, when retracted they do not stop unwanted solar radiation from entering. Louvres, on the other hand, can be used to manage the direction of air flowing if their position is adjusted.
2. Fixed overhangs do not block view while their full shading function is maintained.
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Figure 2.30 The impact of different shading types on vision and ventilation (Baker &
Steemers, Daylight Design of Buildings, 2002)
4. The location of shading devices
Various shading devices are able to be installed either inside or outside the glazing, such as fabric blinds, louvres, and screens. It is preferable to situate these devices outside the building so that the majority of the solar radiation is able to be reflected before it reaches the glazing as shown in Figure 2.31. However, this is not usually the case when a shading strategy is used with roof lights, mostly for the reason that convection upwards keeps the heat generated by absorption away from occupants. In this situation, interior blinds take an extremely low thermal penalty, but provide a major reduction in price (Baker & Steemers, 2002).
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Figure 2.31 External horizontal shading devices with similar performance (Stack,
Goulding, & Lewis, 1999)