solar control & shading devices
TRANSCRIPT
SOLAR CONTROL
& SHADING
NEED FOR SHADING
When ambient temperatures are within or above the comfort zone, any ingress of solar radiation will contribute to discomfort. Shading design must prevent this. However, at cool times of the year, it may be desirable to allow solar radiation to pass directly into the room, to provide a useful heating effect. This response can be provided either by the shading device moveable or by it being geometrically selective.
FUNCTION OF SHADINGSolar radiation entering a room can have three effects :
• Radiation absorbed on to room surfaces will lead to an increase in air temperature.
• Solar radiation falling directly on to an occupant will lead to an increase in the mean radiant temperature experienced.
• High intensities of radiation from direct sun or even the diffused sky can cause discomfort glare, or disability glare where an occupant’s visual performance will actually be impaired.The function of shading is to eliminate these three effects.
FUNCTION OF SHADINGThe function of shading is to eliminate these three effects. i.e.
• to reduce the total amount of radiation entering the room by reflection and absorption.
• to improve the distribution of the light in the room
The functional requirements for shading change with region and climate.
CONSTRAINTS OF SHADINGShading is itself subject to certain constraints :
• Maintenance of air flow through the non-air-conditioned buildings during the cooler hours of the day.
• There will be a need for admitting controlled levels of diffused daylight.
• In most cases there will be a requirement of views out of the window.
USE OF SHADING
Solar controls should be considered for all glazed openings exposed to direct sunlight. Solar control is particularly important on south to west-facing facades, since the solar gains will coincide with the hottest part of the day. Solar control is also vital for lightweight buildings with large areas of glazing.
SOLAR SHADING
When sunlight hits a pane of glass, it splits into three components – that which is reflected, that which is absorbed and that which is transmitted through.
SOLAR SHADING• Reflected component from the glazing has
no thermal effect on the space behind the glazing.
• Absorbed component within the glazing itself heats up the glass. Heat is transmitted inwards and outwards by conduction and long wave radiation.
• Transmitted component of radiation that penetrates through the glass raises the temperature of the surface behind it.
SOLAR SHADINGThe proportion between the three components is determined by the angle at which the solar beam strikes the glazing, i.e. ‘angle of incident’ and by the type of glazing.For most types of glazing, the transmitted component is very small if the angle of incidence is larger than 45º from the normal to the glazing. If the angle is more than 60º , most of the radiation is reflected.
IMPORTANCE OF SHADING
Use of shading device can improve building energy performance, prevent glare, increase useful daylight availability and create a sense of security. Realizing these potential benefits, a no. of shading devices have been invented, such as fixed, manual and automatic moveable, internal and external shading device.
EXTERIOR SHADING DEVICE
Exterior shading device is primary used to control the amount of radiation penetration to the interior of buildings. Some of them are operable, i.e. they can be raised or lowered.Two basic types of exterior shading device are horizontal and vertical .
EXTERIOR SHADING DEVICEIts effectiveness depends on its type and placement relative to glass. When radiation strikes a shading device,
• a part of it is reflected outwards from its surface,
• another part is reflected onto the glazing• remaining part is absorbed by itself,
causing it to heat up.
INTERIOR SHADING DEVICEInterior shading devices are mainly established in order to provide visual comfort by eliminating glare. Further, interior shading devices can contribute to the interior architectural design of rooms without influencing the exterior façade of the building. Because of the heat absorption happening inside the building (the transformation of short wave sunlight energy into long wave heat energy) interior shading devices are not as efficient as exterior shading solutions to reduce solar heat gains.
INTERIOR SHADING DEVICE
Internal shading devices limit the glare resulting from solar radiation. Internal shading devices usually are adjustable and allow occupants to regulate the amount of direct light entering their space. Most commonly these take the form of horizontal or vertical blinds attached above windows.
INTERIOR SHADING DEVICEIn rooms that are oriented north or east without risk for overheating in the summer, the following interior shading devices are in use :
• Venetian blinds – horizontal blade construction, which can be lowered and elevated (1)
• Roller blinds - textile curtains or foils, which are rolled up above the window (2)
• Pleats curtains – textile curtains, which are folded above the window (3)
• Vertical-blades - textile vertical blade curtains (4)• Curtains – textile curtains
Internal Shading devices
COMPARISON
External ShadingExternal shading devices are the most efficient thermally because they intercept the solar energy before it has entered the room. Its effectiveness depends on its type and placement relative to glass. When radiation strikes a shading device,
• a part of it is reflected outwards from its surface, • another part is reflected onto the glazing• remaining part is absorbed by itself, causing it to heat up.
Thus, even if energy is absorbed by them, it is not trapped behind the glass. They carry the disadvantage of having to be weatherproof and are more difficult to control from inside.
Internal ShadingInternal shading is thermally in effective. In such a case , radiation strikes the glazing with no interference, penetrating into the internal space, causing the shading element to heat up, and from there, heating up the room by both long-wave radiation and by conduction. Radiation emitted by the shading device itself is already of the long-wave type, thus it is trapped by the glazing in front of it in the same way as any other long-wave radiation from the interior. It is generally much cheaper to install and is easy for users to control, but is less efficient, for reasons outlined above. It is also vulnerable to damage.
Design Strategy of Shading DevicesThe design strategy of the shading device 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 • 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 , usually no shading is required to this exposure.
• Interior shading devices such as Venetian blinds or vertical blades 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.
Finally, shading and solar control devices have a great potential for architectural expression, adding to the texture and modulation of the facade. They also have the potential (and should) respond to the orientation of the facade, thus visibly reflecting the building’s place in the natural world as well as its urban setting.
Vertical & Horizontal Shading Device
Shading devices should be selected according to the orientation of the window.
• Vertical Shading device is most effective when sun is to one side of the elevation and at low angle, such as eastern or western elevation.
• Horizontal Shading device is most effective when sun is opposite to the building face considered and at high angle, such as for north and south facing walls.
SHADOW ANGLES
Shadow angles are formed by sun shading devices or projections on a wall exposed to the sun. Different design of sun shading devices form different shadow angles.The performance of shading device is specified by two angles :
• Horizontal shadow angle• Vertical shadow angle
These angles depend on the position of the sun and the orientation where the window is facing.
HORIZONTAL SHADOW ANGLE
The horizontal shadow angle (HSA) is required for (or cast by) vertical shading devices. It is the horizontal angle between the normal of the window pane and the azimuth of the sun.
HSA = wall azimuth – solar azimuth
VERTICAL SHADOW ANGLE
The vertical shadow angle (VSA) is required for (or cast by) horizontal shading devices.It is the angle between the ground line and altitude of the sun.
VERTICAL SHADOW ANGLEActually it is measured on a vertical plane normal to the elevation considered. If we imagine a virtual plane between the bottom left-hand and right-hand corners of the window and the sun, then the VSA is the angle this plane forms with the ground plane.
tan VSA = tan(altitude) / cos(HSA)
SOLAR ALTITUDE ANGLE &VSASolar altitude angle describes sun’s position in relation to the horizon, while VSA describes the performance of the shading device. Numerically, the two coincide when, the sun is exactly opposite the wall considered i.e. when solar azimuth and wall azimuth angle are same and HSA = 0. For all other cases, when the sun is sideways from the perpendicular, the VSA is always larger than the solar altitude angle.
Shade DimensionsThese two angles, HSA and VSA, can then be used to determine the size of the shading device required for a window. If the height value refers to the vertical distance between the shade and the window sill, then the depth of the shade and its width from each side of the window can be determined using relatively simple trigonometry.
• Shade Depth : The depth is given by:depth = height / tan(VSA)• Shade Width : The width is given by:width = depth x tan(HSA)
Design Requirements
Requirement of shading largely depends upon the climatic conditions. According to climatic zones, there are three categories of shading requirement :
• Complete year round shading• Complete year round shading but only during
major sunshine hours• Shading during summer months only
Shading Requirements in Different Climatic Zones
Climatic Zone RequirementHot & Dry Complete year round shadingWarm & Humid Complete year round shading but design
should be made such that ventilation is not affected.
Temperate Complete year round shading but only during major sunshine hours
Cold & Cloudy No shadingCold & Sunny Shading during summer months aloneComposite Shading during summer months alone
