AR6303
CLIMATOLOGY UNIT – NO: 2
BY B.HARIHARAN ASSISTANT PROFESSOR MEASI ACADEMY OF ARCHITECTURE
DESIGN OF SOLAR SHADING DEVICES
The Movement of the Sun ▶
The earth rotates on its north south axis in a 24 hour period and orbits the sun in a period of one year.
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The rotating axis is at an angle of 23 degrees. The height at which an observer sees the sun over the horizon (azimuth angle) depends on its location (latitude), the season (position of the earth in its orbit) and on the time of the day (rotation of the earth).
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The maximum or minimum height of the sun respectively, respectively, is reached at noon on the summer and winter solstice. solstice . The azimuth angle expresses the position of the sun over the horizon.
The Sun’s positions ▶
The sun’s position on the sky hemisphere can be specified by two angles:
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Altitude Angle [g] represents the vertical angle at the point of observation between the horizontal plane and the line connecting the sun with the observer.
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Azimuth Angle [α] The angle at the point of observation measured on a horizontal plane between the northerly direction and a point on the horizon circle, where it is intersected by the arc of a vertical circle, going through the zenith and sun’s position.
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North - 0° or 360°
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East - 90°
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South - 180°
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West - 270°
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The Zenith angle (ZEN) is measured between the sun's direction and the vertical and it is the supplementary angle of altitude; ZEN = 90 ° - ALT
Sun path diagrams ▶
There are several methods of projection for representing the sun’s apparent movement two-dimensionally like Equidistant chart, Orthographic projection, Stereographic projection.
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THE STEREOGRAPHIC (OR RADIAL) REPRESENTATION uses the theoretical nadir point as the centre of projection. This is the most widely used method. Such diagrams can be constructed for any latitude and to any desired radius
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The equinox, midsummer and mid-winter sun-path lines are always shown, but the intermediate date lines are arbitrarily chosen. Each sun-path line is valid for two dates: one between December and June and one between June and December.
Sun charts illustrating the variation in the
Reading Sun Path Diagrams ▶
Stereographic sun path diagrams are used to read the solar azimuth and altitude throughout the day and year for a given position on the earth. They can be likened to a photograph of the sky, taken looking straight up towards the zenith, with a 180° fish-eye lens. The paths of the sun at different times of the year can then be projected
onto this flattened hemisphere for any location on Earth.
Reading Sun Path Diagrams ▶
Azimuth Lines - Azimuth angles run around the edge of the diagram.
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Altitude Lines - Altitude angles are represented as concentric circular dotted lines that run from the center of the diagram out.
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Date Lines - Date lines start on the eastern side of the graph and run to the western side and represent the path of the sun on one particular day of the
year. In Ecotect, the first day of January to June are shown as solid lines, while July to December are shown as dotted lines. ▶
Hour Lines/ Analemma - Hour lines are shown as figure-eight-type lines that intersect the date lines and represent the position of the sun at a specific hour of the day. The intersection points between date and hour lines give the position of the sun
Reading the Sun Position ▶ ▶
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Locate the required hour line on the diagram. Locate the required date line, remembering that solid are used for Jan-June and dotted lines for July-Dec.
Find the intersection point of the hour and date lines. Remember to intersect solid with solid and dotted with dotted lines. Draw a line from the very center of the diagram, through the intersection point, out to the perimeter of the diagram. Read the azimuth as an angle taken clockwise from north. In this case, the value is about 62°. Trace a concentric circle around from the intersection point to the vertical north axis, on which is displayed the alt itude angles. Interpolate between the concentric circle lines to find the altitude. In this case the intersection point sits exactly on the 30° line. This gives the position of the sun, fully defined as an azimuth and altitude.
Angle of incidence ▶
Horizontal Component of Angle of Incidence : d: Difference between the solar azimuth and wall azimuth. Eg., Lets assume the azimuth angle to be 239° and
the wall is facing the west – 270° , ▶
d= 270 – 239 = 31°
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Vertical Component of Angle of Incidence is same as the solar altitude angle itself g.
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Angle of incidence β = Angle between a line perpendicular to the wall and the sun’s direction.
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Cos β = Cos d x Cos g
Angle of incidence is required for selecting the appropriate gain factor in heat gain calculations g through window and for calculating incident radiation on an opaque surface. β
Shadow angle: ▶
The Performance of Shading Device is specified by 2 angles.
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HORIZONTAL SHADOW ANGLE d
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VERTICAL SHADOW ANGLE v
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Horizontal Shadow Angle d characterizes a vertical shading device and it is the difference between the solar azimuth and wall azimuth ,
same as the horizontal component for the angle of incidence. ▶
Vertical Shadow Angle v characterizes the horizontal shading device , i.e., a long horizontal projection from the wall and it is measured on a vertical plane normal to the elevation considered.
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Tan v = tan g x sec d
HORIZONTAL SHADOW ANGLE
VERTICAL SHADOW ANGLE
The shadow angle protractor ▶
This is a semi-circular protractor, showing two sets of lines
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RADIAL LINES, marked 0 at the centre, to -90o to the left and+90o
to the right, to give readings of the HSA ▶
ARCUAL LINES, which coincide with the altitude circles along the centreline, but then deviate and converge at the two corners of the protractor; these will give readings of the VSA.
HS A
VS A
The Shadow Angle Protractor
HSA OF A PAIR OF VERTICAL FINS SUPERIMPOSED ON SUN-PATH DIAGRAM
VSA OF A HORIZONTAL DEVICE SUPERIMPOSED ON SUN-PATH SUN-PATH DIAGRAM
Design of shading devices To design a horizontal shading device this way, use the following basic steps: 1. Determine cut-off date: ▶
This is the date before which the window is to be completely shaded s haded and after which the window will be only partially shaded.
2. Determine Start and End Times: ▶
These represent the times of day between which full shading is required. Keep in
mind that the closer to sunrise and sunset these times are, the exponentially larger the required shade. 3. Look up Sun Position: ▶
Use solar tables or a sun-path diagram to obtain the azimuth and altitude of the sun at each time on the cut-off cut -off date.
4. Calculate the Shadow Angles: ▶
Using the methods detailed in the Shadow Angles topic, calculate the
HSA and VSA at each time. 5. Calculate Required Depth and Width: ▶
Once again, using the Shadow Angle methods, calculate the depth
and width of the required shade on each side of the window. window. Shade Depth: ▶
The depth of the shade is given by: depth = height / tan(v tan( v)
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The width is given by: width = depth * tan( d)
Figure 2
SHADING DEVICES : ▶
VERTICAL DEVICES
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HORIZONTAL DEVICES
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EGG – CRATE DEVICES
VERTICAL SHADING DEVICES ▶
Narrow blades with close spacing gives the same shadow angle as broader blades with wider spacing
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Using the shadow angle protractor the ‘shading mask’ of a given device is established. If the shading mask of the device is done on the same scale as the protractor, on tracing paper, it can be laid on the solar chart – shading times for the particular device (dates and hour) can be read off directly.
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It will be seen that this type of device is most effective when the sun is to eastern or western elevation
SHADING DEVICES : HORIZONTAL SHADING DEVICES ▶
Eg. Canopies, horizontal louvers, externally applied venetian blinds
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Vertical shadow angle is used for its design. Shading mask of a segmental type is the most effective when the sun is opposite to the building face & at a high angle for north & south facing walls. It permits a view downwards only
EGG CRATE SHADING DEVICES ▶
These are combination of horizontal & vertical shading device Eg., Grill blocks / jallis & decorative screen work can be effective for any orientation – depending on detail dimensions.
Shading Devices •The use of shading device is an important aspect of many high-performance building design strategies. • It has been proved that the use of shading device could improve building energy performance, prevent glare, increase useful daylight availability (between 100~2000lux) and create a sense of security. • Realizing these potential benefits, a varied of shading configurations have been invented and put in the market, such as fixed, manual and automatic movable, internal and external shading device. • Accordingly, many simulation tools have been developed to support shading design.
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Exterior shading device •Exterior shading device is primary used to control sunlight penetration to the interior of buildings. •Such shading devices are always attached on the mullion as a s eparate component of building envelop, but can also be achieved by disposition of the building floors to create overhangs. •Exterior shading device decrease direct beam penetration by projecting shadow on the window along the sunlit direction;
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Types of exterior shading device •Two basic types of exterior shading device are • Horizontal & Vertical • Various combination of these creates many configurations to accommodate different envelop shapes and orientations.
Each orientation of the building requires a different approach to the design of shading. The north elevation (in the northern hemisphere) essentially does not require shading because except in the summer months in the early morning and late evening, no sun penetration occurs. 66
SOUTH SIDE ORIENTATION . Horizontal shading devices are suited to southern exposures. Roof overhangs can also easily be used to shade southern exposures on low rise buildings. -the length of the projection is determined as a geometric function of the height of the window and the angle of elevation of the sun at solar noon. - Such shading devices can be designed to completely eliminate sun penetration in the summer and allow for complete sun penetration during the winter when such is desired f or passive heat gain.
Basic Shading Strategy for a South Elevation 67
SOUTH SIDE ORIENTATION
In order to obtain shading in the late morning and early afternoon when the sun is not at its high point, the shading device should be extended either side of the window opening .
Elevation of Shading Device Configurations for South Facing Facades
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EAST AND WEST SIDE ORIENTATION The east and west elevations are both difficult to shade. In east direction we receives early morning sun and from west direction we receives evening sun Its angle is also low enough to preclude shading using overhangs.
Shading Devices for Non Southern Exposures
Shading can be provided in the way of landscaping and foliage 69
Other shading devices Besides attaching shading device on exterior of building envelop, many other ways can also prevent sun light from penetrating to the interior of buildings. •Interior shading device •Building self-shading •Shaded by trees and other obstructions
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Shading Devices in IHC The reflectors are installed above the building to provide shade and prevent sun from entering into the building. The reflectors are aligned at an angle which reflect back 70% of the sunlight and change their angle during winter to allow sunlight to fall on the windows.
Sun shading devices are incorporated into the design of the facade and the overhanging floors of each level to provide sufficient shade to ensure a comfortable environment within the dining and office spaces
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Solar shading device for high-rise buildings The external shading system sits like a skin on the outside of a building, moving around on tracks to shield the sun while still allowing daylight to penetrate.
Made from a lightweight frame incorporating photovoltaic cells, the sunshield: •reduces the energy required to light and cool the building by 30%; •produces enough energy to power the movement of the shade and provide up to 10% of the building remaining power needs; •is made from a simple and cost-effective series of modular panels, which can be removed individually without compromising the whole system; and •incorporates a guide rail at each floor level t o provide both structural support and access for cleaning and maintenance.
Solar Louvers minimizes glare & over heating 72
