Solar Chart

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Solar Chart & Design of Shading Devices1Sun Path DiagramsSun-path diagrams or solar charts are the simplest practical tools for depicting the suns apparent movement. The sky hemisphere is represented by a circle (the horizon). Azimuth angles (i.e. the direction of the sun) are given along the perimeter andAltitude angles (from the horizon up) are shown by a series of concentric circles, 90 (the zenith) being the centre.2

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7The sun-path lines are plotted on this chart for a given latitude for the solstice days, for the equinoxes and for any intermediate dates.For an equatorial location (LAT = 0) the diagram will be symmetrical about the equinox sun-path, which is a straight line; for higher latitudes the sun-path lines will shift away from the equator.For a polar position the sun-paths will be concentric circles (or rather an up and down spiral) for half the year, the equinox path being the horizon circle, and for the other half of the year the sun will be below the horizon. The shifting of sun-paths with geographical latitudes is illustrated by the following Figures.The date-lines (sun-path lines) are intersected by hour lines. The vertical line at the centre is noon.

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10Use a Sun Path Diagram to find solar altitude and azimuth for any given time, to help in sizing shading devices.Choose the sun path diagram with latitude closest to your site.Find the intersection of the two curves corresponding to the month and hour of interest.From this point, read solar altitude from scale at right and read solar azimuth from scale below. This is the suns position at that month and hour.11

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13SHADING DEVICESVertical devices, e.g. vertical louvres or projecting fins. These are characterised by horizontal shadow angles (HSA) and their shading mask will be of a sectoral shape. By convention HSA is measured from the direction of orientation (i.e. From the surface normal), positive in clockwise and negative in the anticlockwise direction. The HSA cannot be greater than 90 or less than 90, as that would indicate that the sun is behind the building.These devices may be symmetrical, with identical performance from left and right, or asymmetrical.They are most effective when the sun is towards one side of the direction the window is facing.We may distinguish the device HSA (as above) and the solar HSA, which is the required performance at a given time.14

15Horizontal devices, e.g. projecting eaves, a horizontal canopy or awning, or horizontal louvers and slats. These are characterized by a vertical shadow angle (VSA). One large or several small elements may give the same performance, the same vertical shadow angle. Their shading mask, constructed by using the shadow angle protractor , will be of a segmental shape. They are most effective when the sun is near-opposite to the window considered. Figure shows a canopy with a device VSA of 60.The solar VSA is the same as the ALT (altitude) only when the sun is directly opposite the window (when AZI = ORI, or solar HSA = 0). When the sun is to one side of the surface normal, its altitude must be projected onto a vertical plane perpendicular to the window.16

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18Egg-crate devices, e.g. concrete grille-blocks, metal grilles.These produce complex shading masks, combinations of the above two and cannot be characterized by a single angle.An example of this is shown in Figure.

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20Sizing Overhangs and FinsUse these equations to find starting dimensions for shading elements. Do the calculations to find:depth required for a shading element, orextent of shadow cast by a shading element with given depth.For each facade, select a critical month and time for shading. Suggested: south windows use September noon, east use September 10 am, west use September 3 pm, or ask mechanical engineer for estimate of peak cooling time in east, south, and west zones.Find solar altitude and azimuth for target month/hour from the sun path diagrams.

21Use the formulas below to size overhang, fin, or both. Results are a minimum starting point.If overhang is too big, try breaking it into several smaller elements or dropping part of it down for an equivalent depth.If sizing overhang for east or west window, you may notice that a fin must be added for adequate shading; otherwise overhang becomes unreasonably deep.Improvements: Extend ends of overhang wider than window or use a continuous element. Make overhang deeper or add another horizontal element part-way down the window. Add vertical elements to the scheme.22

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