DIFFERENT FORMS OF SUNPATH DIAGRAM AND THEIR USES IN

FUNCTIONAL DESIGN OF BUILDINGS

K K SRIRAMCE14B026

Not “sun-bath” its “Sun-path”

Definition-layman’s terms.

• Sun path refers to the apparent significant seasonal-and-hourly positional changes of the sun (and length of daylight) as the Earth rotates, and orbits around the sun.

• Sun-path diagram as the name suggests is something that is used to determine the location ,in the sky, of the sun at any point of time during the day, throughout the year.

Use in Functional Design

The most immediate use of a sun-path diagram is that the solar azimuth and the altitude can be read and hence the position can be exactly determined..

• The optimum position and orientation of various sunlight related equipment like solar heaters, ovens is known.

• By studying the sun-path diagram of a place one can identify the optimal orientation of solar panels in a given building.

• By identifying the solar-windows of a particular location, one can design the building such that there is maximum utility of the solar energy by placing thermal mass required for indirect heat gain in the right orientation.

• By tracing down the surface area illuminated, to the greatest extent, by the sun, the location of clerestories and fenestrations of a building can be optimised so that the thermal and visual comfort levels in the building are met.

• The shading devices also can be designed similarly.

• Coming to one of the most important uses, the location of the house so that it receives the maximum shade during summer can be obtained using softwares like autodesk ecotect

Different forms

• For all those who are wondering how a sunpath diagram looks like…

• Wait, this is the 3-D view, how does this look like on paper?

• So people came up with methods to represent it on paper which define the basis for characterization of sun-path diagrams.

• The most popular 2 representations are: polar and cartesian.

Polar sun-path diagrams

What is a polar sun-path diagram??

• Imagine somebody lying on the ground facing the sky and starts taking photographs of the sky all along the day, throughout the year using a fish lens camera.

• All these photographs superimposed forms a polar sun-path diagram.

How does somebody even read that?

• Azimuthal lines: Azimuth angles run around the edge of the diagram in 15° increments. A

point's azimuth from the reference position is measured in a clockwise direction from True North on the horizontal plane. True North on the stereographic diagram is the positive Y axis (straight up) and is marked with an N.

• Altitude lines:Altitude angles are represented as concentric circular dotted lines that

run from the centre of the diagram out, in 10° increments from 90° to 0°. A point's altitude from the reference position is measured from the horizontal plane up.

• Date and month lines:Date lines represent the path of the sun through the sky on one particular day

of the year. They start on the eastern side of the graph and run to the western side. There are twelve of these lines shown, for the 1st day of each month. The first six months are shown as solid lines (Jan-Jun) whilst the last six months are shown as dotted (Jul-Dec), to allow a clear distinction even though the path of the Sun is cyclical.

• Hour Lines: Hour lines represent the position of the sun at a specific hour of the day,

throughout the year. They are shown as figure-8 style lines that intersect the date lines. The intersection points between date and hour lines gives the position of the sun. Half of each hour line is shown as dotted, to indicate that this is during the latter six months of the year.

• Step 1 - Locate the required hour line on the diagram.• Step 2 - Locate the required date line, remembering that solid are

used for Jan-Jun and dotted lines for Jul-Dec.• Step 3 - Find the intersection point of the hour and date lines.

Remember to intersect solid with solid and dotted with dotted lines.• Step 4 - Draw a line from the very centre of the diagram, through the

intersection point, out to the perimeter of the diagram.• Step 5 - Read the azimuth as an angle taken clockwise from North. • Step 6 - Trace a concentric circle around from the intersection point

to the vertical North axis, on which is displayed the altitude angles.• Step 7 - Interpolate between the concentric circle lines to find the

altitude.

Classification of Polar representation

• Depending on the scale of the altitude circles, the projections are again classified into 3 types:

Spherical , Equidistant and Stereographic.

Spherical projection

• In this method, the radial distance from the centre is simply the cosine of the altitude angle. As shown above, the relative change in radius between 75° and 90° is very much greater than between 15° and 0°.

• Uses : This makes such diagrams very good for considering overhead

shading or very tall surrounding buildings.

Equidistant projection

• Using this method the radial distance is simply a linear factor of the altitude angle. Thus the relative change in radius between all angles is the same.

• Uses: There is no bias towards either the zenith or the horizon.

Stereographic projection

• This is a more complex projection in which azimuth lines are first projected back to a reference point located a distance of 1 radius beneath the circle centre. The point where each of these lines intersects the zero axis gives the radial distance.

• Uses: The primary advantage of this method is that it increases the resolution of the diagram at low solar altitudes making it more suitable for the majority of surrounding building overshadowing situations.

Cartesian sun-path diagram

• Similar to the polar sun-path diagrams, somebody takes pictures of the sky and starts collecting the data regarding the position of the sun quantized by the azimuthal and altitude of the sun.

• Now the azimuthal values are plotted along the X-axis and the altitude values are plotted along the Y-axis for different parts of the day and throughout the year.

Reading the cartesian sun-path diagrams.

• Step 1 - Locate the required hour line on the diagram (similar to that in polar).

• Step 2 - Locate the required date line, remembering that solid are used for Jan-Jun and dotted lines for Jul-Dec. In these diagrams, the highest altitude line at noon is always in midsummer (either 1st July or 1st Jan, depending on hemisphere). Each other line represents the 1st of each month, solid Jan-Jun, dotted Jul-Dec.

• Step 3 - Find the intersection point of the hour and date lines. Remember to intersect solid with solid and dotted with dotted lines.

• Step 4 - The azimuth is given by reading off the horizontal axis. • Step 5 - The altitude is given by reading off the vertical axis.

Classification of cartesian representation

• Not all parts of the sky contribute equally to the daylight that arrives at a surface or passes through a window. Different sky conditions can result in variations in the distribution of luminance over the sky dome, with some areas brighter or darker than others. For a flat surface or a window, light from some parts of the sky will arrive at normal incidence whilst others will arrive at almost grazing incidence, with the cosine law determining their relative contribution.

• These effects can be accommodated within cartesian sun-path diagrams by applying non-linear scales to one or both of the axis. This can be done solely for visualization purposes or, with the addition of distributed points or a grid of squares, used as a basis for manually taking off measurements for day-lighting potential or overshadowing effect.

• Those well modified implemented diagrams are called Waldram diagrams.

Thank you.