Sun Charts: Projections of Solar Events and Shadowing from the Sky Dome
The emphasis of this lesson is the Sun Chart tool (or Sun Path). These flat diagrams are found in many solar design tools, but may look completely foreign to the new student in solar energy. How do we interpret the arcs and points plotted on a sun chart? Why do we have two different types of plots (one looks like a rectangle, and one looks like a circle)? Why do some plots go from 0-360°, while others go from -180° to +180°?
What are Sun Charts?
If we want to visually convert our observations of the sky-dome onto a two-dimensional medium, we can either use an orthographic projection or a spherical projection on a polar chart. These projections are useful for calculating established times of solar availability or shadowing for a given point of solar collection.
The Sun Path describes the arc of the sun across the sky in relation to an earth-bound observer at a given latitude and time.
All light incident upon Earth's surface must pass through the atmosphere and be attenuated (lost from absorption or back scattering). In order to simplify the many points of origin of light, we divide the sky and the Earth's surface into components, or spatial blocks of an imaginary hemispherical projection on the sky. The Sky Dome refers to the sum of the components for the entire sky from horizon to zenith, and in all azimuthal directions. In our following sections, a collecting surface is assumed to be horizontal first, as a pyranometer measuring device is mounted horizontally and facing the sky to measure the Global Irradiance/Irradiation in the shortwave band for the sky dome. Most of our solar collectors will be tilted up from horizontal in some way (PV, solar hot water, windows, walls, even your eyes). Those surfaces oriented otherwise are termed a Plane of Array measurement (POA), requiring specific tilt and azimuth information in the description. For those solar collecting surfaces that are not horizontal, the reflectance of the ground is an additional source of light, through the albedo effect. The beam, sky diffuse, and ground diffuse light sources incident upon the tilted collector are estimated using models of light source components.
The sky dome can be projected onto flat surfaces for analysis of shading and sky component behavior.
- Orthographic Projection: takes the sky dome and projects altitude and azimuth values outward onto a surrounding vertical cylinder. The cylinder is then opened flat. Figure 2.16, below, shows the sun rising in the East ( to the left) and setting in the West ( to the right). Proper observation shows that the largest arc in the chart at the top, June 21, is the Northern Solstice, while the smallest, December 21, is the Southern Solstice.
Credit: Jeffrey R. S. Brownson
- Polar Projection: takes the sky dome and projects altitude and azimuth values down onto a circular plane. However, in the polar projection, the arc for December 21 is at the top while the arc for June 21 is at the bottom. This happens because we are effectively lying on the ground with our heads facing south, and holding that large piece of paper straight up to the sky.
Credit: Jeffrey R. S. Brownson
How do We Make and Read Sun Charts?
Go to the University of Oregon Solar Radiation Monitoring Laboratory website. The scientists at the site have provided an excellent tool for plotting sun paths onto orthographic projections or polar/spherical projections. The default page is for creating an orthographic projection of your site of interest. The alternate page for polar projections will use the same data you can input, but will output the alternate form. Note that both use the meteorological standard for azimuth angles, where North is set at 0°, increasing clockwise to 360°.
- Specify the location by and . (Latitude is important for our calculations of sun-observer angles).
- Specify the time zone (software does the correction from UTC to local time zone).
- Choose data to be plotted (Choose to plot hours in local solar time: default). These plots are symmetrical for half of the year when you plot them in arcs of solar time. Hence, when you plot in solar time, "Plot dates 30 or 31 days apart, between solstices, December through June" will look the same as "Plot dates 30 or 31 days apart, between solstices, June through December." Side note: if you do plot in "local standard time," you will observe half of an analemma each hour, where only the Equation of Time ($E_t$) has not been corrected for in the time correction.
- Set chart format parameters: These are your choice to personalize the output file. Be creative, and try to present clear data visualization.
- Choose file format for chart. (I prefer the PDF for working.)
- Enter the code to make sure you are not a web bot roaming about.
- Download the image, print it out, and use it for shading analysis!
What about Shadows?
When designing a solar energy conversion system for any application, we must pay special attention to the occurrence of shadows throughout the year. We discuss a method to assess the shading using 2-D projections.
The next page gives you an opportunity to print and analyze your own sun chart.