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Stone Marmot Goes Solar: Part 3, Shading

By Sid of Stone Marmot

Dec. 6, 2009

As stated in my last article, our house has a lot of shade to the east and west but little to the south. Why does this matter?

Contrary to what some may believe (like whoever installed the City of St. Petersburg's solar powered parking meters), these photovoltaic (PV) panels are SOLAR panels, not SKY panels or UP panels. They won't work too well unless they are directly and completely exposed to the sun, not just facing the sky or facing up. Even if a small corner of a panel is shaded, that panel will have reduced output, if it isn't completely shut down. And when that panel is part of an array of panels, if that panel has reduced output or is completely shut down, the entire array that this panel is part of will also have reduced output. The most common exception to this case, which still isn't too common, is when there is some isolation means, such as a diode in each series string of panels, which are usually referred to as blocking diodes. But these isolation means often introduce some significant losses to the system, so they are rarely included in modern PV arrays.

Virtually all larger modern PV panels have bypass diodes in them. These bypass diodes allow the shaded portion of the panel to be bypassed, possibly allowing the panel to output some energy, depending upon how much of the panel is shaded. But the output is still reduced by the shaded section. If it is reduced below the control range of the inverter, the inverter will operate at a far lower efficiency and may even go offline to protect itself and anything it is connected to. So these bypass diodes don't solve the problem of shading.

I'm talking above about hard shading, such as from a building, mountain, densely leafed tree or shrub, etc. Diffuse shade, such as from a power line located 40 feet from the array, will frequently have little impact on the array output, though this impact will increase with the size of the diffuse shading object, the number of these objects, and how close they are to the PV panels.

But hard shade can come from things you may not think of, such as chimneys, plumbing roof vent pipes, a big wet leaf, a big blotch of bird doo-doo, etc. And some diffuse shade generators, such as lots of dust or salt spray residue to the point of being very visible, can also significantly reduce the output of these PV panels. If you get fairly regular rains in your area, Mother Nature may take care of many of these potential environmentally caused problems for you. If you don't get regular rains or if you notice a sudden drop in PV output, you may need to wash down your panels occasionally to maintain peak performance. But you do need to consider chimneys and such in the placement of your panels.

The “rule of thumb” in determining the impact of shading is that you want the panels to be totally exposed to the sun from 9 AM to 3 PM. That way you will get at least 80 % of the solar energy available to your site during a typical day. But like most “rules of thumb,” this is rather simplistic and has lots of exceptions.

For example, this assumes that solar noon, that is, when the sun is at its highest point in the sky, is at 12 noon on the clock. This is true in the winter in the US when we are on Standard Time. But when we are on Daylight Saving Time, this all shifts by one hour since the sun is now at its highest point around 1 PM. After all, if the change from Standard Time to Daylight Saving Time shifts the sunrises and sunsets by one hour, wouldn't solar noon also change by one hour?

This rule of thumb also assumes you are in the center of your time zone and that center is located at a multiple of 15 degrees from zero degrees longitude, for example, 75 degrees for the US Eastern Time Zone, 90 degrees for the US Central Time Zone, etc. Few major US cities are located at a multiple of 15 degrees from zero degrees longitude. For every degree you are located east or west of this multiple of 15 degrees from zero degrees longitude, you have to subtract or add, respectively, 4 minutes to the time zone noon to find solar noon for your location. For example, Clearwater, Florida, where I live, is located at about 82 degrees, 50 minutes longitude. This is almost 8 degrees west of the expected center of the Eastern Time Zone, which is 75 degrees. So solar noon occurs over a half hour later, or about 12:30 PM Standard Time and 1:30 PM Daylight Saving Time, here in Clearwater than it would in Philadelphia, which is located almost in the middle of the time zone.

Why does all this matter? If you live in Clearwater, Florida, it is August (Daylight Saving Time), and you are trying to see how much shade impacts your potential solar installation, you would not be checking to assure you have no shade problems from 9 AM to 3 PM, but from about 10:30 AM to 4:30 PM. This is a big difference in this case.

Note that since the earth has an elliptical orbit around the sun, the rate at which it goes around the sun changes throughout the year. This changes solar noon up to an additional plus or minus 17 minutes. See http://www.idialstars.com/eot.htm for more information on this variation if you want to account for it.

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