Yesterday was the autumn equinox, which officially marked the end of summer. The day also has unique meaning for designers of solar energy systems. Why, you ask? The sun’s path through the sky is predictable. If you know your location (lat/long), and the time of year, it’s possible to predict with considerable precision where the sun will be at a given moment in the day. Equipped with an understanding of the sun’s daily and seasonal movement, a solar system designer works to position solar panels for maximum exposure to sunlight. This means at least two things, each of which relates to the equinox:

(1) Angle of inclination

All else equal, your typical solar panel absorbs the most sunlight — and therefore produces the most electricity — when it’s oriented perpendicular to the solar resource. But, as just noted, the sun is a moving target. More specifically, the altitude of its path changes throughout the year. We all know, for instance, that in winter the sun doesn’t get nearly as high in the sky at midday as it does in the summer. This disparity is particularly pronounced at higher latitudes — as anyone who has visited Alaska can attest.

So, if the sun’s altitude varies seasonally, how can we position the panels to be perpendicular? There are two answers. One is to literally re-position the panels throughout the year (and even throughout the day). Tracking devices enable just this, improving exposure at all times and thus maximizing performance. Such systems are expensive, however, so much so that small- and medium-scale solar systems rarely have tracking functionality.

The second answer is to forgo the fancy tracking system and instead shoot for an average angle of inclination that captures the bulk of sunlight throughout the year. The most straightforward way of doing this is to incline the panels at a angle that is equal to the project site’s latitude (in degrees). Such an approach aims for consistent system performance throughout the entire year. How?

The sun will achieve its highest position in the sky on June 21 (the summer solstice), and its lowest position on December 21 (the winter solstice). On the spring and fall equinoxes, March 21 and September 21, respectively, the sun’s path will be exactly between these two extremes. See the following diagram of the “solar window,” from the National Renewable Energy Lab.

By inclining the panels at the site’s latitude (give or take a few degrees), the designer is, in effect, trying to approximate a perpendicular orientation throughout most of the year. If the panels are put at an angle steeper that the latitude, they’ll be better tuned for winter months; at a smaller angle, they’ll be better tuned for summer months.

(2) Shading

Notice something in the diagram above? (Hint: they grow in the ground and are made of wood.) That’s right — trees. Shading can result from any nearby, tall object — buildings, radio towers — but trees are common culprits. Excessive shading can be a project killer, because solar PV systems need clear exposure to the sun, particularly during midday.

When a solar designer/installer comes to your home or business, he or she will typically come equipped with a device (like this one, for example) to determine where, and when, shading will occur. After taking some measurements, they’ll be able to determine: (1) the best location, relative to trees and adjacent buildings, for your solar system, and (2) how much annual sunlight, in percentage points, will be lost to shading. If that figure is too high, your project is a non-starter — and the installer will probably tell you as much.

As you can see, the system in the diagram above would experience shading in the afternoon, on/around the winter solstice (December 21). Otherwise, there appears to be plenty of unobstructed exposure. A solar panel system in this particular example would likely be viable option.

Those were my thoughts for yesterday, the fall equinox. If you’ve got any questions on any of the above, don’t hesitate to post them here.

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