Calculate Sun Position for Solar Gain

Understanding sun positioning throughout the year is necessary to maximize solar gain from your panels.


| February 2016



Solar panels

In full sun, about 100 watts of solar energy per square foot reaches the Earth’s surface.


Photo by Fotolia/emieldelange

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On-Site Solar

When calculating solar energy, how much is available is determined by your position on the Earth and cloud cover.  There are tables that will tell you how many sunny days per year in your area.  Further calculations will tell you exactly how many hours of sunshine are available per year, when factoring in the sunrise and sunset times.  Next you must reduce this further by the angle that your solar collectors, either solar-electric or solar hot water, face towards the sun. There is an instrument, similar to a surveyor’s azimuth, called a Solar Pathfinder that will help you orient the collectors correctly.

The nominal angle for solar collectors is equal to your site’s latitude.  However, due to the tilt of the Earth’s axis, during the year the vertical angle to face the sun will change by plus and minus 23.5 degrees.  For example, if your site is located at 45 degrees latitude, then on December 21st (winter solstice) the angle to the sun will be 21.5 degrees and on Jun 21st (summer solstice) it will be 66.5 degrees, only on March 21st and September 21st (spring and fall equinoxes) will the angle be match the latitude (in this example 45 degrees).  That is a range of 47 degrees up and down throughout the year.  East and west, the solar azimuth varies 180 degrees each day during the equinoxes, and varies from non-existent to always on at the Poles and some where in-between for all the other latitudes during the solstices.

As the sun’s vertical angle changes with the seasons, its azimuth angle changes with the time of day.  The bad news is that when a solar collector is disoriented from the sun by 45 degrees either east or west or up or down, the energy received is 25% less.  Further, energy will be reduced by nearby structures or trees that overshadow the collector.

A sun tracking mechanism can be used to maximize the energy collected from solar collectors.  The benefit in energy gained by optimum sun tracking has to be weighed against its maintenance and the power to operate it.  Manual tracking is too labor intensive to capture all variations of the sun angle per the time of day, but it does minimize the loss per the seasonal angle changes.  A simpler idea is to hinge the collectors and manually adjust the angle weekly as the season changes.

peter
2/15/2016 10:34:48 PM

Use http://pvwatts.nrel.gov It takes about 45 seconds to predict (reasonably close) how much energy a solar system anywhere in the country will produce.


peter
2/15/2016 10:34:31 PM

Use http://pvwatts.nrel.gov It takes about 45 seconds to predict (reasonably close) how much energy a solar system anywhere in the country will produce.






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