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Solar declination / solar panel array design

  1. Jun 28, 2010 #1
    I need some help, I am planning on installing a 10 kilowatt solar panel array in the next couple of months.

    I will not be using a solar tracking device. The plan thus far is to determine the solar south bearing and face the panels in that direction and work out an appropriate angle for the panels.

    I have done a bit of research and read a couple forums all ready. It is my understanding that the declination is relative to geographic location. The location is Iroquois, Ontario on the St. Lawrence river. 44°55′00″N 75°16′00″W.

    From this table,


    I declination is listed as 23 degrees and 19 minutes today and from this calculator,


    I get a solar declination for my area of 23.24 degrees. Does this mean that in the solar south direction the sun will always be 23.24 degrees from the plane of the ground?

    Once i understand declination completely and can determine the max and min angles I can determine the lengths of shadows that the tree lines around the lot will cast and the location and length of the shadows from a water tower which is south of the lot. I want to locate the array in an area where the tree line and water tower will never cast shadows on the panels. If possible.

    I also need help with declination so I can determine the best angle to adjust the panels to during the seasons or summer or winter solstice.
    Last edited by a moderator: Apr 25, 2017
  2. jcsd
  3. Jun 29, 2010 #2

    Filip Larsen

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    Gold Member

    I think you got some of the measures mixed up, so just to make it clear: declination is the "latitude" of an object in the sky as measured in the equatorial system and it is (ignoring small small variations like parallax) equal for all earthly observers. Given the declination of the sun for some date, you need to determine the (angular) altitude of the sun for your geographic latitude, which will tell you how high over your horizon the sun will be at noon. If you like, you can read a bit on these and other coordinate systems in [1] and [2].

    Thankfully, even though change of coordinate system is complicated if you want the precise position, it all boils down to a simple formula when you are only interested in the daily maximum altitude:

    max altitude = sun's declination + 90 - geographic latitude

    where all the measures are in degrees. For instance, from the table you linked to you can see the sun's declination today is around +23,3 deg and your latitude is around +44,9 deg so the suns altitude over your horizon at noon will be around

    +23,3 deg + 90 deg - 44,9 deg = 68,4 deg.

    If you look at it over the whole year, you can see the sun has minimum declination of around -23,4 deg in December (winter solstice) and maximum declination +23,4 deg in June (summer solstice) which means the sun for you will vary in altitude from 21,7 deg in the winter to 68,5 deg in the summer.

    By the way, the mean altitude (when sun declination is 0 deg around the equinoxes in March and September) will be "90 deg - latitude" so that is around 45,1 deg for your latitude. Aiming for that angle on your panel will mean the sun will be at a plus/minus 23,4 deg in the vertical at noon all year around which again mean that the peak power will be down to 92% in summer and winter compared to if you could aim it spot on. If you aim the panel to near the lowest altitude of the sun you should get a bit more power in the winter at the cost of lowering the power in the summer down to 68% (everything else being equal).

    If you want to be very precise, you should also note that the sun is not exactly south when its noon, but may shift up to around 4 deg west or east over the course of a year, details in [3]. This is mostly just for information, as I don't think this should have any noticeable effect on your heating compared to other much more significant effects like weather, clouds, tree shadows, material, etc.

    [1] http://en.wikipedia.org/wiki/Celestial_coordinate_system
    [2] http://en.wikipedia.org/wiki/Horizontal_coordinate_system
    [3] http://en.wikipedia.org/wiki/Equation_of_time
    Last edited: Jun 29, 2010
  4. Jun 29, 2010 #3
    Thanks for your response Filip Larson. That really cleared a couple of things up for me. I asked a friend in a renewable energy class and he said declination was the angle of the sun in the sky so thats what must have confused me.

    I am going to use some of this information you with to do some more reading and some calculation and I might be posting back in a couple of days If I have any more questions.
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