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Green Building Bible, Fourth Edition
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Renewable Energy: Direction to point PV panels

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  1.  
    • CommentAuthorCWatters
    • CommentTimeDec 10th 2015
     
    I can see how you can optimise the output at say 3pm by pointing your panels at the sun at 3pm. But suppose you wanted to "maximise power generation in the afternoon". How do you work out which direction to point the panels? For example it wouldn't be as simple as pointing them midway between where the sun is at noon and dusk because the sun is stronger at noon than at dusk. So it should be more like SSW rather than SW.

    Is there a program that can do this?
  2.  
    site specific I guess. find the sun position mid summer at 3pm and point them at it ??
    • CommentAuthorEd Davies
    • CommentTimeDec 10th 2015
     
    Don't know of a specific program to do that. If you just want to maximize annual production after 12:00 each day you could try PVGIS and put in a horizon file which blocks the sun out completely in the morning.
    • CommentAuthorCWatters
    • CommentTimeDec 10th 2015
     
    Thanks for that.
    • CommentAuthortony
    • CommentTimeDec 10th 2015
     
    I would say SSW but do you want this in summer or winter as pitch angle can be varied too, likely only 10 % improvement possible
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 10th 2015
     
    Posted By: CWatters"maximise power generation in the afternoon"
    Do you mean maximum power at 3pm, or maximum generation, or yield, during the afternoon exclusively?

    I think you get the maximum yield, you just slice a sphere up at the appropriate angles for azimuth and altitude and sum the results. Then add in a negative number for the probability of cloud cover.
    • CommentAuthortony
    • CommentTimeDec 11th 2015
     
    Isn't the cloud cover number almost the same in all directions? So it cancels out
    • CommentAuthorskyewright
    • CommentTimeDec 11th 2015
     
    Posted By: Ed DaviesDon't know of a specific program to do that. If you just want to maximize annual production after 12:00 each day you could try PVGIS and put in a horizon file which blocks the sun out completely in the morning.

    &/or use the "Daily Radiation" tab on PVGIS for a range of azimuths, slopes & months (preferably with an accurate horizon file) to see what best suits CWatters' chosen definition of "maximise power generation in the afternoon"?
    • CommentAuthorsnyggapa
    • CommentTimeDec 11th 2015
     
    Posted By: tonyIsn't the cloud cover number almost the same in all directions? So it cancels out


    probably, but it may be different at different times of day - I don't know how clouds work, but if for example clouds were more common in the morning and got "burned off" by the afternoon then you may get an improvement by orientating the panels slightly more to the west to catch more afternoon cloud free time. or, you may not

    people like me were limited to where they could go anyhow, having only 1 suitable roof

    -Steve
    • CommentAuthorCWatters
    • CommentTimeDec 11th 2015
     
    Do you mean maximum power at 3pm, or maximum generation, or yield, during the afternoon exclusively?


    I was asking on behalf of someone else but I believe they meant generation/yield in the afternoon. As would happen if the panels were in full shade before midday.
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 11th 2015
     
    If by midday you mean solar noon, then you can just halve the yield.
    It then comes down to the best angles.

    You have to find out the azimuth angle that get sunlight for the most hours, so that will be SSW or around 200°.
    Then you need the altitude (or slope) angle to get the greatest yield.
    As the Earth varies by 23.5° and the UK is 50 to 58° North, you need to slope the modules somewhere between 27 and 35°.
    So somewhere around 31° is a good compromise. Which not far off what a roof is anyway.

    Play about in PVGIS for your location and see what comes out.

    I hate geometry.
    • CommentAuthorEd Davies
    • CommentTimeDec 12th 2015 edited
     
    Just been playing with PVGIS with a horizon file for this. The horizon starts at east and goes anti-clockwise in 360/number-of-points-given steps. The program then linearly interpolates between them. So I said horizon almost to the zenith for the NE quadrant and the SE quadrant like this:

    89
    89
    89
    89
    89
    89
    89
    89
    89
    0
    0
    0
    0
    0
    0
    0
    0
    0
    0
    0
    0
    0
    0
    0
    0
    0
    0
    89
    89
    89
    89
    89
    89
    89
    89
    89

    The optimum annual production, according to PVGIS, would be with the panels at 36° to the horizontal and 56° west of south (ie, bearing 236° clockwise from north).

    Setting the panels to due south (azimuth 0°) and just letting PVGIS optimize the slope it says the optimum is 23° inclination. This shows a flaw in this horizon scheme: it doesn't block out morning indirect light so it's putting the panels at a shallower slope to catch that.

    Hmmm, more thought needed.

    For the record, here are a few cases. PVGIS optimum first. Inclination, azimuth, annual average daily production and December average daily production.
    Inc Az  Year  Dec
    36  56  1.96  0.52
    60  20  1.72  0.59
    23   0  1.79  0.47
    50  10  1.74  0.57
    • CommentAuthortony
    • CommentTimeDec 12th 2015
     
    So steep is best in winter, you have almost WSW whereas I was thinking SSW.
    •  
      CommentAuthorSteamyTea
    • CommentTimeDec 12th 2015
     
    Ed
    Did you just pick one location for your PVGIS investigation, or did you pick your Somewhere in Leicestershire?
    We are at different ends of the country, how much difference to the slope could that make?
    • CommentAuthorEd Davies
    • CommentTimeDec 12th 2015 edited
     
    Tony, yep, steep is better in winter. Not that those numbers are necessarily winter optimum but just a bit better than year-round optimum. WSW is probably good for year-round but loses a lot in winter when the sun is basically south when it's enough above the horizon to be useful.

    We really need to know why we're optimizing afternoon generation. I suspect that something else is actually required.

    Steamy, It was somewhere on Otmoor, just to the east of Bicester. Arbitrary flat bit of ground in the middle of England though the flatness isn't awfully important given that I was supplying the horizon. Vague idea that CWatters might be around that area but might be confusing him with somebody of a similar name nothing to do with GBF.

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