Shading Question

LHawesLHawes Solar Expert Posts: 46
I have a flat roof and would like set up 3 or 4 rows of panels tilted at around 32 degrees for my San Diego location but I realize each row will shade the row behind it. What I'd like to do is figure out is, by how much? How can I figure the distance that the shadow will be cast behind one row so I may place my second row out of that shadow?

Thanks in advance

Larry

Comments

  • icarusicarus Solar Expert Posts: 5,399 ✭✭✭✭
    Re: Shading Question

    Don't let your panels be the cause of any shading on any other panels in your array.

    Even a little bit of shading of even one panel will dramatically cut the out put of the entire series string. It would be like throwing good money away! You would be way better to set them up flat to avoid shading,,, in most cases!

    Now,, you should be able to do some calcs using the sun angle at various times of the day/year and the size of the panels to determine what/where/when shading will occur.

    I'm sure there are computer models out there,, but you could do it with pencil, paper, ruler (scale) and protractor. Draw out a scale drawing of your roof in cross section, add the know sun angle(s) and play with panel orientation at various times of the day/year (sun angle)

    Tony
  • LHawesLHawes Solar Expert Posts: 46
    Re: Shading Question

    Thanks for the reply Tony and your advice is why I would like to know the details. I thought there might be a 'computer model' out there that would supply the info but perhaps the only way is to set up a model as you suggest? I'd like it to be very accurate for the reasons you stated, so I guess the frst place to look is sun angles throughout various times of day and year?
  • Solar GuppySolar Guppy Solar Expert Posts: 1,962 ✭✭✭
    Re: Shading Question

    As a very general rule, I use 2X the panel length as the needed row spacing .. The higher the panel is over the mounting plane, the more distance needed.

    This is just a ballpark calculation, if you can't get that spacing your going to have to compromise on the angle.
  • LHawesLHawes Solar Expert Posts: 46
    Re: Shading Question

    I think I'm on the right track but would like to see if there any warts on my figuring. The original panel is 62" (5' - 2") long and when set on a 30 degree angle it casts a vertical shadow 55" (4' - 7") long. With the winter solstice angle of 26.29 degrees it looks like the next row of panels will have to be placed 5'4" away. Does this look OK?


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  • Solar GuppySolar Guppy Solar Expert Posts: 1,962 ✭✭✭
    Re: Shading Question

    That's the Sun Angle at Solar noon ... so you will be partially shaded most of the day.

    You are on the right track, but do the calculation each hour of the day to see when shading happens
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Shading Question

    it isn't just the angle of the pvs that's 32 degrees as that is your latitude too and the pv angle is ok as a compromise. that 32 degrees means the sun is 90-32=58 degrees overhead on average. during the winter the sun is about 23.5 degrees lower in the sky or 34.5 degrees for solar noon during the winter solstice. any other time of day during that time of the year the sun will be even lower in the sky and cause shading on the pvs. this may be difficult to keep the shadows off of the pvs with not only spacing them apart, but also raising the rear pvs and doing both is the best option you'll have imo. it has been awhile since i've done math with triangles, but if nobody else pipes in on that in a few days or you can't figure it out i may take a stab at it, but in any case the number of pvs (state what pvs they are) in how many rows are wanted and give the dimensions of the roof that they can fit into.
    btw, i don't recommend that you keep your pvs flat to avoid the shadowing as you will reap very little power from them by doing so, especially in the winter. that may suit somebody with an rv with only 1 or 2 pvs, but not for a larger home system. you have other options somebody with an rv may not want to do.
  • icarusicarus Solar Expert Posts: 5,399 ✭✭✭✭
    Re: Shading Question

    Neil,

    My point was given a choice between significant shading of a series array or flat,,, you'd have to consider flat as an option. Depends on a lot on latitude ,, closer to the equator the less important tilt is.

    T
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Shading Question

    no problem tony, but i wanted him to be sure that it would not be a good idea to normally do that.
    "closer to the equator the less important tilt is"
    it could be important in a way for the sun rises from the east and goes straight overhead and then falls straight to the west. there's no southern aiming. flat might be a real consideration there as an economic compromise, but you could lose a great deal from the straight line the sun follows. i picture a tracking system that would just tilt the pvs, like over the tip of and centered on a triangular mount. best would be 3 positions. i'd guess 45 degrees pointed east to flat to 45 degrees pointed west. tracking might be misleading too as i'd picture some kind of timer, rather that the devices we see now out there, that would energize an arm to move the pvs. i know that would need more thought on getting that to work, but it would be more viable for equatorial zones than trackers we are familiar with.
  • icarusicarus Solar Expert Posts: 5,399 ✭✭✭✭
    Re: Shading Question

    Neil,

    In the other extreme, I'm working with some folks who live at ~69degrees N. Now you have really consider tilt AND tracking. The long days make for some interesting calcs.

    Tony
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Shading Question

    i've seen some pics of a setup inside the arctic circle that just put pvs flat against the side of a tower and are facing all 3 directions of the 3 legged tower. that gets expensive, but saves the troubles encountered in tracking as well as maintenance. night time is a [email protected]^ch way up there for solar though.:cry:
  • LHawesLHawes Solar Expert Posts: 46
    Re: Shading Question

    Thanks for all the responses. I've attached a pic that I hope will help. I have a flat garage that is a little over 20' 6" x 16' plus an alley between the garage and the fence line that can be used and is around 6' wide. I'd would like to maximize the location and tilt of some Sharp 165's that measure 62" x 32.5". I have found that if I tilt them at 30 degrees the distance between panels becomes so great that I can fit MAYBE 16 panels and we have a total of 30 we'd like to implement. We have more room on the house roof, which is also pretty flat, but we'd like to maximize the roof area.

    If we lower the angle to 15 degrees we can fit perhaps 24 panels with a double row over the alley way but am worried about the low pitch for winter sun. Any other thoughts?

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  • mike95490mike95490 Solar Expert Posts: 8,503 ✭✭✭✭✭
    Re: Shading Question

    What is the extra cost of the fancy angled mounts ? I just paved a section of my west faceing roof with more panels to make up for the lame aim.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • LHawesLHawes Solar Expert Posts: 46
    Re: Shading Question

    I don't have them priced exactly but it will be close to $1000 - $1500 for materials unless we build aluminum mounting brackets ourselves.
  • solarteksolartek Solar Expert Posts: 69 ✭✭✭✭
    Re: Shading Question
    LHawes wrote: »
    Thanks for the reply Tony and your advice is why I would like to know the details. I thought there might be a 'computer model' out there that would supply the info but perhaps the only way is to set up a model as you suggest? I'd like it to be very accurate for the reasons you stated, so I guess the frst place to look is sun angles throughout various times of day and year?

    Go download a free copy of Google Sketchup. You can draw the panels and set their angles and inter-row spacing. You can input a latitude and longitude for your location and then use the shading tool do perform a pretty decent analysis including time of day and year.

    Scott.
  • LHawesLHawes Solar Expert Posts: 46
    Re: Shading Question
    solartek wrote: »
    Go download a free copy of Google Sketchup. You can draw the panels and set their angles and inter-row spacing. You can input a latitude and longitude for your location and then use the shading tool do perform a pretty decent analysis including time of day and year.

    Scott.

    That's a great idea. I've got Sketchup I'll try it!
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Shading Question

    well i looked it over somewhat and it's not looking promising. firstly, let me correct you on the the space you indicate the pv will take up. you list it as 1' 7" and what i get is 27.56" or 2' 3.56". the shadow at solar noon for winter solstice will be 25.06" if i figured that part right. note that this will be shaded at any other time of the day around the solstice. adding them together gives nearly 53" per row with shadow. that'll fit 3 rows +1 doubled so a total of 4 can fit. if you were to raise each successive row up at least 8.61" then you can fit 4 rows with an extra piggybacked to the last one for a total of 5. the way i figured it at the winter solstice with it raised like i said more than half of the time will have shading. this is one row short of your goal and will have some shading at the lower points of sunrise and sunset at all times of the year no matter what.
    reducing the angle to 15 degrees may allow more to fit(didn't make any calculations) at the expense of efficiency for being miss aimed, especially during the winter months. that many pvs in that space is a lose-lose situation. i'd also wonder what your city or town will say about pvs hanging over an alley?
    what did you come up with on that google sketch?
  • LHawesLHawes Solar Expert Posts: 46
    Re: Shading Question

    Thanks so much for the follow up. I think our numbers are as close as they can be using two different brains and methods. I've attached the Sketchup graphic and my architectural mock-up which coincide pretty closely with the numbers I got.

    The architectural program (Chief Architect) is pretty accurate though I'm sure we'll have to make some real world adjustments.

    We've decided to install 4 rows of 4 on the roof, 1 row over the alley for a toal of 20 panels (which is actually a side yard that should be no problem with the city) and 2 rows of 5 on the main house. We were trying to avoid the main house because of its distance from the inverter but it's a pretty good location and will take the stress out of trying to stuff all those panels on the garage.

    Angles and exposures are still not ideal but the homeowner is willing to make the compromises to get all 30 panels located.

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  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Shading Question

    i guess i was close enough for government work, eh?:D let us know how well that works out.
    ps. i downloaded that program, but have never worked with it so i don't know what i'm basically doing with it. if one of you guys could explain enough to get me started in using that program it would be appreciated. i was going to ask for it in a pm, but decided here is better to allow more dummies like me to use it.
  • LHawesLHawes Solar Expert Posts: 46
    Re: Shading Question
    niel wrote: »
    i guess i was close enough for government work, eh?:D let us know how well that works out.
    ps. i downloaded that program, but have never worked with it so i don't know what i'm basically doing with it. if one of you guys could explain enough to get me started in using that program it would be appreciated. i was going to ask for it in a pm, but decided here is better to allow more dummies like me to use it.

    The best way by far to learn SketchUp is to use their on line tutorials. It's not a hard program to learn the basics but like anything there's a curve that can eat some time.

    For that simple drawing I did you draw a square using the square tool and type in your dimensions as you draw - 32.5 comma 62 and press enter. You now have a 32.5 x 62" flat 2 dimensional box. Then they have a push pull tool that allows you to add a third dimension by pulling up the flat box. If you type in 1.8 and press enter while pulling you will end up with a 32.5 x 62 x 1.8" cube, or a Sharp 165 Solar panel.

    Hit the protractor to set your angle, which is really kinda tricky and I do it a few times till it works. Set your location and sun time of day - viola. Not quite as easy as I've describe but not too brutal to learn and I think it might really come in handy for answering questions just like this one.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Shading Question

    thanks for the quick rundown on it. i won't be getting to it soon to try it out, but if i remember i'll get to it in a week or so.
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