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E/W fixed azymuth PV panels: output power curve during the day?

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  • E/W fixed azymuth PV panels: output power curve during the day?

    Hello,

    What happens if I mount some PV panels with fixed azimuth (east/west, have not decided yet)? What should be their output power curve during the day?

    For example, If I mount them toward east there is a maximum output power in the morning. The question is, I'll still got some power (and how much, in percent) in the afternoon?

    It's an off-grid installation thus I'm not interested in annual average production but the daily output.
    Last edited by santaclaws; 03-26-2016, 06:53 PM.

  • #2
    The thing that determines the output power of the panel is the strength of the incoming sunlight (insolation) and the angle at which it strikes the panels.
    Away from solar noon the sun is lower in the sky with more atmosphere, dust and weather in the way, so the insolation may be lower.
    And when the sun is coming at an angle to the panel surface the output is lower.
    If you point a panel (azimuth and elevation) so that it points directly at the sun at solar noon you will get the maximum possible output at that time of day.
    If you aim it higher or lower (roof tilt or angle of ground mount) you will get less power at noon. But if you aim it lower you may get slightly more power in morning and evening when the sun is lower in the sky.

    For your area there may be weather conditions that make either morning or evening worse in terms of insolation, so pointing the panel in the "bad" direction can reduce your output a lot compared to the ideal position.
    You can find out exactly what the effect of angle on daily production is for your area (leaving out considerations of local shade) by plugging different panel angles into PVWatts.

    For a panel located near the equator, a single axis tilt with the axis perpendicular to the path of the sun will do much better than a single axis that is set vertically and scans from east to west in a circle.

    Your power in the afternoon will be close to zero once you reach the point where the sun is coming in parallel to the panel surface. It will get very small before that.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

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    • #3
      And in case you didn't know, pvwatts is a free online tool at http://pvwatts.nrel.gov/

      Mounting half your array pointing east, and half pointing west, might make sense if you wanted to maximize the number of hours with significant power.

      Half of my array points south, and half points west, so I get a goodly amount of power in the afternoon, and not as much in the morning. Seems like a good compromise.

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      • #4
        Originally posted by DanKegel View Post
        And in case you didn't know, pvwatts is a free online tool at http://pvwatts.nrel.gov/

        Mounting half your array pointing east, and half pointing west, might make sense if you wanted to maximize the number of hours with significant power.

        Half of my array points south, and half points west, so I get a goodly amount of power in the afternoon, and not as much in the morning. Seems like a good compromise.

        One of the advantages of splitting your panel count into subarrays this way is that you can often get a higher total production using a small GTI or CC that you would need if all of the panels were pointed in the same direction.

        If your off-grid lifestyle includes some large loads during the day, you have more freedom to schedule them to the panel production time if you spread your window out as Dan describes. That could mean less total stress on your battery bank.
        SunnyBoy 3000 US, 18 BP Solar 175B panels.

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        • #5
          Thanks a lot for your input.

          I already have 8 x 230W PV panels pointed south and I want to add 8 x 260W more.

          They are mounted on a flat roof thus I could set any azimuth/tilt I want. During sunny days (especially in the summer), I got more than enough power at noon thus there's no reason to add more pointed south panels anyway (btw, I live in Europe / northern hemisphere).

          As you have suggested, I guess it's better to split the new panels (4 east / 4 west), to extend the battery bank charging time. Usually, the bulk/absorb charging is ended at noon but there isn't always enough time (sun) to fully EQ charge them on a daily basis.

          Btw, I have a 48V/500A FLA battery bank and I want to protect them as much as possible by lower DOD cycles.

          Anyway, seems like the east pointed panels are useless after noon (and the west pointed ones before noon). It's somehow uncomfortable to think that they are standing in the sun doing nothing half of the time.

          I might add only 4 new panels and use an automatic positioning system to change the azimuth during the day but I guess the cost will be the same (and a moving hardware is more prone to failure).

          Using that great tool (PVwatts), seems like I'll get the same annual energy production using 4 panels pointed to south or 8 spilt panels (4E/4W). This sounds logical enough, but I wonder how much is the charging time extended if I use this split system. May I get +2 hours or so?

          It would have been great if that online tool had the posibility to set the time of the day for the actual output power. But, like you've just said, the sun rays are parallel with the east pointed panels surface in the afternoon thus there's no useful power available.

          Btw, it's only the ambient light that matters in this situation. May I get at least 10% of the panel rated power or something?

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          • #6
            For that matter, could it be better to use a SE/SW azimuth instead? Like I've just said, I'm not interested in annual production but to extend the battery charging period during a day.

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            • #7
              PVWatts has an hourly output option that will help answer your questions about charging time. If you set the loss factors to give results that are close to what your south array actually produces, you can get a really accurate model. You might also consider downloading NREL's System Advisor Model... a bit more complicated to use, but you can model the multi-faced array in one shot.

              Your idea is a very good way to increase daily charging capability without exceeding the max safe charge current for your battery.
              CS6P-260P/SE3000 - http://tiny.cc/ed5ozx

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              • #8
                That PVWatts is the best tool ever! I didn't noticed the "hourly" feature (as it's available for the downloaded sheet only).

                I really managed to accurate set the azimuth/tilt to get the most extended charging time. Even there's not much power in the early morning / late evening, you could still get double output power (referenced to south oriented panels) for at least 2-3 hours.

                The next best thing is to find a way to set up a simple tracking system, to be able to adjust the azimuth for all new (8) panels to east then west (after noon).

                The problem is I couldn't use a single large array of panels (to use a single tracking system) because I have to keep the height as low as possible (city regulations). Being mounted on a flat roof, I had to put them in a single row / landscape mode.

                There were no problems with the actual array (south fixed azimuth) but these new panels should be mounted in a single row (too) and most probably they should move independently.

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                • #9
                  Are tracking systems actually economical?

                  Which has better payback time -- 260 watt panels with a tracking system, or 315 watt panels without one?
                  And once mainstream bifacial panels come on the market, would they be a win here?

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                  • #10
                    Originally posted by DanKegel View Post
                    Are tracking systems actually economical?

                    Which has better payback time -- 260 watt panels with a tracking system, or 315 watt panels without one?
                    And once mainstream bifacial panels come on the market, would they be a win here?
                    1.) Usually not.

                    2.) Depends on what a cost analysis would estimate. Every situation is different. Rules of thumb are usually better for measuring thumbs than estimating energy system economic viability.

                    3.) See 2 above.

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                    • #11
                      Originally posted by DanKegel View Post
                      Are tracking systems actually economical?
                      I don't need a MPPT tracker but a simple flip-over mechanism to switch an entire row of panels at once (from east to west orientation).

                      Originally posted by DanKegel View Post
                      Which has better payback time -- 260 watt panels with a tracking system, or 315 watt panels without one?
                      It's not about the payback time but I have limited space available. Of course, instead of fliping over a single row of panels I could put two rows of panels back to back (east/west).

                      Btw, I guess I'll have to use two separate chargers for east/west oriented panels, right? (for MPPT reasons)

                      PS: Speaking of payback time, I've never thought about it as I was going off-grid for "green" reasons only (I have cheap utility power available but I don't want want it anymore).

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                      • #12
                        Or consider mounting bifacial panels vertically; then you'd get morning and evening power without having to move anything...

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