Optimum module angle

solarixsolarix Solar Expert Posts: 713 ✭✭
I've always been told that the optimum module tilt for year round overall production is your latitude angle. However, I've been examining the geometry on an upcoming job and have started to wonder if a lower tilt, favoring the summer, is better - as the summer has more hours in the day. I'm figuring that about 5 degrees lower than the latitude angle is best for a fixed array. Is this right?

Comments

  • petertearaipetertearai Solar Expert Posts: 463 ✭✭✭✭
    Re: Optimum module angle

    My understanding is that it depends if your off grid or grid tied.
    My situation off grid I want the most I can get in the middle of winter so set to latitude + 10 deg.
    But if grid tied you may want more production in the summer latitude - 10 deg.
    ie flatter in summer and steeper in winter.
    2225 wattts pv . Outback 2kw  fxr pure sine inverter . fm80 charge controller . Mate 3. victron battery monitor . 24 volts  in 2 volt Shoto lead carbon extreme batterys. off grid  holiday home 
  • petertearaipetertearai Solar Expert Posts: 463 ✭✭✭✭
    Re: Optimum module angle

    Check this out it may be of interest. http://pvcdrom.pveducation.org/SUNLIGHT/SHCALC.HTM
    2225 wattts pv . Outback 2kw  fxr pure sine inverter . fm80 charge controller . Mate 3. victron battery monitor . 24 volts  in 2 volt Shoto lead carbon extreme batterys. off grid  holiday home 
  • solarixsolarix Solar Expert Posts: 713 ✭✭
    Re: Optimum module angle

    I'm talking the theoretical optimum without weather or external factors like an off-grid bias for winter. In my situation, with a rate plan that is higher rates in the summer, the summer hours are more valuable, so an even lower tilt is desirable.

    Wow! those are fun charts!! Thanks. The 3rd one shows my point how in the summer the days are longer. How come the 1st chart has the same 6am to 6pm day length all year long? (correction - you have to move the chart to a higher latitude, only the same all year at the equator)

    As I type at 6:30am, on almost the longest day of the year the sun has been up for an hour and my 5.5kW system is already putting out 400w. It will do right about 37kWh today (haven't seen clouds for weeks). In the winter it doesn't get started until after 8:00 and the typical daily total is 25kWh. With daily kWh being naturally higher in summer, doesn't it make sense (if your goal is max overall production) to lay the modules lower than the latitude angle to capture more of these longer days? I can see how the latitude angle is the median tilt between the summer max and the winter minimum, but this assumes the hours are the same. And I can see that the modules are only really productive when the sun angle is within 45 degrees or so of perpendicular, so you get the same 6 hours of good sun no matter what season. But while the max power is the same (or less because of the heat) the days are just longer now, making a lot more power. There must be a lot more diffused light in addition to the direct sun rays that the modules capture in making use of these longer days.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Optimum module angle
    solarix wrote: »
    I'm talking the theoretical optimum without weather or external factors like an off-grid bias for winter. In my situation, with a rate plan that is higher rates in the summer, the summer hours are more valuable, so an even lower tilt is desirable.

    Wow! those are fun charts!! Thanks. The 3rd one shows my point how in the summer the days are longer. How come the 1st chart has the same 6am to 6pm day length all year long? (correction - you have to move the chart to a higher latitude, only the same all year at the equator)

    As I type at 6:30am, on almost the longest day of the year the sun has been up for an hour and my 5.5kW system is already putting out 400w. It will do right about 37kWh today (haven't seen clouds for weeks). In the winter it doesn't get started until after 8:00 and the typical daily total is 25kWh. With daily kWh being naturally higher in summer, doesn't it make sense (if your goal is max overall production) to lay the modules lower than the latitude angle to capture more of these longer days? I can see how the latitude angle is the median tilt between the summer max and the winter minimum, but this assumes the hours are the same. And I can see that the modules are only really productive when the sun angle is within 45 degrees or so of perpendicular, so you get the same 6 hours of good sun no matter what season. But while the max power is the same (or less because of the heat) the days are just longer now, making a lot more power. There must be a lot more diffused light in addition to the direct sun rays that the modules capture in making use of these longer days.

    Run some test case through PVWatts (http://rredc.nrel.gov/solar/calculators/PVWATTS/version1/ or http://www.nrel.gov/rredc/pvwatts/grid.html). You might find that you are rearranging deck chairs on the Titanic. ;^)
  • solarixsolarix Solar Expert Posts: 713 ✭✭
    Re: Optimum module angle

    In putting my data into PVwatts, I get the usual results that using the latitude angle for module tile is the optimum. For example it thinks that 34.7 deg produces 8925kWh while 30 deg is only 8904kWh. I'm not saying I don't believe it - I just don't understand it. Yes, this is only a 1/4 percent difference in energy, but the effect becomes bigger as you do lower angles. Besides, though a small amount, we are talking free power here as it is the same cost (generally) to mount the modules at these tilts.
    I haven't waded through all the math in the PVWatts white paper, but am really doubting how the latitude angle can be the optimum module tilt.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Optimum module angle

    Take a look at the Mac's Lab page here: http://www.macslab.com/optsolar.html

    Pretty good explanation of the effect of different angles on panels through the year.
  • BB.BB. Super Moderators, Administrators Posts: 32,005 admin
    Re: Optimum module angle

    Also remember that PV Watts takes local weather conditions into account...

    And, there are losses due to "hot panels".

    For example, Solar Guppy can get more power with his panels pointing east of south. They are more efficient in the cooler morning air, and he has afternoon thunder showers that reduce afternoon production.

    For me, I get paid about 3x for "summer" noon-6pm (weekday) power vs what I get paid at other times/"winter" time. So--I would have been better off if my panels were biased for summer afternoons (roof is facing east of south--so cannot change that).

    All depends on your billing plan/needs for power.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • solarixsolarix Solar Expert Posts: 713 ✭✭
    Re: Optimum module angle

    Ahhh, thank you, that chart from Macslab is what I needed (at least it agrees with me)
    The chart for fixed tilt year-round modules shows the best tilt for me is exactly 5 degrees lower than the latitude angle. They say the angle can be figured by multiplying the latitude by 0.76 then adding 3.1 degrees. Don't know how they came up with that equation, but probably the result of some voluminous calculations. Not big deal perhaps, but casts doubt on the accuracy of the venerated PVWatts tool.
  • solarixsolarix Solar Expert Posts: 713 ✭✭
    Re: Optimum module angle

    I realize there are other factors, I'm just focusing on the tilt angle at the moment.
    But yes, temperature effects and time of use rates are very important as well.

    Because of our time of use rates, the optimum cost effective system size is achieved by only going for about 80% of your usage (grid-tied of course) and orienting the panels straight southwest to make the most of the high afternoon rates (12pm to 7pm on-peak time). Trying to produce 100% of your usage means making enough power to offset your night-time usage which is much less cost effective.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Optimum module angle

    Don't disparage PV Watts. Like Bill said, that takes into account local weather data as well. The Mac's Lab chart is based on "all else being equal", which it rarely is. Local conditions make a large difference in PV output. You could be in a spot where PV Watts says "X" Watt hours but you happen to be on top of the local mountain @ 5,000 feet so you actually get 1.2"X" Watt hours.

    Nothing can predict your power 100%; they can only give estimates based on known data and explain the reasons for the differences. Sort of like this forum. :roll:
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Optimum module angle
    Don't disparage PV Watts. Like Bill said, that takes into account local weather data as well.
    Agreed. I had a chance to run some numbers on a system that had been in place for a while and which had been monitored. When I compared the actual output of the system through 2 years of monitoring with what PVWatts predicted, the bottom line agreed to within about 3%. That's not bad. Would that the Government Accounting Office had a record nearly that good. ;^)
  • solarixsolarix Solar Expert Posts: 713 ✭✭
    Re: Optimum module angle

    Yes, on an absolute basis, PVwatts is the best at taking into account all the factors, even the difficult ones like weather, but on this narrow issue of optimum tilt, I question the accuracy. There clearly seems to be a relative advantage to using a little lower tilt than the latitude angle. And at lower angles like the 4/12 pitch common here, there is a more significant difference in projected yearly power involved with this question. In my calculations of estimated power for my customers, I want to be as accurate as possible especially if I can substantiate higher ones.
    I will agree that the harvest I've obtained from my own system jives very well with the PVwatts projections. Will let you know when I've had it in for an entire year. Of course that means little due to weather variations etc.
  • BB.BB. Super Moderators, Administrators Posts: 32,005 admin
    Re: Optimum module angle

    That ends up being the crux of the problem... I have had similar accuracy with PV Watts (better than 5% when I first installed my system)--But the real variation of the weather itself can be +/- 10-20% (month by month over 20+ year average).

    It is easier to put a +20% larger system in (what I did for my Grid tied system) and not worry.

    And, I have seen my total home power usage increase as the kids got computers, a freezer and overflow fridge in the shed, etc... I still run ~$300 a year credit -- But part of that was planning on an electric car.

    We have very heavy tiered pricing here (from $0.09 to $0.051+ per kWH--the more we use, the higher rates we pay). I planned on my solar system to allow me the option of using more power (never plugged in that window A/C yet, no electric car yet, etc.) and not have our electric bill skyrocket.

    With conservation (and not using the extra fridge/freezer), we would still be under $30 per month for electricity (typically around 200-300 kWH per month). The solar allowed us to be more flexible on our electricity choices.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Optimum module angle
    solarix wrote: »
    Yes, on an absolute basis, PVwatts is the best at taking into account all the factors, even the difficult ones like weather, but on this narrow issue of optimum tilt, I question the accuracy. There clearly seems to be a relative advantage to using a little lower tilt than the latitude angle. And at lower angles like the 4/12 pitch common here, there is a more significant difference in projected yearly power involved with this question. In my calculations of estimated power for my customers, I want to be as accurate as possible especially if I can substantiate higher ones.
    I will agree that the harvest I've obtained from my own system jives very well with the PVwatts projections. Will let you know when I've had it in for an entire year. Of course that means little due to weather variations etc.
    That's what I meant with the "rearranging deck chairs" comment. When you are seeing a variation that is less than the lumped inaccuracy intrinsic to the statistical model (and no predictive model can be absolutely accurate), then there really is no variation predicted by the model. Put another way, variation in output between ostensibly identical arrays set at slightly different tilt angles will be swamped by larger variables in the system. Statistically speaking, 8925kWh and 8904kWh are the same number.
  • solarixsolarix Solar Expert Posts: 713 ✭✭
    Re: Optimum module angle

    I know that no one could prove (or even notice) the difference, but in selling the systems, the more I can get the justifiable energy projection up, the better. And as a practical matter, it doesn't cost me one more dime to install a system at 30deg rather than 35deg, so why not do the best one? (if we can theoretically settle on which one actually is) Just because it is hard to observe the difference, doesn't make it not worth doing.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Optimum module angle

    I will now do my famous Dave Sparks imitation: "For maximum efficiency, install a dual axis tracker!" :p

    Which actually brings up the point that you would have to shift angles all year round for peak harvest. That's usually not practical. In terms of the one angle you can shift, you can judge the best angle using a shadow cast on the panel. Which time of year you pick for this is optional, and the difference it makes will vary greatly with the latitude. Installs at the Equator don't vary much Winter to Summer. Above the Arctic Circle it's another story. :p

    Not promising customers more than they're likely to get is a good practice too. Unfortunately the unpredictable aspects of solar make it difficult to come up with reliable expectations.
  • ggunnggunn Solar Expert Posts: 1,973 ✭✭✭
    Re: Optimum module angle
    solarix wrote: »
    I know that no one could prove (or even notice) the difference, but in selling the systems, the more I can get the justifiable energy projection up, the better. And as a practical matter, it doesn't cost me one more dime to install a system at 30deg rather than 35deg, so why not do the best one? (if we can theoretically settle on which one actually is) Just because it is hard to observe the difference, doesn't make it not worth doing.
    My advice is to pick one, declare it the best, and challenge anyone to prove you wrong. ;)

    It may not cost you a dime to install a system at 30 degrees rather than 35 degrees, but it may not make a dime's worth of difference, either. Some answers are just not worth the effort of dredging them out of the noise.
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