Shading: Conflicting results

[rscott]

Hi all,

My south-facing roof has quite a bit of shade, due to trees that may not be possible to cut down (there is a brook behind the house, so they are in wetlands, and when I checked in 2011 the conservation commission seemed to think it wouldn't be likely to get permission to cut them down for solar). I've had 2 company come to look.

Company 1 used a SunEye (or similar device). Before getting the results, he seemed to think that it would work, but the results were "too low to read accurately" around 32%. He is suggesting using the north-facing roof, with a handful of panels on the south-facing roof where there would be the least shade (but was clear it wasn't ideal).

Company 2 used HelioScope with LIDAR, and seemed to think that it would work. They have a production guarantee (90% of the production estimate). From the numbers, it looks good -- nearly twice the estimated production per dollar spent compared to using the north-facing roof like Company 1 suggested. The company has been around for 6 years, and gets good reviews. It looks like the estimated production is somewhere around 60% of what it would be without shading.

Given that Company 1 didn't think it was worth using the south-facing roof, I'm a bit hesitant with going ahead with Company 2. Obviously they wouldn't offer the guarantee if they didn't trust the numbers, but if the results are really poor, it would be a hassle for me and cost them.

Any thoughts as to why there is a discrepancy like this?
-Scott

[AzRoute66]

Panels are very sensitive to exactly where, and for how long, the shade is present. If you take something the size of a shoebox lid, on a standard grid tie panel, you can place it such that it has very little impact, or by moving it few inches shut it down from a third to nearly 100%. How the panels are wired together will make a huge difference in the effects of shade. Additionally, using micro-inverters or optimizers, as opposed to 'string' controllers will have impacts on the amount of production that is obtained from any given array. Even something like laying out the panels in a landscape versus portrait configuration will change your shade tolerance. As each installer has their own thinking and pet techniques for dealing with shade it is not surprising that different numbers will be offered.

[Mike90250]

Woe unto you if the trees act like trees, and continue to grow, increasing the shade on your roof. Solar may not be feasible in areas near protected trees.

[ButchDeal]

Hi all,

My south-facing roof has quite a bit of shade, due to trees that may not be possible to cut down (there is a brook behind the house, so they are in wetlands, and when I checked in 2011 the conservation commission seemed to think it wouldn't be likely to get permission to cut them down for solar). I've had 2 company come to look.

Company 1 used a SunEye (or similar device). Before getting the results, he seemed to think that it would work, but the results were "too low to read accurately" around 32%. He is suggesting using the north-facing roof, with a handful of panels on the south-facing roof where there would be the least shade (but was clear it wasn't ideal).

Company 2 used HelioScope with LIDAR, and seemed to think that it would work. They have a production guarantee (90% of the production estimate). From the numbers, it looks good -- nearly twice the estimated production per dollar spent compared to using the north-facing roof like Company 1 suggested. The company has been around for 6 years, and gets good reviews. It looks like the estimated production is somewhere around 60% of what it would be without shading.

Well as someone that does EXTENSIVE work with LiDAR and solar design. I will tell you that Company 2 has made a mistake. LiDAR is always OLD (trees were younger and lower when it was gathered years ago), it also comes in many forms based on what time of year the LiDAR was flown. Meaning that if it was flown in winter (most common) that is it leaf off and will show a much lower hight for vegetation. further most uses for LiDAR are NOT looking for vegetation heights but ground level and so they are trying to remove the trees as noise.

Now Company 1 was sitting on your roof with a device that looks at the trees as they are on that day (years after the LiDAR was gathered) and they see the tops of the trees (not statistical elevations based on random light pulse hitting a branch someplace on the tree).

In other words Company 1 is most likely closer to accurate and keep in mind Mikes comment, the trees will continue to grow.

[rscott]

Thank you all for your help.

I talked a bit more with Company 1 (SunEye) and they say they use LiDAR in their estimates as well.

Aurora shows that they use LiDAR data from 2010, and I imagine the two companies I am dealing with don't have anything more recent. Fortunately, the trees are older and likely have not grown much since then.

The gamble with going with Company 2 is very tempting, as even if they produce a bit more than half of what they estimate, the numbers (cost) would work better than Company 1's proposal. And it would also give me the flexibility of cutting/pruning the trees to produce more if possible (with the advantage of producing even more electricity than the Company 1 proposal).

Of course, I'm kicking myself for not factoring in the shading back when I bought the house!
-Scott

[Sunking]

No shade can be tolerated. Clear cut you property or give up on solar. Salesman will tel you anything you want to hear to make a sell. They could careless if it works or not. Salesman do not get a commision telling the truth.

[ButchDeal]

I am very familiar with the LiDAR models of Aurora etc. they still do mostly leaf off LiDAR. which means that the tree heights are lower than reality.

As for your banking in the production guaranty, I would have a very close read in that, production guaranties are slanted towards the installer.

[bcroe]

As discussed, shade kills production. A space is required for panels that is unshaded 3-4 central hours of the
day, to be effective. Your best shot might be to clear your lot, and put panels at the northern border facing south.
Bruce Roe

[FFE]

Most of the information people rely on to give expert advice on shade tolerance is obtained using simulated shade by covering panels. If what is causing your shade is directly touching the panels then that is an accurate model. In real life trees reduce the output of your array, but nothing like the test data.
I had the same dilemma as you. The guarantee was written to my satisfaction. Everything very smart people that post here implied I should not have installed a system. I have two years worth of output that beat the guaranteed number. The payback is much longer than a shade free system. I also have a string inverter and doubt that having panel level MPPT would increase the output enough to offset the potential future issues they may cause. If you believe the output guarantee is written well enough and you believe the company will be there to back it up, go for it. Google Quantifying Shading(apostrophe)s Economic Impact, it tells a slightly different story but still uses the same shading simulation. I have only seen one video using real roof vents and small potted trees to simulate shade. That video correlates to my real life output.

I just figured out why my posts have issues. I am guessing the forum does not like apostrophes.

[ButchDeal]

Most of the information people rely on to give expert advice on shade tolerance is obtained using simulated shade by covering panels. If what is causing your shade is directly touching the panels then that is an accurate model. In real life trees reduce the output of your array, but nothing like the test data.
I had the same dilemma as you. The guarantee was written to my satisfaction. Everything very smart people that post here implied I should not have installed a system. I have two years worth of output that beat the guaranteed number. The payback is much longer than a shade free system. I also have a string inverter and doubt that having panel level MPPT would increase the output enough to offset the potential future issues they may cause. If you believe the output guarantee is written well enough and you believe the company will be there to back it up, go for it. Google Quantifying Shading(apostrophe)s Economic Impact, it tells a slightly different story but still uses the same shading simulation. I have only seen one video using real roof vents and small potted trees to simulate shade. That video correlates to my real life output.

I just figured out why my posts have issues. I am guessing the forum does not like apostrophes.

NREL tests with remote shadows and most models use real shadow tests. I monitor many many sites with real shadows with and without module level .m.pPT and I can tell you a decent model does match very very accurately the shadows and module level MPPT does very much help.
Results will depend on actual shadows as some odd situations can be handled without optimizers.
there are of course many installers using poor models and all models are only as good as the people setting things up,

i have seen many installers that do not (damn forum appostrafie bull ****) know how to set up a model correctly.

treese reduce production, how much depends on the azimuth Of the array and the direction of the tree.

[J.P.M.]

Most of the information people rely on to give expert advice on shade tolerance is obtained using simulated shade by covering panels. If what is causing your shade is directly touching the panels then that is an accurate model. In real life trees reduce the output of your array, but nothing like the test data.
I had the same dilemma as you. The guarantee was written to my satisfaction. Everything very smart people that post here implied I should not have installed a system. I have two years worth of output that beat the guaranteed number. The payback is much longer than a shade free system. I also have a string inverter and doubt that having panel level MPPT would increase the output enough to offset the potential future issues they may cause. If you believe the output guarantee is written well enough and you believe the company will be there to back it up, go for it. Google Quantifying Shading(apostrophe)s Economic Impact, it tells a slightly different story but still uses the same shading simulation. I have only seen one video using real roof vents and small potted trees to simulate shade. That video correlates to my real life output.

I just figured out why my posts have issues. I am guessing the forum does not like apostrophes.

Aside from the fact that the weather more than likely was a large factor in beating the estimate, even if the weather was "average", you'd still beat the vendor's estimate and guarantee. EVERYONE's does. Know why ? - Because estimated output is calculated and then purposely reduced. Seen it probably 50 times. A vendor would be a bigger fool then the customer to do otherwise.

[FFE]

Butch, please post a link to the study. I have been searching for a good independent test. The only thing from NREL I have seen is how they think they figured out how to cover paneled with opaque tarps to simulate shade better.

JPM, that is the same info I gleaned from this site and took it to the bank. This site is also full of info (including this thread) that anything more than a tiny bit of shade is a no go for solar.

rscott, I hope more people with a south facing roof with lots of shade will post their experience here.

[rscott]

Thanks again everyone for your input. I think I've pretty much decided to go ahead with it, knowing that there is risk involved of generating quite a bit less electricity than hoped for.

I've found 13 different satellite images of my house, and it shows that the south-facing roof is nearly completely covered in shade at some times, and nearly or entirely shade-free at others. So with microinverters or power optimizers and bypass diodes, I should be able to generate quite a bit of electricity at certain times of the day/year. If production is really bad, I can work on seeing if I can get the trees cut (or take advantage of the production guarantee). And at any point in the future, I know that I have the ability to "add" electricity by cutting/pruning the trees (which would also help with the conservation commission: having the solar panels in place means a guaranteed *immediate* benefit from cutting trees).


AllPics.png

I'm also going to look into the north-facing roof in addition (as we use a lot of electricity, due to geothermal heating), and see if the numbers would work for that as well.

I will try to remember to post the results here for future reference (if someone sees this in the future and thinks I may have installed the system already, feel free to post, which should generate an E-mail to me).

[ButchDeal]

Butch, please post a link to the study. I have been searching for a good independent test. The only thing from NREL I have seen is how they think they figured out how to cover paneled with opaque tarps to simulate shade better.

JPM, that is the same info I gleaned from this site and took it to the bank. This site is also full of info (including this thread) that anything more than a tiny bit of shade is a no go for solar.

rscott, I hope more people with a south facing roof with lots of shade will post their experience here.

That paper references HOW they came up with the method. The reasons for using the opaque tarps is that trees are not a quantifiable scientific measurement. No two are the same.
Further look at the SolarEdge public sites, you can SEE the shadow of trees move across the arrays.

[ButchDeal]

Thanks again everyone for your input. I think I've pretty much decided to go ahead with it, knowing that there is risk involved of generating quite a bit less electricity than hoped for.

I've found 13 different satellite images of my house, and it shows that the south-facing roof is nearly completely covered in shade at some times, and nearly or entirely shade-free at others. So with microinverters or power optimizers and bypass diodes, I should be able to generate quite a bit of electricity at certain times of the day/year. If production is really bad, I can work on seeing if I can get the trees cut (or take advantage of the production guarantee). And at any point in the future, I know that I have the ability to "add" electricity by cutting/pruning the trees (which would also help with the conservation commission: having the solar panels in place means a guaranteed *immediate* benefit from cutting trees).

AllPics.png


I'm also going to look into the north-facing roof in addition (as we use a lot of electricity, due to geothermal heating), and see if the numbers would work for that as well.

I will try to remember to post the results here for future reference (if someone sees this in the future and thinks I may have installed the system already, feel free to post, which should generate an E-mail to me).

These photos are taken at different times and different years. You can see massive tree size difference in some of them and some are completely leaf off (Winter)
The first and third look like they were taken a long time ago, while the fifth is more recent with more trees.

The bottom row left is more recent (more growth) and taken at about 11:00 am to noon and full leaf ON
Bottom row middle is latter in the year (fall some trees are partial leaf off) and take much later in the day, maybe 4:00 to 5:00 pm look at the shadow from the chimney
Bottom row right is summer again but even latter in the day but the trees are smaller so it is a few years older. Look at the much longer shadow on the chimney. If you had solar at the point of the photo it would generate very little due to the angle of the sun to the pv modules.

Second row second photo in is dead of winter, full leaf off and several years ago due to the small size of the trees to the south west of the house. Also those trees to the south west either do not lose their leaves or lose them late in the year.

What you have to remember is that the production is highest when the sun is most direct to the PV modules. In your case if there were no trees, due to the azimuth of the roof, your peak production would be morning, about 10:00 am and taper off from there. The bad thing is that your shadows are most full at 10:00 am. In winter time the sun is LOWER in the sky, and though the tree is leaf off it doesn't really matter as there are so many shadows breaking up performance is going to tank.

I am not trying to talk you out of going solar but your expectations are off of the results.
Your best sun is near the ridge line though you didn't state your location you likely have fire code set backs forcing you to keep the solar tree feet from the ridge line (not helping production) and for quite obvious reasons you are going to need to install optimized system or micros.
Your north (NNW) facing roof would help some but not all that much in comparison mostly because you have a tree growing to the South west that will block the sun on the NNW roof in the evening. I would discourage getting high performance expensive PV modules. They will not help with the production in your case, but they will hurt the ROI and should a branch fall and crack on would be much more costly to replace.

We have seen worse and installed solar on worse but we set the homeowners expectations of results.