Planning new install - 5 degree metal roof aimed north, need advice

[bob-n]

Tons of great advice, all of it. Thanks to JPM, solarix, Mike90250, littleharbor and Ampster for their usual great guidance.

I'm assuming that snowmonster's system would put the panels very close to the roof, so even if snow falls off the panels, it has no where to go.

Are solar panels engineered to be submerged in snow and stay submerged for a month, as might happen with a shallow angle, a bad snow storm and a few cold weeks? Is there any comparable issue to ice dams, where daytime melting followed by nighttime freeze might cause ice-stress?

Similarly, if the system uses microinverters or other electronics on the roof, is that engineered for snow submersion?

If the building heats the bottom of the snow, it might turn to pooling water, trapped by ice above. Is that an issue for panels or electronics?

[snowmonster]

Tons of great advice, all of it. Thanks to JPM, solarix, Mike90250, littleharbor and Ampster for their usual great guidance.

I'm assuming that snowmonster's system would put the panels very close to the roof, so even if snow falls off the panels, it has no where to go.

Are solar panels engineered to be submerged in snow and stay submerged for a month, as might happen with a shallow angle, a bad snow storm and a few cold weeks? Is there any comparable issue to ice dams, where daytime melting followed by nighttime freeze might cause ice-stress?

Similarly, if the system uses microinverters or other electronics on the roof, is that engineered for snow submersion?

If the building heats the bottom of the snow, it might turn to pooling water, trapped by ice above. Is that an issue for panels or electronics?

The system is going to be a string inverter with optimizers. The Iron Ridge tilt legs will lift the panels off the roof even more so I don't think that will be a problem.

I started learning more about sun angle and azimuth today and then stumbled across this calculator - https://www.rbisolar.com/solar-calculator/ You can play around with time of day, location, panel size, etc. All of these online calculators seem to suggest a lower pitch than what I was calculating (even adjusting for my 5 degree sloped roof in the wrong direction).

As far as self shading goes, the sun angle here isn't even above 20 degrees until noon and drops below that again at 3:30. Combine that with only 2.55 hours of solar insolation on Dec 21, do I even really care about self shading at 9:00 AM or 10:00 AM on Dec 21?

[J.P.M.]

The system is going to be a string inverter with optimizers. The Iron Ridge tilt legs will lift the panels off the roof even more so I don't think that will be a problem.

I started learning more about sun angle and azimuth today and then stumbled across this calculator - https://www.rbisolar.com/solar-calculator/ You can play around with time of day, location, panel size, etc. All of these online calculators seem to suggest a lower pitch than what I was calculating (even adjusting for my 5 degree sloped roof in the wrong direction).

As far as self shading goes, the sun angle here isn't even above 20 degrees until noon and drops below that again at 3:30. Combine that with only 2.55 hours of solar insolation on Dec 21, do I even really care about self shading at 9:00 AM or 10:00 AM on Dec 21?

Using the winter solstice solar elevation angles and hours away from solar noon will pretty much ensure no/min. self shading, but in reality because the POA irradiance on an array is less in intensity as well as frequency (cloudier days) from mid/late fall until say, mid Feb or so, decreasing the row pitch from the no shade ever scenario to one that loses some production away from solar noon during the winter months may actually result in more annual production by enabling a larger array to be placed on the roof. Using 12/21 0800 or 1600 hrs. angles is an example. There are lots of models like SAM for example that will generate a lot more detail and useful info for design than can be explained here.

After doing such things as more than a hobby but less than a job for 45+ yrs., I've learned that treating this subject in a linear mode isn't the best way for me to work on these types of things. Thinking in gradations and keeping a sinusoidal curve in mind helped me.

Any comment on wind loading considerations with respect to the design ?

I'd also respectfully suggest you think about what snow will do, where it will go once an array is in place, and the potential for damage from buildup/leaks/ice ponding and other things that will come with the array.

[Ampster]

..............
FWIW, my numbers for annual output pretty much agree with what you report. I'd note those #'s seem in line w/my comment that, to a 1st approx., the max. output of an unshaded sawtooth array on a horizontal roof is about the same as the output of a horizontal array on that roof.

@Solarix mentioned that angles below 15 degrees from horizontal do not self clean from rainfall. I know you have done some analysis on the affect of dirt on panels. Care to speculate on where the sweet spot may be between tilt angles and panel soiling. I understand that different environments may have different soiling characteristics.

This is a theoretical question since I may have missed an opportunity when I had to remount some panels due to a reroof that my HOA did two years ago. They were originally installed flat because my HOA did not want them seen from the facility. I later realized that that kind of restriction was unenforceable but I was also limited on roof area. When I reinstalled them I considered giving them some small amount of slope but in my haste I did not bother with that detail. It is now a rental and the tenant pays for electricity so it is not a big financial issue for me.

[snowmonster]

I am also curious of the soiling issue. We do get some reasonably hard rains from time to time that I would think would help.

[J.P.M.]

@Solarix mentioned that angles below 15 degrees from horizontal do not self clean from rainfall. I know you have done some analysis on the affect of dirt on panels. Care to speculate on where the sweet spot may be between tilt angles and panel soiling. I understand that different environments may have different soiling characteristics.

This is a theoretical question since I may have missed an opportunity when I had to remount some panels due to a reroof that my HOA did two years ago. They were originally installed flat because my HOA did not want them seen from the facility. I later realized that that kind of restriction was unenforceable but I was also limited on roof area. When I reinstalled them I considered giving them some small amount of slope but in my haste I did not bother with that detail. It is now a rental and the tenant pays for electricity so it is not a big financial issue for me.

Observations and measurements I've done have led me to several theories on panel fouling mechanisms that have led to further observations and measurements and then modified measurements.

But right now, big cooking/baking day for me. Got a roast to cook, a bunch of baking and some ice cream to make.

More on array fouling vs. tilt angle later if you or anyone else responds/wants more mind spoor from me.

Short/anecdotal comment til then: Horizontal panels turn into a sand box/dried mud pit sooner rather than later. Vertical panels stay about as clean as kitchen windows.

With respect to fouling rate vs. tilt angle only and maybe a bit distinct from of a general discussion about fouling rates in general: A bit less anecdotal but still speculative and in any case incomplete preliminary set of observations based on info/readings and observations of cooperative neighbors, what I've read in the journals, and what I think I might know about material transport mechanisms such as, for example, stream bed loading, angles of repose and other esoteric and seemingly unrelated phenomena: Between the two extremes, horizontal and vertical, the rate of fouling for an array might at least partially be some function of the cosine of the array tilt angle raised to some power < 1.0, but there may well be and probably are a dozen or more other factors that come into play, they may or may not be independent of one another, and some may be absent or greater in effect as f(site location, season, other).

[solarix]

Anything over 15 degrees stays quite clean in our area. Not sure exactly what angle actually is. 15 degrees is where I draw the line. Another factor is if you have anything sticky that blows around. Some pollen can be a problem and then of course bird poo doesn't wash of by itself.

[bcroe]

of course bird poo doesn't wash off by itself.

It does here before long, in northern IL. Bruce Roe