Clearing the snow off the roof mounted PV panels

[peakbagger]

Three Northeasters in row had encased my roof top array in 12 to 18" of snow. I had gotten closer to 3 feet but a stiff wind had moved a lot of it off the roof plus the snow had settled. They are on a 2.5/12 slope on second floor roof so getting at them with a roof rake from the ground is not possible. I can get at a strip of asphalt shingles at the lower edge of the array and about 1 foot of the bottom of the panels with a rake. If its warm and sunny that usually lets them melt slowly but with the stretch of cold sunny days in the forecast I got the extension ladder out.

Using an extension ladder to clear an array from below has its limitations and risks. I have worked them out over the years to something I feel comfortable with but its not without its risks. If the snow is not stuck to the panels, there is always the chance that trying to clear off the snow can cause a snow avalanche where the entire mass of the snow decides to slide down the panels. That will knock an extension ladder backwards. This normally happens either with fresh snow or after a rapid warm up. I set my ladder at shallower angle than standard against the roof and make sure its sticks up well past the edge of the roof. I also have solid tie off point under the soffit. If the conditions have the potential, I tie the ladder off to the soffit and then even if there is an avalanche, the ladder breaks the sheet and the snow goes to either side of me.

The snow I had this weekend was definitely not going to slide. I had about foot of powder on top of "Styrofoam". The clearing at the base of the panels plus some residual heat from the attic of the house caused some freeze thaw between the panels and the base of the snow. It freezes pretty tightly. I use a plastic roof rake, its some sort of injection molded plastic and I have never seen any damage to the four brands of panels I have used over the years. I rake one handed with the other hand wrapped around the ladder rail. The soft stuff comes off pretty quickly. The underlying styrofoam not so quickly. Some folks treat panels with kid gloves but my experience over the years is they are pretty robust. Sure they can shatter if hit with a point object but my experience over 16 years is they take a lot of abuse. I end up using the rake and chopping at the dense stuff with the width of the rake and usually it pops off in chunks leaving a frozen layer of ice/snow on the top of the panels. I mostly get the loose stuff off and once the sun gets at this stuff its gone in a couple of hours depending on the amount of sun and the outside temp. The minor PITA is my Iron Ridge racks use top attachments for the panels, that means short threaded stubs with nuts on sit above the panel frames. The rake on occasion catches on them which nicks the rake edge.

Took me about 45 minutes to rake (and chop) off a 2 KW array 2 rows of 4 panels. Even with the ice/snow mix on them, the inverter started up and within 2 hours they were full output (for the low sun angle).

I am not advocating that someone should clean off difficult to access panels every storm and some roofs are just not safe to access. I have a couple of other arrays that are far easier to clean off and usually live with the reduced output until late winter/early spring where the longer days and high sun can really add some generation. It can snow well into April but most storms are followed with warm sunny days so my approach exposing the lower edge of the roof and the bottom of the array from the ground seems to work for most storms. If I do clean it off, I try to do it in the early morning as soon as possible as standing in the avalanche zone on the ground is also not a great idea.

 

[maple1]

What about a walkway thingie, above the panels? Then you could push it off from above? With a couple of permanently fixed ladders to get up there, it might make doing it semi-regularly before it gets too bad, easier.

 

[peakbagger]

There are all sorts of options but the reality it is not worth the hassle during much of the winter as the angle is way off from optimal. They should be about 30 degrees off of vertical for winter and are closer to 60 degrees, great for summer but not so good for winter. With Spring starting tomorrow they should should be at 45 degrees so now it makes sense to clear them. I can adjust the angle on my other arrays but usually hold off a bit due to snow loading with late season snowstorms.

I drove by a big commercial PV field last week in Mass. Half the panels were covered with snow and there was a big lump of snow at the lower edge of the arrays that prevented any snow from sliding down. Those arrays use string inverters, so at best they are getting 1/2 output if someone knew what they were doing when they wired the MPTT circuits.

 

[Circus]

The anxiety of missing a little free energy wears off after a little penciling. I went through the same thing the first snow with my DHW. Now, if labor is needed to make it work, it ain't going to work.

 

[jebatty]

I am a strong believer in clearing snow, about a 15 minute exercise ... and in a ground mount PV system which makes snow clearing very fast and easy. My 46 panel system in north central MN is at 35 degrees tilt, latitude is 47 degrees, winter solstice sun at 19 degrees, summer solstice sun at 62 degrees. Nov - Jan are the low production months due to heavy cloud cover and low sun angle in Nov and Dec and that plus snow in Jan. But, the heavy snow months are Feb, Mar and April. I clear off the snow with a passion, and the kWh production for Feb, Mar and Apr well justify that minimal effort.

Feb, Mar and April typically have increasing amounts of snow, cold temperatures and clear skies. All of those, plus the "snow bounce" irradiation from ground to panels, contribute to very high kWh production, with only June and July regularly exceeding these snowy months. Summer temperatures typically are high, clouds are common, especially in the afternoons, and even "clear" skies are laden with humidity, dust and high altitude smoke from forest fires in the western US and Canada, and these factors reduce irradiation.

The chart shows monthly production for Mar 2018 -19. Not shown on the chart, but true every year, is that highest daily kWh production occurs in Mar-Apr, with late Feb not far behind. As examples, Apr 7 and Apr 17 of 2018 both had kWh production exceeding 90 kWh, while the highest daily production in Jun-Jul 2017 was 82 kWh. Average annual production of my system is about 15,500 kWh, and those two days in Apr contributed 1.2% of annual production. The grid value of those two days was $22, or $11/day. Had the panels been snow covered, production would have been near 0. Earning $11 of electricity for a 15 minute snow clearing exercise works out to $44/hr of tax free income equivalence. Worth clearing snow? Absolutely. Plus, a little outside exercise is nothing to complain about.

I understand the difficulty or impossibility of clearing snow on roof mount systems and that ground mount is not possible for many; also that local conditions vary greatly regarding sun, temperature, snow and atmospheric conditions. Where my system is located, clearing snow make sense and make cents that turn into considerable value.