Any idea's on melting snow off PV panels?

Good luck, my roof panels have a shallow angle but not as shallow as yours (8/14 pitch). Mine are on the second floor roof. Its highly likely that standard construction details to keep ice dams from forming are working against you. If you think of how an ice jam forms, heat from the attic and sun warms up the attic to the point where the snow melts and then runs down the roof to the overhang where there no warm attic and freezes. The goal of most standard roof details is to keep a cold attic by installing soffit vents and ridge vents. If you think of the solar panels they are roughly doing the same thing. They are spaced off the roof to improve summer time air circulation which means cold air enters the bottom and leaves the top so you don't get any melting. If you tighten up the panels for winter by installing closure strips around the edges, then you take a summertime performance hit as the panels will run hotter. Note in order for installations to look nicer and also more vermin proof some installers install these closure strips anyhow and some of the rackless systems I have seen require the closure strips to counteract wind uplift.

The lower edge of my panels sit about 18" back from the edge of the roof and I have found that snow will slide down off the panels and build up on the strip of exposed shingles preventing further movement. Weather dependent this snow can freeze up into the beginnings of an ice dam as the strip is over the overhang and there is some melting off the panels.

I have considered installing a ramp from the lower edge of the panel to the edge of the roof and cutting vents in my attic at the lower edge of the panels but haven't done the vents as cutting any holes in shallow roof is asking for trouble. My current approach is use a plastic roof rake to remove the snow off the shingles at the lower edge of the panels as if this area between the shingles and the lower lip of the panel is blocked the air flow under the panels is restricted. I also think if its sunny out the shingles heat up a bit and contribute to snow melt on the panels. The lower edge of my roof is about 18' from the ground and I can get my rake up to the shingles plus about 18" of the lower panels. I have to careful as the panel mounting bolts stick up above the frames so I can only rake between the panel seams. If I do this the panels melt off much faster than doing nothing. Its especially noticeable after a big snow dump where the panels have 8 or 10 inches of snow on them and the panel gap with the roof is sealed. If I do nothing it may be weeks but if I use the rake the panels clear off after a few sunny days unless the snow crusted over. While writing this, it popped into my head that if I made some black plastic strips out of black polyethylene sheet that covered the shingles it would help in two ways, snow sliding off the panel on the plastic would tend to slide off compared to piling up and once the sun gets to it its going to generate more heat at the base of the panel. I envision I could bend it down over the roof edge and attach the lower edge to the fascia board and have some hooks that hang off under rack mounts to secure the top edge under the panel. I would remove it in the summer

I have a string inverter so its all or nothing for me compared to microinverters. I have a pole mount and wall mount array so I still have winter generation. My pole mount is set in the winter at 30 degrees and I still have to rake it on occasion.

I didn't have a roof option for our PV, and seeing your situation, glad that I didn't. Result was a ground mount, more expensive than roof plus 220' of underground wiring. The benefit, these increasingly sunny winter days are great on kWh production. I'm not sure where you are, but I am near Backus, MN. Here are my charts from Dec - Feb to date. Hope this helps. My system is 12.3 kW. I brush the snow from all of my panels very diligently.

... and February ...

Jeepman401 said:

Thoughts? Maybe I'm suffering from cabin fever and just need a project.

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I keep thinking of some sort of "wiper" system where a bar stored at the array top is pulled down across all the panels pushing the snow off. I picture a manual chain drive or electric worm gear drive.

The panels generate electricity. Could you backfeed them somehow and heat them up?

velvetfoot said:

The panels generate electricity. Could you backfeed them somehow and heat them up?

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This questions comes up on occasion on the various solar forums and the consensus is there really isn't a viable way to do this. If it were attempted, the warranty would be null and void and I expect with microinverter based systems, it would be very complex. String inverters based systems might be bit easier.

The problem is the heat of fusion of the snow. Snow is effectively powdered ice and in order to convert solid ice to liquid water it takes 144btu/lb to make that transition. That heat has to come from somewhere and to do it electrically would take a bit of power.

My casual observations of how my panels melt off is if the panels are covered completely with snow, it takes a warm above freezing day to get the melting started to the point where there is exposed panel. Once the darker panel is exposed generally on the top edge, the snow starts to melt but the layer of snow below the strip of exposed panel will not slide down as its up against the edge of the roof. If someone were to try putting self regulating heat tape under the panel I expect that a strip at the base of the panel along with a transition ramp of poly from the bottom edge of the panel to the roof edge would encourage gravity to allow the snow to slide down over the course of the day. I have seen on some occasions where the snow slides down the center of the each panel while getting caught by the raised clamp bolts between the panels. Since the heat tape would be under the panel I expect that the warranty would also be null and void.

I do have a glycol filled solar hot water system that really doesn't do a lot in the winter (I use if for preheat for DHW). I have considered putting a loop up under the panels but it comes back to I hate cutting holes on shallow roof.

It might also help to clean the panels and apply a silicone based surface treatment (e.g. Rain-X) prior to snow season.
It would be interesting to know whether the primary cause of snow sticking is physical interference (at the frames or elsewhere) or adhesion on the panel faces.

From the looks of the top of your panels the snow wants to slid off. You could try clearing below and perhaps the lower couple inches so the snow can slide. Don't feel too bad, your not missing much power this time of year considering your shallow roof and short days.

Circus said:

From the looks of the top of your panels the snow wants to slid off. You could try clearing below and perhaps the lower couple inches so the snow can slide. Don't feel too bad, your not missing much power this time of year considering your shallow roof and short days.

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I agree. My panels are at 35 degrees so your "slide" might be less. I use a snow roof rake, my roof is about as high as your highest roof. I can only reach about the bottom few feet of the lowest panels, so I clean the snow off them & the shingles underneath. The difference is quite dramatic compared to not cleaning them off. If I leave the snow on, it takes days for the snow to melt from the panels. If I cleaned the bottom third of the lowest panels, the snow would be gone in a day / hours from all the panels. Worth a try

There is thread over on SolarPanelTalk Forum on someone with ice damming issues that line up with this thread

I bet if you install some short shallow solar air heaters against the south wall and duct it to beneath the PV's the heat would keep em clear. Just build it to shed any snow.

PS What are those black panels anyway?

Anyway you could mount a couple of those solar powered mole vibrator spikes to your array? Wondering if that would be enough to keep the snow off?

Yesterdays warm weather and rain cleaned mine right off (until the next snow storm)

How about dribbling windshield washer fluid through holes in strategically placed tubing?

Circus said:

Don't feel too bad, your not missing much power this time of year considering your shallow roof and short days.

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Not exactly true. I'm at 48* N, tilt 35*, and solar production is outstanding on clear days, particularly so when the temps are down low. In the 1st 16 days of February I had one day at 55kWh, two days at about at 60kWh and two days at 70 kWh. At noon today my system already was at 29 kWh and power output at 10,910 watts. My all-time record daily high production was 89kWh.

Contrast that with last July when only seven days were 70kWh or above for the entire month. July 2015 produced 1,770 kWh of electricity, and this February already has produced 576 kWh against an estimated total of 1,160 kWh. Winter sunny days, although short, have clear, dust free skies, and low humidity, all making high insolation value. Summer days, although longer, at least where I live also are dustier, less clear, and the heat definitely reduces panel output. And this last summer saw skies hazy most of the summer from smoke from western US and Canadian forest fires.