It just snowed in Michigan. 8 Inches

[peakbagger]

I have a roof mounted set of solar panels and live in a very snowy area. The town next to me has the highest ground snow load in the state and my house is less than a 1000 feet from the town line. The ground snow load equates to roughly 10 feet of snow on the roof. Unless a house is of new construction, we do need to shovel off roofs or the buildings may collapse. Therefore we have figured out ways of removing snow and they apply to roof mounted solar panels. On a single story roof, a standard roof rake works most of the time unless the snow changed to rain and froze. A steep angle doesn't help sometimes. There are a couple of neighboring homes that have very expensive pole mounted trackers that stow in a vertical plane and as I drove by them yesterday they were completely covered in snow, but tracking nicely. I expect the next warm day the snow will peel off in a big lump. The trick with most panels is to get them started by knocking some or all of the snow from the bottom of the panels. Once the sun gets to part of the panel, it will start warm up and the snow above will start to loosen up. This is actually a dangerous time as at some point the weight of the snow will exceed the attachment to the panels and large sheets of very heavy snow will come flying off the roof. It can injure the raker or anyone walking by and will damage decks and landscaping. The best approach is get it as soon as possible after the storm.

Some people insist that they need a soft edge on the roof rake to prevent scratching the PV panels. I have seen zero damage on any of my three arrays from a plastic roof rake and my original array is around 15 years old.

One of my arrays are on a second story roof. I can just barely get to the lower edge with a roof rake from the ground. A standard roof rake pole is 5' long. With 3 extensions that's about 15 feet. 3 poles are rigid enough to control, add in another pole and it gets very hard to handle as it whips side to side. Adding in a fifth pole is not recommended. Using 4 poles I can reach the strip of roof below the array and the lower foot of the panels That frequently is enough, the sun heats up the strip of asphalt shingles and that heat ends up under the panels. It also heats up that lower strip of PV panel. Usually a few hours of sun later, I will hear rumbling and the will slide down and off the roof. A major caution is that don't use a roof rake anywhere near any incoming power lines. My power comes in underground so its not an issue with me but if it comes in overhead at you place its very important that you dont go near it. Some people argue that the rake poles should be fiberglass or insulated to prevent possible conduction of leakage current from the panel frame to the pole. My arrays are all grounded and given the number of non conductive layers I am wearing I don't worry about it, but that's your call.

If it doesn't let loose its time to decide if the production is worth the risk. If you are not comfortable working off extension ladders you may just want to leave them to the next warm up. I have a permanent tie off point under my soffit. I put the extension ladder in place and tie it off and make sure that the ladder sticks up 4 or 5 feet above the edge of the roof. I then start raking to my right and left. avoiding the snow directly above the ladder . At some point, the snow directly above the ladder will let loose but it will be stopped by the ladder which is tied off. The biggest PITA is that the snow sliding off the roof will fall in around the extension ladder legs and pack in tight. I usually have to dig them out with shovel.

[drbob]

Why shut it off ?

I dunno

[peakbagger]

I took a picture on how snow builds up on panels and my method of getting the snow off of inaccessible roof panels NCM_0070.JPG




As you can see the wall mounted panels which are an adjustable angle don't hold a lot of snow in the winter as they are at 30 degrees from vertical(although I did rake them off shortly before I took the picture. The roof mounted PV panels are another story, My rake pole with 4 sections just barely gets over the edge of the roof. As long as I can expose the asphalt shingles in front of the lower edge early enough in the AM on a reasonably warm day, the shingles will heat up and put some heat under the panels and usually within a day the snow slides off. The shingles were clear just a few minutes before I took the photo so the build up on the edge of the roof had already slid off the panels in just a few minutes

The solar hot water panels are a different story. They are at a steep angle but what usually happens is the snow slides down off the warm panels and then hits the roof and sits there. The lower part of the collector is too cold to do much melting but the upper section where its clear is heating some water just a lot less of it. The snow over the bottom usually freezes and thaws daily and that mound at the base of the panel is pretty well there until a major warm up. I heat with a wood boiler in the winter so the SHW is just preheat for the cold water makeup to the tank.

Note snow sliding off the roof collectors falls just past the lower edge of the wall mount panels and just short of the pickup. This is just early winter, I may end with 4 to 5 feet on the ground some winters. The accumulation on top of the wood pile is just two recent storms worth.

The icicles in the center are formed by heat under the roof radiated by the chimney. I have soffit vents but when I am running the wood boiler the snow melts around the chimney and runs down the roof until it hits the edge of the chimney and then refreezes. I have the entire roof covered with a rubberized product called storm shield so I don't have to worry about ice damming.

[Murby]

Dear Santa,
While you're up there, would you mind cleaning off my solar panels?

[jathans]

I am new to solar topics and came to this forum to find out more about it. And I am surprised to read that sun does not warm up solar panels enough to get rid of the snow as soon as it shines on them for some time. I thought this was one of their great characteristics, to make snow melt faster than surrounding roofing. While now, I am reading the energy generation drops to nothing with the snow cover...Does this mean that countries with a heavy snowfall and roof mounted solar panels cannot supply much power during the long winter months?

[SunEagle]

I am new to solar topics and came to this forum to find out more about it. And I am surprised to read that sun does not warm up solar panels enough to get rid of the snow as soon as it shines on them for some time. I thought this was one of their great characteristics, to make snow melt faster than surrounding roofing. While now, I am reading the energy generation drops to nothing with the snow cover...Does this mean that countries with a heavy snowfall and roof mounted solar panels cannot supply much power during the long winter months?

Unfortunately you have learned the lesson that most people do not know about. Unless you can clean off your solar panels just about any amount of snow, dust, dirt, shade, etc. will block out the light needed to generate electricity.

If you live up North and get snow you will have to remove it to get your panels working again. That is one reason solar is not as efficient in the Northern latitudes as those in the South.

The other reason is the amount of sunlight during winter up North is much less then farther South. Both issues make it harder to justify using solar to power your electrical appliances.

[drbob]

it depends on how much snow you get. if the panels are all covered and no black is showing they will absorb little sunlight. and not thaw out much, if a few black spots are showing they melt off quickly in full sun,

[SunEagle]

it depends on how much snow you get. if the panels are all covered and no black is showing they will absorb little sunlight. and not thaw out much, if a few black spots are showing they melt off quickly in full sun,

I agree that even if a little of the panel is exposed to the sun you will get some heating.

But if the panels do not have a large angle tilt or there is ice at the bottom don't expect the panels to clear themselves completely.

And if even some of the cells are still blocked you can still reduce the output of the panel by over 50%.

[bcroe]

I am new to solar topics and came to this forum to find out more about it. And I am surprised to read that sun does not warm up solar panels enough to get rid of the snow as soon as it shines on them for some time. I thought this was one of their great characteristics, to make snow melt faster than surrounding roofing. While now, I am reading the energy generation drops to nothing with the snow cover...Does this mean that countries with a heavy snowfall and roof mounted solar panels cannot supply much power during the long winter months?

Right, the sales people don't talk about your PV equipment being shut down by snow or clouds. Or
the best angle in different seasons, both for production, and for snow. If best production is desired,
the snow must be cleared immediately. All this is much easier to optimize with ground mounted panels.

Even with these northern IL panels kept clean, June production is about 3 times Dec production. The
loss is due to a combination of shorter days, increased shadowing with a lower sun, and cloudy skies.
If on net metering, the best approach might be to optimize summer production and not worry too much
about Dec. If you are not net metering, the opposite of going for best possible winter output might be
important, there will be plenty in summer. Bruce Roe

[Sunking]

Hey are you guys up north enjoying shoveling Global Warming?

[J.P.M.]

it depends on how much snow you get. if the panels are all covered and no black is showing they will absorb little sunlight. and not thaw out much, if a few black spots are showing they melt off quickly in full sun,

The concept of an energy balance can be useful to help understand what's happening here.

If, or how quickly, snow cover melts from a panel (or anything else for that matter) depends on an energy balance on the element/device.

If the net energy flow is positive (energy flows into the panel, or any device), that is, things are heating up, and energy is being absorbed by, say, a solar device, with the energy gain either from absorbing solar radiation or, as in the case of a solar thermal collector, also by perhaps circulating warm(er) fluid through the collector, the excess energy absorbed will heat the device to the point (a temperature) where energy gains equal the energy losses. if that equilibrium temp. is above the melting point of ice, and the energy balance remains positive, that is, positive energy input continues, some melting of ice/snow will most likely be the result, along with some increase in device temp.

If that excess energy condition continues, as might occur on a sunny winter day, enough melting may occur to expose part or all of the darker solar device with the result being more energy absorption and more heat generation as required to maintain the energy balance between the device and its surroundings. The probable result will be more snow/ice melt, and probably some useful energy production. That condition will continue until the energy inflow becomes less than required to maintain the device temp., at which point the device temp. will drop to maintain the energy balance between the device and the surroundings. If the device temp. drops enough and the surrounding temps. drop or are already low enough, melting will stop and if the surrounding are cold enough, things will begin to freeze up.

Depending on the nature of the snow cover on a solar device, it's depth, and it's ability (or lack of ability) to transmit solar radiation, some solar energy may actually get through some snow cover. A cm. or two of light powder, for example, may actually allow some irradiance to hit the panel surface and initiate some warming. However, that amount of energy addition may well be insufficient to warm the panel enough to initiate any melting and/or the ensuing/likely snow cover movement. It all depends on a heat balance on the device under consideration.

A perhaps unlikely, but possible example: It's -20 C. (= - 4 F.), windless and PV panels are completely covered with a new, uniform and fresh, fluffy snow of a couple of cm. or so depth that limits (but does not altogether stop) solar irradiance absorption to, say 300 W/m^2 P.O.A. Under those conditions, some electricity production may occur, say, maybe 30-50 W/m^2 or so. Under those conditions, even though the non electrical waste heat (~ 250 - 270 W/m^2 or so) may produce some heating of, say, enough to raise the panel temp. by about 10 C. above the ambient temp., that is, up to -10 C., that heating will not be enough to get the panel temp. up to the melting point of the snow cover. However, once/if any portion of the panel does become snow free, the panel temp., or at least those portions of the panel that become snow free panel will get more energy input, and the panel temps. will increase, perhaps somewhat locally, to maintain the energy balance. If the temp. increase required to maintain the energy balance gets to the melting point of the ice/snow cover, some (perhaps only local) of the snow/ice will melt once the heat of fusion of ice is added.

There's a few gory details left out, but that's the Reader's digest version.

[drbob]

The concept of an energy balance can be useful to help understand what's happening here.

If, or how quickly, snow cover melts from a panel (or anything else for that matter) depends on an energy balance on the element/device.

If the net energy flow is positive (energy flows into the panel, or any device), that is, things are heating up, and energy is being absorbed by, say, a solar device, with the energy gain either from absorbing solar radiation or, as in the case of a solar thermal collector, also by perhaps circulating warm(er) fluid through the collector, the excess energy absorbed will heat the device to the point (a temperature) where energy gains equal the energy losses. if that equilibrium temp. is above the melting point of ice, and the energy balance remains positive, that is, positive energy input continues, some melting of ice/snow will most likely be the result, along with some increase in device temp.

If that excess energy condition continues, as might occur on a sunny winter day, enough melting may occur to expose part or all of the darker solar device with the result being more energy absorption and more heat generation as required to maintain the energy balance between the device and its surroundings. The probable result will be more snow/ice melt, and probably some useful energy production. That condition will continue until the energy inflow becomes less than required to maintain the device temp., at which point the device temp. will drop to maintain the energy balance between the device and the surroundings. If the device temp. drops enough and the surrounding temps. drop or are already low enough, melting will stop and if the surrounding are cold enough, things will begin to freeze up.

Depending on the nature of the snow cover on a solar device, it's depth, and it's ability (or lack of ability) to transmit solar radiation, some solar energy may actually get through some snow cover. A cm. or two of light powder, for example, may actually allow some irradiance to hit the panel surface and initiate some warming. However, that amount of energy addition may well be insufficient to warm the panel enough to initiate any melting and/or the ensuing/likely snow cover movement. It all depends on a heat balance on the device under consideration.

A perhaps unlikely, but possible example: It's -20 C. (= - 4 F.), windless and PV panels are completely covered with a new, uniform and fresh, fluffy snow of a couple of cm. or so depth that limits (but does not altogether stop) solar irradiance absorption to, say 300 W/m^2 P.O.A. Under those conditions, some electricity production may occur, say, maybe 30-50 W/m^2 or so. Under those conditions, even though the non electrical waste heat (~ 250 - 270 W/m^2 or so) may produce some heating of, say, enough to raise the panel temp. by about 10 C. above the ambient temp., that is, up to -10 C., that heating will not be enough to get the panel temp. up to the melting point of the snow cover. However, once/if any portion of the panel does become snow free, the panel temp., or at least those portions of the panel that become snow free panel will get more energy input, and the panel temps. will increase, perhaps somewhat locally, to maintain the energy balance. If the temp. increase required to maintain the energy balance gets to the melting point of the ice/snow cover, some (perhaps only local) of the snow/ice will melt once the heat of fusion of ice is added.

There's a few gory details left out, but that's the Reader's digest version.

panels are like blacktop. sun thaws out blacktop, sun thaws out panels. gee whiz, its heat.

[SunEagle]

panels are like blacktop. sun thaws out blacktop, sun thaws out panels. gee whiz, its heat.

"Heat" is relative to the amount of BTU thermal transfer.

Also if a panel can absorb as much heat as a "blacktop" then the solar cell output will be reduced to over heating..

How much heat do you need to melt a lot of snow?

More than just some sun on a freezing winters day.

[Murby]

it depends on how much snow you get. if the panels are all covered and no black is showing they will absorb little sunlight. and not thaw out much, if a few black spots are showing they melt off quickly in full sun,

Yes! This is what I have found to be true. When cleaning them off, it is not necessary to get them spotless or even close to it. If you see black, the panel will do the rest by itself.

[drbob]

"Heat" is relative to the amount of BTU thermal transfer.

Also if a panel can absorb as much heat as a "blacktop" then the solar cell output will be reduced to over heating..

How much heat do you need to melt a lot of snow?

More than just some sun on a freezing winters day.

are you kidding? I live in the north country, northern Wisconsin. it warmed up to -10 yesterday. but there was bright sun.... solar panels did thaw out,, 2 inches of fresh snow melted off. minus 10 f colder than your florida freezer