The coldest days in Minnesota also tend to be the sunniest. Those blasts of air from the North Pole bring exceptionally clear weather, with the intensity of the sun making up (somewhat) for the shortness of the days.

Given that, and knowing that solar cells are more efficient when they're cold, you would think that winter in Minnesota would be pretty good for solar production. And it would be except for the snow. A few inches of snow on the solar panels will bring the production to zero, and this time of year we also usually have at least a few inches of snow on the roof pretty much all the time.

There are a lot of solar installations where you can go online and view the production data. This seems to be a common feature of solar monitoring systems, and many people make their systems public--it's sort of a social networking thing for energy nerds. It's cool because you can go online and find solar installations near you and see how much power people are generating.

It's also a little depressing, though, when the weather is intensely sunny but the nearby solar systems are completely dark. We had our first major snowstorm of the year about ten days ago, followed by an extended period of bright sunshine and extremely cold weather. All these snow-covered solar panels are losing a lot of power! I understand that November and December are the darkest months of the year, and the solar contractors take snow cover into account when calculating how much power you should expect to generate.

Nevertheless, it somehow feels wrong to let that much sun go to waste, even though we shouldn't be expecting to generate much power this time of year, and it's dangerous to get up on the roof to clean the panels.

So I've been thinking about ways to safely and cheaply remove snow from solar panels. The "cheap" is important because removing snow from the solar panels will probably only give an extra 10% generation over the year. It's not worth it to spend a lot of money for that amount of gain.

There are three basic approaches to removing snow and ice from a surface: mechanical, chemical, and thermal. It's not necessary to completely clean the solar panels, just get enough snow off so the dark surface can start absorbing light. The heat of the sun will do the rest--even with the most efficient solar panels, over half the sun's energy goes to heat the panel and not generate electricity. It might be good enough to leave up to an inch of snow and ice on the panels, if the intense sunlight defrosts the rest quickly enough. The slope of the roof and the slipperiness of the glass surface mean that if you can break the adhesion between the snow and the solar panel, it should mostly just tubmle off.

1. Mechanical: Physically removing the snow from the panels is the simplest method, and could be as easy as brushing it off with a roof rake. That could work for the part of our system over the garage, which is relatively close to the ground, but the panels on the roof of the house will be three stories above ground level. Climbing up on a snow- and ice-covered roof is dangerous, so a useful mechanical system has to be something automated. I've seen systems with motorized pushers which move across the solar array, and they definitely work but are too expensive.
2. Chemical: Spraying some sort of deicing fluid on the solar panels should break the bond between the snow and the glass and allow the snow to fall off. The problem here is finding an effective antifreeze which will be safe for both the solar panels and the environment. Salt is a bad idea, since you've got electrical systems involved. Hot water is also a bad idea, since it will freeze in the tubes and make it a one-use system. An alcohol-based system might be environmentally safe, but alcohol can be corrosive to a lot of stuff and it's not clear if it would damage the solar equipment. Sugar water should be safe for the equipment and the environment, but isn't that great as an antifreeze. Propylene glycol should be safe for the equipment, but maybe not the environment. And so on.
3. Thermal: Heating the solar modules would certainly work and be environmentally safe. The problem is that it takes a lot of energy to melt snow and ice, and it's possible that it could take more energy to shed the snow than you would generate. Partly it comes down to whether you need to melt all the snow, or just a little bit to make it slide off. Also, this kind of defroster system would probably have to be built into the solar modules at the factory and bonded to the glass, so it's not something you could easily add on after the fact.

So for now, there's no obvious solution to snow on the solar panels other than the one which the professionals advise: wait for spring.

But at some point someone may invent a clever way to clean the solar array which is cheap, safe, and effective. When that happens, I'm guessing it will sell like crazy in these northern climates, just so we can avoid the heartbreak of seeing all those photons go to waste.