Mar 02 2010
Solar panels in the snow and other shading events
The winter months have brought lots of snowfalls, or as they are known in the world of solar energy, ‘shading events.’ You might be forgiven for wondering what exactly happens to the performance of solar panels when they are covered in snow, or anything else for that matter.
Shading is a big issue for solar arrays. A small amount of shading on one solar panel can result in a big power loss for the entire system. This is because of how they are connected together; a solar panel is made of a number of solar cells connected in series. Each solar cell has a current of around 8 Amps and a small voltage of 0.6V or so when under full sunlight. For those who remember their physics classes from school, this means that when they are connected in series the voltages add up but the current stays equal. Solar panels are then connected together in series to make a string, so the current still stays the same (on large arrays multiple strings are connected in parallel).
What this means is that if one solar panel, or even one cell of one solar panel is affected, it will affect all the others. When a cell is shaded its output current decreases, which means the current for all the other cells and modules is also limited. So one small patch of shade can disproportionately reduce the power output of the whole system.
This effect can be limited by a number of means.
The best way is to make sure your solar panels are not going to be shaded in the first place. This should be checked as part of the site survey, conducted by your MCS accredited installer. You should ensure that nothing will shade the modules during the middle of the day, when your system should be producing the most energy. Shading can be checked using a special design tools that show the path of the sun behind various shading objects. This can be either a lens that shows the horizon and path of the sun in front of you, or a full design software package that uses photographs of the surroundings.
With snow it does help to clear it off. But there isn’t usually much sun when its snowing, and if the sun does come out, the snow melts pretty quickly.
If you cannot eliminate shading as is often the case in built up areas, there are several technologies that can limit the effect of it. Many solar panels now include bypass diodes that disconnect groups of solar cells if they are shaded. It is fairly crude but often works well. When you buy solar panels make sure to ask about bypass diodes.
A second technology that is not available yet in Europe but soon will be is distributed conversion. Here, rather than have power electronics (like the inverter) positioned all in one place, you have some electronics placed on each module. This allows each module to operate independently. One company in the US called Enphase claims this technology increases power output by upto 25 percent.
These are all things to bear in mind when buying a photovoltaic system.
