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Shedding light on solar panels shading
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Shedding light on solar panels shading

15 mins 24 Mar 2021
The fact that shadows are bad for solar panels seems obvious. But how bad are they exactly? Well, in the worst case scenario a small shadow on a corner of one panel could kill the production of the whole array. How do manufacturers fight the problem of shading? What are the most troublesome objects that block the sun? What to do with a nasty tree that has grown up high over the last 10 years next to your solar system? In this article we'll study the problem of shading.

Shading of one solar cell affects the whole panel

So why do solar panels in shade produce much less electricity? Сells of a solar panel are connected in series. It means that the performance of one cell directly influences the production of the next one. Gilbert Masters, former professor of Engineering at Stanford, says in his work "Renewable Energy and Efficient Electric Power Systems" that if one out of 36 cells doesn't get any sunlight, the performance of the whole panel falls down by 75%.

What is worse, it doesn't stop there. Panels are also normally connected in series, which means one shaded panel drags down the performance of the others. Solar panel shading also leads to shorter lifespan of a PV module. It's quite logical: if some cells aren't working, others burn out quicker. Manufacturers try to make their solar panels shade-tolerant. This is why they started adding bypass diodes in the modules. These devices decrease the impact of shading on the production of a panel. Bypass diodes are expensive, so it reflects on the price of a panel itself.

Generally, there are 2-3 bypass diodes in the junction box of a panel. They split the panel into sectors. If a solar cell in one of the sectors is entirely shaded, a bypass diode doesn't allow it to influence cells in other sectors. For example, if there are three bypass diodes in a module and one cell is under the shade, the performance of a panel drops by 33%. However, cells usually aren't shaded completely, so their energy production isn't nullified, but rather decreases.

Objects that block direct sunlight are a real trouble

Panels make use of direct and indirect sunlight: the irradiance that is reflected by clouds, snow, or water onto panels helps them to generate energy too. Objects that are far away from modules primarily block this indirect irradiance. Their effect on the production of solar panels is minor. The real trouble comes from the objects that directly block the sunlight from panels.

Most often panels are overshadowed by:

  • Trees. If the trees are yours, trimming them might help.

  • Clouds. Clouds are actually less of a problem than you might think. While they do cast shade on your panels, sunlight still penetrates them. Even on very cloudy days panels perceive the solar radiation invisible to an eye. Overall, clouds decrease panels production by 25-40%.

  • Panels can overshadow each other. It's often a problem of ground installations. If you can lower the tilt, it's better to do it. Imperfect angle impacts the efficiency of your system much less than solar panel shading.
Other buildings and your own roof. The way to avoid it is to plan the positioning of your panels before installation.

Plan your solar panel system in advance

To avoid shading you should carefully choose a place for your system before installing panels. The so-called shading analysis can be performed by solar experts and with the help of special tools and software.

One of the simplest methods you can use is called "1-3". For example, if something is 10 ft higher than your panels, you want it to be three times further from your array – 30 ft respectively.

It is recommended to mount modules horizontally, rather than vertically. The reason for this is the way that bypass diodes split panels into sectors. In the winter, for example, snow accumulates on top of a solar panel and eventually slides down to its lower half. If a panel is mounted vertically, snow blocks the sunlight for cells in all sectors, greatly decreasing the power output of a panel. If a panel is installed horizontally, only the lower part is covered and half of the panel is still working. Some experts claim that this positioning also leads to smaller losses from natural shading as well.

Panel brand and type matter

Panel brand and type matter

Some manufacturers deal with the problem of shading better than others. The ability of PV modules to perform well in low-light conditions and in shadows becomes one of the selling points. For example, Canadian Solar panels barely lose any efficiency on cloudy days. Uni-solar cells have good shading resistance. Some Panasonic PV modules have four bypass diodes which allows them to retain higher efficiency in shade.

If you know in advance that your panels will most likely be overshadowed – for example, there are lots of tall buildings and trees around – consider thin-film or amorphous panels, because they are less affected by shading.

A lot can happen around you during the standard 25-year lifespan of a solar panel. Trees may grow, high buildings may appear nearby. If initially your panels were free from shade, and now something is always blocking the sun, there are countermeasures to be taken.

Microinverters handle shading better than string inverters

Microinverters handle shading better than string inverters

Residential solar panel systems are often accompanied by inverters to convert DC from panels to AC. The most popular type is a string inverter. Its production, however, suffers greatly when one panel in the string is shaded. The solution to the problem is to use microinverters.

A microinverter goes on top of every panel and converts DC to AC on the spot. This way if one panel is overshadowed, at least it doesn't affect others in the array. Since you need several microinverters for an array, they end up being more expensive than string inverters, but your system becomes much more efficient.

The other option you can use is power optimizers. They limit the effect of shading to one panel which suffers from it. However, they don't convert current by themselves so you'll need to use them together with a string inverter.

Finally, consider connecting your solar panels in parallel. This way the performance of one panel won't affect the others. Keep in mind that with this type of connection voltage stays low, but the current of all panels is cumulative.
Illustrations – Marina Fionova
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