SunWeb: Shading on solar energy collecting devices

The effect of shading on solar panels

Shading on solar panels is bad news. But many people fail to realise just how drastic an effect even a small amount of shading can have on the output of your array. The reasons why shading is so bad are slightly technical - but we'll do our best to explain them here in not-too-technical jargon, so stick with us.

In sunlight, each solar cell in an array acts as a little electron pump, pushing electrons from one side of the cell to the other, and giving a voltage boost to the system as they do so. A single cell isn't very powerful though, so in order to get a useful voltage, you need to put quite a number of cells in series. The output of one cell becomes the input to the next cell.

When a cell is shaded, the number of electrons it can pump from one side to the other drops. That, in itself, wouldn't be too bad you might think - you would just lose out by the power output of one cell. But unfortunately, because it is not pumping so many electrons up to its neighbour now, it limits the number of electrons that the neighbour can pump too. Same for the next cell in the line - and the next, and so on.

The other cells can manage to force some extra electrons through the badly performing cell, so it's not quite the case that the whole system performs as poorly as the worst-performing cell in the string - but it's not all that far off. You might easily see a 50% loss in power from a string of solar cells if just a single cell is shaded.

Fortunately, we can help to some extent by fitting bypass diodes to solar panels. Bypass diodes are fitted in parallel with a string of PV cells, and they do exactly what they say on the tin - they allow current to bypass a poorly performing set of cells.

There are a couple of problems with this though.

▪ It wouldn't be practical to fit bypass diodes to every cell - manufacturers fit at most two or three per panel. So even if a single cell is shaded, you will still lose at least a third of the panel output.

▪ It needs a bit of a shove to get the current from the good cells through the diode - it won't just flow round of its own accord. In fact, quite a lot of the power that the good cells are producing will be used up in forcing the current through the bypass diodes.

▪ Inverters are designed to work with a specific size of solar array, with a given input voltage (or at least a band of input voltages). By the time you have lost the voltage from the shaded cells (and any unshaded neighbours on the same bypass diode), and then reduced the remaining voltage further by the amount needed to force the current through the bypass diodes, the remaining voltage will be a lot less. Often, it will drop below the start-up voltage for the inverter - which will then shut down. Even if it's not low enough to shut down, it certainly won't be working at its design voltage, and at lower voltages its efficiency will be a lot less.

According to one study, which you can download here, the output of a 1400W string - which was fitted with bypass diodes - dropped by 10% when only 4 cells were shaded. When 12 cells were shaded, the power output dropped by more than 50%!

The options you have if you have partial shading are:

▪ Install as normal - and have a poorly performing system. This, unfortunately, is the answer for many installers.

▪ Opt for a smaller array - just don't put panels in the regions which get shaded. Cheaper, and the money you do spend will be well spent. But you lose out on output.

Break the array into chunks, and put an inverter on each or use an inverter with dual MPPT. You can even go to the length of putting an inverter on every single panel - Enecsys micro inverters and Tigo voltage optimisers, for example, work on this principle. This will increase the price of your system, but it will be more effective.

http://www.midsummersolar.co.uk/en/frequently-asked-questions/9-technical-articles/26-the-effect-of-shading-on-solar-panels

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