Well, to a certain extent, yes.
You see, solar cells are connected in series, and when one cell in the series does not work, no current passes through and the entire series of cells does not generate electricity.
Hard shading is created when a physical object, such as a telephone pole, or tree is physically obstructing the sunlight, creating obvious visible regions of lit and unlit cells on the array.
How hard shade affects the electricity output from a PV cell is not easy to determine, as the physical geometry of the shade comes into play. As long as there is a solid strip or channel of illuminated material between two electrodes on a PV cell, there will be some electric current. The current is proportional to the surface area of the cell that was illuminated and the shape of the shadow does not appear to matter as much as the area of the shadow.
However, certain shapes of shading would create narrow areas of illumination in which the current would squeeze and be focused in the narrow illuminated portions of the cell, creating areas of extremely high temperature, known in the industry as “hot spots.” These hot spots in very rare cases have been known to cause burn-outs and small fires within modules, as they may have the current from an entire module being pinched into a very tiny area of solar cell.
If there is no complete illuminated path between electrodes or the entire cell is shaded, then no current will flow through the cell, and its voltage output will collapse. This has the effect of “opening” the circuit, as there is no longer a complete path for the electrons to travel.
Most modern solar cells are designed to lower the odds of this effect by imprinting silver leads across the cell. This is an attempt to provide more possible paths for an electron to take across the silicon, much the same way that a freeway allows more rapid travel through a densely packed city. As a result, as long as these silvered-cells have any light exposed to them, they will generate power.
Let us try to explain how shading and the poor function of one cell can affect the overall output of the module as follows:
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.
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, but 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.
These diodes are not a perfect solution to the problem, but they could significantly cut down the losses from shading that would have happened otherwise.
According to one study, the output of a 1400 W 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 – the so-called micro inverters, but these can be quite expensive.
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