Bypass Diodes and Shade: What’s the Benefit?

The panels are equipped with bypass diodes that can reroute the current if a cell fails. However, when solar cells are shaded, a unique situation arises. A shaded cell isn’t broken, but it doesn’t work as effectively as the adjacent cells that are in the sun. Therefore, some current can flow through the shaded cell, but certainly not as much as through the other cells. In such cases, does the current pass through the cell or is it rerouted via the bypass diode?

The answer is: it can be either, depending on how many cells are shaded. The inverter also plays a significant role. Suppose there is only one cell in the shade (perhaps a leaf from a tree has fallen on it) while all other cells are in the sun. In this case, the current forces its way through the shaded cell. Although the shaded cell cannot pass much current, it is compelled by the others. What happens then is that the shaded cell allows the current to pass through but begins to absorb energy instead of producing it. The energy absorbed by the cell is emitted as heat; the cell becomes much hotter than the other cells. This phenomenon is known as a ‘hotspot’.

When multiple cells are shaded, the current has two options: In one case, it can flow through all these cells, but since some are shaded and thus pass less current, it becomes a mere trickle. The shaded cells dictate how much current flows, which affects the cells in the sun that could otherwise produce much more.

In the other case, the current bypasses the shaded portion and uses the bypass diode route. This prevents the current from being ‘restricted’ by the shaded cells, maintaining a much higher yield. This is the more desirable situation, but how can we achieve it?

The answer varies depending on the situation. Ultimately, it comes down to: How much does it cost to install an optimiser, and how much additional yield does it provide? Some systems with optimisers, such as those from market leader SolarEdge, only work if optimisers are installed behind all panels, regardless of whether they are shaded or not. This can impact the price and may make the system more prone to faults (as there are more components). At the same time, the additional yield from optimisers might not justify the cost; if it’s just a few small shaded areas in the morning and/or evening, when the sun is already weak, the investment in optimisers might not be warranted.

Whether an inverter handles shading effectively depends on the software used for the MPP tracker. All current inverters from SMA are equipped with ‘OptiTrac Global Peak’, which ensures that shaded (parts of) panels are bypassed in the presence of shadow. Some other inverter brands have similar systems, but many do not. Therefore, be sure to pay close attention when selecting an inverter!