Shading and how to mitigate it

Even a little bit of shading can have very detrimental effects, and not just to production. If shading is a factor in your installation, it's essential that you (or a solar consultant) understands it's impacts and the mitigating options that are available.

Understanding Shading

Shading comes in different varieties, based on the type of shading it is. There's no strict naming conventions to define these, although we'll use some common ones below. 

Soft Shading

Also known as objective shading, soft shading is caused by objects that are far away, most notably clouds. While this type of shading is not controllable, it's factored into your yield estimate that solar retailers provide. You'll also find that when it is cloudy, diffuse radiation still hits the panel, enabling a little bit of power generation. Shading from clouds is almost always uniform, and uniform shading is easier to manage. We'll discuss why later. 

Hard Shading

 Hard shading from close object

Hard shading from close object

Also known as subjective shading, hard shading is caused by solid objects close to the panel. Examples of this include flues, chimneys, leaves, dirt, trees, birds, bird droppings, other roof areas, antennas etc. These objects can block more sunlight on effected areas than soft shading. It's incorrect to assume that shading a small portion of the system is not a big issue, as non uniform shading is very difficult to manage. It can also cause unwanted side effects and damage to a panel. To understand this, we need to know how panels and strings work.

How Panels and Strings Work

60 Cell Panel Diagram. Click on the image to enlarge.

Panels are made up of solar cells, most commonly 60 cells. These cells are connected in series, with three bypass diodes installed on each sub-string of 20 cells.

When you have a string inverter in most standard installations, panels are connected in series. The voltage increases for every panel you have in the string, while the current remains the same. String length can vary, but for 60 cell panels they are usually around 6 to 14 panels long, depending on panel voltage and inverter limits. You can also connect strings of equal length (voltage) in parallel. This increases the current, while keeping the voltage the same. To do this, the inverter must be able to handle the added current.

Panels in this configuration are connected to an inverters Maximum Power Point Tracker (MPPT), which determines the most effective operating voltage for the string (or multiple strings) of panels connected to it. Most string inverters 3 kW and above have two MPPTs, which allows groups of panels to be connected and managed separately. This opens up multiple possibilities, like better shade management and added flexibility in design.

What Happens When We Introduce Shade

Loss of Current, Which Means Loss in Power

If a totally opaque object is blocking all the cell, no current is being produced. If a half-opaque object is blocking only half the cell, 25% of the cell current is lost. Current loss is proportional to the amount of sunlight being blocked. Overall loss is higher though, as cells connected in a string can only output at the current of the lowest producing cell. Remember, power equals the voltage multiplied by current, so dropping one of these reduces your power.

When it Really Goes "Pear Shaped"

At a certain point, a cell goes into "reverse bias voltage" where all flow stops and the cell converts current from the other cells into heat, creating a "hotspot". A hotspot can cause a range of unwanted effects, like burn outs, faster degradation and cracking of the cell and/or glass. All panels have bypass diodes installed which prevents hotspots, but they aren't perfect. Let's take a look.

Interesting note here is that only hard shadows will cause the above to occur. Soft shadows will impact all the panels evenly and not block as much sunlight. This is why hard shadows are worse than soft shadows.

Bypass Diodes

Bypass diode has activated. Click on image to enlarge.

All conventional panels these days are fitted with bypass diodes, usually three, which enables current to flow around any sub-strings that have a cell in reverse bias. This prevents hotspots from occurring. It also stops any lower current producing cells from lowering the current of all the cells. There's issues with bypass diodes though.

An activated bypass diode will cut off that entire sub-string, which is 20 of the 60 cells on a conventional panel. This cuts the output of the panel by just over a third (1/3 lost voltage plus a tiny bit of diode resistance). Unfortunately though, bypass diodes are not known for their lifespan, and regularly activating them will not help this. If the diode fails, it will either no longer protect the cells on the sub-string from hotspots (leading to further issues) or it will permanently disable the sub-string.

If you have two strings connected in parallel to one MPPT and one has activated bypass diodes, it's going to create false maximum power points. This is an issue with a string inverter. In this case, the string with the diodes activated will have a lower voltage. When this happens, even intelligent string inverters will lock onto the incorrect operating voltage, resulting in high yield losses. Soft shading, or light variations between parallel strings (like east/west), are not a big issue. These shading effects are uniform and don't activate the bypass diodes, meaning the voltages of each string are similar under these conditions.

Yes, bypass diodes serve an important purpose and are a critical feature on a solar panel. But there are better ways to manage shading in a system.

Panel Level Optimisation

A normal string inverter has to consider all the panels within an MPPT when managing output. Systems with panel level optimisation are able to manage panels individually, resulting in better performance in shaded conditions. Devices and products that enable panel level optimisation include:

  • Micro inverters, like Enphase.

  • Power optimisers, like panels with Tigo optimisers fitted on a standard string inverter.

  • Optimised systems like SolarEdge, which has a power optimiser in every panel.

Devices with panel level optimisation enable more drastic changes to voltage and current on shaded panels, while other panels can operate normally. Bigger changes to the voltage and current of shaded panels not only provides better output, but in some circumstances avoids the bypass diodes activating. Both of these are huge for shade management.

Cell Optimisation

Maxim Integrated's Optimiser (circled)

There's an electrical company called Maxim Integrated, who have developed a tiny chip which manages strings of cells within the panel individually. This enables even better shade management than systems that have panel level optimisation, and it usually costs less.

Unfortunately you can't just add these to any panel (one of our readers tried). Maxim Integrated are selling the chips to panel manufacturers, who embed them into the panel as a replacement for the bypass diodes. This provides the panel with three optimisers, one on each sub-string. Cell optimisers are a very new technology, but one that's growing in popularity with the panel manufacturers. More and more panel manufacturers are releasing a panel that is Maxim Integrated and we expect this to continue.

Shading Scenario And How It's Managed

Click to enlarge - Here we see a string of 12, 280W panels. All panels are in full sunlight, with the exception of one panel that has 40% shading on two cells. These two cells are on separate sub-strings.

How a String Inverter Would Manage This

In this case with only very little shading, the string inverter still has limited options. Given that only two cells are giving 60% output, lowering the overall current to 60% would result in a 40% loss, with the only other benefit being the bypass diodes remain inactive. The inverter will keep the voltage and current high, triggering two bypass diodes to activate, resulting in a production loss of just over 5.5%. As the shaded cells are on different sub-strings, two bypass diodes activate, cutting slightly over 2/3 of one panels production.

How Devices with Panel Level OptimIsation Would Manage This

Any device with panel level optimisation, so a power optimiser, micro inverter or optimised system, would manage this scenario much better. Firstly, all the panels without shading would be operating at their optimal voltage and current. The shaded panel however will operate at 60% of it's current capacity. Overall, this means the loss is less than 3.4% which is over 2% better than the string inverter. Furthermore, no bypass diodes have activated, which increases the lifespan of the panel. This is a much better result from a scenario with very small amount of shading.

How Cell Optimisers Would Manage This

Cell optimisers go a little deeper again. In this case, the shaded panel would have two sub-strings operating at 60%. The third sub-string would operate at full capacity. Now we're looking at a loss of less than 2.3%, which is even better than panel level optimisation.

Summary

The Scenario We Looked At

This scenario is just one of an unlimited amount of possibilities that could occur. However, it shows very small shading. There are many scenarios where the performance difference is greater and even some where it's smaller, it really just depends on the scenario. For example, if you completely shaded one panel, then the panel and cell optimisers are only going to be marginally better (unless you have parallel strings connected to one MPPT). If you had patchy shading across the entire array or multiple strings in parallel, it can get much higher.

Generally though, you can expect a notable production difference in shaded systems between string inverter systems and systems with panel level optimisation. You also reduce bypass diode usage, increasing panel lifespan. There are also gains between cell optimisation and panel level optimisation, but it's not as drastic.

If You Have Shading

A little bit of shading can be managed by the panel and a string inverter, but if panels are being shaded 10-20% or more of the time, you need to consider other options mentioned in this article. If the panel is going to be shaded more than 40-50% of the time, you need to reconsider installing the panel entirely.

Shaded systems require careful planning, which is where you'll depend on your solar retailer. A good retailer and consultant will identify any shading issues early and recommend a solution that manages these conditions effectively.

Not Just Shade Management

The products mentioned in this blog can offer more than just shade management. This includes greater flexibility in design, better performance in ideal conditions, better management of panel degradation and soiling, panel level monitoring etc. This blog was just to look at how this technology manages shading.

Further Reading and a BiG Thanks

We came across some excellent articles and sources of information when putting together this post. We have listed these below. They are quite technical but explain shading in much greater detail. I would also like to thank Rodd Zhang, who clarified some technical points in his excellent article and the effects of shading.