UNDERSTANDING 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. 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 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 affected 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
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.
In a string inverter system, 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
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.
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.
In string systems, because the panels are connected in series, a panel producing less current reduces the current (and therefore the power produced) in the entire string. A completely shaded panel will produce much less voltage. If enough panels in the string are shaded the voltage of the string will drop to the point where the string produces little or no power at all.
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
Solar panels 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 are issues with bypass diodes, however.
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 in a short circuit it will no longer protect the cells on the sub-string from hotspots, leading to further issues. If it fails open circuit 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.
DIODE FAILURE DETECTION
Bypass diodes can and do fail. This can be due to long periods at high current and high temperature when they are actively bypassing shaded cells, or due to their Peak Inverse Voltage rating being exceeded such as when a nearby lightning strike occurs. The practical way to determine the health of Bypass Diodes is with an I-V curve tracer such as the HT Instruments IV400. By analysing PV array, string, or module I-V curves, it is possible to quickly identify anomalies due to short circuit Bypass Diodes.
Failed diodes can be quickly detected using a thermal imaging camera. This photo shows an IR image of the PV module with a failed bypass diode. In the PV module, the solar cells connected to the normal bypass diode and failed bypass diode have different surface temperatures. The reason is that a failed bypass diode constitutes a closed circuit with the connected solar cells, and the current generated from the solar cells induces heat in the solar cells. Therefore, the surface temperature of the solar cells on the module connected to the failed bypass diode is higher than that of the solar cells connected to the normal bypass diode.
PANEL LEVEL OPTIMISATION
A normal string inverter has to consider all the panels within an MPPT when managing output. Enphase systems are able to manage panels individually, resulting in better performance in shaded conditions.
Enphase systems 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 important for shade management.
Importantly these features also reduce stress on the panels, leading to much better reliability. In addition, any panel failures are easily spotted using the Enlighten monitoring.
