Solar Panel Shading - Failing Bypass Diodes
Introduction
Partially shaded solar panels can result in a significant decline in performance. Panels contain internal bypass diodes that help mitigate the effects of shading. However, in certain conditions, years of regular shading can lead to accelerated diode failure and permanent damage to the solar panel. If left in a damaged state for a long time, it can potentially lead to dangerous consequences. In this article, we'll delve into the challenges posed by solar panel shading and explore the potential issues that can occur with failing bypass diodes and how they can be avoided.
Types of shading
Shading can result from trees, nearby buildings or fixed obstructions like chimneys and evaporative cooling systems, plus natural elements like dust accumulation or bird droppings. When a panel is shaded, it significantly reduces the amount of sunlight reaching solar cells, decreasing energy production. However, shading is generally not uniform, and depending on the amount of shading encountered, there will be very different outcomes.
Shading can be over the entire solar array (across all panels), partial shading across some panels, or shade can happen in a small area over some of the cells on individual panels. While shading across a whole array will severely reduce performance, partial shading on one or more panels may not have much impact on the overall performance. However, partial shading is potentially more damaging if it occurs regularly in the exact location. For example, if a rooftop cooler or chimney is located next to or in front of a solar panel. Regular permanent shading can lead to the formation of hot spots (hot cells) and failure of the bypass diodes, which can significantly decrease solar generation and even develop into a potentially dangerous situation after several years. Learn more about hot spots and the various types of solar panel and cell degradation.
Severe shading across several solar panels resulting in poor performance.
How shading affects solar panels
The majority of solar systems use standard string solar inverters. In such systems, partial shading over one or two solar panels will result in a noticeable decline in overall system performance. Understanding why shading poses such a problem requires a basic understanding of how solar systems function. In string solar setups, panels are interconnected in series strings that typically comprise 3 to 14 panels. This configuration is used because panels connected in series generate a higher voltage, optimising the efficiency of the solar inverter in converting DC electricity to AC electricity. Nevertheless, if any panel within a string encounters significant shading (over more than half of the panel), it can adversely impact the performance of the entire string due to reverse current flow into the shaded cells. As I explain below, thanks to bypass diodes, small areas of shading over a few cells do not have a large impact on overall system performance. It is worth noting that solar systems with microinverters do not suffer from the same performance issues when shading is a problem, as these systems allow each panel to operate independently.
Bypass Diodes – The simple solution
When a portion of a solar panel is shaded, the shaded cells will produce less power (low current). Meanwhile, the unshaded cells will be producing full power (high-current), and a reverse current situation will occur where the current can flow back into the shaded cells, resulting in overheating of the cell. This is where bypass diodes save the day. Diodes are small, cheap, long-lasting devices that only allow electrical current to flow in one direction. These are available in many varieties and are used in all kinds of power electronic devices.
How bypass diodes work in a solar panel
Most modern solar panels contain bypass diodes to provide an alternate current path when a cell or multiple cells become shaded or faulty. The diodes are generally located within the junction box on the rear side of the PV module. Diodes are relatively simple devices that allow current to only flow in one direction, enabling current to bypass the solar panel under certain conditions. They do this by opening or closing depending on the voltage bias direction. Most solar panels are divided into three groups of cells connected in series, with each group containing a bypass diode. If it were cost-effective, there would be one bypass diode for each cell, but this is complex and expensive to integrate, so most manufacturers use one diode for a group of series cells.
A basic solar panel diagram showing the 3 bypass diodes (one for each group of 20 cells). Shading some of the cells causes a reverse current and forces the diode to open which bypasses the shaded part of the panel.
How shading can lead to bypass diode failure
When a bypass diode is activated due to severely shaded cells, some energy passing through the diode is dissipated as heat. Continuous operation of bypass diodes under shaded conditions can eventually lead to overheating and potential failure. Diodes are more prone to failure over time due to factors such as temperature fluctuations, humidity, and electrical stress during continuous operation—learn more about bypass diode failure modes. The different types of shading (light or heavy shading) have a big influence over whether or not the diodes will be activated and what amount of stress they will be under. Light shading from nearby trees is less problematic than heavy shading from close, fixed rooftop-mounted objects.
Light shading
If your system suffers from regular shading over a large area due to a nearby tree, the panels will encounter what could be judged as medium shading or light shading, especially if the tree loses its leaves in winter. In this instance, due to the large amount of shade over numerous panels, the bypass diodes may not be forced to activate. Additionally, the string voltage and current will be lower, resulting in less heating if the diodes are activated. In this situation, the diodes may function without issues for several decades.
Heavy shading
On the other hand, it has been found that rigid or fixed shading over a small area on panels installed next to roof vents and chimneys can cause diodes to fail prematurely, allowing reverse current to heat up the shaded cells, resulting in burn marks from extreme hot spots. This is due to two reasons: the panel and junction box temperature is much higher when most of the panel is exposed to sunlight, and voltage and current flowing through the panels and diode is higher when only a small portion of a panel is shaded. These two factors result in increased thermal stress placed on the diode.
What happens when bypass diodes fail?
When a diode fails, it can happen in two ways. It may become permanently open (open-circuit) or permanently closed (short-circuit). In my personal experience, and what other installers have reported, they tend to fail permanently closed (permanently bypassed), resulting in reduced power at all times, even without shading. However, bypass diodes that fail in an open position can have far greater consequences, as the reverse current cannot be blocked when cells are shaded. This can lead to overheating of the cell and a thermal runaway, resulting in internal arcing and burning of the rear polymer back sheet. If left for a long time, it can even lead to fire.
The whole idea of bypass diodes on panels is that they are rarely used. Daily activation due to hard shade from nearby objects, especially during hot summer weather, eventually leads to diode failure.
Can bypass diodes be fixed or upgraded?
An older type solar panel junction box showing the three serviceable bypass diodes in the middle.
Solar panels contain one or more junction boxes mounted on the rear side, which house the bypass diodes and provide a secure connection point for the interconnection cables. Until around 2017, most solar panels were of the standard 60-cell format and had a large single junction box with a removable lid, allowing users to inspect and even replace the diodes. Unfortunately, this design sometimes led to water ingress, and over the years, the junction boxes evolved into smaller enclosures filled with potting compound (polyurethane), which are not serviceable. The diodes used in solar panels are Schottky diodes, common semiconductor-metal based diodes. These low-cost diodes are typically rated at 30A or higher and can withstand up to 1000V.
Unfortunately, it is almost impossible to replace modern diodes. By almost impossible, I personally managed to replace several blown bypass diodes around two years ago, but not without great difficulty. The process involved carefully scrapping out the potting material from within the junction box, cutting out the blown diode, soldering in a new one, and finally refilling the junction box with a fire-resistant filler. Replacing three diodes took several hours, but it was successful, and the panels are still in operation today. Unfortunately, if the panel fails for another reason, it will not be covered by warranty due to my modifications.
Junction boxes in modern solar panels are not designed to be serviceable, so replacing a faulty diode is extremely difficult as they are encased in a potting compound that is almost impossible to remove.
How to test if a bypass diode has failed?
Determining whether a bypass diode has failed in a solar panel can be difficult, especially on modern panels, which do not allow easy access to the diodes. Here are three methods to help detect a faulty bypass diode:
Visual Inspection and Thermal Imaging
Visual Inspection: On older panels with a serviceable junction box, you can conduct a visual inspection of the diode for any signs of damage, such as cracking, discolouration, or melting.
Thermal Imaging: If the bypass diodes are not accessible, using a thermal imaging camera can help detect failed diodes. A solar panel with a faulty diode during normal operation may exhibit abnormally hot cells compared to functioning ones. This method is particularly useful for identifying issues in real-time and can be conducted under normal operating conditions without dismantling the panel.
Electrical Testing with a Multimeter
Diodes in panels with a serviceable junction box can be tested by disconnecting the solar panel from the array and using a multimeter to test the bypass diode directly. A working diode should show low resistance in one direction (forward-biased) and high resistance in the opposite direction (reverse-biased). If the diode shows low resistance in both directions or high resistance in both directions, it is faulty.
Performance Monitoring
Monitoring the performance of the solar panel can indirectly indicate a bypass diode failure. If a particular section of the panel (usually divided based on the number of cells connected to a single diode) is underperforming, producing less power than expected or fluctuating power output, this can be a sign that a diode has failed. This underperformance is typically notable in shaded conditions where a functioning bypass diode would normally mitigate performance loss. Data from performance monitoring systems can be analyzed to detect these anomalies, which could suggest a diode problem, among other potential issues.
Each of these three methods has its advantages and should be chosen based on the tools available and the type of solar installation. However, working on solar arrays should only be done by experienced solar professionals as you can be exposed to dangerous voltages. Additionally, working at heights on rooftops can be very dangerous.
Potential diode improvements
Diode lifespan could be increased by integrating small aluminium heat sinks to dissipate heat, but the problem with this is that any exposed metal could be extremely dangerous and cause electric shock or short-circuit. Another improvement is to use durable, water-proof junction boxes that cover a heatsink and provide airflow. Or even use higher grade TO-220 package Schottky diodes. However, manufacturers are in a constant cost battle, and considering the already low cost of solar panels, the extra costs mean that the numerous competitors will get an advantage. Unless, of course, it is marketed as a premium feature. We can only hope this problem is addressed at some stage with advancements in diode or panel designs. In the meantime, it is clear that panels should always be mounted to avoid direct shading, especially from roof-mounted obstacles.
How to reduce the effects of shading
Check that no trees or branches have grown significantly and are causing shading on one or more panels. Note this may not be obvious during the middle of the day when the sun is much higher in the sky. More importantly, pay particular attention to shadows cast by fixed rooftop obstacles such as chimneys, both in the early morning and later in the afternoon, especially in summer. Depending on your latitude, the sun's path can vary dramatically from summer to winter, so ensure you check throughout the year. Shading in summer can result in much more severe problems due to the higher temperatures, which can accelerate degradation and diode failure.
Solution - If regular shading on a few panels is a problem, it can be overcome by adding power optimisers to the affected panels, such as those from Tigo Energy or Huawei. Power optimisers are small add-on devices attached directly to each solar panel, enabling each panel to operate independently. If significant shading occurs across most of the panel, the optimiser will bypass the entire panel meaning the bypass diodes will not need to be activated as the panel is effectively isolated from the string. Another obvious solution is to relocate the shaded panel or panels to an unshaded area if there is a roof area available. If the panel cannot be relocated, removing a severely shaded panel from the string might be a worthwhile precaution to prevent a diode failure leading to a potential catastrophic failure in the future. Either way, the shaded panel will not be performing well, so removing it completely is a safer option.
During the design stage of a rooftop solar array, if hard shading cannot be avoided, power optimisers should be added to the string. Alternatively, microinverters should be used instead of string inverters.
A power optimiser mounted on the rear side of a solar panel helps avoid diode failure if the panel is partially shaded.
