As rooftop PV systems age, installers need to understand how system performance can be affected by potential induced degradation, writes Michelle McCann.
Potential Induced Degradation (PID) is a failure mechanism for solar panels. Panels that exhibit PID can have a profoundly reduced output power, with complete failure (0% power output) possible.
Potential induced degradation occurs when panels are at a high (negative) electrical potential relative to ground. Essentially, ions in various parts of the panel find themselves free to wander about and a current pathway is created between the solar cells and the frame of the module, leading to current leakage from the module to ground.
PID accelerates with higher temperatures, higher humidity and higher system voltages. While the exact mechanism of module degradation remains a subject of active research, it is clear there are multiple pathways to degradation and that a favoured explanation for at least some of the degradation is due to the migration of sodium atoms from the glass. Regardless of the exact cause, the result of PID is reduced system output.
Ion migration may be the root cause of PID but another way of conceptualising the situation is to acknowledge the fact that the combination of sun, high voltages and outdoor life is quite powerful, really, and if you leave things outside for long enough, changes will occur. These changes usually lead to less power output from the panel.
Solar panels operate in some of the harshest environments our world can throw at them. They undergo high temperatures, high temperature variations (sometimes in very short time periods), extreme weather events, strong UV, are subjected to a high electrical potential and we expect them to perform for decades.
Evidence that the outdoors is a harsh environment for our solar panels exists in the number of degradation mechanisms that are acknowledged: not just PID, but also LID (light induced degradation), LeTID (light and elevated temperature induced degradation) and STBID (soon-to-be-identified-degradation).
Is it a problem?
Since PID can occur in systems above 600V, it can occur on anything above a mid-sized rooftop system.
Domestic systems have two sets of groundings. Firstly, there is equipment or protective grounding, whereby the module frame is connected to earth. This is an excellent idea for safety reasons and is done in all domestic systems.
Secondly, there is system or functional grounding. This can be done to the negative pole of the inverter, or the positive, or occasionally floating between. PID only occurs in panels that are under a high negative potential. It can therefore be avoided completely when the negative side of the inverter is grounded, such that all panels experience a positive potential relative to earth. Grounding is usually done following recommendations of the panel supplier and inverter manufacturer and is subject to specifications of the inverter and panel technology. In many cases this will mean the negative side of the inverter will not be grounded.
In some cases PID can be reversible, but only if symptoms are mild and it is caught in the early stages. Most residential rooftop systems would not warrant monitoring sophisticated enough to capture PID problems.
Given increasing price pressure on manufacturers, it is hardly surprising that shortcuts are sometimes taken in material quality and workmanship. There is no doubt, however, that solar panels can be made to perform for decades. Recent studies from the TISO 10kW plant, which has been in operation in Lugano Switzerland for over 35 years (it was installed in 1982 and was the first grid-connected PV plant in Europe!) show some stellar panels still operating at 95% of their initial power.
If you suspect that modules may have degraded, you can test for this: a combination of electroluminescence and STC power tests can usually determine whether or not degradation has occurred. What can you do to ensure PID won’t be a problem? Purchase carefully and test, test test the products that you buy.
Many module manufacturers offer “PID-free” modules, but not all modules marked PID-free are free from the effects of PID. Testing for PID is a mandatory requirement for most solar farms built globally, but there is currently no possibility to test for PID in Australia.
The standard for PID testing is IEC 62804. There are two tests in this standard, the aluminium foil test and the chamber test. The chamber test is the better of the two and is done in a damp heat chamber. It requires the panel to withstand 60°C at 85% humidity under a bias of 1,000 or 1,500V (according to the module specifications) for 96 hours. A module can be labelled as “PID free” if, following testing, the drop in output power is less than 5%.
Many manufacturers are opting for more stringent testing; up to 600 hours and there are moves to require the test to be done at 85°C. Nonetheless, even a “PID-free” module is not necessarily free from the effects of PID. In many cases a deeper understanding of the test results is warranted.
In response to the fact that there is currently no possibility to test for PID in Australia, PV Lab Australia submitted a grant application to develop PID testing capability. Happily, the application was successful and with assistance from the ACT Government under the Renewable Energy Innovation Fund we have begun to develop this capability.
The work was divided into four stages. In the first stage we worked closely with Dr Peter Hacke from National Renewable Energy Laboratory (NREL) in Colorado. Dr Hacke, one of the leading global experts in the field of PID, led the panel of experts that developed the first IEC standard for PID testing (IEC 62804, released in 2015).
The grant allowed for a research trip to NREL and timing presented an opportunity for us to combine this with attending NREL’s PV Reliability Workshop. The workshop was outstanding value. Following the workshop, we spent some days at NREL with Dr Hacke and various members of his team. We discussed both the aluminium foil test and the chamber test in detail and decided to aim high and develop the chamber test for the Australian market.
The second stage was the tendering for and purchase of equipment. Our equipment is in two parts: the chamber that will house the modules during testing and the electrical rack that connects to the panels and supplies the high voltage during the test. The chamber was purchased from Labec, a specialist manufacturer in Sydney. The rack was built together with assistance from staff at the Australian National University.
We are about to enter stage three, our first tests on commercially available modules. Watch this space for our first results!
Dr Michelle McCann is a partner at PV Lab Australia and had twice held the world record for high-efficiency solar cells.