Without True Standards, The Accuracy Of PID-free Products Must Be Proven
Potential induced degradation (PID) of solar modules has been known in the industry for more than a decade, but it hasn’t been a huge concern in Singapore. That will soon change, as new products and installation techniques enter the market.
But first, let’s start with a definition—what is PID? As Jenya Meydbray, section head of module and inverter testing at DNV GL, explained, PID happens when different components in the same system are at different voltage potentials. Solar cells are usually sandwiched between glass and a metal frame. A potential voltage difference can build between the cell, glass and frame. This allows electrical current to leak and the module loses its peak performance.
“The solar cells in the panel are physically very close to the frame,” he said. “Typical glass is low-ion sodium glass, a standard piece of glass. Sodium ions in the glass can drift in one direction or another. If you have a voltage potential, those ions will slowly drift out. The problem with PID is that those sodium ions penetrate into the cell surface and can damage certain cells.”
These sodium ions move around depending on how a system is grounded. Singapore almost always negatively-grounds its systems, while Europe has been installing ungrounded or “floated” systems. Negatively-grounded systems haven’t had PID concerns because that grounding pulls the sodium away from the solar cells and toward the frame, avoiding damage.
“The sodium will either be driven toward the cell or away from the cell,” Meydbray said. “You can get accumulation of salt around the frame when it’s driven away from the cell. If you have a negatively grounded system, the sodium goes away from the cell and you have no degradation concern.”
PID issues have been more apparent in Europe because their ungrounded systems allow salt to build up on the cells. Solar modules promised to perform for 20-plus years were degrading much more quickly than expected.
Meydbray said the concern for PID has risen because of transformerless inverters. By definition, systems using transformerless inverters are ungrounded and therefore more susceptible to PID—acting more like systems in Europe. Transformerless inverters are more efficient and cheaper, so the trend of installing them is unlikely to reverse. Contractors should therefore be conscious of how different pieces of the solar power system work together.
“Testing is highly doable in a short time frame,” Meydbray said. “For large buyers of PV equipment, they should test their equipment. If there’s a PID issue with a module, then they shouldn’t mate that module with a transformerless inverter. They should do the necessary homework to find modules to work with that inverter.”
While the inverter alone doesn’t contribute to PID, its relationship to the project as a whole opens the door to degradation. Steve Reed, product manager for U.S. Technology at SMA America, said PID concern is exceedingly rare among its inverter customers, and the SMA America Service Center has received zero calls about PID-related production losses.
“Integrators should work with their module suppliers to ensure they select readily available, PID-free solutions,” he said. “Transformerless inverters are not compatible with modules that require grounding. In the case that an integrator is concerned about PID, SMA offers a PV-Offset box, which is a voltage bias device that can be paired with select inverters. However, given the relative absence of PID among a massive European installation base, this unit has rarely been requested by our customers.”
Reed suggested the best action plan is to be diligent with yearly maintenance.
“[Crystalline-silicon] PID degradation is generally reversible and actions can be taken post-operation,” he said. “It usually makes more economic sense to use yearly maintenance measurements to detect the existence or absence of PID and then take action only if necessary.”
But what if a module claims to be PID-free? Will that solve the problem before it even exists? Many module manufacturers have gone through harsh testing conditions to confidently proclaim their modules PID-free. But this testing is merely suggested right now, and nothing is standard.
“There’s not a real certification against this, it’s just that we follow this protocol,” said George McClellan, technical sales manager at REC Group. “I think as an industry, we need to have a definition around what truly is PID-free. It’s kind of like the salt spray or blowing sand test—all of it is kind of subjective, but there is certification around them.”