Although crystalline solar panels are often sold with 25- to 30-year lifespan guarantees, those 30-year-old modules won’t be performing as well as they did on Day 1. Performance declines as solar cells degrade due to unavoidable circumstances like UV exposure and weather cycles. Manufacturers realize this, so solar panels come with a power output or performance warranty that usually guarantees 80% production at 25 years.
Panel companies are only comfortable offering this guarantee because of a 2012 NREL study (“Photovoltaic Degradation Rates—An Analytical Review”) that found solar panels degrade about 0.5% to 3% each year, barring any equipment issues.
So panels degrade automatically; that’s worked into their performance warranties. There are also outside forces that can contribute to a panel’s degradation and possible failure. We talked with Sarah Kurtz, research fellow at NREL and co-author of that oft-cited 2012 study, on how technology and manufacturing changes, along with installation practices, affect degradation rates.
A complex issue
According to NREL, modules can fail because of unavoidable elements like thermal cycling, damp heat, humidity freeze and UV exposure. Thermal cycling can cause solder bond failures and cracks in solar cells. Damp heat has been associated with delamination of encapsulants and corrosion of cells. Humidity freezing can cause junction box adhesion to fail. UV exposure contributes to discoloration and backsheet degradation. These things just happen, and it’s difficult to determine how bad the degradation will be.
“Solar panel degradation and failure is not a clear-cut situation,” Kurtz said. “There are lots of different reasons why they degrade and why they fail.”
Kurtz said module manufacturers are looking into every piece of the solar panel puzzle, all the way down to the encapsulants and adhesion materials, to try to slow degradation rates.
“Companies are figuring out how to change the formulation of the encapsulants so they don’t yellow,” she said. “In my opinion, they’ve made great progress in solving this problem.”
New inverters, higher voltages and PID
If it wasn’t bad enough that solar panels turn on themselves after years in the field, outside products can also contribute to degradation levels. The increased usage of transformerless inverters on U.S. solar projects has raised the threat level of potential induced degradation (PID) of solar panels. PID happens when different components in the same system are at different voltage potentials (such as the frame and the solar cell), which can allow electrical current to leak and modules to lose their peak performance. Often, simply negatively grounding a system removes the concern for PID, but transformerless inverters are ungrounded.
When that electrical current leaks, sodium ions in the glass move toward the solar cell or the frame, depending on how the system is grounded. There’s also an issue with the whole industry moving to higher voltages, because higher voltages make that current pull stronger, and sodium ions move more easily over top solar cells, reducing their output.
Frameless modules can help reduce the PID possibility (since there’s no metal frame to disrupt voltages). And many module manufacturers take extra steps to ensure modules are PID-free now. It’s important for installers to know what products they’re combining into a full system to know if something besides the panel may contribute to degradation.
Cheaper panels and less material
Back in 2015, NREL surveyed New York installers and found that many were having the same issues with new solar modules. As module companies were trying to lower their prices, they made their frames thinner to reduce the aluminum.
“[Installers were] finding that those frames will bend,” Kurtz said. “As snow melts and then refreezes on the edge of the module, that puts quite some strain on the frame. Those newer frames would bend.”
Bent frames can strain the whole panel, and it can be especially bad as panels get thinner and less mechanically robust.
“When people squeeze the cost down, they can find low-cost materials or they’ll try to reduce the total amount of material,” Kurtz said of today’s modules. “As you optimize the cost of the module, you’ll tend to see more mechanical failure mechanisms.”
More, thinner busbars
Solar panels sometimes fail because of busbar solder bond failures. With the trend of more busbars on solar cells, you would think there is a higher chance of solder bond failures. That’s not entirely true.
“Cells can easily break,” Kurtz said. “If you have a big ribbon with a big solder bond, it puts more local stress on the cell and causes them to be more likely to break. By reducing the size of those solder bonds, you can reduce the amount of stress at the point where that ribbon gets connected to the cells.”
With more busbars and more solder bonds, there is a higher probability of solder bond failure. But the importance of one solder bond failure goes down when there are more busbars to pick up the slack. Also, more busbars across a solar cell can decrease the chance of full cell breakage.
Flexible panels and installation
As module companies decrease their costs, they may turn to ultra-thin solar cells that use less silicon. Thinner solar panels are more flexible and not as rigid as older module models, which makes installation a delicate process.
Hand-to-hand transport can affect a module, especially if installers are carrying modules on top of their hardhats. That flexing and bouncing up and down can take a real toll and lead to microcracks in the cells. Same with dropping a module and the biggest no-no—standing or walking on top of solar modules.
“It doesn’t necessarily stop working right away, but it will degrade with some time,” Kurtz said.
What can we do?
Not all new technologies are bad, nor are all modules destined for failure. Kurtz mentioned that recent NREL research has found fewer PV module issues being reported. And although the types of problems may be changing, module warranties are increasing and system lifespans are getting longer.
Smart buying and installation of solar panels and other project components can mitigate potential degradation chances. Using trusted products and installing them with care will ensure a solar system will perform at its best—with no more than 3% power loss each year.