Figure 1: National Renewable Energy Laboratory (NREL) researchers Tim Silverman and Nicole Luna looking at data generated by PLatypus, which is linked to a photovoltaic system at the Flatirons School District.
To help solve the huge problem of climate change facing humanity, we need to build tons of solar power plants. These plants can produce cheap, clean electricity, but only if they can operate reliably over the long term.
That's why Tim Silverman, a senior scientist at the U.S. Department of Energy's National Renewable Energy Laboratory (NREL), and colleagues are detecting problems in existing solar cells with the Platypus system, a camera system designed to detect invisible, stealthy problems in on-site photovoltaic modules.
The Platypus system images photoluminescence, which occurs when light is absorbed and re-emitted at a longer wavelength. Solar cells exhibit photoluminescence, and the stronger the re-emitted light, the better the quality of the solar cell.
"When the inside of a solar cell ruptures due to severe hail weather, we can't see the cracks with our eyes." Silverman describes, "But the crack creates two new interfaces where charge carriers tend to recombine in a way that doesn't produce light. As a result, when we take photos of photoluminescence, the area where the crack exists shows up darker than the surrounding area."
Creating photoluminescent images
To create the photoluminescent images, the researchers shone monochromatic light onto the solar panels and used a camera to collect the re-emitted light. It is best to perform this operation at night to avoid sunlight.
Silverman says, "Photoluminescence intensity is much dimmer than incident light, so we needed to design a system that could emit a lot of light and collect the small amount of light that was returned after the wavelength shift."
It requires powerful LEDs, sensitive cameras and filters to prevent the raw light from appearing in the image.Silverman says, "For a long time, this was a small-scale measurement done in the lab. But fortunately, as the main devices have gotten better and cheaper, the photoluminescence intensity of solar cells has gotten stronger, which allows us to do this today on a large scale and at a low cost."
Despite the fact that most solar power plants work well, the researchers designed their PLatypus tool to detect invisible faults in solar panels.
Silverman says, "I always thought of photoluminescence imaging as a specialized technique used in the lab. Then my friend Will Hobbs did it in his garage with a product he ordered off the Internet, and it was a pleasant surprise. We realized we could apply the device in the garage to use in a real power plant."
Photo Figure 2: National Renewable Energy Laboratory researchers Tim Silverman, Nicole Luna, Byron McDanold, and Josh Parker test a prototype of the Platypus system in a photovoltaic system on the Flatirons campus.
An interesting challenge for the researchers was that midway through their first full-scale demonstration, it started snowing.Silverman and colleagues didn't have an easy way to reschedule their demonstration, so they had to clear the snow from the solar panels as they demonstrated. Their proof-of-concept prototype proved to be snow-proof.
"Proof of concept demonstrated through prototyping that our technology was feasible and that the cost and system complexity were reasonable." Silverman said, "But at the moment it is not a user-friendly tool for everyday use. In future projects, we hope to produce a new version. People can then use Platypus to check for themselves so they don't have to worry about whether there is unseen and invisible damage to the power plant."
