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Ultra-Fast Weathering System Developed
September 29, 2009
NREL's Ultra-Accelerated Weathering System tests products for outdoor durability. Researchers who worked on the project include (left to right), Judy Netter, NREL; Henry Hardcastle, Atlas Material Testing Technology; and Al Lewandowski, Gary Jorgensen and Carl Bingham of NREL. | Scientists at the National Renewable Energy Lab, working with researchers in Russia and Arizona, have developed a way to shine the equivalent of 50 suns on new products that need to be tested for outdoor durability.
Product manufacturers can find out in 10 weeks how new paints or new anti-corrosive materials will look in 10 years.
"What's been done in the past is the Rip Van Winkle approach to testing," NREL senior scientist Gary Jorgensen says. "You put a product on exposure outdoors and go to sleep for 20 years. You wake up and see how long it lasted."
The challenge has been how to simulate the effects of decades of the sun's rays, without getting things so hot that the product is destroyed because it heats up more than it ever would in the real world. If the product gets too hot, it will blister, peel, melt or otherwise fail to function.
Sun damage on outdoor products comes almost exclusively from UV rays, the same light that causes skin cancer in humans. The thermal load itself does little damage. However, the thermal load of 10 or 50 suns certainly would damage the sample.
NREL's Ultra-Accelerated Weathering System (UAWS) can reflect virtually all of the sun's UV rays onto the samples, but attenuate the visible and near-IR rays. The result is a system that quickly gives an accurate portrait of what the sun will do to a product in 10, 20 or 30 years.
The seeds of the idea go back almost 15 years. In the mid-1990s, NREL was involved with accelerated testing that would allow companies to predict with confidence what the service-life should be on their products.
"It occurred to me that at NREL we already had a high-flux solar furnace that could provide exposures of 2,000 suns," Jorgensen recalls.
The furnace wasn't suitable for product testing, though, because it couldn't shine rays evenly over samples. Serendipitously, an NREL scientist working on the furnace also was embarking on a separate project to homogenize the solar flux.
NREL researchers found they needed help and looked to the end of the Cold War to find it. The U.S. government had freed up some money to put former Soviet nuclear weapons scientists back to work. The aim of this initiative was to re-employ the laid-off Soviet scientists so they wouldn't be tempted to sell weapons technology to unstable regimes.
"The Russian group designed the coated mirrors and the faceted mirror array," Jorgensen says. "They worked on a socially redeeming project."
The UAWS has an array of 29 square mirrors with spherical curves. Just as a lighthouse uses numerous pieces of bent glass to magnify light, the UAWS uses the curves of the mirrors to concentrate UV.
Each mirror has 96 layers of vacuum deposited coatings on them, Jorgensen says. Each layer of film uses alternating high- and low-refractive materials. The thickness of each layer helps produce mismatched refractions which transmit some wavelengths and reflect other wavelengths back toward the sample chamber.
The array reflects the UV portion of the solar spectrum back to a chamber that holds the samples. By attenuating most of the thermal and IR spectra the system keeps the heat down to about 35 C. "It's called spectral splitting," Jorgensen says.
The samples still can get hot just with the small sliver of thermal energy allowed in. So, to more precisely simulate the damage the actual sun will do over time, samples being tested are attached by vacuum to chilled copper blocks during exposure. Also, cool air blows across the surface of the samples to keep the temperature down.
NREL scientists have correlated exposure at the UAWS with real-time outdoor exposures and exposures in controlled chambers that use artificial lights sources. With UAWS they've demonstrated product lifetimes of 20 years based on two summer's worth of exposure.
NREL also involved Atlas Materials Testing Technology. "The best acceleration technology they had was at six suns," Jorgensen says. "We told them we could do better than that by a factor of 10. They were interested and are now commercializing the technology."
Source: NREL
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