What will it take for light-emitting diodes to achieve critical mass in the lighting marketplace? The biggest obstacles have been cost and cast. These futuristic nanofiber and nanoparticle technologies could overcome both.
Light-emitting diodes (LEDs) are regarded by the lighting industry as the future of general illumination. LEDs have been around for years, but they still hold only about a seven percent share of the light source market.
Replacing conventional lights with LEDs is a costly proposition that takes years to pay off. LEDs do pay off, eventually, because they save more energy and last many times longer than conventional lamps. But, as with many energy efficiency measures, buyers are hesitant to make bigger investments up front with the promise of saving that amount over the course of a decade or longer.
Despite higher energy efficiency and product longevity, buyers have been slow to switch to LED lighting solutions due to inferior color quality. The brightest, and therefore most efficient, LEDs emit a ghostly blue cast.
Thus far, the accepted method of overcoming that blue cast is to coat LEDs with yellow phosphor. The phosphor absorbs part of the blue light and emits yellow light to create a more natural white — but still not quite as white as incandescent sources.
Bringing the light color closer to natural white requires adding other phosphors to the coating. Those phosphors have another problem: They’re made with rare earth elements for which the market price is unpredictable. That word, “rare,” has something to do with it.
Toxicity is another cost factor. Environmental regulations could make life harder for companies using cadmium nanoparticles, for example, to solve the color problem.
High-priced ingredients could put the kibosh on some techniques to improve LED color quality by adding more exotic (and more expensive) phosphors into the mix.
LumiSands
Seattle-based LumiSands showed me a better idea. Their phosphors are nanoparticles made from silicon — and there’s nothing rare about sand.
LumiSands co-founder Ji Hao Hoo says the semiconductor industry already derives silicon using well-established methods. “Because of the solar boom, the cost of silicon has dropped dramatically,” he says.
“We decide what color we want our nanoparticles to emit, and that determines what size to make them,” Hoo explains. “Then we mix the particles into a silicone polymer, similar to the material in soft contact lenses.”
When the gel is cured it forms a flexible, translucent silicone disc. (LumiSands could use a different polymer to make a rigid lens, but the readily available polymer for them to experiment with happens to be silicone.)
“What we do with that disc is to put it over a conventional white-light LED,” Hoo says. “The nanoparticles in our disc absorb some of the UV produced by the LED and reemit reds, and thus enhance the color composition of the light.”
Hoo estimates his discs will cost less than adding expensive phosphors to today’s white-light LEDs. “Ultimately we want to also present the option of substituting the use of rare-earth phosphors altogether.”
LumiSands recently competed in the University of Washington Environmental Innovation Challenge and got an honorable mention. “Competitions are a great way to meet collaborators and perhaps potential investors,” Hoo says, “but we’re not yet ready to approach investors or manufacturers.”
Hoo’s team needs more time and money to develop its technology before it can offer it to light fixture manufacturers. We might see his material in commercial products a year from now.
NLITe
Another way to correct off-color light sources is to bounce their light off a reflective surface that shifts the spectrum. That’s the idea behind NLITe, a reflective material developed at the Research Triangle Institute in Research Triangle Park, N.C., with funding from the Department of Energy.
NLITe is a technical fabric based on polymer nanofibers. By controlling the composition and structure of these nanofibers while producing a technical fabric, RTI found that it can selectively alter the color of light from luminaires. It doesn’t matter whether LED or some other light source is used; the fabric is tuned to reflect a pleasing spectrum of light.
“Color shifting allows you to turn blue LEDs white, for example,” says Galen Hatfield, vice president of the Commercial Programs Division, Engineering & Technology at RTI. “You can take the light color as it’s coming out of the fixture and adjust it to be more pleasant.”
The material also has higher reflectivity, an even more appealing feature of NLITe, financially speaking at least. RTI says the fabric reflects 98 percent of incoming light. In contrast, traditional reflector materials such as aluminum and paint typically have reflectance values below 80 percent because they absorb some of the light.
That improvement in lumen output translates to savings in the long run. Luminaires deliver more light but use no more energy. Architects can specify fewer lights when the fixtures are more efficient. With fewer fixtures, owners save on relamping and maintenance costs.
RTI develops technologies, they don’t make products. The most likely path to market would be by licensing NLITe to a manufacturer of technical fabrics, who would sell material to light fixture makers.
For now, NLITe is at the prototype stage, Hatfield says. “We’ve done more than a dozen light fixtures and had them analyzed at third-party laboratories and luminaire companies. They validated that we have light efficiency gains as high as 39 percent.”
RTI in 2011 received an R&D 100 Award, given to the top 100 technologies. RTI does $750 million in R&D a year. DOE funds some of RTI’s research into technologies that save energy on lighting.
RTI hasn’t yet determined the eventual cost of NLITe, which could be on the market by 2013.
“The cost of using NLITe is clearly going to be more expensive than just lighting-grade white paint,” Hatfield says. “But when you fold in the value in terms of using fewer fixtures and lower total energy consumption, that’s where you really get the benefit.”
The author was a judge at the University of Washington Environmental Innovation Challenge.
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