Love energy-efficient LED lights, but miss the warm glow of incandescent bulbs? A new nanotech-enabled design by MIT researchers just might breathe new life into incandescent lighting. It promises to increase the efficiency of incandescent light sources by twenty times, surpassing that of LED bulbs.
Incandescents are thermal emitters: they heat up a tungsten filament to such high temperatures that it glows. Only a very small fraction of energy is emitted as visible light. Most of it is lost as infrared radiation. So the luminous efficiency of a typical incandescent is a paltry 2.5 percent compared to 5-10 percent for compact fluorescents and 14-15 percent for state-of-the-art compact LED bulbs.
It is possible to tailor the thermal radiation of a light source so that it emits more visible light and less infrared. This involves putting specially designed periodic nanostructures on the emitter’s surface, says Ognjen Ilic, a postdoctoral researcher in physics at MIT. The structures resonate at specific wavelengths of light, allowing only those wavelengths to be emitted.
However, the approach only works for room temperature light sources. The temperature of tungsten filaments reaches up to 3000 K. “Those nanostructures are delicate and would start degrading at those temperatures,” Ilic says.
So Ilic and his colleagues designed a resonating nanostructure that would surround the filament. They used numerical optimization techniques to design the structure, which is a stack of materials of different refractive indices and nanometer thicknesses. The structure lets through visible wavelengths but reflects infrared light back at the tungsten filament to be reabsorbed.
The researchers built a proof-of-principle device based on the scheme. This reincarnated incandescent source, reported in the journal Nature Nanotechnology, doesn’t look much the long coiled tungsten wire in traditional light bulbs. Here, the filament is a flat, wavy 1-square-centimeter piece of tungsten laser-machined from a thin tungsten sheet. The researchers sandwiched it between two 2-cm2 resonators.
In theory, the scheme should allow a luminous efficiency of 40 percent, Ilic says. But the prototype just is 6.6 percent efficient.That would require a complex stack consisting of 300 layers of four different materials: oxides of silicon, aluminum, tantalum, and titanium. For now, the researchers built a simpler resonator containing a 90-layer stack made of silicon dioxide and tantalum pentoxide.
So, what’s keeping nanotech-enabled incandescents from becoming the next best thing in efficient lighting? “The materials involved are cheap and abundant and the manufacturing process is scalable,” Ilic says. But the nanolayer production costs will have to be brought down before this reincarnated incandescent could compete with LED light bulbs.
“But beyond lighting, another really interesting avenue to use this would be for energy conversion,” he says. Tailoring thermal emission to match the absorption spectrum of photovoltaic cells, he adds, could boost the efficiency of thermo-photovoltaic systems, which combine solar thermal and photovoltaics to achieve extremely high light-to-electricity efficiencies.