Researchers Pursue Efficient Incandescent Bulb

26 January 2016

MIT researchers together with Purdue University researchers have found a way to enhance the efficiency of incandescent bulbs, at the time that global regulations aimed at improving energy efficiency are phasing out incandescent bulbs in favor of compact fluorescent bulbs (CFLs) and light-emitting diode bulbs (LEDs).

Their research for an efficient incandescent device involves a two-stage process in which the first stage uses a conventional heated metal filament, found in a normal incandescent bulb, surrounded by a structure that captures the black body radiation and reflects it back to the filament to be reabsorbed and reemitted as visible light.

Incandescent bulbs work by heating a thin tungsten wire to temperatures of around 2,700 degrees Celsius. The hot wire emits black body radiation, a very broad spectrum of light that provides a warm look and a faithful rendering of all colors in a scene.

But more than 95% of the energy that goes into them is wasted mostly as heat. Globally, country after country has banned, or is phasing out, the inefficient technology.

The light recycling structures of the novel lighting device is a form of photonic crystal and can be made using conventional material-deposition technology.

The second step efficiently converts electricity into light with an increasingly larger luminous efficiency. The efficiency of the human eye of conventional incandescent lights is between 2 and 3% that of fluorescent (including CFLs) is between 7 and 15%, and that of most compact LEDs between 5 and 15%.

The new two-stage incandescent bulbs could reach efficiencies as high as 40% with exceptional reproduction of colors and scalable power, according to the researchers.

Today, the proof-of-concept units have only achieved about 6.6% efficiency, matching the efficiency of some of today’s CFLs and LEDs, but this is already a threefold improvement over the efficiency of today’s incandescents, researchers point out.

The researchers designed a photonic crystal that works for a wide range of wavelengths and angles. It is made of a stack of thin layers, deposited on a substrate. “When you put together layers, with the right thicknesses and sequence, you can get very efficient tuning of how the material interacts with light,” says MIT researcher Ognjen Ilic.

In the new system, the desired visible wavelengths pass right through the material and on out of the bulb, but the infrared wavelengths get reflected and travel back to the filament, adding more heat, that then gets converted to more light. Since only the visible ever gets out, the heat just keeps bouncing back in toward the filament until it finally ends up as visible light.

The work was supported by the Army Research Office, through the MIT Institute for Soldier Nanotechnologies, and the S3TEC Energy Frontier Research Center funded by the U.S. Department of Energy.

The findings are in the journal Nature Nanotechnology.

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