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'Light recycling' enables power-efficient incandescent bulbs

Posted: 14 Jan 2016 ?? ?Print Version ?Bookmark and Share

Keywords:MIT? LED? incandescent? nanophotonic? CFL?

Incandescent bulbs, commercially developed by Thomas Edison (and still used by cartoonists as the symbol of inventive insight), operate by heating a thin tungsten wire to temperatures of about 2,700°C. That hot wire emits what is known as black body radiation, a very broad spectrum of light that provides a warm look and a faithful rendering of all colours in a scene.

However, these bulbs have one major drawback: More than 95 per cent of the energy that goes into them is wasted, most of it as heat. That's why country after country has banned or is phasing out the inefficient technology. Now, researchers at MIT and Purdue University may have found a way to change all that. Traditional light bulbs, thought to be well on their way to oblivion, may receive a reprieve thanks to a technological breakthrough.

Power-efficient incandescent bulbs

A proof-of-concept device built by MIT researchers demonstrates the principle of a two-stage process to make incandescent bulbs more efficient. This device achieves efficiency comparable to some compact fluorescent and LED bulbs. Courtesy of the researchers

The findings are reported in the journal Nature Nanotechnology by three MIT professors: Marin Soljacic, professor of physics; John Joannopoulos, the Francis Wright Davis Professor of physics; and Gang Chen, the Carl Richard Soderberg Professor in Power Engineering, as well as MIT principal research scientist Ivan Celanovic, postdoc Ognjen Ilic, and Purdue physics professor (and MIT alumnus) Peter Bermel PhD '07.

Light recycling

The key is to create a two-stage process, the researchers stated. The first stage involves a conventional heated metal filament, with all its attendant losses. But instead of allowing the waste heat to dissipate in the form of infrared radiation, secondary structures surrounding the filament capture this radiation and reflect it back to the filament to be re-absorbed and re-emitted as visible light. These structures, a form of photonic crystal, are made of Earth-abundant elements and can be made using conventional material-deposition technology.

That second step makes a dramatic difference in how efficiently the system converts electricity into light. One quantity that characterises a lighting source is the so-called luminous efficiency, which takes into account the response of the human eye. Whereas the luminous efficiency of conventional incandescent lights is between two and three per cent, that of fluorescents (including CFLs) is between seven and 15 per cent, and that of most compact LEDs between five and 15 per cent, the new two-stage incandescents could reach efficiencies as high as 40 per cent, the team said.

The first proof-of-concept units made by the team do not yet reach that level, achieving about 6.6 per cent efficiency. But even that preliminary result matches the efficiency of some of today's CFLs and LEDs, they point out. And it is already a threefold improvement over the efficiency of today's incandescents.

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