Reinvented incandescent light bulb offers 21st-century efficiency

Reinvented incandescent light bulb offers 21st-century efficiency

Researchers at MIT (Cambridge, MA) have built a more energy-efficient incandescent light bulb that combines "the warm look of traditional light bulbs with 21st-century energy efficiency."
By eeNews Europe

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The work is in the field of "nanophotonics" and while the focus of the research was on light bulb emission, it may also have applicability to other energy conversion technologies. The new structure is a conventional in-vacuum filament that has an outer coating or surround of a secondary structure that captures radiation outside the visible – presumably, mostly in the infrared (IR) spectrum – and reflects 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. They appear to be acting both as reflector at IR wavelengths and band-pass or high-pass spectral filter, in the visible. The (normally wasted) heat is returned to the filament and is additive with the resistive heating of the electric current in raising the filament temperature.

The reserachers quote theoretical efficiencies of as much as 40% although devices made so far have reached around 6%. That, however, is already comparable with light sources such as some compact fluorescent and present-day LED lamps.

Incandescent bulbs work by heating a thin tungsten wire to temperatures of around 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. More than 95% 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.

The new findings are reported in the journal Nature Nanotechnology ("Tailoring high-temperature radiation and the resurrection of the incandescent source") 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.

The key, say the researchers, is to create a two-stage process. The second step – "recycling" the previously lost energy – 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 2% and 3%, that of fluorescents (including CFLs) is between 7% and 15%, and that of most compact LEDs between 5% and 15%. The new two-stage incandescents could reach efficiencies as high as 40%, the team says.

The first proof-of-concept units made by the team do not yet reach that level, achieving about 6.6% 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.

One key to their success was designing a photonic crystal that works for a very wide range of wavelengths and angles. The photonic crystal itself is made as a stack of thin layers, deposited on a substrate.

"When you put together layers, with the right thicknesses and sequence," says Ilic, you can get very efficient tuning of how the material interacts with light. In their system, the desired visible wavelengths pass right through the material and on out of the bulb, but the infrared wavelengths get reflected as if from a mirror. They then 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 technology involved has potential for many other applications besides light bulbs, Soljacic says. The same approach could "have dramatic implications" for the performance of energy-conversion schemes such as thermo-photovoltaics. In a thermo-photovoltaic device, heat from an external source (e.g., chemical, solar, etc.) makes a material glow, causing it to emit light that is converted into electricity by a photovoltaic absorber.

"LEDs are great things, and people should be buying them," says Soljacic. "But understanding these basic properties" about the way light, heat, and matter interact and how the light’s energy can be more efficiently harnessed "is very important to a wide variety of things."

He adds that "the ability to control thermal emissions is very important. That’s the real contribution of this work." As for exactly which other practical applications are most likely to make use of this basic new technology, he says, "it’s too early to say."

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