MIT Finds a Way to Improve Incandescent Lighting Efficiency

Incandescent lighting was introduced more than a century ago by Thomas Edison and has remained essentially unchanged for all that time. Regulations have come into force the world over with their intention firmly set on improving energy efficiency and have begun to phase out incandescent bulbs, replacing them with more efficient compact fluorescent and LED bulbs.

Incandescent bulbs work by heating a thin tungsten wire to high temperatures, around 2700 degrees Celsius. This causes the filament to emit a broad spectrum of light, known as ‘black body radiation’. This wide band of light is what provides that warm light level that we all know and love and lighting manufacturers have tirelessly worked to reproduce.

High inefficiency is the downfall with this method, although a broad band of light is produced, only a small percentage of it is actually visible to the human eye. 95% of the photons produced are wasted as heat with only the remaining 5% producing useable light.

New research by five professors from Massachusetts Institute of Technology and Purdue University has found a potential solution: a filter that prevents the would be wasted photons from escaping and reflects them back into the filament. These are then reabsorbed and converted back into visible light. Constructed from a form of nanophotonic crystal built up from alternating layers of materials such as silicon dioxide and Tantalum Rod in thicknesses less than 1/100th that of a single human hair. The research claims that this approach could improve the efficiency of incandescent lighting by 10 times, making it more efficient in fact than current commercial compact fluorescent and LED lighting.

This filter works in two ways, a two stage process according to the research conducted. The first stage takes the conventional heated tungsten filament, inefficiencies and all, but instead of losing the heat in the form of infrared radiation, the filter around this filament allows the desired wavelengths to pass through but captures the infrared wavelengths and reflects them like a mirror. They then travel back to the filament and add more heat, causing its temperature to rise further. The more this process is repeated, the higher the maintained temperature and brightness while using far less electricity.

This second stage dramatically affects the luminous efficiency of the bulb. Incandescent bulbs normally have a luminous efficiency of between 2 and 3 percent, whereas today’s CFLs have an efficiency of between 7 and 15 percent and most modern LED bulbs are between 5 and 15 percent. These new two stage incandescent bulbs could reportedly reach luminous efficiency levels of up to 40 percent, but so far, proof of concept units have only reached 6.6 percent efficiency. That said, this level still matches some of today’s CFLs and LEDs, with a threefold improvement on current incandescents.

This same innovation can also be used to improve the efficiency of solar panels, allowing only the wavelengths through that correspond with the semiconductor band gap of the material in the solar cell. This will translate to greater efficiency in energy conversion in the solar cell.

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