By Mark Halper
Posting in Design
Light emitting diodes are known for their efficiency. Now an MIT team takes things to new heights, more than doubling the power out over power in.
A team led by Parthiban Santhanam found that an LED powered by 30 picowatts of electricity emitted 70 picowatts of light power - an efficiency of over 230 percent - according to a synopsis by the American Physical Society.
The same sort of result does not hold true at higher wattages that would be of any use for say, reading a book or lighting a room. No matter how efficiently the LED operates, 70 picowatts - that's 70 trillionths of a watt - won't illuminate many dark corners, or pages of War and Peace. Although today's efficient 20-watt LED bulbs shine as brightly as a 100-watt incandescent, we'd have a long way to go before 70 picowatts will serve any functional purpose.
But that's not the point. What's important is that the MIT team took a new approach to trying to improve LED efficiency. In the conventional quest, scientists try to increase the number of photons emitted per electron that travels across the LED, which is a semiconductor.
Santhanam and crew worked by decreasing the voltage, and found that a reduction in voltage decreased electrical input power more than it decreased outgoing light power (remember, "voltage" is not a measure of power, but of electric potential). They used a particular LED with high conductivity.
"With each halving of the voltage, they reduced the electrical power by a factor of 4, even though the number of electrons, and thus the light power emitted, dropped by only a factor of 2," according to the synopsis.
The website physorg explains that the process tapped excess heat from vibrations in the LED's atomic lattice. That holds potential for designing lights that don't generate heat, it notes, adding, "When used as a heat pump, the device might be useful for solid-state cooling applications, or even power generation."
Santhanam and co-authors Dodd Joseph Gray, Jr. and Rajeev J. Ram reported their findings in Thermoelectrically Pumped Light Emitting Diodes Operating above Unity Efficiency, published by Physical Review Letters.
Photo of blue LEDs from mrbill via Flickr.
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Mar 8, 2012
Witness the sentence about increasing "efficiency" to 230 "percent." Efficiency in power CONVERSION is CAPPED (by DEFINITION) at 100 percent. According to any sane science, there is NO SUCH THING as "230 percent efficiency." If the American Physical Society said there was, they frankly LIED when they put it thus. Now, the EFFECT may indeed be real, but it is NOT due to that impossible "efficiency." Rather, it is due to tapping the thermal vibrations at that very small scale, and using THOSE to generate power that supplements the electrically-caused light. Science is about an approach to absolutes. Please use the CORRECT terminology when reporting on it, because when real effects are couched in FALSE FANTASY LANGUAGE, the less-educated are frustrated in their efforts to understand, because they go away with nonsense "information," about "230 percent efficiency" and using LEDs to "generate" power...