LEDs (Light Emitting Diodes) ; Basically, LEDs are just tiny light bulbs that fit easily into an electrical circuit. But unlike ordinary incandescent bulbs, they don't have a filament that will burn out, and they don't get especially hot. They are illuminated solely by the movement of electrons in a semiconductor material, and they last just as long as a standard transistor. The lifespan of an LED surpasses the short life of an incandescent bulb by thousands of hours.
On October 9, 1962 GE scientist Nick Holonyak illuminated the first practical visible LED. 
Nearly 5 percent of Africans without access to electricity, or some 28.5 million people, now use solar-powered LED lights. That's up from 1 percent five years ago, according to figures released this month by Lighting Africa, a project of the International Finance Corp., the private-sector investment arm of the World Bank. There's a growing market in South Asia, too.
Worldwide, in the past six months, 2.1 million LED-solar products have been sold to people who are unable to plug in to electrical grids, the IFC says. Sales have been growing at a rate of 150 percent annually for several years — a function of both the demand for lighting and the improved quality of LED lamps.ogy with heat exchangers.
"Just as the radiator in your car exchanges heat between the air and a coolant, this system exchanges mass, or water, across a membrane," Banchik says. "There's a method in literature used for sizing heat exchangers, and we borrowed from that idea.”
Benefits of LED lightbulbs
Long-lasting - LED bulbs last up to 10 times as long as compact fluorescents, and far longer than typical incandescents.
Durable - since LEDs do not have a filament, they are not damaged under circumstances when a regular incandescent bulb would be broken. Because they are solid, LED bulbs hold up well to jarring and bumping.
Cool - these bulbs do not cause heat build-up; LEDs produce 3.4 btu's/hour, compared to 85 for ncandescent bulbs. Common incandescent bulbs get hot and contribute to heat build-up in a room. LEDs prevent this heat build-up, thereby helping to reduce air conditioning costs in the home.
Mercury free - no mercury is used in the manufacturing of LEDs.
More efficient - LED light bulbs use only 2-17 watts of electricity (1/3rd to 1/30th of Incandescent or CFL).

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