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The conversion of energy between light and electricity (LEDs/solar cells) and between light and heat (light bulb/greenhouse) is complemented by thermoelectric conversion between electricity and heat.
Here is what my EETimes story said about thermoelectrics: Today's state-of-the-art thermoelectrics are only about 5 percent efficient, but new research indicates that a class of material called skutterudites—plus a new technque for aligning their atoms—could improve thermoelectric efficiencies to as much as 20-percent, enough for commercialization. Such high-efficiency thermoelectric converters on the exhaust pipe of an automobile, for instance, could convert enough heat into electricity to charge the batteries of a hybrid vehicle...
Full Text: http://bit.ly/NextGenLog-dXDm