The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice-versa. http://en.wikipedia.org/wiki/Thermoelectric_effect
Possible Energy source
To say that a solar thermoelectric generator is a simple device understates the case considerably. Benjamin Franklin could have knocked one together in a spare afternoon while waiting for the next thunderstorm to blow in; for that matter, it would not have posed a significant challenge to a skilled craftsperson in ancient Egypt. All you need is the ability to work nonferrous metals and the very basic geometry skills needed to shape a parabolic dish reflector. (Passive Solar) Strictly speaking, the efficiency of heat-to-electricity conversion isn’t that high, but given a more meaningful definition of efficiency – for example, labor and resources input to electricity output – it leaves many other options in the dust, and its sustainability is hard to match; we’re talking, ultimately, about a technology that could conceivably power radio communication and the like for as long as our species endures.