Doug kittredge, USA asked:
Are there materials or solid state devices that convert heat directly into electricity? Wouldn't this be better than even zero emissions, as it would reduce the warming effect of cities?
You canít convert heat itself directly into electricity because you canít create or destroy energy, but you can do useful work with it converting it from one form into another. The forms which energy always converts to eventually are the more disordered forms of energy and heat is one of the most disordered forms.
You can get useful energy, useful electricity, from heat flowing from somewhere which is hot to somewhere which is cold. Weíve been doing that for hundreds of years, itís essentially what a steam engine does. Youíre moving heat from a hot fire to the cold outside world. You can do this just by putting two different metals together, this is called a thermocouple. That produces minute amounts of energy, but it is quite useful for measuring temperatures. You can get much more energy out using semiconductors, essentially you build a diode and the hot electrons can go through diode - then they have to flow all the way around the circuit and back to the other side. As they flow around they can do work.
So you can generate electricity through flows of heat but not from heat itself.