There is a new (not totally but under development) way of converting heat to electricity via acoustic waves! A high pressured gas inside a close cylinder heated at one end and cooled at the other end is capable of producing very high pressured acoustic waves travelling at sonic, super sonic and ultrasonic frequencies. There pressure waves are capable of moving a piston. Like Sterling engines, a linear alternator is used to produce electricity. Very interesting concept.
Visit Los Almost National Lab's home page for futher details if you are interested. I am working on one such machine :o