Thermal Energy to Useful Energy

[highamperage]

Hey, what are the most efficient/effective ways you know of to take thermal (heat) energy and convert it into either electricity or mechanical action in a portable form?

I was thinking peltier devices, but turns out that only has an efficiency of about 10-20%.

Steam power is another option, but there must be a more direct and efficient method.

Ideas?

 

[brewnog]

Internal combustion engines can convert heat to mechanical action at efficiencies approaching 50%.