Nice and simple i was just posting about these in the gaming mug thread, there is a solid state (no moving parts eg turbine etc) electro thermal rectifier (totally not what theyre called)
and its called a peltier unit/module they can use any form of heat differential to make power eg hot CPU on one side moderate heatsink on the other = power down the positive
Take a look: http://uk.farnell.com/peltier-elements-thermoelectric-modules
heres a video from a great channel about them:
moderator: if I need to remove the video pleas ask