An external combustion engine. Or a no-combustion engine, powered by the heat from the sun, or in my case, from the heat of the network switch it is sitting on, or the wood stove in the living room.
The MM-6 Stirling Engine. by American Stirling
This gadget is just amazing. I have had it sitting on a slightly warm ethernet switch in my office for over a year now, and it just keeps silently running. It will run using only the heat from my hands.
The parts are beautifully machined and precise, and the piston slides in and out of the glass cylinder making no noise, but fitting tightly enough that air does not get past it. $359
The Heat Wave Wood Stove Fan. by Thermal Engine Corporation
When you want to move the hot air from the wood stove around the house, what better way than to use a Stirling Engine powered by the heat from the stove?
This little engine really moves, and moves 300 cubic feet of air per minute. It is quite robust, made of heavy gauge welded steel, aluminum and brass, and stands up to abuse, although it doesn't get much abuse sitting on top of the wood stove. We have played with it on a gas range that gets much hotter than the wood stove, and besides the steel getting a nice blue color, no damage was incurred.
We don't let our wood stove get above about 500 degrees Fahrenheit, for safety reasons, but this little gadget can easily take twice that. And since it moves the air around near the stove, it helps to keep the stove cooler and the room warmer at the same time.
It works well, does a useful job, and is a great conversation piece. And it uses no energy except the heat from the wood stove.
You can buy Peltier-junction thermoelectric generators with a motor and fan that are made for the same purpose (moving the hot air from the stove into the room), but ours lasted only about a year before the Peltier junction became intermittent, and the fan stopped working. The Stirling engine in the Heat Wave has had no such problems -- it is built like a tank.