There are a lot of pretty amazing small Sterling cycle engines. I’m most impressed by the LTD – low temperature difference – models that can spin a flywheel on temperature difference between room air and a bit of ice, or room air and a room heating radiator, room air and a cup of hot coffee, or even room air and water slightly cooled by evaporation. According to this page
, some LTD Sterling engines will spin with only a 0.5 C difference between their hot and cold sinks!
Though these little engines are impressive, they produce hardly any power – just enough to overcome the tiny amount of friction of their well-made parts to spin a flywheel.
The power of a well-made Sterling engine is mostly a function of the size of its cold sink and temperature difference – the Sterling engine Wikipedia page
gives 500 W/m2
/K. So to get a lot of power, you either have to have a big cold sink, or a big hot to cold sink temperature difference, or both.
There are some pretty high-power new Sterling engine built on the “both” approach. Sweden has 3 200-foot subs that can run for weeks submerged at 5 knots using a pair of 100 HP liquid oxygen + diesel fueled Sterling engines – the Gotland class
, a serious war machine. More peaceful is the 15 HP (11 KW) CleanEnergy Sunbox
concentrated solar-powered Sterling Engine.
There are some interesting antiques, such as the 180 W (0.25 HP), diesel or gasoline -fueled Philips MP1002CA
“Bungalow generator” from 1951, or the wood/coal/gas fueled Ericsson's water pumping engine
from ca 1880.
Keep in mind that Sterling engines aren’t “something from nothing” – if you use one to generate electricity from a temperature difference produce by your house heater, you’re more than paying for it in increased heating fuel or electricity consumption. If you can find a true waste heat situation – something hot that needs cooling, like a computer heat sink – it could server double duty of cooling what need to be cooled while looking cool, but I think little Sterling engines are good mostly for their impress-you-and-your-friends-with-their-niftiness value.
You can get really nice little ones for around US$35. I’ve been hinting around gifting occasions for years, but no luck so far.