Tagged: stirling

Hello world!

This is my maker blog devoted to sharing my projects and other peoples cool projects. I think stuff like this:

is pretty amazing.

Stirling engines are kind of awesome but at their best, they still under perform compared to combined gas-jet steam cycles. The benefit of Stirling engines though, is that you don’t need to build at an industrial level, as the person in the youtube video so clearly demonstrates.

I have so many questions, like is it cost effective to make a Stirling engine out of plastic rather than metal? The problem is that plastics tend to have thermal conductivity(i.e. how quickly they can give off heat) of between .19 to .5 \frac{W}{mK} whereas iron has a thermal conductivity of 80 \frac{W}{mK}. That’s about 400 times better than the best plastic, which is high density polyethylene. What this means is that to make a Stirling engine out of plastic with a similar power output as an iron engine, you would need 400 times the surface area between the working gas and the hot and cold reservoirs. 

But what if 400 times the material was still cheaper if it was made out of plastic? Plastic typically comes form oil and oil is currently trading at about $93/barrel. Each barrel has about 159 liters in it so the raw materials for plastic cost at least $0.58/liter.  By contrast, iron ore is trading at about $100/metric ton and a ton of iron has about 142 liters of volume.  That’s about $0.70/liter for iron ore. I’m sort of shocked by the outcome. Oil and iron ore are trading for close to the same price per volume. If this translates into the cost of producing a Stirling engine, and this is a big if, then that would mean that a plastic stirling would cost several hundred times more than an iron stirling for the same energy output. Warning! These numbers are so rough that they should hardly be believed. They’re only meant to give some intuitive insight into the cost effectiveness. 

There’s a lot that could be wrong with this picture. Plastics are easier to work with and you might require less plastic than metal, especially for low temperature stirling engines. There are so many questions and so little time!!!……..