I love the idea of solar driven Stirling engines. I have even built a couple of Stirling engines.
I am a little skeptical of Dan Rohas at Greenscience. While he is enthusiastic and his heart is in the right place, I will buy you a steak dinner if we ever see that particular engine and that Fresnel lens produce an honest 100 watts of electricity.
The very very best Stirling designs have approached 50% efficiency. This engine has very little in common with those exotic Stirlings. In order to attain that efficiency, the working gas must be at much higher pressures than atmospheric, and the working gas won't be plain air (typically nitrogen). The size and shape of the focal point of the Fresnel lens must be closely matched to the receiving (hot) end of the Stirling, which it isn't. The cold end of the stirling will need some active cooling or the temperature delta will rapidly fall as the cold end heats up. His only has passive air cooling.
The engine itself in it's current setup, if he is lucky, will be 10% efficient at turning heat into rotary motion. Once he hooks it to an alternator or generator, he will be lucky to convert 33% of the rotary motion into usable electricity. His Fresnel lens is about a meter square, so back of the napkin says 1000 watts * 0.1 * 0.33 = 33 watts at the peak of solar input at solar noon. Before that and after that time of day it will be less.
A Fresnel requires active tracking in two axes to keep the focal point on the receiver, so that's going to take some energy away.
Again, it's a super cool proof of concept and very exciting. Full points for that.
There was, or perhaps maybe is, a Stirling cogenerator that was commercially produced over in europe. The WhisperGen. I tried pretty hard to obtain a North American version, but they were just never built.
Ripasso is making good noises, but I don't see anything you can buy, and they are not scaled for single homeowner.
Dean Kamen (of Seque fame) has been promising a Stirling co-generation unit for a decade. It's always 2-3 years away from commercial production. He talks about a 10kw unit, and a 2kw unit. I'll be at the top of the list when it becomes available.
I will look over their setup several times to see what I can glean.
My first impression is, everything he says is possible and plausible.
Low delta Stirling engines have much greater potential in third world (and rural or residential US) settings because so much cheap low grade "waste" heat is available. Also, you don't need a solar concentrator or a tracking system to produce useful temperatures. These kinds of Stirling engines also do not require exotic and expensive materials like high grade stainless steel and titanium alloys to survive the very high temperatures needed with a solar concentrator system.
The main drawback is lower Carnot efficiency (that's why his Stirling is so big). and relatively low output.
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