Clever, Clean, Fascinating and entirely inconsequential.
I did see a nice marine version for CHP on a small sailboat. But at $15/Watt
you limit your customers.
Another externally "fired" engine that we've all heard about is the Stirling Cycle engine.
We've all seen the demo Stirling engines. Edmund Scientific sells them, so does American Science
Surplus. The little motor sits on top of a hot cup of coffee or tea and off it goes, happily spinning its
little flywheel at 200 RPM or so.
The concept is wonderful. The engineering is a nightmare. Thermal inertia, temperature cycling in
the recuperator, a Goldilocks displacer shoving the gas through a gap that can't be too large or too small, it
has to be just right. No easy task when the one end of the cylinder wants to be at 1000º while the other
end needs to be ice cold. Even the free-piston version which does away with shaft, seals, and much of the
friction, still suffers from thermal conduction across this difference; that heat is lost to the cycle.
Here's the problem, high energy conversion requires high thermal conductivity; high thermal conductivity
means high thermal losses.
These devices make great and wonderful learning toys for kids. I'd argue that the spark of imagination
is worth many times the cost of the toy.