> I haven't seen those but there is a simple trick to make such a thing
> using hollow pressurized blades with a little water or other volatile
> liquid in each. the heat source is applied to one of the lower quadrants
> of the fan wheel and the liquid in the blade that presents itself there
> evaporates (under pressure!). This changes the center of gravity of the
> wheel and it moves a 'blade' upwards (the hot blade moves up - the wheel
> axis is horizontal). After setting up right (enough of the liquid must
> stay a gas until the relevant blade is past the TDC of the wheel), it
> will work probably not at amazing speed. Some damping is required to
> prevent oscillations due to the liquid in the blades. I suppose that a
> larger fan supplies its own damping. If you try to build such a thing
> pay attention to pressurisation. The pressure in the hot blade can
> become very large (use very little liquid if possible).
> 
> A solar powered device using a 5" concave mirror and acetone as working
> fluid was planned by yours truly for last summer but postponed ;-) I
> also think that a device based on phase change could be made using wax
> or even solder as working fluid.

Such designs a frequently found on "free energy" websites, usually with
the "blades" designed as old propane tanks.

Their efficiency is such that you would be better off using the solar energy
to grow food, eating it, and riding a stationary bike with the calories from
the digested food.  They are a cute trick, though.

Mike H.
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist