re: moving mirrors

All high efficiency/high temperature systems move the mirrors.

If you want to be cheap you can use 'trough' mirrors (fixed) for 
preheating and a single parabolic mirror for the final fun. Although the 
major power input is needed at the parabolic mirror.

If you would just switch to another working fluid (other than water), 
like propane (at low temp.), nitrogen, CO2 or whatever, it would be much 
easier imho. Even welding gas (Ar+He) is interesting.

If you go for efficiency you will have to go for high temperature. There 
is no choice imho. Just pick the highest temperature materials you dare 
to use and try for it. Either that or use a solar-chemical processes 
(like ZnO->Zn + 1/2O @2300K ... Zn(s)+H2O(steam)->ZnO+H2, repeat (add 
water vapor, extract hydrogen + oxygen). Obtaining 2300K is going to be 
interesting. Small crucibles sold for casting can take almost as much.

Imho, if you boil water, you lose. It's ok to use water, as a gas 
(steam). This can be achieved, f.ex., by using a vacuum in the system on 
the cold side. Power plants use this.

How hard would it be to make a closed circuit system in which liquid 
water is not allowed to circulate (condensate sump at bottom) and a 
Rankine cycle is used, with a pump or roots blower recirculating cold 
gas (steam) to the heater (hot steam).

Water is great for low temperature systems (up to about 200 deg. C). 
After that it becomes impossibly hard to prevent it from destroying the 
installation. Power plants have entire divisions concerned with feed 
water treatment and hundreds of treatises were written on the theme. 
Keeping feed water clean in a small system is a sisyphic enterprise 
imho. And even so, who is going to work in the garage with 210at steam 
at 350 deg. C ?! Just to drive a point home:

http://www.gcsescience.com/r3.htm

Pick 2nd or 4th from top depending on what your engine is made of.

All I can say is, I hope that the water injection scheme works out. 
Against all the numbers. It has happened before.

Peter
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