Hi Boreades
I wonder if you've done any work on what temperatures would be reached at the focal point of a full-scale Stonehinge design?
I haven't done a great deal of work on the Stonehinge design but the test arrangements indicate that up to about 600C could easily be achieved: The commercial designs are easily capable of getting to 800C plus. I did some verification works on this because that is more or less the temperature that we needed to get water hot enough to drive turbines (turbines need steam at 600C plus because otherwise the droplet size can wear turbine blades).
There's a bit more about the commercial types of arrangement here:
http://www.flyingoven.com: The technical resources have been put into this type of design rather than Stonehenge because it offers a realistic chance of being able to produce fuel by extracting carbon dioxide from the air: A mega-environentally friendly way of energy production (I will probably start to bore everyone rigid if I go too much into this)
Just got a note from a Professor of Archaeology saying that he agrees with some of the conclusions of Solving the Neolithic!