Sorry for replying out of thread, I do not use a threaded mail reader 
and I get digests in the evening normally.

Re: fuel heat value I used in calculation: I used gasoline which is 
rated at 45 MJ/kg and priced as of yesterday in Texas (average about 
$2.6/gal), according to the web. I assumed that the cost of other fuels 
would be cheaper since gas is considered very expensive now.

Re: cost of heating $150/month: this is a figure I got from several 
discussions wrt heating costs on the web, for the last year (2004), for 
somewhere in the central US afair, using kerosene I think. It may be 
double now. I was just trying to get some numbers.

Re: mylar mirror: The vacuum formed mirror is not really new but its 
efficiency is only 0.8 or so afaik. You can make a better mirror using 
the spun cast method or by using a dish as a former for a fiberglass or 
composite concrete copy. The mirror you make need not be round (pita to 
handle), it can be a rectangle inscribed in the mold or a petal 
(triangle as above). That way you lose some surface but the thing is 
much easier to make, handle and array.

The problem with the parabolic shape proposed is, you cannot make an 
array from them (the focal point is way too close). So it turns out that 
just taking plain glass mirrors (at $6 / sq. meter if what I read was 
true) and warping them slightly in a frame (they will warp anyway) will 
yield something closer to the 10 meter focal distance needed for an 
array. Then the secondary mirror can be on a pole, and IT can be 
hyperbolic (using a parabolic dish as casting form and mirroring the 
'wrong' side), and the target on the ground and fixed, where you can get 
at it.

Imho the bad news with the array is, the mirrors need to move. The good 
news is that arrays scale. You can start with 2 mirrors and increase to 
50 later. The bad news is that a moving linear array is subject to cos 
phi/2 losses and that the mirrors shade each other beyond a relatively 
small sun angle (say 45 degrees). But the time of useful sun is only 
about 6-8 hours per day and at that angle cos phi/2 loss is <15%.

Peter

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