On Fri, Sep 23, 2005 at 02:30:29PM -0700, James Newtons Massmind wrote:
> I'm thinking about an array of mirrors, each perhaps 1 foot square (these
> are sold at low cost for decorating in homes), all laying on a flat surface,
> and each tilted at some angle so that sunlight shining on that mirror is
> reflected back to a central point.

Fresnel mirror array. Curnutt solar furnace. The Phoenix Turbine Builders Club
(PTBC) has a description here:

http://phoenixnavigation.com/ptbc/articles/ptbc36.htm

> 
> Yes, the angle may be somewhat more limited.
> 
> Yes, I'm talking about trading efficiency for ease of construction.

But if you keep making such tradeoffs, your efficency will be so low as
to not be useful.

> My contention is that for the cost of moving some number of mirror on
> heliostats, you could build a field of unmoving mirrors some factor larger.
> 
> In the end, I believe the "efficiency with respect to cost" would be higher
> than with moving mirrors. In other words, for X dollars, you could generate
> more total power with non-moving mirrors and a moving target than with
> moving mirrors and a non-moving target. The loss due to angle is offset by
> the increased area of the mirror field.

If you use a larger field, then maybe you can fix different parts of the field
to point at the sun at different times of the day. So you can have an East, South, 
and West Field. 

But the PTCB is looking at a tracking system that's little more than a couple
of sensors, a PIC (16F84: how quaint!), and a stepper motor. That combo is
a heck of a lot cheaper than enlarging the mirror array.

> 
> The exceptions are probably due to a very high cost of moving the target.
> Such as when there are high pressure or caustic materials involved.

The target isn't the problem here, the sun is. I was arguing that moving the
target along with the mirror(s) would simplify tracking the mirror box at the
focal point. Simply mount everything on a big boom that swings east to west 
during the day. Since the target would be directly below the focal mirror box
on the boom, no additional adjustments from the mirror array to the target would
need to be done except for a weekly or bi-weekly elevation adjustment.

> I don't think very large, and therefore difficult to move targets count
> against this idea because in most cases, the target could be split into many
> smaller targets, each moveable.

But what I can't figure out here is why there is an additional cost of moving the
mirror if you're moving the target anyway? I know that you're thinking about the
stability of the mirror array. But enclosing it in a clear top enclosure should
mitigate virtually all of the wind effects.

BAJ
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