Re: [Phys-l] fire starter from the sun - revisited

[Brian Whatcott <betwys1@sbcglobal.net>]

At 10:51 PM 4/24/2006, Mike,  you wrote:
> A gross of 1-foot-square mirror tiles.  That would be 144 square feet.
> How well could these be aimed?  If you could get them all pointed at the
> same square-foot of target, no small feat, the solar concentration would
> be 144:1.
>
> Now take a 100-mm diameter f/2 magnifying glass.  The area is 7850 mm^2
> and the minimum solar spot would be a diameter of 1.86 mm which would be
> an area of 2.72 mm^2.  This solar concentration would be 7850:2.72 which
> reduces to 2880:1.  That would be about 20 times more concentrated than
> the gross of mirrors if you could actually get all the mirrors aimed at
> the same 1-ft^2 of target.  It would appear to me you would need a few
> thousand 1-foot mirrors to have any hope of reaching the temperature of
> a 100-mm f/2 lens, and that assumes the holders could keep them aimed
> onto a one-foot spot.
>
>
> Michael D. Edmiston, Ph.D.


Mike offers a concentration factor of 144:1 for the mirror tiles, and
2900:1 for the 4 inch lens. He concludes that the tile target temperature
 would be much lower than the lens target.

Harking back to the need to evaluate the admitted power by including
some consideration of the area of the collector,
I see the tiles pass some fraction of  13 X 700 watts
and the lens passes some fraction of  8 / 1000   X 700 watts.

The temperature rise one might expect is  proportional to power input
per unit target area
13 X 700 watts    versus  2900 X 6  watts
or  9 kW via tiles vs 18 kW for the lens for a comparable area.

This might seem like a modest advantage in favor of the lens.
Unfortunately, the lens solution is impractical on several counts:
1) you don't get to hold a lens 8 inches away from an enemy fleet.
2) The smaller spot size of the lens allows much more quenching
by the surrounding cold mass. (A factor already adverted to)
3)    The Ancients could make shiny flats, they could not make
 spherical lenses.


Brian Whatcott    Altus OK    Eureka!