Robin wrote: > What would it add to the cost of a sq. meter, > to sandwich the lens material between two flat > sheets of polycarbonate, to add structural rigidity?
Thanks for the interest, Robin. This is the perfect question. Polycarbonate is a pricey plastic, shatter resistant, but kind of saggy. Therefore, I would guess the cost would go up about 20 times the price of my super cheap fresnel lens film. I used to buy quite a bit of polycarbonate, but haven't priced it lately, so it's just an estimate. There's more cost than just money, too. There's about 5% loss by reflection per surface. With four more surfaces it's almost 20% of the available solar energy gone. Conversely, imagine this cheap stuff stapled to a cheap wooden or maybe tubular plastic frame. You blast it with a heat gun or perhaps even hold it over an open fire and the film shrinks tight as a drum, giving you a flat lens probably more rigid than if you sandwiched it between thicker materials. Cheap, cheap, cheap. Did I mention cheap? Now since you probably want to hold something still at the focus of the fresnel, imagine the lens as the base of a pyramid and the apex of the pyramid as the focus. This is a more or less upside down pyramid with the base aimed at the sun and the apex near the ground. This gives you a very rigid open frame structure at very low cost. Cheap. I like cheap. I would estimate that the optimum fresnel film thickness for a one square meter device to be 50 microns or about .002 inch. Even very large arrays would probably never need the film to be thicker than 125 microns or about .005 inch. This means cheap and easy transport of the lenses because they weigh almost nothing. Yep, you can roll 'em up and mail 'em. Hope I'm not boring all you vorts with my over-enthusiastic fresnel blather. M. _______________________________________________ Join Excite! - http://www.excite.com The most personalized portal on the Web!
