About: Georgia Tech Spiral Solar Cooker

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After cutting the spiral from a 4-foot (1.2m) square sheet, pairs of holes were drilled at four points along each turn of the spiral. The holes were plotted in a special pattern, so that if the spiral were "wound up" somewhat like a torsion spring, the holes could be aligned, and wire passed through them to tie the spiral to a supporting X-frame. The winding set up spring tension in the turns of the spiral with the result that the continuous strip tilted inward toward the center, to varying degrees. The resulting geometry constituted a fresnel-type reflector, of depth less than 2" (5cm), yet approximating the effect of a paraboloid shape.

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rdfs:label	Georgia Tech Spiral Solar Cooker
rdfs:comment	After cutting the spiral from a 4-foot (1.2m) square sheet, pairs of holes were drilled at four points along each turn of the spiral. The holes were plotted in a special pattern, so that if the spiral were "wound up" somewhat like a torsion spring, the holes could be aligned, and wire passed through them to tie the spiral to a supporting X-frame. The winding set up spring tension in the turns of the spiral with the result that the continuous strip tilted inward toward the center, to varying degrees. The resulting geometry constituted a fresnel-type reflector, of depth less than 2" (5cm), yet approximating the effect of a paraboloid shape.
dcterms:subject	Solar cooker designs Fresnel solar cooker designs
dbkwik:solarcookin...iPageUsesTemplate	dbkwik:resource/fNTiBPcj8fe2KcC8wF_ung== dbkwik:resource/oHtQwMp34zyeWUyPt0neVw==
abstract	After cutting the spiral from a 4-foot (1.2m) square sheet, pairs of holes were drilled at four points along each turn of the spiral. The holes were plotted in a special pattern, so that if the spiral were "wound up" somewhat like a torsion spring, the holes could be aligned, and wire passed through them to tie the spiral to a supporting X-frame. The winding set up spring tension in the turns of the spiral with the result that the continuous strip tilted inward toward the center, to varying degrees. The resulting geometry constituted a fresnel-type reflector, of depth less than 2" (5cm), yet approximating the effect of a paraboloid shape. For low cost, The Suntwist Company recommended using fiberboard substrate and a glued-on aluminum foil reflective surface. While these materials satisfied the performance requirements of the concentrator, the practical issue of material durability was amplified in significance by the design. In particular the anchor holes, necessarily located quite near the inner edges of the spiral's turns, were at risk of breaking through to the edge under stress from the wire, during construction and with handling/use. This would result in de-alignment of the spiral and degradation of the focus. Construction using sheet metal could overcome the issue, but increased cost and tooling needed for construction. In the example pictured below, the sheet metal is stainless steel. While stainless steel offers corrosion resistance, reflectivity is only ~ 65%. Aluminum offers significantly better reflectivity, but is structurally weaker.

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