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A solar funnel is the basis for many solar cooking designs. In it, light is funneled by flat reflectors to a pot or other surface to be heated which is located on the other end of the funnel. This differs from designs where the pot sits between the sun and the reflectors, as in most parabolic cookers. A solar funnel is the flat version of the more idealized Compound Parabolic Concentrator.
Most sources of information on the internet about solar funnel type solar concentrators are vague about the details of tilting the reflectors. If the reflectors are opened too wide, then light bounces immediately out of the funnel. If the reflectors are too narrow, then the opening is small and very little light is collected.
Where reflector tilt angles are suggested for solar cookers, the angles suggested are often non-ideal and based on intuition.
The ideal angle at which to incline the reflectors is 75 degrees (or, 15 degrees from the solar axis). This is steeper than usually intuited.
A good length for the reflectors to be is twice the width of the target opening.
A review of the theory of solar funnels may be found at the Permafacture Institute's page of solar funnel design... except that's a dead link. Correct links: Ray tracing for concentrator design and open source solar concentrator design and better solar concentrator design.