Vinay (Hexa yurt project) from the Solar Turbine email group (part of OSE’s project to develop an open source solar thermal electricity generation contraption) asked that I do some ray tracing of a Solar Funnel type solar concentrator. This is the type most commonly used in solar cookers, where flat aluminum-foil-over-cardboard reflectors funnel light into an oven.
In order to test the ray tracing program under open development, and so that I could get the hang of the commands before embarking on something more complicated, I fiddled with it. However, the funnel started to seem more interesting than I had originally thought. It became so interesting in fact that I decided to write a report on the subject. This is motivated by the fact that the design suggested by ray tracing varied significantly from the common implementation.
To my knowledge, which is limited to only two or three people who have researched and built one, the ideal angle at which to incline the reflectors is 60 degrees. Perhaps by widening to as broad as 45 degrees, the oven can be turned less frequently. One internet site claimed that by steepening the sides a few degrees, one could get slightly higher temperatures.
Ray tracing and geometric arguments inspired by ray tracing suggest a significantly steeper funnel of 75 degrees. With proper depth (which is not achieved with difficulty), a solar funnel of this design could reach concentration ratios of 12x, where the 60 degree model is maxed out at 4x. Even with such steep sides, the oven need not be adjusted more than once an hour. This study found no non-trivial situations in which a 75 degree tilt was inferior to a 60 degree tilt. Builders of solar ovens, take heed.
The report can be found here