Last Single-Mirror Solar Oven Simulation Test

by Curlydock
Nov 24, 2007

In my previous couple of posts I showed some of the results of tests of a program I wrote to simulate what I call a “kaleidoscopic” type solar oven. In earlier posts I detail the actual oven I use to bake bread. I wanted to see how to build a better oven of this type, so I wrote the simulation for ray-tracing various reflector sizes, shapes, quantities and configurations.

This post will cover what I hope is the last of the one-reflector tests. I wish to begin tests of two-mirror simulations in the next post.

nov1026a.jpg

Graph “nov1026a” shows the results of five runs with a single equilateral triangular reflector in the plane of the horizon. That reflector I frequently refer to as “R3”. Five sweeps of the sun from zero degrees (horizon) to 90 degrees (zenith) are shown, one sweep for each size of R3. The sides are all equal and are measured in cavity diameters. The smallest R3 is 2.0 and the largest is 8.0 cavity diameters on a side. The cavity bounding sphere is represented by the ball that is aways in the corner farthest from the sun when the sun is on the horizon.

Observe that the gain never exceeds 2.0 and never falls below 1.0. This is consistent with my expectations of what happens with only one mirror and increases my confidence in the simulator.

Rule 1026a

Also observe that the increment of improvement in gain decreases as R3 gets larger. Large mirrors increase the gain when the sun is at lower angles but the reflector has to get perhaps impractically large to produce these gains. At solar angles above about 20.0 degrees, it hardly seems worth having a floor reflector larger than 4.0 cavity diameters on a side. This is more than my prejudice up until now, which was that 3.0 cavity diameters on a side was the practical limit. The actual oven I made has only 3.0. So, the next time I build one of these ovens I will probably go with 4.0. It depends on what else we learn when more reflectors are in play.

“Rule 1026a” is, then: the triangular reflector always parallel with the plane of the horizon, that is the one the cavity is above, should be sized to about 4.0 cavity diameters on a side, physical practicality permitting, and probably not less than 3.0.

Now I combine this rule with rule 1010a from the previous post, which said the cavity should be elevated. That results in chart “1027a”:

nov1027a.jpg

For this run I kept the mirror sized to 8.0 cavity diameters on a side and elevated the bottom of the cavity from just touching R3 to where the bottom of the cavity is 0.5 cavity diameters over R3. Otherwise the run and chart calibration are the same.

Again, the gain never exceeds 2.0 and never falls below 1.0, which is good.

Also note that elevating the cavity improves gain when the sun is at higher angles. This is consistent with what we learned in the prior post and was to be expected. The simulation still seems to work. For this configuration, with the cavity elevated, the range of sun angles with the maximum gain of 2.0 is much larger.

In a prior post I expressed some doubt about whether the simulation was following the ray trace through an arbitrary number of reflections or breaking off too soon. I later discovered that was indeed a problem. But it only affected simulations of more than one mirror, none of which I had published yet. I fixed the bug and am now confident that I am ready to do multi-mirror tests. In the next post we will see some results using two reflectors. Those reflectors are the ones I have named “R1” and “R2” in prior posts. They are linked by a vertical hinge and open like a book over the plane of R3. R3 will be put aside while we consider only R1 and R2 and will return probably much later when three-mirror tests begin.