In the first installment, I mentioned that I built a device for measuring the best angle for a solar box oven reflector. This is a description of that device and how it is used.
The device has three physically coupled sub-units: a miniature solar box “oven” with one reflector, a protractor for measuring the reflector angle and a pinhole “camera” for aligning the device properly with the solar rays. The substrate is made of foam-filled poster board held together with plastic package sealing tape. The reflector is aluminum foil glued shiny-side out. Black gaffers tape and black permanent marker were used to control undesired light reflections.
Box “Oven” sub-unit
The miniature box “oven” is not really an oven, but it is similarly constructed. It has a cube volume of 5 cm on each edge. The reflector is a square of 13 cm on each edge. Instead of measuring internal temperature, as in an oven, we will use a photocell driving an electrical meter to measure light intensity. Temperature measurements take too long to stabilize. The photocell I used was scavanged from a defective solar powered calculator. I coverd it with a piece of paper to diffuse and reduce the amount of light. An analog meter was used because a digital meter makes it too difficult to search for a peak response. One has to wait too long for the digits to stabilize.
Protractor sub-unit
A clear plastic protractor is taped to the device. It is used to measure the angle of the reflector after the peak light response is found.
Pinhole “camera” sub-unit
The pinhole “camera” is not really a camera. But it does focus an image of the sun on a screen. When the device is properly aligned so that the solar rays enter the box aperture at a 90 degree angle, then the sun’s image will be seen in the center of a cross-hair that is drawn on the projection screen. For this to work, much care needs to be used in the construction of the device so that all the edges are straight, corners are square and lengths are accurate. Use a sharp knife to cut the poster board. The lens of the camera is a pinhole in a piece of aluminum foil. Most of the foil is covered with flat black fabric tape to keep light reflected from that surface from getting in the oven and causing an error in the reading.
How to Use the Device
1. Align the whole unit with the sun so that the image of the sun, a tiny white dot, is in the center of the target cross-hair.
2. Adjust the angle of the reflector for a peak response in light intensity.
3. Read the angle of the reflector from the protractor.
4. Repeat the first three steps several times and compute the average of the readings. The average will be more accurate than any single reading.
Results
I took five readings. They were: 118, 121, 120, 122 and 121 degrees. The average is 120.4 degrees. Converting that to “reflector angle”, as defined in the first installment, gives: 180.0 – 120.4 = 59.6 degrees.
Conclusions
The result for the best angle for a solar box oven reflector being 59.6 degrees is only 0.4 of a degree away from 60.0 degrees, the angle used in many actual designs. The spread between the largest and smallest measurement was: 122 -118 = 4, or +/- 2 degrees. In the first installment, I wondered if the reflector angle (90 + 45) / 2 = 67.5 degrees might be a better angle than 60.0 degrees. But 67.5 falls significantly outside the result of 60.0 +/- 2.0 degrees. So, for whatever reason, 60.0 degrees appears to be the best angle.
The reason for this might be found in a more complicated analysis taking into account such things as the fact that much of the light entering the box oven comes from directions other than directly from the sun. Any reflector in this type of design will not only concentrate light coming from the sun but also block the entry of refllected and diffused light from other parts of the sky and terrain.
In a subsequent experiment I covered the reflector with a sleeve made of flat black paper. I could slide the sleeve up and down to test the effect of reflector length on the outcome. The results were still consistant with 60.0 degrees as long as the reflector was at least as long as the box aperture was wide. When the reflector was a fraction of the box width, these were the results:
fraction ; best angle
0.333 ; 55.0
0.625 ; 56.0
0.833 ; 58.0
1.000 ; 60.0
>1.000 ; 60.0
My intention is to use the best angle I found for each of the four reflectors in a pyramidal consentrator design. I have almost completed that oven and will test it soon.
Green Man Waiting 