Box Oven Design Basics

I’ve been cooking about twice a week with a Sun Oven for two years in Arizona. Here are my comments.

Reflectors.
Here is a glass panel entry into an insulated chamber into which (concentrated) sunlight is directed. The sunlight heats the chamber and due to insulation, the heat is trapped and does not easily escape. Some web sites indicate that with just the insulation and not the concentration, the heat is enough to cook (ie above 212 F).  This requires much manual attention to keep turning the unit. So we start with the enhanced design – with a wider reflector assembly capable of higher heat, allowing a shorter cooking time, closer to typical oven baking recipes. This is an image of the commercially available Solar cooker, with reflectors, we purchased for $190.

from https://www.sunoven.com/usa/images/global_sun_oven_thumb.jpg
from https://www.sunoven.com/usa/images/global_sun_oven_thumb.jpg

This is a cutaway image of this basic design (described in many places on the web) where reflective panels direct sunlight into an open window. There is a frame that holds the square glass window, some rectangular reflective surfaces on the side of the opening, and triangular reflective surfaces that connect the sides.

 Cutaway of reflectors into glass window
Cutaway of reflectors into glass window

100_solar_reflectors_2_

Tilt angle.
At what angle should the side reflector be tilted? There is discussion of this angle on a number of web sites – here is our rationale. If it were tilted up 45, light coming straight on would hit the reflector and be bounced straight sideways, hit the other mirror, and go back toward the sun. But for every degree the reflector is tilted up, the reflected light would come at the glass at a two degree difference – that is if the reflector were angled up at 46 degrees, the reflected light would not be horizontal but at a 2 degree slant. At 47 degrees, the light would differ from the horizontal at a 4 degree slant, and so on. According to Snell’s law, a low angle of incidence with the glass would cause the light to be reflected off (in actuality, reflected off the lower surface of the glass) and not enter, pretty much like a rock skipping across water. Based on densities of glass and air, the formula says you need to hit it by at least 16 degrees to penetrate, so half of that (8 degrees) says the minimum tilt up must be 53 degrees.
Tilt angle considerations.
Other sites talk about losses through the glass and so on, and conclude that a better angle (to compensate for losses) would be 60 degrees. Other sites choose this 60 degree angle because it makes for easier construction. So for our purposes, we will start the design with the tilt up angle at 60 degrees for the side reflector. Below, on this page is a calculatore to allow the experimenter to try different angles and show the dimensions of the various reflector pieces. We hope to experiment with angles around 53 degrees and see if there is any significant difference in performance.Some other web sites indicate the size of the wall area is significant (more wall loses more heat), but other sites indicate not much is lost through the walls and you can ignore insulation. In other words, there are ranges of opinions as to the relative importance of various factors in the design. In this discussion of angles of sunlight, keep in mind these theoretical calculations apply for the couple of minutes the cooker is adjusted to point directly at the sun, but does not take into account a 2-3 hour solar trajectory during which a meal is cooked. Most of us solar cooks may make an adjustment or two to the orientation — if we are home during the cooking — but a fun problem would be to design the reflectors so one could put one’s food in the (unattended) cooker when leaving for work, and come home to a warm, cooked meal.
Reflector component sizing.
Assuming the opening to the heat box is square, the Rectangle reflectors will nominally be  the same width as the square. For the maximum useful height of a rectangle, consider that at some point in length, the reflected sunlight will over-shoot the hole and not be captured as show in the diagram at the right. Computing this height requires trigonometry or something equivalent, which the reader could figure out. The calculator below computes this number for various angles. As an example, a 60 degree tilt with an 18″ wide window would indicate the maximum useful height would also be 18″. (Again remember these calculations are for the sun coming at the window “straight on”, so making the rectangles larger in this dimension might have some value.)For the Triangle reflectors, the sides will be as long as the rectangle reflector sides. The angle of the point where they meet the window would be interesting to know in order to cut pieces for later assembly. In our example of 60 degrees and 18″ square, this angle would be 41.4 degrees. (Also the base of each triangle would be 12.73 inches.)

At some point, more reflector over-shoots
At some point, more reflector over-shoots

Lastly, it might be interesting to know the angle formed where the rectangular reflectors meet the triangle reflectors. This would be useful, for example, if you were fabricating channels to hold the reflectors. (If the design is to create the reflectors and just tape them together, this information is less useful.) In our example, the angles between the reflectors is 139.1 degrees.