An Experiment in Solar Cooking - page 4

A cooker with extended time between adjustments
The purpose of this experiment was to build a cooker with good performance, but requiring less frequent adjustment, more like the comparison cooker. Again, this design used the same 18"x18"x28" (End flaps not included) 3M cardboard storage box as before. With the sun at either extreme of its range, each opposite end acts as a corner reflector. When the sun nears the middle, the center section takes over. The center sections of the reflectors are at about 40 degrees from vertical, the ends about 25 degrees. The last design was meant to be adjusted every half to one hour. This one could hopefully go for two or three hours. I've made the assumption that someone can be available for oven adjustments, but with this design, the interval between adjustments could be longer. This one is also foldable. The reflector halves fold up, and in use they are temporarily tied to the base with string.

  

Construction - See above drawing
I cut two rectangular, 18" by 28" pieces for the base. Glued them together for strength.

Cut two 18" by 47" sides from box.
Drew a line along one side (about 1/4" from the edge) with a straight-edge to use as a straight reference line. Cut along the line. All measurements will be made from this line.
Drew a line from top to bottom, midway between ends.
Drew a line 7" away from center on either side (These are fold lines).
Drew a line 13 3/8 away from center on either side.
Drew a line 3 3/4" away from cut edge from end to end.
Made a mark 9 1/2" from cut edge on each outside edge of ends.
Made a line from "A" to "B" and from "B" to "C" on each side of center (See above drawing ).
Cut from "A" to cut edge.
Cut from "A" to "B" to "C" (The 14" x 3 3/4" tab left will be bent outward to secure the reflectors to the base).
Made the second reflector the same way.
Coated the base and reflector sections with heavy-duty aluminum foil.
When dry, made 7 holes at 2" intervals from "B" to "C" about 1/2" from edge (A cardboard strip could be glued along that edge on the outside for re-enforcing).
Made bends inward along the 7" lines. Bent the 14" x 3 3/4" tab outward 90 degrees.
Laid the two reflectors together (Shiny side facing) and ran a string from bottom to top through the holes.
Set the reflectors on the base and pushed the ends toward each other until the bottoms lay flat on the base. The distance between the the top of the center sections of the reflectors is about 30".
Adjusted the slack out of the strings to hold the ends together but still let it fold flat.
Made holes through the tabs into the base and used string to tie the reflectors down.

Overall length of cooker - 38", width - 30"
Center section angles about 40 degrees from vertical, ends angled about 25 degrees
Reflector surface of fold-up design has about 18% more area than comparison cooker

Test #1 - 2 hour water-heating test of fold-up cooker vs. comparison cooker - July 30, 2007
This was a test for heating from the cookers limit.
The sun gets overhead about 1:30 pm. Both cookers were rotated to where they would be properly adjusted for that time (About 25 degrees from the sun's position at the start of the test).
Heating test of 1 1/2 cups of tap water in black painted glass canning jars (Slightly loosened lids). Air temp. was 77 F, light breeze, no clouds.

10:35 am  Water temp. 77 F (Start of test)
12:35 pm  Water temps. - Fold-up cooker 181 F, Comparison cooker  144 F
Air temp. was 89 F at end of test.

Test #2 - 1 hour water-heating test of fold-up cooker vs. comparison cooker - July 30, 2007
This was a test for heating with the cookers near mid-day.
Lowered elevation of cookers, but didn't rotate the cookers. Air Temp. was 89 F, light breeze.

12:40 pm  Water temp. 83 F (Start of test)
1:40 pm  Water temps. - Fold-up cooker 164 F, Comparison cooker  137 F
Air temp. was 92 F at end of test. The sun was at its peak somewhere around 1:15 to 1:30 pm.

    Comparison cooker is on left, fold-up cooker on right

Aug 2, 2007, 11:00 am. - Put a one-third pound hamburger patty in an 8-inch diameter, painted-black covered aluminum pan. Set pan in the center of the fold-up cooker. Cooker was aimed about 10 degrees away from the sun. One-half hour later I checked it and the burger was thoroughly cooked in its own juices (Overcooked, actually). Next time I'd use a grille-type support with short legs inside the pan to keep food up out of the liquid, and also check it sooner.

Summary
This fold-up design performed much better than the comparison cooker I used as a reference. The symmetrical reflectors capture more sunlight than the open-sided design, and the 18% increase in surface area probably helped. I believe it could have gone for three hours without adjusting, if necessary.
The angle of the ends (25 degrees in this case) ultimately determines the limit that the cooker will work at. As the sun begins approaching 90 degrees to the corner reflector surface, less and less light is reflected into the cooker. This cooker has a maximum useable window of about 50 degrees (25 degrees from vertical on each end). It would be better to use the center 40 degrees for best heating. The angle I chose for the corner reflectors ended up being a compromise between widest range and good heating.
An angle adjustment, like a board, with something to prop up one side is useful if the sun doesn't track directly overhead (like the Twin Cities area). Whatever design is ultimately chosen to cook with, there will be a learning curve to see how much time different foods require.

In this design, as well as the last one, food could be suspended by a simple tripod instead of sitting it on the base of the cooker to avoid possible problems with spilled food when the base is tilted.
 

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