There is another shape which concentrates sunlight very well and it's much easier to make than a parabola. This is a cone at 45 degrees. Cones have been used for more than 100 years to concentrate sunlight. They are simple to make and can accomplish many of the same tasks as the more difficult-to-make parabola.
The French mathematics professor, Augustin Mouchot, invented the conical cooker in the 1870's. The cooker that he used in Africa was a truncated cone with a glass-enclosed metal pot in the middle. The cooker concentrates light into a line that forms the central axis of the cone. Telkes cookers concentrate sunlight to a plane, 45 degree cones concentrate to a line, and parabolics concentrate to a point. Although parabolics have the greatest amount of concentration, a cone focusing to a line is quite sufficient for boiling up the tea, or even running a steam engine.
Easy construction is a major advantage of conical cookers. To build one you need to draw a wedge of 105.5 degrees out of a circle. Add a couple of inches to one side of the wedge for a flap that will join the cone together. Cut out the piece and fasten the edges together. This method quickly makes a 45 degree cone.
After forming the cone, you can screw two plywood plates a couple of inches apart to the sheet metal. Pieces of 2 by 4 fill in the gap between the plywood plates. The stand surrounds the cone and two large bolts form an axle for rotating the apparatus to the proper sun angles.
A quart sized can with a hole punched into the top holds the water. It's held in the middle of the cooker by a cylinder, open at the top, made of sheet metal. The can full of water
slides about one quarter its length into the cylinder where it's held in place. A one gallon glass jar is inverted over the can and cylinder, providing glazing and insulation.
The interior of the lid from the one gallon jar is cut away, leaving the threaded part. The cutaway lid slides down over a quart sized can and is fitted into a circular hole cut in a piece of lumber attached to the plywood. The one gallon jar screws into the lid and can be taken off and be replaced, using pot holders, a towel, etc. The jar completely encloses the water in its can. (Please see the drawing.)
The conical boiler is aimed by the "nail method". A long nail or dowel is nailed into the plywood base and is fitted so that it's parallel to the central axis of the collector. When the nail throws no shadow, the collector is aimed right at the sun!
The sheet metal for our 54" conical concentrator cost about 17 dollars. It was made from part of a 4' by 8' panel of sheet metal. We covered ours with mylar but aluminum foil would also work. We used contact cement as the glue. We've also made conical cookers out of tin cans wired together. Bands of bamboo held the cone together.
The intent in trying the conical was to see if we could make a solar cooker that would boil water in an easier way than, say, a wood fired stove. And it actually is easier to boil small amounts of water in this device, since you just fill the can and aim. There's no fire to start, etc. A pint of water will
usually boil in less than 12 minutes.
When much more hot water is wanted, it's probably easier to fill a whole kettle and wait for an hour or two until it boils in the Telkes. The problem is that the sun moves enough that for good results, the conical must be re-aimed at the sun about every 15 minutes. Many people might find this more trouble than planning ahead and using the slower Telkes. But, for "tea for two", in a hurry, there's nothing like a Mouchot conical!
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