Solar Cookers and Cooking
There are a great many solar cooker designs, plans, and fully built units available for purchase on the Internet. There are two general design types, parabolic reflector cookers, and box cookers. The parabolic reflector type cookers include multi-facet flat panel designs and true parabolic reflectors, and have the advantage of being light in weight and easily portable. Many are collapsible, so they can store in a small space. However, most can only heat one pot at a time. The box cookers tend to be heavy, bulky, and may not get as hot as the parabolic cookers, but they have the advantage of allowing use of multiple cooking pots if the box is built large enough to accommodate them, so a whole meal can be prepared at one time in a box cooker.
Around children the box type cooker is the preferred type due to its superior safety. Parabolic designs focus the Sun's rays into a very intense concentrated area of light that could damage a child's eyes, or possibly burn his/her skin, where a box cooker is a more passive design, and the heat is contained inside of the cooker box away from a child's reach. The Sun's light is not focused into a potentially damaging point or area in box cookers, although, like looking directly at the Sun, looking at reflected sunlight from a box cooker's cover or wing mirrors could certainly damage eyes, but it won't burn the skin. The cover glass does get hot enough to burn skin if touched however.
The box type solar cooker I built is a little more elaborate and refined than most designs you will see on the Internet. I added two features that I have not seen in any of the box designs on the Internet. I wanted the cooker to achieve temperatures high enough to easily boil water, and that is not likely with a single tilt-cover mirror, so I added two additional wing mirrors that can be angled in any direction to focus the sunlight into the box. Also, due to the Earth's rotation, the solar cooker has to be adjusted in direction about every 15-30 minutes to track the Sun and remain focused. It is surprising how quickly the temperature will drop off if this is not tended to. It is not easy to rotate a heavily loaded cooking box to keep the mirror(s) focused on the Sun, so I mounted the box on a large industrial weight "lazy-Susan" bearing. It now rotates so easily that I have to use a wood wedge, see image below, under it to prevent it from drifting out of alignment if a light breeze strikes it.
I will comment further about my design, but if you would like to look at a large collection of cooker designs, visit the following URL.
Here are just three example designs from the many on the above linked page. Parabolic and flat facet reflector cookers come in a large variety of designs, as do the box cookers.
Below is a picture of my demo design for a solar box cooker. It can hold two medium pots or one large one. With its numerous rotation points it is fairly complex to build, as you can easily see in this picture. However, most people do not need to be nearly so detailed, my wife Gretchen would say "anal," in how they build one, so the construction can be much less complex and difficult. I am a blacksmith/decorative metals artist, and I have a very well equipped shop, so making the complex metal hardware is easy for me to do, so I elected to make this demo unit a cut above most home built box cookers you might see. BTW, the angled support arm for the wing mirror on the right is longer than the one on the left. That was intentional in order to keep the right wing mirror clear of the cover mirror's slide/lock bracket that is visible on the right side.
The cooker was going through a test run on my shop's entry slab when this picture was taken. The temperature was holding at 234°F when I took this picture. The six quart pot inside the cooker box is full of boiling water (started out cold), thus you see the clear cover glass lid is steamed up. I ended up drilling a small vent hole in the back upper right corner of the box to allow the escape of steam. Now I must use care not to be burned by the mostly invisible jet of steam that is emitted.
Below is a picture of the temperature probe thermometer read-out showing the box temperature, not the temperature of the water in the pot. The water can't be any hotter than 212° at atmospheric pressure. The yellow arrow points to the temperature. The highest temperature I was able to achieve without the wing mirrors was 196°F.
Please realize that the cooker box above is a demo unit only, built only for our group demonstration. It can most certainly be used to cook food, but a more practical design would allow four to six cooking pots to fit in the box so that an entire meal can be prepared at one time. My design (not including the wing mirrors or rotating base) is based on an international award winning design that is currently being produced for use in 3rd world countries where they have used up all their available cooking fuel firewood.
If I were to build another cooker box to be used for daily/emergency cooking needs, I think I would try using a double pane glass top in order to increase the insulation value of the glass cover. The box is fairly well insulated due to the 1"+ box wall thickness, but the single layer 3/16" thick cover glass allows a significant heat loss. I have no doubt that given a dual pane top it would reach and hold temperatures in excess of 250°.
The old fruit box I used is lined with 1" thick pine on its sides and bottom and has heavy weight aluminum foil on its inner sides, shiny side facing in. The bottom of the box floor is painted black with high temperature stove paint. Also, all cooking pots and lids need to be spray painted black on their outside surfaces to absorb the heat more effectively. I also learned as I experimented with the cooker that it needs to have a drip pan placed in the bottom in case a pot boils over. I am using an old rusty rectangular steel baking pan, ideal for this use due to its dark color. In the drip pan I have placed two 5/8" square steel bars to elevate the cooking pots above the floor. Elevating the pots above the floor of the cooker box allows the hot air to circulate underneath, and will more quickly bring the pots and contents up to cooking temperature.
Feel free to phone me or visit me at my shop if you have questions regarding my solar cooker design. :-)
Ron Reil