Solar Cooking Ovens

Solar ovens can come in handy if there is a disaster of sorts that your electricity is been severed off and your oven isn't working any more or maybe you have been forced to leave your home and wouldn't have sources for fuel to cook.  Having a solar oven would come very handy for your emergency preparedness.  Many poorer countries in the world use solar cookers for cooking and have benefited from them greatly.  Especially when lacking other fuel resources.

There are many types of solar ovens that can be purchased or made depending on your finances or ambition to have one made.  There are many sites online with free plans in making solar ovens or if you just want to purchase one, there are sites also available to do so.

There are three common solar ovens that are made and used:        

Box Cookers    These are great for cooking large amounts of food being that they cook more slow and evenly.  Many differ in making of the slanting the face toward the sun and the number of reflectors they have. You can figure that food in a single-reflector box cooker will take about twice as long as in a conventional oven . However, since you can't really burn your food, you don't have to watch the cooker or stir any food as it cooks. You can just put in a few pots with different foods and then come back later in the day and each pot will cook to perfection and then stay hot until you take it out.

 

Panel Cookers    It is made with various flat panels concentrate the sun's rays onto a pot inside a plastic bag or under a glass bowl. The advantage of one of these is that they can be built in an hour or so for very little. Panel cookers cook smaller portions, usually only in a single pot, but often they cook slightly faster. Some people have reported the need to stir food every once in a while when using this kind of cooker to assure that the food heats evenly. In Kenya, these are being manufactured for the Kakuma Refugee Camp for very little.

 

Parabolic Cookers    These ovens usually made having a concave disks that focus the light onto the bottom of a pot. They cook foods about as fast as on a conventional stove. Since the concentrated sunlight shines directly on the bottom of a pot, the pot heats up and cooks very quickly. The food will burn though, so you have to stir it and watch it carefully. The disadvantage is that they are quite complex to make and you have to constantly follow the sun to keep a good focus.  They will cause burns and eye injury if not used correctly.

Here are some useful information to keep in mind when building or using solar ovens.

1.  The temperature reached by box cookers and panel cookers depends primarily on the number and size of the reflectors used. A single-reflector box cooker usually tops out at around 150E° C (300E° F) as the food approaches being done. High temperatures, however, are not needed for cooking. Your oven will cook just fine as long as it gets up to about 90E° C (200E° F) or so. Higher temperatures cook larger quantities, cook faster, and allow for cooking on marginal days; However, many people prefer to cook at lower temperatures, since then they can leave the food to cook while they go about their business. With a single-reflector box cooker, once the food is cooked, it just stays warm and doesn't scorch. It's good to keep in mind that no food can go above 100E° C (212E° F) at sea level anyway, unless a pressurized cooking vessel is used. The high temperatures you see in cookbooks for conventional ovens are just for convenience and for special effects such as quick browning.

2.  As far as having the cookers face the sun the box cookers with one back reflector don't need to be turned unless you are cooking beans which take up to 5 hours. Panel cookers need to be turned more often than box cookers, since they have side reflectors that can shade the pot. Parabolic cookers are the most difficult to keep in focus. These need to be turned every 10 to 30 minutes, depending on the focal length.

4.  Building a box cooker out of plywood or glass works great unless you need a cooker that can stay outside even in the rain, you'll do just fine with a cardboard cooker. Cardboard is much easier to work with and holds heat just as well. Some people have used the same cardboard box cooker for over 10 years.

5.  Using mirrors are more reflective than simpler materials such as aluminum foil, but the added reflectiveness is probably not worth the increased cost and fragility involved with using  mirrors.

6.  Some people prefer to paint the walls black thinking that the oven will get hotter. It seems, however, that the walls will get hotter, but the food won't necessarily get hotter. Its more preferable to cover the inner walls with aluminum foil to keep the light bouncing until it hits either the dark pot or the dark bottom tray. Since the bottom tray is in contact with the pot, the heat the tray collects will move into the pot easily. You can buy flat-black spray paint that says "non-toxic when dry" on the label but otherwise, black tempera paint works, but you have to be careful not to wash it off when you wash the pot.

7.  The best kind of pot to use are dark, light-weight, shallow pot that is slightly larger than the food you will cook in it. Metal pans seem to cook best. Hardware stores in the US usually carry dark, speckled, metal pans called Graniteware. Shiny aluminum pots--so common in developing countries--can be painted black or can be blackened in a fire. Cast iron pots will work, but extra solar energy is used to heat up the pot as well as the food, so they will not work in marginal conditions.

8.  For insulating the walls of a box cooker, fiberglass or Styrofoam is usually not recommended since they give off ill-smelling gases as they heat up. Natural substances such as cotton, wool, feathers, or even crumpled newspapers work well. Many people, however, leave the walls empty of any stuffing, preferring instead to place a piece of foiled cardboard as a baffle inside the wall airspace. This makes a lighter cooker and seems to be adequate. Most of the heat loss in a box cooker is through the glass or plastic, not through the walls. This is why a few percentage points of efficiency here or there in the walls doesn't effect the overall temperature and cooking power that much.

9.  All three types, water can be brought to a boil. A little-known fact, however, is that to make water safe to drink, it only has to be pasteurized, not sterilized. Pasteurization takes place at 65E° C (150E° F) in only 20 minutes. This treatment kills all human disease pathogens, but doesn't waste the energy needed to bring the water to a boil. One reason that people are told to boil their water is that thermometers are not readily available in many places and the boiling action serves as the temperature indicator

10.  If wanting to use the solar box for canning it's best to only can fruit.  Vegetables or meats need to be canned under pressure.

11.  You can cook pasta in a solar box cooker but use two separate pans to keep the pasta from getting pasty.  Heat the dry pasta with oil in one pan; heat the liquid with herbs in another. Fifteen to 20 minutes before eating, combine the two. If you are going to use a sauce, heat that in a third container.

12.  Building or using a box cooker out of cardboard works fine and won't catch fire.  Paper burns at 451E° F (233E° C) so your cooker won't get hot enough to burn.

13.  Depending on where you live will depend when you can cook.  In tropical regions and in the southern US you can cook all year depending on the weather. In areas as far north as Canada you can cook whenever it is clear except during the three coldest months of the year.

14.  You have a lot of different items you can cook with the cooker and a good first food to try is a small quantity of rice, since it is fairly easy to cook and looks very different cooked than it does raw.  Chicken or fish is also very easy to cook.

15.  If your cooker gets only up somewhere around 250E° F (121E° C) and your recipe calls for 350E°F (177E° C) or even 450E° F (232E° C) you will still be able to cook.  A temperature of 250E° F (121E° C) is plenty hot enough for all kinds of cooking. Remember that water cannot get hotter than 212E° F (100E° C). Thus if you are cooking food that contains water, it cannot get hotter than this either. Conventional cookbooks call for high temperatures to shorten the cooking time and for browning. Food just takes longer in most solar cookers, but since the sun is shining directly on the lid of the pot, the food browns just about as well as in a conventional oven.

16.  If the sun goes in front of the clouds while I'm cooking your food will continue to cook as long as you have 20 minutes of sun an hour (using a box cooker). It is not recommended that you cook meats unattended when there is a possibility of substantial cloudiness.  If you can be sure that the sky will stay clear though, you can put in any type of food in the morning, face the oven to the south, and the food will be cooked when you get home at the end of the day.