Last updated: 13 May 2016
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Why doesn't my solar cooker work?
Here are some common reasons:
- Whenever we hear reports of people having trouble using a solar cooker, we most often find that they were using normal shiny pots whose finish reflects the light away from the pot instead of absorbing it. You need to use a dark pot with a dark lid. The lid is required even when baking bread.
- If you have built a box cooker, make sure that it is not too tall. It's best to make the cooker only slightly taller than the tallest pot you plan to use.
- You are cooking on a sunny day, but there is a lot of haze. To judge how good of a solar cooking day you have, just look at the color of the sky. If it is blue, you have a good day. If it is white, then you will have more trouble cooking.
- Your cooker is too small for the amount of food you are trying to cook. For example, there should be at least 5 cm (2 inches) of space around the pot in a box cooker.
Does my cooker get hot enough to cook?
New solar cooks often report that they test the temperature of their solar cooker while empty and only see it reach temperatures around 120 °C (248 °F). They wonder if this is hot enough to cook when recipes call for 175 °C (347 °F) or even 230 °C (446 °F). This lower temperature (120 °C) is hot enough for almost all kinds of cooking. Remember that water cannot get hotter than 100 °C (212 °F) unless it is in a pressure cooker. Thus if you are cooking food that contains water, it cannot get hotter than this either. Conventional cookbooks call for higher temperatures to shorten the cooking time and for browning. Food does take longer to cook in most solar ovens, but since the sun is shining directly on the lid of the pot, which is often very close to the top surface of the food being cooked, browning occurs almost as well as in a conventional oven.
Is there any way to speed up the time it takes to solar cook something?
Cooking speed and temperature vary greatly based on the type of solar cooker being used. Parabolic cookers can reach the highest temperatures, cook quickly, and are able to fry/grill. Panel cookers cook at lower temperatures over longer periods of time, similar to how an electric slow cooker functions. Box cookers are somewhere in the middle and are most similar in temperature capability to a traditional oven.
Almost all solar cookers work best when the sun is high in the sky—the hours between 10:00AM and 2:00PM are usually the best. Food that is started at 9:00 or 10:00 in the morning will cook much faster than food that is started at 1:00 or 2:00 in the afternoon.
If you make a solar box cooker, the cooking speed can be increased by adding more or larger reflectors on top of the box. If you do this, you may need to adjust the cooker towards the sun more frequently to avoid having the reflectors cast shadows on the cooker window.
When using a panel cooker, such as the CooKit, the there are a few things which can speed up cooking. Ensure the front reflectors are adjusted properly to direct the maximum amount of light onto the pot. Food will also cook quicker if the pot is elevated a few centimeters off the bottom of the cooker with a small stand. Use a stand that does not block too much light to the bottom of the pot though; a simple wire stand or a shallow glass glass bowl work well. Make sure the pot is stable on the stand to avoid any spills during cooking. When using a panel cooker, the plastic bag that insulates the pot must be sealed tightly to avoid heat loss which can cause food to cook more slowly. Solar Cooking The Netherlands - KoZon reports that an increase in temperature of 20 °C (68 °F) can be achieved by placing the pot inside two plastic bags. Regardless of cooker type, use a dark colored cooking pot/pan—black is best—with a good lid for maximal efficiency.
For most solar cookers, smaller quantities of food will cook more quickly than larger ones. With box cookers, food will cook faster if it is divided into several smaller pots instead of having it all in one large pot. See Solar cooking for large groups.
Another way to speed cooking is to preheat one pot while you are cooking the other.
Check out these plans for other cookers which can be hand-made from cardboard like the CooKit, some of which can cook a little faster than the CooKit (Heaven's Flame box cooker and the Twelve-Sided Parvati cooker).
What happens if clouds go in front of the sun while I'm cooking?
Your food will continue to cook as long as you have 20 minutes of sun per hour (using a box cooker). It is not recommended that you cook meats unattended when there is a possibility of significant cloud cover. More information on this can be found on the Food safety page.
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.
If cloud cover of significantly more than 25-30% moves in and looks like it will stay, you need to save the food by moving it to some other kind of cooker or placing it in a Heat-retention cooker.
What are the basic kinds of solar cookers?
There are three basic kinds:
This type of cooker has the advantage of slow, even cooking of large quantities of food. Variations include slanting the face toward the sun and the number of reflectors.
In this design, various flat panels concentrate the sun's rays onto a pot inside a plastic bag or under a glass bowl. The advantage of this design is that they can be built in an hour or so for next to nothing. In Kenya, these are being manufactured by Solar Cookers International for US$5 each. There are many other groups manufacturing panel cookers, expecially the CooKit.
These are usually concave disks that focus the light onto the bottom of a pot. The advantage is that foods cook about as fast as on a conventional stove. The disadvantage is that they are complicated to make, they must be turned often to follow the sun.
Other design types
- Institutional solar cooker designs
- Fresnel solar cooker designs
- Solar wall oven designs
- Hybrid solar cooker designs
- In-the-ground solar cooker designs
- Photovoltaic cooker designs
- Conical solar cooker designs
Who made the first solar cooker?
The first solar cooker we know of was invented by Horace de Saussure, a Swiss naturalist experimenting as early as 1767.
Where are solar ovens being used the most?
There are reliable reports that there are more than 500,000 cookers in use in both India and China. There are also tens of thousands of solar panel cookers in use by the Darfur refugees in the camps in Chad. We are aware of solar cooking projects in most of the countries of the world. See our country by country resources page for information on the use of solar cookers in each country.
How hot do solar ovens get?
Place an oven thermometer in the sunny part of the oven to get a reading similar to what the cooking pot is "feeling". 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 50 °C (122 °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 90 °C (194 °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 food containing moisture cannot go much above 100 °C (212 °F), 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.
How long does it take to cook a meal?
As a rule of thumb, you can figure that food in a panel cooker like the CooKit or a single-reflector box cooker will take about twice as long as in a conventional oven. However, when the time required to obtain fuelwood and tend the fire are considered, solar ovens usually demand less of the cook’s time. Also, since your food very seldom burns in a box cooker or panel cooker, you don't have to watch the cooker or stir any food as it cooks.
Using a single-reflector box cooker, you can just put in a few pots with different foods and then come back later in the day and in general the food in each pot will be cooked and kept warm until you take it out. Of course fresh vegetables will definitely overcook and become very soft if left in the cooker too long.
Panel cookers like the CooKit 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.
Cooking with a parabolic cooker is very similar to cooking on one burner of 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 can burn though, so you have to stir it and watch it carefully.
Can you fry or cook food on a griddle?
Yes you can. However the typical box cookers and panel cookers that most solar cooks use are best suited to baking, boiling and roasting food, and require little tending while cooking. A parabolic cooker is typically designed to direct heat from below, focused up at the bottom of the cooking pot. These cookers work well for stir frying, and with a griddle, can do a good job at preparing flatbreads like tortillas and ingera easily reaching temperatures above 250 °C (482 °F). Parabolic cookers require frequent reorientation to the sun.
Do you have to turn the cooker to follow the sun?
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. Of course turning these cookers more often to follow the sun would result in faster cooking. 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.
How do I cook in a season when the sun is very low in the sky?
Make sure you are using a light-weight, dark-colored pot not much bigger than the food to be cooked. It also must have a dark lid. Box cookers shouldn’t be much deeper inside than the pot is high. You’ll need to prop the back of the cooker up to lean it toward the sun and adjust the reflector very carefully so that you can see that light illuminating the inside of the cooker. If there is wind, put the cooker in a sheltered location. The Fun-Panel cooker captures the sun well at low angles as does the Heaven's Flame box cooker. The All Season Solar Cooker allows for cooking at any sun angle.
Can you cook pasta in a solar box cooker?
To keep the pasta from getting pasty, use two pans. 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.
If solar ovens are so good, why isn't everyone using one?
There are many factors at work here. First and foremost, the vast majority of the world's population does not even know that it is possible to cook with the sun. When they find out about it there is almost universal enthusiasm, especially in regions where the gathering of cooking fuel and the process of cooking over a smoky fire is a great burden. There are many factors that need to be in place to make it possible for poor people to solar cook on an on-going basis. The most successful projects have been ones where the need was the greatest, the weather the most favorable, and where the solar cooking promoters have taken a long-range approach to the transition.
If you build a box cooker out of cardboard, won't it catch fire?
No. Paper burns at 451° F (233° C) and your cooker won't get that hot.
How much of the year can you cook?
What foods should I try first in my new Cooker?
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. See also Tips and tricks.
Apples are another easy first project. Just dice or slice enough good cooking apples to fill your pot or cooking jar. Layer them in your cooking container with a bit of sweetener of choice and some sprinkles of cinnamon and put in your cooker for a few hours, until they tenderize and settle. Depending on the heat of your cooker, you'll end up with something between stewed apples and chunky applesauce.
Do solar cookers work at high altitudes?
Yes. In fact you can cook faster at higher altitudes. Solar radiation is typically much higher at higher altitudes due to thinner atmosphere that doesn't filter the sunlight as much as it does when cooking at sea level. Therefore at higher altitude the food is heated up more quickly, so you gain time there. It might take somewhat longer to cook some foods since water boils at a lower temperature at higher altitudes. See: Boil 3, Add 1 Method
Where can I buy a solar cooker?
Solar cookers may be purchased from companies in many countries of the world:
I'm planning to do a science project on solar cooking. What should I study?
If you're planning a science project, Solar Cooker International wants you to know that your research can help extend the world's knowledge of solar cooking and be of great help to people around the world. You should be aware that it's easy to build a high-performance solar cooker if you have access to modern materials. However, the more than a billion poor people in the world, who could really benefit from having a solar cooker, don't have access to such materials. This means that your research will be most useful if it concentrates on the simplification of cooker design or on the use of low-tech, local materials.
How are the solar ovens received in developing countries?
Can you cook for large groups with solar cookers?
What are the challenges faced in getting people to change the way they cook?
Myriad. However, the work of solar cooking promoters isn't "getting people to change the way they cook." Our role, more appropriately, is to help build the consciousness and the practical systems to make solar cooking known, available and affordable widely throughout the world where climates are appropriate and, especially, where people are feeling a need for relief from current problems in cooking. When people are equipped with knowledge and opportunity, they can decide for themselves whether they want to solar cook. One challenge, when planning the introduction of a program, is to make sure the proposed cookers are appropriate for enough of the staple and special foods of that culture (for example, introducing CooKits in a culture based very heavily on deep-fried or stir-fried foods might be less than successful). Trainers need to understand when solar foods require a different technique than cooking over a fire or stove (as is the case with cornmeal porridge (see Hard porridge). Then they can offer people the opportunity to save time and money with a new way of cooking, while continue to eat the foods they like and are accustomed to, well-prepared. People will often change themselves, if they can see clearly that it is to their advantage to do so and if they can also retain some of the familiar rather than making too many changes at a time.
That is not to say that we do not make efforts to explain the advantages, demonstrate the advantages and encourage people to practice, improve and make the most of their solar cookers.
What's the future potential for solar ovens in developing countries?
What has to happen for solar ovens to become more widespread?
Where can I find solar cooking recipes
How can people earn money by making and selling solar cookers?
What resources are available online?
Solar cooker components
Should I build a box cooker out of plywood or is cardboard good enough?
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 we know have used the same cardboard box cooker for over 10 years. You can also make the cardboard cooker more durable by painting it on the outside. See Waterproofing.
Would a mirror make a better reflector?
While mirrors are more reflective than simpler materials such as aluminum foil, the added gain is probably not worth the increased cost and fragility involved with using a mirror. Also remember that the light bouncing off of a mirror has to go though the mirror's glass sheet twice, each time losing strength.
Does it help to paint the walls black?
In a solar box cooker 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. It is generally recommended 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.
What type of paint should I use?
In developed countries you can buy flat-black spray paint that says "non-toxic when dry" on the label. Otherwise, black tempera paint works, but you have to be careful not to wash it off when you wash the pot. Solar cookers in Uganda report that they use aluminum pots that have been blackened on the outside by fire.
Is glass better than plastic for the window of a box cooker?
It has been reported that glass provides about 10% better performance than plastic. And there is reason to believe that under windy conditions, glass is preferred since it doesn't flap in the wind and pump heat out of the cooker. Plastic, however, is often recommended since it is much less fragile and easier to transport and works plenty well. Plastic glazings will have to be replaced periodically though since they are broken down by UV light. One excellent, easily-obtained plastic film is that used to make oven cooking bags. These are for sale in grocery stores and cost less than US$1 per bag. Other plastics will also work. Plexiglas also works well.
What kind of pots work best?
Ideally, you want to use a 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 USA 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 very well on a good day when you start cooking early, but extra solar energy is used to heat up the pot as well as the food, so they will not work in marginal conditions. Some of the newer backpacking pots and pans are black inside and out (or the pot is black and only the top of the lid needs to be painted), have tight lids, and work very well in solar cookers.
What is the best insulation to use?
If you wish, you can insulate the walls of a box cooker with various substances. Fiberglass or Styrofoam is usually not recommended since they give off foul-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 inside each wall to divide the space there into two compartments. This greatly improves the insulating power of the walls without the added weight of some other insulating substance that you might use to fill the air space. 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.
Could I use high-tech materials to make a more efficient solar cooker?
You may find that creating a high-performance cooker using fancy materials will make solar cooking more attractive to people in developed countries. In these countries, cooking only makes up a small percentage of daily energy use, but this is because people in developed countries consume enormous amounts of energy for other purposes (driving, lighting, air conditioning, etc.). But often, energy needs are also at odds in developed countries, where meals are being prepared in an air-conditioned kitchen. Heating and cooling the space at the same time is not a sustainable strategy. Energy costs, particularly in the southwestern USA, have risen dramatically recently, and there are reports of families having to spend $600/month for electricity. Homes that have a south facing kitchen wall could incorporate a Solar Wall Oven, providing the expected convenience of a typical modern kitchen. Solar cooking and drying clothes outside on a line are the simplest, least expensive ways to use solar energy to offset some of this high energy consumption. This will hopefully open them to the possibility of using alternative energy in other ways.
Millions of poor people around the world, however, still cook over a smoky fire everyday. To find wood for the fire, they have to walk many hours everyday. Other poor city dwellers don't have access to wood, so they have to spend up to half of their income on cooking fuel. These people could never afford an oven made of high-tech materials.
So it's up to you to decide which population you want to serve. You could work on creating the most practical solar cooker for people in developed countries to help lead them into a greener future, or you can investigate how to make cookers out of cheap, locally-available materials for people in poor countries who can't afford more.
Can you sterilize water in a solar oven?
Yes. In 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 65° C (150° F). This treatment kills all germs that cause disease in humans, 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.
Can you use a solar box cooker for canning?
Yes, but for fruits only! Do not can vegetables or meat in a solar box cooker, since these foods need to be canned under pressure!