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Solar cooking frequently-asked questions

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If your question isn't answered below, try searching in the box above. If the answer doesn't turn up, please send an email to webmaster@solarcooking.org. See also articles on the components of a solar cooker.

Troubleshooting

Why doesn't my solar cooker work?

Here are some common reasons:

  • Whenever we hear reports of people having trouble cooking in a solar cooker we most often find that they were using normal 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. You need to use the lid even if you are baking bread.
  • You have built a Kyoto Box solar cooker. To make this cooker cook you need to cut off the little reflectors and replace them with a single back reflect such as in the Minimum Solar Box Cooker.
  • Your box cooker is 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.

My cooker only gets up to 250°F (121°C). Is this hot enough to cook when recipes call for 350°F (177°C) or even 450°F (232°C)?

Thermo2

A temperature of 250° F (121° C) is hot enough for almost all kinds of cooking. Remember that water cannot get hotter than 212°F (100°C) unless you use a pressure cooker. 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 does take longer in most solar cookers, 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.

Solar cooking usually takes longer than cooking on a conventional stove. Is there any way to speed it up?

From what I know of solar cookers, it seems that solar cookers that are low-cost and fairly easy to make tend to cook slowly. The solar cookers that are fast usually are more expensive, or at least more complicated to make.

I believe that making solar cookers that people won't use is not productive. I believe that most cooks will not use solar cookers often unless they experience real benefits from the solar cookers. I do not believe that solar cookers have to be perfect in every way--but they will not become popular if they are not better than the available alternatives.

I think some discussion is needed with those complaining. For example, if the problem is that the cookers are slow, could the solution be that cooks start cooking the meals earlier? If that is not a good solution, why isn't it? What benefits do the users get from solar cooking? Freedom from smoke? Freedom from having to spend hours collecting firewood--or from having to spend money buying charcoal or firewood? Freedom to do other things while the solar cooker cooks the food without the cook having to be there to stir the food or add sticks to the fire? Do they feel that starting the meal earlier is more trouble than these benefits are worth?

Almost all solar cookers work best when the sun is high in the sky--the hours between 10 in the morning and 2 in the afternoon are usually the best. Food that is started at 9 or 10 in the morning will cook much faster than food that is started at 1 or 2 in the afternoon.

That is one way to think about the problem. Another way is to consider the type of cooker and how it is used. If you make a box cooker, the cooking speed could be increased by adding more reflectors or larger reflectors on top of the box. If this is done, it might mean that the box will have to be adjusted more frequently to point towards the sun and avoid having the reflectors cast shadows on the cooker window. Would the families accept such a change?

If the cooker type is similar to the CooKit there is not a lot that you can do to make it cook faster. One small step is to make sure the cooks are adjusting the front flap of the cooker properly. The food in the pot will also cook more rapidly if it is elevated a few centimeters above the bottom of the cooker. Some people have made little stands for their pots using rabbit wire or chicken wire for this purpose. With these stands, there can be an increased possibility that the pot of food will be knocked over and spilled--but the food does cook a little faster. With the CooKit type of cooker, one has to be sure that the plastic bag that insulates the pot is sealed tightly--if a lot of hot air escapes from the bag, the food will cook more slowly. The cooking pot must be a dark color and have a good lid--the color black is best. Solar Cooking Netherlands reports that an increase in temperature of 40°F (20°C) can be achieved by placing the pot inside two plastic bags.

If people are using large pots of food in the CooKit, making the CooKits larger will provide some help. However, very large amounts of food are difficult to cook in a Cookit.

For most solar cookers, smaller quantities of food will cook more quickly than larger quantities. I think that cooks cannot decide to simply give their families less food--perhaps the solution is that each family needs 2 cookers, each cooking a quantity of food that is appropriate to the cooker, rather than overloading one cooker with more food than it can cook in a reasonable amount of time.

With box cookers, the food will cook more quickly if it is divided into several smaller pots instead of having it all in one large pot.

The Solar Cookers World Network has [Plans|plans for many cookers] that can be hand-made from cardboard like the CooKit but which can cook a little faster. Unfortunately, they take more work to make. These cookers cook a faster than the CooKit: the Heaven's Flame box cooker, the Double-Angled Twelve Sided, 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 an hour (using a box cooker). It is not recommended that you cook meats unattended when there is a possibility of substantial cloudiness. More information on this see Food safety. 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 serious cloud cover significantly more than 25-30 % moves in and looks like it's there to stay, you need to save the food by moving it to some other kind of cooker. If your food is basically done (or near to and still very hot), and you're just keeping it warm enough to be safe and tasty until mealtime, you can tuck it into a box (or leave it in the box cooker, if that's what you're using) with insulation such as old quilts or comforters or sleeping bags tucked around it before closing the box and it will stay hot for at least a few hours. If the food definitely needs more cooking or has cooled into the danger zone, what will best emulate the gentlle heat of your solar cooker is an oven turned low (between 200 and 250F, depending on how much cooking the food still needs and how much time you have to reach it in). Electric slow cookers (aka crockpots) can also provide a good save for underdone foods such as soups, stews, etc. You can also use a stove burner, but if you want to retain some of that special flavor of solar cooked foods, keep the heat on the low side or use a diffuser on the burner. None of those options use as much power or heat up your kitchen as much as burners on high or higher oven termperatures.

Basic information

What are the basic kinds of solar cookers?

There are three basic kinds:

Minimum Solar Box Cooker Photo
Box cookers

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. You'll find an article discussing solar box cooker designs on the Solar Cooking Archive.

Solar-cooker-design-Bernars -cookit
Panel cookers

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.

Solar-cooker-designs-photo8-photor small
Parabolic cookers

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 focused often to follow the sun, and they can cause burns and eye injury if not used correctly. Some of these concerns have recently been reduced by Dr. Dieter Seifert's design.

The Compendium of solar cooker designs shows many variations on these themes. You can also listen to a good good introduction to solar cooking on the Solar Cooking Archive.

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. See this article for more info.

Where are solar ovens being used the most?

There are reliable reports that there are over 500,000 cookers in use in both India and China. There are also tens of thousands of solar panael 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 150° C (300° 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 (200° 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?

Cooking factors
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.

See our foods index for more specific directions for each food type.

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, focussed up at the bottom of the cookpot. These cookers work well stir frying, and with a griddle, can do a good job at preparing injera flatbread easily reaching temperatures above 250°C (400°F). Parabolic cookers require frequent reorientation to the sun, and care must be taken to avoid looking directly at the cooker from its focal point.

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.

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?

Solarex lg small-1-
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. Click the picture to see a map showing the amount of sunlight each part of the world receives.

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 Solar cooking hints.

Apples are another easy first project. Just dice or slice enough good cooking apples (not Red Delicious, which are better eaten fresh) 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, and the taste will be so fresh you may never want apples cooked any other way!

If the sun is out and you can't decide what to cook or don't have anything you want to cook that day, put some water on to boil. You can learn a lot about your cookers and what level of sun produces what results by boiling water. You can make tea with your water if you like, but even if don't use the water for food or cooking (you can water your plants with it later when it's cool, or put it to some other use), you will learn a lot from just boiling water. If you have made more than one cooker, this is a way to see which one heats up faster. Remember, the power is free and the water need not be wasted, so you can experiment all you like!

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, fewer particles (both air pollution and moisture particles) in the air that will impede, reflect, absorb and scatter sunlight. Water vapor absorbs energy and at higher altitudes humidity is usually lower. Therefore at higher altitude the food is heated up more quickly, so you gain time there. Depending on what type of food you cook (amount of moisture) or what type of pot you use, may result in a "steady state" lower temperature due to water evaporating. However the energy to the pot is higher.

Where can I buy a solar cooker?

Solar cookers may be purchased from companies in at least thirteen countries on five continents.

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?

Of course! It may just take a wee bit longer than a conventional oven or anyother electric cooking appliance, but if you don't have the time, just have others make their own and have a large cookout.

What are the challenges faced in getting people to change the way they cook?

Myriad. However, I don't think of the work of Solar Cookers International as "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.

That said, there are many challenges.

See also

What's the future potential for solar ovens in developing countries?

What has to happen for solar ovens to become more widespread?

Please see Promoting solar cooking

Where can I find solar cooking recipes

You can use conventional recipes as long as you follow these cooking guidelines. You can also use these recipes.

How can people earn money by making and selling solar cookers, when some solar cookers are so easy to make that almost anyone can do it?

See Income generation.

What resources are available online?

You will find the largest collection of solar cooking info on the Solar Cookers World Network wiki (the site you are currently on). Join the Network to share information. You can also participate in these online discussion groups.

Solar cooker components

Should I take the time to build a box cooker out of "real" materials like plywood and glass 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.

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?

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. See Plastic bags.

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 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 very well on a good day and started 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.

Main article: Cooking pots

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.

Main article: Insulation

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.

Other uses

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.

Main article: Water pasteurization

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! See also Canning.

See also

See this article in our foreign language wikis

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