The Science of Solar Cooking

HOW SOLAR COOKERS WORK

What are the basic kinds of solar cookers?

There are three basic kinds:

  • Box cookers
    This type of cooker has been 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 here
  • Panel cookers
    This recent development was sparked by Roger Bernard in France. 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 for the Kakuma Refugee Camp project for US$2 each.
  • 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.

There is a detailed document here showing a large number of variations on these themes. You can also listen to a good introduction to solar cooking here.

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 100,000 cookers in use in both India and China. We are aware of solar cooking projects in most of the countries of the world. Solar Cookers International has recently had a breakthrough in Kenya using the CooKit panel cooker. More than 5000 families are now solar cooking there.

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 no food can go above 100° C (212° 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.
 
What's a Greenhouse Enclusure?
 
A heat-resistant bag (or similar device) surrounds the pot, acting like a greenhouse by allowing sunlight to hit the pot and preventing heat from escaping. Clear vases, or pyrex bowls can also be used for greenhouse enclosures.  

 
 
 
 
Solar cookers like the DATS cooker at the top of the home page can reach temps of over 300 degrees in the right conditions, but most solar cookers' temps hov
er in the 200 degrees to 300 degrees range. These temperatures are high enough to cook just about anything you might want to cook. Solar cookers work by using direct sunlight to heat up dark pots and pans. Different types of solar cookers work in slightly different ways. The article below is from http://www.solarcookers.org and it explains how solar cookers work.

Types

The three most common types of solar cookers are heat-trap boxes, curved concentrators (parabolics) and panel cookers. Hundreds — if not thousands — of variations on these basic types exist. Additionally, several large-scale solar cooking systems have been developed to meet the needs of institutions worldwide.


girl with box cooker
Box cookers

This type of cooker has been 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.

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.


woman with parabolic cooker

woman with panel cooker
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.


Principles

Most solar cookers work on basic principles: sunlight is converted to heat energy that is retained for cooking.


sun


Fuel: Sunlight

Sunlight is the "fuel." A solar cooker needs an outdoor spot that is sunny for several hours and protected from strong wind, and where food will be safe. Solar cookers don't work at night or on cloudy days.


Convert sunlight to heat energy

Dark surfaces get very hot in sunlight, whereas light surfaces don't. Food cooks best in dark, shallow, thin metal pots with dark, tight-fitting lids to hold in heat and moisture.

black pot absorbing sun's rays white pot reflecting sun's rays


Retain heat

A transparent heat trap around the dark pot lets in sunlight, but keeps in the heat. This is a clear, heat-resistant plastic bag or large inverted glass bowl (in panel cookers) or an insulated box with a glass or plastic window (in box cookers). Curved concentrator cookers typically don't require a heat trap.

black pot with bag black pot in box cooker


Capture extra sunligh
t

One or more shiny surfaces reflect extra sunlight onto the pot, increasing its heat potential.

panel cooker with sun's rays box cooker with sun's rays
parabolic cooker with sun's rays
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