Baking in the Sun

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“It is as unnecessary to use combustibles for fuel under a brilliant sun as it is foolish to put a solar oven out in the rain,” states Darwin Curtis, a partner in Solar Household Energy, Inc. (SHE). “Yet in many tropical areas of the world, more than 40 percent of families’ disposable income is spent on fuel for cooking. Sometimes more is spent on fuel than on food.”
 

   
 

Photograph of a Solar Oven

Several years ago Curtis and his colleagues recognized that the distribution of solar ovens in underdeveloped countries was not going well. (As the term suggests, a solar oven is a cooking device powered solely by sunlight.) At that time, Curtis explains, the entire effort to make solar ovens available to developing nations—parts of the world where this technology is needed desperately—was in the hands of governments, non-government organizations (NGOs), and the United Nations.

“The problem was much too large to be dealt with by charitable organizations,” says Curtis. “The only way adequately to address the problem is to induce private enterprise to get involved.” Thus, Curtis and his colleagues founded SHE to support developing world entrepreneurs in the manufacture and merchandising of solar cooking equipment.

Curtis says the first hurdle is to get people to accept this radically different technology. They must overcome the perception that solar ovens are too inefficient for people who are accustomed to more conventional means of cooking, and too expensive for those people who have little or no disposable income. People have generally regarded affordable solar ovens as unacceptable, and acceptable ovens as unaffordable, he summarizes.

With this understanding, SHE began a research and development initiative to construct a new solar oven of maximum commercial viability. Their goal is to have a prototype by early 2002. Curtis knows that in order for this model to succeed, it must meet four criteria: higher efficiency, durability, minimal cost, and attractive appearance. (If it doesn’t look right, it won’t sell.)

“Our objective is to increase by 50 percent the efficiency over comparable ovens,” Curtis explains. “The most widely affordable solar ovens now in use cost $2 or $3. They are remarkably effective but they are made of cardboard. We must use materials that can last a decade or more. And the ovens must retail for less than $30.”

Curtis says NASA satellite data are helping SHE determine which parts of the world receive sufficient sunlight to promote solar cooking. He reckons that SHE should focus on places that receive at least 4 kilowatt hours per square meter per day on average. Luckily, he says, the areas of the world where many people in greatest need for this technology live have solar insolation above 4, and as high as 6 kilowatt hours per square meter per day.
 

  Solar ovens are a simple and effective way to provide the energy needed for cooking to people without access to other energy sources. The ovens concentrate solar energy by reflecting sunlight through a transparent layer such as glass or plastic and onto a dark cooking pot. The sunlight is converted to heat on the dark surface of the pot, and the heat is trapped by the transparent material. Some solar ovens reach temperatures as high as 350° F (177° C), which is why you don’t want a car with black seats. (Photograph courtesy SSE project)
 

Palette for Solar Insolation

So how do solar ovens work? The concept depends upon two things: a means of converting solar energy into heat, and a means of trapping this heat around a cooking vessel. Curtis explains this is done most simply by using a black or dark-colored vessel surrounded by some sort of transparent envelope. Sunlight passes through the transparent envelope and strikes the dark pot, which absorbs the sunlight and converts it into heat. The transparent envelope traps the heat to achieve cooking temperatures.

“This is often referred to in literature, as ‘the greenhouse effect,’” Curtis says. “In cheap solar ovens, 250°F (121°C) can be achieved. In the expensive models, 350°F (177°C) is not unusual.”

He points to a 1997 United Nation’s Food and Agriculture Organization report that states there are 2 billion people in the world who are experiencing shortages of fuel to cook with. Curtis acknowledges that solar ovens are not a comprehensive solution for all of these people because there are days when the sun doesn’t shine. But he points out that in many places there are well over 300 days of sunshine in a year. The use of solar ovens in these circumstances can reduce dramatically the need for other fuels and can therefore help improve the quality of life.

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  Many of the regions most in need of energy, such as West Africa and the highlands of South America, receive an abundance of sunlight. The animation at left shows the average amount of sunlight a given area of the Earth receives each month. Four kilowatt hours per square meter per day of solar energy is enough to meet most basic energy requirements. Click once on the animation to stop it; double-click to resume play. (Animation courtesy SSE Project)
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