How Solar Cooking Works

­Using stoves and ovens, we can cook foods like meat, vegetables, beans, rice, bread and fruit in just about any way. We can bake, stew, steam, fry and braise. Using a solar cooker, we can do the same things, but by using sunlight instead of gas or electricity.

Sunlight isn't hot in and of itself. It's just radiation, or light waves -- basically energy generated by fluctuating electric and magnetic fields. It feels warm on your skin, but that's because of what happens when those light waves hit the molecules in your skin. This interaction is similar to the concept that makes one form of solar cooker, the box cooker, generate high temperatures from sunlight.

At its simplest, the sunlight-to-heat conversion occurs when photons (particles of light) moving around within light waves interact with molecules moving around in a substance. The electromagnetic rays emitted by the sun have a lot of energy in them. When they strike matter, whether solid or liquid, all of this energy causes the molecules in that matter to vibrate. They get excited and start jumping around. This activity generates heat. Solar cookers use a couple of different methods to harness this heat.

The box cooker is a simple type of solar cooker. At maybe 3 to 5 feet (1 to 1.5 meters) across, it's essentially a sun-powered oven -- an enclosed box that heats up and seals in that heat. At its most basic, the box cooker consists of an open-topped box that's black on the inside, and a piece of glass or transparent plastic that sits on top. It often also has several reflectors (flat, metallic or mirrored surfaces) positioned outside the box to collect and direct additional sunlight onto the glass.

­To cook, you leave this box in the sun with a pot of food inside, the pot sitting on top of the black bottom of the box. When sunlight enters the box through the glass top, the light waves strike the bottom, making it scorching hot. Dark colors are better at absorbing heat, that's why the inside is black. The molecules that make up the box get excited and generate more heat. The box traps the heat, and the oven gets hotter and hotter. The effect is the same as what goes on in a standard oven: The food cooks.

Box cookers can reach up to 300 degrees F (150 degrees C) [source: SHEI]. That's hot enough to safely cook meat.

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A parabolic cooker can get even hotter, up to 400 degrees F (204 degrees C), which is hot enough to fry food or bake bread. This slightly­ more complicated design uses curved, reflective surfaces to focus lots of sunlight into a small area. It works a lot like a stove, and it's big, sometimes up to several feet across.

A pot of food sits on an arm that holds it in the center of the curved reflectors, suspended slightly above the bottom point of the oven, where all the light is concentrated. This small point gets so hot -- and the molecules vibrate so much -- that the heat waves move upward in a steady stream to strike the bottom of the pot.

Both parabolic and box cookers are quite large, making them difficult to carry around. And box cookers are heavy because of the glass. A panel cooker, which uses parabolic reflectors positioned above a box-type oven, tends to be smaller and lighter. The cooking pot goes in a plastic bag while it cooks, which acts as a heat trap (like the transparent top on a box cooker). People sometimes use these types of cookers in ­camping.

Camping is something of a side job for solar cookers, though. The more central applications have to do poverty, hunger and disease.

How can cooking with sunlight help?

­Around the globe, hundreds of millions of people have limited access to cooking fuels [source: SCI]. In most cases, electricity and gas are out of the question; only charcoal and firewood are within reach, and even charcoal can be too expensive. So we're left with wood. The problem is that in many poor, rural areas in places like Africa and India, trees are scarce. It takes a lot of wood to cook meals for an entire family every day, and what few wood sources there are continue to dwindle. Families have to walk for hours to collect cooking wood, and they end up spending what little money they have on their fuel, leaving less money to buy food.

The result is not only hunger, but also disease.

When 1 billion people are without access to clean water, the ability to easily pasteurize (heat to the point at which microbes die) is crucial [source: SCI]. Two million people die every year from bacterial illnesses that could have been prevented by pasteurizing drinking water [source: SCI]. Heating meat, as well as vegetables grown with contaminated water, is also important for health. All of these nutritional necessities can contain harmful bacteria, worms and viruses that cause potentially deadly diseases like hepatitis A, giardia and E. coli sickness.

It doesn't take a whole lot of heat to pasteurize. Water is pasteurized at just 150 degrees F (65 degrees C), and food is pasteurized at 180 degrees F (82 degrees C) [source: SCI]. Solar cookers, which can cost as little as $10 a unit for a simple design, easily reach the upper temperatures of pasteurization, and they cook a meal in a few hours, depending on the cooker and the type of food. Best of all, people don't have to walk for miles to get the fuel they need. They simply go outside and use the sun -- for free. Using a solar cooker for a single year can eliminate the need for 1 ton (0.9 metric tons) of firewood [source: Justus].

­Another problem that can be solved by solar cooking has to do with the simple act of burning wood. Fires release pollution into the air. This smoke, filled with particulates, is bad for the environment, but it's even worse for the people who are breathing that air. When people use open fires to cook indoors, they end up inhaling microparticles that can cause all sorts of health problems, including both lung and heart disease. One estimate puts the number of people who die from this type of air pollution at 1.5 million per year [source: Madrigal]. A solar cooker eliminates the need for an open flame, meaning cleaner air.

While solar cookers are a great solution for many developing countries, there are a few drawbacks. The very trait that makes solar cookers so beneficial -- sunlight as fuel -- also makes it problematic. Solar cooking on a broad scale is really only viable for countries that have a dry, sunny climate for at least half the year. Areas of India, Brazil, Kenya and Ethiopia are some of the ideal locations for this cooking method [source: SCI].

Perhaps the bigger problem is that even in places like India, the sun isn't always shining. Solar cookers won't work at all in nighttime or on cloudy days. That means firewood is still a necessity for the sun's off hours. Still, cooking most meals with light instead of wood makes a big dent in the problems facing the fuel-deprived masses.

For more information on solar cooking and related topics, look over the links on the next page.