In order to understand how solar ovens work you will first need to understand a few science facts. We like to call them The 3 C's of Solar Cooking!
There is an old saying that the sun shines on both the good and the bad. That is just a poetic way of saying that sunlight falls evenly everywhere, and it is reasonably accurate. So if you were to place an object out in the sun it would receive the same amount of sunlight no matter where you placed it, as long as you stay out of the shade.
The first "C" of solar cooking is "Concentrate," and that means we need to get more sunlight in an area than would fall there naturally. To do that we need reflectors like the mirror shown in the picture to the left. Notice how the are where the mirror's reflection hits is almost twice as bright as the surrounding sidewalk. Using reflectors, you can concentrate the sunlight from a much larger area into a smaller area and dramatically increase the usable power of the sun's rays.
Both panel ovens and our giant parabolic stoves use reflective surfaces to concentrate the sunlight from large areas into the cooking area where we place the food.
Another method that can be used is magnifying lenses. Have you ever used a magnifying glass to start a fire? The magnifying glass bends the light rays from a larger area and focuses them on a much smaller area. Even a small magnifying glass can create intense heat at its focus point.
One of the ovens that we use in our demonstrations is a large Fresnel lens in a frame. A Fresnel lens is essential a flattened version of a magnifying glass lens. The lens we use collects the sunlight from about 1 square meter and concentrates it to an area of approximately one square centimeter. That makes the heat and light at the focal point one thousand times (1000x) more intense. The focal point is so hot it can melt a penny, burn a whole in a cement brick, or carve a burnt out hole in your food if you are not careful. And you should never look directly at the light at the focal point without welding goggles as it could cause permanent eye damage because of how intensely bright it is.
Although the sunlight falls around you, what it does once it gets there is often a matter of the color of the object it hits. The "white light" of sunlight is actually made up of many different wavelengths of light. Wavelengths is just a fancy way of saying different colors. When sunlight passes through a prism the white light is spread out to show all the colors of the rainbow. In fact, when sunlight bounces around inside raindrops they act like little, miniature prisms. That is actually what causes the rainbows we see during thunderstorms.
Because there are different colors of light in sunlight, different things happen when it hits objects of different colors. Some of the colors of light will be reflected and some colors will be absorbed. The light that is absorbed will help to heat up the object. The light that is reflected is just bounced away back into the environment. The color of an object is actually determined by the colors it reflects, not the colors it absorbs. That may seem counterintuitive, but it is true.
Plants absorb red light and blue light to conduct photosynthesis. The green light is mostly reflected away. That is why most plants appear green. In fact, if you tried to grow a plant under pure green light, it would die of "light starvation" because it would not be getting the red and blue light it needs to make its own food!
Objects that are white reflect most of the light that strikes them back out into the environment. Because they reflect all the colors of sunlight they appear white to us. It also means that they absorb very little of the Sun's energy that hits them. You probably already know this, but you may not have considered it before. Clouds reflect the sunlight during the day, so the temperature is cooler. (Even though the bottoms look dark to us because no light is getting through, the tops are always white.) A white car will be cooler during the summer than a black car. Even the Arctic and Antarctic are colder than they would otherwise be because the snow near the poles reflects away much of the Sun's warmth that does reach them.
Black objects absorb nearly all of the light that hits them. That is why they appear dark to us. This power to absorb the Sun's energy makes the color black very important to solar cooking. The pots and pans we use with our solar ovens may be made from a variety of materials, and some will work better than others, but the most important quality in solar cookware is that the color should be dark, preferably black. Black cookware will simply absorb more of the Sun's energy and convert it into heat than any other color.
Objects of Other Colors
Objects of other colors will reflect the wavelengths of light with the same color as the object appears to us. So a red ball reflects only the red light and absorbs all the other colors. A blue ball would reflect only blue light and absorb the rest.
In reality, that is an over simplification of what really happens. Most objects reflect varying amounts of light from across the spectrum. Take the teal colored ball at right: It reflects only 30% of the red light, nearly 100% of the green light, and about 70% of the blue light. When those colors of light mix in those percentages, the color we perceive is teal.
The important thing to understand is that light that is absorbed gets converted into heat while light that is reflected away will not help you cook with the Sun.
The final principal to learn about is how to capture and retain the heat that you have concentrated and converted. An efficient solar oven works on the same principles as a greenhouse used for growing plants. Glass or plastic is used to allow the visible light through to the cooking vessel, but once that light is converted to heat (infrared radiation) the glass or plastic will act like an insulating blanket and keep the heat from escaping quickly. Not all of the heat is captured, but by capturing as much of the heat as you can, your solar oven will be able to reach and sustain higher cooking temperatures. And higher temperature means shorter cooking times. Take a look at this illustration of a greenhouse below.
A large parabolic cooker only concentrates the incoming energy, but it's large size makes it hot enough to cook with even a shiny, reflective stainless steel pot!
CONCENTRATE & CONVERT
A Fresnel lens is like a giant magnifying glass! This large, flat lens is made of plastic and concentrates the light to an intensely bright spot behind the lens. The black pan absorbs the radiant energy of all of the incoming light and converts it into heat. Darker colored objects absorb energy, and lighter colored objects reflect the energy away. That is why a white car stays cooler in the summer than a black car does.
CONCENTRATE, CONVERT, & CAPTURE
The glass enclosure of this panel oven allows the light to pass through to the cooking vessel, but it captures the hot air inside to minimize heat loss to the outside environment. The reflectors on a panel oven concentrate extra light on the dark colored cooking vessel where it is absorbed and converted into heat.
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Many of the images are made from Public Domain source images. We would like to offer our sincere appreciation to the authors who have shared their talents with the world, especially the hard-working employees of our federal government.
The original grant that made this project possible was provided from NASA educational endowment funds administered through the Idaho Space Grant Consortium. We are truly grateful to NASA for it's dedication to making science both exciting and accessible to America's students. A heartfelt thanks goes out to all the staff at the Idaho Space Grant Office for their help and support in making this possible.