S'more Energy, Please!
by Nick D'Alto
Hungry for alternatives to fossil fuels? Try cooking with sunshine, and learn how solar ovens are producing energy around the world!
In a refugee camp in rural Kenya, tribal villagers are preparing their dinner using an atomic stove—an oven powered by a thermonuclear fusion reactor. Is this scene for real?
Absolutely. The stove is the UNESCO-sponsored “CooKit”—a portable solar oven made from simple materials and powered by sunshine. These small, inexpensive devices are improving the lives of people around the world, by letting them use a distant nuclear reactor—Earth's sun—to cook food, distill fresh drinking water, and even sterilize medical instruments and supplies.
It may sound unlikely. (“Yeah, right,” you say.) After all, how much energy could there possibly be inside one square yard of sunshine? Believe it or not, over 1.4 kilowatts—enough energy to power more than a dozen light bulbs!
In fact, the only reason that the sun doesn't fry us all is that cloud cover, Earth's atmosphere—plus, a relatively low azimuth (the sun's angle in the sky) reduces the average amount of solar power that reaches the ground (called the mean solar flux) to just more than that given off by a 100-watt light bulb. Find a way to catch that still respectable amount of energy by building a solar oven—just some reflectors, a black cooking surface, and a glass cover—and you've got a free, no-pollution way to replace fossil fuel.
Gravity is so powerful at the sun's core that each second, billions of tons of hydrogen atoms undergo nuclear fusion, generating 400 billion billion megawatts of power—the stuff you call sunshine.
Now, imagine riding on a stream of the sun's photons (the term for “packets” of light energy) as they travel 93 million miles to Earth. At 186,000 miles per second (the speed of light), it takes just eight seconds for the sun's rays to reach your solar oven.
Lucky thing—sunlight has an average wavelength (that's the distance between “waves” of energy) that lets it pass right through the oven's glass cover. (By the way, that's why glass is transparent.) Next, the light strikes the black cooking surface. Too bad—its luck just ran out. The light's energy gets absorbed. In fact, that's why a black surface appears black to our eyes—light goes in, but it can't get out!
Inside the surface's walls, light energy is converted to molecular vibration; the surface gets hot. And because heat waves have a much longer wavelength than light waves, they can't pass through the glass cover. Like an energy “jail,” the heat stays trapped.
- Nuclear fusion on the sun liberates photons of light energy.
- Light's wavelength lets it pass right through the oven's glass.
- Light gets converted to molecular vibration (heat) in the cooking pot.
That's the science behind solar ovens. Sound familiar? It's the same reason parked cars get hot in summer. Also, it's why too much CO2 makes Earth's temperature rise. Thermodynamicists (physicists who study heat conversion into other energy forms) call this “the greenhouse effect”—bad when it happens in the environment, but good inside a solar oven.
Engineers can make this process even better by incorporating insulation (materials that slow heat flow) in the oven's walls. They can also catch more power by improving the collector's reflectivity (its shininess) and its orientation to the sun. Collectors called heliostats can follow the sun in the sky, while parabolic (curved) collectors focus light to a single point. Forget roast chicken—make them big enough, and these solar furnaces can melt steel!
However, high-tech isn't always best. Appropriate technology (using only indigenous—native—materials) enables rural peoples around the world to create their own solar appliances. In India, ingeniously designed solar ovens are preventing the destruction of acres of forest each year that previously were cut for firewood and then burned, contributing to pollution. That's how solar solutions can help solve the energy crisis—and maybe even save the planet.
So, are you ready to build your own “nuclear-powered” toaster oven? It's easy!
- 4 pieces of cardboard (each a foot square)
- Roll of aluminum foil
- Sheet of black construction paper (your “cooking surface”)
- Slice of apple
- Supermarket oven-roasting bag (your “oven”)
- To make reflectors, simply cover the sheets of cardboard with the aluminum foil.
- Wait for a bright, sunny day and place your cooking surface in direct sunlight (preferably on a flat rock or picnic table).
- Position the reflectors at angles around the cooking surface to catch the sun. (WARNING: To protect your eyes, NEVER look into the reflectors or at the sun!)
- Pop a slice of apple into your oven and place it on your cooking surface.
- Begin cooking!
- Come back 20 minutes later, and your apple will be baked!
As a solar engineer, you can boost your oven's efficiency. For example, can you angle the reflectors to generate more heat? Try different reflector geometries and then test your heat factor by cooking different foods (mmmm!). You can also use an oven thermometer. Evaluate the benefits of insulation by elevating your black cooking surface (try a few small stones) to let a layer of air pass beneath it. Does your oven's temperature stay consistently hotter?
Hungry for s'more? Then try some of our sizzling “solar snacks” (below). Using the brilliance of photonic science, your next snack can be “atomic-powered.” And all because you live just eight seconds from a nuclear power plant (traveling at the speed of light, that is!).
- A person who escapes to another place to find protection or shelter from danger or trouble.
- You put a cup of water and a cup of flour inside a solar oven. Which do you think will heat up more quickly? Why? Write a sentence or two to explain your answer.
[anno: A cup of flour would heat more quickly than a cup of water because flour is a solid and water is a liquid. The Sun heats solids, such as land, more quickly than it heats liquids, such as lakes and oceans.]
- Both the cup of water and the cup of flour heat up to 100 degrees F. The Sun goes down, and the temperature drops to 65 degrees F. If you took the temperature of the water and flour every few minutes, what do you think you would find?
[anno: The water cools more slowly than the flour. Both would reach 65 degrees F, but it would take the water longer to reach that temperature than the flour.]