We’ll show you how to build a solar oven, so you can create savory meals while cooking without fuel.
A rhubarb pie and an apple tart are baked in two simple, handmade solar ovens.
By Philip Reynolds
Summer’s arrived and the heat is inescapable. You don’t want to turn on the stove to make dinner, which will heat up the house even more. If you’re like me and don’t have air conditioning — or if you’re energy conscious and keep the AC low — cooking indoors can be unbearable. Instead, why not use the source of all this heat to your advantage?
Solar radiation is the most prolific source of energy on our planet. About 84 billion kilowatt-hours of light reach Earth every day — more than four times our global energy consumption. The challenge is to efficiently harness this energy. Most people settle for gathering solar energy by eating vegetables from their gardens or catching its reflection with their cameras. Trap that energy in an insulated box with some food — then you’ll really be cookin’!
The functioning principles of a solar oven are simple: concentrate, convert, contain. Sunlight — or visible light — is concentrated by several reflective surfaces to pass through a glass lid into an insulated box. A pot of food you put inside the box will absorb the light and convert it into longer-wavelength infrared energy, or heat. The insulation will inhibit the heat from escaping, and the wavelengths will be too long to pass back through the glass lid. So, they’ll bounce around and heat up your food. Ever leave your car windows closed on a bright, warm day? Then you’ll recognize the basic principles of solar cooking.
Nearly 20 years ago, as I was helping a friend build his straw bale house (at which point he figured out I was into “weird stuff”), he mentioned that some people he knew were teaching a solar-oven-building workshop. I was intrigued, so I sent in my 50 bucks to attend. Three weeks later, I found myself on a homestead out in the Kentucky woods, looking at the piles of plywood, cardboard, tinfoil and glass that were to become solar ovens. The instructors, Mark and Andy, had just returned from a nonprofit-sponsored trip to Peru, where they taught people how to build and use solar cookers. Many of Peru’s mountain villages suffer from deforestation, so solar cooking offers a good alternative to cooking with wood.
That day, using only hand tools, I built my own “yard appliance.” This solar cooker requires only cheap materials and the design is so simple: square, with an inner box and an outer box separated by 1 or 2 inches of insulation, and a glass cover on the inner box to let in sunlight. Additionally, four reflectors are arrayed at obtuse angles to the glass to focus more light into the box and raise its internal temperature enough to make cooking possible. We used plywood for the outer box for rigidity and durability, and crumpled newspapers for insulation. A variety of materials can serve as insulation, including sawdust, chicken feathers or fiberglass batting. Insulation is key, because you want to trap heat in the box as effectively as possible to offset shading by clouds that will invariably show up to block the sun. In optimal conditions, the cookers we built can heat to 400 degrees Fahrenheit in about an hour, which is hot enough to cook anything from a casserole to biscuits. That’s a pretty good appliance for only a $50 investment. For me, the deal was even sweeter — I married Mark’s sister a few years later, and we’ve been solar cooking together ever since.
You can easily manage a solar cooker by keeping these elements in mind:
Sun. Don’t put the oven where a shadow will fall across it. If you use recycled glass for the top, make sure it hasn’t been treated with UV-resistant coating.
Time. Cooking times are longer than in a conventional oven because average temperatures fluctuate during the day. As long as the oven temperature remains above 200 degrees, your food will still cook.
A sense of adventure. You never really know what the weather will do.
For best results, preheat the oven for at least 30 minutes — allow it to heat while empty or with an empty pot inside. Also, someone should be around to adjust the cooker now and then. The reflectors will angle sunlight into the box for about two hours of the sun’s path across the sky, after which you should rotate the cooker to follow the sun. With this method, solar cooking will take about twice as long as cooking with electricity or gas, but cooking time will decrease dramatically if you rotate the oven every 30 minutes.
Cover reflectors with basic aluminum foil, Mylar tape, acrylic mirror or other highly reflective products. The top of your solar cooker should be inclined, either by design or by tilting the box on blocks. The best angle is based on your latitude and the declination of the sun, but in the Northern Hemisphere it should be about 30 degrees during summer and 60 degrees during winter. To increase your solar oven’s ability to convert light to heat, paint the bottom of its interior with black, high-heat paint. To improve heat-holding ability, add some thermal mass, such as a large rock or brick. On days with strong sun and no cloud cover, two to three hours is enough to cook almost anything, from a pot of rice to a loaf of bread, without adjusting the cooker.
I put my oven to use right away and had fun figuring out what worked. When cooking grains or beans, you only need a little more than half as much water as on a stovetop. Cooking veggies in the slow, even heat results in incredibly savory dishes. Food rarely burns, and only ever on top, so nothing ever sticks to the bottom of the pot. Even bread, pastries and meat will do well, although you’ll need to maintain a high average temperature, so plan in advance, and cook on days forecasted to be completely clear and sunny. Frying is difficult as the temperature typically isn’t high enough and you’ll lose heat every time you open the cooker to stir the pan — but you could experiment with quick-cooking foods, such as eggs.
Dark-colored cookware works best — dark objects convert sunlight into heat energy more easily than light-colored or reflective objects. Cast iron, black enamel and dark ceramic are good options, and lids help hold in heat. But, when cooking colorful veggies, don’t use a clear lid. The concentrated sunlight will bleach the vegetables’ color.
I painted the outside of a couple of wide-mouth, quart-sized Mason jars flat black with high-heat paint (including the outsides of the lids), and these became my rice- and bean-cooking jars. They heat up quickly, are space-efficient, and double as storage jars for leftovers. Never fill the jar more than half-full, including water. A cup of brown rice needs 11⁄2 cups of water and is usually done after 70 minutes. Don’t tighten the lid all the way when cooking in a jar or you’ll risk an explosion.
I used that first oven for five years before my wife and I replaced it with a much larger unit. We mounted the new oven on the roof of our earth-bermed house. We live in the woods, and our roof is the only place with reliable sun — but the cooker is easy to access because we can walk right onto its surface from the rear of the house. We use the solar cooker for the majority of our hot meals from April through October. The sun is too weak, spotty and low in the sky in our area to rely on for solar cooking during winter. And by then, we’re using our woodstove for heat, so we often cook on that. We tarp the solar cooker during winter to prolong its life span. For a small investment and a little know-how, you, too, can get cookin’!
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Joel Dufour lives in an electricity-free home he built with his wife, Chris, in the woods near Frankfort, Ky. He’s been solar cooking since 1994. He owns and operates Earth Tools, a gardening-equipment supply company.
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