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Solar Cookers          A Critique of Solar Cookers from the June 2007 issue of Cook’s Ilustrated Magazine
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Solar cookers use just the sun's rays to work—they don't require fuel or emit smoky fumes, and they use minimal water. They're environmentally friendly, but are they really useful?

Before we started to test solar cookers, we were skeptical—could anything simply left in a pot or box to cook in the sun actually taste good? But we were proved wrong—at least partially. We also didn't anticipate just how much fun solar cookers would be. Whenever we had a sunny day, we wanted to try cooking something new. But these results still beg the question: Are solar cookers more than just a toy for a food hobbyist?

Background:
Cooking in the sun has a long history. The contemporary impetus for using solar cookers is largely economic and environmental. Solar cookers don't require any fuel, they don't create smoke pollution, and they use minimal water; all factors that make them attractive for use in developing regions around the world. Over 100,000 are reportedly used in India and China. Most solar cookers are produced by nonprofit organizations; profits from cookers sold in the U.S. or Europe subsidize cookers shipped elsewhere. In the U.S., sales are geared to several audiences: those who advocate environmentally sound practices, people in sunny climates who want a fuel-efficient way to cook that doesn't heat up the kitchen, outdoor enthusiasts who like their portability, and food hobbyists who enjoy experimental cooking.

There are two main types of solar cookers: concentrating or parabolic cookers and hot boxes or ovens. We tested one of each, as well as a hybrid form (see photos below). The Hot Pot Solar Cooker ($100) is a large, insulated pot that sits on and is surrounded by reflector shields (metallic panels that concentrate the sun's rays at the pot). Its temperature stayed below 200 degrees. Our hot box model, the SOS Sport Solar Oven ($140), is a box with a specially molded insulating lid and insulated sides—its internal temperature reached 250 degrees in our tests. The Sun Oven ($200) is a combination of both types: The box has insulated sides and a glass top, and it’s ringed by reflectors. It reached an internal temperature of 350 degrees.

To cook effectively, it is necessary to understand some of the science that makes solar cookers work. It’s not the sun's heat that cooks the food, but rather the sun's ultraviolet rays. The sun must be high in the sky in order for the ultraviolet radiation to penetrate the atmosphere. For example, from November through March in the Northern Hemisphere, when the sun is low on the horizon, its light passes through more atmosphere to reach the earth. This screens out most of the UV rays—that's why it’s difficult to get a tan in the winter. When the sun is overhead, light rays pass through less atmosphere, so less UV radiation is screened out. A solar cooker works like a one-way lobster trap. It lets UV light rays in and then converts them to longer infrared light rays that can't escape. Infrared radiation has the right energy to make the water, fat, and protein molecules in food vibrate vigorously and heat up.

Practically, this means that successful cooking requires a clear sky, with the sun at least 45 degrees or more above the horizon for a significant amount of time (depending on what you're cooking, from two to eight hours). There are two easy ways to test for suitable conditions: 1) if your shadow is shorter than your height or 2) if the UV Index in your area is 7 or higher (see below for an online source for the daily Index, as well as other information).


Testing:
Our test kitchen is located just outside Boston, Mass., which is not the ideal location for solar cookers (our UV Index rarely gets above 7, and cloudless, haze-free days are not a given), but with a little planning, we had ample testing days. One additional challenge was finding a place in our urban setting that had uninterrupted sun for at least six hours—we finally set up our open-air kitchen on the roof. All three cookers were easy to assemble and use. The Hot Pot required no additional equipment and the SOS Sports Oven came with two pots that we used in the SOS and Sun Oven models. Ideally, you should use a dark, lightweight, shallow metal pot—Graniteware is a good choice. To conduct each testing, we set up the reflectors on the Hot Pot and Sun Ovens, put the food in the pots, put the pots on the reflector base or in the ovens, and oriented the cookers toward the sun. Then once every hour or so we repositioned the ovens to follow the sun and checked on the food.

Over several weeks, we tested (and retested) a variety of food, including rice, whole chickens, broccoli, marinated pork, roasted garlic, potatoes, and chocolate chip cookies. Our conclusions? What the cookers cooked well, they cooked very well, especially food that appreciated long, slow cooking and could generate sufficient internal moisture to stay moist and tender, like the whole chicken, baked potatoes, garlic, and marinated pork. But time-sensitive and/or drier food presented a challenge, especially since the cookers proved very sensitive to temperature fluctuation, either from being opened to check the food’s progress or from passing clouds. The cookies were the only time-sensitive food we liked, largely because we could check on their progress without opening the ovens (we could see through the covers). But despite checking frequently, we never got the rice right—we couldn't catch it before it was blown out and starchy—nor the broccoli, which turned army green and smelled skunky by the time it was tender.

Recommendations:

  • We found solar ovens to be fun and educational, but they have quite a few limitations. In sunny parts of the country, solar cooking can be an entertaining and environmentally responsible hobby.
  • Timing requires a lot of trial and error. Stews or soups that are pretty forgiving work best.
  • Cooking at midday isn’t terribly practical if you work or for evening meals. Holding the food until dinnertime raises food safety issues (see below).
  • Planning is difficult; even a partially cloudy day can slow or prevent cooking.

The bottom line? Solar cookers are surprisingly good at cooking certain things, but overall they’re unreliable. They won’t replace our indoor ovens or our charcoal grills anytime soon. When someone figures out how to store and better control the energy the ovens capture from the sun, they’ll become a lot more useful.

For those tempted to purchase one,
we would recommend the SOS Sport Oven. It had the greatest capacity—it could hold a small baking sheet, while we had to use the pot lid to bake the cookies in the Sun Oven—and kept the most level temperature. Since it doesn't have reflectors, it was less sensitive to passing clouds and less finicky about being positioned to follow the sun. The Hot Pot is simply too limiting. It can't bake and it did the worst job of holding heat against a passing cloud or when not oriented properly. With its reflectors, the Sun Oven attained the hottest temperature, but it was more susceptible to temperature fluctuations, making it difficult to judge cooking time. It actually overcooked food. The panels could not be removed, making it awkward to clean, and the size of the oven was limiting. It was also $60 more expensive than the Sport.

A Word about Food Safety:
We reserved cooking protein for cloudless days and during peak cooking times (between 10 a.m. and 2 p.m.), which kept our food safe (internal food temperature should not remain between 40 degrees and 140 degrees for more than two hours). However, during less-than-perfect days, we would have reservations if you’re not able to monitor internal food temperature either before it starts cooking or while the food stays in the cooker until dinnertime. The USDA has published a caution about solar cookers, noting possible problems. See Do New Consumer Products Enhance or Endanger Food?.

Sources for Further Information
Purchasing:
We purchased all three solar cookers from one source.

Reflections
PO Box 1163
Rough and Ready, CA 95975
(530) 273-9378
www.solarovens.net

Information:
Solar Cookers International is a nonprofit organization dedicated to spreading the word about solar cookers and supplying developing nations with solar cookers. Their website has an extensive amount of information, a newsletter, and ovens and other products available for purchase.
solarcookers.org

The Solar Oven Society manufactures the Sport Solar Oven. It is also a nonprofit organization that promotes solar cooking in the U.S. as a way to raise funds to encourage solar cooking overseas. You can buy the Sport directly from this site. It is also a good source for recipes—the SOS was the only solar cooker that came with a cookbook.
Contact:
Bill Potts, Solar Oven Society
Marketing and Sales Consultant, Volunteer
952-890-0574
www.solarovens.org


For more information about solar ovens, including the technology, from an independent source, see Solar Cooking, a Technical Brief from The Schumacher Centre for Technology and Development in the UK (available in both a text and PDF version). See www.practicalaction.org, click on Practical Answers, Energy, Solar, and Solar Cooking.

To determine local UV Index:
The EPA has a site that will calculate the local UV Index for that day based on zip codes:
UV Index. See http://www.epa.gov/sunwise1/uvindex.html

 

Photo of magazine covers

Articles and photographs about the Sport Solar cooker have been featured in recent issues of the following magazines:

Cook's Illustratedt: Junel, 2007
Click to read the complete article

Cooking Light: April, 2006
Click to read the complete article

Gourmet January, 2005
Click to read the editor’s letter and see the story.

Cabela’s Outfitter Journal: December, 2004
Click to read the complete article.

Southwinds: November, 2004
Click to read the complete article.

NY Times Sunday Magazine, August 21, 2005
Click to read the complete article.

Boating World; December, 2004
Click to read the complete article.

Blackpowder Guns & Hunting,Winter, 2004
Click to read the complete article

 

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