The Disadvantages Of Solar Cooking

Solar cookers seem like a no-brainer. Instead of using increasingly scarce fossil fuels, which add pollutants to the atmosphere and contribute to climate change, they take advantage of the sun's energy, which is free, clean and abundant. In fact, many people do use solar cookers, but most live in developing countries.

There's a reason for that. Most developing countries are in the tropics, where sunny and hot weather is common. Regrettably, solar cooking technology is imperfect, and one limitation of the solar cooker, as much as its main advantage, is the sun.

The position of the sun in the sky is suitable for cooking for only a fraction of each day, and cloudy days don't count. This means that solar cookers often don't fully cook what you put in them, and that can be dangerous.

Since the development of the first solar oven by Swiss physicist Horace Bénédict de Saussure in 1767, there have been a number of improvements in solar cookers. Whether you're shopping for your home, looking to outfit an expedition or making a donation to a rural community, you can now choose from four main types of solar cooker, each with its advantages and disadvantages.

The oven that de Saussure developed was made mostly of glass and wood, and although it did achieve high temperatures, it wasn't a practical design for everyday use in the modern world. Contemporary solar cookers are, on the whole, more lightweight and compact, and they are generally cheap to build.

• The Hot Box: The closest to de Saussure's design, the hot box is essentially an insulated square or rectangular box with a glass or plastic lid. It has one or more reflective panels that fold out to focus sunlight into the interior, which is painted flat black to better absorb and radiate the heat. 

• The Panel Cooker: Like a box cooker without the box, the panel cooker has several reflective panels that fold out to create a lightweight enclosure. The easiest cooker to build and transport, the panel cooker is the one you want for your camping trip. 

• The Parabolic Dish: This model takes advantage of geometry to increase cooking temperature and reduce cooking time. Instead of flat panels, it features a parabolic dish that works like a lens to focus sunlight to a point. It can achieve temperatures in the range of 250 degrees C and can fry and grill food as well as simply cook it. 

• The Vacuum Tube Cooker: A fairly recent innovation, the vacuum tube cooker actually consists of a pair of tubes, one inside the other. The outer tube is sealed, and the inner one, where the food goes, is painted black. A vacuum between the tubes acts as a near-perfect heat insulator, ensuring that the heat that radiates through the reflectors on the outer tube and gets absorbed by the inner ones stays inside the cooker long after the sun goes down. 

Solar cookers, like solar panels, need sunlight to operate, but unlike panels, you can't hook up solar cookers to batteries and store energy for use when the sun goes down. The best you can do is create a closed, insulated space that retains heat, but few cookers, even when insulated, can maintain temperatures high enough for cooking when there's no sun.

Another problem with solar cookers is that, with the exception of the vacuum tube type, they have to be periodically realigned with the sun, and the sun moves. Consequently, someone has to keep adjusting the oven to keep it aligned.

An innovative way around this is to equip the cooker with a sundial-like rod which allows the user to align the cooker to optimize the amount of sunlight over a several-hour period. Even with this innovation, however, manual realignment is required at some point to take full advantage of available sunlight and maintain cooking temperature.

A third limitation of the solar cooker is that the sun's position in the sky is optimal for cooking at around noon, but you probably want to eat dinner in the evening. Cooking generally takes about three hours, so you must find way to keep the food warm for several hours. This is difficult to do, and it's just as difficult to reheat the food when the sun is low in the sky, so you may have to adjust you meal schedule to compensate.

Box solar cooker advantages and disadvantages are among the easiest to explore for yourself. If you do a solar cooker project for high school, this is probably the type you'll build.

You'll find that sealing the box to thermally insulate it can be challenging, and on a cold, windy day, you may have trouble generating enough heat to cook even a small plate of food. Some box cookers use bricks to store heat, but this makes them heavy and difficult to transport, and it reduces available cooking space.

Lack of insulation is even more of a problem with panel and parabolic cookers, because they typically don't have any enclosure at all. Panel cookers may be easy to build and carry around, but they make up for this by taking the longest time to cook food. In cold weather, a panel cooker my not generate enough heat to fully cook your food, and partially cooked food, especially meat, can make you sick.

Parabolic cookers generate much more heat and cook food the fastest of all cookers, but even that comes with a price. Parabolic cookers generate so much heat that they can ignite anything placed in the cooking area. Besides the dangers associated with such high heat, parabolic cookers generally aren't portable.

The vacuum tube cooker can work even when the sun is low, and because it absorbs ultraviolet radiation, it also works on moderately cloudy days. The temperature inside the cooker gets as hot as a parabolic cooker, around 250 degrees C (480 degrees F), so food cooks in about an hour, and because the cooker retains heat, the vacuum tube cooker will keep your food warm until you're ready to eat it. You can also fry and grill food.

There is a lot to like about the vacuum tube cooker, but it isn't without its disadvantages:

• The vacuum tube cooker is expensive. A model large enough to cook a meal for eight people costs about $600, about ten times more than a hot box. 

• You can't build one yourself. The vacuum tube has to be factory sealed, and it is fragile. Don't even think about dropping it.  

• The cooker is cylindrical and probably won't accommodate something as large as a turkey, although it's great for vegetables and smaller pieces of meat. 

• It works in moderate cloud cover but not on extremely cloudy days, and of course, it won't work at night, so you have to time your cooking to accordingly. 

All in all, the perfect solar cooker has yet to be invented, and the best approach to solar cooking is to have an electric, gas or wood-fueled backup cooker available for dishes a solar cooker can't handle or days when it won't work.