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Cooking eggs with a solar cooker reflector.

Solar cookers

by Chris Woodford. Last updated: November 27, 2016.

Next time you zap your dinner in the microwave or fire up the gas in your stove, spare a thought for the two billion or so people (almost a third of the world's population) who don't enjoy the same luxury. Many people in developing countries still have to burn piles of wood fuel to cook their food, which might cost a quarter or more of their income or take several hours to collect for free (with the burden typically falling on women and young children). Burning wood is bad for people's health and can be bad for the environment too if it destroys forested areas that aren't replaced. Solar cookers are a promising alternative with huge advantages and relatively few drawbacks. Using mirrors to build up heat and glass or plastic to trap it, they cook a meal with nothing more than the power of the Sun. Let's take a closer look at how they work!

Photo: Cooking eggs for free—with a solar reflector. The dish focuses the Sun's rays on the frying pan! Photo by Warren Gretz courtesy of US DOE/NREL (US Department of Energy/National Renewable Energy Laboratory).

What is a solar cooker?

Have you ever tried that old camping trick with a magnifying glass to burn a hole in paper? What you're doing is using a lens to concentrate rays of sunlight into a tiny area so you raise the temperature of the paper beyond its ignition point—in other words, so it catches fire. Sunlight is a bit of a misnomer: the energy that comes from the Sun is a wide spectrum of electromagnetic radiation that contains heat as well as light, in the shape of infrared light (just too red for our eyes to see) and ultraviolet radiation (just beyond the blue-violet end of the spectrum we can see).

Solar cookers work in a similar way to camp-fire magnifying glasses, but instead of lenses (which are relatively heavy and expensive) they use cheap mirrors to concentrate heat on a dark-colored cooking pot (dull black materials absorb the Sun's heat best). Other simple materials such as plastic and glass help to trap and retain the heat the mirrors have collected.

Types of solar cookers

There are three main types of solar cookers, though since many are self-built or modified using locally available materials, there are literally hundreds of variations.

Box cookers

These are the most common kind of solar cooker. They're sturdy wooden boxes lined with heat-reflective aluminum foil and they have a hinged lid made from glass. Simpler box cookers can be made from single thicknesses of sturdy cardboard with foil on the inside, but a more effective design has double wooden walls with insulation in between to trap heat more effectively. How does it work? You put your dark-colored cooking pot inside the cooker and close the lid. The Sun's rays enter through the glass and the foil concentrates them on the pot. Like the glass in a greenhouse, the lid allows sunlight in but stops heat from escaping (it also prevents any wind from cooling the pot). In an alternative design of box cooker, the lid is lined with foil and you prop it open so it concentrates extra sunlight down onto the pot. In this case, you also need to put the cooking pot inside a heat-resistant plastic bag to stop the heat from escaping. Box cookers are particularly good for slow-cooking a family meal over a period of several hours.

Parabolic reflectors

Chinese people boil water with a kettle on a parabolic-reflector type of solar cooker.

These are sometimes called solar kettles because they're often used for boiling water. Instead of a closed box, you have a mirror shaped like a parabola (a cone-shaped curve) so it concentrates sunlight at its focus, maybe 30cm–1m (1–3ft) in front of its midpoint. That's where you suspend your kettle or cooking pot. Parabolic reflectors are the most effective at concentrating heat so they generally achieve the highest temperatures and cook fastest (without the need for a heat-retaining plastic bag). However, they're quite large and cumbersome and (unlike slow, steady box cookers) need constant attention (especially if there are young children nearby).

Photo: Chinese people boil water using a kettle placed at the focus of a parabolic reflector. Photo by Simon Tsuo courtesy of US DOE/NREL (US Department of Energy/National Renewable Energy Laboratory).

Panel reflectors

A panel-reflector type of solar cooker being used to bake cookies.

These are like a cross-between box cookers and parabolic reflectors. Using a number of flat reflectors hinged together, you enclose an area and stand your cooking pot (covered with a sealed plastic bag) in the middle of it. Panel reflectors can be made simply and cheaply (for just a few dollars) from cardboard and aluminum foil, and since they're quick and easy to assemble they're often supplied by humanitarian organizations to people in developing countries. Unlike parabolic reflectors, they can be folded up, stored in very little space, and easily transported.

Photo: This panel cooker mixes elements of box cooker and reflector: the panels collect heat that builds up inside the box. Photo by Warren Gretz courtesy of US DOE/NREL (US Department of Energy/National Renewable Energy Laboratory).

What are the advantages and disadvantages of solar cookers?

Solar cookers can save the world's poorest people the expense of buying wood fuel or the time-consuming chore of collecting it. (It's often women who gather firewood in difficult or dangerous conditions, while children who have to walk for hours to collect wood or water have less time for school or helping their families in other ways.) Solar cookers also bring health benefits. Cooking on blazing fires can be dangerous and unpleasant and often causes respiratory problems, while solar cookers are safer (they use no fire) and are entirely smoke-free. Solar cookers can be better for people's health in other ways. Food cooked more slowly is more nutritious, and solar cookers can be used to pasteurize food or water (so it's safer to keep it for longer) and kill germs. There are environmental benefits too. Reducing the need to gather wood-fuel can help to protect forests and the biodiversity they contain.

There are a few drawbacks, however. Solar cookers obviously rely on sunlight so they can be used only at certain times of day in good weather, and not necessarily when people want or need to cook. The food also has to be cut up small so it will fit in a pot, and since solar cookers don't provide the warmth, heat, and community focal point that a fire may give, using them can involve a certain amount of cultural adjustment. These may or may not be minor details for people who've always cooked on open fires and whose home lives revolve around them. Then again, using a solar cooker isn't obligatory; the most important point is that it provides an alternative that wasn't there before.

Who invented solar cookers?

Diagram of solar cooker invented by Harry Cherrier in 1915, from US patent 1,158,175.

Solar cooking is one of those things that no-one can really claim to have invented; doubtless people in tropical countries have been cooking with sunlight for as long as they've been cooking! Having said that, European scientist and alpine explorer Horace-Bénédict de Saussure (1740–1799) is generally credited as the inventor of the modern, box-type of solar cooker.

In more recent times, the earliest US patent I've found for a solar cooker was filed by Harry Cherrier of Redlands, California in February 1915 and granted in October the same year. Cherrier explains how his invention "will utilize the sun's rays for cooking purposes" in preparing dried fruits, vegetables, "jellies, jams, etc." in such a way that they are "much superior" to those cooked the usual way. Unlike many modern solar cookers, Cherrier's invention speeds up the cooking process by sucking in a steady draft of air, heating it up, and passing it through—so it's a little bit like a modern, fan-assisted oven: "To properly prepare such material it is necessary that the air supply be continuous, abundant, and hot so that the moisture in the material to be cooked is absorbed by the air and rapidly taken away."

You can explore many more designs for solar cookers with a quick search for "solar oven patents" at the US Patent and Trademark Office (or on Google Patents).

Artwork: Harry Cherrier's "fan-assisted" solar cooker from 1915. Like a conventional solar oven, it cooks using sunlight, but the cooking process is speeded up by air drawn into the cooker, heated by the outer casing, passed through the central cooking chamber, then back out again through a flue. From US Patent 1,158,175: Solar cooker by Harry Cherrier, artwork courtesy of US Patent and Trademark Office.

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Woodford, Chris. (2010/2012) Solar cookers. Retrieved from http://www.explainthatstuff.com/solar-cookers.html. [Accessed (Insert date here)]

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