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Solar cooking under clouds

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Last updated: 18 November 2016      

Many effective types of cooker have been developed that work when the sun is shining. But the sun does not shine all the time, even during daylight hours. In many parts of the world, it shines only very rarely. So can we design a cooker that would work under cloudy conditions?

Of course it is possible. Photo-Voltaic (PV) cells generate some electricity even when the light intensity is low. Put enough of them together, and make them power an electric oven, and you've got a solar cooker that will work in dim light.

Evacuated tube solar cooker designs can cook food in diffuse light. The general idea is that the food is in a dark-coloured tube that is suspended inside a larger glass tube, and the space between the tubes is evacuated. The inner tube is heated by incoming light, and the heat cannot easily escape so the temperature rises, even when the light level is low.

Photo-voltaic cells

Photovoltaic cells produce some electricity even in dim light, so if enough of them are connected together, they would be able to power an electric oven that would cook food using the energy of daylight on a cloudy day. There are a few possible variations of the idea. Instead of a resistive heating element, a Peltier-effect heat pump might be used. This would improve the efficiency. Also, thermo-electric devices might be used instead of the PV cells. This might reduce the cost.

Cost is the big drawback of this idea. At present prices, the PV cells that would be needed to run a useful cooker would cost thousands of dollars. This would be prohibitive in most situations. However, the prices of PV devices keep falling. It is possible that, in the not so distant future, this kind of cooker may become affordable.

Heat-storage cookers

In many locations the weather is often cloudy, but the sun does shine sometimes. It would be possible to use the direct sunshine, concentrated by a parabolic reflector or something of the sort, to heat materials to a high temperature. Rocks might be used, or water, or molten salt, or many other possibilities. The hot material would be stored in an insulated container, and could be used later, when the sun is not shining, to provide heat for cooking.

Insulated heat-accumulators

Main article: Heat storage

When light falls on a dark-coloured object, the temperature rises, If there is very good thermal insulation, such as a vacuum, preventing the escape of the heat to the environment, the temperature will rise to a high value even if the incoming light is dim. Solar ovens using this principle can be made that will work in diffuse sunlight. Whether they would work, if constructed with affordable materials, under dark cloudy conditions remains to be seen. Morris Dovey, who is a highly skilled experimenter, proposes to test this idea soon.

Burning vegetation

Main article: Biomass briquette

Plants absorb sunlight and use it to produce organic material even if the light is dim. If the plants are cut, dried and burned on a cooking fire, the food is cooked using the accumulated solar energy. Of course, this method has been used in practice for thousands of years. As traditionally done, it has major disadvantages, including deforestation and atmospheric pollution. But it should be possible to minimize these problems with good design. Instead of cutting down forests, plants might be deliberately grown for this purpose. They would be selected for fast growth. They might, for example, be algae growing in water. The burning could be done in a stove that releases very little pollution into the atmosphere. CO2 would be released, of course, but that would just equal the amount that the plants removed from the air as they grew. The ash from the fire would be returned to the ground or to the water, recycling minerals.

Some science

Clouds consist of tiny droplets of water or crystals of ice. Usually, they are between about 10 and 100 micrometres in size. (A micrometre, abbreviated µm, is a millionth of a metre, or a thousandth of a millimetre.) In heavy rainclouds, the droplets can be several millimetres in size, but nobody would want to do solar cooking in a downpour! Visible light has wavelengths between 0.4 and 0.7 µm.

Light will pass fairly easily through clouds if its wavelength is large compared with the size of the droplets or crystals in the clouds. To pass through a cloud with droplets 10 µm in size, the wavelength of the light would have to be larger than that. If the droplets are larger, the wavelength would also have to be larger.

So, in order for light to get through normal clouds, its wavelength would have to be at the very least about 15 times longer than the longest wavelength in visible light. This means that the light would have to be "far infrared". The "far" means that it's a long way from the visible region of the spectrum.

Our eyes have evolved to be able to make the best use of the light that is available. This means that the visible part of the spectrum is where the sun's output is greatest. The sun does emit light in the infrared and ultraviolet regions, but almost all of it is close to the visible region. So the sun does emit infrared light, but it's almost all "near infrared", close to the visible. Only a tiny fraction of the sun's light is "far infrared", which could penetrate clouds. Even this small amount is absorbed high in the earth's atmosphere, unless its wavelength is so long that it would be categorized as radio waves. So the amount of cloud-penetrating infrared sunlight that reaches the ground is essentially zero.

That's why conventional solar cookers, which cook well in direct sunshine, are useless if the sky is overcast. And it's why, if we want to be able to solar-cook in cloudy conditions, we must design and use different kinds of cooker.

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