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Solar cooking under clouds

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Solar cooking is a topic of wide interest and utility. 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 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.

But is there a simpler and cheaper way to do it? I just looked at the "Solar cooker" page on Wikipedia, and saw under "solar kettles" a mention of a technology using evacuated tubes that 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.

This sounds cheaper than using a whole lot of PV cells, but can we think of (and build) something simpler and cheaper still? Let's brainstorm about this on the Talk (Discussion) page, or under the heading "More discussions" on this page. Please sign your posts.

DOwenWilliams 18:23, October 15, 2010 (UTC) David Williams

Photo-voltaic cells

This is the idea that I mentioned in the initial article here. PV 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.

More discussions

What is implied in "Cooking under clouds"

For solar cooking purposes, Sunlight can be considered as having two components viz visible rays and infra red rays.

They occur in roughly equal proportions.

When there is a cloud cover, portions of visible rays are being blocked by the clouds.

Howver, clouds do not interfere with infra red rays in any significant maneer and that portion is avaialable for cooking.

Infra red rays can directly heat food when they hit it.

So cloud cover cooking implies cooking with something like 60% to 70% of the heat strength of a clear day cooking.

Cooking Times will go up proportionately as there is less heat available.

My hypotheis is that final temperature attained in the cooker will go down more than proportionately as heat losses due to convection and conduction remain the same.

Unless proper care is taken, chances are that the final temperature attained is within the range in which bacteria thrives.

Ashok Mathur2 01:20, October 8, 2010 (UTC)

Hmmm... When the sun is shining, I can feel its heat on my skin. That's the effect of the infra-red radiation. But when the sky is overcast, I don't feel the heat, so it seems to me that the infra-red is greatly reduced if not completely eliminated.

It is true that infra-red can penetrate cloud to some extent, but the clouds are not really transparent to it. The amount of infra-red that gets through is only a small fraction of the sun's output.

As an experiment, try using a focusing device, such as a convex lens or a concave mirror, to focus sunlight on a piece of paper. When the sun is shining, you will easily set the papar on fire. But if the sky is cloudy, you won't have any effect on the paper.

Or just try cooking something with a conventional solar cooker which cooks effectively when the sun is shining. You are saying that it will put about 60% or 70% as much heat into the food on a cloudy day as on a sunny one. Try it. If the sky is overcast, I bet the food won't even get warm.

DOwenWilliams 02:00, October 8, 2010 (UTC) David Williams

Some science

I did a bit more research on this....

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.

DOwenWilliams 18:09, October 11, 2010 (UTC) David Williams

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