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solar cooking under clouds

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Posted by dow on October 3, 2010, 2:33 pm
 
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...

              dow

Posted by Morris Dovey on October 3, 2010, 3:35 pm
 
On 10/3/2010 9:33 AM, dow wrote:


It depends on the location, on how hot the food needs to be heated, on
the food itself, and on how cloudy.

I built a flat passive solar heating panel that discharged air at 185F
(85C/358K) on a clear summer day at 42N - and that's hot enough to
safely cook any locally-produced foodstuff. (For the intended purpose of
space-heating, the panel was judged a failure - but was a good learning
experience).

If airflow through that panel were completely blocked (stagnation), I
think a temperature well in excess of the boiling point of water could
be achieved without difficulty under the same conditions.

It'd never manage to cook a Christmas dinner north of the Arctic Circle,
but might work year round in the tropical zone with moderate overcast,
for some definition of "moderate". :)

--
Morris Dovey
http://www.iedu.com/DeSoto/
PGP Key ID EBB1E70E


Posted by dow on October 3, 2010, 5:00 pm
 
I'm a bit confused.(Easily done.) You said you tested your panel on a
clear summer day, but does that mean that the sun was shining? If so,
it's not rally relevant to this topic.

Cooking by the light of the Aurora Borealis is probably not possible.
But maybe we could cook Christmas dinner using diffuse sunlight south
of the Antarctic Circle.

          dow


Posted by Morris Dovey on October 3, 2010, 10:51 pm
 On 10/3/2010 12:00 PM, dow wrote:


In general, a collector designed to focus the sun's energy works well at
focusing the energy from a point source - and very poorly when the
incoming energy is diffuse. In this regard, concentrating collectors are
nearly binary devices.

A flat panel can collect a high percentage of the available radiant
energy without regard for whether the source is an apparent point or the
whole sky.

As soon as clouds, overcast, fog, etc are introduced into the discussion
there arises a need to quantify their effect: do they block 1% of the
radiation, 10%, 99%? How much of a diffusing effect do they have - is
the sun a clearly-defined bright disk or does the entire sky seem
equally bright?


Agreed - but I doubt we'll have much success with solar cooking south of
the Antarctic Circle, too. :)

--
Morris Dovey
http://www.iedu.com/DeSoto/
PGP Key ID EBB1E70E


Posted by dow on October 4, 2010, 12:37 am
 
I'm not sure about that. As seen from a point on the Antarctic Circle,
the noon-day sun on the summer solstice is about 47 degrees above the
horizon. That should be high enough to cook Christmas dinner.

Ideally, I'd like the cooker to work when the light is completely
diffuse, with no visible sign of the sun's position in the sky. And it
would be good if it would work when the overall light intensity is
only 10%, or even 1%, of the intensity of full sunlight.

I've been vaguely thinking about several kinds of designs. Maybe a
battery of PV cells could drive a Peltier effect heat pump, instead of
just a resistive oven. That might improve the overall efficiency. And
maybe the PV cells could be substituted by thermo-electric devices.
The vacuum-flask idea might be used to keep the junctions warm.

This is all higher-tech than most solar cookers. I can't think (yet)
of any way to do it with simple stuff.

      dow

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