Posted by Morris Dovey on October 4, 2010, 2:21 am
On 10/3/2010 7:37 PM, dow wrote:
>> On 10/3/2010 12:00 PM, dow wrote:
>>
>>> 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.
>>
>> 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?
>>
>>> 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.
>>
>> Agreed - but I doubt we'll have much success with solar cooking south of
>> the Antarctic Circle, too. :)
>
> 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.
Hmm - I think I must be out of phase today. From a point on the
Antarctic Circle, the noon-day sun on the summer solstice (sometime
around the third week of June) should be right on the horizon, yes?
> 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.
That would be good. In clear sun we can approximate insolation as
perhaps 1 kW/m^2. If the intensity were reduced to 10% then that would
drop to 100 W/m^2, and at 1% we'd be working with 10 W/m^2.
If we build our collector with unobtanium (a perfect thermal insulator
and perfectly transparent in only one direction) we may still need
somewhere close to 100 m^2 of collector to cook our Christmas penguin.
Let's move that feast back to the Arctic Circle and make polar bear stew
for our summer solstice celebration.
> 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.
Admittedly, I haven't looked seriously at Peltier devices for some time
- but resistance heat is already 100% efficient and the heating elements
are (probably) still a lot less expensive.
> This is all higher-tech than most solar cookers. I can't think (yet)
> of any way to do it with simple stuff.
It really does sound as if you're looking for a one-size-fits-all solar
solution. Methinks we might be more productive if we start picking the
low-hanging fruit first. There's a lot that can be done (and isn't) that
doesn't require high-tech solutions.
--
Morris Dovey
http://www.iedu.com/DeSoto/
PGP Key ID EBB1E70E
Posted by Malcom \"Mal\" Reynolds on October 4, 2010, 12:04 am
> 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
When I was doing such things we had anecdotal evidence that using a "gold" foil
instead of a "silver" foil would increase performance during overcast
conditions. YMMV
--
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blandit.
Posted by Josepi on October 4, 2010, 12:20 am
The PV setup would be more expensive (probably) but would be useful for more
than cooking. A solar stove would only be useful when needed.
Solar burner would need to have a tracking mechanism.
Solar burner designed for no direct sun could be a problem if direct sun
hit..ie. burning bottom, over boiling, exploding mechanism etc..
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 Josepi on October 4, 2010, 12:25 am
Under extreme low light conditions, obviously an energy storage mechanism
may be necessary to boost the power level delivered compared to power level
received. I am not sure how this could be accomplished using a solar thermal
arrangement.
Possibly slowly heating a large thermal mass to more than a desired
temperature and then placing the pot in contact with the large thermal mass
to cook for a lessor time?
PV may still work better for extreme conditions provided your batteries work
in the Antarctic.
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 4, 2010, 2:37 am
On 10/3/2010 7:25 PM, Josepi wrote:
> PV may still work better for extreme conditions provided your batteries work
> in the Antarctic.
I have difficulty digesting this. Solar thermal has no conversion losses
(because there is no conversion).
PV loses most of the energy in the initial radiation-to-electrical
conversion, with additional losses in the electrical-to-chemical and the
chemical-to-electrical conversions at the battery. Charge controller and
inverter, if present, lose still more...
--
Morris Dovey
http://www.iedu.com/DeSoto/
PGP Key ID EBB1E70E
XML site map
>>
>>> 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.
>>
>> 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?
>>
>>> 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.
>>
>> Agreed - but I doubt we'll have much success with solar cooking south of
>> the Antarctic Circle, too. :)
>
> 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.