Posted by Morris Dovey on October 5, 2010, 4:12 pm
On 10/4/2010 9:23 PM, dow wrote:
By the time I looked last night it was dark. I'm looking at the 10:35 am
"Camera 71" image - "silver sky" overcast with light rain (pavement
looks wet), and I'd expect a good flat panel cooker to reach 100°C.
This isn't really as difficult as you'd probably expect, since the
collector only heats a small volume of air and continuously cycles that
air through the absorber. At each cycle, more heat is added to the air.
Since the warmest of the air travels through the oven, the operation is
very much like (actually /is/) a convection oven.
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Posted by dow on October 5, 2010, 6:57 pm
This is very interesting. Have you made these things? Do you have one
that could be used as a demo? It sounds far better than most of the
solar cookers that are usually described.
Posted by Morris Dovey on October 5, 2010, 8:04 pm
On 10/5/2010 1:57 PM, dow wrote:
I have something that comes close - a passive flat panel designed to
draw cool air from near the floor and discharge warmed air at the top
for space heating. In clear sky conditions at approx 70°F/21°C the
discharge was right at 185°F/85°C - and I have no doubt whatever that if
it were configured as a closed circuit the temperature would go well
higher than the boiling point of water.
The cooker I sketched uses an 8-foot/2438 mm high (long?) capture area,
and the heating panel I built was only 6-foot/1828 mm, so the sketch
illustrates a design that would produce significantly higher
temperatures under the same conditions. The 8-foot cooker /might/
satisfy the 300°F/150°C benchmark, but I wouldn't dare make that claim
without building and testing a prototype first.
The heating panel was a failure because the temperature was much too
high for the intended purpose, but it provided some good lessons.
I haven't paid much attention to solar cookers (nor to solar water
heaters) because I was up to my ears with (first) the space heating
problem and (later) with the solar pump problem.
As far as I've been able to tell, almost all amateur solar thermal
development efforts have been hampered by a mindset that heat and
temperature are somehow the same thing. I suspect that if "most solar
cookers" aren't better than my failed heating panel, it's because the
developers had that mindset. :(
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Posted by dow on October 5, 2010, 8:41 pm
I'm worried by your description of testing your device under "clear
sky conditions". Does that mean in direct sunshine? Suppose it reaches
an equilibrium temperature 100C above ambient in full sunshine, what
would it do in light that is only 10% as intense as full sun? Would
the equilibrium temperature then be only 10C above ambient? That might
make a useful egg incubator, but not a cooker!
Posted by Morris Dovey on October 5, 2010, 9:36 pm
On 10/5/2010 3:41 PM, dow wrote:
You'll hear that a lot from me. Consider yourself invited to derate as
seems appropriate to the situation you're modeling. Simply put, the only
two unambiguous points are "clear sky" and "full dark".
Worse, "clear sky" may indicate a higher insolation than "direct sun".
An equilibrium condition is not reached until the space (oven or
structure) reaches a temperature at which energy is being lost to the
environment as rapidly as energy is being harvested.
That equilibrium temperature is a function of how well the system is
insulated. If energy is added and none is lost, the system will heat to
the point of self-destruction.
Insolation (solar intensity) affects only how rapidly that equilibrium
(or self-destruction) point is reached - it determines neither the
equilibrium point nor the self-destruct point.
The difference between the heating panel and the cooker is that the heat
from the panel was immediately discarded, while the heat in a closed
cooker is accumulated.
I suppose I should mention that most of the solar cookers I've seen
don't even attempt to accumulate heat (except in the cooking vessel and
the food). That's a lot of heat being thrown away!
Try putting your hooks into this: as air passes through the absorber,
enough heat is added to it to raise its temperature from 70°F to 185°F
for an increase of 115°F. Now 185°F isn't bad, but it's not what we
really want - so let's run that same air through the absorber again. The
second pass adds as much heat as did the first pass, so we'll see
_another_ increase in temperature. The third pass does it again and...
Anyway, each pass turns out to take only seconds - and the oven begins
preheating with the first light of dawn. If your insolation drops to
50%, it merely doubles the time required to reach the equilibrium
temperature - but the equilibrium temperature itself isn't much affected.
Better poke a hole in that eggshell.
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