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Posted by dow on October 6, 2010, 2:03 am

As you say, the device will reach an equilibrium temperature at which
the rate of loss of heat to the environment equals the rate at which
heat is supplied, by absorption of light in htis case.

I remember long ago learning "Newton's Law of Cooling", which states
that if heat is lost by conduction and/or convection, the rate of loss
of heat is proportional to the difference in temperature between the
object that is losing heat and its surroundings. So if the temperature
difference is changed by a factor of ten, the rate of loss of heat
will also change by a factor of ten.

This implies that if the rate at which heat is supplied changes by a
factor of ten, the difference between the equilibrium temperature and
the ambient temperature will also change by a factor of ten. If the
oven reaches thermal equilibrium 100C above ambient when the sun is
shining on it, then if the light level is only one-tenth of full
sunlight, the equilibrium temperature will be only 10C above ambient.

Certainly, the amount of thermal insulation will affect the
proportionality constant. But is it practical to make the insulation
so good that the equilibrium temperature is 100C above ambient when
the light level is low, as under clouds? In theory, it should be
possible, but is it in practice?


Posted by Morris Dovey on October 6, 2010, 6:13 am
On 10/5/2010 9:03 PM, dow wrote:

A quallified "yes" and an unqualified "yes". :)

This is essentially the problem I set out to solve for a lower
temperature range with my passive solar space-heating panels.

To be completely honest with you, the practical solution involves more
than just insulation...

What we're really trying to do is cobble together a black (or adequately
dark gray) body - a construct that absorbs very much more readily than
loses energy.

Cooker construction should be tight to prevent convection losses. This
isn't very challenging and is largely a quality of construction issue.

The cooker needs materials that insulate adequately. I can affirm that
such materials are available and that, in general, the cost is dependent
on the maximum temperature they need to survive. A good choice will
minimize radiation losses from the body.

Our gray/black body requires an aperture to admit energy from outside,
absorb it, and not re-radiate more than some minimum back to the

In this case the aperture consists of a highly-transmissive and
highly-insulating glazing in combination with an adequately efficient
(meaning that it doesn't reflect or re-radiate more than a minimum of
the incoming energy back to the environment) absorber/heat exchanger.

Perfection is frequently the enemy of good, and that's really true here
- a thermally perfect cooker, as I mentioned in an earlier post, would
self-destruct. We don't need perfection. The keyword is "adequate".

Morris Dovey

Posted by dow on October 6, 2010, 3:20 pm
In reality, I guess we should also think about whether the device is
affordable by the people who might want to use it.

The PV idea would certainly be very expensive to implement. It sounds
like the materials that would be needed to make your thing work under
really cloudy conditions would also be expensive. The only possibility
we've seen so far that might be cheap enough to use in the "third
world" is the one you don't like, growing and burning vegetation.

Perhaps we should think further outside the box.


Posted by Morris Dovey on October 6, 2010, 5:20 pm
 On 10/6/2010 10:20 AM, dow wrote:

That's always an important consideration.

PV does look like a poor option for many regions - and not just because
of the purchase price. I'm hoping that it does become a viable option,
but I don't think it will become so in the short to intermediate term.

I think I can produce an inexpensive solar oven capable of operating in
the 350-400F (176-204C) bread-baking range. I may spend a little
time on the design this winter, and build a prototype next year.

If the context is changed from personal use to communal use, the entire
cost/benefit analysis shifts significantly.

I once lived in a place where all bread was baked on (the outside of) a
communal oven, and where date palm fronds were the only available fuel.
IIRC, they didn't get much mileage from the fuel - and didn't have all
that much fuel to work with.

If you like to cook with fire, I suspect that a small hibachi and a wok
with a fitted clay stove would be difficult to beat.

Of course. Problems don't always come with solutions attached.

I've cut and pasted excerpts from our conversation into a (draft) web
page at http://www.iedu.com/DeSoto/Oven/  in the hope that this
discussion will encourage other folks to think about possible solutions.
Perhaps by spring it'll be an already solved problem so I won't have to
build anything. :)

Morris Dovey

Posted by dow on October 9, 2010, 9:08 pm
If you can make a solar oven that would bake bread in really cloudy
conditions, I think it would be very valuable. Let us know how it


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