Posted by dow on October 6, 2010, 2:03 am
> On 10/5/2010 3:41 PM, dow wrote:
> >> On 10/5/2010 1:57 PM, dow wrote:
> >>> 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.
> >> 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 70F/21C the
> >> discharge was right at 185F/85C - 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 300F/150C 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. :(
> >> --
> >> Morris Doveyhttp://www.iedu.com/DeSoto/
> >> PGP Key ID EBB1E70E
> > 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!
> 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 70F to 185F
> for an increase of 115F. Now 185F 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.
> --
> Morris Doveyhttp://www.iedu.com/DeSoto/
> PGP Key ID EBB1E70E- Hide quoted text -
> - Show quoted text -
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?
dow
Posted by Morris Dovey on October 6, 2010, 6:13 am
On 10/5/2010 9:03 PM, dow wrote:
> 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?
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
environment.
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
http://www.iedu.com/DeSoto/
PGP Key ID EBB1E70E
Posted by dow on October 6, 2010, 3:20 pm
> On 10/5/2010 9:03 PM, dow wrote:
> > 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?
> 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
> environment.
> 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 Doveyhttp://www.iedu.com/DeSoto/
> PGP Key ID EBB1E70E
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.
David
Posted by Morris Dovey on October 6, 2010, 5:20 pm
On 10/6/2010 10:20 AM, dow wrote:
> In reality, I guess we should also think about whether the device is
> affordable by the people who might want to use it.
That's always an important consideration.
> 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.
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°-400°F (176°-204°C) 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.
> Perhaps we should think further outside the box.
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
http://www.iedu.com/DeSoto/
PGP Key ID EBB1E70E
Posted by dow on October 9, 2010, 9:08 pm
> On 10/6/2010 10:20 AM, dow wrote:
> > In reality, I guess we should also think about whether the device is
> > affordable by the people who might want to use it.
> That's always an important consideration.
> > 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.
> 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.
> > Perhaps we should think further outside the box.
> Of course. Problems don't always come with solutions attached.
> I've cut and pasted excerpts from our conversation into a (draft) web
> page athttp://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 Doveyhttp://www.iedu.com/DeSoto/
> PGP Key ID EBB1E70E
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
goes!
dow
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> >> On 10/5/2010 1:57 PM, dow wrote:
> >>> 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.
> >> 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 70F/21C the
> >> discharge was right at 185F/85C - 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 300F/150C 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. :(
> >> --
> >> Morris Doveyhttp://www.iedu.com/DeSoto/
> >> PGP Key ID EBB1E70E
> > 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!
> 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 70F to 185F
> for an increase of 115F. Now 185F 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.
> --
> Morris Doveyhttp://www.iedu.com/DeSoto/
> PGP Key ID EBB1E70E- Hide quoted text -
> - Show quoted text -