Posted by Tim Jackson on March 27, 2009, 8:20 am
Don T wrote:
>
> On the other hand, how about the idea of the black oxidized Ag tube as
> the collector? Any improvement over Cu in the corrosion resistance
> characteristics?
>
I'm not saying that copper is particularly good for this application,
I'm just saying it's got a higher reflectivity at fIR than at optical
wavelengths, and not many materials do.
Yes you can improve any good IR reflector's ratio by applying an
optically absorbent layer that is thinner than the emitted wavelength.
Someone mentioned for example blackened nickel - that should work, so
should silver, but you have be able to ensure the layer is thin enough
not to raise the fIR emissivity, not much more than a micron.
Tim
Posted by Tim Jackson on March 29, 2009, 5:22 pm
Don T wrote:
>> Don T wrote:
>>
>>> How about a tube made of Ag with a polished inner surface and a black
>>> oxidized outer surface? Will the polished inner surface reduce the
>>> conductivity of Silver of the absorbed heat to the fluid inside
>>> while at the same time reflecting heat from the fluid back to the
>>> Silver tube? IIRC English tea pots were of Silver and the inside
>>> surface was highly polished while the outside may have been highly
>>> ornamented and less highly polished. The theory being that the
>>> polished inner surface would maintain the temperature of the fluids
>>> inside hot longer.
>>>
>>
>> OMG, have you ever SEEN the inside of an English silver teapot?
>>
>> It is inevitably brown, tea coloured. Even the best butler can't
>> clean out the residue after every use (you just can't get the staff
>> these days). :) And besides it would affect the taste - a good teapot
>> needs breaking in.
>>
>> What do you think we have tea-cosies for anyway.
>>
>> And IIRC, not being a tea drinker myself, my grandmother's was matt
>> inside like most silverware.
>>
>>
>> Tim
>
> Well you did a good job of shooting a WAG in the butt. I don't know
> where the original thought came from but I am sure that it was in
> something I read once upon a time. In any case I have an ancient tea pot
> in my wife's knick knack cubby, the inside is well polished but not a
> mirror finish so I will guess it was not -quite- a matte finish
> originally.
>
Here is a picture of the offending if not offensive item - my
grandmothers old silver teapot - I tracked it down. The inside hasn't
been cleaned for decades (she died in '87 & I don't think it's been used
often since). The inside of the *lid* got polished nicely though.
www.tim-jackson.co.uk/gifs/teapot.jpg
Incidentally I've got a Russian samovar that looks pretty much the same
inside, and that's just limescale. So I imagine this is tea-stained
limescale.
Tim
Posted by Curbie on March 26, 2009, 4:57 pm
wrote:
>Tim Jackson wrote:
>> Morris Dovey wrote:
>>> Tim Jackson wrote:
>>>> Morris Dovey wrote:
>>>>>
>>>>> Nope. I've already /measured/ that half of that trough width can
>>>>> produce 725F. Doubling the trough width while maintaining the same
>>>>> "bright line" width will deliver twice the energy to the same area -
>>>>> so I expect a temperature at least close to double the 725F, or 1450F.
>>>>
>>>> That is flawed physics. First you need to be thinking about absolute
>>>> temperature not relative, and second, in radiation the temperature
>>>> goes as the fourth root of energy.
>>>
>>> Might work for you, but I'm working with old-fashioned temperature
>>> which represents energy density per unit mass.
>>>
>>> I think of and talk about temperature in units with which the person
>>> I'm talking is familiar - and yes, convert (whatever) to Kelvins for
>>> calculations.
>>>
>>> Just out of curiosity, do /you/ have a thermometer graduated in Kelvins?
>>>
>>> :)
>>>
>>
>>
>> No I don;t have a thermometer calibrated in kelvins, nor one that will
>> measure 1450F. But nonetheless when you are talking about radiation you
>> have to convert to absolute temperature to do any useful calculations.
>>
>> I'm not criticising your units, but I'm giving you a C- for your physics.
>>
>> It works like this.
>>
>> The maximum temperature you can achieve in your target is that
>> temperature where the self-radiated power is equal to the incident
>> power. This is calculated by Stefan's Law.
>> (http://en.wikipedia.org/wiki/Stefan%27s_law )
>> which says that the emitted power is proportional to the fourth power of
>> the temperature.
>>
>> So if you double the incident power you will increase the absolute
>> temperature by about 20%
>>
>> If you started with 725F, that's 658K, doubling the power will get you
>> 783K, which I make to be 950F.
>>
>> This of course is the 'ideal' situation, and does not account for any
>> power you may be removing to drive an engine, this of course has to be
>> subtracted from the incident power before doing the temperature
>> calculation.
>>
>> So for example if you were capturing a kilowatt of sunlight and getting
>> 725F at your target without load, then it was radiating a kilowatt back
>> into space. If you doubled the power by using a bigger mirror around
>> the same target then you could draw off one kilowatt while maintaining
>> 725F. Two kilowatts come in, one gets radiated, just the same, the other
>> goes into your engine.
>My apologies - I think I misunderstood what you were saying. Yes, I
>understood that the re-radiation will be a problem - and no, I did not
>have an adequate physics background to be able to quantify it.
>A full kW into the engine wouldn't really be a bad starting point.
>Obviously, more would be better (which is why I'm puzzling over ways to
>make the tube [my hot head] both a better absorber and a poorer radiator).
>In the real world, I have to work with whatever heat/temperature I get -
>and if it's not enough for some particular purpose, scale up until it
>/becomes/ adequate.
>The method isn't physics or engineering at its finest, but then the
> overall problem doesn't seem to have captured the interest of either
> of those communities.
Over the past 30 years McDonnell Douglas, Power Kinetics, the Jet
Propulsion Laboratories, Sandia National Laboratories, Cummins Engine
Company, SunLab, and others have all big buck wacks at EXACTLY this
idea "Solar powered Stirling". All have engaged members of BOTH
physics and engineering communities, having varying degrees of success
none of which was commercial.
I knew all this before I spent a huge amount of time playing with this
idea, it did stop me and I hope it won't stop you. Most discoveries
come from people who are not looking for exact thing they find, who
knows what you'll find by trying, for certain, you'll find nothing by
NOT trying. I just think your going to have to scale-up your collector
or scale-down your output power expectations.
But who cares what I think (or anyone else), keep the costs down, have
blast doing it, and no matter the results you'll be better off for the
experience.
Best wishes.
Curbie
>The most dismaying aspect for me is that sometime in the near future I'm
>going to need to shop for a torch and regulators; and learn to weld and
>braze sufficiently to cobble this monstrosity together.
>Hmm - I just realized that not only did the Stirling engine originate in
>the UK, but so has much of the physics help I've gotten. :)
>Thanks for posting - and for your patience and persistence!
Posted by Morris Dovey on March 26, 2009, 7:53 pm
Curbie wrote:
> Over the past 30 years McDonnell Douglas, Power Kinetics, the Jet
> Propulsion Laboratories, Sandia National Laboratories, Cummins Engine
> Company, SunLab, and others have all big buck wacks at EXACTLY this
> idea "Solar powered Stirling". All have engaged members of BOTH
> physics and engineering communities, having varying degrees of success
> none of which was commercial.
That's worth knowing - and I'll guess that the knowledge doesn't deter
me from inquiring for exactly the same reasons it didn't deter you. :)
> I knew all this before I spent a huge amount of time playing with this
> idea, it did stop me and I hope it won't stop you. Most discoveries
> come from people who are not looking for exact thing they find, who
> knows what you'll find by trying, for certain, you'll find nothing by
> NOT trying. I just think your going to have to scale-up your collector
> or scale-down your output power expectations.
I don't mind that (much). The original goal was to produce a
solar-powered pump that could be deployed inexpensively off-grid in
undeveloped parts of the world. At this point it appears that goal will
be met within the next few months. It /isn't/ particularly efficient,
and it doesn't qualify as a technical wonder - but it'll fill a need.
Serendipity, like fortune, passes everywhere. :) When I tried not
painting my flat panel absorbers flat black and left them shiny, the
output jumped. It was a physicist in Scotland who figured out what was
going on and who sent me off to read up on black body radiation (my head
hurt for weeks!), and by the time I'd made mods to take real advantage
of the effects, that "jump" looked almost trivial.
The large corporations /must/ focus on developments that contribute
substantially to their bottom line - and there just isn't much profit in
developing a device that a farmer without any kind of technical training
can build for himself. Hmm - or is there? Maybe we need to think about
that some more...
...people with full bellies and healthy families probably aren't
inclined to become either terrorists or willing victims.
> But who cares what I think (or anyone else), keep the costs down, have
> blast doing it, and no matter the results you'll be better off for the
> experience.
Eh? I care what you think - and it makes a (positive) difference in my
result when you share your thoughts and make me re-examine my own. And
you're right, I'm a lot better off than I would be if I decided to spend
my final years in a rocking chair, puffing on my pipe, being bored out
of my (shrinking) mind.
--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/
Posted by daestrom on March 26, 2009, 11:01 pm
> Tim Jackson wrote:
<snip>
>> So for example if you were capturing a kilowatt of sunlight and getting
>> 725F at your target without load, then it was radiating a kilowatt back
>> into space. If you doubled the power by using a bigger mirror around the
>> same target then you could draw off one kilowatt while maintaining 725F.
>> Two kilowatts come in, one gets radiated, just the same, the other goes
>> into your engine.
> My apologies - I think I misunderstood what you were saying. Yes, I
> understood that the re-radiation will be a problem - and no, I did not
> have an adequate physics background to be able to quantify it.
> A full kW into the engine wouldn't really be a bad starting point.
> Obviously, more would be better (which is why I'm puzzling over ways to
> make the tube [my hot head] both a better absorber and a poorer radiator).
> In the real world, I have to work with whatever heat/temperature I get -
> and if it's not enough for some particular purpose, scale up until it
> /becomes/ adequate. The method isn't physics or engineering at its finest,
> but then the overall problem doesn't seem to have captured the interest of
> either of those communities.
> The most dismaying aspect for me is that sometime in the near future I'm
> going to need to shop for a torch and regulators; and learn to weld and
> braze sufficiently to cobble this monstrosity together.
One more point about temperatures and Carnot. It isn't how hot the outside
of the cylinder/tube gets, it's how hot the working fluid temperature gets.
Of course you wouldn't do this, but if your collector tube were made of
insulation, the outside could get 1000F but if the working fluid inside only
rises to 80F, then that would be the temperature you plug into Carnot (after
converting things to absolute).
So on the one hand you want a nice small 'collector' to focus the sunlight
on so it reaches a nice high temperature. But on the inside of the tube you
might want a cross piece or something to increase the surface area between
the tube and the working fluid (air) inside. The name of the game is
getting the heat from the tube into the working fluid as quickly as
possible.
daestrom
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> On the other hand, how about the idea of the black oxidized Ag tube as
> the collector? Any improvement over Cu in the corrosion resistance
> characteristics?
>