Posted by Morris Dovey on March 26, 2009, 11:30 pm
daestrom wrote:
> 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.
Yes - getting the heat /inside/ the hot head was, and to some extent
still is, a difficulty with the low-temperature engine.
> 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).
Yup - that's the reason I was willing to spring for copper...
> 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.
Why didn't /I/ think of that! I went through a ray-tracing exercise a
while back and noticed that more heat would be applied from the sides
than from elsewhere. It seems like the design should be able to take
advantage of this...
Thank you!
--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/
Posted by Bruce Richmond on March 27, 2009, 12:50 am
> daestrom wrote:
> > 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.
> Yes - getting the heat /inside/ the hot head was, and to some extent
> still is, a difficulty with the low-temperature engine.
> > 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).
> Yup - that's the reason I was willing to spring for copper...
> > 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.
> Why didn't /I/ think of that! I went through a ray-tracing exercise a
> while back and noticed that more heat would be applied from the sides
> than from elsewhere. It seems like the design should be able to take
> advantage of this...
> Thank you!
> --
> Morris Dovey
> DeSoto Solar
> DeSoto, Iowa USAhttp://www.iedu.com/DeSoto/
From a posting in alt.solar.thermal
http://groups.google.com/group/alt.solar.thermal/msg/2fa4fead14ce1285?hl==
en
There was this link for spray paint.
http://www.dampney.com/Products/Products.asp?ProductID=28
Don't know anything about it but it sounds like what you were looking
for. For all I know bare copper is better.
For increasing surface area of the tube to the working fluid, I seem
to remember seing inserts with a cross section like * that you could
slide inside a pipe. Might even be able to find a source for pipe
with them pre-installed. Would be best to slip a cold insert into a
hot pipe so they would have better contact for heat transfer.
Might be able to make a cheap vacume tube out of a used floresent
light tube.
While you may get the high temp you are looking for by focusing to a
3/8" line on a 1" pipe, to heat the working fluid inside the pipe you
need as much area as possible hot. So the heat from that 3/8" strip
will flow out to the rest of the pipe, spreading the heat out and
dropping the temp. Might be better off not bringing it to such a
narrow beam to start off with.
Putting a small reflector on the back side of the pipe would not only
reduce losses due to re-emission, if the beam is made a bit wider than
the pipe it could be used to heat the back side of the pipe.
I don't *know* that the ideas above will help but they might be worth
thinking about.
Bruce
Posted by Morris Dovey on March 27, 2009, 2:11 am
Bruce Richmond wrote:
> There was this link for spray paint.
>
> http://www.dampney.com/Products/Products.asp?ProductID (
I should have bookmarked it then. Have it now - thank you.
> For increasing surface area of the tube to the working fluid, I seem
> to remember seing inserts with a cross section like * that you could
> slide inside a pipe. Might even be able to find a source for pipe
> with them pre-installed. Would be best to slip a cold insert into a
> hot pipe so they would have better contact for heat transfer.
I'll dig around and see what I can find. I like the idea, but am
beginning to think that I might try this thing out first with an empty
tube just to see how that plays.
One of the trade-off factors here is that the viscosity of air increases
with temperature, and the air in this long narrow hot head will need to
move /fast/. I do want to distribute the heat as efficiently as
possible, but I don't want to waste power overcoming flow resistance.
It'll be an interesting task to find the balance point...
> Might be able to make a cheap vacuum tube out of a used fluorescent
> light tube.
I read that thread, too - but I think when I get to that point I'll just
buy one of the tubes used in solar water heaters...
> While you may get the high temp you are looking for by focusing to a
> 3/8" line on a 1" pipe, to heat the working fluid inside the pipe you
> need as much area as possible hot. So the heat from that 3/8" strip
> will flow out to the rest of the pipe, spreading the heat out and
> dropping the temp. Might be better off not bringing it to such a
> narrow beam to start off with.
I've had that same thought - and what seems to make most sense is to set
up the mounts (they're just U-bolts) for the tube so that the tube can
be raised so that the focus is at the bottom of the tube, and lowered to
the point where the focus is coincident with the center of the tube.
> Putting a small reflector on the back side of the pipe would not only
> reduce losses due to re-emission, if the beam is made a bit wider than
> the pipe it could be used to heat the back side of the pipe.
I'll file that one for more thought, but the current geometry won't let
me do that (I'd need to raise the focus, which would complicate the
construction.
> I don't *know* that the ideas above will help but they might be worth
> thinking about.
Until I'm sure that it can't be further improved and still be built at
reasonable cost, /everything/ is worth thinking about.
Thanks!
--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/
Posted by sno on March 29, 2009, 6:09 pm
Morris Dovey wrote:
> Curbie wrote:
>> Morris,
>>
>> Two things jump through my foggy recollection of Striling cycles and
>> solar concentration, here is what is confusing me:
>> 1) Gas law is Gas law. P = (nRT) ÷ V Whether your moving a solid (a
>> piston) or liquid (water) you still have to heat the volume of gas to
>> the desired pressure, for each cycle, to the work. 2) Volume of heat
>> from a parabola is a function of the area of the
>> collector and temperature of heat is a function of the "Concentration
>> Ratio" (geometric or ideal) of the collector aperture area / receiver
>> aperture area. I don't see how you create the volume of heat or
>> temperatures cited to heat the gas to pressurize enough cycles to
>> create 1hp (or anything like it)?
>>
>> More math cheese:
>> 8' x 4' = 32 Ft^2
>> 250 BTU per Ft^2 (on a reasonably clear day)
>> 32 (Ft^2) x 250 (BTU) = 8000 BTU per hour
>>
>> Plug the 8000 BTU per hour into gas law and chop the pressure into any
>> bore, stroke, and frequency per hour combination you like and I can't
>> find the Hp.
>>
>> I have a spread sheet on gas law into Hp if or interested, maybe it's
>> all goofed up and you point out my mistake, won't be the first time.
>
> Here's another way to come at the problem (different units): figure
> 1kW/m^2 for total input. The trough uses a 4x8 sheet of mirror, but the
> trough width is noticably less due to curving the mirror sheet.
>
> Still, we end up with more than 2 m^2 of capture area which should
> provide 2kW of input power. We're going to have losses at the focal tube
> and losses due to Carnot cycle efficiency limitations. In the best of
> all possible worlds [ :) ] the efficiency losses will be close to 26%
> (at 70F/1450F head temperatures).
>
> Those 26% efficiency losses drop our 2kW to 1.48kW, and I'm hoping
> (because I don't yet have enough experience to predict) that the losses
> at the focal tube leave enough to get at least 746W out as mechanical
> energy.
>
> Not factored into any of this is heat re-cycled by the fluidyne's
> regenerator, possible water jacket cooling of the cold head to improve
> Carnot efficiency, or collection efficiency boost resulting from tricky
> (artsy-fartsy) sun side insulation and/or reflector shielding of the
> collection tube.
>
> In summary - I think there /should/ be enough juice to squeeze out
> somewhere /around/ a horsepower - less if I do a shoddy job - and maybe
> more if I can both design and build well - which is certainly still in
> question, since I'm not an ME or physicist, and I don't have any
> thermodynamics or heat transfer background. :P
>
> I have a Stirling cycle web page (let me know if you spot errors) at
>
> http://www.iedu.com/DeSoto/Projects/Stirling/StirlingCycle.html
>
> that tries to describe the Stirling cycle in both prose and formulas. I
> used those formulas (formulae?) to build a software simulator - which
> still doesn't account for everything it should, but does seem to agree
> more or less with what I've been able to measure.
>
When I was a kid we had a neon tube sign maker where I lived....he had
glass tubes about 8 ft long....
Have often wondered if you could not increase the efficiency of a home
built concentrating collector by using one of these tubes around the
collector....you can draw a vacuum in them....and could coat one side
with a reflective material to bounce back radiation....
Do not know how you would seal the ends...maybe melting the tube around
the collector...??
Do not know if you can still get these glass tubes or how much they
cost....or their diameters ....
May or may not be practical....
have fun.....sno
Posted by Morris Dovey on March 29, 2009, 6:41 pm
sno wrote:
> When I was a kid we had a neon tube sign maker where I lived....he had
> glass tubes about 8 ft long....
>
> Have often wondered if you could not increase the efficiency of a home
> built concentrating collector by using one of these tubes around the
> collector....you can draw a vacuum in them....and could coat one side
> with a reflective material to bounce back radiation....
>
> Do not know how you would seal the ends...maybe melting the tube around
> the collector...??
>
> Do not know if you can still get these glass tubes or how much they
> cost....or their diameters ....
>
> May or may not be practical....
>
> have fun.....sno
Thanks - I, too, remember a local shop where such magic was worked. As a
kid I'd have been underfoot for hours at a time if I'd been allowed. :)
I think Neon John is the 'local' expert these days - but I think I'll
probably just buy an off-the-shelf tube when I get to that point.
I /like/ your idea of the reflective coating on one half of the tube.
--
Morris Dovey
DeSoto Solar
DeSoto, Iowa USA
http://www.iedu.com/DeSoto/
> outside of the cylinder/tube gets, it's how hot the working fluid
> temperature gets.