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Glass pipe or aluminium? | 7 comments (7 topical, editorial)

Re: Glass pipe or aluminium? (3.00 / 0) (#1)
by GaryGary (gary@BuildItSolar.com) on Tue Sep 2nd, 2008 at 08:33:52 AM MST
(User Info) http://www.BuildItSolar.com

Hi,
Basically you want two things from a good solar air heating collector absorber.

The first is that it absorbs sun well. Just about anything painted black (or even a dark color) will do this well. Black painted metal sheet absorbs 95% of the sun that shines on it.

The 2nd (and harder) thing is that you want good heat transfer from the hot absorber to the air. Since air does not have a high heat capacity, you have to move a fair bit of air over each square foot of absorber area to get good performance.

There are quite a few absorber configurations that work well:

Plain sheet metal with the air flowing behind the metal in a narrow channel and with baffles to make sure the air scrubs all the absorber. This is a back pass collector and requires a fan.
I use 2 layers of ordinary black aluminum window screen with the flow path organized so that the air has to flow through the screen. This does not need a fan for good performance. This works well, and is cheap, but does require good sized vent holes in the wall. http://www.builditsolar.com/Projects/SpaceHeating/solar_barn_project.htm
Some people use furnace filter media painted black. The air flows through the absorber and the furnace filter media has a lot of heat transfer area. Along the same line, some people use black polyester felt -- have to be careful about not melting it at stagnation. These collectors need a fan.
The beer can collectors have become pretty popular, and (I think) they are pretty efficient. They seem like a lot of work to build, but I guess the prep work is enjoyable. These collectors need a fan.
The inside of just about any room works well. That is, sun shining through a window is about as good a solar collector as you can have as long as you have some kind of insulating window treatment to prevent heat loss at night.

One thing an absorber does not need (and is actually bad) is mass in the collector -- it should have as low a thermal mass as practical. I mention this, because people tend to want to add mass inside the collector, and it just degrades the performance.

Some more material on all of these including some new material on a nicely done beer can collector here:
http://www.builditsolar.com/Projects/SpaceHeating/Space_Heating.htm#Passive
This is a mix of both air and water heating collector plans, so scan trough all of them to find the plans for the air heating ones.

I don't see any particular advantage to glass tubes, but I'm not sure exactly how you would use them?

Gary
Gary gary@BuildItSolar.com www.BuildItSolar.com

Re: Glass pipe or aluminium? (3.00 / 0) (#2)
by jaskiainen (jaskiainen74inhottestmailcompany) on Tue Sep 2nd, 2008 at 09:07:23 AM MST
(User Info)

Hi!

You are right. The preparing phase should be enjoyable atleast if i go with aluminium.
I'm not sure how do the glass tubes work but there must be some advantage of using them.
Otherwise industrial made ones would use some other material?
I think tho that difference between good and exellent heater is quite minimal.

Air does not have good heat capacity, but if you use glass tube sealed around
aluminium pipe and vacuum the air out and fill it with some gas wich will conduct
the heat better would it perform better?

Anyway this was just a thought that seems to me a bit too complicated if
the perform doesn't get much, much better...
It seems to me that i'm gonna have a plenty of good times to come ) I had something like this in mind:

"http://www.freeweb.hu/napenergia/gyak/szp/sztgyi_en.htm">http://www.freeweb.hu/napenergia/gyak/szp/sztgyi_en.htm Best regards jaskiainen [ Parent ]

Re: Glass pipe or aluminium? (3.00 / 0) (#3)
by carlb23 on Tue Sep 2nd, 2008 at 11:14:49 AM MST
(User Info)

I suspect that the commercial ones you are looking are evacuated tube collectors. These collectors use a tube in the glass with a liquid in the tube which is heated to steam and rises to the top of manifold which is filled with a water/glycol solution.

I have never seen a hot air collector using glass tubes. I have built a small one (7'x8' 56 sq/ft)using flexible corrugated aluminum dryer exhaust tubing in a multi-pass design using a 180 cfm blower and it works quite well. inlet temp 68 degrees outlet temps 138 degrees.

I have also built one 3'x37' 111 sq/ft using light weight corrugated roofing material for the collector and a 465 cfm blower that will raise the temps in mid winter 32 degrees outside from 68 degrees inlet temp to 145 degree outlet temps.

Carl

[ Parent ]

Glass pipe or aluminium? | 7 comments (7 topical, 0 editorial)

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