Glass pipe or aluminium?

[jaskiainen]

Hi all!

I'm gonna try to build a solar air heater for my friends "green energy" house.

Just want to know your opinion of should i use glass pipes inside it or

should i start drinking beer heavily to get aluminium beer cans?

Industrial made heaters often comes with glass pipes so i assume there is some benefit with it?

I hope that there is someone with more experience to point me in right direction.

If it is glass then i can do even vacuum pipes at work.

If the glass don't give me any significient advantage i think i will go slow with

it and start collecting beer and soda cans

Regards jaskiainen

[GaryGary]

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

[jaskiainen]

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

[carlb23]

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

[spinningmagnets]

I would suggest that the glass tubes might not work well for the home enthusiast. So far, I have seen them used two ways.

Just as carlb23 said there are vertical evacuated (vacuum) tube heat transfer glass cylinders. the priciple is that although water experiences a liquid/gas phase change (boils) at 212F at sea level, if you go to a higher altitude (lower air pressure/partial vacuum), it boils at a lower temp.

If the tube is 1/4 filled with liquid such as freon, the sun boils it into a gas that rises, taking heat with it. The tops of the vertical tubes are set inside a horizontal pipe run of water you want to heat. As the heat goes from the hot gas to the cooler water (through the glass to maintain vacuum), the freon condenses and runs back down to the bottom.

The other way I have seen glass tubes used for heat gathering is with the California desert SEGS steam/electric plant. Rows of trough collectors focus the sun onto horizontal black pipes filled with a flowing salt syrup.

This carries and stores heat to a heat storage tank, but since the temps get up to 900F (salt syrup is used so it wont boil) much heat was initially radiated out into the air and lost along the way. To insulate the hot pipes (as much as is possible), they were covered with glass tubes, and a vacuum maintained between the glass and the black pipe.

"Avoid clichés like the plague, they're old hat" -Steven Wright

[jaskiainen]

I know that this might be a bit hard for me to make but i thought something like this:

http://www.freewebs.com/solarwyse/solar_tubesspecs.html

As they say in page the performance in cold weather is much better than regular

aluminium pipe because of the vacuum insulation. (no heat losses)

And this is something i can do only not sure where can i get that absorption

material. Maybe some very thin aluminium tube around that inner glass tube?

My first idea was only aluminiun pipe with vacuumed glass pipe around it.

There is still a small problem with it. I'm not sure how can i get that glass pipe

around the aluminium airtight? Vacuuming them is not a problem and even filling

them with some sort of gas is doable for me. Only if it helps.

This one seems much easier for me to do. We have different dia glass pipes

at work (we make neon lights) and all the equipments that i need to do these.

So i think i atleast ask them how much would some glass pipes cost for me.

If the price is right i think i will go with glass/aluminium.

I think there will be many of you here who are interested of the results

if everything goes like it should.

[GaryGary]

Hi,

The SRCC tests both regular flat plate collectors (both air and water heating) and evacuated tube collectors.  The test reports are here:

http://www.solar-rating.org/RATINGS/RATINGS.HTM

In the Directory of SRCC certified solar collector ratings.

You can also use the calculator to check the comparitive efficiency of evacs and flats:

http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm#efic

The bottom line is that there is very little difference.  

What you want to do sounds like an interesting experiment, and I would certainly like to hear how it comes out, but I would not expect a whole lot of performance gain for the vacuum insulation.

Gary

[SteveCH]

Disclaimer to start: I do not have a space-heating solar system other than the fact our attached greenhouse does that job thru fan-assisted vents.

I agree, though, from knowing other people with systems, the evacuated glass is not something I've seen on a home system.

Not that it won't be fascinating to watch along if you experiment.

However, if you do this stuff for your friend, I would advise to aim over toward the simplest ideas. I have learned the hard way that this is best in most cases. Unless the friend is really into fooling around with the system, in which case he or she would probably be doing this themselves. I've set up or helped set up various alternative power systems for friends and neighbors, and by far the most successful outcomes and happiest friends are those which involved a very simple, almost-cannot-screw-it-up-after-I-leave ones. As you research, I predict you'll find that there are gains in performance for the fancier, more complex ideas over the simple, basic ones but that those gains are not so significant they are worth it.

But, for it in any case and let us know how it turns out. Good luck.