Solar Evac Tube Question

[David Iannone]

Hello all, I am new to this forum and need some advice from all you experts. I recently bought a house and just installed my Tetco geothermal and am loving it (especially the lack of a fuel bill :-) ). I've been wanting to go further with the green resources and impliment evacuated solar tubes for my domestic hot water and in addition, the emergency backup heating for the geothermal using a heat exchanger. So I purchased what are supposedly high-end evacuated tubes from a guy on Ebay. I believe he is currently still selling them, they were used in some government building and actually have gold inside the tube absorbers which he says makes them better efficiency then most of the brands out there. (dunno if this rings any bells to anyone that has seen these specific ones on ebay). Anyway, these tubes I have purchased do not have heat pipes in them, they are only the glass tube part. My question is what are my options for using these in a system? The guy has links on how to build your own heat pipes out of copper tubing but I have also been looking into the direct exchange system where the fluid flows in and out a u-tube pipe placed inside the evacuated tube. What are the advantages and disadvantages and what would be easier to build? Hope I'm in the right forum and thanks in advance for any help.

[Vasile Brindus]

...Anyway, these tubes I have purchased do not have heat pipes in them, they are only the glass tube part. My question is what are my options for using these in a system? The guy has links on how to build your own heat pipes out of copper tubing but I have also been looking into the direct exchange system where the fluid flows in and out a u-tube pipe placed inside the evacuated tube.

Hi!

AFAIK, evacuated tubes have vacuum in the interior of the glass pipe, so I'm not sure how you can use them since you have only the glass part. Do you mind posting the ebay auction link/ID or the links provided by the seller?

Regards,
VasileB

[David Iannone]

Here is the link to a current auction of his:

http://cgi.ebay.com/Gold-Mirror-Solar-Thermal-Energy-Collector-Tubes-VA_W0QQitemZ290284070925QQcmdZViewItemQQptZElectri cal_Solar_US?hash=item290284070925&_trksid=p3286.c0.m14&_trkparms=72%3A1205%7C66%3A2%7C65%3A12%7C39%3A1%7C 240%3A1318%7C301%3A0%7C293%3A1%7C294%3A50

Correct me if I'm wrong but I'm pretty sure the glass tube has an inner and an outer section that is fused together and the evacuated vaccuum is in between the 2 not inside the inner tube. That is why you can replace one tube on a manifold system if you had to, just slide it out. Sound right?

[Rob Beckers]

Hi David,

All the tubes I've seen are double wall with the vacuum between walls. So, the inside where the collector goes is regular air pressure. However, to echo Vasile, I have heard about tubes that have the collector itself in a vacuum flask, with some sort of manifold sticking out the top. Why not just send a note to the EBay seller and ask?

-RoB-

[David Iannone]

Rob, The guy that sold me the tubes has been very helpful and actually has a very informative website on different methods of green power. He recommends building heat tubes myself out of copper, where the fluid would be heated at the end of the copper tube....I believe that would be the indirect transfer method. Since I can never do anything normal and always have to experiment with stuff :-) I was wondering if I could run the fluid directly inside the tube. My idea is pretty simple, no heat tube, instead have the water go into the evac tube through a center tube and then pick up the heat on the way out surrounding that inlet tube. Anyone know if this would work and how it would compare to the indirect method?

[Joe Blake]

If you decide to use copper tubing this may not be a good idea, but adding salt to the water to make brine can increase the "heat capacity" of the system.

Joe

[Rob Beckers]

Rob, The guy that sold me the tubes has been very helpful and actually has a very informative website on different methods of green power. He recommends building heat tubes myself out of copper, where the fluid would be heated at the end of the copper tube....I believe that would be the indirect transfer method. Since I can never do anything normal and always have to experiment with stuff :-) I was wondering if I could run the fluid directly inside the tube. My idea is pretty simple, no heat tube, instead have the water go into the evac tube through a center tube and then pick up the heat on the way out surrounding that inlet tube. Anyone know if this would work and how it would compare to the indirect method?

Maybe...
You would still need a collector (black surface) to convert all the radiation coming from the sun into heat. Water itself is a great heat transfer medium, but would make a very poor collector. The other issue is pressure. I'm fairly sure those vacuum flasks were not meant to handle anything beyond atmospheric pressure, so you can't pressurize the fluid.

-RoB-

[Brian McGowan]

I have been thinking about this. My thought was to use food coloring or black water color paint to make the water black and then it is the collector. I'm not sure what issues you will have with keeping the inside tube clean. You may be better off running copper pipe painted black with high temperature paint like they sell to paint your grill or something. Leave it open at the end furthest into the tube. Use a rubber stopper at the open end with 2 holes. The pipe enters one hole and and water travells all the way to the end of the copper pipe picking up heat the entire way then exits and has to travel back along between the outside of the pipe and the inside of the tube picking up more heat until it exits at the rubber stopper. Sealing may be problematic.
Brian

[David Iannone]

Hmmm, Well my evacuated tubes have a black/goldish absorber built into the vacuum sealed part, seems like absorbing the heat wouldn't be an issue. Even the indirect tubes use just a copper heat tube and a transfer medium like a metal mesh type material but it is not darkened or anything, from what I've seen almost all evacuated tubes are dark in color.

I just realized that you're right, I can't run domestic pressure through these, guess I will be doing some sort of low pressure antifreeze based closed loop with a heat exchanger...anyone have any ideas/links on building one? Thanks for your help....

[Rob Beckers]

David, can you post a picture of the tubes (or a few, also showing the connection part at the 'top').

-RoB-

[Paul Bailey]

Hi David : this is a photo of fellow GPT member Ashley Berwicks Low/no pressure evac tube system during the install for a total of 75 tubes. Most of these systems are from China and as usual they all have there pro and cons. Paul

[Vasile Brindus]

There are many principles to get the heat out of the the tubes, see this (http://books.google.ca/books?id=7-7AkRrbnN4C&pg=PA66&dq=evacuated+solar+tube+philips&client=firefox-a#PPA40,M1) or this (http://books.google.ca/books?id=F4LTw6HOaIgC&pg=PA97&dq=evacuated+solar+tube+philips&client=firefox-a#PPA101,M1). The ones that I recently saw had some black absorber plate in center the flask, thermally connected to the heat transfer pipe. I also saw another type, with an U-shaped pipe in the flask.

Yours seem different, the absorber is a film on the flask wall. Can you confirm where is the "absorber coating" located? It makes some sense to have it on the inner tube's vacuum side, to be protected. But on the inner side would make more sense, if it's not corroded by the liquid - hence the possible explanation of it being "gold plated". BTW, it's at least funny to see a solar collector advertised as having a "gold mirror" :-)

In their description is a phrase you can easily overlook, since it doesn't relate to what they are showing later: "Water flows in the central tube and collector space, surrounded by two outer tubes with a vacuum between them. The water gets so hot that it will boil, so water must be kept moving by a circulation pump." So it seems that here (http://books.google.ca/books?id=9OmnhfL_4rAC&pg=PA72&lpg=PA72&dq=Owens-Illinois+Glass+Company+solar+evacuated&source=web&ots=l2Rm-0z-P6&sig=pt1gux4EQJY8cKe5mbXqSSxA4Ms&hl=en&sa=X&oi=book_result&resnum=10&ct=result#PPA72,M1) is the designed working principle. Even the name is pretty close (Owings-Illinois in the auction vs Ovens-Illinois in the book). Also, search Google for "Sunpak evacuated solar tubes" if you want more reading.

If so, what the seller recommends is a hack, they inserted some transfer medium (metal sponge or similar) to get the heat from the wall to the heat pipe and then farther to the manifold. In my opinion, the efficiency is a lot lower than the original design (flushing the whole wall with liquid). But the original design also had some major inconveniences: you couldn't use water because it could freeze (and you couldn't just drain it) and the inner tube needed also to be sealed (which adds some maintainance issues down the road). Probably that's why they were taken down after a some years.

Anyway, using coloured water doesn't help at all, since the absorber gets almost all of the sun radiation.

[Stewart Corman]

David,
Let me put on my chemical engineer's cap and chime in here:
I went to the eBay auction posting and looked at his photos.

I see a four foot long sealed 2 1/8" double wall evaculated glass tube open on both ends, whose inside surface of the inner wall is coated with a proprietary coating ( probably not gold) to absorb heat.

The reason they talk about using copper pipes is that their is no convenient way to connect a liquid (antifreeze?) delvery system to the top and bottom of the glass tubing ...a pipe will just slide inside and all the connection are simple sweat fittings. If that is the preferred configuration, then a matte black paint finish applied to the outside of the copper would be a benefit. A small circ pump would be needed, but pressure from a domestic hot water system is fine since it is all sweat copper.

It appears that they used a large rubber stopper to center the copper tubing and in a slight variation, that is another means of delivering fluid by simply having a short copper stud at top and bottom stuck through a large stopper ...but this cannot support any pressurized domestic system.

A possible easier alternative could be a short 1" PVC pipe ( what is inside ID?), with two groves turned on a lathe for O rings then simply inserted into the glass envelope. This would require no alignment nor concern for expansion/movement. A small circ pump like that used for a fountain could supply enough pressure at the bottom to overcome the head of 4 feet of liquid without blowing any seals in a non-pressurized sytem. The distribution manifold header becomes trivial PVC fittings.

If you got a bundle of 7 tubes (photo shows 19 wrapped together) for $165 ..that is a small investment. IMHO, I would build a shallow box frame, either procure cardboard tubes (to be sliced in half) from local rug store or cast plaster with a 4 or 6" PVC pipe mold impression to enable some light gathering and line it with aluminum foil as the back reflector.

Keep us posted on your project ...too bad eBayer wasn't closer to me or I'd buy two sets of these tubes to play with. Then again, this dealer has a unit that looks very similar to Les's Schott unit that is 60inches long with 12 tubes and manifold for $525:

http://cgi.ebay.com/Solar-Hot-Water-Collector-Evacuated-Glass-12-Tube-Panel_W0QQitemZ110326548070QQihZ001QQcategoryZ3240 QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

BTW, above has a link that gives some numbers for how much heating to expect

Stew Corman from sunny Endicott

[David Iannone]

Hey guys, I'm back....thank you for the replies, I was subscribed to this thread but apparently didn't get an email so I didn't think there were any other posts. Anyway, I will respond to a few of the posts and then ask a multitude of questions lol.

There are many principles to get the heat out of the the tubes, see this (http://books.google.ca/books?id=7-7AkRrbnN4C&pg=PA66&dq=evacuated+solar+tube+philips&client=firefox-a#PPA40,M1) or this (http://books.google.ca/books?id=F4LTw6HOaIgC&pg=PA97&dq=evacuated+solar+tube+philips&client=firefox-a#PPA101,M1). The ones that I recently saw had some black absorber plate in center the flask, thermally connected to the heat transfer pipe. I also saw another type, with an U-shaped pipe in the flask.

Yours seem different, the absorber is a film on the flask wall. Can you confirm where is the "absorber coating" located? It makes some sense to have it on the inner tube's vacuum side, to be protected. But on the inner side would make more sense, if it's not corroded by the liquid - hence the possible explanation of it being "gold plated". BTW, it's at least funny to see a solar collector advertised as having a "gold mirror" :-)

In their description is a phrase you can easily overlook, since it doesn't relate to what they are showing later: "Water flows in the central tube and collector space, surrounded by two outer tubes with a vacuum between them. The water gets so hot that it will boil, so water must be kept moving by a circulation pump." So it seems that here (http://books.google.ca/books?id=9OmnhfL_4rAC&pg=PA72&lpg=PA72&dq=Owens-Illinois+Glass+Company+solar+evacuated&source=web&ots=l2Rm-0z-P6&sig=pt1gux4EQJY8cKe5mbXqSSxA4Ms&hl=en&sa=X&oi=book_result&resnum=10&ct=result#PPA72,M1) is the designed working principle. Even the name is pretty close (Owings-Illinois in the auction vs Ovens-Illinois in the book). Also, search Google for "Sunpak evacuated solar tubes" if you want more reading.

If so, what the seller recommends is a hack, they inserted some transfer medium (metal sponge or similar) to get the heat from the wall to the heat pipe and then farther to the manifold. In my opinion, the efficiency is a lot lower than the original design (flushing the whole wall with liquid). But the original design also had some major inconveniences: you couldn't use water because it could freeze (and you couldn't just drain it) and the inner tube needed also to be sealed (which adds some maintainance issues down the road). Probably that's why they were taken down after a some years.

Anyway, using coloured water doesn't help at all, since the absorber gets almost all of the sun radiation.

As far as I know, these are just like any other standard evacuated solar tube. Mine have a glass outer tube fused to a glass inner tube, with the vacuum in between. And to answer your question the black/gold absorber coating is on the vacuum side of the inner glass tube, so it is protected. As far as absorbing heat, wow, solar tubes are amazing....if I hold this out in the sun you can feel the heat radiating from inside within a minute, so they are definitely doing their job.

Regarding the description the seller had posted, yeah I think some of his statements are misleading or at least confusing. I don't know how these were originally set up but I find it hard to believe the fluid actually contacted the glass tube itself, maybe I'm wrong though. As you mentioned it looks like the seller made up his own heat pipes but I'm going a different route that I'll get to below.


David,
Let me put on my chemical engineer's cap and chime in here:
I went to the eBay auction posting and looked at his photos.

I see a four foot long sealed 2 1/8" double wall evaculated glass tube open on both ends, whose inside surface of the inner wall is coated with a proprietary coating ( probably not gold) to absorb heat.

The reason they talk about using copper pipes is that their is no convenient way to connect a liquid (antifreeze?) delvery system to the top and bottom of the glass tubing ...a pipe will just slide inside and all the connection are simple sweat fittings. If that is the preferred configuration, then a matte black paint finish applied to the outside of the copper would be a benefit. A small circ pump would be needed, but pressure from a domestic hot water system is fine since it is all sweat copper.

It appears that they used a large rubber stopper to center the copper tubing and in a slight variation, that is another means of delivering fluid by simply having a short copper stud at top and bottom stuck through a large stopper ...but this cannot support any pressurized domestic system.

A possible easier alternative could be a short 1" PVC pipe ( what is inside ID?), with two groves turned on a lathe for O rings then simply inserted into the glass envelope. This would require no alignment nor concern for expansion/movement. A small circ pump like that used for a fountain could supply enough pressure at the bottom to overcome the head of 4 feet of liquid without blowing any seals in a non-pressurized sytem. The distribution manifold header becomes trivial PVC fittings.

If you got a bundle of 7 tubes (photo shows 19 wrapped together) for $165 ..that is a small investment. IMHO, I would build a shallow box frame, either procure cardboard tubes (to be sliced in half) from local rug store or cast plaster with a 4 or 6" PVC pipe mold impression to enable some light gathering and line it with aluminum foil as the back reflector.

Keep us posted on your project ...too bad eBayer wasn't closer to me or I'd buy two sets of these tubes to play with. Then again, this dealer has a unit that looks very similar to Les's Schott unit that is 60inches long with 12 tubes and manifold for $525:

http://cgi.ebay.com/Solar-Hot-Water-Collector-Evacuated-Glass-12-Tube-Panel_W0QQitemZ110326548070QQihZ001QQcategoryZ3240 QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

BTW, above has a link that gives some numbers for how much heating to expect

Stew Corman from sunny Endicott

Yes, these are sealed/fused at the one end. As far as I know, most solar tubes are that way so either a heat pipe or a u-pipe circuit would have to be used. As you stated, with a sealed end there would be no way to do a flow-through design. And as I mentioned above, these do have a special black coating on the inside tube and somehow a gold coating is implemented in it. From what it appears, you can actually the gold and the guy had told me that some guys were actually buying them and smashing them open just to use them for the gold itself. Anyway, he would only sell me a maximum of 49 for this reason so I am hoping I can generate a good amount of hot water.

Now, the latest progress on the project...I decided to go with a direct flow system as opposed to a heat pipe system. For one, I was leary about making my own heat pipes. I contacted a few solar companies in China and spoke with them about making custom heat pipes for me, it did not seem to be a problem at all. The prices were dirt cheap (of course) but the ocean freight shipping cost was too high. So I decided to make a direct flow system completely out of copper, all sweat fittings. What I came up with is a 3/8" copper tube inside a 7/8" copper tube. The 7/8" copper tube is capped at the end and inserted into the collector. The 3/8" tube is open at the end where the 7/8" cap is and slightly shorter...cold water goes in the collector via the 3/8" tube towards the capped end then out in between the two tubes and collects the heat on the way out. Hope that makes sense because it sounds confusing lol. And I used stainless steel wool in between the inside of the glass collector and 7/8" tube to transfer the heat. What do you guys think?

The other day I put together an entire assembly of 3 tubes, manifold and all and ran in on a sunny afternoon. I was kind of disappointed as it only raised the water (about 6 gallons) temp about 10-15 degrees. But I wasn't sure if the pump I had set up was inhibiting it. I had it running constant the whole time, a slow flowrate but constant. The water definitely takes the heat away, as soon as I turned it on the water was steaming but then it cooled down as it ran and remained that way. Would the constant pumping not allow the collectors to build enough heat? I wasn't sure it there was more advantage to having the pump turn on and off based on collector temperature. Any input on that?

Regarding the rest of the system, I was debating between a separate glycol loop or a drainback/freezeproof system and I decided to go with the latter, straight domestic water at household water pressure (50 psi) and am either going to design a drainback circuit or a way to relieve the collector of the water when freezing conditions are encountered. I was able to find a 120 gallon stainless steel tank hot water heater tank and will control a pump system either with a solar control or a PLC (if I can figure out how to program it). So that's where I'm at now. I am mostly concerned if you guys think this system will work as I have it designed and be efficient enough to justify the time and money I have invested. Any input at all is greatly appreciated!

[Stewart Corman]

David,
Air is a great insulator, so any gap between your copper tube and the inside wall of the glass is presenting a barrier to heat transfer. Painting the copper matte black could make a small improvement.

as soon as I turned it on the water was steaming but then it cooled down as it ran and remained that way. Would the constant pumping not allow the collectors to build enough heat? I wasn't sure it there was more advantage to having the pump turn on and off based on collector temperature. Any input on that?As far as circulation: heat transfer is a function of the difference in temperature across any boundary.... SO, the cooler you can keep the circulating fluid, the higher the net transfer ...this means you keep the water circulating as much as possible to the storage tank.

And I used stainless steel wool in between the inside of the glass collector and 7/8" tube to transfer the heat. What do you guys think?Not positive ....where did you get that idea?? in electronics, they use heat conducting gel ..the idea is to have a liquid to solid interface ie no air...you might consider filling the gap with petroleum jelly(?) and use rubber gasket at the bottom to keep it from drooling out? Parafin might be a good choice as it becomes clear when it melts at fairly low temp. The heat transfer rate from air to solid is only 1/10 the transfer rate from liquid to solid ..measured in BTU/hr/ft^2/degree F

hope some of this helps

Stew

[Vasile Brindus]

I don't know how these were originally set up but I find it hard to believe the fluid actually contacted the glass tube itself, maybe I'm wrong though. As you mentioned it looks like the seller made up his own heat pipes but I'm going a different route that I'll get to below.



Believe it or not :-) that's exactly what it should happen. If you read the book I pointed to (page 72), you'll see that the liquid fills the inner glass tube through a central pipe. What they describe it's similar but not exactly the same build as you have, their inner tube is copper instead of glass, so the heat is even better transferred from the coating to the liquid... Probably that's why your efficiency is so low: the heat isn't properly transferred from the black coating to the water via the steel wool!

I think you can easily do a test: fill a tube directly with water and use another one with your copper pipes+steel wool as a reference. Mount them side by side in the same location and check the temperature after a few minutes and you'll find if the bad transfer is the culprit. Probably there's no need for a water flow, convection may be enough.


Regarding the rest of the system, I was debating between a separate glycol loop or a drainback/freezeproof system and I decided to go with the latter, straight domestic water at household water pressure (50 psi) and am either going to design a drainback circuit or a way to relieve the collector of the water when freezing conditions are encountered.

Drainback? You'll have to put the tubes with the sealed end up, so the water drains from the pipes.

Regards,
Vasile

[David Iannone]

David,
Air is a great insulator, so any gap between your copper tube and the inside wall of the glass is presenting a barrier to heat transfer. Painting the copper matte black could make a small improvement.

As far as circulation: heat transfer is a function of the difference in temperature across any boundary.... SO, the cooler you can keep the circulating fluid, the higher the net transfer ...this means you keep the water circulating as much as possible to the storage tank.

Not positive ....where did you get that idea?? in electronics, they use heat conducting gel ..the idea is to have a liquid to solid interface ie no air...you might consider filling the gap with petroleum jelly(?) and use rubber gasket at the bottom to keep it from drooling out? Parafin might be a good choice as it becomes clear when it melts at fairly low temp. The heat transfer rate from air to solid is only 1/10 the transfer rate from liquid to solid ..measured in BTU/hr/ft^2/degree F

hope some of this helps

Stew

Well, since there is a gap between the copper and inner glass tube I used the stainless wool to transfer the heat. Like I had mentioned, the tubes seem to be effective, and I would imagine that since I turn the water on and it comes out hot right away then eventually cools down to an average then the wool is transferring the heat pretty well. I actually got the idea from the guy that sold me the tubes. He or a partner has a website www.fossilfreedom.com where he had used steel wool as his transfer medium. Most of the solar tube systems I have seen, even with heat pipes, use an S or 6-shaped aluminum fin that basically contacts inside the inner tube and connects to the heat pipe. Nothing fancy so I figured the wool would work just as well. A liquid would be nice but probably much more of a hassle...

Believe it or not :-) that's exactly what it should happen. If you read the book I pointed to (page 72), you'll see that the liquid fills the inner glass tube through a central pipe. What they describe it's similar but not exactly the same build as you have, their inner tube is copper instead of glass, so the heat is even better transferred from the coating to the liquid... Probably that's why your efficiency is so low: the heat isn't properly transferred from the black coating to the water via the steel wool!

I think you can easily do a test: fill a tube directly with water and use another one with your copper pipes+steel wool as a reference. Mount them side by side in the same location and check the temperature after a few minutes and you'll find if the bad transfer is the culprit. Probably there's no need for a water flow, convection may be enough.



Drainback? You'll have to put the tubes with the sealed end up, so the water drains from the pipes.

Regards,
Vasile

I did check out that link but that is not the tube I have, even though it definitely seems to be the same manufacturer. I have actually never seen an evacuated tube like that with a copper inner tube, all the different brands I have seen are a glass tube within another glass tube and a coating somewhere in between. If I did have that copper inner tube I definitely would have come up with a more direct contact design but mine seems to be different then what is pictured there.

You may be right about the transfer but it seems like it is working from another aspect. If I sit the setup out in the sun with no water, the copper pipe and even the manifold get hot fast! So there is some kind of transfer going on. Then when the water pumps through it takes that heat away very quickly and the output is no longer hot but just warm. Again, this is just 3 tubes, so I am not sure how much output to expect. I will try that single tube test though as soon as I can, at least it will tell me if there are any transfer problems like you mentioned.

As far as drainback goes, if all goes well the tubes will actually be horizontal. On the roof I am putting them on that will be perpendicular to the sun's axis and still allow the majority of the water to run out when the pressure is released. If I get them close to level most of it should drain out (I hope).

Hopefully I will have some time on my hands tomorrow so I may just build a 25 tube manifold and see what happens. In any case keep the input and suggestions coming! I appreciate the help and I'll keep you guys posted....