Bad experience with evacuated tubes.

[tandrews]

Heat tube vacuum

I will suggest that if your heat tubes have burst, you should not replace them with the same type.
Rather, drain them and refill with acetone, then solder them shut.
I say this because if you put hot coffee in a thermos bottle and leave it outside (sealed) in -20 it WILL freeze. Evacuated tubes (refer to the glass, not the heat tube) are fabulous insulators, but they are not perfect insulators. An exposed heat tube neck will accelerate the process, but an unexposed one is still in -20. Replace water filled tubes that failed with water filled tubes and expect the same results - eventually.

This talk of vacuum in a heat tube/pipe is directly relative to the temperature applied.
That is, when I made mine, the application of vacuum was impossible, until I had the second end cap soldered and the tube cooled.
See link earlier in this thread for the method I used. (End cap soldered, 8cc acetone added, heat bottom to push 02 out, then solder second cap as tube cools and goes through zero ambient pressure)

If an evacuated (glass) tube fails, the inner tube will very quickly lose it's coating to oxidation, or at very least become distinct in appearance from it's sealed neighboring tubes.
If a heat tube fails, efficient heat transfer drops off - it does not cease.

If you have failed HEAT tubes, try this:

thaw and drain fluid from tube
Heat tube with a heat gun or torch to drive all water out of tube.
Use a syringe to inject 8cc(ish) acetone into split.
Rotate tube to put acetone in bulb/manifold end at the bottom end.
Work split closed with a vice, pliers, or whatever method is easiest for you - no need to flatten pipe.
Prep split area for soldering with sandpaper and flux
have a helper heat bottom of tube while a finger seals the split (neoprene, latex gloves will keep greasy human oils out of the mix)
Allow the pressure from boiling acetone to push the air out past your finger at least a couple times to ensure most 02 is removed
Apply heat to split and seal with solder - this is the artsy bit as it needs to happen when acetone is neither pressurizing nor pulling air into split
Expect failure of that last step more then once, but don't worry about some solder in the bottom of your tube. Skill is required, but there is no huge learning curve.

Wa la. repaired heat tube that will never freeze.

Nay sayers will comment that solder will never hold.
They should consider this is the very bottom of the temp gradient in the heat tube/pipe, and the cost to do this repair is mostly time.

Boiling acetone will not burn without oxygen.
If you get a flame at split when torching to solder split, this means you have removed oxygen from tube - good for you - now seal that sucker before it starts sucking 02 in.

If you fear repairing a split, get some HVAC 3/8" end caps and cut the split off the heat tube.
While one end is open, rinse with acetone to get any other crud out.
If heat pipe length is critical to ensure manifold retention, replace the lost length with copper scrub pads under the heat pipe at the bottom of the evacuated glass tube.

Step last - pass it on if it saved you X dollars.
Happy Holidays.

[Designer_Mike]

Can I get some clarification about your set up please. And you could also confirm my understanding of them below.

My understanding of evacuated tubes is that there are two tubes under vacuum:
The (external) glass tubes are under nearly full vacuum. A tube inside a tube sealed at both ends and coated in high heat absorbing material. This provides an isolation from the exterior temperature and wind extremes that kill productivity.

The internal (heat tube) which is what has failed, I was told is under a partial vacuum to allow the water inside to "boil" at a lower temperature and transfer the energy to the water header above.

Obviously if the heat tube splits, it is game over. If the glass tube fails and the heat tube is still in tact, I would think it would still function but at a greatly diminished efficiency.

When you did your experimenting with different fluids and water, did you apply any type of vacuum to the heat tube? Sorry if you mentioned it in one of the posts, but I didn't have time to completely read the entire thread.

Acetone boiling point is about 135 deg F which is roughly equivalent to the boiling point of water in 25in of vacuum.
Of course the minute you get any water boiling, the vacuum will be reduced and as the temperature increases you will get increasing steam pressure which is part of how the whole thing works.

It would be very interesting to take some of the Acetone tubes and put them under a modest amount of vacuum and test production changes. Since I believe you still have a valve mounted to the end of the tube, it should be fairly easy to use a small hand vacuum pump and evacuate the heat tubes and simply close the valve.

The vacuum does not affect the freezing temperature of the water.

Does anyone have any idea how much vacuum the heat tubes are. It would be very interesting to have a pressure gauge on the tubes as they function.

[mkel]

speculation

That's some great history you posted !

here is some speculation ... on #5.

If the solution in the heat tube is water and the tube has lost it's vacuum I doubt it would be capable of vaporizing and transmitting heat to the bulb. Allthough it may turn to steam at some point, it's effectiveness would be greatly reduced...... purely my own thoughts ...
mike

5. Not all of the heat pipe failures are from freezing. A high percentage of pre-2006 Apricus tubes stop functioning without any external evidence.

[LucMan]

I have seen no vintage 70's version of evacuated tube systems . I have seen several 70's flat plate systems, among those only a few drain back systems were still operational.
All of the glycol systems were dead.
Has anyone run into a silicon fluid based system that still is operational?

[Art VanDelay]

Evacuated tube technology originated in the US in the 70's

This thread contains a juicy irony:

I can't find a US engineer selling heat pipe evacuated tubes who knows for sure how they are freeze protected. I've gotten a few folks to speculate, and I don't mind speculating either. Pointed questions to the Chinese manufacturers result in the "trade secret" answer.

A few things are pretty clear, however:

1. Quality problems or design errors have resulted in a high historic heat pipe failure rate.

2. Since some heat pipes work and don't freeze break, it is possible to make the technology work for North America.

3. All the large Chinese manufacturers use water in the heat pipe as the working fluid.

4. Overfilling of the heat pipes with water can cause freeze breaks.

5. Not all of the heat pipe failures are from freezing. A high percentage of pre-2006 Apricus tubes stop functioning without any external evidence.

So, be sure you get an enforceable warranty from your supplier. Better yet, get the supplier to explain how their freeze protection works and how they maintain the quality to ensure they are manufactured correctly.

For you "Evacuated Tube Geeks", here's a quote from a forthcoming NREL report on the topic of Chinese collectors:

"Production and use of evacuated tube collectors is growing worldwide; the nature of glass tube production favors high degrees of automation. In the late 1970

[mkel]

reply to Art

copied from my previous post about fluid found in (27) unbroken tubes.
Since my tubes are on the roof I don't plan on going up there anytime soon ... but next time I'm up there I'll take a pic and see if you know who made mine. Really doesn't matter to me anymore as I modified them.

>>> 5. In this process I cut all the tubes that had not burst. I found that every tube contained a different amount of fluid with a variance of as much as 8cc. Some tubes had almost nothing.. maybe 2cc. The fluid appeared to be dirty water and I suspect the copper dust is not a product of engineering but residue from poor manufacturing of deburing or pipe prep. The amounts of residue went from a trace to quite a bit and some had none. <<<

Mkel,

Did you find any copper powder in the failed tubes? Did you cut open an undamaged tube to see what the factory fluid is?

Another American solar engineer has postulated that the copper powder provides "nucleation sites" for crystallization to begin.

If you provide a good closeup photo of the header near the outlet, I might be able to identify the manufacturer if it's one of the "big 3".

[Art VanDelay]

Photo? Copper Powder?

Mkel,

Did you find any copper powder in the failed tubes? Did you cut open an undamaged tube to see what the factory fluid is?

Another American solar engineer has postulated that the copper powder provides "nucleation sites" for crystallization to begin.

If you provide a good closeup photo of the header near the outlet, I might be able to identify the manufacturer if it's one of the "big 3".

[mkel]

thanks

The guy I got mine from is from china and travels back and forth monthly. He told me he is surprised that so many in the USA think these are some new technology. He said they are everywhere in china and have been for many years. Somehow I get calls from people all over that are installing these and want to ask questions. They seem to becoming a rather common installation at resorts in Costa Rico and Panama. I've had several calls from there. BTW .. I'm an experimenter -- not an expert by any means.
mike

Great post mkel!
I don't think that you will have any failures with the shraeder cores as long as you use the refrigeration type if you leave the cap on. Let us know how the solder holds up.
I think that the US & Canada exporting Chinese companies have rectified the problems with their tubes freezing. Most tubes were originally made for the southern China market, where temperatures are mild. Some offer 2 different tubes, low temp rated and standard. They had no idea how cold it gets here in the US and Canada.
I recently returned from a trip to southern China and was surprised at the amount of thermosyphon systems installed on rooftops. Thousands of installations in the Shanghai area, it seemed as if all the newer buildings had multiple rooftop systems. I was also amazed at the amount of mini split a/c & heatpump systems hanging from the sides of 25 floor apartment houses. I'm still envisioning how the serviceman rappels 15 floors down from the roof to repair a unit!

[mkel]

don't know

Hi Art,
I ran into a guy who lives here in USA but is from china and imports for his business. He wanted to start selling evac collectors here and brought over some in one of his containers. I bought mine from him over a year ago. Unfortunately I don't know who made them as they have his company name on them. They are very well built and until this heat tube problem they really worked well.
I hope I don't have issues with the solder. I have several tubes leaning up outside with no header. To keep rain from getting down the glass tube I taped a soup can over the bulb and glass tube leaving a small opening at the bottom. I wanted to see what would happen if they just cooked. they have been out there about 6 weeks with no issues so far. Thanks for your comments.

PS .. You probably know this but some out there may not ... coiling the tube and putting it in the freezer will not tell you anything unless you absolutely know all of the liquid is concentrated in the bottom of the tube and it is placed vertical or close to the angle it would be in the collector. Coiling the tube allows the fluid to disperse along a longer length of the tube and would never be able to burst the tube ... rendering the test useless.

Mkel,

Great job showing your problem and your solutions.

I think that most of the evacuated tube collectors being imported these days to the US don't have this freeze breaking problem. I've been testing SunMaxx and not had any trouble yet, and I've put them in my chest freezer also.

Hence my question of who made yours and when. Pre 2006 Apricus heat pipes had a high failure rate. This information is tough to find online, which makes these forums the best place to find it.

I don't think solder can handle the dry stagnation temperatures inside the evacuated tube. I've seen temperatures at 400F up at the header, which means the absorber was much higher. That's why the heat pipes are brazed.

[LucMan]

Great post mkel!
I don't think that you will have any failures with the shraeder cores as long as you use the refrigeration type if you leave the cap on. Let us know how the solder holds up.
I think that the US & Canada exporting Chinese companies have rectified the problems with their tubes freezing. Most tubes were originally made for the southern China market, where temperatures are mild. Some offer 2 different tubes, low temp rated and standard. They had no idea how cold it gets here in the US and Canada.
I recently returned from a trip to southern China and was surprised at the amount of thermosyphon systems installed on rooftops. Thousands of installations in the Shanghai area, it seemed as if all the newer buildings had multiple rooftop systems. I was also amazed at the amount of mini split a/c & heatpump systems hanging from the sides of 25 floor apartment houses. I'm still envisioning how the serviceman rappels 15 floors down from the roof to repair a unit!

[Art VanDelay]

Whose Tubes and what year of manufacture?

Mkel,

Great job showing your problem and your solutions.

I think that most of the evacuated tube collectors being imported these days to the US don't have this freeze breaking problem. I've been testing SunMaxx and not had any trouble yet, and I've put them in my chest freezer also.

Hence my question of who made yours and when. Pre 2006 Apricus heat pipes had a high failure rate. This information is tough to find online, which makes these forums the best place to find it.

I don't think solder can handle the dry stagnation temperatures inside the evacuated tube. I've seen temperatures at 400F up at the header, which means the absorber was much higher. That's why the heat pipes are brazed.

[mkel]

temp where I live

Hi Hamish,
I added some pics to my post above... not sure why I got a thumbnail of the same pic.
It does get cold at times here in Missouri USA ... last winter we had several nights at -15 and windchill in the area of -30 degrees. Normal winter cold snap would be around 10 degrees.
My system is a total hybrid that I built.. making some changes now but when I have time I'll post pictures of everything.
mike

Hi mkel,
thanks for sharing your experiences and experiments with the heat pipes. I like your idea of fitting a valve to the heat pipe end and being able to manage the heat pipe indefinitely.
Please post some pics, and wellcome to the forum. How low do the temps go where you are.

[hamish]

Hi mkel,
thanks for sharing your experiences and experiments with the heat pipes. I like your idea of fitting a valve to the heat pipe end and being able to manage the heat pipe indefinitely.
Please post some pics, and wellcome to the forum. How low do the temps go where you are.

[russ]

Hi mkel,

Even more fun when you get it worked out in the end!

Well done!

[mkel]

reply to Russ

Hi Russ,
While it did give me measurements like you indicate I found that the heat tubes with less fluid transfered less heat to the header and gave me pretty much the same reading as the failed tubes. I found the most acurate way to find out if the tube was failed was to pull the glass tube and pull the heat tube up about a foot. If the heat tube was black in color it was failed. If it is copper or gold in color it is still good. However....... even this method wasn't 100%.
No matter what ... this is great fun playing with these systems.
broken-tubes-opt.jpg

Hi mkel - Welcome to Solar Panel Talk!

Good post and a lot of research it seems!

The IR temp measuring device should show a big temp difference as in many degrees comparing bad tube to good ones. The tube is collecting heat and the vacuum doesn't let it escape.

I generally think the IR devices are useless for measuring temperature but they should be great for a go/no go indicator in a situation like this.

They rely totally on the emissivity setting and that is rarely adjusted correctly by the user.

It seems to make sense to use a liquid with a low melting point rather than water -