Tuesday, November 08, 2011

More about evacuated tubes for parabolic troughs

My interest is in using the glass evacuated tubes not as a component of a commercial collector, but as an insulated collector for a DIY parabolic trough concentrating collector. With the addition of a rotatable tracking parabolic trough behind a single evacuated tube, it is possible to increase the heat capture significantly, up to 8-15x safely. In this manner, it would be possible to do things that are not possible with a flat plate or an evacuated tube system - making steam for example. In a recent demonstration I showed that temperatures of over 630°F are possible on an experimental DIY basis.

Please see Wikipedia for a backgrounder on evacuated tube solar collectors or Google "evacuated tube solar" or "solar vacuum tubes". Much has been written about this method of solar heating that seems to be popular elsewhere than North America.

(click any picture to enlarge)

Searching the web reveals numerous manufacturers and truly, there are many. Evacuated tube collectors are widely produced in large volumes, mostly in China and India. Here are some that I found which give some specific information on tube specs and various details about the tubes.

These are not my recommendations, just suggested links for further information:

First, a video from Apricus showing manufacture of evacuated tubes in their Chinese plant:

  

Links to other manufacturers websites:

  • Haining Jixang Solar Energy Co. Ltd. Haining, Zhejiang. Schroll down the page for the specs. Note the diameters and lengths OD 47mm and 58mm, lengths of 1500 and 1800mm - you will be seeing these again. Select "heat pipe" on the left menu and you will see the specs for the heat pipe. Note Transfer power:≥150W. Check out the wide variety of their systems.
  • Haining Qiruite Photoelectric Co. Ltd. (Qirui) Haining, Zhejiang. A bit hard to understand in the English, but the same sizes can be seen about half way down the page. Do they make a 70mm dia? The length looks like a mistake.
  • Jiaxing Jinyi Solar Energy Technology Co., Ltd. (Jinyi) Jiaxing City, Zhejiang. Produce standard (JVN series) evacuated tubes with IDs of 37mm, 47mm, 58mm, lengths 500mm 800mm 1500mm 1800mm 1900mm 2000mm 2100mm. Have advanced (JVT series) three coating tubes.
  • Zhejiang Qianjiangcho Luminous Energy Co. Haining, Zhejiang. Seem to be redoing their web page, seems less complete than when I last visited and got specific sizes and lengths. Have three different types of coating systems available in their "vacuum tubes". ID's 37mm, 47mm, 58mm all +/- 0.7mm, corresponding ODs 47mm, 58mm, 70mm, standard lengths 1200, 1500, 1700-1800 and 2000mm all +/-5mm.
  • Linuo Ritter (Sino German joint venture) Gives specs, but does not give sizes of their standard offerings.

Two places to buy evacuated tubes on-line although these venders only offer "shorty" tubes, 20.5"/500mm long:

Patrick Ward of Fossil Freedom, Denver, CO? sells recycled US Government surplus evacuated tubes 45.75"/ 1160mm long.

If you like the idea of an evacuated tube which is open at both ends with expansion bellows fitted: Dezhou Mingnuo New Energy Co., Ltd., Dezhou City, China offers a 4000mm receiver tube of this type.UPDATE: The link no longer works. I have written to the company to get info. "Sun Island" Haining Chaoda Solar Collector Tubes, Ltd is another manufacturer. These do not have bellows, or an absorptive coating. Open both end tubes are rare and you would have to import a case lot. The open at one end tubes are much more commonly available, at least in North America. I searched "open both ends solar evacuated tube" to find these.

The (open one end) tubes that I have measure ID 43.5mm OD 58mm and 71 inches (1803mm) including the pinch off but not including the heat pipe. This seems to be a size common to at least three of the manufacturers listed above, so we could assume this size might be also available from others? I cannot find a specific international specification for the tubes alone. If anyone knows this spec, please let me know.

For me, the length of 1800mm (5.9') is a convenient size for my standard parabolic reflector with a 4 foot length. About a foot of evacuated tube sticks out both ends of the concentrator which helps to suspend it in place at the focus. Fluid and electrical connections are made to the internal absorber at the open end. The diameter is critical for the max size of absorber I can insert into the evacuated tube. 43-47mm seems to be a fairly standard size.

I can only give pictures of the construction of the evacuated tubes that I have at hand but others I have seen, although different in some of the details, are similar. Here I am removing the "guts" of the tube by pulling on the bulb of the heat pipe. With this particular tube, the innards pulled out readily. With others, the heat pipe (the copper tube assembly in the center) comes out of the aluminum absorber leaving it behind in the evacuated tube. With those, I had to pick away at the fiber glass "bung" until I could grab the absorber with pliers and pull it out.

Here is an end view of the aluminum absorber. The heat pipe is held in the center of the evacuated tube, supported down it's entire length by this formed sheet aluminum part with also improves heat coupling from the inside wall of the evacuated tube to the heat pipe.

In this view, I have shown the bulb end of the heat pipe. The heat pipe dia is 0.317" / 8.04mm. The bulb dia is 0.943" / 23.97mm and has a length of approximately 3.905" / 100mm. The fiberglass bung can also be seen.

Here is the other end of the heat pipe, the aluminum absorber support and the evacuated tube. The evacuated tube has a shiny metallic "getter" on the inside which will be familiar to readers old enough to have worked with electronic vacuum tubes. The getter is a metallic coating which removes impurities in the vacuum after the tube has been sealed. It is also a diagnostic for the quality of the vacuum since it will turn to a powdery white appearance as it is depleted. If the vacuum is good the getter will be bright and shiny like this one.

The bung which seals the top of the evacuated tube is a powdery brittle compressed bit of what looks to be fiberglass? Hopefully it is not asbestos? It basically flakes apart when you remove it so is not reusable. Winding fiberglass pipe wrap into a cylinder was found to be a useful replacement for the bung and also able to accommodate an inlet and an outlet tube as was done for the test of an insulated concentrator.

So where do you get yourself some evacuated tubes for your own experiments? I don't think that you will yet find these at your local home center unless you are in the southwest USA or Mexico.

You could begin by writing to the companies listed above and any others you find and asking them about dealer/installers in your area. Look at your local sources for solar domestic water heating. You might be interested in a complete system, but for the purposes of a DIY project, we only really need a source for a few tubes, initially at least.

The tubes are relatively inexpensive. A solar dealer installer will typically offer this information readily if you ask what happens if a tube breaks (what if my kid throws a baseball through one? or a hail storm breaks some?), replacements will be required. The installer should keep a supply of spare evacuated tubes for this possibility and he/she will probably tell you something like "don't worry the tubes are only $X dollars each". I have heard $10-25. Find out what sizes the dealer carries in stock and have a look if you can at the heat pipe construction and coupling to the glass tube. Find out if they use a non-freezing fluid or water with some chemicals. Use the information you get from the dealer or several to determine the price you are willing to pay in your area and the most convenient supplier and then ask to buy a few tubes. You will get raised eyebrows perhaps, but you should get your tubes at a reasonable price. We don't really need the heat pipe unless you want to use it that way.

The tubes are certainly breakable and long and thin so getting them locally is a plus unless you want to buy a box lot as I did and deal with shipping costs.

CAUTION: Evacuated tubes are certainly breakable and it happens all of a sudden. They are pretty robust but like all glass, are delicate to certain types of shock. Being under a vacuum, they IMPLODE rather than EXPLODE but the effect is certainly sudden, makes a mess and is quite probably hazardous with all the VERY SHARP large and small shards of glass which result. After I broke this one, I started to wear safety glasses when handling evacuated tubes. Be careful!

Index - Comparing concentrator to flat plate solar collector

20 comments:

Mike said...

I have wondered how fragile these are. In your estimation what size of hail would break them?

george plhak said...

Good question that I don't have an answer for. I did see on the web some spec for hail resistance but can't point to it just now. I suggest that you ask your supplier, particularly if you are in an area where destructive hail is more common. Around here, it is rare (thankfully).

george plhak said...

I just came across this interesting thread "Problems with evacuated tubes" about failure of heat pipes which contain water in very cold situations:
http://www.solarpaneltalk.com/showthread.php?4096-Bad-experience-with-evacuated-tubes
So if you are going to use the heat pipes, you need to know what fluid they contain, or replace them with a tube loop as I am planning to do and run a fluid that will not freeze at your expected winter low temp.

IanB said...

Thanx for doing this blog I am in Dryden Ontario Canada as you know we have harsh winters here and i am looking into how these systems perform in winter with lots of snow and cold temps. I was wondering can you enclose the parobolic set up? it would be easier removing snow off it in winter and would it lose performance doing this? you know they have parobolic systems in place in africa they runs in series up to 400 meter lengths with a different patented mixture for high temp heat transfer they get temps upto 600f and make steam then drive turbines that make electricity cool stuff...

george plhak said...

Thanks IanB. I will post new stuff as I get it done. Thanks for your interest. If you have a web link to the work in Africa, I would like to know about that - thanks!

mtmtntop said...

re: evacuated tubes in cold climates. i have a 75 tube array in montana. you need to use glycol that you can get at any plumbing/hvac store. the stuff i got has burst protection to -100 f and cost $61 for a 5 gallon bucket. i have been in several nasty hail storms with no damage. the array stands at a 70 degree angle. i am considering building a parabolic trough with these tubes using georges "pipe is stationary" design because the tubes have a nipple you need to plug into a manifold of some type. the manifold has glycol circulating in it picking up the tubes heat.

IanB said...

http://www2.env.uea.ac.uk/gmmc/energy/energy_links/Riffel.pdf

Some info in africa they use synthethic oil as fluid gets temps up to 300-325f

Bernardo Aranha said...

Great job, George!
Look i've found:
http://jxsolar.en.made-in-china.com/product/JbIntUjuVRhH/China-U-Pipe-Solar-Vacuum-Tube.html
I want to buy it. How it works? I mean paymen(paypal?), shipping(air/sea?). Do u hv some tip? I'm waiting for seller answer.
Regards,
Bernardo.

george plhak said...

Thank you Bernardo. Let me know their reply.

Kitchen Helper said...

George, any thoughts on how to make a diy manifold to replace the commercial one utilizing the heat pipe? Would it be possible to use 1" copper pipe?

george plhak said...

Hello Kitchen - Patrick Ward at Fossil Freedom shows how he makes manifolds for the heat pipes in evacuated tubes:
http://www.fossilfreedom.com/manifolds.html

usdoc1 said...
This comment has been removed by a blog administrator.
Chris Gates said...

why wouldnt the tubes have something like clear plexiglass covering them so they dont break?

David R said...

@Chris Gates: Evacuated tubes already have 2 layers of glass. More glass/plastic means less sun reaching the heat pipe.

Evacuated tube systems sold in Australia have to meet a national standard AS/NZS 2712 which includes a hail test. The test hail is 25mm (1") diameter travelling at 25 m/s (82 ft/sec).

Amey Hukkeri said...

hi...i am manufacturing a parabolic trough collector...
I am getting the vacuum tube used for flat plate collector.Can this tube be used for parabolic trough collector?
else where can i get the vacuum tube for parabolic trough collector??
I live in the city of Pune in India.

George Plhak said...

Amey: I do not know about vacuum tubes used in flat plate collectors so I do not know if what you are getting will be suitable. I am sorry my article does not suggest sources of supply for you, there are numerous manufacturers listed and ideas about how/where to find the tubes. Thank you and good luck. George

40 Watt Solar Panel Kit said...

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Ryan said...

Hi George. Awesome information, thank you.
Would it make sense to put a fiberglass or other cover in a parabolic shape OVER the entire device, opposite to the parabolic mirror (ie. forming a 360 degree outer shell between the fiberglass cover and the mirror, with the collector in the middle)? I would think this would aid in heat collection and also would keep the vacuumed tube safe from hail and other elements, and also from debris collecting within the mirrored area. Any thoughts?

George Plhak said...

Thank you Ryan. A cover would not, in my opinion, help in heat collection but would actually reduce heat because the cover would block some of the solar radiation. The heat by the way, gets trapped inside the evacuated tubes so making an enclosed space would not really help. As far as reducing contamination, a cover might help but having the collectors on an angle (to match the latitude of your location) helps wash them clean in my experience (except perhaps near the equator). Did you see my parabola cleaner?
As far as breakage, none of mine have broken due to objects from the sky but rather from my own clumsiness. If you look at the commercial use of these evacuated tubes, none of them are covered. The tubes are inexpensive enough to replace the odd one that breaks.
Thanks for your kind words and interest. George

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