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

Friday, November 04, 2011

Concentrator absorber condition after stagnation test

After doing the stagnation test with my DIY solar concentrator a couple weeks ago I had the copper absorber and the evacuated tube used sitting on my workbench. I wanted to get a close look for damage after the interior of the evacuated tube reached a temperature of over 630°F.

You should be able to see quite clearly the discoloration of the copper between the two yellow arrows (click any picture to enlarge). The two yellow arrows are located near the ends of the concentrator reflector and show where the heat originated.

As I wrote in about the stagnation test, the collector does not normally operate with no liquid flow to cool the absorber yet the system must be designed so that it is capable of withstanding this condition without damage.

Near the bottom, you can see that my home-made return bend has not suffered the same heat as did the center section. I was worried about the solder melting. Silver solder (hard solder) apparently has a defined melting temperature of over 850°F but alloys vary between manufacturers. I do not have technical information on the silver solder that I used so I will need to find some for the next build that has a known, defined melting temperature.
Near the middle, the copper tubes and the copper mesh are well and truly scorched. The oxidation of both is clearly evident in this picture. I have not pressure tested the absorber, but I am confident that the damage that has occured is to the surface only and that the absorber could probably endure another episode of extreme heating without failure.

Still, the effect of the stagnation heat are quite stunning. Clearly this is something that should not happen regularly.

In this view at the top can be seen the thermocouple sensor in the approximate location it was during the test. If I do the test again, I will have another thermocouple down at the middle as well as this one at the top. Although I measured +630°F at this location, I suspect that it may have been hotter further down as evidenced by the heat discoloration.

The fibreglas bung has deteriorated somewhat from the neat roll I originally inserted in the end of the evacuated tube. This is because I had partially removed the absorber from the evacuated tube after the test and left it partially out for a couple of days of wind and rain which battered the fiberglass somewhat. Normally the bung would be fully inside the end of the evacuated tube.

In this closeup of the top can be seen a black smear on the copper tube. This is actually a remnant of the plastic film that is bonded to the fiberglas wrap I used. This plastic melted from the heat and got onto the copper tube and the inside of the evacuated tube. I will need to find fiberglass wrap without the plastic film or remove the plastic film before using the fiberglass wrap the next time.

The absorber overall is about five feet long and my impression of it sitting on the bench is of several electric kettle heaters unwound and straightened out. I am estimating that the absorber was subjected to about 800 watts of power over several hours in an enclosed space (the evacuated tube). The effect on the copper tube and mesh is dramatic.

Index - Comparing concentrator to flat plate solar collector