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
3 comments:
George,
Love all the data. I'm looking at solutions for hsolar ydronic heat, and thinking the concentrator might be a reasonable way to get more heat transferred on a cloudy Oregon winter day. In summer I could generate steam an make electricity...just kidding, that's more control and complexity than I want to maintain. Tesla had a nifty patent with iron pipes and steam for generating electricity, if considered trying to replicate that on a small lab scale to see if i could make a DVM show current generation with an oscillating stream heat /cooling cycle in an iron pipe wrapped in copper (I recall this was Teslas idea).
I was surprised that the tracker on a flat plate was so close to the collector performance.
To generate steam, are you planning on linking the concentrators in series, or just reducing the media flow rate?
Hello Ross and thanks for your comments and interest.
The concentrator unfortunately does not work as well as a flat plate in cloudy conditions, it needs hard sun.
I would like to read more about Tesla's work that you mentioned. Can you send me a link?
I was pleased and a bit surprised also at the comparisons. I had been asked how the concentrator compares so I tried to do a detailed set of comparisons to answer that question.
For steam generation, I will definitely reduce the flow rate. I haven't decided on parallel vs. series yet.
Dear George
I'm going to make a solar trough which produce steam. I intend to fill thermal liquid oil in side a vacuum tube. So I need to close the open of the tube by bung & the water need to pump inside the copper tube continuously. It is so complicate to control the process. Do you know a heat oil pump? If it's cheap, i will use it to circulate the oil for heat transfer to make steam. Otherwise, I will place water tank at the heigh level the same at the open of the tube
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