Monday, October 10, 2011

5 Comparing concentrator to flat plate solar collector - stagnation

The test today involved measuring the stagnation temperature of both the flat plate and the concentrating solar collectors used up to now in this series of comparative tests. The stagnation temperature is the temperature reached with no fluid flow such as when the pump fails or the owner forgets to turn on the system. It is important for solar collectors to be able to withstand the stagnation temperature without damage. Another interpretation of the stagnation temperature might be the highest temperature that can be reached with fluid speed of near zero.

As this might have been a destructive test, I left it to nearer the end of my tests.

Truthfully, the test today was conceived and executed in a bit of a hurry. I did not plan to do testing today. But when I noticed shortly before solar noon that the polyethylene hoses had melted from the brass fittings on the concentrator that I began to wonder just how hot it was inside the glass evacuated tube? and inside the flat plate box? I had not been running the pumps so this was truly stagnation on a bright, hot clear fall solar day.

I could not use the HOBO date recorder for this test since the probes that I have are spec'd only to 212°F and likely the temperatures would be higher than this. Fortunately, I had at hand my trusty Fluke 52II dual channel thermometer and a couple of high temperature thermocouple probes rated to 1000°F.

I drilled a hole in the plastic horizontal closure strip on the top of the flat plate collector and inserted one of the thermocouple probes so that is was resting on the aluminum absorber plate about 6" from the top of the collector.

For the concentrator, I pulled out the fiberglass bung and the copper tubing loop far enough to unwind the bung down to where the copper tubes passed through and laid in the other thermocouple wire so that the probe was inside about 6" from the top of the evacuated tube and then rewound the bung and reinserted it into the evacuated tube.

The tracker was out of focus for the concentrator for the time of day. So the beginning results might be indicative of the stagnation temperature of an evacuated tube as normally used, without a reflector. Clearly from the starting temperature (378°F) the water inside had long since boiled away.

I was working on something else nearby and periodically took pictures of the Fluke display for both the concentrator (channel 1) and the flat plate (channel 2). I used the time of day recorded by the camera to construct the graph using Microsoft Excel. The time of day is not daylight savings, so to compare to the other tests one hour should be added.

(click to enlarge) As the concentrator came into focus, the temperature increased dramatically to a maximum of 636.1°F! By this time, the sky contained high altitude haze that limited any further increase.

The flat plate collector on the other hand stayed at about 200°F for the entire test, dropping off earlier than the concentrator which caused me to look at the sky and to notice the haziness. Ambient air temperature during the test was between 76-80°F. The air was still, there was no wind at all. Both the concentrator and the flat plate collector were tracking the sun.

Over six hundred degrees inside the concentrator collector! Wow!

I will pull out the copper tubing loop tomorrow to check the soldered connections.

No damage seems apparent to the flat plate. I was worried about the foam polystyrene insulation but it seems not to have melted.

Index - Comparing concentrator to flat plate solar collector


mtmtntop said...

Have you done any more work with evacuated tubes and the parabolic mirror? i am thinking about adapting your design to accommodate the different size of evacuated tubes. also, everything would have to be in copper. i have a 75 evacuated tube array now that shares a boiler loop to heat an "endless pool" and a hydronic floor. It stagnates about 375 (F) I am considering your collector and evacuated tubes to share the boiler loop that heats the rest of my house. ( I bought your book) I would probably only use this in the winter. i'm in montana

george plhak said...

I am thinking hard about it but instead doing some winter projects that need doing here. What size are your evacuated tubes? Thanks for your interest. George.

harishdarji said...

I am very much interested in your design of parabolic trough. It is possible to use this parabolic trough for industrial application. In oil heating , for steam generation ,air heating and so many thermal process heating. can you give some more detail about the energy generation form parabolic trough. Can we use this technology in India.

Mike MacDonald said...

Where is your little steam engine then ??

That temp is enough to give you dry steam and therefore drive a piston to generate power.

Get Cracking man !! I wanna see a Steam engine !!!

George Plhak said...

Mike - Please see my article:
How to make solar superheated steam.
Thanks for your interest.

Thea Rasins said...

Thank you for posting this - I have a evacuated tube that is about 3 feet long and looks like a giant silver/black test tube. Is there a resource I can obtain pipe that will allow me to circulate water into and out of the tube (thus heating it)? I am also interested in learning more about designing sealed circulation systems powered by heated water (to provide radiant heating for a home), but I need to understand thermodynamics more. What I learned on Gilligan's Island does not seem to have been quite detailed enough, and I have no coconuts.

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