Monday, October 10, 2011
5 Comparing concentrator to flat plate solar collector - stagnation
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.
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.