Sunday, October 09, 2011

3 Comparing concentrator to flat plate solar collector

In this test, both solar collectors in the solar test jig are insulated. The flat plate collector is fixed in position in the same way as these are normally used. The concentrating parabolic trough collector rotates automatically to face the sun as these are usually used (and the way they must be used for them to operate properly).

Details of the insulation for each collector are given in previous postings, for the flat plate and for the concentrator. More background on the test methodology and the collectors is here.

Temperatures in the two separate reservoirs (red picnic coolers) were logged automatically throughout the day as was the ambient temperature in a shaded location near the solar test jig. Each reservoir holds a small aquarium pump that circulates the fluid through each collector and back to it's reservoir. The flow rates of the fluid (water) were monitored on two flow gauges and balanced to be the same 0.85 LPM. The quantity of water in each system was the same 45 lbs (20.5 kg). The plumbing runs are identical lengths and sizes for the two systems.

The day was a perfect solar day (rare here at this time of year): clear blue sky, no clouds and only a slight breeze 10-20 KPH during part of the day.

Here are the overall results for the day. Both systems start to heat rapidly as the direct sun hits them about 09:45. The concentrator has a slight lead until about 13:45 when they begin to run almost identical temperature profiles until about 15.20 when the flat plate abruptly begins to cool while the concentrator continues to rise in temperature.

The tracker, which in this test is controlling only the concentrator position, reaches the end of it's travel at just after 16:00. The concentrator collector remains in the beam for over a half an hour further and the temperature of the concentrator curcuit continues to rise until about 16:45 when the collector finally leaves the beam and the concentrator temperature begins to fall.

The temperature of both systems falls rapidly until the end of the test at 19:30 at which time the sun is setting.

At the beginning of the day, both collector temperatures rise with the ambient although the concentrator does not rise as quickly as the flat plate. This may indicate that the insulation of the concentrator collector might be slighly better than the insulation of the flat plate if the other factors of each system were truly the same.

The solar test jig is shielded from the rising sun by a row of trees so the exposure as the sun rises is not even, hence the undulating solid line of the concentrator temperature from about 08:50 until full exposure to the sun occurs at about 09:40 and indeed the flat plate temperature from about 09:30 to 09:40.

The pumps, by the way, have been running all night to even out the temperatures at startup.

In this zoom on the period around solar noon, you can see that the two systems seem to track each other from about 14:00 to 15:20. This would seem to indicate that when near normal to the sun at its zenith, the efficiencies of the two solar collectors I am testing are more or less the same under the test conditions.

The peak concentrator temperature of 129.876F occurs at 16:45. This is 50.2 degrees F above ambient. For the flat plate, the peak of 128.998F occurs earlier, at 15:17. This is 49 degrees F above ambient.

Tomorrow, I will test again, with both the flat plate and the concentrator tracking the sun.

Index - Comparing concentrator to flat plate solar collector

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