BTU Output of Flat Plate Vs. Heat Pipe Evacuated Tubes in Sunny Conditions

BTU Output of Flat Plate Vs. Heat Pipe Evacuated Tubes in Sunny Conditions:

Following is data on a test of the BTU production of 4' by 10' flat plate collector versus a 30 evacuated tube heat pipe array. The evidence clearly shows superior performance of heat pipe evacuated tubes when compared to similarly sized flat plate collector.


The Test: Testing was conducted over several days, in generally sunny weather conditions. The goal of the test was to define the output differences between these two types of solar collectors.

Both collectors were set side by side, and equal angle and declination. A single 50 gallon tank with a single pump was connected to a flow splitter. The flow splitter would send water either to the evacuated tube or the flat plate collectors. Both returns were sent to the single tank. The flow splitter would alternatively send water to the flat plate and then evacuated tube collectors. The pump operated continuously throughout the tests. A flow meter allowed visual verification that flows were always identical.

Separate T1 sensors were set on the exit manifolds of both collectors. These T1 sensors were first calibrated on a single manifold to determine that sensors accurately described temperatures. A single T2 sensor was set on near the pump, measuring tank temperature of water drawn from the bottom of the 50 gallon tank. An additional sensor recorded outside temperature.

The Setup: One single 4' x 10' flat-plate collector vs. a 30 evacuated tube heat pipe array.

The Data:







The Conclusion

Flat plate and evacuated tube collectors each have an appropriate application and use. Care should be taken when selecting the type of collector, depending on a host of variables.

Generally,

1) Tubes enjoy a significantly longer and more productive solar day. Evacuated tubes begin BTU production earlier in the day, and continue to produce BTU later in the day than flat plate collectors. The round shape of a tube allows solar gain no matter what the angle of the sun. In contrast, a flat plate collector has a low solar angle in the morning and afternoon, with direct angle occurring only at the middle of the day.

2) Tubes operate at higher temperatures than flat plate collectors. The old rule of thumb for flat plate collectors is hot water at approximately 50 degrees above ambient temperature. This rule of thumb appears accurate. Evacuated tubes greatly exceed this threshold.

3) For a high temperature tank or for winter radiant demand, evacuated tubes perform better.

This is one of several photos of evacuated tube and flat plate collectors in snowy conditions.

The conventional wisdom is that flat plates melt snow, tubes stay covered. We disagree.

Snow after snow, location after location, the tubes clear. This picture was taken in Perry, KS the morning after a 6" snowfall. The outside temperature was 13 degrees, no wind, sun brightly shining. The manifold temp of the evacuated tube was 359 degrees, the flat plate was 83.

As the day went on, the situation stayed the same. Snow on flat plates, no snow on tubes, tubes running hot.