Evacuated Tube Versus Flat Plate Solar Hot Water Panels – Who Has The Upper Hand?

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I was recently discussing the different types of solar hot water panels with a business owner who is strongly considering the investment of a solar water heating system. He had heard a lot lately about evacuated tube collectors but he knew Röhrenkollektor the system I was offering him used flat plate collectors. (“Collectors” is just another name for panels because they collect solar energy.) So he asked me which type of panel was better. “The answer is simple,” I told him. “Both.” He scratched his head and looked back at me puzzled.

Internal water temperature

Of course it’s not quite that cut and dried. The real answer is found by plotting the slope-versus-intercept curve of both types of collectors on the same graph. This graph will show that flat plate collectors are about 80% efficient when the internal water temperature is equal to the outside (ambient) air temperature. As the temperature difference between the water and the air increases, the efficiency of a flat plate collector falls. Evacuated tube collectors are only about 50% efficient when the water and air temperatures are equal. But their efficiency does not fall off as fast at higher temperatures.

Plotted on the same graph, the two curves cross each other (crossover point) around the 140oF differential mark. So, before the crossover (to the left on the graph) flat plate collectors are more efficient. To the right of the crossover, evacuated tube collectors are more efficient.

For example, let’s say it is 40oF degrees outside. A flat plate collector will be more efficient as long as the water temperature is below 180oF. An evacuated tube collector will be more efficient as the water temperature rises above 180oF. On a day where the outside temperature is 0oF, the efficiency crossover point occurs when the water temperature hits 140oF.

Generally most domestic hot water (DHW) and space heating (SH) applications occur before the crossover point on the graph. For applications that require higher temperature, like absorption chillers, evacuated tube collectors will typically perform better. Since evacuated tubes are generally more expensive than flat plates on a cost per square-foot basis, you can go beyond the crossover point a little ways and still be better off with flat plate collectors.

For the most part, there is no good reason I can think of to use evacuated tube collectors on residential applications in all Röhrenkollektor but the coldest of areas. There is a long list of commercial applications that are best served by flat plate panels. Here are just a few examples:

  • College or University Dormitories
  • State or County Detention Centers
  • Retirement Communities
  • Nursing Homes
  • Carwashes

Evacuated tube collectors are a better fit for industrial processes requiring temperatures 180oF and above for sanitation purposes.