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

Jan 25
08:48

2011

Dr. Ben Gravely

Dr. Ben's http://www.solarhotwater-systems.com Blog Spot was created with one primary purpose in mind. That is to make publicly available reliable information on best practices, installation techniques, and design strategies regarding solar water heating systems to architects, engineers, installers, and owners. The format of this website is interactive so please feel free to leave your thoughts, insights, comments, and questions for discussion on Ben's Blog.

I was recently discussing the different types of solar hot water panels with a business owner considering the purchase of a system. He had heard a lot about evacuated tube collectors but he knew 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.”

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. For higher temperature applications, like absorption chillers, evacuated tube collectors 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.

There’s no good reason I can think of to use evacuated tube collectors on residential applications. 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.

It certainly helps to be working with a solar professional who understands the differences between these two types of panels and is not just trying to promote one over the other to make the sale. So, when it comes to choosing between evacuated tube and flat plate solar hot water panels, the most critical factor is the temperature at which they will operate for a given application.

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