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Solar Thermal 101: Evacuated Tube Solar Collectors

This is the third in a series of posts written by Free Hot Water’s co-founder and senior mechanical engineer, Gal Moyal. We’ll be posting this series every Wednesday, so please make it a date. Some of the information in future posts may be very technical, but if you have any questions at all, please feel free to contact us. We sincerely want to help. If you would like to have a more hands-on experience, explore our certified Free Hot Water training courses.  –Solar Fred.


Evacuated Tube Collectors consists of several glass tubes, each of which has concentric inner and outer walls. The inner space is evacuated and the vacuum helps keep the inner tube isolated.

The system works on the same principal as a Thermos® bottle. Just like a thermos, the solar evacuated tube allows most of the heat loss to be eliminated. That makes evacuated tube design a preferred system in cold climate areas. However, you always need to pay attention to the header design. This is the weakest area of the Evacuated Tube collector design.

How it Works

The sun’s energy is trapped by the absorbing strips and transferred to a specialized fluid that is sealed within the internal copper tubing. When heated, this fluid changes from liquid to vapor and rises toward the top of the tube, and into a manifold assembly located at the top of the collector.

Heat conducts though this copper capsule into a fluid circulating along the manifold were it heats the water that flows into the manifold. Once the fluid cooled, it condenses back to a liquid and flows back to the bottom of the tube, ready to repeat the cycle.

Evacuated Tube Solar Collector (Click to Enlarge)

Evacuated Tube Solar Collector (Click to Enlarge)

That’s it for this week.  Next Wednesday will be much more complicated. Gal will be explaining collector efficiency and how that effects the choice of panels. Warning: There will be calculus.  If you have any questions about any post, you can always contact Gal, and he’ll be happy to explain.  To keep up with the latest posts, you can also join our Facebook page and start a discussion there.  Thanks.

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