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Old 09-25-20, 04:10 AM   #8
Hong Kong
Join Date: May 2010
Location: Hong Kong
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Originally Posted by jeff5may View Post
Drain back is not generally recommended with evacuated tube collectors. Mainly two reasons:
First, stagnation temperature can literally melt down gasket and sealing materials. When the sun shines brightly and there's no fluid to absorb the heat, temperatures in the collector can skyrocket to over 150 degC. If there's any liquid pooled in the collector, it expands to steam, which causes more things to cook and melt.

The other big problem is cold shocking. When the collector is under full sun, and fluid pumping stops, you get the previous paragraph. Stagnation and steam generation and pressure and yadda yadda yadda. When the fluid starts pumping again, all of the hot collector plumbing gets a sudden blast of cold fluid. At first, your manifold acts as a large coffee maker boiler, boiling and spewing shots of hot water with steam downstream. If the pump is strong, it pushes the hot froth down the pipe, overcomes the boiling action, then the boiling froth is replaced by lower temperature fluid. This heat shocks the whole plumbing all the way to a mixing vessel (if there is one).
Thank you for that insight, Jeff5May.

I will have more than one sensor inside a manifold (that is, not on the exit, but built into the panels themselves. I will have plenty of user capacity to sink the thermal energy. If for some reason the system drains back, I won't let the liquid back until the temperature is under control. Ideally, the liquid would be flowing before it heats up at all.

I will even do more, like using compound parabolic mirrors, inside an insulated box. I am looking for the most heat resistant tubes but I do expect that I will blow some tubes, during testing.

The point of these panels are that they will be operating even at very low (and diffuse light), which is the most significant drawback of both electric at thermal solar panel. At least with current technology. I intend to change that, and yes, there isn't any RD without failures and mistakes. I am asking here first, though, to avoid as many mistakes as possible and thus, your feedback is most appreciated.

This is too important to develop that I won't let myself be held back by challenges, but rather figure out how to best overcome them.
Space heating/cooling and water heating by solar, Annual Geo Solar, drainwater heat recovery, Solar PV (to grid), rainwater recovery and more ...
Installing all this in a house from 1980, Copenhagen, Denmark. Living in Hong Kong. Main goal: Developing "Diffuse Light Concentration" technology for solar thermal.
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