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Old 09-10-20, 07:02 AM   #1
Hong Kong
Join Date: May 2010
Location: Hong Kong
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Default Drain-back system and evacuated tubes

I am researching about drain-back, and how to make the most of it:

Efficiency and simplicity (no heat exchangers, no glycol anti-freeze, no freezing, no boiling, no auto-air-extractors, less lag and so on.)

I don't have any experience yet with drain-back, but I am aware of some of the classic mistakes.

Also, I don't have experience with using or installing evacuated tubes. Are they generally suitable for drain-back? I need to be absolutely sure there are no chance of damage from water NOT draining, thence freezing when it gets cold outside.

I will not have a hot water tank, but a hot water buffer. The difference mainly being that it's the bulk of water in the tank that will be common with the solar, the radiators, and underfloor heating, while the domestic hot water is what will be in a coil, inside the tank. This way, the hot water will be "freshly produced", always flowing (when in use), so there is no "sump of half rotten water". Well, the half rotten water will be that circulated in the system, not what I shower in.

The bit of it about the hot water buffer I am quite confident about, but not so much that of evacuated tubes and drainback. I am going to make an installation of compound parabolic mirrors for the evacuated tubes, and there will a specially designed filter in front, which helps to align diffuse light, for even higher efficiencies in challenging situations.

My main goal is to optimise efficiency, but especially the efficiency when it's 1) cold 2) overcast/foggy and 3) low light. Making usable solar thermal energy, in freezing overcast weather might sound impossible - but it's actually not. Especially not for radiant underfloor heating, all you really need is 30 C / 85 F from the solar panels. You will get a lot more on a sunny day, of course, but that's not the point. The point is - how much can you get on a cold and overcast day? Can you store enough thermal energy to avoid any auxiliary energy source?

My point is, if the technology is right, and the area is sufficiently large - useful solar can be made in almost all situations. Snow storms will probably be the exception, as well as obviously night time (I am not going for a broad spectrum solution, like infrared). But with a combination of diffuse light concentration, evacuated tubes, thermal storage and a few other added tricks, I just need to make it thought the winter.

Again, my main question is about drain-back and evacuated tubes - can I rely on the liquid section of the evacuated tubes manifold to drain properly?

Thank you all for your drainback. I mean, feedback!

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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