View Single Post
Old 06-28-15, 07:01 PM   #23
Mikesolar
Master EcoRenovator
 
Mikesolar's Avatar
 
Join Date: Aug 2012
Location: Toronto
Posts: 958
Thanks: 40
Thanked 158 Times in 150 Posts
Default

There are a few reasons why I don't like tubes. The biggest one is that there is a HUGE temp difference between the inner and outer glass and nature hates a vacuum so it cannot help but fail faster than a flat panel.

Aside from that, there are 2 main types of vacuum tubes, one has the heat pipe (much described elsewhere on here) which has a very low liquid volume and only in the header, and the other one is a U tube or annular tube in tube (most common being the U tube. You cannot use the U tube as a drainback as the liquid stays stuck in the U.....duh (bit self explanatory) and the glycol/water will not steam out of the lower part so it will degrade very fast.

Vann, the picture you have shown is the best for a drainback system (or pressurized glycol). The stagnation temps are seldom above 200C where with the better vacuum tubes are around 300C and you cannot use anything above 100C anyway. A flat panel with a painted absorber will stagnate at, perhaps, 160C and still give good performance when the water temps are around 80C (the most usable temp), even nearly as good as the highly selective surface (as used by Viessmann, Wagner and many others).

The Germans have this crazy desire to get the most out of a panel so they made them so they will absorb 95% of the sunlight and emit 5% back to the atmosphere. A painted panel will absorb 95% but emit 20% back. In the end, it doesn't matter as the panel will still get up to 80C with ease. The rest is about insulation.

My belief is that if you want 6m2 of high efficiency panel area, put in 8m2 of not so efficient panel and have a system that runs at a lower overall temp, glycol/panel/pump etc, will last a lot.....longer.
Mikesolar is offline   Reply With Quote