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Old 06-25-15, 05:53 PM   #21
vann
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Mike, thanks for your insight, and for the tips.
I did some reading after your post, trying to learn some more. I still don't understand much of the stuff. I have some questions, if you're willing to explain.

You say not to use tubes when doing drain back system, only flat panels. Is this because flat panels are more heat resistant (to stagnant phases)?
As far as I understand, flat plate collectors are copper pipes running through a glass covered collector. Through these copper pipes runs the heat exchange fluid (water & glycol mixture, or just water in drain back systems).
Do I get it right? Are the copper pipes empty until fluid is pumped inside, or are they sealed with some proprietary fluid inside? (like evacuated tubes)



Since evacuated tubes works differently, I guess that's why they are not as good to use with drain back solar systems.
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You need a 20-30L tank (placed as high as possible) as a water reservoir for 6m2 of flat panels (do not use tubes)

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Old 06-28-15, 09:02 AM   #22
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I think if you drainback evacuated tubes and then the sun comes out, the glass overheats and destroys the tubes because there is no place for the heat to go. Can anyone confirm?
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Old 06-28-15, 08:01 PM   #23
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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.
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Old 06-28-15, 08:08 PM   #24
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Quote:
Originally Posted by MN Renovator View Post
I think if you drainback evacuated tubes and then the sun comes out, the glass overheats and destroys the tubes because there is no place for the heat to go. Can anyone confirm?
That is essentially correct. There is a very high failure rate among tubes.
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Old 07-15-15, 05:46 PM   #25
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Default Stagnation is NOT an issue

My company made Drainback Pro panels and tanks in North America, before finding an improved solar hybrid method called SunPump.

Drainback is not subject to stagnation, because water is not left in the panels by default, it is pumped through when the conditions are right. The panels are empty during temperatures outside the parameters, freezing and boiling extremes are not possible when the absorber pipes have drained and only contain air.

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Old 07-17-15, 06:43 AM   #26
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Given the low cost of PV, I am seriously looking at PV based water heating. I have looked at the other European refrigerant based systems like the SunPump, and my installer friends over there put it on par in terms of efficiency, cost and overall installation issues with glycol based thermal. This is not to say that they do not have their place but for the complexity of ANY fluid based system compared to PV, it is tough to recommend.

It used to be that there was few technologies to accomplish water heating. Not any more.
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Old 07-18-15, 02:10 AM   #27
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Regarding the previous post. I've had various different chats with local solar folks on this and their suggestion has spend the money and/or space you'd use on solar water heating on PV if grid-tied. The idea is if you go with a heat pump water heater or if you go with a tankless heater with small diameter hot pipes (1/2 or 3/8) to increase FPM water travel to reduce time to delivery and reduce surface area of the pipe that you'd save more than using a solar hot water tank with an electric backup heater because the PV will capture energy when the hot water system might otherwise not be able to capture it while maintaining a lower initial cost(equipment, piping, pumps, labor, water permit) and get a more reliable setup with less maintenance. The cost of PV has dropped but water heat hasn't. I suppose in areas other than MN where the sun shines more consistently with less snow cover it might make more sense but at the same time that PV would capture more too. Solar hot water with integrated space heat would be an exception to this because you're likely to use more of the energy available for capture, although a system sized for that would be more of a waste in the summer unless you have a process heat use or absorption chiller.
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Old 07-18-15, 02:38 AM   #28
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Quote:
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The cost of PV has dropped but water heat hasn't.
I think you have put your finger on the key point here. When I installed my solar thermal system a few years ago the economics clearly showed that was a better option than PV. A significant drop in PV prices, plus improvements in heat pump technology, has apparently changed that.

Quote:
I suppose in areas other than MN where the sun shines more consistently with less snow cover it might make more sense
Snow cover is good, in the right situation. My panels are on the south-facing wall of my house and the ground slopes up to the house. At the time of year when there is snow cover the sun is very low in the sky and is reflected off a large area of snow onto my panels. I get maximum production on bright days when there is snow cover, despite there being fewer daylight hours at that time of year.

Quote:
Solar hot water with integrated space heat would be an exception to this because you're likely to use more of the energy available for capture, although a system sized for that would be more of a waste in the summer unless you have a process heat use or absorption chiller.
My system provides mainly space heat. In my climate there is a relatively short period when no space heat at all is required, so this makes more sense here than somewhere with a longer summer.
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Old 07-18-15, 04:49 AM   #29
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I am still riveted on the possibilities of the hybrid pv thermal panels. In a place like Minnesota or Alaska, a hybrid setup could do wonders if configured for heating only. The pv could run a compressor, the thermal could help out the evaporator. When the system was not active, the pv could source other power needs, and the thermal could be stored. When the unit got called into action, the thermal store would drastically improve efficiency until the stored heat was extracted. At the very least, the added thermal energy would lengthen the time the heater operated before it needed to defrost.
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Old 07-18-15, 06:43 AM   #30
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Jeff, this is exactly why I built my first PVT panels a few years ago. The real issue now is the amount of equipment needed to do the job. As we reduce our heat load, the equipment can make sense but still, there is a lot of piping, pumps and storage to put in. Economics come in to play so we found that the best is to do pool heating with it (so far). Using it with a heat pump was always on my mind though.

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