Durability of the solar panels/collectors in Drain Back Solar Hot Water Systems

[vann]

I am considering drain back solar system for domestic hot water, and was wondering about durability of the solar panels (collectors) when used in such system.

This video seems to do good explanation of the Drain Back Solar Systems principle:
http://youtu.be/qOXcbsjUOZw

As I understand the principle of Drain Back Solar Systems, once the set high limit temperature for the drainback tank water/glycol is reached - the controller shuts off the pump and stops the water flow. This stops the tank water from overheating, but then the collector itself is not transferring the collected temperature. Since this can/will happen often (depending of the drainback water tank versus the solar collector ratio), how safe is this prolonged "dry stagnation" for the collector condition and durability?

It seems that solar panels/collectors intended for use with Drain Back Solar Hot Water Systems, should be made with bigger tolerance for high temperatures. Or any collector will do?
Can somebody share some insight or experience, how durable are solar panels/collectors when used in drain back system?

[AC_Hacker]

Welcome aboard!

I think that you are the first person from Macedonia on this forum.

I must admit that I do not have direct experience with drain-back systems. However, I have not heard of solar collectors failing from the stagnant phase.

I was under the impression that the drain-back cycle would kick in at night, and also protect the collectors from freezing.

-AC

[vann]

Tnx AC, and wow - a reply from the author of the famous "The Homemade Heat Pump Manifesto"

Originally Posted by AC_Hacker

...I have not heard of solar collectors failing from the stagnant phase.

I was under the impression that the drain-back cycle would kick in at night, and also protect the collectors from freezing.

-AC

Yes, but drain back phase could also occur at daytime, depending on the system dimensioning. Namely, one could aim to design the whole system so that collector power is such that on the perfect sunny conditions, the water in the drainback tank does not get above 175-195F (80-90C). When the conditions are not as good, drainback tank water temperature would be lower - and that is good compromise to live with, considering the alternatives and costs of heat dump part of the system.
But still even when designing to avoid heat dump, one should aim to be close to 175-195F (80-90C) temperature range. With the certainty of some water evaporating eventually, there could be possibilities of daily occurrences of the drain back (stagnant ) phase.

I'd very much like to hear some do's and don'ts on designing Drain Back Solar Hot Water Systems. Yours are excellent advices AC, shame that you don't have experience with drainback systems. But hopefully somebody else can help.
If the thread does go in that direction, maybe admins or moderators could rename this thread.

[jeff5may]

There are many materials that will work with a drainback collector. If you are purchasing panels, the manufacturer will have specs and charts to help you do your design homework. However, if you DIY the collectors, that means more design homework up front for you. There are lots of case studies and plans at build it solar where others have tread the path you are taking.

If you install too many panels for your heat load or storage capacity, stagnated panels will exist. If any panels aren't strong enough, you will find out fast. If you don't have a relief mechanism, the system could explode from the high temp and pressure. Not a trial and error friendly rig. Extra homework up front and conservative overbuilding practice will eliminate bad days for you.

[SDMCF]

My solar hot water system is not a drain-back system, but maybe some of my experience may apply anyway.

I have had the system stagnate & overheat when the power failed - my system relies on a pump for circulation - and this would be the same overheating panel scenario you are concerned about. The system did indeed overheat, to the point where it was producing hammer in the pipework and I was genuinely concerned about an explosion or at the least a burst. Thinking about it later I don't understand why the pressure relief valve did not activate. Either it is faulty or there I have made a design error in the layout.. Either way, it was not a nice experience. The end result was no major damage, just one evacuated tube lost all its thermal covering on the pipe inside the glass. It could have been a lot worse.

It could be a good idea to design in a method of losing any excess heat produced when your tank temperature gets up to your required maximum. If the power is on I have 2 possibilities. One is to lose the heat into my central heating system and the other is to send the heat to my front steps which I keep clear of snow using solar heat in the winter. Not sure how much heat the steps can take but the central heating can certainly take anything the panels can produce. Neither of these options is effective if the power is off though. Think about that in your system design.

[vann]

I will definitely be purchasing commercial made panels. But the thing is, there are really slim pickings at local shops in my country.
I'm still researching and learning about differences between collectors compatible with drawback system, and those intended only for closed loop systems. It seems there are certain requirements needed for collectors that are to be used in drawback system. Like sturdiness to high temperatures for withstanding the stagnant daily phases, and absence of places/pockets where water can stay and not drain down in the drainback reservoir.

I'm trying to find local contractor company to design and deploy the solar system for me, but thing is there aren't none here that do drainback solar systems. And not many such here that are even willing to talk about trying.
So I'm left with DIY option, which means I would have to design the system myself and install it - hopefully with help of local installer.

Originally Posted by jeff5may

If you don't have a relief mechanism, the system could explode from the high temp and pressure. Not a trial and error friendly rig. Extra homework up front and conservative overbuilding practice will eliminate bad days for you.

Yes some kind of relief mechanism must be present. After startup from a sunny stagnant phase, I guess there will be quite some steam - when the water hits the collector heating elements.

[vann]

Originally Posted by SDMCF

Thinking about it later I don't understand why the pressure relief valve did not activate. Either it is faulty or there I have made a design error in the layout..

My guess is that the pressure relief valve was faulty. Ether that or it was specified for activation on bigger pressure than your system is able to produce when overheated.
Yes, I'd say that this is very important point. One that could maybe be simulated when installing the system (dunno if the glycol can be reused after?)

Your example is why I want to go with drainback solar system design, instead of standard pressurized closed loop with glycol that everybody and their mom has. When public electric power is off, to address the glycol overheating problem some kind of backup power for the pump is needed, but sometimes it's possible that power will be off longer than the backup capacity (UPS with inverter, or something like that). And than you will have the same (big) problem.

That's the beauty of drainback system, when public electric power is off - the exchange fluid (destiled water in this case, instead of glycol) will simply drainback down in the drainback water tank. Yes, the collectors themselves will be exposed to much increased heating - but the rest of the system is safe.

Originally Posted by SDMCF

It could be a good idea to design in a method of losing any excess heat produced when your tank temperature gets up to your required maximum. If the power is on I have 2 possibilities. One is to lose the heat into my central heating system and the other is to send the heat to my front steps which I keep clear of snow using solar heat in the winter. Not sure how much heat the steps can take but the central heating can certainly take anything the panels can produce. Neither of these options is effective if the power is off though. Think about that in your system design.

You need some kind of a heat dump for overheating protection. But when the public electric power is off, I don't think it will be the boiler that will overheat - only the heat exchange fluid (glycol I guess) in the upper part of the tubing. Not sure how corrosive glycol is, but it will for sure be quite hot (and not funny for anybody using your steps at the time).

[NiHaoMike]

What about power the pumps from a PV panel?

[vann]

Was thinking about that also, but than I'd have to source the PV panel and the DC powered pump from ebay/aliexpress... and I prefer to source locally, for the guaranties (and possibly for the installation/design service from a local installer/supplier company).

After all, drainback variant seems so much more simple and easier for maintenance. And cheaper too, with glycol needing replacement after some exploitation period, and closed loop systems needing more expensive electronics in general (at least, that is as how I understand things so far).

[vann]

Originally Posted by jeff5may

If you don't have a relief mechanism, the system could explode from the high temp and pressure. Not a trial and error friendly rig. Extra homework up front and conservative overbuilding practice will eliminate bad days for you.

After looking again the bellow video, it seems that pressure relief mechanism is not needed with drain back systems. Because the system is not pressurized. As I understand it, the drainback water tank is not pressurized.
http://youtu.be/VlAfTYTA0Z0