Drain-back system and evacuated tubes

[jeff5may]

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).

[SDMCF]

Jeff, how do you see such problems happening? I have no experience with drainback systems so I can only guess.

It seems to me there are 2 ways: a failure to pump fluid into the collectors because of pump or power failure, or collector temperature rising rapidly before the system can react and pump fluid. Have I missed something?

Failure to pump is a problem in any system, so not unique to tubes or drainback.

Slow reaction at first seems surprising, but thinking about it I can understand how this might happen. The temperature sensors on mine are at the (outlet) end of the manifold. Without fluid in the system to dampen the temperature rise the end of each tube could conceivably get very hot before the temperature rises is conducted as far as the temperature sensor. If I hold in my hand the end of a tube that is normally inserted into the manifold, with the tube in full sun the end quite quickly becomes too hot to handle.

I had not considered that before, so I was lucky that I did not (could not with my layout) opt for a drainback solution.

[osolemio]

Originally Posted by SDMCF

Jeff, how do you see such problems happening? I have no experience with drainback systems so I can only guess.

It seems to me there are 2 ways: a failure to pump fluid into the collectors because of pump or power failure, or collector temperature rising rapidly before the system can react and pump fluid. Have I missed something?

Failure to pump is a problem in any system, so not unique to tubes or drainback.

Slow reaction at first seems surprising, but thinking about it I can understand how this might happen. The temperature sensors on mine are at the (outlet) end of the manifold. Without fluid in the system to dampen the temperature rise the end of each tube could conceivably get very hot before the temperature rises is conducted as far as the temperature sensor. If I hold in my hand the end of a tube that is normally inserted into the manifold, with the tube in full sun the end quite quickly becomes too hot to handle.

I had not considered that before, so I was lucky that I did not (could not with my layout) opt for a drainback solution.

There are advantages and disadvantages to both systems. If drainback is done right, I sincerely believe it will eventually be simpler in construction, more reliable, more failsafe, cheaper to build and maintan and even more efficient.

I am really tired of watching energy lost in heat exchangers. I need a better system, and that is what I will build.

[osolemio]

Originally Posted by jeff5may

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.

[solarhotairpanels]

Originally Posted by jeff5may

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).

Jeff how are you doing?

I've had my drain back system in place for over 7 years now and no issues at all even with stag temps going off the wall..
Just wanted to chime in on your thoughts

2 - 20 tube evacuated tube collectors
1 - 6 gallon used electric hot water heater for drain back tank with site glass (electric disconnected
1 - 40 gallon used Superstor storage tank with heat coil in bottom connected to collector piping
1 - Grundfo's circulator pump
Resol solar controller... allow me to slow pump / water speed down which works beautiful. // uses less watts at lower speed and water picks heat quickly running thru evacuated tube collector manifolds.

Couldn't be happier..
Hardly no maintainence at all other then flushing system with vinagar now and then to remove rusty water from solar loop and now and then I have to add water to the loop because it evaporates over time.