Steamback System Design

Sunnovations

Steamback, PEX and Sunnovations

Great Thread!



The Sunnovations system also uses overheat protection mechanism similar to the steamback. However, in closed loop glycol systems, the system pressure can still be quite high - and hence increase the boiling point of the fluid - so care should be taken to keep it under the critical temperature for glycol breakdown (~275'F).



In the Sunnovations system the fluid is purged from the collectors the moment the pressure in the system exceeds ambient pressure (i.e. ~2 psi). This definitely keeps the maximum boiling point below 220 F (for a 50/50 mix) - so far away from the critical temperature for glycol.



As the pressure in the system never exceeds a few psi - PEX can safely be used in the Sunnovations system (by the way: PEX can still handle 80 psi at 200'F).



The reasons for using PEX are: lowest material cost for the pipe run (compared to copper or corrugated stainless tubing) - while the flexibility of the PEX tubing and lack of need for fittings in the solar pipe run make it easy to install.

Kevin_in_Denver_2

Great Steamback Article by Fortunat

http://www.renewableenergyworld.com/rea/news/article/2011/08/steamback-shows-promise-for-solar-water-overheating

thoughtfulTom

Don't ever use PEX on solar!

Most seem to have learned this, but still I see people pushing it to "save" money (one explosion and you lose every cent you ever saved, plus your reputation). I let a general contractor talk me into it on the cold run to a drainback system. It MIGHT have worked. But the load wasn't enough (mis-sized heat exchanger) and the system heated up. I saw what was happening and turned it off.



It exploded while draining down!!



Solar and PEX do not mix!

thoughtfulTom

life of evacuated tubes

Hot Rod asks how long evacuated tubes last. My research (and two different evacuated tube manufacturers) tell me 15 years max.



We have switched to flat panels. Rumors of a lifetime guaranteed tube, but it isn't here yet.



Still have to figure out what to do with the customers that have evacuated tubes. We will survive the warranty, but we design/intend lifetime systems, not 15 year systems.

Sunnovations

PEX on Solar?

I generally agree one should not use PEX on Solar systems. But the as explained: the Sunnovations system can not run a positive pressure (so nothing can "explode") and temperatures are kept under 200 F (by physical principles). We have never had a problem with PEX bursting in any of the many systems installed - even fine in Phoenix, AZ.

Kevin_in_Denver_2

Vented or closed Drainback?

A closed drainback system can build up pressure, but an atmospheric tank wouldn't.

So this must have been a sealed system?



We are very interested in hearing more about evacuated tube reliability. Please start a new thread, and by all means, name the manufacturers.

thoughtfulTom

Drainback

Kevin,

Yes, it took 3 errors on my part to cause a problem - the obvious one was I let myself be bullied by the contractor into doing shoddy work (PEX in a solar loop).



2. I undersized the heat exchanger



3. I used a standard 150PSI pressure gauge. I had specified a 50 pound gauge, but my crew installed a standard gauge.



I like that it take 3 mistakes on the same point for a drainback to cause you grief. Compare that to the multitude of simple mistakes that can KO a glycol system.



But when the pipe blew up, it was me that paid for it and the drywall and the paint, even though the root cause was the contractor decision to require PEX. Shoulda coulda woulda put that in writing...

thoughtfulTom

life of evacuated tubes

My thought process was 1)glass sags (meaning it moves) and 2)Nature abhors a vacuum. Endless web searches later I began to get the idea that eventually the vacuum would be diminished by air molecules moving through the glass (I speak here of two wall evacuated glass tubes - the best solution to the dissimilar materials problem).



At some point the vacuum will become so diluted as to lose its roughly R-45 value. Even at that point, you still have a collector as good as a a flat panel (but now the whole concept of net aperture area becomes very, very important).



Anyways, my thoughts were verified by a couple of manufacturers. One puts it in their marketing material:

http://www.americansolarworks.com/Marketing%20Documents/ASW-58A-22_30.pdf



The other doesn't seem to publicly state the limitation (I haven't looked that hard). It is a question of physics, so it applies to all borosilicate tubes, as far as I know.



I don't know that there is much more to say about it.

oldbarns

drainback EV tube with PV pump

in the interests of simplicity im thinking of installing a draindown system using a PV driven pump with Evacuated tubes

I have found a great pump  [u][size=12][color=#0000ff]http://www.solarco-op.net/pv_pump_spec.htm[/size][/color][/u]   eco15pv

Can you have a look at the attached please as to the design

 

The Tubes and the PV will be just above the tank  (yet to purchase but I have been told 30 odd at 58mm would be best) (plus the PV)

 

The tank is in and working and the input and output pipes are available,  the connections are open to the tank, no coils for this

My woodburner also has a pair of connections open to the full 350l

 

I have a loop in the tank from an oil boiler for backup    and 1 loop for UFH   and a further loop with mixer for mains hot water

 

The water in the tank has anti corrosive treatment but no antifreeze

 

This is a drainback system based on the diaphragm pump offering minimal resistance to the drainback and the inverted breather pipe above the manifold giving opportunity for venting and air ingress during drainback

 

I know its simple and may not be quite as efficient as a managed system but with a pair of 3 way T valves (12vDC) managed by a stat on the tank I cant see any great issues but im looking for a second opinion please in case I have missed the point

Is there anything in your opinion majorly wrong with this setup please?

 

Regards rob