Steamback System Design

Kevin_in_Denver_2

"Steamback" is the word now being used to describe what happens to closed-loop solar systems during stagnation. If you correctly size your expansion tank and relief valve, the fluid in the solar panels will vaporize and push all the liquid out of them, thereby protecting the glycol from high temperatures.



Siggy's first article on the design criteria for these systems was published in PME a few months ago, but the original research was done in 2003 in Germany by Hausner & Fink.



Besides protecting the glycol, it provides an elegant means of overtemperature protection, and eliminates the need for inelegant "heat dumps". The controller simply lets the panels stagnate, and the excess heat is thereby rejected before it is collected.



I think I know why we didn't think of this way back in the 80's: We were used to thinking of solar panels as boilers and were scared to death at the thought of "dry firing".



My questions:



1. Has anyone done a system like this, and are you satisfied with it?

2. Who was the person who coined the term "Steamback"?

hot_rod

Schuco

is where I first heard of the concept.



My concern is how well the glycol can handle stagnation temperatures, regardless of steaming back or sitting in the collector. At 25 psi the glycol flashes to steam around 250F, I think.



Do they require a special fluid to "steam" and does, will this fluid pass the FDA low toxicity requirement?



I'm interested in the concept but i don't hear much about it. I wonder why more solar companies are not pushing it? I will quiz some of the German engineers and solar folks I know.



hr

Kevin_in_Denver_2

SunMaxx

SunMaxx just started teaching it:

http://www.solarwebinars.com/11082010-fundamentals-in-steamback-design/



HR, I just read your post from 2007: http://72.3.142.104/forum-thread/104424/Buffer-tank-question



Sounds like Shuco started putting the German research right to work. I believe they are saying that the water boils before the glycol, and the water steam pushes liquid glycol out. So during the steamback event, only water steam is in the panel. Some glycols are apparently rated for continuous operation over 350F.

Fortunat

not sure, but I'll claim it if noone else does

Kevin,



We've installed close to 1,000 solar hot water systems using this controlled stagnation strategy in the last 5 years. As long as you get the details right (expansion tank size, location, and plumbing geometry), it works great.



Back in 2006 we were struggling with systems that would overheat during peak summer times when families went on vacation and because we install lots of evacuated tube systems, the conventional techniques of leaving the pump on at night etc failed to fix the problem and adding a dump zone to every DHW system looked pretty unattractive.



We did a lot of research and after I came upon a series of articles by Hausner and Fink et al, we built a test rig in our shop (we called it the 'solar roller' because it was an evacuated tube collector plumbed in a closed loop with just a site glass, plumbing and an expansion tank and  mounted on wheels). For the summer of 07, we put this rig in and out of stagnation in the parking lot out front about 100 times and tested the glycol pH every now and then and also calculated steam volume and how far down the pipe the steam goes etc. Once we were satisfied that it worked well, we started making this standard on all our SHW systems.



At the risk of over-reaching, it is possible that I coined the phrase 'Steamback', though I wouldn't argue if someone else claims it instead. For sure, I believe that I am the one who posted that Hausner and Fink article (and the one who renamed it when I downloaded it to say 'steamback' so I could find it again in my archives). For a while we also called it 'vapor drainback' or 'intrinsically safe system design', but Steamback proved the most popular and catchy around the shop so we've been calling it that ever since.



For the longest time, I couldn't find anyone in this country who would agree that it made any sense, but everything I read about the german solar industry refers to it as standard practice (i happen to read german so I read a lot of european solar literature). For example, all the stuff from the IEA regarding solar combi systems tends to refer to this method and most expansion tank sizing guidelines refer to implicitly (by adding collector volume to the required expansion volume).



 Recently, I've seen a bunch of manufacturer's make reference to it in their literature in this country too. A couple of those I think I can take credit for having turned on to the concept, but certainly not most of them.



If anyone has a good reason why NOT to do this in a typical SHW system, I'd be intersted to hear it.



~Fortunat

www.revisionenergy.com

Fortunat

very easy on the glycol

Rod,



our experience is that 'steamback' is actually  very easy on the glycol (certainly compared to keeping the fluid in the collector at high temp). That is the whole reason we do it.



If you read the literature you'll see they often refer to 'collector emptying behavior', or systems where a small amount of vapor that develops in the collector can push the rest of the fluid our of the collector before it also boils.



 For systems that have this kind of good emptying behavior, only a very very small volume of glycol is actually ever vaporized. Considering that the steam to water volume ratio is something like 1000 to 1, for most stagnation events, less than a teaspoon of fluid actually boils.



In our 'solar roller' testing we also confirmed what others have said that it appears that even what actually boils is actually mostly the water (sort of a partial distillation event). Because our solar roller had a couple site glasses on either side of the collector, we watched the fluid recondense each time we pushed it back inside. Interestingly, the condensing fluid was almost totally clear in color, which indicates to me that what boiled was mostly water (not the glycol).



Anyway, I can't say for absolute certain that there is no negative effect on the glycol, but if there is, we can't measure it. In addition to the 'solar roller' test, we've put our shop system into stagnation a couple hundred times over the last few years and after that the glycol pH is still exactly the same as it was the day it was commissioned.



Having said all that, if the collectors don't empty well, the glycol turns nasty fast. That is an important lesson to learn.



~Fortunat

www.revisionenergy.com

hot_rod

what type of tubes

are you using, heat pipe or U tube? Will the steamback work with all types? What about stagnation inside an empty evac tube header, what temperatures do you see and any long term effect of those temperatures on the components? Some of the Euro testing indicates stagnation temperatures in excess of 500F!



I'm concerned with 5, 10, 15 years down the road. The ugly glycol systems I have worked on were installed in the 90's. Unmaintained no doubt, but when did the glycol go bad along that time frame. I sure it is related to how often and how long they were in stagnation conditions.



I am still of the opinion a non glycol system best serves the overheat and stagnation issues. A water based drainback is simple and most efficient.



Are there any drainback evac tube systems being marketed by manufacturers?



hr

Fortunat

Definiely NOT U type

Rod,



The U tube evacuated tube collectors are NOT a good match for steamback (or drainback) systems. The heat pipe tubes generally are.



Likewise, on flat plates, the typical Harp type flow pattern tends to have good emptying behavior while the modified double harp (like the Stiebel Eltron SOL25) tends to have poor emptying behavior, except if installed upside down (which makes them hard to purge).



Basically the literature shows (and our experience seems to confirm) that if the collector geometry is favorable, that only a very small amount of fluid ever actually has to boil in order to clear the collectors completely. On the other hand if either the collector geometry (or the near collector piping) is unfavorably configured, then the boiling phase lasts a long time and lots of fluid is exposed to those high temperatures. In those cases the glycol degrades very rapidly (not to mention that the steam travels a long distance back in the piping and often the system makes a lot of noise in this phase).



As far as I can tell, once the vaporization phase is done, the collector is basically in the same condition as it would be in a drainback system (except that instead of being filled with moist air, it is filled with a dry superheated steam). So as far as I can tell, if a collector is designed to withstand dry stagnation in a drainback system, it should also be ok in the steamback condition.



The non glycol option is proven and works fine, but I think pitched collectors and piping often look like crap on the roof and they make installation a pain, especially for larger systems installed on flat roofs. Most of the old drainback systems we maintain have been 'antifreezed' in the meantime as a backup protection and at that point, it seems like you might as well be in a closed loop, pressurized glycol system.



Especially for DHW systems which might only see this stagnation condition a couple hours a year, steamback seems like a great and cost effective option for overheat control.



by the way, the Sept issue of SunWind Energy magazine (www.sunwindenergy.com) has an article on this topic which pretty much sums up the overheat control options and comes to similar conclusions as we have. Of course their perspective is a European one and drainback is still largely unknown and unused in Europe.





~Fortunat

www.revisionenergy.com

hot_rod

double pane flat plate collectors

are another concept getting more attention. I see a glass manufacturer in Michigan is building double pane windows with a vacuum in between instead of argon, krypton or xenon gas. They measure 3/16 inch thick or about 1/2 of a typical double glass window. Small glass beads or "pillars" keep the panes from sucking together.



This could bring the high operating temperatures and lower loss to flat plate collectors. The plate style collectors would present more absorber area to the sun compared to an evac tube array of the same gross area.



Exciting times in the solar thermal business.



hr

Kevin_in_Denver_2

Other "Not so Hot" Ideas

I guess the Europeans are a few years ahead on this one.  An article from last year's Solar Professional  investigates some US overtemperature protection schemes that are ugly or inelegant or both, see attachment.



No disrespect to the brilliant guys in the article, it's just that steamback is simpler.

Kevin_in_Denver_2

SunMaxx is OK with Drainback and Evac Tubes

SunMaxx engineers won't discourage drainback  with their evacuated tubes, but they aren't heavily promoting  it either.  FWIW, I couldn't break a SunMaxx with thermal shock, but I did break an Apricus tube.  The Apricus was from 2005, I'm not sure if they've updated their design.



Although dry stagnation may go to 500F, "steamback stagnation" won't be nearly that hot because the boiling/condensing process can move a lot of heat away from the header to system piping.  I've never seen anything higher than 380F.



I am concerned about the effect that 380F has on the foam used in the typical evac tube header.   The SRCC stagnation test isn't a long-term test.

Kevin_in_Denver_2

and the winner is...

It looks like Fortunat did coin the term, if not the theory behind it.   Sometimes you need a catchy name to make an innovation catch on.



And I apologize for not listening so well.   Fortunat's been trying to explain it to us for at least a year:  http://www.heatinghelp.com/forum-thread/128278/Glycol-turns-to-steam

Fortunat

which ET collectors are foam insulated

I am concerned about the effect that 380F has on the foam used in the

typical evac tube header.   The SRCC stagnation test isn't a long-term

test.



Kevin,



Are SunMaxx collector headers foam insulated? Apricus is rock wool, which I think is fine to about 350 deg C or higher.



But I too wonder about long term effects of collectors sitting at high temperature (either in drainback systems or in the steamback configuration). I don't know of anyone (manufacturers or third parties) who do long term testing of this type.



In single wall evacuated tubes, I've seen some systems with very high tube failure rates. The manufacturer (which subsequently went bankrupt) speculated that it may have been the result of high collector temperature accelerating the off-gassing of the copper heat pipe, which can result in loss of vacuum. That was the first I'd ever heard of that. This is one of the collector types that claims to have built in stagnation control via thermally activated snap disc in the heat pipe, but ironically while that lowers manifold temperature during stagnation, i think that device actually increases the stagnation temperature at the absorber.



 This particular mechanism obviously isn't a problem in the double wall ET collectors because there is no copper parts within the vacuum, but their may be other issues lurking.



I hope that sometime soon the EN or SRCC standards and tests will start to incorporate some kind  of accelerated life testing of collectors to give installers and end users confidence that not only will the collectors perform as advertised the day they are installed, but that they'll also stay that way for the 20 year project life of most solar thermal systems.



~Fortunat

www.revisionenergy.com

Kevin_in_Denver_2

Dry stagnation OK for flat plate collectors

I have a test bed going since 1982, dry stagnating most of  the time, and the panels are fine.   They have fiberglass insulation between the absorber and polyiso foam insulation, tempered glass, brazed copper absorber with black chrome selective surface, EPDM gasket, and aluminum box frame.



Thanks for pushing the envelope, Fortunat, eventually the manufacturers will integrate everything for minimum cost and maintenance, and for maximum longevity.   It's been a strange 40 years of product development.  Thanks also for educating us.



http://greenbuildingindenver.blogspot.com/

hot_rod

I'm always asking installers

if they can point me to any 15, 20 year or older evac tube systems. There were a handful of evac tube manufactured and installed in the 80's. I've only had one taker and he worked for the company that built the old Sunmaster tubes. Although he admitted to replacing some over the years.



If evac tube start failing in 10- 15 years or less it will not look good for the solar industry, and will turn into an expensive lesson for homeowners and installers.



I'm not sure how to do accelerated testing under conditions that are expected on rooftops around the world?







hr

Mark Eatherton

Interesting concept....

But like with drain back solar systems, what happens if a load shows up at 2:30 PM, and the panels are in stagnation? Does it sound like a 747 is landing on the roof? That can't be good for ANY component...



Give me good ol' drain back technology. If I can't do a drain back, then maybe steam back is applicable, but looks like its asking for trouble to me...



I think the primary reason the Germans invented steam back is because their collectors are typically series looped, which is not conducive to the drain back principles. No?



And granted, drain back doesn't work in every application, but it is a proven means of stagnation issue avoidance with very little negative consequences.



ME

Fortunat

Controls

Well that isn't really a problem is it?

just like a drainback system, if you are using steamback then you should set up the controller so that it won't pump if the collector exceeds the heat transfer fluid's boiling point. Pretty much every controller out there has that functionality, right?



~Fortunat

www.revisionenergy.com

Kevin_in_Denver_2

Accelerated Aging Testing is common

"Accelerated aging testing is performed on products to determine information on shelf life and to document expiration dates.



Accelerated aging is a testing method used to estimate the useful

lifespan of a product when actual lifespan data is unavailable. This

occurs with products that have not existed long enough to have gone

through their useful lifespan: for example, a new type of car engine or a

new polymer for replacement joints or medical products and packaging.



The test is carried out by subjecting the product to unusually high

levels of stress, designed to accelerate the effects of normal use.

Mechanical parts are run at a very high speed, far in excess of what

they would receive in normal usage. Polymers are often kept at elevated

temperatures, in order to accelerate chemical breakdown.



Also, the device or material under test can be exposed to rapid (but

controlled) changes in temperature, humidity, pressure, strain, etc. For

example, cycles of heat and cold can simulate the effect of day and

night for a few hours or minutes.



Real time aging must be performed in conjunction with any accelerated

aging study to correlate the results found during accelerated aging."

Mark Eatherton

Wasted energy....

Somehow, my original response never made it her.



I see a half a days worth of wasted FREE energy not being harvest due to stagnation lockup.



Why not have a soft start pump program if a load re-appears during stagnation conditions?



ME

Kevin_in_Denver_2

"Stagnation Lockup" Wouldn't be Common for DHW only

With a solar DHW system, stagnation lockup, either in a drainback or a steamback system, would only happen during a summer vacation, pump failure, control failure, or power failure.



In the case of summer vacation, the uncollected energy is a non-issue because the load would only return if the family came home from vacation during the day and took showers right away.



The other cases are too rare to worry about.



In most families, 80% of the DHW load (showers) occurs just before collection starts, and if the system isn't oversized, that means the tank will stay below the 160-180F maximum.



Similarly, with a combisystem, the extra uncollected heat isn't needed in the summer, and you rarely get much extra during the heating season. That is, during heating system you rarely max out the tank temp.



Mark, by "soft start" do you mean the pump starts slowly ? Wouldn't you still get noisy steam hammer?

Mark Eatherton

I don't honestly know Kevin...

Might have to schedule some lab time up at RRCC's solar lab. :-)



In a drain back system, it would be difficult, if even possible to start slow and go up slow. You probably wouldn't be able to establish a siphon on the down comer due to all the things working against you.



And you're right, I am probably over thinking it. I just remember all those stagnated systems at solar noon I use to service, and it makes the hair stand up on the back of my neck thinking about what happens if one attempts to service it during peak insolation and stagnation. Come to think of it, I think I remember trying to overcome the steam bubble in a closed loop system with the system circulator, to no avail.



In the words of the badly missed Rossane Rosanna Danna, "NEVER MINND...."



I'll go back to the basement now and stare at the computer screen of my highly controllable boiler.... ;-)



Thanks for the conversation.



ME

hot_rod

the newest solar controllers

have a bunch of drainback functions to help with some of these issues. They have the ability to use a single high head pump on a variable speed function or two pumps in series with one dropping out after the siphon is started. I'm using the variable speed with a 15-100 Grundfos with five 4X8 collectors currently.



In the drainback mode you have a filling period which runs the pump at 100% speed to establish the siphon.



The stabilization mode, which ignores a shut off condition to eliminate short cycling while the filling cycle takes place.



An initalization period to assure there is a switch on condition.



The booster pump function fires that second pump at 100% speed for filling then drops to the one pump on a variable speed function.



New is a drain impulse setting. This allows the circ to run for a timed period, after a turn off condition, to establish head and help "suck" the collectors dry, sort of a second draindown function. Seems a bit overkill, but there must be a reason this was added? Maybe sluggish drainback water in Germany



In Wisconsin glycol drainbacks are encouraged by the local solar association. With glycol in a ladder style collector they claim there is no need to slope the collectors on the roof. Maybe 90% or more of the fluid drains out of the collector if they are mounted level, and at a 45 degree pitch, anyways. The glycol protects against installer error provides extra protection for piping in cold areas with in-adequate pitch.



I'm not 100% on board with glycol drainbacks, as the best feature of drainback is the plain water! But it does provide that extra insurance. One contractor I meet has been monitoring older collectors to see if they get a "cooked glycol" coating after years of service.



hr

Mark Eatherton

You mean my C-30 controller is a dinosaur???

Also, you said "Maybe sluggish drainback water in Germany " Don't they call that stuff Grappa. I remember (vaguely) drinking that stuff on a tour bus in Germany.



That guy that is looking for baked on glycnoids wouldn't happen to be from Denver would he? DBD?



ME

Kevin_in_Denver_2

How far away from the collector is the steam?

Fortunat,



During a prolonged steamback event, with well-insulated piping, how far away from the Apricus header does the the steam bubble push the liquid?



If we figure out this distance, then we know how far away the PEX has to be.



I've already had a PEX blowout within 5 feet of the Apricus.

Mark Eatherton

PEX in a solar loop?

Seriously???



I would not.



I'd be leary of using it within 5' of the systems storage tanks on the potable side.



ME

Fortunat

not worth it

I agree with mark...it isn't worth the danger of failure to do PEX anywhere in the solar loop. We categorically refuse to do it for our customers. see photo...we have about a dozen of these laying around the shop that we use to convince customers that they are better off spending a couple hundred extra dollars on a copper pipe run.



 I will, sheepishly, admit to having PEX in my own solar loop at home, but in my defense I didn't know better at the time and the collectors are out on the barn where a failure isn't so damaging. Also, I don't charge myself to go and repair and re pressurize it after it fails (as it has pretty much every other year once...I am slowly replacing pex with copper back from the manifold, one 6 ft stick at a time).



Our testing shows that with an evacuated tube system and 1/2 inch pipe run, a single 30 tube collector can push fluid as much as 6 ft down each pipe. The equilibrium condition seems to be something less, but during the phase change phase it seems to push down fairly far in the pipe.



If you know your expansion tank precharge pressure and your system pressure, you can calculate the total steam volume by watching the pressure gauge during the steamback event and working Boyle's law.



But I still wouldn't do PEX there. What happens, for example, if while the collectors are in stagnation some chucklehead forces the pump on? In that condition you make a lot more steam (and noise) as you keep splashing glycol into the hot collector.



so in conclusion...'do as I say, not as I do'. (as a new dad, I'm practicing my hypocrisy early and often)



Hope that is helpful,



~Fortunat

www.revisionenergy.com

Mark Eatherton

That is UUUUuuggly...

And I too tried PEX in my own flat pate system. I knew it was going to fail, but it is MY experiment, and I can do as I please :-)



I knew exactly the day it failed, because both of my dogs were extremely skittish and velcroish (stuck to my side).



Must have been one hell of a steam explosion :-)



That sample you have is very graphic. It'd convince me.



ME

hot_rod

guilty also

of trying pex on solar on my shop. Here is a piece I used to connect between the collectors. Also note how the insulation melted to the tube.



I used Wheatland Tube Mega Thread steel pipe to pipe up the collectors this summer, and 1" fiberglass insulation.



hr

Mark Eatherton

I'm betting that one made a pretty good sound when it cut loose too...

BOOOooommmmmm PHSSSSssssss.....



Cause any goats to fall over?



ME

Kevin_in_Denver_2

It's Unanimous

No PEX in a closed loop collector loop.



OK for atmospheric drainback collector loop, if you isolate the PEX somehow from the stagnation temperature conducting its way down the piping to the plastic.



There are many possible ways to do that, but that's another thread.

hot_rod

why even risk pex in solar?

not knowing where that high temperature may end in the piping, It is always a guesstimate. On a typical residential system how much $$ would be saved by using pex, over a round trip of maybe 120'? Consider also the potential for rodents to nibble on pex exposed in attics.



Other choices... copper, steel, stainless, maybe even soft aluminum coil tubing seeing as new aluminum absorbers collectors are hitting the market.



hr

Bob Gagnon plumbing and heating

Stagnation

Wouldn't all causes of stagnation be eliminated with a second pump, 12 volt to a battery, and a second high limit switch up in the collectors, just about cover everything?



Thanks, Bob Gagnon

Kevin_in_Denver_2

Properly Sized Expansion Tank

Bob,



I think that a larger expansion tank is a better and simpler solution than an extra pump and battery.

hot_rod

collectors will stagnate

when the tank(s) reach set point also. This could happen when the load (family) is gone on summer vacation, for example. 0 load highest insolation.



hr

Bob Gagnon plumbing and heating

Stagnation

Hot Rod you talked about using a three way thermostatic valve, tied to a piece of baseboard,  to prevent overheating, a guy from Maine showed me a drawing at SolarFest last year that looked like it would work. Simply add that into the loop with the redundant pump. With steamback, I wonder how big an expansion tank I would need with 120 evac tubes, and how the loop would purge the air out itself. You would think if the pump was trying when the sun was out, the anti freeze would flash to steam also?



Thanks, Bob Gagnon

Fortunat

doable but is it sensible?

Bob,



I don't think there is any question about whether or not you can protect a system from stagnation using methods other than steamback. In fact, it is what all of us were doing before we learned how to do what we are doing now. The question is, does it make sense?



So far in your system we've added:



- a second pump

- a thermostatic control at the collector to start that second pump

- a battery to power the pump

-a battery charger to keep the battery charged

- a thermostatic valve or three way zone valve for the dump zone

- the dump zone itself



With those components, you could make a nice, robust system that would have a fairly limited risk of a stagnation event. But you've also added at least $1,000 to the system as well as half a dozen new failure points (what happens when the three way valve stops seating and starts to leak through the dump zone all the time). That might be worth it in some cases, but for most residential jobs I see, that extra cost would sink the project in a hurry.



>With steamback, I wonder how big an expansion tank I would need with 120 evac tubes



depends on the collector volume, system pressure and a couple other factors. But the short answer is: bigger, but not as big as you might think. For a typical 120 tube system we use a model 90 tank, which i think is 14 G total volume.



>You would think if the pump was trying when the sun was out, the anti freeze would flash to steam also?



If you set the collector max temperature appropriately in your solar controller, then the pump won't run when the collectors are full of steam.



~Fortunat

www.revisionenergy.com

hot_rod

I agree

how many parts and how much technology do you want to add to prevent problems from ann over-design issue?



You use it or you lose it! Somehow. Personally on large arrays with lopsided load profiles drainback is really the best answer. Running a pump to dump heat that you didn't need to collect in the first place seems silly



Every pump, valve, control adds energy consumption plus more failure points. Here is a valve schematic that was sent to me. I have not tried this method but I know a few installers that have.



hr

Bob Gagnon plumbing and heating

I Don't See It

First does this system purge itself? Will all the steam turn back to water without needing a service call to get air out? I am a big fan of collecting as much energy as possible using a large storage tank, so a heat dump is not needed on my system. The large cooler tank collectors much more energy. The other problem is my large collector system, and really large heat exchanger may require me to purchase 4- #90 expansion tanks, which would cost me more than a circulator pump and high limit switch. I have an extra marine battery for my boat and who doesn't have a battery charger?  I can't see a pump and high limit switch needing that much service, I hardly ever change out circulators anymore, but I do change expansion tanks quite often.



Thanks, Bob Gagnon

Kevin_in_Denver_2

Siggy's Steamback Article

Bob,



Most of your questions and the engineering behind the answers can be found in Siggy's PME article attached.



In answer to the first question, steam will cause a bubble, yes, but there isn't any air in that bubble, only water vapor, which condenses when the collector cools off. Boiling isn't a problem at that point if the controller is set correctly. The current generation of controllers have a setpoint that prevents collector pump activation if the collector is above, say, 205F.

scott markle_2

stagnation

The achilles heal of closed loop glycol is stagnation. Conservative sizing and night cooling (flat plate only) lower the risks but a summer brown out will do the trick, even areas with good power will experience an outage in the lifespan of a solar system. Therefore we must consider stagnation as a design condition.



Since tubes without slip disk limiters can get so hot that they would surly ruin even the best glycol it's nice to believe that this steam pocket is an effective way of evacuating glycol from the header. Still I would imagine that the irregular interior shape of a tube header would leave enough fluid behind to create some undesirable residues. I think boiling a capful of glycol in a clean pan would be a good way of gauging this.



I have been looking a Shuco recently. I think Shuco and Schot (not available here) are the only companies that are manufacturing both thermal and PV collectors. I like that Shuco makes mounting racks that work with PV and thermal.



Shuco only makes flat plate serpentine collectors, the ones designed for residential applications have no headers so I imagine you can't put more than two or three in series. I think this is smart because it means very low fluid volume I also seems like it would work well with this steam-back concept.



Anyway I have a feeling that the supper low volume in these Shuco serpentine header-less flat-plates combined with the fact that flat plate collector stagnation temperatures are lower, and this steam effect could make occasional stagnation an acceptable part of a systems normal operation.



My question is this, when the steam condenses and the void it created collapses will the fluid refill this void in a way that doesn't require any manual purging can a steam generating stagnation occur without any imeadiate maintenance being required ?

Kevin_in_Denver_2

A Steam Bubble Has No Air - Always

Scott - when a steam bubble condenses, there is no air to be purged.



The whole point of steamback is to prevent a maintenance event after any length of  stagnation event.

hot_rod

solar expansion tanks

look for a brand that has a high temperature bladder, designed for solar use.



Even the solar specific tanks seem to be failing prematurely on tube systems with the elevated temperatures. Use a tank that contains the fluid in the steel vessel, not the bladder or bag, as they cool better with the fluid in the steel.



I noticed some of the tank manufacturers in Europe suggest an intermediate tank between the piping and the bladder tank. it is just a plain steel tank, looks like an expansion tank, but the large surface area helps cool down the fluid before it hits the bladder tank.



Zilmet call this a SolarPlus tank and they offer it in a one vessel version, SolarPlus Safe also. Caleffi partners with this Italian company to provide our solar expansion vessels.



Remember too that solar expansion tanks need to hold some fluid, and enough precharge pressure to accomodate cold temperature pressure reduction. Just as pressure increases when you heat a fluid, when the fluid cools to low ambient temperature condition the pressure drops. You need to maintain a minimum for your NPSH.



If I were a evac tube, closed system installer I'd certainly install a dual tank system.



Also using a long pipe to connect the tank offers some cooling, we send a long length of corrugated solarFlex with our solar packages to provide a cool down section in the piping. with plate collectors this seems to be sufficient.



Check out the zilmet.it site for tank sizing formula and explanation.



hr