Stiebel Eltron Collectors Shedding Snow?

matt_sunwaysolar

I am currently servicing an "abandoned" commercial system with Stiebel Eltron SOL25 Plus Collectors (flat plate). Was up on the roof today doing some work after Chicago got a few inches of snow overnight. It was a very sunny day for winter (albeit very cold) and I couldn't believe that not a single collector shed its snow all day. Collector pitch is 45deg.

I am used to using flat plate collectors by Solar Skies and AET, and these things shed snow like crazy when the sun comes out. Now I'm curious if this is something to expect for all of winter production on this system. Has anyone here worked with these and/or noticed this issue?

Thanks in advance.

icesailor

I've never seen one of these Stiebel Elton collectors. I guess they are water collectors.

The most common reason for snow not falling off roofs or slanted surfaces are ice dams. If the snow builds up to the bottom of the panel, the melting snow flows down and starts to build a ice dam block at the bottom. It can have a lot to do with how much moisture the snow contains. If it is really cold out, the bottom of the roof or collector can promote freezing and cause the frozen ice dams. Usually, the melting snow water flows down to the glass and lubricates the panel. Allowing the snow/ice to fall off. If it is still really cold, there may not be enough water in the snow to make the snow too heavy to stay on the panel. Think avalanches in the mountains. When the snow gets to heavy and unbalanced, it flows down. Like rocks on a cliff.

It won't take much to catch the snow on the bottom. Something as simple as a upturned bottom.

FWIW.

matt_sunwaysolar

No ice dam on this one. It's a flat roof and they're elevated about a foot off the roof surface.

matt_sunwaysolar

Maybe a little bit of ice at the bottom of the collector, but shouldn't be enough to stop any melting of snow from taking place.

Mark Eatherton

I have panels at 55 degrees and experience the same problem, but I have a control logic which allows me to send heat back up to the collectors, and de-ice/snow them. When I am there (mountains) I prefer to use manual labor (ground mounted) but if need be, I can thaw them out. I think there are a number of factors that contribute to them holding snow. If it was warm when it started snowing, and then froze, you have good adhesion. If it was cold (below freezing) when it started snowing, then you have poor adhesion and the snow slides off fairly easily.

ME

icesailor

Looks like some ice damming to me. In the second photo, notice the ice dam that starts about 18" up from the bottom and curves around to the left and rises up? Classic symptom of ice damming. The open space is the real tip-off. As the upper snow melted, it ran under the snow and stopped there it couldn't travel to the bottom. That's why the open space between the two patches of snow. If you look inside the collector, you might see something that allows the water to freeze and stop there. The snow tried to slough off the plate but was stopped by the build up of snow being melted. The lower patch is very thin at the top and gets thicker at the bottom.

I'm no authority on solar collectors, but if there is a thyristor that keeps the pumps from running at night when the collector is cold, perhaps it is keeping the pumps from running when it is warming or not at all. Once that panel starts to get some sun, it should shed snow like a big oak tree in the fall. If they are still like that, look under the snow above the ice dam and see if it is wet on the bottom. And ice where that hump is. Its probably not as much of a long time problem as it might appear today.

IMO

hot_rod

Is it a single glass covering? Typically the copper absorber warms and that heat energy warms the glass to release the snow.

If it is a double layer glass, think of a thermal pane window, or a vacuum tube there is less heat transfer to the glass to release the frozen bond.

Are they drainback of glycol? A clear path at the bottom for the snow to slide off?

A catch 22 to sending hot water up to warm the collector, on a cold, windy, sunny day in Chicago that collector may only gain 12- 15,000 BTU over the course of the day. If you send more heat energy than that up to "defrost" them, plus the energy required to run the pump, you're actually going backwards, expending energy, not collecting it. I don't think you win that game:)

Give them time they will work when conditions are right.

Where is the sensor located, strapped onto the external piping, or into a well in the collector?

Make sure you controller is set up properly, is the OCN or OCF function enabled. Most controllers have a collector min. setting as well as an anti freeze function.

icesailor

@hot rod:

A good example of that is with Velux skylight "roof windows". It can take days for the snow and ice to melt off them after a snow. They are heavily insulated.

matt_sunwaysolar

I've attached another pic here from the next day. They have finally started shedding the snow. The last two days in Chicago have been about as sunny as can be hoped for in winter, but it has been extremely cold. By comparison, my shop has AET collectors at 52deg, with snow gone by midday the same day I took the first Stiebel Eltron pics and the storage tank at 130 deg.

The pics I posted originally are a little misleading on the ice dam, as the strip about 1/3 of the way up that looks like melting and freezing was actually a hand running over the snow to try to facilitate movement and melting. Obviously, a failed attempt. But it probably suggests there is an ice layer on the glazing anyhow since the snow went nowhere.

I also looked at the specs for the collectors again, and there is no double layer of glass, just tempered, low E, like I believe most other flat plate collectors have. Obviously, tempered glass is the standard; anyone know if low E is also?

I'm pretty opposed to sending fluid up to melt the snow with programming due to added complexity and loss of energy to do so. Anyhow, while charging system, it ran "in reverse" (with storage heating collectors) for several hours, with the collector sensor reading around 70deg and full winter sun and did nothing to melt anything.

To answer the other questions, it is a pressurized glycol system and the sensor is installed in with outlet of one of he collector banks in a proprietary well provided by Stiebel Eltron. Stiebel Eltron collectors are also plumbed in series (which is not my preference).

If I didn't have a lot of experience with (other) flat plates and how they perform in winter, I suppose I wouldn't be so surprised. But I've never seen collectors take more than 2 days of full sun to shed snow. I suppose I'll chalk it up to "super insulated" or bad design.

SWEI

Low-e glazing would render solar collectors nearly useless. You need that IR energy to generate heat. Tempered low iron glass is the standard.

matt_sunwaysolar

Sorry, spec sheets say "low iron", not "low e". Anyone familiar with the difference, if there is one?

hot_rod

low iron content glass is a clearer glass.

Typical glass has a green hue. Notice this when you look at the edge of glass, for example.

Solar glass is often a lower iron "clear" glass.

You'll also notice this when someone buys replacement glass from a glass company, it will not match any collectors alongside it

I have a pic somewhere of an array of DB collectors next to glycol, the DB collectors warm faster, lose snow quicker, due to no fluid, actually starting an hour earlier sometimes. This was a Wisconsin apt install.

icesailor

You mentioned "Ice Damming". That was the only thing I was commenting on. I think it is unusual to have that much sun shining on those collectors without the slow sliding off. I mentioned Velux "roof window" skylights. They have "Low E" glass and the snow slides off in a day or so, no matter how cold. Maybe is helped by heat inside the structure.

But in the last photo you just sent, on the featured collectors, why is there that line about 12" up from the bottom? It isn't on the other collectors. I can see the ones behind where you ran your hand.

Snow and ice is very "sticky" and adheres to almost anything. Is the top side of the glass ribbed? The snow may want to stick more. Try spraying one panel with 100% silicone spray. That might break the surface tension of the glass to the water and make it want to slide off. See what happens. Black painted and waxed automobiles will still have snow and ice stuck to it. Even with the sun out.

Is the space between the bottom of the glass and the black reflective of the inside of the panel greater on those panels that the others you have experienced?

I'm just asking out of curiosity.

matt_sunwaysolar

I've looked at the pics a few times and I'm not sure what line you're referencing.

The glass is smooth on the outside, no stipple.

As far as I can tell, there is not any considerable amount of space between the glazing and absorber plate in comparison with other collectors. All told they are only a little over 3" thick.

icesailor

In all of the first three pictures and in the last one, there is a line of bare snow, between layers of snow above and below the line of bare glass, a foot or so above the bottom of the panel.

I don't know, I was only trying to add to the conversation.

What does the manufacturer of the panels have to say? You probably aren't the only one to have noticed this with their panels.

What's interesting about this is that the colder the ice gets, the more it expands. While the glass is shrinking the colder it gets. The fact that the sun should be getting through the snow and the black plates should be absorbing energy from the Sun and reflecting it back is interesting. The pumps aren't supposed to be running when the panel is cold. But is there any possibility that the fluids inside the panels and the piping connections are flowing by gravity, locally? That would remove the reflected heat that would normally heat up the underside of the glass and make the snow fall off. You said that you weren't happy with how it was piped. Is it possible that it will gravity feed/circulate within its area? If you understand what I am saying? Stranger things have happened.

Also, in the last photo, almost all of the snow is gone. That looks more like evaporation of the snow/moisture by the wind. I don't know if you appreciate the effect of wind on evaporation of snow and ice. Water vapor can be as low as -40 degrees while water may be freezing from +39 to +32 degrees. When the snow slides off en-mass, it is usually a big pile of snow below the edge where it fell off.

hot_rod

Usually the bottom side of solar glazing is textured, sometimes called a "cracked ice" texture, not unlike shower door, privacy glass.
There are many patterns to choose from as far a textures go. This textured surface cuts down on glare for aircraft flying over-head and some say helps diffuse the sunlight entering the collector.

I have seen the glass installed with the rough side out, by mistake, I suspect that would limit the snow slide. Drag your fingernails across the glass and you will know which side is facing up

matt_sunwaysolar

IS:
I'm not even going to reach out to the manufacturer on this one. First, the system was designed and installed by others and the equipment may be out of warranty. Second, I have enough experience with a number of different solar thermal manufacturers to know that they'll typically point the finger anywhere else on a more "vague" issue like this one. First, they'll ask/point to the installation and design and wonder if the installer did something wrong. Usually you end up in circles like that for a while and, at the end of the day, it's not a warranty issue or an equipment malfunction, so the issue will ultimately hit a brick wall after some wasted time.
I'll continue to monitor the situation and plan on the system producing better in summer than winter, like all systems, but to a larger variation here.

Thanks for all the insight, everyone.

icesailor

Its my experience with this subject that if not for intelligent and experienced folks like you, the manufacturers would be out of business. You do all their re-engineering and troubleshooting for them and you pay them for the opportunity to educate them.

Then, they have to give an argument to their paying students and blame the failures on us, rather than them, who made the mistake in the first place.

I still think that you are getting some form of circulation in the panels. If there was absolutely no movement of water/fluid through the panels, the sun would warm up the panels by the black flat surface getting hot by absorbing the heat from the sun. There would be enough solar energy to heat the space and radiate enough heat energy to warm the glass surface. At the Chicago temperatures I see on the weather TV, it wouldn't take much flow to take back any solar gain.

Is it possible to completely shut off the supply and returns to the solar arrays so that there is no possibility of unwanted circulation? And wait and see if the snow falls off?

We Icesailors love to see natures Zamboni. A layer of smooth snow on the ice. And the sun to melt the snow to water, and no wind to make ripples. Then a night or two of no wind and clouds. Radiational cooling is a beautiful thing. Cold OAT, hard ice, 10 to 20 MPH of wind and sharp skates? Almost as good as sex. But the most fun you will ever have with your clothes on.

matt_sunwaysolar

IS:

While I won't completely guarantee it, I'm pretty convince that thermosiphoning is not taking place. First, we do have a check valve installed to prevent that issue. Second, the collector sensor was buried between 0 - 25deg all day the first day and until the snow melted from the tops of the collectors the second day. We also installed flow meter/ balancing valves on each collector bank (6 total) in order to balance the flow through the collectors properly and verifiably. No flow being detected when the system is OFF.

Like I said earlier, I'm chalking this up to a bad collector design. I'd be more concerned if I had sold the equipment and these were the results, but I was brought in after the fact to rectify the situation as best as possible. In a perfect world, we'd tear everything out and start over, but I haven't figured out a way to get the customer to agree to that.

hot_rod

Wonder what type of back and edge insulation is inside. It could be loosing most of it's heat out the frame and not allowing the glass to warm up. A sensor into the collector would help determine what's going on.

Also infiltration takes a big chunk on a windy day especially if the corners are not sealed.

If it's a SRCC certified collector all that should have shown up in the testing they go through, and show up in the performance rating numbers.

Collectors with loose, batt type insulation sometimes allow all
the insulation to shift during transit.

Not sure how far you want to go with this. Any other collectors in the neighborhood to compare to?

Many of the solar controllers have hour meters built in under the view menus hP for hours of pump run. If the owner is concerned he could keep track of the pump run hours on cold days.

icesailor

@matt_sunwaysolar:

"" While I won't completely guarantee it, I'm pretty convince that thermosiphoning is not taking place. First, we do have a check valve installed to prevent that issue. Second, the collector sensor was buried between 0 - 25deg all day the first day and until the snow melted from the tops of the --- ""

You know far more about these things that I do. I'm in no way questioning your knowledge of what you are doing.

It wouldn't take much thermo-syphoning/leaking by check valves to make that happen. The colder it is, the harder it is to gain any heat from the sun, enough heat that the cold of the fluid can't suck it back out if the air temperature in the panels.

WIth my experience with water well check valves, I know that not all check valves are the same.

<<<This was written yesterday before the big Internets crash from the goings on in France. This site and most others went down. >>>

Hot Rod's comments are spot on.

Mark Eatherton

Check valves don't necessarily stop thermosiphoning. In fact, I've had a heat exchanger on top of a 500 gallon open tank connected to a solar array 12 stories above the storage tank that kept freezing and breaking the heat exchagner. Come to find out that the collectors were FORWARD thermosphoning, bringing back glycol below freezing and breaking tube in teh shell and tube exchanger. Fix was simple, drilled a 1/4" hole in the water return side of the HX to allow the water to drain back and eliminate the thermosiphon connection.

THe difference between low e and low iron is that low e glass has a super thin coating of metal called TCO, or Tin Coating Oxide that is sprayed into the glass oven and allowed to actually penetrate into the surface of the glass, giving the glass the ability to reflect long wave IR transmission back to the source. Low iron, as HR pointed out, has less iron and absorbs less short wave energy on its way through the glass. The TCO is what we use as a conductor on our radiant window glass. It has a sheet resistance.

We have had PV manufacturers who were interested in using our heated glass to ensure snow free glass and productivity. I think they were looking at the potential loss of revenue from having the array blocked for 3 or more days, and the loss of energy from defrosting was less than the energy they COULD have harvested had the glazing been clear.

ME

hot_rod

You really need a check on both supply and return side of the solar loop. There is a huge driving force when you have a very cold collector and a hot solar storage tank, mother nature really tries hard, and often wins, to balance that temperature difference.

The new solar pump stations come with two of these clever triple duty valve. It serves as a shut off valve, isolation for the pump for repairs, a spring check (aka gravity gate) , and a temperature gauge well.

Also if you turn the lever half way between open and close, it holds open the check. That is the only way you can drain down a collector it to force the check valves open as you have one on both sides.

See the cutaway I made to show how the edge of the ball lifts the check open. In the summer months the checks can be forced open if the system is un-used for periods of time. This allows all the energy it collects during the day, to siphon back at night and cool down the tank. A free gravity powered over-heat protection, so you don't cook the glycol. Just remember to turn them back when you come back from vacation

Truth be known we have had several jobs where these spring checks don't hold the thermosiphon. If your collector is warmer than outside temperature, or at midnight you have thermosiphon. Yet another reason to embrace drainback.

These checks have a very light spring so the check can pop open even with the circ pump running at 30% speed in V/S mode.

We have an additional 1/2 psi check that screws onto the top of this pump station. Lock the pump to 40% minimum speed so it has enough "fizz' to pop a 1/2 psi check.

Live and learn just when you think you have seen it all.

SWEI

That is a very clever valve design.

Happy we live in drainback country. I have an abiding faith in gravity.