Attic Tank For Solar Storage

Andrew Hagen_2

" A Golden Thread" is more of a history book than a technical manual. I recommend downloading <a href="http://www.caleffi.us/caleffi/en_US/Site/Technical_library/Idraulica_magazine/index.sdo"target="_blank">Caleffi's idronics 3</a>. Pages 16 & 17 break down some of the pros and cons of glycol versus drainback.

Yes, drainback solar design absolutely protects against freezing or overheating. Pay careful attention to the solar panel design if using drainback. Some collectors are not designed to be used this way and will not naturally drain completely. I am not sure how many on/off cycles it would take to stress the copper header or heat pipe to failure in a heat pipe evac tube used in a drainback system. My gut feeling is that it would take many years for this to occur if it ever did. Then again, my gut has been wrong before.

gasfolk

Attic Tank Problems

Some solar designs (e.g. thermosiphon) seem to require storage tank installation near or above the height of the solar panels. I am trying to learn whether an attic tank can work with a drain-back solar system.

Has anyone installed an attic tank or any storage tank above the basement?? Are there any problems with placing a solar storage tank in a conditioned attic?? Are concerns about freezing and structural issues satisfactorily answered as follows??:

Freezing: The risk of freezing appears remote--the top of this house can freeze only if the heat goes off and the entire house (including plumbing) also freezes. No??

Structural: If the tank can be installed horizontally, the load per square foot would appear acceptable--less than a comparable, third-floor bath tub. No??

I would be grateful if anyone can point out other potential problems with attic tanks.

Thanks,

gf

scott markle_2

The drain back tank is sized to hold the volume of water that is held in the collectors, you don't have to put the storage tank in the attic too. From what I understand it's a good idea to get the drain-back tank closer to the collectors so that the pump has less lift to overcome when the system starts running.

I'v seen some pictures posted here of a glycol drain-back system that incorporated the drain-back tank in the collector, can't remember the manufacturer.

hot_rod

not sure about your question

thermosiphon systems will typically have the tank built right into the collector. These are called ICS or" intergrated collector storage". Some have the tank in the space below the panel.

In either case you need to have enough stuctural strength to support that system. Usually a 40 or 50 gallon tank. Rheem Solarhart is one example of an ICS. 50 gallons of water, the tank and panel will exceed 500 lbs.

This system is very common in warm climates like Spain, Israel, southern Italy are all big ISC markets. Solar has been required in Israel since the '80's There is actually a huge replacement market there as the entire system is replaced when they spring a leak.

Drainbacks are a different system altogether. As Scott mentioned only the Drainback tank needs to be elevated only, IF you are trying to cut down pump lift. Many, most, mount the drainback tank at the top of the storage.

If you have a multiple story building, locating the drainback tank up near the panel will reduce the pump size. 28- 32" of lift is do-able with a common high head circ, like a Grundfoe 26-99, taco equivlent. I believe the Wilo Star 32 is a specific drainback pump with a high steep curve.

www.energylabsinc.com is one good place to look for drainback info. HTP builds a drainback tank. HWP in WI sells drainback components. Lots of choices out there for components to build your own. Look to see more DB packages hitting the market soon.

Residential DB tanks are usually in the 6-15 gallon capcity as all they hold is the panel and piping volume and an air space for expansion. Pressurized DB is the way to go.

Some DB proponents use glycol in their systems. This eliminates any freeze issues, lets you mount the panels and piping level, also. However the glyciol adds expense, leak issues, higher pump head, fluid maintenanc, etc.

Many well designed and installed DB systems are out there with 20, 30 year history of troublefree performance.

The system and type needs to be built to your wants and desires, there are many ways to do solar thermal properly and safely.

hr

Andrew Hagen_2

Dead Load

Obviously the structural question is very case-specific. Some attics are framed almost like a 3rd floor, while others are not. Freezing is also very case-specific.

There is absolutely no reason this could not be done if the attic floor can support the load.

You should install a pan under the equipment that is piped to a drain in case something leaks or the relief valve blows. That shouldn't happen, particularly with a drainback system, but you never know.

Larry Weingarten

Day and Night...

... commonly did attic storage with the collector on the roof in the 1920s. (get a copy of "A Golden Thread" by Butti and Perlin) They built false chimneys to put the tank in vertically. It promoted stratification in the tank. Ultimately, the problem was leaks and collapsed ceilings. So, knowing the tank will leak, protect against it.

It may be my semantics, but my understanding of the differences between Integral Collector Storage (ICS), or breadbox or batch heaters and thermosyphon, is the latter separate collector and storage functions. Storage is done in an insulated tank that looses heat far slower at night than the ICS systems that combine both functions in one place. One ICS example uses blackened 4" copper pipe as both collector and storage. It's directly under glass, which is good during the day and bad at night. ICS systems however are much more freeze tolerant than the thermosyphon collectors. Day and Night got it's name because it was the first solar to be able to deliver hot water both day and night :~)

Yours, Larry

scott markle_2

I took a look at the Solahart offerings, it wasn't clear to me if the entire line of this Australian made product is available here thru Rheem.

One of the configurations is described- "closed circuit design makes it suitable for frost prone and poor water quality areas." I'm guessing that the flat plate has a solution that thermosyphons to an exchanger in the tank? but what about expansion etc. It's just not clear to me how this works. Open systems in warm climates with good water quality make a lot of sense. I was a bit surprised when the instructor in an ASHRAE class I took said that this simple design was the type of system he had on his house in florida for the last 20 years. I guess a good engineer understands KISS.

Personally I'm curious about the approach (popular in china) of the heat pipe that is directly connected to an insulated storage tank. The tubes are obviously frost proof and they transfer heat passively to the insulated tank, no glycol, no drain back, no pump. Odd that this approach which is hugely popular in china is not more widely distributed here, especially in moderate climates or for summer camps, out door showers etc. If you want to buy a shipping container of these things there very reasonable, but they seem hard to find otherwise, one distributer that deals with Sunda tubes said they had quality issues with the tanks and no longer distribute these, too bad.

scott markle_2

HR, just had a peek at the energylabs link and I noticed that the pictured systems were all piped in pex. What's up with this? Does the ability to quickly shut down a drain-back system make pex acceptable for all but the near collector pipe work? Is this something new, or just a convenient (low cost) way to illustrate piping?

gasfolk

Does Drain-Back Protect From Overheating??

Thanks for the good ideas. I have requested "A Golden Thread" by Butti and Perlin, from our public library, and checked out the websites recommended above. This approach may be promising for our project, especially if we can limit the DB tank size to reduce space and structural (dead load) issues.

Another site describes DB systems as protecting against overheating because once the storage tank is full no water is sent to the panels to stagnate.

1) Is that correct??
2) If so, how reliable is it??
3) If not, is there a formula for calculating the minimum size needed for an attic DB tank to prevent stagnation overheating of the output of the DB tank (transfering heat either to a larger storage tank or directly to the heat distribution system or DHW supply line)??
4) Working with evac tubes, can you size an attic DB tank to reliably keep its output temp less than 180F (i.e. within pex operating limits)?? Is 40 gallons necessary or sufficient??

Thank you all again,

gf

(Edited for clarity.)

gasfolk

BTW...

A Google search for overheating issues returned the information above.

gf

hot_rod

good trivia, Larry

I hadn't heard the vertical tank detail before.

The Day and Night company went on to build gas fired water heaters as natural gas became available and inexpensive in California. I also heard a freak freeze in So. California took out a lot of the non freeze protected solar systems and many owners went with gas fired HW tanks instead of replacing or repairing the solar.

hr

gasfolk

From Heat Transfer Products...

"The SuperStor Drain Back Tank is designed...to protect the system from both freezing and overheating. The Drain Back Tank comes with either an internal heat exchanger for use with a storage tank or without a heat exchanger to be connected to a tank with an internal heat exchanger or plate frame heat exchanger. Drain back systems are a smart choice...when overheating during the warmer months is a problem. Drain back systems have less components, so maintenance is reduced, compared to a pressurized glycol system. Drain back systems also provide protection where water quality may be a problem." http://www.htproducts.com/superstordrainbacktank.html

So, same questions with a better reference.

Thanks,

gf

hot_rod

A DB evac tube system??

I keep hearing rumors about evac tube drainbacks, but have yet to get any first hand experience or history.

I do know those heat pipe type evac tube condensers can get incrediably hot in summer no flow conditions. I've read numbers in excess of 600F.

Seems sending even 100F water up against that temperature would cause some real thermal expansion, stress, etc.

hr

gasfolk

Very helpful observations

Hi HR,

A search turned up the following site, which suggests some interesting compatibility on DB and at least a few evac tubes:

http://alternativepowerdistribution.com/

This HO is not climbing three stories onto a slate roof for any reason, and my hat's off to PRO's and DIYers with the skills to do this work. This HO's challenge is to understand enough to find a solar installer who can maximize performance while working with our plumber and roofer and the limitations of existing equipment, slate roof, historic district, etc. So, there seems a lot to learn.

Thanks to everyone for offering guidance. I had hoped this would be a quick decision but am now searching the Wall and elsewhere for the best books on solar thermal.

Thanks again,

gf