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Solar thermal system and collector choices

Torrboy | Posted in Mechanicals on

I’m renovating a house in the Seattle area. Part of the renovation includes changing the roof lines for good solar exposure. Heating will be hydronic radiant upper floor (1900 ft2) and hydronic wall panels in finished daylight basement (1400 ft2). Floors will be connected with an open staircase. Air sealing and high levels of insulation are being incorporated. Being on Puget Sound, we’re sacrificing insulated west walls for an expansive view of the Sound using a lot of Serious windows. Very efficient windows but windows none the less. On the rare Western Washington sunny winter afternoon, they’ll help in heating the house.

Energy system involves the following:
PV (2 arrays)
Solar thermal (liquid)
Daikin Altherma air-water heat pump with Daikin DHW tank.

I’ve been presented an extensive energy package proposal. The solar thermal system proposed is a closed loop pressurized propylene glycol system. It comprises (8) Buderus SKS 4.0 collectors and a 200 gallon PL750/2S Buderus storage tank. This integrates with the Daikin Altherma system with the Daikin hot water tank. After doing some reading and calculating the relatively small amount of DHW I might use per day (sometimes 15-20 over a couple weeks), I broached the subject of a heat dump. It took some coaxing but it was finally agreed that a heat dump system was required. The number of panels may be reduced as well. After doing more reading and research, I really like the idea of a closed loop drainback system. With care taken for proper drainage, overtemp, freezing, and power failure problems are resolved.

The company I’m working with is relatively new and comprised of 3 persons from different backgrounds. Radiant system installer and controls engineer, electrician with extensive PV experience, and a structural engineer. They’re good to work with and are open to other ideas. Some of the engineering is being done by a distributor for Buderus so the proposal is probably skewed. I think they’re open to building the best system possible and that’s what I’m after.

With all this said, I need help in selecting the best type of collector (plate or tube) and if a drainback system is better than a pressurized glycol system. Any other advice is appreciated as well.
Thanks to all in advance.

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  1. GBA Editor
    Martin Holladay | | #1

    It's hard to compare drainback systems to pressurized glycol systems, because each system has advantages and disadvantages.

    I don't know if you'll find it useful, but here's a link to one of my articles: Solar Hot Water. In that article, I noted that the best book on designing and installing a solar hot water system is Solar Hot Water Systems by Tom Lane. If you haven't bought Tom's book yet, I urge you to buy it and read it.

    Tom is a big fan of drainback systems.

  2. Mike Eliason | | #2

    4 thoughts...
    1. flat plate will work fine here in seattle, i'd avoid the tubes, even though i realize they look better.

    2. if going for triple pane windows... skip serious and go with cascadia (locally produced) w/ the cardinal 179 or better yet 180 LoE #2#5. between the service issues and appearance of the glazing - i could never utilize serious on my own projects.

    3. i'd dump the daikin. we looked into this pretty seriously for some high end projects, and there are better uses of funds until the system comes down in price...

    4. all that work - any thoughts on a passivhaus retrofit? wouldn't need the hydronic heating, and here in the mild NW - highly glazed facades can be worked around (even w/ our lack of sun)

  3. Torrboy | | #3

    Martin, I'll review your article and check into Mr. Holladay's book. Maybe comparing is easy but choosing is difficult? Thanks very much.

    1. I'm holding out on my decision on the collectors. I've heard the evacuated tubes generate more heat during cloudy days but are more prone to failing and and cost more. The more robust flat plate collector may be what I end up with.
    2. I'll check into the Cascadia windows. Our company actually builds tooling for Cardinal so that's another reason to check them out.
    3. I need a heating system and the Daikin package, although expensive, seems to integrate everything well. I also am not a fan of tossing away good money so I'll check into alternatives.
    4. No, I haven't thought of a design geared specifically for passive solar. I've read an article or two in Fine Homebuilding on the subject. The remodel does have a highly-glazed west facade. Highly glazed! External screens for the summer time and interior, insulated blinds for winter nights.

    Again, thanks for the advice. Much appreciated. If anyone else wants to chime in, please do so.

  4. WilliamLi | | #4

    Reading Tom Lane's book definitely swayed me towards a drainback system here in Vancouver, BC. as well as away from evacuated tubes for the Pacific Northwest. We've had a system in with flat plate collectors mounted on our house for the past couple of years, and so far it seems to be working well and has contributed to drastically reducing our energy bills. Especially in the recent warm summer nights, it's been quite cool to have the system running, even at night, after the kids drain enough out of the storage tank to force some cold water in.
    Our system was a Zen tank installed by Taylor Munro. Unfortunately, neither company seems to have survived, but the system is working well.
    Tom Lane makes some pretty convincing arguments for drainback, not the least of which are removing the requirement for a heat dump and the ability to use water as the working fluid, improving the working lifetime of the collectors and reducing the need for annual drain-and-flush. Not near as energy-saving as any of the passive systems, but probably safer for the Pacific Northwest with our occasional cold snaps but overall mild weather.

  5. Torrboy | | #5

    Of course, Tom Lane's book and "Mr. Holladay's" article. Sorry about that Martin.
    Thanks William for your first-hand experiences. Great information!

  6. kevin_in_denver | | #6

    Tom Lane's book is excellent, but I think he needs to update his opinions about evacuated tube collectors, and glycol systems.

    In the 70's and 80's, all the evacuated tube designs failed and were extremely expensive.

    Well, they've gotten all the bugs out and the cost per square foot is competitive with flat plates, and there is the potential of going lower than conventional flat plates. This is mainly due to lighter weight, less copper and insulation needed. Unfortunately, all the reliable and economical evacuated tube collectors come from China.

    I am also a fan of drainback systems, having pioneered various drainback designs in the 80's. I still prefer drainback over glycol, mainly because they usually perform better at a lower cost. However, all of the old problems caused when glycol systems overheat have been solved in the last decade through "steamback" design principles. Steamback allows the elimination of those inelegant heat dumps.

    For more information on steamback, see this thread at HeatingHelp:

  7. WilliamLi | | #7

    Interesting link, Kevin. The other arguments of Tom's which swayed my decision were the ones about the corrosiveness of glycol over water and the the need for regular replacement of working fluid as the glycol degrades. I'm curious what your thoughts are on these issues. As I'm just a user and not an installer, my experience with these systems is limited to my one system plus some reading.

  8. kevin_in_denver | | #8

    Well, with a properly designed steamback system the glycol will last a really long time and won't get corrosive because it basically won't ever be exposed to temps much above boiling. I've seen 15 year old glycol still in fine shape.

    Some drainback designs must be monitored due to loss of fluid through evaporation. Some open systems require regular topping off.

    Other than that, I prefer drainback, especially in larger systems.

  9. Torrboy | | #9

    The fun never ends. Another dilemma is local contractors who may not be familiar with drainback or steamback systems, figuring a way to get them up to speed, and have them build the system you want and build it correctly.
    GBA is a great source of information. Thanks to all for your input. Hopefully I'll remember to post an update.

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