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Flat Panels or Evac tubes?

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  • Wayco Wayne_2
    Wayco Wayne_2 Member Posts: 2,472
    Hot Road

    You just said a good reason to go tubes. Little or no wind problems. Put it on the list for pros for tubes and cons for plates. WW

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  • hot_rod
    hot_rod Member Posts: 14,786
    yes, less wind

    lift issues goes in the plus column for tubes.

    Heat transfer grease, and tube to dry well connection longevity and transfer-ability will go as a minus in the tube column :)

    No grease or slip fit thermal transfer connections with flat panels.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Ted_5
    Ted_5 Member Posts: 272
    3/4\" hail stone

    Viessmann has a white paper of a test performed on the ET and FP, it past having a 3/4" hail stone traveling at 60 mph! I can find it and pass it on if anyone would like it, just e-mail me directly.

    Ted
  • Brendon- all this tilting?

    You make it sound like you will be up there each day. Many systems will only need to be adjusted two or three times a year. And if you make it worst case scenario, like on a 5 story building with a very steep pitch, tilting would be very difficult, but most systems would be much easier to tilt, and those are the ones that should be considered for a tilt mount. I have a steep pitch to my collectors but I notice in the spring and fall the sun is much higher and I could use more solar energy for space heating. One tilt a year up to about 45 Degrees would be all I need, I have plenty of domestic in the summer. Surely we can find fasteners that will keep the collectors on the roof. Like Hot Rod said about the evac tubes, my 120 tube array is a false roof over a deck and I have had 70 MPH winds with no problems, it's all in the proper design. When you say a fixed collector is best, I think you mean it is easiest. I think the system that collects the most energy is best. It's like when people say that using solar only for domestic hot water is best, they mean it is the easiest. So if you want to wait around for the high temp domestic water with a fixed collector, the same way everyone has been doing it for years, go ahead. But I think we have to improve these systems.

    Thanks, Bob Gagnon

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  • bob,you're doing awesome stuff, no doubt. it's great to see! but I say "solar DHW is best", personally, I say it because it's the only cost effective use of solar thermal in a residence in most areas of the country today. not because it's "easiest" but because the additional cost of collectors and storage and integration is unlikely to ever be recouped (integration being the small money there)

    that might change if $5/gallon oil is a reality for this winter. but I doubt it will be (not this year).

    your idea of a "partial load" house heat dump is pretty cool though. I wonder if it would be best to just dump whatever heat you collect in the winter that is over domestic temp? certainly an idea worth considering if maximum fuel economy is the goal. I would agree most of us are a little more fixated on comfort perfection than that, but your idea makes a lot more sense for keeping the fuel bill down. Use it or lose it, as HR says.

    keep experimenting, tough guy ;)
  • Theealchemist
    Theealchemist Member Posts: 59
    Flat Panels or Evac tubes?

    The Evacuated tube systems usually have less of a dead load on the roof when compared to flat panels. Those older 2x6 roofs with on center rafters at 2 feet sure like to sag sometimes under additional weight......
  • Partial Load House Heat Dump

    Great name Rob and thanks for the kind words. But I think instead of dumping all the heat into the house that is over the domestic temperature, we should keep our storage tanks at as low a temperature as possible, and we can harvest and use many more BTU'S. You can collect three times as many BTU'S at 95 degree temps than you can at 165 degree temps, that is huge. Picture a day during the heating season, when the sun is shining, as soon as the temps in your storage tank reach 95 degrees you circulate that water through a separate radiant panel, not integrated, heating your house, while maintaining the lowest possible temperatures in the storage tank, allowing your collectors to harvest the most BTU'S. You run the tank down to about 80 degrees as the sun goes down. You take your showers using that 80 water in the storage tank to pre heat your domestic, and the temp might drop to 60 degrees in that tank before the sun comes up the next day. Now your collector can start collecting the easy low temperature BTU'S, the low hanging fruit. You won't need super large storage tanks if you are dumping the heat into the house and the radiant panel won't have to be that big, because you could run it for 24 hours, while you might only collect solar energy for 8 hours. If your going up on the roof anyway, adding a few more collectors won't be that much more work or that much more expensive. In Massachusetts we spend 17% of our home energy dollar on domestic hot water and 52% on heating. Spend some time and think about this because I think it would make a big difference. I think now, more than ever we have to work to improve these systems.

    Thanks, Bob Gagnon

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  • You know what Bob... I think this might really be brilliant. I've been pushing the on demand boost concept hard to utilize every BTU possible on the DHW end, but this just takes the concept to a whole new level... you could be "dumping" whenever the water is over room temperature and pushing your collector water temps from 110 to 120 right down to 80 or so..

    Thinking less about either/or and more about BTU utilization is surely worthwhile. It's probably even worth letting those daytime temperatures rise to higher levels, like 80 degrees. Might as well "store" the BTUs in the house and collect more.

    I will think about this a lot more Im' sure, but great idea bob, really. thanks for sharing it.
  • hot_rod
    hot_rod Member Posts: 14,786
    Cedar Mountain Solar in NM

    has been designing and installing solar radiant without "wet" tank storage for many years. They have a method to store that energy in radiant slabs via priority controls, etc.

    Remember though with super efficient homes ICF or SIPs for example, you may have design loads down 10 BTU/ sq foot or less. So even a 75- 78 slab temperature could lead to overheating. The key would be to have a zone somewhere that could handle some overshoot without becoming uncomfortable.

    Bristol Stickney, a very sharp solar engineer, of Cedar Mountain Solar will have a monthly "solar" column in PHC News magazine. I suspect we will be learning more about their low or no storage solar heating methods. Stay tuned! Here is their "coming out" article.

    www.phcnews.com/june_08/bristol_feature.php

    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Peak Solar

    Thanks Rob, but this is just basic stuff. Not only will you gain more hour to hour but you will eliminate what I call "Peak Solar" (What's really cool is that we are naming this stuff and figuring it out as we go. Gary Wallace likened it to the Wild West) Peak solar is the time in the afternoon, maybe 1:00 or 2:00 PM, when my domestic solar tank maxes out, at about 120-150 degrees, for my high temperature domestic water system. The collectors can't make water hotter than the water in the tank, and the collectors just sit there for the rest of the day. By using the system for heating, and keeping the tank temps low, you can harvest lower temp energy all day, down to about 80 degrees. I do this with my low temp heating tank. This will add another big efficiency increase in addition to the hour by hour efficiency increase. We can make these systems a lot more efficient.

    Thanks, Bob Gagnon

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  • hot_rod
    hot_rod Member Posts: 14,786
    120- 150F DHW

    max temperature, Bob? With 120 evac tubes? what size DHW tank is connected to that array?

    I had my 4- flat panel (160 square feet) 80 gallon system at over 200 a few weeks back. By mistake :)

    Seems you should be able to get much higher temperature than 150F in the DHW tank, even in winter?

    Most of the new electronic solar controllers out there have an over heat function that allows the tank to run up higher than typical setpoint tank temperature of 135F or so. 205F is an adjustable range, just below T&P pop.

    The Calffi I solar and most of the Resol controllers have a "tube collector special function" It is to help prevent overheating of the collectors and allows the tank to run up by pulsing the pump for 30 seconds at 100% pump speed. It keeps recalculating and will again pulse the pump if the temperature exceeds 4Ra.--

    There is also a "collector temperature limitation" function it has a factory default of 285F and is adjustable from 230- 400F!

    They also have an evening "recool" function to dump some heat, to the cool evening air, down to the "adjusted minimum tank temperature" that you program in.

    Max tank temperature is factory at 140F but adjustable up to 205F.

    Maybe a control like this would be a way to leverage more of your solar harvest? With the proper ASSE 1016/ 1017 mixer you could run that DHW storage much higher. Sounds like you have plenty of HP to do it.

    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Wayco Wayne_2
    Wayco Wayne_2 Member Posts: 2,472
    Good stuff Bob

    You got me thinking. I've got radiant floors in my 1st floor of the house. The thermostat is a programmable. Why not turn up the set point while were at work instead of turning it down? Get the heat into the house and lower the storage tank temperature. Do you have a web page with pictures of your system? I'd be interested to see. WW

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  • Wayco Wayne_2
    Wayco Wayne_2 Member Posts: 2,472
    Actually

    I just found your "find a professional ad and links. My Home owners association wouldnt like your tube array. :) I personally think it's beaautiful. WW

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  • Bob,

    My problem with this design is that in the summer when %100 solar DHW is easily attainable from larger arrays like you have, this plan may require supplemental input. Unless you have a very effective HX in that 200 gal tank, In-order to get 110 I'm guessing you might need to keep the 200 gal tank fairly hot.

    The system seems fine provided there is a heating load. I just question the efficiency of all that storage with it's associated losses during DHW only periods. A well insulated house with some minor passive solar features May only need active heating inputs for 6 months of the year. Those storage tanks if within the envelope may also be contributing to cooling loads.

    I'm curious about your summertime operation. What sort of temperatures are you maintaining in the storage tanks? How cold is your incoming water and how hot does it leave your tanks at shower flow rates of say 2 gpm? What is your immersion coil made of?


  • Viessmann's copper counter-flow header design uses a dry spring loaded connection. No grease, no corrosion, excellent transfer, reverse return advantages including the clean look of pipes entering from one side. Leave if Viessmann for these sort of metallurgical manufacturing marvels, Payback is another question.
  • Wayco Wayne_2
    Wayco Wayne_2 Member Posts: 2,472
    Is there

    a rule of thumb to compare flat plates to tubes? Something like 2 sq ft of flat plate equals 1 tube or the like. It would be a useful number to tuck in the old gray matter. WW

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  • Thoughts

    Not all tubes have the same (per tube) surface area, So a flat plate to tube conversion number is meaningless unless we are specifying manufacturers.

    There is no magic that makes the the surface area of a tube collector vastly more efficient than a flat plate. The surface area of a collector and available radiation are the starting points. Evac tubes perform better at higher operating temperatures and lower ambient temperatures, (less heat is reradiated to the environment in these conditions), It's a bit silly to say which is best without defining the conditions of operation. Even in florida there are high temperature applications that tubes are better suited for than plates. Because a typical DHW system will operate under such a wide variety of conditions seasonally it becomes difficult to make blanket determinations on comparative efficiency. However I think H.R has provided a useful rule of thumb number with the 5-8% boost that he says can be achieved with tubes. Obviously "it all depends".

    To my mind the tube advantage (for residential applications) comes mainly down to:the possibility of one man installation, reduced weight, and wind shear.

    Perhaps the marketing appeal of something that looks new and different from those ugly boxes that the neighbor had taken off his roof years ago is also a factor. I'm being a bit cynical , but I do believe there is something to this. The Prius outsells the civic hybrid not because it's performance is vastly different but because it looks different, it's futuristic, it sends a message, it's fashionable.

    Like it or not our industry is subject to the same influences, and I suspect that one of the advantages of tubes is that they look different from the solar technology of the 70's and 80's. For many in the general public the systems from this period are perceived as not having worked very well, lots of reasons for this not the least of which was a period of historically low energy prices that made even good systems somewhat irrelevant in strictly economic
    terms.

    The affordability and reliability of electronic controls and perhaps wet-rotor circulators have improved, lots of experience has been gained, but fundamentally we are working with a established technology,new tube designs,pump stations etc. aside, this is nothing new. What's new (hopefully) is better implementation, Lets try and keep the hype down, solar thermal has huge untaped potential but it's not a silver bullet. Until I see wide spread utilization of solar DHW I'm reluctant to get excited about active space heating.

    Bob, your energy and thought on the subject impressive, I agree that serving the low temperature loads first is an excellent design concept. However I think you focus to much on the winter side of collection. Loosing 50% of a the january capacity sounds really bad, but given that solar resources are so low at this time of year (at our latitude) this is not as big a chunk of what's seasonally available as you might think. I'm more put off by the Summer over capacity than limited winter utilization.As far as adjustable pitch is concerned, Why would someone with a system sized for a space heating application want a 10deg summer angle anyway?

    I believe that the most practical and environmentally sound solar system is matched to 100% of anticipated summer DHW loads. In shoulder seasons it may be more effective to put the solar input to a low temperature heating load than to the DHW, show me a relatively simple schematic without massive storage that can do this and I'll get excited.
  • hot_rod
    hot_rod Member Posts: 14,786
    tubes run a bit heavier

    then the same amount of flat panel apeture, generally.

    Apricus AP 30, 40.85 sq. ft weight 182 lbs.

    A 4X10 Solar skies, 37.85 sq. ft 153 lbs.

    I agree it is easier for one man to move 30 tubes onto the roof (30 trips up and down the ladder) compared to a 4x10 flat panel.

    I work alone for the most part when I install flat panels, even one, I hire a crane truck. Sign companies around here charge $150 for one hour or less. The operators are always willing to help sling and rig the panel. most ask a lot of questions about solar.

    $150 is a bargin compared to ropeing them up :)

    hr
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • WV EGBERT_2
    WV EGBERT_2 Member Posts: 98
    wayco wayne, NRT

    Hey guys if you have access to Viessmann Solar tech manuals . They have both flat panels and tubes. I doubt they care if they sell one or the other so they may not be biased.
    That said , in the manuals you will fine charting info on the two: if you look at the tube chart for let's say the 20 tube array and designing for 60% coverage (DHW) then look at their flat panels chart for the same 60% coverage, you'll find that more square footage with flat is needed. But as others have said, it depends.
    The tech manual will have also, optical efficiency, heat loss, weight, etc...
    Viessmann does not require grease , and the tubes can be replaced (if neededed) without the need to drain. As a matter of fact, this thread got me thinking, and I went up on the roof and rotated the tubes, come winter I'll go back up and rotate them again to caputure more in the east-west axis.
    HR had mentioned a diff control that seems a bit advanced, I'm pretty sure a a premium price. My opinion is to put the money elsewhere. A simple Goldline works also, for far cheaper.
    That said if you start looking at price for panels , I found the $2200+ difference for two flat vs. one evac array as stated above a bit off.
    And this does not include the tax incentives. Which in my state total 55% with Fed and state incentives. These incentives have no caps for commercial and $5000 for residential, plus NY also has no sales taxes on anything solar related, so if you are doing an array with a new boiler, some radiant etc, you give your supplier the NY state exempt form, they should not charge you sales tax. Pass that on to your clients. check out dsireusa.org for all the incentives and tax exemptions in you state.
    Bottom line, do something, flat or plates or both !!
    And if you home owner's association won't let you put something on the roof, then put it on the ground in the back yard.

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  • Scott- Simple Solar Schematic

    Simply run the delta tee controller of the larger, cooler tank during the heating season to harvest the maximum amount of low temp BTU'S. Leave the thermostat up to about 75 degrees on the radiant loop to use this energy and to keep your tank at as low a temp as possible. Have a low limit relay to break the radiant curcuit when the temp in the tank drops below 85 degrees so it's all automatic. For the warmer months run the delta tee controller off the smaller tank to get higher temps for the domestic load. You will sacrifice harvesting some of the low temp BTU'S but the hotter water in the smaller tank will allow you to go 100% solar for a longer period of time. If you have a tilting collector, or a smaller collector array, you may not need a 1000 gallon tank to prevent overheating, but the larger tank will get you through longer periods of cloudy weather without using fossil fuels.

    Thanks, Bob Gagnon
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  • Wayco Wayne_2
    Wayco Wayne_2 Member Posts: 2,472
    Wow! Good stuff

    The reason I was asking about area used of tubes vs plates is because of my roof. I went up and sat up on the apex and watched the sun set and imagined the angles of the sun. I know my roof is slightly west of true South. About 10 degrees as a matter of fact. But then I looked at my tree line. In the Winter the lower angled sun may be diffused through the trees even though they will have lost their leaves. I was originally thinking of using some AET 4 x 10 flat plates, But if I used some Thermomax tubes I'd be able to keep more absorbing ability up higher on the roof away from any tree line influence. I'm meeting with my solar guy tomorrow for a site visit. I'm sure he can tell me what is best. He may even have one of those neato siting instruments that helps you visualize the path of the sun. WW

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  • I generally find viessmann documentation about as fun to read as this site pounded through babelfish in 3 languages ;)

    but I'll bite the bullet for this, thanks for the tip devan.
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  • Scott, My Solar Storage Tank

    Does have a good heat exchanger, I made it out of 5 coils of 1/2" X 60' copper. The hottest my primary domestic tank gets is about 160 degrees because I dump the heat from my solar collectors into my larger cooler storage tank, after the first pass through the primary domestic tank. The large storage plus the steep pitch of my collectors prevent overheating. The domestic tank doesn't have to be that hot, I get a good shower even if it is running about 115 or 120 degrees, remember the domestic water is pre heated by the large storage tank, which may run 90 to 110 in the summer, and it is also pre heated by my grey water system before it even gets to the solar tanks. With all this pre heating the domestic may be 100 degrees before it gets to the primary domestic tank. Staging the tanks like this give me 100% of my domestic hot water, there are only a handful days per year when the water is not quite as hot as I would like. The tanks are well insulated with 3" to 8" of foam. My basement does get a little warm and I may have to run my geothermal radiant cooling a little longer in the summer because of the heat loss from the tanks, but that is a renewable energy system also.

    Wayne, I also have trees cutting into my winter collection, but I don't notice that much difference when the leaves are off.

    Thanks, Bob Gagnon

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