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looking for some direction on solar collectors for dhw

Post in 'The Green Room' started by 2.beans, Oct 6, 2008.

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  1. 2.beans

    2.beans Minister of Fire

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    im sure there already a post but input would be nice thanks

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  2. rhetoric

    rhetoric Member

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  3. myzamboni

    myzamboni Minister of Fire

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    domestic hot water
  4. DaveM195352

    DaveM195352 New Member

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    tons of material to read on-line - google solar hot water, etc
    the best site I have found is:
    builditsolar.com
    good luck and keep us posted

    Dave from Maine
  5. daveswoodhauler

    daveswoodhauler Minister of Fire

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  6. 2.beans

    2.beans Minister of Fire

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    i spoke to a solar salesman and he recommend the stiebel eltron flat plate solar collector. pexsupply sells them for 850$ delivered . anybody running these maybe? i think i really want to try some panels instead of messing with my wood boiler during the shoulder months.
  7. steam man

    steam man Minister of Fire

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    I am just now finishing up installing my system. I installed it myself-120 evacuated tube for DHW first and space heating to come. Only 60 tubes are in for now. The tubes I got from here-www.siliconsolar.com. I worked out the design criteria myself. It was not a simple installation for one man. I have been making some heat for about 2 days. I'll post some pictures later today.

    Mike
  8. peakbagger

    peakbagger Minister of Fire

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    I dont know where you are in NH but there is a solar co-op in the Plymouth area that does bulk buys of Apricus evacuated tube collectors and has weekend installs. Their website is http://www.plymouthenergy.org/. They have several installs coming up and each install, has an open house.

    I have flat plate collectors with a DC pumping system run by a 40 watt solar panel. It is very good at generating lots of warm water but not so good in the winter at generating hot enough water. The evacuated tube collectors are less efficient but are better at making hot water year round. Bascially a flat plate system will generate water no more than 80 F warmer than outdoor temp, versus evacuated tube can go 200F over the outside temp. So if you want smaller volumes of hot water year round directly off a solar panel go evacuated tube, if you use larger amounts of hot water and have a efficient means of stepping up the temp, go flat plate.

    Some trade offs for evacuated tubes are

    More fragile, (consider that the tubes are basically thick fluorescent tubes laying out flat on your roof).

    You need to find a way to dump the heat in summer as the fluid will boil and pressurize. Alternately you need to come up with a way of covering the panels when you dont need the heat.

    You want to make sure your pumping system is robust as overheated stagnant fluid in the tube header will breakdown quickly.

    As for DC pumping with a PV panel, it sounds neat and runs during a power outage, but it adds $400 to the system cost (note, most of the kits that are sold with DC pumping use a 20 watt panel which is insufficent to run a system in NH). I would install a 110 AC Taco cartridge circulator with a differential controller. The DC systems usually sell themselves as no need for controls "when the sun is out the pump runs", that is not fully true, it will work but you will lost some heat in the mornings and evenings when the tank temp is higher than the collector temp, yet the pump runs. There is a DC controller built by a guy in Maine (artec?) that works well to get around this issue.

    And lastly, make sure that you have mutiple check valves in the system and inspect their operation on a annual basis. Most of the failed systems from years past were taken out of service due to reverse thermosyphoning on a real cold night. If the check valve fails open, the system will send all the heat to the collectors on the roof and freeze the potable water side of the heat exchanger (usually bursting it).

    Drop me a PM if you have more questions
  9. steam man

    steam man Minister of Fire

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    Quote-Some trade offs for evacuated tubes are

    More fragile, (consider that the tubes are basically thick fluorescent tubes laying out flat on your roof).

    You need to find a way to dump the heat in summer as the fluid will boil and pressurize. Alternately you need to come up with a way of covering the panels when you dont need the heat.

    You want to make sure your pumping system is robust as overheated stagnant fluid in the tube header will breakdown quickly.



    I have a few comments on the above points. The evacuated tubes seem to be very robust. I man-handled them while installing and if the didn't break then I can't see them breaking from anything. Keep in mind flat plate panels having glazings that can break if enough force hits them. Certified evacuated tubes pass a test simulated to heavy hail hitting them. Losing the vacuum seal will be a greater concern. However, one tube loss does not shut down the whole system.

    Correctly sizing the system will minimize overheating. Proper design parameters and high quality heat transfer fluid must be used that is meant for evacuated tube temperatures.

    The temperature load should dictate what to use. Roughly, lower temp systems can utilize flat plate while higher temp's required should use evacuated tubes.

    Its just after 10am and my tubes have been on for almost two hours making heat in northern Maine.

    Mike
  10. 2.beans

    2.beans Minister of Fire

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    steam man what size storage tank do you use and what have you been able to get your tank temp up to?
  11. steam man

    steam man Minister of Fire

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    I've had it up to 110 deg or so today and yesterday. However, I still have pipes to insulate, some tweaking of control parameters, etc. We have already done laundry and showers so it is a little hard to tell. I want to try and use the heated water at the lowest temp during the day since solar collection is much more efficient at those temps. The heat calculator on the controller says I have made about 5kw every hour today minus .15 KW-hr for the pump operation. That's about 17,000 btu/hr. average. I still have to verify some parameters so it is hard to say. The 120 gallon solar preheat tank is hot though. I actually turned off my 40 gallon indirect it feeds into. YEAH!!!
    I only have 60 out of 120 tubes in for right now and possibly will go to 180 tubes. I want to be sure I don't over do it. I will have a large tank for radiant heat collection going in. I do have my pool that can take the excess heat. But this is the off season for that, of course. I'll go take some pictures and see about getting them on here.

    Mike
  12. steam man

    steam man Minister of Fire

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    My tank just hit 120 deg at 2:00 PM. Collector temp is 133 deg.

    Below is my installation.
  13. steam man

    steam man Minister of Fire

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    Made a mistake. I hope this works.

    Attached Files:

  14. steam man

    steam man Minister of Fire

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    4:00 PM tank is 128 deg. Collector about 138 deg

    Mike
  15. 2.beans

    2.beans Minister of Fire

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    could you let me know what you lose in temp in the overnight. looks good
  16. steam man

    steam man Minister of Fire

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    We'll be using some hot water tonight. Ideally, you should use it when it gets hot and have the tank relatively cool by sun up the next day for more efficient heat collection. Even using hot water during during the day when the tank gets hot should help collect more energy. Its like I tell my wife-living with solar requires a little timing change in our lifestyles. No big problem.

    Mike
  17. steam man

    steam man Minister of Fire

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    3:00 AM tank was 125 def F. Filled large whirlpool and temp went to 110 deg. 8:00 tank was about 98 deg. What I have an issue with is that having the tank sensor on the outside of the tank under the insulation probably gives it a slow response time, especially since it is a stone-lined tank. If I could fabricate an exceptionally long thermowell I could get the sensor inside the tank.

    Mike
  18. 2.beans

    2.beans Minister of Fire

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    what are you doing up at 3AM? ive built some long thremowells but its easyer to show in a picture than trying to describe it. ill try to post a picture this week of the thermo well.if your temp sensor is on the outside i bet your inside temps are higher. im wondering what i would need to heat up 1000 gallons for useable hot water. i can shut off one tank so id only have 500 gallons though, which may be easyer. keep up the posts, thanks
  19. steam man

    steam man Minister of Fire

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    Had to take the Pups out for their thing. Actually, I have to get on an early flight today.

    Tank made it to about 126 deg. I did notice the outside "skin " temp was 4 deg hotter than the the temp coming directly out of the exchanger for a little while. That shut down the circ so I do have a temp response issue. I have a 3/4" pipe plug at the top of the tank if I could put an extended thermowell down through there. What have you got in mind?

    Mike
  20. Hansson

    Hansson Feeling the Heat

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    I know that people here are building systems so the can reach stagnation.

    I have try to find some info in English
    http://www.aee-intec.at/0uploads/dateien48.pdf
  21. steam man

    steam man Minister of Fire

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    I am going to read the article with some interest. I am an engineer so I have looked at every detail of the design criteria for trying to do just that. Survive stagnation. Every component subject to those temperatures would have to be rated for it. The biggest obstacle I have found is the heat transfer fluid. Any "safe" propylene glycol product is probably not going to work. Other heat transfer fluids have toxicity problems, usually above 400 deg F not to mention flammabilty issues. Most importantly, finding a fluid that has the heat carrying capacity is also an issue. Ones I have looked had less than half the heat carrying capacity of water. There's also material capatibilty problems. Hopefully I will get that problem worked out. I do have heat dump capacity but there is always a weak link such as having one pump. I was thinking if the tubes could be low there could be a gravity dump zone above them. I'll read the article. If you get any more info on this subject I would appreciate you posting it.

    Mike
  22. Hansson

    Hansson Feeling the Heat

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  23. steam man

    steam man Minister of Fire

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    Thanks Hansson. I'll check it out.

    Mike
  24. steam man

    steam man Minister of Fire

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    Here's an update on my solar dhw heating. I have plans to connect the tubes to some radiant floor tubing as a dump but haven't got around to it yet. So......I figured with the shorter days I could just install most of the tubes and use my 160 g of storage without much trouble. I got 103 out of the 120 tubes installed. Seeing the tank temp approach 140-150, I ran the dishwasher, and then two loads of laundry. The tank temp didn't drop much but the collector temp dropped and started cycling the pump. This told me that the cold water in the bottom of the tank was affecting the glycol temp but the tank skin temp has a slow response. That was reading up to 149 deg. I fabricated an extended thermowell that reached from the top down to the bottom 1/4 of the tank. That temp now read 167 deg F-an 18 deg difference. That shut the pump down since I had the high temp limit set at 160. It was late in the day so the sun was going down. Introducing cold water into the tank also gave a much quicker tank temp response. Looks like I'll have to get the floor tubes going.

    Anyone know what the max temp a dhw tank should store water at? I don't see anything stated right off hand.

    Mike
  25. Westernmainer

    Westernmainer New Member

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    [This question is for Mike?


    How is your system working now with the recent cold weather?

    We are looking at systems for our house in Maine




    'v
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