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Solar HW how to plumb the HX?

Post in 'The Green Room' started by Bad Wolf, Aug 10, 2012.

  1. Bad Wolf

    Bad Wolf Feeling the Heat

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    I'm finally to the point of hooking up my panels to the coil I placed in my unpressurized storage tank when I installed it 4 years ago.
    Question: Should the HW from the panels go to the top of the coil or the bottom? This is a 150' copper coil similar to the ones for my boiler HX and DHW coils. With the boiler I go into the top so as to maintain stratification, i.e. the hottest water enters at the top and sheds heat as it goes toward the bottom.
    But I was thinking that since there is usually a 15 degree differance between the top and bottom of the tankand my controller is set for 10 degrees higher than the top of the tank the panels would have to achive at least 25 degree rise or everything would shut down.
    Sending the HW to the bottom seems counter to the way I do the boiler and would cause some turnover as the heated water on the bottom rises to the top.

    Which is the proper way for a solar panel set up?

    This is mainly meant to be a summer DHW set up only, once I get to burning I doubt the panels would ever be able to come on since I keep the tank between 150 and 180 in the winter.

    Thanks

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  2. GaryGary

    GaryGary Feeling the Heat

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    If you pull the water for the collectors out of the bottom of the coil, the collectors will run at a somewhat lower temperature than if you pulled the water out of the top of the coil -- this will make your collectors more efficient, since the average absorber temperature will be lower and the absorber will lose less heat out the collector glazing.

    For a good collector with 40 F ambient temp, lowering the average absorber temp from 120F down to 110F results in the efficiency going from 53.4% up to 55.9% under full sun conditions -- so, a modest, but worthwhile gain.
    http://www.builditsolar.com/References/Calculators/Collector/ColEfic.htm

    Seems like returning the hotter water from the collector outlet to the top of the coil/tank will encourage stratification in the tank, which is good.

    Gary
  3. semipro

    semipro Minister of Fire

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    From what I can gather from research, heat exchangers operate more efficiently when flow is configured so that temp differential between the two different media is maximized. This makes sense as heat flows faster when across a great temperature difference. (This is why setback thermometers save energy.)
    In your case this would mean introducing water from your collectors to the bottom, colder portion (end) of your storage tank.
    And, while stratification in your storage tank may be good for some reasons, any movement of the tank fluids will tend to break down the boundary layer that forms at the coil surface, again increasing the temp. diff. and enhancing heat transfer.

    Gary's point is a good one too though. Returning the coldest water you can to your collector enhances heat transfer efficiency there so you'd want this water coming from the cold end of your storage tank.

    I guess the question is then how do you maximize heat transfer at both the collector and storage tank?

    Since there are multiple inter-dependent processes going on here that seem to be counter-effective I'd be inclined to try it both ways and monitor the storage tank inlet/outlet temps. Whichever plumbing configuration gives you the highest average temp at your storage tank given the same exposure conditions at the collector (ambient temp, radiant energy) wins.

    I'm just thinking out loud here. Someone, (like Gary ;)) has probably already done such testing and I just didn't come across it.
  4. GaryGary

    GaryGary Feeling the Heat

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    Good point -- I had not thought about that.
    Not sure which of the two sets of benefits would be the most important.
    I guess one thing is that if the heat exchanger coil is 150 ft long its quite a bit of heat exchanger area, so its probably going to be hard to keep it from working well :)

    How many sqft of collector area do you have? and, what is the diameter of the heat exchanger coil? -- these numbers would allow comparing yours to the usual recommendation for collector heat exchanger coil area for collectors.

    Gary
  5. mmudd

    mmudd Member

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    My system pumps actual tank water thru the collectors as it is a drain down system and is not pressurized. I pump out of bottom of 200 gal storage tank thru 168 sq ft of collector and return from collectors to top of tank to encourage statification and keep hottest water on top. My DHW heat exchanger is a loop of copper that sits in top of tank. It sounds like your system is different from mine in that your solar collectors will be pressurized loop running thru a copper coil heat exchanger to heat up your non pressurized storage. I would still think you would want to hot line returning from collector to enter at top of coil to encourage stratification.

    By the way, my collectors turn on even when my wood boiler has supplied heat to the storage. My storage tank and collector water are the same, with no heat exchanger. Because my solar controller is really a differential control , it is all relative. Each trip thru collector water will gain heat, so if bottom of my storage was at 120 the collector water returning would be 15 degrees or so hotter at 135, which was usually a few degrees hotter than top of storagetank temp, . If storage was at 150, collector water would come back at 165 etc. It was not a big factor of heat gain in winter but every bit helps. If water temp was at high limit of solar controller, it would obviously not run, but that never happened at bottom of storage tank. Not sure if the use of pressuized solar system with heat exchanger in an open storage would really make any difference.

    Since sun is lower in sky this time of year and outside temps have been around 70, I have been getting to 170 and higher temps in 200 gal storage after 2 days of good sun
  6. pdf27

    pdf27 Member

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    You're missing something here: you're trying to minimise the return water temperature, not maximise the heat transfer when the water first enters the heat exchanger. If you put the hot in at the bottom, it will rapidly cool down at first but will also rapidly approach the (warmer) temperature of the water higher up the tank, thus causing a major drop off in heat transfer. If you put the hot in at the top, the return will be exposed to a lower temperature before it leaves the tank and should therefore be cooler. That's the way most commercial heat exchangers work - with the two fluid flows going in opposite directions.

    Maintaining stratification in a solar tank is also a good thing - it means in a marginal case you've got half a tank of warm water instead of a whole tank of tepid water.

    Having said that, the difference is pretty minor - if one is easier than the other I'd be pretty tempted just to plumb it in whichever way is easier.
  7. tom in maine

    tom in maine Minister of Fire

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    Solar hx should always be in the bottom of the tank, unless you want to make the solar system perform with less perceived storage.
    If you install the solar hx nearer to the top of the tank, the solar system will never heat the entire tank and you could compromise your DHW hx if it is
    covering an area where solar does not reach.
  8. Bad Wolf

    Bad Wolf Feeling the Heat

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    All of my coils sit on the bottom and go all the way to the top of the tank. I have six 3x7 panels or 126 sq.ft. coils are about 24" in dia.
    So far this set up has worked well. On the boiler side (double coil) my return water is almost always the same temp as the water at the bottom of the tank. Thats telling me that its able to shed all of its heat in the tank. I'll plumb it coming in at the top and going out the bottem just like my boiler coils. I figure this will be mostly a shoulder or summer time system. If I get any benifit in the winter great but I'm not counting on it.

    Once I get up and running I'll report back.
    HPIM2638.JPG
  9. mmudd

    mmudd Member

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    Looks like there is space between each pipe in the coil so you you could compress it down. I would compress your solar coils as close as possible to get them as low in the tank as possible, since you are adding heat to the tank fron the coils, you would want them in the colder bottom water of the tank as much as possible I would think to get the heat transfer to tank water. Either way it should work.

    That looks like a lot of storage for the collector area. 1200 gallons vs 126 sq ft collector. You won't get much in winter, but it will run. My first solar system was 64 ft collector and 450 gallons storage. After 10 days of high 90 temps at peak of heat season, Top of tank was 135, bottom much cooler. In winter, could not get noticable gain, but maybe some maintanance heat. I now have 168 ft of collector and 200 gal non pressurized storage. Yesterday, after 3 good sun days, tank was near 180 degrees with about 10 degrees diff at bottom. That will last me about 3+ days for DHW needs, depending on use. Much better performance. It works good in autumn even though ambiaent temps are lower with sun at lower angle.

    I'm in process of hooking up 2 coils in solar tank to wood boiler system . One at bottom will keep min temp for DHW from wood boiler pressuized storage tank, one at top will be able to supply heat to my boiler storage. Like you, not sure what to expect as far as output, but every little bit helps. Good luck
  10. semipro

    semipro Minister of Fire

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    Getting into to these discussion about which way works best makes me just want to hook up some sensors and a data logger and try different configurations.
    Maybe one day I'll get around to it....or someone like Gary at BuildItSolar will beat me to it.
  11. Thomas Anderson

    Thomas Anderson Member

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    If you have flat plate collectors, they might have trouble keeping up with 180 degrees, but evacuated tubes will go basically to boiling if you don't keep them cooled by continually circulating them, even in the dead of winter. The correct configuration for maximum heating in any case is to both deposit and extract at the top of the tank with maximum stratification. Since your coils go the full depth, I would start with the hottest water at the top of the coil and expect the output of the coil at the bottom to be the same temperature as the bottom of the tank. That sets you up for both good stratification and also helps ensure the best absorption from your panels by sending them the coldest water possible. If you go the reverse direction, you send your panels hot water (the temperature at the top of the tank) and you'll lose more heat to the outside. The coils going to the house should be the opposite -- the last water out should be from the top so that the house gets the hottest water available.
  12. Thomas Anderson

    Thomas Anderson Member

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    In my own system, I'm installing a mixing pump which will only turn on if the top of the tank exceeds 180::F. This way, the tank will be stratified until the top gets too hot, then it will mix the contents to increase the total heat storage. My backup propane on-demand hot water heater kicks on if the temperature of water going to the house is below 167::F, so that provides a decent margin. And by keeping the top cooler than the rating of the liner, I'll extend the life of the tank. But if the top is under 180::F, stratification saves on propane.
  13. woodgeek

    woodgeek Minister of Fire

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    An interesting approach.

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