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SolarInterested

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Reply with quote  #21 
Quote:
Originally Posted by stmbtwle
... My test procedure will be as follows (when I get it together): Set up the water-side to run through a 5-gal bucket.  Record start temperatures of ambient air, collector air near the top, and water. ...

Input/ideas/suggestions are welcome.


Could you add the temperature of the surface of the glazing? Circulating air inside the collector should be increasing losses through the glazing as would be indicated higher temperatures on the outside of the glazing when the fan is running.

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stmbtwle

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Reply with quote  #22 
You have a valid point, but considering the surface of the glazing is exposed to ambient air, I'm not sure there'd be much difference in outside temperature as the outside air will cool the surface.  I can try it, though.

Running the fan probably will increase losses through the glazing, but it should also increase transfer through the heat exchanger.   We won't know till I try it.

I'm using twinwall, so I may be able to stick a probe into the edge and see what I get there, too.

I've ordered a DC speed control, so I'll be able to reduce the fan speed to see if there's any difference. 


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stmbtwle

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Reply with quote  #23 
Re the fans:  While the output IS water, in reality this thing is an AIR collector (though you might call it a hybrid).  If it had two ducts running to and from an external heat exchanger I'm sure you'd call it an air collector, and most would agree that the fans are necessary in that case.  The only real difference I see is that the heat exchanger is internal.

The fans are there to transfer the heat from the air to the water via the heat exchanger.   They should also help the air flow along the hot collector surface, and recover more heat.  If we lose some to the glazing in the process, so be it.  As long as we gain more than you lose, we're ahead of the game.  We'll find out soon enough.

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stmbtwle

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Reply with quote  #24 
Well it's built.  It went together quickly, probably a lot more quickly than a CPVC/aluminum collector.   Pics:
http://s71.photobucket.com/user/stmbtwle/library/Solar%20collector
Notes:

Overall dimensions are 24"x48"x7" 
Effective collector area about 7.27 sq ft.  / .675 sq meters.
Heat exchanger unit 10"x14" is located midway between front and back of collector, with aluminum baffles on each side to prevent "short circuit" air flow.
Fans(2) rated at 40 cfm each, 80 cfm total
Pump rated at 1 gpm.

For the moment it is an open "black box" per the original post.
Possible improvements:  
     Screen attached to back to increase turbulence
     Screen absorber down center
     Metal divider/absorber down center, possibly corrugated
     Metal divider with screen absorber in front.
     Run hoses inside collector to pick up more heat (may have to go to cpvc).

Tomorrow we get to "play"

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solardan1959

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Reply with quote  #25 
stmbtwle,
   First, I applaud your effort to do this test but I disagree with a lot of things you are saying.  The radiator should be, well is, the absorber.  The sun and lots of hot air all around the radiator will heat it up, it does not stay cool and only warms up as you blow hot air across it.  There really is no thermo syphon action as the hot absorber, (the radiator), is cooled by the active pumping process.  The fan will only circulate the cooler air from the bottom and mix it with the hotter air that will naturally be at the top heating up the radiator anyway, so just blowing more cooled air across the radiator is not going to transfer any more heat than having the hotter air that already fully surrounds the radiator does.
  But lets see how your test goes and thanks for doing it.  I would like to build a double radiator set up so the whole collector has radiators in it with no open space on the bottom.

   On the other hand if we just took the hot air out of a collector or blew it past the radiator as it exited then while not being very efficient on the water side, you would get hot air for heating and warm water.  With that configuration I would agree with what you were saying but I would also then put the fans at the input to the collector and effectively blow air across the radiator and not directly blow air right onto the radiator.

  But this is all guessing and only testing will prove it so I look forward to your results.  I would focus on the water temp not the collector temp but both are nice to know.

Dan
stmbtwle

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Reply with quote  #26 
Dan.. first, I have to construct something approximating the original plans for the ARETHA collector, otherwise what's the point? If I can improve on it later, fine.

Ok the radiator is the "absorber" in that it absorbs heat from the air and transfers it to the water. In other words a heat exchanger.  But by itself it's not going to "absorb" much heat directly from the sun, it's too small.  The collector IMO is the air box with whatever appurtenances are installed. The radiator is just a means of transferring the heat from the air to the water.

Sure you could build a collector that is all radiators, but you will then have a finned water-tube collector, not all that different from any other. It will probably be more efficient from a thermal point of view, but it will also be more expensive. The original post talks about 5 sq. meters, that's going to take a LOT of radiators, and drive the cost up severalfold. The way I read it, the driving factor here is cost. So in my mind the collector is the air box and always will be. If I want to collect twice the heat, I simply double the size of the box (cheap) and may be able to use the same heat exchanger. If I can improve the box with a screen or a divider, I think that's "legal".

Others... bear in mind that the radiator does NOT function as a radiator, but as a heat exchanger to cool the air and heat the water, quite the reverse of its usual function. Will it work? I'm sure it will. How WELL will it work? Only testing will tell. Could you take any air collector and duct the air through a radiator/heat exchanger? Sure you could and it might even be better. I encourage you and the other members to try it.

I've seen a couple threads where someone has installed a "baseboard heater" in the top of an air collector, to get hot or at least warm water.   This is a slightly different approach to the same idea.  Is the fan an improvement or a detriment? we don't know at this stage.





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Budwyzer

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Reply with quote  #27 
That is sexy cool.  Good job on it!


I have a question, Where do you get your glazing? 
stmbtwle

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Reply with quote  #28 
Results of the first real test:
1542 (3:42 pm to you landlubbers)  Air temp 86.  Water temp 84.  Water volume 6 US gal.  Collector temp 153F  Start fans.
1543   Collector temp 144F  (Internal temperature drops at first when fans are started, possibly due to mixing)
1546   Collector temp 153F  (temp is starting to climb again)
1553   Collector temp 167F  (this thing heats up FAST)  Start pump.
1600   Collector temp 106F. Water temp 88F  (the "radiator" really works at transferring heat)
1620   Collector temp 109F  Water temp 93F  Turn off fan to see what happens, pump still on.
1624   Collector temp 120F, water temp 93F   (without fans collector heats up, heat exchange not as good)  Turn on fans
1630   Collector temp 113F, Water 95F
1700   Collector temp 115F, Water 102F
1730   Collector temp 120F, Water 108F


Other notes: 

Once stabilized, collector temperature is 10-20F warmer than water input temperature.  As the input water increases, so does the collector temp.   Reducing water flow should increase the output temp, if not the storage temp.

Stagnation could be a real problem with this type of collector, some provision for venting the heat is needed.

The heat exchanger definitely works.  The air side might be improved; I don't know what it would take to get say, a water temp of 120F.

I don't have equipment to determine the actual water temp rise in the collector, working on that.  Nor do I have speed controls installed on the fans or the pump.  Reducing speed on either should increase temperatures (the whole object of this exercise IS hot water, after all)...

My calculations indicate something on the order of 1200 BTU gain in 1.8 hours, or 663 btu/hr for a 2x4 collector.  I don't know if this is something to get excited about or not...


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stmbtwle

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Reply with quote  #29 
Observations:

This collector should have no problem attaining the temperatures mentioned by Paleometeo.   I've reached 135F (57C) and it was still climbing.

The fans are required.   Without them the air temperature inside the collector gets to dangerous levels.  A different box design might help.

With fans and pump running, steady-state: 
     Air temp Ambient: 86F / 30C  before fan:  113F / 45C   After:    99F / 37C      Change: -14F / -8C
     Water temp:                        Input:          90F / 32C   Output: 91F / 33C       Change: +1F / +1C

Can anyone tell me from those temp differences whether I need more/less fan or pump?


With fans off
     Air temp (not stable)            before fan   127F / 71C   After:  160F / 71C   (and rising)


Here in Florida the sun goes nearly overhead, so I have to point the collector East in the morning and West in the afternoon (forget South).  Maybe I should try laying it flat on the ground?

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solardan1959

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Reply with quote  #30 
stmbtwle,
   Sounds like you need a bigger tank and more flow.  That starting at 86 ambient doesn't hurt either.  Another radiator and a kiddie pool would help to try and cool that down.  Be interesting to see what it does when its much cooler like around 32.
  Up here, (Northern Minnesota), I ran about 150 to 160 inside my closed collector with outside air under 70.  I think my flow was about 3 gallons a minute.  With temps like yours people normally vent the collectors or pretty much leave the top and bottom open.  I wonder what kind a job the radiator would do if there was no glazing with your heat.

Dan
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