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Re: testing pool solar panels

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Posted by nicksanspam on June 8, 2007, 12:36 pm

OK. If it's 60 F outdoors and the pool is 70 F and 250 Btu/h-ft^2 of sun
is hitting 80 ft^2 of unglazed pool heaters 2' above the water surface,
how much water should we pump through the heaters to maximize the COP?

It looks like the answer is zero, with zero heat gain for the pool :-)

If 250x80 = 20K Btu/h = poolgain + airloss, and poolgain = 500gpm(Tf-70)
and airloss = (T-60)80ft^2x2Btu/h-F-ft^2, with final and average heater
temps Tf and T = (70+Tf)/2, Tf = (24K+35Kgpm)/(80+500gpm), which makes
COP = 89.5K/(gpm+6.25gpm^2), with a min COP = 0 at infinite gpm.

20 HEAD=2'pool heater head (feet)
30 FOR GPM=1 TO 5'heater flow
40 HP=HEAD*8.33*GPM/60/550'pump horsepower
50 PEt6*HP*3.412'pump power (Btu/h)
60 TF=(24000+35000!*GPM)/(80+500*GPM)'final heater water temp (F)
70 PS`*8.33*GPM*(TF-70)'pool solar gain (Btu/h)
80 COP=PS/PE'coefficient of performance

1        101.7241      15855.72      12338.92
2        87.03704      17030.23      6626.459
3        81.64557      17461.37      4529.477
4        78.84616      17685.23      3440.661
5        77.13178      17822.32      2773.866

Why pump more than 2 gpm? Going to 4 increases the pool heat gain by 4%
while halving the COP (to a thousand times more than an AC COP of 3 :-)


Posted by gfretwell on June 8, 2007, 5:16 pm
On 8 Jun 2007 08:36:36 -0400, nicksanspam@ece.villanova.edu wrote:

If it is 60f outside and you have unglazed collectors you are wasting
your money pumping water up there in the first place.

Posted by nicksanspam on June 8, 2007, 5:53 pm

No, in full sun. This is a Small Matter Of Physics.


Posted by gfretwell on June 8, 2007, 8:06 pm
 On 8 Jun 2007 13:53:54 -0400, nicksanspam@ece.villanova.edu wrote:

The physics I am thinking of is the heat loss to the air.
Outdoor pool?
You might actually see some small net gain in water temperature but
you won't get it warm enough to swim in and that is the point isn't
I suppose if you are one of those "polar bear club" folks you can but
most folks I know won't get in water that is much below 80f.
I have about 70% solar to pool surface in South Florida and I have a
hard time maintaining the ambient air temp with the pool uncoverd,
covered will get me about 10 degrees above ambient air but that drops
like a stone as soon as you take the cover off.  

Posted by Steve on June 9, 2007, 12:30 pm

Interesting but I dont know what the hell it all means... :-)
Panels are not "up there" they are 2 feet above the ground on a 4' x 20'
plywood stand. No glazing just 2 panels with 50 1/4" black plastic tubes per
I'm in New England, 80 would be great but I'd be happy to get to the mid
70's. Now that I have the setup with controls I need to get a sunny day
while I'm at home to run some tests with the amount of flow.

You bring up a good point.. I was thinking low flow with higher heat
transfer but I see the point of having higher water turnover with just a few
degrees rise. Its not how hot I can get the water coming out of the
collectors. the point is to increase the whole pools temp.... That may be
better with the higher volume lower temp increase???

Now wheres the sun !!!! Forcast is for clouds and rain all week...




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