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Old 04-08-12, 07:55 PM   #1
RSquirrel
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Default Collector Circ Pump for Small Pool

I am planning to fabricate an open-loop, 4'x8' solar collector utilizing 81 feet of serpentined 1/2" ePEX tubing (yes, the tubing will be totally protected from direct sunlight). The PEX tubing has an inside diameter in the range of 0.461" to 0.509". At this length, the total volume within the collector computes to approximately 0.70 to 0.86 gallons of water.

Initially, I will use just one collector panel, with the option of adding a second one, based on need and degree of success.

I will be mounting the collector(s) in an optimally-pointed location (with adjustable angle) on the roof of the pool house for a small (~3000 gallon) above-ground pool (3 psi head - top of collector to surface of pool). So, I will need a small circulation pump (no, I cannot use the system pump - it does not have sufficient capacity beyond what is needed for filtration).

What formulas and/or rules-of-thumb can anyone suggest to determine minimum, optimum, maximum flow rate (gph) for said pump, in order to maximize solar gain (or more specifically, transfer of heat into the pool)?

If a second collector is added (same elevation), should it be in parallel or series with the first?

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Old 04-08-12, 10:10 PM   #2
Xringer
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In electronics, resistance in parallel increases the flow.
Picture the pump's output hose changed to twice the diameter.
My guess, that series would make the pump work harder.

I think the heat inside (under the glass) of the last panel of a series array, is the hottest.
So, it seems logical that series will give you hotter water, but at less GPM.
Whereas Parallel would give you more water that's less hot.

Kinda seems like the amount of BTUs per sq foot is going to be the same, regardless of configuration.
Except for one thing. If water is already really hot in the last collector,
then perhaps it's not going to be as easy for the sun to add more heat.

Picture water coming into a collector that's already outrageously hot.
It seems like water coming into a coil that's lukewarm would be easier to heat.

Picture putting 5 pound block of copper on the stove, and heating it to 400F.
Not too hard to do, shouldn't take too long.

Then, After it's at 400F, leave the gas setting the same and see how long it takes to get the block on up to 800F.
My guess is, it's going to take a lot longer than it did to get it to 400F.
(I've never done this.. So, I might be wrong)!
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Old 04-10-12, 06:20 PM   #3
herlichka
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I have a similar setup heatig my small (1700 Imp. gallon) pool. It consists of about 300 feet of 1" poly, fed by a submersible marine bilge pump powered by a 20 watt solar panel.

There are no control devices, the pump runs when it's sunny, and doesn't when it clouds over.

A couple of hydraulic fundamentals will help you in your design. The head elevation will be offset equally by the fall as the water leaves your collector and returns to the pool, a siphon effect if you will. So the power required to pump "up" will be supplemented by the gain of the falling water on the return side. This is assuming that there is no vent in the loop, this would break the siphon.
Parallel conductors reduce the "friction loss", allowing a pump to pump a lot more water. Splitting the flow into two same diameter, same length pipes will allow four times the water to flow while the pump does virtually the same work. It may be to your advantage to split the pex within your collector. More water will flow through your collector, and your pump will not have to work as hard.
I don't really know if flow rate are that critical- slow flows will produce hotter water, faster flows will produce more water, but at a lower temperature. The net energy gain is the same, but it seems more rewarding to feel hotter water flowing into your pool. The only other consideration is that you can use a smaller pump to pump at lower flow rate, maybe saving some money on the purchase.
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Old 10-04-12, 09:23 AM   #4
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You could always just go with a purpose-built solar pool pump like the Lorentz PS600 system, but there are flow issues at low irradiance levels (not enough flow through solar panels to get desired heating performance).

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