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Old 09-28-12, 12:19 PM   #11
GaryGary
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Hi,
Just some thoughts on sizing the pump and supply and return line for this 200 sf collector.

Heliodyne recommends a flow rate between 0.025 gpm/sf of collector to 0.075 gpm/sf of collector.

From the point of view of thermal efficiency of the collector, within reason, more flow is always going to make for higher collector efficiency. More flow means the temperature rise over the collector absorber is going to be smaller, so the average absorber temperature is lower, so the heat loss out the glazing is smalerl, so the efficiency is better. From the collectors point of view, as far as I can see, there is no "ideal" flow -- the more flow the better the efficiency. Eventually you end up with a pump that requires a lot of power and flow velocities that are too high for copper, and I suspect that is what sets Heliodyne's upper limit.


Look at a Low and and High flow rate:
At the low end of this range -- say 0.03 gpm/sf or 6 gpm (for 200 sf)
At the high end of this range -- say 0.05 gpm/sf or 10 gpm:

Pressure Drops:
If the collectors are 100 ft from the house, that's 200 ft return, and maybe add an extra 50 ft for other loses (turns, valves, manifolds)... just to get a rough idea.

Pressure drop for 1 inch pex at 10 gpm is 32 ft of head, and at 6 gpm, 12.6 ft of head.
Pressure drop for 3/4 cpr at 10 gpm is 60 ft of head, and at 6 gpm, 23.6 ft of head.
Pressure drops from:
http://www.everhotinc.com/barrier-pex-tech-specs.pdf
Pressure Loss of Water Due to Friction in Copper Tubes

The 3/4 seems a bit small to me even at the lower flow? This is a very long pipe run.

Efficiency:
This page tries to give a quantitative answer to what happens to collector efficiency as you raise the flow rate. Its based on the logic listed above -- that is, more flow rate means lower collector temperature rise, which means lower heat loss and greater efficiency.
Determining Solar Water Heating Collector Flow Rate

The values in the table of collector efficiency vs flow rate are calculated using this collector efficiency calculator: Solar Collector Efficiency Calculator
The only thing that changes for each flow rate is that the average absorber temperature goes down a little as flow rate goes up -- this causes efficiency to increase a bit. The table assumes full sun, 40F ambient temp, and 100F collector input temp.

So, if you buy that argument,
Collector Efficiency at 0.03 gpm/sf is 49.4%
Collector Efficiency at 0.05 gpm/sf is 50.1%

Heat output:
Heat out for 200 sf at 0.03 gpm/sf = 29640 BTU/hr or 8687 watts
Heat out for 200 sf at 0.05 gpm/sf = 30060 BTU/hr or 8810 watts.

So, under full sun conditions, the higher flow rate produces about 120 watts more in heat output.

This 120 watts of increased heat out for the higher flow rate can be compared to the increase in pump power needed for the higher flow rate.
If they are close to a push, than I'd say go with the smaller pump.

Just looking at a couple of Taco pumps:
New Page 1
At the 0.03 gpm (6 gpm) and 12.6 ft of head
A Taco 008 is a good match and does the 6 gpm at 14 ft of head.
power consumption is 92 watts for the Taco 008.
http://www.taco-hvac.com/uploads/Fil...ry/100-1.9.pdf


At the 0.05 gpm/sf (10 gpm) and 32 ft of head,
The Taco 013 comes pretty close -- just a bit short on head at 10 gpm.
power consumption is 230 watts.
http://www.taco-hvac.com/uploads/Fil...ry/100-6.4.pdf

So, the pump power increase to go up to 0.05 gpm/sf is roughly 230-92 = 138 watts compared to the 120 watts for added heat output for the higher flow. So, it seems like the lower flow rate and smaller pump is the clear winner here?

This is the first time I've done one of these calcs where the higher flow rate did not win -- I guess because of the fairly large pressure drop along the long supply and return pipes.


I'd appreciate anyone's thoughts on these sizing methods.

Gary

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