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Old 09-28-12, 01:19 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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Old 09-28-12, 01:31 PM   #12
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Thank you very much for running through the calculations Gary. I think the only thing you forgot is that I'm running a closed loop system with probably a 50/50 mix of polypropolene, so that flow rate must increase to get the same heat out.

According to your site your recommend a 15% higher flow rate, and say your pressure head will increase by ~30%.
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Last edited by Daox; 09-28-12 at 01:34 PM..
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Old 09-28-12, 04:25 PM   #13
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Quote:
Originally Posted by GaryGary View Post
Hi,
Just some thoughts on sizing the pump and supply and return line for this 200 sf collector...Gary
Wow Gary, great job!

Thanks for including all your references.

I know when I was trying to calculate why my loop field pump was using so much power, and what the fix was, I nearly went blind shifting from PDF to PDF, and trying to co-relate all the info.

There must be a spreadsheet or program that would inter-relate all those variables.

Best,

-AC
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Old 09-28-12, 06:27 PM   #14
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Gary, that is great. I just didn't have time to do the calculations.

What it does show is that the the flow rate is on the high side. Remember that the flow rate has a relationship to the amount of heat collected on the absorber so that is what your flow should depend on. As you don't know this, with this panel, I would start with a RESOL controller with variable speed where you can set the parameters. That way you will get the optimal flow rate.

Doax, you only need 40% propylene glycol not 50% so you pumping should be a bit easier.
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Old 09-29-12, 09:14 AM   #15
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Thanks for the help guys. I'll look into reducing the flow rate and using a smaller pump.

According to my NoBurst bucket, 50% polypropylene will give me 'burst protection' to -60F, and 'flow protection to -10F'. Its really not the clearest. Does that means its going to start turning to slush at -10F?
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Old 09-29-12, 11:38 AM   #16
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Hi,
Just wondering how you plant to insulate the 100 ft pipe?

Gary
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Old 09-29-12, 12:06 PM   #17
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Here is the Tyfocor propylene glycol report for Bosch (Buderus).

http://www.google.ca/url?sa=t&rct=j&...FqWeIw&cad=rja

It is a 2mb file and I didn't know how else to show it. I use this stuff almost exclusively and it is available from Viessmann or Buderus dealers as well as Calefi. Bar none, it is the best one out there.

If you have a way to make sure that the panels never get above 90C, I would say to use the PEX but there are dual PEX pipes available in a plastic case with urethane insulation meant for underground that you could get. It would ensure that you had minimal heat loss and keep it sealed. Remember that 100ft one way is a long distance.
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Old 09-29-12, 12:11 PM   #18
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viscosity for any oil gets higher when the temp gets colder but there are systems running in the Canadian arctic with 50% glycol. I have done a number of systems in Fort Francis, Ontario with 40% and AFAIK there has not been any issues.

How deep can you bury the piping and can you place a 2" sheet of SM over the tubing to allow the ground heat to come up closer to the tubing? It's just one way to help it out.
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Old 09-29-12, 05:51 PM   #19
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Quote:
Originally Posted by GaryGary View Post
Hi,
Just wondering how you plant to insulate the 100 ft pipe?

Gary
It will be surrounded by 3" of XPS similar to what you did with yours.



Quote:
If you have a way to make sure that the panels never get above 90...
That can be done. I am building my own differential controller to control the system so I can do whatever I want.

Quote:
...I would say to use the PEX but there are dual PEX pipes available in a plastic case with urethane insulation meant for underground that you could get. It would ensure that you had minimal heat loss and keep it sealed. Remember that 100ft one way is a long distance.
I looked into commercial insulated options. IMO they don't offer nearly enough R value, and they're incredibly expensive. Most of the stuff I saw had only around R3 (where mine is R15), and it cost 3X more than what I am doing.
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Old 09-29-12, 05:55 PM   #20
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Quote:
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It will be surrounded by 3" of XPS similar to what you did with yours.





That can be done. I am building my own differential controller to control the system so I can do whatever I want.



I looked into commercial insulated options. IMO they don't offer nearly enough R value, and they're incredibly expensive. Most of the stuff I saw had only around R3 (where mine is R15), and it cost 3X more than what I am doing.
R3? Ive never seen one that low but if you are boxing it in that is fine. I would still put armaflex around it as well.

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