Proper flow for solar

[mxdad777]

I recently replaced my 27 year old 2 hp single speed pump with a new Intelliflo variable speed pump. I have a solar system and was wondering what is the best way to determine the optimum RPM of the motor to efficiently flow the water through the solar panels? Is it just trial and error, or is there a method or formula to it? My solar panels are on top of the house (single story) approximately 50 feet from the pump.

 

[Bama Rambler]

If you can post the size and make of the panels and the plumbing layout (length, number of suctions, number of returns) and size and all equipment as well as the filter pressure when running solar and not. Maybe Mark (mas985, master of all things hydraulic) will come along and give you an estimate.

 

[mas985]

Rule of thumb for solar is about 0.1 GPM/sqft of panels. As for the RPM to get that, I need some more information:

Current Pump Make/Model
Height of pump relative to the pool water level
Diameter and number of individual suction pipes from the equipment to the pool
Diameter and number of individual return pipes from the equipment to the pool
Diameter and number of return eyeballs
Filter type and backwash valve model if known
Filter PSI with and without solar at 2500 RPM or higher
Heater - yes or no
Pool/Spa Valve - yes or no (If there is a spa, the number of jets and size)
Check Valves - Number and locations

 

[mxdad777]

Rule of thumb for solar is about 0.1 GPM/sqft of panels. As for the RPM to get that, I need some more information:

Current Pump Make/Model
Pentair Intelliflo VS 011018
Height of pump relative to the pool water level
12" above water line
Diameter and number of individual suction pipes from the equipment to the pool
One skimmer line and one main drain both 1 1/2" diameter
Diameter and number of individual return pipes from the equipment to the pool
One 1 1/2" return line
Diameter and number of return eyeballs
5 eyeballs 1" diameter
Filter type and backwash valve model if known
48 sq ft DE filter with Pentair 2" slide valve
Filter PSI with and without solar at 2500 RPM or higher
18 PSI @ 3110 RPM w/o solar (I'll have to check tonight w/solar)
Heater - yes or no
no
Pool/Spa Valve - yes or no (If there is a spa, the number of jets and size)
no
Check Valves - Number and locations
One 2" just after the filter and I think there are one or two others. I'll have to check

 

[MattM]

It depends also on how much you care about getting the max amount of heat out of the solar panels versus pump energy use.
I recently had my solar system reviewed by hot sun industries, and they felt that there were just a few parameters to keep in mind when determining a speed:
a) if you have multiple sets of panels across various sides of the roof, try to limit water in the colder roof sides so that they all have even heating (one of them shouldn't be tremendously hotter than the other).
b) start by finding the lowest speed that moves water onto the roof and see if air gets evacuated from all the panels, if not...bump up 50 rpm, shut off pump and let water drop from panels, and try again.
c) When you get to a point where all air is getting out of the panels, make sure it isn't taking too long....ideally all air removed from panels in first 5 minutes, but up to 30 minutes or so is fine...you need to take temp readings on the panels themselves rather than just watching the return valves (although the valves are helpful). Panels that don't have full water flowing will be much hotter than panels that do. To get an idea of what that temperature is, take an ambient reading when pump is not running.
d) find out how hot the panels are or put something on the pipes returning to the pool from solar and see how much the water has been heated. It should be at least 2 degrees warmer than the pool and not more than 8 degrees. The greater the differential, the less efficient solar heating will be as energy will be lost to the environment(air/wind/etc) rather than going to pool. On the other hand, if the value is too low...than you might have issues with the solar actually cooling the water on non-optimal days. There is some temp loss in just moving water up to the roof and back and the heating has to make that back and then some extra to making heating worthwhile. Thats why you should always try to see at least a 2 degree gain.
e) try to measure all the above between noon-2pm on a sunny day to get the most accurate results.
f) check solar threshold settings in intellitouch/whatever automation you are using
g) make sure the temp sensor on the roof is positioned correctly (there are numerous gotchas that can significantly impact solar performance if it is not setup right).

 

[mxdad777]

If you can post the size and make of the panels

Manufacture of solar panels is Fafco. There are 7 4x10 panels located on the roof. Panels are located at 12' above ground, but pipes running to and from panels go over a 19' peak. Pipe size to and from panels is 2" and total distance of run (one way) is 58'. Here is a picture of the panels I have. This isn't my roof, but they look the same. Basically it is a bunch of spaghetti tubing running between two pipes. Currently I have my pump running at 2000 RPM but I think I should be able to cut it back further. The flow seems pretty strong to me at the eyeball returns at the 2000 RPM.

 

[mas985]

Sorry, I seem to have lost track of this thread but your flow rate without solar should be: Flow Rate (GPM) = RPM / 36

What was the RPM and filter pressure with solar? (A higher RPM for calibration would be better)

As for flow rate with solar, you should only need about 28 GPM to maintain efficiency but that is not likely to be enough to keep the VRV closed so I would just step down RPM until you start to see air in the returns. But at 12' rise, the filter pressure should be above 6 PSI. Priming flow rate could also be a limiting factor so some trial and error might be necessary.