# Thread: Flow rate to solar panels

1. ## Flow rate to solar panels

I just installed a Pentair intelliflo variable speed pump. I have the Pentair Solartouch solar controller connected to it, as well as a flowvis flow meter. I'm trying to set the recommended flow rate for the Fafco Sunsaver panels that I have installed (2 story house). From what I've read, the optimal flow rate is 4 GPM per panel. I have 7 panels. When I adjust the speed of the pump so that it shows 28 GPM on the Flowvis, it doesn't seem as if the pump is pushing enough water, as I start to see air in the flowvis, and get bubbles in the pool. To get to the point of no air bubbles, I have to set the speed to around 2500 RPM, which is showing 50 GPM on the Flowvis. Am I missing something, or am I stuck with having a higher flow rate due to my setup?

2. ## Re: Flow rate to solar panels

The problem is that you don't have enough pressure in your panels at the lower flow rate since the 2-story return to the pool is negative pressure that needs to be overcome. Otherwise, the valve at the top of the panels opens up and lets air in. One way of handling this is to restrict the flow at the bottom of the return from the panels so that the pressure will be higher. You can have the same flow rate, but the restriction will increase the pressure. It's similar to what happens when a pump is connected to narrower piping or to something with a flow restrictor. To get the same flow rate, the pressure must be higher.

If your panels even when hot could handle negative pressure without collapsing, then a different sort of valve that only opened to let air in when the pressure exceeded a certain negative pressure (negative with respect to air; i.e. lower than air pressure) could be used, but that is not normally done (I write later on about moving the air intake to a lower height instead of at the top of the panes). Instead, flow restriction is added to increase pressure in the panels. Ideally, such flow restriction would be dynamic so that at higher flow rates (pump RPM) less restriction would be used (I think there are spring valve restrictors that do this, but others know about such things better than I).

If each story is 12 feet and your panels are on a roof that is another 6 feet high at the top, then that's 30 feet which is 13 PSI. The formula I use for my Intelliflo relating Head (feet of water) to RPM and GPM is the following:

Head (feet of water) = (RPM/350)2 - GPM2/470

With 2500 RPM and 50 GPM I get 45.7 feet of head which is obviously more than 30 and likely higher than the height at the top of your panels where the air release valve is likely placed. You didn't say the RPM when you are at 50 GPM, but if I assume 1500 RPM at 28 GPM then I get a Feet of Head of only 16.7 which obviously is not enough.

From an energy efficiency point of view for the pump and from an efficiency point of view for the solar panels, not adding flow restrictions and instead just having a higher flow rate is better. Adding flow restrictions needlessly wastes pump energy through the friction/turbulence from the restrictor. There is probably a compromise you can make where you could lower the point of the air intake for draining of the panels since it may not need to be quite at the top of the panels since some amount of negative pressure there is probably OK. Then you could somewhat lower your pump speed and flow rate for more energy savings.

Mark (mas985) is the expert in this area and hopefully he'll come along and give you some advice.

3. ## Re: Flow rate to solar panels

The Fafco manual seems to indicate that there is a max flow rate of 8 GPM per panel - which ends up being about 56 GPM. I would think that running the panels at 50 GPM should be OK as it's within specs. I'm just wondering how much it will shorten the life of the panels....

4. ## Re: Flow rate to solar panels

Originally Posted by opto_isolator
To get to the point of no air bubbles, I have to set the speed to around 2500 RPM, which is showing 50 GPM on the Flowvis. Am I missing something, or am I stuck with having a higher flow rate due to my setup?
Without mods, you are basically stuck with that and that operating point sounds about right too for a two story house.

Exactly where is the VRV installed?

Another option as chem geek alluded to is to lower the VRV. This will allow you to run at a lower RPM and keep it closed. However, there is a minimum flow rate of about 40 GPM to prime solar panels on a two story house so you will need at least that for a short period of time before dropping to the lower RPM. It looks like the Suntouch would support this. You can lower the VRV to as low as about 6 feet above the pad and still have it work fine.

However, with a lower VRV there is a slight risk of pipe collapse under certain conditions. If the solar valve shuts off AND the pump RPM is high enough to keep the panels filled with water, the water in the panels will continue to heat up. When the pump is shut off, this hot water can soften the PVC return pipe and since the pipe is under negative pressure, it has the potential to collapse. This is a pretty rare event and can be prevented by running the pump only on a lower RPM when solar is shut off so the panels can drain some.

The Fafco manual seems to indicate that there is a max flow rate of 8 GPM per panel - which ends up being about 56 GPM. I would think that running the panels at 50 GPM should be OK as it's within specs. I'm just wondering how much it will shorten the life of the panels....
I don't see why it would shorten the life of the panels. Pressure at the panels is probably more of a concern than flow rate. But since you are setting the RPM so the VRV closes, the pressure is close to zero so that should not be a concern. So I don't think you have anything to worry about.

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