Solar Panel plumbing and flow rate

[Slachey]

I just purchased (2) 4' x 20' FACO solar panels for my pool, but have a question about water flow to the panels. Although I have purchased a (3) way diverter valve to divert a portion of the water to the panels and the remainder directly to the pool, I am not sure what my best bet is to ensure that I am getting the appropriate flow to the panels without having a flow meter installed. My pump is a single speed unit capable of producing upwards of 40 to 50 gpm and I am told that the panels should accept ~ 4 gpm each ( I have them installed in parallel). This tells me that I should be diverting ~ 8 gpm of the available 40 to 50 gpm to the panels. Without simply guessing, or adjusting the 3 way diverter to have an opening size of slightly less than 25% going to the panels, is there a better way of adjusting the flow?

As an FYI, the plumbing is setup in an "H" arrangement with the 3 way diverter on the left side of the "H", a PVC Tee section to the right", and check valves on the pool water inlet side of the diverter and the solar panel inlet side of the Tee.

Thank you all in advance for your help!

[swimdaddy]

Do you have a check valve on the return side of the solar panel? If you do, you can disconnect your line from the pressure side of the check valve (this is where quick release connections are nice), turn your system on and allow the solar heated water to drain into a bucket with gallon markings and time how long it takes to fill. Adjust you 3-way valve to where you want it and mark the valve for future reference. When your done you can pour the water back in the pool and reconnect.

That being said, I am no pool or hydraulics expert, and the above test does not acount for the pressure in the system when it is connected. I would assume that the gpm would be higher when it is disconnected and lower when it is connected because of back pressure at the return side? I am sure one of the mods can answer this with greater clarity and expertise.

[CarlD]

I am not a hydraulics expert either, but the more water you can run through your panels, the better. Your limiting factors should be the pump's capacity and the pressure the panels can tolerate without leaking.
Remember: you want the panels to generate heat, not temperature. If the panels are in direct sun, and water is flowing through them, yet they are cool to the touch, then the heat energy they are absorbing is being transferred to the water at maximum or near-maximum efficiency.

[Slachey]

Swimdaddy,

Yes, I have a check valve on the return side plumbing of the solar panel. As I am using Flex PVC that will connect to rigid PVC at the valving manifold I will be using PVC Unions that can be disassembled. With this said, I like your thought to simply fill a bucket and adjust flow accordingly via the valve until I can at least determine flow rate (gpm) at various valve settings.

CarlD,

Although I completely agree with your second sentence, without knowing the pressure that the panels can tolerate, or a method to measure such pressure, I think that I will use the manufacturers gpm rating as a starting point and measure flow via Swimdaddy's suggestion.

Thank you both for your suggestions!

[mas985]

You can usually go by pressure rise. A 2-4 PSI rise is typical but it depends on the type of panels you have. If these are the kind of panels that have both the supply and return on the same side of the panels, they have a lot more head loss so a PSI rise of 5 may be more appropriate.

But the method that swimdaddy outlined will not give you a very accurate flow rate measurement. You are changing the plumbing which changes the head loss and flow rate.

[swimdaddy]

You can usually go by pressure rise. A 2-4 PSI rise is typical but it depends on the type of panels you have. If these are the kind of panels that have both the supply and return on the same side of the panels, they have a lot more head loss so a PSI rise of 5 may be more appropriate.

But the method that swimdaddy outlined will not give you a very accurate flow rate measurement. You are changing the plumbing which changes the head loss and flow rate.

Ya, that's what I thought might happen, thanks for clarifying!

Combining what Carl and you said, The panels seem to start feeling cool with a 2 PSI increase. I have a similar solar panel heater that is 4.5ft by 20ft with 4 loops all in series.

[Slachey]

Guys,

My 4 x 20 panels have the internal manual valve that allow one panel to divert flow down one half, then the 2nd half, but I am not using them in this fashion. The (2) panels are installed horizontally and in parallel, one above the other, with the left side headers coupled and the right side headers coupled. The inlet is at the lower left on panel 1 (lower panel) and the water flows through the left header of panel 1 and panel 2 before going through the panels and discharging at the upper right header of panel 2 (upper panel). I believe this is the proper configuration based on what I have been reading on this site. It has been cloudy here, so I have not been doing much testing, but there was a few hours of sun over the weekend. The panels seemed quite hot when the 3-way valve was adjusted to ~ 25% opening towards the panels and 75% towards the pool, so I kept adjusting the valve until the panels become cool. This occurred when the valve was nearly 100% towards the panels. I also checked the filter gauge and did not see any increase in pressure at all. In the pool, the water coming from the return was ~ 2 degrees warmer than the surrounding water, so it sounds like it is working properly. The fact that there is no pressure rise is quite surprising to me. Is this common? My pump is a 2 hp single speed unit.

[mas985]

At 100% flow, no pressure rise would be very unusual. Almost impossible given the panel size. Are you sure that there is no other path for the water to flow besides through the panels? Also, is the pump running on full speed? On low speed of a two speed would be very low pressure rise.

[Slachey]

The pump is a 2 hp single speed unit as I mentioned in my post. My plumbing manifold is in a horizontal "H" configuration. On the upper horizontal leg, the flow is from right to left from the filter. In the middle of the leg is the 3-position diverter valve, and to the left is the plumbing to the panels. On the lower horizontal leg the flow is from left to right from the panels and through a "T" section. To the right of the "T", the flow goes back to the pool. The vertical leg between the diverter valve and "T" is the bypass, allowing flow directly back to the pool. I have the diverter valve adjusted so that there is zero flow going through the vertical leg and 100% flow going to the left towards the panels. Location wise, the pump / filter is located directly next to the pool and I have 50' of flex PVC going to and from the 20' long panels.

[mas985]

I should have asked this before but what is the make/model of the pump?

Also, where are the panels installed roof (1 or 2 story) or ground?

Filter pressure with solar on?