I have seen this discussed quite a few times and often wondered where the sweet spot is on solar panel water flow rates. Is it better for the water to sit there and cook a little and get hot? Most folks here say no. But, can water moving quickly through the panels really strip heat out of the panel that fast? Like if you run your hand through a flame fast enough it won't get hot at all, but if you let sit there a few extra seconds ... ouch!
It does not seem logical to me that heated glycol can enter at 160 degrees and comes out at 115 degrees and the pool water doesn't go up a few degrees at least. I agree that the heat has to go somewhere, but it doesn't appear that it went into the pool water in this case.
My solar panels are HOT, like 130+ (can't remember for sure), in summer and the water coming out of the pool returns is noticeably warmer than the pool water, assume at least several degrees. I can gain 6+ degrees in a day. But, I never experimented with pump flow rate to see if it helped, I was more interested in energy consumption and trying to find the lowest speed that seemed like enough flow. This summer I am going to try to pay extra attention and try turning up the pump speed when solar is on to see if I can get more overall pool temp gain by increasing water flow.
p.s., I am neither a hydraulic nor a thermal engineer. Just a guy who likes my pool water between 85 and 87.