Domestic pool turnover rates

[CarlD]

Maybe Carl, that's what I was waiting for clarification over. It's normally a pressure required to lift the water not necessarily a flow rate but in this case that 48 GPM maybe where the pump flow supplies sufficient pressure to reach the panels.
It takes time to transfer the solar energy into the water so to fast and the water won't absorb enough heat so the excess flow is just a waste of energy, there will be a point where to slow and the panel stays to hot so not supplying enough heat to the pool. Finding the sweet spot is the desirable point.

I'm sorry Teapot, but this is completely wrong and in contradiction to the laws of physics. It's a common mistake and INTUITIVELY it seems right, but, in fact, is not.

Remember: Transference of heat in or out is the same. By your logic you will cool your car's engine down FASTER by slowing down the water pump, the coolant flow, and by not going as fast through the air. Clearly, this is not true--you want to move the coolant through the system as fast as possible without cavitation or leakage in order to cool the engine down. And, the radiator fan goes ON when you slow down to keep air moving as fast as possible over the radiator fins and coils.
(Of course, if you don't want the engine to cool so much this isn't true--which is why there's a thermostat for those conditions).

What you are transferring is heat energy, not temperature. BTUs/Kcals, not degrees. In fact, the MOST efficient panels will NOT be getting much hotter than the pool water because they are transferring such a high percentage of heat to the water.

I have 30 solar panels, all 2'x4'. Originally, as built, they were all in a serial, one after another. My pool would heat up, but slowly and I had to keep the total water flow super-low so as not to cause leaks. Several years back, I re-plumbed the system so now there are two circuits of 15 panels each. I was able to increase the water flow, and I found my pool heats up about 40% faster than it did before. Greater flow=greater heat energy transfer.

Don't confuse heat energy with temperature. Would you rather have a flow of 10 gpm with water that's 5 degrees (F) warmer, or 1gpm of water that's 25 degrees warmer? Which has more BTUs?

[chem geek]

Carl is correct. Faster flow rates are more efficient for heat transfer. The reason is actually very easy to understand. If the flow rate is slower, then the solar panels get hotter because the heat absorbed from the sun is not getting carried away as fast. In other words, the water temperature gets hotter, but that also makes the panels hotter. Hotter panels re-radiate and thermally conduct heat to the air especially when there is any wind. So the efficiency of the panels drops as the flow rate is lowered. With little to no wind, at 4 GPM per panel they are 80% efficient transferring 80% of the sun's total energy (which during peak noontime sun in the middle of summer on a clear day is 1000 Watts per square meter) to the water. At 3 GPM per panel they drop to being 75% efficient (I was wrong in what I wrote before -- I misread the graph). At 2 GPM they are 70% efficient. At 1 GPM they are about 60% efficient.

So yes, I could lower the flow rate through the panels and not lose too much heat transfer efficiency but save a reasonable amount of pump electricity energy as I noted. I don't actually have to have 48 GPM to keep the VRV open so to get to the roof. I could use a lower flow rate but haven't experimented to see how low I could go. See this post for details on GPM, RPM, PSI for my pool. My Watts are higher at roughly the same GPM and RPM as teapot because my pool plumbing and fixtures are not nearly as efficient. On the suction side, I've got two long (probably almost 100') 1.5" pipe with one going to pool drains (split near the end to two pipes to two separate drains) and one to a skimmer. On the pressure side, I've got one long (probably around 60-70') 2" pipe that splits at the pool to three 1.5" pipes for each of 3 returns.

With no solar at 26 GPM I'm at 1500 RPM and 275 Watts with around 4-5 PSI, but with solar at 48 GPM I'm at 3000 RPM and 1500 Watts with around 24 PSI. At 30 GPM for both, no solar is 6.5 PSI while solar is 14 PSI so you can see how much solar is adding in resistance with the very long pipe runs using 2" pipe and headers. If the VRV is 20 feet high then that would only need around 9 PSI or so (at the base; must be higher at the filter since pressure will drop along the pipe run) to keep the valve closed so I should be able to operate at 3 GPM per panel so 36 GPM total with 75% (instead of 80%) efficiency and with 2500 RPM, 900 Watts and 16 PSI. I could probably go even lower if I wanted to.