Experts say that the optimum temperature for drinking tea is 60ºC (140ºF). We have all experienced the joy of a hot cup of tea in cold hands and the pleasant and quick warming effect it has. Our hands warm up quickly because the temperature between the cup and our hands is significant. As our hands warm up, the rate at which our hands warm slows down, and eventually reach equilibrium with the cup. At this point, no additional heat transfer occurs. Once the tea cools off, the warming capability still exists, but the warmth transferred to your hands occurs much slower.
This effect highlights the science of solar thermal technology and heat transfer. People in the biz refer to this a “Delta T,” or the difference between the ambient temperature and the temperature of the fluid to be heated. The larger the temperature difference, the more heat is transferred in a given time. I wrote previously about slowing water down to improve solar pool heating. It doesn’t work — for the same reason a tepid cup of tea doesn’t warm your hand much.
What you want to do is move a lot of water through solar pool heating panels, heating a lot of water a little bit as it circulates. Another way of looking at this is that you want to keep the water in the panels cool relative to the surface temperature of the solar collectors. Maintaining a high flow rate continually introduces cool pool water into the panels, allowing them to work more efficiently.
It may sound counter-intuitive at first, but once you think about relative temperatures and how heat is transferred, it becomes common sense — a higher flow rate results in more pool heating overall.
The graph below shows the efficiency of a typical solar pool heating panel based on test conditions at various flow rates. The recommended flow rate for most panels is 3-5 gallons per minute. Once you reach these flow rates, additional flow results in small marginal increases in performance, and the additional cost of water pumping becomes uneconomical. A variable speed pump is the ideal solution to achieve proper flow, but valves can also be used to control flow rates. In most pools, allowing full flow through the panels is desirable unless you have a very powerful pump.
If you are looking for the total flow rate, just multiply the number of panels by the desired flow rate. For example, if you have 8 panels and want 4 gallons per minute, the total system flow should be at least 32 GPM. It is advisable to go a little over to compensate for filters getting dirty and flow rates decreasing. Always start with a clean, preferably new, filter cartridge.