Occasionally we get a call from a client that says the solar heated water coming out of their pool jets does not feel very hot. Our response is pretty simple: “Good!”
It’s not that we are being dismissive. If you feel a large temperature difference in the heated water coming from the solar panels, that usually means the flow rate is too low. The best way to heat a pool is a large amount of water heated a small amount each pass through the solar panels.
It’s all about flow rate!
Pool Solar Panel Heating Performance Relies On High Flow
A typical solar panel holds under 4 gallons of water and the optimum flow rate is over 4 gallons per minute. That means water molecules in the solar panel have less than 60 seconds to heat up. You shouldn’t expect it to get much warmer than the pool water that enters them.
And that’s a good thing.
The reason is that solar panels perform best when there is a large difference in the surface temperature and the water temperature inside. This is called Delta T, or the difference in temperature. Heat us transferred more quickly when there is a bigger difference. If you allow water to move slowly through the solar panels it get’s hotter, but the solar panel’s ability to transfer heat to the water diminishes quickly.
Don’t Slow Down Your Flow Rate!
We have seen people slow down their flow rate by closing off valves or reducing variable speed pump speeds. It may seem counterintuitive, but that strategy is counterproductive!
Mathematically:
Flow x Temperature Rise = BTUs Delivered
What we want to do is deliver maximum BTUs (British Thermal Units) to the water. You can do that by increasing temperature rise or flow. As we mentioned previously, temperature rise is greater when flow is greater. So you deliver more total BTUs when you increase the flow rate!
Since all we want to do is deliver the most heat, you should focus on increasing flow and not worry so much about the temperature of the water coming out of the jets. It will take time, but solar panels will heat your pool more efficiently this way.
Comments
I’ve tested flow rates and slower seems to work better. I’ve had my pump running normal speed and it didn’t do squat. I’ve had to restrict the valves so that less water runs through the panels and has more time to heat up and it works.
Hi Jon,
You might think this is better because the water coming from the pool jets is warmer, but what you are doing is actually counterproductive to overall heating. Heat transfer is temperature rise times flow rate. Sure, you might double the temperature rise at a lower flow rate, but if the flow rate is 1/3, for example, you will get 33% less overall heat transfer. This relationship will differ for varying conditions.
Take a close look at the following heating efficiency curve for a solar pool heating panel (test results from the manufacturer). At a flow rate of 1 GPM the panel is 62% efficient, but at 3 GPM it is 80% efficient. You get 30% more heating performance by tripling the flow rate in this case. (0.80/0.62 = 1.3).
The math doesn’t lie. This relationship is summed up in Fourier’s Equation, which uses the second law of thermodynamics to explain how there is faster heat transfer from conduction when there is a larger difference in temperature between surfaces. That’s why we want to keep the water flowing faster through the panel – to keep the water in the panels cooler so the heat can transfer faster from the solar collector surface.
Now of course there is a limit to this. As you can see, the curve flattens out around 5 GPM. You will see little increase in efficiency beyond this flow rate. Running your pump any faster will just cause more energy use without much additional solar heat transfer. And the curve represents efficiency at a given set of conditions. There may be times when the curve flattens out at lower flow rates. However, faster flow will always result in more heat transfer. That said, in really cold conditions it is possible/plausible that a very small temperature rise could result in water returning to the pool cooling down substantially before it reaches the pool. But in this type of condition, it’s highly unlikely that you will want to be swimming anyway.
Ok, so that’s the longwinded way of telling you that your perception of more heating is not the reality of overall heat transfer performance. Faster flow rates are always better for solar pool heating, but no so fast that you are wasting energy with little extra benefit.