Replacing solar pool heater panels

[Dirk]

Not entirely. Higher RPM over the same run time can reduce the life of the motor bearings.

But that was not my main point. My main point is that precise flow rate is not that critical to heat gain. You would never notice a 10% swing in flow rate because the change in efficiency is so small. So trying to nail the manufactures spec is pointless IMHO. As long as you are close, you can expect those published efficiencies. Also, it is important to understand that those specs are based upon a single set of inlet temperatures and environmental conditions. As you pointed out, these do change and that is why I think it is more important to measure temperature difference than flow rate.

Also, if you are trying to squeeze out as much heat as possible, you should be running a maximum flow rate specs. Efficiency increases even past the manufactures recommendations. It is a continuous curve. It just doesn't change as much.

Understood! I didn't mean to speak for you, just for why increased efficiency was worth pursing for me and my pool. I did understand that the efficiency curve is "endless." The higher the flow, the more heat you'll get. But it is a curve, and eventually what you are gaining in heat is so little that it's not worth the additional cost of running the pump (whether that's in electricity or wear'n'tear).

I've been assuming that the manufacturer's spec accounts for that, and they base their recommended flow rate not so much on the ultimate efficient number for their panel, but rather what is a reasonable trade off between heat and cost for most users. Any chance I'm close on that notion?

 

[mas985]

From what I have seen, it seems that they simply pick the 80% efficiency number for flow rates but again, this efficiency is based upon a specific set of assumptions. Most if not all basically use the 0.1 GPM/sq-ft metric. Here is the curve you will likely see most often for 4x10 panels (note how it say "Typical Conditions"):




You could target 90% efficiency if you really wanted to. Most manufactures spec maximum pressure and if the panels are on the roof, they will probably never get to that pressure even at full speed so you could run at full speed if you wanted to.

When I first installed my pool and solar, I had a single speed pump at the time so was running at 100% all the time. Wasn't an issue and I didn't bother bypassing any of the water either (lower energy use).

It is a trade off of panel efficiency, motor energy consumption and motor wear and tear. Every pool owner with solar should make these trade offs themselves since priorities differ from person to person and there is nothing set in stone that you have to operate at any particular flow rate.

 

[Dirk]

It is a trade off of panel efficiency, motor energy consumption and motor wear and tear. Every pool owner with solar should make these trade offs themselves since priorities differ from person to person and there is nothing set in stone that you have to operate at any particular flow rate.

Agree!

Does a larger or VS pump help with the wear and tear component of the math? My solar RPM setting is 2210 (about 600 watts) to achieve Heliocol's recommended flow rate. I've got a 3HP VS pump. So I'm nowhere near maxing my pump RPMs. Of course, half of that 2210 would be better for my wear and tear factor, but how much better? Is running at my recommended flow rate taking a year off my pump's life? A month? Cutting it in half? That might end up a rhetorical question, or at least have no accurate answer, but it's got me curious.

 

[mas985]

For bearings, it is really about RPM and a larger pump should allow for lower RPM but the wear should be proportional to RPM^2 as that is proportional to the energy due to friction. So 1/2 the RPM is 4x the bearing life. However, bearing failure may not be the first thing to fail or it may not be due to friction (e.g. seal failure).

 

[Dirk]

For bearings, it is really about RPM and a larger pump should allow for lower RPM but the wear should be proportional to RPM^2 as that is proportional to the energy due to friction. So 1/2 the RPM is 4x the bearing life. However, bearing failure may not be the first thing to fail or it may not be due to friction (e.g. seal failure).

Whether it comes off as me agreeing or disagreeing with you sometimes, I always learn a lot! Thanks for your input!