Domestic pool turnover rates

[Anonymous [GDPR] European in the UK]

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.

[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.

[Anonymous [GDPR] European in the UK]

Hi Richard and Carl, sorry been away for a while.
Yes Carl, understand the physics and apologies for not explaining clearly what I meant although I get the feeling Richard understood.
It's tricky to use a the car engine analogy as there is far more heat available to get rid off so transference would be faster courtesy of 1500 high energy, high temperature explosions taking place every minute but the thermodynamic rules are essentially the same I believe. It's more that actual application on a swimming pool where the solar energy is considerably less as is the temperature (geographical situation may of course alter that if you are in Arizona, California or over my part of the world).
Carl you said "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".
Obviously we are looking for the maximum BTU's where possible but are those actual figures for what you receive or just a statement to explain your point?

My point is that in Richards example the flow rate is 48 GPM which maybe more efficient in terms of the energy gain from panels only (though I doubt it) but doing that costs him on the pump electricity an additional 1275 watts (equal to over 6kw via a air to water heat pump). Slowing the water flow through the panels as Richard has shown reduces the panels heat efficiency by a small amount but like wise reduces the electricity by a larger amount so the overall efficiency is greater.

Richard assuming the 100% or near as is achieved at 48 GPM on your setup then 80% at 4GPM/panel then the electrical saving by slowing down the flow would save considerably more although the panels would get a bit warmer they won't go passed the outside temperature as that would be equilibrium?

Finding via a graph the best point for low electricity usage to the pump and the best flow for the panel is really what I was driving at so the slower flow around the panels gains more heat energy is absorbed which may not be as efficient as faster but is more efficient when the pump electricity is also considered.

On the best pool setup I have done (not the cheapest in capital outlay terms) the flow rate on filtration is 52.8 US GPM on 81 watts with plumbing lengths about half of yours Richard. The owner is planning to use solar (Evacuated tubes and heat exchanger) so this will impact on the overall efficiency so your data is helpful, I would prefer him to use a heat pump as the headloss is a lot less so allowing him to keep the running cost to minimal but some data is needed to back up that discussion.

[CarlD]

I'm sorry but this statement you made:

"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."

is simply flat-out wrong.

However, the rest of your argument, that the wattage consumed pumping the water up to the roof-top panels may exceed the wattage used by a properly sized and programmed heat pump, is a valid question. Richard shows that he may be able optimize his heat gain vs his pumping wattage expenditure by slowing his pump or usage but that does NOT mean the panels themselves will work more efficiently.

The car model is actually a perfect example. The cooling system is virtually identical to a solar panel system except that the variables are different. The amount of heat energy generated by the engine may be no more than a large solar panel system, although the temperature is far higher, and pressurized to increase the boiling point of the coolant. Running your panels at night to cool your pool is exactly the same exercise as the car's engine so the same questions are valid.

Yeah, I pull the numbers out of my ...(hat)...as an example, no more.

So you are conflating apples and oranges. The question of how many watts do you burn for resultant BTU gains of solar panels vs a heat pump is valid. The assertion that moving the water faster through the panels loses efficiency (short of cavitation or leakage) isn't. They are differently things.

I'm in a different situation than Richard as my solar panels are less than a foot above my returns, so I don't have to expend huge amounts of pump energy fight gravity. Plus, the panels' own limiting factor on flow is simply failure from too much pressure. The noticeable gains in water temp from pushing the system are annoyingly offset by the cost and effort of repairing leaks or worse, replacing panels at about $70/each. When running my pump at full speed I must barely crack the valve for each circuit or risk leakage failure. Only at low speed can I full open the valves.

[Anonymous [GDPR] European in the UK]

Yes Carl, I'll agree to that bad wording on my part.
Like to know more on your system, how about re plumbing to 3 banks of 10, rather than 2 of fifteen may reduce the problem you are having when running up fast? The pressure issue is due to dynamic head pressure as the flow can't get through easily enough. On my system I get loads of flow but very little pressure (0.25 PSI). That would cause me a problem with Richard's setup as there wouldn't be enough grunt in the pump to gain much flow so I would have to up the size of my pump. but yours as you say is hugely different being closer to the ground, variable speed pump allows you to tailor to specific needs. have you any flow, pressure temp data you can pass on? What panels do you have?

[CarlD]

I don't have the detailed data Richard has. I'm not a whiz with calculating head either. For me, investing in a pressure flow meter just isn't necessary.
The system is a FantaSea--you can see many pictures in my re-build thread. The decking is the solar system so walking on it is comfortable as the water running through them cools the panels. The panels are only sold by LeisureLiving.com which is also poolsupplies.com.

The system has advantages of keeping the deck cool, not requiring additional space for solar panels, hiding in plain sight so it's invisible, and always not much higher than the pump. My wife insisted that the pool and solar system could not be "ugly" and she's always been pleased with its appearance. Individual panels are easily replaced, being only 2'x4'. Remove a few screws, tip the panel up, disconnect the hoses, connect the new panel and put it in place.

The disadvantages however are myriad. There is only one source for the panels. They are expensive to replace. They require a LOT of plumbing--hoses and clamps, all of which can leak. Leaks crop up far too easily, both at joints and in the panels. Winterization is a tedious process. I need to break each circuit into 3 sections and blow them out with a shop vac, which takes hours to complete. Spring opening is easier as I just have to re-make the junctions I disconnected.

The original system, which FantaSea never changed, was ALL the panels were in serial, giving very poor flow, but still heating the pool. The system's return was very close to the main return--after the water had circulated 120' around it. I created a manifold to separate the northern and western panels from the southern and eastern ones, with a valve for each, and a main cutoff for both of them. I then cut in two new returns at the diagonal corner from the main return. The old solar return was converted into an overflow drain so the pool cannot be overfilled (simply ran plumbing from the old return up to the maximum water level, then down to a yard drain).

Splitting it further would be problematic because I thought about it. I'm not sure I'd gain enough from that to justify the additional plumbing and issues.