Incremental energy costs for solar pool heating

[ocdave]

I have talked to a few vendors about installing a solar pool heating system and when I ask about the incremental energy cost of running the system, the answer I get is that it is basically zero since I would have to run my pool pump anyway. Rather than dumping water directly back into the pool, they are running it through the solar collectors before dumping it back into the pool so I get the solar benefit without any incremental energy cost. Sounds a bit too good to be true. Surely, this is going to cost me more to run since my variable speed pump would have to be run at a higher RPM? Any real world experience out there to help me get a non-sales answer?

Thanks!

[OvertheSun]

Our solar pool heater is located on the roof of our second story house. The pump definitely uses more power to pump the water up to the panels when the solar is on then when it isn't, and we've had various energy efficiency experts and contractors target that as something we could do to reduce our electic bill, in addition to upgrading to a variable speed pump.

I can't tell you how much of an effect it has because we haven't done a comparison with it on versus off yet. But it only makes sense that it would. It might be less of an effect if the panels were on the ground than on the roof.

[russ]

Any extra lift the pump has to do (to a roof for example) requires more power - any additional restriction in the line such as panels requires more power. For anyone to say otherwise is a bit silly. In many cases it won't be much.

[inetdog]

Our solar pool heater is located on the roof of our second story house. The pump definitely uses more power to pump the water up to the panels when the solar is on then when it isn't, and we've had various energy efficiency experts and contractors target that as something we could do to reduce our electic bill, in addition to upgrading to a variable speed pump.

I can't tell you how much of an effect it has because we haven't done a comparison with it on versus off yet. But it only makes sense that it would. It might be less of an effect if the panels were on the ground than on the roof.

In the ideal world, the extra energy required to pump the liquid up to the panels comes right back in terms of suction as the liquid descends on the return leg.
That means that the only real difference will be the length of the pipe.
If it is a drainback system, there will be some initial early power required just to fill the upper collector loop, and you will not get that back.

[russ]

In the ideal world, the extra energy required to pump the liquid up to the panels comes right back in terms of suction as the liquid descends on the return leg.
That means that the only real difference will be the length of the pipe.
If it is a drainback system, there will be some initial early power required just to fill the upper collector loop, and you will not get that back.

In a closed loop - not in an open loop like a pool

[inetdog]

In a closed loop - not in an open loop like a pool

Even in an open system, as long as the return pipe stays full and the height differential between collector and pool water level does not exceed the maximum siphon height or suction head (between 25 and 34 feet at sea level depending on some other factors and who you believe), then you will get back all the energy you put in raising the water. See http://www.onestopfire.com/head.htm for some definitions and explanatory material.
In a closed system you can even exceed the suction limit because you can pressurize the system to at least the difference in pressure between top of collector and the lowest point in the system.

[russ]

Even in an open system, as long as the return pipe stays full

A full pipe I agree but that is not a given.

[Naptown]

Since all piping originates and terminates under the surface of the water it is essentially a closed loop. Yes some additional power is needed to initially pump the water up to the roof but once filled it should balance out and only have the additional static head.
The only way to be sure is to plug the pump into a kill a watt meter or use an amp meter to see what if any difference there is in consumption during non heating mode, pump up to collectors and filled collectors circulating.

[russ]

Is the pipe constantly 100% full or not - where it terminates is meaningless

[J.P.M.]

Is the pipe constantly 100% full or not - where it terminates is meaningless

I'd suggest it's only meaningless if air can get into the system by virtue of the location of the termination(s) - a spigot outlet for example without a u bend, that sort of thing. The way I learned it, if air cannot enter the system once full, designed to stay that way and operating normally, the pumping losses will be only those losses normally associated with any system pumping fluids around, viscosity, pipe friction, velocity head changes, valves or other restrictions, etc. Attempting to fill a system with a large elevation change with a low head pump might be fun however.

[russ]

Depends on pipe diameter and layout.

[Naptown]

I have installed hundred of these. There is a vacuum breaker at the top of the system. as long as the pressure remains only marginally higher than a vacuum no air enters system. If you have bubbles in the poo after the initial purge you have trouble.

[russ]

I have installed hundred of these. There is a vacuum breaker at the top of the system. as long as the pressure remains only marginally higher than a vacuum no air enters system. If you have bubbles in the poo after the initial purge you have trouble.

We are talking about generic pumping systems - you are talking about a specific small sector under special conditions - how many of those you have installed is of no consequence.

[Naptown]

We are talking about generic pumping systems - you are talking about a specific small sector under special conditions - how many of those you have installed is of no consequence.

What I am talking about is the specific system question the OP asked not a generic system that may or may not apply to his situation.
Why are we talking about things that do not apply to the original question.

[LFM]

The flow rate of water returning to the pool after the solar panel cancels the energy needed to lift the water to the panel. The flow rate of the water exiting the solar panel/panels in most cases exceeds the lift flow rate. So a throttling valve needs to be installed in the effluent line of the solar panel, to insure the panels and return line stay full of water during the heating cycle. If the effluent line of the panel is not throttled the bleeder valve at the top of the system will not be under a positive pressure and will pull in air. This is true in a closed loop where the influent and effluent lines run to and from the pool return or open loop the water returns from the panel direct back into the pool with a air gap. If your using a variable speed pump (your already saving 60 to 70% on energy cost) it should not be necessary to make any chances to flow rate.