two-speed with solar panels

[mas985]

Is there really a difference between "negative head" and "vacuum"? As I see it, unless there is still positive head at the top of the panels, the water on its way down is pulled by gravity and produces a "suction" at the top. By suction I mean a pressure lower than atmospheric. If the system is sealed, that negative pressure "sucks" the water up from the inlet. But if there is a valve, the suction will make the valve open. As I said, it's possible that the valve will stay closed with a small amount of suction, but eventually it will open, and I would imagine the margin to be too small to be of use.

Thanks!
Luigi

Yes there is a difference between negative head and a vacuum. The primary difference is the reference. Vacuum usually refers to pressure below atmospheric or negative PSI. Your solar valve on the roof will not release until the pressure in the pipes is below atmospheric which requires the pump to be off and water to be draining out of the pipes. This causes suction.

However, with the pump on, there is a net positive pressure within the pipes usually between 10-20 PSI above atmospheric pressure (i.e. your filter PSI reading). This keeps the valve closed. So while the water going down the pipe offsets the water going up the pipe, this is called static head, there is still a substantial positive pressure to keep that valve closed. There just isn't any head added to the system because of elevation. The only pressure difference is the dynamic head loss experienced in pipe and solar panels. The water falling simply makes it such that less pressure is needed to push the water up the pipes.

The easiest way to see this is when the solar is first turned on. You will see the PSI rise substantially and then after the pipes are filled, the PSI will drop slightly but still higher than without solar.

So in formula terms:

Filter PSI Solar on = PSI Solar off + PSI elevation Rise - PSI elevation fall + PSI Dynamic Head Loss Panels and Pipe

PSI elevation Rise - PSI elevation fall nets to 0 so there is no PSI impact due to elevation.

[rilesworld]

I have been trying to learn a little more about solar options and have a couple of basic questions.

Can you use PV solar panels to heat your pool? I guess I thought that you either had a PV system that generates electricity that can create units that can be sold or you have a "water heating" system that just creates hot water. Is that accurtate?

It seems like having solar for heating the pool is beneficial early and late in the year to extend the swim season, but does anybody use their system for anything during all those other months? I know during peak summer it seems like you can use it to cool your pool, but what about all that sun during the summer and winter months? Just curious.

Some of you that have a PV system. What size system do you have and how much energy are you able to generate over the year?

Thanks,

Riles

[mas985]

I have been trying to learn a little more about solar options and have a couple of basic questions.

Can you use PV solar panels to heat your pool? I guess I thought that you either had a PV system that generates electricity that can create units that can be sold or you have a "water heating" system that just creates hot water. Is that accurtate?

You could use them to generate electricity and then run an electric heater but that would be very inefficienct. The better bet is to use the electricity to run the pump and then use passive solar for heating.

It seems like having solar for heating the pool is beneficial early and late in the year to extend the swim season, but does anybody use their system for anything during all those other months? I know during peak summer it seems like you can use it to cool your pool, but what about all that sun during the summer and winter months? Just curious.

Really depends on where you are. Most of the year, I use the solar to keep the pool between 85-90 degrees. There is onlyabout three weeks in the summer where I don't need it. Also, you need to add more heat if you don't use a solar cover. Although our days here in CA can get quite hot, the nights seem to be fairly cool and that is when you lose a lot of heat.

Some of you that have a PV system. What size system do you have and how much energy are you able to generate over the year?

Thanks,

Riles

Some thoughts.

[cleancloths]

A PV system generates electricity, what you do with that electricity is up to you. CA like NJ has some very good incentives to install PV. PV provides electricity year-round - so I find it a much better use of my roof space. My 54 panel system generates over 12,000 kwh per year.

[tphaggerty]

I second mas985. My controller stays on all summer. After it rains or we have a few cold nights, it will kick on, even in July & August.

Extending the season isn't anything to sneeze at, though. We opened about 3 weeks ago, the water in now 85+. We will swim until almost the end of October in 80+ water. No one else, and I mean no one, will do that around us (unless they get solar!!)

[cleancloths]

I second mas985. My controller stays on all summer. After it rains or we have a few cold nights, it will kick on, even in July & August.

Extending the season isn't anything to sneeze at, though. We opened about 3 weeks ago, the water in now 85+. We will swim until almost the end of October in 80+ water. No one else, and I mean no one, will do that around us (unless they get solar!!)

I've done it for years in Northern NJ with my heatpump. The key issue is more of using a solar cover then how you heat. The heat loss in April/May and Sept/Oct at night is HUGE if you don't use a cover.

[tphaggerty]

I've done it for years in Northern NJ with my heatpump. The key issue is more of using a solar cover then how you heat. The heat loss in April/May and Sept/Oct at night is HUGE if you don't use a cover.

True enough. We also use a "solar" cover religiously. When we first got the pool, we bought a very expensive, very ugly commerical reel. We were going to return it because it is very big, but after getting used to it (we can roll it over the diving board with no problem and can reel the cover up without bending over), we decided to keep it.

Many people we talked to about their pools complained about the price of heating, but didn't bother to use the cover. Sort of like heating a house and leaving all the windows open at night when it's still getting down into the 40s.

[semenzato]

Oh, I see what you mean. The "suction" from the falling water reduces the pressure at the top, but not quite enough to make it negative (that is, below atmospheric). The term "negative head" confused me. I would think of it more in terms of "head reduction".

However, I am still not sure this works. You're saying that I would use the pump at high speed to fill the pipe, then I can switch to low speed. But you also agree that even with low speed there is higher-than-atmospheric pressure in the pipe at its highest point (where the valve is located). That means that at low speed the pump still has enough power to fill the pipe, and I don't need two speeds, just the low speed.

Anyway, thanks for your help and advice. I will have to do some measurements and calculations and see if I can get by with a smaller pump.

[mas985]

Oh, I see what you mean. The "suction" from the falling water reduces the pressure at the top, but not quite enough to make it negative (that is, below atmospheric). The term "negative head" confused me. I would think of it more in terms of "head reduction".

However, I am still not sure this works. You're saying that I would use the pump at high speed to fill the pipe, then I can switch to low speed. But you also agree that even with low speed there is higher-than-atmospheric pressure in the pipe at its highest point (where the valve is located). That means that at low speed the pump still has enough power to fill the pipe, and I don't need two speeds, just the low speed.

Anyway, thanks for your help and advice. I will have to do some measurements and calculations and see if I can get by with a smaller pump.

Not quite. Static head, which is basically the rise in elevation, does not depend upon the rate of water flow (GPM). Dynamic head (pipe friction loss) does depend on the rate of water flow. A pump at high speed has a pump head curve such that it can take care of both the static and dynamic head together (i.e. high head curve) quite easily. See here:

http://www.poolplaza.com/WhisperFlo-techspecs.shtml

The pump at low speed, which is basically half speed and half flow, only has enough power to handle the lower dynamic head (notice the head curves are much lower). So at low speed, the flow rate is 1/2 of the high speed so the dynamic head is 1/4 of that at high speed. Since the pipe is primed with full speed, at half speed there is no longer any static head so it will have no problem with the head loss that remains.

Here is what would happen in a typical pool.

No Solar with full speed filter PSI = 15, head loss about 45 feet
With Solar at full speed filter PSI = 20, head loss about 52 feet
With Solar at low speed filter PSI = 5, head loss about 13 feet

To ease your mind, people with pools and two speed pumps have been operating on low speed with solar for years so no worries.


Here are a few more links you can learn about pumps and hydraulics.

http://www.powermat.com/pools/index.html
http://poolplaza.com/pool-pump-sizing-2.shtml
http://www.zoeller.com/zep/techbrief/JF1article.htm
http://www.eere.energy.gov/industry/.../pumpintro.pdf

and I have much more technical version when you are ready.

[semenzato]

OK. I will indulge myself by being doggedly stubborn and incapable of understanding this.

The pump at low speed, which is basically half speed and half flow, only has enough power to handle the lower dynamic head (notice the head curves are much lower). So at low speed, the flow rate is 1/2 of the high speed so the dynamic head is 1/4 of that at high speed. Since the pipe is primed with full speed, at half speed there is no longer any static head so it will have no problem with the head loss that remains.

Here is what would happen in a typical pool.

No Solar with full speed filter PSI = 15, head loss about 45 feet
With Solar at full speed filter PSI = 20, head loss about 52 feet
With Solar at low speed filter PSI = 5, head loss about 13 feet


OK. But can this work if the high point in the loop is higher than 13 feet over the pool water level, and there is a valve at that point that opens at negative pressure? (Taking atmospheric pressure as the origin.)

If you think this works, then it means that the pump at low speed generates enough head to get the water all the way to the top, and there is no need for the high speed.

More specifically: you say the low-speed dynamic head is 13ft, or 5psi (by the way, 13ft is closer to 6psi). So on the way up to the panels, the pressure drops, both because of friction and height. What's the pressure in the pipe at the top? Is it 2psi? 1psi? Or -1 psi? (The reference point is atmospheric pressure, about 15psi at sea level, so -15psi is the lowest value.) If you give me a positive number (or even 0 as a limit case), I'll tell you that in this case 5psi is enough to get the flow started, not just maintained. If you don't agree, please explain why. If you give me a negative number, I'll tell you that that's not possible, because the valve will open. If you don't agree, please explain why.

To ease your mind, people with pools and two speed pumps have been operating on low speed with solar for years so no worries.

I am sure it doesn't hurt to have the higher speed available. I still don't see why it's necessary. I agree completely that the two-speed pump makes sense if there is a siphon effect. But the valve prevents that effect. What's wrong with this reasoning?

By the way, I just took readings of the pressure at my filter. It's 16 psi without solar. When solar kicks in, the reading rises to 24 psi, then drops down to 23 psi. That's well over the static head for the panels, so I am positive I could run at much less power and flow.