Pump Efficiency

[mas985]

Richard,

I live in CA as well and pay the same rates as you so I feel your pain. There is more pain to come as California is going to time of day charging so for those of us that have solar and would like to run during the day, our rates will stay high or may even increase.

Over the past couple of years, I have done a lot of research on pumps, hydraulics and energy usage. I cannot speak for all pumps, but I have done quite a bit of analysis on the Hayward Northstars since this is what I have.

First, it is difficult to know the exact efficiency of the pump since you would need the actual power efficiency curve which is difficult to come by. I have not found anyone at Hayward willing to give it to me. Power draw varies with head load so unless you create your own curve using a power meter or power company meter, you are stuck using the maximum draw. Also, current meters are inaccurate since they do not give you the phase between the voltage and current (i.e. power factor). Without a watt meter, the most accurate method is to use your power company's electric meter. Actually, this may be more accurate than a power meter since this is how they charge you so it includes any errors. I have counted the spins per minute with pump on and off and got a pretty accurate usage. You have to find the gearing ratio for your meter to convert to kwh.

Second, the kw rating for Hayward pumps are in euro-units for HP. You are correct that this does not include the pump efficiency and is the output power of the pump.

So using the maixmum current load for the pump and the formula:

Efficiency = (Head * GPM) / (3960/.746 * Max Input KW),

I estimated that my 1 HP Norhstar peaks at about 54% efficiency for 60 feet of head (86 GPM) which is pretty good. It is actually probably a bit better than that since the current draw will probably be less than the max.

I have not done this analysis for too many pumps but I know that the Northstar is one of the most efficient pumps out there.

Also, I have solar on a second story house but did not need to go with a high head pump since most high efficiency pumps are fairly high head to begin with. Anyway it probably does not mater since most pumps deal quite well with static head on the pressure side. During startup, head loss is basically the static lift since water flow is low until prime is complete which for a two story house is usually less than 30 feet. Nearly every pump should handle this quite well for the panel priming. After that, negative static compensates for the positive static so you left with only the dynamic friction loss in the panels themselves.

You seem to have quite a bit of friction loss if your solar system and may benefit somewhat with a solar bypass. A 15 PSI increase is not typical for a solar system which is usully between 4-8 psi. It could be due to your pump or probably the solar system. You don't need much flow through the panels to get good heat exchange so if you bypass some of the water, your friction loss will drop and flow rates will increase. This way you don't need to worry so much about high head pumps or extra strain on the solar panels.

A couple of useful formulas:

For Pumps

http://www.mcnallyinstitute.com/02-html/2-01.html

Note that when you lower the braking HP of the pump, the flow rates will be lower and thus the head loss will decrease. However, if you go with a 3/4 HP Northstar, for example, the braking HP is acutally 1.23 which is not much different than what you have but I suspect the power draw will be much less. So you should see a drop in energy use without increasing your turnover rate.

For the plumbing system, you can use Darcy-Weisbach equation which seems to be a bit more accurate than Hazen-Williams for swimming pool plumbing:

http://www.engineeringtoolbox.com/da...ion-d_646.html

Head loss ~ GPM ^2 / D ^5 (D = Pipe Diameter, ~ proportional)

This allows you see what effects there are from changing either flow rates or pipe diameter on head loss. The operating point for the plumbing system and the pump is where the head curve crosses the plumbing curve.

For even more energy savings, you might want to consider a 2 speed pump. Energy rate drops by 2/3 and flow rates drop by only 1/2 so for the same turnover, your energy use should drop by a 1/3 or 33% savings.

Poolplaza.com gives all of the pump head curves and some of the current draws for the pumps. For Northstars, I asked Hayward for the current draws but they would only give me the maximums for each. Here are the current draws for the full rated pumps.

HP SF BHP Kw
3/4 1.85 1.3875 1.265
1 1.85 1.85 1.794
1 1/2 1.6 2.4 2.346
2 1.35 2.7 2.714
2 1/2 1.35 3.375 3.3925

Note that a good approximation is that BHP = KW. So if you don't know the current draw, you can estimate it from BHP.

So from the head curves, here are the efficiencies:

Energy Efficiency

HP/Head 40 50 60 70 80 90
3/4 49% 54% 54% 44%
1 48% 53% 54% 51% 42%
1 1/2 43% 50% 52% 51% 45% 29%
2 44% 50% 52% 51% 47% 34%
2 1/2 40% 45% 49% 50% 48% 42%

(oops wrong table replaced with the correct one)

Good luck in your quest for energy reduction.