Sizing PV System for Well Pump

[John Pitts]

We've purcahsed 17 acres with an existin well. The pump is a Goulds 18LS15422 2 wire 1 1/2 HP with a well depth of 260 feet and a pumping depth of 186 ft.

I want to put this on a solar PV system but have no idea how to size the system. Right now the pump is on a 60 Amp 240 volt connection.

Any ideas short of replacing the pump with a proper solar friendly pump?

[Joe Blake]

I've never worked in a well situation, but from watching TV and reading, it's not a simple question. Eg, where is the pump? Is it down the well, at the side, on top? There'll be that physical dimension to take into account.

How many gallons per hour do you want it to pump?

Do you want a DC pump (12/24 volts) or an AC pump using an inverter?

Do you wish to have a bank of batteries?

Would it be possible to use the existing pump with an inverter? (At 240 volt/60 amps that would be over 14,000 watts of power, which is HUGE inverter).

Have you considered a wind turbine with a storage tank, so it pumps water while the wind is blowing, and this fills the tank?

As I say, there are many questions that you'd need to answer before deciding which pump to buy.

I'll leave a more detailed analysis to somebody who's actually got a pump.

:laugh::laugh:

Joe

[Joe Blake]

Whoops,

I misread your thread. Apparently you DON'T want to replace the pump. My bad.

Well, in a sense that makes it a little simpler. But still some questions.

Firstly, since the term "horsepower" is pretty imprecise (electrical hp is different to mechanical hp is different to etc.)

(Cut and pasted from Wikipedia)

Mechanical horsepower ≡ 33,000 ft·lbf/min

= 550 ft·lbf/s
= 745.6999 W
Metric horsepower ≡ 75 kgf·m/s

= 735.49875 W (exactly)
Electrical horsepower ≡746 W
Boiler horsepower ≡ 33,475 Btu/h

=9809.5 W
Hydraulic horsepower =Flow Rate(US gal/min) * Pressure(psi) / 1714

So if you can work out some way of converting to watts/kilowatts that may be easier, since solar panels generally express their output in these measures.

How many hours per day are you expecting or want to pump?

(1) If you want to pump your water 24 hrs per day, then you'll need sufficient battery storage to hold power to run the pump during night/overcast periods and sufficient solar panels to run your pump AND recharge your batteries to carry you through. (I'd be guessing but I'd say 2.5 times the power requirements of your pump in PV power - at LEAST.)

(2) If you only want to pump a few hours per day that'll probably mean a smaller bank of PV panels, but you'll still need batteries to run the pump.

(3) If you only want to pump during daylight hours with no batteries then you'll need to have at LEAST sufficient PV panel output to match the requirements of the pump.

So assuming (for ease of calculation) your motor is 1 KW then you'll need at least 1 KW and probably an extra 10-20% to make up for losses in the inverter just to run in daylight. And remember an inverter will consume power even while it's only on standby.

If you could calculate a KWatt rating for your pump, I'd be able to be a little more precise perhaps.

Oh, and by the way, welcome to the board.

:bigsmile:

Joe

[Rob Beckers]

Hi John,

I'm no specialist on motors and pumps, so hopefully someone with more hands-on experience will chime in. With that caveat:

According to the docs on your motor that I've found it has a locked-rotor-amp rating of 56 Ampere (and by the way, it's supposed to be fused or have a breaker that's rated at 35A according to the manufacturer). What that means is that during startup it may, ever so briefly, draw 56 Ampere. This makes it a pretty large pump, and your source has to be able to deliver that.

To run this pump off-grid means batteries and an inverter (a 240V split-phase inverter or a stack of single-phase inverters). The inverter has to be able to deliver a surge current of around 60A (or more) to reliably start the pump (That's 14.4 kW at 240V). That's quite a feat for the average inverter; for example the Outback series tops out at a surge current of 50A. Same for the Xantrex XW series.

As I said, don't take what I wrote as gospel. Talk to people that have installed off-grid pumps. Maybe LRA current ratings are such a worst-case that a much smaller inverter can reliably start the pump, I don't know.

-RoB-

[Joe Blake]

Maybe LRA current ratings are such a worst-case that a much smaller inverter can reliably start the pump, I don't know.

-RoB-

Rob,

I don't think it's "worst case" - it's the norm. Even small electric motors such as found in computer hard disk drives require a "big" draw of current to get started from rest. (But of course the computer has the added disadvantage of the processor itself running flat out with all the boot up testing etc). Although my computer draws less than 70 watts normally running, on boot up, a 150 watt inverter struggles.

But I digress.

I think your figures are in the ball park.

One oddball idea occurs to me to overcome this, would be to have a "pump priming tank" whereby water is pumped into a high tank, and when the pump starts up, water could be released to act as a "kick starter" to turn the pump rotor from stationary, thereby reducing the necessity for a high start up amperage.

Joe

On Edit:

Or perhaps a Humongous capacitor to store the start-up surge power.

JB

[Ralph Day]

Hi John
Starting that 186 foot tall slug of water moving against pressure is what requires such a surge of power. Most submersible pumps start with an enormous surge compared to their running load. I moved from a pump like that to a Grundfos soft start submersible and it's much more friendly on the Xantrex 4048 inverter. The old pump would spike at almost 40 amps 120vac on startup, then run at 11. The new pump starts at 0 amps and ramps up to 8 or so.

That's not really your question though. You'd have to have a monster system to run that pump. Cheaper by far to replace with a soft start pump than size PV batteries and inverter to use the existing. Just ballparking: new pump $1000. PV, batteries inverters $20,000. Scary thought. If off=-grid is your long term goal then replacing the pump in the initial outlay of money is the way to go. I wish i had.

Last suggestion, 1000gallon holding tank, generator sized to run the pump load, charging etc, and a little booster pump (Dankoff) to pressurize the holding tank water for the house. THis is my summer setup and the booster pump only delivers 2gallons per minute, but that's fine at 50psi. Everything depends on your household setup regards wind, solar, batteries, grid tied etc. Some more information would help those here to advise.

Ralph

[John Pitts]

A hearty thanks to all who answered. The response is pretty much in line with the answers I've been getting from the local folks.

Lools like a pump replacement is in order.

By the way, the ultimate goal is totally off-grid capability with a grid intertie as the middle step ( it will take some time to get up the cash for the battery bank).

Thanks again. As my first experience posting on this site, I will definitely continue.

[Ralph Day]

Hi John
Just be sure your intermediate inverter (the grid tie one) is also able to be off-grid too. Not all are able to do both. Most manufacturers have very good web sites and forums

Ralph

[Renic Gunderson]

A hearty thanks to all who answered. The response is pretty much in line with the answers I've been getting from the local folks.

Lools like a pump replacement is in order.

By the way, the ultimate goal is totally off-grid capability with a grid intertie as the middle step ( it will take some time to get up the cash for the battery bank).

Thanks again. As my first experience posting on this site, I will definitely continue.

If you live in a place with net metering, you will get a much better return on investment by simply connecting a grid tied solar system to the meter feeding the pump. What state, or country, are you in?