# calculations?



## Nimrod (Jun 8, 2010)

I mentioned on another post that I am building a solar system to power water pumping to water an orchard. I am trying to keep it cheap and simple. Line losses and inefficiencies will be ignored during the calculations because I don't know what they will be and they will be relativily small. The system will consist of solar panels, a charge controller, a 12 volt pump, and a battery bank.

The solar pump draws 10 amps and will run for 1 hour a day maximum. I assume I need to provide it with 2 to 3 times the power when it starts up? This means 30 amps or so.

The charge controller and the battery bank are needed to power the pump on cloudy days. 

The Morningstar PS 30 charge controller sounds good. It has a load connection that allows up to a 30 amp load and shuts the pump off if the battery voltage gets too low so the batteries won't be damaged. The charge amperage and the solar panel input are both 30 amps max too. The PS 30 has the normal charge rates and an automatic equalise charge too. It's not an MPPT charge controller so the panells have to be 17 volt max panells or a lot of power will be wasted.

According to wy white wolf the maximum discharge rate of a battery is C/2 so if it were pulling the full 30 amps the battery has to be rated for at least 60 amp hours. I should be able to get by with a 100 amp hour battery.

Also according to www the minimum charge rate is c/12 so a 100 amp hour battery has to have a minimum of 8.3 amps to charge. The minimum panel wattage to charge the battery is 8.3 amps X 17 volts = 141 watts. The charge controller will handel 30 amps at 12 volts of input from the panels but the panels are rated at 17 volts so they have to be rated at 30 amps X 17 volts so the maximum wattage of the panels is 432 watts. 

If we factor in a bit for innefficiencies, the panels can be from 150 watts to 450 watts and work. I will probably start with 2 80 watt panels and expand to up to 6 if I need to run other things.

Is this correct or am I way off?


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## TnAndy (Sep 15, 2005)

Think you have it pretty much right, my question is how much water are you going to pump in one hour ? Assuming a couple gallons/minute, that isn't much water for an orchard in one hour.


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## 12vman (Feb 17, 2004)

IIRC.. A sandpoint well isn't going to recover real fast so you're going to be limited to the recovery of the well as for how much flow you have to your storage tank. I assume a 1"-2" casing? (hand driven) Not much reserve in a casing that size. Think low flow over time..


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## Nimrod (Jun 8, 2010)

TNAndy, 
I am watering the orchard here a couple of times a week for a half hour via a drip system with the water under pressure. It works fine. The off grid system will have to have one pump suck water out of the sandpoint and fill a 700 gallon water tank and another pump to pump the water out of the tank to water the orchard. I can set it up so both pumps are not running at the same time. The watering pump will pull 4 gallons a minuit out of the water tank so in half an hour it will drain 120 gallons out. The pump that fills the tank will be set to come on for an hour every day. It will shut off when the tank is full and when the tank is full it won't come on. Having it come on every day ensures that at least some days will be sunny enough to refill the tank. The reserve capacity in the tank is another failsafe that the orchard will be watered on time. 

12vman,
The ground both here and on the new place is mostly sand. A 1 1/4 inch sandpoint here will run continously with a 1/2 hp, 110 volt pump that is rated 4.5 gallons per minuit at 40 psi. The new place has a little bit of wetland on it so I know the watertable is close enough to the surface for a sandpoint. I don't think there will be a flow rate problem at the new place but I'll have to try it.

Another consideration is the discharge of the battery. I rated the system to have one 10 amp pump running for 1 hour a day. This is probably more than will actually draw but let's use that figure anyway. This means the load on the battery is 10 amp hours a day. I am not sure when the low voltage load shut off on the charge controller kicks in but it looks like the system could run for 4 or 5 days with no charging before reaching the 50% discharge on the battery. I expect the low voltage shut down would happen at about that point too. Mn is pretty sunny so it's unlikely we would have 5 days in a row where there was not enough sunshine to charge the battery so it should not be damaged.

Thanks for the help. now I just have to wait for the ground to thaw out.


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## Jim-mi (May 15, 2002)

The start surge for the circulating pump is going to be dependent on how much back pressure it has to work against. (Going up hill)......
More pressure more surge.current......

The LVD (low voltage disconnect) you mentioned is 30 amps . . . . .how much leeway is there before it blows. . .??

You cannot assume that the battery voltage will always be "up" . . .topped off.

When / if the battery voltage is down (not charged for awhile) the start up surge of the circulating pump will exacerbate the lowered voltage --voltage dip--and you will have a try-to-start-shut-down go around. and around as the LVD does its job......

To solve this you need much more than your proposed minimum battery and PV...........


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## idahodave (Jan 20, 2005)

I would suggest you reconsider using 80 watt panels, you can buy a 200 watt for almost the same price, but it will need a MPPT controller. 

If you ever expand the system you'll be way ahead.


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## Nimrod (Jun 8, 2010)

Thanks to everyone for all the help I'm getting here. I respect your opinions.

Jim-mi,

The pumps and the water tank will be at the same level. The well pump will get "back pressure" from lifting the water from the well. If I put it at a low spot on the property it should not have to lift the water more than 10 feet. The sprinkling pump will have to pump the water up hill. I think I allowed 3 time the amps to accomodate the surge draw. Isn't this enough?

The low voltage disconnect is about 8 volts, not 30 amps. The battery will be charged whenever the sun is out while the pumps will only run a maximum of a hour a day and probably considerably less. I suppose it is possible that if there was no sun for 4 days in a row that the battery voltage could fall below 8 volts when a pump tries to start up and the LVD will shut off the pump. It's also true that if the pump tries to start when the battery is low, and there is sun, then the panels will add their output to the battery and it's less likely that the pump won't start. 

I can see where you might get where a pump tries to start and the battery voltage dips below 8 volts and shuts the pump down and reset and repeat. Would this happen often? Would it hurt anything if it did?

Idahodave,

You are right but the MPPT controler costs about $300 more.


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## Jim-mi (May 15, 2002)

apples and oranges...
You say the controller will handle a "30 amp load"
Nothing to do with 8 volts and 30 amps

Most charge controllers I am aware of have a LVD of around 10.5 volts..
Got to pity your poor very short lived battery if you let it go down to 8 volts. . . . .Very poor...

There are way too many variables here to put a bottom line on what *start currant* would be needed for your described circulation pump.......

Would this cycling happen often...? yes . .dragging the battery further down . . .
Hard to say how the internals of the controller would survive this cycling.....
But the *life* of the battery would be predictably short.........


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## Nimrod (Jun 8, 2010)

I looked up the specs on a Morningstar PS-30 and I had not remembered correctly. The LVD point is 11.4 volts. 

I guess I will have to try it the way I speced it out and if it doesn't work I can add batteries and/or panels until it does.


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