# Question on solar water pumps and AC-DC conversion



## icentropy (Jul 5, 2013)

Hi all I posted this topic up on the water forum but i think it's turning into an electrical issue more than a water one.

I have a hand dug well / spring in a ravine 110' below my future cabin spot. I'm putting in a road to get to that location along the hill side. The road length will be around 730' total. I will be installing a 1000g underground cistern at the cabin.

I need to find a way to get the water from the well up to the cabin off grid. I went through a few different pump ideas and due to budget i think I'm going to go with a Shurflo 9300 submersible pump. This will pump 1.7gpm to the cistern. I thought I had to install a seperate solar array at the pump due to the distance but now that I see that it only draws [email protected] I figured if i can step it up to 120V at the cabin, send it down and step it back down to 30V at the pump I could use 12AWG wire which isn't too bad in price. I'd spend an extra $300 in cable plus the cost of the converter at the pump but i'd gain a lot more sunlight and consolidate my solar array in one place. 

My main questions are:

What are the typical losses when switching from 24V DC to 120AC then back to 30V DC? 

Does anyone know why they rate the pump at 30V DC? I thought all solar panels ran 12V and could be wired in series to get 24V.....? 

and lastly did i screw the numbers up some how?


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## Harry Chickpea (Dec 19, 2008)

First. There is no free lunch. Whenever dealing with energy - ESPECIALLY alt energy - keep that in mind.

DC motors have a range of voltages that they work at, depending on the wiring and load and design. The important concept of DC motors is the way they handle loads. Traction motors in particular, but all DC motors in general, have balancing points and load ratings.

When a DC motor is run without a load, it runs at high speed and when voltage and amperage is measured across it, it will be at supply voltage and use low amps. As load is applied, the motor slows down, the voltage measurement decreases, and the amperage (which is current and supplies the strength in torque) increases until the balancing point is met. This property makes DC superior in many motor applications to AC. As the load increases, more amperage is drawn and the motor begins to heat. The load rating is how much load the motor can take without the heat burning it up. Some larger motors can go over-rating for short periods of time as long as there is thermal cut-out protection.

So, what does this have to do with your situation? Line loss and conversion losses. Line loss at low amperage is minimal. Line loss at high amperage is a challenge.

If the pump is rated at 2.8A @ 30V, I expect that it is designed to be run off a 36 or (more likely) 48 volt supply, and it would inherently "lug down" to about 30 volts at the end of the run while in use.

You will take a hit with each conversion. Off the cuff, 10% to 15% each conversion would be expected. If wire is expensive and the converters cheap AND the pump has to run continuously, it might be a way to go.

There is a sneaky cheat though since you only have a 1000 gal cistern. Place a storage battery (with a low voltage cutout) near the pump and use a timer circuit.

During the day, the solar recharges the battery near the pump. That recharging doesn't take a lot of amperage, and the losses while using a smaller wire would be acceptable. At about 11 AM, the pump comes on to fill your tank and then shuts off, either when the tank is full or when the low voltage cutout pulls it off the battery. The rest of the day the solar is refilling the battery to keep it from resting in a discharged state. 

A trolley system I researched used a similar idea to extend the range of the trolley. Power generated overnight was stored in Edison cells a mile from the end of the line. The next day, the power stored in those cells augmented the weak power from the power station about ten miles away.


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## 12vman (Feb 17, 2004)

How about a simple shallow well pump and a generator. Fill the cistern when needed. Just run a water line from the well area to your cistern. Connect the pump, prime, and fire it up.. 

You may only need to do this 1-2 times a month. I store ~800 gal. but I don't do laundry here and if I'm a little conservative, it will last us (me and the wife) 2 months. (No well.. Collect rain water)


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## oldasrocks (Oct 27, 2006)

110 head pressure will take a submersible pump at least a 1/2 hp, maybe 3/4hp

Ever think of the option to pump a trailer tank full and pull it up the hill and repump into the holding tank? small 12 V pump on a car battery would fill the bill then.


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## wy_white_wolf (Oct 14, 2004)

icentropy said:


> Hi all I posted this topic up on the water forum but i think it's turning into an electrical issue more than a water one.
> 
> I have a hand dug well / spring in a ravine 110' below my future cabin spot. I'm putting in a road to get to that location along the hill side. The road length will be around 730' total. I will be installing a 1000g underground cistern at the cabin.
> 
> ...


 With what little I see of your idea you're better off to run solar direct off a panel mounted close to the pump. If you use the LCB that shurflo sells it has a place to hook up a tank float. This controls a relay for the pump so it only requires a few milliamps to be sent all that distance.

WWW


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