An unusual hookup....?

Okay, here's a problem involving direct, long-distance distribution to multiple loads...

We are implementing a solar water pumping system utilizing approximately 5kW of PV energy. We need to distribute this power directly from the PV panels to 5 separate pumps located on a slope approximately 700 meters long.

However, the pumps are only needed for 6 months out of the year, so we would like to consolidate our panels in one location so that we can connect them to power a house located at the bottom of the slope while the pumps aren't running. This means that we then have to run power from our 5kw array to a distance of between 15-700 meters and distribute it to 5 separate loads along the way. Does anyone have any insight as to the best means of conducting this transmission? Running AC vs. DC lines (our Grundfos Sq Flex pumps can handle either)? What voltage would be most efficient (and still safe)? Are there other factors that we should consider?

Any advice here would be greatly appreciated... thanks.

Comments

  • mike95490mike95490 Solar Expert Posts: 7,283 ✭✭✭✭
    Re: An unusual hookup....?

    Running High voltage AC (via 240V transformers) - whats the amp draw of the pump @ 240VAC

    You have to price out the delta between copper wire, and iron transformers.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
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  • BB.BB. Super Moderators Posts: 26,837 admin
    Re: An unusual hookup....?

    Check the voltage rating of the pumps/controllers... I believe that they may be rated for 30-300 VDC... If so, you can avoid the AC conversion and transformers to 240 VAC and just send out 200-300 VDC to the pump motors/controllers.

    This would be a very nice solution--especially if you don't need batteries and can live with just daylight pumping.

    On the other hand, a 0.7 kM long wiring run may not be the best thing in the world. You need to check with the pump manufacturer--but if you are connecting just to solar panels, you may have to run each pump with its own panels. The controllers could get the MPPT confused (Maximum Power Point Tracking--matching solar panel voltage/current output to the needs of the pump motor for maximum pumping flow) with multiple pump motors connected to one large solar array (they may work fine--I just don't know).

    You would probably be better off just mounting a few panels next to each pump location. You might rack the panels so you can truck them back to the home during the off-season... But you run the risk of breaking them while doing this (PV panels are just large tempered glass panels).

    To look at wiring losses (size of copper wire required to run 700 meters), there is a wiring spread sheet here. Remember that the wire runs are 2x (+ and -) as long--or 1.4 kM in this case.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: An unusual hookup....?

    "You need to check with the pump manufacturer--but if you are connecting just to solar panels, you may have to run each pump with its own panels."

    we contacted grundfos today and their initial response is that if we want to power all the pumps from the same array we need to have them feed into a battery bank first so that the pumps first so that the pumps receive constant voltage rather than have it fluctuate as it would when powered directly by panels. their concern was that when the power supply from the panels dipped down, the pumps would be competing for the scraps of energy. i'm not exactly sure why this is a problem so i'm waiting for clarification. it seems to me that these sqflex pumps just wait until they receive enough energy and then just turn on anyway. plus when the power supply dips down they just run at a slower rate which may not be a huge issue for our watering schedule.

    i'm still trying to understand which kind and gauge of wire would be required to achieve the lowest possible losses for long line runs. most wire charts only seem to go up to 48V copper wire.

    i think we're starting to wonder if it's worth centralizing the panels at all and if just putting them next to each pump might be best.
  • System2System2 Posts: 6,290 admin
    Re: An unusual hookup....?
    mike90045 wrote: »
    Running High voltage AC (via 240V transformers) - whats the amp draw of the pump @ 240VAC

    You have to price out the delta between copper wire, and iron transformers.

    The amp draw of the pumps at 240V is 8.4 and the max Wattage each pump can take is only 1.4 kW. However, there are some instances where the multiple pumps will be running so drawing a higher amperage could be helpful.
  • BB.BB. Super Moderators Posts: 26,837 admin
    Re: An unusual hookup....?

    Actually, this series of pumps has a MPPT controller (Maximum Power Point Tracking) as I understand it...

    Basically, a solar panel wants to put out a constant voltage, but the current varies as the amount of sun light changes...

    A standard motor is, more or less, a constant power device. As the voltage falls, the motor itself wants to increase current draw to keep V*I=constant-power.

    The controllers in this pumps attempts to match the I*V characteristics to that of the pump. In this case, on the DC input side, it varies the current until it finds the peak power (V*I=Ppeak). On the motor side, generally they require more current as the voltage falls--so the output of the controller attempts maximize current to the motor (using Ppeak=V*I of the motor).

    So, in this case, the pump motor will operate at reduced speed (and reduced flow) to get some water pumped when there is less than maximum sunlight.

    In the case of connecting multiple pumps to one solar panel array--each would be attempting to find the Ppeak of the solar array by varying the current drawn--and each of the pumps would be doing this on its own--so the feedback to the pump electronics would be confusing--(Voltage changes would not track the local controller's changes in current).

    By adding batteries, the voltage will remain the same, and the controllers can just maximize the I*V to the pump motor--as required. (I hope that this all makes sense).

    However, adding batteries adds cost to the system (batteries, charge controller, some sort of low voltage cutout, ~20% additional losses due to batteries, etc.). If, the pumps move enough water during the 4-5 hours of daylight--you should not need batteries. If, however, you need to pumps operational more than 4 hours per day, or during storms (for example, the difference between irrigation pumping vs storm water pumping) -- then you may need to add the batteries to increase pump operational times.

    Regarding wire size--the system losses usually just boil down to how many volt drop from input to output you can withstand. For example, 12 volt system with 2 volt drop (10 volt output) would probably be too much voltage drop for the average 12 volt appliance (and 2v drop / 12 volts=17% power loss).

    However, at 120 volts (ac or dc), a 2 volt drop gives 118 volts at the pump--not a big deal at all and the losses are much less (2 v / 120 v = 1.7% voltage drop).

    Also, the voltage drop itself is proportional to the current going through the wires.... V=I*R

    So, if your voltage is 10x (120 vs 12 volts), the current is 1/10 as large... and the voltage drop is 1/10 as large too. (12 volts at 10 amps = 120 watts; 120 volts at 1 amp = 120 watts). So you can use 1/10 the size of of wire (and cost) to run a "high voltage" system.

    Lastly, wiring power loss goes with the square of the square of the current.... P=I^2*R

    So, for the same size wire, if you double the voltage (example, from 60 to 120 volts), the current halves. The square of the current is (1/2*1/2=1/4), so you actually reduce the power loss by 75% (if the wire gauge remains the same).

    I know that this is mostly in US units... But using the spread sheet link above, I used 1.4km = 4,260 feet of 8 awg wire (at 50C), set the solar panel voltage to 240 vdc, and set the panel amps to 10 amps (guessing a 2kW array/pump). I got 32 volts or ~14% voltage drop (240-32=208 volts at the pump).

    Normally, 14% loss due to wiring is considered to be too large of loss for a solar system... But you can easily price the difference to add 14% more solar panels, or the price of running 8 awg wire vs 2 awg (3.3% losses), or moving the array next to the pump and use 46 feet of 14 awg wire for 0.5% losses... (of course, you would probably use aluminum wiring strung on poles vs buried copper wire--but the calculations are similar).

    In the end, 1.4 km of 8 (or 2) awg copper wire is probably too expensive--and moving the panels next to the pump is better solution (also, up here we are now having problems with stolen copper wires from buildings and such).

    Does this help?

    -Bill

    By the way, 240 VAC vs 240 VDC should not be a difference in wiring losses in this situation... Using a high voltage array saves the battery/inverter/transformer losses and the pumps will probably run better on their own MPPT DC controllers anyway.
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: An unusual hookup....?

    Thanks Bill. We really appreciate all your help on this. I think we're going to take your advice and stick to keeping the panels and the pumps together. Trying to work out all the details of the other system is getting complicated and sounding rather expensive. But for what it's worth, your advice was really informative and helpful. Thanks again...

    Cheers,
    The Aurora Project team
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