Grundfos 11 SQF 2 question

chetwynd · September 28

I see that it performs best at over 100 VDC, so should I just wire let’s say four 36V panels in series and that’s it? It doesn’t need more amps?

thanks!

BB. · September 28

The short answer is yes. Just add more panels in series (or batteries, etc.) to reach the >100 VDC needed to "optionally" drive the pump motor.to

I sometimes use an analogy of an automatic transmission vs MPPT. The "Gasoline Engine" side of the automat ic transmission keeps the engine's RPM, Torque, etc. in the operational zone (say 1, 000 RPM to 6,000 RPM. And adjusts the actual engine operating point based on how much power (torque, RPM/MPH) is needed at the wheels.

For MPPT, buasically this is Voltage and Current from the array (Pmp=Vmp*Imp). If you "need" 400 Watts of power to drive the pump (GPM, dynamic head/water column height+pressure needed at surface), you could have:

So, you you have 5 amps @ 100 VDC = 500 Watts
Or 2.5 amps @ 200 VDC = 500 Watts

The "magic" of MPPT. Matches what is "available" from the solar array (or any DC power source) and what the "pump motor" needs to run efficiently... And if the available solar wattage is less, then the MPPT simply slows the motor RPM down to match the available solar/DC power.

Vmp of panels say 30 VDC
100 VDC / 30 VDC = 3.3 panels rounded up to 4 panels
400 Watt of power needed / 4 panels - 100 Watts per panel in- series
Vmp-array-std = 4 * 30 Vmp-std = 120 Vmp-array-std

Here is a nice (complicated to read at first glance) performance chart of GPM of water flow vs water head:

https://product-selection.grundfos.com/us/products/sqflex/11-sqf-2-95027335?pumpsystemid=2472901387&tab=variant-curves

Practially speaking, hot solar panels can have Vmp-panel drop to ~80% of standard (data sheet) voltage, so 30 Vmp panel would actually be:

30 volts * 0.80 panel derating = 24 VDC
100 VDC / 24 VDC Vmp per panel = 4.166 or 4-5 panels in series (if you want "max performance" in Hot weather

Then there is the actual wattage of a panel... Say you want 400 Watts, then derated array should be:

400 Watts / 0.80 hot panel Vmp derating = 500 Watt derated array

Then there is the issue that you probably want to pump for a large part of the day that is not solar noon... So you need a solar array that is X times larger to pump from 9 am to 3 pm at "rated power capacity" (GPM, Pressure)...

So perhaps you want 200 to 400 Watt panels to get more pumping during morning/evening times (various ways to calculate this).

Or perhaps you are willing to take a 20% loss in electrical efficiency so you can run a 48 VDC battery bank + Solar charger + Solar array for 24 hour per day pressurized water availability... Vs using a pressure storage tank, tower, pumping uphill to a pond/tank, or using AC power from solar/Genset/etc... (or perhaps "skip" the DC system and use the your existing AC solar power system (Grundfos pumps are very "solar friendly" in that they have virtually "no" starting surge current--Unlike standard AC powered pumps which can have 2x or more Starting surge current).

I am not a solar pumping person... Lots of questions that you may have already answered (say need water pumping during the sunny day for irrigation/stock watering/etc. and don't need backup power for poor weather/winter vs summer, etc....

Trying to not make the answer "too complicated" here without knowing your needs, knowledge, and decision points up to now.

-Bill

JRHill · October 6

chetwynd said:

I see that it performs best at over 100 VDC, so should I just wire let’s say four 36V panels in series and that’s it? It doesn’t need more amps?

My water well using the same pump worked good as gold on a 48vdc bank of deep cells via 800 watts of panels to charge the batteries. It world draw ~ 780 watts at load and of course the pump is soft start so there are is no 'surge' concern. As a comparison if I plugged into 120vac (via the IO 101 box and that is HIGHLY recommended) the pump would draw slightly higher wattage, ~ 840 as I recall and of course there was slightly more volume. So you say "that it performs best at over 100 vdc", I'm not sure where you get that impression.

You do not mention any batteries/charge controller are involved. But I have seen solar panels that directly power a Grundfos well pump for a livestock tank in the field. I have no experience of what happens for example on a moderately cloudy day when there are frequent clouds that would have the pump drop out at low input just to restart and then fall out again ad nauseum. I wouldn't consider that optimum. All I can say is that you definitely want to panel (2s2p or 4s) on the generous side for the total knowing that you aren't going to be producing the panel's nameplate output most of the time anyway. Say, 1000 watts minimum? That pump is darned expensive not to mention down time and pulling and resetting the replacement. Without any more information on your system design I'm not sure what else to add here. Except:

After the 2nd bank of batteries reached end of life I said to-heck-with the batteries/solar and just plugged the Grundfos pump into the 120vac service from the house's off grid system and haven't looked back. The system has performed wonderfully for 15 years with a reliable supply of power whether AC or DC. At the time I eliminated the batteries I did add a 2nd 80g tank (60-75psi).

Grundfos 11 SQF 2 question

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