Will This System Operate Without Batteries?

[GrizzlyGuy]

To keep my question simple I'm going to leave out the details of the application that I have in mind. Let's just say that I had built a nifty hybrid system that worked great, but somehow today I managed to destroy my entire battery bank (and somehow managed to live to tell about it). On this particular day Mr. Murphy is on quite a roll: the grid is down, my generator is out of fuel, the wind turbine is a tangled mess, and for whatever reason no improvement to the situation can be made for weeks.

I'd still like to run a particular load for a 1/2 hour or so whenever it is bright and sunny. I reconfigure my system as follows with everything connected in series as listed:

PV Array (suitably sized, voltage appropriate for MPPT)
Xantrex XW MPPT80 600 High Voltage Controller
Xantrex XW6048-120/240-60 Hybrid Inverter-Charger
240V Load (30A start-up, 15A steady state)

Will that system actually run while the sun is high in the sky? Or would it not run at all because (for example) the charge controller is unhappy with having zero volts across my non-existent "battery" and shuts itself down? Obviously I don't want the MPPT controller to control any charges (no batteries), I just want it to provide sufficient current/power at 48V to the inverter.

Would it make any difference if I had functionally equivalent OutBack equipment in place of the Xantrex equipment?:

PV Array (suitably sized, voltage appropriate for MPPT)
Outback Flexmax 80 Amp MPPT Solar Charge Controller
Outback Radian GS8048 8,000 Watt Inverter/Charger
240V Load (30A start-up, 15A steady state)

If none of that would work, then are there any other equipment combinations that would?

[Mike90250]

Sadly, most of the MPPT controllers, need battery power to startup, and then they can begin charging. You could just get 4, 12V "box store" (Kmart, Sams, Costco) batteries, and use that as an emergency bank. But you HAVE to have batteries for the charger to function, and to supply surges to the inverter, otherwise the inverter will trip offline from under-voltage, nearly all the time.

Just get 4 batteries - $90 each.

To run your stated load [ 240V, 15A ] you will need a bit over 4,000w PV harvest. That's 2 charge controllers !

That thought is essentially what I do with my irrigation pump, it's on a timer, and just before noon, it kicks in, and batteries are already in Absorb, so the pump is running off solar, not the batteries.

[GrizzlyGuy]

Sadly, most of the MPPT controllers, need battery power to startup, and then they can begin charging. You could just get 4, 12V "box store" (Kmart, Sams, Costco) batteries, and use that as an emergency bank. But you HAVE to have batteries for the charger to function, and to supply surges to the inverter, otherwise the inverter will trip offline from under-voltage, nearly all the time.

Just get 4 batteries - $90 each.

To run your stated load [ 240V, 15A ] you will need a bit over 4,000w PV harvest. That's 2 charge controllers !

That thought is essentially what I do with my irrigation pump, it's on a timer, and just before noon, it kicks in, and batteries are already in Absorb, so the pump is running off solar, not the batteries.

Very interesting, thanks!

Your actual load sounds a lot like the mystery load that I described: a 3 HP submersible well pump sitting down around 750' and driven by a variable speed/constant flow controller (Goulds calls it a "Balanced Flow" controller). That's what our well guys are recommending to replace the current pump that is on its last legs. The 240V current requirements I stated are probably a little high, but not too out of line.

I wonder if it would be possible to build a bootstrap circuit to replace the batteries? Maybe a 47V voltage regulator hanging off the charge controllers' inputs and tied to their outputs via a relay (along with a relay-switched low resistance dump path to ground) with a 555 timer controlling the relay. Aw the heck with it, for $360 in cheap batteries that would only be used in an emergency, it probably wouldn't be worth the experimentation time and warranty-voiding efforts to find out.

[Mike90250]

.... Aw the heck with it, for $360 in cheap batteries that would only be used in an emergency, it probably wouldn't be worth the experimentation time and warranty-voiding efforts to find out.

Yow, a 3hp pump - you should be in 3 phase land for that. They make pretty inexpensive Single-3phase Vari-speed controllers (VFD/VSD) for that. http://en.wikipedia.org/wiki/Variable-frequency_drive

My 1/2 hp pump draws 1,000 w (even!) as reported by the inverter (xw-6048) 3hp would be 6000w running, maybe a bit less, and if it was 3phase via a smart controller, you can select a 5 sec start-up ramp to cut down on the surge, and after a minute, you can have it throttle back and burn only 5KW instead of 6. (smart VFD/VSD controllers are nice)

vendor:http://www.automationdirect.com/adc/...Catalog/Drives

[GrizzlyGuy]

Yow, a 3hp pump - you should be in 3 phase land for that. They make pretty inexpensive Single-3phase Vari-speed controllers (VFD/VSD) for that. http://en.wikipedia.org/wiki/Variable-frequency_drive

Yes, based on the specs and description of the pump controller the well guy recommended (this BF-30) I think it actually is a VFD with fewer bells and whistles. The well guy didn't specify a particular pump model or manufacturer in his estimate (he called it a "10FA3" pump) but it should be a 3-phase pump or else the BF-30 wouldn't be able to drive it.

Since I don't know the particular pump that he has in mind, it is tough to make a closer estimation of the power requirements. Those BF-30 specs mention a CentriPro 3HP motor with 10.1 SFA @230V, and a Franklin 3HP motor with 10.9 SFA at the same voltage. The full load current will be less than SFA, and since it is variable speed it won't always be running at full load (unless we're sucking water at max flow rate), so the actual power consumption could be considerably less than what my initial SWAG above would calculate out to. No worries, I can size things more precisely later if my wife and I actually decide to pursue a solar option. I think it would need to be a 5HP pump instead of a 3HP to get up into the 5-6 kW range you are talking about(?).

FYI our current pump is also 3-phase, but uses the old mechanical-style phase converter rather than a controller. It is a beast, all or nothing, and is only pumping 4 gpm at 40 psi. The new pump will (supposedly) give us 10 gpm at 60 psi.

[Naptown]

Maybe I'm missing something here but a constant pressure pump wouldn't that also mean constant current draw?

[GrizzlyGuy]

Maybe I'm missing something here but a constant pressure pump wouldn't that also mean constant current draw?

I think it would if the flow was held constant along with the pressure. But the BF-30 controller documentation says:

Constant Pressure: Provides consistent pressure even as flow requirements vary.

My understanding (could be wrong) is that if you had a situation where the only demand on the pump was providing water for a single open faucet, the pump might be loafing along at 50% output or less. If you opened a second faucet, the pump would rev up so as to keep the pressure constant at both faucets. The flow would double but the pressure would remain the same. The increased flow would lead to it consuming more power so as to provide that increased flow.

A crazy(?) idea just popped int my head: the well guy has to pull the old pump to install the new one. While installing the new one, maybe he could also drop in a Grundfos 6 SQF-3 solar pump (specs here) with it's own wiring and output pipe? If the main pump is down at 750', the Grundfos could sit up at 700' (or wherever as long as it was sufficiently below the water line). If the main pump failed or we had an extended power outage, I could run the Grundfos pump from a much smaller generator or solar system, with "smaller" being relative to what it would take to run the big pump. It will run on either AC or DC which is nice.

Down at 700' the Grundfos would only pump about 4-5 gpm at max power, and doesn't have the "oomph" to do that and also pump into my pressure tanks, but I'd at least have water available in a non-pressurized storage tank. I could even pump from the storage tank into the pressure tanks with a boost pump if I wanted to. The well guy's test report shows the well diameter as 6 5/8", the Grundfos is only 3" dia, so there should be room to have it sitting next to the 1 1/4" drop pipe coming up from the main pump.

There is probably some good reason why doing this is impossible/illegal/whatever, but I don't know what it would be.

[Mike90250]

.... The well guy's test report shows the well diameter as 6 5/8", the Grundfos is only 3" dia, so there should be room to have it sitting next to the 1 1/4" drop pipe coming up from the main pump.

There is probably some good reason why doing this is impossible/illegal/whatever, but I don't know what it would be.

1) It's an EXPENSIVE pump, and needs a fancy controll box too .$$$


2) A small pump, in a large bore, needs a torque plate, or it will wind up (twist) the drop pipe when starting and stopping !! That will be difficult to solve, but if you have the $$$ perhaps the well guy can make it happen.

Just locate a high spot, even 15 ' higher than your water faucets, and put in a 1,500 gallon tank. That will provide pressure, not a lot, but usable. Pump into that. I have my tank farm 160' high on a hill, and a couple thousand feet of 1.5" & 3" pipe, for domestic, and for Fire & Irrigation.

[GrizzlyGuy]

1) It's an EXPENSIVE pump, and needs a fancy controll box too .$$$

Expensive? This is a solar energy forum, what isn't?

It's actually not that bad: $1900 for the pump, another $1100 for the #10 2-conductor submersible wire, and $200 for the 1" plastic pipe. It doesn't need a controller box, just an on/off switch. The MPPT and overload protection circuitry is built into the body of the pump itself. You could wire directly from solar panels to the pump if you wanted to. An Aquatec Pressure Pump ($130) would let me pump its output from a small reservoir into my existing pressure tanks if I wanted to.

An above-ground 1,500 gallon tank would run me around $600, but freezing would be a problem up here in the mountains. I could bury a tank like they do in Alaska but that would be expensive to do. I think the torque issue could be handled by clamping the pump to the steel drop pipe going down to the main pump, or by modifying a torque boot to fit the configuration.

If the Grundfos pump was running at its max power of 1200w, their performance curves say it should be pumping about 4.5 gpm from down at 750'. If I estimate an 800' run of a #10 AWG wire pair, and assume that the pump is running at its Imax of 8.4A (worst case for wire loss) then I'd lose about 67W in each leg of the wiring, or 134W total loss. Total power consumption would then be 1334W. That works out to 296W/gpm worst case. The best case would be for the pump running at its max of 300 VDC, which would be 4A at its max power of 1200w, so the total wire loss would drop to 30W which works out to 273W/gpm. Neither of those values nor the cost seem so bad when you consider that you wouldn't have to incur the additional expense and loss from an upstream inverter, charge controller or even batteries if 'run while the sun is up' is all you need in a grid-down scenario.