East/West Facing Solar Panels

[oztules]

30 seconds is a life time in cloudy conditions. It will pay to try it on a clear day where the solar insolation does not vary at all in a 30 second block... impossible to guarantee in clouds.

If it still varies considerably, then you will have to think algorithms in the classic are not linear, ie behave differently as the power increases.... likely to do with the power curve of the panels, and where the programmer thought you should be on that power curve for the voltage they see... ie depends how they calculate the MPPT point, jump table or fuzzy logic perhaps.

If their MPPT is real time fuzzy logic, then they should be the same, as max power is max power.... if they use some other formula.... then all bets are off.

Failing that.... maybe we will never know??



............................oztules

[ChrisOlson]

The thing is, though, I did it twice and got pretty much the same results.  I didn't write down Vmp as showed on the controllers though.  Forgot to do that.  But I think it was right around 90 in all cases anyway, give or take a couple volts.

They got three different things in the controller for solar arrays.  One they call "Fast Auto & Timed I-V PV Sweep".  That's the default and what I am using.

But they also got "Legacy P&O" and "Dynamic" you can choose from.  I've tried them other ones and can't see much difference.

I guess they do have a fourth one too where you can choose your own operating voltage as a percentage of Voc.  I've tried that before too, and by playing with the percentage value can get it close to the output of using "Fast Auto & Timed I-V PV Sweep".  But it don't stay constant and all it takes is for a cloud to go over and it's way off.  So I never use that.
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Chris

[ghurd]

I agree with Oz about clods, or those days when the sky is all exactly the same shade of gray.  It changes too fast to get anything very meaningful.

Clouds and angles can do some strange things to PVs.
I tested matched pairs (as matched as possible, consecutive SNs, or at least from the same case) of smaller panels for Commanda.  The test was to see how balanced real PVs were for a large project of her's.

Amorophous, mono and poly.  12V.  Wired 2 in series for 24V, so the same current.  Measuring the voltage of each panel shows whats what.  Tried it both with fixed resistances, and batteries.

One always did better than the other, which I expected.
Almost always, the one that did best in full sun didn't do as well as the other in overcast conditions.
At an angle, usually one dropped considerably more than the other.
The output voltage even had some effect.
The percentages were a lot larger than I expected.

I would think all that variation would have some effect on MPPT logic... somehow... not sure how.
G-

[ChrisOlson]

I would think all that variation would have some effect on MPPT logic... somehow... not sure how.

My interest in it being somewhat piqued, I tried a different experiment a bit ago.  We have grey overcast with some sleet coming down.  I know spring is around the corner - just haven't seen it yet.

I wired the roof array into the single Classic 150 thru its disconnect breaker.  So I can shut off either the roof array, or the shop array while watching the screen on the Classic.

Very poor output, as would be expected in these conditions - 526 watts and the Classic is running the arrays at 87 volts in the poor light.  This is 100 watts/kW installed capacity.

Shutting off the breaker for the house roof array causes it to drop to 352 watts output and the Vmp does not change.  This figures out to an output of about 94 watts/kW installed capacity.  Turning the roof array back on makes it come up to 525 - about the same as before.  Shutting off the shop array makes it drop to 143 watts and the Vmp showed 88.  This figures out to an output of 95 watts/kW installed capacity for the 1.5 kW east/west roof array.  Turn everything back on and it came up to 530 watts and 87 Vmp, so no basic change from all three readings with everything on.

So if I add those figure up for the individual outputs; 143 + 352 = 495 watts.  This is vs ~525 with everything on.  That's a 6% boost today with the grey sky and some precipitation coming down.  Not the same as the ~13% that I measured yesterday with what I call "Cloud Shine", but still get more power with the entire works on one controller.

The other thing I noticed yesterday is that with the whole works on one controller I was still making 68 watts after the sun had set for the day!  If I turned off even breaker in a combiner the Classic would go to zero output and not come back.  Flip that breaker back on and it went to 68 watts with the sun below the horizon.  LOL!

I don't know what the explanation is, but I am concluding there is some sort of "boost" from connecting multiple arrays together as one big unit.  On the perfect day I will get less power by doing this because the Classic can only handle about 4.8 kW on a 48V system, so it's going to amp-limit itself and "waste" part of the array.  But the thing is, on those days I don't need the power because the bank is floating by 2:00 in the afternoon anyway.  I want to optimize output on the less than perfect day because that's when we need the power the most.

The Classic's power measurement to the batteries is very accurate - in the neighborhood of +/- 1% with the type of shunt they use in there.  So I'm satisfied that it's not erroneous readings.  I'm going to leave it all on one Classic for now and shut off the individual breakers from time to time over the next few days in various conditions and write down what I get.  If it proves out that I can get even 6% more on the bad day, it's worth it to take the "hit" on the good day simply because on the good day the controllers would have the whole works throttled back in Float anyway.

I'm thinking it has something to do with the arrays being combined, being able to collect more available light, and sending it down one pipe, eliminating the losses in an additional MPPT controller.  But that's just my theory on it.
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Chris

[Wolvenar]

Maybe the controllers back down because with both working  because the apparent battery voltage is higher to each from the incoming power from the other?

[ChrisOlson]

Maybe the controllers back down because with both working  because the apparent battery voltage is higher to each from the incoming power from the other?

I don't think that's the cause of it, Wolv.  They adjust for battery voltage even if that was the case.  And a power input of 500-600 watts with our bank and loads is like peeing in the ocean.  So it just doesn't change it.  These are the types of average loads our system carries almost 24 hours a day, with daytime peak loads sometimes 4-5x that, and nightime average loads around 500-800 watts.



So basically, two controllers supplying 500 - 1,000 watts is like insignificant and doesn't change the bank voltage, or voltage that the controllers "see", by even .1 volts.
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Chris

[oztules]

"
So if I add those figure up for the individual outputs; 143 + 352 = 495 watts.  This is vs ~525 with everything on.  That's a 6% boost today with the grey sky and some precipitation coming down.  Not the same as the ~13% that I measured yesterday with what I call "Cloud Shine", but still get more power with the entire works on one controller."

Grey sky and precipitation... and still getting 10%  is what I like to see........ 500w is the average my house uses over a 24hr period..... for you probably 1kw plus a bit.

Our previous discussion about solar is starting to have some teeth.... for the cost of your new inverter, you could spring for another 5kw, and see 1kw power on grey rainy days... and that starts to make a difference to the power input required even in inclement weather.....

Cheap solar really makes the difference now days.



Still don't know why your seeing the differences, unless the classic is a bit heavy on the idle current, and that makes it look better with a larger output.... bit like that inverter I modified from 200watts idle to 20 odd watts idle.... but I can't see how a buck would use up magnetizing currents like that.....mystery

 
All power is good power, and the powers that be and must be obeyed, will find a use for it .....have no doubt.


....................oztules

[ChrisOlson]

Our previous discussion about solar is starting to have some teeth.... for the cost of your new inverter, you could spring for another 5kw, and see 1kw power on grey rainy days... and that starts to make a difference to the power input required even in inclement weather.....

Yes, I certainly could.  But on the other hand the wind turbines pretty easily produce 3-5 kW on those grey rainy days too because they normally associated with sort of front that makes the wind blow hard.  So I'm trying to build a balance of systems that works well in all weather and minimizes our generator run time in the long term.

With careful design and selection of components we're able to use a very small generator for our primary standby, compared to what most would consider necessary for a 30 kWh/day off-grid all-electric home.  That has saved tremendously on fuel in the generator.  We use it primarily for peak load management in combination with Generator Support, which saves on buying extra inverter capacity to run big load, and saves on buying batteries to run the big loads.

At the same time, using the Gen Support takes the load off the batteries and RE system so it can do the job of charging batteries instead of using a generator to charge batteries.  Using a generator for on-demand peak load is quite efficient, actually.  The gen power goes right to the loads, and properly set up, it runs the generator at its prime power rating where it's the most efficient.  Using a generator into an inverter charger to charge batteries that are at best 85% efficient, then converting it back to AC to run the loads is horribly inefficient use of a generator.

Over the long term, after getting this set up, I only recall three times since the first of the year that our inverter has started the gen for battery charging.  And in the new inverter I got a STOP V trigger set so it shuts it off when the bank voltage reaches 56, and from that point let the RE system do its job.

So that's what I've been trying to do - find that balance of systems that's the best for us.  We're getting there, and this summer will tell if we have the need for more solar power yet.
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Chris

[ChrisOlson]

I got all 6 kW of solar capacity on a single Classic 150 with 3.75 kW facing south at 30° tilt, 1.5 kW facing east on a 22.5° roof slope, and .75 kW facing west on the other 22.5° roof slope. Today was the first perfectly clear day we've had in weeks. I logged the output from 7:00 AM to 5:00 PM with the Classic and graphed the CSV export from the Status Panel in Excel. This is what it looks like, with some comments on it below.



From about 8:15AM to 8:45AM there's a dip in the power due to shading on the east facing array from a wind turbine tower.

That same tower causes a dip in the output from 10:30 AM to 12:30 PM due to shading on the south-facing array. Today the turbine was running and the blades caused shadow flicker on the south-facing array, which really screwed it up for about two hours as the shadow moved across the array, knocking out series strings of three panels one at a time for two hours.

Damned 90 foot tall wind turbines.  But this won't be a problem as we get into summer more because the sun is at higher elevation then and the shadow flicker from the rotor on that turbine doesn't affect the south-facing array then.

The bank went into absorb at around noon and was done with absorb at 3:00 PM where the big dip in amps occurs. Although the controller did hit darned near 90 amps (about 5,400 watts) for a very brief time, the FET's got hot and it de-rated pretty fast. Then you can see the arrays being "throttled" most of the afternoon in absorb - the bank and loads didn't require the full output of what it could do. At about 2:20 PM you can see a spike where it goes up to 88 amps or so. That was due to me doing some welding in the shop and the big load on the system caused the controller to put out max power for a little bit.

The other up's and down's are caused by varying loads in the house - 'fridge and freezer starting and stopping, wife doing whatever - turning her stuff on and off, etc.. And that's why the morning curve is smoother - it was basically requiring everything the arrays and controller could put out - right up to where the turbine tower caused some problems. In the afternoon, with the arrays "throttled", the controller would adjust output as necessary to meet charging requirements and loads on the inverter.

So I'm going to say putting the whole outfit on one controller works fine.  28.4 kWh from it and it could produce more if we had the loads for it today.
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Chris