More MPPT Confusion and Question

[BFL13]

Still trying to understand my MPPT set-up and what would be the effect of changing things a bit.

My 230w (30.0 Vmp x 7.7 Imp) poly panel has a Vmp of 30.0v with a measurement tolerance of +/- 3%
So actual Vmp could be anywhere from 29.1 to 30.9

My wiring zoo from panel to controller, measured in action this summer showed panel at EG 29.3 and at controller input 29.0

Voltage drop was therefore 0.3 at about ??7.7a Imp?? (Rated Isc is 8.3a and was getting that panel disconnected)

(I have the controller output wattage then at 203w (12.7Vbat x 16a) but that is after controller efficiency loss and whatever else)

So the question is, where it is trying to stay at Vmp at the "knee" of the IV curve, how bad is that 0.3 voltage drop? What would my controller output be if I cut that voltage drop in half say?

I am thinking (backwards? getting mixed up here) that the controller will "see" the panel voltage as 0.3v lower than it is. But on the IV curve that lower voltage would make the amps higher but if it is lower then amps are way down the knee drop off

Rating max is 230w (30 x 7.7) I measure 29.3v at the panel which is where the controller has set the voltage thinking that is the MPP?

But the controller input voltage is 29, not 29.3 so does that mean the controller's idea of MPP is 0.3 low or 0.3 high?

The IV curve where Vmp is at the knee which has a huge drop off in amps so a small change in voltage must have a big effect, but which way?

I am trying to calculate how much I can improve my wattage as seen at the controller output by reducing that 0.3v loss to the input by x amount and see if it is "worth it." But I am all mixed up 😞

[red31]

I've used local data and govt data from several hundred miles away (closest) on clear days.

http://www.nrel.gov/midc/

[JiminDenver]

i have a app for the cell phone as a light meter. I can't say how accurate it is but it does let me know when the illumination is up or down.

[jrnymn7]

Is there an inexpensive device for measuring irradiance? Perhaps that would help eliminate the possibility of fluctuating/inconsistent readings? A more controlled test, if you will.

[09FLSTC]

BFL13 wrote:

mena661 wrote:
I think you need to invest in a clamp on meter. You'd use it quite a bit.

I would, but then Salvo would have me up for holding it wrong, and I would still be accused of "measurement error."

If she comes back next year, maybe your mermaid sister could hold it steady in her flippers so the numbers would be right?

That made my day. Knowing why Ralph at Bogart passed on MPPT for his new popular PWM controller has me leaning that way again on my newest set up.

[mena661]

I understand what you're saying now. Guess I was confused too. 🙂

[BFL13]

I am confused as to what makes Salvo think there is something wrong with the controller. I think he got mixed up reading my pile of numbers.

He thinks there is a "discrepancy" because he says the MPPT is not out-performing what PWM could do in the case I gave by 5-10%

Mine was typically 15.5a MPPT vs 14.5a PWM so about 1/14.5 = 6.9% in the Spring. That was with the 44C panel and its estimated 7% loss from that much heating.

Then he suggests that "discrepancy" could be because the irradiance was lower. In fact, if that were lower then the input would be less and that would make the controller's output look better! Oh well. It is easy to get mixed up sometimes reading all this stuff.

I can't take any more measurements till April so I am still hoping somebody else with solar working now can take measurements as outlined above so we can see what might be "normal" to compare with what I got.

Salvo wrote:
There's three possibilities for this discrepancy.

1. Irradiance fluctuates. You could be measuring mppt current when irradiance was lower.



2. Measurement error.

3. Your mppt controller is sub par.

If everything is working correctly, you should see 5 to 10% more current with mppt.

BFL13 wrote:

Salvo that is part of the big puzzle to me. The Isc was over the rated Isc of 8.3 so to me that means the panel is at least at STC for irradiance, allowing for the high temperature raising Isc but lowering Voc.

I just figure if the Isc is over its rating that means the insolation is ok, and any losses are from something else. No?

[mena661]

BFL13 wrote:

If she comes back next year, maybe your mermaid sister could hold it steady in her flippers so the numbers would be right?

LOL! How did I miss this? She might be willing to help if you let her inside the fence. And bring more fish!

And I agree with Salvo. The proper measurements need to be done. I think you will have a better understanding doing it that way. AND no guesswork or extrapolations would be needed.

[BFL13]

I would like to get my Imp to resolve this. However, I do already have enough info to estimate my controller efficiency AFAIK.

Trying that again---

In the Spring example, I have 44C panel temp which is a 7% loss of power on the usual graph that shows 10% loss at 50C. 230w panel with 7% ( 16w) off is 214w.

I have a 1% measured voltage drop panel-controller so 1% of 214 is 2 so ASSumed input is 212w. With measured 29v Vmp, that makes ASSumed Imp 7.3 (Panel rating is 30 x 7.7)

Measured output at that time was 205w (Vbat 13.5 , amps 15) where the Vbat was taken with meter and also seen on Trimetric. Amps from Trimetric and solar controller display agree. So lots of cross-checking there confirms controller watts display of 205w output.

212-205 = 7 and 7/212 is a 3.3% loss so controller efficiency is 96.7% using that input.

For controller efficiency to be worse, the input would have to be higher, not lower. We know the output is not lower.

For the input to be higher than 212 you have to get the panel higher than its 230 or ignore temp loss and voltage drop loss or both. Insolation was near STC as shown by the Isc being just above its rating as it should be with the panel temperature being above 25C at 44C.

I have no doubt the reality is a little different with the real Imp, but it can't be much different, so I just can't see where the controller could be faulty. It appears to be making the same sort of amps other controllers do.

People with the Morningstar 15L on their 230w panels report 15 amps (might be getting a more but has a 15a cut-off) and I get 15a in the Spring. PWM 230w would get 14.5a so that also shows my 15 is about right too.

So what's wrong?

[Salvo]

You haven't taken the right measurements. You need to measure input and output power to the charge controller. No one can set you straight until you do this.

Besides the graph I posted that refutes your results, here's a few more:
First Law of Thermodynamics
Ohm's Law
Watt's Law

BFL13 wrote:
Salvo, I took a zillion measurements and that's what I got.
...
If anyone has some data that proves otherwise, please post it.

[CJW8]

The accuracy of a $500 Fluke clamp on DC ammeter is only +-2%, I suspect my $100 Craftsman is not as good as that.

My Rogue display's input as well as output if I remember correctly. I can't perform any test at this time.

[BFL13]

It is possible the Isc is above rating when new.

I have had Sharp, Sun, and Hanwha panels (so far 🙂 ) Their warranties are about not losing more than a certain amount over the years. Not to stay at as new ratings. The ratings you get when new are plus or minus already though.

A dealer told me that means when he gets a shipment of panels the average panel in the shipment of say 130w panels must be 130w, but any individual "130w" panel could be more or less that. Luck of the draw what yours might be out of the pile when you buy one.

It would be interesting to monitor Imp and see how that related to Isc, etc.

Mena is right that a clamp-on meter would be handy to get Imp if you can get at the wire. Do the fancy controllers display inputs as well as outputs? My controller only displays output.

If somebody here has that, please post some actual figures where we can see what Vmp and Imp are compared with output power. That would show the controller's actual efficiency.

To complete the picture also note the wattage of the array and wattage limit of the controller so we know where we should be on the efficiency curve.

Also be good to know at the same time as those figures are taken what the panel temperature is and ambient temperature, and the panel Isc and Voc at the moment and of course the panel's ratings as printed on it.

Throw in the voltage at the panel and at the controller input (should be near what the controller thinks the Vmp is but with line loss showing.)

With that info, we can compare it to what I got except for the Imp I don't have (except by assumption from the other figures). That should clear up whether my figures are "normal" too.

[CJW8]

A residential solar installer told me that new panels have a higher Isc measurement than spec so that they are still within spec after their 20 year warranty. This may differ between manufacturers.

[BFL13]

I already have the Solar30, so I would just need JiminDenver skills to trade a $230 230w panel for two 120s at $240 each, and I am good to go. 🙂 My MPPT controller could be my "loss leader" so to speak. 😞

EDIT--whatever the atmospherics, I was getting over the rated Isc of 8.3a on the 230w while all this was going on, so IMO I was getting insolation as at STC. (not NOTC) Side by side my 130w was also getting its Isc rating of 8.2 or more so that is a confirmation cross-check of the insolation.

[NinerBikes]

JiminDenver wrote:
BFL

Is it possible that with the higher temperatures also comes higher humidity or haze? The difference in temperature is more like a half a amp for me but it's a lot dryer here too.

He's on an island surrounded by ocean, he probably has a lot of onshore moisture flow, daily.