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 😞

[JiminDenver]

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.

[NinerBikes]

I'd be more inclined to go back to a pair of 17V Vmax 120W panels and your Solar 30 bungholer PWM Charge controller, and get what you get with the controller putting out 15.0V to your Sweeper batteries, and call it good. Probably 14 - 14.5 Amps, maybe a bit more, with fat enough wire. $250 US in panels, and $31 for the PWM, and what ever for fat wire to minimize losses. Seems tried and true. You get 210 watts at the battery, pretty consistently and you know where the other 30 watts went. No brainer, for me at least.

What do you have now in the MPPT controller and the 30V 230 watt panel, $$$ wise, that makes it worth how temperature sensitive it is for amps output, or amps missing? Seems like a solution looking for a problem?

[BFL13]

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?

[mena661]

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

[BFL13]

Salvo, I took a zillion measurements and that's what I got. We stopped all solar as usual at the end of September and will re-start next mid-April.

I understand "measurement error" as a factor, but I think I have got past that by taking enough samples. It is what it is (was)

If anyone has some data that proves otherwise, please post it. I would like to improve my set-up if there were any way to do that. If it is indeed maxed out as some have said, then that's that.

[Salvo]

It's a good time to take measurements.

[BFL13]

09FLSTC wrote:

MrWizard wrote:
I think your trying too hard
Learn to live with it , with what it is..any gain will be insignificant

Salute

That's no fun. It means I am doomed with no hope. Gottaluv the MPPT or just crawl into a corner! 🙂

[09FLSTC]

MrWizard wrote:
I think your trying too hard
Learn to live with it , with what it is..any gain will be insignificant

Salute

[BFL13]

EDIT--re the above, I was getting 44C back of tilted up panel with 16C ambient. and 51C panel at 25C ambient.

It is true I have never measured the current input to the controller, only taken the voltage there. I know that the Isc at the controller's input is the same as at the panel and is at or above rating so I believe insolation is good.

AFAIK, the input current is whatever it comes to with the power there divided by the Vmp and that is the Imp of the moment.

Working backwards, knowing output power is say 200w and controller efficiency is say 93%, then input power would be 215w. Measured voltage is 29.3 at panel and 29 at controller input. 215/29 = 7.4 amps while rated Imp is 7.7a

If something is wrong with the controller so that it's efficiency is 85% instead of 93, then with 200w output (actual) then input would be 235w, which is too high. (Panel Voc at 34 instead of rated 36.8 due to heating so its power must be down from 230w too)

So that indicates the controller efficiency at 93% when panel is near max power rating of controller is reasonable. So again, I don't think the controller is the problem.

I think what I am seeing must be fairly normal. It would be good to get comparable figures for a $600 MPPT controller in the same situation and see if they are any better.

In the comparison, if the $600 controller were rated for 500w instead of my 250w one, then it would be higher on the efficiency curve using the 230w panel. Have to fudge for that, and then see.

[pianotuna]

Hi,

For those of us who do not acronym well NOTC = Normal Operating Cell Temperature ratings. NOCT recognizes a bit of reality and assumes the following: 800 watts per square meter of Sunlight Irradiance, an average of 20'C (68'F) Air Temperature, an average wind velocity of 1 meter per second (2.24 miles per hour), with the back side of the solar panel open to that breeze (as opposed to being on a roof where heats builds up under the panels).

Given those conditions, what is the Temperature of the Solar CELL (not the air)? The average was found to be about 48'C (118.4'F). Some panels are a little higher and some a little lower. What you need to know is that ALL solar panels experience a voltage drop as they heat up. Since Volts x Amps = Watts, the power of the solar panel diminishes as they heat up.

[Salvo]

There's lots of speculation, but it sounds like the mppt controller has issues. You need to measure solar current and voltage as well as battery charging current and voltage. How many watts are they apart?

A good mppt controller has better than 95% efficiency at max current.

Regarding the temperature effect: Even in spring time the panel should still get quite hot. Look at the numbers, solar is only 17& efficient. Most of the remaining 83% goes to heating the panel. If you have a meter squared sized panel with 1000W/m^2 irradiance, then 0.83 * 1000W = 830 W goes to heat. Solar is a huge heat collector.

If you have 1000W irradiance then the panel temperature will be greater than 25C and the panel wattage will be lower. Isc will remain about the same.

You really need to look at the NOTC spec, not the STC spec. For example the 140W Kyrocera panel has 140W (STC) and 101W (NOTC). Granted NOTC is at 800W/m^2, but panel temperature is allowed to go up.

[BFL13]

The solar controller may be sub-par but it does better in the Spring and Fall than in Summer. My amps to the battery seasonally change with panel temperatures. ( IR gun pointing up under the panel) The panel is the thing that loses power with high temps, not the controller. So I believe the controller is not the culprit as such.
It is the way the amps are created from the power with an MPPT controller that is the problem, compared with getting Isc with PWM

Another possible is in that Tracer owner's manual it has graphs of controller efficiency vs array wattage, where efficiency falls off into the lower 90s from mid-90s as you get up in wattage toward the controller's max rating.

With my 230w panel and the controller rated for 250w, that puts me at the lower end of controller efficiency, which would account for a few more of my missing watts at the output, Spring or Summer.

The best output watts I have seen in the Spring is about 205-210w of the rated 230w- goes with about 15.5 amps. In Summer I mostly see 180w or so with about 13.5 amps.

EDIT: 27w/13.5v = 2 amps. So do the above figures make sense?
207-180 = 27 and 15.5-13.5 = 2. Ta da!

And where 10% of panel power is lost at 50C, and 10% of 230w is 23w = 1.7 amps

With my PWM and same array wattage it would be 14.5 amps every time despite the change in panel temperature. (based on actual results with 200w and 210w scaled to 230w)

This is all with ambient around 16C for the "good" results and only 25C for the "bad" results, so I would not like to be trying my MPPT set-up at 35C ambient like down there gets. 😞

I have no comparable figures to compare with mine where Brand X is using a different controller in the same conditions.

Salvo's direct plot above with his 135w comes out the same as I got with my 130w and PWM so that is a cross-check on that.

[Salvo]

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?

[BFL13]

Ok, I am seeing the light (I think) Thanks guys. Hit me on the head with a 2 by 4 enough and gradually things become clear 🙂

[LScamper]

The controller will look at the voltage at the controller input and the current at the controller input. The controller will multiply this current times this voltage to determine Watts. It will keep doing this at different volts and until it sees a maximum at the controller input. This is the voltage it will run at. It will keep checking this at what ever rate the designer programmed into it.