Answering some solar panel with MPPT controller ?

[ktmrfs]

Here is some real world data on solar panels when used with a MPPT controller to answer some question I, BFL and others have.
This report is focused on the questions we had regarding solar panel use with MPPT controllers.

AND FOR THOSE WHO MAY WANT TO START A MPPT VS. PWM CONTROLLER DISCUSSION PLEASE DO NOT TURN THIS INTO ANOTHER MPPT VS. PWM DISCUSSION, FLAME ETC. IF YOU WANT TO CUSS OR DISCUSS PWM VS. MPPT CONTROLLERS GO START ANOTHER THREAD.

OK, now to the questions I was looking to answer for:

1)BFL’s question; “Can you look for a peak in Voc or Isc to determine optimum panel orientation?”

2)Can one predict accurately output power by measuring MPPT input voltage and panel output current?

3)In late fall at high latitudes what % of nominal power can I expect to get from the panel under optimal light?

4)Can one predict power loss between the panel and controller and predict the effect on output power (charging current and watts) with reasonable accuracy?

5)If one knows the resistance and current between the panel and controller can one predict the power to the battery with Series vs. parallel panels and longer or higher resistance paths between the panel and controller? (really an expansion of the above question)

6)Can one measure Temperature effects on Panel output power?

7)What is typical MPPT controller efficiency, or for this paper, the efficiency of a Morningstar sunsaver 15 MPPT controller operating in the 130 W region???

8)What if I just lay the panels flat, how far from peak power will I be?

9)With series connected panels what is the effect of shading on one of the panels?

10)Since I also have another trailer with PWM controller and use the same portable panels with this trailer, I was curious what effect a long (90ft) run between the panels and PWM controller has when used with a PWM controller. In this case I don’t have the choice of going series connected panels, that’s counterproductive. So staying with parallel connected panels is the only choice. Can I predict the results?

Rather than one LONG……. Post, I will break the post into sections. The next section is focused on assumptions, test conditions, test equipment, and test methodologies.

Then there are individual posts for each of the 10 questions.

And for those reading now, please delay any responses till you see a “That’s All folks” in my last post so my posts can be in sequence uninterrupted. I should have them all posted within an hour.

[ktmrfs]

Post #2

Background:
During the testing the maximum battery acceptance current was higher than the maximum panel output current. I.E., the batteries were discharged enough that the solar panel can’t output the max current the battery can accept at its state of charge.
Once the battery acceptance current drops below what the panel and controller can put out, most of the configurations I have analyzed should converge and all should give virtually identical output currents to the battery. But I’ve done enough for the last few days, and we are heading to some partial clouds, so as one of my favorite professors used to say. “The proof is left to the reader”.

Measurement resolution:
Due to measurement tools resolution and accuracy, some of the end results may seem and are somewhat suspect for absolute accuracy. MPPT controller efficiency is one example. Please don’t get mired down in the numbers, but look at the end results for what the comparison is saying. E.G MPPT conversion efficiency seems to be very good, rather than It can’t possibly be 98%, it can’t be better than 97%. Sorry, but my measurement tools aren’t good enough to get close than probably 2-4%. All I can say from my measurements is that conversion efficiency appears to be very high.
Unfortunately the resolution of the trimetric and my DC current probe is really not sufficient when one is making comparisons between two large numbers that are expected to give only slightly different values for current, voltage or power. The trimetric has 0.1V and 0.1A resolution and the current probe 0.1A resolution. And while I know the accuracy of my DMM and current probe, I don’t know the accuracy of the trimetric for current and voltage measurements. This really showed up in trying to calculate converter efficiency.
Since data without test conditions, test equipment, and actual configuration are pretty meaningless, I’ll start with outlining the setup my trailer has and the equipment I used to make measurements.

Trailer, solar Panels, and solar panel extension cables:
4 T-125 Golf cart batteries
Morningstar Sunsaver 15A MPPT controller
Morningstar 10A PWM controller
Battery monitored with a Trimetric battery monitor
All removable interconnect between the Solar panel and controller is via Anderson 50A connectors. Very low contact resistance and very repeatable.
Trailer connector for the MPPT controller is a 50A Anderson connector with 5’ of #10AWG wire between the connector and controller
Controller output is 2’ of #10 wire to a junction box and from there 8ft of #3/0 AWG wire to the +and – battery terminals.
Each Solar panel has 5’ of #10 wire to solar MC4 connectors
Panel output is combined (either series or parallel) with a combiner consisting of MC4 connectors on one end and an Anderson 50A connector on the other. Wire is 4’ of #10
Panel extension cords consist of sections of 30A RV cord with the ground soldered to one of the other wires at both ends. This translates into a cord with one conductor #7AWG, the other #10AWG. Cables are 50’, 25’ and 15’ in length. 50A Anderson connectors on each end.
Calculated combined resistance at 90ft is 0.15ohm

Solar panels are two each 80w panels, Voc 21V, Vmp 17.5V, Isc 4.98A, Imp 4.58A
During the test, the trailer batteries were at 80%SOC per the trimetric. The goal was to get the batteries discharged enough to allow the batteries to easily accept whatever current the controller could output.

Solar Conditions:

Completely clear skies with not even a jet contrail!!! Very unusual for western Oregon, but it does happen. And it stayed that way during the several hours of test both days.
Temperature 70F
Latitude 46N
Altitude: 175ft (basically sea level)
Date of Tests: October 14th and 15th, 2013

Test equipment:
Tektronix TX3 DMM NIST calibrated, Resolution: Volts 0.001V, Current 0.001A
Tektronix DCM330 AC/DC True RMS clamp on current meter, resolution 0.1A, Max current 1000A.
Trimetric battery monitor system. Resolution; volts 0.1V, current 0.1A

Measurement uncertainty:
The trimetric voltage resolution is 0.1V, current resolution 0.1A. Accuracy is unknown.
The clamp on current meter also has 0.1A resolution and accuracy at low current levels is not specified. So, the first order of business was to compare the current measured with the clamp on meter to that of the TX3DMM at low currents. I measured Isc of a single panel, and with panels in parallel and with the TX3 and DCM330. In both cases, the DCM330 matched the TX3 to the 0.1A resolution of the current probe. I repeated the measurements several times and go consistent results. So it gave me confidence that using the current probe would be reliable and accurate enough.

The posts that follow present my results and conclusions for each of the questions.

And to repeat: FOR THOSE WHO SEEM TO WANT TO START A MPPT VS. PWM CONTROLLER DISCUSSION PLEASE DO NOT TURN THIS INTO ANOTHER MPPT VS. PWM DISCUSSION, FLAME ETC. IF YOU WANT TO CUSS OR DISCUSS PWM VS. MPPT CONTROLLERS GO START ANOTHER THREAD.


So now off to some measurements.