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Measurements for three solar panels

May 18, 2019 — BarryK

Today is Saturday 18th May, 2019, Federal voting day here in Australia. I got back from the voting booth about 9.00am and saw that the sky looks quite blue, just a few wisps of cloud, barely visible, no wind at all, very cool, ambient about 14 degrees C. Looks like a good day for testing my solar panels.

I now have test instruments to perform more accurate measurements, and a variable resistive load to more exactly locate the maximum-power point.

I tested three panels...

100W flexible panel

I purchased this panel in 2017, see blog report:

At that time, got a very low current out of the panel. I think now, that there was something wrong with the regulator. Well, something was wrong, as now getting much better performance.

Although the sky looked blue, the sun was low in the sky so more atmosphere to go through, this being almost mid-winter, and I read 800W/m2 irradiance (note, I have the habit of referring to the sun intensity as "insolation", though technically that is the solar energy over a certain time period, whereas "irradiance" is the instantaneous intensity, so the latter is correct when quoting W/m2). Here is the setup:


...the panel is set as close to directly facing the sun as I could judge. Notice the current-shunt (with crocodile clips attached), used to measure short-circuit current, and the brick holding down the ends of the nichrome wire. Testing at about 9.45am, ambient 14degC, 800W/m2:


...this graph shows output voltage versus current drawn from the panel. The peak-power point is at the knee of the curve, which is 4.96*16.95 -> 84.07W.

This is pretty good, considering that the cell temperature will be above 25 degC, and irradiance is only 800W/m2. The panel is rated at 100W @ 25degC and 1000W/m2.

So, if I perform some magic calculations, attempt to estimate what the power output would be at cell-temp of 25degC and 1000W/m2 irradiance...
Delta-temp: 46 - 25 -> 21 -> +5 for actual cell temp -> 26
Power change, temp: 0.4%/degreeC (0.4/100)*84.072*26 = 8.74W
Power compensation, irradiance: (1000/800)*84.072 -> 105.09W
Extimated power o/p of panel, at cell temp 25 degC @ irradiance 1000W/m2:
105.09 + 8.74 -> 113.85W I think that this estimation is too generous. It is based on the actual cell temperature being 5degC higher than the reading on the back of the panel (46degC), the power changing 0.4%/degC, and the power o/p being linear with irradiance. Also, I am assuming that the sun-power meter is reading accurately.

I definitely have to revisit these assumptions, as it is pretty obvious that I would not be getting power output above what the manufacturer is claiming!

250W Atem-Power folding panel

I had previously tested this, and got a disappointing power output, see this blog post:

Hoping now to get more power out of it! Here is the setup, panel angled to directly face the sun:


Testing one half of the panel only, at 10.30am, ambient 16degC, 850W/m2, temp at back of panel 50degC:


Peak power is 4.85A @ 16.21V -> 78.62W
As this is for half of the panel, multiple by 2: 157.2W

I am not going to apply my estimated corrections, as they are too generous. Instead, I want to do another test when solar irradiance is close to 1000W/m2, and will attempt to get the cell temp close to 25degC (possibly by shading the panel until just before taking a reading).

120W Powertech folding panel

This is a traditional glass-fronted folding panel, and very heavy -- the weight is what motivated me to look for something else. Don't recall where I purchased it, think it was in 2016.

EDITED 2019-05-23:
Ah, my faulty memory! Hunted through old blog posts, found a reference to using my folding panel in 2012, a trip to Windy Harbour. So that's when I bought it, late 2011 or early 2012. Had feedback from Jon, the Powertech brand is still sold, through Jaycar -- don't think that I bought mine from them though.

This is what it looks like:


Testing both panels, at 11.00am, irradiance 850W/m2, ambient 16degC, back of panel 47degC:


The peak power point is 6.48A @ 16.58V -> 107.4W

Again, will hold off with applying corrections.


I want to hold off with conclusions until I have sorted out the compensations for cell temperature and solar irradiance. For now, just comparing the readings as measured:

100W flexible
250W Atem-power
120W Powertech

The specification plates on all three panels state that the figures are at cell temp of 25degC and irradiance of 1000W/m2. My readings are slightly higher cell temperature and lower irradiance. The third column shows percentage output relative to that claimed.

Note, irradiance was slightly lower, at 800W/m2, when measuring the 100W panel, than when measuring the other two, at 850W/m2, so that 84% really should be a tad higher.

These results are very good, higher than I expected.  Except, the Atem-Power panel is still disappointing. 

Tags: nomad