Mostly for Solar Geeks; a short novella

[12thgenusa]

We just got home from a short camping trip to Moraine Park CG in the only open camping loop in all of Rocky Mountain NP (with about six other hardy souls mostly tent camping). Elevation is 8,200 ft. Yesterday it snowed on and off all day with some periods of clear, but it was mostly high, thin clouds, lower cumulus clouds and lots of moisture in the air. Relative humidity was 75 percent and the temperature was between 32 and 38 degrees F all day.

The day before yesterday I had run the battery bank to about 75 percent (~110Ah used). That day had started well for solar but was solid clouds by early afternoon. Even so I made 80 Ah (1.11kWh) with a peak of 421 watts. But we watched several movies, had the sound system going most of the day, worked the furnace pretty hard, made two pots of coffee with the coffee maker, and used the electric toaster along with lots of led lights. The next morning (yesterday) the bank was at a deficit of 135 Ah.

With the conditions described above I still made 106 Ah (1.50 kWh) with a peak output of 456 watts. This is measured at the controller so is downstream of wiring losses and from a pair of panels rated at 370 watts combined. The panels were aligned about 7 degrees West of true South and were tilted at 28 degrees. Even so this represents a premium of 23 percent over panel rating. The temperature difference of 25C below STC would account for about 12 percent of that at .47 percent/degree C. The rest I am not sure of. The air was definitely not clear and dry. Maybe there was considerable edge cloud lens effect from the many passing cloud banks. However I have always thought that was mostly a short duration burst effect with little cumulative value.

Then today I lowered the panels to flat about 10AM. We left camp at about noon and arrived home at 1PM. Today was mostly sunny with a few snow flurries and was sunny after we arrived home. The totals for today were 114.9 Ah (1.62 kWh) with a peak of 485 watts. The temperature was in the high 40s for most of the day. The 485 watts is a 31 percent premium over rated panel wattage. I didn't have my computer connected to see a graphical display of the charge data, but only the data points mentioned above that are logged at the end of the day's production.

So what do you all think are the reasons for the seeming exceptional production? Higher elevation with cooler, cleaner, thinner air certainly helps. But especially yesterday with the heavy, moisture laden air, periods of snow with lots of clouds doesn't seem to make for a good solar day.

BTW my controller is temp compensated so the voltage may have gotten to 15+ V, but it is output limited to 30 amps. I have heard from others that it will output 31 amps before clipping so I might have been "up against the stops" part of the time.

[12thgenusa]

Well I understand the effect of cell temperature on solar production. I understand that at higher elevations there is less atmosphere above so there is less scattering of photons, producing more direct radiation and less diffuse radiation. I occasionally view the webpage of the NREL facility in Golden, CO which is at about 5600 ft. You will quite often see irradiance above the 1kW/M^2 standard.

What I don't quite understand is that the air was not clear, there was a lot of high overcast, there were lots of heavy clouds, it was snowing. These factors should have overwhelmed the temperature and elevation components.

Most of my camping is above 8000 ft. and I have camped in this same location many times in periods of better weather and haven't seen this before although I have occasionally seen 400+ watts from my 370 watt system.

Gee, I wonder what it would do if I were up there when it was 32 degrees and "severe clear"?

[red31]

I little south of there the data @ south park mt, this is horizontal panel data.

DATE (MM/DD/YYYY),TOT Global LI-200 [kW-hr/m^2],MAX Global LI-200 [W/m^2]
4/24/2016,6.7963,1225.0000
4/25/2016,6.8105,1084.0000
4/26/2016,4.7257,1233.0000
4/27/2016,5.8879,1172.0000

[JiminDenver]

Welcome to solar in the Rockies.

[pianotuna]

At 18000 feet 50% of the atmosphere is below you. Pressure is 7.34 psi.

At 8200 feet 30% is below you. Pressure is about 10.2 psi

At sea level that is 14.7 psi

Add that to the cold and the temperature sensor and the readings make lots of sense.

[pconroy328]

Hmm - quick google shows one paper. They also talk about how lowering humidity helps.

[pconroy328]

I too wonder if 8200' between you and the ocean waves was a big contributor? I'll bet there's some research papers out there that talk about solar cell production and elevation.

[pianotuna]

There is less atmosphere for the photons to be absorbed in. Going high and cold are both great for increasing panel output. The temperature sensor on the battery bank is the icing on the cake.

Great to hear about how well your system is performing.