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Solar Array Data, January Through August

In my Solar Array Surprises post about the SAS solar farm, one of the surprises was a midday dip in the power output, for which commenters supplied several possible explanations.

That data was from April, and we could only speculate what the summer data would look like. But now summer is over (by some accounts), and we can look at eight months of solar output data.

Looking at the summer data brings a new surprise: It's very noisy.

solar array data for July 2009

I know we had a fair amount of rain this summer, but I remember some dry spells, too. I guess the clouds were always around. At least July 14 looks clear and sunny, which is better than I can say for any day in June or August.

Getting back to the midday dip, here's a Graph Builder plot using the Data Filter to show only the most sunny day in each month (hand-picked).

solar data for most sunny days in 2009

A few observations from the plot:

  • Power output is greater in the summer (height of the curve).

  • Daylight lasts longer in the summer (width of the curve).

  • The midday dip is more pronounced in the winter.

  • Summer output is noisier.

  • Points 1 and 2 are obvious. The third point supports the idea that the horizontal axis of the panel rotation is causing the dip, since it is more pronounced when the sun is lower in the sky. The fourth point could just be the weather, but it might be exacerbated by the power output being near capacity for the array.

    Ambient temperature was also mentioned as a possible factor in the power output, but I haven't analyzed it. I was hoping to find at least one cool sunny day and one warm sunny day from each season for comparison. It's still possible the temperature accounts for the slightly higher output in the morning than the afternoon since solar cells are more efficient at lower temperatures.

    UPDATE (09-09-09): The solar array data is now available from the JMP File Exchange. Scroll down to the bottom of my author page to find the file titled "Solar Array Data Jan - Aug 2009."

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    Jerry Williams wrote:

    I believe the alignment of our array using a single access tracking system is the culprit. Fixed mounted systems typically face South. The SAS system is oriented more toward the East and follows the path of the sun to the West. The path of the sun in winter months is at a lower angle in horizon which limits the solar irradiance to face of panels. Thus, the dip in production when panels are in flat position and sun is lower in sky and more to the side . The panels face the sun more directly in the morning and afternoon. Collectively, kwh production is maximized.

    In the summer, the angle of the sun is higher and solar exposure is good. The more direct exposure and increase in temperature causes the â noiseâ at peak. Typical solar panels experience about 1% annual degradation cell productivity. The summer "noise" is degradation.


    Jeremy wrote:

    I would be interested to see this compared to daily (or even hourly) cloud cover and humidity levels during the year. Humidity is not a constant, and while it is constantly more humid during the summer months in Cary, NC than in the winter, I will bet that it is particularly less humid during midday during the winter, which could be causing more light to be bouncing off of the solar array rather than being refracted off of the water droplets in the air and captured by the array as we see happening during the summer months. Also, the noisiness during the summer is likely due to the thicker, and lower, cloud cover that is more effectively reflecting sunshine away from the array intermittently throughout a summer day.

    All of this is clearly conjecture at this point, but that's my hypothesis: look into humidity and cloud cover.


    Xan Gregg wrote:

    It's been submitted to the JMP File Exchange and should show up in the next day or so. Have fun!


    Douglas M Okamoto (Data to Information to Knowledge) wrote:

    Would you please post monthly SAS Solar Farm data in the JMP file exchange as you the April data? On April 16, 2009, time series for irradiance (in kilowatts per square meter) show peaks at mid-day for Array A and Array B. First order differencing of time series for power (in kilowatts) makes the apparent drop-off in wattage at mid-day look like white noise.