Autumn and Winter Generation Results

Our solar PV generation had many interesting aspects over this winter. Now that spring has come and there is no longer snow on the panels, we are able to see many influences of cloudy weather, snow-covered panels, and the brightening days of spring. Also, we have gone through the fall and winter seasons and are able to see some trends. I’ll discuss autumn accumulated energy and its trend, impact of snow coverage, winter results, and energy trends in the early spring. I’ll show the results of RETScreen model, and finally I’ll analyze the instantaneous generation results. I’ll cover these all over the next several blog entries which I hope to post over this week.
Today’s entry is the daily power generation and revenues. Recall, out system is 4.9 kW of SolarWorld 245 watt panels and a Fronius 5100 string inverter [5,000 watts]. There are 20 panels on two strings. The panels are on a 40 degree slope and azimuth of 135 degrees [SE]. We have a microFIT contract at $0.802/kWhr
Daily Electricity Generation
I’ve plotted out the daily generation from 24/08/2010 [day system was grid-tied] to end of March 2011. The plot is shown below and my observations follow.

Solar PV electricity generation at 243 Farley Dr, Guelph, ON, to 31 March 2011.

1. Autumn daily generation varies considerably due to clouds: The maximum generation during the first week was between 26 and 28 kWhr but then less than 25 kW until mid November. The maximum values decreased into December to a maximum of 16 kWhr. There are several factors that affect the amount of power generated besides the clouds. The snow on the ground reflects sun onto the panels. The sun angles change the amount of time and intensity of light that strikes the panels. Colder temperatures make the panels more efficient. The most cloudy days would be producing the lowest values in the chart between August and mid-November – between 2 and 3.8 kWhrs per day on the cloudiest days. On the sunniest days the average hours of light is 4.9 hours [24 kWhr/4.9 kW of panels = 4.9 hrs] and on the cloudiest days it is 0.41 hours. 
2. Snow cover means little generation: Observe the data from 22/11 to 7/01. This month and a half period saw considerable number of days with either partial of full snow coverage on the panels. Many of these days were 0.3 to 0.8 kWhrs only! Ground mounted systems or panels that are accessible with a long broom could sweep off some of the snow to improve generation during these periods. Our panels are too high to reach. Brushing off the panels may result in scratching and damage.
   The snow was melted from the panels in early January. Several days generated over 10 kWhr, but were the snow returned mid-January. 
3. Cold, clear days increased generation: The bright clear days, the highly reflective snowy ground, and the cold temperature resulted in very high output. The peak output was up to 35 kWhrs on several days. The panels were putting out up to 5200 watts, that is 260 W from panels rated at 245 W. This really surprised me. These days were crystal clear blue sky, between -5C and +5C with a gentle breeze. The panels would have remained cold thereby increasing their electrical efficiency. This was very pleasing to see after the dark low energy days of December were not so long ago.