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The Effect of Sky Conditions on Solar Panel Power Output

By Richard G. Born, Northern Illinois University, Operations Management and Information Systems

Increasing interest in going “green” by the use of alternative energy sources makes the study of solar energy especially suitable for students of physics. Solar panel systems on rooftops can cost thousands of dollars with payback periods on the order of decades. Yet some people are willing to pay the price to play their part in protecting our planet.

Many questions face the homeowner contemplating installation of solar panels. One of the most predominant questions is “What is the effect of sky conditions on solar panel power output?” How much will a heavily clouded overcast sky reduce output? Do fair-weather cumulus clouds result in significant reduction of power output when compared to a clear blue sky? What kind of reduction in output can be expected from shadows from nearby trees?

In order to investigate these questions the circuit shown below was constructed. It consists of a small (9″x12″) 9-watt solar panel ($139 from SolarMade.com) commonly used for 6-volt and 12-volt charging systems, a Vernier current probe, and a load.

Solar panel circuit

Two suggested loads are (1) eight tiny incandescent Christmas tree lights in series cut from an old string of 50 lights, or (2) the two 68-ohm resistors on the Vernier circuit board hooked in series, for a total resistance of 136 ohms. The advantage of the lights is that the student can clearly see that the panel is producing power, but their disadvantage is that they are not ohmic and hence do not provide constant resistance for computation of power by the use of P=I2R.

The Vernier voltage probe could not be used here, along with the equation P=VI, as the voltage from the solar panel was around 17 volts peak, higher than the maximum of 10 volts allowed by the voltage probe. The advantage of the two 68-ohm resistors is that they are ohmic and one can therefore use P=I2R to compute power output from the solar panel. Driven by a current of about 0.13A from the solar panel these 5-watt resistors were producing a little more that 2 watts of power at peak and were quite warm to the touch, providing the student a different means of being convinced that the solar panel is producing power.

The picture below shows the setup that was used. The cardboard box provides shade for the Vernier LabQuest and lights (or alternatively, the Vernier circuit board with its two 68-ohm resistors). The solar panel rests on two boards to provide convective air flow to keep the temperature of the panel as low as possible. (Research studies have shown that heat reduces the power output from solar panels.)

Data collection setup

The experiment was run for 15 hours on a day that provided a variety of sky conditions, from roughly sunrise to sunset. Current readings were taken by the current probe every 15 seconds. The results are summarized and annotated in the Excel chart shown below. The chart was created from data exported from a Logger Pro file collected by LabQuest.

Current vs Time graph

For each of the annotated regions on the Excel chart, the solar panel output in Joules/Hour was computed, and finally the percentage of solar panel energy output for each of the sky conditions was computed as it compared to the PEAK output at “high-noon”.

Data table

The following observations are noted:

  1. Shortly after sunrise, when the sun was hidden by both trees and a heavy overcast sky, it is seen that the solar panel was producing power at only 3% of its peak. Once the sun was above the trees, but still hidden by heavy overcast clouds, the panel was producing power at about half of its peak capability.
  2. Blue sky with occasional fair-weather cumulus clouds resulted in nearly identical solar panel output in Joules/hour as compared to clear blue sky. It is reasonable that these cumulus clouds may have made the “peak” a bit lower than it would have been if the sky was clear during the high-noon time span.
  3. The mid-to-late afternoon frequent, heavy dark clouds in an otherwise blue sky resulted in about a 30% reduction in solar power production.
  4. Before sunset, when the sun was again hidden by trees, but in an otherwise clear sky, the solar panel was producing power at 15% of its peak.

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