Experiments in Dust

A week or so after installing my photovoltaic plant I began to wonder about the effects of dust accumulating on the panel surfaces. So I devised an experiment to quantify the degradation.

I allowed the surfaces to accumulate dust for six weeks in preparation for the experiment. More detrimental soil, such as bird droppings, were promptly removed.

Originally the array consisted of four sub-arrays, each with its own inverter. I recently replace the original inverters and in doing so reorganized the array into three equal sub-arrays, each with six bays of four panels.

My approach to quantifying the dust extinction factor

I know no way to quantify the degree of surface soil other than to state the six-week period and include a photograph that shows before and after. The picture also reveals where the dust originates (our drought-ravaged soil). The distant six bays (West) and the nearest six bays (East) are unwashed. Click on the thumbnail to see a larger image.

The experiment is based on selectively cleaning a sub-array and observing its relative performance before and after cleaning. The West sub-array was the reference control for the Mid and East sub-arrays, and the East sub-array was the reference control for the West sub-array.

The raw data

Below are four charts of instantaneous output power for each sub-array over a two-hour period around solar noon. The sample rate was 4 times per minute. The earliest, August 17, shows performance before any cleaning was done. August 20 shows performance on the day the Mid sub-array was cleaned. August 22 shows the East cleaning, and August 24 shows the West cleaning. Cleaning was quickly done at sunrise.

Examining the charts, note how the Mid sub-array output moved above West on August 20, the day it was cleaned. Then East widens its lead over Mid and West on August 22. Finally on August 24 West regains its superiority over Mid and the relative ranks are as they were before cleaning.

Perhaps a more meaningful presentation is shown below. During the first five days of the chart, the west sub-array suffered some curtailments, reportedly because the inverter sensed a grid frequency disturbance. The broken line shows the likely performance for those days. This was probably a malfunction since the other two inverters did not curtail.

The delivered energy can be seen to improve after each cleaning. It is difficult to see, but Mid clearly improved relative to West on the day it was cleaned. Likewise East shows an increasing slope relative to Mid and West on its cleaning day. West, slightly below Mid before cleaning, moved noticeably above after cleaning.

Determining an improvement value

While the above charts clearly indicate that cleaning begat improvement, the degree of the improvement is not well expressed. Below is another chart to help assess this.

The calculations get a bit tricky when a control sub-array is cleaned. I applied a factor to compensate for the increased output of the control reference. A 2-day delay between sub-array cleanings provided an alignment spline to determine the factor. Absolute accuracy is not the goal here, rather it is relative performance before and after cleaning. In every case the clean-day transitions were bounded before and after by an undisturbed reference control.

Here is shown the relative delivered energy, before and after cleaning, for each sub-array referenced to the output of a different sub-array. West acted as control for Mid and East, and East was the control for West. The broken lines are running average trendlines for the sub-arrays.

This has been a difficult evaluation, and by no means a precise one. But from the chart above it seems reasonable to state that cleaning produced an improvement of between 2-1/2% and 3%. With only six weeks of soil accumulation, one might deduce that a much longer exposure period would accumulate enough soil to significantly reduce performance. But I also suspect that my array, due to its location, accumulates soil faster than a rooftop array would.

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