Overall Installation Output Reductions Induced by Soiling

Power losses ranging up to 15-30% have been observed in horizontally mounted modules during lengthy dry periods in Southern Europe and North Africa [4.20]. At the end of dry seasons in arid regions of Africa, massive dust accumulation has resulted in power losses ranging as high as 80% in certain cases [Wag06]. Periodic solar generator cleaning is a necessity at such sites and should be factored into the installation’s upkeep costs.

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Figure 4.95 Soiling striations can also occur on modules with high edges and steeper tilt angles. The tilt angle of the module shown here is b = 65°. Insolation during the winter is somewhat lower here since g = —61°

The degree of soiling and the power loss observed are strongly influenced by local conditions. The Bern University of Applied Sciences PV Lab has been monitoring the performance of a large number of PV installations in Burgdorf since 1992. While only minor permanent soiling has been observed at many of these installations after a number of years of operation, some of them have exhibited soiling-induced power reductions ranging up to 10%.

In particularly severe cases, however, elevated soiling-induced power losses of the order of 30% can also occur in PV plants in moderate climates. Over the course of 2005, a substantial decline in specific energy yield was observed in a PV installation that was installed in 1997 on the roof of a Burgdorf fast food restaurant located on a heavily trafficked thoroughfare and a railway line, and is also in close proximity to a sawmill. In October 2005, the PV Lab conducted a more detailed investigation of the installation.

During the first six months of 2005, a tall building adjacent to this installation was renovated, whereupon not only the lower edges but also the entire surface of the installation’s framed modules were severely fouled (see Figures 4.96 and 4.97). In this case, the natural cleansing effect of rain was not sufficient to remove this soiling, which was probably induced by construction dust.

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Figure 4.96 View of the solar generator (b = 30°, 3.3 kWp) comprising M55 modules and mounted on the roof of a fast food restaurant in Burgdorf. The front modules were cleaned just before this picture was taken. Unlike the modules shown in Figures 4.92 and 4.93, the modules in this picture exhibit the same level of soiling across their entire surface, which is why the power loss measured for them was far larger

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Figure 4.97 Heavily fouled solar cell (fi — 30°) from the PV system in Figure 4.96, with 30% output loss. Apart from the soiling that is also visible on the lower edge in Figure 4.93, the entire remaining surface of the cell exhibits a homogeneous degree of soiling. Taken together, these two instances of soiling result in acute output loss

The installation was cleaned in order to measure the power loss that it had incurred as the result of this soiling. Prior to and following this cleaning, measurements were realized using a characteristic curve measuring device, accompanied by synchronous measurement of irradiance G and the module temperature of the I-G characteristic curve for the installation as a whole and its individual strings; these results were then standardized based on STC (see Figure 4.98). An overall output reduction of around 29% was observed. Inasmuch as all of the installation’s strings were functioning normally, this output reduction could only have been attributable to soiling, and not to the failure of specific strings. Another particularly deleterious factor that probably has a negative effect on this solar generator (comprising M55 modules) is that it is exposed to kitchen exhaust from the restaurant, in addition to the ambient emission load.

I-V-Characteristics (at STC) before and after Cleaning

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Voltage V in Volts

Figure 4.98 I-V characteristic curve standardized at STC for the PV installation in Figure 4.96, before and after cleaning (nominal output 3.3kWp). The pre-cleaning output Pmpp — 2154W was 28% lower than the 3025W measured after cleaning

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Figure 4.99 Trees can induce shading in some PV installations with the passing years. This picture taken on a late November morning displays relatively severe shading of a PV system that was installed in the mid 1990s

Updated: August 8, 2015 — 7:49 am