Inverter undersizing not universally effective to reduce soiling losses

An international research team explained that, although inverter clipping is initially effective in mitigating soiling losses, these losses could become more visible with time, as solar module degradation makes clipping less frequent. They also warned that inverter undersizing alone may not be enough to mitigate soiling losses and suggested cleaning as an additional way to reduce them.

Internal view of a solar inverter

Image: Russell Neches, Wikimedia Commons

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An international research team has investigated the effect of inverter clipping on mitigating soiling losses in PV systems and has found that this strategy may not be as effective as commonly thought.

Inverter clipping occurs when a PV system's DC energy is larger than the maximum input size of the inverter. This saturates the inverter and the excess DC energy is not converted into AC. “Because of this masking effect, inverter undersizing has often been suggested as a practical soiling mitigation strategy,” the research group stated. “Indeed, the soiling losses are not visible from the AC side during clipping if they are not bigger than the difference between the energy rating of the modules and the capacity of the inverter.”

In the paper “Quantifying the impact of inverter clipping on photovoltaic performance and soiling losses,” published in Renewable Energy, the scientists explained that their theoretical work aimed at answering the typical question that PV modelers and soiling experts are often asked: “Isn’t inverter clipping enough to mitigate the effects of soiling on photovoltaic systems?”

“We addressed this question by modeling the PV performance and losses for a grid of locations across the United States, considering the typical configuration of PV systems,” the research's lead author, Leonardo Micheli, told pv magazine. “Our findings suggest that relying solely on a high inverter-loading ratio (ILR) may not be sufficient to effectively mitigate soiling losses. This is particularly true over the long-term because, as the degradation and performance loss raise, the frequency of clipping decreases, making the impact of soiling more visible.”
The academics collected irradiance and weather data from NREL's National Solar Radiation Database (NSRDB), daily precipitation data from PRISM, and hourly data estimating potential accumulated soiling from NASA's MERRA-2, which is the latest version of global atmospheric reanalysis for the satellite era produced by NASA Global Modeling and Assimilation Office (GMAO). With these data, they generated maps via the Cartopy package to evaluate the magnitude of clipping across the United States.
Their analysis showed that clipping is more frequent in regions with higher solar radiation, such as the non-coastal Southwestern states like Arizona, Colorado, Nevada, New Mexico, and Utah, which the academics said is attributable to the higher solar resources compared to the Eastern states and the lower temperature and lower soiling compared to the coastal West.
“Fixed and horizontal single-axis tracker (HSAT) systems in these states clip for more than 7% and 8% of the time, respectively. Colorado and New Mexico are the two states in which clipping occurs the most, with averages above 8% in both mounting configurations,” the group stressed. “By calculating a simple arithmetic mean, an average national clipping time of 4.0% and 3.5% per year can be estimated for fixed and HSAT PV systems, respectively.”
The analysis also showed that clipping can lower the visible soiling losses by less than 0.1% on average, and that a larger relative reduction in losses can be achieved in those areas where losses are already limited.
“For example, the highest relative reductions are found in New Mexico and Colorado (>40%), where, however, the soiling losses are not greater than 1.15%/year and 0.6%/year, respectively,” the researchers emphasized. “On the other hand, in those areas where soiling losses are more intense, such as southern California and Arizona, the impact of clipping is limited. Indeed, the maximum absolute reduction in soiling losses does not exceed 1%, even in those locations where the losses are higher than 4%, and can reach up to 9%/year.”
The scientists concluded that inverter undersizing has a “limited effect” for reducing soiling losses in PV systems across the contiguous United States. They also suggested increasing cleaning frequency as an additional soiling mitigation strategy. “In the operational phases, soiling should be regularly monitored in order to assess whether anti-soiling actions, such as periodic cleanings, should be put in place, independent of the initial mitigating effects of inverter undersizing,” they stated.

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