Impacts of Soiling on Utility-Scale PV System Performance

The value of energy produced from utility-scale PV systems deployed throughout the Desert Southwest depends on the systems’ ability to match the seasonal and time-of-day utility loads. During times of increased demand, some utilities charge more per kilowatt-hour—effectively increasing the value of each kilowatt-hour produced by a PV facility—whether it is realized as an avoided cost or sold. As a result, project developers, engineers, owners and utility operators focus on designing and maintaining systems that maximize production during these high-value periods.

Coincidently, the Desert Southwest experiences a long dry season that corresponds with these periods of high demand and increased energy values. In the absence of significant rain events or regular cleaning, production losses due to soiling, also known as soiling losses, increase. In 2006 at the IEEE 4th World Conference on Photovoltaic Energy Conversion, PowerLight released an article titled “The Effect of Soiling on Large Grid-Connected Photovoltaic Systems in California and the Southwest Region of the United States.” The authors confirmed “a gradual but marked decrease in system performance through the dry season for systems in arid climates” and concluded that “performance losses due to system soiling are not constant through time, rather they depend on the amount and frequency of rain that falls on the array.”

Despite PowerLight’s conclusions, several of the commonly used PV production modeling tools, and thus system owners, assess the impacts of soiling on an annual basis. Consequently, system owners have typically addressed array soiling in one of two ways—either wash the array on a regular basis to limit the losses due to soiling, or forgo array cleaning and rely on rain events to keep soiling losses to a minimum. When based on annual soiling losses—as opposed to seasonal or monthly—these decisions fail to address short-term soiling impacts that may justify the cost of cleaning an array to maximize production during the high-value kilowatt-hour periods.

In this article, I review key soiling characteristics with historical weather data to simulate site-specific soiling losses on a monthly and annual basis for two sites in the Desert Southwest. The results confirm variation in soiling losses throughout the year for both locations, with average monthly losses in June and July—the high-value period—well above the annual average. In addition, prescribed cleanings are introduced to the model to understand their impact on annual soiling losses.

Measuring Soiling Losses

Soiling can be measured as either the rate at which contaminants accumulate on the module surface or the resulting decrease in production. Ultimately, we need to determine the decrease in system performance due to soiling loss. Assuming all other factors remain constant, comparing actual production values between a control subject and a soiled array is one way to determine soiling losses for a given site.

To simulate soiling losses over time, we must determine the rate at which soiling accumulates. Although soiling rates can be calculated in a variety of ways, a soiling rate that represents the daily percent decrease in production is most valuable for the purposes of PV production modeling. Once a soiling rate for a site has been established, it can be used with rainfall data to estimate past, present and future soiling losses.

As demonstrated in the PowerLight study, the “measured soiling rate” represents the slope of a linear fit curve applied to performance data between rain events. In other words, PowerLight’s study assumes that the percent change in performance over time—in the absence of rain or cleaning—equals the percent change in soiling losses over time.

In January 2013, First Solar published a paper in the IEEE Journal of Photovoltaics titled “Direct Monitoring of Energy Lost Due to Soiling on First Solar Modules in California” that details an alternative soiling measurement technique to determine site-specific soiling rates. The technique is based on a methodology proposed in “Solar Cell Arrays: Degradation Due to Dirt,” which was published in the Proceedings of the American Section of the International Solar Energy Society in 1989, and is intended to be “practical and automated … foregoing complex equipment such as IV curve tracers.” Rather than equating soiling rates to the increase in production losses per unit of time, the method First Solar uses compares production levels among a control module, a module that is not cleaned on a regular basis and the expected performance based on typical irradiance and temperature readings.


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