The PPL E.W. Brown Solar Facility located near Harrodsburg, KY has been in operation since September 2016. The 10-MW facility spans roughly 38.5 acres and consists of a total of 44,500 photovoltaic (PV) modules with a net generating capacity of 19,000 MWh. The modules are divided into 10 sections (1A to 6B; refer to Figure 1) and each section contains its own inverter unit.
![EWB Solar Farm](/sites/idea/files/inline-images/image_2.png)
This facility is situated near to the E.W. Brown coal-fired power plant and surrounded by ash ponds that were in operation during the early years of the solar farm. The ash ponds have since been decommissioned and are currently inactive. In 2021, areas near to and within the solar farm were converted to a pollinator habitat with various blooms starting around May 2021. Since April 2020, there is also a small flock of sheep grazing the field annually, starting as early as mid-March and ending as late as mid-November, depending on the weather.
Over time, soiling and volatile organic compound (VOC) materials deposited and formed a thin film cementation on the surface of the solar panels. Based on the surrounding conditions, various compounds from the power plant, dust, pollen, fibers, etc. are expected to be in the soiling materials, as well as general air impurities (sulfates, nitrates, organic carbon, elemental carbon, crustal components) found in the central KY region.
The objective of this study is to quantify the derating of solar panel performance, which results from both soiling and solar-cell material degradation. Because solar panels in Section 6B were professionally washed, using a mechanical brush and water, on June 15, 2022, the individual effects of panel soiling and material degradation that make up the total performance reduction can be decoupled via comparing the washed and unwashed panels. Consequently, performance loss associated with soiling was quantified and the benefits of cleaning were determined. To gain insight into the soiling formation mechanism, soiling materials were collected and analyzed to characterize and identify the components of the soiling materials. If the sources of the soiling materials can be traced, potential preventive measures might be designed, planned, and implemented.
Historical data, starting from September 2016, of current and power output, as well as weather conditions are available and provided by PPL. For the purpose of this study, a string (6BS1) consisting of 228 modules was selected from Section 6B (washed). For comparison purposes, a reference string (5BS6) consisting of 304 modules was selected from Section 5B (unwashed). Their locations are marked in Figure 1 and labeled “Data used (strings)”. Minute data containing current outputs from these two strings and converter power outputs from both sections, along with weather related data, collected at the two meteorological stations on site, marked as Met 1 and Met 2 in Figure 1, are used in the analyses.
Soiling samples from solar PV modules in Sections 5B and 6B as well as corresponding soil samples from two ground locations were collected; with locations shown in Figure 1 and labeled as “Soiling Samples (panels)”, “Soil sample (ground)”, and “Soil sample (gravel road)”, respectively. All samples were analyzed by scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD).
Performance ratio is a metric commonly used to measure how effectively a plant converts sunlight collected by the solar panels into alternating current (AC) energy relative to the expected energy based on the panel nameplate rating. However, performance ratio is also affected by the solar PV cell temperature and is often corrected to the standard reporting temperature of 25 °C. Guided by Ishii et al (Prog. Photovolt: Res. Appl. 2011; 19:170–179) and the National Renewable Energy Laboratory (NREL/TP5200-57991, 2013), a temperature-adjusted performance ratio (PRadj) defined in Eq (1) below is used to study the soiling and solar cell material degradation.