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Photovoltaic (PV) technologies directly convert sunlight into electricity and are one of the most diffused and economically viable renewable energy sources. Unlike traditional energy generation resources, performance of renewable energy sources is constantly being affected by external environmental factors. One such underestimated factor is the process of soiling, where organic or inorganic contaminants accumulate on the surface of PV modules and lower the energy generation.


Soiling is the process where dirt, dust, pollen, soot, aerosols, snow, and other contaminants are deposited on the PV modules. Soiling absorbs, reflects, and scatters part of the irradiance, reducing the intensity of the sunlight reaching the PV cell for converting into electricity. Soiling has a major effect on the efficiency of the PV-Solar system. Negative impacts of soiling for the return on investment (ROI) of a utility scale PV-Solar project, can be two sided. Both a reduction in electricity output revenue and incurring operational and maintenance (O&M) costs for undertaking the cleaning of modules.

With the increasing size of the PV-solar power plants, it's not possible to avoid soiling altogether. Soiling losses are reversible however and can be removed naturally or artificially. It’s important to continuously monitor soiling buildup to make sure that module cleaning is scheduled at the right time. With suitable remedial actions, it’s possible to maximize energy performance. The energy lost annually from soiling amounts to as much as 7% in parts of the United States to as high as 50% in the Middle East.

Soiling metrices used to quantify the amount of soiling on a PV array are Soiling Ratio (SRatio) and Soiling Rate (Srate).

Per IEC 61724-1 standard, the Sratio is calculated by dividing the actual electrical output of a PV array under soiling conditions, with the expected output of the clean PV array. The electrical outputs should be measured and estimated at the same time and the same conditions should also be considered according to the recorded irradiance and temperature values. The soiling loss called ‘‘Soiling Level’’ in the IEC standards, can be calculated as 1-SRatio.

SRate is the daily rate at which the soiling ratio decreases while soiling deposits on the PV surfaces. It is a measure of the slope of the soiling ratio profile for a specific period and is expressed in %/day.


The current IEC 61724-1 standard recommends calculating the soiling ratio using only data measured within ±2h of solar noon for fixed tilt systems or for angles of incidence under 35° for active tracking systems. This way, the losses are calculated only for the hours of highest irradiance and energy production. It also recommends monitoring soiling where the expected annual losses are higher than 2%, with at least two soiling sensors for PV sites greater than 5 MW.


There are not a one size fits all method that can be used for mitigating the effect of soiling. Soiling can be mitigated by cleaning the PV modules when the energy losses are higher than the cleaning costs. This means that an accurate soiling monitoring system is required to properly address this issue that increases the energy yield, while minimizing the operating and maintenance costs. It’s commonly assumed that the soiling will be uniform across all the modules in a plant. But soiling being an amalgamation of various factors like environment, climate of a location, site configuration, and design of modules used, this may not always be the case. In some instances, different strings and modules within the same plant may have different soiling losses. Soil monitoring is an essential soiling mitigation tool as it helps to detect extreme soiling conditions and to adapt the cleaning schedule depending on the interannual variability of the climatic conditions or other exceptional soiling events such as road or building works. For large PV-Solar plants it’s even more important to closely monitor soiling to identify sections that are economically worth cleaning.


Various passive anti-soiling techniques are also being utilized like Anti-soiling Coatings, Tilt Angle and Solar Trackers, Heating of Surfaces Preventing Water Condensation, Electrodynamic Screens, PV Module Design, Site Selection, Adaption, and Monitoring. Though it is worth noting that none of the passive anti-soiling technology can eliminate the need for cleaning. Various factors influence the path towards optimal cleaning technology which include: soiling type and deposition rates, water availability, accessibility of the site, system configuration (such as tracking versus fixed tilt angle), roof mount versus ground mount, labor costs, equipment required, and feed-in contract conditions.


The most commonly deployed equipment for soil monitoring of a PV-Solar power plant is soiling stations, along with optical soiling measurement (OSM) sensors, with the least common method being Soiling image-analysis (SIA) sensors.


Soiling Stations are by far the most common commercially available solution to monitor soiling. This solution comes with two PV modules (like the production modules) one of which is left to soil naturally and the other one is cleaned manually or automatically at regular intervals. The Soiling ratio is calculated by comparing electrical output of these two modules. Although they are a simple solution, Soiling stations do not eliminate the need for cleaning and maintenance, which can be expensive in remote locations. The most common models seen in the market are the Atonometrics RDE300 series and the Campbell Scientific’s CR-PVS1.


Optical Soiling Measurement (OSM) sensors are used to estimate the soiling ratio from the optical characteristic of the soiling accumulated on a PV glass. These were intended to lower the cost and increase the reliability of soiling monitoring. These need little or no maintenance as there are no moving parts and do not need to be cleaned. The most common models seen in the market are Atonometrics MARS, Kipp & Zonen DustIQ.


Soiling Image-Analysis (SIA) sensors are an emerging class of soiling monitors. These are distinct from the OSM, as they are designed to operate through the analysis of PV module’s aerial photos. In this case, the brightness, and the contrast of darker areas (cells) and of the lighter areas (ribbons and gaps) of the PV module’s surface are compared.


Even with optimized cleaning scenarios its estimated that soiling reduces global solar power production by 3%–4% approx. 3 to 5 billion dollars in lost revenue. These losses are estimated to be between 4% - 7% in 2023, with up to 7 billion dollars lost worldwide every year. The main objective of PV monitoring is to provide a real-time diagnostic of the energy production and to identify possible faults and energy losses, to maximize the energy yield, minimize the O&M costs, and avoid safety hazards. Soiling is one of the most relevant environmental factors, along with irradiance and temperature, that can significantly affect the performance of a PV-Solar plant.

Whatever soil monitoring solution you decide to use for your projects, Nor-Cal can facilitate custom pre-configured Meteorological stations to capture soiling data and data logger, equipped on every MET station. Please contact the Nor-Cal team to discuss the details of your existing or upcoming projects.

Nashvinder Singh

Written by Nashvinder Singh

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