External contamination (“soiling”) of the incident surface is a major limiting factor for solar technologies. A 5-year field glass coupon study was conducted to better understand external contamination and its effects; compare cleaning methods and the use of preventative coatings; and explore the abrasion resulting from cleaning to advise on accelerated abrasion testing. Test sites included the cities of Dubai (UAE), Kuwait City (Kuwait), Mesa (AZ), Mumbai (India), and Sacramento (CA). Through the 5-year cumulative study, dry brush, water spray, and wet sponge and squeegee cleaning methods were compared to no cleaning. Optical microscopy was used to obtain images, including representative color images, grayscale images for object analysis, and oblique images for coating integrity assessment. A thresholding protocol was developed to analyze and distinguish specimens using the ImageJ software. Optical performance was quantified using a spectrophotometer, including comprehensive optical characterization (transmittance, reflectance, and absorptance in addition to forward- and back-scattering). Atomic force microscopy was used to verify the abrasion damage morphology, including the width and depth of surface scratches. Analysis of the results included correlation of optical performance and particle area coverage, rank order (by coating or location), and the acceleration factor for abrasion damage. The efficacy of external cleaning was more readily distinguished from the effectiveness of antisoiling coatings. The acceleration factor for dry brush cleaning of a porous silica coating was found to be on the order of unity.

Even though soiling of photovoltaic (PV) modules can substantially reduce power production, cost effective mitigation remains a challenge. The differences in soiling type, environmental conditions, and module properties all contribute to substantial soiling variability between sites and even within sites. Here, we develop tools to evaluate the production data from individual modules and observe specific incidences where nonuniform soiling produces an imbalance of light throughput resulting in a “hot cell” which induces a module string diode to shut down power delivery from that cell string. Thus, demonstrating that even a small amount of nonuniform soiling can substantially reduce power production.

NREL will work with the Participant to perform accelerated long-term durability and other standard durability tests for prototypes to help demonstrate the reliability of the technology for PV applications. NREL will test commercially relevant-sized prototypes under realistic environmental soiling conditions to correlate dust removal efficacy for improving the PV output under standard solar conditions. Measurements will include voltage-current characterization in solar simulators before and after dust cleaning.

Photovoltaic (PV) modules work best in the sunniest environments. Unfortunately., often the sunniest places also have substantial amounts of airborne “dust” that deposits on the front surface of the modules and blocks the sunlight; reducing energy output. In fact, natural soiling has reduced the energy output of PV systems since the technology was first used, and viable mitigation strategies have remained elusive ever since. With the ever-increasing deployments around the world, especially in dusty environments, soiling is becoming a billion-dollar problem, worldwide. While substantial work has been done to examine and resolve some of the issues with PV soiling, often mitigation comes down to physically cleaning the modules. However, a more systematic evaluation of the different module properties correlations to soiling mitigation needs to be done. In many instances, the causal connections between module properties and soiling are simply not known. This lack of knowledge results in a substantial increase in time and effort to evaluate and qualify appropriate soiling mitigation protocols based on site specific issues and the intrinsic module properties that are typically not optimized for mitigating soiling in a given environment. Thus, module property protocols and/or standards are needed to more quickly help identify appropriate module and site-specific mitigation. Thus, in this paper, we will present a review of the different issues between module properties and their relationship to soiling mitigation, and then outline a roadmap of the issues that still need to be resolved with additional research and development. Issues from frameless modules to anti-fungal glass compositions will be discussed.

Data for thousands of systems by multiple sources were transmitted to NREL for evaluation.Initial automated data quality assurance (QA) checks identified hundreds of systems (QA Tier 1) with high-quality meteorological and AC production data and hundreds of QA Tier 2 systems with adequate data quality.A standard RdTools analysis was conducted for these systems, evaluating performance loss on an annualized basis.To date with the systems evaluated so far, we find the median performance loss rate (aka Rd or degradation rates)is in line with historical degradation rates previously published for modules and systems (-0.5% to -0.9% / yr, Jordan et al. 2016).

Natural soiling and the subsequent requisite cleaning of photovoltaic (PV) modules result in abrasion damage to the cover glass. The durability of the front glass has important economic consequences, including determining the use of antireflective and/or antisoiling coatings as well as the method and frequency of operational maintenance (cleaning). The abrasion of coatings and glass has been explored in a field study, including the soiling-prone locations of Dubai (United Arab Emirates), Kuwait City (Kuwait), Mesa (Arizona), Mumbai (India), and Sacramento (California). Dry-brush-cleaned specimens will be compared with those subjected to artificial-brush testing. The characteristics of material integrity, surface energy, optical transmittance, surface roughness, and scratch size were examined using an optical microscope, contact goniometer (for water), spectrophotometer, interferometer, and atomic force microscope, respectively. The findings of this article will be used to provide feedback regarding the cleaning equipment, cleaning methods, and coatings used in the PV industry. The study here will also be used to aid in developing an abrasion standard for the PV industry.

The US Department of Energy's PV Fleet Performance Data Initiative has been launched in order to collect and evaluate production data across multiple PV fleet partners. Performance statistics are anonymized, aggregated and shared to represent a snapshot of the US commercial and utility-scale fleet. Production data have been collected from over 1500 systems representing more than 1.3 GWdc capacity. Because this project is still adding system data and updating methodology, degradation trends and statistics will be detailed in future publications. Preliminary analysis to date indicates median performance loss rates are in line with previous publications of system degradation, on the order of -0.6%/yr to -0.9%/yr. These values are higher than module-only degradation rates which are often used in proforma estimates of project performance and economics, potentially exposing owner/operators to increased risk if systems under-perform over time.

We report for the first time on direct measurements using atomic force microscopy (AFM) of electric field induced attraction and adhesion forces associated with soiling on photovoltaic (PV) modules. Real dust particles and silica spheres as surrogate to simulating dust particles were glued to AFM probe cantilevers. The electric field induced force (Fes) was measured via AFM force-distance (f-z) curves, where the electric field was generated by applying a voltage (Vs) to a simulated PV module. Fes and van der Waals (Fvw) force contributions could be separated with the f-z curves. The results show that Fes similar to 2.5 mu N on dust particles is similar to 5 times larger than Fvw similar to 0.5 mu N at even Vs = -100 V (i.e., similar to large-module operating voltages). Fes increases by an order of magnitude as the applied potential increases from Vs = -100 V to Vs = -500 V. These adhesion forces are by far the strongest that we have measured using the AFM technique for "initial" contact of particles. Furthermore, unlike the more typical short-range forces of Fvw and liquid bridge, Fes extends sub-millimeters to a millimeter beyond the PV module surface, creating large attraction forces to even uncharged dust particles (i.e., via induced dipoles) in the air. These results indicate that the high voltages typically used with PV arrays today will attract more dust particles from the air, hold the dust particles to the surface very strongly, and potentially induce other PV module surface effects, all of which could increase the power production losses because of soiling.

An indoor soil deposition chamber has been designed and constructed at the Arizona State University Photovoltaic Reliability Lab (ASU-PRL) to simulate photovoltaic (PV) module soiling in the field. This paper discusses the next set of revisions to the design, which are organized into three categories: design improvements, cost reduction, and new features. The design process was carried out with the intention of working toward the development of a national or international soiling standard. The improvements made to the chamber are described in detail to provide a guide for other laboratories or research facilities who wish to study photovoltaic (PV) soiling.

Natural soiling has reduced the energy output of PV systems since the technology was first used, and viable mitigation strategies have remained elusive ever since. With the ever-increasing deployments around the world, especially in dusty environments, soiling is becoming a billion-dollar problem, worldwide. Furthermore, as plant operators continue to look for ways to increase revenue, the PV operating voltages have increased to between 1000 V and 1500 V when the sun is shining. This has resulted in some unforeseen consequences nominally combined into what is termed “Potential Induced Degradation.” 1 Recent work by Jiang et. al., 2 at NREL using Atomic Force Microscopy has demonstrated that these large potentials also affect soiling by substantially increasing the attraction of dust to the surface, but also by increasing the adhesion force. Jiang et. al., have also shown that these higher soiling attraction and adhesion forces continue long into the night when the PV is no longer producing power. In this paper, we present a set of field results that demonstrate enhanced soiling rates that is due to the strong electric fields induced by these high voltage PV arrays. This includes observation of enhanced soiling rates measured in the field when a module is held at ±1000 V. This is critical information for installation operators because soiling losses may be higher on some panels than what is measured by typical soiling stations, and because the high voltages are not uniform across an array, some modules may have more soiling than others, leading to potential issues with non-uniform soiling problems at the array level. We present this set of compelling electric field induced soiling results in this paper.

The natural soiling of photovoltaic cover glass has recently been shown to include both an inorganic and organic particulate matter. Under favorable growth conditions, the latter can lead to the growth of dense colonies of filamentous fungi, which potentially leads to measurable performance losses over time. Herein, we report on a field study where glass coupon samples were deployed in soiling-prone locations, which focused on Dubai (United Arab Emirates) and Mumbai (India). For each site location, clear differences in the soiling were observed. The samples from Mumbai were contaminated with an abundance of filamentous fungi, whereas the samples from Dubai had primarily inorganic contamination. The effectiveness of soiling mitigation strategies, which include cleaning techniques and glass coatings, are discussed in detail.

The natural soiling of photovoltaic cover glass has recently been shown to include both inorganic and organic particulate matter. Under favorable growth conditions, the latter can lead to the growth of dense colonies of filamentous fungi, potentially leading to measurable performance losses over time. Herein, we report on a field study where glass coupon samples were deployed in soiling-prone locations, focusing on Dubai (United Arab Emirates) and Mumbai (India). For each site location, clear differences in the soiling were observed. The samples from Mumbai were contaminated with an abundance of filamentous fungi whereas the samples from Dubai had primarily inorganic contamination. The effectiveness of soiling mitigation strategies, including cleaning techniques and glass coatings, are discussed in detail.

An indoor soil deposition method has been developed to simulate natural soil deposition on glass coupons or one-cell and multicell photovoltaic (PV) modules. This method uses variable ambient humidity, coupon/module temperature, and dust composition within a single custom-made chamber to create a natural and uniform soil deposition layer. Antisoiling (AS) coatings from two different manufacturers were applied on two one-cell monocrystalline silicon modules. Three layers of Arizona road dust have been deposited on the one-cell modules with AS coatings and an uncoated one-cell reference module at varied humidity levels. The soiled modules were exposed to an open-circuit subsonic wind tunnel at varying speeds and the effectiveness of AS coatings have been quantified using the transmittance gain. Transmittance loss resulting from the AS coating has been measured and compared with the transmittance of the uncoated reference module using a reflectance spectrophotometer. Reflectance measurements have also been taken to compare the transmittance loss of Arizona road dust and soil collected from PV modules’ superstrates. The soiled one-cell modules were then exposed to rain from a rain simulator. The transmittance gain due to rain exposure is quantified using a rain gain and rain coefficient. These tests cumulatively may be used to help develop a test standard for evaluating the effectiveness of AS coatings.

The contamination of solar photovoltaic cover glass can significantly reduce the transmittance of light to the surface of the photovoltaic cell, reducing the module's power output. The solar industry has been developing antireflection (AR) and antisoiling (AS) surface coatings to enhance light transmittance and mitigate the impacts of soiling. Although uncoated glass has been field tested for decades, minimal data exist to demonstrate the durability of AR and AS coatings against abrasion and surface erosion, including from: natural weathering, airborne sand, and industry cleaning practices. Coupons 75 mm square of varying types have been field-deployed to gather long-term data on coating durability; the initial results are presented here after 1 year of outdoor exposure near Sacramento, California. Duplicate sets of coupons were cleaned monthly per four different cleaning practices. All coupons demonstrated inorganic soiling as well as microscale biological contamination, regardless of cleaning method. Additionally, full-sized, field-aged modules from other areas of the world presented with similar types of contamination as the field-aged coupons; micrographs and results from genomic sequencing of this contamination are included here. Optical microscopy, scanning electron microscopy, atomic force microscopy/energy-dispersive spectroscopy, surface roughness, transmittance, and surface energy analysis of representative specimens and cleaning practices are presented.

Natural soiling has reduced the energy output of PV systems since the technology was first used. Projecting even a small ~4% average annual soiling loss (found in some places in the U.S.), translates to ~10 GW of power loss worldwide, which correlates today to ~$2 billion in lost revenue annually, worldwide. Production losses due to soiling may be even higher in high soiling environments, substantially increasing the levelized cost of electricity (LCOE). Furthermore, while soiling has been discussed in the literature for more than 70 years, solutions to many problems are still needed. NREL is working with the PV industry to develop the tools/knowledge so that the effects of soiling can be predicted for different environmental conditions and cost effective mitigation can be implemented. For this paper, we will describe our efforts to (1) predict PV module soiling rates based on environmental factors at a PV installation and from its energy production data, (2) quantitatively measure the adhesion forces to understand the physics enabling soiling, and (3) develop related standards on PV module coatings and artificial soiling. For example, NREL has used soiling station data to identify the most important environmental factors that are correlated with average soiling losses, and is using this information to develop models that accurately predict soiling losses at prospective sites without the need for local soiling stations. Furthermore, by providing a detailed understanding of soiling mechanisms and the corresponding requirements for PV module coatings, we are providing valuable information about what improvements in performance may be possible at a given PV site. Ultimately, this effort will help reduce the uncertainty in PV plant power output and maintenance requirements, and thus reduce costs.

Monitoring of photovoltaic (PV) systems can maintain efficient operations. However, extensive monitoring of large quantities of data can be a cumbersome process. The present work introduces a simple, inexpensive, yet effective data monitoring strategy for detecting faults and determining lost revenues automatically. This was achieved through the deployment of Raspberry Pi (RPI) device at a PV system's combiner box. The RPI was programmed to collect PV data through Modbus communications, and store the data locally in a MySQL database. Then, using a Gaussian Process Regression algorithm the RPI device was able to accurately estimate string level current, voltage, and power values. The device could also detect system faults using a Support Vector Novelty Detection algorithm. Finally, the RPI was programmed to output the potential lost revenue caused by the abnormal condition. The system analytics information was then displayed on a user interface. The interface could be accessed by operations personal to direct maintenance activity so that critical issues can be solved quickly.

As part of efforts to reduce photovoltaic (PV) costs and improve reliability/durability, temperature control needs to be closely examined in a number of ways including novel passive cooling technology. Modest PV module operating temperature reductions can increase PV power output by 10% or more. The higher the operating temperature of a typical PV device, the lower the overall PV conversion efficiency. Because some incident light is converted to heat in PV cells, the PV module heats up (typically 25°C above ambient temperature in full sun), and the overall energy output of the PV system is reduced (typically a 10% power loss for every 20°C to 30°C increase in temperature). We are integrating material on the PV module that adsorbs water from the air at night when the PV module temperature is cool and the relative humidity in the air is typically high. This sorbed water evaporates as the PV module heats up during the day, taking the excess heat with it and thus effectively cooling the PV module (see Fig. 1). Initial experiments with this novel passive cooling have demonstrated that PV module operating temperatures may be reduced nearly 30°C with evaporative cooling. In this paper, we describe 1) novel materials development, 2) initial modeling that predicts how the cooling will work in different environments, and 3) results demonstrating the PV operating temperature reductions achieved to date.

Long-term acquisition of solar panel performance parameters, for panels operated at maximum power point in their real environment, is of critical importance in the photovoltaic research sector. However, few options exist for the characterization of non-standard panels such as concentrated photovoltaic systems, heavily soiled or shaded panels or those operating under non-standard spectral illumination; certainly, it is difficult to find such a measurement system that is flexible and affordable enough to be adopted by the smaller research institutes or universities. We present here an instrument aiming to fill this gap, autonomously tracking and maintaining any solar panel at maximum power point while continuously monitoring its operational parameters and dissipating the produced energy without connection to the power grid. The instrument allows periodic acquisition of current-voltage curves to verify the employed maximum power point tracking approach. At the same time, with hardware schematics and software code being provided, it provides a flexible open development environment for the monitoring of non-standard generators like concentrator photovoltaic systems and to test novel power tracking approaches. The key issues, and the corresponding solutions, encountered in the design are analyzed in detail and the relevant schematics presented.

Decreasing LCOE with predictive soiling loss models (using site data to predict annualized energy loss), quantification of different soiling mechanisms (using AFM-based characterization), and developing standards for PV module coatings.

This review is intended to identify the method or methods--and the basic details of those methods--that might be used to develop an artificial abrasion test. Methods used in the PV literature were compared with their closest implementation in existing standards. Also, meetings of the International PV Quality Assurance Task Force Task Group 12-3 (TG12-3, which is concerned with coated glass) were used to identify established test methods. Feedback from the group, which included many of the authors from the PV literature, included insights not explored within the literature itself. The combined experience and examples from the literature are intended to provide an assessment of the present industry practices and an informed path forward. Recommendations toward artificial abrasion test methods are then identified based on the experiences in the literature and feedback from the PV community. The review here is strictly focused on abrasion. Assessment methods, including optical performance (e.g., transmittance or reflectance), surface energy, and verification of chemical composition were not examined. Methods of artificially soiling PV modules or other specimens were not examined. The weathering of artificial or naturally soiled specimens (which may ultimately include combined temperature and humidity, thermal cycling and ultraviolet light) were also not examined. A sense of the purpose or application of an abrasion test method within the PV industry should, however, be evident from the literature.