Patrick D. Burton

Application and Characterization of an Artificial Grime for Photovoltaic Soiling Studies

Effective evaluation and prediction of photovoltaic performance loss due to soiling require consistent test methods. Natural soil accumulation is time-consuming and location specific, and thus does not provide reproducible results across different geographic regions. Therefore, we have demonstrated a laboratory technique to artificially apply soil to a specimen and quantify the resulting effects of the film on the transmission of incident light. An artificial soil analogue was formulated with NIST-traceable components and applied to the specimen using an aerosol spray gun. This approach produced consistent soil coatings, which were directly correlated to electrical performance loss of multicrystalline Si cells in a laboratory setting. Two independent measurement techniques were used to quantify the influence of the layer of artificial soil on the spectral transmission of light. It was found that the performance loss due to deposited soil could be effectively predicted over a range of mass loadings. Furthermore, it was demonstrated that the composition of the blend, termed “standard grime,” had a significant and repeatable influence on performance loss. The methods presented here provide the basis for further study of the influence of specific soil types on the performance loss of PV systems. It is envisioned that these laboratory studies could be coupled with field studies to better understand these effects.

Spectral Sensitivity of Simulated Photovoltaic Module Soiling for a Variety of Synthesized Soil Types

The accumulation of soil on photovoltaic (PV) modules may introduce a spectral loss due to the color profile of the accumulated material. In order to compare the spectral and total losses experienced by a cell, soil analogs were formulated to contain common mineral pigments (Fe2O3 and göthite) with previously developed “standard grime” mixtures. These mixtures simulated a wide range of desert soil colors and were applied to glass test coupons. The light transmission through the deposited film was evaluated by UV/vis/NIR spectroscopy and by placing the coupon over a test cell in a 1-sun simulator and quantum efficiency test stand. Distinct peaks in the 300-600-nm range were observed by UV/vis/NIR spectroscopy corresponding to the Fe2O3 and göthite. Approximately analogous features were noted in the QE measurement. Overall comparisons were made by integrating the response of a soiled coupon relative to a clean reference. Soils rich in red pigments (Fe2O3) caused a greater integrated response than soils rich in yellow pigment (göthite). The yellow soils caused a greater attenuation in a specific region of the spectrum (300-450 nm), which may have significant implications to specific devices, such as multijunction and CdTe technologies.

Quantification of a Minimum Detectable Soiling Level to Affect Photovoltaic Devices by Natural and Simulated Soils

Soil accumulation on photovoltaic (PV) modules presents a challenge to long-term performance prediction and lifetime estimates due to the inherent difficulty in quantifying small changes over an extended period. Low mass loadings of soil are a common occurrence but remain difficult to quantify. In order to more accurately describe the specific effects of sparse soil films on PV systems, we have expanded upon an earlier technique to measure the optical losses due to an artificially applied obscurant film. A synthetic soil analog was sprayed onto glass coupons at very brief intervals with a high-volume, low-pressure pneumatic sprayer. Light transmission through the grime film was evaluated using a quantum efficiency test stand and UV/vis spectroscopy. A 0.1-g/m2 grime loading was determined to be the limit of mass measurement sensitivity, which is similar to some reports of daily soil accumulation. Predictable, linear decreases in transmission were observed for samples with a mass loading between 0.1 and 0.5 g/m2. A similar change was observed for soiled coupons from an outdoor monitoring station. Collected soil from the field coupons was analyzed to develop a compositional analog for indoor studies. Natural and synthetic soils produced similar decreases in transmission.

Effects of photovoltaic module soiling on glass surface resistance and potential-induced degradation

The sheet resistance of three soil types (Arizona road dust, soot, and sea salt) on glass were measured by the transmission line method as a function of relative humidity (RH) between 39% and 95% at 60°C. Sea salt yielded a 3.5 orders of magnitude decrease in resistance on the glass surface when the RH was increased over this RH range. Arizona road dust showed reduced sheet resistance at lower RH, but with less humidity sensitivity over the range tested. The soot sample did not show significant resistivity change compared to the unsoiled control. Photovoltaic modules with sea salt on their faces were step-stressed between 25% and 95% RH at 60°C applying -1000 V bias to the active cell circuit. Leakage current from the cell circuit to ground ranged between two and ten times higher than that of the unsoiled controls. Degradation rate of modules with salt on the surface increased with increasing RH and time.

Determination of a minimum soiling level to affect photovoltaic devices

Soil accumulation on photovoltaic (PV) modules presents a challenge to long-term performance prediction and lifetime estimates due to the inherent difficulty in quantifying small changes over an extended period. Low mass loadings of soil are a common occurrence, but remain difficult to quantify. In order to more accurately describe the specific effects of sparse soil films on PV systems, we have expanded upon an earlier technique to measure the optical losses due to an artificially applied obscurant film. A synthetic soil analogue consisting of AZ road dust and soot in acetonitrile carrier solvent was sprayed onto glass coupons at very brief intervals with a high volume, low pressure pneumatic sprayer. Light transmission through the grime film was evaluated using a QE test stand and UV/vis spectroscopy. A 0.1 g/m2 grime loading was determined to be the limit of mass measurement sensitivity, which is similar to some reports of daily soil accumulation. Predictable, linear decreases in transmission were observed for samples with a mass loading between 0.1 and 0.5 g/m2. Reflectance measurements provided the best means of easily distinguishing this sample from a reference.

Reliability model development for photovoltaic connector lifetime prediction capabilities

This paper describes efforts to characterize different aspects of photovoltaic connector reliability. The resistance variation over a population of connections was examined by measuring 75 connectors from three different manufacturers. The comparison shows differences in average resistance of up to 9% between manufacturers. The standard deviation of resistance among the same manufacturer ranged from 6%-11%. In a separate experiment, the corrosive effects of grime on the connector pins during damp heat accelerated testing at 85°C/85% RH were studied. We observed a small resistance increase in the first 100 hours of damp heat and no further changes up to the current 450 hours of available data. With the exception of one connector, the effects of grime on connector performance during accelerated testing could not be measured during this time period.

Pattern Effects of Soil on Photovoltaic Surfaces

The texture or patterning of soil on PV surfaces may influence light capture at various angles of incidence (AOI). Accumulated soil can be considered a microshading element, which changes with respect to AOI. Laboratory deposition of simulated soil was used to prepare test coupons for simultaneous AOI and soiling loss experiments. A mixed solvent deposition technique was used to consistently deposit patterned test soils onto glass slides. Transmission decreased as soil loading and AOI increased. Dense aggregates significantly decreased transmission. However, highly dispersed particles are less prone to secondary scattering, improving overall light collection. In order to test AOI losses on relevant systems, uniform simulated soil coatings were applied to split reference cells to further examine this effect. The measured optical transmission and area coverage correlated closely to the observed ISC. Angular losses were significant at angles as low as 25°.

Macro- and microscale particle size effects of soil on photovoltaic surfaces

The texture or patterning of soil on PV surfaces may influence light capture at various angles of incidence. Accumulated soil can be considered a micro-shading element, which changes with respect to AOI. While scattering losses at this scale would be significant only to the most sensitive devices, micro-shading could lead to hot spot formation and other reliability issues. Indoor soil deposition was used to prepare test coupons for simultaneous AOI and soiling loss experiments. A mixed solvent deposition technique was used to consistently deposit patterned test soils onto glass slides. Transmission decreased as soil loading and AOI increased. Highly dispersed particles are less prone to secondary scattering, improving overall light collection.

Final Technical Report: Advanced Measurement and Analysis of PV Derate Factors.

The Advanced Measurement and Analysis of PV Derate Factors project focuses on improving the accuracy and reducing the uncertainty of PV performance model predictions by addressing a common element of all PV performance models referred to as “derates”. Widespread use of “rules of thumb”, combined with significant uncertainty regarding appropriate values for these factors contribute to uncertainty in projected energy production.

A Handbook on Artificial Soils for Indoor Photovoltaic Soiling Tests

This manuscript is intended to serve as a practical guide to conducting repeatable indoor soiling experiments for PV applications. An outline of techniques, materials and equipment used in prior studies [1–3] is presented. Additional recommendations and practical guidance has been presented. Major sections include techniques to formulate soil simulants, (‘standard grime’) and feedstocks from traceable components, spray application, and quantitative measurement methodologies at heavy and minimal soil loadings.