Bruce Hardison King

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.

HCPV Characterization: Analysis of Fielded System Data.

Sandia and Semprius have partnered to evaluate the operational performance of a 3.5 kW (nominal) R&D system using 40 Semprius modules. Eight months of operational data has been collected and evaluated. Analysis includes determination of Pmp, Imp and Vmp at CSTC conditions, Pmp as a function of DNI, effect of wind speed on module temperature and seasonal variations in performance. As expected, on-sun Pmp and Imp of the installed system were found to be ∼10% lower than the values determined from flash testing at CSTC, while Vmp was found to be nearly identical to the results of flash testing. The differences in the flash test and outdoor data are attributed to string mismatch, soiling, seasonal variation in solar spectrum, discrepancy in the cell temperature model, and uncertainty in the power and current reported by the inverter.. An apparent limitation to the degree of module cooling that can be expected from wind speed was observed. The system was observed to display seasonal variation in performance, li...

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.

New data set for validating PV module performance models

A new publicly available data set was completed for use in validating models that estimate the performance of flat-plate photovoltaic (PV) modules. The data were collected for one-year periods at three climatically diverse locations (Cocoa, Florida; Eugene, Oregon; and Golden, Colorado) and for PV modules representing all technologies available in 2010 when the work began. The same makes and models of PV modules were tested at all locations and common data acquisition systems were used with calibrations performed at the National Renewable Energy Laboratory. For use in determining model parameters and coefficients, baseline and post-deployment measurements were performed indoors with solar simulators, including per the requirements of IEC 61853 Part 1: Irradiance and Temperature Performance Measurements and Power Ratings. Outdoors, the PV modules were characterized per the requirements of the Sandia array performance model. A users manual describes the contents of the data set and how to access the data.

Systems Long Term Exposure program: Analysis of the first year of data

While indoor accelerated testing of products is important for development, it is equally important to conduct field-testing to determine performance and degradation under real world conditions. To address this need, Sandia has developed an outdoor small systems evaluation laboratory. The Systems Long Term Exposure (SLTE) project spans three geographic locations: one in a hot/dry climate, one in a hot/humid climate and one in a cold climate. Identical systems representing three commercial technologies are installed at each location. In this paper we present the results and analysis from the first year of monitoring of these systems.

Characterzing aerosol Jet ® multi-nozzle process parameters for non-contact front side metallization of silicon solar cells

High efficiency solar cells can be produced by applying the front side metallization in a two-step process. A seed layer is printed followed by an electroplating step to increase thickness and reduce current loss. While traditional screen-printing can be used to print the seed layer, greater benefit is realized by utilizing a direct-write approach to simultaneously reduce the width and thickness of the seed layer. Aerosol Jet Printing is a non-contact direct-write approach that has been shown to have advantages for printing the seed layer. A multi-nozzle print head was developed to increase throughput to a level comparable to screen-printing. A modified screen-printing paste was used to print a seed layer for collector lines on 156 × 156 mm multi-crystalline silicon wafers. By adjusting print process parameters such as gas flow rates, operating temperature, and nozzle orifice openings, line width was controlled over a range of approximately 35–70 microns. In parallel, a single-nozzle print head was developed for printing busbars of around 1mm width in a single step.

Recent advancements in outdoor measurement techniques for angle of incidence effects

Reflection losses from a PV module become increasingly pronounced at solar incident angles >60°. However, accurate measurement in this region can be problematic due to tracker articulation limits and irradiance reference device calibration. We present the results of a measurement method enabling modules to be tested over the full range of 0-90° by articulating the tracker in elevation only. This facilitates the use of a shaded pyranometer to make a direct measurement of the diffuse component, reducing measurement uncertainty. We further present the results of a real-time intercomparison performed by two independent test facilities ~10 km apart.

Photovoltaic system fault detection and diagnostics using Laterally Primed Adaptive Resonance Theory neural network

Cost effective integration of solar photovoltaic (PV) systems requires increased reliability. This can be achieved with a robust fault detection and diagnostic (FDD) tool that automatically discovers faults. This paper introduces the Laterally Primed Adaptive Resonance Theory (LAPART) artificial neural network to perform this task. The present work tested the algorithm on actual and synthetic data to assess its potential for wide spread implementation. The tests were conducted on a PV system located in Albuquerque, New Mexico. The system was composed of 14 modules arranged in a configuration that produced a maximum power of 3.7kW. The LAPART algorithm learned system behavior quickly, and detected module level faults with minimal error.

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.