Larry Pratt

The effect of uncertainty in modeling coefficients used to predict energy production using the Sandia Array Performance Model

Predicting photovoltaic array performance is an important part of system design and monitoring, so its important to quantify the uncertainty associated with the predictions. The Sandia Array Performance Model [1] is one of many tools used to predict annual energy production, but the effect of the uncertainty in model coefficients has not been fully investigated. This paper quantifies the relative importance of voltage and current temperature coefficients, as well as the coefficients relating voltage and current to solar irradiance, for crystalline silicon modules. Using the coefficient variation observed in the Sandia module database and computer simulation, the effect of the uncertainty was quantified in terms of the range in predicted annual energy production relative to actual energy production by three small grid-connected PV systems. The relative importance of each coefficient by month of the year was also determined in order to understand the seasonal behavior of the performance model.

Optimized emitter wrap-through cells for monolithic module assembly

Migration from the previous generation of Advent Solar emitter wrap-through (EWT) cells to the current technology platform based on 243 cm2 multicrystalline Si cells and monolithically interconnected cell and encapsulated modules has brought with it significant performance increases. Extraction of current from multiple contact pads distributed on the cell rear, optimization of the emitter through-holes, and reduction of through-hole lengths associated with moving to thinner silicon wafers has led to significantly reduced series resistance. Ag metallization costs are also lowered because of the reduction of grid finger lengths. Improved isolation between interdigitated p and n regions on the cell rear has lead to negligible shunt resistance losses. This, in combination with understanding of diode recombination losses, passivation, and optimization of Si material for the cell design through appropriate choice of base resistivity has lead to a greater than one percent absolute efficiency improvement over previous generation EWT cells, with best multicrystalline Si cells producing 17% conversion efficiency using the regular production processes. Additional improvements to efficiency are obtained with the implementation of texturing. These efficiency gains are further leveraged on the module level by the reduction of series resistance losses associated with the backplane interconnect design compared to conventionally interconnected front-side contacted cells.

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.

Successful transfer of Sandia National Laboratories' outdoor test technology to TÜV Rheinland Photovoltaic Testing Laboratory

Sandia National Laboratories (Sandia) has observed an increased demand for high accuracy outdoor photovoltaic (PV) module characterization using Sandias Photovoltaic Array Performance Model [1]. To meet this demand, Sandia entered into a competitively-bid agreement in May 2009 with TÜV Rheinland Photovoltaic Testing Laboratory (TÜV-PTL) to transfer Sandias capability to fully characterize standard, commercial-scale PV modules. Sandia and TÜV-PTL worked closely on two round-robin experiments and months of subsequent work and discussions that resulted in module performance output calculations agreeing to within +/−2.5%.

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.

Correcting bias in measured module temperature coefficients

Temperature coefficients for PV modules describe the change with temperature of current, voltage and power. Coefficients are commonly determined by linear regression using measured module output at fixed irradiance and varying temperatures. We compare temperature coefficients determined for the same modules from both outdoor and indoor measurements. We find systematic bias in the temperature coefficients for voltage and power, with values derived from indoor measurements consistently smaller in absolute value than values derived from outdoor testing during which the module temperature is measured as specified in IEC 61853-1. Our work suggests that the bias results from a corresponding bias in the estimated module temperature. However we have not identified an alternative arrangement of a few thermocouples that would result in consistent values for temperature coefficients from either indoor or outdoor measurements.

Procedure to Determine Coefficients for the Sandia Array Performance Model (SAPM)

The Sandia Array Performance Model (SAPM), a semi-empirical model for predicting PV system power, has been in use for more than a decade. While several studies have presented comparisons of measurements and analysis results among laboratories, detailed procedures for determining model coefficients have not yet been published. Independent test laboratories must develop in-house procedures to determine SAPM coefficients, which contributes to uncertainty in the resulting models. Here we present a standard procedure for calibrating the SAPM using outdoor electrical and meteorological measurements. Analysis procedures are illustrated with data measured outdoors for a 36-cell silicon photovoltaic module.

Rating of CPV modules: Results of module round robins

The results of three CPV module round robins are presented. Ten test labs around the world participated to the round robins in total. Each round robin used a different CPV module technology (Daido Steel, Soitec, Suncore). The data gathered at the test labs was used to test CSOC power rating procedures as basis for the IEC draft standard 62670-3. The deviation between the minimum and the maximum power output rated at the test labs was in average 4.4 % with a standard deviation of 1.8 %abs. This underlines that power ratings or CPV modules are reliable and reproducible.

Validated Method for Repeatable Power Measurement of CIGS Modules Exhibiting Light-Induced Metastabilities

We report on the validation of a stabilization procedure designed to minimize variations in repeated power measurements at standard test conditions caused by transient light-induced metastabilities in copper indium gallium diselenide (CIGS) modules. Such metastable effects frustrate the repeatable and accurate measurement of a modules performance in the electrical state to which it stabilizes under normal operation outdoors. The procedure studied here is based on a light exposure followed by forward electrical bias as the module cools to the measurement temperature. The procedure was tested in a lab-to-lab intercomparison involving five different labs. Results show that the procedure is effective in yielding repeatable measurements and that the variations due to metastabilities are of roughly the same magnitude as those associated with variations in illumination conditions between different flash simulators. We also find that temperature-corrected measurements made immediately upon completion of the light exposure are less repeatable than those made after the module has cooled to 25°C under bias.

Outdoor test and analysis procedures for generating coefficients for the Sandia Array Performance Model

The Sandia Array Performance Model (SAPM), a semi-empirical model for predicting PV system power, has been in use for more than a decade. While several studies have presented laboratory intercomparisons of measurements and analysis, detailed procedures for determining model coefficients have never been published. Independent test laboratories must develop in-house procedures to determine SAPM coefficients, which contributes to uncertainty in the resulting models. In response to requests from commercial laboratories and module manufacturers, Sandia has formally documented the measurement and analysis methods as a supplement to the original model description. In this paper we present a description of the measurement procedures and an example analysis for calibrating the SAPM.