This paper investigates the feasibility of using fixed resistors instead of active maximum power tracking as electrical loads for long-term exposure testing of photovoltaic modules and arrays. The method of investigation was to compare resistive versus active loading on two modules for which historic current-voltage data over time were available. Also, a small amorphous silicon array was installed with a resistive load and the performance has been monitored versus time. The major conclusion of this work is that fixed resistive loading is an inexpensive and viable means of loading photovoltaic devices for exposure testing if the resistance value used is close to the ratio of the voltage to the current at the maximum power point under Standard Test Conditions. Copyright (C) 2006 John Wiley & Sons, Ltd.

In this paper, we present a comparison of maximum power degradation rates of individual modules under outdoor conditions in Golden, Colorado. Test modules include single- and polycrystalline-Si (x-Si, poly-Si), amorphous Si (a-Si, single, dual, and triple junction), CdTe, Cu-In-Ga-Se-S (CIS), and c-Si/a-Si heterostructure, from nine difference manufacturers. From monthly blocks of output power data, ratings were determined using multiple regressions to Performance Test Conditions (PTC). Plotting the power ratings versus time allowed degradation rates to be calculated from linear regressions. We also include a summary of module degradation rates obtained from the open literature over the past five years. Compared with the common rule-of-thumb value of 1% per year, many modules are seen to have significantly smaller degradation rates. A few modules, however, degrade significantly faster

As part of the work conducted in the PV Systems Reliability and Performance R&D Task, a 1.5-kWdc photovoltaic (PV) array consisting of 36 Solarex MST-43MV dual-junction a-Si modules was installed and its performance monitored for almost six years (September 1999 through May 2005) at the National Renewable Energy Laboratory (NREL) Outdoor Test Facility (OTF). This paper describes the system and its performance based on the PV for Utility-Scale Applications (PVUSA) power rating method.

This paper presents third year results of a round robin exposure experiment designed to assess the performance of thin-film amorphous silicon (a-Si) solar modules operating in different climatic conditions. Three identical sets of commercially available a-Si PV modules from five different manufacturers were simultaneously deployed outdoors in three sites with distinct climates (Arizona -USA, Colorado - USA and Florianopolis - Brazil). Every year all PV module sets were sent to the National Renewable Energy Laboratory (NREL) for standard testing conditions measurements under a SPIRE simulator. The four-year experiment aims to determine the light-induced degradation and stabilization characteristics of a-Si regarding specific history of exposure, and to monitor and compare degradation rates in different climates. We present results from the first three years of measurements, showing that while most of the manufacturers underrate their products by 20 to 25% to account for the light-induced degradation, outdoor exposure temperature seems to be what will ultimately determine the stabilized performance level of a-Si.

The National Renewable Energy Laboratory (NREL) has been monitoring the performance of a 1- kW/sub AC/ United Solar Systems Corporation (USSC) roofing system over the 6-year period from October 1998 to September 2004. This paper investigates the performance and reliability of this system. The annual degradation and seasonal fluctuation of the system's power output are calculated using the PVUSA power rating regression model. The system performance is also examined using the additional performance parameters of yield, reference yield, and performance ratio, which allow for a somewhat less complicated data collection and analysis. The data indicate that the system has exhibited stable performance over time, with an overall degradation rate comparable to that found in crystalline silicon systems.

The use of appropriate performance parameters facilitates the comparison of grid-connected photovoltaic (PV) systems that may differ with respect to design, technology, or geographic location. Four performance parameters that define the overall system performance with respect to the energy production, solar resource, and overall effect of system losses are the following: final PV system yield, reference yield, performance ratio, and PVUSA rating. These performance parameters are discussed for their suitability in providing desired information for PV system design and performance evaluation and are demonstrated for a variety of technologies, designs, and geographic locations. Also discussed are methodologies for determining system a.c. power ratings in the design phase using multipliers developed from measured performance parameters.

The PV Systems Performance and Reliability R & D group currently has seven grid-tied 1-2 kilowatt PV systems deployed at NREL's Outdoor Test Facility (OTF) and two 6 kilowatt systems mounted on the roof of NREL's Solar Energy Research Facility (SERF). The systems, which employ several PV module technologies including crystalline silicon (c-Si), amorphous silicon (a-Si), cadmium telluride (CdTe), and copper indium diselenide (CIS), are being monitored to determine the long-term performance and reliability of the modules and arrays under actual field conditions. The length of observation ranges from 2 months for our newest system to 11 years for our oldest systems. The annual degradation and seasonal fluctuation of the systems' power output are calculated using the PV for Utility-Scale Applications (PVUSA) power rating regression model.

This paper provides the changes in performance ratings of two photovoltaic (PV) systems located on the roof of the Solar Energy Research Facility (SERF) building at the National Renewable Energy Laboratory (NREL) in Golden, Colorado. For the period of May 1994 to April 2002, the performance rating of the two PV systems decreased at the rate of 1% per year. Most of the changes in performance rating are attributed to changes in the performance of the PV arrays. But about a fifth of the observed changes were from the inverter not tracking the peak-power as effectively as the PV arrays aged.

This paper reviews a testing program conducted at NREL for the past two years that applied voltage, water vapor, and light stresses to thin-film photovoltaic (PV) modules with SnO2:F transparent conducting oxides (TCOs) deposited on soda-lime glass superstrates. Electrochemical corrosion at the glass-TCO interface was observed to result in delamination of the thin-film layers. Experimental testing was directed toward accelerating the corrosion and understanding the nature of the resulting damage.