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Module Measurements

NREL researchers measure module current vs voltage (I-V) under simulated and outdoor conditions. Key capabilities are discussed below.

Simulated Module Current vs Voltage

We use two I-V measurement systems to assess the performance parameters for PV modules under simulated conditions: a Spire 5600 pulsed solar simulator and a large-area continuous solar simulator (LACSS).

  • Spire 5600 SLP Pulsed Solar Simulator. This simulator uses a long-arc pulsed xenon lamp, with a 30- to 100-ms flash duration, that is filtered to simulate the global reference spectrum (IEC 60904-3), and whose irradiance has a spatial nonuniformity of ±1% over an area of 2000 cm × 1370 cm. Temperature measurements are made with a spring-loaded platinum resistance temperature detector (RTD) in contact with the back of the module. The original software has been modified to store data and update the directory in our standardized tab-delimited text format and to have complete control over the bias direction and other sweep parameters. Temperature coefficients can be measured using an adjustable heating blanket. The I-V characteristics as a function of lamp intensity in the range of 0.2 to 1.3 suns can be measured by changing the lamp voltage. A spectral correction is applied to the data based on the lamp voltage.

  • Large-Area Continuous Solar Simulator. The LACSS is a custom-designed system that is used for measuring module performance. It uses a Spectrolab model X200 continuous 25-kW xenon short-arc lamp to provide a filtered light source to simulate the global spectrum (IEC 60904-3). It can measure modules as large as 170 cm × 130 cm with spatial nonuniformity of ±3%. Sample temperature is measured with a spring-loaded RTD at the back of the module.

The following table is a condensed list of characteristics for module I-V measurements under simulated light.

0.2- to 1-sun module I-V 30- to 100-ms flash; sweep both directions 2 Xe flash lamps 200 cm × 137 cm 1 mV /250 V 0.1 mA / 25 A
1-sun modules 100% duty-cycle continuous light allows the sample to be in any state prior to measurements; spectral irradiance measured; user-controlled bias 25-kW Xe 170 cm × 130 cm 5 mV / ±300 V ± 1 µA to ±60 A

Outdoor Module Current vs Voltage

We use three I-V measurement systems to assess the performance parameters for PV modules under natural sunlight: the standard outdoor measurement system (SOMS), the NREL-developed custom prototype concentrator evaluation test bed, and the Daystar DS-10/125 portable I-V curve tracer.

  • Standard Outdoor Measurement System. The SOMS is our principal system for measuring module performance under prevailing outdoor conditions. We attempt to make these measurements under a clear sky, with the irradiance between 950 and 1,050 W/m2, and as close to 25°C as possible. For each measurement, we use a pyranometer and a silicon reference cell in a module package to determine the total irradiance, and a spectroradiometer to determine spectral content of the sunlight between 350 and 1,100 nm. Once the operator has set the parameters, the system measures the I-V characteristics and the temperature (using a thermocouple at the back of the module), and calculates other performance characteristics.

  • Prototype Concentrator Evaluation Test Bed. We developed this test bed to be able to evaluate I-V characteristics throughout the day of prototype concentrator cells and modules. The site includes instruments to measure sample and air temperature, wind speed, and direct-beam irradiance. The system also records data from the Reference Meteorological Irradiance System with the individual module I-V curves. The data are analyzed for performance as a function of time, temperature, and light level. This test bed data set is also used to evaluate translation equations and energy-rating methods.

  • Daystar DS-10/125 Portable I-V Curve Tracer. Like the SOMS, this system is used to make module I-V measurements under prevailing outdoor conditions. But typically, we only use it when the sample is too cumbersome or impractical to disassemble and remount on the SOMS test bed. Total irradiance is measured using a pyranometer mounted in the same plane as the sample..

The following table is a condensed listing of characteristics for module I-V measurements instrumentation.

Flat-plate and concentrator I-V measurements under outdoor conditions 2-axis positioning; meteorological parameters; spectral irradiance measured; user-controlled bias Sunlight 200 cm × 300 cm 5 µV to ±300 V ± 1 µA to ±60 A
Unattended periodic I-V measurements of prototype concentrator modules and cells mounted in a 2-axis tracker; translation equations, stability, and performance of concentrator Up to 10 samples at a time; spectral irradiance and meteorological parameters; multiple temperature channels Sunlight 61 cm × 120 cm 0.01 mV to 100 V 20 nA to 20 A
I-V measurements under outdoor conditions Portable; may be powered either with a 120-Vac line or with a 12-Vdc battery. Sunlight Unlimited 12 mV to 600 V 5 mA to 125 A