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Outdoor Module Current Versus Voltage (I-V)

The National Renewable Energy Laboratory (NREL) Device Performance group uses three I-V measurement systems to assess the performance parameters for photovoltaic (PV) modules under natural sunlight: the standard outdoor measurement system, the Daystar DS-10/125 portable I-V curve tracer, and the NREL-developed custom prototype concentrator evaluation test bed.

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

Major Instrumentation for Outdoor Module I-V Measurements

System Typical Applications Special Features Light Source Test Bed Voltage Resolution/Limit Current Resolution/Limit
Standard outdoor measurement system 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
Daystar DS-10/125 portable I-V curve tracer I-V measurements under outdoor conditions Portable; may be powered either with a 120Vac line or with a 12Vdc battery. Sunlight unlimited 12 mV to 600 V 5mA to 125A
Prototype concentrator evaluation test bed 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 Bare cells under a Fresnel lens or modules less than 2 feet by 4 feet 0.01 mV to 100 V 20 nA to 20 A

Standard Outdoor Measurement System

The standard outdoor measurement system (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 1050 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 1100 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.

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.

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.

For additional information contact Keith Emery, 303-384-6632.