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How to Measure Irradiance with a Reference Module?

The irradiance incident on a PV module is proportional to its temperature-corrected short-circuit current. Therefore, you can use a reference PV module to measure total effective irradiance. Using a reference module for irradiance measurements can be advantageous for thin-film technologies, where spectral mismatch between the module and another irradiance sensor is a concern, and for bifacial systems, where you need to measure total effective front plus rear-side irradiance.

In the past, using reference modules required dedicated standalone modules for measurement. But now, our RDE300i PV Module Measurement System lets you make measurements on modules that are in-line and operating in your array, turning any module in your array into a reference module.

In order to perform an irradiance calculation, you must first obtain a traceable calibration value for short-circuit current. Once the calibration value is obtained, you may accurately calculate irradiance from a PV module with RDE300i.

How to Calibrate your PV Module

The goal of calibrating a PV module is to measure its short-circuit current at standard test conditions (STC), which are defined as 25 degrees Celsius and 1000 W/m2 under a standard solar spectrum (AM1.5 or IEC 60904-3). Calibration can be performed at many laboratories that offer module calibration services. The temperature coefficient of short-circuit current is also required for the calibrated module; this value is typically obtained from manufacturer datasheets.

How to Measure Irradiance

After determining the calibration of a PV module, irradiance may be calculated using the equation shown with a measured short-circuit current and module temperature. In this equation, Isc is the measured short-circuit current and T is the measured module temperature. Isc,ref represents the known short-circuit current at the reference condition, such as STC, while Tref denotes the temperature at the reference condition (25 °C). Irrref represents the reference condition irradiance, which is typically 1000 W/m2, and 𝛼 represents the temperature coefficient of short-circuit current.

Irradiance Equation

How to Measure Irradiance on a PV Module

RDE300i Product Image

RDE300i can measure effective irradiance for both monofacial and bifacial solar modules. Simply input the calibration data for your PV module during initial configuration. Then, once properly installed on your reference module, RDE300i automatically calculates effective irradiance every two seconds using the equation shown above. This is achieved through rapid measurements of the module’s short-circuit current and back-of-module temperature with an adhesive-backed RTD.