PHOTOVOLTAIC MODULES PERFORMANCES MEASUREMENTS Use of pyranometer as reference instrument Author: ing. Giuseppe Terzaghi Head of Albarubens testing laboratory Rome - October,

13 - October Photovoltaic modules performances measurement 2 Background Photovoltaic energy is one of the world fastest increasing market: Moore’s like law: costs decrease and performances increase Oil is polluting and not renewable People like this technology Huge plants are in progress: the biggest in Italy will be 72MW, this year. The international module price is base on POWER, not on ITEMS The power measurement is critical: If overestimated, clients pay more If underestimated, clients pay less The commercial accepted tolerance actually is +/- 3% This value appears as very poor, but it’s very hard to obtain.

13 - October Photovoltaic modules performances measurement 3 Standard requirements Photovoltaic modules qualification is regulated by different international standards: Crystalline modules: IEC EN Thin-film modules: IEC EN Concentrating modules: IEC EN The standard asks to measure the IV trace of modules, determining on it some values, including the maximum power. Of course, the electricity produced by PV modules depends on light radiation: null in the night! So the standards fix some standardized measurement conditions, to easily compare different modules: 1000 W/m2 of global irradiance on module, normal direction 25°C of module temperature Standardized light spectrum (AM1.5  sunshine in blue sky)

13 - October Photovoltaic modules performances measurement 4 Procedures There are two ways to apply irradiance to a PV module: Using natural sunlight (waiting for good weather conditions) Using a lamp simulator (typically a xenon flash) Using a steady-state simulator (like new generation LED types) Use of “flasher” of “LED” is practically mandatory for factory measurements: it’s not possible to stop a production line in case of rain. Natural sunlight is the best choice for laboratory grade measurements. Modules characterized in natural sunlight can be used to calibrate measurements in solar simulator. In any case, the procedure is this: Place the module orthogonal to radiant source Measure the electric power produced by the module Acquire the real irradiance and normalize the measured power to the standard value (1000 W/m2) Acquire the real temperature and normalize the measured power to the standard value (25°C) Acquire the real spectrum and normalize the measured power to the standard value (AM 1.5) Not considering the electricity meter (typically an electronic load), you need a thermometer, an irradiance meter and, perhaps, a spectroradiometer. And a radiant source.

13 - October Photovoltaic modules performances measurement 5 Measurement uncertainty Keeping in mind the goal of a good power measurement, it’s necessary to list the errors affecting it: Temperature: the typically temperature coefficient of module power is 0.4%/°C; Electrical power: very easy to measure in natural sunlight (using seconds exposition time), affected by some parasitic effect in case of millisecond measurements (flash simulator) Spectrum: the natural sun light spectrum is same of the standard one (of course), with practically negligible errors in standard measurement conditions (around 12 o’clock in a sunny day); a spectral mismatch correction becomes mandatory in case of use of solar simulator; Irradiance: the module power depends practically linearly by irradiance; an error of 5% in irradiance means 5% in power. Instability of source: if irradiance varies during the measurement, its correction may become hard. Stability is absolute using LED, impossible using flasher, depending on clouds in natural sunlight. Summarizing, perfect measurements are possible in natural sunlight only, using the best possible instrument to measure it.

13 - October Photovoltaic modules performances measurement 6 Irradiance meters Photodiodes: Very fast measurement, suitable for flasher; Spectral dependent response, requiring mismatch correction; Temperature dependent response, requiring temperature compensation; Cosine corrected optics available; Reference cells: Similar to photodiodes in behavior; Same correction necessities; Optical response similar to PV modules; Pyranometers: No corrections required (temperature compensated, flat spectral response in range of interest) Very low uncertainty (down to 1.2%); Cosine corrected optic by default; Speed enough for short-term variations (clouds), not for flasher; Suitable for new generation LED solar simulators;

13 - October Photovoltaic modules performances measurement 7 Pyranometer considerations Albarubens laboratory uses Kipp&Zonen pyranometers to measure solar radiation during PV performances measurement, in natural sunlight. We currently use different versions: CMP03 – low requirements measurement only (don’t cry in case of crash); CMP11 – good enough for standard power measurements, everyday use; CMP22 – mainly for calibration purpose, also interesting for UV sensitivity; Pyranometer electrical signal is very low (typically 1000 W/m2); we use special instruments connected to lap-top: Kipp&Zonen SolRad Integrator for long time performances measurements; Agilent 34401A for standard acquisition; Laboratory-grade calibration is available at WRC (World Radiation Center), if you can wait many month. Our national accreditation authority (Sinal/Accredia) now accepts calibration service form Kipp&Zonen, with a little increase estimated errors. The Albarubens accredited uncertainty, on the whole PV power measurement, is at the moment 1.8% (k=2 / 95%): the best in Italy, very far from the standard value obtained in solar simulator (>3%).

13 - October Photovoltaic modules performances measurement 8 Direct sunlight measurement

13 - October Photovoltaic modules performances measurement 9 Low irradiance measurement

13 - October Photovoltaic modules performances measurement 10 Conclusions Pyranometers are not suitable for traditional flash simulators, but perfect for natural sunlight and new LED type. The calibration is easily refereed to international standards. The use is easy and need not any special compensation. The uncertainty is low and the response time is good enough for the application. Pyranometers are the best choice to measure irradiance for photovoltaic applications.