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Energy rating of photovoltaic modules improves certainty for investors

The Fraunhofer Institute for Solar Energy Systems in Freiburg said its new measurement procedures can offer more certainty to photovoltaic project developers and investors.

The energy rating enables a precise yield prognosis for each location worldwide, therefore improving the bankability.

Real environmental effects are taken into account in the testing laboratories and flow into the energy yield assessment, said Fraunhofer ISE.

“Up to now the data sheet and the manufacturer’s specifications have served as the basis for the energy yield assessment when planning photovoltaic systems,” said Boris Farnung, head of the calibration team at Fraunhofer ISE. “In the practice, however, there is often a discrepancy between the manufacturer’s specifications and the laboratory measurements.

“Only with uniform power rating measurements is it possible to determine benchmarks for different module types at a given location, added Farnung. “When the solar irradiation over a day sinks to one fifth the standard value of 1000 W/m², the efficiency decreases two to five percent. In addition, losses due to higher operating temperatures are measureable at Central European locations and become more evident in southern locations.”

For the energy rating, the dependence on irradiation and the temperature behavior are measured for a relevant number of modules, chosen randomly, said the institute. With these measurements, the energy yield of a module can be determined for any given location using the meteorological data at the site. The measurements require highly complex and cost intensive equipment that few laboratories worldwide can afford.

After undergoing a complete modernization of its measurement technology, Fraunhofer ISE said it now fulfills the requirements over and above those set down in the international standard IEC61853 Part 1. For example, the radiation incident on the test module is extremely homogeneous, while in the climate chamber the preset module temperature remains stable.

With a measurement uncertainty of 1.8 percent for crystalline silicon photovoltaic modules, Fraunhofer ISE said it now offers the highest precision worldwide.

At Fraunhofer ISE, the importance lies not only in the electrical measurements. For projects in which thousands of modules are to be installed, clever statistical methods to check the nominal power are needed. Also, special analysis procedures such as electroluminescence are applied to locate hidden faults, like micro cracks. 

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