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Sensors measure the stresses occurring in the solar modules. Material fatigue can be calculated from this data.
© Fraunhofer IWM
Test method for photovoltaic panel
22.10.2013

Predicting material fatigue of solar modules

Snow loads, temperature fluctuations and wind gusts: solar modules are exposed to a variety of environmental factors which lead to material fatigue over the years. Researchers at the Fraunhofer Institute for Mechanics of Materials IWM have developed a method of calculating the effects of these influences. Their simulation model predicts when a material will break or peel off.

Module manufacturers give their customers a commercial guarantee of up to 25 years but they cannot always provide any reliable data about the service life of the module. Before photovoltaic panels can be approved they must meet certain standards. To achieve this, the modules undergo various experiments and are exposed to high temperatures or strong mechanical stresses. "These results only testify to the robustness of a new unit fresh from the factory over short periods of extreme stress", explained Alexander Fromm, a scientist from the Fraunhofer Institute for Mechanics of Materials IWM in Freiburg. However, for the actual service life of the materials, only age-related effects, such as material fatigue, are relevant but they only occur over the course of time.

Fromm is working on the „Zuverlässigkeit von PV-Modulen II“ ("Reliability of PV Modules II") project funded by the German Federal Ministry for the Environment to find a new procedure that can be used for predicting the service life of solar modules, "Our two-pronged principle combines real measurement data with a numerical simulation." He initially investigated how mechanical stresses affect the system in a field test. Snow loads, temperature fluctuations and wind gusts generate mechanical stresses on the modules. This leads to material fatigue. The plastic embedding material and the cell connectors that connect the solar cells together are particularly vulnerable. "This is just like repeatedly bending a paper clip. Eventually it will break", said Fromm.

Breezes cause vibrations in the module

Fraunhofer researchers equipped a complete solar module with sensors to record the impact on the material. During the evaluation, they noted that light winds are sufficient to generate vibrations in the module. The higher the ambient temperature, the more pronounced these vibrations are. Furthermore, the vibration frequency increases over time because the plastic material becomes more rigid and brittle from UV radiation. "The interesting question now is how these influences impact the service life of the components in the long term. This is where our simulation model comes into play," explained Alexander Fromm.

A detailed 3D simulation model of the solar module was created to do this. Numerical calculations can then be derived from the measurement results of the field test as to how environmental influences impact on the module components and what mechanical stresses the materials are under in the long term. "Thanks to the simulation, we have found, for example, that the UV-induced embrittling plays a far greater role in material fatigue than previously assumed," said Fromm. In order to predict the service life of a module, researchers combined measurement values from the field test with known strength characteristic values of the corresponding materials. Taken together, these figures show at what load the material is expected to break or peel off.

Improving geometry and materials

To create optimal and reliable forecasts, the developers need material characteristic data that is as detailed as possible and information about the geometry of the module to be tested. Based on their calculations, the researchers are not only able to make statements about the expected service life, but they are also able to see potential improvements in the geometry and materials and predict the effects of different materials on the mechanical stresses in the module.

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