PVSEC Hamburg, 5 September 2011 – Potential induced degradation has recently been identified as a key factor impacting the energy yield of solar modules. Output of a solar module can be reduced if the module is exposed to high negative voltage between solar cells and the ground during operation, especially in major plants. Basically, this effect is reversible. It can also be avoided by implementing additional system design measures. The more economical option, however, is using a technology that provides for modules or cells that are resistant against PID.
A broadly accepted or even standardised test for PID resistance, which would facilitate quick and easy assessment, is not available at present. This also means that there is little information on how severely PID affects normal solar modules on the market.
Four independent institutes – the Fraunhofer Institute for Solar Energy Systems (ISE), Photovoltaik-Institut Berlin (PI-Berlin), TÜV Rheinland and VDE Testing and Certification Institute – and the German solar companies Q-Cells, Schott Solar and Solon have defined conditions to test crystalline PV modules for their PID sensitivity. These conditions were comprehensively checked for applicability and accuracy in separating PID-free products from items that are prone to PID.
The inspection was carried out at room temperature (25° Celsius) to provide a simple test that would not require expensive special equipment. According to the established parameters, negative voltage of 1,000 volts is to be applied to the cells of a module for seven days (168 hours) through the junction box.
The front panels of all modules are covered with aluminium foil or a continuous film of water and grounded, ensuring full comparability. A module will be deemed resistant against PID if power output decreases by less than 5% during the test. Solon had produced special modules that were prone to PID to confirm the test conditions’ relevance. Significant losses in output were observed when testing these modules.
When they were inspected at the institutes’ laboratories, PID-protected glass-foil standard modules from series production at Q-Cells, Solon and Schott achieved output reductions of less than 5%, proving that they are resistant against PID. A cross check with an unrepresentative sample of modules from other brand manufacturers procured in the market revealed a loss of output of significantly more than 50% for some modules. This means a preliminary test design has been developed, which assesses crystalline solar modules’ sensitivity to PID in a simple, specific and relevant manner and separates modules that are prone to PID from their PID-resistant counterparts.
However, it is not yet possible to draw conclusions on the long-term performance of installed modules on the basis of these test conditions, as it will largely depend on module wiring and climatic factors. The involved institutes and companies are committed to further developing PID tests into a general standard.