Climate specific accelerated ageing tests and evaluation of ageing induced electrical, physical, and chemical changes

Abstract

As the PV market shows enormous potential with huge growth rates especially in climatic-sensible regions, specific artificial ageing test procedures are a key point for an efficient and fast product development of new PV modules/materials optimized for the use in specific climatic regions. Based on the definition of four climate profiles (dry and hot—arid, moderate, humid, and hot—tropical and high irradiation—alpine), a program was worked out with 14 climate specific test conditions for accelerated ageing tests. The big challenge in this respect was the adaption/advancement of existing standard procedures for PV modules/components testing in a way that reliable testing for certain climatic conditions optimized PV modules is possible. The time-dependent repeated application of combined climatic and environmental stresses (temperature, temperature cycles, humidity, irradiation, mechanical load, salt mist) was used to induce performance losses, material degradation, and failures in test modules which resemble those effects occurring in real-life PV installations under comparable climatic and environmental conditions. For this demanding task, a large number of identical test modules with respect to composition and module design was manufactured. A detailed nondestructive analysis/characterization of all modules was performed: (1) before; (2) during (six intermediate stages); and (3) after the accelerated ageing test. The nondestructive characterization methods used to follow the module's ageing processes throughout the whole accelerated ageing procedure were current-voltage characteristics measurements and electroluminiscence images for the electrical performance evaluation and ultraviolet fluorescence (spectroscopic and imaging) measurements, Fourier transform infrared spectroscopy as well as colour measurements of the backsheets outer layer for recording of chemical changes of the encapsulant and backsheet. The electrical and material characterization data were incorporated in an optimized database. As stated above, a set of three identical modules was exposed together in the respective climate specific ageing tests and subsequently characterized in order to increase statistical reliability of the measuring results. The analysis of the data and first approaches of advanced data treatment have already clearly shown that the electrical and material degradation of the test modules is dependent on (1) the type and combination, (2) duration, and (3) mode (sequential versus constant) of the stresses applied. On the one hand, the simulation of environmental stresses like heavy snow and wind load and enhanced frequency of temperature cycling resulting in cell cracks and cell connector breakage could be demonstrated. Additional treatment in salty atmosphere, on the other hand, did not show an accelerating effect on degradation on the electrical or material side. The accelerating effect of enhanced temperature, humidity, or additional irradiation on the degradation of power and materials could be shown very well. Direct evidence for the formation of acetic acid and lead-acetate in the encapsulant ethylene vinyl acetate after prolonged damp-heat exposure could be found and directly related to cell corrosion effects paralleled by power losses.