Outdoor weathering of PV modules — Effects of various climates and comparison with accelerated laboratory testing

Understanding the degradation behavior of PV modules during long-term operation and developing realistic and effective procedures for accelerated testing that can simulate 25 years of operation requires a reliable database for outdoor weathering and for accelerated testing in the laboratory. To assess the effects of real-life weathering, we set up four test sites covering a wide range of climatic effects in terms of irradiation, ambient temperature and relative humidity. The test sites are located in Cologne, Germany (moderate climate), in Serpong, Indonesia (tropical climate), on the Zugspitze in the German Alps (high mountain climate) and in Sede Boqer in the Negev Desert of Israel (arid climate). The monitoring program surveyed the relevant meteorological parameters and temperatures of various crystalline silicon PV modules for open-rack mounting [1]. Our database covers a period of 3 years and suffices for comparing the weathering effects in the different climates. The database also allows comparisons of load levels under real-life conditions and under the test conditions during the accelerated testing of PV modules as defined by test standard IEC 61215. In an experimental study we subjected various crystalline silicon PV modules to extended thermal cycling and damp heat-testing, exceeding 200 cycles and 1000 hours respectively, in order to assess weaknesses in the module construction. Extended qualification testing revealed enhanced degradation after 2000 hours of testing with damp heat and after 400 cycles in thermal cycling. Standard IEC test conditions therefore do not suffice for determining weaknesses in module construction. The correlation between accelerated laboratory testing and outdoor weathering still requires further research. One particular problem is the question of tolerance ranges in product quality during module production.

INTRODUCTION

During their lifetime PV modules are exposed to various harsh environmental conditions that can affect their functionality. The major processes in PV module degradation are: •

Diffusion processes, in particular water vapour ingress in the encapsulation. The long-term effect is corrosion leading to the deterioration of the electrical contacts.

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Thermo-mechanical stress caused by the alternation between day and night, causing cracks in the electrical circuitry or delamination at the interfaces between materials due to variations in the coefficient of thermal expansion.

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Photo-degradation and thermal degradation of polymers leading to a loss of elasticity, optical transmission, bonding strength and insulation properties.

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Static and dynamical mechanical stress caused by snow or wind.