ANALYSIS OF WEATHERED c-Si PV MODULES D. Chianese, A. Realini, N. Cereghetti, S. Rezzonico, E. Bur, G. Friesen, A. Bernasconi LEEE-TISO, University of Applied Sciences of Southern Switzerland (SUPSI), DCT Via Trevano, 6952 Canobbio, Switzerland Phone: +41/91/935 13 56, Fax: +41/91/935 13 49, www.leee.dct.supsi.ch ABSTRACT The LEEE-TISO testing centre for PV components has, since 1991, carried out systematic tests, under real operating conditions, on the most important modules currently on the market. PV c-Si modules coming from production shows a typical degradation in performance when exposed to light of up to -5%. This initial degradation of c-Si modules takes place during the first hours of exposure. During the first year of operation some of the tested modules shows almost no further degradation and some a very small degradation. The long term degradation of c-Si modules has been extensively studied on one of the LEEE-TISO plants. In 21 years of service, the Arco Solar ASI 16-2300 modules of the 10 kW TISO plant showed several signs of physical degradation. Yellowing of PVB encapsulant and hot-spots affected the module efficiency. Nevertheless, the results of the indoor performance measurements of all plant modules indicate that the ASI 16-2300 modules are still working in a very satisfactory manner. 1 INTRODUCTION At the LEEE-TISO Test Centre, the most commonly sold modules on the market undergo a series of tests in order to examine their quality and reliability in terms of energy production and power degradation over time [1,2]. The laboratory is also monitoring the oldest grid- connected PV power plant in Europe (TISO 10kWp sc-Si). The two test installations allowed to observe degradation mechanisms that take part in c-Si modules in short- medium- and long-term. 1.1 The LEEE-TISO Test Stand In the test stand (see Figure 1), the modules were selected from those most commonly found on the Swiss market or which had interesting innovations. In order to guarantee impartiality and neutrality regarding measurements, the modules were purchased anonymously. Two modules for each type were acquired. The LEEE-TISO testing procedure includes regular indoor measurements at standard test condition (STC). The performance measurements of all modules are repeated every 3 months. Before 2001 the indoor measurements has been executed at the ESTI laboratory of the Joint Research Centre in Ispra. Since 2001 The LEEE- TISO laboratory has his own class A solar simulator, accredited ISO 17025, to execute indoor measurements at STC. The modules of the test stand are measured indoor before installation (Pa), after a first outdoor exposure of 20kWh/m2 (P 0 ) and then every 3 months. A complete test cycle, executed under real environmental conditions and at maximum power point operation, has a duration of 15 months. In this paper results about performance degradations of the test cycles 5, 6, 7, 8 and 9 will be presented. In the test period going from 1997 until 2003 98 modules of 48 different types has been tested for at least 1 year. 1.2 The 10kWp TISO plant In May 1982, the first European grid-connected PV plant came into operation (see Figure 1). The first objective of this array was to provide a technologically advanced facility of medium size, giving practical information for the planning of future photovoltaic plants. In April 2000, in proximity to the 20-year design life of the plant, a collaboration between the LEEE-TISO and the ESTI Laboratory (JRC Ispra) started to determine the Mean Time Before Failure (MTBF) of the system and to investigate the physical degradation mechanism in action. Figure 1: Test stand of the LEEE-TISO testing centre (left) and the TISO 10kWp grid-connected PV power plant (right). 2 DEGRADATION AFTER FIRST EXPOSURE Almost all standard c-Si PV modules tested in cycle 5 to 9 showed a degradation in performance when exposed for the first time to sun light. Such power degradation occurs during the first hours of exposure (H=2.5 kWh/m2) In order to avoid first degradation effects influences the determination of module energy yield, the test procedure of the LEEE-TISO laboratory, has been modified in 2001 adding a period of light soaking of 20kWh/m 2 followed by 3 months of stabilisation [1]. In this first outdoor exposure period the mean degradation of the 98 c-Si modules tested at the LEEE-TISO is -3.0%. The degradation is between -0% and -5% with respect to the initial power P a [1]. The performance loss is mainly due to degradation of the short circuit current Isc (see