ABSTRACT An increased consciousness of the reliance on conventional patterns of energy at the advent of the 21st century has stimulated an increasing interest in sustainable development and renewable energies, such as photovoltaic systems. The overall efficiency of such systems is strongly affected by specific degradation modes that should be detected and diagnosed by applying responsible monitoring techniques. This study focuses on the effect of hot-spot heating, a discontinuity that is commonly reported during an integrated photo- voltaic system’s lifetime operation. In the presented approach, thermal images of an operating photovoltaic array, obtained by field infrared thermographic measurements, were analyzed by means of temperature line profiles and image histogram features. The results revealed the presence of several hot spots in the four modules of the array that were all associated with specific discontinuous cells. The quantification of each hot spot’s impact to the cell’s electrical characteristics is a key issue in evaluating the performance of each photovoltaic module under inspection. Thus, a simple simulation model based on the module’s equivalent electrical circuit was also developed to assess the overall module’s current and perform- ance deterioration due to each detected hot spot. KEYWORDS: photovoltaic module thermometry, thermal image analysis, solar cell discontinuities, hot-spot heating, simulated photovoltaic module modeling. Introduction Cost-effective photovoltaic applications comprise a major building block in the future of sustainable energy supply scenarios. In fact, significant global research focuses on achieving high utilization factors and optimum payback time for such applications by minimizing and isolating potential sources of any dysfunction, failure or abnormally subdued operation of a photovoltaic system (Botsaris and Tsanakas, 2010). Discontinuities, such as cracks or interconnection mismatches in solar cells, are a genuine problem for photo- voltaic modules (Figure 1). They are hard to avoid, and up until now, it has been basically impossible to quantify their impact on a module’s performance during its lifetime (Kontges et al., 2010). Ideally, discontinuous cells or cell strings are identified and rejected during the early stages of a module’s manufacturing process, using, for example, ultra- sonic techniques, thermal flux thermography or electrolumi- nescence imaging (Breitenstein and Rakotoniaina, 2005; Dallas et al., 2007; Fuyuky et al., 2005; Hoyer et al., 2009; Kasemann et al., 2006; Van der Borg and Burgers, 2003). However, even if this is done perfectly, new discontinuities may occur either within the stage of string/module assem- bling or during the lifetime of an operating module. Practically, the presence of a crack may have only a marginal effect on a new module’s operation, as long as the different parts of the discontinuous cell remain electrically connected. However, as the module ages and is subjected to thermal and mechanical stresses, the repeated relative movement of the cracked cell parts usually results in a complete electrical separation and, inevitably, in inactive cell parts and significant hot-spot heating effects. ME TECHNICAL PAPER w x On the Detection of Hot Spots in Operating Photovoltaic Arrays through Thermal Image Analysis and a Simulation Model by J.A. Tsanakas * and P.N. Botsaris † * Democritus University of Thrace, School of Engineering, Department of Production Engineering and Management, Faculty of Materials, Processes and Engineering, Vas. Sofias 12, Central University Campus, Building 1, Xanthi, 67100, Thrace, Greece; 30 2541079878; fax 30 2541079878; e-mail itsanaka@ee.duth.gr. † Democritus University of Thrace, School of Engineering, Department of Production Engineering and Management, Faculty of Materials, Processes and Engineering, Vas. Sofias 12, Central University Campus, Building 1, Xanthi, 67100, Thrace, Greece; 30 2541079878; fax 30 2541079878; e-mail panmpots@pme.duth.gr. APRIL 2013 • MATERIALS EVALUATION 457