Characterization and Analysis of Photovoltaic Modules and the Solar Resource Based on In-Situ Measurements in Southern Norway
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- Institutt for elkraftteknikk 
In broad terms, this thesis is concerned with outdoor testing of individual photovoltaic (PV) modules and characterization of the solar resource in Southern Norway. The work aims to evaluate the irradiance and temperature dependency of the efficiency for various types of PV technology (mostly crystalline-silicon, c-Si), and to get insight into and explain using device physics the performance differences within the generic c-Si class which have been reported in the literature. In addition, the thesis discusses and compares the local irradiation results obtained in this and earlier studies, and the predictions of the Photovoltaic Geographical Information System (PVGIS) of the European Commission’s Joint Research Centre (JRC). Two research installations have been built for regular measurement of currentvoltage characteristics (I-V curves) of ten c-Si and one thin-film CuInGaSe2 (CIGS) modules, and the key environmental variables (solar irradiance in the plane of the modules, ambient and module temperatures). Data recorded during one full year (2011) are used to characterize each of the tested devices in terms of identification of their I-V curve model parameters and their relative efficiency (with respect to that at standard testing conditions, STC). The solar resource in the city of Grimstad is analyzed from several perspectives including the annual irradiation, its seasonal distribution, magnitudes and durations of overirradiances due to cloud enhancement, the optimal azimuthal orientation of a PV array, and solar energy lost due to clouds. This is complemented by a study of local meteorological data from fifteen consecutive years which are publically available on the Internet. The work reveals and quantifies the opposite effects of the series resistance RS and the ideality factor n on a module’s performance at different levels of illumination. It shows the inevitable trade-off faced by PV manufacturers when optimizing the cell structure, e.g., using two instead of three front busbars in screen-printed c-Si cells increases the annual energy yield per kilowatt-peak installed at the expense of reduced STC power. These results contribute to a better scientific understanding of PV performance and to the industry by providing quantitative guidelines for product design. A key finding reported in the thesis is that cloud enhancement of sunlight can boost the plane-of-modules irradiance well beyond the extraterrestrial levels even at such a high latitude as Southern Norway, almost at sea level. A burst of 1528 W/m2 has been recorded in the year 2012. Statistics of overirradiance magnitudes and durations are presented. The work also shows that the clouds block nearly a half of the generally available solar energy in Grimstad, and that the useful fraction exceeds the predictions of the PVGIS by 10 %. These findings increase the scientific knowledge of the terrestrial sunlight in general and the PV potential in Southern Norway in particular. Furthermore, the thesis documents new or improved methods for I-V curve analysis and for measurement of the environmental variables: identification of model parameters; calculation of the equivalent cell temperature (ECT) from open-circuit voltage; and the use of the short-circuit currents of multiple co-planar PV devices to improve the accuracy of irradiance measurements. The thesis also demonstrates some limits of the applicability of the widely used 1- and 2-exponential I-V curve models.