STUDY ON THE PERFORMANCE OPTIMIZATION OF THIN FILM SILICON AND BIFACIAL PHOTOVOLTAIC MODULES THROUGH OUTDOOR VARIABLES

Main object of this Ph.D. work has been the performance optimization of thin film Silicon and bifacial photovoltaic (PV) devices, by means of an in-depth study of their behavior by varying the outdoor variables affecting their electrical performance. The first part of such work has been focused on the study of performance instability phenomena affecting tandem amorphous/microcrystalline Si mini - modules under variable working conditions in terms of illumination, voltage stresses (both in DC reverse bias and in direct bias in Maximum Power Point) and temperature. An accurate analysis of the causes of such phenomena has been carried out, by thoroughly studying the effects on the performance of such PV devices in actual outdoor operation, due to variations of solar spectrum and of solar cell operating temperature. The relevant role played by the latter outdoor variable on the power conversion efficiency of such devices has been highlighted and demonstrated. Considering the crucial importance which the study of electrical behavior of high efficiency bifacial PV devices is currently assuming, essentially due to the fast development and diffusion of such promising devices, the second part of this work has concerned the performance optimization of bifacial PV systems. For such purpose, a novel analytical model to accurately predict the electrical performance of bifacial PV modules, usable for any geographical location, has been developed and its accuracy validated through experimental tests outdoor. This has allowed, on one hand, to analyze in-depth the behavior of bifacial devices and their performance optimization, by varying the main outdoor variables which determines such performance in real operating conditions. On the other hand, it has been possible to predict the electricity generation of bifacial PV systems with and without solar trackers for different geographical locations, by highlighting whether and where the solar tracker adoption is convenient for optimizing the performance of these systems. This Doctoral thesis consists of eight chapters, in which the main topics are treated as specified below. Chapter 1 constitutes an introduction to such work. Initially, the crucial importance of the renewable energy sources’ exploitation for a sustainable development is highlighted and discussed, by introducing the efforts made so far in such context and the future challenges. Subsequently, attention is paid to the photovoltaic technology, which is one of the most attractive and advanced renewable energy technologies. An overview on the current development and future perspectives of such technology is presented. Here, the different generations of PV devices available today (both in the market and at research and pre-commercial demonstration phase) are described, by highlighting the ones considered most promising. Afterwards, a focus is made on the PV devices which are the main subject of this thesis, namely tandem amorphous/microcrystalline Si solar cells and silicon based high efficiency bifacial solar cells. The main features of such devices and the related open issues are introduced. Finally, the motivations which have led to the present thesis study and what are its fundamental goals are described. Chapter 2 summarizes the main findings of previous studies in the scientific literature regarding performance instability phenomena of amorphous silicon-based solar cells, in order to present an overview on this topic and facilitate the reading of the following chapter 3. After a focus on the well-known Staebler-Wronski Effect and on the main thermal effects able to influence the performance of the mentioned solar cells (by allowing their recovery/improvement), a synthesis of the already known basic dynamics allowing to improve the electrical performance of such cells is provided. In this regard, we focus on the effects (observed during experiments indoor) triggered by the application of DC reverse bias stresses to such PV cells under illumination. Finally, some instability effects observed on the mentioned devices, induced by solar spectrum variations, are illustrated. Chapter 3 is focused on the results of new experimental activities, carried out in this work on tandem a-Si/µc-Si photovoltaic mini - modules, with the main goal of further investigating the performance instability phenomena found in the literature on a-Si based solar cells (summarized in chapter 2). Firstly, we carefully explain the aims of such activities and we describe the experimental setups prepared and used, in addition to the experimental methods applied. Subsequently, the main findings of the experiments performed are highlighted and discussed in depth. Initially, we present the results of the experiments performed by subjecting different groups of specimens of the mentioned mini - modules to different reverse bias DC stress conditions (both outdoor under the sunlight, and indoor under the illumination provided by a solar simulator). Here we show the effects induced on the specimens by such stress conditions and we discuss on the convenience to apply them. Then, we show and discuss performance instability phenomena observed on stabilized specimens working outdoor in their Maximum Power Point, under variable temperature and illumination conditions. Finally, the causes of such phenomena are individuated and thoroughly analyzed, by studying the effects on the mini – modules performance induced by solar cell temperature and solar spectrum variations. Chapter 4 outlines the major conclusions based on the experimental research activities carried out on tandem a-Si/µc-Si photovoltaic mini - modules, illustrated and discussed in the previous chapter 3. In particular, we focus on the relevant rule played by the PV cell temperature variations on the electrical performance of the mentioned devices, by highlighting the power conversion efficiency improvements due to the cell thermal annealing, obtainable in hot climates. Chapter 5 concerns a state of the art regarding the modeling of bifacial photovoltaic devices performance and their behavior. An introductory section contains general considerations about the relevance of accurately predicting the electrical performance of the mentioned devices and illustrates the fundamental general structure of the main models so far proposed in the scientific literature. In the following sections, an overview regarding the optical, thermal and electrical modeling of bifacial solar devices is provided, as proposed in different studies in the literature. Contextually, the main findings of such studies, concerning the behavior foreseen for bifacial PV devices (by varying the main parameters able to affect their performance), are summarized. Finally, a focus is made on the accuracy so far achieved by the main models in the literature. Chapter 6 is dedicated to the activities of development and experimental validation of a novel model for predicting the performance of bifacial photovoltaic module and to the presentation of the main simulation results provided by such model. Initially, the model proposed is described. Afterwards, the main validation results of such model are shown, by highlighting the very good agreement observed among the simulated results and the experimental ones (obtained by experiments performed outdoor in Sicily, on a bifacial Silicon mini - module consisting of four solar cells in series). Contextually, the results obtained by varying the main parameters affecting the electrical performance of a bifacial PV module (i.e. its elevation from the ground, its tilt angle, and the ground reflectivity) are illustrated and commented in detail. Chapter 7 concerns the energy generation prediction of bifacial and mono-facial PV strings with and without solar tracker, installed at different geographical locations. Once the needed upgrades to the model presented in chapter 6 and the main assumptions for the calculation of the mentioned energy generation are specified, the most relevant results provided by the model are presented and discussed. In particular, we present a detailed investigation on perimeter effects able to influence a bifacial PV string performance as well as on the effect induced by the latitude on such performance. Contextually, the convenience of adopting uniaxial horizontal solar tracker for the mentioned PV strings is investigated too. Chapter 8, as conclusive section of this Doctoral thesis, summarizes the most relevant findings of such Ph.D. work, based on the global set of the experimental activities carried out, providing novel information to understand the electrical behavior both of tandem amorphous/microcrystalline silicon PV mini - modules and high efficiency silicon based bifacial PV devices and systems. Finally, some recommendations and possible future developments of this work are provided. This work has been prevalently carried out in collaboration with the researchers of the Energy Conversion Device Group of CNR-IMM Catania Headquarter under the supervision of Dr. Salvatore Antonino Lombardo. Concerning the PV devices manufacturing and providing, the research activity has benefited from the collaborations with Enel Green Power and in particular with the 3SUN R&D group leaded by Dr. Cosimo Gerardi. The Ph.D. candidate Fabio Ricco Galluzzo, during the Ph.D. course, attended numerous educational and seminar activities on characterization techniques of materials and devices, both at the University of Catania and at the University of Palermo.