Modeling and Analysis of the Operation of PV Power Generators Under Varying Atmospheric Conditions

Abstract

Photovoltaic (PV) technology permits us to harness and transform solar radiation into electricity. However, PV power generators are still a minor share in the global power generation capacity. One of the main reasons for it is that PV systems are greatly dependent on the atmospheric conditions affecting their operation. Furthermore, series connection of PV cells is prone to power losses when the electrical characteristics of the cells are dissimilar or the cells operate under non-uniform operating conditions. Especially during changing atmospheric conditions, the operation and control of PV generators is complicated and there is a demand for improvement since today’s inverters do not reach their best performance.

In this thesis, a state of the art inclusive thermal and electric simulation model of PV generators is proposed and validated with data measured at the Tampere University of Technology (TUT) solar PV power station research plant. The dynamic thermal and electric behaviors of the PV modules are first modelled separately theoretically based on previous authors’ work. Subsequently, these models are further improved by analyzing module temperature measurements and the electric behavior of the PV modules operating under varying meteorological conditions. All the relevant climatic and site specific parameters, heat transfer mechanisms and parasitic resistive effects are considered without major simplifications to obtain the highest possible accuracy. Finally, the separate thermal and electric models are integrated and the result is a comprehensive simulation model that predicts the thermal and electric performance of PV generators operating under varying atmospheric conditions. This simulation model is intended, among other things, to assist in the inverter design and development of maximum power point (MPP) tracking algorithms, especially to improve their efficiency and operation under non-ideal and fast changing environmental conditions.

Partial shading affects the electrical characteristics of PV generators, causing them to operate away from their MPP and thus complicating the task to reach the maximum power production. This task is normally carried out by the power electronic converters interfacing the PV generators. Furthermore, partial shading conditions generally cause mismatch losses too. In this thesis, a method to generate a spatial irradiance map from a set of irradiance measurements is proposed and utilized to analyze the effect of moving clouds on the mismatch losses on several PV generator configurations and layouts. The mismatch losses are studied for several sizes of generators in which both series and parallel connection of PV modules are considered. The results indicate that the mismatch power losses caused by non-uniform operating conditions due to moving clouds can be reduced by locating PV modules of the generator as close to each other as possible. Furthermore, parallel connection of PV modules should be favored with respect to series connection.