Power Electronics in Renewable Energy Systems

Over the past century, global energy consumption has experienced a consistent upward trend. According to official estimates, global energy consumption is projected to rise by 44 percent between 2006 and 2030 [1]. Fossil fuels, including liquid fuels, coal, and natural gas, can be regarded as the predominant energy sources of the modern world. The ongoing processes of urbanization, industrialization, and the expanding access to electricity have resulted in an extraordinary reliance on fossil fuels. Currently, the primary issues associated with fossil fuels are the emission of greenhouse gases and the irreversible exhaustion of natural resources. According to the US Government's official Energy figures, worldwide carbon dioxide emissions would increase by 39% between 2006 and 2030, reaching 40.4 billion metric tons [1]. Greenhouse gas emissions, as well as the threat of global warming and the depletion of fossil fuel supplies, have highlighted the significance of alternate and greener energy sources. The current electricity transmission and distribution infrastructure is also significantly impacted by the search for cleaner and more dependable energy sources. Transmission lines are typically used to generate and distribute the majority of the power to the significant load centers. Power was always transferred from the utilities to the customers in a single direction. Renewable energy sources alone cannot sustain the entire grid in the near future [1]. They must be connected to the main grid to serve as auxiliary power sources, lessening the load on major power producing units. They could also be used to serve load units that are separated from the main grid. A distributed power generation (DG) system is one that uses wind turbines, fuel cell-based sources, micro generators, and solar systems in addition to the main power grid. End users in a distributed generation system do not have to be passive consumers; they can also be active grid suppliers. Traditionally, big power generation units (often made up of synchronous generators) monitor and manage critical power delivery characteristics such as frequency and voltage. In DG systems, the power electronic interface must regulate the voltage, frequency, and power to connect the energy source to the grid. The emphasis will be on high power density, robust dc-ac and ac-ac modules with sophisticated control and safety requirements. This paper provides some of the power electronic interface requirements as they apply to wind, fuel cell, and solar power production units, as well as a qualitative examination of the existing power electronic topologies that can be utilized. Energy storage is also critical for DG; however, this study focuses primarily on the power electronics components of DG. Section II provides an overview of wind power generating and its attendant issues. Sections III and IV include overviews of power generation using fuel cells and photovoltaics, as well as the implications for the accompanying power electronic circuits. Section V offers the conclusion.

Wind Energy Systems

The majority of renewable energy is produced by wind [1], [3]. The cost of power generated by wind energy-based systems has decreased to one-sixth of its value in the early 1980s, and grid-connected wind capacity has more than doubled over the past 20 years [3]. A wind energy conversion system's salient characteristic is:

• Available wind energy

• Wind turbine type

• Electric generator and power electronic circuits for grid connection

Fig. 1. Variable speed wind energy conversion system

Wind energy – Wind speeds, air pressure, atmospheric temperature, earth surface temperature etc., are highly inter-linked parameters. Due to the inherent complexity, it is unrealistic to expect an exact physics-based prediction methodology for wind intensity/sustainability. However, distribution-based models have been proposed, and employed to predict the sustainability of wind energy conversion systems [4]. This document is not intended to provide a detailed description of wind energy resources. According to studies, the variance in mean output power over a 20-year period has a standard deviation of less than 0.1 [4]. It is plausible to conclude that wind energy is a reliable source of clean energy. The basic physics of wind energy can be described as follows. Wind power can be extracted as

where P is Power extracted from the wind (Watts), ρ is Air density (typically ~1.225 kg/m³ at sea level), A is Swept area of the turbine blades (A=πr2

The tip speed ratio is defined as the ratio of the tip speed to wind speed.

Where ω = Angular velocity (radians per second), R is Rotor radius (length of the blade, in meters) and RPM is Rotations per minute of the turbine [3].  Cp? versus λ characteristics are important indicators of the aerodynamic efficiency of a wind turbine. Based on the aerodynamic blade theory, there is an optimal λ opt corresponding to the maximum power. Wind turbines are characterized as drag-based or lift-based, depending on the aerodynamic concept used. Wind turbines are characterized as horizontal or vertical axis based on their mechanical structure. Wind turbines are characterized as fixed or variable speed turbines based on the rotation of the rotor. The current focus is on horizontal axis, lift-based variable speed wind turbines [2], [3]. Power electronic circuits play an important part in variable speed wind energy conversion systems. Fixed-speed wind turbines are simple to operate, dependable, and durable. However, the grid frequency determines the speed of the rotor. As a result, they are unable to maintain the optimal aerodynamic efficiency. Fixed-speed wind turbines are unable to determine the optimal power extraction point Cp? max when wind speeds vary. In variable speed wind turbines, power electronic circuitry partially or entirely separates the rotor mechanical frequency from the grid electrical frequency, allowing for variable speed operation. The power electronic (PE) interface's needs are determined by the type of electric generator used and grid conditions. Figure 1 shows a variable-speed wind energy conversion system. Doubly-fed induction generators are often used in partially variable speed wind energy conversion systems [5].  Figure 2 displays a doubly-fed induction generator in which the power converter system controls the rotor circuit via slip rings while the stator circuit is connected to the grid. This strategy is useful because the power converter must handle around 25% - 50% of the system's total power [5]. As seen in Fig. 2, the power converter system consists of a rotor-side ac-dc converter, a dc link capacitor, and a dc-ac inverter connected to the grid.  The power converter enables vector control of the field which facilitates active/reactive power control.