Research Paper: Wind Turbines the Depleting Fossil Fuel

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Wind Turbines

The depleting fossil fuel, sky rocketing oil prices, polluting air and global warming facilitates the demand for pollution free renewable energy resources. Among the various sources of renewable energy, wind energy is visualized as much promising because the recent advances in technology have reduced its cost to a level wherein it has become competitive with other sources of conventional fuel. The wind turbines exists in varied sizes and in varied capacity to generate power, operating over a wide range of wind speeds and they can be established by individual owners of property or by a society grouped together to create a wind energy farm linked to a utility. (Rodman; Meentemeyer, 2006)

Over the decades, the United States and European countries are increasingly turning to wind power. Of late, wind power generation has witnessed considerable up scaling in terms of the size of individual turbine and the scale of typical projects. In case of the modern wind turbines of the multi-megawatt class, the nacelle height as also the rotor diameter comes to about 100m. Therefore at the vertical position, the blade tip can attain heights of about 150m. The German Enercon E-12 model of wind turbine having a rotor diameter of 112m is currently the largest wind turbine with a power generation capacity of 4.5MW. A diagram of the German Enercon E-12 wind turbine is shown in Exhibit -- I. (Slootweg, 2003)

(1) How Wind Turbines Work:

The working principle of a wind turbine includes two conversion processes that are done by two primary components viz the rotor and generator. While the rotor extracts kinetic energy from the wind and converts it into generator torque, the generator converts this torque into electricity and supplies it into the grid. A diagram showing the working principle is shown in Exhibit -- II. (Slootweg, 2003) a wind turbine is a complex system that draws knowledge from aerodynamics, mechanical, electrical and control engineering. For the generating system, almost every wind turbine in operation across the world use either one of the following system (i) Squirrel-cage induction generator (ii) Doubly-fed or wound motor induction generator and (iii) Direct drive synchronous generator. Squirrel cage induction systems were the first system consisting of a conventional, directly grid-coupled squirrel-cage induction generator. The rotor speed of this type of generator varies with the amount of power generated. (Slootweg, 2003)

These types of induction generators are capable of running at two different but constant speeds by altering the number of poles of the stator windings. The other two, i.e. doubly fed and the 'direct drive synchronous generator' makes use of variable speed systems and is used in variable speed turbines. For variable speed operation, the electrical grid frequency and mechanical rotor speed should be decoupled for which power electronics is being used. In case of the "doubly fed induction generator, a continuous voltage source converter feeds the three phase rotor winding." (Slootweg, 2003)

In this manner, the mechanical and electrical rotor frequency is decoupled and the rotor frequency and electrical stator are able to independently match the mechanical rotor speed. In case of the Direct-Drive Synchronous Generator, the generator is fully decoupled from the grid through a power electronics converter attached to the stator winding. The grid portion of this converter is a voltage source converter; the generator part could be a voltage-source converter or a diode rectifier. The direct drive generator is made functional through application of permanent magnets or an excitation winding. A diagram showing the working principles of the three types of wind turbines is given in Exhibit -- III. (Slootweg, 2003)

Now let us have an understanding of Horizontal Axis Wind Turbines -- HAWT. 'Horizontal axis wind turbines' are the successful wind energy harnessing machines in the world. The current trend towards high variety of wind turbine rotors brings new challenges to wind turbine manufacturers. These varied wind turbine rotors mainly designed for efficient field performance can tolerate high loads due to higher strength weight ratios. As a result, flow properties over the blade surface may affect their performance. (Rajendran; Madhu; Tide; Kanthavel, 2011)

Wind energy converters having their axis of rotation placed in a horizontal state are realized almost exclusively on the propeller-like concepts. Presently this is the prevailing design principle used in European wind turbines and American wind turbines. This is due to the reasons stated as follows (i) in case of design of propellers, the power output and rotor speed are capable of being controlled by pitching the rotor blades in their longitudinal axis i.e. blade pitch control. Besides, rotor blade pitching is by far the most effectual safeguard against over-speed and high wind speed, particularly in large turbines. (ii) the shape of the rotor blade can be optimized aerodynamically and has been demonstrated to attain its maximum efficiency in situations when the aerodynamic lift is tapped to a maximum degree. Together, these benefits are the reasons why almost all of the wind turbines for generation of electricity which has been manufactured to this day have horizontal-axis motors. (Hau, 2005)

Nothing expresses more regarding the potential of wind turbine than relating to its diameter -- i.e. 'the shorthand for the area which is swept by the rotor.' (Schaeffer; Pratt, 2001) the wind turbine with the bigger rotor will catch more of the speed of the wind and thus generate more electricity in comparison to a turbine having a smaller turbine with a smaller rotor, regardless of their respective generator ratings. (Schaeffer; Pratt, 2001)

(2) Comparison with one other technology (Solar energy technology):

Wind and Solar energy are renewable forms of energy which can be made use for generation of electricity. (Yiew; Singh; Singh, 2011) Similar to the wind energy, the solar energy is also one of the most promising sources of renewable energy considering its unlimited potentiality. (Ajao; Oladosu; Popoola, 2011) Solar energy has a lot of applications apart from the global greenhouse effect. Few among them are solar thermal factors such as solar distillation, flat plate collectors, microclimate greenhouse technology used for crop drying and generation of solar electric viz solar photovoltaic -- PV system. The Energy Payback Time -- EPBT for solar thermal and solar electric -- PV system is about 3 and 15 years respectively. (Tiwari, 2006)

The radiation of energy of the sun is measured to be about 3.8 x 10 KW per second. The radiation of most of this energy is conveyed through electromagnetic radiations that come to about 1.5Kw/m at the atmosphere's boundary. Passing through the atmosphere the surface of the earth gets about 1KW of solar power per square meter that averages mostly to .5 over all hours of daylight. Studies with regard to the availability of the solar energy source of power have depicted its feasibility for practical application. In case of solar photovoltaic applications, the solar radiation is transformed directly to electricity. (Ajao; Oladosu; Popoola, 2011)

The usual mode of performing this is by the application of silicon solar cells. The power generating unit is the solar module that involves various solar cells electrically connected on a plate platform. The major elements of a photovoltaic system incorporate the arrays that involves "photovoltaic conversion devices, their interconnection and support, power conditioning equipment" which transform DC to AC and supplies controlled generation of voltage and current, regulator, that normally handles the operation of the total system along with the additional storage for 'standalone (non-grid) systems.' (Ajao; Oladosu; Popoola, 2011)

Solar flat plate collector is amongst the most accepted and cheap collection system of solar energy to absorb solar energy, transform it into heat and then transfer the absorbed heat to a stream of liquid of gas. The collector plate is typically installed on the roof of building or any other structure having a greater height. Tracking of the sun is not required and it functions on low maintenance. This is capable of being used at high operating temperatures by only altering the configuration of the absorber through an evacuated tube and therefore it is called as evacuated tubular collector. Annual energy saving can be increased by (i) enhancing insolation (ii) increasing the time period sunshine exposure and (iii) reducing the total heat loss. (Tiwari, 2006)

Irrespective of its advantages, the barriers to solar energy are rather associated to what could be generally termed as "implementation." Particularly, they incorporate the following:

• Higher Costs: The concentrating solar power (CSP) and solar photovoltaic (PV) generating plants, to illustrate, generate electricity at costs considerably more than for electricity generated from wind or fossil fueled power plants. (Komor, 2009)

• Transmission: The supply of electricity from power plants to industries, cities, and other places are performed through transmission lines. Compared to fossil-fueled plants, utility-scale solar power plants are more remotely situated. They therefore, necessitate erection of new, costly and controversial transmission lines -- and this in reality has been found to be really difficult. (Komor, 2009)

• Intermittency/Variability: The source of solar energy is a variable one, intending that it varies with the changes in weather patterns, clouds and… [END OF PREVIEW]

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