TECHNIQUES FOR IMPROVING THE EFFICIENCY OF WIND TURBINES Rajnish Anand 1, Himanshu Kumar Mohit 2.

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1 International Journal of Mechanical, Robotics and Production Engineering. Volume VI, Special Issue, 2016, ISSN , TECHNIQUES FOR IMPROVING THE EFFICIENCY OF WIND TURBINES Rajnish Anand 1, Himanshu Kumar Mohit Department of Mechanical Engineering, R.V.S.C.E.T, Jamshedpur. ABSTRACT: One of the major issues for the future of humanity is ensuring the continuation of energy Supply. Conventional fossil fuels have been the main source of energy of the modern Society for the last two centuries, while at the same time they have created devastating prospects for the environment. Inevitably, the existing and undiscovered reserves, will be depleted or become too expensive to extract. A future that relies on renewable energy will guarantee the future of the generations to come. Wind energy has gained prominence as a means of generating electricity without emitting air pollutants or greenhouse gases. As the wind spins a wind turbine's blade assembly, known as a rotor, a generator connected to the rotor generates electricity. Large wind turbines generate electricity at a lower cost and higher efficiency than smaller ones, because longer rotor blades capture the energy from a larger cross-section of the wind, known as the rotor-swept area, and because taller towers generally provide access to stronger winds. The greater and more consistent the wind, the more electricity is produced. Two research directions of wind turbines performance are pursued, optimizing a wind turbine performance and optimizing a wind farm performance. The goal of single wind turbine optimization is to improve wind turbine efficiency and its life-cycle. The performance optimization of a wind farm is to minimize the total cost of operating a wind farm based on the computed turbine scheduling strategies. The methodology presented in the dissertation is applicable to processes besides wind industry. Keywords: Turbine Efficiency, Performance, Optimization, Control Model, AFC (Air Flow Control). [1] INTRODUCTION Wind power has been growing dramatically since the beginning of the 21st century. The global installed capacity at the end of 2013 was around 318 GW, up from 18 GW at the end of the Around 35 GW of new wind capacity were added in 2013, the lowest growth since 2008, after 44 GW in Over the last couple of years, wind s centre of growth has been moving from Europe and North America to Asia, which emerged as the global leader. In fact China Rajnish Anand and Himanshu Kumar Mohit

2 became the first market in terms of total installed capacity in a very short time, overtaking the United States in A number of recent important policy measures and programs have emerged in support of the expansion of the wind market, and many of the new policy developments concern offshore wind. The contribution of wind power to the energy supply has reached a substantial share even on the global level. According to World Wind Energy Association (WWEA), based on the current growth rates, in 2016, the global capacity is expected to increase up to 450,000 MW. By the end of 2020, at least 700,000 MW are expected to be installed globally. Recently, thanks to the development of small wind turbines, quiet and specified for urban use, it is possible to harness wind power for on-site energy generation or domestic production or agricultural districts. These turbines, reaching a maximum of 20 kw of power, can also find space on rooftops or in gardens; they have relatively little visual impact and are able to produce energy even from modest wind flows. Moreover, in contrast to large wind turbines, such plants do not require major infrastructures for electricity transmission from utilities and lend themselves to distributed generation of electricity. Small wind systems can be used both as stand-alone systems or as grid connected systems, and both can be paired with other energy conversion systems, such as photovoltaic. [2] VARIOUS TECHNIQUES TO IMPROVE THE EFFICIENCY OF WIND TURBINE (a) VAWT USING CYCLOIDAL BLADE SYSTEM CWT (Cycloidal Wind Turbine) introduces the cycloidal blade system to the classical VAWT. Figure 1 shows the cycloidal blade system that has been studied at the University of Washington, NACA, etc. since the early 1900s. It produces thrust by varying the cyclic pitch angle of several blades rotating in the direction parallel to the rotating axis. CWT differs from the classical Darrieus rotor, which use a blade of fixed pitch angle. It maintains efficient power generation by active blade control according to the change of wind direction and wind speed. Pitch angle and phase angle can be controlled in the cycloidal blade system. This paper describes the research for the development of the 1 KW class wind turbine. Fig 1 - Cycloidal blade system Rajnish Anand and Himanshu Kumar Mohit 2

3 In Figure 1, when the wind direction is upper part to lower part, the rotor rotates in the counter clockwise direction. Pitch angle θ is the angle between the tangent line and the camber line of blade. It is varied periodically and maximized when the angle φ is 90* and 270* at the setting of Figure 1. At this position, maximum pitch angle is defined; η is the phase angle and defined as the angle between the wind direction and the line connecting the two positions of the maximum pitch angle. It is increased in the counter clockwise direction. Figure 2 shows the basic concept of the control mechanism. The magnitude of eccentricity e is defined as the distance from the centre of rotation O to the point of the eccentricity point P, as shown in Figure 2. The phase angle of eccentricity ε is defined as the angle between the line OP and the vertical line. The magnitude and the phase angle of the eccentricity are used to adjust the magnitude and direction of the rotor. Fig. 2 - Basic concept of CWT control mechanism (b) ACTIVE FLOW MODIFICATIONS ON WIND TURBINE BLADES The invention relates generally to the field of wind turbines, and more specifically to active flow modification in wind turbines for reducing loads, reducing aerodynamic losses, improving energy capture, reducing noise, and combinations of there.wind turbines are increasingly gaining importance in the area of renewable sources of energy generation. In recent times, wind turbine technology has been applied to large-scale power generation applications. Of the many challenges that exist in harnessing wind energy, one is maximizing wind turbine performance while minimizing system loads in given wind conditions. Non-limiting examples of improved wind turbine performance parameters, which lead to minimized cost of energy, include maximized aerodynamic efficiency, maximized energy output, minimized wind turbine system loads, minimized noise, and combinations thereof. Examples of wind turbine system loads include extreme loads (operational and parked/idling) and fatigue loads. In general, flow separation over wind turbine blades leads to stall, which is often a limiting factor in wind turbine blade design. When stall occurs, lift generated by the blade decreases significantly and a large component of the torque, which is the driving force imparted by the wind to the wind turbine, is lost. Solutions that provide an ability to control (diminish or delay) separation will allow the wind turbine blade to maximize lift. Some passive flow control solutions, for example, vortex generators, have been applied to remedy the boundary layer separation problem, but in such solutions there is no provision to stop the flow control when the flow control becomes unnecessary or undesirable. Rajnish Anand and Himanshu Kumar Mohit 3

4 For example, one of the principal constraints in wind turbine design is that caused by system loads. When a separation control solution is being used to enhance lift, the blade experiences higher loading that can reach failure-inducing levels if the wind conditions change beyond normal operational or expected levels. One effective approach for increasing the energy output of a wind turbine is to increase the swept area of the blades, for example, by increasing rotor size (diameter). The higher systems loads on a larger rotor (thicker and larger chord length) due to structural and material limitations, and blade/tower clearances typically constrain this growth. Another challenge is posed by changing wind conditions such as wind gusts or storms that lead to an undesired loading of the wind turbine blade as the lift being generated fluctuates or increases to very large levels. These loads constraints often lead to increased cost of the blade and other components of the wind turbine system, which can reduce or cancel the benefits of growing the rotor in terms of a system-level metric like cost of energy. (c) Active Flow Control Techniques Flow control, in contrast to turbine control, improves the performance of wind turbines by not manipulating the loads caused by the airflow, but rather by manipulating the flow around the blades. The main focus is to reduce extreme loads and to mitigate fatigue loads along the blade. In other words, the objective of flow control techniques is to change the local aerodynamic characteristics of each of the air foils that compose the blades. Fundamentally, this is achieved by adopting one of the following approaches:- a) delay or advance laminarto-turbulent-transition, b) Suppress or enhance turbulence and c) Prevent or promote separation. Active load control or active flow control (AFC) is the manipulation of the airflow around a blade with the use of external energy in order to improve the aerodynamic characteristics of the air foil. In order to achieve this, a network of smart devices including sensors, controllers and actuators is placed along the span of the blade. AFC developed as the size of wind turbines was growing, making load control essential for rigidity of the structure. The aerodynamic performance of an air foil depends on its shape and angle of attack which define its polar: lift coefficient (C1) and drag coefficient (Cd) as a function of Reynolds number. This is known as lift curve. It is important to clarify that an AFC device is considered successful when it can effectively mitigate loads generated on the blades. According to this argument a preferable AFC device is the one that can manipulate the aerodynamic characteristics for a large range of angles of attack and achieve the maximum change in lift coefficient for that range. This is irrelevant to whether the change in lift refers to increase or decrease compared to the original state. C1 Delay Stall ΔC1 C1 Inc. Lift Rajnish Anand and Himanshu Kumar Mohit 4

5 Dec. Lift (a) α (b) α Figure 3: Adjustments in lift curve due to active flow control techniques: a) DS devices, b) I/D devices When considering the lift curve, two possible approaches (figure 3) can be detected, in which AFC devices are able to adjust the lift curve: By shifting the entire lift curve upwards or downwards in respect to the baseline curve. A complete redesign of the physical shape of the airfoil (camber) is required. By increasing the critical lift point by a fraction ( Cl), past the stall angle. In other words these techniques delay separation by delaying stall and eventually they reattach a naturally detached flow. The AFC devices should have certain characteristics in order to be considered effective: They should be small and scalable. They should have fast response time. They should require low power input. They should be effective under wide range of operating conditions. They should be reliable and have low maintenance cost. In case they fail, they should not affect the proper turbine operation. Also, they should be easily and inexpensively replaced. (c) PASSIVE CONTROL TECHNIQUE: In passive flow control the turbine is able to adjust against changes in the flow passively, without the need for external power input. They typically consist of surface modifications that can be either fixed or freely moving Rajnish Anand and Himanshu Kumar Mohit 5

6 components. Their activation is caused by excitation of the surface flow. Early small wind turbines of a few KWs were using passive stall control having blades at a fixed pitch angle. As the wind speed was increasing, so did the angle of attack, eventually leading to stall. Passive control techniques are simple but can be effective for a narrow range of operating conditions. (d) PLASMA ACTUATUR:- Plasma is the state of ionized matter which is a mixture of free electrons, positive ions and neutral particles. The generation of plasma in air can be created by a strong electric or electromagnetic field. Both require a substantial amount of external energy in order to excite the molecules from a stable state to electronic state. The goal of plasma actuators is to control separation by reattaching a naturally detached airflow or by detaching a naturally attached airflow. To do so, they modify the airflow profile in the boundary layer by accelerating the airflow tangentially and very close to the wall. When plasma is generated in the leading edge of an air foil operating in the past-stall range of angles of attack, the separation point moves further towards the trailing edge. The result of increased attached flow around the air foil surface due to plasma effect leads to a consequent increase in the critical lift C1. The aimed manipulation of this C1 along the turbine blades can be used as AFC mechanism. There are several types of plasma actuators, but four are the most commonly used for research purposes: DC surface corona discharge plasma actuators. Single Dielectric Barrier Discharge (SDBD) actuators. Sliding discharge actuators. Directional plasma wall micro-jet actuators. In principle, a plasma actuator installed on a surface, transfers high momentum fluid from the free stream into the boundary layer. When a high voltage difference is applied between two electrodes in air medium, an electric field is created. This leads to the generation of charged particles in the electrodes. Due to Coulomb forces the positive ions attract to the negative ones and vice versa. During their motion they collide with air molecules of the boundary layer and exchange momentum. As a result, they induce a flow between the electrodes, called ionic wind or electric wind, composed of charged particles and air molecules. This ionic wind (plasma) adds momentum to the flow and ultimately modifies the boundary layer profile of the surface. Rajnish Anand and Himanshu Kumar Mohit 6

7 Figure 4: Sketch of the airflow behavior generated by plasma actuator operation: (left) wall jet formation under quiescent air conditions and (right) manipulation of an existing boundary layer Fundamentally, if applied in stationary flow field (ambient air) the plasma actuator acts as a wall jet above the electrodes along the actuator surface. Then, according to the measured average ionic wind induced over this surface is typically around 7m/s at a distance of 0.5mm from the wall. The generated body force is around 0.15 MN per Watt of input electrical power. When applied in a typical flow it can be used as flow control mechanism. [3] ADVANTAGES The concept of plasma actuators for AFC is a relatively novel technique for aerodynamic applications. They have received considerable attention and extensively studied in the recent years due to their advantages over fluidic and mechanical devices:- 1.Their main advantage is that they directly convert electric energy into kinetic energy which is capable of instantly modifying the flow. 2. Another very important aspect is that they have no moving parts which significantly improves reliability and eliminates the probability of them being source of vibration or noise. 3. A key advantage is their fast response time that enables a real-time control over a broad range of actuation frequencies (wide bandwidth). 4. Plasma actuators are robust and lightweight devices that have low profile when mounted in aerodynamic surfaces. 5. Other aspects that have drawn attention for flow control research are their simple integration on different structures, as well as their low cost and low power require- mints. [4] DISADVANTAGES 1. Low performance in flow control for high wind velocities. 2. Low conversion efficiency from electric to kinetic energy. A large fraction of input energy is lost in the form of gas heating losses. 3. High voltage requirements. 4. Degradation of dielectric material for SDBD plasma actuators. Rajnish Anand and Himanshu Kumar Mohit 7

8 5. Generation of extraneous gases such as ozone in the air. [5] CONCLUSION The above discussed techniques in this paper are used to improve the efficiency of wind turbine. The plasma actuator is the latest technique to enhance the efficiency of wind turbine. These techniques are cost effective and economical. By using these methods the utilization of wind turbine will be automatically increase all around the globe and this can lead to the advancement of this type of energy source. The effective and economical techniques suggested in this paper can boost the utilization of non- conventional source of energy and decrease the dependence on conventional source of energy and fossil fuels. REFERENCES [1] Master of Science Thesis ACTIVE FLOW CONTROL USING PLASMA ACTUATORS APPLICATION ON WIND TURBINES by Athanasios Dialoupis dated 25 August [2] Paper on ACTIVE FLOW MODIFICATIONS ON WIND TURBINE BLADES By Seyed Gholamali Saddoughi, Anurag Gupta and Philippe Giguere, published on 10 June [3] Rajnish Anand and Himanshu Kumar Mohit 8