Question 3 and 4 1. List and explain different wind turbine types. Use electrica

Question

Question 3 and 4

1. List and explain different wind turbine types. Use electrical system designs to briefly describe their functionality. Outline the advantages and disadvantages of these turbines. Which turbine would you select for a stand-alone operation supplying to an island with a constant wind speed? Explain with reasons. (20 Marks) 2. What are the three basic steps to start generation using a wind turbine? Describe your answer for a full scale wind turbine. (10 Marks) 3. Answer the following with regards to wind turbine components: (a) What factors decide the size of an electric generator? Why would you choose an Interior Permanent Magnet generator for a full scale wind turbine? (10 Marks)

Explanation / Answer

Q.1 List and explain different wind turbine types. Use electrical system designs to briefly describe their functionality. Outline the advantages and disadvantages of these turbines. Which turbine would be selected for a stand-alone operation supplying to an island with a constant wind speed?

Explain with reasons.                                                            

Q.2 What are the three basic steps to start generation using a wind turbine?

Describe your answer for a full scale wind turbine.

Q.3 Answer the following with regards to wind turbine components (a) What factors decide the size of an electric generator?

Q.4 Why would you choose an interior permanent magnet generator for a full scale wind Turbine?

Answer –

Ans. Q.1 –   

TYPES OF WIND TURBINE WITH THEIR FUNCTIONS –

Wind turbines can be separated into two basic types determined by which way the turbine spins. Wind turbines that rotate around a horizontal axis are more common (like a wind mill), while vertical axis wind turbines are less frequently used (Savonius and Darrieus are the most common in the group).

Horizontal Axis Wind Turbines (HAWT)

Horizontal axis wind turbines, also shortened to HAWT, are the common style that most of us think of when we think of a wind turbine. A HAWT has a similar design to a windmill, it has blades that look like a propeller that spin on the horizontal axis.

Horizontal axis wind turbines have the main rotor shaft and electrical generator at the top of a tower, and they must be pointed into the wind. Small turbines are pointed by a simple wind vane placed square with the rotor (blades), while large turbines generally use a wind sensor coupled with a servo motor to turn the turbine into the wind. Most large wind turbines have a gearbox, which turns the slow rotation of the rotor into a faster rotation that is more suitable to drive an electrical generator.

Since a tower produces turbulence behind it, the turbine is usually pointed upwind of the tower. Wind turbine blades are made stiff to prevent the blades from being pushed into the tower by high winds. Additionally, the blades are placed a considerable distance in front of the tower and are sometimes tilted up a small amount.

Downwind machines have been built, despite the problem of turbulence, because they don't need an additional mechanism for keeping them in line with the wind. Additionally, in high winds the blades can be allowed to bend which reduces their swept area and thus their wind resistance. Since turbulence leads to fatigue failures, and reliability is so important, most HAWTs are upwind machines.

HAWT advantages

HAWT disadvantages

Cyclic stresses and vibration

When the turbine turns to face the wind, the rotating blades act like a gyroscope. As it pivots, gyroscopic precession tries to twist the turbine into a forward or backward somersault. For each blade on a wind generator's turbine, force is at a minimum when the blade is horizontal and at a maximum when the blade is vertical. This cyclic twisting can quickly fatigue and crack the blade roots, hub and axle of the turbines.

Vertical axis

Verticalaxis wind turbines, as shortened to VAWTs, have the main rotor shaft arranged vertically. The main advantage of this arrangement is that the wind turbine does not need to be pointed into the wind. This is an advantage on sites where the wind direction is highly variable or has turbulent winds.

With a vertical axis, the generator and other primary components can be placed near the ground, so the tower does not need to support it, also makes maintenance easier. The main drawback of a VAWT generally create drag when rotating into the wind.

It is difficult to mount vertical-axis turbines on towers, meaning they are often installed nearer to the base on which they rest, such as the ground or a building rooftop. The wind speed is slower at a lower altitude, so less wind energy is available for a given size turbine. Air flow near the ground and other objects can create turbulent flow, which can introduce issues of vibration, including noise and bearing wear which may increase the maintenance or shorten its service life. However, when a turbine is mounted on a rooftop, the building generally redirects wind over the roof and this can double the wind speed at the turbine. If the height of the rooftop mounted turbine tower is approximately 50% of the building height, this is near the optimum for maximum wind energy and minimum wind turbulence.

VAWT subtypes

Darrieus wind turbine

Darrieus wind turbines are commonly called "Eggbeater" turbines, because they look like a giant eggbeater. They have good efficiency, but produce large torque ripple and cyclic stress on the tower, which contributes to poor reliability. Also, they generally require some external power source, or an additional Savonius rotor, to start turning, because the starting torque is very low. The torque ripple is reduced by using three or more blades which results in a higher solidity for the rotor. Solidity is measured by blade area over the rotor area. Newer Darrieus type turbines are not held up by guy-wires but have an external superstructure connected to the top bearing.

Savonius wind turbine

A Savonius is a drag type turbine, they are commonly used in cases of high reliability in many things such as ventilation and anemometers. Because they are a drag type turbine they are less efficient than the common HAWT. Savonius are excellent in areas of turbulent wind and self starting.

VAWT advantages

VAWT disadvantages

              

Turbine selected for a stand-alone operation supplying to an island with a constant wind speed –

A stand-alone power system (SAPS or SPS), also known as remote area power supply (RAPS), is an off-the-grid electricity system for locations that are not fitted with an electricity distribution system.

The hybrid power plant is a complete electrical power supply system that can be easily configured to meet a broad range of remote power needs. There are three basic elements to the system - the power source, the battery, and the power management center. Sources for hybrid power include wind turbines, diesel engine generators, thermoelectric generators and solar PV systems. The battery allows autonomous operation by compensating for the difference between power production and use. The power management center regulates power production from each of the sources, controls power use by classifying loads, and protects the battery from service extremes.

Hence, For such a system of large power requirement Horizontal Axis Wind Turbines (HAWT) is best because in this turbine the tall tower base allows access to stronger wind in sites which raise the wind speed can increase by 20% and the power output by 34%.

Answer Q.2 –

Generation using a wind turbine –

A turbine, like the ones in a wind farm, is a machine that spins around in a moving fluid (liquid or gas) and catches some of the energy passing by. All sorts of machines use turbines, from jet engines to hydroelectric power plants and from diesel railroad locomotives to windmills. Even a child's toy windmill is a simple form of turbine.

The huge rotor blades on the front of a wind turbine are the "turbine" part. The blades have a special curved shape, similar to the airfoil wings on a plane. When wind blows past a plane's wings, it moves them upward with a force we call lift; when it blows past a turbine's blades, it spins them around instead. The wind loses some of its kinetic energy (energy of movement) and the turbine gains just as much. As you might expect, the amount of energy that a turbine makes is proportional to the area that its rotor blades sweep out; in other words, the longer the rotor blades, the more energy a turbine will generate. Obviously, faster winds help too: if the wind blows twice as quickly, there's potentially eight times more energy available for a turbine to harvest. That's because the energy in wind is proportional to the cube of its speed.

Wind varies all the time so the electricity produced by a single wind turbine varies as well. Linking many wind turbines together into a large farm, and linking many wind farms in different areas into a national power grid, produces a much more steady supply overall.

How does a wind turbine work?

Answer Q.3

Factors decide the size of an electric generator –

Wind speed and a wind turbine size are the factors that determine the power generation capacity of a wind turbine installation. Usually, wind resource as sessment is done prior to a wind system’s construction.

The power (energy/second) available in the wind will be given by the formula Power

= 0.5 x rotor swept area (m2) x density (kg/m3) x velocity3(m/s)

It can be noted that thepower generated is cube of the wind velocity and because of this, even a small difference in wind speed will bring about a large difference inavailable energy and in electricity produced and therefore, a large difference in the cost of electricity generated.

The ability of a turbine to generate electric power is measured in Watts.

Answer Q.4 –

Interior permanent magnet generator for a full scale wind Turbine –

Among the many types of generators used in wind energy systems, permanent magnet synchronous

generators are used for variable speed operation. There has been an increasing interest in using IPMSG with direct driven wind turbine over the traditional externally excited synchronous machine due to a significant reduction in magnet prices as well as magnetic material characteristics improvement.

Thank You.

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