Wind Energy is Renewable Energie

Development

Since 1990 wind turbine technology has developed rapidly. In just five years time the weight of wind turbines could be reduced by half. The noise emissions were cut down by half in just three years.

The yearly power production of a turbine has increased a hundredfold in 15 years. A few years ago the 500 kW turbine was a sensation and today the 1.5 - 2 MW turbines are standard.

A prototype of a turbine with 4.5 MW that will be able to produce electricity for 3,000 households will go into operation in 2002.

Reliability of Energy Supply

No single power plant can guarantee the supply of electricity. Many power plants are networked in order to be able to ensure the reliable delivery of power.

Since wind does not blow at the same speed at the same time everywhere, there are fluctuations in the electricity production. However, the modern generators are becoming more and more flexible and are able to smooth out these fluctuations in order to produce a more even supply of electricity.

Just like this fluctuating electricity production, electrical plants must deal with a fluctuating electricity consumption. The more wind turbines on the grid, the better short fluctuations can be evened out with the fluctuations of other turbines.

Types of Wind Turbines

The classical horizontal axis wind turbine with three rotor blades is predominant in the market.

In times when wind turbine technology was just developing, experiments were made with vertical axis turbines (disadvantages: tower in way of wind, wind swirls) and with horizontal turbines with one, two or three rotor blades.

The dynamic of the three bladed rotor is easiest to manage. The moment of inertia of a three bladed rotor referring to the tower does not change during rotation. This results in less problems due to oscillation as is caused by a two or one bladed rotor. Other reasons the three bladed rotor is better accepted by the public is that it produces less noise since it rotates at a slower speed at the blade tips. It also is optically quieter due to it turning at a slower pace.

About 90% of the installed wind turbines today have three rotor blades.

Pitch-Control = Control of the rotor blades angle

On a pitch controlled wind turbine the turbine´s electronic controller checks the power output of the turbine constantly. When the power output becomes too high, it sends an order to the blade pitch mechanism which immediately pitches (turns) the rotor blades slightly out of the wind. When the wind is less strong the blades are turned back into the wind. The rotor blades have to be able to turn around their longitudinal axis.

This system does not only prevent damage to the turbines during strong winds but also maximises their power output during these strong winds.

The Tauernwindpark is pitch controlled.

Stall-Control

Stall controlled wind turbines have the rotor blades bolted onto the hub at a fixed angle. The rotor blade profile has been aerodynamically designed to ensure that the moment the wind speed becomes too high, it creates turbulence on the side of the rotor blade which is not facing the wind. This stall prevents the lifting force of the rotor blade from acting on the rotor.

Advantages of the stall control system is that moving parts in the rotor itself are avoided and a complex control system is not necessary. On the other hand, stall control involves a very complex aerodynamic design and related design challenges in the structural dynamics of the whole wind turbine for instance to avoid stall-induced vibrations.

Active-Stall-Control

The rotor blades are able to be pitched like the pitch controlled wind turbines. The difference is that when the machine reaches its rated power, the blades will pitch in the opposite direction, increasing their angle to the wind and going into a deeper stall. Thus the excess energy in the wind can be avoided.

An advantage of the active stall controlled machines is that the power output can be more accurately controlled than the passive stall controlled ones. The generator is not strained during gusty winds.

The turbines can run at rated power at all high wind speeds. This is not the case with the stall controlled ones which usually have a drop in the electrical power output for higher wind speeds, as the rotor blades go into deeper stall.

The active stall control system is often installed in the large turbines (1 MW and more).

Gearbox

In order to transform the moving energy of the rotor into electrical energy with the help of a generator, a rotor speed of 1,500 revolutions per minute (rpm) is necessary. Therefore with a rotor speed of about 30-50 rpm, a gearbox must be used. With a gearbox you convert between slowly rotating, high torque (moment or turning force) power which you get from the wind turbine rotor - and high speed, low torque power, which you use for the generator. The degree of efficiency is 98% per gear phase, the loss of energy due to the rubbing of the gears is given off in the form of heat and noise.

Turbines without a gearbox are also manufactured (e.g. Enercon). These machines are built with a many poled circular generator, the result is a direct-current circuit which can be operated on a variable speed control system. A gear ratio is not necessary and the gearbox can be left out.

Direct Grid Connection

Most wind turbines run at almost constant speed with direct grid connection. The speed of the generator and the rotor is given by the grid. Therefore the rotor cannot always work with the optimal aerodynamic efficiency unless the rotor blade can be mechanically controlled.

Another possibility is a generator with a pole switching system. This allows for operation under reduced speed and a better adaptation of the current on the rotor during low wind speeds.

Offshore Wind Parks

The utilisation of wind energy in offshore areas presents a very promising new perspective, especially for countries with a dense population. Due to the low roughness of the ocean surface and the consistency of the blowing wind, the potential of wind energy at the ocean is much higher than on land. A disadvantage is the high costs of installation but research work is being carried out and technology improving which should help to reduce these costs.

Progress has been made in the manufacturing of the foundations. In the foreseeable future it looks like it may even be economical to install offshore turbines in water with a depth of 15 metres.

Great Britain is the country in Europe with the most potential for offshore wind parks. The largest offshore wind park at this time is Middelgrunden which is near Copenhagen (40 MW installed capacity, predicted annual energy yield: 99,000 MWh, in operation since May 2001).

Denmark is planning to have installed an offshore capacity of 4,100 MW by the year 2003. That would amount to 40% to 50% of Denmark´s electricity consumption.

Types of Generators

The wind turbine generator converts mechanical energy to electrical energy. It is different compared to other generating units attached to the electrical grid because it has to work with a power source (the wind turbine rotor) which supplies very fluctuating mechanical power (torque). The wind turbine generator efficiency amounts to between 90% and 98%. There are two kinds of types of generators which are used for wind turbines:

Asynchronous (Induction) Generators
Synchronous Generators

Indirect Grid Connection

With indirect grid connection, the wind turbine generator runs in its own, separate mini alternating current (AC) grid. This grid is controlled electronically (using an inverter), so that the frequency of the alternating current in the stator of the generator may be varied. In this way it is possible to run the turbine at variable rotational speed. The turbine will generate alternating current at exactly the variable frequency applied to the stator.

The AC current is converted into direct current (DC) by using thyristors or large power transistors. The fluctuating DC is converted to AC with exactly the same frequency as the public electrical grid. This AC current is not smooth and the waves can be smoothed out by using inductances and capacitors in a so-called AC filter mechanism.

Disadvantages of the indirect grid connection are that it is costly, it seems that availability rates tend to be somewhat lower that conventional machines and that the power electronics may induce harmonic distortion.

Power Control of Wind Turbines

Wind turbines are designed to produce electrical energy as cheaply as possible. They are therefore generally designed so that they yield maximum output at wind speeds around 15 meters per second. It does not make sense to design turbines that maximise their output at stronger winds because such strong winds are rare. In case of stronger winds it is necessary to reduce the output of the turbine in order to avoid damaging it.

There are three different power control systems on modern wind turbines:

- Pitch-Control
- Stall-Control
- Active-Stall-Control.

Asynchronous (Induction) Generator

Asynchronous generators are very reliable and tend to be comparatively inexpensive. They synchronise easily with the grid and need little repair. A very useful mechanical property of the generator is that it will increase or decrease its speed slightly if the torque varies. This means less wear and tear on the gearbox. This is one of the most important reason for using an asynchronous generator rather than a synchronous generator on a turbine which is directly connected to the electrical grid. However, the efficiency coefficient is lower than by a synchronous generator.

Synchronous Generator

Synchronous generators have a high efficiency coefficient and can be directly connected to the grid. They normally use electromagnets in the rotor which are fed by direct current from the electrical grid. Since the grid supplies alternating current, they first have to convert alternating current to direct current before sending it into the coil windings around the electromagnets in the rotor. Disadvantages of the synchronous generator are that the magnet which is necessary for synchronisation is expensive and tends to become demagnetised by working in the powerful magnetic fields inside a generator.