Jochen Twele

Offshore wind farms

In the future, wind energy will be playing a dominant role in raising the share of renewables in electricity generation. This will significantly reduce the emission of carbon dioxide and the use of fossil fuels. Land use limitations in areas with high population density are hindering the installation of new large wind farms. Offshore, however, there is an enormous wind resource that has the advantages of both abundant space and dense winds. In 1995, a study found that the exploitable offshore wind resource theoretically exceeds the total consumption of electricity in Europe (Fig. 16-1).

Blade geometry according to Betz and Schmitz

With the help of the Betz or the Schmitz (Glauert) theory [1, 2, 7], designing a wind turbine is relatively straightforward. These theories provide the blade chord and the blade twist relative to the radius, after the design tip speed ratio, the aerodynamic profile and the angle of attack or the lift coefficient have been specified.

Scaling wind turbines and rules of similarity

Wind turbines are used in a variety of applications with very different performance requirements. In terms of power supply, a small holiday cottage requires electrical energy of approx.1.5 to 2 kW, a medium-sized restaurant approx. 75 kW with a base load of approx. 15 to 25 kW, and a large farm approx. 50 to 100 kW. In the first case, given a rated wind speed of v = 9 m/s, a turbine with a rotor diameter of 3.5 m would be sufficient. For the base load of the restaurant, a rotor diameter of 7 to 8 m would be required, whereas for the 50 to 100 kW, a turbine rotor with a diameter of 15 to 20 m is needed. Large turbines with a diameter of 80 to 100 m, can supply power of up to 3 MW.

Wind turbines - design and components

Wind turbines are energy converters. Independent of their application, type or detailed design all wind turbines have in common that they convert the kinetic energy of the flowing air mass into mechanical energy of rotation. As already discussed in chapter 2, two aerodynamic principles are suitable for this purpose, the lift and the drag, see Fig. 3-1. Drag driven rotors reach only, as mentioned, moderate power coefficients and are of no major importance to the technical applications, apart from the anemometers.

Concepts of electricity generation by wind turbines

Wind turbines for power generation may be characterized according to their type of application - Grid-connected wind turbines - Wind turbines for stand-alone operation and - Wind turbines for hybrid systems, e.g. wind-diesel or wind-photovoltaic systems. Grid-connected wind turbines, section 13.1, have the advantage that their produced power can be fed into the grid at all times. The storage problem for excess electrical energy, e.g. at night, is transferred to the grid and is solved there often by using hydro power plants with pump storage facilities. These have in Germany an installed capacity of approx.

Planning, operation and economics of wind farm projects

This chapter is structured according to the chronology of the individual working phases of a wind farm project: preliminary and project planning, erection and operation. Each phase may be differentiated according to the following aspects: Technical aspects - Aspects of the permission regulation and - Economic aspects.

Wind pump systems

The application of wind mills for water pumping is of lesser importance today than in the past centuries. However, for several reasons, it is useful to discuss this type of wind energy application in a wind energy book targeted at development and planning engineers as well as students.

Supervisory and control systems for wind turbines

The Western mill was the first wind turbine which operated completely automatic, “without a human supervisor”. But the tasks of the control system and the supervisory system were strongly interwoven. The main vane which adjusts the rotor to the wind direction is in fact a simple “all-in-one” control system: it is sensor to register the deviation between the wind direction and the rotor axis, and at the same time it is the actuator producing the forces for the correction of the deviation from the demand value, Fig. 12-1. Together with the transverse vane it controls the turbine in the entire operating range from standstill to storm protection: For very strong winds the spring extends and the rotor is turned to increase the angle between rotor axis and wind direction. Rotational speed, power extraction and thrust are reduced, see also Fig. 12-7 (yaw angle β), and annex I of this chapter. In this annex additional examples of simple mechanical control systems for small wind turbines are presented.

Wind turbines for electricity generation - basics

Presently wind turbines are used primarily for electrical power generation. Threephase alternators (AC generators) are used almost exclusively. Even for applications that require DC the lower-cost alternator/rectifier configuration has superceded the DC-generator. When a three-phase generator feeds directly into a grid, that operates at a fixed frequency (e.g. 50 Hz in Europe, 60 Hz in the U.S.A.) the angular velocity of the generator is fixed - or almost fixed. In this situation the power generation capability of the wind turbine will be fully utilized for just one value of wind speed (approximately 8 m/s in Fig. 11-1a). Thanks to the highly developed converter technology of today it is now possible to operate with variable generator speeds even for grid-connected generators (see Fig. 6-18). This yields better utilization of the wind turbine; during gusty winds it also substantially decreases the mechanical stresses in the blades and the shaft between turbine and generator.

Wind turbine operation at the interconnected grid

On the one hand, the grid-connected operation of wind turbines places demands on the wind turbines’ operational behaviour and its technical equipment for grid connection. On the other hand, the operation of the interconnected grid is increasingly influenced by wind power production. Some years ago, the effects were negligible because the installed wind power was small compared to the grid capacity. But nowadays, at least in Germany, Denmark and Spain, the grid integration of wind energy has become a technical and economical challenge.