Wind Energy Handbook. Michael Barton Graham

Wind Energy Handbook - Michael Barton Graham


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is rather low, 800 times less than water, which powers hydro turbines, and this leads inevitably to the large size of a wind turbine. Depending on the design wind speed that is chosen, a 3.5 MW wind turbine may have a rotor that is 100 m in diameter. The power coefficient describes that fraction of the power in the wind that may be converted by the turbine into mechanical work. It has a maximum value of 16/27 or 0.593, and rather lower peak values are achieved in practice (see Chapter 3). Incremental improvements in the power coefficient are continually being sought by detailed design changes of the rotor, and by operating at variable speed it is possible to maintain the maximum power coefficient over a range of wind speeds. However, these measures will give only a modest increase in the power output. Major increases in the output power can only be achieved by increasing the swept area of the rotor or by locating the wind turbines in higher wind speeds.

Schematic illustration of the largest commercially available wind turbines.

      In the past a number of studies were undertaken to determine the optimum size of a wind turbine by balancing the complete costs of manufacture, installation, and operation of various sizes of wind turbines against the revenue generated (Molly et al. 1993). However, these early estimates would now appear to be too low, and more recent studies indicate that the lowest cost of energy is obtained with rotors approaching 150 m diameter, although smaller turbines may be preferred on some sites for reasons of environmental impact and difficulty of transporting very large components to the site. Even larger turbines give the lowest cost of energy offshore, where the foundation and cabling costs of individual turbine are high and the very large blades can be transported by ship directly from the factory to the site.

      Until around the year 2000, the installed wind turbine generating capacity was so low that its output was viewed by electricity Transmission System Operators simply as negative load that supplied energy but played no part in the operation of the power system and maintaining its stability. Since then, with the greatly increased capacity of wind generation, turbines are required to contribute to the operation of the power system. The requirements for their performance are defined through the Grid Codes, issued by the Transmission System Operators (Roberts 2018). Compliance is mandatory and must be demonstrated before connection to the network is allowed. Compliance with the Grid Code requirements is difficult to achieve with simple fixed speed induction generators using the Danish concept, and these regulations have been a major driver for the use of variable‐speed generators.

      The use of wind energy to generate electricity is now well accepted, with a large industry manufacturing and installing up to 50 GW of new capacity each year. Although there are exciting new developments, particularly in very large offshore turbines, and many challenges remain, there is a considerable body of established knowledge concerning the science and technology of wind turbines. This book records some of this knowledge and presents it in a form suitable for use by students (at final year undergraduate or post‐graduate level) and by those involved in the design, manufacture, or operation of wind turbines. The overwhelming majority of wind turbines presently in use are horizontal axis connected to a large electricity network. These turbines are the subject of this book.

      The book attempts to record well‐established knowledge that is relevant to wind turbines that are currently commercially significant. Thus, it does not discuss a number of interesting research topics or areas where wind turbine technology is still evolving. Although they were investigated in considerable detail in the 1980s, large vertical axis wind turbines have not proved to be commercially competitive and are not currently manufactured in significant numbers. Hence, the particular issues of vertical axis turbines are not dealt with in this text.

      There are presently around one billion people in the world without access to reliable mains electricity, and, in conjunction with other generators (e.g. batteries, diesel engines, and solar photovoltaic units), wind turbines may in the future be an effective means of providing some of them with power. However, autonomous power systems (sometimes known as autonomous microgrids) are extremely difficult to operate reliably, particularly in remote areas of the world and with limited budgets. A small autonomous microgrid has all the technical challenges of a large national electricity system but, due to the low inertia of the generators, requires a very fast, sophisticated control system to maintain stable operation as well as a store of energy. Over the last 40 years there have been a number of attempts to operate


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