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at power ratings of up to 1.5 MW. However, at larger rotor diameters and generator ratings, the architecture ceases to be effective because, with larger rotors, aerodynamic stall is increasingly difficult to predict, and the larger induction generators were no longer able to provide enough damping and torsional compliance in the drive train. Also, the requirements of the electrical Transmission System Operators for connecting wind farms to the network become very difficult to meet with a simple fixed speed induction generator. Hence, as the size of commercially available turbines approached or exceeded that of the large prototypes of the 1980s, the concepts investigated then of variable‐speed operation and full‐span control of the blade pitch as well as advanced materials and modern control systems were increasingly adopted. The evolution of modern wind turbines is described in Serrano‐González and Lacal‐Arántegui (2016).

      In 1991, the first offshore wind farm, consisting of 11 450 kW wind turbines, was constructed at Vindeby, 3 km off the coast of Denmark. Throughout the 1990s, small numbers of offshore wind turbines were placed close to shore, while in 2002, the Horns Rev, 160 MW wind farm, was constructed some 20 km off the western coast of Denmark. At the time of writing (2020), there was around 29 GW of offshore wind energy capacity in operation (Global Wind Energy Council 2020), concentrated mainly off the coasts of northern Europe and eastern China. There are a number of offshore wind farms of capacity greater than 500 MW, and even larger installations are under construction or planned. The wind turbines installed in the early offshore wind farms were marinised conventional designs that had been proved onshore. More recently, very large wind turbines designed specially for transport directly by sea from the factory to the offshore site have been installed. Further, the possibility of higher blade tip speeds because of more relaxed noise constraints and a reduced emphasis on the visual appearance of wind farms far from land continue to lead to interest in the development of very large, lower solidity rotors (Jamieson 2018).

      The stimulus for the development of wind energy in 1973 was the increase in the price of oil and concern over limited fossil fuel resources. From around 1990, the main driver for the use of wind turbines to generate electrical power has been the very low CO₂ emissions, over the entire life cycle of manufacture, installation, operation, and de‐commissioning, and the potential of wind energy to help mitigate climate change. In 2007, the European Union established a policy that 20% of all energy should be from renewable sources by 2020. Because of the difficulty of using renewable energy for transport and heat, this implies that in some countries 30–40% of electrical energy should come from renewables, with wind energy playing a major part. Energy policy continues to develop rapidly, with the European Union extending its target for the share of energy to come from renewables by 2030 to 32% and many countries now adopting a commitment to reduce or eliminate greenhouse gas emissions before 2050.

Figure 1.1 shows the remarkable growth in the installed capacity of wind power worldwide over 15 years to 2019. The typical annual rate of increase of capacity was more than 10%. Figure 1.2 shows the growth of wind energy capacity by country, dominated by China and the USA. Figure 1.3 summaries current capacity (2019) by country and region of the world.

Figure 1.1 Wind power capacity worldwide (World Wind Energy Association 2020).

Figure 1.2 Wind power capacity by country (US Energy Information Administration 2019; REN21 2020).

Figure 1.3 Installed onshore wind power capacity in countries with more than 10 GW, regions, and total offshore (Global Wind Energy Council 2020).

The development of wind energy in some places has been more rapid than in others, and this cannot be explained simply by differences in the wind speeds. Important factors include the financial support mechanisms for wind generated electricity, access to the electrical network, the permitting process by which the local civil authorities give permission for the construction of wind farms, and the perception of the general population, particularly with respect to visual impact. The development of offshore sites, although at considerably increased cost, is in response to these concerns over the environmental impact of onshore wind farms.

Figure 1.4 Onshore wind turbines in flat terrain.

      Source: Stockr/Shutterstock.com.

Figure 1.4 shows modern wind turbines in flat open terrain, and Figure 1.5 shows an offshore wind farm.

      When it was a new electricity generation technology, wind energy required financial support for some years to encourage its development and stimulate investment from private companies. Such support was provided in many countries in recognition of the contribution that wind generation makes to mitigating climate change and the security of national energy supplies. Feed‐in Tariffs continue to be offered in a number of countries. These are fixed prices paid for each kWh generated from renewable sources with different rates for wind energy, photovoltaic solar energy, and other renewable energy technologies. This support mechanism has the benefit of giving certainty of the revenue stream from a successful project and is credited by its supporters for the very rapid development of wind energy, and other renewables, in these countries.

Large wind farms are now often supported through competitive auctions that establish a price that a project developer can expect for the electrical energy. This acts to reduce uncertainty and hence project financing costs. The cost of generating electricity from wind power continues to fall and is now below the retail price of electricity in most countries and lower than the cost of generation from alternative sources of energy under favourable conductions of high site wind speed and low wind farm constriction costs. These cost reductions mean the need for national subsidies is rapidly reducing.

Figure 1.5 Offshore wind farm.

      Source: fokke baarssen/Shutterstock.com.

1.2 Modern wind turbines

      The power output from a wind turbine is given by the well‐known expression:

       ρ is the density of air (1.25 kg/m3)

       Cp is the power coefficient

       A is the rotor swept area

       U is the free wind speed

The density

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