INTRODUCTION

The world's reliance on electricity to meet its needs for heat, light, motive power and the exchange of information continues to increase. World consumption of electrical energy grew from 17 500 TWh in 2004 to 24 000 TWh in 2015, an annual growth rate of around 3.4%. Reducing emissions of CO2 to limit climate change will require a transition from fossil fuels to electricity for many services including for heating (e.g. from gas boilers to heat pumps) and for transport (e.g. from internal combustion engines to electric vehicles). This will further increase the demand for electricity.

A traditional power system is supplied by large fossil fired or hydro units with synchronous generators. The operation of these units is well understood and, because of the energy stored in the fossil fuel or in large reservoirs, they are relatively easy to control. In contrast, many renewable generators, e.g. wind and solar power units, respond to the instantaneous renewable resource and are connected to the network through static power electronic interfaces which have quite different operating characteristics to those of large rotating generators.

The power generated by a large renewable generator such as a wind farm or hydro generator must be transported through transmission and distribution networks to the load. Smaller generators, e.g. domestic PV systems, are connected close to the customers but rely on the central power system to provide a stable reference of voltage and frequency. Depending on their location and power output with respect to the local load, renewable generators that are distributed over the network will have a greater or lesser impact on the voltages and the frequency of the power system.

With an increasing penetration of renewables the operation of the power system becomes more difficult. However, many smaller national electricity systems (e.g. Ireland) operate routinely with more than 50% of their electrical power being supplied from wind energy. Traditionally a power system has attempted to supply any load when electricity is demanded. With a high fraction of renewable generation operating on the system it becomes necessary to take control of the load, particularly in the event of an unexpected failure of a generator or an error in forecasting the renewable resource.

INTRODUCTION

Generating electricity from the wind is one of the more cost effective ways of harnessing renewable energy and increasing numbers of wind turbines are being installed in many countries. By 2015 there was more than 430 GW of wind turbine capacity installed worldwide. For comparison, peak electricity demand in Great Britain is around 55 GW and in the continental USA around 800 GW.

Modern wind turbines can be very large with rotor diameters greater than 100 m and similar tower heights. Advanced composite materials are used to resist the forces of the wind as well as those created by gravity acting on these large rotating structures. The most important loads are caused by extreme wind speeds and fatigue loads that are created either by turbulence in the wind or the very large number of rotations and hence stress reversals experienced by a wind turbine blade during its life. All large modern wind turbines operate at variable rotational speed and are connected to the electrical grid though power electronic converters. As wind turbines become larger their structures are increasingly dynamic using active control to manage the loads through sophisticated control systems.

As with all renewable energy technologies, the resource determines if a scheme will be profitable and this chapter includes a description of the wind resource and how energy from a wind farm can be estimated.

In many countries the environmental impact of a wind farm, particularly visual impact and noise, will determine if permission for its construction will be obtained from the civil authorities. In response to concerns over environmental impact some wind turbines are being sited offshore, particularly around the coasts of Europe. Locating turbines offshore leads to a higher cost of the electricity but very large offshore wind farms can be built with limited environmental impact.

This chapter describes the wind energy resource, wind turbines and wind farms.