Wind power generation relies on special turbines to convert the kinetic energy of the wind into electrical energy. Because of its dependence on wind, which experiences fluctuations due to gusts and temporary calms, wind power has an intermittent power output. A well-sited wind turbine can reach a “capacity factor” of above 40% – that is, it will produce about 40% as much energy as a generator that was able to run 24 hours a day.
The amount of power produced by a wind turbine also depends on the strength of the wind at a particular site: doubling the wind speed increases the power output of a turbine by a factor of eight. However, for a wind turbine to attain its highest performance, its design must match the weather conditions it will face during its lifetime. Three dimensions, including wind speed, extreme gusts and turbulence, determine the wind classes for which turbines are designed for. The International Electrotechnical Commission (IEC) sets international standards for the conditions each wind class encompasses.
Wind power generators can either be connected to the local power grid (“on-grid” systems), or used as standalone systems with a backup generator that runs on diesel (“off-grid” systems). More recently, wind power projects are being commissioned, together with energy storage systems, in an effort to manage the intermittency of wind power and secure the stability of the electrical grid.
Wind turbine operation
At very low wind speeds, there is insufficient torque exerted by the wind on the turbine blades to make them rotate. As the speed increases, the wind turbine will begin to rotate and generate electrical power. The speed at which the turbine first starts to rotate and generate power is called the cut-in speed. As the wind speed rises above the cut-in speed, the output power rises until it reaches the limit that the electrical generator is capable of. The limit to the generator output is called the rated power, and the wind speed at which this occurs, is called the rated wind speed.
At higher wind speeds than the rated speed, the design of the turbine limits the power to this maximum level and there is no further rise in the output power. As the speed increases above the rated power wind speed, the forces on the turbine structure continue to rise and, at some point, there is a risk of damage to the rotor. As a result, a braking system is employed to bring the rotor to a standstill. This is called the cut-out speed.
Onshore wind power systems consist of a turbine positioned on top of a tower, placed on land. A turbine is a device in which a bladed wheel is turned by a force (i.e. the wind) and connected by a shaft to a generator to produce electricity. Currently, approximately 560 GW of installed capacity exists worldwide for onshore wind systems. Onshore wind power systems typically cost about USD $1.6 million per installed MW. They consist of the following components:
Rotor: the rotor is at the hub of a wind turbine, attached to the blades. The rotor converts the linear motion of the wind into rotational energy, which then drives a generator. Nearly all commercial wind turbines in use today are horizontal-axis wind turbines (HAWTs), with a propeller-type rotor.
Blades: the rotor blades capture the wind and transfer its power to the rotor hub, causing it to rotate and drive the generator. Doubling the length of the blades can produce a fourfold increase in power. However, lengthening the blades also increases their weight, making them harder to move, less stable and more expensive to produce. Advances in blade materials and manufacturing have enabled the production of lightweight blades more than 80 metres long. High-stress areas of the blades are often composed of costly carbon fibre composites, among the strongest lightweight materials available. The rest of the blade is usually composed of less costly fiberglass composite. Today, most turbines contain three blades due to considerations around cost performance, power rating, tip speed, loadings on the hub, and aesthetical preferences.
Drivetrain: the drivetrain is the heart of a wind system’s mechanical-to-electrical conversion machinery, and costs almost as much as the blades. The drivetrain connects the rotor to a gearbox, increasing the rotational speed by about a ratio of 100 to 1. The output shaft on the gearbox drives a conventional synchronous generator. In most modern turbines the drivetrain is variable speed, so that the output speed can vary with the speed of the wind, resulting in more efficient conversion and lower dynamic loads on the turbine systems. The drivetrain is a source of major operational challenges with wind systems due to gearbox failure. As such, direct drive systems, i.e. using no gearbox at all, have started gaining traction in the market recently.
Nacelle: the nacelle is the outer covering containing the key components of the turbine, including the drive train.
Tower: a typical commercial-size wind turbine will sit atop a tower that is 70 metres to 150 metres high. Current designs usually use a single tubular (solid) tower.
Ancillary components: turbines also include other components involved in the production and transfer of power, such as a brake, a yaw mechanism that moves the rotor to face into or out of the wind, electronic controllers, a hydraulics system, a cooling unit, a wind vane, sensors and power converters that convert the turbine’s variable-output DC (direct current) electricity to constant-frequency AC (alternating current).
Balance of system: in the context of wind power, the “balance of system” refers to everything in the windfarm outside of the turbine towers – such as transformers, cables, and substations.
Offshore wind turbines
Wind turbines can also be installed in the ocean where winds are stronger and more persistent, and where more room exists for large-capacity wind farms. Offshore systems employ similar turbine technology to their onshore equivalents, with the exception that the offshore systems are installed on a support structure and they are designed to withstand the higher-speed wind environment and the added wave-induced loading on the turbines. Offshore wind is the anticipated approach for future wind deployment in Europe, where there is less land area rich in wind resources than in the US. Currently, there is an installed capacity of offshore wind farms of approximately 20 GW, predominantly in Europe. Offshore systems currently cost about a third more than their onshore equivalents – USD$2.3 million to USD$2.6 million per installed MW.
A major drawback of wind power systems is the intermittent nature of the wind. In many regions, there simply is not enough wind close to the ground to make wind competitive with standard forms of power generation.
At higher altitudes – hundreds or thousands of feet in the air – there are almost always steady winds that blow at relatively constant speeds. Some companies are developing “flying” or “lighter than air” wind turbines, which are tethered to the ground via a cable that conducts the electricity generated by the turbine back to earth. The hope is that these systems will be able to operate at peak capacity between 70% and 90% of the time.
Offshore wind installations have started moving away from shore motivated by the shortage of locations with shallow waters. As such, new floating structures are emerging to overcome restrictions around water depth and ground conditions, while enabling the exploitation of larger wind resources.
Small-scale wind turbines are also attracting interest from developers due to their potential to supply electricity at scales under 50 kW. This could potentially enable novel business models for wind-based distributed generation for commercial and even residential buildings.