Draft PPS 18: Renewable Energy
Annex 1 Wind Energy: Technology
A3. There are essentially two types of wind turbine – those that have rotors that rotate about a vertical axis, and horizontal axis machines whose rotating shafts are aligned horizontally. Most wind turbines installed today are of the latter type and this is likely to remain the case for the foreseeable future. The remainder of this section refers primarily to horizontal axis machines.
A4. Whilst wind turbines are sometimes used to generate mechanical power, particularly for pumping water, this section deals only with the electricity producing variety. Such wind turbines convert the kinetic energy of the wind that passes through the swept area of the rotor into electrical energy by means of a rotor (generally comprising 3 blades), a mechanical drive train (usually including a gearbox) and an electrical generator. These are all mounted on a tower. The height of the tower is normally at least twice the length of a blade. The blades need to be far enough from the ground to minimise turbulence and to maximise the energy capture of the wind turbine. Normally solid tubular towers are used rather than lattice constructed towers.
A5. Wind turbines are defined by the size (diameter) of the rotor and rated power or capacity in kilowatts (kW) or megawatts (MW). The rated capacity of a wind turbine is a measure of the maximum output of the electricity generator which will generally be achieved in wind speeds greater than 12-15m/s at the hub height of the rotor. There are two things worth noting:
- an increase in the rotor diameter of a wind turbine will result in a greater than proportional change in rated power (see figure 1);
- an increase in wind speed will result in a greater than proportional change in rated power. Rated power is proportional to the cube of the wind speed, and hence a doubling of wind speed will result in a roughly eight-fold increase in power output.
A6. Technological advances have led to a wide range of wind turbine designs. The smallest turbines, some with a rotor diameter of less than one metre, are usually used for charging batteries although recent mains-connected micro-turbines have been introduced to the market. At the other end of the scale turbines with rotor diameters of greater than 100m are now being deployed.
Approximate sizes of typical three-bladed turbines by installed capacity, also showing approximate annual energy output based on an average capacity factor of 0.3, the figure for the number of homes supplied is based on the average UK household consumption of 4100 kWh/year (OFGEM)
A7. The blades are usually of a glass-fibre reinforced plastic construction. Other materials used include wood-epoxy laminates and carbon fibres. These may both become more prevalent as current wind turbine designs are scaled up. They are generally the largest single item that is transported to a wind farm during construction. Smaller turbines (less than 50kW) may use blades made of a variety of other materials such as plastics, metal or wood.
A8. The blades are attached to the hub, which is in turn attached to the main shaft that drives the generator, usually but not always via a gearbox.
A9. The generator, gearbox and yaw drive that turns the rotor to face the wind are the main components housed within the nacelle. For large, grid-connected turbines the rotor alignment with the oncoming wind is always controlled actively via the yaw drive and they are designed so that the blades see the wind before the tower does. Such a design is known as an upwind rotor with active yaw control. Smaller turbine designs may use upwind or downwind rotors and may use active or passive yaw control. Vertical axis machines require no yaw control by virtue of their design.
A10. The nacelle is mounted on the tower, which for large grid-connected turbines is normally of a tubular steel construction. Smaller turbines (less than 50kW) may be mounted on similarly designed towers, but may equally use lattice or guyed towers. Turbines designed specifically for micro-generation may be mounted directly onto existing structures, such as roofs.
Main components of a wind turbine
A11. There are a number of technical differences amongst the wind turbines that are currently available. The most obvious difference is in the number of blades. Most machines now have three blades, but there are some two-bladed machines in operation. Other than this the two most important differences are the way in which a turbine regulates its power capture above rated wind speed (pitch or stall regulation) and whether the machine operates at a fixed or variable rotor speed.
A12. The turbine is controlled by its own computer system, which provides both operational and safety functions. In addition to controlling blade angle and rotor speed, a wind turbine’s control system must also align the rotor with the oncoming wind. This is achieved by rotating the nacelle in relation to the tower top with a yaw gear mechanism.
A13. Modern wind turbines also continuously monitor their own performance and if atypical vibrations caused by component imbalances are detected, or if connection to the local electricity grid infrastructure is lost, all turbines must be capable of emergency stops. Most modern wind turbines undergo test certification procedures, which must conform to the guidelines laid down by the International Electro-technical Commission (IEC).
A14 Wind turbines can be deployed singly, in small clusters, or in larger groups known as wind farms. Factors that may influence the size of a development include the physical nature of the site, the capacity of the local electricity distribution network and the organisation undertaking the development. It is likely that the Region’s wind resource will be harnessed most satisfactorily using a mixture of these types of development.
A15. The direction of rotation of the wind turbine rotors will be common across a wind farm. Wind turbines are usually semi-matt white, off white or grey in colour, often as a condition of planning permission. The colours of the blades, nacelle and towers are normally the same.
This section contains the following sub-categories:
- Spacing of Turibines
- Other Infrastructure
- Connection to the Electricity Grid
- Operation and Maintenance
- Wind Resource