A turbine is a means of extracting work from a fluid as it changes from a high pressure to a low pressure. A turbine consists of a shaft carrying a number of vanes or blades, and there is a transfer of energy between the fluid and the rotor. In a turbine, the fluid does work on the rotor.
For gas, steam or water turbines, the fluid is accelerated in a set of fixed nozzles, and the resulting high-speed jets of fluid then change their direction as they pass over a row of curved blades attached to a shaft. A force is exerted on the blades equal to the rate of change of momentum of the fluid, and this produces a torque at the rotor shaft. The momentum of the fluid in the tangential direction is changed and so a tangential force on the rotor is produced. The rotor therefore rotates and performs useful work, while the fluid leaves it with reduced energy. The velocity of the fluid is reduced to somewhere near the value it possessed before entering the nozzles.
For any turbine, the energy held by the fluid is initially in the form of pressure. For a turbine in a hydro-electric scheme, water comes from a high-level reservoir: in a mountainous region, several hundred metres head may thus be available, although water turbines are in operation in other situations where the available head is as low as three metres or less. For a steam turbine, the pressure of the working fluid is produced by the addition of heat in a boiler; in a gas turbine, pressure is produced by the chemical reaction of fuel and air in a combustion chamber.
The impulse (or constant pressure) turbine has one or more fixed nozzles, in each of which the pressure is converted to the kinetic energy of an unconfined jet. The jets of fluid then impinge on the moving blades of the rotor, where they lose practically all their kinetic energy and, ideally, the velocity of the fluid at discharge is only just sufficient to enable it to move clear of the rotor.
In a reaction turbine, the change from pressure to kinetic energy takes place gradually as the fluid moves through the rotor and, for this gradual change of pressure to be possible, the rotor must be completely enclosed and the passages in it entirely full of the working fluid.
It was during the oil crisis of the early 1970s that modern interest in wind turbines took form. From this period, the basic wind energy conversion system for power generation has gradually taken shape. Today, the basic system starts with a large rotor comprising two, three or four blades mounted on a horizontal shaft at the top of a tall tower. The blades interest the wind and capture the energy it contains, energy which causes them to rotate in a vertical plane about the shaft axis. The slow rotation of the shaft is normally increased by use of a gearbox, from which the rotational motion is delivered to a generator.