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A steam turbine is a device that is able to convert the thermal energy in pressurized steam into rotary motion. This rotary motion is often utilized in power plants to generate electricity, though it may have other applications as well. The basic principle behind steam turbines was first written about as early as the first century AD, while the modern steam turbine was not designed until 1884.
The first modern steam turbine was designed by Sir Charles Parsons. It was utilized, in conjunction with a dynamo, to generate 7.5 kW of power. Shortly after its invention, the device found primary use in power plants around the world and on board naval vessels. Earlier reciprocating steam engines were both less efficient and more difficult to adapt for use on board ships, requiring complicated systems rather than a simple direct drive mechanism.
Steam turbines utilize the principle of allowing pressurized steam to expand in controlled stages. Each turbine can be composed of a number of impulse and reaction turbines, each of which allows the steam to expand and turn the blades, or buckets, within the device. Most steam turbines employ both of these variations in concert, with the impulse turbines operating under high pressure, and low pressure being utilized for the reaction turbines. The difference between the two is that the impulse variety uses a nozzle to introduce high velocity steam into the rotors, while the rotors in reaction turbines are themselves a type of nozzle.
The arrangement of impulse and reaction turbines within a steam turbine can effectively make the system very efficient. By making use of both high and low pressure, and having the steam do work at each stage of its expansion, the process may remain highly isentropic. This simply means that the entropy that goes into the system is similar to that which comes out. The other benefit of the design is that the spinning of the turbines creates rotary motion, which can be ideal for both power generation and turning paddles or other drive mechanisms on ships.
A large percentage of all the power generated in the world makes use of steam turbine generators. They are present in both fossil fuel and nuclear power plants, with the former burning coal or oil to heat water to steam, and the latter relying on a nuclear fission reaction to generate the steam. Additionally, some power is generated utilizing a process known as focused solar energy. This type of power plant can make use of the sun to produce steam that may then turn steam turbines.