Steam is used in most large scale power plants to drive the turbines that work the generator. The turbines are just one part of the steam cycle that is required with this method of electricity generation.

Steam

OK, lets get one thing straight first. The stuff that comes out of your kettle that you call steam is NOT steam, its water vapour. Water vapour is lots of little droplets of water in the air - in technical terms that is a suspension of liquid water in air. You can see it either as a distortion of whatever is behind or, when the droplets are bigger, as a cloud.

Steam is a gas and, like air, you can't see it. The steam used to drive turbines is superheated, which means it is heated far above its boiling temperature. Good quality turbine steam can be about 600°C and starts at quite a high pressure.

A basic Steam Cycle

A basic steam cycle consists of a pump, a boiler, a turbine and a condenser. The pump puts work into the system to move the water around. The water flows into the boiler which puts heat into the system and turns the water into steam. The steam flows into the turbine which takes work out of the system for generating electricity. The steam then flows into the condenser which takes heat out of the system and turns the steam back into water. The water then flows back to the pump and starts round again.

The 1st Law of Thermodynamics

The 1st Law of Thermodynamics is an axiom. It can't be proved but it has never failed to be true. It is the law of conservation of energy and says that in all processes, total energy is conserved. Energy can not be produced or destroyed only changed from one form to another.

Steam cycles are closed systems. This means that they have no real beginning or end and will carry on going round until something happens to stop them. Their only inputs and outputs are work and heat. The pump does work on the steam and the steam does work on the turbines. The boiler gives heat to the steam and the condenser removes heat. Work and heat are both energy transfers between the system and its surroundings.

The 1st Law of Thermodynamics states that in closed systems, the net work out must be equal to the net heat in. In other words the energy(work) removed by the turbines minus the energy(work) provided by the pump should be equal to the energy(heat) gained in the boiler minus the energy(heat) lost in the condenser.

The work put in by the pump is whatever energy is required to over come all the friction in the system and keep the water/steam moving at the required speed. In most cases the heat provided by the boiler is fixed by the heat source (coal, oil, nuclear, etc...). This means that the work done by the turbines depends on the heat lost by the condensers.

Real Steam cycles

Real steam cycles contain far more than just the four main elements. Many have more than one turbine and sometimes steam is reheated in another boiler before going through secondary turbines. There may be deaerators and other cleaning devices to remove air that can leak into turbines. Sometimes steam is tapped off to start heating the water out of the condensers before it reaches the boiler. Sometimes water is tapped off to cool other parts of the system.

Whatever strange diversions and contraptions are included, the 1st Law of Thermodynamics must always apply.