Conventional Energy Generation
The first practical electricity generating system using a steam turbine was designed and made by Charles Parsons in 1887 and used for lighting an exhibition in Newcastle. Since then, apart from getting bigger, turbine design has hardly changed and Parson's original design would not look out of place today. Despite the introduction of many alternative technologies in the intervening 120 years, over 80 percent of the world's electricity is still generated by steam turbines driving rotary generators.
The Energy Conversion Processes
Electrical energy generation using steam turbines involves three energy conversions, extracting thermal energy from the fuel and using it to raise steam, converting the thermal energy of the steam into kinetic energy in the turbine and using a rotary generator to convert the turbine's mechanical energy into electrical energy.
Raising steam (Thermal Sources)
Steam is mostly raised from fossil fuel sources, three of which are shown in the above diagram but any convenient source of heat can be used.
Chemical Transformation
In fossil fueled plants steam is raised by burning fuel, mostly coal but also oil and gas, in a combustion chamber. Recently these fuels have been supplemented by limited amounts of renewable biofuels and agricultural waste.
The chemical process of burning the fuel releases heat by the chemical transformation (oxidation) of the fuel. This can never be perfect. There will be losses due to impurities in the fuel, incomplete combustion and heat and pressure losses in the combustion chamber and boiler. Typically these losses would amount to about 10% of the available energy in the fuel.
Nuclear Power
Steam for driving the turbine can also be raised by capturing the heat generated by controlled nuclear fission. This is discussed more fully in the section on Nuclear Power.
Solar Power
Similarly solar thermal energy can be used to raise steam, though this is less common.
Geothermal Energy
Steam emissions from naturally occurring aquifers are also used to power steam turbine power plants.
The Steam Turbine (Prime Mover)
Working PrinciplesHigh pressure steam is fed to the turbine and passes along the machine axis through multiple rows of alternately fixed and moving blades. From the steam inlet port of the turbine towards the exhaust point, the blades and the turbine cavity are progressively larger to allow for the expansion of the steam.
The stationary blades act as nozzles in which the steam expands and emerges at an increased speed but lower pressure. (Bernoulli's conservation of energy principle - Kinetic energy increases as pressure energy falls). As the steam impacts on the moving blades it imparts some of its kinetic energy to the moving blades.
There are two basic steam turbine types, impulse turbines and reaction turbines, whose blades are designed control the speed, direction and pressure of the steam as is passes through the turbine.
Impulse Turbines
The steam jets are directed at the turbine's bucket shaped rotor blades where the pressure exerted by the jets causes the rotor to rotate and the velocity of the steam to reduce as it imparts its kinetic energy to the blades. The blades in turn change change the direction of flow of the steam however its pressure remains constant as it passes through the rotor blades since the cross section of the chamber between the blades is constant. Impulse turbines are therefore also known as constant pressure turbines.
The next series of fixed blades reverses the direction of the steam before it passes to the second row of moving blades.
Reaction Turbines
The rotor blades of the reaction turbine are shaped more like aerofoils, arranged such that the cross section of the chambers formed between the fixed blades diminishes from the inlet side towards the exhaust side of the blades. The chambers between the rotor blades essentially form nozzles so that as the steam progresses through the chambers its velocity increases while at the same time its pressure decreases, just as in the nozzles formed by the fixed blades. Thus the pressure decreases in both the fixed and moving blades. As the steam emerges in a jet from between the rotor blades, it creates a reactive force on the blades which in turn creates the turning moment on the turbine rotor, just as in Hero's steam engine. (Newton's Third Law - For every action there is an equal and opposite reaction)
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