Electricity Generation

Electricity is produced at a an electric power plant. Some fuel source, such as coal, oil, natural gas, or nuclear energy produces heat. The heat is used to boil water to create steam. The steam under high pressure is used to spin a turbine. The spinning turbine interacts with a system of magnets to produce electricity. The electricity is transmitted as moving electrons through a series of wires to homes and business.

Electric Power Plants:

Electric Power Plants have a number of components in common and are an interesting study in the various forms and changes of energy necessary to produce electricity.

Boiler Unit: Almost all of power plants operate by heating water in a boiler unit into super heated steam at very high pressures. The source of heat from combustion reactions may vary in fossil fuel plants from the source of fuels such as coal, oil, or natural gas. Biomass or waste plant parts may also be used as a source of fuel. In some areas solid waste incinerators are also used as a source of heat. All of these sources of fuels result in varying amounts of air pollution, as well as, the carbon dioxide ( a gas implicated in global warming problems).

In a nuclear power plant, the fission chain reaction of splitting nuclei provides the source of heat.

Turbine-Generator: The super heated steam is used to spin the blades of a turbine, which in turn is used in the generator to turn a coil of wires within a circular arrangements of magnets. The rotating coil of wire in the magnets results in the generation of electricity.

Cooling Water: After the steam travels through the turbine, it must be cooled and condensed back into liquid water to start the cycle over again. Cooling water can be obtained from a nearby river or lake. The water is returned to the body of water 10 -20 degrees higher in temperature than the intake water. Alternate method is to use a very tall cooling tower, where the evaporation of water falling through the tower provides the cooling effect.

Creating Electricity using a Generator:
If a magnetic field can create a current then we have a means of generating electricity. Experiments showed that a magnetic just sitting next to a wire produced no current flow through that wire. However, if the magnet is moving, a current is induced in the wire. The faster the magnet moves, the greater the induced current.

This is the principal behind simple electric generators in which a wire loop is rotated between to stationary magnetics. This produces a continuously varying voltage which in turn produces an alternating current .

Diagram of a simple electric generator is shown on the right.

To generate electricty then, some (mechanical) mechanism is used to turn a crank that rotates a loop of wire between stationary magnets. The faster the crank turns, the more current that is generated.

In hydroelectric, the falling water turns the turbine. The wind can also turn the turbine. In fossil fuel plants and nuclear plants, water is heated to steam which turns the turbine.

Electricity Generation in Power Plants

There are many methods to generate electricity. The most common and simple method is by a battery, a very familiar household item. This is ideal for very small capacities. To meet the entire domestic, commercial and industrial demand electricity generation has to be done in a large scale in power plants.

Most of the electricity generated in the world is by using an electrical generator. An electrical generator uses the principle of Faraday to produce electricity. Faraday found that a copper coil when moved in a magnetic field produces a voltage across the coil. The electric generator consists of a magnet called the ‘rotor’, which can be rotated inside a copper coil called the 'stator' to generate electricity.

To generate electricity we have to mechanically rotate the ‘rotor’. Energy is required to rotate the generator. Energy available in various forms like coal, oil and gas has to be converted to mechanical energy to rotate the generator. This conversion can take place in many different ways. The power plant does this conversion from the primary energy source to mechanical rotation and then to electricity. A small form of this generator is what we have in our car, the alternator. The car’s engine rotates the alternator which generates power for the car. A slightly bigger version is the portable generator driven by a petrol or diesel engine, normally used as a standby in case of a mains power failure.

Moreover, in thermal power plants, a steam turbine rotates the generator. This requires the continuous flow of high-pressure steam. Burning fuels like coal, oil or gas produces heat. This heat boils the water in the boiler to produce high-pressure steam. This steam rotates the turbine, which in turn rotates the generator. Burning of other fuels like bio-mass or municipal waste also produces steam for use in the turbine. Waste heat from other process can also produce steam.

Nuclear power plants also use a steam turbine to rotate the generator. Steam generators produce steam by utilising the heat from nuclear fission taking place in a nuclear reactor. Geo-thermal power plants use the steam available from underground geo-thermal reservoirs to rotate the turbine.


It is also possible to rotate the generator directly without a steam turbine using other engines. Internal combustion engines, diesel or petrol driven, can directly turn the generators to produce electricity. The gas turbine power plant uses a gas turbine. The thrust produced from combustion of natural gas rotates the gas turbines, which in turn rotates the generator. This is something like using the thrust from the jet engines of an airplane. In Hydroelectric power plants the energy of water stored at a height in reservoirs created by dams, turn hydro turbines and connected generators to generate electricity. In a wind energy farms, force of the wind turns the wind turbines, which inturn rotates the generators attached to them to produce electricity.

There are other methods to generate electricity without a generator using electrochemical reactions. The most common is the battery in which chemical reaction converts to electricity. In a solar plant, energy from the sunlight is converted directly to electricity by using Solar cells. In a hydrogen fuel cell, chemical reaction between hydrogen and oxygen is used to generate electricity.

All types of power plants cannot be installed everywhere. Availability of the primary energy source is the primary concern. For example, the installation of Wind turbines is done only in places with consistently good wind throughout the year. Also of great concern is the efficiency of conversion from the primary source to final electric output.

The attached mindmap shows the conversion of primary energy sources to electricity.
 
Mindmap

 

Steam Turbine Electricity Generation Plants

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 fuelled 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 Principles

High 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

Steam Turbine Electricity Generation Plants

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)


 

Electric Generation Using Natural Gas

Natural gas, because of its clean burning nature, has become a very popular fuel for the generation of electricity. In the 1970s and 80s, the choices for most electric utility generators were large coal or nuclear powered plants; but, due to economic, environmental, and technological changes, natural gas has become the fuel of choice for new power plants. In fact, in 2000, 23,453 MW (megawatts) of new electric capacity was added in the U.S. Of this, almost 95 percent, or 22,238 MW were natural gas fired additions. The graph below shows how, according to the Energy Information Administration (EIA), natural gas fired electricity generation is expected to increase dramatically over the next 20 years, as all of the new capacity that is currently being constructed comes online.
There are many reasons for this increased reliance on natural gas to generate our electricity. While coal is the cheapest fossil fuel for generating electricity, it is also the dirtiest, releasing the highest levels of pollutants into the air. The electric generation industry, in fact, has traditionally been one of the most polluting industries in the United States. Regulations surrounding the emissions of power plants have forced these electric generators to come up with new methods of generating power, while lessening environmental damage. New technology has allowed natural gas to play an increasingly important role in the clean generation of electricity. For more information on the environmental benefits of natural gas, including its role as a clean energy source for the generation of electricity.