It was stated earlier that when two bodies are at different potentials an electric force exists between them. This force can cause an electron flow between the bodies if a conducting path exists between them.
The electric force or pressure is termed an electromotive force (e.m.f.). It is measured by the potential difference in volts between two points.
If points of different potential are joined by, an electrical conductor, the electrons will be forced from the point where there is a surplus of electrons to a point where there is a deficiency. That is, electrons will flow from a point of higher electron potential to one of lower electron potential whenever a conducting path exists between those points. These issues give rise to three factors:
- Potential difference
Once the potential difference between two points has been reduced to zero, the flow of electrons stops. For practical purposes this irregular flow of electrons is unsatisfactory. To ensure a continuous flow of electricity a continuous source of electromotive force is required.
- Electrical materials
Electrical energy needs to be guided to where it can be used effectively. This is done with materials that exhibit specific characteristics. One type of material conducts the electrical energy to where it can be used (a conductor), while another type prevents any unwanted flow from other parts of an electrical circuit (an insulator).
- Electron flow
The rate of flow of electricity is governed by the above two factors. That is, a high potential difference tends to increase the flow and a poor conductor tends to decrease the flow.
Sources of electromotive force
The great bulk of electrical energy is still generated in the same manner as it was a hundred years ago. Fossil fuel is burned to obtain heat, in order to convert water to steam for mechanical devices to drive electrical generators. The major concern now is for future supplies of fossil fuels to continue the process, or the replacement of fossil by nuclear fuels to generate steam. There are many other methods for producing electricity, but none of them at the present stages of development can produce anything like the quantities available from the rotating generator.
The generator employs the principle of forcing conductors through a magnetic field to generate a voltage. Depending on the type of machine, the resulting supply may be alternating or direct. The generation of alternating voltages is discussed in Chapter 8, and that of direct current in Chapter 12.
The only real variations are in the type of prime mover required. The driving unit may be a steam turbine, water turbine, gas turbine or diesel engine, each having particular advantages. The electromechanical method is used wherever large quantities of electrical power are supplied from a common source for large cities and their associated industries. A typical operating efficiency could be 40% although this figure is highly variable.
Development of the generator
Faraday’s experiments with induction led to the development of rotating machinery to produce dynamic electricity. Initially rotating machinery was of open construction and primitive by today’s standards.
Figure 2.6 shows early versions in the development of, a direct current generator. It can be seen that there has been little obvious change in direct current generators during the last 60 years. However, there has been refinement of the internal construction and materials used.
Modern power generation methods
It was soon realised that alternating current offered many advantages in both the generation and transmission of electrical power. The changeover from direct current to alternating current power was a progressive one taking many years and meant that both systems were in use for a considerable time. Methods of generating electrical power were rather stable, in that steam-driven engines originally used to drive low-speed generators were used to drive alternating current generators instead.
Steam-driven turbines were a later development and alternators became larger and larger.
The most common way of generating power in the mainland states of Australia is with steam turbines. In the majority of’ cases, steam is produced by burning either coal, oil or gas. The methods for producing steam may vary from place to place and may be governed by local conditions. Steam-powered generating stations are often sited adjacent to large quantities of water for cooling and condensation purposes.
Gas-fired steam generators are noted for high efficiency and their ability to produce quantities of steam quickly. They have the ability to get alternators onto line for additional power supplies at peak times and make excellent standby plants for emergencies. The heat from the exhaust is returned to preheat water and air as well as driving the unit’s air-compressors to force air into the steam generator.
Italy has been generating electrical energy for many years by harnessing steam emerging from the earth. The steam is cleaned and fed to low-pressure turbines driving alternators. Precautions have to be taken to ensure that finely divided solids and `wet’ steam are prevented from reaching the turbines. Large volumes of steam are required, owing to the relatively lower temperature and pressure.
The USA and New Zealand have commenced using this method for power generation comparatively recently.
France and other European countries have committed their future to nuclear energy for the generation of steam to drive turbines. They have no access to cheap supplies of oil, coal or gas, so have to look to alternative means for the generation of electrical energy. The nuclear energy is used to produce steam which is then fed to turbines as in steam-powered generating stations.
France and England have done innovative work in collecting sea water by tidal movement for generating power with hydroelectric turbines. In some coastal areas these two countries have tidal movements of between 6 m and 9 m. On the north-west coastal regions of Australia there are similar variations in sea level. Because of low population densities it is currently not an economic proposition in Australia.
The principle is to contain as much water as possible at high tide and release it at low, tide through turbines. Generating electrical power this way is of course somewhat limited to times of low tide levels.
In Scandinavia, Canada, Tasmania, and the Snowy mountains in mainland Australia, hydroelectric power is generated by running large volumes of water through low-speed turbines. The water is collected at as high a level as possible in reservoirs. A power station is then built at a lower level.
As the water `falls’ down to the power station it gains sufficient energy to drive the turbines. Tasmania has ample supplies of water (most of the time) but no easy access to coal, oil or gas. The water run-off from the power station can be channelled to power stations at lower levels, used for irrigation, or red into the water supply of adjacent towns. (See Fig. 2.7.)
In remote areas, diesel-driven alternators are more common. The power is generated in a somewhat similar fashion although on a larger scale than that of the portable generator. It is a more expensive method for generating electrical power but if other fuel supplies such as water and coal or furnace oil are not available at an economical rate to generate steam, there is no alternative.