Strain energy
When a force is applied to a structure and causes it to distort, the point of application of the force is changed and work is done. This can be stored ii structure as strain energy or resilience (see section 8.11). If the force is removed this energy may be recovered and converted to other forms. In the operation of a mechanical pinball machine, for instance, strain energy is stored in the spring and then released in the form of kinetic energy or potential energy of the ball.
Heat
All substances consist of molecules, which are in motion, and between which there are forces of attraction. In everyday language we call this molecular energy heat. In thermodynamics it is termed internal energy and the word ‘heat’ is used t mean the energy that is transferred from one body to another by conductivity convection or radiation.
Chemical energy
Fuels such as oil and coal contain the elements carbon, hydrogen and sometimes sulphur, each of which combines with oxygen when the fuel is burned, releasing energy in the process. The amount of energy available from the fuel is called its calorific value and it is usually quoted for 1 kg. In the case of gaseous fuels it may be given in terms of volume. The energy released during combustion may be used directly for heating purposes or it may be converted to other forms.
Electrical energy
Electricity is not a form of energy itself but it is often a very convenient means of transferring energy from one body to another. We use the term ‘electrical energy’ for the energy being transferred. The energy transferred in this way can be stored in electrochemical batteries as potential energy.
Sources of Energy
Solar energy
Enormous amounts of energy reach the outer atmosphere of the Earth from the Sun. The quantity reaching the surface depends upon the ozone layer, dust, clouds and latitude. Even in temperate latitudes it can amount to 500 W/m2. This energy is readily available for producing hot water and spa heating using simple solar panels. Installation costs are high, however, and the energy available decreases in winter when the requirement is greatest. Much higher temperatures are needed for power generation and these can l~ achieved by focusing the Sun’s rays with concave mirrors.
Solar cells convert the Sun’s energy directly into electricity. These have been used to power small experimental cars through electric motors of about 1 hp. Photoelectric cells are already widely used in light meters and solar powered calculators where very small currents are sufficient but they are n yet economic for power generation.
Ocean thermal energy
The Sun causes the surface temperatures of oceans to be considerably high than that at great depths. This temperature difference could be used in a thermodynamic cycle to generate electricity.
Tidal energy
Seawater can be used to drive turbines in a barrage built across a bay or estuary using the potential energy that exists when the levels are different on the two sides. In its simplest form, such a scheme could only generate electricity for part of the day but constant generation is possible using two basins. One tidal generating scheme has been built on a river estuary in northern France.
The use of tidal energy is not new. The tidal mill at Woodbridge in Suffolk had been working for 200 years when it closed in 1957 and the first such mill on the site was built in about 1170.
Hydroelectric power
In mountainous countries substantial amounts of power can be generated from the potential energy of water stored by dams. Although the construction costs can be very high the direct cost of production is very small.
Wave energy
The vertical motion of the sea can be utilised to generate power in several ways. One system uses the relative movements of floating rafts to convert the wave energy. Very large amounts of energy are theoretically available, particularly in the winter when the requirement is greatest.
Geothermal energy
We know that very high temperatures exist within the Earth and in some places heat in practical quantities is available at the surface. Hot springs have been used at least since Roman times. There are installations in several countries, which use geothermal energy to generate electricity.
Wind energy
Wind power has been harnessed for many centuries and is now seriously considered for the generation of electricity. We can make a rough estimate of the power available from a windmill by considering the kinetic energy of the air approaching the circle swept out by the blades.
