Energy sources

Energy is available from a number of sources:

Pumped storage system

The water flows down through the turbines during the day and is then pumped back uphill overnight during periods of low electricity demand.

Fossil fuels

Coal
Oil
Gas

Since there is a limit to the reserves of fossil fuels considerable work is being done to develop other alternative sources of energy.
The major need of the western world is to use one of the primary sources of energy to generate electricity. Some of the various primary sources are considered below.

Wood

Gasification of wood produces a gas that can be used for combustion.

Biochemical energy

Alcohol can be produced from cane sugar, maize, cassava, sago palms, yams and other root crops and then used to generate heat and finally electricity. The growth of some bacteria may also be a way of producing flammable gases.

Geothermal energy

This results from the flow of heat up through the Earth's crust from hotter rocks beneath the surface. In Iceland 30 per cent of domestic hot water is produced using geothermal energy. In Britain the average thermal gradient appears to be about 40 ^oC km^-1 and the average potential for heat extraction is some 6 x 10^-2 W m^-2
The maximum value found globally is some 10W m^-2.

Wind

Offshore 'wind farms' are now a source of renewable energy being favourably considered. Windmills with vanes of diameter 130 m can give up to 7.2 MW. A very large windfarm, with an area of over 500 km² is scheduled to be constructed on the Dogger Bank off the east coast of England starting in 2016. This is the first of two area to be developed each with a capacity of 1.2 GW.

Hydroelectricity

Hydroelectricity now generates some 20-25 per cent of the world's electricity and there are about 100 schemes of 100 MW capacity throughout the world. The pumped storage system as used in Snowdonia is a useful variation of the normal hydroelectric power.

Waves

Wave energy is thought to be able to generate about 5 kW m^-1 in accessible positions from the shore. On this basis about 8 GW may be achieved from the shores of the United Kingdom.

Tides

The tidal barrage across the river Severn is still being considered as a potential large energy source. This scheme, which would be the biggest of its kind in the world would use 192 turbines with an installed capacity of over 7000 MW. A smaller scheme for the River Mersey would have a capacity of some 500MW.

There would be major environmental impact from such a large dam so submersible turbines have been considered. The problem with these is that the water flow through them needs to be laminar. In the case of the turbulent waters of the Severn estuary it is likely that the turbine blades would break apart due to instability.

Solar energy

The heat from the Sun may be used to produce steam, and this steam used to drive turbines. One array of mirrors in Southern California has an area of 7.3 x 10⁴ m² and the steam produced by it gives an electrical output of 10 MW. This facility assumes 300 cloudless days a year! An alternative form of solar power is to convert the sunlight to electricity directly using photovoltaic cells. Cost is a problem here (at the time of writing it is some $0.7 per watt) but this method may still be attractive in very sunny positions.

Nuclear energy

The use of nuclear fission for the generation of electricity has become very important in this century. Many countries in the world such as the USA, Great Britain, France, Canada and the Soviet Union have a well-developed nuclear fission reactor programme although the reactors are of different types. The nuclear fusion reaction may be a source of energy in the future although this may not be until after the year 2020.

The large requirement of energy means that these alternative energy facilities have to be large if they are to replace conventional plant to any substantial extent. For example, to generate the 1000 MW produced by many large power stations would need one of the following:

300 windmills each with a 100 m diameter rotor
a large tidal barrage
a very large dam in a hydroelectric scheme
a 400 km² solar collector
a 100 km ocean wave-powered generator.

Osmotic pressure

It is hoped to use the osmotic pressure the could be produced in estauries to give usable energy. osmotic power is a clean, renewable energy with a global capacity of some 1600 TWh - equal to the annual consumption of China.

Power 'used' by an adult human

Almost everything that happens in the world is a result of a change of energy from one form to another, although the conversion is never 100 per cent efficient. The following table shows the amounts of power used by an adult male of mass 76 kg when doing various activities.

Activity	Power used (W)		Activity	Power used (W)
Sleeping	83		Swimming (breaststoke 1.6 km hr-1)	475
Sitting	120		Skating	535
Riding in a car	140		Climbing stairs (116 steps min-1)	685
Walking (4.8 kmh-1)	265		Tennis	440
Cycling (15 kmh-1)	410		Cycling (21.3 kmh-1)	700
Basketball	800		Running (100 m at 36 kmh-1)	1500

Energy types

Mechanical (Tensile, kinetic, gravitational potential)
Magnetic
Heat (radiant and kinetic)
Nuclear
Sound (kinetic)
Magnetic
Chemical
Electrical