Chapter 24 – Alternative Energy Sources

Nuclear Energy – Fission Reactors Large quantities of energy are released when the nucleus of the uranium isotope U splits in response to being bombarded by neutrons. The nuclei split in a process called nuclear fission. New elements are formed so it is described as a nuclear reaction. The neutrons produced in this process then split more uranium atoms, so more energy and neutrons are produced. This process continues causing a chain reaction.

Fission Reactors cont… The energy released from the fission of uranium- 235 can be used as a heat source for generating electricity in a power station. 1kg of uranium produces the same electrical energy as 2500 tonnes of coal. Reactor designs vary. In some nuclear reactors, control rods containing neutron absorbers such as boron are inserted between fuel rods containing uranium to control the reaction rate.

Fission Reactors cont… The reactor vessel is in water, which cools it and also absorbs neutron to moderate the chain reaction below: n + U  Ba + Kr + 3 n + energy Other fission reactions for uranium-235 include: n + U  Ba + Kr + 2 n + energy n + U  Ba + Kr + 3 n + energy n + U  Te + Zr + 3 n + energy In these fission reactions, a small amount of mass is converted to energy. The amount of energy obtained when mass is converted to energy is given E = mc 2.

Nuclear Waste The presence of radioactive isotopes in spent fuel requires long term waste solutions. Steel and concrete containers are used to transport nuclear waste by road and rail. Nuclear waste gives off a considerable amount of heat and must be stored for about five years to all short-lived radioactive wastes to decay and the material to cool down. The waste containing longer lived isotopes must then be stored safely for several thousand generations.

Nuclear Waste cont… Methods used to store waste include: – Burial in sealed tanks or stockpiling in deep mines in geologically sound regions. – Stockpiling in air conditioned warehouses. – Dumping sealed containers of waste in deep ocean trenches. – Sealing it in a special type of glass.

Advantages/Disadvantages ADVANTAGESDISADVANTAGES Accident rates are very low at nuclear plants Release of radioactive substances by accident, hijacking or sabotage. Nuclear plants are not sources of air pollution. Can cause release of radioactive particles in large amount, ie Chernobyl Conserve fossil fuels and as a consequence reduce acid rain and production of greenhouse gases. Radioactive material designed for good purposes could be used in the manufacture of weapons Long construction time and a limited working life. Greenhouse gases produced during the mining, transport and refining of uranium and in the construction of the stations.

Nuclear Fusion When a larger nucleus is formation from smaller ones it is call nuclear fission. For example, at high temperatures, hydrogen nuclei can combine to form helium nuclei. Nuclear fusion occurs in the Sun and is the source of solar energy. It also occurs when a hydrogen bomb is detonated, but such a rapid release is not suitable for a power station. It is were possible to control nuclear fusion it would offer clean and safe power, with 1kg of fusionable material producing the energy available from 10,000,000kg of fossil fuel.

Nuclear Fusion cont… There have been two main problems in the development of this energy source: – Sustaining the temperatures of 100,000,000°C required for the reaction. – Containing material at such high temperatures.

Renewable Energy Sources Renewable energy sources are those that are continually being replaced by natural processes.

Biochemical Fuels Biodiesel is derived from the triglycerides in vegetable oils and animal fats that undergo a transesterification reaction with methanol to produce a mixture of methyl esters of the fatty acids. The glycerol and esters are separated. The mixture of esters of the fatty acids is sold as biodiesel. Waste plant material can be converted into the mixture of carbon monoxide and hydrogen known as syngas, which can be used to generate heat and electricity. It can also be converted into diesel.

Solar Energy The sun provides the Earth with about J of energy per hour. Solar efficient design of builidings dramatically reduces heating, cooling and lighting costs. Electricity can also be generated by using parabolic collectors or dishes to focus the Sun’s energy. A liquid is used to transfer the heat from this point to a turbine and generator, as in a coal-fired power station. The disadvantages of generating electricity using solar cells are that: – They are expensive – They are less effective in cloudy weather and do not operate at night – Large areas of land are needed for collection because of their relatively low efficiency as energy converters.

Hydroelectricity Hydroelectricity is obtained by using the energy of falling water to drive turbines connected to electricity generators. In a sense, hydroelectric power stations are solar powered. Solar energy evaporates water, transferring it from low to higher altitudes in the form of rain. Gravitational potential energy is converted to kinetic energy as rainwater in rivers flows downhill.

Hydroelectricity cont… Hydroelectricity power stations supply about 25% of the world’s and 10% of Australia’s electricity. Further development of this energy resource is possible in Australia, but will be restricted because of: – A limited number of suitable sites – Concerns about the environmental impact of building large dams.

Wind Power Wind-powered electricity generators are a more recent development. They are usually near the coast to make the most of the higher wind speeds. Average wind speeds exceeding 5m/s are regarded as suitable for electricity generation. The disadvantages of wind-powered electricity generation are that: – The turbines are highly visible and their effect on the landscape must be considered – The moving blades of the turbines produce some noise so they must be away from populous areas – Electricity production is dependent on wind speeds and is not more reliable than the wind – Very large number of turbines are needed to equal the output of a coal-fired power station

Tidal Power Average wave heights of 2.5 metres off the Australian coast are ideal for wave-energy production. Tidal power stations are best located where there is a large drop between high and low tides. These are several potential sites in Australia.

Geothermal Power In volcanic regions of the world, heat flowing from hot underground rocks to underground water may cause superheated steam to reach the surface. The energy from the steam can be harnessed to produce electricity. Geothermal power stations are a proven, cheap and relatively clean form of power, however, it is restricted to suitable sites.