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Alternative sources of energy

• Renewable energy sources (可再生能源 )are derived from natural process and can be generated in a relatively short time.

Photovoltaic cells A wind turbine光伏電池

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A hydroelectric power plant A geothermal(地熱 ) power plant

A tidal(潮汐 ) power plant A biomass(生物質能 )factory

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• Non-renewable energy sources(不可再生能源 ) cannot be regenerated in a relatively short time.

Uranium fuel

Petrol, a derivative from petroleum Coal

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• Solar energy is the energy we obtained directly from the Sun as it is shining over the Earth all the time.

• Solar radiation power per unit area received at the top of the Earth’s atmosphere is the solar constant太陽常數(≈1360 W m–2).

Solar energy

Example 4.1

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• Solar energy can be used to cook or heat up water.

Solar heating(太陽能加熱 )

Solar water heater

Solar collector

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• Solar electricity usually refers to the use of a solar cell, or a photovoltaic cell, to generate electricity.

Solar electricity

A solar cell and its structure

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When light shines on the junction, an electron leaves an atom and moves to the n-side. On losing an electron, the atom becomes a positively charged ‘hole’.

At the p-n junction, an electric fieldpoints from the n-side to the p-side.

The electric field causes the electrons from the neighbouring atoms to fill up the vacant sites, so the positive hole appears to move to the p-side.

Formation of an electron-hole pair at the p-n junction

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Example 4.2Checkpoint (p.109) O

Current is produced when sunlight is incident on the solar cell.

Working principle of a solar cell

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• Wind is produced by the movement of air from regions of high pressure to regions of low pressure.

• A modern wind turbine captures wind to rotate its blades and drive the gearbox behind.

• The kinetic energy of the wind is thus converted into electrical energy.

Wind power

Structure of a wind turbineWind turbine

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• The maximum power obtained from a wind turbine is

where ρ and v are the density and velocity of air respectively, and A is the swept area of the wind turbine.

Example 4.3

In time t, the volume of air passing through the wind turbine is Avt.

Checkpoint (p.113) O

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• Hydroelectricity is currently the largest renewable energy source in the world.

• It makes use of the conversion of potential energy of the water to the electrical energy.

Hydroelectricity (Hydroelectric power)

Water gains kinetic energy when running down.

Hydroelectric power

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Example 4.4Checkpoint (p.115) O

Schematic structure of a hydroelectric power plant

A hydroelectric power plant

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• There is a very strong attractive force which holds the nucleons together in a nucleus.

• This force is associated with a large amount of energy, which is known as the binding energy Eb of a nucleus.

• The average binding energy per nucleon is found from dividing the binding energy Eb of a nucleus by the mass number A.

Nuclear power

An atom consists of a massive nucleus surrounded by a cloud of electrons.

14Binding energy curve

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• In nuclear fission, an atomic nucleus splits into two or more smaller nuclei involves the splitting of atomic nuclei.

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• In nuclear fusion, two or more atomic nuclei merge to form a larger nucleus.

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• The amount of energy released can be calculated from the mass defect of the reaction.

Example 4.5 Example 4.6

18Chain reaction of uranium-235

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• Fuel rods contain 3% of uranium-235 for fission chain reaction to take place.

• Control rods are used to absorb excessive neutrons and slow down the reaction.

• Moderator is used to slow down the fast neutrons and as an absorber of heat from the fuel rods.

• Coolant is used to cool down the steam or water passed through the turbine.

Structure of the nuclear reactor in a nuclear power plant

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Schematic diagram of a nuclear power plant

Checkpoint (p.122) ONuclear power plant

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Energy, society and environment• The use of energy is crucial to the development

of a modern society.

Factories consume a lot of electricity.

Modern transportation allows people and resources to move around at high speeds across the globe.

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• Because of the depletion (消耗 )of fossil fuels, we make use of alternative energy sources to replace the fossil fuel.

We depend heavily on fossil fuels such as oil.

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• Nuclear power is an abundant energy source which produces virtually no emissions, and its energy cost is comparable to that of fossil fuel power plants.

A nuclear power plant

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• However, the disposal of nuclear waste and risk of nuclear disaster are still major problems in the industry.

The Chernobyl disasterRadioactive waste must be treated with due care.

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• Most renewable energy sources are clean and sustainable, but they are more expensive to use.

Costs of generating electricity using different energy sources in 2007

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• We also have to consider the potential risks and problems the use of energy sources pose on our environment and the extra costs for handling these issues.

Hydroelectricity Wind power Solar electricity

Building hydroelectricity power plant affects the aquatic ecology.

Building a wind farm requires a large space with high enough wind speed.

Generating electricity with solar cells is not cost effective and has low efficiency.

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• Burning fossil fuels brings pollution problems.• Some power plants and vehicles release pollutants

such as carbon dioxide, sulphur dioxide, nitrogen oxides and particulates.

Pollution problems caused by the use of energy

Many pollutants are emitted from a coal-burning power plant.

Photochemical smog is a result of air pollution.

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• Scientists have discovered that the average temperature on the Earth has been rising since the 19th century. This is known as global warming.

Rise in average temperature of the Earth in the last 120 years

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• Global warming has been associated with the rapid growth in the amount of CO2 in the atmosphere caused by burning fuels.

Rise in carbon dioxide concentration during the last 200 years

Checkpoint (p.130) O

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Energy end-use data in Hong Kong

• Statistics of energy use in Hong Kong can be found in Hong Kong Energy End-use Data.

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• For commercial energy end-use, air conditioning and lighting account for the largest energy consumption.

Commercial energy end-use data in 2007

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• For residential energy end-use, air conditioning, cooking and lighting account for the largest energy consumption.

Residential energy end-use data in 2007

33Checkpoint (p.132) O

Energy use in different sectors from 1997 to 2007