Transcript Alternative sources of energy

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 tidal(潮汐) power plant
A geothermal(地熱) 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.
Petrol, a derivative from petroleum
Coal
Uranium fuel
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Solar energy
• 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).
Example 4.1
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Solar heating(太陽能加熱)
• Solar energy can be used to cook or heat up water.
Solar water heater
Solar collector
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Solar electricity
• Solar electricity usually refers to the use of a solar
cell, or a photovoltaic cell, to generate electricity.
A solar cell and its structure
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At the p-n junction, an electric field
points from the n-side to the p-side.
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’.
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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Current is produced when sunlight is incident on the solar cell.
Working principle of a solar cell
Example 4.2
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Wind power
• 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 turbine
Structure of a wind 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.
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Hydroelectricity (Hydroelectric power)
• 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.
Water gains kinetic energy
when running down.
Hydroelectric power
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A hydroelectric power plant
Schematic structure of a hydroelectric power plant
Example 4.4
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Nuclear power
• 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.
An atom consists of a massive
nucleus surrounded by a cloud
of electrons.
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Binding 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
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Chain 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
Nuclear 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.
Radioactive waste must be treated
with due care.
The Chernobyl disaster
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• Most renewable energy sources are clean and
sustainable, but they are more expensive to use.
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Costs of generating electricity using different energy sources in 2007
• 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
Building hydroelectricity
power plant affects the
aquatic ecology.
Wind power
Building a wind farm
requires a large space
with high enough wind
speed.
Solar electricity
Generating electricity
with solar cells is not
cost effective and has
low efficiency.
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Pollution problems caused by the use of energy
• Burning fossil fuels brings pollution problems.
• Some power plants and vehicles release pollutants
such as carbon dioxide, sulphur dioxide, nitrogen
oxides and particulates.
Photochemical smog is a result
of air pollution.
Many pollutants are emitted from
a coal-burning power plant.
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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
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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 Enduse 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
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Energy use in different sectors from 1997 to 2007
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Checkpoint (p.132)