Transcript Alternative energy sources (Nuclear energy)

Some basic concepts of energy
(A) Conservation of energy
Energy cannot be created nor destroyed. It can be
transformed from one form to another, but the total
amount of energy remains unchanged.
Examples
 A light bulb changes electrical energy into light
energy and heat.
 A motor changes electrical energy into kinetic energy.
 An electric generator changes mechanical energy into
electrical energy.
(B) Units
 In physics, all forms of energy are measured in
joules (J).
 However, kilowatt-hour (kWh) and British thermal
unit (Btu) are also units of energy.
1 kWh = 3.6 x 106 J or 3.6 M J
1 Btu = 1.1 x 103 J or 1.1 k J
(C) Degradation
 Some forms of energy are more useful than others
because they are more suitable for doing work and
being transformed into other forms of energy.
 Electrical and chemical energy are in this category
and are called high-grade energy. Internal energy is
low-grade energy that is not easily transformed into
anything else.
 By the law of conservation of energy, energy can be
transformed from one form to another. Eventually, all
energy will be transformed to the low-grade internal
energy, such as work done against friction. This
process is called degradation of energy.
(D) Energy density
 In an energy source such as Uranium fuel and
coal, which has a high energy density, the
energy is concentrated and the converter need
to release it (i.e. nuclear reactor and a furnace)
is relatively small.
 On the contrary, the energy density of solar
energy is low and as a result a solar converter
must be larger for the same power output.
Non-renewable sources and
Renewable sources
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Non-renewable energy refers to energy that
cannot be renewed or can only be recreated in
nature in a period of time much longer than
that human beings consume the energy.
Examples
Coal, petroleum oil, natural gas and other
fossil fuels.
Nonrenewable energy
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Coal is formed by dead plants covered by sand
and mud. It contains mainly carbon.
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Oil is formed by dead marine plants and
animals covered by sand and mud. It contains
mainly hydrocarbons and is the largest energy
source in the world.
Renewable energy
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Renewable energy can be regenerated or
recycled through natural processes. While this
energy must be finite, the supply is so large
that there is no foreseeable end in our or our
children’s lifetimes.
Examples
Solar energy, wind energy, hydroelectric
energy, tidal energy, geothermal energy,
biomass energy etc.
Solar energy
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The sun’s energy is produced by nuclear fusion.
It consists of electromagnetic radiation ranging
from short-wavelength X-rays to longwavelength radio waves
99% of the energy is in the form of light,
infrared and ultraviolet radiation.
direct solar heating
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There is direct solar heating in which the light
energy is allowed to heat a house through
transparent windows or heat a mass of water which
can then be circulated to heat the home.
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There are photovoltaics or
solar cells where the light
energy is transferred into
electrical energy by the use
of solar panels. For example,
these panels can be attached
to the roof of a house. Also
many satellites use solar
panels for their power needs.
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Finally there is solar power station where the sun light
is reflected and focused using a huge parabolic mirror.
This reflected light energy can then be used to heat water
to create steam which can be used to drive a turbine to
produce electricity just like in a conventional power
plant.
Wind energy
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Wind energy is the kinetic energy contained in
the movement of a mass of air molecules. The
wind can be used to turn a turbine, which can
then produce electricity using a generator.
Advantages:
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Wind energy is unlimited and does not cause any
pollution problems.
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Building and running windmills are relatively cheap.
Disadvantages:
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A large number of windmills are required to produce
appreciable amounts of energy.
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The supply of wind energy is not steady since wind
may be weak in some days.
Hydroelectric power
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Potential energy of water in a high-level
reservoir becomes kinetic energy when it flows
from a dam. The flowing water turns a turbine
at the bottom of a dam to generate electricity.
Hydroelectric power
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Advantages:
Hydroelectric energy is unlimited.
Disadvantages:
The hydroelectric power stations are only suitable to
be built in mountainous regions having high rainfall.
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Tides rise and fall each day. When tides come into the
shore, they can be trapped in reservoirs behind dams.
When the tide drops, the water behind the dam can be
let out just like in a regular hydroelectric power plant.
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In order for tidal energy to work well, at least a 16feet difference between low tide and high tide is
needed. There are only a few places where this tide
change occurs around the earth.
One plant in France makes enough energy from tides
(240 megawatts) to power 240,000 homes.
However, the aquatic life may be affected by
interfering the rise and fall of tides.
Geothermal energy
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Geothermal energy is heat
energy that comes from the
molten interior of the earth.
There are places where this
molten material is close
enough to the surface.
Geothermal energy
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A long tunnel is drilled
into the ground to reach
the hot rock. Cold water is
pumped into the tunnel
and is heated up under the
ground. The heated water
or steam will emerge from
the tunnel to generate
electricity by turning a
turbine.
Biofuels
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To many people, the most familiar forms of
renewable energy are the wind and the sun.
But biomass (plant material and animal waste)
supplies almost 15 times as much energy in the
United States as wind and solar power
combined and has the potential to supply much
more
Biofuels
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Biomass is a renewable energy
source because the energy it
contains comes from the sun.
Through the process of
photosynthesis, chlorophyll in
plants captures the sun’s energy by
converting carbon dioxide from the
air and water from the ground into
carbohydrates, complex compounds
composed of carbon, hydrogen, and
oxygen. When these carbohydrates
are burned, they turn back into
carbon dioxide and water and
release the sun’s energy they contain.
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Biomass energy can also generated by the decay of
large masses of plant or animal material or waste
which forms methane and other combustible gases.
These gases contain chemical energy, which when
burned can be used to generate electricity.
Alternative energy sources (Nuclear
energy)
Nuclear energy from controlled nuclear fission can be
used to generate electricity.
Advantages:
 1. Unlike coal- or oil-fired power stations, nuclear
fuel is small in size and so there is no fuel
transportation problem.
 2. It is cheaper than coal or oil for generating
electricity.
 3. It is clean and causes little environmental
pollution while coal- and oil- fired power stations
emit large amount of fly-ash.
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Disadvantages
 1. However, nuclear energy constitutes an
unacceptable hazard to the public.
 2. Although the chance of an accident happening is
very small, the consequence is extremely serious.
 3. Large sums of money have to spend on
maintaining and upgrading the safety standards of the
reactor.
 4. Besides, the wide spread use of nuclear energy
will lead to the growth of nuclear weapons.
Efficiency of energy transform
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Consider a light bulb which changes electrical energy
into light energy.
Electrical energy (100 J) ⇒ Light energy (30 J)+
Heat (70 J)
Efficiency
= (useful energy output)/(total energy input) x 100%
Efficiency
= (useful energy output)/(total energy input) x 100%
Efficiency of the light bulb = 30 / 100 x 100%
Note:
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1. The efficiency of an ideal transformer is
100% while that of a practical one is about
90%.
2. In a real case, the efficiency must be
smaller than 100% as some energy is lost, such
as work done against friction.
Device
Efficiency
Energy transfer
Large electric motor
90 %
Electrical to mechanical
Large electric
generator
90 %
Mechanical to electrical
Domestic gas boiler
75 %
Chemical to internal
energy
Steam turbine
45 %
Internal energy to
mechanical
Car engine
25 %
Chemical to mechanical
Example 1
Calculate the efficiency of a 1250 W electrical kettle if it takes 15
minutes to boil 2.2 kg of water from 20 oC to 100 oC. It is given
that the specific heat capacity of water is 4200 J kg-1 oC-1.
Explain why the efficiency is not equal to 100 %.
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Solution:
Energy input (electrical energy) = Pt
=1250 x 15 x 60 = 1125000 J
Energy output (heat) = mcDT
= 2.2 x 4200 x 80 = 739200 J
Efficiency of the kettle
= 739200 / 1125000 x 100% = 65.7 %