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8.5 Electricity Generation and Transmission

Generation of Electricity

Electrical energy is not a natural or primary source of energy an it cannot be stored very easily.

Electrical energy is generated by several ways:-

(i) Stored energy (fossil fuel, nuclear and biomass) is converted to thermal energy to make steam that turns the turbine attached to the generator.

(ii) Gravitational potential energy (Hydro) is converted to kinetic energy of water to turn the turbine

(iii) Kinetic energy of wind turns a wind turbine to generate electrical energy

(iv) Solar energy transforms directly into electrical energy by solar cells

(v) Chemical energy transforms directly into electrical energy by accumulators

Sources of energy

Sources of energy can b divided into two

classifications:

(i) Non-renewable sources

– Fossil fuels

– Nuclear fuel

(ii) Renewable source

– Hydropower

-Solar energy

-Biomass

-Wind power

-Geothermal energy

-Wave energy

-Ocean thermal energy

-Tidal energy

Generation of Electricity from VariousSources

(1) Thermal power stations

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Fuels such as coal, petroleum natural gas and uranium are used in the thermal power stations.

Heat energy is produced by burning coal or oil in a furnace or from the fission of uranium nuclei in the core of a nuclear reactor.

Water absorbs the heat energy in a boiler or heat-exchanger and is turned into steam at a high pressure.

The high-pressure steam converts heat energy into mechanical energy as it turns the turbine.

The generator converts the mechanical energy into electrical energy.

Efficiency : 30% – 35% of the energy stored in the fuel is transformed into electrical energy.

The disadvantages of this energy source:

(i) Air pollutants

Burning of fuels produces smoke, dust and toxic acid in the air

(ii) Acid rain

Burning of fuel also produced acidic gases such as sulphur dioxide and nitrogen oxides. These gases dissolve in water in the atmosphere to form acids which results in acid rain.

(iii) Greenhouse effect

The excessive carbon dioxide in the atmosphere gives rise to higher temperatures in the environment.

(iv) Non-renewable

Their supply is limited and they will eventually run out.

(v) Expensive

(vi) Harmful effects of radioactive

radiation on humans and environments

(viii) Bad smell

A bad smell is released from the biomass

The advantages of this energy source:

(i) Can develops a new town ( coal, petroleum)

(ii) Can recycle the waste matter (biomass)

(iii) Small amount of radioactive is required

(2) Hydroelectric power station

clip_image004

In hydroelectric stations there is no need to heat steam at all.

Water is collected in a high reservoir and possesses a high gravitational potential energy.

When the water is felt through a pipe , its potential energy changes to kinetic energy.

The kinetic energy of water changes to electrical energy when the water turns

the blades of a turbine.

The disadvantages of this energy source:

(i) Disturbs the equilibrium of the ecosystem in the environment.

A large area of forest land has to be destroyed include flora and fauna.

(ii) Displaces local population

(iii) Highly cost

The advantages of this energy source:

(i) Clean and does not emit pollutants to the environment.

(ii) Renewable

(iii) Can develops a recreation area

(iv) Control flood

(3) Solar energy

Solar energy can be converted into electrical energy by two methods:

(i) Solar furnace

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A concave paraboloidal reflector can be used to focus and concentrate the radiant heat energy from the sun. At the focus the heat energy is as concentrated as that produced by burning fossil fuels. The heat energy is used to produce steam whose kinetic energy is used to drive the turbine of an electrical generator.

clip_image008Solar panels use sun energy to generate thermal energy to heat water at home.

(ii) Solar cell

clip_image010

Solar cells are made of semiconductor materials which develop an e.m.f. when exposed to sunlight.

Solar cell are used in calculator, lamps, wrist watch , heating water and in satellite.

The disadvantages of this energy source:

(i) Requires a large area to collect the Sun’s ray.

(ii) The efficiency of conversion is quite low (about 5%) and the cell production are expensive to produce.

(iii) The intensity of Sun’s ray collected is not consistent , depends on weather condition and time of day.

The advantages of this energy source:

(i) Clean and does not emit pollutants to the environment.

(ii) Renewable

(iii) Free

(4) Wind energy

clip_image012

The kinetic energy of wind rotates blades connected to the rotor of an electrical generator to produce the electrical energy.

The disadvantages of this energy source:

(i) Requires a large area to construct a wind turbine.

(ii) The speed of rotation of the rotor is not consistent, depends on weather conditions.

(iii) Produces noise.

The advantages of this energy source:

(i) Clean and does not emit pollutants to the environment.

(ii) Renewable

(iii) Free

(5) Wave energy

clip_image014

clip_image016

The ‘Salter duck ‘ , is a specially shaped float which rocks back and forth as the waves strikes it. This rocking motion can be used to drive a turbine and generate electricity. The whole wave energy generator has a row of ducks lined up facing the oncoming waves.

The disadvantages of this energy source:

(i) The high cost of building and maintaining the wave generators.

(ii) Very vulnerable and difficult to defend.

The advantages of this energy source:

(i) Clean and does not emit pollutants to the environment.

(ii) Renewable

(iii) Free

Transmission of Electricity

The major problems to transmit the electrical energy from a power station to the users is the power loss in the cables as heat energy.

Power loss in the cables ,

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P = Power loss in the cables

I = Current flows in the cables

R = Resistance of the cables

To find the current flows in the cable;

clip_image020

I = Current flows in the cables

P = Power transmits through the cables

V = Voltage transmits through the cables

Reducing power loss in transmission lines

(1) Using thick cables with a large cross-sectional area to reduce the resistance because P α R

Aluminium alloys are used because they are cheaper and much lighter than copper.

(2) Reducing the current through the cables because P α I2.

To overcome this problem, electrical energy is transmitted at high voltage because

clip_image022

Transformers are used to increase or decrease the voltage and the transformer can only function with the voltage of an alternating current.

This is the reason why alternating current is used in the transmission of electricity.

Example 1

A power station generates 40 kW of electric power using a power transmission line with resistance of 8W.

What is the power wasted due to the resistance of transmission cable when the voltage is transmitted at

(a) 500 V (b) 20 kV

Solution

Example 2

A power station generates 80 MW of electric power at a voltage 80 kV through a cable of resistance 5W .

Determine

(a) The power loss due to the transmission cable

(b) The percentages of the power loss

(c) The efficiency of the power transmission

(d) The potential difference along the cable

Solution:

Model of an Electricity Transmission System

clip_image024

To set up a model power lines in the laboratory these following ideas should be followed:

(1) Wires must have a low resistance to reduce power loss

(2) Electrical power must be transmitted at low currents to reduce power loss.

(3) To carry the same power at low current we must use a high voltage

(4) To step up to a high voltage at the beginning of a transmission line and to step down to a low voltage again at the end we need transformers.

(5) Transformers only work when they are supplied with alternating current.

Example 3

In a model of power line , a 12V a.c. supply is connected by wires of total resistance 4Ω to a lamp of resistance 6Ω.

Calculate

(a) The current flowing in the wires

(b) The power loss in the wires

(c) The voltage

       i. drop along the power line

       ii. available to the lamp at the end of the line

(d) The power converted in the lamp

Solution

National Grid Network

clip_image026

The National Grid Network is a network of transmissions lines which link all the major power stations in the country with all the major users such as our homes, offices, cities and factories.

Electrical energy generated in power stations where its voltage is increased by using a step-up transformer to reduce energy loss in the cable before transmission via the national grid.

At a sub-stations, before reaching the various consumers , the high voltage is reduced by using step-down transformer.

Advantages of the National Grid Network

(1) Easy to manage.

        Regional control and switching centres enable power to be sent where and when it is needed.

(2) Less interruptions and continuously

       Regional control and switching centres allow some stations and lines to be shut down for maintenance work without cutting off the consumers.

(3) Reducing cost of generation

       High voltage transmission can reduce the power loss in transmission cables. The cost of production is reduced.

(4) Easy to control and regulate

     At peak periods of electricity usage , more generators can be switched on. During periods of low demand ,some generators can be turned off.

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