3.5 Understanding the generation and transmission of electricity

Sources of energy for the generation of electricity

Non-renewable sources Renewable sources

• Fossil fuels: coal, petroleum and natural gas•Nuclear fuel

•Hydropower•Biomass•Solar energy•Wind power•Tidal energy•Wave energy•Geothermal energy

Thermal power stations using Fossil Fuels

Using coal, petroleum and natural gas in a furnace to produce heat to boil later in boiler. Steam at high pressure is formed.

The steam drives a turbine which rotates the a.c. generator

The lower pressure exhaust steam is condensed back to water by cooling water in a condenser

The cooling water absorbs latent heat from the steam and is sent to the cooling tower

Heat is lost to the atmosphere in the form of warm air and the cooled water is recirculated for further use

Effects on the environment(1) Burning fossil fuel produces smoke, dust and toxic gasses (NO, SO2) causing air pollution.(2) Excessive CO2 causes greenhouse effect and gives rise to global warming(3)Toxic gasses form acid rain affect the equilibrium of the earth’s ecosystem(4) Expensive and non-renewable

Nuclear Power Reactor

Pressurized Water Reactor (PWR)

In a PWR, the heat is removed from the reactor by water flowing in a closed pressurized loop. The heat is transferred to a second water loop through a heat exchanger. The second loop is kept at a lower pressure, allowing the water to boil and create steam, which is used to turn the turbine-generator and produce electricity. Afterward, the steam is condensed into water and returned to the heat exchanger.

AdvantagesHigh energy density (small amount release large quantity energy)Minimum discharge of toxic gas and CO2

Reactor also can used to produce useful radioisotopes for industry, medicine and agricultureDisadvantagesHigh cost of designing and buildingWaste highly radioactiveRisk accident lead to leakage to environmentNuclear fuel enrich to produce weaponHot water discharged causes thermal pollution

In a BWR, water boils inside the reactor itself, and the steam goes directly to the turbine-generator to produce electricity. Here, too, the steam is condensed and reused

Boiling Water Reactor (BWR)

A stable atom of Uranium-235 becomes an unstable (radioactive) atom of Uranium-236 when it absorbs a neutron. The nucleus of an unstable U-236 atom breaks apart spontaneously, releasing a large amount of energy, nuclei of two lighter elements, and three neutrons.

Other U-235 atoms can absorb the neutrons that are released in the reaction, forming more unstable atoms. As increasing numbers of unstable atoms experience fission, more energy and neutrons are released, resulting in a chain reaction of fission.

Fission of uranium is carried out at a controlled rate. The heat energy released in the fission process is used to heat water to produced high-pressure steam to turn a turbine of a power generator

Hydroelectricity

Energy from water is created by the force of water moving from a higher elevation to a lower elevation through a large pipe (penstock). When the water reaches the end of the pipe, it hits and spins a water wheel or turbine. The turbine rotates the connected shaft, which then turns the generator, making electricity. The turbine and generator change mechanical energy into electrical energy.

AdvantagesFree and renewable, clean and does not pollute environmentWater in dam can be used to irrigate farms and control floods

DisadvantagesBuilding of dams can upset the balance of the ecosystemNot suitable in earthquake regionNot suitable in flat and dry regions

Solar Energy

Solar One power station uses the Sun's heat to make steam, and drive a generator to make electricity. Mirrors are arranged in -circles around the "power tower". the mirrors turn to keep the rays focused on the tower, where oil is heated to 3,000 degrees Celsius, The heat from the oil is used to generate steam, which then drives a turbine, which in turn drives a generator capable of providing 10MW of electrical power.

Advantages (a) Freely available and renewable(b) Clean and does not pollute environment.

Disadvantages(a) Require very large space to collect sufficient energy(b) Lower efficiency in electricity generation(c) Intensity of sunlight is dependent on seasonal changes, climate and latitude of the region

Biomass

Sugar cane can be fermented to make alcohol, which can be burned to generate power

Other solid wastes, can be burned to provide heat, or used to make steam for a power station. "Bioconversion" uses plant and animal wastes to produce fuels such as methanol, natural gas, and oil. We can use rubbish, animal manure, woodchips, seaweed, corn stalks and other wastes.

Advantages(a) Renewable(b) Liquid biofuels from biomass are lead free and sulphur-free(c) Reduces problem of disposal of organic waste

Disadvantages(a) Require large storage space far from human population if the biomass is animal dung or sewage

How it works

Wind

The wind blows the propeller round, which turns a generator to produce electricity. We tend to build many of these towers together, to make a "wind farm" and produce more electricity. The more towers, the more wind, and the larger the propellers, the more electricity we can make.

Advantages(a) Renewable and free(b) Clean and does not pollute environment(c) Suitable at isolated places such as mountainous regions and islands

Disadvantages(a) Requires many windmills which cover a large area(b) Generates high level of noise(c) Spoil the natural beauty of the region

Electricity Transmission

He electrical resistance of the cables causes heat to be produced in the cable during transmission. The heat energy lost is equal to I2 R To overcome the energy loss, electrical energy is transmitted at a high voltage, to reduce the current and therefore reduce the I2 RTransformers play an important role in the transmission of electricityThe power to be transmitted by the cables is given by P=IVI is inversely proportional to the voltage for a certain value of power

Example:A power station produces electricity power of 100 kW. This power is transmitted at a voltage of 2 kV by a cable with a resistance of 5 . Determine :a). The current in the cableb). Power loss. (Due to the heating of the cable)c). The potential difference across the end of the cable.d). Power loss, if it is transmitted at a voltage of 200 kV

Answer:

Answer:

Transmitted by 2kV meant, the output voltage of the generator transformer is 2kV. The 5 wire will be back to the generator transformer. (Because of the resistance of the wire which causes a power loss, the potential difference across the ends of the wire is not 2 kV)

a). P =IV I = P/V =[100 x 103 ]/[2 x103] = 50 A (current in the cable)b). Power loss P = I2 R = 502 x 5 = 12500 W (current in the cable causes a heating effect and energy is lost as heat)

c). Potential across the cable V= IR =50 x 5 = 250 V

d). I = P/V = [100 x 103 ]/[200 x103] = 0.5 A (current in the cable)Power loss P = I2 R = 0.52 x 5 = 1.25 W

GET IT RIGHT

[ Incorrect ]

Students use the formula, Ploss = V2/R to find the energy loss in a cable

[Correct]

Students must use the formula P=I2R , as the current, I in the cable is known. The value of V, the potential difference across the ends of the cable is not known.

The voltage for the transmission = Voltage across the cable (to and fro) + Input voltage for the transformer at the other end

National Grid Network

It is a network of electrical cable connecting electrical power stations and substation to consumers of electricity in a closed network

The electricity that is generated at 25 kV is stepped up to 275 kV to be sent over long distances on Supergrid.

This very high voltage is stepped down in stages by substation transformers to local consumers

Advantages of the National Grid Network

1. Efficient and reliable energy distributionGeneration and distribution of energy can be controlled according to Requirements. Power stations in regions of low demand can supplyextra power to region of high demand

2. Backup for power failure or station maintenanceIf there is a break down or interruption of supply at a particularpower station, power can be supply by others stations to meet the demand

The Importance of Renewable Energy

1. Non renewable energy are depleting and becoming more costly

2. Renewable energy sources are natural, clean and do not pollute the

environment

3. Renewable energy sources can be replenished, do not run out and

can be replaced by natural processes.

4. Non renewable energy source like fossil fuel has harmful effects

on the environment and causes global warming