Topic 4 How should we generate the electricity we need?
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In the exam you need to know:
In most power stations an energy source is used to heat water. The steam produced drives a turbine which is coupled to an electrical generator. Common energy sources include coal, oil and gas, which are burnt to produce heat and uranium/plutonium, in which nuclear fission produces heat. Energy from renewable energy sources can be used to drive turbines directly. Renewable energy sources used in this way include wind, the rise and fall of water due to waves and tides, and the falling of water in hydroelectric schemes. Electricity can be produced directly from the Sun’s radiation using solar cells. In some volcanic areas hot water and steam rise to the surface. The steam can be tapped and used to drive turbines. This is known as geothermal energy. Using different energy resources has different effects on the environment. These effects include the release of substances into the atmosphere, noise and visual pollution, and the destruction of wildlife habitats. The advantages and disadvantages of using fossil fuels, nuclear fuels and renewable energy sources to generate electricity. These include the cost of building power stations, the start-up time of power stations, the reliability of the energy source, the relative cost of energy generated and the location in which the energy is needed.
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Key Words Fossil Fuel Power Station Nuclear Alternative (Renewable) Energy
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Fossil Fuels
Fossil Fuels are the remains of plants and animals that lived many millions of years ago. They captured energy from the Sun while alive. When they died they were buried and compressed to form either coal or oil.
Fossil fuels are very concentrated forms of energy, which means that we can get out lots of energy from a small amount of the fuel. 50 litres (40 kg) of petrol will take a car 800 km. A 100 kg battery will take a care about 200 km.
Fossil fuels will run out. They are non-renewable. Although new reserves are being found, there will eventually be a time when there are not enough to meet the demand.
Fossil fuels give out carbon dioxide, which contributes to global warming.
Coal-fired Power Stations
Electricity is made in huge quantities in Power Stations. The most common type of power station is powered by coal. The picture below shows the general idea.
Coal (1) is crushed in a mill. It is then blown in a stream of air (2) to be burned in a large boiler (3) which produces large amounts of steam. The steam turns a turbine (4) which turns a generator (5). The generator is connected to the step-up transformer (see Topic 3) by very heavy wires which can carry 100 000 A at 25 000 V.
To make the power station more efficient, the steam is cooled in a condenser (6). The waste heat is carried to the cooling tower (7), where it returns as cool water (8). The clouds you see around cooling towers are not smoke; they are water vapour.
Here is a simple diagram of a turbine and generator.
Question 1: What is the energy chain in a coal fired power station? ANSWER
Coal fired power stations have the following advantages:
They use coal which is relatively plentiful;
They are less expensive to build and run compared to nuclear power stations;
They do not produce dangerous waste.
They have disadvantages:
They need trainloads of coal every day;
They produce carbon dioxide which is a greenhouse gas, involved in global warming;
They produce sulphur dioxide which makes acid rain.
Nuclear Power Stations
A nuclear station is identical to a coal-fired station in the way that it uses steam to turn a turbine. The boiler is different and is called a reactor.
The fuel is uranium whose nuclei split by a process called fission. Fission releases lots of energy. I kg nuclear fuel is equivalent in energy to 25 tonnes of coal.
Question 2: What is the energy chain in a nuclear power station? ANSWER
Advantages are:
There are no waste gases, therefore no pollution;
Much less fuel is needed;
Some reactors can generate material that can be used in other reactors.
Disadvantages are:
Nuclear waste is dangerous;
Nuclear power stations are very expensive to build;
They are even harder to dismantle;
The radioactive waste remains dangerous for a long time.
Nuclear power remains controversial. There were high hopes that they would produce electricity so cheaply that it would be free. However they did not live up to those hopes.
Although nuclear power stations have a good safety record, there have been some well-known accidents such as Chernobyl. In this case an unauthorised experiment was carried out to find out what would happen in a worst case failure; they found out. The reactor "ran away" and there was a chemical explosion that blew the lid of the reactor and tipped it on its side, spilling out a large quantity of radioactive muck.
It may be that nuclear power is used in the future when fossil fuels run out. There will be a lot of debate about it.
Alternative Energy Sources
These are often called renewable or sustainable energy sources. They do not depend on fossil fuels. They mostly capture energy from the Sun as it arrives.
Wind Turbines
The UK is one of the windiest places in Europe. Therefore it makes sense to tap energy from the wind. However lots of turbines are needed to replace one conventional power station. Also on a calm day, they don't work.
Although wind turbines are clean, many people don't want them in their area.
Hydroelectric Power
The energy from falling water can produce a lot of electricity in a hydroelectric power station.
Question 3: How do you think this power station works? ANSWER
These power stations can only be built in hilly areas with lots of rainfall. Big dams need to be built. Large reservoirs destroy large areas of countryside and wildlife habitats. Switzerland generates most of its electricity by hydroelectric power.
Pumped storage schemes use large amounts of electricity at night to make the generators act as huge motors to pump water back into the reservoir. At times of peak demand they can quickly reverse to allow water to drive the turbines as generators.
Tidal Power
The rise and fall of the tides are driven by the moon. Large estuaries can be dammed and the flowing water can be used to drive a turbine.
Question 4: When do you think these power stations generate most power and least power? ANSWER
Solar Power
Solar panels capture energy directly from the sun to be used in two different ways:
to heat water in pipes (you often see them on roofs);
to generate electricity with photovoltaic cells;
This car uses electricity from the Sun to drive a small motor.
Solar panels are not very efficient. To make reasonable amounts of electricity, you need large panels.
Biofuels
Crops can be grown to make oil or alcohol which can in turn be used to run an engine.
Some rapidly growing trees can be harvested to burn as fuel to heat water.
Biological waste (e.g. food scraps and the material you flush away when you sit on the lavatory) decompose to form methane gas, which can be burned in machine like a large car engine which drives a generator.
Question 5: These fuels give out carbon dioxide. Do you think they contribute to global warming? ANSWER
Geothermal Power
This uses energy from hot rocks below the Earth's surface. Water is pumped down to the rocks and turned to steam. It then comes through another pipe to drive a turbine. Waste heat is often used to heat people's houses.
Question 6 Does this energy come from the Sun? ANSWER
Websites you could look at:
http://www.cat.org.uk/index.tmpl?refer=index&init=1 (Centre for Alternative Technology at Machynlleth, Mid Wales)
http://www.knowledgehound.com/topics/altenerg.htm To find out more detail about some of the renewable energy sources.
Now try Question 7
Now try Question 8 which is a crossword.
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Summary
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Now try the Topic Quiz.