Core Physics Topic 7 - Generating Electricity
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 car about 200 km. Although electric cars are marketed as very modern, they were actually on the roads before the petrol car. Some bright spark had the idea of detaching the horse, sticking a battery under the carriage, and running the vehicle with an electric motor. Their range was limited. All in all, a horse was cheaper.
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.
Graphic from www.nothingnerdy.wikispaces.com
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 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.
This picture shows a turbine and generator.
What is the energy chain in a coal fired power station?
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.
Graphic by US N.R.C, Wikimedia Commons
The fuel is uranium whose nuclei split by a process called fission. Fission releases lots of energy. 1 kg nuclear fuel is equivalent in energy to 25 tonnes of coal.
What is the energy chain in a nuclear power station?
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.
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.
Another major accident occurred at Fukoshima Daiichi (Fukoshima No 1), in which the reactors shut down in response to an earthquake. As it was cooling, a tsunami swamped the emergency diesel generators and the reactors overheated.
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.
Graphic by Ssgxnh, Wikimedia Commons
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.
Large numbers of wind-farms are being built, many out at sea. Although wind turbines are clean, many people don't want them in their area.
The energy from falling water can produce a lot of electricity in a hydroelectric power station.
Graphic by Tomia, Wikimedia.com
Switzerland generates most of its electricity by hydroelectric power.
How do you think this power station works?
These power stations can only be built in hilly areas with lots of rainfall. Big dams need to be built, which is every expensive. Large reservoirs destroy large areas of countryside and wildlife habitats. The failure of dam is a major disaster.
Many people are now setting up small hydro-electric schemes that might provide the energy needs of a house. That is nothing new. The engineer and businessman, William George Armstrong, built one such installation in his new house at Cragside in Northumberland in 1890. There are some small-scale hydro-electric power stations that take water from a river or a millpond.
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.
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.
When do you think these power stations generate most power and least 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;
A solar-powered car uses electricity from the Sun to drive a small motor. Some road-signs are lit by electricity generated by a solar cells and stored in a battery.
Solar panels are not very efficient. To make reasonable amounts of electricity, you need large panels.
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. Or the gas can be used to cook with.
Graphic by SVG, Wikimedia Commons
These fuels give out carbon dioxide. Do you think they contribute to global warming?
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.
Graphic by Energy Information Administration, USA, Wikimedia Commons
Does this energy come from the Sun?
Do this interactive fill-in-the-space question.
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