How should we generate the electricity we need?
(http://www.antonineeducation.co.uk/physics_gcse/Unit_1/Topic_4/topic_4_how_should_we_generate_t.htm)
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.
Summary
. Most of our energy comes from fossil fuels;
. Coal is burned in power stations to make electricity;
. Nuclear power is used to make electricity;
. Nuclear power produces dangerous waste;
. Fossil fuels will run out;
. Therefore alternatives will need to be found;
Alternative (renewable) energy sources include wind, hydroelectric, tidal, solar, biofuels, and
geothermal power.
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.
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.
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.
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.
. Let's use a wind turbine as a general example. The structure of most wind turbines in use today is a horizontal axis
wind turbine, which looks like a distant cousin to a windmill. The blades are held out from the main structure and
are connected to rotating gears by a steel shaft. Most often the blades themselves are made out of lightweight
materials such as combinations of aluminum and plastic. These materials are tough enough that the wind will spin
the blades, but so lightweight that the wind will pull them with even greater ease.
. Process
Wind is the movement of air. When the air moves across a wind turbine's blades it turns the blades. As the blades turn,
the steel shaft they're connected to spins as well. The movement of the shaft turns gears inside the housing at the top of
the wind turbine and creates electricity.
Hydroelectric Power
The energy from falling water can produce a lot of electricity in a hydroelectric power station.
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.
Hydroelectricity is electricity generated by hydropower, i.e., the production of power through use of the gravitational
force of falling or flowing water. It is the most widely used form of renewable energy. Once a hydroelectric complex is
constructed, the project produces no direct waste, and has a considerably lower output level of the greenhouse gas
carbon dioxide (CO2) than fossil fuel powered energy plants. Worldwide, hydroelectricity supplied an estimated 816 GWe
in 2005. This was approximately 20% of the world's electricity, and accounted for about 88% of electricity from renewable
sources.
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.
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;
Solar panels are not very efficient. To make reasonable amounts of electricity, you need large
panels. Silicon is a very common element, but is normally bound in silica, or silica sand. Processing silica (SiO2) to
produce silicon is a very high energy process - at current efficiencies, it takes one to two years for a conventional solar
cell to generate as much energy as was used to make the silicon it contains.
Solar technologies are broadly characterized as either passive solar or active solar depending on the way they capture,
convert and distribute sunlight. Active solar techniques include the use of photovoltaic panels and solar thermal collectors
(with electrical or mechanical equipment) to convert sunlight into useful outputs. Passive solar techniques include
orienting a building to the Sun, selecting materials with favorable thermal mass or light dispersing properties, and
designing spaces that naturally circulate air.
Greenhouses
greenhouse (also called a glasshouse or hothouse) is a building where plants are cultivated.
Main article: solar greenhouse (technical)
A greenhouse is a structure with a glass or plastic roof and frequently glass or plastic walls; it heats up because incoming
solar radiation from the sun warms plants, soil, and other things inside the building faster than heat can escape the
structure. Air warmed by the heat from hot interior surfaces is retained in the building by the roof and wall.
Solar sails
Light Tubes
light tubes, also called solar tubes, which are placed into a roof and admit light to a focused area of the interior. These
somewhat resemble recessed light fixtures in the ceiling. They do not allow as much heat transfer as skylights because
they have a less exposed surface area.
Solar Cooking
A solar oven or solar cooker is a device which uses sunlight as its energy source. Because they use no fuel and they
cost nothing to run, humanitarian organizations are promoting their use worldwide to help slow deforestation and
desertification, caused by using wood as fuel for cooking.
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.
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.
Geothermal energy originates from the original formation of the planet, from radioactive decay of minerals, and from solar
energy absorbed at the surface. It has been used for space heating and bathing since ancient roman times.
Geothermal power is cost effective, reliable, and environmentally friendly, but has previously been geographically limited
to areas near tectonic plate boundaries. Recent technological advances have dramatically expanded the range and size
of viable resources, especially for direct applications such as home heating. Geothermal wells tend to release
greenhouse gases (carbon dioxide & nitrous oxide plus any hot water may contain mercury, arsenic, and antimony which
if disposed of into rivers, can render their water unsafe to drink) trapped deep within the earth, but these emissions are
much lower than those of conventional fossil fuels.
Electricity generation requires high temperature resources that can only come from deep underground. The heat must be
carried to the surface by fluid circulation, either through magma conduits, hot springs, hydrothermal circulation, oil wells,
drilled water wells, or a combination of these. This circulation sometimes exists naturally in the most favorable areas
where the crust is thin: magma conduits bring the heat close to the surface, and naturally occurring hot springs bridge the
last gap to the surface.
If no hot spring is available, a well must be drilled into a hot aquifer. Away from tectonic plate boundaries the geothermal
gradient is 25-30°C per km of depth in most of the world, and wells would have to be drilled several kilometers deep to
permit electricity generation.
In ground that is hot but dry, or where water pressure is inadequate, it is possible to inject a fluid to stimulate production.
Two boreholes are bored into a candidate site, and the deep rock between them is fractured by explosives or high
pressure water. Water is pumped down one borehole and steam comes up the other. Liquefied carbon dioxide may also
be used. This concept is called hot dry rock geothermal energy in Europe, or enhanced geothermal systems in North
America. A much greater resource potential may be available from this approach than from conventional tapping of
natural aquifers.