Boiler, turbine, generator: the chain behind most electricity

1. Most large power stations do the same three things in the same order, and differ only in what supplies the energy at the start.
2. A boiler transfers energy to water and turns it into high-pressure steam.
3. The steam rushes through a turbine, doing mechanical work on the angled blades and spinning them at high speed.
4. The turbine shaft drives a generator, which transfers energy electrically to the grid.
5. Fossil fuels, biofuels, nuclear fuel and geothermal resources all use this full chain, because all four begin by producing thermal energy.
6. Hydroelectric, tidal, wave and wind stations have no boiler: the moving water or air turns the turbine directly.
7. Solar cells use neither boiler nor turbine nor generator. They transfer energy from light straight into an electrical output.

Burning a chemical store: fossil fuels and biofuels

1. Fossil fuels are coal, oil and natural gas. Burning them releases the chemical store laid down by plants and marine organisms hundreds of millions of years ago, and the hot gases heat water in the boiler.
2. They are non-renewable, because they form far more slowly than we consume them.
3. They are highly reliable and available on demand, which is why they still carry so much of the world's electricity supply, and they can be built almost anywhere fuel can be delivered.
4. Burning them releases carbon dioxide, which contributes to the enhanced greenhouse effect, along with sulfur dioxide, which causes acid rain.
5. Biofuels are wood, ethanol from sugar or maize, and biogas from decaying waste. They release a chemical store in exactly the same way, and feed the same boiler.
6. Biofuels are renewable, because a new crop can be grown within a season, and the growing crop absorbs carbon dioxide as it does so.
7. Their drawbacks are scale and land use: producing enough fuel takes large areas of farmland that could otherwise grow food.

Energy from moving and falling water

1. Hydroelectric stations hold water behind a dam, high above the turbines. The water has a large gravitational potential store, and letting it fall drives the turbines directly.
2. They are renewable and very reliable, and can be brought up to full output within seconds, which makes them ideal for covering sudden demand.
3. They need a suitable valley and reliable rainfall, so availability is limited by geography, and building the reservoir floods land and displaces habitats and communities.
4. Tidal barrages trap water at high tide and release it through turbines as the tide falls, using the kinetic and gravitational potential stores of the sea.
5. Tides are driven by the Moon and the Sun, so they are extremely predictable, though a barrage only generates for part of each tidal cycle and disrupts estuary ecosystems.
6. Wave generators float on the surface and use the up-and-down kinetic store of the waves. They are renewable but currently small in scale, and must survive storms.

Heat from inside the Earth

1. Geothermal resources use the internal energy of hot rocks a few kilometres underground, kept hot by the radioactive decay of elements inside the Earth.
2. Cold water is pumped down one borehole, is heated by the rock, and returns up a second borehole as steam, which drives a turbine and generator.
3. Geothermal energy is renewable, reliable and available day and night, and it releases almost no carbon dioxide.
4. Its limitation is availability: it is only practical where hot rock lies close enough to the surface, typically in volcanic regions such as Iceland and New Zealand.

Splitting heavy nuclei: nuclear fuel

1. A nuclear power station releases the nuclear store of uranium by fission, in which a large nucleus splits into smaller ones and releases energy.
2. The reactor takes the place of the boiler's furnace: the energy released heats water, and from there the chain is the familiar boiler, turbine, generator.
3. A very small mass of fuel releases an enormous amount of energy, so the resource offers large scale and high reliability, and produces no carbon dioxide while running.
4. Uranium is non-renewable, the stations are expensive to build and to decommission, and fission produces radioactive waste that stays hazardous for thousands of years.

Three ways of using the Sun

1. Solar cells transfer energy from light directly into an electrical output. There are no moving parts, so they are silent and need little maintenance, but their output stops at night and falls on cloudy days.
2. Solar panels are a different device entirely. They use infrared and other electromagnetic waves from the Sun to heat water flowing through pipes behind a dark absorbing surface, for washing and heating rather than for electricity.
3. Wind turbines use the kinetic store of moving air. The blades turn a shaft that drives a generator, with no boiler in the chain.
4. Wind exists because the Sun heats the Earth's surface unevenly. Warm air rises over the hotter regions and cooler air flows in to replace it, and that horizontal flow is the wind.
5. All three are renewable and release no carbon dioxide while operating, but solar and wind are unreliable, since neither sunshine nor wind can be scheduled.
6. Wind farms also need large areas of land or shallow seabed for a modest output, and some people object to their appearance and noise.

Weighing the resources against each other

1. Comparisons are judged on five things, and a good answer picks a different one for each point rather than repeating the same idea.
2. Renewability: wind, solar, hydroelectric, tidal, wave, geothermal and biofuels are renewable; fossil fuels and nuclear fuel are not.
3. Availability: geothermal, hydroelectric and tidal need particular geography, while fossil and nuclear stations can be sited almost anywhere.
4. Reliability: fossil, nuclear, geothermal and tidal supplies are dependable; wind, wave and solar depend on the weather.
5. Scale: fossil and nuclear stations deliver the largest outputs, while solar cells and single turbines deliver comparatively little.
6. Environmental impact: burning fuels releases carbon dioxide, fission leaves radioactive waste, and dams and barrages flood land and disturb ecosystems.

Trace almost anything back and you reach the Sun

1. Radiation from the Sun is the original source of energy for almost every resource we use.
2. Fossil fuels and biofuels store energy captured by plants through photosynthesis, separated only by how long ago the plants grew.
3. The Sun evaporates water, which falls as rain and refills the reservoirs behind hydroelectric dams.
4. Uneven solar heating drives the wind, and the wind whips up the waves, so both are solar in origin.
5. Only three resources are not: geothermal, which comes from radioactive decay inside the Earth, nuclear, which comes from the fuel's own nuclei, and tidal, which comes from the gravitational pull of the Moon and the Sun.