Energy sources

Homes, transport and industry need energy for heating, movement and electrical devices. An energy resource is something we can use to meet those needs. Some resources are burned to provide heat directly. Others are used to generate electricity, which can then be delivered to where it is needed. Electricity is a way of transferring energy, not a resource that must be found in nature.

The key distinction is whether a resource is replenished on a useful timescale. Renewable resources can be replaced naturally or remain available for the foreseeable future. Non-renewable resources have finite supplies that are used much faster than they form. This does not mean that energy disappears when a fuel is used: energy is conserved, but the usable supply of that fuel is depleted.

Fuels: heat directly or electricity through steam

Fossil fuels—coal, oil and natural gas—are non-renewable. They formed from ancient living material over millions of years. Burning them releases energy from their chemical stores. Fuel can be used directly, such as gas for heating or oil-derived fuel for transport. In a fossil-fuel power station, burning fuel heats water to make steam; the steam turns a turbine, which drives a generator to produce electricity. Fossil fuels provide concentrated, transportable stores of energy, but burning them releases carbon dioxide. Coal- and oil-burning can also release sulfur dioxide, which contributes to acid rain. A gas-fired station can respond relatively quickly when electricity demand changes.

Nuclear fuel is also non-renewable because supplies of fuels such as uranium are finite. In a nuclear power station, splitting atomic nuclei releases energy that heats water. As in a fossil-fuel station, steam turns a turbine and generator. Nuclear fission does not burn fuel and does not directly release carbon dioxide or sulfur dioxide during electricity generation, but it produces radioactive waste that must be managed safely.

Biofuels are made from recently living material. Wood can be burned for heating or to raise steam for electricity generation; oil from crops can be processed into fuel for vehicles. Unlike fossil fuels, plants used to make biofuel can be grown again on a human timescale, so biofuel can be renewable if it is replaced as it is used. Burning it still releases carbon dioxide. Growing replacement plants can take in carbon dioxide, but it would be misleading to assume that every biofuel has no environmental impact.

Using movement, water and sunlight

Wind is a renewable resource. Moving air turns wind-turbine blades, which drive a generator. Wind power needs no fuel to be burned, but its electrical output depends on wind conditions.

Hydroelectricity uses water stored high behind a dam. As the water falls, energy from its gravitational store is transferred to its movement; the flowing water turns a turbine and generator. The water supply is replenished through the water cycle, although the amount available depends on rainfall. Tidal power also uses moving water to turn turbines, for example as water flows through a tidal barrage. Tides are renewed by gravitational effects, principally involving the Moon. Unlike wind, their timing is predictable, although a barrage can affect shipping and aquatic habitats.

The Sun provides a renewable flow of energy. Solar cells convert sunlight directly into electricity, without a steam turbine. Their output falls when there is less sunlight and stops at night. Sunlight also drives processes behind several other resources: it heats the air that produces wind, powers the water cycle that replenishes hydroelectric reservoirs, and enables the plant growth behind biofuels. Energy stored in fossil fuels also ultimately came from ancient living things that depended on sunlight. Nuclear fuel and tides have different origins.

Choosing a resource for a purpose

The method of use matters as much as the renewable label. A vehicle can carry a concentrated fuel for direct use, while a wind turbine or solar cell generates electricity where conditions allow. Fossil-fuel, biofuel and nuclear stations can use heat to make steam and generate electricity; wind, hydroelectric and tidal systems use moving air or water to turn generators; solar cells make electricity directly. Comparing resources therefore involves asking what form of energy is needed, whether the resource is available when needed, and what consequences its use has.