Fossil fuel energy

Fossil fuels, such as coal, oil and natural gas, store chemical energy. When a fuel burns, combustion releases energy as heat. This heat can be used directly, for example to heat buildings.

Fossil fuels in power stations

In a fossil-fuel power station, combustion heats water in a boiler to produce steam. The steam turns a turbine, which is connected to an electrical generator. As the turbine turns the generator, the generator transfers energy electrically.

The energy transfer can be followed from the fuel to the electricity: chemical energy in the fuel is released as heat, the steam turns the turbine, and the generator produces electrical energy.

Availability and environmental effects

Fossil fuels are widely available and can provide reliable, controllable power on a large scale. However, their supplies are finite. They are non-renewable because they are used much faster than they form.

Burning fossil fuels releases carbon dioxide, which contributes to climate change. Combustion can also release air pollutants, while extracting fossil fuels can damage environments.

Biofuel energy

Biofuels are fuels made from recently living material. They contain a store of chemical energy that can be released by burning them. The energy released can be used for heating or to power an engine.

Biofuel can also be used to generate electricity. Burning the fuel heats water in a boiler, producing steam. The steam turns a turbine connected to a generator, which transfers energy electrically.

Biofuel supply and environmental effects

Biofuels can be renewable if the source material is replaced as it is used. Their availability, however, depends on a sufficient supply of suitable material. The amount produced may also be limited by the land available to grow it. Unlike a source that depends on immediate weather conditions, stored biofuel can be burned to provide power when needed.

Burning biofuel releases carbon dioxide and may cause air pollution. Growing replacement material takes in carbon dioxide, but this does not mean that using biofuel has no environmental impact: cultivation, processing and transport also have environmental effects.

Wave power

Wave-power devices use the motion of water at the sea surface to generate electricity. As waves move a device, energy is transferred from the moving water to a generator, which produces electricity.

Wave energy is renewable: waves are continually produced by natural processes. However, the power available depends on wave conditions. When waves are weaker, less power is available, so electrical output is not continuously reliable.

Wave-power locations and devices

Useful generation requires a suitable coastal or offshore location with sufficient wave activity. Several devices can be combined to produce a larger output. Installing and maintaining them at sea can be difficult because they must operate in a marine environment.

Wave-power devices produce no combustion emissions during operation because they do not burn fuel. However, they may affect marine habitats, obstruct navigation or alter coastal views.

Tidal power

Tidal power uses the movement or changing height of seawater as tides rise and fall. Moving water turns turbines, which are connected to generators that produce electricity.

Tidal turbines and barrages

Tidal turbines can be placed where seawater flows strongly as the tide changes. The moving water turns the turbine, driving its generator.

A tidal barrage is a barrier built across an area of water. It can hold water on one side, creating a difference in water level between the two sides. Water flowing through turbines in the barrage then drives generators.

Benefits and limitations of tidal power

Tidal energy is renewable because tides continue to occur naturally. Tides are also predictable, so the timing of electricity generation can be anticipated. However, output changes during the tidal cycle: tidal power is not continuously available.

Tidal schemes can produce substantial power without combustion emissions during operation. However, suitable sites are limited, and large installations can be costly to build. Schemes may also alter habitats, change how sediment moves and affect navigation.

Hydroelectric power

A hydroelectric scheme uses water stored at height to generate electricity. A dam holds water in a reservoir, where the water has gravitational potential energy because of its height.

When water is released through the dam, it flows downwards and its gravitational potential energy is transferred to the kinetic energy of moving water. The water turns a turbine, which is connected to a generator. The generator converts energy from the turning turbine into electrical energy.

Hydroelectric power: supply and control

The water cycle replenishes the water supply, so hydroelectric power is renewable when managed sustainably. However, generation depends on rainfall and on how much water is available in the reservoir.

Water stored behind the dam can be released when electricity is needed. This allows the power output to be controlled and increased quickly. Large schemes can supply substantial power, but they require suitable sites where water can be stored and released from a height.

Effects of hydroelectric schemes

Operating a hydroelectric scheme does not involve burning fuel. However, creating a dam and reservoir can flood land and displace communities. A dam can also alter river habitats by changing the flow of water.

Geothermal energy

Geothermal energy uses thermal energy from the Earth’s interior. In some places, underground water is heated by the surrounding rocks. Hot water or steam brought to the surface can provide useful heating.

Geothermal energy can also produce electricity. Steam turns a turbine connected to a generator. As the turbine turns the generator, the generator produces electricity. Where underground heat is used to produce the steam, it heats water without the need to burn fuel.

Geothermal availability and impacts

Geothermal energy is renewable when heat and water are replenished sufficiently as they are used. It can provide reliable power because it does not depend on changing weather conditions.

Geothermal generation is most suitable where high underground temperatures are accessible. Such conditions are not available everywhere, limiting suitable locations and the scale of generation.

Geothermal installations can disturb land and may release underground gases or mineral-rich water. However, they do not need to burn fuel during operation.

Nuclear power

A nuclear power station uses energy released from nuclear fuel to heat water and produce steam. The steam turns a turbine, which is connected to a generator. As the turbine turns the generator, electrical power is produced.

The energy that heats the water comes from nuclear fission: the splitting of atomic nuclei in the fuel. The fuel is not burned to provide the heat.

Nuclear power: benefits and limitations

Nuclear fuel is non-renewable: its supply is finite and cannot be replaced as quickly as it is used. Nuclear stations can provide reliable electrical power on a large scale. Because the fuel is not burned, they do not release carbon dioxide through fuel combustion during operation.

Nuclear stations require substantial construction and safety measures. They also produce radioactive waste, which must be managed to limit exposure to radiation. Accidents at a nuclear station can have serious environmental consequences.

Solar cells

A solar cell transfers energy from sunlight directly into electrical energy. Unlike a power station that uses heat to turn a turbine, it does not need a boiler or a turbine to generate electricity.

Solar cells can be used individually or connected together in a larger group called an array. Arrays can be installed at different scales, from panels on buildings to installations covering large areas.

Solar cell output

Sunlight is a renewable energy source, but the electrical output of solar cells is variable. It depends on the amount of daylight reaching them, which changes with the weather and location. Solar cells cannot generate electricity at night because there is no sunlight.

Environmental effects of solar cells

Solar cells produce no combustion emissions while generating electricity because they do not burn fuel. However, manufacturing and disposing of them have environmental impacts. Large solar installations also use land.

Solar water heating

A solar water-heating panel absorbs infrared and other electromagnetic radiation from the Sun. The absorbed radiation heats the panel, which transfers energy to water, providing useful hot water.

This is direct heating: the panel supplies thermal energy rather than generating electricity. It is different from a solar cell, which converts radiation into electrical energy.

Solar water heating: availability and limitations

Solar radiation is renewable, but the heat a panel can provide depends on the sunlight reaching it. This varies with daylight, weather and location. When solar input is insufficient to provide enough hot water, another heat source may be needed.

Panels can be fitted to buildings, but the useful heat available is limited by the sunlight received and the area available to collect it. A larger collection area can absorb more radiation when sunlight is available.

Solar water heating: environmental impacts

Solar water-heating panels produce no combustion emissions during operation because they do not burn fuel. Their manufacture and installation do, however, have environmental impacts.

Wind power

Wind carries energy because air is moving. The Sun heats the Earth’s surface unevenly, causing differences in air temperature and pressure that contribute to air movement. The Sun is therefore the ultimate energy source for wind power.

A wind turbine transfers energy from moving air to a generator. Wind turns the turbine’s blades, which drive the generator to produce electricity.

Wind turbines and their output

Wind is renewable because it is continually replenished by natural processes. However, wind speed varies, so a turbine’s electrical output is intermittent: it cannot be relied on to produce the same output all the time. Wind turbines therefore need suitable locations where wind is available often enough to make their use worthwhile.

A turbine can operate individually, or many turbines can be grouped together in a wind farm. Wind farms can be built on land or offshore.

Environmental effects of wind power

Wind turbines produce no combustion emissions while operating because they do not burn fuel. However, turbines can change the appearance of a landscape, create noise and harm wildlife. Their construction and installation also have environmental impacts.

Energy transfer efficiency

A process is useful when it transfers supplied energy in the way intended. Efficiency describes the proportion of the total supplied energy that is transferred usefully. A more efficient process transfers a greater proportion of its input energy to the intended useful output.

The remaining energy is transferred in less useful ways, often to the surroundings as thermal energy. It is not destroyed: it is simply not transferred to the intended useful output.

Calculating energy transfer efficiency

Efficiency can be calculated from the useful energy transferred and the total energy supplied:

Efficiency is a ratio with no unit. Multiply the ratio by $$100$$ to express it as a percentage. An efficiency of $$100%$$ means all the supplied energy is transferred usefully.

For example, if a process is supplied with $$100\ \text{J}$$ and transfers $$40\ \text{J}$$ usefully, its efficiency is $$40/100 = 0.40$$, or $$40%$$. The remaining $$60\ \text{J}$$ is transferred in less useful ways.

Efficiency using power

Power is the rate at which energy is transferred. Efficiency can also be calculated by comparing the useful output power with the total input power:

As with the energy ratio, multiply the result by $$100$$ to express efficiency as a percentage. The power comparison gives the same efficiency as the energy comparison when the energy transfers are measured over the same time interval.

Nuclear fusion energy

Nuclear fusion occurs when light atomic nuclei combine to form a heavier nucleus. Energy is released in the process.

In the Sun, the temperature is high enough for light nuclei to fuse. The energy released is eventually radiated into space as light and other forms of radiation.

Using fusion to generate electricity

Researchers are investigating how fusion energy could be harnessed to produce electrical energy on a large scale. One approach would use the released energy to provide heat, which could drive a turbine and generator.

Fusion is different from nuclear fission, the process used in existing nuclear power stations. In fission, a heavy nucleus splits into smaller nuclei; in fusion, light nuclei combine.

Efficiency calculations

Efficiency measures the fraction of the total energy input that becomes useful energy output. It can also be calculated using useful power output and total power input.

The result is a ratio, usually written as a decimal. To express it as a percentage, multiply it by $$100%$$. For example, an efficiency of $$0.75$$ is $$75%$$.

Use the same units for both energy quantities, or the same units for both power quantities. Convert units where necessary before dividing.

Calculating efficiency

Suppose a device receives $$2\text{ kJ}$$ of energy and produces $$1500\text{ J}$$ of useful energy. First convert the input to $$2000\text{ J}$$, then divide the useful output by the total input:

Finding an output or input

The efficiency relationship can be rearranged to find a missing quantity. These forms work for either energy or power, provided the input and output are both the same kind of quantity:

Use efficiency as a decimal ratio in these calculations. For the device above, $$75% = 0.75$$. An input of $$2000\text{ J}$$ therefore gives a useful output of $$0.75 \times 2000\text{ J} = 1500\text{ J}$$. Conversely, a useful output of $$1500\text{ J}$$ at that efficiency requires an input of $$1500\text{ J} \div 0.75 = 2000\text{ J}$$.