Renewable energy involves the generation of energy used in the production of electricity and heat, and in transport, from sources where the carbon emissions resulting from such energy generation are at least offset by the absorption of carbon from the atmosphere in the same timeframe. For example, the burning of coal to produce electricity involves the rapid release of carbon that became trapped away through natural processes occurring over millions of years. There is therefore a net release of carbon into the atmosphere over the life of the power station. Wind energy on the other hand replaces fossil fuelled generation, saving more carbon than was consumed in the production and installation of the wind farm and resulting in a net reduction in carbon over its life.

What is renewable energy?

The production and use of renewable energy arises when the rate of use of energy does not exceed the rate of production of the energy source. For example, when wind turbines capture the kinetic energy of the wind to create electrical energy, the conversion only occurs as wind energy is available. As the wind is a constant resource, this is considered 'renewable'. Coal, on the other hand, is the result of the conversion of the chemical energy stored in coal, which has been formed over many millions of years as a result of sedimentation, pressure and heat, to electrical energy through combustion. The time required to extract, burn and use this energy is significantly faster than the time required for its creation. As a result, stores of energy (in coal) are being used faster than they are being produced, and will eventually run out. This is non-renewable, or finite, energy.

Types of renewable energy

The conversion of energy from its natural stores is primarily required by man to produce the electricity, heat and transport required to meet the demands of society. This section explores each of these different uses, alongside the technologies that have been developed to convert energy from its natural sources (such as wind, sun, geothermal) to electricity, heating or transport requirements; and the potential benefits and costs of each compared with more traditional, finite sources of energy.

• Renewable electricity is converted from:
  • kinetic energy in wind which is captured and converted in wind turbines;
  • solar energy contained in the light from the sun, which is captured in a chemical and electrical process in solar panels;
  • chemical energy stored in biomass and wastes, which can be converted to electrical energy through combustion to produce heat and steam or the direct production of a gas which can be combusted in anaerobic digestion or gasification;
  • the thermal energy of deep underground rocks (deep geothermal), which is captured through heating water to power a steam turbine;
  • the kinetic energy of rivers or tides, which is captured and converted in water turbines;
• Heat from renewable sources comes from:
  • thermal energy contained in deep rocks (deep geothermal), the air (air source heat pumps) or the shallow ground (ground source heat pumps);
  • the thermal energy of the sun directly;
  • the production of renewable gases from organic material (such as animal slurry) which can be combusted to produce heat;
• Fuels for renewable transport come from:
  • biofuels, which convert the chemical energy in sustainably sourced plants to a chemical energy that can be used in vehicles; and
  • electric and natural gas powered vehicles, where the electricity or gas being used is generated by renewable sources (as above).

This is summarised in the diagram below:

Benefits and costs of renewable energy

Renewable technologies have a number of costs and benefits.

Benefits

The clear benefit to renewable technologies is the availability of a sustainable resource to produce energy, which will not run out as it is used. This provides long-term security of supply for technologies that have limited input costs associated with the natural energy source (for example wind and solar have no 'fuel' costs).

Costs

To date, the investment requirement required to build and operate renewable technologies are higher than for the same capacity of traditional energy generation. This has meant that government support has been required, often in the form of revenue subsidies, to make renewables economic to build and operate. See the policy directory for more detail of these, and other such policies. Further, renewable technologies such as wind and solar are intermittent (i.e. they only produce electricity when the natural resource is available) and therefore cannot, on their own provide energy all of the time. This is particularly an issue for electricity, which cannot easily be stored at a large scale. Finally, certain renewables, in particular offshore wind and large-scale solar, have been critisised for the visual impacts they have on the environment.