National and global energy resources: A renewable energy resource is one that is being (or can be) replenished as it is used.

The idea

A renewable energy resource is replenished while it is used, so the rate at which it is replaced is at least as fast as the rate at which people use it. This matters because non-renewable resources such as coal, oil and natural gas are used far faster than they form, so their stores will eventually run out. Renewable resources include solar, wind, hydroelectric, wave, tidal, geothermal and biomass. For example, sunlight arrives continuously, wind blows because of pressure differences, and biomass can be regrown. Some renewables are not available all the time, so they may need storage or back-up supplies. The key idea is replenishment rate compared with use rate, not whether the resource is natural or clean.

What earns the marks

• States that a renewable resource is replaced or replenished as it is used, so it is not used up permanently.
• Compares the rate of replenishment with the rate of use, noting that replenishment must keep pace with use.
• Gives a valid example such as solar, wind, hydroelectric, wave, tidal, geothermal or biomass and explains why it is renewable.
• Contrasts renewable resources with non-renewable resources such as coal, oil and natural gas, which form over very long periods.
• Recognises that some renewable resources are intermittent, so energy may need to be stored or supplied from another source when demand is high.

Common mistakes and how to put them right

• Thinking that any natural resource is renewable.

  Put it right: A resource is renewable only if it is replenished at least as fast as it is used.

• Believing that renewable means zero environmental impact.

  Put it right: Renewable resources can still have environmental effects, for example land use for wind farms or flooding for hydroelectric reservoirs.

• Assuming biomass is non-renewable because it is burned.

  Put it right: Biomass is renewable when the plants or crops are regrown at the same rate as they are used.

Exam tips

• Define renewable in terms of replenishment rate compared with use rate, then give one named example and explain why it fits the definition.
• When comparing resources, use the phrase 'used faster than it is replaced' for non-renewables and 'replaced as fast as it is used' for renewables.
• If asked to evaluate, mention reliability as well as renewability, because a renewable resource can still be unavailable at times.