Energy hierarchy

It is a similar approach to the waste hierarchy for minimising resource depletion, and adopts a parallel sequence.

This category includes eliminating waste by turning off unneeded lights and appliances and by avoiding unnecessary journeys.

Losses are incurred when energy is harvested from the natural resource from which it is derived, such as fossil fuels, radioactive materials, solar radiation or other sources.

The first class of renewables derive from climatic or elemental sources,[14] such as sunlight, wind, waves, tides or rainfall (hydropower).

These are treated as being inexhaustible because most derive ultimately from energy emanating from the sun, which has an estimated life of 6.5 billion years.

[15] The other main class of renewables, bioenergy,[16] derives from biomass, where the relatively short growing cycle means that usage is replenished by new growth.

[17] Bioenergy sources can be solid, such as wood and energy crops; liquid, such as biofuels; or gaseous, such as biomethane from anaerobic digestion.

[citation needed] The next priority in the hierarchy covers energy sources that are not entirely sustainable, but have a low environmental impact.

Some also place nuclear energy in this category, rather than the one above, because of the required management/storage of highly hazardous radioactive waste over extremely long (hundreds of thousands of years or more) timeframes [19] and depletion of uranium resources.