The Energy Hierarchy is a classification of energy options. It is a similar approach to the waste hierarchy for minimizing resource depletion, and adopts a parallel sequence. The highest priorities cover the prevention of unnecessary energy use both by eliminating waste and improving energy efficiency. The sustainable production of energy resources is the next priority. Depletive and waste-producing energy generation options are the lowest priority. For an energy system to be sustainable: the resources applied to producing the energy must be capable of lasting indefinitely; energy conversion should produce no harmful by-products, including net emissions, which can not be fully recycled; and it must be capable of meeting reasonable energy demands.
The top priority under the Energy Hierarchy is energy conservation or the prevention of unnecessary use of energy. This category includes eliminating waste by turning off unnecessary dayneys. Heat loss from buildings is a major source of energy wastage, so improvements to building insulation and air-tightness can make a significant contribution to energy conservation. Many countries have agencies to encourage energy saving.
The second priority is to ensure that energy is used and consumed efficiently. Energy efficiency has two main aspects.
Energy efficiency is the ratio of the productive output of a device to the energy it consumes. Energy efficiency was a low priority when energy was low and environmental impact was low. In 1975 the average fuel economy of the United States was under 15 miles per gallon Incandescent light bulbs, which were the most common type of late 20th century, waste 90% of their energy, with only 10% converted to useful light. More recently, energy efficiency has become a priority. The last reported average fuel efficiency of US cars had almost doubled from the 1975 level; LED lighting is now more efficient than incandescent. Many household appliances are now required to display their energy efficiency.
Fossil fuels, radioactive materials, solar radiation or other sources. Most electricity production is in thermal power stations, where much of the energy source is lost as heat. The average efficiency of world electricity production in 2009 was c.37%. A priority in the energy hierarchy is to improve the efficiency of energy conversion, whether in traditional power stations or by improving the performance ratio of photovoltaic power stations and other energy sources. Overall efficiency and sustainability can also be improved by capacity-or fuel-switching from less efficient, less sustainable resources to better ones; but this is mainly covered under the fourth level of the hierarchy.
Renewable energy naturally occurs, theoretically inexhaustible sources of energy. These sources are treated as inexhaustible, or naturally replenished, and fall into two classes.
The first class of renewables derives from climatic or elemental sources, such as sunlight, wind, waves, tides or rainfall (hydropower). Geothermal energy from the earth’s heat also falls in this category. These are treated as being inexhaustible because they are most economically viable from the sun, which has an estimated life of 6.5 billion years.
The other main class of renewables, bioenergy, derived from biomass, where the short-term growth cycle is increased by new growth. Bioenergy is usually converted by combustion, and therefore gives rise to carbon emissions. It is treated as a carbon neutral overall, because an equivalent amount of carbon dioxide will be extracted from the atmosphere during the growing cycle. Bioenergy sources can be solid, such as wood and energy crops; liquid, such as biofuels; or gaseous, such as biomethane from anaerobic digestion.
The next priority in the hierarchy covers energy sources that are not entirely sustainable, but have a low environmental impact. These include the use of fossil fuels with carbon capture and storage. Nuclear energy is sometimes treated as a low-impact source, because it has low carbon emissions.
The lowest priority is the energy production using unsustainable sources, such as unabated fossil fuels. Some of the more important things in this category, because of the need management of storage of highly hazardous radioactive waste and the depletion of uranium resources. There is a consensus that the share of such energy sources must decline. Within this tier, there are possibilities for adverse adverse impacts by switching from the most damaging fuel sources, such as coal, to less emissive sources, such as gas. Many suggest that when such high impact energy use has been minimized, the effects of any unavoidable residual use should be counterbalanced by emissions offsetting.
The Energy Hierarchy was first proposed in 2005 by Philip Wolfe, when he was Director General of the Renewable Energy Association. This first version had three levels; energy efficiency, renewables and traditional energy production. It was endorsed and adopted in 2006 by a consortium of institutions, associations and other bodies in the Sustainable Energy Manifesto. Subsequently, the concept has been adopted and refined by others in the energy industry and in government.