The Energy Hierarchy is a classification of

energy In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...

options, prioritised to assist progress towards a more

sustainable energy Energy is sustainable if it "meets the needs of the present without compromising the ability of future generations to meet their own needs". Most definitions of sustainable energy include considerations of environmental aspects such as greenh ...

system. It is a similar approach to the

waste hierarchy Waste hierarchy is a tool used in the evaluation of processes that protect the environment alongside resource and energy consumption from most favourable to least favourable actions. The hierarchy establishes preferred program priorities based ...

for minimising

resource depletion Resource depletion is the consumption of a resource faster than it can be replenished. Natural resources are commonly divided between renewable resources and non-renewable resources (see also mineral resource classification). Use of either ...

, and adopts a parallel sequence. The highest priorities cover the prevention of unnecessary energy usage both through eliminating waste and improving

energy efficiency Energy efficiency may refer to: * Energy efficiency (physics), the ratio between the useful output and input of an energy conversion process ** Electrical efficiency, useful power output per electrical power consumed ** Mechanical efficiency, a ra ...

. 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 Energy transformation, also known as energy conversion, is the process of changing energy from one form to another. In physics, energy is a quantity that provides the capacity to perform work or moving, (e.g. Lifting an object) or provides heat. ...

should produce no harmful by-products, including net emissions, nor wastes which cannot be fully recycled; and it must be capable of meeting reasonable energy demands.

Energy saving

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 unneeded lights and appliances and by avoiding unnecessary journeys.

Heat loss Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...

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 Energy conservation is the effort to reduce wasteful energy consumption by using fewer energy services. This can be done by using energy more effectively (using less energy for continuous service) or changing one's behavior to use less service (f ...

Energy efficiency

The second priority under the energy hierarchy is to ensure that energy that is used is produced and consumed efficiently.

Energy efficiency Energy efficiency may refer to: * Energy efficiency (physics), the ratio between the useful output and input of an energy conversion process ** Electrical efficiency, useful power output per electrical power consumed ** Mechanical efficiency, a ra ...

has two main aspects.

Conversion efficiency of energy consumption

Energy efficiency is the ratio of the productive output of a device to the energy it consumes. Energy efficiency was a lower priority when energy was cheap and awareness of its environmental impact was low. In 1975 the average fuel economy of a car in the US was under 15 miles per gallon Incandescent light bulbs, which were the most common type until the late 20th century, waste 90% of their energy as heat, with only 10% converted to useful light. More recently, energy efficiency has become a priority. The last reported average

fuel efficiency Fuel efficiency is a form of thermal efficiency, meaning the ratio of effort to result of a process that converts chemical potential energy contained in a carrier (fuel) into kinetic energy or work. Overall fuel efficiency may vary per device, wh ...

of US cars had almost doubled from the 1975 level; LED lighting is now being promoted which are between five and ten times more efficient than incandescents. Many household appliances are now required to display

labels A label (as distinct from signage) is a piece of paper, plastic film, cloth, metal, or other material affixed to a container or product, on which is written or printed information or symbols about the product or item. Information printed dir ...

to show their energy efficiency.

Conversion efficiency of energy production

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. Most electricity production is in thermal power stations, where much of the source energy 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 A photovoltaic power station, also known as a solar park, solar farm, or solar power plant, is a large-scale grid-connected photovoltaic power system (PV system) designed for the supply of merchant power. They are different from most building- ...

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.

Sustainable energy production

Renewable energy describes naturally occurring, theoretically inexhaustible sources of energy. These sources are treated as being inexhaustible, or naturally replenished, and fall into two classes.

Elemental renewables

The first class of renewables derive from climatic or elemental sources, such as sunlight, wind, waves, tides or rainfall (

hydropower Hydropower (from el, ὕδωρ, "water"), also known as water power, is the use of falling or fast-running water to Electricity generation, produce electricity or to power machines. This is achieved by energy transformation, converting the Pot ...

Geothermal energy Geothermal energy is the thermal energy in the Earth's crust which originates from the formation of the planet and from radioactive decay of materials in currently uncertain but possibly roughly equal proportions. The high temperature and pres ...

from the heat of the earth's core also falls in this category. These are treated as being inexhaustible because most derive ultimately from energy emanating from the

sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

, which has an estimated life of 6.5 billion years.

Bio-energy

The other main class of renewables,

bioenergy Biomass is plant-based material used as a fuel for heat or electricity production. It can be in the form of wood, wood residues, energy crops, agricultural residues, and waste from industry, farms, and households. Some people use the terms biom ...

, derives from biomass, where the relatively short growing cycle means that usage is replenished by new growth. Bioenergy is usually converted by combustion, and therefore gives rise to carbon emissions. It is treated as carbon neutral overall, because an equivalent amount of carbon dioxide will have been extracted from the atmosphere during the growing cycle. Bioenergy sources can be solid, such as wood and

energy crop Energy crops are low-cost and low-maintenance crops grown solely for energy production by combustion (not for food). The crops are processed into solid, liquid or gaseous fuels, such as pellets, bioethanol or biogas. The fuels are burned to g ...

s; liquid, such as biofuels; or gaseous, such as biomethane from anaerobic digestion.

Low impact energy production

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 Carbon capture and storage (CCS) or carbon capture and sequestration is the process of capturing carbon dioxide (CO2) before it enters the atmosphere, transporting it, and storing it (carbon sequestration) for centuries or millennia. Usually th ...

. Nuclear energy is sometimes treated as a low impact source, because it has low carbon emissions.

High impact energy production

The lowest priority under the energy hierarchy is energy production using unsustainables sources, such as unabated fossil fuels. Some also place nuclear energy in this category, rather than the one above, because of the required management/storage of highly hazardous

radioactive waste Radioactive waste is a type of hazardous waste that contains radioactive material. Radioactive waste is a result of many activities, including nuclear medicine, nuclear research, nuclear power generation, rare-earth mining, and nuclear weapons r ...

over extremely long (hundreds of thousands of years or more) timeframes and depletion of uranium resources. There is a consensus that the share of such energy sources must decline. Within this tier, there are possibilities for limiting 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 usage has been minimised, the effects of any unavoidable residual usage should be counterbalanced by emissions

offsetting A carbon offset is a reduction or removal of emissions of carbon dioxide or other greenhouse gases made in order to compensate for emissions made elsewhere. Offsets are measured in tonnes of carbon dioxide-equivalent (CO2e). One ton of carbon ...

Origins of the energy hierarchy

The Energy Hierarchy was first proposed in 2005 by Philip Wolfe, when he was Director General of the

Renewable Energy Association The Association for Renewable Energy and Clean Technology, previously known as Renewable Energy Association (REA), is a renewable energy and clean technology trade association in the UK encompassing all of renewables industry in the United K ...

. 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.

References

{{reflist, 30em Emissions reduction Energy conservation Energy policy Renewable energy Hierarchy