Sustainable energy

"Green power" redirects here. For other uses, see Green power (disambiguation).

Sustainable energy is energy produced and used in such a way that it "meets the needs of the present without compromising the ability of future generations to meet their own needs."

The term "sustainable energy" is often used interchangeably with the term "renewable energy". In general, renewable energy sources such as solar, wind, and hydroelectric energy are widely considered to be sustainable. However, particular renewable energy projects, such as the clearing of forests for the production of biofuels, can lead to similar or even worse environmental damage than using fossil fuel energy. Nuclear power is a safe zero-emission source, but its sustainability is debated because of nuclear waste. The concept of sustainable energy is similar to the concepts of green energy and clean energy in its consideration of environmental impacts, however formal definitions of sustainable energy also include economic and socio-cultural impacts.

The energy transition to meet the world's needs for electricity, heating, cooling, and power for transport in a sustainable way is widely considered to be one of the greatest challenges facing humanity in the 21st century. Worldwide, nearly a billion people lack access to electricity, and around 3 billion people rely on smoky fuels such as wood, charcoal or animal dung in order to cook. These and fossil fuels are a major contributor to air pollution, which causes an estimated 7 million deaths per year. Production and consumption of energy emits over 70% of human-caused greenhouse gas emissions.

Proposed pathways for limiting global warming to 1.5 °C describe rapid implementation of low-emission methods of producing electricity and a shift towards more use of electricity in sectors such as transport. The pathways also include measures to reduce energy consumption; and use of carbon-neutral fuels, such as hydrogen produced by renewable electricity or with carbon capture and storage. Achieving these goals will require government policies including carbon pricing, energy-specific policies, and phase-out of fossil fuel subsidies.

Moderate amounts of wind and solar energy, which are intermittent energy sources, can be integrated into the electrical grid without additional infrastructure such as grid energy storage. These sources generated 8.5% of worldwide electricity in 2019, a share that has grown rapidly. Costs of wind, solar, and batteries are projected to continue falling due to economies of scale from increased investment.

Definitions

Buildings in the Solar Settlement at Schlierberg incorporate rooftop solar panels and are built for maximum energy efficiency. As a result, they produce more energy than they consume.

The concept of sustainable development was described by the World Commission on Environment and Development in its 1987 book Our Common Future. Its definition of "sustainability", now used widely, was, "Sustainable development should meet the needs of the present without compromising the ability of future generations to meet their own needs." In its book, the Commission described four key elements of sustainability with respect to energy: the ability to increase the supply of energy to meet growing human needs, energy efficiency and conservation, public health and safety, and "protection of the biosphere and prevention of more localized forms of pollution."

Various definitions of sustainable energy have been offered since then which are also based on the three pillars of sustainable development, namely environment, economy, and society.

• Environmental criteria include greenhouse gas emissions, impact on biodiversity, and the production of hazardous waste and toxic emissions.
• Economic criteria include the cost of energy, whether energy is delivered to users with high reliability, and effects on jobs associated with energy production.
• Socio-cultural criteria include energy security, such as the prevention of wars over the energy supply.

The organizing principle for sustainability is sustainable development, which includes the four interconnected domains: ecology, economics, politics and culture.

The United Nations Economic Commission for Europe defines sustainable energy based on three pillars: environment, energy security and quality of life.

Background

Clean Energy in Kenya. USAID connects a Kenyan dairy farmer to biogas that enables him to light his new home with clean energy.

Providing sustainable energy is widely viewed as one of the greatest challenges facing humanity in the 21st century, both in terms of meeting the needs of the present and in terms of effects on future generations. Bill Gates said in 2011:

If you gave me the choice between picking the next 10 presidents or ensuring that energy is environmentally friendly and a quarter as costly, I'd pick the energy thing.

Worldwide, 940 million (13% of the world) people do not have access to electricity, and 3 billion people rely on dirty fuels for cooking. Air pollution, caused largely by the burning of fuel, kills an estimated 7 million people each year. United Nations Sustainable Development Goal 7 calls for "access to affordable, reliable, sustainable and modern energy for all" by 2030.

Energy production and consumption are major contributors to climate change, being responsible for 72% of annual human-caused greenhouse gas emissions as of 2014. Generation of electricity and heat contributes 31% of human-caused greenhouse gas emissions, use of energy in transportation contributes 15%, and use of energy in manufacturing and construction contributes 12%. An additional 5% is released through processes associated with fossil fuel production, and 8% through various other forms of fuel combustion. As of 2015, 80% of the world's primary energy is produced from fossil fuels.

In developing countries, over 2.5 billion people rely on traditional cookstoves and open fires to burn biomass or coal for heating and cooking. This practice causes harmful local air pollution and increases the danger from fires, resulting in an estimated 4.3 million deaths annually. Additionally, serious local environmental damage, including desertification, can be caused by excessive harvesting of wood and other combustible material. Promoting usage of cleaner fuels and more efficient technologies for cooking is, therefore, one of the top priorities of the United Nations Sustainable Energy for All initiative. As of 2015, efforts to design clean cookstoves that are inexpensive, powered by sustainable energy sources, and acceptable to users have been mostly disappointing.

In 2020, the International Energy Agency warned that the economic turmoil caused by the coronavirus outbreak may prevent or delay companies from investing in green energy. The outbreak could potentially spell a slowdown in the world's clean energy transition if no action is undertaken.

Proposed pathways for climate change mitigation

Bangui Wind Farm in the Philippines.

Workers construct a solar panel array structure in Malawi

Cost–benefit analysis work has been done by a disparate array of specialists and agencies to determine the best path to decarbonizing the energy supply of the world. The IPCC's 2018 Special Report on Global Warming of 1.5 °C says that for limiting warming to 1.5 °C and avoiding the worst effects of climate change, "global net human-caused emissions of CO₂ would need to fall by about 45% from 2010 levels by 2030, reaching net zero around 2050." As part of this report, the IPCC's working group on climate change mitigation reviewed a variety of previously-published papers that describe pathways (i.e. scenarios and portfolios of mitigation options) to stabilize the climate system through changes in energy, land use, agriculture, and other areas.

The pathways that are consistent with limiting warning to approximately 1.5 °C describe a rapid transition towards producing electricity through lower-emission methods, and increasing use of electricity instead of other fuels in sectors such as transportation. These pathways have the following characteristics (unless otherwise stated, the following values are the median across all pathways):

• Renewable energy: The proportion of primary energy supplied by renewables increases from 15% in 2020 to 60% in 2050. The proportion of primary energy supplied by biomass increases from 10% to 27%, with effective controls on whether land use is changed in the growing of biomass. The proportion from wind and solar increases from 1.8% to 21%.
• Nuclear energy: The proportion of primary energy supplied by nuclear power increases from 2.1% in 2020 to 4% in 2050. Most pathways describe an increase in use of nuclear power, but some describe a decrease. The reason for the wide range of possibilities is that deployment of nuclear energy "can be constrained by societal preferences."
• Coal and oil: Between 2020 and 2050, the proportion of primary energy from coal declines from 26% to 5%, and the proportion from oil declines from 35% to 13%.
• Natural gas: In most pathways, the proportion of primary energy supplied by natural gas decreases, but in some pathways, it increases. Using the median values across all pathways, the proportion of primary energy from natural gas declines from 23% in 2020 to 13% in 2050.
• Carbon capture and storage: Pathways describe more use of carbon capture and storage for bioenergy and fossil fuel energy.
• Electrification: In 2020, around 20% of final energy use is provided by electricity. By 2050, this proportion more than doubles in most pathways.
• Energy conservation: Pathways describe methods to increase energy efficiency and reduce energy demand in all sectors (industry, buildings, and transport). With these measures, pathways show energy usage to remain around the same between 2010 and 2030, and increase slightly by 2050.

Energy efficiency

Main article: Efficient use of energy

Energy efficiency and renewable energy are often considered the twin pillars of sustainable energy. The International Energy Agency estimates that 40% of greenhouse gas emission reductions needed for the Paris agreement can be achieved by increasing energy efficiency. Opportunities for improvement on the demand side of the energy equation are as diverse as those on the supply side, and often offer significant economic benefits. For instance, there is significant potential to increase energy efficiency of cooking in developing countries, which would also help decrease mortality from air pollution. Improved energy efficiency also increases energy security for importing countries, as they rely less on oil producing regions.

Between 2015 in 2018, each year saw less improvements in energy efficiency compared to the previous. In transport, consumer preferences for bigger cars is part of the driver of the slowdown. Globally, governments did not strongly increase their ambition level for energy efficiency policy over this period either. Policies to improve efficiency include building codes, performance standards, and carbon pricing. Efficiency slows down energy demand growth so that rising clean energy supplies can make deep cuts in fossil fuel use. A 2011 historical analysis demonstrated that the rate of energy efficiency improvements was generally outpaced by the rate of growth in energy demand, due to continuing economic and population growth. Becaus this e carbon emissions over the period studied were coupled with total energy use, despite energy efficiency gains total carbon emissions continued to increase. Thus, given the thermodynamic and practical limits of energy efficiency improvements, slowing the growth in energy demand has been said to be essential.

Renewable energy sources

Rise in renewable energy consumption from 1965 to 2019

Main article: Renewable energy

When referring to sources of energy, the terms "sustainable energy" and "renewable energy" are often used interchangeably, however particular renewable energy projects sometimes raise significant sustainability concerns. Renewable energy technologies are essential contributors to sustainable energy as they generally contribute to world energy security, and reduce dependence on fossil fuel resources thus mitigating greenhouse gas emissions.

Solar power

11 MW solar power plant near Serpa, Portugal 38°1′51″N 7°37′22″W

Main article: Solar power

In 2019, solar power provided around 3% of global electricity. Most solar power uses photovoltaic (PV) cells to convert light into electricity. Solar panels can be integrated on buildings or used in solar parks connected to the electrical grid. Concentrated solar power produces heat to drive a heat engine. Solar power was initially used for small-scale energy: powering calculators and providing electricity to remote areas. Although typically warranted for 25 years a solar panel will usually generate for longer, although at reduced efficiency, and almost all of it can be recycled. Typical panels convert less than 20% of the sunlight that hits them into electricity, as higher efficiency materials are either too expensive or their lifetime is not guaranteed. But market growth is mostly driven not by efficiency gains, but by the falling cost per unit of electricity generated and stored. The cost of electricity from new solar farms is competitive with, or in many places cheaper than, existing coal plants.

MIT's Solar House#1 built in 1939 used seasonal thermal energy storage (STES) for year-round heating.

Solar heating

Sketch of a parabolic trough collector

This section is an excerpt from Solar water heating.[edit]

Solar water collectors installed in Spain

Solar water heating (SWH) is heating water by sunlight, using a solar thermal collector. A variety of configurations are available at varying cost to provide solutions in different climates and latitudes. SWHs are widely used for residential and some industrial applications.

A Sun-facing collector heats a working fluid that passes into a storage system for later use. SWH are active (pumped) and passive (convection-driven). They use water only, or both water and a working fluid. They are heated directly or via light-concentrating mirrors. They operate independently or as hybrids with electric or gas heaters. In large-scale installations, mirrors may concentrate sunlight into a smaller collector.

As of 2017, global solar hot water (SHW) thermal capacity is 472 GW and the market is dominated by China, the United States and Turkey. Barbados, Austria, Cyprus, Israel and Greece are the leading countries by capacity per person.

Wind power

Wind power: worldwide installed capacity

Main article: Wind power

Wind turbines are turned by the kinetic energy of wind and, in 2019, their electric generators provided approximately 6% of the global electricity supply. Wind power is a sustainable and renewable energy, and has a much smaller impact on the environment compared to burning fossil fuels. Wind farms consist of many individual wind turbines, which are connected to the electric power transmission network. New onshore wind is often competitive with, or in some places cheaper than, existing coal plants.

Onshore wind farms also have an impact on the landscape, as typically they need to be spread over more land than other power stations and need to be built in wild and rural areas, which can lead to "industrialization of the countryside" and habitat loss. Offshore wind power has less visual impact. After about 20 years wind turbine blades need replacing with larger blades, and research continues on how best to recycle them and how to manufacture blades which are easier to recycle. Although construction and maintenance costs are higher at sea some analysts forecast that, because the winds are steadier and stronger than on land, with future larger blades offshore will become cheaper than onshore wind in the mid-2030s.

Hydropower

Main article: Hydropower

Hydroelectric dams are one of the most widely deployed sources of sustainable energy.

Among sources of renewable energy, hydroelectric plants have the advantages of being long-lived—many existing plants have operated for more than 100 years. Also, hydroelectric plants are clean, have few emissions and can compensate for variations in wind and solar power. Criticisms directed at large-scale hydroelectric plants include: dislocation of people living where the reservoirs are planned, and release of greenhouse gases during construction and flooding of the reservoir. However, it has been found that high emissions are associated only with shallow reservoirs in warm (tropical) locales, and innovations in hydropower turbine technology are enabling efficient development of low-impact run-of-the-river hydroelectricity projects.

In 2019, hydropower supplied 16% of the world's electricity, down from a high of nearly 20% in the mid-to late 20th century. It produced 60% of electricity in Canada and nearly 80% in Brazil. As of 2017, new hydropower construction has stopped or slowed down since 1980 in most countries except China.

Geothermal

One of many power plants at The Geysers, a geothermal power field in northern California, with a total output of over 750 MW.

Main articles: Geothermal power and Geothermal heating

Geothermal energy is produced by tapping into the thermal energy created and stored within the earth. It arises from the radioactive decay of an isotope of potassium and other elements found in the Earth's crust. Geothermal energy can be obtained by drilling into the ground, very similar to oil exploration, and then it is carried by a heat-transfer fluid (e.g. water, brine or steam). Within these liquid-dominated systems, there are possible concerns of subsidence and contamination of ground-water resources. Therefore, protection of ground-water resources is necessary in these systems. This means that careful reservoir production and engineering is necessary in liquid-dominated geothermal reservoir systems. Geothermal energy is considered sustainable because that thermal energy is constantly replenished.

Geothermal energy can be harnessed to for electricity generation and for heating. Technologies in use include dry steam power stations, flash steam power stations and binary cycle power stations. As of 2010, geothermal electricity generation is used in 24 countries, while geothermal heating is in use in 70 countries. International markets grew at an average annual rate of 5 percent over the three years to 2015.

Geothermal power is considered to be a sustainable, renewable source of energy because the heat extraction is small compared with the Earth's heat content. The greenhouse gas emissions of geothermal electric stations are on average 45 grams of carbon dioxide per kilowatt-hour of electricity, or less than 5 percent of that of conventional coal-fired plants.

Biomass

Main article: Biomass

Sugarcane plantation to produce ethanol in Brazil

A CHP power station using wood to provide electricity to over 30.000 households in France

Biomass is biological material derived from living, or recently living organisms. As an energy source, biomass can either be burned to produce heat and to generate electricity or converted to modern biofuels such as biodiesel and ethanol.

Biomass is extremely versatile and one of the most-used sources of renewable energy. It is available in many countries, which makes it attractive for reducing dependence on imported fossil fuels. If the production of biomass is well-managed, carbon emissions can be significantly offset by the absorption of carbon dioxide by the plants during their lifespans. However this "carbon debt" may be paid back too late, or (especially in the United States) not properly accounted for. If the biomass source is agricultural or municipal waste, burning it or converting it into biogas also provides a way to dispose of this waste. Bioenergy production can be combined with carbon capture and storage to create a zero-carbon or negative-carbon system, but it is doubtful this can be scaled up quickly enough.

If biomass is harvested from crops, such as tree plantations, the cultivation of these crops can displace natural ecosystems, degrade soils, and consume water resources and synthetic fertilizers. In some cases, these impacts can actually result in higher overall carbon emissions compared to using petroleum-based fuels. According to the UK Committee on Climate Change in the long term all uses of biomass must maximise carbon sequestration, for example by using wood in construction or bio-energy with carbon capture and storage (BECCS): : "and away from using biofuels in surface transport, biomass for heating buildings, or biomass for generating power without CCS".

Biofuels

Main article: Biofuel

Biofuels are fuels, such as ethanol, manufactured from various types of biomass, such as corn or sugar beet. Biofuels are usually liquid and used to power transport, often blended with liquid fossil fuels such as gasoline, diesel or kerosene. As of 2020 which are sustainable biofuel is being debated.

Cellulosic ethanol has many benefits over traditional corn based-ethanol. It does not take away or directly conflict with the food supply because it is produced from wood, grasses, or non-edible parts of plants. However, as of 2020, there have been few commercial plants of cellulosic ethanol, mostly concentrated in Europe.

Use of farmland for growing fuel can result in less land being available for growing food. Since photosynthesis is inherently inefficient, and crops also require significant amounts of energy to harvest, dry, and transport, the amount of energy produced per unit of land area is very small, in the range of 0.25 W/m² to 1.2 W/m². In the United States, corn-based ethanol has replaced less than 10% of motor gasoline use since 2011, but has consumed around 40% of the annual corn harvest in the country. In Malaysia and Indonesia, the clearing of forests to produce palm oil for biodiesel has led to serious social and environmental effects, as these forests are critical carbon sinks and habitats for endangered species. In 2015, annual global production of liquid biofuels was equivalent to 1.8% of the energy extracted from crude oil. According to the UK Committee on Climate Change, due to limited supply the best uses of biomass in the long term may include using biomass for aviation biofuel, providing that some carbon is captured and stored during manufacture of the fuel.

Marine energy

Main article: Marine energy

Marine energy is mainly tidal power and wave power. As of 2020, a few small tidal power plants are operating in France and China, and engineers continue to try and make wave power equipment more robust against storms.

Non-renewable energy sources

Nuclear power

Main article: Nuclear power debate

Electricity generation related CO2 emissions in France as of 27 May 2020 with overall CO2 intensity of 52 gCO2eq/kWh.

Electricity generation related CO₂ emissions in France as of 27 May 2020 with overall CO₂ intensity of 52 gCO₂eq/kWh. Source: electricitymap.org

Nuclear power plants have been used since the 1950s to produce a zero emission, steady supply of electricity, without creating local air pollution. In 2012, nuclear power plants in 30 countries generated 11% of global electricity. The IPCC considers nuclear power to be a low-carbon energy source, with lifecycle greenhouse gas emissions (including the mining and processing of uranium), similar to the emissions from renewable energy sources. As of 2020 nuclear power provides 50% of European Union low-carbon electricity and 26% of total energy production in Europe.

There is considerable controversy over whether nuclear power can be considered sustainable, with debates revolving around the risk of nuclear accidents, the cost and construction time needed to build new plants, the generation of radioactive nuclear waste, and the potential for nuclear energy to contribute to nuclear proliferation. These concerns have spurred the anti-nuclear movement and led to a decrease in the contribution of nuclear energy to the global electricity supply since 1993. At a global level, opposition to nuclear energy stood at 62 percent in 2011. Public support for nuclear energy is often low as a result of safety concerns, however for each unit of energy produced, nuclear energy is far safer than fossil fuel energy. The uranium ore used to fuel nuclear fission plants is a non-renewable resource, but sufficient quantities exist to provide a supply for hundreds of years.

Electricity generation related CO₂ emissions in Germany as of 27 May 2020 with overall CO₂intensity of 257 gCO2eq/kWh. Source: electricitymap.org

Traditional environmental groups such as Greenpeace and the Sierra Club are opposed to all use of nuclear power. Individuals who have described nuclear power as a green energy source include philanthropist Bill Gates and environmentalist James Lovelock.

Safety assessment of already operating nuclear power plants to extend their lifetimes, perhaps up to 80 years, continues. Regardless of past accidents, nuclear power remains the safest energy source available per unit of energy as compared to other sources.

Some newer nuclear reactor designs are capable of extracting energy from nuclear waste until it is no longer (or significantly less) dangerous, and have design features that greatly minimize the possibility of a nuclear accident. These designs (for instance the molten salt reactor) have yet to be commercialized. Thorium is a fissionable material used in thorium-based nuclear power. The thorium fuel cycle claims several potential advantages over a uranium fuel cycle, including greater abundance, superior physical and nuclear properties, better resistance to nuclear weapons proliferation and reduced plutonium production. Therefore, it is sometimes referred as sustainable.

Fusion

Main article: Nuclear fusion power

A prospective energy source is nuclear fusion (as opposed to nuclear fission used today). It is the reaction that exists in stars, including the Sun. Fusion reactors currently in construction (ITER) are expected to be inherently safe due to lack of chain reaction and do not produce long-lived nuclear waste. The fuel for nuclear fusion reactors are very widely available deuterium, lithium and tritium.

(Fossil) fuel switching

See also: Sustainable Development Goal 7

On average for a given unit of energy produced, the greenhouse gas emissions of natural gas are around half the emissions of coal when used to generate electricity, and around two-thirds the emissions of coal when used to produce heat: however reducing methane leaks is imperative. Natural gas also produces significantly less air pollution than coal. Building gas-fired power plants and gas pipelines is therefore promoted as a way to phase out coal and wood burning pollution (and increase energy supply in some African countries with fast growing populations or economies), however this practice is controversial. Opponents argue that developing natural gas infrastructure will create decades of carbon lock-in and stranded assets, and that renewables create far less emissions at comparable costs. The life-cycle greenhouse-gas emissions of natural gas are around 40 times the emissions of wind and nuclear energy. Switching cooking from dirty fuels such as wood or kerosene to LPG has been criticised and biogas or electricity has been suggested as an alternative.

Sustainable energy system

Sectors

Power

Compared to heating and transport, renewables grew significantly faster in the power sector. As of 2018, about a quarter of all power came from modern renewable sources.

Heating and cooling

Further information: Renewable heat

A large fraction of the world population cannot afford sufficient cooling or live in poorly designed houses. In addition to air conditioning, which requires electrification and additional power demand, passive building design and urban planning will be needed to ensure cooling needs are met in a sustainable way. Similarly, many households in the developing and developed world suffer from fuel poverty and cannot heat their houses enough. Existing heating practices are often polluting. Alternatives to fossil fuel heating include waste heat, solar thermal, geothermal, electrification (heat pumps, or the less efficient electric heater) and biomass. The costs of all these technologies strongly depend on location, and uptake of the technology sufficient for deep decarbonisation requires stringent policy interventions.

Transport

There are multiple ways to make transport more sustainable. Public transport usually requires less energy per passenger than personal vehicles such as cars. In cities, transport can be made cleaner by stimulating nonmotorised transport such as cycling. Energy efficiency of cars has increased significantly, often due to regulation-driven innovation. Electric vehicles use less energy per kilometre, and as electricity is more easily produced sustainably than fuel, also contribute to making transport more sustainable. Hydrogen vehicles may be an alternative for larger vehicles which have not yet been widely electrified, such as long distance lorries.

Carbon capture and storage

See also: CCS and climate change mitigation

In theory, the greenhouse gas emissions of fossil fuel and biomass power plants can be significantly reduced through carbon capture and storage (CCS), although this process is expensive. When CCS is used to capture emissions from burning biomass in a process known as bioenergy with carbon capture and sequestration (BECCS), the overall process can result in net carbon dioxide removal from the atmosphere. The BECCS process can also result in net positive emissions depending on how the biomass material is grown, harvested, and transported.

As of 2014, the lowest-cost mitigation pathways for meeting the 2 °C target typically describe massive deployment of BECCS. However, using BECCS at the scale described in these pathways would require more resources than are currently available worldwide. For example, to capture 10 billion tons of CO₂ per year (GtCO₂/y) would require biomass from 40 percent of the world's current cropland.

To properly compare the costs of wind and solar power with that from natural gas with CCS it is necessary to estimate not just the LCOE but the whole system cost. These will depend considerably on the location due to different: carbon prices, grid enhancements needed for flexibility, availability of suitable geology for carbon dioxide storage etc.

Managing intermittent energy sources

See also: Capacity factor

In a pumped-storage hydroelectricity facility, water is pumped uphill electricity generation exceeds demand. The water is later released to generate hydroelectricity.

Solar and wind are variable renewable energy (VRE) sources that supply electricity intermittently depending on the weather and the time of day. Overall intermittency can be reduced by combining these sources, and in some places batteries can also be combined to eliminate intermittency completely, so that the whole installation produces dispatchable power.

Most electric grids were constructed for non-intermittent energy sources such as coal-fired power plants. Half the world's electricity will need to be wind and solar by 2030, to limit the global rise in temperature to well below 2 °C by 2050. As larger amounts of solar and wind energy are integrated into the grid, it becomes necessary to make changes to the overall system to ensure that the supply of electricity is matched to demand.These changes can include the following:

• Using hydroelectricity, natural gas plants or nuclear plants to produce backup power
• Using grid energy storage to store excess solar and wind energy and release it as needed. The most commonly used storage method is pumped-storage hydroelectricity, which is feasible only at locations that are next to a large hill or a deep underground mine. Batteries are being deployed widely. Other storage technologies such as power-to-gas have been used in limited situations.
• Trading electricity with other locations in a regional grid or through long-distance transmission lines
• Reducing demand for electricity at certain times through energy demand management and use of smart grids.
• Energy market, or more specifically electricity market, changes so that power supply flexibility is better paid

As of 2019, the cost and logistics of energy storage for large population centers is a significant challenge, although the cost of battery systems has plunged dramatically. For instance, a 2019 study found that for solar and wind energy to replace all fossil fuel generation for a week of extreme cold in the eastern and midwest United States, energy storage capacity would have to increase from the 11 GW in place at that time to between 230 GW and 280 GW, depending on how much nuclear power is retired.

Pumped-storage as well as load following coal, fossil gas and nuclear power plants are the most widespread techniques for balancing intermittent energy sources as of 2020.

Energy storage

Some technologies provide short-term energy storage, while others can keep it for much longer. Bulk energy storage is currently dominated by hydroelectric dams, both conventional as well as pumped. Grid energy storage is a collection of methods used for energy storage on a large scale within an electrical power grid.

Common examples of energy storage are the rechargeable battery, which stores electricity as chemical energy readily reconvertible to electricity to return to the grid, the hydroelectric dam, which stores energy in a reservoir as gravitational potential energy, and ice storage tanks, which store ice frozen by cheaper energy at night to meet peak daytime demand for cooling.

Hydrogen

Metal Hydride for Hydrogen Storage-Ovonic

Main articles: Hydrogen economy, Hydrogen fuel, and Hydrogen storage

Hydrogen is a zero-emission fuel that can be produced by using electrolysis to split water molecules into hydrogen and oxygen. Hydrogen can play a role for storing energy in a sustainable energy system if the electricity used to produce it is generated from sustainable sources, such as wind or solar. Hydrogen can be produced when there is a surplus of intermittent renewable electricity, then stored and used to generate heat or to re-generate electricity. As of 2018, very little of the world's supply of hydrogen is created from sustainable sources. Nearly all hydrogen is created by steam methane reforming (SMR), which results in high greenhouse gas emissions but is currently cheaper than creating hydrogen via electrolysis. Although some carbon from SMR could be captured the process can be made more sustainable by using autothermal reforming with carbon capture and storage technologies to remove most of the carbon dioxide that is emitted.

Twenty per cent hydrogen can be mixed into natural gas pipelines without changing pipelines or appliances, but as hydrogen is less energy-dense this would only save 7% of emissions. As of 2020 trials are underway on how to convert a natural gas grid to 100% hydrogen, in order to reduce or eliminate emissions from residential and industrial natural gas heating. It can be used to power vehicles that have hydrogen fuel cells. As it has a low energy to volume content, it is easier to use in hydrogen-powered ships or heavy road vehicles than in cars and airplanes.

Electrification

Main article: Electrification

Electrification is a key part of using energy sustainably, as many mainstream sustainable energy technologies are electrically powered, in contrast to the technologies they replace. Specifically, massive electrification in the heat and transport sector may be needed to make these sectors sustainable, with heat pumps and electric vehicles playing an important role.

As of 2018 860 million people are estimated to have no electricity, of whom 600 million are in sub-Saharan Africa. According to a 2019 report by the IEA, in sub-Saharan Africa "current and planned efforts to provide access to modern energy services barely outpace population growth" and would still leave over half a billion people without electricity and over a billion without clean cooking by 2030. But according to the report this can be vastly improved, in part by accelerating electrification.

Government energy policies

Main articles: Energy policy and Economics of climate change mitigation

Comparing trends in worldwide energy use, the growth of renewable energy to 2015 is shown by the green line

According to the IPCC, both explicit carbon pricing and complementary energy-specific policies are necessary mechanisms to limit global warming to 1.5 °C.

Energy-specific programs and regulations have historically been the mainstay of efforts to reduce fossil fuel emissions. Successful cases include the building of nuclear reactors in France in the 1970s and 1980s, and fuel efficiency standards in the United States which conserved billions of barrels of oil. Other examples of energy-specific policies include energy-efficiency requirements in building codes, banning new coal-fired electricity plants, performance standards for electrical appliances, and support for electric vehicle use. Nevertheless, fossil fuel subsidies remain a key barrier to a transition to a clean energy system.

Carbon taxes are an effective way to encourage movement towards a low-carbon economy, while providing a source of revenue that can be used to lower other taxes or to help lower-income households afford higher energy costs. Carbon taxes have encountered strong political pushback in some jurisdictions, whereas energy-specific policies tend to be politically safer. According to the OECD climate change cannot be curbed without carbon taxes on energy, but 70% of energy-related CO2 emissions were not taxed at all in 2018. Some studies estimate that combining a carbon tax with energy-specific policies would be more cost-effective than a carbon tax alone.

See also

• Environmental impact of the energy industry
• Fossil fuel divestment
• Life-cycle greenhouse-gas emissions of energy sources
• Sustainable Energy for All initiative

Bibliography

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• REN21 (2020). Renewables 2020: Global Status Report (PDF). Paris. ISBN 978-3-948393-00-7.{{cite book}}: CS1 maint: location missing publisher (link) CS1 maint: numeric names: authors list (link)
• Smil, Vaclav (2017a). Energy Transitions: Global and National Perspectives. Santa Barbara, California: Praeger, an imprint of ABC-CLIO, LLC. ISBN 978-1-4408-5324-1. OCLC 955778608. {{cite book}}: Invalid |ref=harv (help)
• Smil, Vaclav (2017b). Energy and Civilization : A History. Cambridge, Massachusetts: The MIT Press. ISBN 978-0-262-03577-4. OCLC 959698256. {{cite book}}: Invalid |ref=harv (help)
• Tester, Jefferson (2012). Sustainable Energy : Choosing Among Options. Cambridge, MA: MIT Press. ISBN 978-0-262-01747-3. OCLC 892554374. {{cite book}}: Invalid |ref=harv (help)