Fossil fuel phase-out

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https://en.wikipedia.org/wiki/Fossil_fuel_phase-out

 

Investment: Companies, governments and households have been investing increasing amounts in decarbonisation, including renewable energy, electric vehicles and associated infrastructure, energy storage, energy-efficient heating systems, carbon capture and storage, and hydrogen energy.

 

Cost: With increasingly widespread implementation of renewable energy sources, the levelised cost of energy has declined, most notably for energy generated by solar panels.

Fossil fuel phase-out is the gradual reduction of the use and production of fossil fuels to zero, to reduce deaths and illness from air pollution, limit climate change, and to strengthen energy independence. It is part of the ongoing renewable energy transition.

Although many countries are shutting down coal-fired power stations, electricity generation is not moving off coal fast enough to meet climate goals. Many countries have set dates to stop selling gasoline and diesel cars and trucks, but a timetable to stop burning fossil gas has not yet been agreed.

Current efforts in fossil fuel phase-out involve replacing fossil fuels with sustainable energy sources in sectors such as transport and heating. Alternatives to fossil fuels include electrification, green hydrogen and biofuel. Phase-out policies include both demand-side and supply-side constraints. Whereas demand-side approaches seek to reduce fossil-fuel consumption, supply-side initiatives seek to constrain production to accelerate the pace of energy transition and reduction in emissions. It has been suggested that laws should be passed to make fossil fuel companies bury the same amount of carbon as they emit. The International Energy Agency estimates that in order to achieve carbon neutrality by the middle of the century, global investments in renewable energy must treble by 2030, reaching over $4 trillion annually.

Scope

While crude oil and natural gas are also being phased out in chemical processes (e.g. production of new building blocks for plastics) as the circular economy and biobased economy (e.g. bioplastics) are being developed to reduce plastic pollution, the fossil fuel phase out specifically aims to end the burning of fossil fuels and the consequent production of greenhouse gases. Therefore, attempts to reduce the use of oil and gas in the plastic industry do not form part of fossil fuel phase-out or reduction plans.

Types of fossil fuels

Coal

Main article: Coal phase-out

See also: Beyond Coal

 

The annual amount of coal plant capacity being retired increased into the mid-2010s. However, the rate of retirement has since stalled, and global coal phase-out is not yet compatible with the goals of the Paris Climate Agreement.

 

In parallel with retirement of some coal plant capacity, other coal plants are still being added, though the annual amount of added capacity has been declining since the 2010s.

Coal use peaked in 2013 but to meet the Paris Agreement target of keeping global warming to well below 2 °C (3.6 °F) coal use needs to halve from 2020 to 2030. However as of 2017, coal supplied over a quarter of the world's primary energy and about 40% of the greenhouse gas emissions from fossil fuels. Phasing out coal has short-term health and environmental benefits which exceed the costs, and without it the 2 °C target in the Paris Agreement cannot be met; but some countries still favour coal, and there is much disagreement about how quickly it should be phased out.

As of 2018, 30 countries and many sub-national governments and businesses had become members of the Powering Past Coal Alliance, each making a declaration to advance the transition away from unabated coal power generation. As of 2019, however, the countries which use the most coal have not joined, and some countries continue to build and finance new coal-fired power stations. A just transition from coal is supported by the European Bank for Reconstruction and Development.

In 2019 the UN Secretary General said that countries should stop building new coal power plants from 2020 or face 'total disaster'.

In 2020, although China built some plants, globally more coal power was retired than built: the UN Secretary General has said that OECD countries should stop generating electricity from coal by 2030 and the rest of the world by 2040.

Oil

The 2010 Deepwater Horizon oil spill discharges 4.9 million barrels (780,000 m³)

 

See also: Peak Oil

Crude oil is refined into fuel oil, diesel and gasoline. The refined products are primarily for transportation by conventional cars, trucks, trains, planes and ships. Popular alternatives are human-powered transport, public transport, electric vehicles, and biofuels.

Natural gas

See also: Natural gas § Environmental effects, and Peak gas

 

Natural gas well in Germany

Natural gas is widely used to generate electricity and has an emission intensity of about 500g/kWh. Heating is also a major source of carbon dioxide emissions. Leaks are also a large source of atmospheric methane.

In some countries natural gas is being used as a temporary "bridge fuel" to replace coal, in turn to be replaced by renewable sources or a hydrogen economy. However this "bridge fuel" may significantly extend the use of fossil fuel or strand assets, such as gas-fired power plants built in the 2020s, as the average plant life is 35 years. Although natural gas assets are likely to be stranded later than oil and coal assets, perhaps not until 2050, some investors are concerned by reputational risk.

Natural gas phase-out is progressing in some regions, for example with increasing use of hydrogen by the European Network of Transmission System Operators for Gas (ENTSOG) and changes to building regulations to reduce the use of gas heating.

Reasons

Commonly cited reasons for phasing out fossil fuels are to:

• reduce deaths and illness caused by air pollution
• limit climate change
• reduce fossil fuel subsidies
• strengthen energy independence – countries with low or no fossil fuel deposits often transition away from fossil fuels to gain energy independently

Health

See also: Air pollution § Health effects

Most of the millions of premature deaths from air pollution are due to fossil fuels. Pollution may be indoors e.g. from heating and cooking, or outdoors from vehicle exhaust. One estimate is that the proportion is 65% and the number 3.5 million each year. According to Professor Sir Andy Haines at the London School of Hygiene & Tropical Medicine the health benefits of phasing out fossil fuels measured in money (estimated by economists using the value of life for each country) are substantially more than the cost of achieving the 2-degree C goal of the Paris Agreement.

Climate change mitigation

Fossil-fuel phase-out is the largest part of limiting global warming as they account for over 70% of greenhouse gas emissions, but as of 2020 needs to move 4 times faster to meet the goals of the Paris Agreement. To achieve the climate goal the vast majority of fossil fuel reserves owned today by countries and companies must remain in the ground.

Employment

The renewable energy transition can create jobs through the construction of new power plants and the manufacturing of the equipment that they need, as was seen in the case of Germany and the wind power industry.

This can also be seen in the case of France and the nuclear power industry. France receives about 75% of its electricity from nuclear energy and hundreds of jobs have been created for developing nuclear technology, construction workers, engineers, and radiation protection specialists.

Energy independence

Countries which lack fossil fuel deposits, particularly coal but also petroleum and natural gas, often cite energy independence in their shift away from fossil fuels.

In Switzerland the decision to electrify virtually the entire railway network was taken in light of the two world wars (during which Switzerland was neutral) when coal imports became increasingly difficult. As Switzerland has ample hydropower resources, electric trains (as opposed to those driven by steam locomotives or diesel) could be run on domestic energy resources, reducing the need for coal imports.

The 1973 oil crisis also led to a shift in energy policy in many places to become (more) independent of fossil fuel imports. In France the government announced an ambitious plan to expand nuclear power which by the end of the 1980s had shifted France's electricity sector almost entirely away from coal gas and oil and towards nuclear power.

The trend towards encouraging cycling in the Netherlands and Denmark also coincided with the 1973 oil crisis and aimed in part at reducing the need for oil imports in the transportation sector.

Phase-out of fossil fuel subsidies

See also: Fossil fuel subsidies § Phase-out

Significant fossil fuel subsidies are present in many countries. Fossil fuel subsidies in 2019 for consumption totalled USD 320 billion spread over many countries. As of 2019 governments subsidise fossil fuels by about $500 billion per year: however using an unconventional definition of subsidy which includes failing to price greenhouse gas emissions, the International Monetary Fund estimated that fossil fuel subsidies were $5.2 trillion in 2017, which was 6.4% of global GDP. Some fossil fuel companies lobby governments.

Phasing out fossil fuel subsidies is very important. It must however be done carefully to avoid protests and making poor people poorer. In most cases, however, low fossil fuel prices benefit wealthier households more than poorer households. So to help poor and vulnerable people, other measures than fossil fuel subsidies would be more targeted. This could in turn increase public support for subsidy reform.

Economic theory indicates that the optimal policy would be to remove coal mining and burning subsidies and replace them with optimal taxes. Global studies indicate that even without introducing taxes, subsidy and trade barrier removal at a sectoral level would improve efficiency and reduce environmental damage. Removal of these subsidies would substantially reduce GHG emissions and create jobs in renewable energy.

The actual effects of removing fossil fuel subsidies would depend heavily on the type of subsidy removed and the availability and economics of other energy sources. There is also the issue of carbon leakage, where removal of a subsidy to an energy-intensive industry could lead to a shift in production to another country with less regulation, and thus to a net increase in global emissions.

In developed countries, energy costs are low and heavily subsidised, whereas in developing countries, the poor pay high costs for low-quality services. It is difficult to measure energy subsidies, but there was some evidence in 2001 that coal production subsidies had declined in several developing and OECD countries.

A plan has been put forward to power 100% of the world's energy with wind, hydroelectric, and solar power by the year 2030. It recommends transfer of energy subsidies from fossil fuel to renewable, and a price on carbon reflecting its cost for flood, cyclone, hurricane, drought, and related extreme weather expenses.

Excluding subsidies the levelised cost of electricity from new large-scale solar power in India and China has been below existing coal-fired power stations since 2021.

A study by Rice University Center for Energy Studies suggested the following steps for countries:

1. Countries should commit to a specific time frame for a full phaseout of implicit and explicit fossil fuel subsidies.
2. Clarify the language on subsidy reform to remove ambiguous terminology.
3. Seek formal legislation in affected countries that codifies reform pathways and reduces opportunities for backsliding.
4. Publish transparent formulas for market-linked pricing, and adhere to a regular schedule for price adjustments.
5. Phase-in full reforms in a sequence of gradual steps. Increasing prices gradually but on a defined schedule signals intent to consumers while allowing time to invest in energy efficiency to partially offset the increases.
6. Aspire to account for externalities over time by imposing a fee or tax on fossil energy products and services, and eliminating preferences for fossil fuels that remain embedded in the tax code.
7. Use direct cash transfers to maintain benefits for poor segments of society rather than preserving subsidised prices for vulnerable socioeconomic groups.
8. Launch a comprehensive public communications campaign.
9. Any remaining fossil fuel subsidies should be clearly budgeted at full international prices and paid for by the national treasury.
10. Document price and emissions changes with reporting requirements.

Studies about fossil fuel phase-out

The countries most reliant on fossil fuels for electricity vary widely on how great a percentage of that electricity is generated from renewables, leaving wide variation in renewables' growth potential.

In 2015, Greenpeace and Climate Action Network Europe released a report highlighting the need for an active phase-out of coal-fired generation across Europe. Their analysis derived from a database of 280 coal plants and included emissions data from official EU registries.

A 2016 report by Oil Change International, concludes that the carbon emissions embedded in the coal, oil, and gas in currently working mines and fields, assuming that these run to the end of their working lifetimes, will take the world to just beyond the 2 °C limit contained in the 2015 Paris Agreement and even further from the 1.5 °C goal. The report observes that "one of the most powerful climate policy levers is also the simplest: stop digging for more fossil fuels".

In 2016, the Overseas Development Institute (ODI) and 11 other NGOs released a report on the impact of building new coal-fired power plants in countries where a significant proportion of the population lacks access to electricity. The report concludes that, on the whole, building coal-fired power plants does little to help the poor and may make them poorer. Moreover, wind and solar generation are beginning to challenge coal on cost.

A 2018 study in Nature Energy, suggests that 10 countries in Europe could completely phase out coal-fired electricity generation with their current infrastructure, whilst the United States and Russia could phase out at least 30%.

In 2020, the Fossil Fuel Cuts Database provided the first global account of supply-side initiatives to constrain fossil fuel production. The latest update of the database recorded 1967 initiatives implemented between 1988 and October 2021 in 110 countries across seven major types of supply-side approaches (Divestment, n=1201; Blockades, n= 374; Litigation, n= 192; Moratoria and Bans, n= 146; Production subsidies removal, n=31; Carbon tax on fossil fuel production, n=16; Emissions Trading Schemes, n= 7).

The GeGaLo index of geopolitical gains and losses assesses how the geopolitical position of 156 countries may change if the world fully transitions to renewable energy resources. Former fossil fuel exporters are expected to lose power, while the positions of former fossil fuel importers and countries rich in renewable energy resources is expected to strengthen.

Multiple decarbonisation plans that get to zero CO₂ emissions have been presented.

A Guardian investigation showed in 2022, that big fossil fuel firms continue to plan huge investments in new fossil fuel production projects that would drive the climate past internationally agreed temperature limits.

Renewable energy potentials

In June 2021 Dr Sven Teske and Dr Sarah Niklas from the Institute for Sustainable Futures, University of Technology Sydney found that "existing coal, oil and gas production puts the world on course to overshoot Paris climate targets." In co-operation with the Fossil Fuel Non-Proliferation Treaty Initiative they published a report entitled, Fossil Fuel Exit Strategy: An orderly wind down of coal, oil, and gas to meet the Paris Agreement. It analyses global renewable energy potential, and finds that "every region on Earth can replace fossil fuels with renewable energy to keep warming below 1.5°C and provide reliable energy access to all."

Assessment of extraction prevention responsibilities

In September 2021, the first scientific assessment of the minimum amount of fossil fuels that would need to be secured from extraction per region as well as globally, to allow for a 50% probability of limiting global warming by 2050 to 1.5 °C was provided.

Challenges of fossil fuel phase-out

After recovering from the COVID-19 pandemic, energy company profits increased with greater revenues from higher fuel prices resulting from the Russian invasion of Ukraine, falling debt levels, tax write-downs of projects shut down in Russia, and backing off from earlier plans to reduce greenhouse gas emissions. Record profits sparked public calls for windfall taxes.

The phase-out of fossil fuels involves many challenges, and one of them is the reliance that the world currently has on them. In 2014, fossil fuels provided 81.1% of the primary energy consumption of the world, with approximately 465 exajoules (11,109 megatonnes of oil equivalent). This number is composed by 179 EJ (4,287 Mtoe) of oil consumption; 164 EJ (3,918 Mtoe) of coal consumption, and 122 EJ (2,904 Mtoe) of natural gas consumption.

Fossil fuel phase-out may lead to an increment in electricity prices, because of the new investments needed to replace their share in the electricity mix with alternative energy sources.

Another impact of a phase-out of fossil fuels is in employment. In the case of employment in the fossil fuel industry, a phase-out is logically undesired, therefore, people employed in the industry will usually oppose any measures that put their industries under scrutiny. Endre Tvinnereim and Elisabeth Ivarsflaten studied the relationship between employment in the fossil fuel industry with the support to climate change policies. They proposed that one opportunity for displaced drilling employments in the fossil fuel industry could be in the geothermal energy industry. This was suggested as a result of their conclusion: people and companies in the fossil fuel industry will likely oppose measures that endanger their employment, unless they have other stronger alternatives. This can be extrapolated to political interests, that can push against the phase-out of fossil fuels initiative. One example is how the vote of United States Congress members is related to the preeminence of fossil fuel industries in their respective states.

Other challenges include ensuring sustainable recycling, sourcing of the required materials, disruptions of existing power structures, managing variable renewable energy, developing optimal national transition policies, transforming transportation infrastructure and responsibilities of fossil fuel extraction prevention. There is active research and development on such issues.

According to the people present at COP27 in Egypt, Saudi Arabian representatives pushed to block a call for the world to burn less oil. After objections from Saudi Arabia and a few other oil producers, summit's final statement failed to include a call for nations to phase out fossil fuels. In March 2022, at a United Nations meeting with climate scientists, Saudi Arabia, together with Russia, pushed to delete a reference to "human-induced climate change" from an official document, disputing the scientifically established fact that the burning of fossil fuels by humans is the main driver of the climate crisis.

Major initiatives and legislation to phase out fossil fuels

China

China has pledged to become carbon neutral by 2060, which would need a just transition for over 3 million workers in the coal-mining and power industry. It is not yet clear whether China aims to phase-out all fossil fuel use by that date or whether a small proportion will still be in use with the carbon captured and stored. In 2021, coal mining was ordered to run at maximum capacity.

EU

At the end of 2019, the European Union launched its European Green Deal. It included:

• a revision of the Energy Taxation Directive which is looking closely at fossil fuel subsidies and tax exemptions (aviation, shipping)
• a circular economy action plan,
• a review and possible revision (where needed) of the all relevant climate-related policy instruments, including the Emissions Trading System
• a sustainable and smart mobility strategy
• potential carbon tariffs for countries that don't curtail their greenhouse gas pollution at the same rate. The mechanism to achieve this is called the Carbon Border Adjustment Mechanism (CBAM).

It also leans on Horizon Europe, to play a pivotal role in leveraging national public and private investments. Through partnerships with industry and member States, it will support research and innovation on transport technologies, including batteries, clean hydrogen, low-carbon steel making, circular bio-based sectors and the built environment.

The European Investment Bank contributed over €81 billion to help the energy industry between 2017 and 2022, in line with EU energy policy. This comprised nearly €76 billion for initiatives related to power grids, energy efficiency, and renewable energy throughout Europe and other parts of the world.

India

India is confident of exceeding Paris COP commitments. In the Paris Agreement, India has committed to an Intended Nationally Determined Contributions target of achieving 40% of its total electricity generation from non-fossil fuel sources by 2030.

Japan

Japan has pledged to become carbon neutral by 2050.

United Kingdom

See also: Energy policy of the United Kingdom

The UK is legally committed to be carbon neutral by 2050, and moving away from the heating of homes by natural gas is likely to be the most difficult part of the country's fossil fuel phase out.

Legislation and initiatives to phase out coal

Further information: Coal phase-out

Phase-out of fossil fuel power plants

See also: Alternative energy, energy transition, and 100% renewable energy

 

Bloomberg NEF reported that in 2022, global energy transition investment equaled fossil fuels investment for the first time.

 

In 2020, renewables overtook fossil fuels as the European Union's main source of electricity for the first time.

Alternative energy refers to any source of energy that can substitute the role of fossil fuels. Renewable energy, or energy that is harnessed from renewable sources, is an alternative energy. However, alternative energy can refer to non-renewable sources as well, like nuclear energy. Between the alternative sources of energy are: solar energy, hydroelectricity, marine energy, wind energy, geothermal energy, biofuels, ethanol and hydrogen.

Energy efficiency is complementary to the use of alternative energy sources, when phasing-out fossil fuels.

Renewable energy

Renewable energy is energy from renewable resources that are naturally replenished on a human timescale. Renewable resources include sunlight, wind, the movement of water, and geothermal heat. Although most renewable energy sources are sustainable, some are not. For example, some biomass sources are considered unsustainable at current rates of exploitation. Renewable energy is often used for electricity generation, heating and cooling. Renewable energy projects are typically large-scale, but they are also suited to rural and remote areas and developing countries, where energy is often crucial in human development. Renewable energy is often deployed together with further electrification, which has several benefits: electricity can move heat or objects efficiently, and is clean at the point of consumption.

From 2011 to 2021, renewable energy has grown from 20% to 28% of global electricity supply. Use of fossil energy shrank from 68% to 62%, and nuclear from 12% to 10%. The share of hydropower decreased from 16% to 15% while power from sun and wind increased from 2% to 10%. Biomass and geothermal energy grew from 2% to 3%. There are 3,146 gigawatts installed in 135 countries, while 156 countries have laws regulating the renewable energy sector. In 2021, China accounted for almost half of the global increase in renewable electricity.

Globally there are over 10 million jobs associated with the renewable energy industries, with solar photovoltaics being the largest renewable employer. Renewable energy systems are rapidly becoming more efficient and cheaper and their share of total energy consumption is increasing, with a large majority of worldwide newly installed electricity capacity being renewable. In most countries, photovoltaic solar or onshore wind are the cheapest new-build electricity.

Many nations around the world already have renewable energy contributing more than 20% of their total energy supply, with some generating over half their electricity from renewables. A few countries generate all their electricity using renewable energy. National renewable energy markets are projected to continue to grow strongly in the 2020s and beyond. According to the IEA, to achieve net zero emissions by 2050, 90% of global electricity generation will need to be produced from renewable sources. Some studies have shown that a global transition to 100% renewable energy across all sectors – power, heat, transport and industry – is feasible and economically viable. Renewable energy resources exist over wide geographical areas, in contrast to fossil fuels, which are concentrated in a limited number of countries. Deployment of renewable energy and energy efficiency technologies is resulting in significant energy security, climate change mitigation, and economic benefits. However renewables are being hindered by hundreds of billions of dollars of fossil fuel subsidies. In international public opinion surveys there is strong support for renewables such as solar power and wind power. In 2022 the International Energy Agency asked countries to solve policy, regulatory, permitting and financing obstacles to adding more renewables, to have a better chance of reaching net zero carbon emissions by 2050.

Hydroelectricity

Chief Joseph Dam near Bridgeport, Washington, US, is a major run-of-the-river station without a sizeable reservoir.

In 2015, hydroelectric energy generated 16.6% of the world's total electricity and 70% of all renewable electricity. In Europe and North America environmental concerns around land flooded by large reservoirs ended 30 years of dam construction in the 1990s. Since then large dams and reservoirs continue to be built in countries like China, Brazil and India. Run-of-the-river hydroelectricity and small hydro have become popular alternatives to conventional dams that may create reservoirs in environmentally sensitive areas.

Wind power

Wind power is the use of wind energy to generate useful work. Historically, wind power was used by sails, windmills and windpumps, but today it is mostly used to generate electricity. This article only deals with wind power for electricity generation. Today, wind power is almost completely generated with wind turbines, generally grouped into wind farms and connected to the electrical grid.

In 2022, wind supplied over 2000 TWh of electricity, which was over 7% of world electricity and about 2% of world energy. With about 100 GW added during 2021, mostly in China and the United States, global installed wind power capacity exceeded 800 GW. To help meet the Paris Agreement goals to limit climate change, analysts say it should expand much faster - by over 1% of electricity generation per year.

Wind power is considered a sustainable, renewable energy source, and has a much smaller impact on the environment compared to burning fossil fuels. Wind power is variable, so it needs energy storage or other dispatchable generation energy sources to attain a reliable supply of electricity. Land-based (onshore) wind farms have a greater visual impact on the landscape than most other power stations per energy produced. Wind farms sited offshore have less visual impact and have higher capacity factors, although they are generally more expensive. Offshore wind power currently has a share of about 10% of new installations.

Wind power is one of the lowest-cost electricity sources per unit of energy produced. In many locations, new onshore wind farms are cheaper than new coal or gas plants.

Regions in the higher northern and southern latitudes have the highest potential for wind power. In most regions, wind power generation is higher in nighttime, and in winter when PV output is low. For this reason, combinations of wind and solar power are suitable in many countries.

Solar

Main articles: Solar power and Solar energy

In 2017, solar power provided 1.7% of total worldwide electricity production, growing at 35% per annum. By 2020 the solar contribution to global final energy consumption is expected to exceed 1%.

Solar photovoltaics

The 71.8 MW Lieberose Photovoltaic Park in Germany

 

Main article: Photovoltaics

Main article: List of photovoltaic power stations

Solar photovoltaic cells convert sunlight into electricity and many solar photovoltaic power stations have been built. The size of these stations has increased progressively over the last decade with frequent new capacity records. Many of these plants are integrated with agriculture and some use innovative tracking systems that follow the sun's daily path across the sky to generate more electricity than conventional fixed-mounted systems. Solar power plants have no fuel costs or emissions during operation.

Concentrated solar power

The 150 MW Andasol solar power station is a commercial parabolic trough solar thermal power plant, located in Spain. The Andasol plant uses tanks of molten salt to store solar energy so that it can continue generating electricity even when the sun isn't shining.

 

See also: Concentrated solar power

Concentrating Solar Power (CSP) systems use lenses or mirrors and tracking systems to focus a large area of sunlight into a small beam. The concentrated heat is then used as a heat source for a conventional power plant. A wide range of concentrating technologies exists; the most developed are the parabolic trough, the concentrating linear fresnel reflector, the Stirling dish and the solar power tower. Various techniques are used to track the Sun and focus light. In all of these systems a working fluid is heated by the concentrated sunlight, and is then used for power generation or energy storage.

Nuclear energy

Main article: Nuclear power

The 2014 Intergovernmental Panel on Climate Change (IPCC) report identifies nuclear energy as one of the technologies that can provide electricity with less than 5% of the lifecycle greenhouse gas emissions of coal power. There are more than 60 nuclear reactors shown as under construction in the list of Nuclear power by country with China leading at 23. Globally, more nuclear power reactors have closed than opened in recent years but overall capacity has increased. China has stated its plans to double nuclear generation by 2030. India also plans to greatly increase its nuclear power. The Manhattan 2 Project has presented a report that describes how to significantly increase nuclear power via factory automation.

Several countries have enacted laws to cease construction on new nuclear power stations. Several European countries have debated nuclear phase-outs and others have completely shut down some reactors. Three nuclear accidents have influenced the slowdown of nuclear power: the 1979 Three Mile Island accident in the United States, the 1986 Chernobyl disaster in the USSR, and the 2011 Fukushima nuclear disaster in Japan. Following the March 2011 Fukushima nuclear disaster, Germany has permanently shut down eight of its 17 reactors and pledged to close the rest by the end of 2022. Italy voted overwhelmingly to keep their country non-nuclear. Switzerland and Spain have banned the construction of new reactors. Japan's prime minister has called for a dramatic reduction in Japan's reliance on nuclear power. Taiwan's president did the same. Shinzō Abe, prime minister of Japan since December 2012, announced a plan to restart some of the 54 Japanese nuclear power plants and to continue some nuclear reactors under construction.

As of 2016, countries such as Australia, Austria, Denmark, Greece, Malaysia, New Zealand and Norway have no nuclear power stations and remain opposed to nuclear power. Germany, Italy, Spain and Switzerland are phasing-out their nuclear power. Despite this, most pathways for spurring a fossil fuel phase-out that keeps pace with global electricity demands include the expansion of nuclear power, according to the IPCC. Likewise, the United Nations Economic Commission for Europe has stated that global climate objectives would likely not be met without nuclear expansion.

Cost overruns, construction delays, the threat of catastrophic accidents, and regulatory hurdles often make nuclear power plant expansion practically infeasible. Some companies and organisations have proposed plans aimed at mitigating the cost, duration, and risk of nuclear power plant construction. NuScale Power, for example, has received regulatory approval from the Nuclear Regulatory Commission for a light-water reactor that would theoretically limit the risk of accidents and could be manufactured for less than traditional nuclear plants. The Energy Impact Center's OPEN100, a platform that provides open-source blueprints for the construction of a nuclear plant with a 100-megawatt pressurised water reactor, claims that its model could be built in as little as two years for $300 million. In both plans, the ability to mass manufacture small modular reactors would theoretically cut down on construction time.

Biomass

Main article: Biomass

Biomass is biological material from living, or recently living organisms, most often referring to plants or plant-derived materials. As a renewable energy source, biomass can either be used directly, or indirectly – once or converted into another type of energy product such as biofuel. Biomass can be converted to energy in three ways: thermal conversion, chemical conversion, and biochemical conversion.

Using biomass as a fuel produces air pollution in the form of carbon monoxide, carbon dioxide, NOx (nitrogen oxides), VOCs (volatile organic compounds), particulates and other pollutants at levels above those from traditional fuel sources such as coal or natural gas in some cases (such as with indoor heating and cooking). Use of wood biomass as a fuel can also produce fewer particulate and other pollutants than open burning as seen in wildfires or direct heat applications. Black carbon – a pollutant created by combustion of fossil fuels, biofuels, and biomass – is possibly the second largest contributor to global warming. In 2009 a Swedish study of the giant brown haze that periodically covers large areas in South Asia determined that it had been principally produced by biomass burning, and to a lesser extent by fossil fuel burning. Denmark has increased the use of biomass and garbage, and decreased the use of coal.

Energy efficiency

Main article: Efficient energy use

 

Costs of producing renewable energy have declined significantly, with 62% of total renewable power generation added in 2020 having lower costs than the cheapest new fossil fuel option.

Moving away from fossil fuels will require changes not only in the way energy is supplied, but in the way it is used, and reducing the amount of energy required to deliver various goods or services is essential. Opportunities for improvement on the demand side of the energy equation are as rich and diverse as those on the supply side, and often offer significant economic benefits.

A sustainable energy economy requires commitments to both renewables and efficiency. Renewable energy and energy efficiency are said to be the "twin pillars" of sustainable energy policy. The American Council for an Energy-Efficient Economy has explained that both resources must be developed to stabilise and reduce carbon dioxide emissions:

Efficiency is essential to slowing the energy demand growth so that rising clean energy supplies can make deep cuts in fossil fuel use. If energy use grows too fast, renewable energy development will chase a receding target. Likewise, unless clean energy supplies come online rapidly, slowing demand growth will only begin to reduce total emissions; reducing the carbon content of energy sources is also needed.

The IEA has stated that renewable energy and energy efficiency policies are complementary tools for the development of a sustainable energy future, and should be developed together instead of being developed in isolation.

Phase-out of fossil fuel vehicles

Main article: Phase-out of fossil fuel vehicles

 

Sales of electric vehicles (EVs) indicate a trend away from gas-powered vehicles that generate greenhouse gases.

Many countries and cities have introduced bans on the sales of new internal combustion engine vehicles, requiring all new cars to be electric vehicles or otherwise powered by clean, non-emitting sources. Such bans include the United Kingdom by 2035 and Norway by 2025. Many transit authorities are working to purchase only electric buses while also restricting use of ICE vehicles in the city center to limit air pollution. Many US states have a zero-emissions vehicle mandate, incrementally requiring a certain per cent of cars sold to be electric. The German term de: Verkehrswende ("traffic transition" analogous to "Energiewende", energetic transition) calls for a shift from combustion powered road transport to bicycles, walking and rail transport and the replacement of remaining road vehicles with electric traction.

Biofuels

Main article: Biofuel

Biofuels, in the form of liquid fuels derived from plant materials, are entering the market. However, many of the biofuels that are currently being supplied have been criticised for their adverse impacts on the natural environment, food security, and land use.

Public opinion

Protest at the Legislative Building in Olympia, Washington. Ted Nation, a long-time environmental activist beside protest sign.

Those corporations that continue to invest in new fossil fuel exploration, new fossil fuel exploitation, are really in flagrant breach of their fiduciary duty because the science is abundantly clear that this is something we can no longer do.

— Christiana Figueres, executive secretary of the United Nations Framework Convention on Climate Change

Opinion polls

Gallup

In 2013, the Gallup organisation determined that 41% of Americans wanted less emphasis placed on coal energy, versus 31% who wanted more. Large majorities wanted more emphasis placed on solar (76%), wind (71%), and natural gas (65%).

Prominent individuals supporting a coal moratorium

• US politician Al Gore stated:

If you're a young person looking at the future of this planet and looking at what is being done right now, and not done, I believe we have reached the stage where it is time for civil disobedience to prevent the construction of new coal plants that do not have carbon capture and sequestration.

Prominent individuals supporting a coal phase-out

• Eric Schmidt, then CEO of Google, called for replacing all fossil fuels with renewable sources of energy in twenty years.

Mitigation of peak oil

The standard Hubbert curve, plotting crude oil production of a region over time.

 

World energy consumption, 1970–2025. Source: International Energy Outlook 2004.

The mitigation of peak oil is the attempt to delay the date and minimize the social and economic effects of peak oil by reducing the consumption of and reliance on petroleum. By reducing petroleum consumption, mitigation efforts seek to favorably change the shape of the Hubbert curve, which is the graph of real oil production over time predicted by Hubbert peak theory. The peak of this curve is known as peak oil, and by changing the shape of the curve, the timing of the peak in oil production is affected. An analysis by the author of the Hirsch report showed that while the shape of the oil production curve can be affected by mitigation efforts, mitigation efforts are also affected by the shape of Hubbert curve.

For the most part, mitigation involves fuel conservation, and the use of alternative and renewable energy sources. The development of unconventional oil resources can extend the supply of petroleum, but does not reduce consumption.

Historically, world oil consumption data show that mitigation efforts during the 1973 and 1979 oil shocks lowered oil consumption, while general recessions since the 1970s have had no effect on curbing the oil consumption until 2007. In the United States, oil consumption declines in reaction to high prices.

Key questions for mitigation are the viability of methods, the roles of government and private sector and how early these solutions are implemented. The responses to such questions and steps taken towards mitigation may determine whether or not the lifestyle of a society can be maintained, and may affect the population capacity of the planet.

Alternative energy

The most effective method of mitigating peak oil is to use renewable or alternative energy sources in place of petroleum.

Iceland was the first country to suggest transitioning to 100% renewable energy, using hydrogen for vehicles and its fishing fleet, in 1998, but the actual progress has been very limited.

Transportation fuel use

Because most oil is consumed for transportation most mitigation discussions revolve around transportation issues.

Fuel substitution

Main article: Future energy development

While there is some interchangeability, the alternative energy sources available tend to depend on whether the fuel is being used in static or mobile applications.

Biofuel

A sample of biodiesel

Biofuel is a fuel that is produced over a short time span from biomass, rather than by the very slow natural processes involved in the formation of fossil fuels, such as oil. Biofuel can be produced from plants or from agricultural, domestic or industrial biowaste. The climate change mitigation potential of biofuel varies considerably, from emission levels comparable to fossil fuels in some scenarios to negative emissions in others. Biofuels are mostly used for transportation, but can also be used for heating and electricity. Biofuels (and bioenergy in general) are regarded as a renewable energy source.

The two most common types of biofuel are bioethanol and biodiesel. The U.S. is the largest producer of bioethanol, while the EU is the largest producer of biodiesel. The energy content in the global production of bioethanol and biodiesel is 2.2 and 1.8 EJ per year, respectively. Demand for aviation biofuel is forecast to increase.

Bioethanol is an alcohol made by fermentation, mostly from carbohydrates produced in sugar or starch crops such as maize, sugarcane, or sweet sorghum. Cellulosic biomass, derived from non-food sources, such as trees and grasses, is also being developed as a feedstock for ethanol production. Ethanol can be used as a fuel for vehicles in its pure form (E100), but it is usually used as a gasoline additive to increase octane ratings and improve vehicle emissions.

Biodiesel is produced from oils or fats using transesterification. It can be used as a fuel for vehicles in its pure form (B100), but it is usually used as a diesel additive to reduce levels of particulates, carbon monoxide, and hydrocarbons from diesel-powered vehicles.

Static installations

Although oil and diesel still generates a small share of global electricity, some Middle East oil producing countries are replacing that with solar power, as it is more profitable to export the oil.

Mobile applications

See also: Biofuel, Hydrogen economy, Methanol economy, and Hydrogen-powered aircraft

Due to its high energy density and ease of handling, oil has a unique role as a transportation fuel. There are, however, a number of possible alternatives. Among the biofuels the use of bioethanol and biodiesel is already established to some extent in some countries.

The use of hydrogen fuel is another alternative under development in various countries, alongside, hydrogen vehicles though hydrogen is actually an energy storage medium, not a primary energy source, and consequently the use of a non-petroleum source would be required to extract the hydrogen for use. Though hydrogen is currently outperformed in terms of cost and efficiency by battery powered vehicles, there are applications where it would come in useful. Short haul ferries and very cold climates are two examples. Hydrogen fuel cells are about a third as efficient as batteries and double the efficiency of gasoline vehicles.

Electric vehicles powered by batteries are another alternative, and these have the advantage of having the highest well-to-wheels efficiency rate of any energy pathway and thus would allow much greater numbers of vehicles than any other methods. In addition, even if the electricity was sourced from coal-fired power plants, two advantages would remain: first it is cheaper to sequester carbon from a few thousand smokestacks than it is to retrofit hundreds of millions of vehicles, and second encouraging the use of electric vehicles allows a further pathway for scaling up of renewable energy sources.

Alternative aviation fuel

The Airbus A380 flew on alternative fuel for the first time on 1 February 2008. Boeing also plans to use alternative fuel on the 747. Because some biofuels such as ethanol contains less energy, more "tankstops" might be necessary for such planes.

The US Air Force is currently in the process of certifying its entire fleet to run on a 50/50 blend of synthetic fuel derived from the Fischer–Tropsch process and JP-8 jet fuel.

Conservation

When alternative fuels are not available, the development of more energy efficient vehicles becomes an important mitigant. Some ways of decreasing the oil used in transportation include increasing the use of bicycles, public transport, carpooling, electric vehicles, and diesel and hybrid vehicles with higher fuel efficiency.

More comprehensive mitigations include better land use planning through smart growth to reduce the need for private transportation, increased capacity and use of mass transit, vanpooling and carpooling, bus rapid transit, remote work, and human-powered transport from current levels. Rationing and driving bans are also forms of reducing private transportation. The higher oil prices of 2007 and 2008 caused United States drivers to begin driving less in 2007 and to a much greater extent in the first three months of 2008.

In order to deal with potential problems from peak oil, Colin Campbell has proposed the Rimini protocol, a plan which among other things would require countries to balance oil consumption with their current production.

Axial tilt

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Axial_tilt

 

The positive pole of a planet is defined by the right-hand rule: if the fingers of the right hand are curled in the direction of the rotation then the thumb points to the positive pole. The axial tilt is defined as the angle between the direction of the positive pole and the normal to the orbital plane. The angles for Earth, Uranus, and Venus are approximately 23°, 97°, and 177° respectively.

In astronomy, axial tilt, also known as obliquity, is the angle between an object's rotational axis and its orbital axis, which is the line perpendicular to its orbital plane; equivalently, it is the angle between its equatorial plane and orbital plane. It differs from orbital inclination.

At an obliquity of 0 degrees, the two axes point in the same direction; that is, the rotational axis is perpendicular to the orbital plane.

The rotational axis of Earth, for example, is the imaginary line that passes through both the North Pole and South Pole, whereas the Earth's orbital axis is the line perpendicular to the imaginary plane through which the Earth moves as it revolves around the Sun; the Earth's obliquity or axial tilt is the angle between these two lines.

Over the course of an orbital period, the obliquity usually does not change considerably, and the orientation of the axis remains the same relative to the background of stars. This causes one pole to be pointed more toward the Sun on one side of the orbit, and more away from the Sun on the other side—the cause of the seasons on Earth.

Standards

There are two standard methods of specifying a planet's tilt. One way is based on the planet's north pole, defined in relation to the direction of Earth's north pole, and the other way is based on the planet's positive pole, defined by the right-hand rule:

• The International Astronomical Union (IAU) defines the north pole of a planet as that which lies on Earth's north side of the invariable plane of the Solar System; under this system, Venus is tilted 3° and rotates retrograde, opposite that of most of the other planets.
• The IAU also uses the right-hand rule to define a positive pole for the purpose of determining orientation. Using this convention, Venus is tilted 177° ("upside down") and rotates prograde.

Earth

Further information: Ecliptic § Obliquity of the ecliptic

See also: Earth's rotation and Earth-centered inertial

Earth's orbital plane is known as the ecliptic plane, and Earth's tilt is known to astronomers as the obliquity of the ecliptic, being the angle between the ecliptic and the celestial equator on the celestial sphere. It is denoted by the Greek letter ε.

Earth currently has an axial tilt of about 23.44°. This value remains about the same relative to a stationary orbital plane throughout the cycles of axial precession. But the ecliptic (i.e., Earth's orbit) moves due to planetary perturbations, and the obliquity of the ecliptic is not a fixed quantity. At present, it is decreasing at a rate of about 46.8″ per century (see details in Short term below).

History

Earth's obliquity may have been reasonably accurately measured as early as 1100 BCE in India and China. The ancient Greeks had good measurements of the obliquity since about 350 BCE, when Pytheas of Marseilles measured the shadow of a gnomon at the summer solstice. About 830 CE, the Caliph Al-Mamun of Baghdad directed his astronomers to measure the obliquity, and the result was used in the Arab world for many years. In 1437, Ulugh Beg determined the Earth's axial tilt as 23°30′17″ (23.5047°).

During the Middle Ages, it was widely believed that both precession and Earth's obliquity oscillated around a mean value, with a period of 672 years, an idea known as trepidation of the equinoxes. Perhaps the first to realize this was incorrect (during historic time) was Ibn al-Shatir in the fourteenth century and the first to realize that the obliquity is decreasing at a relatively constant rate was Fracastoro in 1538. The first accurate, modern, western observations of the obliquity were probably those of Tycho Brahe from Denmark, about 1584, although observations by several others, including al-Ma'mun, al-Tusi, Purbach, Regiomontanus, and Walther, could have provided similar information.

Seasons

Main article: Season

 

The axis of Earth remains oriented in the same direction with reference to the background stars regardless of where it is in its orbit. Northern hemisphere summer occurs at the right side of this diagram, where the north pole (red) is directed toward the Sun, winter at the left.

Earth's axis remains tilted in the same direction with reference to the background stars throughout a year (regardless of where it is in its orbit) due to the gyroscope effect. This means that one pole (and the associated hemisphere of Earth) will be directed away from the Sun at one side of the orbit, and half an orbit later (half a year later) this pole will be directed towards the Sun. This is the cause of Earth's seasons. Summer occurs in the Northern hemisphere when the north pole is directed toward the Sun. Variations in Earth's axial tilt can influence the seasons and is likely a factor in long-term climatic change (also see Milankovitch cycles).

Relationship between Earth's axial tilt (ε) to the tropical and polar circles

Oscillation

Short term

Obliquity of the ecliptic for 20,000 years, from Laskar (1986). The red point represents the year 2000.

The exact angular value of the obliquity is found by observation of the motions of Earth and planets over many years. Astronomers produce new fundamental ephemerides as the accuracy of observation improves and as the understanding of the dynamics increases, and from these ephemerides various astronomical values, including the obliquity, are derived.

Annual almanacs are published listing the derived values and methods of use. Until 1983, the Astronomical Almanac's angular value of the mean obliquity for any date was calculated based on the work of Newcomb, who analyzed positions of the planets until about 1895:

ε = 23°27′8.26″ − 46.845″ T − 0.0059″ T² + 0.00181″ T³

where ε is the obliquity and T is tropical centuries from B1900.0 to the date in question.

From 1984, the Jet Propulsion Laboratory's DE series of computer-generated ephemerides took over as the fundamental ephemeris of the Astronomical Almanac. Obliquity based on DE200, which analyzed observations from 1911 to 1979, was calculated:

ε = 23°26′21.448″ − 46.8150″ T − 0.00059″ T² + 0.001813″ T³

where hereafter T is Julian centuries from J2000.0.

JPL's fundamental ephemerides have been continually updated. For instance, according to IAU resolution in 2006 in favor of the P03 astronomical model, the Astronomical Almanac for 2010 specifies:

ε = 23°26′21.406″ − 46.836769″ T − 0.0001831″ T² + 0.00200340″ T³ − 5.76″ × 10⁻⁷ T⁴ − 4.34″ × 10⁻⁸ T⁵

These expressions for the obliquity are intended for high precision over a relatively short time span, perhaps ± several centuries. J. Laskar computed an expression to order T¹⁰ good to 0.02″ over 1000 years and several arcseconds over 10,000 years.

ε = 23°26′21.448″ − 4680.93″ t − 1.55″ t² + 1999.25″ t³ − 51.38″ t⁴ − 249.67″ t⁵ − 39.05″ t⁶ + 7.12″ t⁷ + 27.87″ t⁸ + 5.79″ t⁹ + 2.45″ t¹⁰

where here t is multiples of 10,000 Julian years from J2000.0.

These expressions are for the so-called mean obliquity, that is, the obliquity free from short-term variations. Periodic motions of the Moon and of Earth in its orbit cause much smaller (9.2 arcseconds) short-period (about 18.6 years) oscillations of the rotation axis of Earth, known as nutation, which add a periodic component to Earth's obliquity. The true or instantaneous obliquity includes this nutation.

Long term

Main article: Formation and evolution of the Solar System

Main article: Milankovitch cycles

Using numerical methods to simulate Solar System behavior over a period of several million years, long-term changes in Earth's orbit, and hence its obliquity, have been investigated. For the past 5 million years, Earth's obliquity has varied between 22°2′33″ and 24°30′16″, with a mean period of 41,040 years. This cycle is a combination of precession and the largest term in the motion of the ecliptic. For the next 1 million years, the cycle will carry the obliquity between 22°13′44″ and 24°20′50″.

The Moon has a stabilizing effect on Earth's obliquity. Frequency map analysis conducted in 1993 suggested that, in the absence of the Moon, the obliquity could change rapidly due to orbital resonances and chaotic behavior of the Solar System, reaching as high as 90° in as little as a few million years (also see Orbit of the Moon). However, more recent numerical simulations made in 2011 indicated that even in the absence of the Moon, Earth's obliquity might not be quite so unstable; varying only by about 20–25°. To resolve this contradiction, diffusion rate of obliquity has been calculated, and it was found that it takes more than billions of years for Earth's obliquity to reach near 90°. The Moon's stabilizing effect will continue for less than 2 billion years. As the Moon continues to recede from Earth due to tidal acceleration, resonances may occur which will cause large oscillations of the obliquity.

Long-term obliquity of the ecliptic. Top: for the past 5 million years; note that the obliquity varies only from about 22.0° to 24.5°. Bottom: for the next 1 million years; note the approx. 41,000-year period of variation. In both graphs, the red point represents the year 1850. (Source: Berger, 1976).

Solar System bodies

See also: Poles of astronomical bodies § Poles of rotation

All four of the innermost, rocky planets of the Solar System may have had large variations of their obliquity in the past. Since obliquity is the angle between the axis of rotation and the direction perpendicular to the orbital plane, it changes as the orbital plane changes due to the influence of other planets. But the axis of rotation can also move (axial precession), due to torque exerted by the Sun on a planet's equatorial bulge. Like Earth, all of the rocky planets show axial precession. If the precession rate were very fast the obliquity would actually remain fairly constant even as the orbital plane changes. The rate varies due to tidal dissipation and core-mantle interaction, among other things. When a planet's precession rate approaches certain values, orbital resonances may cause large changes in obliquity. The amplitude of the contribution having one of the resonant rates is divided by the difference between the resonant rate and the precession rate, so it becomes large when the two are similar.

Mercury and Venus have most likely been stabilized by the tidal dissipation of the Sun. Earth was stabilized by the Moon, as mentioned above, but before its formation, Earth, too, could have passed through times of instability. Mars's obliquity is quite variable over millions of years and may be in a chaotic state; it varies as much as 0° to 60° over some millions of years, depending on perturbations of the planets. Some authors dispute that Mars's obliquity is chaotic, and show that tidal dissipation and viscous core-mantle coupling are adequate for it to have reached a fully damped state, similar to Mercury and Venus.

The occasional shifts in the axial tilt of Mars have been suggested as an explanation for the appearance and disappearance of rivers and lakes over the course of the existence of Mars. A shift could cause a burst of methane into the atmosphere, causing warming, but then the methane would be destroyed and the climate would become arid again.

The obliquities of the outer planets are considered relatively stable.

Extrasolar planets

The stellar obliquity ψ_s, i.e. the axial tilt of a star with respect to the orbital plane of one of its planets, has been determined for only a few systems. But for 49 stars as of 2012, the sky-projected spin-orbit misalignment λ has been observed, which serves as a lower limit to ψ_s. Most of these measurements rely on the Rossiter–McLaughlin effect. So far, it has not been possible to constrain the obliquity of an extrasolar planet. But the rotational flattening of the planet and the entourage of moons and/or rings, which are traceable with high-precision photometry, e.g. by the space-based Kepler space telescope, could provide access to ψ_p in the near future.

Astrophysicists have applied tidal theories to predict the obliquity of extrasolar planets. It has been shown that the obliquities of exoplanets in the habitable zone around low-mass stars tend to be eroded in less than 10⁹ years, which means that they would not have seasons as Earth has.

Coordination polymer

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Coordination_polymer

 

Figure 1. An illustration of 1- 2- and 3-dimensionality.

A coordination polymer is an inorganic or organometallic polymer structure containing metal cation centers linked by ligands. More formally a coordination polymer is a coordination compound with repeating coordination entities extending in 1, 2, or 3 dimensions.

It can also be described as a polymer whose repeat units are coordination complexes. Coordination polymers contain the subclass coordination networks that are coordination compounds extending, through repeating coordination entities, in 1 dimension, but with cross-links between two or more individual chains, loops, or spiro-links, or a coordination compound extending through repeating coordination entities in 2 or 3 dimensions. A subclass of these are the metal-organic frameworks, or MOFs, that are coordination networks with organic ligands containing potential voids.

Coordination polymers are relevant to many fields, having many potential applications.

Coordination polymers can be classified in a number of ways according to their structure and composition. One important classification is referred to as dimensionality. A structure can be determined to be one-, two- or three-dimensional, depending on the number of directions in space the array extends in. A one-dimensional structure extends in a straight line (along the x axis); a two-dimensional structure extends in a plane (two directions, x and y axes); and a three-dimensional structure extends in all three directions (x, y, and z axes). This is depicted in Figure 1.

History

The work of Alfred Werner and his contemporaries laid the groundwork for the study of coordination polymers. Many time-honored materials are now recognized as coordination polymers. These include the cyanide complexes Prussian blue and Hofmann clathrates.

Synthesis and propagation

Coordination polymers are often prepared by self-assembly, involving crystallization of a metal salt with a ligand. The mechanisms of crystal engineering and molecular self-assembly are relevant.

Figure 2. Shows planar geometries with 3 coordination and 6 coordination.

Intermolecular forces and bonding

Forces that determine metal-ligand complexes include van der Waals forces, pi-pi interactions, hydrogen bonding, and stabilization of pi bonds by polarized bonds in addition to the coordination bond formed between the metal and the ligand. These intermolecular forces tend to be weak, with a long equilibrium distance (bond length) compared to covalent bonds. The pi-pi interactions between benzene rings, for example, have energy roughly 5–10 kJ/mol and optimum spacing 3.4–3.8 Ångstroms between parallel faces of the rings.

Coordination

The crystal structure and dimensionality of the coordination polymer is determined by the functionality of the linker and the coordination geometry of the metal center. Dimensionality is generally driven by the metal center which can have the ability to bond to as many as 16 functional sites on linkers; however this is not always the case as dimensionality can be driven by the linker when the linker bonds to more metal centres than the metal centre does linkers. The highest known coordination number of a coordination polymer is 14, though coordination numbers are most often between 2 and 10. Examples of various coordination numbers are shown in planar geometry in Figure 2. In Figure 1 the 1D structure is 2-coordinated, the planar is 4-coordinated, and the 3D is 6-coordinated.

Metal centers

Figure 3. Three coordination polymers of different dimensionality. All three were made using the same ligand (4,5-dihydroxybenzene-1,3-disulfonate (L)), but different metal cations. All of the metals come from Group 2 on the periodic table (alkaline earth metals) and in this case, dimensionality increases with cation size and polarizability. A. [Ca(L)(H₂O)₄]•H₂O B. [Sr(L)(H₂O)4]•H₂O C.[Ba(L)(H₂O)]•H₂O. In each case, the metal is represented in green.

Metal centers, often called nodes or hubs, bond to a specific number of linkers at well defined angles. The number of linkers bound to a node is known as the coordination number, which, along with the angles they are held at, determines the dimensionality of the structure. The coordination number and coordination geometry of a metal center is determined by the nonuniform distribution of electron density around it, and in general the coordination number increases with cation size. Several models, most notably hybridization model and molecular orbital theory, use the Schrödinger equation to predict and explain coordination geometry, however this is difficult in part because of the complex effect of environment on electron density distribution.

Transition metals

Transition metals are commonly used as nodes. Partially filled d orbitals, either in the atom or ion, can hybridize differently depending on environment. This electronic structure causes some of them to exhibit multiple coordination geometries, particularly copper and gold ions which as neutral atoms have full d-orbitals in their outer shells.

Lanthanides

Lanthanides are large atoms with coordination numbers varying from 7 to 14. Their coordination environment can be difficult to predict, making them challenging to use as nodes. They offer the possibility of incorporating luminescent components.

Alkali metals and alkaline earth metals

Alkali metals and alkaline earth metals exist as stable cations. Alkali metals readily form cations with stable valence shells, giving them different coordination behavior than lanthanides and transition metals. They are strongly affected by the counterion from the salt used in synthesis, which is difficult to avoid. The coordination polymers shown in Figure 3 are all group two metals. In this case, the dimensionality of these structures increases as the radius of the metal increases down the group (from calcium to strontium to barium).

Ligands

In most coordination polymers, a ligand (atom or group of atoms) will formally donate a lone pair of electrons to a metal cation and form a coordination complex via a Lewis acid/ base relationship (Lewis acids and bases). Coordination polymers are formed when a ligand has the ability to form multiple coordination bonds and act as a bridge between multiple metal centers. Ligands that can form one coordination bond are referred to as monodentate, but those which form multiple coordination bonds, which could lead to coordination polymers are called polydentate. Polydentate ligands are particularly important because it is through ligands that connect multiple metal centers together that an infinite array is formed. Polydentate ligands can also form multiple bonds to the same metal (which is called chelation). Monodentate ligands are also referred to as terminal because they do not offer a place for the network to continue. Often, coordination polymers will consist of a combination of poly- and monodentate, bridging, chelating, and terminal ligands.

Chemical composition

Almost any type of atom with a lone pair of electrons can be incorporated into a ligand. Ligands that are commonly found in coordination polymers include polypyridines, phenanthrolines, hydroxyquinolines and polycarboxylates. Oxygen and nitrogen atoms are commonly encountered as binding sites, but other atoms, such as sulfur and phosphorus, have been observed.

Ligands and metal cations tend to follow hard soft acid base theory (HSAB) trends. This means that larger, more polarizable soft metals will coordinate more readily with larger more polarizable soft ligands, and small, non-polarizable, hard metals coordinate to small, non-polarizable, hard ligands.

Structural orientation

1,2-Bis(4-pyridyl)ethane is a flexible ligand, which can exist in either gauche or anti conformations.

Ligands can be flexible or rigid. A rigid ligand is one that has no freedom to rotate around bonds or reorient within a structure. Flexible ligands can bend, rotate around bonds, and reorient themselves. These different conformations create more variety in the structure. There are examples of coordination polymers that include two configurations of the same ligand within one structure, as well as two separate structures where the only difference between them is ligand orientation.

Ligand length

A length of the ligand can be an important factor in determining possibility for formation of a polymeric structure versus non-polymeric (mono- or oligomeric) structures.

Other factors

Counterion

Besides metal and ligand choice, there are many other factors that affect the structure of the coordination polymer. For example, most metal centers are positively charged ions which exist as salts. The counterion in the salt can affect the overall structure. For example, silver salts such as AgNO₃, AgBF₄, AgClO₄, AgPF₆, AgAsF₆ and AgSbF₆ are all crystallized with the same ligand, the structures vary in terms of the coordination environment of the metal, as well as the dimensionality of the entire coordination polymer.

Crystallization environment

Additionally, variations in the crystallization environment can also change the structure. Changes in pH, exposure to light, or changes in temperature can all change the resulting structure. Influences on the structure based on changes in crystallization environment are determined on a case by case basis.

Guest molecules

The addition and removal of guest molecules can have a large effect on the resulting structure of a coordination polymer. A few examples are (top) change of a linear 1D chain to a zigzag pattern, (middle) staggered 2D sheets to stacked, and (bottom) 3D cubes become more widely spaced.

The structure of coordination polymers often incorporates empty space in the form of pores or channels. This empty space is thermodynamically unfavorable. In order to stabilize the structure and prevent collapse, the pores or channels are often occupied by guest molecules. Guest molecules do not form bonds with the surrounding lattice, but sometimes interact via intermolecular forces, such as hydrogen bonding or pi stacking. Most often, the guest molecule will be the solvent that the coordination polymer was crystallized in, but can really be anything (other salts present, atmospheric gases such as oxygen, nitrogen, carbon dioxide, etc.) The presence of the guest molecule can sometimes influence the structure by supporting a pore or channel, where otherwise none would exist.

Applications

Coordination polymers are commercialized as dyes. Particularly useful are derivatives of aminophenol. Metal complex dyes using copper or chromium are commonly used for producing dull colors. Tridentate ligand dyes are useful because they are more stable than their bi- or mono-dentate counterparts.

One of the early commercialized coordination polymers are the Hofmann compounds, which have the formula Ni(CN)₄Ni(NH₃)₂. These materials crystallize with small aromatic guests (benzene, certain xylenes), and this selectivity has been exploited commercially for the separation of these hydrocarbons.

Research trends

Molecular storage

Although not yet practical, porous coordination polymers have potential as molecular sieves in parallel with porous carbon and zeolites. The size and shapes of the pore can be controlled by the linker size and the connecting ligands' length and functional groups. To modify the pore size in order to achieve effective adsorption, nonvolatile guests are intercalated in the porous coordination polymer space to decrease the pore size. Active surface guests can also be used contribute to adsorption. For example, the large-pore MOF-177, 11.8 Å in diameter, can be doped by C₆₀ molecules (6.83 Å in diameter) or polymers with a highly conjugated system in order to increase the surface area for H₂ adsorption.

Flexible porous coordination polymers are potentially attractive for molecular storage, since their pore sizes can be altered by physical changes. An example of this might be seen in a polymer that contains gas molecules in its normal state, but upon compression the polymer collapses and releases the stored molecules. Depending on the structure of the polymer, it is possible that the structure be flexible enough that collapsing the pores is reversible and the polymer can be reused to uptake the gas molecules again. The Metal-organic framework page has a detailed section dealing with H₂ gas storage.

Luminescence

Luminescent coordination polymers typically feature organic chromophoric ligands, which absorb light and then pass the excitation energy to the metal ion. Coordination polymers are potentially the most versatile luminescent species due to their emission properties being coupled with guest exchange. Luminescent supramolecular architectures have recently attracted much interest because of their potential applications in optoelectronic devices or as fluorescent sensors and probes. Coordination polymers are often more stable (thermo- and solvent-resistant) than purely organic species. For ligands that fluoresce without the presence of the metal linker (not due to LMCT), the intense photoluminescence emission of these materials tend to be magnitudes of order higher than that of the free ligand alone. These materials can be used for designing potential candidates for light emitting diode (LED) devices. The dramatic increase in fluorescence is caused by the increase in rigidity and asymmetry of the ligand when coordinated to the metal center.

Electrical conductivity

Structure of coordination polymers that exhibit conductivity, where M = Fe, Ru, OS; L = octaethylporphyrinato or pthalocyaninato; N belongs to pyrazine or bipyridine.

Coordination polymers can have short inorganic and conjugated organic bridges in their structures, which provide pathways for electrical conduction. example of such coordination polymers are conductive metal organic frameworks. Some one-dimensional coordination polymers built as shown in the figure exhibit conductivities in a range of 1x10⁻⁶ to 2x10⁻¹ S/cm. The conductivity is due to the interaction between the metal d-orbital and the pi* level of the bridging ligand. In some cases coordination polymers can have semiconductor behavior. Three-dimensional structures consisting of sheets of silver-containing polymers demonstrate semi-conductivity when the metal centers are aligned, and conduction decreases as the silver atoms go from parallel to perpendicular.

Magnetism

Coordination polymers exhibit many kinds of magnetism. Antiferromagnetism, ferrimagnetism, and ferromagnetism are cooperative phenomena of the magnetic spins within a solid arising from coupling between the spins of the paramagnetic centers. In order to allow efficient magnetic, metal ions should be bridged by small ligands allowing for short metal-metal contacts (such as oxo, cyano, and azido bridges).

Sensor capability

Coordination polymers can also show color changes upon the change of solvent molecules incorporated into the structure. An example of this would be the two Co coordination polymers of the [Re₆S₈(CN)₆]⁴⁻ cluster that contains water ligands that coordinate to the cobalt atoms. This originally orange solution turns either purple or green with the replacement of water with tetrahydrofuran, and blue upon the addition of diethyl ether. The polymer can thus act as a solvent sensor that physically changes color in the presence of certain solvents. The color changes are attributed to the incoming solvent displacing the water ligands on the cobalt atoms, resulting in a change of their geometry from octahedral to tetrahedral.