Entropic gravity

From Wikipedia, the free encyclopedia

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

The theory of entropic gravity abides by Newton's law of universal gravitation on Earth and at interplanetary distances but diverges from this classic nature at interstellar distances.

Entropic gravity, also known as emergent gravity, is a theory in modern physics that describes gravity as an entropic force—a force with macro-scale homogeneity but which is subject to quantum-level disorder—and not a fundamental interaction. The theory, based on string theory, black hole physics, and quantum information theory, describes gravity as an emergent phenomenon that springs from the quantum entanglement of small bits of spacetime information. As such, entropic gravity is said to abide by the second law of thermodynamics under which the entropy of a physical system tends to increase over time.

The theory has been controversial within the physics community but has sparked research and experiments to test its validity.

Significance

At its simplest, the theory holds that when gravity becomes vanishingly weak—levels seen only at interstellar distances—it diverges from its classically understood nature and its strength begins to decay linearly with distance from a mass.

Entropic gravity provides an underlying framework to explain Modified Newtonian Dynamics, or MOND, which holds that at a gravitational acceleration threshold of approximately 1.2×10⁻¹⁰ m/s², gravitational strength begins to vary inversely linearly with distance from a mass rather than the normal inverse-square law of the distance. This is an exceedingly low threshold, measuring only 12 trillionths gravity's strength at Earth's surface; an object dropped from a height of one meter would fall for 36 hours were Earth's gravity this weak. It is also 3,000 times less than the remnant of Earth's gravitational field that exists at the point where Voyager 1 crossed the solar system's heliopause and entered interstellar space.

The theory claims to be consistent with both the macro-level observations of Newtonian gravity as well as Einstein's theory of general relativity and its gravitational distortion of spacetime. Importantly, the theory also explains (without invoking the existence of dark matter and tweaking of its new free parameters) why galactic rotation curves differ from the profile expected with visible matter.

The theory of entropic gravity posits that what has been interpreted as unobserved dark matter is the product of quantum effects that can be regarded as a form of positive dark energy that lifts the vacuum energy of space from its ground state value. A central tenet of the theory is that the positive dark energy leads to a thermal-volume law contribution to entropy that overtakes the area law of anti-de Sitter space precisely at the cosmological horizon.

Thus this theory provides an alternative explanation for what mainstream physics currently attributes to dark matter. Since dark matter is believed to compose the vast majority of the universe's mass, a theory in which it is absent has huge implications for cosmology. In addition to continuing theoretical work in various directions, there are many experiments planned or in progress to actually detect or better determine the properties of dark matter (beyond its gravitational attraction), all of which would be undermined by an alternative explanation for the gravitational effects currently attributed to this elusive entity.

Origin

The thermodynamic description of gravity has a history that goes back at least to research on black hole thermodynamics by Bekenstein and Hawking in the mid-1970s. These studies suggest a deep connection between gravity and thermodynamics, which describes the behavior of heat. In 1995, Jacobson demonstrated that the Einstein field equations describing relativistic gravitation can be derived by combining general thermodynamic considerations with the equivalence principle. Subsequently, other physicists, most notably Thanu Padmanabhan, began to explore links between gravity and entropy.

Erik Verlinde's theory

In 2009, Erik Verlinde proposed a conceptual model that describes gravity as an entropic force. He argues (similar to Jacobson's result) that gravity is a consequence of the "information associated with the positions of material bodies". This model combines the thermodynamic approach to gravity with Gerard 't Hooft's holographic principle. It implies that gravity is not a fundamental interaction, but an emergent phenomenon which arises from the statistical behavior of microscopic degrees of freedom encoded on a holographic screen. The paper drew a variety of responses from the scientific community. Andrew Strominger, a string theorist at Harvard said "Some people have said it can't be right, others that it's right and we already knew it – that it’s right and profound, right and trivial."

In July 2011, Verlinde presented the further development of his ideas in a contribution to the Strings 2011 conference, including an explanation for the origin of dark matter.

Verlinde's article also attracted a large amount of media exposure, and led to immediate follow-up work in cosmology, the dark energy hypothesis, cosmological acceleration, cosmological inflation, and loop quantum gravity. Also, a specific microscopic model has been proposed that indeed leads to entropic gravity emerging at large scales. Entropic gravity can emerge from quantum entanglement of local Rindler horizons.

Derivation of the law of gravitation

The law of gravitation is derived from classical statistical mechanics applied to the holographic principle, that states that the description of a volume of space can be thought of as ${\displaystyle N}$ bits of binary information, encoded on a boundary to that region, a closed surface of area ${\displaystyle A}$. The information is evenly distributed on the surface with each bit requiring an area equal to ${\displaystyle {\ell }_{\text{P}}^2}$, the so-called Planck area, from which ${\displaystyle N}$ can thus be computed: $${\displaystyle N=\frac{A}{{\ell }_{\text{P}}^2}}$$ where ${\displaystyle {\ell }_{\text{P}}}$ is the Planck length. The Planck length is defined as: $${\displaystyle {\ell }_{\text{P}}=\sqrt{\frac{\hslash G}{c^3}}}$$ where ${\displaystyle G}$ is the universal gravitational constant, ${\displaystyle c}$ is the speed of light, and ${\displaystyle \hslash }$ is the reduced Planck constant. When substituted in the equation for ${\displaystyle N}$ we find: $${\displaystyle N=\frac{A{c}^3}{\hslash G}}$$

The statistical equipartition theorem defines the temperature ${\displaystyle T}$ of a system with ${\displaystyle N}$ degrees of freedom in terms of its energy ${\displaystyle E}$ such that: $${\displaystyle E=\frac{1}{2}N{k}_{\text{B}}T}$$ where ${\displaystyle k_{\text{B}}}$ is the Boltzmann constant. This is the equivalent energy for a mass ${\displaystyle M}$ according to: $${\displaystyle E=M{c}^2.}$$

The effective temperature experienced due to a uniform acceleration in a vacuum field according to the Unruh effect is: $${\displaystyle T=\frac{\hslash a}{2\pi c{k}_{\text{B}}},}$$ where ${\displaystyle a}$ is that acceleration, which for a mass ${\displaystyle m}$ would be attributed to a force ${\displaystyle F}$ according to Newton's second law of motion: $${\displaystyle F=ma.}$$

Taking the holographic screen to be a sphere of radius ${\displaystyle r}$, the surface area would be given by: $${\displaystyle A=4\pi r^2.}$$

From algebraic substitution of these into the above relations, one derives Newton's law of universal gravitation: $${\displaystyle F=m\frac{2\pi c{k}_{\text{B}}T}{\hslash }=m\frac{4\pi c}{\hslash }\frac{E}{N}=m\frac{4\pi c^3}{\hslash }\frac{M}{N}=m4\pi \frac{GM}{A}=G\frac{mM}{r^2}.}$$

Note that this derivation assumes that the number of the binary bits of information is equal to the number of the degrees of freedom. $${\displaystyle \frac{A}{{\ell }_{\text{P}}^2}=N=\frac{2E}{k_{\text{B}}T}}$$

Criticism and experimental tests

Entropic gravity, as proposed by Verlinde in his original article, reproduces the Einstein field equations and, in a Newtonian approximation, a ${\displaystyle \frac{1}{r}}$ potential for gravitational forces. Since its results do not differ from Newtonian gravity except in regions of extremely small gravitational fields, testing the theory with earth-based laboratory experiments does not appear feasible. Spacecraft-based experiments performed at Lagrangian points within our solar system would be expensive and challenging.

Even so, entropic gravity in its current form has been severely challenged on formal grounds. Matt Visser has shown that the attempt to model conservative forces in the general Newtonian case (i.e. for arbitrary potentials and an unlimited number of discrete masses) leads to unphysical requirements for the required entropy and involves an unnatural number of temperature baths of differing temperatures. Visser concludes:

There is no reasonable doubt concerning the physical reality of entropic forces, and no reasonable doubt that classical (and semi-classical) general relativity is closely related to thermodynamics [52–55]. Based on the work of Jacobson [1–6], Thanu Padmanabhan [7–12], and others, there are also good reasons to suspect a thermodynamic interpretation of the fully relativistic Einstein equations might be possible. Whether the specific proposals of Verlinde [26] are anywhere near as fundamental is yet to be seen – the rather baroque construction needed to accurately reproduce n-body Newtonian gravity in a Verlinde-like setting certainly gives one pause.

For the derivation of Einstein's equations from an entropic gravity perspective, Tower Wang shows that the inclusion of energy-momentum conservation and cosmological homogeneity and isotropy requirements severely restricts a wide class of potential modifications of entropic gravity, some of which have been used to generalize entropic gravity beyond the singular case of an entropic model of Einstein's equations. Wang asserts that:

As indicated by our results, the modified entropic gravity models of form (2), if not killed, should live in a very narrow room to assure the energy-momentum conservation and to accommodate a homogeneous isotropic universe.

Cosmological observations using available technology can be used to test the theory. On the basis of lensing by the galaxy cluster Abell 1689, Nieuwenhuizen concludes that EG is strongly ruled out unless additional (dark) matter-like eV neutrinos is added. A team from Leiden Observatory statistically observing the lensing effect of gravitational fields at large distances from the centers of more than 33,000 galaxies found that those gravitational fields were consistent with Verlinde's theory. Using conventional gravitational theory, the fields implied by these observations (as well as from measured galaxy rotation curves) could only be ascribed to a particular distribution of dark matter. In June 2017, a study by Princeton University researcher Kris Pardo asserted that Verlinde's theory is inconsistent with the observed rotation velocities of dwarf galaxies. Another theory of entropy based on geometric considerations (Quantitative Geometrical Thermodynamics, QGT) provides an entropic basis for the holographic principle and also offers another explanation for galaxy rotation curves as being due to the entropic influence of the central supermassive blackhole found in the center of a spiral galaxy.

In 2018, Zhi-Wei Wang and Samuel L. Braunstein showed that, while spacetime surfaces near black holes (called stretched horizons) do obey an analog of the first law of thermodynamics, ordinary spacetime surfaces — including holographic screens — generally do not, thus undermining the key thermodynamic assumption of the emergent gravity program.

In his 1964 lecture on the Relation of Mathematics and Physics, Richard Feynman describes a related theory for gravity where the gravitational force is explained due to an entropic force due to unspecified microscopic degrees of freedom. However, he immediately points out that the resulting theory cannot be correct as the fluctuation-dissipation theorem would also lead to friction which would slow down the motion of the planets which contradicts observations.

Entropic gravity and quantum coherence

Another criticism of entropic gravity is that entropic processes should, as critics argue, break quantum coherence. There is no theoretical framework quantitatively describing the strength of such decoherence effects, though. The temperature of the gravitational field in earth gravity well is very small (on the order of 10⁻¹⁹K).

Experiments with ultra-cold neutrons in the gravitational field of Earth are claimed to show that neutrons lie on discrete levels exactly as predicted by the Schrödinger equation considering the gravitation to be a conservative potential field without any decoherent factors. Archil Kobakhidze argues that this result disproves entropic gravity, while Chaichian et al. suggest a potential loophole in the argument in weak gravitational fields such as those affecting Earth-bound experiments.

Nuclear renaissance

From Wikipedia, the free encyclopedia

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

Timeline of commissioned and decommissioned nuclear capacity since 1970

Since about 2001 the term nuclear renaissance has been used to refer to a possible nuclear power industry revival, driven by rising fossil fuel prices and new concerns about meeting greenhouse gas emission limits.

The term emerged in a context brought about by a worldwide slowdown in the rollout of new nuclear projects. The quantity of nuclear electricity generated worldwide had previously had a marked increase in the period from the late 1970s to the mid-1990s. This was brought about by massive nuclear programs in countries such as the US and France (see graph). With spiralling costs and a decline in the public acceptability of nuclear projects brought about in the aftermath of the Chernobyl nuclear accident in 1986, the speed of the rollout dwindled rapidly, leading to growing questions about the future of the industry.

In the 2000s, the principal vehicle of industry growth was thought to be the project then known as the European Pressurised Reactor (EPR), led jointly by the French and German governments. However, initial implementation of the project was poor, with costly delays and overruns being met with in France, Finland and China.

In 2011, the Fukushima nuclear accident renewed fears about nuclear safety worldwide. Several countries, including Germany, announced a complete withdrawal from nuclear electricity generation. By 2012, the World Nuclear Association reported that nuclear electricity generation was at its lowest level since 1999.

In the 2010s, industry growth was led by advances in China, as seen on the graph below. In 2015, for instance, 10 nuclear reactors were connected to the grid, the highest number recorded since 1990. However, expanding Asian nuclear programs were balanced by retirements of aging plants and nuclear reactor phase-outs with 7 reactors permanently decommissioned that same year. By that time, 67 new nuclear reactors were under construction, including four EPR units.

During the same period, the nuclear industry in Europe and the US was beset with industrial difficulties. In 2015, French nuclear giant Areva, the then-world leader in reactor construction, collapsed, forcing a government-sponsored takeover by utility provider EDF. The restructuring caused further delays in EPR rollout. In 2017, the American producer of the AP1000 reactor Westinghouse Electric Company filed for Chapter 11 bankruptcy protection. Together with delays and cost overruns, the bankruptcy caused cancellation of the two AP1000 reactors under construction at the Virgil C. Summer Nuclear Generating Station.

Despite these concerns, growing apprehension over the energy transition has led in recent years to a reappraisal of the role of nuclear energy as a reliable and carbon-free source of electricity. In particular, the 2022 global energy crisis brought renewed interest in nuclear energy due to low carbon emissions, less need for fuel import and a stable power supply compared to wind and solar. Countries began reversing or delaying nuclear phase-outs and greater attention being given to newer technologies such as Small Modular Reactors alongside increased incentives, such as the European Union listing nuclear energy as green energy.

History

By the year 2009, annual generation of nuclear power had been on a slight downward trend since 2007, decreasing 1.8% in 2009 to 2558 TWh with nuclear power meeting 13–14% of the world's electricity demand. A major factor in the decrease has been the prolonged repair of seven large reactors at the Kashiwazaki-Kariwa Nuclear Power Plant in Japan following the Niigata-Chuetsu-Oki earthquake.

In the same year's World Energy Outlook, the International Energy Agency stated that:

A nuclear renaissance is possible but cannot occur overnight. Nuclear projects face significant hurdles, including extended construction periods and related risks, long licensing processes and manpower shortages, plus long‐standing issues related to waste disposal, proliferation and local opposition. The financing of new nuclear power plants, especially in liberalized markets, has always been difficult and the financial crisis seems almost certain to have made it even more so. The huge capital requirements, combined with risks of cost overruns and regulatory uncertainties, make investors and lenders very cautious, even when demand growth is robust.

In March 2011 the nuclear accident at Japan's Fukushima I Nuclear Power Plant and shutdowns at other nuclear facilities raised questions among some commentators over the future of nuclear power generation. Platts reported that "the crisis at Japan's Fukushima nuclear plants has prompted leading energy-consuming countries to review the safety of their existing reactors and cast doubt on the speed and scale of planned expansions around the world".

Following the accident, the government of Germany announced a phaseout of nuclear power in the near future. As a result, German constructor Siemens exited the sector, and supported the German government's planned energy transition to renewable energy technologies, leaving Areva as the sole actor in EPR construction. China, Switzerland, Israel, Malaysia, Thailand, the United Kingdom, Italy and the Philippines reviewed their nuclear power programs. Indonesia and Vietnam still planned to build nuclear power plants. Countries such as Australia, Austria, Denmark, Greece, Ireland, Latvia, Liechtenstein, Luxembourg, Portugal, Israel, Malaysia, New Zealand, and Norway remained opposed to nuclear power.

Following the accident, the International Energy Agency halved its estimate of additional nuclear generating capacity built by 2035.

The World Nuclear Association has reported that “nuclear power generation suffered its biggest ever one-year fall through 2012 as the bulk of the Japanese fleet remained offline for a full calendar year”. Data from the International Atomic Energy Agency showed that nuclear power plants globally produced 2346 TWh of electricity in 2012 – seven per cent less than in 2011. The figures illustrate the effects of a full year of 48 Japanese power reactors producing no power during the year. The permanent closure of eight reactor units in Germany was also a factor. Problems at Crystal River, Fort Calhoun and the two San Onofre units in the USA meant they produced no power for the full year, while in Belgium Doel 3 and Tihange 2 were out of action for six months. Compared to 2010, the nuclear industry produced 11% less electricity in 2012.

As of July 2013, "a total of 437 nuclear reactors were operating in 30 countries, seven fewer than the historical maximum of 444 in 2002. Since 2002, utilities have started up 28 units and disconnected 36 including six units at the Fukushima Daiichi nuclear power plant in Japan. The 2010 world reactor fleet had a total nominal capacity of about 370 gigawatts (or thousand megawatts). Despite seven fewer units operating in 2013 than in 2002, the capacity is still about 7 gigawatts higher". The numbers of new operative reactors, final shutdowns and new initiated constructions according to International Atomic Energy Agency (IAEA) in 2010 are as follows:

Overview

A total of 72 reactors were under construction at the beginning of 2014, the highest number in 25 years. Several of the under construction reactors are carry over from earlier eras; some are partially completed reactors on which work has resumed (e.g., in Argentina); some are small and experimental (e.g., Russian floating reactors); and some have been on the IAEA's “under construction” list for years (e.g., in India and Russia). Reactor projects in Eastern Europe are essentially replacing old Soviet reactors shut down due to safety concerns. Most of the 2010 activity ― 30 reactors ― is taking place in four countries: China, India, Russia and South Korea. Turkey, the United Arab Emirates and Iran are the only countries that are currently building their first power reactors, Iran's construction began decades ago.

Eight German nuclear power reactors (Biblis A and B, Brunsbuettel, Isar 1, Kruemmel, Neckarwestheim 1, Philippsburg 1 and Unterweser) were permanently shutdown on August 6, 2011, following the Japanese Fukushima nuclear disaster.

Various barriers to a nuclear renaissance have been suggested. These include: unfavourable economics compared to other sources of energy, slowness in addressing climate change, industrial bottlenecks and personnel shortages in the nuclear sector, and the contentious issue of what to do with nuclear waste or spent nuclear fuel. There are also concerns about more nuclear accidents, security, and nuclear weapons proliferation.

New reactors under construction in Finland and France, which were meant to lead a nuclear renaissance, have been delayed and are running over-budget. China has 22 new reactors under construction, and there are also a considerable number of new reactors being built in South Korea, India, and Russia. At the same time, at least 100 older and smaller reactors will "most probably be closed over the next 10–15 years". So the expanding nuclear programs in Asia are balanced by retirements of aging plants and nuclear reactor phase-outs.

A study by UBS, reported on April 12, 2011, predicts that around 30 nuclear plants may be closed worldwide, with those located in seismic zones or close to national boundaries being the most likely to shut. The analysts believe that 'even pro-nuclear countries such as France will be forced to close at least two reactors to demonstrate political action and restore the public acceptability of nuclear power', noting that the events at Fukushima 'cast doubt on the idea that even an advanced economy can master nuclear safety'. In September 2011, German engineering giant Siemens announced it will withdraw entirely from the nuclear industry, as a response to the Fukushima nuclear disaster in Japan.

The 2011 World Energy Outlook report by the International Energy Agency stated that having "second thoughts on nuclear would have far-reaching consequences" and that a substantial shift away from nuclear power would boost demand for fossil fuels, putting additional upward pressure on the price of energy, raising additional concerns about energy security, and making it more difficult and expensive to combat climate change. The reports suggests that the consequences would be most severe for nations with limited local energy resources and which have been planning to rely heavily on nuclear power for future energy security, and that it would make it substantially more challenging for developing economies to satisfy their rapidly increasing demand for electricity.

John Rowe, chair of Exelon (the largest nuclear power producer in the US), has said that the nuclear renaissance is "dead". He says that solar, wind and cheap natural gas have significantly reduced the prospects of coal and nuclear power plants around the world. Amory Lovins says that the sharp and steady cost reductions in solar power has been a "stunning market success".

In 2013 the analysts at the investment research firm Morningstar, Inc. concluded that nuclear power was not a viable source of new power in the West. On nuclear renaissance they wrote:

The economies of scale experienced in France during its initial build-out and the related strength of supply chain and labor pool were imagined by the dreamers who have coined the term ‘nuclear renaissance’ for the rest of the world. But outside of China and possibly South Korea this concept seems a fantasy, as should become clearer examining even theoretical projections for new nuclear build today.

Economics

See also: Economics of new nuclear power plants

Nuclear power plants are large construction projects with very high up-front costs. The cost of capital is also steep due to the risk of construction delays and obstructing legal action. The large capital cost of nuclear power has been a key barrier to the construction of new reactors around the world, and the economics have recently worsened, as a result of the global financial crisis. As the OECD's Nuclear Energy Agency points out, "investors tend to favor less capital intensive and more flexible technologies". This has led to a large increase in the use of natural gas for base-load power production, often using more sophisticated combined cycle plants.

Accidents and safety

Newer reactor designs that are intended to provide increased safety have been developed over time. The next nuclear plants to be built will likely be Generation III or III+ designs, and a few are being built in Japan. However, safety risks may be the greatest when nuclear systems are the newest, and operators have less experience with them. Nuclear engineer David Lochbaum explained that almost all serious nuclear accidents occurred with what was at the time the most recent technology. He argues that "the problem with new reactors and accidents is twofold: scenarios arise that are impossible to plan for in simulations; and humans make mistakes".

Controversy

See also: Nuclear power debate and Anti-nuclear movement

A nuclear power controversy has surrounded the deployment and use of nuclear fission reactors to generate electricity from nuclear fuel for civilian purposes. The controversy peaked during the 1970s and 1980s, when it "reached an intensity unprecedented in the history of technology controversies", in some countries.

In 2008 there were reports of a revival of the anti-nuclear movement in Germany and protests in France during 2004 and 2007. Also in 2008 in the United States, there were protests about, and criticism of, several new nuclear reactor proposals and later some objections to license renewals for existing nuclear plants.

Public opinion

Global public support for energy sources, based on a survey by Ipsos (2011) taken 2 months after the Fukushima Disaster.

Main article: Public opinion on nuclear issues

In 2005, the International Atomic Energy Agency presented the results of a series of public opinion surveys in the Global Public Opinion on Nuclear Issues report. Majorities of respondents in 14 of the 18 countries surveyed believe that the risk of terrorist acts involving radioactive materials at nuclear facilities is high, because of insufficient protection. While majorities of citizens generally support the continued use of existing nuclear power reactors, most people do not favour the building of new nuclear plants, and 25% of respondents feel that all nuclear power plants should be closed down. Stressing the climate change benefits of nuclear energy positively influences 10% of people to be more supportive of expanding the role of nuclear power in the world, but there is still a general reluctance to support the building of more nuclear power plants. After the Fukushima Disaster, Civil Society Institute (CSI) found out that 58 percent of the respondents indicated less support of using nuclear power in the United States. Two-thirds of the respondents said they would protest the construction of a nuclear reactor within 50 miles of their homes.

There was little support across the world for building new nuclear reactors, a 2011 poll for the BBC indicates. The global research agency GlobeScan, commissioned by BBC News, polled 23,231 people in 23 countries from July to September 2011, several months after the Fukushima nuclear disaster. In countries with existing nuclear programmes, people are significantly more opposed than they were in 2005, with only the UK and US bucking the trend. Most believe that boosting energy efficiency and renewable energy can meet their needs.

By region and country

Main articles: Nuclear power by country and Nuclear energy policy by country

Africa

As of March 2010, ten African nations had begun exploring plans to build nuclear reactors.

Egypt

Egypt's first nuclear power plant, El Dabaa Nuclear Power Plant, a Russian built VVER is under construction as of 2021.

South Africa

South Africa (which has two nuclear power reactors), however, removed government funding for its planned new PBMRs in 2010.

Nigeria

In 2021 it was announced that the first nuclear power plant of Nigeria, an OPEN100-based pressurized water reactor (PWR) with 100 Megawatts electric power, would be built. The Open100 design is a small modular reactor with open source blueprints making use of decades of experience in PWR construction operation and maintenance across dozens of reactor types around the globe.

Americas

Canada

With several CANDU reactors facing closure selected plants will be completely refurbished between 2016 and 2026, extending their operation beyond 2050.

United States

See also: Nuclear renaissance in the United States

George W. Bush signing the Energy Policy Act of 2005, which was designed to promote US nuclear reactor construction, through incentives and subsidies, including cost-overrun support up to a total of $2 billion for six new nuclear plants.

Between 2007 and 2009, 13 companies applied to the Nuclear Regulatory Commission for construction and operating licenses to build 30 new nuclear power reactors in the United States. However, the case for widespread nuclear plant construction was eroded due to abundant natural gas supplies, slow electricity demand growth in a weak US economy, lack of financing, and uncertainty following the Fukushima nuclear disaster. Many license applications for proposed new reactors were suspended or cancelled. Only a few new reactors will enter service by 2020. These will not be the cheapest energy options available, but they are an attractive investment for utilities because the government mandates that taxpayers pay for construction in advance. In 2013, four aging, uncompetitive, reactors were permanently closed: San Onofre 2 and 3 in California, Crystal River 3 in Florida, and Kewaunee in Wisconsin. Vermont Yankee, in Vernon, is scheduled to close in 2014, following many protests. New York State is seeking to close Indian Point Energy Center, in Buchanan, 30 miles from New York City.

Neither climate change abatement, nor the Obama Administration's endorsement of nuclear power with $18.5 billion in loan guarantees, have been able to propel nuclear power in the US past existing obstacles. The Fukushima nuclear disaster has not helped either.

As of 2014, the U.S. nuclear industry began a new lobbying effort, hiring three former senators — Evan Bayh, a Democrat; Judd Gregg, a Republican; and Spencer Abraham, a Republican — as well as William M. Daley, a former staffer to President Obama. The initiative is called Nuclear Matters, and it has begun a newspaper advertising campaign.

Locations of new US reactors and their scheduled operating dates are:

• Tennessee, Watts Bar unit 2 is in operation since October 2016;
• Georgia, Vogtle Electric unit 3 became operational in 2023, unit 4 is now planned to be operational in 2024.

On 29 March 2017, parent company Toshiba placed Westinghouse Electric Company in Chapter 11 bankruptcy because of US$9 billion of losses from its nuclear reactor construction projects. The projects responsible for this loss are mostly the construction of four AP1000 reactors at Vogtle in Georgia and V. C. Summer in South Carolina. The U.S. government had given $8.3 billion of loan guarantees on the financing of the four nuclear reactors being built in the U.S. The plans at V. C. Summer have been cancelled. Peter A. Bradford, former U.S. Nuclear Regulatory Commission member, commented "They placed a big bet on this hallucination of a nuclear renaissance".

Asia

As of 2008, the greatest growth in nuclear generation was expected to be in China, Japan, South Korea and India.

China

As of early 2013 China had 17 nuclear reactors operating and 32 under construction, with more planned. "China is rapidly becoming self-sufficient in reactor design and construction, as well as other aspects of the fuel cycle." However, according to a government research unit, China must not build "too many nuclear power reactors too quickly", in order to avoid a shortfall of fuel, equipment and qualified plant workers.

India

Despite some opposition after the Fukushima Incident India continues to expand its nuclear power generation with installed nuclear power capacity increasing from 4780MW in 2014 to 6780MW by 2021 and is expected to reach 22480 MW by 2031.

In 2022 India introduced the long-term low-emission development strategy (LT-LEDS) at the COP27 Climate Conference in which in addition to the tripling of nuclear capacity by 2032 India pledged to increase the role of Nuclear power and is currently exploring the use of Small Modular Reactors as well as the use of nuclear power for desalination and hydrogen fuel production.

South Korea

South Korea is exploring nuclear projects with a number of nations.

Australia

Australia is a major producer of uranium, which it exports as uranium oxide to nuclear power generating nations. Australia has a single research reactor at Lucas Heights, but does not generate electricity via nuclear power. As of 2015, the majority of the nation's uranium mines are in South Australia, where a Nuclear Fuel Cycle Royal Commission is investigating the opportunities and costs of expanding the state's role in the nuclear fuel cycle. As of January 2016, new nuclear industrial development (other than the mining of uranium) is prohibited by various acts of federal and state legislation. The Federal government will consider the findings of the South Australian Royal Commission after it releases its findings in 2016.

Europe

Main article: Nuclear power in the European Union

EPR Flamanville 3 project (seen here in 2010) originally expected to be operational in 2012, has been repeatedly delayed to at least 2024 due to "both structural and economic reasons," and the project's total cost has ballooned well above the original estimates. Similarly, the cost of the identical EPR being built at Olkiluoto, Finland has escalated dramatically, and the project is also well behind schedule. The initial low cost forecasts for these megaprojects exhibited "optimism bias".

On 18 October 2010 the British government announced eight locations it considered suitable for future nuclear power stations. This has resulted in public opposition and protests at some of the sites. In March 2012, two of the big six power companies announced they would be pulling out of developing new nuclear power plants. The decision by RWE npower and E.ON follows uncertainty over nuclear energy following the Fukushima nuclear disaster last year. The companies will not proceed with their Horizon project, which was to develop nuclear reactors at Wylfa in North Wales and at Oldbury-on-Severn in Gloucestershire. Their decision follows a similar announcement by Scottish and Southern Electricity last year. Analysts said the decision meant the future of UK nuclear power could now be in doubt.

The 2011 Japanese Fukushima nuclear disaster has led some European energy officials to "think twice about nuclear expansion". Switzerland has abandoned plans to replace its old nuclear reactors and will take the last one offline in 2034. Anti-nuclear opposition intensified in Germany. In the following months the government decided to shut down eight reactors immediately (August 6, 2011) and to have the other nine off the grid by the end of 2022. Renewable energy in Germany is believed to be able to compensate for much of the loss. In September 2011 Siemens, which had been responsible (through its subsidiary Kraftwerk Union) for constructing all 17 of Germany's existing nuclear power plants, announced that it would exit the nuclear sector following the Fukushima disaster and the subsequent changes to German energy policy. Chief executive Peter Loescher has supported the German government's planned energy transition to renewable energy technologies, calling it a "project of the century" and saying Berlin's target of reaching 35% renewable energy sources by 2020 was feasible.

On October 21, 2013, EDF Energy announced that an agreement had been reached regarding new nuclear plants to be built on the site of Hinkley Point C. EDF Group and the UK Government agreed on the key commercial terms of the investment contract. The final investment decision is still conditional on completion of the remaining key steps, including the agreement of the EU Commission.

In February 2014, Amory Lovins commented that:

Britain's plan for a fleet of new nuclear power stations is … unbelievable ... It is economically daft. The guaranteed price [being offered to French state company EDF] is over seven times the unsubsidised price of new wind in the US, four or five times the unsubsidised price of new solar power in the US. Nuclear prices only go up. Renewable energy prices come down. There is absolutely no business case for nuclear. The British policy has nothing to do with economic or any other rational base for decision making.

Belgium

In light of the 2022 Russian invasion of Ukraine the government of Belgium announced its intention to delay the nuclear phaseout originally planned for 2025. The two remaining nuclear power plants - Tihange 3 and Doel 4 - are to continue providing power to reduce dependency on fossil fuels.

Bulgaria

In 2022 the government of Bulgaria announced a deal to build a new nuclear power plant with Greece acting as the guaranteed costumer of much of the electricity.

Czech Republic

In March 2022 the Czech government launched a tender for a new reactor in the 1-1.6 Gigawatt range at the existing Dukovany Nuclear Power Plant site. The EPR, the AP1000 and the APR-1400 are the finalists for the contract estimated to cost some €6 billion

Finland

After numerous delays and cost overruns the first EPR type reactor to start construction (but not the first to be finished) was connected to the grid in March 2022. The reactor with a net capacity of 1600 Megawatts cost €8.5 billion to build but was sold by Areva/Framatome to Teollisuuden Voima (TVO) at a fixed price of €3.5 billion.

France

During the 2022 French presidential election campaign French President Emmanuel Macron announced a program of new nuclear power plants in addition to the under construction Flamanville 3 reactor. The new power plants are planned to supplement and/or replace aging reactors built in the 1980s during the "plan Messmer" and to allow for a fossil fuel phaseout.

Netherlands

In late 2021 the government of Prime Minister Mark Rutte announced plans to extend the life of the sole existing nuclear power plant at Borssele and to build two new nuclear power plants in the coming years in order to maintain energy supply during and after the planned 2030 coal phaseout.

Poland

Main article: Nuclear power in Poland

As of 2020, Poland was having plans to create 1.5 GW of nuclear capacities and eventually reach 9 GW by 2040. In 2022, Poland announced the construction of three AP1000 reactors from the American supplier Westinghouse until 2033 at Choczewo, near the Baltic Sea.

Russia

Main article: Nuclear power in Russia

In April 2010 Russia announced new plans to start building 10 new nuclear reactors in the next year.

Russia is (as of 2022) the only country to have commercial scale fast breeder reactors. However, the blueprints of these reactors (BN-600 & BN-800) were hacked and leaked by Ukrainian aligned forces in 2022.

United Kingdom

In addition to the EPR under construction at Hinkley Point the government of Boris Johnson announced "big new bets" on nuclear power in light of the 2022 Russian invasion of Ukraine. In addition the existing Sizewell B plant is to be given a lifetime extension.

Currently UK plans to increase its nuclear capacity from 8GW in 2020 to around 24GW by 2050.

Middle East

United Arab Emirates

In December 2009 South Korea won a contract for four nuclear power plants to be built in the United Arab Emirates, for operation in 2017 to 2020.

The first commercial nuclear reactor in the country was connected to the grid in 2020 at Barakah nuclear power plant. Unit 2 started operating in 2021 and as of 2021 Unit 3 was expected to become operational in 2022.

Israel

As of November 2015, the Ministry of National Infrastructure, Energy and Water Resources is considering nuclear power in order to reduce greenhouse gas emissions 25% by 2030.

Views and opinions

In June 2009, Mark Cooper from the Vermont Law School said: "The highly touted renaissance of nuclear power is based on fiction, not fact... There are numerous options available to meet the need for electricity in a carbon-constrained environment that are superior to building nuclear reactors".

In September 2009, Luc Oursel, chief executive of Areva Nuclear Plants (the core nuclear reactor manufacturing division of Areva) stated: "We are convinced about the nuclear renaissance". Areva has been hiring up to 1,000 people a month, "to prepare for a surge in orders from around the world". However, in June 2010, Standard & Poor's downgraded Areva's debt rating to BBB+ due to weakened profitability.

In 2010, Trevor Findlay from the Centre for International Governance Innovation stated that "despite some powerful drivers and clear advantages, a revival of nuclear energy faces too many barriers compared to other means of generating electricity for it to capture a growing market share to 2030".

In January 2010, the International Solar Energy Society stated that "... it appears that the pace of nuclear plant retirements will exceed the development of the few new plants now being contemplated, so that nuclear power may soon start on a downward trend. It will remain to be seen if it has any place in an affordable future world energy policy".

In March 2010, Steve Kidd from the World Nuclear Association said: "Proof of whether the mooted nuclear renaissance is merely 'industry hype' as some commentators suggest or reality will come over the next decade". In 2013 Kidd characterised the situation as a nuclear slowdown, requiring the industry to focus on better economics and improving public acceptance.

In August 2010, physicist Michael Dittmar stated that: "Nuclear fission's contribution to total electric energy has decreased from about 18 per cent a decade ago to about 14 per cent in 2008. On a worldwide scale, nuclear energy is thus only a small component of the global energy mix and its share, contrary to widespread belief, is not on the rise".

In March 2011, Alexander Glaser said: "It will take time to grasp the full impact of the unimaginable human tragedy unfolding after the earthquake and tsunami in Japan, but it is already clear that the proposition of a global nuclear renaissance ended on that day".

In 2011, Benjamin K. Sovacool said: "The nuclear waste issue, although often ignored in industry press releases and sponsored reports, is the proverbial elephant in the room stopping a nuclear renaissance".

Luminescent solar concentrator

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Luminescent_solar_concentrator

A luminescent solar concentrator

A luminescent solar concentrator (LSC) is a device for concentrating radiation, solar radiation in particular, to produce electricity. Luminescent solar concentrators operate on the principle of collecting radiation over a large area, converting it by luminescence (specifically by fluorescence) and directing the generated radiation into relatively small photovoltaic solar cells at the edges.

LSC scheme diagram

Design

Initial designs typically comprised parallel thin, flat layers of alternating luminescent and transparent materials, placed to gather incoming radiation on their (broader) faces and emit concentrated radiation around their (narrower) edges. Commonly the device would direct the concentrated radiation onto solar cells to generate electric power.

Other configurations (such as doped or coated optical fibers, or contoured stacks of alternating layers) may better fit particular applications.

Structure and principles of operation

The layers in the stack may be separate parallel plates or alternating strata in a solid structure. In principle, if the effective input area is sufficiently large relative to the effective output area, the output would be of correspondingly higher irradiance than the input, as measured in watts per square metre. The concentration factor is the ratio between output and input irradiance of the whole device.

For example, imagine a square glass sheet (or stack) 200 mm on a side, 5 mm thick. Its input area (e.g. the surface of one single face of the sheet oriented toward the energy source) is 10 times greater than the output area (e.g. the surface of four open sides) - 40000 square mm (200x200) as compared to 4000 square mm (200x5x4). To a first approximation, the concentration factor of such an LSC is proportional to the area of the input surfaces divided by the area of the edges multiplied by the efficiency of diversion of incoming light towards the output area. Suppose that the glass sheet could divert incoming light from the face towards the edges with an efficiency of 50%. The hypothetical sheet of glass in our example would give an output irradiance of light 5 times greater than that of the incident light, producing a concentration factor of 5.

Similarly, a graded refractive index optic fibre 1 square mm in cross section, and 1 metre long, with a luminescent coating might prove useful.

Concentration factor versus efficiency

The concentration factor interacts with the efficiency of the device to determine overall output.

• The concentration factor is the ratio between the incoming and emitted irradiance. If the input irradiance is 1 kW/m2 and the output irradiance is 10 kW/m2, that would provide a concentration factor of 10.
• The efficiency is the ratio between the incoming radiant flux (measured in watts) and the outgoing wattage, or the fraction of the incoming energy that the device can deliver as usable output energy (not the same as light or electricity, some of which might not be usable). In the previous example, half the received wattage is re-emitted, implying efficiency of 50%.

Most devices (such as solar cells) for converting the incoming energy to useful output are relatively small and costly, and they work best at converting directional light at high intensities and a narrow frequency range, whereas input radiation tends to be at diffuse frequencies, of relatively low irradiance and saturation. Concentration of the input energy accordingly is one option for efficiency and economy.

Luminescence

The above description covers a wider class of concentrators (for example simple optical concentrators) than just luminescent solar concentrators. The essential attribute of LSCs is that they incorporate luminescent materials that absorb incoming light with a wide frequency range, and re-emit the energy in the form of light in a narrow frequency range. The narrower the frequency range, (i.e. the higher the saturation) the simpler a photovoltaic cell can be designed to convert it to electricity.

Suitable optical designs trap light emitted by the luminescent material in all directions, redirecting it so that little escapes the photovoltaic converters. Redirection techniques include internal reflection, refractive index gradients and where suitable, diffraction. In principle such LSCs can use light from cloudy skies and similar diffuse sources that are of little use for powering conventional solar cells or for concentration by conventional optical reflectors or refractive devices.

The luminescent component might be a dopant in the material of some or all of the transparent medium, or it might be in the form of luminescent thin films on the surfaces of some of the transparent components.

Theory of luminescent solar concentrators

Various articles have discussed the theory of internal reflection of fluorescent light so as to provide concentrated emission at the edges, both for doped glasses and for organic dyes incorporated into bulk polymers. When transparent plates are doped with fluorescent materials, effective design requires that the dopants should absorb most of the solar spectrum, re-emitting most of the absorbed energy as long-wave luminescence. In turn, the fluorescent components should be transparent to the emitted wavelengths. Meeting those conditions allows the transparent matrix to convey the radiation to the output area. Control of the internal path of the luminescence could rely on repeated internal reflection of the fluorescent light, and refraction in a medium with a graded refractive index.

Theoretically about 75-80 % of the luminescence could be trapped by total internal reflection in a plate with a refractive index roughly equal to that of typical window glass. Somewhat better efficiency could be achieved by using materials with higher refractive indices. Such an arrangement using a device with a high concentration factor should offer impressive economies in the investment in photovoltaic cells to produce a given amount of electricity. Under ideal conditions the calculated overall efficiency of such a system, in the sense of the amount of energy leaving the photovoltaic cell divided by the energy falling on the plate, should be about 20%.

This takes into account:

• the absorption of light by poorly transparent materials in the transparent medium,
• the efficiency of light conversion by the luminescent components,
• the escape of luminescence beyond the critical angle and
• gross efficiency (which is the ratio of the average energy emitted to the average energy absorbed).

Practical prospects and challenges

The relative merits of various functional components and configurations are major concerns, in particular:

• Organic dyes offer wider ranges of frequencies and more flexibility in choice of frequencies emitted and re-absorbed than rare earth compounds and other inorganic luminescent agents.
• Doping organic polymers is generally practical with organic luminescent agents, whereas doping with stable inorganic luminescent agents usually is not practical except in inorganic glasses.
• Luminescent agents configured as bulk doping of a transparent medium have merits that differ from those of thin films deposited on a clear medium.
• Various trapping media present varying combinations of durability, transparency, compatibility with other materials and refractive index. Inorganic glass and organic polymer media comprise the two main classes of interest.
• Photonic systems create band gaps that trap radiation.
• Identifying materials that re-emit more input light as useful luminescence with negligible self-absorption is crucial. Attainment of that ideal depends on tuning the relevant electronic excitation energy levels to differ from the emission levels in the luminescent medium.
• Alternatively the luminescent materials can be configured into thin films that emit light into transparent passive media that can efficiently conduct towards the output.
• The sensitivity of solar cells must match the maximal emission spectrum of the luminescent colorants.
• Increase the probability of transition from the ground state to the excited state of surface plasmons increases efficiency.

Luminescent solar concentrators could be used to integrate solar-harvesting devices into building façades in cities.

Advances

Transparent luminescent solar concentrators

In 2013, researchers at Michigan State University demonstrated the first visibly transparent luminescent solar concentrators. These devices were composed of phosphorescent metal halide nanocluster (or Quantum dot) blends that exhibit massive Stokes shift (or downconversion) and which selectively absorb ultraviolet and emit near-infrared light, allowing for selective harvesting, improved reabsorption efficiency, and non-tinted transparency in the visible spectrum. The following year, these researchers demonstrated near-infrared harvesting visibly transparent luminescent solar concentrators by utilizing luminescent organic salt derivatives. These devices exhibit a clear visible transparency similar to that of glass and a power conversion efficiency close to 0.5%. In this configuration efficiencies of over 10% are possible due to the large fraction of photon flux in the near-infrared spectrum.

Quantum dots

LSCs based on cadmium selenide/zinc sulfide (CdSe/ZnS) and cadmium selenide/cadmium sulfide (CdSe/CdS) quantum dots (QD) with induced large separation between emission and absorption bands (called a large Stokes shift) were announced in 2007 and 2014 respectively.

Light absorption is dominated by an ultra-thick outer shell of CdS, while emission occurs from the inner core of a narrower-gap CdSe. The separation of light-absorption and light-emission functions between the two parts of the nanostructure results in a large spectral shift of emission with respect to absorption, which greatly reduces re-absorption losses. The QDs were incorporated into large slabs (sized in tens of centimeters) of poly(methyl methacrylate) (PMMA). The active particles were about one hundred angstroms across.

Spectroscopic measurements indicated virtually no re-absorption losses on distances of tens of centimeters. Photon harvesting efficiencies were approximately 10%. Despite their high transparency, the fabricated structures showed significant enhancement of solar flux with the concentration factor of more than four.