Energy development

Energy development[1][2][3] is a field of endeavor focused on making available sufficient primary energy sources[4] and secondary energy forms to meet the needs of society.[5][6][7][8][9] These endeavors encompass those which provide for the production of conventional, alternative and renewable sources of energy, and for the recovery and reuse of energy that would otherwise be wasted. Energy conservation[note 2] and efficiency measures[note 3] reduce the impact of energy development, and can have benefits to society with changes in economic cost and with changes in the environmental effects.

Contemporary industrial societies use primary and secondary energy sources for transportation and the production of many manufactured goods. Also, large industrial populations have various generation and delivery services for energy distribution and end-user utilization.[note 4] This energy is used by people who can afford the cost to live under various climatic conditions through the use of heating, ventilation, and/or air conditioning. Level of use of external energy sources differs across societies, along with the convenience, levels of traffic congestion, pollution sources[10] and availability of domestic energy sources.

Thousands of people in society are employed in the energy industry, of which subjectively influence and impact behaviors. The conventional industry comprises the petroleum industry[note 5] the gas industry,[note 6] the electrical power industry[note 7] the coal industry, and the nuclear power industry. New energy industries include the renewable energy industry, comprising alternative and sustainable manufacture, distribution, and sale of alternative fuels. While there is the development of new hydrocarbon sources,[11] including deepwater/horizontal drilling and fracking, are contentiously underway, commitments to mitigate climate change are driving efforts to develop sources of alternative and renewable energy.

Colloquially, and in non-scientific literature, the terms power,[note 8] fuels, and energy can be used as synonyms, but in the field of energy technology they possess different distinct meanings that are associated with them. An energy source is usually in the form of a closed system, the element that provides the energy by conversion from another energy form; However, the energy can be quantitative, the balance sheet is capable of containing open system energy transfers.[note 9] Illustrative of this can be the emanations from the sun, which with its nuclear fusion is the most important energy source for the Earth[note 10] and which provides its energy in the form of radiation.

The natural elements[note 11] of the material world exist in forms that can be converted into usable energy and are resources from which society can obtain energy to produce heat, light, and motion (among the many uses). According to their nature, the power plants can be classified into:

Primary : They are found in nature: wind, water, solar,[note 12] wood, coal, oil, nuclear.
Secondary : Are those obtained from primary energy sources: electricity, gas.
Classified according to the energy reserves of the energy source used and the regeneration capacity with:

renewable: When the energy source used is freely regenerated in a short period and there are practically limitless reserves; An example is the solar energy that is the source of energy from the sun, or the wind[note 13] used as an energy resource. Renewable energies are:
original solar
natural wind (atmospheric flows)
natural geothermal
oceanic tidal
natural waterfall (hydraulic flows)
natural plant: paper, wood
natural animal: wax, grease,[note 14] pack animals and sources of mechanical energy[note 15]
nonrenewable: They are coming from energy limited sources on Earth in quantity and, therefore, are exhaustible. The non-renewable energy sources include, non-exclusively:
fossil source: petroleum, natural gas, coal
original mineral/chemical: uranium, shale gas[note 16]
So, for example, shale gas is secondary non-renewable. Wind is a primary renewable.

The principle stated by Antoine Lavoisier on the conservation of matter applies to energy development:[note 17] "nothing is created." Thus any energy "production" is actually a recovery transformation of the forms of energy whose origin is that of the universe.

For example, a bicycle dynamo turns in part from the kinetic energy (speed energy) of the movement of the cyclist and converting it into electrical energy will transfer in particular to its lights producing light, that is to say light energy, via the heating of the filament of the bulb and therefore heat (thermal energy). But the kinetic energy of the rider is itself biochemical energy (the ATP muscle cells) derived from the chemical energy of sugars synthesized by plants who use light energy from the sun, which runs from the nuclear energy produced by fusion of atoms of hydrogen, the material itself constitute a form of energy, called "mass energy".

Fossil fuels[edit]

The Moss Landing Power Plant burns natural gas to produce electricity in California.

Natural gas drilling rig in Texas.
Main articles: Fossil fuel and Peak oil
Fossil fuel (primary non-renewable fossil) sources burn coal or hydrocarbon fuels, which are the remains of the decomposition of plants and animals. There are three main types of fossil fuels: coal, petroleum, and natural gas. Another fossil fuel, liquefied petroleum gas (LPG), is principally derived from the production of natural gas. Heat from burning fossil fuel is used either directly for space heating and process heating, or converted to mechanical energy for vehicles, industrial processes, or electrical power generation.

Fossil energy is from recovered fossils (like brown coal, hard coal, peat, natural gas and crude oil) and are originated in degradated products of dead plants and animals. These fossil fuels are based on the carbon cycle and thus allow stored (historic solar) energy to be recycled today. In 2005, 81% were of the world's energy needs met from fossil sources.[12] Biomass is also derived from wood and other organic wastes and modern remains. The technical development of fossil fuels in the 18th and 19th Century set the stage for the Industrial Revolution.

Fossil fuels make up the bulk of the world's current primary energy sources. The technology and infrastructure already exist for the use of fossil fuels. Petroleum energy density in terms of volume (cubic space) and mass (weight) ranks currently above that of alternative energy sources (or energy storage devices, like a battery). Fossil fuels are currently economical, and suitable for decentralized energy use.

Dependence on fossil fuels from regions or countries creates energy security risks for dependent countries.[13][14][15][16][17] Oil dependence in particular has led to war,[18] funding of radicals,[19] monopolization,[20] and socio-political instability.[21] Fossil fuels are non-renewable, un-sustainable resources, which will eventually decline in production[22] and become exhausted, with consequences to societies that remain dependent on them. Fossil fuels are actually slowly forming continuously, but are being consumed quicker than are formed.[note 18] Extracting fuels becomes increasingly extreme as society consumes the most accessible fuel deposits. Extraction in fuel mines get intensive and oil rigs drill deeper (going further out to sea).[23] Extraction of fossil fuels results in environmental degradation, such as the strip mining and mountaintop removal of coal.

Fuel efficiency is a form of thermal efficiency, meaning the efficiency of a process that converts chemical potential energy contained in a carrier fuel into kinetic energy or work. The fuel economy is the energy efficiency of a particular vehicle, is given as a ratio of distance travelled per unit of fuel consumed. Weight-specific efficiency (efficiency per unit weight) may be stated for freight, and passenger-specific efficiency (vehicle efficiency per passenger). The inefficient atmospheric combustion (burning) of fossil fuels in vehicles, buildings, and power plants contributes to urban heat islands.[24]

Conventional production of oil has peaked, conservatively, between 2007 to 2010.[note 19] In 2010, it was estimated that an investment in non-renewable resources of $8 trillion would be required to maintain current levels of production for 25 years.[25] In 2010, governments subsidized fossil fuels by an estimated $500 billion a year.[26] Fossil fuels are also a source of greenhouse gas emissions, leading to concerns about global warming if consumption is not reduced.

The combustion of fossil fuels leads to the release of pollution into the atmosphere. The fossil fuels are mainly based on organic carbon compounds. They are according to the IPCC the causes of the global warming.[27] During the combustion with oxygen in the form of heat energy, carbon dioxide released. Depending on the composition and purity of the fossil fuel also results in other chemical compounds such as nitrogen oxides and soot and fine dust alternativey. Greenhouse gas emissions result from fossil fuel-based electricity generation. Typical megawatt coal plant produces billions of kilowatt hours per year.[28][note 20] From this generation, the carbon dioxide, sulfur dioxide, small airborne particles, nitrogen oxides (NOx) (ozone (smog)), carbon monoxide (CO), hydrocarbons, volatile organic compounds (VOC), mercury, arsenic, lead, cadmium, other heavy metals, and uranium traces are produced.[29][30]

Nuclear power, or nuclear energy, is the use of exothermic nuclear processes,[31] to generate useful heat and electricity. The term includes nuclear fission, nuclear decay and nuclear fusion. Presently the nuclear fission of elements in the actinide series of the periodic table produce the vast majority of nuclear energy in the direct service of humankind, with nuclear decay processes, primarily in the form of geothermal energy, and radioisotope thermoelectric generators, in niche uses making up the rest. Nuclear (fission) power stations, excluding the contribution from naval nuclear fission reactors, provided about 5.7% of the world's energy and 13% of the world's electricity in 2012.[32] In 2013, the IAEA report that there are 437 operational nuclear power reactors,[33] in 31 countries,[34] although not every reactor is producing electricity.[35] In addition, there are approximately 140 naval vessels using nuclear propulsion in operation, powered by some 180 reactors.[36][37][38] As of 2013, attaining a net energy gain from sustained nuclear fusion reactions, excluding natural fusion power sources such as the Sun, remains an ongoing area of international physics and engineering research. More than 60 years after the first attempts, commercial fusion power production remains unlikely before 2050.[39]

There is an ongoing debate about nuclear power.[40][41][42] Proponents, such as the World Nuclear Association, the IAEA and Environmentalists for Nuclear Energy contend that nuclear power is a safe, sustainable energy source that reduces carbon emissions.[43] Opponents, such as Greenpeace International and NIRS, contend that nuclear power poses many threats to people and the environment.[44][45][46]

Nuclear power plant accidents include the Chernobyl disaster (1986), Fukushima Daiichi nuclear disaster (2011), and the Three Mile Island accident (1979).[47] There have also been some nuclear submarine accidents.[47][48][49] In terms of lives lost per unit of energy generated, analysis has determined that nuclear power has caused less fatalities per unit of energy generated than the other major sources of energy generation. Energy production from coal, petroleum, natural gas and hydropower has caused a greater number of fatalities per unit of energy generated due to air pollution and energy accident effects.[50][51][52][53][54] However, the economic costs of nuclear power accidents is high, and meltdowns can take decades to clean up. The human costs of evacuations of affected populations and lost livelihoods is also significant.[55][56]

Along with other sustainable energy sources, nuclear power is a low carbon power generation method of producing electricity, with an analysis of the literature on its total life cycle emission intensity finding that it is similar to other renewable sources in a comparison of greenhouse gas(GHG) emissions per unit of energy generated.[57] With this translating into, from the beginning of nuclear power station commercialization in the 1970s, having prevented the emission of approximately 64 gigatonnes of carbon dioxide equivalent(GtCO2-eq) greenhouse gases, gases that would have otherwise resulted from the burning of fossil fuels in thermal power stations.[58]

As of 2012, according to the IAEA, worldwide there were 68 civil nuclear power reactors under construction in 15 countries,[33] approximately 28 of which in the Peoples Republic of China (PRC), with the most recent nuclear power reactor, as of May 2013, to be connected to the electrical grid, occurring on February 17, 2013 in Hongyanhe Nuclear Power Plant in the PRC.[59] In the USA, two new Generation III reactors are under construction at Vogtle. U.S. nuclear industry officials expect five new reactors to enter service by 2020, all at existing plants.[60] In 2013, four aging, uncompetitive, reactors were permanently closed.[61][62]

Japan's 2011 Fukushima Daiichi nuclear accident, which occurred in a reactor design from the 1960s, prompted a rethink of nuclear safety and nuclear energy policy in many countries.[63] Germany decided to close all its reactors by 2022, and Italy has banned nuclear power.[63] Following Fukushima, in 2011 the International Energy Agency halved its estimate of additional nuclear generating capacity to be built by 2035.[64][65]

Fission economics[edit]
Main article: Economics of new nuclear power plants
The economics of new nuclear power plants is a controversial subject, since there are diverging views on this topic, and multi-billion dollar investments ride on the choice of an energy source. Nuclear power plants typically have high capital costs for building the plant, but low direct fuel costs.

In recent years there has been a slowdown of electricity demand growth and financing has become more difficult, which has an impact on large projects such as nuclear reactors, with very large upfront costs and long project cycles which carry a large variety of risks.[66] In Eastern Europe, a number of long-established projects are struggling to find finance, notably Belene in Bulgaria and the additional reactors at Cernavoda in Romania, and some potential backers have pulled out.[66] Where cheap gas is available and its future supply relatively secure, this also poses a major problem for nuclear projects.[66]

Analysis of the economics of nuclear power must take into account who bears the risks of future uncertainties. To date all operating nuclear power plants were developed by state-owned or regulated utility monopolies[67][68] where many of the risks associated with construction costs, operating performance, fuel price, and other factors were borne by consumers rather than suppliers. Many countries have now liberalized the electricity market where these risks, and the risk of cheaper competitors emerging before capital costs are recovered, are borne by plant suppliers and operators rather than consumers, which leads to a significantly different evaluation of the economics of new nuclear power plants.[69]

Two of the four EPRs under construction (in Finland and France) are significantly behind schedule and substantially over cost.[70] Following the 2011 Fukushima Daiichi nuclear disaster, costs are likely to go up for currently operating and new nuclear power plants, due to increased requirements for on-site spent fuel management and elevated design basis threats.[71]

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Hydroelectricity

Hydroelectricity is the term referring to electricity generated by hydropower; the production of electrical power through the use of the gravitational force of falling or flowing water. It is the most widely used form of renewable energy, accounting for 16 percent of global electricity generation – 3,427 terawatt-hours of electricity production in 2010,[1] and is expected to increase about 3.1% each year for the next 25 years.

Hydropower is produced in 150 countries, with the Asia-Pacific region generating 32 percent of global hydropower in 2010. China is the largest hydroelectricity producer, with 721 terawatt-hours of production in 2010, representing around 17 percent of domestic electricity use. There are now four hydroelectricity plants larger than 10 GW: the Three Gorges Dam and Xiluodu Dam in China, Itaipu Dam across the Brazil/Paraguay border, and Guri Dam in Venezuela.[1]

The cost of hydroelectricity is relatively low, making it a competitive source of renewable electricity. The average cost of electricity from a hydro plant larger than 10 megawatts is 3 to 5 U.S. cents per kilowatt-hour.[1] It is also a flexible source of electricity since the amount produced by the plant can be changed up or down very quickly to adapt to changing energy demands. However, damming interrupts the flow of rivers and can harm local ecosystems, and building large dams and reservoirs often involves displacing people and wildlife.[1] Once a hydroelectric complex is constructed, the project produces no direct waste, and has a considerably lower output level of the greenhouse gas carbon dioxide (CO
2) than fossil fuel powered energy plants.[2]

Hydropower has been used since ancient times to grind flour and perform other tasks. In the mid-1770s, French engineer Bernard Forest de Bélidor published Architecture Hydraulique which described vertical- and horizontal-axis hydraulic machines. By the late 19th century, the electrical generator was developed and could now be coupled with hydraulics.[5] The growing demand for the Industrial Revolution would drive development as well.[6] In 1878 the world's first hydroelectric power scheme was developed at Cragside in Northumberland, England by William George Armstrong. It was used to power a single arc lamp in his art gallery.[7] The old Schoelkopf Power Station No. 1 near Niagara Falls in the U.S. side began to produce electricity in 1881. The first Edison hydroelectric power plant, the Vulcan Street Plant, began operating September 30, 1882, in Appleton, Wisconsin, with an output of about 12.5 kilowatts.[8] By 1886 there were 45 hydroelectric power plants in the U.S. and Canada. By 1889 there were 200 in the U.S. alone.[5]

At the beginning of the 20th century, many small hydroelectric power plants were being constructed by commercial companies in mountains near metropolitan areas. Grenoble, France held the International Exhibition of Hydropower and Tourism with over one million visitors. By 1920 as 40% of the power produced in the United States was hydroelectric, the Federal Power Act was enacted into law. The Act created the Federal Power Commission to regulate hydroelectric power plants on federal land and water. As the power plants became larger, their associated dams developed additional purposes to include flood control, irrigation and navigation. Federal funding became necessary for large-scale development and federally owned corporations, such as the Tennessee Valley Authority (1933) and the Bonneville Power Administration (1937) were created.[6] Additionally, the Bureau of Reclamation which had began a series of western U.S. irrigation projects in the early 20th century was now constructing large hydroelectric projects such as the 1928 Hoover Dam.[9] The U.S. Army Corps of Engineers was also involved in hydroelectric development, completing the Bonneville Dam in 1937 and being recognized by the Flood Control Act of 1936 as the premier federal flood control agency.[10]

Hydroelectric power plants continued to become larger throughout the 20th century. Hydropower was referred to as white coal for its power and plenty.[11] Hoover Dam's initial 1,345 MW power plant was the world's largest hydroelectric power plant in 1936; it was eclipsed by the 6809 MW Grand Coulee Dam in 1942.[12] The Itaipu Dam opened in 1984 in South America as the largest, producing 14,000 MW but was surpassed in 2008 by the Three Gorges Dam in China at 22,500 MW. Hydroelectricity would eventually supply some countries, including Norway, Democratic Republic of the Congo, Paraguay and Brazil, with over 85% of their electricity. The United States currently has over 2,000 hydroelectric power plants that supply 6.4% of its total electrical production output, which is 49% of its renewable electricity.[6]
Generating methods
Turbine row at Los Nihuiles Power Station in Mendoza, Argentina
Cross section of a conventional hydroelectric dam.
A typical turbine and generator
Conventional (dams)
See also: List of conventional hydroelectric power stations

Most hydroelectric power comes from the potential energy of dammed water driving a water turbine and generator. The power extracted from the water depends on the volume and on the difference in height between the source and the water's outflow. This height difference is called the head. The amount of potential energy in water is proportional to the head. A large pipe (the "penstock") delivers water to the turbine.[13]
Pumped-storage
Main article: Pumped-storage hydroelectricity
See also: List of pumped-storage hydroelectric power stations

This method produces electricity to supply high peak demands by moving water between reservoirs at different elevations. At times of low electrical demand, excess generation capacity is used to pump water into the higher reservoir. When there is higher demand, water is released back into the lower reservoir through a turbine. Pumped-storage schemes currently provide the most commercially important means of large-scale grid energy storage and improve the daily capacity factor of the generation system. Pumped storage is not an energy source, and appears as a negative number in listings.[14]

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Photosensitivity

Photosensitivity is the amount to which an object reacts upon receiving photons, especially visible light. In medicine, the term is principally used for abnormal reactions of the skin, and two types are distinguished, photoallergy and phototoxicity.[1][2] The photosensitive ganglion cells in the mammalian eye are a separate class of light-detecting cells from the photoreceptor cells that function in vision.
Sensitivity of the skin to a light source can take various forms. People with particular skin types are more sensitive to sunburn. Particular medications make the skin more sensitive to sunlight; these include most of the tetracycline antibiotics, heart drugs amiodarone, and sulfonamides. Particular conditions lead to increased light sensitivity. Patients with systemic lupus erythematosus experience skin symptoms after sunlight exposure; some types of porphyria are aggravated by sunlight. A rare hereditary condition xeroderma pigmentosum (a defect in DNA repair) is thought to increase the risk of UV-light-exposure-related cancer by increasing photosensitivity.
Veterinary medicine
Main article: Photosensitivity in animals

Photosensitivity occurs in multiple species including sheep, bovine, and horses.

Photosensitizations are classified as primary if an ingested plant contains a photosensitive substance, like hypericin in St John's wort poisoning in sheep, or buckwheat plants (green or dried) in horses.[3]

In hepatogenous photosensitization, the photosensitzing substance is phylloerythrin, a normal end-product of chlorophyll metabolism. [4] It accumulates in the body because of liver damage, reacts with UV light on the skin, and leads to free radical formation. These free radicals damage the skin, leading to ulceration, necrosis, and sloughing. Non-pigmented skin is most commonly affected.
Electronics

Certain electronic devices, such as photodiodes and charge-coupled devices, are designed to be sensitive to light. They are constructed to take advantage of the photoelectric effect, the emission of electrons from matter upon the absorption of electromagnetic radiation. When light (one form of electromagnetic radiation) impinges on the active surface of such a device, electrical current flowing through or electrical charge stored in the device will increase or decrease in proportion to the intensity and wavelength of the light, although there is an upper limit to the amount of electrons released vs the increased intensity of the light, this comes out of quantum mechanics. This trait allows the device to perform regulating and sensing functions of many kinds. For example, a photoresistor circuit may sense ambient light to turn on a street lamp at dusk. Digital cameras use an array of photodiodes whose extreme sensitivity to light allows them to convert incoming photons into varying electrical charges with great accuracy. The varying charges are then encoded in a binary file which can be stored and later viewed on a computer screen or other medium.
Interpretation in chemistry

Chemicals that are photosensitive may undergo chemical reactions when exposed to light. These chemicals, such as hydrogen peroxide and many prescription drugs, are stored in tinted or opaque containers until they are needed to prevent photodegradation. Devices that are photosensitive include the human retina and photographic film; their photosensitive materials undergo a chemical reaction when struck by light.

Typical substances that are photosensitive are alkali salts and silver halides.

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Artificial photosynthesis

Artificial photosynthesis is a chemical process that replicates the natural process of photosynthesis, a process that converts sunlight, water, and carbon dioxide into carbohydrates and oxygen. The term is commonly used to refer to any scheme for capturing and storing the energy from sunlight in the chemical bonds of a fuel (a solar fuel). Photocatalytic water splitting converts water into Hydrogen Ions and oxygen, and is a main research area in artificial photosynthesis. Light-driven carbon dioxide reduction is another studied process, replicating natural carbon fixation.

Research developed in this field encompasses design and assembly of devices (and their components) for the direct production of solar fuels, photoelectrochemistry and its application in fuel cells, and engineering of enzymes and photoautotrophic microorganisms for microbial biofuel and biohydrogen production from sunlight. Many, if not most, of the artificial approaches are bio-inspired, i.e., they rely on biomimetics.

Overview

The photosynthetic reaction can be divided into two half-reactions (oxidation and reduction), both of which are essential to producing fuel. In plant photosynthesis, water molecules are photo-oxidized to release oxygen and protons. The second stage of plant photosynthesis (also known as the Calvin-Benson cycle) is a light-independent reaction that converts carbon dioxide into glucose. Researchers of artificial photosynthesis are developing photocatalysts to perform both of these reactions separately. Furthermore, the protons resulting from water splitting can be used for hydrogen production. These catalysts must be able to react quickly and absorb a large percentage of solar photons.[1]

Whereas photovoltaics can provide direct electrical current from sunlight, the inefficiency of fuel production from photovoltaic electricity (indirect process) and the fact sunshine is not constant throughout time sets a limit to its use.[2][3] A way of using natural photosynthesis is via the production of biofuel through biomass, also an indirect process that suffers from low energy conversion efficiency (due to photosynthesis' own low efficiency in converting sunlight to biomass), and clashes with the increasing need of land mass for human food production.[4] Artificial photosynthesis aims then to produce a fuel from sunlight that can be stored and used when sunlight is not available, by using direct processes, that is, to produce a solar fuel. With the development of catalysts able to reproduce the key steps of photosynthesis, water and sunlight would ultimately be the only needed sources for clean energy production. The only by-product would be oxygen, and production of a solar fuel has the potential to be cheaper than gasoline.[5]

One process for the creation of a clean and affordable energy supply is the development of photocatalytic water splitting under solar light. This method of sustainable hydrogen production is a key objective in the development of alternative energy systems of the future.[6] It is also predicted to be one of the more, if not the most, efficient ways of obtaining hydrogen from water.[7] The conversion of solar energy into hydrogen via a water-splitting process assisted by photosemiconductor catalysts is one of the most promising technologies in development.[citation needed] This process has the potential for large quantities of hydrogen to be generated in an ecologically sound method.[citation needed] The conversion of solar energy into a clean fuel (H2) under ambient conditions is one of the greatest challenges facing scientists in the twenty-first century.[8]

Two approaches are generally recognized in the construction of solar fuel cells for hydrogen production:[9]

    A homogeneous system is one where catalysts are not compartmentalized, that is, components are present in the same compartment. This means that hydrogen and oxygen are produced in the same location. This can be a drawback, since they compose an explosive mixture, demanding further gas purification. Also, all components must be active in approximately the same conditions (e.g., pH).
    A heterogeneous system has two separate electrodes, an anode and a cathode, making possible the separation of oxygen and hydrogen production. Furthermore, different components do not necessarily need to work in the same conditions. However, the increased complexity of these systems makes them harder to develop and more expensive.

Another area of research within artificial photosynthesis is the selection and manipulation of photosynthetic microorganisms, namely green microalgae and cyanobacteria, for the production of solar fuels. Many strains are able to produce hydrogen naturally, and scientists are working to improve them.[10] Algae biofuels such as butanol and methanol are produced both at laboratory and commercial scales. This approach has benefited with the development of synthetic biology,[10] which is also being explored by the J. Craig Venter Institute to produce a synthetic organism capable of biofuel production.[11][12]
History

In the late 60s, Akira Fujishima discovered the photocatalytic properties of titanium dioxide, the so-called Honda-Fujishima effect, which could be used for hydrolysis.[13]

The Swedish Consortium for Artificial Photosynthesis, the first of its kind, was established in 1994 as a collaboration between groups of three different universities, Lund, Uppsala and Stockholm, being presently active around Lund and the Ångström Laboratories in Uppsala.[14] The consortium was built with a multidisciplinary approach to focus on learning from natural photosynthesis and applying this knowledge in biomimetic systems.[15] Research in artificial photosynthesis is undergoing a boom at the beginning of the 21st century.[2] In 2000, Commonwealth Scientific and Industrial Research Organisation (CSIRO) researchers publicize their intent to focus on carbon dioxide capture and conversion to hydrocarbons.[16][17] In 2003, the Brookhaven National Laboratory announced the discovery of an important intermediate step in the reduction of CO2 to CO (the simplest possible carbon dioxide reduction reaction), which could lead to better catalyst designing.[18][19]

One of the drawbacks of artificial systems for water-splitting catalysts is their general reliance on scarce, expensive elements, such as ruthenium or rhenium.[2] With the funding of the United States Air Force Office of Scientific Research,[20] in 2008, MIT chemist and head of the Solar Revolution Project Daniel G. Nocera and postdoctoral fellow Matthew Kanan attempted to circumvent this issue by using a catalyst containing the cheaper and more abundant elements cobalt and phosphate.[21][22] The catalyst was able to split water into oxygen and protons using sunlight, and could potentially be coupled to a hydrogen-producing catalyst such as platinum. Furthermore, while the catalyst broke down during catalysis, it could self-repair.[23] This experimental catalyst design was considered a major breakthrough in the field by many researchers.[24][25]

Whereas CO is the prime reduction product of CO2, more complex carbon compounds are usually desired. In 2008, Princeton chemistry professor Andrew B. Bocarsly reported the direct conversion of carbon dioxide and water to methanol using solar energy in a highly efficient photochemical cell.[26]

While Nocera and coworkers had accomplished water splitting to oxygen and protons, a light-driven process to produce hydrogen from protons still needed to be developed. In 2009, the Leibniz Institute for Catalysis reported inexpensive iron carbonyl complexes able to do just this.[27][28] In the same year, researchers at the University of East Anglia also used iron carbonyl compounds to achieve photoelectrochemical hydrogen production with 60% efficiency, this time using a gold electrode covered with layers of indium phosphide to which the iron complexes were linked.[29] Both these processes used a molecular approach, where discrete nanoparticles are responsible for catalysis.

Visible light water splitting with a one piece multijunction cell was first demonstrated and patented by William Ayers at Energy Conversion Devices in 1983.[30] This group demonstrated water photolysis into hydrogen and oxygen, now referred to as an "artificial leaf" or "wireless solar water splitting" with a low cost, thin film amorphous silicon multijunction cell directly immersed in water. Hydrogen evolved on the front amorphous silicon surface decorated with various catalysts while oxygen evolved off the back metal substrate which also eliminated the problem of mixed hydrogen/oxygen gas evolution. A Nafion membrane above the immersed cell provided a path for proton transport. The higher photovoltage available from the multijuction thin film cell with visible light was a major advance over previous photolysis attempts with UV sensitive single junction cells. The group's patent also lists several other semiconductor multijunction compositions in addition to amorphous silicon.

In 2009, F. del Valle and K. Domen showed the impact of the thermal treatment in a closed atmosphere using Cd1-xZnxS photocatalysts. Cd1-xZnxS solid solution reports high activity in hydrogen production from water splitting under sunlight irradiation.[31] A mixed heterogeneous/molecular approach by researchers at the University of California, Santa Cruz, in 2010, using both nitrogen-doped and cadmium selenide quantum dots-sensitized titanium dioxide nanoparticles and nanowires, also yielded photoproduced hydrogen.[32]

Artificial photosynthesis remained an academic field for many years. However, in the beginning of 2009, Mitsubishi Chemical Holdings was reported to be developing its own artificial photosynthesis research by using sunlight, water and carbon dioxide to "create the carbon building blocks from which resins, plastics and fibers can be synthesized."[33] This was confirmed with the establishment of the KAITEKI Institute later that year, with carbon dioxide reduction through artificial photosynthesis as one of the main goals.[34][35]

In 2010, the DOE established, as one of its Energy Innovation Hubs, the Joint Center for Artificial Photosynthesis.[36] The mission of JCAP is to find a cost-effective method to produce fuels using only sunlight, water, and carbon-dioxide as inputs.  JCAP is led by a team from Caltech, led by Professor Nathan Lewis and brings together more than 120 scientists and engineers from Caltech and its lead partner, Lawrence Berkeley National Laboratory. JCAP also draws on the expertise and capabilities of key partners from Stanford University, the University of California at Berkeley, UCSB, UCI, and UCSD, and the Stanford Linear Accelerator.  In addition, JCAP serves as a central hub for other solar fuels research teams across the United States, including 20 DOE Energy Frontier Research Center.  The program has a budget of $122M over five years, subject to Congressional appropriation[37]

Also in 2010, a team led by professor David Wendell at the University of Cincinnati successfully demonstrated photosynthesis in an artificial construct consisting of enzymes suspended in frog foam.[38]

In 2011, Daniel Nocera and his research team announced the creation of the first practical artificial leaf. In a speech at the 241st National Meeting of the American Chemical Society, Nocera described an advanced solar cell the size of a poker card capable of splitting water into oxygen and hydrogen, approximately ten times more efficient than natural photosynthesis.[39] The cell is mostly made of inexpensive materials that are widely available, works under simple conditions, and shows increased stability over previous catalysts: in laboratory studies, the authors demonstrated that an artificial leaf prototype could operate continuously for at least forty-five hours without a drop in activity.[40] In May 2012, Sun Catalytix, the startup based on Nocera's research, stated that it will not be scaling up the prototype as the device offers few savings over other ways to make hydrogen from sunlight.[41]

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Solar architecture

Solar architecture is the integration of solar panel technology with modern building techniques. The use of flexible thin film photovoltaic modules provides fluid integration with steel roofing profiles that enhances the building's design. Orienting a building to the Sun, selecting materials with favorable thermal mass or light dispersing properties, and designing spaces that naturally circulate air also constitute as solar architecture.
Initial development of solar architecture has been limited by the rigidity and weight of standard solar power panels. The continued development of photovoltaic (PV) thin film solar has provided a lightweight yet robust vehicle to harness solar energy to reduce a building's impact on the environment.

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Solar thermal energy

Solar thermal energy (STE) is a form of energy and a technology for harnessing solar energy to generate thermal energy or electrical energy for use in industry, and in the residential and commercial sectors. The first installation of solar thermal energy equipment occurred in the Sahara Desert approximately in 1910 and was a steam engine without a kettle and fire but with a mirror system for sun light collection to heat water for the needed steam pressure. Because of the influence of World War I, liquid fuel was better developed and the Sahara project was abandoned, only to be reused several decades later.

Solar thermal collectors are classified by the United States Energy Information Administration as low-, medium-, or high-temperature collectors. Low-temperature collectors are flat plates generally used to heat swimming pools. Medium-temperature collectors are also usually flat plates but are used for heating water or air for residential and commercial use. High-temperature collectors concentrate sunlight using mirrors or lenses and are generally used for fulfilling heat requirements up to 300 deg C / 20 bar pressure in industries, and for electric power production. However, there is a term that used for both the applications. Concentrated Solar Thermal (CST) for fulfilling heat requirements in industries and Concentrated Solar Power (CSP) when the heat collected is used for power generation. CST and CSP are not replaceable in terms of application. The 377 MW Ivanpah Solar Power Facility is the largest solar power plant in the world, located in the Mojave Desert of California. Other large solar thermal plants include the SEGS installation (354 MW), also in the Mojave, as well as the Solnova Solar Power Station (150 MW), the Andasol solar power station (150 MW), and Extresol Solar Power Station (100 MW), all in Spain.

Systems for utilizing low-temperature solar thermal energy include means for heat collection; usually heat storage, either short-term or interseasonal; and distribution within a structure or a district heating network. In some cases more than one of these functions is inherent to a single feature of the system (e.g. some kinds of solar collectors also store heat). Some systems are passive, others are active (requiring other external energy to function).[1]

Heating is the most obvious application, but solar cooling can be achieved for a building or district cooling network by using a heat-driven absorption or adsorption chiller (heat pump). There is a productive coincidence that the greater the driving heat from insulation, the greater the cooling output. In 1878, Auguste Mouchout pioneered solar cooling by making ice using a solar steam engine attached to a refrigeration device.[2]

In the United States, heating, ventilation, and air conditioning (HVAC) systems account for over 25% (4.75 EJ) of the energy used in commercial buildings and nearly half (10.1 EJ) of the energy used in residential buildings.[3][4] Solar heating, cooling, and ventilation technologies can be used to offset a portion of this energy.

In Europe, since the mid-1990s about 125 large solar-thermal district heating plants have been constructed, each with over 500 m2 (5400 ft2) of solar collectors. The largest are about 10,000 m2, with capacities of 7 MW-thermal and solar heat costs around 4 Eurocents/kWh without subsidies.[5] 40 of them have nominal capacities of 1 MW-thermal or more. The Solar District Heating program (SDH) has participation from 14 European Nations and the European Commission, and is working toward technical and market development, and holds annual conferences.[6]
Low-temperature collectors
Main article: Solar thermal collector

Glazed Solar Collectors are designed primarily for space heating and they recirculate building air through a solar air panel where the air is heated and then directed back into the building. These solar space heating systems require at least two penetrations into the building and only perform when the air in the solar collector is warmer than the building room temperature. Most glazed collectors are used in the residential sector.
Unglazed, "transpired" air collector

Unglazed Solar Collectors are primarily used to pre-heat make-up ventilation air in commercial, industrial and institutional buildings with a high ventilation load. They turn building walls or sections of walls into low cost, high performance, unglazed solar collectors. Also called, "transpired solar panels", they employ a painted perforated metal solar heat absorber that also serves as the exterior wall surface of the building. Heat conducts from the absorber surface to the thermal boundary layer of air 1 mm thick on the outside of the absorber and to air that passes behind the absorber. The boundary layer of air is drawn into a nearby perforation before the heat can escape by convection to the outside air. The heated air is then drawn from behind the absorber plate into the building's ventilation system.

A Trombe wall is a passive solar heating and ventilation system consisting of an air channel sandwiched between a window and a sun-facing thermal mass. During the ventilation cycle, sunlight stores heat in the thermal mass and warms the air channel causing circulation through vents at the top and bottom of the wall. During the heating cycle the Trombe wall radiates stored heat.[7]

Solar roof ponds are unique solar heating and cooling systems developed by Harold Hay in the 1960s. A basic system consists of a roof-mounted water bladder with a movable insulating cover. This system can control heat exchange between interior and exterior environments by covering and uncovering the bladder between night and day. When heating is a concern the bladder is uncovered during the day allowing sunlight to warm the water bladder and store heat for evening use. When cooling is a concern the covered bladder draws heat from the building's interior during the day and is uncovered at night to radiate heat to the cooler atmosphere. The Skytherm house in Atascadero, California uses a prototype roof pond for heating and cooling.[8]

Solar space heating with solar air heat collectors is more popular in the USA and Canada than heating with solar liquid collectors since most buildings already have a ventilation system for heating and cooling. The two main types of solar air panels are glazed and unglazed.

Of the 21,000,000 square feet (2,000,000 m2) of solar thermal collectors produced in the United States in 2007, 16,000,000 square feet (1,500,000 m2) were of the low-temperature variety.[9] Low-temperature collectors are generally installed to heat swimming pools, although they can also be used for space heating. Collectors can use air or water as the medium to transfer the heat to their destination.
Heat storage in low-temperature solar thermal systems
Main article: Seasonal thermal energy storage

Interseasonal storage. Solar heat (or heat from other sources) can be effectively stored between opposing seasons aquifers, underground geological strata, large specially constructed pits, and large tanks that are insulated and covered with earth.

Short-term storage. Thermal mass materials store solar energy during the day and release this energy during cooler periods. Common thermal mass materials include stone, concrete, and water. The proportion and placement of thermal mass should consider several factors such as climate, daylighting, and shading conditions. When properly incorporated, thermal mass can passively maintain comfortable temperatures while reducing energy consumption.

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Photovoltaics

Photovoltaics (PV) is a method of generating electrical power by converting sunlight into direct current electricity using semiconducting materials that exhibit the photovoltaic effect. A photovoltaic system employs solar panels composed of a number of solar cells to supply usable solar power. Power generation from solar PV has long been seen as a clean sustainable[1] energy technology which draws upon the planet’s most plentiful and widely distributed renewable energy source – the sun. The direct conversion of sunlight to electricity occurs without any moving parts or environmental emissions during operation. It is well proven, as photovoltaic systems have now been used for fifty years in specialized applications, and grid-connected PV systems have been in use for over twenty years.[2]

Driven by advances in technology and increases in manufacturing scale and sophistication, the cost of photovoltaics has declined steadily since the first solar cells were manufactured,[2][3] and the levelised cost of electricity (LCOE) from PV is competitive with conventional electricity sources in an expanding list of geographic regions.[4] Net metering and financial incentives, such as preferential feed-in tariffs for solar-generated electricity, have supported solar PV installations in many countries.[5] With current technology, photovoltaics recoup the energy needed to manufacture them in 1.5 (in Southern Europe) to 2.5 years (in Northern Europe).[6]

Solar PV is now, after hydro and wind power, the third most important renewable energy source in terms of globally installed capacity. More than 100 countries use solar PV. Installations may be ground-mounted (and sometimes integrated with farming and grazing) or built into the roof or walls of a building (either building-integrated photovoltaics or simply rooftop).

In 2013, the fast-growing capacity of worldwide installed solar PV increased by 38 percent to 139 gigawatts (GW). This is sufficient to generate at least 160 terawatt hours (TWh) or about 0.85 percent of the electricity demand on the planet. China, followed by Japan and the United States, is now the fastest growing market, while Germany remains the world's largest producer, contributing almost 6 percent to its national electricity demands.[7][8][9]
The term "photovoltaic" comes from the Greek φῶς (phōs) meaning "light", and from "volt", the unit of electro-motive force, the volt, which in turn comes from the last name of the Italian physicist Alessandro Volta, inventor of the battery (electrochemical cell). The term "photo-voltaic" has been in use in English since 1849.[10]
Solar cells
Solar cells produce electricity directly from sunlight
Global solar potential
Main article: Solar cell

Photovoltaics are best known as a method for generating electric power by using solar cells to convert energy from the sun into a flow of electrons. The photovoltaic effect refers to photons of light exciting electrons into a higher state of energy, allowing them to act as charge carriers for an electric current. The photovoltaic effect was first observed by Alexandre-Edmond Becquerel in 1839.[11][12] The term photovoltaic denotes the unbiased operating mode of a photodiode in which current through the device is entirely due to the transduced light energy. Virtually all photovoltaic devices are some type of photodiode.

Solar cells produce direct current electricity from sun light which can be used to power equipment or to recharge a battery. The first practical application of photovoltaics was to power orbiting satellites and other spacecraft, but today the majority of photovoltaic modules are used for grid connected power generation. In this case an inverter is required to convert the DC to AC. There is a smaller market for off-grid power for remote dwellings, boats, recreational vehicles, electric cars, roadside emergency telephones, remote sensing, and cathodic protection of pipelines.

Photovoltaic power generation employs solar panels composed of a number of solar cells containing a photovoltaic material. Materials presently used for photovoltaics include monocrystalline silicon, polycrystalline silicon, amorphous silicon, cadmium telluride, and copper indium gallium selenide/sulfide.[13] Copper solar cables connect modules (module cable), arrays (array cable), and sub-fields. Because of the growing demand for renewable energy sources, the manufacturing of solar cells and photovoltaic arrays has advanced considerably in recent years.[14][15][16]

Solar photovoltaics power generation has long been seen as a clean energy technology which draws upon the planet’s most plentiful and widely distributed renewable energy source – the sun. The technology is “inherently elegant” in that the direct conversion of sunlight to electricity occurs without any moving parts or environmental emissions during operation. It is well proven, as photovoltaic systems have now been used for fifty years in specialised applications, and grid-connected systems have been in use for over twenty years.

Cells require protection from the environment and are usually packaged tightly behind a glass sheet. When more power is required than a single cell can deliver, cells are electrically connected together to form photovoltaic modules, or solar panels. A single module is enough to power an emergency telephone, but for a house or a power plant the modules must be arranged in multiples as arrays.

Photovoltaic power capacity is measured as maximum power output under standardized test conditions (STC) in "Wp" (Watts peak).[17] The actual power output at a particular point in time may be less than or greater than this standardized, or "rated," value, depending on geographical location, time of day, weather conditions, and other factors.[18] Solar photovoltaic array capacity factors are typically under 25%, which is lower than many other industrial sources of electricity.[19]

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Solar thermal collector

A solar thermal collector collects heat by absorbing sunlight. A collector is a device for capturing solar radiation. Solar radiation is energy in the form of electromagnetic radiation from the infrared (long) to the ultraviolet (short) wavelengths. The quantity of solar energy striking the Earth's surface averages about 1,000 watts per square meter under clear skies, depending upon weather conditions, location and orientation.

The term "solar collector" commonly refers to solar hot water panels, but may refer to installations such as solar parabolic troughs and solar towers; or basic installations such as solar air heaters. Solar power plants usually use the more complex collectors to generate electricity by heating a fluid to drive a turbine connected to an electrical generator.[1] Simple collectors are typically used in residential and commercial buildings for space heating.
Heat collectors

Solar collectors are either non-concentrating or concentrating. In the non-concentrating type, the collector area (i.e., the area that intercepts the solar radiation) is the same as the absorber area (i.e., the area absorbing the radiation). In these types the whole solar panel absorbs light. Concentrating collectors have a bigger interceptor than absorber.[2]

Flat-plate and evacuated-tube solar collectors are used to collect heat for space heating, domestic hot water or cooling with an absorption chiller.
Flat plate collectors
Flat plate thermal system for water heating deployed on a flat roof.

Flat-plate collectors, developed by Hottel and Whillier in the 1950s, are the most common type. They consist of (1) a dark flat-plate absorber, (2) a transparent cover that reduces heat losses, (3) a heat-transport fluid (air, antifreeze or water) to remove heat from the absorber, and (4) a heat insulating backing. The absorber consists of a thin absorber sheet (of thermally stable polymers, aluminum, steel or copper, to which a matte black or selective coating is applied) often backed by a grid or coil of fluid tubing placed in an insulated casing with a glass or polycarbonate cover. In water heat panels, fluid is usually circulated through tubing to transfer heat from the absorber to an insulated water tank. This may be achieved directly or through a heat exchanger.

Most air heat fabricators and some water heat manufacturers have a completely flooded absorber consisting of two sheets of metal which the fluid passes between. Because the heat exchange area is greater they may be marginally more efficient than traditional absorbers.[3] Sunlight passes through the glazing and strikes the absorber plate, which heats up, changing solar energy into heat energy. The heat is transferred to liquid passing through pipes attached to the absorber plate. Absorber plates are commonly painted with "selective coatings," which absorb and retain heat better than ordinary black paint. Absorber plates are usually made of metal—typically copper or aluminum—because the metal is a good heat conductor. Copper is more expensive, but is a better conductor and less prone to corrosion than aluminum. (See: Copper in solar water heaters). In locations with average available solar energy, flat plate collectors are sized approximately one-half to one square foot per gallon of one day's hot water use. Absorber piping configurations include:

    harp – traditional design with bottom pipe risers and top collection pipe, used in low pressure thermosyphon and pumped systems;
    serpentine – one continuous S that maximizes temperature but not total energy yield in variable flow systems, used in compact solar domestic hot water only systems (no space heating role);
    flooded absorber consisting of two sheets of metal stamped to produce a circulation zone;
    boundary layer absorber collectors consisting of several layers of transparent and opaque sheets that enable absorption in a boundary layer. Because the energy is absorbed in the boundary layer, heat conversion may be more efficient than for collectors where absorbed heat is conducted through a material before the heat is accumulated in a circulating liquid.[citation needed]

Polymer flat plate collectors are an alternative to metal collectors and are now being produced in Europe. These may be wholly polymer, or they may include metal plates in front of freeze-tolerant water channels made of silicone rubber. Polymers are flexible and therefore freeze-tolerant and can employ plain water instead of antifreeze, so that they may be plumbed directly into existing water tanks instead of needing heat exchangers that lower efficiency. By dispensing with a heat exchanger, temperatures need not be quite so high for the circulation system to be switched on, so such direct circulation panels, whether polymer or otherwise, can be more efficient, particularly at low light levels. Some early selectively coated polymer collectors suffered from overheating when insulated, as stagnation temperatures can exceed the polymer's melting point.[4][5] For example, the melting point of polypropylene is 160 °C (320 °F), while the stagnation temperature of insulated thermal collectors can exceed 180 °C (356 °F) if control strategies are not used. For this reason polypropylene is not often used in glazed selectively coated solar collectors. Increasingly polymers such as high temperate silicones (which melt at over 250 °C (482 °F)) are being used. Some non polypropylene polymer based glazed solar collectors are matte black coated rather than selectively coated to reduce the stagnation temperature to 150 °C (302 °F) or less.

In areas where freezing is a possibility, freeze-tolerance (the capability to freeze repeatedly without cracking) can be achieved by the use of flexible polymers. Silicone rubber pipes have been used for this purpose in UK since 1999. Conventional metal collectors are vulnerable to damage from freezing, so if they are water filled they must be carefully plumbed so they completely drain using gravity before freezing is expected, so that they do not crack. Many metal collectors are installed as part of a sealed heat exchanger system. Rather than having potable water flow directly through the collectors, a mixture of water and antifreeze such as propylene glycol is used. A heat exchange fluid protects against freeze damage down to a locally determined risk temperature that depends on the proportion of propylene glycol in the mixture. The use of glycol lowers the water's heat carrying capacity marginally, while the addition of an extra heat exchanger may lower system performance at low light levels.

A pool or unglazed collector is a simple form of flat-plate collector without a transparent cover. Typically polypropylene or EPDM rubber or silicone rubber is used as an absorber. Used for pool heating it can work quite well when the desired output temperature is near the ambient temperature (that is, when it is warm outside). As the ambient temperature gets cooler, these collectors become less effective. Most flat plate collectors have a life expectancy of over 25 years.

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Solar energy

Solar energy is radiant light and heat from the sun harnessed using a range of ever-evolving technologies such as solar heating, solar photovoltaics, solar thermal electricity, solar architecture and artificial photosynthesis.[1][2]

Solar technologies are broadly characterized as either passive solar or active solar depending on the way they capture, convert and distribute solar energy. Active solar techniques include the use of photovoltaic panels and solar thermal collectors to harness the energy. Passive solar techniques include orienting a building to the Sun, selecting materials with favorable thermal mass or light dispersing properties, and designing spaces that naturally circulate air.

In 2011, the International Energy Agency said that "the development of affordable, inexhaustible and clean solar energy technologies will have huge longer-term benefits. It will increase countries’ energy security through reliance on an indigenous, inexhaustible and mostly import-independent resource, enhance sustainability, reduce pollution, lower the costs of mitigating climate change, and keep fossil fuel prices lower than otherwise. These advantages are global. Hence the additional costs of the incentives for early deployment should be considered learning investments; they must be wisely spent and need to be widely shared".[1]

The Earth receives 174 petawatts (PW) of incoming solar radiation (insolation) at the upper atmosphere.[3] Approximately 30% is reflected back to space while the rest is absorbed by clouds, oceans and land masses. The spectrum of solar light at the Earth's surface is mostly spread across the visible and near-infrared ranges with a small part in the near-ultraviolet.[4]

Earth's land surface, oceans and atmosphere absorb solar radiation, and this raises their temperature. Warm air containing evaporated water from the oceans rises, causing atmospheric circulation or convection. When the air reaches a high altitude, where the temperature is low, water vapor condenses into clouds, which rain onto the Earth's surface, completing the water cycle. The latent heat of water condensation amplifies convection, producing atmospheric phenomena such as wind, cyclones and anti-cyclones.[5] Sunlight absorbed by the oceans and land masses keeps the surface at an average temperature of 14 °C.[6] By photosynthesis green plants convert solar energy into chemical energy, which produces food, wood and the biomass from which fossil fuels are derived.[7]
Yearly Solar fluxes & Human Energy Consumption
Solar     3,850,000 EJ      [8]
Wind     2,250 EJ      [9]
Biomass potential     ~200 EJ      [10]
Primary energy use (2010)     539 EJ      [11]
Electricity (2010)     ~67 EJ      [12]
1 Exajoule (EJ) is 1018 Joules or 278 billion kilowatt-hours (kW·h).

The total solar energy absorbed by Earth's atmosphere, oceans and land masses is approximately 3,850,000 exajoules (EJ) per year.[8] In 2002, this was more energy in one hour than the world used in one year.[13][14] Photosynthesis captures approximately 3,000 EJ per year in biomass.[15] The technical potential available from biomass is from 100–300 EJ/year.[10] The amount of solar energy reaching the surface of the planet is so vast that in one year it is about twice as much as will ever be obtained from all of the Earth's non-renewable resources of coal, oil, natural gas, and mined uranium combined,[16]

Solar energy can be harnessed at different levels around the world, mostly depending on distance from the equator.[17]
Early commercial adaption
A 1917 patent drawing for Shuman's parabolic trough solar energy system

In 1897, Frank Shuman, a U.S. inventor, engineer and solar energy pioneer built a small demonstration solar engine that worked by reflecting solar energy onto square boxes filled with ether, which has a lower boiling point than water, and were fitted internally with black pipes which in turn powered a steam engine. In 1908 Shuman formed the Sun Power Company with the intent of building larger solar power plants. He, along with his technical advisor A.S.E. Ackermann and British physicist Sir Charles Vernon Boys,[18] developed an improved system using mirrors to reflect solar energy upon collector boxes, increasing heating capacity to the extent that water could now be used instead of ether. Shuman then constructed a full-scale steam engine powered by low-pressure water, enabling him to patent the entire solar engine system by 1912.

Shuman built the world’s first solar thermal power station in Maadi, Egypt between 1912 and 1913. Shuman’s plant used parabolic troughs to power a 45-52 kilowatt (60-70 H.P.) engine that pumped more than 22,000 litres of water per minute from the Nile River to adjacent cotton fields. Although the outbreak of World War I and the discovery of cheap oil in the 1930s discouraged the advancement of solar energy, Shuman’s vision and basic design were resurrected in the 1970s with a new wave of interest in solar thermal energy.[19] In 1916 Shuman was quoted in the media advocating solar energy's utilization, saying:

    We have proved the commercial profit of sun power in the tropics and have more particularly proved that after our stores of oil and coal are exhausted the human race can receive unlimited power from the rays of the sun.
    —Frank Shuman, New York Times, July 2, 1916[20]
Applications of solar technology
Average insolation showing land area (small black dots) required to replace the world primary energy supply with solar electricity (18 TW is 568 Exajoule, EJ, per year). Insolation for most people is from 150 to 300 W/m2 or 3.5 to 7.0 kWh/m2/day.

Solar energy refers primarily to the use of solar radiation for practical ends. However, all renewable energies, other than geothermal and tidal, derive their energy from the sun.

Solar technologies are broadly characterized as either passive or active depending on the way they capture, convert and distribute sunlight. Active solar techniques use photovoltaic panels, pumps, and fans to convert sunlight into useful outputs. Passive solar techniques include selecting materials with favorable thermal properties, designing spaces that naturally circulate air, and referencing the position of a building to the Sun. Active solar technologies increase the supply of energy and are considered supply side technologies, while passive solar technologies reduce the need for alternate resources and are generally considered demand side technologies.[21]

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Energy-plus-house

An energy-plus house (also called: Plus-Energy House, Efficiency-Plus House) produces more energy from renewable energy sources, over the course of a year, than it imports from external sources. This is achieved using a combination of microgeneration technology and low-energy building techniques, such as: passive solar building design, insulation and careful site selection and placement. A reduction of modern conveniences can also contribute to energy savings, however many energy-plus houses are almost indistinguishable from a traditional home, preferring instead to use highly energy-efficient appliances, fixtures, etc., throughout the house.

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Zero-energy building

A zero-energy building, also known as a zero net energy (ZNE) building, net-zero energy building (NZEB), or net zero building, is a building with zero net energy consumption, meaning the total amount of energy used by the building on an annual basis is roughly equal to the amount of renewable energy created on the site. These buildings still produce greenhouse gases because on cloudy (or non-windy) days, at night when the sun isn't shining, and on short winter days, conventional grid power is still the main energy source. Because of this, most zero net energy buildings still get half or more of their energy from the grid. Buildings that produce a surplus of energy over the year may be called "energy-plus buildings" and buildings that consume slightly more energy than they produce are called "near-zero energy buildings" or "ultra-low energy houses".

Traditional buildings consume 40% of the total fossil fuel energy in the US and European Union and are significant contributors of greenhouse gases.[1][2] The zero net energy consumption principle is viewed as a means to reduce carbon emissions and reduce dependence on fossil fuels and although zero-energy buildings remain uncommon even in developed countries, they are gaining importance and popularity.

Most zero-energy buildings use the electrical grid for energy storage but some are independent of grid. Energy is usually harvested on-site through a combination of energy producing technologies like solar and wind, while reducing the overall use of energy with highly efficient HVAC and lighting technologies. The zero-energy goal is becoming more practical as the costs of alternative energy technologies decrease and the costs of traditional fossil fuels increase.

The development of modern zero-energy buildings became possible not only through the progress made in new energy and construction technologies and techniques, but it has also been significantly improved by academic research, which collects precise energy performance data on traditional and experimental buildings and provides performance parameters for advanced computer models to predict the efficacy of engineering designs. Zero Energy Building is considered as a part of smart grid. Some advantages of these buildings are as follow:

    Integration of renewable energy resources
    Integration of plug-in electric vehicles
    Implementation of zero-energy concepts

The zero-energy concept allows for a wide range of approaches due to the many options for producing and conserving energy combined with the many ways of measuring energy (relating to cost, energy, or carbon emissions).

Despite sharing the name "zero net energy", there are several definitions of what the term means in practice, with a particular difference in usage between North America and Europe.[3]

Zero net site energy use
    In this type of ZNE, the amount of energy provided by on-site renewable energy sources is equal to the amount of energy used by the building. In the United States, “zero net energy building” generally refers to this type of building.

Zero net source energy use
    This ZNE generates the same amount of energy as is used, including the energy used to transport the energy to the building. This type accounts for losses during electricity transmission. These ZNEs must generate more electricity than zero net site energy buildings.

Net zero energy emissions
    Outside the United States and Canada, a ZEB is generally defined as one with zero net energy emissions, also known as a zero carbon building or zero emissions building. Under this definition the carbon emissions generated from on-site or off-site fossil fuel use are balanced by the amount of on-site renewable energy production. Other definitions include not only the carbon emissions generated by the building in use, but also those generated in the construction of the building and the embodied energy of the structure. Others debate whether the carbon emissions of commuting to and from the building should also be included in the calculation.

Net zero cost
    In this type of building, the cost of purchasing energy is balanced by income from sales of electricity to the grid of electricity generated on-site. Such a status depends on how a utility credits net electricity generation and the utility rate structure the building uses.

Net off-site zero energy use
    A building may be considered a ZEB if 100% of the energy it purchases comes from renewable energy sources, even if the energy is generated off the site.

Off-the-grid
    Off-the-grid buildings are stand-alone ZEBs that are not connected to an off-site energy utility facility. They require distributed renewable energy generation and energy storage capability (for when the sun is not shining, wind is not blowing, etc.). An energy autarkic house is a building concept where the balance of the own energy consumption and production can be made on an hourly or even smaller basis. Energy autarkic houses can be taken off-the-grid.

Net zero-energy building
    Based on scientific analysis within the joint research program “Towards Net Zero Energy Solar Buildings” [4] a methodological framework was set up which allows different definitions, in accordance with country’s political targets, specific (climate) conditions and respectively formulated requirements for indoor conditions: The overall conceptual understanding of a Net ZEB is an energy efficient, grid connected building enabled to generate energy from renewable sources to

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Building insulation

Building insulation refers broadly to any object in a building used as insulation for any purpose. While the majority of insulation in buildings is for thermal purposes, the term also applies to acoustic insulation, fire insulation, and impact insulation (e.g. for vibrations caused by industrial applications). Often an insulation material will be chosen for its ability to perform several of these functions at once.
Thermal insulation

Thermal insulation in buildings is an important factor to achieving thermal comfort for its occupants. Insulation reduces unwanted heat loss or gain and can decrease the energy demands of heating and cooling systems. It does not necessarily deal with issues of adequate ventilation and may or may not affect the level of sound insulation. In a narrow sense insulation can just refer to the insulation materials employed to slow heat loss, such as: cellulose, glass wool, rock wool, polystyrene, urethane foam, vermiculite, perlite, wood fibre, plant fibre (cannabis, flax, cotton, cork, etc.), recycled cotton denim, plant straw, animal fibre (sheep's wool), cement, and earth or soil, Reflective Insulation (also known as Radiant Barrier) but it can also involve a range of designs and techniques to address the main modes of heat transfer - conduction, radiation and convection materials.[1][2]

The effectiveness of Reflective Insulation (Radiant Barrier) is commonly evaluated by the Reflectivity (Emittance) of the surface with airspace facing to the heat source.

The effectiveness of bulk insulation is commonly evaluated by its R-value, of which there are two - metric (SI) and US customary, the former being 0.176 times the latter. For attics, it is recommended that it should be at least R-38 (US customary, R-6.7 metric).[3] However, an R-value does not take into account the quality of construction or local environmental factors for each building. Construction quality issues include inadequate vapor barriers, and problems with draft-proofing. In addition, the properties and density of the insulation material itself is critical.
Planning

How much insulation a house should have depends on building design, climate, energy costs, budget, and personal preference. Regional climates make for different requirements. Building codes specify only the bare minimum; insulating beyond what the code requires is often recommended.

The insulation strategy of a building needs to be based on a careful consideration of the mode of energy transfer and the direction and intensity in which it moves. This may alter throughout the day and from season to season. It is important to choose an appropriate design, the correct combination of materials and building techniques to suit the particular situation.

To determine whether you should add insulation, you first need to find out how much insulation you already have in your home and where. A qualified home energy auditor will include an insulation check as a routine part of a whole-house energy audit.[4]
In the USA

An initial estimate of insulation needs in the United States can be determined by the US Department of Energy's ZIP code insulation calculator.
Russia

In Russia the luxury of cheap gas has led to poorly insulated, overheated and inefficient consumers of energy. The Russian Center for Energy Efficiency found that Russian buildings are either over- or under-heated, often consume up to 50 percent more heat and hot water than needed. Fifty-three percent of all carbon dioxide (CO2) emissions in Russia are produced through heating and generating electricity for buildings.[5] However, Green House Gas (GHG) emissions from the Soviet Bloc are still below their 1990 levels).[citation needed]

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Efficient energy use

Efficient energy use, sometimes simply called energy efficiency, is the goal to reduce the amount of energy required to provide products and services. For example, insulating a home allows a building to use less heating and cooling energy to achieve and maintain a comfortable temperature. Installing fluorescent lights or natural skylights reduces the amount of energy required to attain the same level of illumination compared with using traditional incandescent light bulbs. Compact fluorescent lights use one-third the energy of incandescent lights and may last 6 to 10 times longer. Improvements in energy efficiency are generally achieved by adopting a more efficient technology or production processes [2] or by application of commonly accepted methods to reduce energy losses.

There are many motivations to improve energy efficiency. Reducing energy use reduces energy costs and may result in a financial cost saving to consumers if the energy savings offset any additional costs of implementing an energy efficient technology. Reducing energy use is also seen as a solution to the problem of reducing carbon dioxide emissions. According to the International Energy Agency, improved energy efficiency in buildings, industrial processes and transportation could reduce the world's energy needs in 2050 by one third, and help control global emissions of greenhouse gases.[3]

Energy efficiency and renewable energy are said to be the twin pillars of sustainable energy policy[4] and are high priorities in the sustainable energy hierarchy. In many countries energy efficiency is also seen to have a national security benefit because it can be used to reduce the level of energy imports from foreign countries and may slow down the rate at which domestic energy resources are depleted.
Energy efficiency has proved to be a cost-effective strategy for building economies without necessarily increasing energy consumption. For example, the state of California began implementing energy-efficiency measures in the mid-1970s, including building code and appliance standards with strict efficiency requirements. During the following years, California's energy consumption has remained approximately flat on a per capita basis while national U.S. consumption doubled.[5] As part of its strategy, California implemented a "loading order" for new energy resources that puts energy efficiency first, renewable electricity supplies second, and new fossil-fired power plants last.[6]

Lovins' Rocky Mountain Institute points out that in industrial settings, "there are abundant opportunities to save 70% to 90% of the energy and cost for lighting, fan, and pump systems; 50% for electric motors; and 60% in areas such as heating, cooling, office equipment, and appliances."[citation needed] In general, up to 75% of the electricity used in the U.S. today could be saved with efficiency measures that cost less than the electricity itself. The same holds true for home-owners, leaky ducts have remained an invisible energy culprit for years. In fact, researchers at the US Department of Energy and their consortium, Residential Energy Efficient Distribution Systems (REEDS) have found that duct efficiency may be as low as 50-70%. The US Department of Energy has stated that there is potential for energy saving in the magnitude of 90 Billion kWh by increasing home energy efficiency.[7]

Other studies have emphasized this. A report published in 2006 by the McKinsey Global Institute, asserted that "there are sufficient economically viable opportunities for energy-productivity improvements that could keep global energy-demand growth at less than 1 percent per annum"—less than half of the 2.2 percent average growth anticipated through 2020 in a business-as-usual scenario.[8] Energy productivity, which measures the output and quality of goods and services per unit of energy input, can come from either reducing the amount of energy required to produce something, or from increasing the quantity or quality of goods and services from the same amount of energy.

The Vienna Climate Change Talks 2007 Report, under the auspices of the United Nations Framework Convention on Climate Change (UNFCCC), clearly shows "that energy efficiency can achieve real emission reductions at low cost."[9]

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Light is radiant energy

Light is radiant energy, usually referring to electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight.[1] Visible light is usually defined as having a wavelength in the range of 400 nanometres (nm), or 400×10−9 m, to 700 nanometres – between the infrared, with longer wavelengths and the ultraviolet, with shorter wavelengths.[2][3] These numbers do not represent the absolute limits of human vision, but the approximate range within which most people can see reasonably well under most circumstances. Various sources define visible light as narrowly as 420 to 680[4][5] to as broadly as 380 to 800 nm.[6][7] Under ideal laboratory conditions, people can see infrared up to at least 1050 nm,[8] children and young adults ultraviolet down to about 310 to 313 nm.[9][10][11] Primary properties of visible light are intensity, propagation direction, frequency or wavelength spectrum, and polarisation, while its speed in a vacuum, 299,792,458 meters per second, is one of the fundamental constants of nature. Visible light, as with all types of electromagnetic radiation (EMR), is experimentally found to always move at this speed in vacuum.[citation needed] In common with all types of EMR, visible light is emitted and absorbed in tiny "packets" called photons, and exhibits properties of both waves and particles. This property is referred to as the wave–particle duality. The study of light, known as optics, is an important research area in modern physics. In physics, the term light sometimes refers to electromagnetic radiation of any wavelength, whether visible or not.[12][13] This article focuses on visible light. See the electromagnetic radiation article for the general term. The speed of light in a vacuum is defined to be exactly 299,792,458 m/s (approximately 186,282 miles per second). The fixed value of the speed of light in SI units results from the fact that the metre is now defined in terms of the speed of light. All forms of electromagnetic radiation move at exactly this same speed in vacuum. Different physicists have attempted to measure the speed of light throughout history. Galileo attempted to measure the speed of light in the seventeenth century. An early experiment to measure the speed of light was conducted by Ole Rømer, a Danish physicist, in 1676. Using a telescope, Rømer observed the motions of Jupiter and one of its moons, Io. Noting discrepancies in the apparent period of Io's orbit, he calculated that light takes about 22 minutes to traverse the diameter of Earth's orbit.[14] However, its size was not known at that time. If Rømer had known the diameter of the Earth's orbit, he would have calculated a speed of 227,000,000 m/s. Another, more accurate, measurement of the speed of light was performed in Europe by Hippolyte Fizeau in 1849. Fizeau directed a beam of light at a mirror several kilometers away. A rotating cog wheel was placed in the path of the light beam as it traveled from the source, to the mirror and then returned to its origin. Fizeau found that at a certain rate of rotation, the beam would pass through one gap in the wheel on the way out and the next gap on the way back. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, Fizeau was able to calculate the speed of light as 313,000,000 m/s. Léon Foucault used an experiment which used rotating mirrors to obtain a value of 298,000,000 m/s in 1862. Albert A. Michelson conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's methods in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. The precise measurements yielded a speed of 299,796,000 m/s.[15] The effective velocity of light in various transparent substances containing ordinary matter, is less than in vacuum. For example the speed of light in water is about 3/4 of that in vacuum. As an extreme example of the nature of light-slowing in matter, two independent teams of physicists were able to bring light to a "complete standstill" by passing it through a Bose-Einstein Condensate of the element rubidium, one team at Harvard University and the Rowland Institute for Science in Cambridge, Mass., and the other at the Harvard-Smithsonian Center for Astrophysics, also in Cambridge.[16] However, the popular description of light being "stopped" in these experiments refers only to light being stored in the excited states of atoms, then re-emitted at an arbitrary later time, as stimulated by a second laser pulse. During the time it had "stopped" it had ceased to be light.

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Nuclear power in India

From Wikipedia, the free encyclopedia
Nuclear power in India is located in India
BARC
BARC
Kaiga
Kaiga
Kakrapar
Kakrapar
Madras
Madras
Koodankulam
Koodankulam
Narora
Narora
Rajasthan
Rajasthan
Tarapur
Tarapur
Jaitapur
Jaitapur
Atomic Power Stations in India (view)
Location dot red.svg Active plants
Location dot blue.svg Planned plants

Nuclear power is the fourth-largest source of electricity in India after thermal, hydroelectric and renewable sources of electricity.[1] As of 2013, India has 21 nuclear reactors in operation in 7 nuclear power plants, having an installed capacity of 5308 MW[2][3] and producing a total of 30,292.91  GWh of electricity[4] while seven other reactors are under construction and are expected to generate an additional 6,100 MW.

In October 2010, India drew up "an ambitious plan to reach a nuclear power capacity of 63,000 MW in 2032",[5] but, after the 2011 Fukushima nuclear disaster in Japan, "populations around proposed Indian NPP sites have launched protests, raising questions about atomic energy as a clean and safe alternative to fossil fuels".[6] There have been mass protests against the French-backed 9900 MW Jaitapur Nuclear Power Project in Maharashtra and the Russian-backed 2000 MW Kudankulam Nuclear Power Plant in Tamil Nadu. The state government of West Bengal state has also refused permission to a proposed 6000 MW facility near the town of Haripur that intended to host six Russian reactors.[6] A Public Interest Litigation (PIL) has also been filed against the government’s civil nuclear programme at the Supreme Court.[6][7]

Despite this opposition, the capacity factor of Indian reactors was at 79% in the year 2011-12 compared to 71% in 2010-11. Nine out of twenty Indian reactors recorded an unprecedented 97% Capacity factor during 2011-12. With the imported uranium from France, the 220 MW Kakrapar 2 PHWR reactors recorded 99% capacity factor during 2011-12. The Availability factor for the year 2011-12 was at 89%.

India has been making advances in the field of thorium-based fuels, working to design and develop a prototype for an atomic reactor using thorium and low-enriched uranium, a key part of India's three stage nuclear power programme.[8] The country has also recently re-initiated its involvement in the LENR research activities,[9] in addition to supporting work done in the fusion power area through the ITER initiative.
India's first research nuclear reactor and its first nuclear power plant were built with assistance from Canada.^[10][11] The 40 MW research reactor agreement was signed in 1956, and CIRUS achieved first criticality in 1960. This reactor was supplied to India on the assurance that it would not be used for military purposes, but without effective safeguards against such use.^[10][11] The agreement for India's first nuclear power plant at Rajasthan, RAPP-1, was signed in 1963, followed by RAPP-2 in 1966. These reactors contained rigid safeguards to ensure they would not be used for a military programme. The 200 MWe RAPP-1 reactor was based on the CANDU reactor at Douglas Point and began operation in 1972. Due to technical problems the reactor had to be downrated from 200 MW to 100 MW.^{[citation needed]} The technical and design information were given free of charge by AECL to India.^{[citation needed]} The United States and Canada terminated their assistance after the detonation of India's first nuclear explosion in 1974.

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Energy policy of India

From Wikipedia, the free encyclopedia
The energy policy of India is largely defined by the country's burgeoning energy deficit[1] and increased focus on developing alternative sources of energy,[2] particularly nuclear, solar and wind energy.
The energy consumption in India is the fourth biggest after China, USA and Russia.[3] The total primary energy consumption from crude oil (29.45%), natural gas (7.7%), coal (54.5%), nuclear energy (1.26%), hydro electricity (5.0%), wind power, biomass electricity and solar power is 595 Mtoe in the year 2013. In the year 2013, India's net imports are nearly 144.3 million tons of crude oil, 16 Mtoe of LNG and 95 Mtoe coal totalling to 255.3 Mtoe of primary energy which is equal to 42.9% of total primary energy consumption.[4] About 70% of India's electricity generation capacity is from fossil fuels, with coal accounting for 40% of India's total energy consumption followed by crude oil and natural gas at 28% and 6% respectively.[1] India is largely dependent on fossil fuel imports to meet its energy demands — by 2030, India's dependence on energy imports is expected to exceed 53% of the country's total energy consumption.[1] In 2009-10, the country imported 159.26 million tonnes of crude oil which amounts to 80% of its domestic crude oil consumption and 31% of the country's total imports are oil imports.[1][5] The growth of electricity generation in India has been hindered by domestic coal shortages[6] and as a consequence, India's coal imports for electricity generation increased by 18% in 2010.[7]

Due to rapid economic expansion, India has one of the world's fastest growing energy markets and is expected to be the second-largest contributor to the increase in global energy demand by 2035, accounting for 18% of the rise in global energy consumption.[8] Given India's growing energy demands and limited domestic fossil fuel reserves, the country has ambitious plans to expand its renewable and nuclear power industries. India has the world's fifth largest wind power market[9] and plans to add about 20GW of solar power capacity by 2022.[8] India also envisages to increase the contribution of nuclear power to overall electricity generation capacity from 4.2% to 9% within 25 years.[10] The country has five nuclear reactors under construction (third highest in the world) and plans to construct 18 additional nuclear reactors (second highest in the world) by 2025.[11]
Total installed Power generation Capacity (June 2014)^[12]

Source	Total Capacity (MW)	Percentage
Coal	148,478.39	59.51
Hydroelectricity	40,730.09	16.33
Renewable energy source	31,692.14	12.70
Natural Gas	22,607.95	9.06
Nuclear	4780	1.92
Oil	1,199.75	0.48
Total	249,488.32

Sector	Total Capacity (MW)	Percentage
State Sector	93,540.7	37.49
Central Sector	68,324.63	27.38
Private Sector	87,622.99	35.12
Total	249,488.32

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Thermal energy

Thermal energy is a term sometimes used to refer to the internal energy present in a system in a state of thermodynamic equilibrium by virtue of its temperature.[1] The average translational kinetic energy possessed by free particles in a system of free particles in thermodynamic equilibrium (as measured in the frame of reference of the center of mass of that system) may also be referred to as the thermal energy per particle.[2]

The internal energy of a system may include other forms of energy in a thermodynamic system in addition to thermal energy, namely forms of potential energy that do not influence temperature and do not absorb heat, such as the chemical energy stored in its molecular structure and electronic configuration, and the nuclear binding energy that binds the sub-atomic particles of matter.[citation needed]

Microscopically, the thermal energy may include both the kinetic energy and potential energy of a system's constituent particles, which may be atoms, molecules, electrons, or particles in plasmas. It originates from the individually random, or disordered, motion of particles in a large ensemble, as consequence of absorbing heat. In ideal monatomic gases, thermal energy is entirely kinetic energy. In other substances, in cases where some of thermal energy is stored in atomic vibration, this vibrational part of the thermal energy is stored equally partitioned between potential energy of atomic vibration, and kinetic energy of atomic vibration. Thermal energy is thus equally partitioned between all available quadratic degrees of freedom of the particles. As noted, these degrees of freedom may include pure translational motion in gases, in rotational states, and as potential and kinetic energy in normal modes of vibrations in intermolecular or crystal lattice vibrations. In general, due to quantum mechanical reasons, the availability of any such degrees of freedom is a function of the energy in the system, and therefore depends on the temperature (see heat capacity for discussion of this phenomenon).[citation needed]

Macroscopically, the thermal energy of a system at a given temperature is proportional to its heat capacity. However, since the heat capacity differs according to whether or not constant volume or constant pressure is specified, or phase changes permitted, the heat capacity cannot be used to define thermal energy unless it is done in such a way as to insure that only heat gain or loss (not work) makes any changes in the internal energy of the system. Usually, this means specifying the "constant volume heat capacity" of the system so that no work is done. Also the heat capacity of a system for such purposes must not include heat absorbed by any chemical reaction or process.[citation needed]





Differentiation from heat[edit]
Heat, in the strict use in physics, is characteristic only of a process, i.e. it is absorbed or produced as an energy exchange, always as a result of a temperature difference. Heat is thermal energy in the process of transfer or conversion across a boundary of one region of matter to another, as a result of a temperature difference.[3] In engineering, the terms "heat" and "heat transfer" are thus used nearly interchangeably, since heat is always understood to be in the process of transfer. The energy transferred by heat is called by other terms (such as thermal energy or latent energy) when this energy is no longer in net transfer, and has become static.[4] Thus, heat is not a static property of matter. Matter does not contain heat, but rather thermal energy, and even the thermal energy is subject to transformations into and out of other types of energy, and so can be considered to be "conserved" only when these processes are small. The heat transfer rate or heating rate is the amount of energy per unit time being transferred as heat, or the heat power.

When two thermodynamic systems with different temperatures are brought into diathermic contact, they spontaneously exchange energy as heat, the exchange being transfer of thermal energy from the system of higher temperature to the colder system. Heat may cause work to be performed on a system, for example, in form of volume or pressure changes. This work may be used in heat engines to convert thermal energy into other forms of energy. When two systems have reached athermodynamic equilibrium, they have attained the same exact temperature and the net exchange of thermal energy vanishes, and heat flow ceases.

Definitions[edit]
Thermal energy is the portion of the thermodynamic or internal energy of a system that is responsible for the temperature of the system.[1][3] The thermal energy of a system scales with its size and is therefore an extensive property. It is not a state function of the system unless the system has been constructed so that all changes in internal energy are due to changes in thermal energy, as a result of heat transfer (not work). Otherwise thermal energy is dependent on the way or method by which the system attained its temperature.[citation needed]

From a macroscopic thermodynamic description, the thermal energy of a system is given by its constant volume specific heat capacity C(T), a temperature coefficient also called thermal capacity, at any given absolute temperature (T):

U_{thermal} = C(T) \cdot T.
The heat capacity is a function of temperature itself, and is typically measured and specified for certain standard conditions and a specific amount of substance (molar heat capacity) or mass units (specific heat capacity). At constant volume (V), CV it is the temperature coefficient of energy.[5] In practice, given a narrow temperature range, for example the operational range of a heat engine, the heat capacity of a system is often constant, and thus thermal energy changes are conveniently measured as temperature fluctuations in the system.

In the microscopical description of statistical physics, the thermal energy is identified with the mechanical kinetic energy of the constituent particles or other forms of kinetic energy associated with quantum-mechanical microstates.[citation needed]

The distinguishing difference between the terms kinetic energy and thermal energy is that thermal energy is the mean energy of disordered, i.e. random, motion of the particles or the oscillations in the system. The conversion of energy of ordered motion to thermal energy results from collisions.[6]

All kinetic energy is partitioned into the degrees of freedom of the system. The average energy of a single particle with f quadratic degrees of freedom in a thermal bath of temperature T is a statistical mean energy given by the equipartition theorem as

E_{thermal} = f \cdot \tfrac 1 2 kT \,\!
where k is the Boltzmann constant. The total thermal energy of a sample of matter or a thermodynamic system is consequently the average sum of the kinetic energies of all particles in the system. Thus, for a system of N particles its thermal energy is[7]

U_{thermal} = N \cdot f \cdot \tfrac{1}{2} kT.
For gaseous systems, the factor f, the number of degrees of freedom, commonly has the value 3 in the case of the monatomic gas, 5 for many diatomic gases, and 7 for larger molecules at ambient temperatures. In general however, it is a function of the temperature of the system as internal modes of motion, vibration, or rotation become available in higher energy regimes.[citation needed]


Uthermal is not the total energy of a system. Physical systems also contains static potential energy (such as chemical energy) that arises from interactions between particles, nuclear energy associated with atomic nuclei of particles, and even the rest mass energy due to the equivalence of energy and mass.[citation needed]

Thermal energy of the ideal gas[edit]
Thermal energy is most easily defined in the context of the ideal gas, which is well approximated by a monatomic gas at low pressure. The ideal gas is a gas of particles considered as point objects of perfect spherical symmetry that interact only by elastic collisions and fill a volume such that their mean free path between collisions is much larger than their diameter.

The mechanical kinetic energy of a single particle is

E_{kinetic} = \tfrac 1 2 m v^2 \,\!
where m is the particle's mass and v is its velocity. The thermal energy of the gas sample consisting of N atoms is given by the sum of these energies, assuming no losses to the container or the environment:

U_{thermal} = \tfrac 1 2 N m \overline{v^2} = \tfrac{3}{2} N k T,
where the line over the velocity term indicates that the average value is calculated over the entire ensemble. The total thermal energy of the sample is proportional to the macroscopic temperature by a constant factor accounting for the three translational degrees of freedom of each particle and the Boltzmann constant. The Boltzmann constant converts units between the microscopic model and the macroscopic temperature. This formalism is the basic assumption that directly yields theideal gas law and it shows that for the ideal gas, the internal energy U consists only of its thermal energy:

 U = U_{thermal}.\;
Historical context[edit]
In an 1847 lecture entitled On Matter, Living Force, and Heat, James Prescott Joule characterized various terms that are closely related to thermal energy and heat. He identified the terms latent heat and sensible heat as forms of heat each effecting distinct physical phenomena, namely the potential and kinetic energy of particles, respectively.[8] He describes latent energy as the energy of interaction in a given configuration of particles, i.e. a form of potential energy, and the sensible heat as an energy affecting temperature measured by the thermometer due to the thermal energy, which he called the living force.

Distinction of thermal energy and heat[edit]
In thermodynamics, heat must always be defined as energy in exchange between two systems, or a single system and its surroundings.[9] According to the zeroth law of thermodynamics, heat is exchanged between thermodynamic systems in thermal contact only if their temperatures are different, as this is the condition when the net exchange of thermal energy is non-zero. For the purpose of distinction, a system is defined to be enclosed by a well-characterized boundary. If heat traverses the boundary in direction into the system, the internal energy change is considered to be a positive quantity, while exiting the system, it is negative. As a process variable, heat is never a property of the system, nor is it contained within the boundary of the system.[3]

In contrast to heat, thermal energy exists on both sides of a boundary. It is the statistical mean of the microscopic fluctuations of the kinetic energy of the systems' particles, and it is the source and the effect of the transfer of heat across a system boundary. Statistically, thermal energy is always exchanged between systems, even when the temperatures on both sides is the same, i.e. the systems are in thermal equilibrium. However, at equilibrium, the net exchange of thermal energy is zero, and therefore there is no heat.

Thermal energy may be increased in a system by other means than heat, for example when mechanical or electrical work is performed on the system. No qualitative difference exists between the thermal energy added by other means. Thermal energy is not a state function, although it may be closely related to the internal energy of some systems, which is a state function. There is also no need in classical thermodynamics to characterize the thermal energy in terms of atomic or molecular behavior. A change in thermal energy induced in a system is the product of the change in entropy and the temperature of the system.

Heat exchanged across a boundary may cause changes other than a change in temperature. For example, it may cause phase transitions, such as melting or evaporation, which are changes in the configuration of a material. Since such an energy exchange is not observable by a change in temperature, it is called a latent heat and represents a change in the potential energy of the system.

Rather than being itself the thermal energy involved in a transfer, heat is sometimes also understood as the process of that transfer, i.e. heat functions as a verb.

Today's narrow definition of heat in physics contrasts with its use in common language, in some engineering disciplines, and in the historical scientific development of thermodynamics in the caloric theory of heat. The phenomenon of heat in these instances is today properly identified as the entropy.[5]

The origin of heat energy on Earth[edit]
Earth's proximity to the Sun is the reason that almost everything near Earth's surface is warm with a temperature substantially above absolute zero.[10] Solar radiationconstantly replenishes heat energy that Earth loses into space and a relatively stable state of near equilibrium is achieved. Because of the wide variety of heat diffusion mechanisms (one of which is black-body radiation which occurs at the speed of light), objects on Earth rarely vary too far from the global mean surface and air temperature of 287 to 288 K (14 to 15 °C). The more an object's or system's temperature varies from this average, the more rapidly it tends to come back into equilibrium with the ambient environment.

Thermal energy of individual particles[edit]

The term thermal energy is also often used as a property of single particles to designate the kinetic energy of the particles. An example is the description of thermal neutrons having a certain thermal energy, which means that the kinetic energy of the particle is equivalent to the temperature of its surroundings.

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