A mode of transport is a solution that makes use of a particular type of vehicle, infrastructure and operation. The transport of a person or of cargo may involve one mode or several modes, with the latter case being called intermodal or multimodal transport. Each mode has its advantages and disadvantages, and will be chosen for a trip on the basis of cost, capability, route, and speed.

Human-powered

Human powered transport is the transport of people and/or goods using human muscle-power, in the form of walking, running and swimming. Modern technology has allowed machines to enhance human-power. Human-powered transport remains popular for reasons of cost-saving, leisure, physical exercise and environmentalism. Human-powered transport is sometimes the only type available, especially in underdeveloped or inaccessible regions. It is considered an ideal form of sustainable transportation.

Although humans are able to walk without infrastructure, the transport can be enhanced through the use of roads, especially when enforcing the human power with vehicles, such as bicycles and inline skates. Human-powered vehicles have also been developed for difficult environments, such as snow and water, by watercraft rowing and skiing; even the air can be entered with human-powered aircraft.

Animal-powered

Animal-powered transport is the use of working animals for the movement of people and goods. Humans may ride some of the animals directly, use them as pack animals for carrying goods, or harness them, alone or in teams, to pull sleds or wheeled vehicles. Animals are superior to people in their speed, endurance and carrying capacity; prior to the Industrial Revolution they were used for all land transport impracticable for people, and they remain an important mode of transport in less developed areas of the world.

Air

A fixed-wing aircraft, commonly called airplane, is a heavier-than-air craft where movement of the air in relation to the wings is used to generate lift. The term is used to distinguish from rotary-wing aircraft, where the movement of the lift surfaces relative to the air generates lift. A gyroplane is both fixed-wing and rotary-wing. Fixed-wing aircraft range from small trainers and recreational aircraft to large airliners and military cargo aircraft.

Two things necessary for aircraft are air flow over the wings for lift and an area for landing. The majority of aircraft also need an airport with the infrastructure to receive maintenance, restocking, refueling and for the loading and unloading of crew, cargo and passengers. While the vast majority of aircraft land and take off on land, some are capable of take off and landing on ice, snow and calm water.

The aircraft is the second fastest method of transport, after the rocket. Commercial jets can reach up to , single-engine aircraft . Aviation is able to quickly transport people and limited amounts of cargo over longer distances, but incur high costs and energy use; for short distances or in inaccessible places helicopters can be used. WHO estimates that up to 500,000 people are on planes at any time.

Rail

Rail transport is where a train runs along a set of two parallel steel rails, known as a railway or railroad. The rails are anchored perpendicular to ties (or sleepers) of timber, concrete or steel, to maintain a consistent distance apart, or gauge. The rails and perpendicular beams are placed on a foundation made of concrete, or compressed earth and gravel in a bed of ballast. Alternative methods include monorail and maglev.

A train consists of one or more connected vehicles that run on the rails. Propulsion is commonly provided by a locomotive, that hauls a series of unpowered cars, that can carry passengers or freight. The locomotive can be powered by steam, diesel or by electricity supplied by trackside systems. Alternatively, some or all the cars can be powered, known as a multiple unit. Also, a train can be powered by horses, cables, gravity, pneumatics and gas turbines. Railed vehicles move with much less friction than rubber tires on paved roads, making trains more energy efficient, though not as efficient as ships.

Intercity trains are long-haul services connecting cities; modern high-speed rail is capable of speeds up to , but this requires specially built track. Regional and commuter trains feed cities from suburbs and surrounding areas, while intra-urban transport is performed by high-capacity tramways and rapid transits, often making up the backbone of a city’s public transport. Freight trains traditionally used box cars, requiring manual loading and unloading of the cargo. Since the 1960s, container trains have become the dominant solution for general freight, while large quantities of bulk are transported by dedicated trains.

Road

A road is an identifiable route, way or path between two or more places. Roads are typically smoothed, paved, or otherwise prepared to allow easy travel; though they need not be, and historically many roads were simply recognizable routes without any formal construction or maintenance. In urban areas, roads may pass through a city or village and be named as streets, serving a dual function as urban space easement and route.

The most common road vehicle is the automobile; a wheeled passenger vehicle that carries its own motor. Other users of roads include buses, trucks, motorcycles, bicycles and pedestrians. As of 2002, there were 590 million automobiles worldwide.

Automobiles offer high flexibility and with low capacity, but are deemed with high energy and area use, and the main source of noise and air pollution in cities; buses allow for more efficient travel at the cost of reduced flexibility. Road transport by truck is often the initial and final stage of freight transport.

Water

Water transport is the process of transport a watercraft, such as a barge, boat, ship or sailboat, makes over a body of water, such as a sea, ocean, lake, canal or river. The need for buoyancy unites watercraft, and makes the hull a dominant aspect of its construction, maintenance and appearance.

In the 1800s the first steam ships were developed, using a steam engine to drive a paddle wheel or propeller to move the ship. The steam was produced using wood or coal. Now most ships have an engine using a slightly refined type of petroleum called bunker fuel. Some ships, such as submarines, use nuclear power to produce the steam. Recreational or educational craft still use wind power, while some smaller craft use internal combustion engines to drive one or more propellers, or in the case of jet boats, an inboard water jet. In shallow draft areas, hovercraft are propelled by large pusher-prop fans.

Although slow, modern sea transport is a highly effective method of transporting large quantities of non-perishable goods. Commercial vessels, nearly 35,000 in number, carried 7.4& billion tons of cargo in 2007. Transport by water is significantly less costly than air transport for trans-continental shipping; short sea shipping and ferries remain viable in coastal areas.

Other

Pipeline transport sends goods through a pipe, most commonly liquid and gases are sent, but pneumatic tubes can also send solid capsules using compressed air. For liquids/gases, any chemically stable liquid or gas can be sent through a pipeline. Short-distance systems exist for sewage, slurry, water and beer, while long-distance networks are used for petroleum and natural gas.

Cable transport is a broad mode where vehicles are pulled by cables instead of an internal power source. It is most commonly used at steep gradient. Typical solutions include aerial tramway, elevators, escalator and ski lifts; some of these are also categorized as conveyor transport.

Spaceflight is transport out of Earth’s atmosphere into outer space by means of a spacecraft. While large amounts of research have gone into technology, it is rarely used except to put satellites into orbit, and conduct scientific experiments. However, man has landed on the moon, and probes have been sent to all the planets of the Solar System.

Suborbital spaceflight is the fastest of the existing and planned transport systems from a place on Earth to a distant other place on Earth. Faster transport could be achieved through part of a Low Earth orbit, or following that trajectory even faster using the propulsion of the rocket to steer it.

Adapted from the Wikipedia article Transport, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Cores

Laminated steel cores

Transformers for use at power or audio frequencies typically have cores made of high permeability silicon steel. The steel has a permeability many times that of free space, and the core thus serves to greatly reduce the magnetizing current, and confine the flux to a path which closely couples the windings. Early transformer developers soon realized that cores constructed from solid iron resulted in prohibitive eddy-current losses, and their designs mitigated this effect with cores consisting of bundles of insulated iron wires. Later designs constructed the core by stacking layers of thin steel laminations, a principle that has remained in use. Each lamination is insulated from its neighbors by a thin non-conducting layer of insulation. The universal transformer equation indicates a minimum cross-sectional area for the core to avoid saturation.

The effect of laminations is to confine eddy currents to highly elliptical paths that enclose little flux, and so reduce their magnitude. Thinner laminations reduce losses, but are more laborious and expensive to construct. Thin laminations are generally used on high frequency transformers, with some types of very thin steel laminations able to operate up to 10& kHz.

One common design of laminated core is made from interleaved stacks of E-shaped steel sheets capped with I-shaped pieces, leading to its name of “E-I transformer”. Such a design tends to exhibit more losses, but is very economical to manufacture. The cut-core or C-core type is made by winding a steel strip around a rectangular form and then bonding the layers together. It is then cut in two, forming two C shapes, and the core assembled by binding the two C halves together with a steel strap. They have the advantage that the flux is always oriented parallel to the metal grains, reducing reluctance.

A steel core’s remanence means that it retains a static magnetic field when power is removed. When power is then reapplied, the residual field will cause a high inrush current until the effect of the remaining magnetism is reduced, usually after a few cycles of the applied alternating current. Overcurrent protection devices such as fuses must be selected to allow this harmless inrush to pass. On transformers connected to long, overhead power transmission lines, induced currents due to geomagnetic disturbances during solar storms can cause saturation of the core and operation of transformer protection devices.

Distribution transformers can achieve low no-load losses by using cores made with low-loss high-permeability silicon steel or amorphous metal alloy. The higher initial cost of the core material is offset over the life of the transformer by its lower losses at light load.

Solid cores

Powdered iron cores are used in circuits (such as switch-mode power supplies) that operate above main frequencies and up to a few tens of kilohertz. These materials combine high magnetic permeability with high bulk electrical resistivity. For frequencies extending beyond the VHF band, cores made from non-conductive magnetic ceramic materials called ferrites are common. Some radio-frequency transformers also have movable cores (sometimes called ‘slugs’) which allow adjustment of the coupling coefficient (and bandwidth) of tuned radio-frequency circuits.

Toroidal cores

Toroidal transformers are built around a ring-shaped core, which, depending on operating frequency, is made from a long strip of silicon steel or permalloy wound into a coil, powdered iron, or ferrite. A strip construction ensures that the grain boundaries are optimally aligned, improving the transformer’s efficiency by reducing the core’s reluctance. The closed ring shape eliminates air gaps inherent in the construction of an E-I core. The cross-section of the ring is usually square or rectangular, but more expensive cores with circular cross-sections are also available. The primary and secondary coils are often wound concentrically to cover the entire surface of the core. This minimizes the length of wire needed, and also provides screening to minimize the core’s magnetic field from generating electromagnetic interference.

Toroidal transformers are more efficient than the cheaper laminated E-I types for a similar power level. Other advantages compared to E-I types, include smaller size (about half), lower weight (about half), less mechanical hum (making them superior in audio amplifiers), lower exterior magnetic field (about one tenth), low off-load losses (making them more efficient in standby circuits), single-bolt mounting, and greater choice of shapes. The main disadvantages are higher cost and limited power capacity (see “Classification” above). Because of the lack of a residual gap in the magnetic path, toroidal transformers also tend to exhibit higher inrush current, compared to laminated E-I types.

Ferrite toroidal cores are used at higher frequencies, typically between a few tens of kilohertz to hundreds of megahertz, to reduce losses, physical size, and weight of switch-mode power supplies. A drawback of toroidal transformer construction is the higher labor cost of winding. This is because it is necessary to pass the entire length of a coil winding through the core aperture each time a single turn is added to the coil. As a consequence, toroidal transformers are uncommon above ratings of a few kVA. Small distribution transformers may achieve some of the benefits of a toroidal core by splitting it and forcing it open, then inserting a bobbin containing primary and secondary windings.

Air cores

A physical core is not an absolute requisite and a functioning transformer can be produced simply by placing the windings near each other, an arrangement termed an “air-core” transformer. The air which comprises the magnetic circuit is essentially lossless, and so an air-core transformer eliminates loss due to hysteresis in the core material. The leakage inductance is inevitably high, resulting in very poor regulation, and so such designs are unsuitable for use in power distribution. They have however very high bandwidth, and are frequently employed in radio-frequency applications, for which a satisfactory coupling coefficient is maintained by carefully overlapping the primary and secondary windings. They’re also used for resonant transformers such as Tesla coils where they can achieve reasonably low loss in spite of the high leakage inductance.

Windings

The conducting material used for the windings depends upon the application, but in all cases the individual turns must be electrically insulated from each other to ensure that the current travels throughout every turn. For small power and signal transformers, in which currents are low and the potential difference between adjacent turns is small, the coils are often wound from enamelled magnet wire, such as Formvar wire. Larger power transformers operating at high voltages may be wound with copper rectangular strip conductors insulated by oil-impregnated paper and blocks of pressboard.

High-frequency transformers operating in the tens to hundreds of kilohertz often have windings made of braided Litz wire to minimize the skin-effect and proximity effect losses. Large power transformers use multiple-stranded conductors as well, since even at low power frequencies non-uniform distribution of current would otherwise exist in high-current windings. Each strand is individually insulated, and the strands are arranged so that at certain points in the winding, or throughout the whole winding, each portion occupies different relative positions in the complete conductor. The transposition equalizes the current flowing in each strand of the conductor, and reduces eddy current losses in the winding itself. The stranded conductor is also more flexible than a solid conductor of similar size, aiding manufacture.

For signal transformers, the windings may be arranged in a way to minimize leakage inductance and stray capacitance to improve high-frequency response. This can be done by splitting up each coil into sections, and those sections placed in layers between the sections of the other winding. This is known as a stacked type or interleaved winding.

Both the primary and secondary windings on power transformers may have external connections, called taps, to intermediate points on the winding to allow selection of the voltage ratio. In power distribution transformers the taps may be connected to an automatic on-load tap changer for voltage regulation of distribution circuits. Audio-frequency transformers, used for the distribution of audio to public address loudspeakers, have taps to allow adjustment of impedance to each speaker. A center-tapped transformer is often used in the output stage of an audio power amplifier in a push-pull circuit. Modulation transformers in AM transmitters are very similar.

Certain transformers have the windings protected by epoxy resin. By impregnating the transformer with epoxy under a vacuum, one can replace air spaces within the windings with epoxy, thus sealing the windings and helping to prevent the possible formation of corona and absorption of dirt or water. This produces transformers more suited to damp or dirty environments, but at increased manufacturing cost.

Coolant

High temperatures will damage the winding insulation. Small transformers do not generate significant heat and are cooled by air circulation and radiation of heat. Power transformers rated up to several hundred kVA can be adequately cooled by natural convective air-cooling, sometimes assisted by fans. In larger transformers, part of the design problem is removal of heat. Some power transformers are immersed in transformer oil that both cools and insulates the windings. The oil is a highly refined mineral oil that remains stable at transformer operating temperature. Indoor liquid-filled transformers are required by building regulations in many jurisdictions to use a non-flammable liquid, or to be located in fire-resistant rooms. Air-cooled dry transformers are preferred for indoor applications even at capacity ratings where oil-cooled construction would be more economical, because their cost is offset by the reduced building construction cost.

The oil-filled tank often has radiators through which the oil circulates by natural convection; some large transformers employ forced circulation of the oil by electric pumps, aided by external fans or water-cooled heat exchangers. Oil-filled transformers undergo prolonged drying processes to ensure that the transformer is completely free of water vapor before the cooling oil is introduced. This helps prevent electrical breakdown under load. Oil-filled transformers may be equipped with Buchholz relays, which detect gas evolved during internal arcing and rapidly de-energize the transformer to avert catastrophic failure. Oil-filed transformers may fail, rupture, and burn, causing power outages and losses. Installations of oil-filled transformers usually includes fire protection measures such as walls, oil containment, and fire-suppression sprinkler systems.

Polychlorinated biphenyls have properties that once favored their use as a coolant, though concerns over their environmental persistence led to a widespread ban on their use. Today, non-toxic, stable silicone-based oils, or fluorinated hydrocarbons may be used where the expense of a fire-resistant liquid offsets additional building cost for a transformer vault. Before 1977, even transformers that were nominally filled only with mineral oils may also have been contaminated with polychlorinated biphenyls at 10-20 ppm. Since mineral oil and PCB fluid mix, maintenance equipment used for both PCB and oil-filled transformers could carry over small amounts of PCB, contaminating oil-filled transformers.

Some “dry” transformers (containing no liquid) are enclosed in sealed, pressurized tanks and cooled by nitrogen or sulfur hexafluoride gas.

Experimental power transformers in the 2& MVA range have been built with superconducting windings which eliminates the copper losses, but not the core steel loss. These are cooled by liquid nitrogen or helium.

Insulation drying

Construction of oil-filled transformers requires that the insulation covering the windings be thoroughly dried before the oil is introduced. There are several different methods of drying. Common for all is that they are carried out in vacuum environment. The vacuum makes it difficult to transfer energy (heat) to the insulation. For this there are several different methods. The traditional drying is done by circulating hot air over the active part and cycle this with periods of vacuum (Hot Air Vacuum drying, HAV). More common for larger transformers is to use evaporated solvent which condenses on the colder active part. The benefit is that the entire process can be carried out at lower pressure and without influence of added oxygen. This process is commonly called Vapour Phase Drying (VPD).

For distribution transformers which are smaller and have a smaller insulation weight, resistance heating can be used. This is a method where current is injected in the windings and the resistance in the windings is heating up the insulation. The benefit is that the heating can be controlled very well and it is energy efficient. The method is called Low Frequency Heating (LFH) since the current is injected at a much lower frequency than the nominal of the grid, which is normally 50 or 60 Hz. A lower frequency reduces the affect of the inductance in the transformer and the voltage can be reduced.

Terminals

Very small transformers will have wire leads connected directly to the ends of the coils, and brought out to the base of the unit for circuit connections. Larger transformers may have heavy bolted terminals, bus bars or high-voltage insulated bushings made of polymers or porcelain. A large bushing can be a complex structure since it must provide careful control of the electric field gradient without letting the transformer leak oil.

Adapted from the Wikipedia article Transformer, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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The best known type of radiostation are the ones that broadcast via radiowaves. These include foremost AM and FM stations. There are several subtypes, namely commercial, public and nonprofit varieties as well as student-run campus radio stations and hospital radio stations can be found throughout the developed world.

Although now being eclipsed by internet-distributed radio, there are many stations that broadcast on shortwave bands using AM technology that can be received over thousands of miles (especially at night). For example, the BBC has a full schedule transmitted via shortwave to Africa and Asia. These broadcasts are very sensitive to atmospheric conditions and solar activity.

Also, many other non-broadcast types of radio stations exist. These include base stations for police, fire and ambulance networks, military base stations, dispatch base stations for taxis, trucks, and couriers, emergency broadcast systems, and amateur radio stations.

Arbitron, the United States based company that reports on radio audiences, defines a “radio station” as a government-licensed AM or FM station; an HD Radio (primary or multicast) station; an internet stream of an existing government-licensed station; one of the satellite radio channels from XM Satellite Radio or Sirius Satellite Radio; or, potentially, a station that is not government licensed.

Over radiowaves=

Shortwave

See Shortwave for the differences between shortwave, medium wave and long wave spectra. Used largely for international broadcasts by organs of state propaganda, religious organizations, militaries and others.

AM

AM stations were the earliest broadcasting stations to be developed. AM refers to amplitude modulation, a mode of broadcasting radio waves by varying the amplitude of the carrier signal in response to the amplitude of the signal to be transmitted.

Many countries outside of the U.S. use a similar frequency band for AM transmissions. Europe also uses the long wave band. In response to the growing popularity of FM radio stereo radio stations in the late 1980s and early 1990s, some North American stations began broadcasting in AM stereo, though this never gained popularity, and very few receivers were ever sold.

Advantages

One of the advantages of AM is that its unsophisticated signal can be detected (turned into sound) with simple equipment. If a signal is strong enough, not even a power source is needed; building an unpowered crystal radio receiver was a common childhood project in the early years of radio. Another advantage to AM is that it uses a narrower bandwidth than FM.

AM broadcasts occur on North American airwaves in the medium wave frequency range of 530 to 1700 kHz (known as the “standard broadcast band”). The band was expanded in the 1990s by adding nine channels from 1620 to 1700& kHz. Channels are spaced every 10& kHz in the Americas, and generally every 9& kHz everywhere else.

Disadvantages

The signal is subject to interference from electrical storms (lightning) and other EMI.

AM transmissions cannot be ionospherically propagated during the day due to strong absorption in the D-layer of the ionosphere. In a crowded channel environment this means that the power of regional channels which share a frequency must be reduced at night or directionally beamed in order to avoid interference, which reduces the potential nighttime audience. Some stations have frequencies unshared with other stations in North America; these are called clear-channel stations. Many of them can be heard across much of the country at night. ”(This is not to be confused with Clear Channel Communications, merely a brand name, which currently owns many U.S. radio stations on both the AM and FM bands.)” During the night, this absorption largely disappears and permits signals to travel to much more distant locations via ionospheric reflections. However, fading of the signal can be severe at night.

AM radio transmitters can transmit audio frequencies up to 15& kHz (now limited to 10& kHz in the US due to FCC rules designed to reduce interference), but most receivers are only capable of reproducing frequencies up to 5& kHz or less. At the time that AM broadcasting began in the 1920s, this provided adequate fidelity for existing microphones, 78 rpm recordings, and loudspeakers. The fidelity of sound equipment subsequently improved considerably, but the receivers did not. Reducing the bandwidth of the receivers reduces the cost of manufacturing and makes them less prone to interference. AM stations are never assigned adjacent channels in the same service area. This prevents the sideband power generated by two stations from interfering with each other. Bob Carver created an AM stereo tuner employing notch filtering that demonstrated that an AM broadcast can meet or exceed the 15& kHz baseband bandwidth allocted to FM stations without objectionable interference. After several years, the tuner was discontinued. Bob Carver had left the company and the Carver Corporation later cut the number of models produced before discontinuing production completely. AM stereo broadcasts declined with the advent of HD Radio.

FM

FM refers to frequency modulation, and occurs on VHF airwaves in the frequency range of 88 to 108 MHz everywhere (except Japan and Russia). Japan uses the 76 to 90& MHz band. Russia has two bands widely used by the Soviet Union, 65.9 to 74& MHz and 87.5 to 108& MHz worldwide standard. FM stations are much more popular in economically developed regions, such as Europe and the United States, especially since higher sound fidelity and stereo broadcasting became common in this format.

FM radio was invented by Edwin H. Armstrong in the 1930s for the specific purpose of overcoming the interference (static) problem of AM radio, to which it is relatively immune. At the same time, greater fidelity was made possible by spacing stations further apart. Instead of 10& kHz apart, as on the AM band in the US, FM channels are 200& kHz (0.2& MHz) apart. In other countries greater spacing is sometimes mandatory, such as in New Zealand, which uses 700& kHz spacing (previously 800& kHz). The improved fidelity made available was far in advance of the audio equipment of the 1940s, but wide interchannel spacing was chosen to take advantage of the noise-suppressing feature of wideband FM.

Bandwidth of 200 kHz is not needed to accommodate an audio signal — 20& kHz to 30& kHz is all that is necessary for a narrowband FM signal. The 200& kHz bandwidth allowed room for ±75& kHz signal deviation from the assigned frequency, plus guard bands to reduce or eliminate adjacent channel interference. The larger bandwidth allows for broadcasting a 15& kHz bandwidth audio signal plus a 38& kHz stereo “subcarrier”—a piggyback signal that rides on the main signal. Additional unused capacity is used by some broadcasters to transmit utility functions such as background music for public areas, GPS auxiliary signals, or financial market data.

The AM radio problem of interference at night was addressed in a different way. At the time FM was set up, the available frequencies were far higher in the spectrum than those used for AM radio – by a factor of approximately 100. Using these frequencies meant that even at far higher power, the range of a given FM signal was much shorter, thus its market was more local than for AM radio. The reception range at night is the same as in the daytime.

The original FM radio service in the U.S. was the Yankee Network, located in New England. Regular FM broadcasting began in 1939, but did not pose a significant threat to the AM broadcasting industry. It required purchase of a special receiver. The frequencies used, 42 to 50& MHz, were not those used today. The change to the current frequencies, 88 to 108& MHz, began after the end of World War II, and it was to some extent imposed by AM radio owners so as to attempt to cripple what was by now realized to be a potentially serious threat.

FM radio on the new band had to begin from the ground floor. As a commercial venture it remained a little-used audio enthusiasts’ medium until the 1960s. The more prosperous AM stations, or their owners, acquired FM licenses and often broadcast the same programming on the FM station as on the AM station (“simulcasting”). The FCC limited this practice in the 1970s. By the 1980s, since almost all new radios included both AM and FM tuners, FM became the dominant medium, especially in cities. Because of its greater range, AM remained more common in rural environments.

Amateur radio

Independent “ham” radio operators, largely hobbyists, licensed by respective national bodies and assigned callsigns. See Amateur radio.

Citizens band radio

Citizens’ band radio or CB is usually unlicensed broadcasting over frequencies set aside for that purpose. It is often used by truck drivers to communicate to one another.

Other types of radio communication over radiowaves

Radio communications have been and are used for all variety of data transmissions. The earliest radio application, Morse code, can still be heard today. Experiments in sending pictures and text date back to the early days of radio. A variety of clock signals are also broadcast. Another early use of radio was coded transmission of information by national governments in peace and war. During the Cold War the USSR and allied governments had national programs to block shortwave and other frequency transmissions by using jamming techniques. One signal known as Russian woodpecker suddenly appeared on July 4, 1976 and just as suddenly disappeared at the end of 1989, and is still something of a mystery. More and more radio frequencies are being used to send digital packets of information of varying degrees of complexity.

An early form of digital radio broadcasting was packet radio, which combines digital information with traditional radio broadcasting over the air.

Digital radio broadcasting has emerged, first in Europe (the UK in 1995 and Germany in 1999), and later in the United States, France, the Netherlands, South Africa and many other countries worldwide. The most simple system is named DAB Digital Radio, for Digital Audio Broadcasting, and uses the public domain EUREKA 147 (Band III) system. DAB is used mainly in the UK and South Africa. Germany and Holland use the DAB and DAB+ systems, and France use the L-Band system of DAB Digital Radio.

In the United States digital radio isn’t used in the same way as Europe and South Africa. Instead, the IBOC system is named HD Radio and owned by a consortium of private companies that is called iBiquity. An international non-profit consortium Digital Radio Mondiale (DRM), has introduced the public domain DRM system.

Satellite

Satellite radio broadcasters are slowly emerging, but the enormous entry costs of space-based satellite transmitters, and restrictions on available radio spectrum licenses has restricted growth of this market. In the USA and Canada, just two services, XM Satellite Radio and Sirius Satellite Radio exist. Both XM and Sirius are owned by Sirius XM Radio, which was formed by the merger of XM and Sirius on July 29, 2008, whereas in Canada, XM Radio Canada and Sirius Canada remain separate companies.

Adapted from the Wikipedia article Radio broadcasting, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Many races in the galaxy are not under the governance of the Citadel. They either refuse to recognize the authority of the Citadel (the batarians), have been removed as member species by the Council (the krogan and quarians), or have never officially established contact with Citadel space (the vorcha). A few races are openly antagonistic towards Citadel member races and non-members alike (the Collectors, geth, and Reapers).

Batarians

The batarians are a race that are socially similar to humanity, and as such have a political rivalry with the Human Systems Alliance, with whom they compete for unclaimed territory. The batarians have a slimmer build than the average human but are about the same height, on average, as a human. They have four eyes, one inner set located approximately where that of a human’s would be, and a second pair on top of the first pair’s browline. Their noses have eight openings, four on each side of the head. Their skin is a cream color with a purple stripe running down their chin and a chocolaty brown area where hair would be on a human. Batarians used to have an embassy on the Citadel, but they closed it in protest of the council’s decision to allow human colonization to continue in the Skyllian Verge, an area that the batarians considered within their sphere of influence. As a result, the batarians have essentially placed themselves at odds with most of the other Citadel Council races and particularly the Systems Alliance. This rivalry has led them into something of a proxy war with the Alliance, with the batarians using their influence in the chaotic Terminus Systems (especially in areas bordering Alliance space) to foment political instability and criminal activity that could eventually cause problems for the Alliance. As a result, batarians have many contacts with and in many cases are in direct control of galactic organized crime syndicates and terrorist groups.

Collectors

The Collectors are an insectoid race long believed by Citadel races to be a myth, but are occasionally sighted outside Citadel space. Their given name comes from their unusual trade requests; Collectors have been known to trade highly advanced technology in exchange for members of other species with specific traits, such as quotas of seven left-handed quarians or sixteen sets of batarian twins. More recently, the Collectors have requested human biotics and have been behind the disappearance of entire human colonies, harvesting them for unknown purpose. As their technology is based on Reaper technology, the Collectors are believed to be working for the Reapers. The Collectors are one of the central enemies of ”Mass Effect 2”. Over the course of the plot of ”Mass Effect 2”, the Collectors are revealed to be the remains of the Protheans, genetically modified by the Reapers to serve as a virtual slave race. They are identified by EDI by their DNA which bears distinct similarities with the Protheans.

The Collectors are often remote controlled by the Reaper called ”Harbinger”, he seems to have possessed the Collector General who monitors the drones in the field and can extend possession to any one of them at will. Possession causes bright yellow eyes, increased power, and yellow lines all over a drones body, they are then directly controlled by Harbinger and serve as an avatar for him until destroyed or released from control. ”Harbinger” is familiar with Shepard and often taunts the player during combat through a possessed Drone.

Geth

The geth are a race of collectively intelligent automatons that live beyond the Perseus Veil, an enormous nebula that obscures dozens of solar systems. The geth were created by the quarians to serve as laborers and proxy soldiers. The geth function through a neural network, a form of collective intelligence; through the network, the intelligence of geth increases proportionately to the number of geth in close proximity. As a result, where a single geth might only be capable of thought patterns analogous to instinctual drives, a group will be capable of abstract thought and reasoning. The quarians underestimated the power of this neural network and continued to make modifications to geth programming to allow them to take on more complex tasks. After a level of complexity in the neural network had been reached, the geth began to question their nature and purpose, achieving sentience.

Fearing this development, the quarians began deactivating and liquidating the geth, who realized what was happening and rebelled. After a bloody war, the quarians were driven off their home world by the geth. The other races of Citadel space initially feared a geth invasion would follow, but none occurred. Instead, the geth retreated behind the Perseus Veil and were not seen in Citadel space again for nearly 300 years, when ”Mass Effect” takes place. During this time of isolation, the geth continued to evolve and create increasingly advanced subtypes such as the “hopper”, extraordinarily agile due to an advanced locomotor system.

It is revealed in ”Mass Effect 2” that the geth of the first game were one of two opposing geth factions; the “Heretics” and the true geth. The Heretics believe in serving the Reapers, referred to by the Geth as “The Old Machines”, and were the primary antagonists of the first game. The other geth are not necessarily hostile to the rest of the Galaxy, and over the course of the plot of ”Mass Effect 2”, the player has an opportunity to recruit the Geth infiltrator Legion to serve as part of his crew. Legion will always refer to itself as “us”, as there are more than a thousand individual programs running on its single platform. If questioned about this, Legion will answer it’s platform requires these many programs in order for it to have the cognitive abilities necessary for it to operate effectively independent from other Geth in Citadel Space (Legion also asserts an average Geth platform has about a dozen to a hundred or so programs operating in it, depending on its purpose).

The long-term goal of the geth is the construction of a “mega-structure”, a massive mainframe capable of housing every geth program in existence simultaneously, and thus, achieving the peak of their processing capacity. It’s believed that the Heretics had been promised a Reaper body to serve as a mega-structure by Sovereign. They call this the Geth’s Future, the non-Heretics believed that the geth should achieve their future through their own means and that the process is as important as the results. Legion reveals that the non-Heretics intend to do this by building a massive Dyson Sphere.

Krogan

The krogan are a species of large reptilian bipeds native to Tuchanka, a world known for its harsh environment, scarce resources, and over-abundance of vicious predators. Once hailed as the saviors of the galaxy for their successful destruction of a dangerously xenophobic and powerful spacefaring insectoid race known as the rachni, the krogan are now a shadow of their former prominence. Flush with confidence following their defeat of the rachni and the recognition of the Citadel races and their subsequent leniency towards the krogans’ obvious ambition for an empire of their own, the krogan began colonizing worlds at a dangerous rate, eventually forcibly colonizing inhabited worlds and encroaching upon Citadel territories, drawing the attention of the Citadel Council. Unwilling to curtail their activities and relinquish those worlds and territories belonging to other races, the krogan rebelled against the Citadel. Initially successful against the combined forces of the Citadel and the newly-discovered turians, the krogan were defeated by a salarian-developed genophage which rendered only one out of every thousand krogan births viable.

Once infected by the turians with this weapon, the krogan were unable to maintain their numbers despite their short seven week gestation period and through attrition were eventually defeated. Due to the continuing effects of the genophage and their belligerent nature, the krogan are a dying species and are trapped in a downward spiral of meaningless violence. Some krogan hire themselves out as mercenaries, assassins or muscle for various organized crime syndicates; others manage to make out a living through brigandage and piracy. Many krogan still consider themselves at war with the galaxy at large and are still incorporated into small war parties or clans traditionally led by warlords. Increasingly, however, the krogan have taken to fighting amongst themselves for territory, resources, and even over those few krogan females who are still capable of producing offspring. Due to their slow extinction, most krogan are becoming increasingly pessimistic and self-centered. Few krogan have any interest in anything other than fighting or the acquisition of material wealth, even to the point of ignoring any possible way of countering the genophage. It is revealed in ”Mass Effect 2” that the genophage was updated, as krogan genetics were slowly overcoming the restrictions of the genophage. A salarian STG team led by Mordin Solus modified the genophage back to its previous mortality rate. This was supposedly carefully tailored to allow the krogan to maintain a viable population. However, given the warlike nature of the krogan, more combat deaths are occurring than are being sustained in population growth.

Biologically, the krogan are a hardy species, able to live for millennia; survivors of the Krogan Rebellions like Urdnot Wrex are still alive by the time of ”Mass Effect”. The large shoulder humps on a krogan store fluids and nutrients, which enable them to go for long periods without food or water. Krogan also possess multiple instances of major organs, in which secondary organs serve as backups should a main organ fail or be damaged. A krogan individual possesses a thick hide, which is extremely hardy and very resistant to cuts, scrapes, and contusions. Krogan are also highly resistant to radiation, poisons, and extreme temperatures. Biotic individuals are rare, though those who do possess the talent are typically quite strong in their abilities and are referred to as Battlemasters.

Most Krogan stand over 7 feet tall, and weigh over a ton in armor.

Quarians

The quarians are a nomadic species of humanoid aliens. Quarians are generally shorter and of slighter build than humans. They dress in full-body environmental suits designed to prevent infections caused by viruses and bacteria. After being expelled by the geth from their homeworld, quarians have had to live in starships for almost 300 years, thereby causing their immune systems to become weaker, and as a result, cannot remove their life-support systems even when they return to their home fleet. Due to their limited amount of living space and resources, quarians may only have as many children as the Conclave decides fit. If the population of the Migarant fleet drops too low, the Conclave rewards families to have multiple children. While if the population increases too high, the Conclave inacts a one child per family law. All young quarians are required to solitarily embark upon a Pilgrimage, a rite of passage where they leave their home ships and set out to discover and bring back something of value or use, whether it be an artifact, equipment, or even knowledge, which they would present to the captain of the ship of their choice. Quarians are required to move to and live on a different ship than the one they grew up on, in order to promote genetic diversity and eliminate genetic damage through inbreeding. Once the gift is accepted, the quarian is accepted into the ranks of the ship; gifts are rarely declined, as most captains are bound by tradition to accept anything that can be of use, but a stigma is attached to those who offer substandard gifts. Their name denotes whether they have embarked on their pilgrimage or not. As with the character Tali, she was named Tali’Zorah nar Rayya before her pilgrimage and Tali’Zorah vas Neema after. Nar Rayya indicates that she was originally from a ship called the ”Rayya” and vas Neema indicates that she chose to offer her findings from her Pilgrimage to the captain of the ”Neema”.

The quarians are divided politically into two branches of government: the Conclave, a civilian body that represents the majority of the people on the various ships of the Migrant Fleet, and the Admiralty Board, composed of the five highest-ranking naval officers in the fleet. The Conclave is subdivided into councils on each ship who advise the captains of the individual vessels; however, the captains still have the final say on all issues and all matters of jurisprudence. Captains who override their respective councils on too regular a basis are either ordered by the Admiralty Board to settle their disputes on their own or relinquish command. The Admiralty Board has a great deal of influence on all matters pertaining to the fleet, has direct command of the fleets’ military forces and has veto power that overrides any decision the Conclave makes that is seen to be detrimental or dangerous to the fleet. However, the decision must be unanimous and once this veto is invoked, the entire Admiralty Board must resign their seats immediately, to prevent any possible abuse of power. Any Admiralty Board member who refuses to relinquish his or her seat is subject to arrest.

The quarians are looked down upon by the Citadel races, mainly due to their creation of the geth three hundred years prior to the game’s timeline. After creating the geth and failing to quell the subsequent geth insurrection, the quarians were finally forced off of their homeworld by the geth and relegated to roaming the galaxy in an increasingly threadbare and derelict migrant flotilla. The quarians are generally unwanted and ostracized throughout the galaxy. The fleet’s demand for resources and the quarian tendency to take whatever employment they can find, often at the expense of native inhabitants, further harms their reputation, and the leaders of any colonies or systems through which the Migrant Fleet might pass are often inclined to donate any spare items of use to the quarians as a bribe to keep them from visiting. Quarians are grateful for the assistance and have never abused this tendency, but many feel insulted by the motivations behind these “gifts”; Despite all this, their skill at electronics, engineering, and cybernetics make them ideal workers for major corporations and mining firms; quarians are considered to be among the best and brightest of the galaxy when it comes to technological and geological aptitude.

Reapers

Reapers are a hyper-advanced machine race and creators of mass relays and the Citadel, resembling the species that their initial genetic material has been taken from, that periodically awaken to destroy all advanced organic life in the galaxy and are the primary antagonists of the Mass Effect trilogy. The term “Reaper” is not actually a self-designation. According to ”Sovereign” (a vanguard left behind to ensure the Reapers’ return), it is a Prothean name given to them, stating they have no name and that they “simply are”. The Reapers hibernate in the dark space that lies beyond the galaxy’s outer rim, and the Citadel itself is a gigantic mass relay that allows them to return to the galaxy. The Citadel’s location at the mass relay network hub, along with its formidable defenses, make it an ideal location for the capital of galactic civilization. Upon the Citadel’s activation, the Reapers quickly attack the Citadel and seize control of the mass relays, decapitating the government command structure and isolating individual systems. With any advanced organic civilizations in disarray, the Reapers then proceed with their genocide by methodically invading each system, exterminating or enslaving populations as they advance. This cycle of destruction has repeated for hundreds of thousands, if not millions of years; however, the Reapers have no known motive for this act beyond it being for reasons organic minds cannot comprehend. The primary plot of the game involves a race against time to prevent the Reapers’ return. It is however possible that the reapers were a now extinct race that somehow invented reaper technology and liquified their entire race.

In the first ”Mass Effect”, it is originally believed that ”Sovereign” is simply a super massive dreadnought of unknown origin about two kilometers long (the largest warship class within game canon is standardized at one kilometer); controlled by rogue Spectre Saren Arterius. Later ”Sovereign” reveals itself to actually be a huge sentient ship, a Reaper, and is the true power behind Saren. Reapers generate an “indoctrination” field, an array of signals that progressively and permanently damage higher-order functions in organic brains. It is revealed that ”Sovereign” uses this to exert influence over its organic charges, to varying degrees (total mind control on one end, suggestion on the other), including Saren Arterius. Sovereign’s design resembles a squid, with a long round hull strong enough to take no noticeable damage when it rammed into a frigate-class vessel, and large multi-jointed limbs equipped with powerful weapons. In conversation with Commander Shephard Sovereign claims that the Reaper race is “infinite” and has “always existed”, and has no creators; being a mechanical machine race these claims are irrational but still Reaper psyche is logically bound. These claims show that Sovereign believes in non rational concepts akin to religious beliefs, though in the case of the Reapers the beliefs of Sovereign are megalomaniacal, as Sovereign (and by extension presumably the entire Reaper race) consider themselves to be immortal.

In ”Mass Effect 2”, a Reaper named ”Harbinger” directs the Collectors to capture entire human colonies. The genetic material (liquefied human bodies) from the captured colonists is used to create a human based Reaper. It is made clear in ”Mass Effect 2” that Reapers are modeled after the organic race that constitutes a Reaper’s organic components. In the subsequent conversation that follows the revelation of the existence of a human-Reaper “larva” it is implied that the harvesting and cyclical exterminations of all sapient life committed by the Reapers is part of a “reproductive process” whereby the Reapers acquire material needed to create new Reapers. The character Legion describes ”Nazara” (”Sovereign’s” real name) as “one ship, one will, many minds”, insinuating the minds of the organics used in a Reaper’s construction are still active in their new form, though it may suggest a geth-like assembladge of programs.

At the end of Mass Effect 2, Harbinger is seen activating the dormant Reaper fleet which appears to consist of hundreds of thousands of ships which share Sovereign’s basic cuttlefish-like appearance. The fleet then appears to be moving towards the galaxy thus setting the stage for Mass Effect 3.

Vorcha

The vorcha appear in ”Mass Effect 2”. The vorcha originate from a small and overcrowded planet which has been largely stripped of natural resources by successive generations of this fast-breeding, savage species. The lack of resources has resulted in a tight-knit clan based society in which rival clans wage constant war against one another for control of scarce resources. Even as their population grows, the vorcha constantly fight each other in fierce competition over basic necessities. This constant warfare has had the twin effects of making each generation of vorcha stronger and more aggressive than its predecessor. However, their continual lack of resources have kept vorcha society extremely primitive.

The vorcha are the most short-lived sapient species currently known, with an average lifespan of only 20 years. The vorcha are known for a rather unique biology that differentiates them from other known species and which carries with it a striking set of advantages and disadvantages. The vorcha have clusters of non-differentiated cells which allow the vorcha limited regenerative abilities, as well as the ability to adapt quickly to its environment, such as developing thicker skin after being burned or increased musculature to survive in high gravity. When a vorcha is injured or in distress, these cells move to the affected area and rapidly mature to specialized forms that will alleviate the issue.

Vorcha society is built around combat. In fact, the vorcha use combat, both singly and in groups, as their default form of communication. The vorcha are a clan based people who prefer living in communal environments with others of their species to living alone or in the company of alien races. When a clan population grows too large, younger members will depart to start a new clan elsewhere. The vorcha are extremely aggressive, both against rivals of their own species and against any alien who stands in their way. Vorcha who have managed to escape their homeworld did so by hiding within the ships of spacefaring races that visited their planet in the past, and have a tendency to occupy uninhabited areas of space stations or larger spaceships. Many of them have found employment in krogan-controlled gangs.

Adapted from the Wikipedia article Races of the Mass Effect universe, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Far field methods achieve longer ranges, often multiple kilometer ranges, where the distance is much greater than the diameter of the device(s). With radio wave and optical devices the main reason for longer ranges is the fact that electromagnetic radiation in the far-field can be made to match the shape of the receiving area (using high directivity antennas or well-collimated Laser Beam) thereby delivering almost all emitted power at long ranges. The maximum directivity for antennas is physically limited by diffraction.

Beamed power, size, distance, and efficiency

The size of the components may be dictated by the distance from transmitter to receiver, the wavelength and the Rayleigh criterion or diffraction limit, used in standard radio frequency antenna design, which also applies to lasers. In addition to the Rayleigh criterion Airy’s diffraction limit is also frequently used to determine an approximate spot size at an arbitrary distance from the aperture.

The Rayleigh criterion dictates that any radio wave, microwave or laser beam will spread and become weaker and diffuse over distance; the larger the transmitter antenna or laser aperture compared to the wavelength of radiation, the tighter the beam and the less it will spread as a function of distance (and vice versa). Smaller antennae also suffer from excessive losses due to side lobes. However, the concept of laser aperture considerably differs from an antenna. Typically, a laser aperture much larger than the wavelength induces multi-moded radiation and mostly collimators are used before emitted radiation couples into a fiber or into space.

Ultimately, beamwidth is physically determined by diffraction due to the dish size in relation to the wavelength of the electromagnetic radiation used to make the beam. Microwave power beaming can be more efficient than lasers, and is less prone to atmospheric attenuation caused by dust or water vapor losing atmosphere to vaporize the water in contact.

Then the power levels are calculated by combining the above parameters together, and adding in the gains and losses due to the antenna characteristics and the transparency and dispersion of the medium through which the radiation passes. That process is known as calculating a link budget.

Radio and microwave

The earliest work in the area of wireless transmission via radio waves (electromagnetic waves) was performed by Nikola Tesla but he did not publish his work immediately. Later on, Guglielmo Marconi used a radio transmission patent from Nikola Tesla and presented as his own. Nikola Tesla appealed and after many years of court battles The United States Supreme Court awarded the radio transmission and reception patent exclusively to Nikola Tesla.

Japanese researcher Hidetsugu Yagi also investigated wireless energy transmission using a directional array antenna that he designed. In February 1926, Yagi and Uda published their first paper on the tuned high-gain directional array now known as the Yagi antenna. While it did not prove to be particularly useful for power transmission, this beam antenna has been widely adopted throughout the broadcasting and wireless telecommunications industries due to its excellent performance characteristics.

Power transmission via radio waves can be made more directional, allowing longer distance power beaming, with shorter wavelengths of electromagnetic radiation, typically in the microwave range. A rectenna may be used to convert the microwave energy back into electricity. Rectenna conversion efficiencies exceeding 95% have been realized. Power beaming using microwaves has been proposed for the transmission of energy from orbiting solar power satellites to Earth and the beaming of power to spacecraft leaving orbit has been considered.

Power beaming by microwaves has the difficulty that for most space applications the required aperture sizes are very large due to diffraction limiting antenna directionality. For example, the 1978 NASA Study of solar power satellites required a 1-km diameter transmitting antenna, and a 10& km diameter receiving rectenna, for a microwave beam at 2.45& GHz. These sizes can be somewhat decreased by using shorter wavelengths, although short wavelengths may have difficulties with atmospheric absorption and beam blockage by rain or water droplets. Because of the Thinned array curse, it is not possible to make a narrower beam by combining the beams of several smaller satellites.

For earthbound applications a large area 10& km diameter receiving array allows large total power levels to be used while operating at the low power density suggested for human electromagnetic exposure safety. A human safe power density of 1& mW/cm2 distributed across a 10& km diameter area corresponds to 750 megawatts total power level. This is the power level found in many modern electric power plants.

;High power

Wireless Power Transmission (using microwaves) is well proven. Experiments in the tens of kilowatts have been performed at Goldstone in California in 1975 and more recently (1997) at Grand Bassin on Reunion Island.

These methods achieve distances on the order of a kilometer.

Laser

In the case of electromagnetic radiation closer to visible region of spectrum (10s of microns to 10s of nm), power can be transmitted by converting electricity into a laser beam that is then pointed at a solar cell receiver.

This mechanism is generally known as “powerbeaming” because the power is beamed at a receiver that can convert it to usable electrical energy.

There are quite a few unique advantages of laser based energy transfer that outweigh the disadvantages.

#collimated monochromatic wavefront propagation allows narrow beam cross-section area for energy confinement over large ranges.

#compact size of solid state lasers-photovoltaics semiconductor diodes allows ease of integration into products with small form factors.

#ability to operate with zero radio-frequency interference to existing communication devices i.e. wi-fi and cell phones.

#control of Wireless Energy Access, instead of omnidirectional transfer where there can be no authentication before transferring energy.

These allow laser-based wireless energy transfer concept to compete with conventional energy transfer methods.

Its drawbacks are:

# Conversion to light, such as with a laser, is moderately inefficient (although quantum cascade lasers improve this)

# Conversion back into electricity is moderately inefficient, with photovoltaic cells achieving 40%-50% efficiency. (Note that conversion efficiency is rather higher with monochromatic light than with insolation of solar panels).

# Atmospheric absorption causes losses.

# As with microwave beaming, this method requires a direct line of sight with the target.

The laser “powerbeaming” technology has been mostly explored in military weapons and aerospace applications and is now being developed for commercial and consumer electronics Low-Power applications. Wireless energy transfer system using laser for consumer space has to satisfy Laser safety requirements standardized under IEC 60825.

To develop an understanding of the trade-offs of Laser (“a special type of light wave”-based system):

#Propagation of a laser beam (on how Laser beam propagation is much less affected by diffraction limits)

#Coherence and the range limitation problem (on how spatial and spectral coherence characteristics of Lasers allows better distance-to-power capabilities )

#Airy disk (on how wavelength fundamentally dictates the size of a disk with distance)

#Applications of laser diodes (on how the laser sources are utilized in various industries and their sizes are reducing for better integration)

Geoffrey Landis is one of the pioneers of solar power satellite and laser-based transfer of energy especially for space and lunar missions.

The continuously increasing demand for safe and frequent space missions has resulted in serious thoughts on a futuristic space elevator

that would be powered by lasers. NASA’s space elevator would need wireless power to be beamed to it for it to climb a tether.

NASA’s Dryden Flight Research Center has demonstrated flight of a lightweight unmanned model plane powered by a laser beam. This proof-of-concept demonstrates the feasibility of periodic recharging using the laser beam system and the lack of need to return to ground.

[http://www.lasermotive.com "Lasermotive"] demonstrated laser powerbeaming at one kilometer during NASA’s 2009 powerbeaming contest. Also [http://lhdev.com "Lighthouse DEV"] (a spin off of NASA Power Beaming Team) along with [http://www.ece.umd.edu/about"University of Maryland"] is developing an eye safe laser system to power an small UAV. Since 2006, [http://www.powerbeaminc.com "PowerBeam"] which originally invented the eye-safe technology and holds all crucial patents in this technology space, is developing commercially ready units for various consumer and industrial electronic products.

Electrical conduction

Adapted from the Wikipedia article Wireless energy transfer, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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The location of colonization can be on a physical body or free-flying:

* On a planet, natural satellite, or asteroid

* In orbit around the Earth, Sun, Lagrangian point or other object

Planetary locations

Some planetary colonization advocates cite the following potential locations:

Mars

The surface of Mars is about the same size as the dry land surface of Earth. The ice in Mars’ south polar cap, if spread over the planet, would be a layer 12 meters (39& feet) thick and there is carbon (locked as carbon dioxide in the atmosphere).

Mars may have gone through similar geological and hydrological processes as Earth and therefore contain valuable mineral ores. Equipment is available to extract ”in situ” resources (e.g., water, air) from the Martian ground and atmosphere. There is interest in colonizing Mars in part because life could have existed on Mars at some point in its history, and may even still exist in some parts of the planet.

However, its atmosphere is very thin (averaging 800 Pa or about 0.8% of Earth sea-level atmospheric pressure); so the pressure vessels necessary to support life are very similar to deep space structures. The climate of Mars is colder than Earth’s. Its gravity is only around a third that of Earth’s; it is unknown whether this is sufficient to support human beings for extended periods (all long-term human experience to date has been at around Earth gravity or one g).

The atmosphere is thin enough, when coupled with Mars’ lack of magnetic field, that radiation is more intense on the surface, and protection from solar storms would require radiation shielding.

Locations in space would necessitate a space habitat, also called space colony and orbital colony, or a space station which would be intended as a permanent settlement rather than as a simple waystation or other specialized facility. They would be literal “cities” in space, where people would live and work and raise families. Many designs have been proposed with varying degrees of realism by both science fiction authors and scientists.

A space habitat would serve as a proving ground for a generation ship which could function as a long-term home for hundreds or thousands of people. Such a space habitat could be isolated from the rest of humanity but near enough to Earth for help. This would test if thousands of humans can survive on their own before sending them beyond the reach of help.

Earth orbit

Compared to other locations, Earth orbit has substantial advantages and one major, but solvable, problem. Orbits close to Earth can be reached in hours, whereas the Moon is days away and trips to Mars take months. There is ample continuous solar power in high Earth orbits, whereas all planets lose sunlight at least half the time. Weightlessness makes construction of large colonies considerably easier than in a gravity environment. Astronauts have demonstrated moving multi-ton satellites by hand. 0g recreation is available on orbital colonies, but not on the Moon or Mars. Finally, the level of (pseudo-) gravity is controlled at any desired level by rotating an orbital colony. Thus, the main living areas can be kept at 1 g, whereas the Moon has 1/6 g and Mars 1/3 g. It’s not known what the minimum g-force is for ongoing health but 1 g is known to ensure that children grow up with strong bones and muscles.

The main disadvantage of orbital colonies is lack of materials. These may be expensively imported from the Earth, or more cheaply from extraterrestrial sources, such as the Moon (which has ample metals, silicon, and oxygen), Near Earth Asteroids, comets, or elsewhere. Other disadvantages of orbital colonies are orbital decay, and atmospheric pollution in the case of Earth.

As of 2009, the International Space Station provides a temporary, yet still non-autonomous, human presence in Low Earth orbit.

Lagrange points

Another near-Earth possibility are the five Earth-Moon Lagrange points. Although they would generally also take a few days to reach with current technology, many of these points would have near-continuous solar power capability since their distance from Earth would result in only brief and infrequent eclipses of light from the Sun.

The five Earth-Sun Lagrange points would totally eliminate eclipses, but only and would be reachable in a few days’ time. The other three Earth-Sun points would require months to reach.

However, the fact that Lagrange points and tend to collect dust and debris, while – require active station-keeping measures to maintain a stable position, make them somewhat less suitable places for habitation than was originally believed. Additionally, the orbit of – takes them out of the protection of the Earth’s magnetosphere for approximately two-thirds of the time, exposing them to the health threat from cosmic rays.

Statites

Statites or “static satellites” employ solar sails to position themselves in orbits that gravity alone could not accomplish. Such a solar sail colony would be free to ride solar radiation pressure and travel off the ecliptic plane. Navigational computers with an advanced understanding of flocking behavior could organize several statite colonies into the beginnings of the true “swarm” concept of a Dyson sphere.

Adapted from the Wikipedia article Space colonization, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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There are a variety of types of solar cookers: over 65 major designs and hundreds of variations of them. The basic principles of all solar cookers are:

* Concentrating sunlight: Some device, usually a mirror or some type of reflective metal, is used to concentrate light and heat from the sun into a small cooking area, making the energy more concentrated and therefore more potent.

* Converting light to heat: Any black on the inside of a solar cooker, as well as certain materials for pots, will improve the effectiveness of turning light into heat. A black pan will absorb almost all of the sun’s light and turn it into heat, substantially improving the effectiveness of the cooker. Also, the better a pan conducts heat, the faster the oven will work.

* Trapping heat: Isolating the air inside the cooker from the air outside the cooker makes an important difference. Using a clear solid, like a plastic bag or a glass cover, will allow light to enter, but once the light is absorbed and converted to heat, a plastic bag or glass cover will trap the heat inside. This makes it possible to reach similar temperatures on cold and windy days as on hot days.

* Plastic Sheet: Uses plastic sheets to assure that liquids do not seep through into the oven. Also to prevent staining of the underlying sheet in the oven.

Alone, each of these strategies for cooking something using solar energy is fairly ineffective, but most solar cookers use two or all three of these strategies in combination to get temperatures sufficient for cooking.

The top can usually be removed to allow dark pots containing food to be placed inside. One or more reflectors of shiny metal or foil-lined material may be positioned to bounce extra light into the interior of the oven chamber. Cooking containers and the inside bottom of the cooker should be dark-colored or black. Inside walls should be reflective to reduce radiative heat loss and bounce the light towards the pots and the dark bottom, which is in contact with the pots.

Box cookers

The inside insulator for the solar box cooker has to be able to withstand temperatures up to 150°C (300 °F) without melting or off-gassing. Crumpled newspapers, wool, rags, dry grass, sheets of cardboard, etc. can be used to insulate the walls of the cooker, but since most of the heat escapes through the top glass or plastic, very little insulation in the walls is necessary. The transparent top is either glass, which is durable but hard to work with, or an oven cooking bag, which is lighter, cheaper, and easier to work with, but less durable. If dark pots and/or bottom trays cannot be located, these can be darkened either with flat-black spray paint (one that is non-toxic when warmed), black tempera paint, or soot from a fire.

The solar box cooker typically reaches a temperature of 150 °C (300 °F). This is not as hot as a standard oven, but still hot enough to cook food over a somewhat longer period of time. Food containing a lot of moisture cannot get much hotter than 100 °C (212 °F) in any case, so it is not always necessary to cook at the high temperatures indicated in standard cookbooks. Because the food does not reach too high a temperature, it can be safely left in the cooker all day without burning. It is best to start cooking before noon, though. Depending on the latitude and weather, food can be cooked either early or later in the day. The cooker can be used to warm food and drinks and can also be used to pasteurize water or milk. If you use an indoor stove for your actual cooking, you can save significant fuel by using the solar cooker to preheat the water to be used for cooking grains, soups, etc., to nearly boiling.

Solar box cookers can be made of locally available materials or be manufactured in a factory for sale. They range from small cardboard devices, suitable for cooking a single meal when the sun is shining, to wood and glass boxes built into the sunny side of a house. Although invented by Horace de Saussure, a Swiss naturalist, as early as 1767, solar box cookers have only gained popularity since the 1970s. These surprisingly simple and useful appliances are seen in growing numbers in almost every country of the world. An index of detailed wiki pages for each country can be found here.

Panel cookers

Panel solar cookers are very inexpensive solar cookers that use shiny panels to direct sunlight to a cooking pot that is enclosed in a clear plastic bag. A common model is the CooKit. Developed in 1994 by Solar Cookers International, it is often produced locally by pasting a reflective material, such as aluminum foil, onto a cut and folded backing, usually corrugated cardboard. It is lightweight and folds for storage. When completely unfolded, it measures about three feet by four feet (1 m by 1.3 m). Using materials purchased in bulk, the typical cost is about US$5. However, CooKits can also be made entirely from reclaimed materials, including used cardboard boxes and foil from the inside of cigarette boxes.

The CooKit is considered a low-to-moderate temperature solar cooker, easily reaching temperatures high enough to pasteurize water or cook grains such as rice. On a sunny day, one CooKit can collect enough solar energy to cook rice, meat or vegetables to feed a family with up to three or four children. Larger families use two or more cookers.

To use a panel cooker, it is folded into a bowl shape. Food is placed in a dark-colored pot, covered with a tightly fitted lid. The pot is placed in a clear plastic bag and tied, clipped, or folded shut. The panel cooker is placed in direct sunlight until the food is cooked, which usually requires several hours for a full family-sized meal. For faster cooking, the pot can be raised on sticks or wires to allow the heated air to circulate underneath it.

High-temperature plastic bags (oven roasting bags) can be re-used for more than a month, but any plastic bag will work, if measures (such as sticks or wires) are taken to keep the bag from touching the hot cooking pot and melting to it. The purpose of the plastic bag is to trap heated air next to the pot; it may not be needed on very bright, windless days.

A recent development is the HotPot developed by US NGO Solar Household Energy, Inc. The cooking vessel in this cooker is a large clear pot with a clear lid into which a dark pot is suspended. This design has the advantage of very even heating since the sun is able to shine onto the sides and the bottom of the pot during cooking. An added advantage is that the clear lid allows the food to be observed while it is cooking without removing the lid. The HotPot provides an alternative to using plastic bags in a panel cooker.

Solar kettles

Solar kettles are solar thermal devices that can heat water to boiling point through the reliance on solar energy alone. Some of them use evacuated solar glass tube technology to capture, accumulate and store solar energy needed to power the kettle. Besides heating liquids, since the stagnating temperature of solar vacuum glass tubes is a high 220 °C (425 °F), solar kettles can also deliver dry heat and function as ovens and autoclaves. Moreover, since solar vacuum glass tubes work on accumulated rather than concentrated solar thermal energy, solar kettles only need diffused sunlight to work and needs no sun tracking at all. If solar kettles use solar vacuum tubes technologies, the vacuum insulating properties will keep previously heated water hot throughout the night.

Cookers with parabolic reflectors

Although these types of solar cookers can cook as well as a conventional oven, they are difficult to construct. Parabolic cookers reach high temperatures and cook quickly, but require frequent adjustment and supervision for safe operation. Several hundred thousand exist, mainly in China. They are especially useful for large-scale institutional cooking.

Parabolic reflectors that have their centres of mass coincident with their focal points are useful. They can be easily turned, to follow the sun’s motions in the sky, rotating about an axis that passes through the focus. The cooking pot therefore stays stationary. If the paraboloid is axially symmetrical and is made of material of uniform thickness, this condition occurs if the depth of the paraboloid is 1.8478 times its focal length.

Using two parabolic troughs to simulate a paraboloid

It is possible to use two parabolic troughs, held with their axes perpendicular but not co-planar, to bring sunlight to a point focus as does a paraboloidal reflector. The incoming light strikes one of the troughs, which sends it toward a line focus. The second trough intercepts the converging light and focuses it to a point. A diagram that shows the principle is at:

http://kmr.nada.kth.se/files/pointfocus/PointFocus/PointFocus-cyl-1+2-rays.jpg

Compared with a single paraboloid, using two partial troughs has important advantages. The troughs are “single curves”, which can be made by bending a sheet of metal without any need for cutting, crumpling, or stretching. Also, the light that reaches the target – the cooking pot – is directed approximately downward, which reduces the danger of damage to the eyes of anyone nearby. On the other hand, there are disadvantages. More mirror material is needed, increasing the cost, and the light is reflected by two surfaces instead of one, which inevitably increases the amount that is lost.

Experimental arrangements of this kind have been made, and have worked well. The two troughs have been held in a fixed orientation relative to each other by being both fixed to a wooden frame, The whole assembly of frame and troughs has to be moved to track the sun as it moves in the sky. http://kmr.nada.kth.se/files/pointfocus/pics/Mirror-cradle.jpg

However, this idea does not yet seem to have been tried in a practical cooker.

Cookers with spherical reflectors

The Solar Bowl is a unique concentrating technology used by the Solar Kitchen in Auroville, India. Unlike nearly all concentrating technologies that use tracking reflector systems, the solar bowl uses a stationary spherical reflector. This reflector focuses light along a line perpendicular to the sphere’s surface and a computer control system moves the receiver to intersect this line. Steam is produced in the solar bowl’s receiver at temperatures reaching 150& °C and then used for process heat in the kitchen where 2,000 meals are prepared daily.

Hybrid cookers

A hybrid solar oven is a solar box cooker equipped with a conventional electrical heating element for cloudy days or nighttime cooking. Hybrid solar ovens are therefore more independent. However, they lack the cost advantages of some other types of solar cookers, and so they have not caught on as much in third world countries where electricity or fuel sources simply do not exist.

A hybrid solar grill consists of an adjustable parabolic reflector suspended in a tripod with a movable grill surface. These outperform solar box cookers in temperature range and cooking times. When solar energy is not available, the design uses any conventional fuel as a heat source, including gas, electricity, or wood.

Adapted from the Wikipedia article Solar cooker, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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On freight corridor, the electrification of Bafgh—Bandarabbas in Persian Gulf is planned as BOT Build-Operate-Transfer base with future continuation to Tehran and Intermodal freight transport at both ends plus the Classification yard.

The rate of return on investment would be very acceptable especially for international investment List of countries by gross fixed investment as percentage of GDP because of enough demand between Bandarabbas to Tehran with only 10% share of railway freight train because of capacity limitation.

Bandarabbas is close to Gheshm Island that has 100& km length and could be connected by bridge or tunnel (like the Channel Tunnel) or Marmaray in Turkey.

One of the big advantages of this line is the big loading gauge and axle load that increases the productivity of the rail transport especially for intermodal freight transport by Well Car for Double stack container from port Shahid Rajaee [http://www.shahidrajaeeport.ir/].

This line also can help to transit oil as a good start to the planned transit pipeline transport.

Iron ore is one of the main commodities on this line, especially from Golgohar mine near Sirjan, Choghart and Chadormaloo near Bafgh to Isfahan for two steel plants and steel mills named Zobahan (Esfahan Steel Company) and Mobarakeh.

The possible use of roadrailers or rolling highway are some other options.

As the rail connection with Zahedan will be inaugurated in near future, it will make it possible to have a direct transit line from Central Asia to Pakistan and India possibly with variable gauge bogies and could be completed with a link to Chabahar port in the south east of the country.

The business plan to justify the electrification and supplying locomotives is going to be prepared.

Cost effective projects

For low density lines by doing cost-benefit analysis it is considered to use Dc elec. loco and the power from city substations for more Cost effectiveness of projects.

For Lorestan province that there is more than 60& km tunnel, the DC diesel locomotive will be converted to electric.

For northern line (that has the grade of 3%) to Mazandaran and Golestan provinces one of the solutions could be third rail for using the existing diesel locomotives and dual-mode transit.

The local production capability in electric energy sector makes the cost effectiveness better. For example the contact wire is produced a locally and also different insulators and there is god companies to wind transformers.

With electrification, the need for shunting will be reduced and the Switcher locomotive as well.

To improve the economic result of electrification it is foreseen to convert the diesel locomotives to electric one both in DC like GE E60 [http://www.rrmuseumpa.org/about/roster/e60.shtml] and AC like AD43C.

Value chain is a theory that could be used for this.

Electrification investment

For approving the electric locomotive or EMU it is mandatory to have some tests in real conditions in test sites like Wildenrath in Germany owned by Siemens [http://www.mobility.siemens.com/mobility/en/pub/urban_mobility/rail_solutions/service/testcenter_for_railway_systems/test_und_validationcenter/railway_track.htm], Old Dalby Test Track in England and Velim railway test circuit in the Czech Republic or Transportation Technology Center in USA [http://www.aar.com/] .

Electrification Simulation

Electrification simulation and rail transport modelling is important to be sure about the results that could be expected.

Electrification prototyping

To ensure the electrification leads to considered results the pilot experiment project, as a pilot plant, prototype and mockup can help for evaluation and specially verification and validation phases.

Electrification machinery

Electrification machinery are those equipments that are used to perform, check and monitor the overhead systems in railway like [http://www.plassertheurer.com/en/production_range/machines_for_renewal.htm ''Machines for Renewal and Installation of Catenary''], Doctor Yellow in Japan railway or NMT New Measurement Train in British Rail.

Electrification & metro links

With electrification the passenger traffic will increase dramatically and to cope with the new demand it is important to have better connection to other mode of transports like Tehran Metro, Mashhad metro…

International Rankings of Iran in Transport defines the situation of railway with other modes.

Electrification effect on safety

The electrification generally could have a reverse effect on safety but the consequences of it may increase safety a lot specially by increasing reliability as the first element of RAMS (reliability, availability, maintainability & safety), for example we can refer to the accident in Neishabur as in Nishapur train disaster in List of rail accidents and derailment (Classification of railway accidents) in tight curves and high cant could be reduced by electrification.

We have to remember the effect of Scheffel Meter-Gauge Bogie in South African railway with Radial axle like Radial steering truck in locomotives with radial bogie as well.

Railway Electrification Education

For higher education, it is considered to have MS level in railway faculty in IRAN University of science and technology.[http://www.iust.ac.ir/index.php?slc_lang=en&sid=18]

Adapted from the Wikipedia article Railway electrification in Iran, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Comparison with internal combustion engines

In contrast to internal combustion engines, Stirling engines have the potential to use renewable heat sources more easily, to be quieter, and to be more reliable with lower maintenance. They are preferred for applications that value these unique advantages, particularly if the cost per unit energy generated ($/kWh) is more important than the capital cost per unit power ($/kW). On this basis, Stirling engines are cost competitive up to about 100& kW.

Compared to an internal combustion engine of the same power rating, Stirling engines currently have a higher capital cost and are usually larger and heavier. However, they are more efficient than most internal combustion engines. Their lower maintenance requirements make the overall ”energy” cost comparable. The thermal efficiency is also comparable (for small engines), ranging from 15% to 30%. For applications such as micro-CHP, a Stirling engine is often preferable to an internal combustion engine. Other applications include water pumping, astronautics, and electrical generation from plentiful energy sources that are incompatible with the internal combustion engine, such as solar energy, and biomass such as agricultural waste and other waste such as domestic refuse. Stirlings have also been used as a marine engine in Swedish Gotland class submarines. However, Stirling engines are generally not price-competitive as an automobile engine, due to high cost per unit power, low power density and high material costs.

Basic analysis is based on the closed-form Schmidt analysis.

Advantages

* Stirling engines can run directly on any available heat source, not just one produced by combustion, so they can run on heat from solar, geothermal, biological, nuclear sources or waste heat from industrial processes.

* A continuous combustion process can be used to supply heat, so most types of emissions can be reduced.

* Most types of Stirling engines have the bearing and seals on the cool side of the engine, and they require less lubricant and last longer than other reciprocating engine types.

* The engine mechanisms are in some ways simpler than other reciprocating engine types. No valves are needed, and the burner system can be relatively simple. Crude Stirling engines can be made using common household materials.

* A Stirling engine uses a single-phase working fluid which maintains an internal pressure close to the design pressure, and thus for a properly designed system the risk of explosion is low. In comparison, a steam engine uses a two-phase gas/liquid working fluid, so a faulty release valve can cause an explosion.

* In some cases, low operating pressure allows the use of lightweight cylinders.

* They can be built to run quietly and without an air supply, for air-independent propulsion use in submarines.

* They start easily (albeit slowly, after warmup) and run more efficiently in cold weather, in contrast to the internal combustion which starts quickly in warm weather, but not in cold weather.

* A Stirling engine used for pumping water can be configured so that the water cools the compression space. This is most effective when pumping cold water.

* They are extremely flexible. They can be used as CHP (combined heat and power) in the winter and as coolers in summer.

* Waste heat is easily harvested (compared to waste heat from an internal combustion engine) making Stirling engines useful for dual-output heat and power systems.

Disadvantages ==

Size and cost issues

* Stirling engine designs require heat exchangers for heat input and for heat output, and these must contain the pressure of the working fluid, where the pressure is proportional to the engine power output. In addition, the expansion-side heat exchanger is often at very high temperature, so the materials must resist the corrosive effects of the heat source, and have low creep. Typically these material requirements substantially increase the cost of the engine. The materials and assembly costs for a high temperature heat exchanger typically accounts for 40% of the total engine cost.

* All thermodynamic cycles require large temperature differentials for efficient operation. In an external combustion engine, the heater temperature always equals or exceeds the expansion temperature. This means that the metallurgical requirements for the heater material are very demanding. This is similar to a Gas turbine, but is in contrast to an Otto engine or Diesel engine, where the expansion temperature can far exceed the metallurgical limit of the engine materials, because the input heat source is not conducted through the engine, so engine materials operate closer to the average temperature of the working gas.

* Dissipation of waste heat is especially complicated because the coolant temperature is kept as low as possible to maximize thermal efficiency. This increases the size of the radiators, which can make packaging difficult. Along with materials cost, this has been one of the factors limiting the adoption of Stirling engines as automotive prime movers. For other applications such as ship propulsion and stationary microgeneration systems using combined heat and power (CHP) high power density is not required.

Power and torque issues

* Stirling engines, especially those that run on small temperature differentials, are quite large for the amount of power that they produce (i.e., they have low specific power). This is primarily due to the heat transfer coefficient of gaseous convection which limits the heat flux that can be attained in a typical cold heat exchanger to about 500& W/(m2·K), and in a hot heat exchanger to about 500–5000& W/(m2·K). Compared with internal combustion engines, this makes it more challenging for the engine designer to transfer heat into and out of the working gas. Because of the Thermal efficiency the required heat transfer grows with lower temperature difference, and the heat exchanger surface (and cost) for 1kW output grows with second power of 1/deltaT. Therefore the specific cost of very low temperature difference engines is very high. Increasing the temperature differential and/or pressure allows Stirling engines to produce more power, assuming the heat exchangers are designed for the increased heat load, and can deliver the convected heat flux necessary.

* A Stirling engine cannot start instantly; it literally needs to “warm up”. This is true of all external combustion engines, but the warm up time may be longer for Stirlings than for others of this type such as steam engines. Stirling engines are best used as constant speed engines.

* Power output of a Stirling tends to be constant and to adjust it can sometimes require careful design and additional mechanisms. Typically, changes in output are achieved by varying the displacement of the engine (often through use of a swashplate crankshaft arrangement), or by changing the quantity of working fluid, or by altering the piston/displacer phase angle, or in some cases simply by altering the engine load. This property is less of a drawback in hybrid electric propulsion or “base load” utility generation where constant power output is actually desirable.

Gas choice issues

The used gas should have a low heat capacity, so that a given amount of transferred heat leads to a large increase in pressure. Considering this issue, helium would be the best gas because of its very low heat capacity. Air is a viable working fluid, but the oxygen in a highly pressurized air engine can cause fatal accidents caused by lubricating oil explosions. Following one such accident Philips pioneered the use of other gases to avoid such risk of explosions.

* Hydrogen’s low viscosity and high thermal conductivity make it the most powerful working gas, primarily because the engine can run faster than with other gases. However, due to hydrogen absorption, and given the high diffusion rate associated with this low molecular weight gas, particularly at high temperatures, H2 will leak through the solid metal of the heater. Diffusion through carbon steel is too high to be practical, but may be acceptably low for metals such as aluminum, or even stainless steel. Certain ceramics also greatly reduce diffusion. Hermetic pressure vessel seals are necessary to maintain pressure inside the engine without replacement of lost gas. For high temperature differential (HTD) engines, auxiliary systems may need to be added to maintain high pressure working fluid. These systems can be a gas storage bottle or a gas generator. Hydrogen can be generated by electrolysis of water, the action of steam on red hot carbon-based fuel, by gasification of hydrocarbon fuel, or by the reaction of acid on metal. Hydrogen can also cause the embrittlement of metals. Hydrogen is a flammable gas, which is a safety concern if released from the engine.

* Most technically advanced Stirling engines, like those developed for United States government labs, use helium as the working gas, because it functions close to the efficiency and power density of hydrogen with fewer of the material containment issues. Helium is inert, which removes all risk of flammability, both real and perceived. Helium is relatively expensive, and must be supplied as bottled gas. One test showed hydrogen to be 5% (absolute) more efficient than helium (24% relatively) in the GPU-3 Stirling engine. The researcher Allan Organ demonstrated that a well-designed air engine is theoretically just as ”efficient” as a helium or hydrogen engine, but helium and hydrogen engines are several times more ”powerful per unit volume”.

* Some engines use air or nitrogen as the working fluid. These gases have much lower power density (which increases engine costs), but they are more convenient to use and they minimize the problems of gas containment and supply (which decreases costs). The use of compressed air in contact with flammable materials or substances such as lubricating oil, introduces an explosion hazard, because compressed air contains a high partial pressure of oxygen. However, oxygen can be removed from air through an oxidation reaction or bottled nitrogen can be used, which is nearly inert and very safe.

* Other possible lighter-than-air gases include: methane, and ammonia.

Adapted from the Wikipedia article Stirling engine, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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Advantages

Economics

The major advantage of hydroelectricity is elimination of the cost of fuel. The cost of operating a hydroelectric plant is nearly immune to increases in the cost of fossil fuels such as oil, natural gas or coal, and no imports are needed.

Hydroelectric plants also tend to have longer economic lives than fuel-fired generation, with some plants now in service which were built 50 to 100 years ago. Operating labor cost is also usually low, as plants are automated and have few personnel on site during normal operation.

Where a dam serves multiple purposes, a hydroelectric plant may be added with relatively low construction cost, providing a useful revenue stream to offset the costs of dam operation. It has been calculated that the sale of electricity from the Three Gorges Dam will cover the construction costs after 5 to 8 years of full generation.

CO2 emissions

Since hydroelectric dams do not burn fossil fuels, they do not directly produce carbon dioxide. While some carbon dioxide is produced during manufacture and construction of the project, this is a tiny fraction of the operating emissions of equivalent fossil-fuel electricity generation. One measurement of greenhouse gas related and other externality comparison between energy sources can be found in the ExternE project by the Paul Scherrer Institut and the University of Stuttgart which was funded by the European Commission. According to this project, hydroelectricity produces the least amount of greenhouse gases and externality of any energy source. Coming in second place was wind, third was nuclear energy, and fourth was solar photovoltaic. The extremely positive greenhouse gas impact of hydroelectricity is found especially in temperate climates. The above study was for local energy in Europe; presumably similar conditions prevail in North America and Northern Asia, which all see a regular, natural freeze/thaw cycle (with associated seasonal plant decay and regrowth).

Other uses of the reservoir

Reservoirs created by hydroelectric schemes often provide facilities for water sports, and become tourist attractions themselves. In some countries, aquaculture in reservoirs is common. Multi-use dams installed for irrigation support agriculture with a relatively constant water supply. Large hydro dams can control floods, which would otherwise affect people living downstream of the project.

Disadvantages =

Ecosystem damage and loss of land

Large reservoirs required for the operation of hydroelectric power stations result in submersion of extensive areas upstream of the dams, destroying biologically rich and productive lowland and riverine valley forests, marshland and grasslands. The loss of land is often exacerbated by the fact that reservoirs cause habitat fragmentation of surrounding areas.

Hydroelectric projects can be disruptive to surrounding aquatic ecosystems both upstream and downstream of the plant site. For instance, studies have shown that dams along the Atlantic and Pacific coasts of North America have reduced salmon populations by preventing access to spawning grounds upstream, even though most dams in salmon habitat have fish ladders installed. Salmon spawn are also harmed on their migration to sea when they must pass through turbines. This has led to some areas transporting smolt downstream by barge during parts of the year. In some cases dams, such as the Marmot Dam, have been demolished due to the high impact on fish. Turbine and power-plant designs that are easier on aquatic life are an active area of research. Mitigation measures such as fish ladders may be required at new projects or as a condition of re-licensing of existing projects.

Generation of hydroelectric power changes the downstream river environment. Water exiting a turbine usually contains very little suspended sediment, which can lead to scouring of river beds and loss of riverbanks. Since turbine gates are often opened intermittently, rapid or even daily fluctuations in river flow are observed. For example, in the Grand Canyon, the daily cyclic flow variation caused by Glen Canyon Dam was found to be contributing to erosion of sand bars. Dissolved oxygen content of the water may change from pre-construction conditions. Depending on the location, water exiting from turbines is typically much warmer than the pre-dam water, which can change aquatic faunal populations, including endangered species, and prevent natural freezing processes from occurring. Some hydroelectric projects also use canals to divert a river at a shallower gradient to increase the head of the scheme. In some cases, the entire river may be diverted leaving a dry riverbed. Examples include the Tekapo and Pukaki Rivers in New Zealand.

Flow shortage

Changes in the amount of river flow will correlate with the amount of energy produced by a dam. Lower river flows because of drought, climate change or upstream dams and diversions will reduce the amount of live storage in a reservoir therefore reducing the amount of water that can be used for hydroelectricity. The result of diminished river flow can be power shortages in areas that depend heavily on hydroelectric power.

Methane emissions (from reservoirs)

Lower positive impacts are found in the tropical regions, as it has been noted that the reservoirs of power plants in tropical regions may produce substantial amounts of methane. This is due to plant material in flooded areas decaying in an anaerobic environment, and forming methane, a very potent greenhouse gas. According to the World Commission on Dams report, where the reservoir is large compared to the generating capacity (less than 100 watts per square metre of surface area) and no clearing of the forests in the area was undertaken prior to impoundment of the reservoir, greenhouse gas emissions from the reservoir may be higher than those of a conventional oil-fired thermal generation plant. Although these emissions represent carbon already in the biosphere, not fossil deposits that had been sequestered from the carbon cycle, there is a greater amount of methane due to anaerobic decay, causing greater damage than would otherwise have occurred had the forest decayed naturally.

In boreal reservoirs of Canada and Northern Europe, however, greenhouse gas emissions are typically only 2% to 8% of any kind of conventional fossil-fuel thermal generation. A new class of underwater logging operation that targets drowned forests can mitigate the effect of forest decay.

In 2007, International Rivers accused hydropower firms for cheating with fake carbon credits under the Clean Development Mechanism, for hydropower projects already finished or under construction at the moment they applied to join the CDM. These carbon credits – of hydropower projects under the CDM in developing countries – can be sold to companies and governments in rich countries, in order to comply with the Kyoto protocol.

Relocation

Another disadvantage of hydroelectric dams is the need to relocate the people living where the reservoirs are planned. In February 2008, it was estimated that 40-80 million people worldwide had been physically displaced as a direct result of dam construction. In many cases, no amount of compensation can replace ancestral and cultural attachments to places that have spiritual value to the displaced population. Additionally, historically and culturally important sites can be flooded and lost.

Such problems have arisen at the Aswan Dam in Egypt between 1960 and 1980, the Three Gorges Dam in China, the Clyde Dam in New Zealand, and the Ilisu Dam in Turkey.

Failure hazard

Because large conventional dammed-hydro facilities hold back large volumes of water, a failure due to poor construction, terrorism, or other causes can be catastrophic to downriver settlements and infrastructure. Dam failures have been some of the largest man-made disasters in history. Also, good design and construction are not an adequate guarantee of safety. Dams are tempting industrial targets for wartime attack, sabotage and terrorism, such as Operation Chastise in World War II.

The Banqiao Dam failure in Southern China directly resulted in the deaths of 26,000 people, and another 145,000 from epidemics. Millions were left homeless. Also, the creation of a dam in a geologically inappropriate location may cause disasters like the one of the Vajont Dam in Italy, where almost 2000 people died, in 1963.

Smaller dams and micro hydro facilities create less risk, but can form continuing hazards even after they have been decommissioned. For example, the small Kelly Barnes Dam failed in 1967, causing 39 deaths with the Toccoa Flood, ten years after its power plant was decommissioned in 1957.

Comparison with other methods of power generation

Hydroelectricity eliminates the flue gas emissions from fossil fuel combustion, including pollutants such as sulfur dioxide, nitric oxide, carbon monoxide, dust, and mercury in the coal. Hydroelectricity also avoids the hazards of coal mining and the indirect health effects of coal emissions. Compared to nuclear power, hydroelectricity generates no nuclear waste, has none of the dangers associated with uranium mining, nor nuclear leaks. Unlike uranium, hydroelectricity is also a renewable energy source.

Compared to wind farms, hydroelectricity power plants have a more predictable load factor. If the project has a storage reservoir, it can be dispatched to generate power when needed. Hydroelectric plants can be easily regulated to follow variations in power demand.

Unlike fossil-fuelled combustion turbines, construction of a hydroelectric plant requires a long lead-time for site studies, hydrological studies, and environmental impact assessment. Hydrological data up to 50 years or more is usually required to determine the best sites and operating regimes for a large hydroelectric plant. Unlike plants operated by fuel, such as fossil or nuclear energy, the number of sites that can be economically developed for hydroelectric production is limited; in many areas the most cost effective sites have already been exploited. New hydro sites tend to be far from population centers and require extensive transmission lines. Hydroelectric generation depends on rainfall in the watershed, and may be significantly reduced in years of low rainfall or snowmelt. Long-term energy yield may be affected by climate change. Utilities that primarily use hydroelectric power may spend additional capital to build extra capacity to ensure sufficient power is available in low water years.

Adapted from the Wikipedia article Hydroelectricity, under the G. N. U. Free Documentation License. Please also see http://en.wikipedia.org/wiki

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