2015-04-01
1201
Generating an Electric Current
The first method used in producing an electric current was chemical in nature. Credit for its discovery is given to an Italian physician named Aloisio Galvani One day while engaged in dissecting a frog, Galvani noticed the leg muscles contract whenever a nearby electric machine was in operation. Further investigation showed the same twitching effect to be obtained by simply connecting the nerve and muscle of the leg to dissimilar metals. But no such result was obtained if only one metal was used or if non-conductors were employed. There were obviously two possible sources of the phenomenon. Hither the current was set up at the junction of the two metals or it was a property of the animal tissues. Galvani favoured the latter view and in 1791 announced his discovery, attributing the current to what he called "animal electricity". The scientist is known to become so prejudiced in favour of his animal magnetism theory that it was quite impossible for him to view objectively later evidence which definitely contradicted it and finally caused it to be discarded.
Another Italian, Alessandro Volta, a professor of physics in the University of Pavia, established the true source of the electric current. He demonstrated that it could be produced by the action of dissimilar metals without the presence of animal tissue of any sort.
In the course of his experiments in 1800 he developed the first electric battery, a device known as a voltaic pile. Although he tried a number of different materials he found that the best results were obtained when he used silver and zink as the two metals. The pile consisted of a series of small discs of these and of cardboard, the latter having been soaked in a salt solution. Then he piled the discs up one on another in the order silver, zink, cardboard, and so forth, ending with zinc. By connecting wires to the top and bottom discs he was able to get continuous electric currents which were of substantial size.
All the essentials of a modern electric cell or battery were presented in the voltaic pile. Developments since (hat time have been largely directed toward making cells more convenient to use and toward eliminating various undesirable chemical reactions.
Примечания:
credit for its discovery is given - честь его открытия принадлежит
twitching effect - эффект сокращения мышц
animal tissues - живая ткань
Faraday's Discoveries
Michael Faraday, who was born in 1791 and died in 1867, gathered together and set in order all the work of the scientists who had worked on electrical problems before him.
In 1823 he discovered how to make an electrical motor. In 1831 he built the first generator, then called it dynamo. The modern car has both a starting motor and a generator. The starting motor draws electric current from the car battery to start the powerful gasoline engine. The generator is driven by the gasoline engine to recharge the battery and to furnish electrical power for all the electrical conveniences in the car.
Faraday's experiments of August 29, 1831, gave us the principle of the electric transformer, without which the later discoveries of that fateful year could have little real practical application. For to convey the electric current over long distances, say to supply a town, or feed an electric railway, it is necessary to generate it at a very high voltage, or force. By means of transformers based on Faraday's induction coil discovery, it is simple for a current direct from a power-station of say 132.000 volts to be stepped down for the electric train to 600 volts and for household use to 240 volts. The procedure is quite simple. The current is fed into the transformer across the primary, or input coil, which corresponds to Faraday's right-hand coil on his induction ring. The resultant induced current is taken from the secondary, of output coil, which corresponds to Faraday's left-hand coil. If this secondary coil has more windings of wire than the primary coil, the voltage will be stepped down.
So the two related discoveries of 1831 provided not only the means of making electricity easily and cheaply, on as large a scale as required, without any cumbersome batteries, but also the way of using it in a safe and practical way.
In 1833 Faraday discovered the effects of passing an electric current through certain solutions, he called this effects the laws of electrolysis. This has made possible the refinement of metals, silver and gold plating, and the manufacture of many chemical products.
Electromagnetic Machines
Before Faraday's discoveries the only usable source of electricity was the galvanic battery, and it made possible some practical applications, including the electric light and the electric telegraph. But the practical supply of electricity on a large scale was only possible by the development of electromagnetic machines, generators and transformers. For the use of electricity to produce mechanical power where it is wanted, another electromagnetic machine — the electric motor — still remains the most effective method.
What made all this possible? It needed not only the discovery and understanding of the basic laws (by Faraday), but also the discovery of materials with suitable properties. It is really very fortunate, that high magnetic fields can be sustained in a material as cheap as iron. Without iron, the whole economics of electromagnetic machines and of electrical-power applications would be quite different.
The electromagnetic machine is still developing in other respects. Using iron, it is cheap to produce the magnetic field, but an important limitation is imposed by saturation. This limit can be overcome by using superconductors at very low temperatures to carry very high currents and produce much stronger magnetic fields — without using iron. This development opens up a new field for machine designs and applications, and it offers a different set of limits from those of the copper-iron machine. Nevertheless, the copper-iron machine is so simple and reliable that it is likely to continue for a very longtime as the main method of producing mechanical power.
For many applications, the dominant factors are not efficiency and power/weight ratio but convenience and cleanliness, and with electricity one is really buying convenience rather than power. It seems likely that the main advances in domestic applications will be by developments of control and programming to give even greater convenience, a good present example being the automatic washing machine. The electric motor is a superb machine to provide power, and its applications must expand for that reason alone.
Примечание:
power/weight ratio - мощность на единицу веса (двигателя)
a voltaic pile - вольтов столб (гальваническая батарея)
The Development of Electric Motor
As early as 1822 Michael Faraday outlined the way in which an electric motor could work: by placing a coil, or armature, between the poles of an electromagnet; when a current is made to flow through the coil the electromagnetic force causes it to rotate.
In 1823 Faraday discovered how to make an electrical motor. In 1831 he built the first generator, then called it dynamo. The modern car has both a starting motor and a generator. The starting motor draws electric current from the car battery to start the powerful gasoline engine. The generator is driven by the gasoline engine to recharge the battery and to furnish electrical power for all the electrical conveniences in the car.
The Russian physicist, Jacobi built several electric motors during the middle decades of the XlX-th century. Jacobi even succeeded in running a small, battery-powered electric boat on the Neva river in St. Petersburg. All of them, however, came to the conclusion that the electric motor was a rather uneconomical machine so long as galvanic batteries were the only source of electricity. It did not occur to them that motors and generators could be made interchangeable.
In 1888, Professor Galileo Ferraris in Turin and Nikola Tesla in America invented, independently and without knowing of each other's work, the induction motor. This machine, a most important but little recognized technical achievement, provides no less than two-thirds of all the motive power for the factories of the world, and much of modern industry could not do without it. Known under the name of "squirrel-cage " — because it resembles the wire cage in which squirrels used to be kept—it has two circular rings made of copper or aluminium joined by a few dozen parallel bars of the same material, thus forming a cylindrical cage.
Although the induction motor has been improved a great deal and its power increased many times ever since its invention, there has never been any change of the underlying principle. One of its drawbacks was that its speed was constant and unchangeable. Some years later a two-speed induction motor was developed. The speed change was achieved.
Fossil Fuels
Coal, Oil and Gas are called "fossil fuels" because they have been formed from the fossilized remains of prehistoric plants and animals. '
They provide around 66% of the world's electrical power, and 95% of the world's total energy demands (including heating, transport, electricity generation and other uses).
Coal is crushed to a fine dust and burnt Oil and gas can be burnt directly Coal provides around 28% of our energy, and oil provides 40%.
Natural gas provides around 20% of the world's consumption of energy, and as well as being burnt in power stations, is used by many people to heat their homes. It is easy to transport along pipes, and gas power stations produce comparatively little pollution.
Other fossil fuels are being investigated, such as bituminous sands and oil shale.
The difficulty is that they need expensive processing before we can use them.
The steam that has passed through the power station's turbines has to be cooled, to condense it back into water before it can be pumped round again. This is what happens in the huge "cooling towers" seen at power stations.
Some power stations are built on the coast, so they can use sea water to cool the steam instead. However, this warms the sea and can affect the environment, although the fish seem to like it.
Advantages of fossil fuels are:
1. Very large amounts of electricity can be generated in one place using coal, fairly cheaply. 2.Transporting oil and gas to the power stations is easy.3.Gas-fired power stations are very efficient. 4. A fossil-fuelled power station can be built almost anywhere, so long as you can get large quantities of fuel to it.
Disadvantages of fossil fuels are:
1. Basically, the main drawback of fossil fuels is pollution. Burning any fossil fuel produces carbon dioxide, which contributes to the "greenhouse effect", warming the Earth. 2. Burning coal produces more carbon dioxide than burning oil or gas it also produces sulphur dioxide, a gas that contributes to acid rain. We can reduce this before releasing the waste gases into the atmosphere. 3.Mining coal can be difficult and dangerous. Strip mining destroys large areas of the landscape. 4.Coal-fired power stations need huge amounts of fuel, which means train-loads of coal almost constantly. In order to cope with changing demands for power, the station needs reserves. This means covering a large area of countryside next to the power station with piles of coal.
Fossil fuels are not a renewable energy resource. Once we've burned them all, there isn't any more, and our consumption of fossil fuels has nearly doubled every 20 years since 1900. This is a particular problem for oil, because we also use it to make plastics and many other products.
Hydrogen and Future Energy Sources
Fossil fuels were formed before and during the time of the dinosaurs - when plants and animals died. Their decomposed remains gradually changed over the years to form coal, oil and natural gas. Fossil fuels took millions of years to make. We are using up the fuels formed more than 65 million years ago. They can't be renewed; they can't be made again. We can save fossil fuels by conserving and finding ways to harness energy from seemingly "endless sources," like the sun and the wind.
We can't use fossil fuels forever as they are a non-renewable and finite resource. Some people suggest that we should start using hydrogen.
Hydrogen is a colorless, odorless gas that accounts for 75 percent of the entire universe's mass. Hydrogen is found on Earth only in combination with other elements such as oxygen, carbon and nitrogen. To use hydrogen, it must be separated from these other elements.
Today, hydrogen is used primarily in ammonia manufacturing, petroleum refining and synthesis of methanol. It's also used in some space program as fuel for the space shuttles, and in fuel cells that provide heat, electricity and drinking water for astronauts. Fuel cells are devices that directly convert hydrogen into electricity. In the future, hydrogen could be used to fuel vehicles and aircraft, and provide power for our homes and offices.
Hydrogen can be made from molecules called hydrocarbons by applying heat a process known as "reforming" hydrogen. This process makes hydrogen from natural gas. An electrical current can also be used to separate water into its components of oxygen and hydrogen in a process called electrolysis. Some algae and bacteria, using sunlight as their energy source, give off hydrogen under certain conditions.
Hydrogen as a fuel is high in energy, yet a machine that burns pure hydrogen produces almost zero pollution.
Fuel cells are a promising technology for use as a source of heat and electricity in buildings, and as an electrical power source for vehicles.
Auto companies are working on building cars and trucks that use fuel cells. In a fuel cell vehicle, an electrochemical device converts hydrogen (stored on board) and oxygen from the air into electricity, to drive an electric motor and power the vehicle.
Although these applications would ideally run off pure hydrogen, in the near term they are likely to be fueled with natural gas, methanol or even gasoline. Reforming these fuels to create hydrogen will allow the use of much of our current energy infrastructure - gas stations, natural gas pipelines, etc. - while fuel cells are phased in.
In the future, hydrogen could also join electricity as an important energy carrier. An energy carrier stores, moves and delivers energy in a usable form to consumers.
Wind Energy
Wind can be used to do work. The kinetic energy of the wind can be changed into other forms of energy, either mechanical energy or electrical energy. This is one form of work. Farmers have been using wind energy for many years to pump water from wells using windmills. The Babylonians and Chinese were using wind power to pump water for irrigating crops 4,000 years ago, and sailing boats were around long before that. Wind power was used in the Middle Ages, in Europe, to grind corn, which is where the term "windmill" comes from. In Holland, windmills have been used for centuries to pump water from low-lying areas.
Today, the wind is also used to make electricity. The Sun heats our atmosphere unevenly, so some patches become warmer than others. These warm patches of air rise, other air blows in to replace them - and we feel a wind blowing. We can use the energy in the wind by building a tall tower, with a large propeller on the top. The wind blows the propeller round, which turns a generator to produce electricity. We tend to build many of these towers together, to make a “wind farm” and produce more electricity. The mote towers, the more wind, and the larger the propellers, the more electricity we can make.
In order for a wind turbine to work efficiently, wind speeds usually must be above 12 to 14 miles per hour. Wind has to be this speed to turn the turbines fast enough to generate electricity. The turbines usually produce about 50 to 300 kilowatts of electricity each. The best places for wind farms are in coastal areas, at the tops of rounded hills, open plains and gaps in mountains - places where the wind is strong and reliable. Wind is blowing in many places all over the earth. The only problem with wind is that it is not windy all the time. To be worthwhile, you need an average wind speed of around 25 km/h.
Isolated places such as farms may have their own wind generators. In California, several "wind farms" supply electricity to homes around Los Angeles. Most wind farms in the UK are in Cornwall and Wales.
Geothermal Energy
Geothermal Energy has been around for as long as the Earth has existed. The centre of the Earth is around 6000 degrees Celsius - hot enough to melt rock. Even a few kilometers down, the temperature can be over 250 degrees Celsius. In general, the temperature rises one degree Celsius for every 36 meters you go down. In volcanic areas, molten rock can be very close to the surface. Geothermal energy has been used for thousands of years in some countries for cooking and heating. The name "geothermal" comes from two Greek words: "geo" means "Earth" and "thermal" means "heat".
Today, people use the geothermal heated hot water in swimming pools and in health spas. Or, the hot water from below the ground can warm buildings for growing plants, like in a green house. In San Bernardino, in Southern California, hot water from below ground is used to heat buildings during the winter. The hot water runs through miles of insulated pipes to dozens of public buildings which are heated this way.
Hot water or steam from below ground can also be used to make electricity in a geothermal power plant. A geothermal power plant is like in a regular power plant except that no fuel is burned to heat water into steam. The steam or hot water in a geothermal power plant is heated by the earth. It goes into a special turbine. The turbine blades spin and the shaft from the turbine is connected to a generator to make electricity. The steam then gets cooled off in a cooling tower.
Geothermal energy does not produce any pollution, and does not contribute to the greenhouse effect. But the big problem is that there are not many places where you can build a geothermal power station and you need hot rocks of a suitable type, at a depth where we can drill down to them. The type of rock above is also important, it must be of a type that we can easily drill through.
The first geothermal power station was built at Landrello, in Italy, and the second was at Wairekei in New Zealand. Others are in Iceland, Japan, the Philippines and the United States.
Hydro Power
When it rains in hills and mountains, the water becomes streams and rivers that run down to the ocean. The moving or falling water can be used to do work. Energy is the ability to do work. So moving water can be used to make electricity. Hydro means water. Hydro-electric means making electricity from water power.
For hundreds of years, moving water was used to turn wooden wheels that were attached to grinding wheels to grind (or mill) flour or corn. These were called grist mills or water mills. The first use of water to generate electricity was in 1882 on the Fox river, in the USA, which produced enough power to light two paper mills and a house.
Today, moving water can also be used to make electricity. Hydroelectric power uses the kinetic energy of moving water to make electricity. Dams can be built to stop the flow of a river. Water behind a dam often forms a reservoir. Dams are also built across larger rivers but no reservoir is made. The river is simply sent through a hydroelectric power plant.
The water behind the dam flows through the intake and into a pipe called a penstock. The water pushes against blades in a turbine, causing them to turn. The turbine spins a generator to produce electricity. The electricity can then travel over long distance electric lines to your home, to your school, to factories and businesses.
Nowadays there are many hydro-electric power stations, providing around 20% of the world's electricity. Hydro is one of the largest producers of electricity in the United States. Water power supplies about 10 percent of the entire electricity that we use. In states with high mountains and lots of rivers, even more electricity is made by hydro power. In California, for example, about 15 percent of all the electricity comes from hydroelectric. The state of Washington leads the nation in hydroelectricity. About 87 percent of the electricity made in Washington state is produced by hydroelectric facilities. Some of that electricity is exported from the state and used in other states.
Solar Energy
We have always used the energy of the sun as far back as humans have existed on this planet. As far back as 5,000 years ago, people "worshipped" the sun. Ra, the sun-god, who was considered the first king of Egypt. In Mesopotamia, the sun-god Shamash was a major deity and was equated with justice. In Greece there were two sun deities, Apollo and Helios. The influence of the sun also appears in other religions - Roman religion, the Druids of England, the Aztecs of Mexico, the Incas of Peru and many Native American tribes.
We know today, that the sun is simply our nearest star. Without it, life would not exist on our planet. We use the sun's energy every day in many different ways. When we hang laundry outside to dry in the sun, we are using the sun's heat to do work - drying our clothes. Plants use the sun's light to make food. Animals eat plants for food. Decaying plants hundreds of millions of years ago produced the coal, oil and natural gas that we use today. So, fossil fuels is actually sunlight stored millions and millions of years ago.
Indirectly, the sun or other stars are responsible for all our energy. Even nuclear energy comes from a star because the uranium atoms used in nuclear energy were created in the fury of a nova - a star exploding.
Solar energy can also be used to make electricity. Solar cells convert sunlight directly into electricity. They are made of semiconducting materials similar to those used in computer chips. When sunlight is absorbed by these materials, the solar energy knocks electrons loose from their atoms, allowing the electrons to flow through the material to produce electricity. This process of converting light (photons) to electricity (voltage) is called the photovoltaic effect. In a sunny climate, you can get enough power to run a 100W light bulb from just one square meter of solar panel.
Solar cells provide the energy to run satellites that orbit the Earth. These give us satellite TV, telephones, navigation, weather forecasting, the Internet and all manner of other facilities.
three feet away - на расстоянии трех футов
Solar Thermal Heat
The major applications of solar thermal energy at present are heating swimming pools, heating water for domestic use, and space heating of buildings. For these purposes, the general practice is to use flat-plate solar-energy collectors with a fixed orientation (position).
Where space heating is the main consideration, the highest efficiency with a fixed flat-plate collector is obtained if it faces approximately south and slopes at an angle to the horizon equal to the latitude plus about 15 degrees.
Solar collectors fall into two general categories: no concentrating and concentrating.
In the no concentrating type, the collector area (i.e., the area that intercepts the solar radiation) is the same as the absorber area (i.e., the area absorbing the radiation).
In concentrating collectors, the area intercepting the solar radiation is greater, sometimes hundreds of times greater, than the absorber area. Where temperatures below about 200º F are sufficient, such as for space heating, flat-plate collectors of the no concentrating type are generally used.
There are many flat-plate collector designs but generally all consist of a flat-plate absorber, which intercepts and absorbs the solar energy, a transparent cover that allows solar energy to pass through but reduces heat loss from the absorber, a heat-transport fluid (air or water) flowing through tubes to remove heat from the absorber, and a heat insulating backing.
Solar space heating systems can be classified as passive or active. In passive heating systems, the air is circulated past a solar heat surfaces and through the building by convection (i.e., less dense warm air tends to rise while more dense cooler air moves downward) without the use of mechanical equipment. In active heating systems, fans and pumps are used to circulate the air or the heat absorbing fluid.
Ocean Energy
The ocean can produce two types of energy: thermal energy from the sun's heat, and mechanical energy from the tides and waves.
Oceans cover more than 70% of Earth's surface, making them the world's largest solar collectors. The sun's heat warms the surface water a lot more than the deep ocean water, and this temperature difference creates thermal energy. Just a small portion of the heat trapped in the ocean could power the world.
Ocean thermal energy is used for many applications, including electricity generation. There are three types of electricity conversion systems: closed-cycle, open-cycle, and hybrid. Closed-cycle systems use the ocean's warm surface water to vaporize a working fluid, which has a low-boiling point, such as ammonia. The vapor expands and turns a turbine. The turbine then activates a generator to produce electricity. Open-cycle systems actually boil the seawater by operating at low pressures. This produces steam that passes through a turbine/generator. And hybrid systems combine both closed-cycle and open-cycle systems.
Ocean mechanical energy is quite different from ocean thermal energy. Even though the sun affects all ocean activity, tides are driven primarily by the gravitational pull of the moon, and waves are driven primarily by the winds. As a result, tides and waves are intermittent sources of energy, while ocean thermal energy is fairly constant. Also, unlike thermal energy, the electricity conversion of both tidal and wave energy usually involves mechanical devices.
A barrage (dam) is typically used to convert tidal energy into electricity by forcing the water through turbines, activating a generator. For wave energy conversion, there are three basic systems: channel systems that funnel the waves into reservoirs; float systems that drive hydraulic pumps; and oscillating water column systems that use the waves to compress air within a container. The mechanical power created from these systems either directly activates a generator or transfers to a working fluid, water, or air, which then drives a turbine/generator.
Biomass Energy
Biomass is matter usually thought of as garbage. Some of it is just stuff lying around - dead trees, tree branches, yard clippings, left-over crops, wood chips, and bark and sawdust from lumber mills. It can even include used tires and livestock manure.
Your trash, paper products that can't be recycled into other paper products and other household waste are normally sent to the dump. Your trash contains some types of biomass that can be reused. Recycling biomass for fuel and other uses cuts down on the need for "landfills" to hold garbage.
This stuff nobody seems to want can be used to produce electricity, heat compost material or fuels. Composting material is decayed plant or food products mixed together in a compost pile and spread to help plants grow.
California produces more than 60 million bone dry tons of biomass each year. Of this total, five million bone dry tons is now burned to make electricity This is biomass from lumber mill wastes, urban wood waste, forest and agricultural residues and other feed stocks.
If all of it was used, the 60 million tons of biomass in California could make close to 2,000 megawatts of electricity for California's growing population and economy. That's enough energy to make electricity for about two million homes!
How biomass works is very simple The waste wood, tree branches and other scraps are gathered together in big trucks The trucks bring the waste from factories and from farms to a biomass power plant Here the biomass is dumped into huge hoppers. This is then fed into a furnace where it is burned. The heat is used to boil water in the boiler, and the energy in the steam is used to turn turbines and generators.
Using biomass can help reduce global warming compared to a fossil fuel-powered plant. Plants store carbon dioxide (C02) when they grow. C02 stored in the plant is released when the plant material is burned or decays. By replanting the crops, the new plants can use the C02 produced by the burned plants. So using biomass and replanting helps close the carbon dioxide cycle. However, if the crops are not replanted, then biomass can emit carbon dioxide that will contribute toward global warming.
So, the use of biomass can be environmentally friendly because the biomass is reduced, recycled and then reused. It is also a renewable resource because plants to make biomass can be grown over and over.
Today, new ways of using biomass are still being discovered. One way is to produce ethanol, a liquid alcohol fuel. Ethanol can be used in special types of cars that are made for using alcohol fuel instead of gasoline. The alcohol can also be combined with gasoline. This reduces our dependence on oil - a non-renewable fossil fuel.
