SAVITA

PHYSICS PROJECT

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TOPICS THAT ARE COVEREDGENERATING ELECTRONSTHERMAL GENERATING PLANTSFOSSIL-FUELED PLANTSCOGENERATIONCOMBINED-CYCLE AND BIOMASS PLANTSNUCLEAR PLANTSKINETIC GENERATING PLANTSALTERNATIVE GENERATIONTRANSMISSION IN ELECTRICITYOVERHEAD TRANSMISSIONUNDERGROUND TRANSMISSION

INTRODUCING

VARIOUS MODES OF GENERATING ELECTRICITY AND

ITS TRANSMISSION

Generating ElectronsThere are a many ways to produce electricity. Electrons can flow between certain different materials providing a current, as in a common battery. While reliable and portable, chemical batteries run down quickly. To provide the large amounts of steady power demanded by modern societies, large power plants have been built. Most power plants make electricity with a machine called a generator.

Generators have two important parts: the rotor (which rotates) and the stator (which remains stationary). Generators use the principle of electro-magnetic induction, which exploits the relation between magnetism and electricity. In large AC generators, an outer shell with powerful magnets rotates around a stationary "armature" which is wound with heavy wire. As they move, the magnets induce an electric current in the wire.

It is important to recognize that electricity is not mined or harvested, it must be manufactured. And since it's not easily stored in quantity, it must be manufactured at time of demand. Electricity is a form of energy, but not an energy source. Different generating plants harness different energy sources to make electric power. The two most common types are "Thermal Plants" and "Kinetic Plants".

Westinghouse Turbine Rotor, 1925

Thermal Generating PlantsThermal plants use the energy of heat to make electricity. Water is heated in a boiler until it becomes high-temperature steam. This steam is then channeled through a turbine, which has many fan-blades attached to a shaft. As the steam moves over the blades, it causes the shaft to spin. This spinning shaft is connected to the rotor of a generator, and the generator produces electricity.

Diagram of a thermal (oil burning) plant in the Hydro-Québec system

FOSSIL-FUELED PLANTSFossil-fuels are the remains of plant and animal life that lived long ago. Exposed to high temperatures and pressures for millions of years underground, these remains have been transformed into forms of carbon: coal, oil, and natural gas. Unlike electricity itself, fossil fuels can be stored in large quantities. After 100 years of research and development, fossil-fueled plants are generally reliable, and problems that do occur are usually confined to a local area. Many electric utilities have operated fossil-fuel plants for decades, and these plants (now fully paid for) are very profitable to run. This not only increases profits to the utility, but keeps down the direct cost to users.

However, fossil-fuel plants can create serious environmental problems. Burning these fuels produces sulfur-dioxide and nitric-oxide air-pollution requiring expensive scrubbers. Wastewater from the used steam can carry pollutants into water-sheds. Even with very good pollution controls, there is still waste material produced. Carbon-dioxide gas, and ash are the current concerns.

Also, fossil-fuels are not renewable. They took millions of years to make, and at some point they will run out. Extracting and transporting them for use has created environmental problems. Strip-mining of coal and oil-spills at sea can produce catastrophic impacts on ecosystems.

CogenerationOil has become too expensive for most power plants. Coal and natural gas are currently cheap in the US, and are being used more often. These two fuels are being used more efficiently in "cogeneration" plants. Cogeneration is not a new idea, and takes advantage of the way many large electricity users operate. Many factories use steam in their production process. Utilities often make and sell steam for these customers, as well as for running their own generators.

Rather than simply condensing and exhausting waste-steam after it has passed thru the turbine, "top-cycle" cogenerators pipe this usable commodity to nearby customers. "Bottom-cycle" cogenerators operate in reverse and use the waste steam from industrial processing to drive turbines. By reusing steam, thermal-efficiency at cogeneration plants can exceed 50%.

Recently developed cogeneration plants use new materials and designs to improve reliability, and control both thermal and atmospheric pollution. Since these new technologies are designed into plants from the start, they are less expensive to install. The economy and capability of cogeneration technology allows many plants to return to burning coal without exceeding air-quality standards. "Circulating Fluidized-Bed" boilers, "Selective Catalytic (and Non-catalytic) Reduction", and "Zero-Discharge" water treatment systems are examples of technologies being used to control various environmental problems.

COMBINED-CYCLE AND BIOMASS PLANTS

Some natural gas plants can produce electricity without steam. They use turbines very much like those on jet-aircraft. Instead of burning jet-fuel and producing thrust, however, these units burn natural gas and power a generator. Gas-turbine generators have been popular for many years because they can be started quickly in response to temporary demand surges for electricity. A newer twist is the "Combined-Cycle" plant which uses gas-turbines in this fashion, but then channels the hot exhaust gas to a boiler, which makes steam to turn another rotor. This substantially improves the overall efficiency of the generating plant.

In addition to these innovations some thermal plants are being designed to burn "biomass." (Shown is a biomass plant in Florida, image copyright US Generating). The term applies to waste wood or some other renewable plant material. For example, Okeelanta Cogeneration Plant in Florida burns bagasse waste from surrounding sugar-cane processing operations during one part of the year, and waste wood during the growing season.

NUCLEAR PLANTSA nuclear power plant is a thermal power station in which the heat source is a nuclear reactor. As is typical in all conventional thermalpower stations the heat is used to generate steam which drives a steam turbine connected to a generator which produces electricity.

Nuclear Power Stations use a fuel called uranium, a relatively common material. Energy is released from uranium when an atom is split by a neutron. The uranium atom is split into two and as this happens energy is released in the form of radiation and heat. This nuclear reaction is called the fission process

In a nuclear power station the uranium is first formed into pellets and then into long rods. The uranium rods are kept cool by submerging them in water. When they are removed from the water a nuclear reaction takes place causing heat. The amount of heat required is controlled by raising and lowering the rods. If more heat is required the rods are raised further out of the water and if less is needed they lower further into it.

KINETIC GENERATING PLANTSHydro-electric Plants

Two basic types of hydro-electric plants are in service. One type, a "run-of-river" plant, takes energy from a fast moving current to spin the turbine. The flow of water in most rivers can vary widely depending on the amount of rain-fall. Hence, there are few suitable sites for run-of-river plants.

A special type of hydro-power is called "Pumped Storage". Some non-hydro plants can take advantage of periods of low demand (and low costs) by pumping water into a reservoir. When demand rises, some of this water is channeled through a hydro-turbine to generate electricity. Since "peak-load" generating units (used to meet temporary demand surges), are generally more expensive to run than "base-load" units (which run most of the time), pumped-storage is one way to boost system efficiency.

Wind PowerWind-farms do not need reservoirs and create no air pollution. Small wind-mills can provide power to individual homes. Air carries much less energy than water, however, so much more of it is needed to spin rotors. One needs either a few very large wind-mills or many small ones to operate a commercial wind-farm. In either case, construction costs can be high.

Like run-of-river hydro-plants, there are a limited number of suitable locations where the wind blows predictably. Even in such sites, turbines often have to be designed with special gearing so that the rotor will turn at a constant speed in spite of variable wind speeds. Some find less technical problems with installations that can turn a scenic ridge or pass into an ugly steel forest, or that can take a toll on birds.

ALTERNATIVE GENERATIONGeothermal Plants

Pressure, radioactive decay, and underlying molten rock make the deep places in the Earth's crust hot indeed. A vivid example of the heat available underground is seen when geysers erupt, sending steam and hot water high in the air. Natural sources of steam and hot water have attracted the attention of power engineers since early in this century.

By tapping this naturally-created thermal energy, geothermal plants provide electricity with low levels of pollution. There are several different varieties of plants, and the product from a geothermal site is used for heating as well as electricity production. Finding suitable sites can be difficult, although as technical innovations occur, more sites are made practical. Tapping geothermal sources can also have the effect of "turning off" natural geysers, and this possibility must be taken into account during the planning stage.

Solar PowerSolar-cells or "photo-voltaics" do not use a generator; they are the generator. Usually arranged in panels, these devices take advantage of the ability of light to cause a current to flow in some substances. A series of cells are wired together and the current flows from the panel when sunlight strikes it. They produce no pollution when operating, and most scientists predict that the fuel supply will last at least 4 billion years.

Solar panels have been relatively expensive to make, and of course they won't work at night or in foul weather. Some of the processes needed to manufacture them have recently been called into question environmentally. Not all of the sunlight striking a solar cell is converted into electricity, and boosting efficiency has been slow work. Yet, the idea of harnessing all of that free sunlight remains a powerful driver for solar power.

Fuel CellsValued for their usefulness on space-craft, fuel cells combine substances chemically to generate electricity. While this might sound very similar to a battery, fuel-cells are powered by a continuous flow of fuel. In the US Space Shuttle, for example, fuel cells combine hydrogen and oxygen to produce water and electricity.

Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to electrical substations located near demand centers. This is distinct from the local wiring between high-voltage substations and customers, which is typically referred to as electric power distribution.

TRANSMISSION OF ELECTRICITY

High-voltage overhead conductors are not covered by insulation. The conductor material is nearly always an aluminium alloy, made into several strands and possibly reinforced with steel strands.

Conductor sizes range from 12 mm2 750 mm2 , with varying resistance and current-carrying capacity. Thicker wires would lead to a relatively small increase in capacity due to the skin effect, that causes most of the current to flow close to the surface of the wire. Because of this current limitation, multiple parallel cables (called bundle conductors) are used when higher capacity is needed. Bundle conductors are also used at high voltages to reduce energy loss caused by corona discharge.

Overhead Transmission

UNDERGROUND TRANSMISSIONElectric power can also be transmitted by underground power cables instead of overhead power lines. Underground cables take up less right-of-way than overhead lines, have lower visibility, and are less affected by bad weather. However, costs of insulated cable and excavation are much higher than overhead construction. Faults in buried transmission lines take longer to locate and repair. Underground lines are strictly limited by their thermal capacity, which permits less overload or re-rating than overhead lines. Long underground cables have significant capacitance, which may reduce their ability to provide useful power to loads.

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