نوع مطلب :جزوات مهندسی برق ،
نوشته شده توسط:مهدی حسن پور
Section One: Reading ComprehensionThere are five sources of energy which together account for nearly all the world's electricity. They are coal, oil, natural gas, hydroelectric power and nuclear energy. Coal, oil and nuclear plants use the steam cycles to turn heat into electrical energy, in
Section One: Reading Comprehension
There are five sources of energy which together account for nearly all the world's electricity. They are coal, oil, natural gas, hydroelectric power and nuclear energy. Coal, oil and nuclear plants use the steam cycles to turn heat into electrical energy, in the following way. The steam power station uses very pure water in a closed cycle. First it is heated in the boilers to produce steam at high pressure and high temperature, typically 150 atmospheres and 550°C in a modern station. This high-pressure steam drives the turbines which in turn drive the electric generators, to which they are directly coupled. The maximum amount of energy will be transferred from the steam to the turbines only if the latter are allowed to exhaust at a very low pressure, ideally a vacuum. This can be achieved by condensing the outlet steam into water. The water is then pumped back into the boilers and the cycle begins again. At the condensing stage; a large quantity of heat has to be extracted from the system. This heat is removed in the condenser which is a form of heat exchanger. A much larger quantity of cold impure water enters one side of the condenser and leaves as warm water, having extracted enough heat from the exhaust steam to condense it back into water. At no point must the two water systems mix. At a coastal site the warmed impure water is simply returned to the sea at a point a short distance away. A 2 GW station needs about 60 tons of sea water each second. This is no problem on the coast, but inland very few sites could supply so much water all the year round. The alternative is to recirculate the impure water. Cooling towers are used to cool the impure water so that it can be returned to the condensers, the same water being cycled continuously. A cooling tower is usually a large concrete structure like a very broad chimney and acts in a similar way, in that it induces a natural draught. A large volume of air is drawn in round the base and leaves through the open top. The warm, impure water is sprayed into the interior of the tower from a large number of fine jets, and as it falls it is cooled by the rising air, finally being collected in a pond under the tower. The cooling tower is really a second heat exchanger where the heat in the impure water is passed to the atmospheric air; but unlike the first heat exchanger, the two fluids are allowed to come into contact and as a consequence some of the water is lost by evaporation.
The cooling towers are never able to reduce the impure water temperature right down to the ambient air temperature, so that the efficiency of the condenser and hence the efficiency of the whole station is reduced slightly compared with a coastal site. The construction of the cooling towers also increases the capital cost of building the power station. The need for cooling water is an important factor in the choice of sites for coal, oil and nuclear plants. A site which is suitable for a power station using one type of fuel is not necessarily suitable for a station using another fuel.
Coal-Fired Power Stations
Early coal-burning stations were built near the load they supplied. A station of 2 GW output, consumes about 5 million tons of coal in a year. In Britain where most power station coal is carried by rail, this represents an average of about 13 trains a day each carrying 1000 tons. This means that large coal-fired stations need a rail link unless the station is built right at the pit head.
Oil-Fired Power Stations
Power station oil can be divided into crude oil which is oil as it comes from the well, and residual oil which remains when the more valuable fractions have been extracted in the oil refinery. The cost of moving oil by pipeline is less than that of moving coal by rail, but even so stations burning crude oil are often sited near deep-water berths suitable for unloading medium-sized tankers. Stations burning residual oil need to be sited near to the refinery which supplies them. This is because residual oil is very viscous and can only be moved through pipelines economically if it is kept warm.
Nuclear Power Stations
In contrast to coal and oil the cost of transporting nuclear fuel is negligible because of the very small amount used. A 1 GW station needs about 4.5 tons of uranium each week. This compares very favorably with the 50,000 tons of fuel which would be burnt each week in a comparable coal-fired power station. Present nuclear stations use rather more cooling water than comparable coal-fired or oil-fired plants due to their lower efficiency. All nuclear stations in Britain, with one exception, are situated on the coast and use sea water for cooling.
Hydroelectric Power Stations
Hydroelectric power stations must be sited where the head of water is available, and as this is often in mountainous areas, they may need long transmission lines to carry the power to the nearest load center or link up with the grid. All hydroelectric schemes depend on two fundamental factors: a flow of water and a difference in level or head. The necessary head may be obtained between a lake and a nearby valley, or by building a small dam in a river which diverts the flow through the power station, or by building a high dam across a valley to create an artificial Lake.
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