【文章內(nèi)容簡介】 h all of these require the boiler to be operated to generate steam, each has its own requirements and constraints. Setting the demand in powerstation applications A boiler producing steam for an operating turbogenerator has to ensure that the machine continually delivers the required electrical energy to the load. With a binedcycle gasturbine plant it is frequently the case that the power generated by the gas turbines is adjusted to meet the demand,with the steam turbine making use of all of the waste heat from the turbines. With all types of powergenerating plant, however, the requirement for generation will be set, directly or indirectly, by the gridcontrol centre (or the 39。central dispatcher39。), and the amount of power that is generated will be related to the local or national demand at that time. In national works, power stations are linked together to generate electrical power in concert with one another. Together they must meet a demand that is made up of the bined needs of all the users that are connected to the system (domestic, mercial, agricultural, industrial etc.). The overall demand will vary from minute to minute and day to day in a way that is systematic or random, dictated by economic, operational and environmental factors. This pattern of use relates to the entire work, and the fact that a large number of power generators and users are linked via the work has little bearing on the overall demand, although the extreme peaks and troughs may well be smoothed out. The interlinking does, however, have operational implications. For example, a sudden failure of one generating plant will instantly throw an extra demand on the others. In a cold or temperate climate the demand will be based predominately on the need for light, heat and motive power. In warmer climates and developed areas it will also be determined by the use of airconditioning and, possibly, desalination plant (for drinkingwater production). 華北電力大學本科畢業(yè)設(shè)計（論文） 9 Figure shows how the total electrical demand on the United Kingdom39。s Grid system varies from hour to hour through the day, and from a warm summer day to a cold winter day. Clearly, in addition to being affected by normal working patterns, the demand is determined by the level of daylight and the ambient temperature, both of which follow basic systematic patterns but which may also fluctuate in a very sudden and unpredictable manner. Similar profiles can be developed for eachcountry and will be determined by climate as well as the country39。s industrial and mercial infrastructure. These days, the demand for electricity in a developed nation is also affected quite dramatically by television broadcasts. During a major sporting event such as an international football match, sudden upsurges in demand will occur at halftime and full time, when viewers switch on their kettles. In the UK this can impose a sudden rise in demand of as much as 2 GW, which is the equivalent to the total output of a reasonably large. Figure Typical electrical demand in the United Kingdom Such a pattern of usage can be pr