Book - School of Science and Technology

Basic considerations 691at full or any other loading but rather to maintain a balance for as much of the year aspossible.Suitable prime movers fall into three general categories: steam turbines,gas turbinesand reciprocating engines. Of these,the steam turbine has the highest heat-to-power ratioat some ten or more to one. It follows that such equipment is most suited to applicationswhich have large thermal demands vis-aÁ-vis electric power requirements: these will probablylie in an industrial field,where a large process load may be accompanied by a lesserelectrical power requirement. Gas turbines have heat-to-power ratios of the order of threeto one but the manner of application can increase this ratio materially by addingsupplementary direct firing to the waste heat boiler. The third group includes industrialgas engines,diesels and spark-ignition engines which are in many cases derived from theautomotive field: items in this group have heat-to-power ratios of between one and a halfand two to one. In exceptional circumstances,of course,the load profiles of a singlebuilding or a building complex may be matched more accurately by the heat-to-powercharacteristics of a CHP group made up from a combination of prime movers.The following two paragraphs provide no more than brief notes on the characteristicsof steam and gas turbines since they are more appropriate to either large site areas or toindustrial applications and are thus not in context with current financial limitations or thesize of the single site building projects which are the subject of this chapter.Steam turbinesElectrical generating stations on a national scale employ high pressure steam boilers andsophisticated techniques to obtain the maximum possible electrical supply from the fuelinput,small though it is in relative terms. However,thermal-electric stations of comparablegenerating capacity,designed to produce heat as well as power,would be designedfor quite different pressure and operating conditions and would,in general,tend toemploy less complex and cheaper steam raising plant. In such cases,back pressureturbines are most likely to be used or,where some proportion of higher pressure steamis required for process use,this may be obtained by pass-out from the turbine at anintermediate pressure. Typically,4 kg/s of steam per MW would be available at a pressureof about 200 kPa which represents the heat-to-power ratio of 10:1 mentioned previously.Gas turbinesThe full-load thermal efficiencies of typical open-cycle gas turbine sets are such that thegeneration of electricity without subsequent heat recovery,leads to an efficiency of lessthan 20%. In fact,industrial gas turbine sets have full load heat ratios varying from aboutfive or seven to one,which represent an efficiency range of 14±20%. However,the exhausttemperature of a gas turbine at full load will be between 450 and 550 C and thus,usingtwo or more stages of heat transfer to utilisation circuits,an overall thermal efficiency ofapproaching 85% may be achieved. By their very nature,gas turbines tend to be noisy andplant rooms thus require a good deal of acoustic treatment. A line diagram of a gasturbine heat recovery source is given in Figure 24.3.Reciprocating enginesWhilst the open-cycle gas turbine has the great merit of rejecting waste heat in a singlefluid stream and at a relatively high temperature,the shaft power produced for generationof electricity is comparatively low. The various forms of reciprocating engine,on the otherhand,are able to convert an equivalent amount of fuel into approximately double the