Book - School of Science and Technology

692 Combined heat and power (CHP)Figure 24.3 Line diagram of a gas turbine total-energy systemshaft power obtained from a gas turbine. Among the types of reciprocating enginegenerally suitable for combined heat and power (CHP) systems are the following:. Industrial spark-ignition gas engines manufactured for stationary use and of ruggedconstruction,with first class in-built facilities for such maintenance as is necessary.. Industrial diesel engines having similar characteristics to the above,many initiallydesigned for use as prime movers for standby generators.. Automotive derived engines,i.e. lorry (diesel) and motor car (petrol) engines whichhave been modified and de-rated for this application.The first two of these categories are both familiar and proven: they need little furthercomment except to add that their operational life is long,extending in known instance ofas much as 50 000 hours. Figure 24.4 illustrates how the output of a large industrial enginemight be employed. The production-line engines of the third category will normally havebeen modified as to pistons,valve gear,ignition equipment and sometimes cylinder headsto suit operation with natural gas as a fuel. Since they will run as stationary unit inrelatively clean plant rooms and at constant speed,their life may be expected to exceedthat of their road-service equivalents by a considerable margin.Generators and controlsEquipment for electrical generation,normally taking the form of three-phase 415 Valternators,will not be discussed here at any length since electrical equipment is notwithin the scope of this book. The leading characteristics of the two generic typesavailable, synchronous and asynchronous,must be noted,however,since they are fundamentalto system economics and operation. The former are significantly more expensivein first cost and require a complex control system to maintain a standard frequency: forparallel operation with the public mains supply,further synchronisation equipment isessential. Synchronous machines are,however,battery started and thus self-sufficient:they do not normally require power factor correction and they may be used in the role ofstandby generators without modification except to switch gear.