Book - School of Science and Technology

Basic considerations 693Figure 24.4 Line diagram of a diesel engine CHP systemAn asynchronous machine is similar to an induction motor with the excitation current,at a low leading power factor,taken from the public mains supply. In fact,they are oftenused in the role of a motor for starting the prime mover,taking over as a generator whenthe running speed is reached and the prime mover is able to provide the required power.For this starting duty,care must be taken that the `motor' is not overloaded. In smallsizes,induction generators are some 3±4% more efficient than the alternative but capacitorsfor power factor correction are required. A major advantage therefore is simplicity,since no complex synchronisation gear is necessary. A disadvantage however is that,unless a suitable auxiliary supply is available to provide the excitation current,anasynchronous machine cannot act as a standby generator in the event of failure of thepublic mains supply.Generation equipment of any type must be provided with all those controls appropriateto the starting,running and shut-down sequences and also with the necessary sensors andactuators to suit the monitoring circuits associated with mains failure,etc. The neutralearth connection,required when generation is wholly independent of the public supply,must be isolated by a safety interlock when exporting power and running in parallel withthe public mains supply.Heat recovery equipmentHeat recovery from reciprocating machines may be better appreciated by reference toFigure 24.5 which is a Sankey diagram representing energy input and the various componentsof energy output. As will be seen,over 50% of input is recoverable at a temperatureadequate for use in low pressure hot water systems. If a suitable use for low grade heat isavailable,such as a swimming pool or a preheater for domestic hot water,a further 10%