Thermodynamics

Chapter 6 | 283Energy source(such as a furnace)Q inSystem boundaryBoilerW inPumpTurbineW outCondenserQ outEnergy sink(such as the atmosphere)W outW net,outFIGURE 6–10Schematic of a steam power plant.The net work output of this power plant is simply the difference betweenthe total work output of the plant and the total work input (Fig. 6–11):W net,out W out W in 1kJ2(6–1)The net work can also be determined from the heat transfer data alone. Thefour components of the steam power plant involve mass flow in and out, andtherefore they should be treated as open systems. These components, togetherwith the connecting pipes, however, always contain the same fluid (not countingthe steam that may leak out, of course). No mass enters or leaves this combinationsystem, which is indicated by the shaded area on Fig. 6–10; thus, itcan be analyzed as a closed system. Recall that for a closed system undergoinga cycle, the change in internal energy U is zero, and therefore the net workoutput of the system is also equal to the net heat transfer to the system:W net,out Q in Q out 1kJ2(6–2)Thermal EfficiencyIn Eq. 6–2, Q out represents the magnitude of the energy wasted in order tocomplete the cycle. But Q out is never zero; thus, the net work output of a heatengine is always less than the amount of heat input. That is, only part of theheat transferred to the heat engine is converted to work. The fraction of theheat input that is converted to net work output is a measure of the performanceof a heat engine and is called the thermal efficiency h th (Fig. 6–12).For heat engines, the desired output is the net work output, and therequired input is the amount of heat supplied to the working fluid. Then thethermal efficiency of a heat engine can be expressed asNet work outputThermal efficiency Total heat input(6–3)HEATENGINEW inFIGURE 6–11A portion of the work output of a heatengine is consumed internally tomaintain continuous operation.Heat input100 kJ 100 kJ1Waste heat80 kJSOURCENetworkoutput20 kJSINK2Waste heat70 kJη th,1 = 20% η th,2 = 30%Networkoutput30 kJFIGURE 6–12Some heat engines perform better thanothers (convert more of the heat theyreceive to work).