Thermodynamics

560 | ThermodynamicsTurbine work output:w turb,out h T w s,turb,out h T 1h 5 h 6s 2 0.87 13583.1 2115.32 kJ>kg 1277.0 kJ>kgBoiler heat input:q in h 4 h 3 13647.6 160.12 kJ>kg 3487.5 kJ>kgThus,w net w turb,out w pump,in 11277.0 19.02 kJ>kg 1258.0 kJ>kgh th w netq in1258.0 kJ>kg 0.361 or 36.1%3487.5 kJ>kg(b) The power produced by this power plant isW # net m # 1w net 2 115 kg>s2 11258.0 kJ>kg2 18.9 MWDiscussion Without the irreversibilities, the thermal efficiency of this cyclewould be 43.0 percent (see Example 10–3c).T22'11'Increase in w net434'P'4 < P 4FIGURE 10–6The effect of lowering the condenserpressure on the ideal Rankine cycle.s10–4 ■ HOW CAN WE INCREASE THE EFFICIENCYOF THE RANKINE CYCLE?Steam power plants are responsible for the production of most electricpower in the world, and even small increases in thermal efficiency can meanlarge savings from the fuel requirements. Therefore, every effort is made toimprove the efficiency of the cycle on which steam power plants operate.The basic idea behind all the modifications to increase the thermal efficiencyof a power cycle is the same: Increase the average temperature atwhich heat is transferred to the working fluid in the boiler, or decrease theaverage temperature at which heat is rejected from the working fluid in thecondenser. That is, the average fluid temperature should be as high as possibleduring heat addition and as low as possible during heat rejection. Nextwe discuss three ways of accomplishing this for the simple ideal Rankinecycle.Lowering the Condenser Pressure (Lowers T low,avg )Steam exists as a saturated mixture in the condenser at the saturationtemperature corresponding to the pressure inside the condenser. Therefore,lowering the operating pressure of the condenser automatically lowers thetemperature of the steam, and thus the temperature at which heat is rejected.The effect of lowering the condenser pressure on the Rankine cycle efficiencyis illustrated on a T-s diagram in Fig. 10–6. For comparison purposes,the turbine inlet state is maintained the same. The colored area onthis diagram represents the increase in net work output as a result of loweringthe condenser pressure from P 4 to P 4 ¿ . The heat input requirements alsoincrease (represented by the area under curve 2-2), but this increase is verysmall. Thus the overall effect of lowering the condenser pressure is anincrease in the thermal efficiency of the cycle.