Thermodynamics

562 | ThermodynamicsT21Criticalpoint43Operating pressures of boilers have gradually increased over the yearsfrom about 2.7 MPa (400 psia) in 1922 to over 30 MPa (4500 psia) today,generating enough steam to produce a net power output of 1000 MW or morein a large power plant. Today many modern steam power plants operate atsupercritical pressures (P 22.06 MPa) and have thermal efficiencies ofabout 40 percent for fossil-fuel plants and 34 percent for nuclear plants.There are over 150 supercritical-pressure steam power plants in operation inthe United States. The lower efficiencies of nuclear power plants are due tothe lower maximum temperatures used in those plants for safety reasons.The T-s diagram of a supercritical Rankine cycle is shown in Fig. 10–9.The effects of lowering the condenser pressure, superheating to a highertemperature, and increasing the boiler pressure on the thermal efficiency ofthe Rankine cycle are illustrated below with an example.FIGURE 10–9A supercritical Rankine cycle.sEXAMPLE 10–3Effect of Boiler Pressureand Temperature on EfficiencyConsider a steam power plant operating on the ideal Rankine cycle. Steamenters the turbine at 3 MPa and 350°C and is condensed in the condenser ata pressure of 10 kPa. Determine (a) the thermal efficiency of this powerplant, (b) the thermal efficiency if steam is superheated to 600°C instead of350°C, and (c) the thermal efficiency if the boiler pressure is raised to 15MPa while the turbine inlet temperature is maintained at 600°C.Solution A steam power plant operating on the ideal Rankine cycle is considered.The effects of superheating the steam to a higher temperature andraising the boiler pressure on thermal efficiency are to be investigated.Analysis The T-s diagrams of the cycle for all three cases are given inFig. 10–10.TTTT 3 = 600°C33T 3 = 600°C3 MPa3T 3 = 350°C3 MPa15 MPa2110 kPa42110 kPa42110 kPa4(a)s(b)s(c)sFIGURE 10–10T-s diagrams of the three cycles discussed in Example 10–3.