A Vapor Power CycleOptimization of a Vapor Power PlantPractical Problems Associated with a Power Plant Using a Carnot CycleConsider a modified cycle - A Rankine cycleIdeal Rankine Cycle - Energy analysisExample - Ideal Rankine CycleSolutionSolution (cont.)Thermal Efficiency – How to enhance it?ReheatingReheat Rankine CycleRegenerationRegenerative CycleRegenerative Cycle - AnalysisA Vapor Power CycleT2341Pv1234BoilerTurbineCompressor(pump)Heat exchanger(Condenser)1234QoutQinWoutWinOptimization of a Vapor Power PlantObjectives: design an optimal vapor power cycle–use idealized Carnot cycle as the model –consider all theoretical and practical limitations and redesign the cycle accordingly Idealized Rankine Cycle–Optimize the Rankine cycle using concepts of superheating, reheating and regeneration–Discuss ways of increasing the efficiency of an idealized Rankine cycle. Carnot cycleTs2341ORT2341(a)(b)sModifed: 11/5/01Practical Problems Associated with a Power Plant Using a Carnot Cycle• Maximum temperature limitation for a cycle (a). What is the maximum temperature in the cycle?• Isentropic expansion in a turbine from 3-4. What is the quality of the steam inside the turbine? Will high moisture content affect the operation of the turbine? • Isentropic compression process in a pump from 1-2. Can one design a condenser and a transmission line system that precisely controls the quality of the vapor in order to achieve an isentropic compression? • Even if we can, is it practical to handle two-phase flow (liquid + vapor) using such a system? • The latter two problems can be resolved by the use of cycle b from previous slide. • However, cycle b requires the compression (1-2)of liquid at a very high pressure (exceeding 22 MPa for steam; Q: where do we get this number?) and that is not practical. • Also, to maintain a constant temperature above the critical temperature is also difficult since the pressure will have to change continuously.Consider a modified cycle - A Rankine cycle•To avoid transporting and compressing two-phase fluid:– We can try to condense all fluid exiting from the turbine into saturated liquid before compressed it by a pump. • When the saturated vapor enters the turbine, as its temperature and pressure decreases, condensation occurs, leading to liquid. These liquid droplets can significantly damage the turbine blades due to corrosion and/or erosion. • One possible solution: superheating the vapor. • It can also increase the thermal efficiency of the cycle (since TH ).Ts3412BoilerTurbineCompressor(pump)Heat exchanger(Condenser)1234QoutQinWoutWinIdeal Rankine Cycle - Energy analysis•Assumptions: steady flow process, no generation, neglect KE and PE changes for all four devices,•First Law: 0 = (net heat transfer in) - (net work out) + (net energy flow in) 0 = (qin - qout) - (Wout - Win) + (hin - hout) Ts1234• 1-2: Pump (q=0)  Wpump = h2 - h1 = v(P2-P1)• 2-3: Boiler (W=0)  qin = h3 - h2• 3-4: Turbine (q=0)  Wout = h3 - h4• 4-1: Condenser (W=0)  qout = h4 - h1Thermal efficiency  = Wnet/qin = 1 - qout/qin = 1 - (h4-h1)/(h3-h2)Wnet = Wout - Win = (h3-h4) - (h2-h1)Example - Ideal Rankine CycleTurbinepumpcondenser1234QoutQinWoutWinboilerConsider the Rankine power cycle as shown. Steam enters the turbine as 100% saturated vapor at 6 MPa and saturated liquid enters the pump at a pressure of 0.01 MPa. If the net power output of the cycle is 50 MW. Determine (a) the thermal efficiency, (b) the mass flow rate of the system, (c) the rate of heat transfer into the boiler, (d) the mass flow rate of the cooling water from the condenser, in kg/s, if the cooling water enters at 20°C and exits at 40°C.Ts1234Solution• At the inlet of turbine, P3=6MPa, 100% saturated vapor x3=1, from saturated table A-5, h3=hg=2784.3(kJ/kg), s3=sg=5.89(kJ/kg K)• From 3-4, isentropic expansion: s3=s4=5.89 (kJ/kg K)• From 4-1, isothermal process, T4=T1=45.8°C (why?)From table A-5, when T=45.8°C, sf4=0.6491, sfg4=7.5019, hf4=191.8, hfg4=2392.8x4 = (s4-sf4)/sfg4 = (5.89-0.6491)/7.5019 = 0.699h4 = hf4+x4* hfg4 = 191.8+0.699(2392.8) = 1864.4 (kJ/kg)• At the inlet of the pump: saturated liquid h1=hf1=191.8qout = h4-h1=1672.6(kJ/kg)• At the outlet of the pump: compressed liquid v2=v1=vf1=0.00101(m3/kg)work input to pump Win = h2-h1 = v1 (P2-P1) = 0.00101(6000-10) = 6.05h2 = h1 + v1 (P2-P1) =191.8 + 6.05 = 197.85 (kJ/kg)• In the boiler, qin=h3-h2=2784.3-197.85=2586.5(kJ/kg)Solution (cont.)(a) The thermal efficiency = 1-qout/qin= 1-1672.6/2586.5=0.353=35.3%(b) Net work output dW/dt=50MW=(dm/dt)(Wout-Win)=(dm/dt)((h3-h4)-(h2-h1)) mass flow rate (dm/dt)=50000/((2784.3- 1864.4 )-(197.85-191.8))=54.7(kg/s)( c) heat transfer into the boiler qin = (dm/dt)(h3-h2)=54.7(2586.5)=141.5(MW)(d) Inside the condenser, the cooling water is being heated from the heat transfered from the condensing steam.q cooling water = qout = (dm/dt)(h4-h1) = 54.7(1672.6) = 91.49 (MW)(dm/dt)cooling water Cp (Tout - Tin) = q cooling water C p, water = 4.177(kJ/kg K) (dm/dt)cooling water = 91490/(4.177*(40-20)) = 1095.2 (kg/s)Very large amount of cooling water is needed Thermal Efficiency – How to enhance it?Thermal efficiency can be improved by manipulating the temperatures and/or pressures in various components(a) Lowering the condensing pressure (lowersTL, but decreases quality, x4 )(b) Superheating the steam to a higher temperature (increases TH but requires higher temp materials)(c) Increasing the boiler pressure (increases TH but requires higher temp/press materials)Ts1234(a) lower pressure(temp)sT12(b) Superheating (c) increase pressuresT1234Low qualityhigh moisture content124Red area = increase in W netBlue area = decrease in W netReheating•The optimal way of increasing the boiler pressure without increasing the moisture content in the exiting vapor is to reheat the vapor after it exits from a first-stage turbine and redirect this reheated vapor into a second turbine.boilerhigh-PturbineLow-Pturbinepumpcondenser123456Ts123564high-Pturbinelow-Pturbine4Reheat Rankine Cycle•Reheating allows one to increase the boiler pressure without increasing the moisture content in the vapor exiting from the turbine.•By reheating, the average temperature of the vapor entering the turbine is increased, thus, it increases the thermal efficiency of the cycle.•Multistage reheating is possible but not practical. One major reason is because the