Qin A Vapor Power Cycle 2 3 Boiler Win Compressor (pump) Wout

Qin A Vapor Power Cycle 2 3 Boiler Win Compressor (pump) Wout Turbine Heat exchanger (Condenser) 1 4 Qout T P 2 1 3 4 3 2 1 4 v

Optimization of a Vapor Power Plant Objectives: 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 cycle 3 T T 2 3 OR (a) 1 Modifed: 11/5/01 2 (b) 1 4 4 s s

Practical 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. Qin 2 3 Boiler Wout Win Compressor (pump) 1 3 T 2 Turbine Heat exchanger (Condenser) 1 4 4 Qout 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 ). s

Ideal 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) 1-2: Pump (q 0) Wpump h2 - h1 v(P2-P1) 3 T 2-3: Boiler (W 0) qin h3 - h2 2 1 3-4: Turbine (q 0) Wout h3 - h4 4 s 4-1: Condenser (W 0) qout h4 - h1 Thermal efficiency Wnet/qin 1 - qout/qin 1 - (h4-h1)/(h3-h2) Wnet Wout - Win (h3-h4) - (h2-h1)

Qin 2 Win Example - Ideal Rankine Cycle 3 boiler Turbine pump condenser 1 4 Qout T 3 2 1 4 s Wout Consider 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.

Solution 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 2392.8 x4 (s4-sf4)/sfg4 (5.89-0.6491)/7.5019 0.699 h4 hf4 x4* hfg4 191.8 0.699(2392.8) 1864.4 (kJ/kg) At the inlet of the pump: saturated liquid h1 hf1 191.8 qout 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) 6.05 h2 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) T 3 2 2 1 T (c) increase pressure 3 (b) Superheating 1 2 4 T 4 1 4 s (a) lower pressure(temp) Low quality high moisture content 2 s Red area increase in W net Blue area decrease in W net 1 s

Reheating 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. high-P turbine T 3 high-P turbine Low-P turbine boiler low-P turbine 4 4 5 6 pump 1 condenser 4 2 1 2 5 3 6 s

Reheat 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 vapor exiting will be superheated vapor at higher temperature, thus, decrease the thermal efficiency. Why? Energy analysis: Heat transfer and work output both change qin qprimary qreheat (h3-h2) (h5-h4) Wout Wturbine1 Wturbine2 (h3-h4) (h5-h6)

Regeneration From 2-2’, the average temperature is very low, therefore, the heat addition process is at a lower temperature and therefore, the thermal efficiency is lower. Why? Use a regenerator to heat the liquid (feedwater) leaving the pump before sending it to the boiler. This increases the average temperature during heat addition in the boiler, hence it increases efficiency. Lower temp heat addition T 3 T higher temp heat addition 2’ 4 2 2 1 5 4 s Extract steam @ 6 From turbine to provide heat source in the regenerator 6 3 1 7 s Use regenerator to heat up the feedwater

Regenerative Cycle Improve efficiency by increasing feedwater temperature before it enters the boiler. Two Options: – Open feedwater : Mix steam with the feedwater in a mixing chamber. – Closed feedwater: No mixing. Open FWH 5 T boiler 6 (y) Open FWH 4 Pump 2 3 5 4 7 (1-y) 2 2 (y) 6 (1-y) 3 1 7 s Pump 1 1 condenser

Regenerative Cycle - Analysis Assume y percent of steam is extracted from the turbine and is directed into open feedwater heater. Energy analysis: qin h5-h4, qout (1-y)(h7-h1), Wturbine, out (h5-h6) (1-y)(h6-h7) Wpump, in (1-y)Wpump1 Wpump2 (1-y)(h2-h1) (h4-h3) (1-y)v1(P2-P1) v3(P4-P3) In general, more feedwater heaters result in higher cycle efficiencies.