Power plant cycle


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Power Plant cycle ( Rankine Cycle)

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    2. 2. THERMAL POWER PLANT A Thermal Power station is a Power plant in which the prime mover is steam driven .Water is heated, turns into steam and spins a steam turbine which drives an electrical generator .After it passes through the turbine , the steam is condensed in a condenser ,and recycled to where it was heated .The greatest variation in the design of thermal power stations is due to the different fuel sources .Some thermal power plants also deliver heat energy for industrial purposes , for heating or for desalination of water as well as delivering electrical power . The Basic Energy Cycle Involved : Chemical Energy Mechanical Energy Electrical Energy
    5. 5. Carnot Cycle operates reversibly and consist of two isothermal steps connected by two adiabatic steps .In the isothermal step at high temp TH , heat |QH| is absorbed by the working fluid of the engine and in the isothermal step at lower temp Tc , heat |Qc| is discarded by the fluid . The efficiency is given by Efficiencies of practical heat engines are lowered by irreversibilities , it is still true that efficiency can be increased when TH increased , TC decreased .
    6. 6. STEP 12 isothermal heat absorption process at TH Vaporization process occurs also at constant pressure Produce sat steam from sat liq . STEP 23 Reversible adiabatic expansion of sat vap to a pressure at which T sat = Tc. Isentropic expansion process represented by vertical line on T-S diagram . Produces WET vap .•STEP 34 Isothermal heat rejection step at TC . •Represented by horizontal line . STEP 41 takes cycle back to its original state . Producing sat water at point 1.
    7. 7. Though we get the HIGHEST POSSIBLE EFFICIENCY , there are severe practical difficulties : STEP 23 TURBINE that take Sat steam produces an exhaust with a high liquid content ,which cause severe erosion . STEP 41 difficulties in the design of a pump that takes a mix of liq and vap (point4) and discharges a sat liq (point 1) . For these reasons an alternative Model Cycle is taken as STANDARAD , atleast for fossil fuel burning power plants , It ‘s called RANKINE CYCLE .
    8. 8. RANKINE CYCLE Who is Rankine and What is Rankine Cycle? A Scottish CIVIL ENGINEER, physicist and mathematician. He was a founding contributor, with Rudolf Clausius and William Thomson, to the science of thermodynamics, particularly focusing on the first of the three thermodynamic laws. The Rankine cycle is a cycle that converts heat into work. The heat is supplied externally to a closed loop, which usually uses water. This cycle generates about 90% of all electric power used throughout the world
    9. 9. TYPES OF RANKINE CYCLE  Ideal Rankine Cycle  Re-heat Rankine Cycle  Re-generation Rankine Cycle
    10. 10. STEP 12 A constant pressure heating process Consist if 3 section :  Heating if subcooled liquid water to its sat temp . Vaporization at constant temp and pressure . SUPER HEATING if vapor to a temp well above its sat temp. STEP 23 Isentropic Expansion of vap in a turbine to the pressure of the condenser . STEP crosses the saturation curve .producing a wet exhaust . The super heating in step 12 shifts the vertical line far enough to the right so that moisture content is not too large.
    11. 11. STEP 41 Isentropic pumping of Sat liquid to the pressure of the boiler , producing compressed liquid . The vertical line is very short (Temp rise associated with compression of liquid is very small).  Energy analysis: steady flow process, no generation, neglect KE and PE changes for all four devices,  0 = (net heat transfer in) - (net work out) + (net energy flow in)  0 = (qin - qout) - (Wout - Win) + (hin - hout) • PROCESS: • 4-1: Pump (q=0)  Wpump = h4 - h1 = v(P4-P1) • 1-2: Boiler(W=0)  qin = h2 – h1 • 2-3: Turbine(q=0)  Wout = h2 – h3 • 3-4: Condenser(W=0)  qout = h3 – h4  Thermal efficiency h = Wnet/qin =  1 - qout/qin = 1 - (h3-h4)/(h2-h1)  Wnet = Wout - Win = (h2-h3) - (h4-h1)
    12. 12. REHEAT RANKINE CYCLE How can we take advantage of the increased efficiencies at higher boiler pressures without facing the problem of excessive moisture at the final stages of the turbine? 1. Superheat the steam to very high temperatures before it enters the turbine. This would be the desirable solution since the average temperature at which heat is added would also increase, thus increasing the cycle efficiency. This is not a viable solution, however, since it requires raising the steam temperature to metallurgically unsafe levels. 2.Expand the steam in the turbine in two stages, and reheat it in between. In other words, modify the simple ideal Rankine cycle with a reheat process. Reheating is a practical solution to the excessive moisture problem in turbines, and it is commonly used in modern steam power plants.
    13. 13. boiler high-P turbine Low-P turbine pump condenser T s high-P turbine 2 3 4 5 6 1 expansion process takes place in two stages. In the first stage (the highpressure turbine), steam is expanded isentropically to an intermediate pressure and sent back to the boiler where it is reheated at constant pressure, usually to the inlet temperature of the first turbine stage. Steam then expands isentropically in the second stage (low-pressure turbine) to the condenser pressure. LOW press
    14. 14. The incorporation of the single reheat in a modern power plant improves the cycle efficiency by 4 to 5 percent by increasing the average temperature at which heat is transferred to the steam. The average temperature during the reheat process can be increased by increasing the number of expansion and reheat stages  Energy analysis: Heat transfer and work output both change qin = qprimary + qreheat = (h3-h2) + (h5-h4) Wout = Wturbine1 + Wturbine2 = (h3-h4) + (h5-h6)
    16. 16. The purpose of Feed Water Heating is to rise the average Temperature at which heat is added to the boiler .This increases the thermal Efficiency of the plant , Which is said to operate on a REGENERATIVE CYCLE . T s 1 2 2’ 3 4 Lower temp heat addition T s 1 2 3 4 5 6 7 Use regenerator to heat up the feedwater higher temp heat addition Extract steam from turbine to provide heat source in the regenerator
    17. 17. HOW CAN WE INCREASE THE EFFICIENCY OF THE RANKINE CYCLE? Increase the average temperature at which heat is transferred to the working fluid in the boiler, or decrease theaverage temperature at which heat is rejected from the working fluid in the condenser. SUPER CRITICAL RANKINE CYCLE : Today many modern steam power plants operate at supercritical pressures (P > 22.06 MPa) and have thermal efficiencies of about 40 percent for fossil-fuel plants and 34 percent for nuclear plants. There are over 150 supercritical-pressure steam power plants in operation in the United States.
    18. 18. THANK YOU