POWER PLANT CYCLES

PRESENTED BY :

SUVAMAY SAHOO ROLL -001010301030BCHE -IV

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

POWER CYCLES :

CARNOT CYCLERANKINE CYCLEOTTO CYCLE DIESEL CYCLEBRAYTON CYCLE

SIMPLE STEAM POWER PLANT

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 .

STEP 12 isothermal heat absorption process at TH Vaporization process occurs also at constant pressure Produce sat steam from sat liq .

STEP 23 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 34 Isothermal

heat rejection step at TC .•Represented by horizontal line .

STEP 41 takes cycle back to its original state .Producing sat water at point 1.

Though we get the HIGHEST POSSIBLE EFFICIENCY , there are severe practical difficulties :

STEP 23 TURBINE that take Sat steam produces an exhaust with a high liquid content ,which cause severe erosion .

STEP 41 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 .

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

TYPES OF RANKINE CYCLE

Ideal Rankine Cycle

Re-heat Rankine Cycle

Re-generation Rankine Cycle

STEP 12 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 23 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 12 shifts the vertical line far enough to the right so that moisture content is not too large.

STEP 41 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)

REHEAT RANKINE CYCLE

How can we take advantage of the increased efficiencies at higher boilerpressures without facing the problem of excessive moisture at the finalstages of the turbine?1. Superheat the steam to very high temperatures before it enters theturbine. This would be the desirable solution since the average temperatureat which heat is added would also increase, thus increasing the cycle efficiency.This is not a viable solution, however, since it requires raising thesteam temperature to metallurgically unsafe levels.2.Expand the steam in the turbine in two stages, and reheat it inbetween. In other words, modify the simple ideal Rankine cycle with areheat process. Reheating is a practical solution to the excessive moisture problem in turbines, and it is commonly used in modern steam power plants.

boiler

high-Pturbine

Low-Pturbine

pump

condenser

T

s

high-Pturbine

2

3

4

5 6

1

expansion process takes place in two stages. In the first stage (the highpressureturbine), steam is expanded isentropically to an intermediate pressureand sent back to the boiler where it is reheated at constant pressure,usually to the inlet temperature of the first turbine stage. Steam then expandsisentropically in the second stage (low-pressure turbine) to the condenserpressure.

LOW press

The incorporation of the single reheat in a modern power plant improvesthe cycle efficiency by 4 to 5 percent by increasing the average temperatureat which heat is transferred to the steam.The average temperature during the reheat process can be increased byincreasing 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)

REGENERATIVE RAKINE CYCLE

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

s1

2

2’

3

4

Lower tempheat addition

T

s1

23

4

5

6

7

Use regenerator to heat up the feedwater

higher tempheat addition

Extract steam fromturbine to provideheat source in theregenerator

HOW CAN WE INCREASE THE EFFICIENCYOF 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.

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