POWER PLANT CYCLES
PRESENTED BY :
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
The Basic Energy Cycle Involved :
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
Produces WET vap .•STEP 34 Isothermal
heat rejection step at TC .
horizontal line .
STEP 41 takes cycle back to its original
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 .
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
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)
• 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 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.
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
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
qin = qprimary + qreheat = (h3-h2) + (h5-h4)
Wout = Wturbine1 + Wturbine2 = (h3-h4) + (h5-h6)
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
Use regenerator to heat up the feedwater
Extract steam from
turbine to provide
heat source in the
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.