Thermodynamic laws, process and cycles in thermal power plants

1. THERMODYNAMIC
LAWS IN THERMAL
POWER PLANT
By SUGAM PARNAMEE

2. The basic energy cycle involved in the plant
is as follows :
Chemical Energy
Mechanical Energy
Electrical Energy
Coal, Steam, Fossil
fuels, Uranium, Thorium,
Natural Gas etc.
Steam generation by
boiler and flowing
through turbine or
alternator
By Turbine, Alternator
THERMAL POWER PLANT

3. A Layout of COAL POWER PLANT
Next is Thermodynamic
Processes

4.  Isobaric processes. (P=0)
 Isothermal Processes. (T=0)
 Adiabatic processes. (Q=0)
 Isentropic Processes. (S=0)
 Isochoric processes. (V=0)
 Throttling.
Throttling process involves the
passage of a higher pressure fluid
through a narrow constriction.
• The effect is the reduction in
pressure and increase in volume.
• This process is adiabatic, but not
isentropic and is irreversible.
• The entropy of the fluid actually
increases.
Such a process occurs in a flow
through a porous plug, a partially
closed valve and a very narrow
The throttling process is
commonly used for the following
purposes :
1. For determining the condition of
steam (dryness fraction)
2. For controlling the speed of the
turbine
3. Used in refrigeration plants
4. Liquefaction of gases.
5. In the petrochemical industry
6. In many cryogenic applications.
THERMODYNAMIC PROCESSES
Next is Thermodynamic Laws

5. LAW
1
s
t
0
t
h
Let’s take a look on each
one by one
LAWS OF THERMODYNAMICS
LAWS
1st
0th
3rd
2nd
 The zeroth law of thermodynamics:
recognizes that if two systems are in
thermal equilibrium with a third, they are
also in thermal equilibrium with each
other, thus supporting the notions of
temperature and heat.
 The first law of thermodynamics
distinguishes between two kinds of
physical process, namely
• Energy transfer as work and
• Energy transfer as heat.
 Principle of conservation of energy
∆Q= ∆U + p.dv
 The second law of thermodynamics:
distinguishes between reversible and
irreversible physical processes.
 It says that the full conversion of heat to
the equivalent amount of work is not
possible.
 That means some energy need
to sink at some lower
temperature.
 The third law of thermodynamics:
concerns the entropy of a perfect
crystal at absolute zero temperature,
and implies that it is impossible to cool
a system to exactly absolute zero.
Next is Power Cycles

6.  CARNOT CYCLE
 RANKINE CYCLE
 BRAYTON CYCLE
 COMBINED CYCLES
Let’s take a look on each
one by one
POWER CYCLES

7. T-s diagram of Carnot vapor cycles.
1-2 isothermal heat
addition in a boiler
2-3 isentropic expansion
in a turbine
3-4 isothermal heat
rejection in a condenser
4-1 isentropic
compression in a
compressor
 The Carnot cycle can be thought of as the most efficient (and Idealised) heat
engine cycle allowed by physical laws.
 It consists of two isothermal processes and two adiabatic processes.
 When the second law of thermodynamics states that not all the supplied heat in a
heat engine can be used to do work, the Carnot efficiency sets the limiting value
on the fraction of the heat which can be so used.
 In order to approach the Carnot efficiency, the processes involved in the heat
engine cycle must be reversible and involve no change in entropy. This means
that the Carnot cycle is an idealization
CARNOT CYCLE
W is the work done by the system (energy exiting the system as work),
QH is the heat put into the system (heat energy entering the system), TC is
the absolute temperature of the cold reservoir,
and TH is the absolute temperature of the hot reservoir.
Next is Rankine
Cycle

8.  The Rankine cycle most closely describes the process by which steam-
operated heat engines most commonly found in power generation plants to
generate power.
Next is Efficiency of Rankine
Cycle
Let’s take a look on each
one by one
RANKINE CYCLE

9. 
 Heat Input = Q23 = H3 –H2
 Heat Rejected = Q41 = H4 –H1
 Work Output = W34 = H3 – H4
 Work done by Pump = W12 = H2 – H1
Work output – Pump work W34 – W12
Heat Input Q23
“the rankine cycle has a lower efficiency compared to corresponding
Carnot cycle with the same maximum and minimum
temperatures.”
η = =
Thermal Efficiency of Rankine Cycle:
Next is 2nd Law analysis for Rankine
Cycle

10.  The Rankine cycle followed by powerplants is not a totally reversible
cycle, it is onlyinternally reversible, since heat transfer through a
finite temperature difference (between the furnace and the boiler or
between the condenser and the external medium) can results in
irreversibilities.
 The second law of thermodynamics can be used in order to reveal the
regions where the largest irreversibilities within Rankine cycleoccur.
 It will be possible, therefore, to act on these regions to reducethe
irreversibilities.
 To do this we must compute the exergy destruction for each component of
the cycle.
Fluid friction and heat loss to the
surroundings are the two
common sources of
irreversibilities.
2nd LAW ANALYSIS OF RANKINE
CYCLE
Exergy is the maximum useful work
that can be obtained from a system
till the system achieve its dead state.
(Now for dead state, refer Thermodynamics book)
Increase in reversibility cause
decrease in Exergy
Next is “How to improve thermal efficiency of Rankine
Cycle

11. The basic idea behind all the modifications is to increase the thermal efficiency
of a power cycle.
Increase the average temperature at which heat is transferred to the working
fluid in the boiler, or decrease the average temperature at which heat is rejected
from the working fluid in the condenser.
HOW TO IMPROVE EFFICIENCY
• Lowering the Condenser Pressure (Lowers Tlow,avg)
• Superheating the Steam to High Temperatures (Increases Thigh,avg)
• Increasing the Boiler Pressure (Increases Thigh,avg)Side effect: Lowering the condenser
pressure increases the moisture content of
the steam at the final stages of the turbine.
• Superheating the Steam to High Temperatures (Increases Thigh,avg)
 Both the net work and heat input increase as a
result of superheating the steam to a higher
temperature. The overall effect is an increase in
thermal efficiency since the average temperature
at which heat is added increases.
 Superheating to higher temperatures
decreases the moisture content of the
steam at the turbine exit, which is desirable.
• Increasing the Boiler Pressure (Increases Thigh,avg)
Side effect:For a fixed turbine inlet
temperature, the cycle shifts to the left and
the moisture content of steam at the turbine
exit increases.
Overcome: This side effect can be corrected
by reheating the steam.
Today many modern steam power plants
operate at supercritical pressures (P >
22.06 MPa) and have thermal efficiencies
of about 40% for fossil-fuel plants and 34%
for nuclear plants.
Next is Brayton Cycle
Let’s take a look on each
one by one

12. BRAYTON CYCLE
 Also known as Gas turbine Cycle because turbines are rotated on the expanse of
natural gases using this cycle in an open system usually.
 Three main parts:-Compressor, Combustor, Turbine.
 4 Processes are involved here in this cycle.
 All four processes of the Brayton cycle are executed in “Steady Flow Devices”.
 Thermal Efficiency () of an ideal Brayton cycle depends on the pressure ratio of
the gas turbine and the specific heat ratio of the working fluid.
1-2: Isentropic compression
(in a compressor)
2-3: Isobaric heat addition
3-4: Isentropic expansion (in
a turbine)
4-1: Isobaric heat rejection
Such devices uses steady flow processes.
These process include:
“Inlet mass flow rate=Outlet mass flow rate
Inlet Energy flow rate= Outlet Energy flow rate”
in a volume controlled open system.
These are interconvertible into each other
Next is Combined
Cycle

13. A gas turbine generator generates electricity and waste heat
is used to make steam to generate additional electricity via a
steam turbine.
i.e., We are using both Brayton as well as Rankine cycle
together.
By doing this definitely efficiency increases but involve higher
one time cost and maintenance cost.
COMBINED CYCLE
WHAT is this actually!
It’s nothing special but we
are just reusing the
remaining part of what we
have invested
i.e., waste heat energyGas turbine burns fuel:
The fast-spinning turbine drives a generator that converts a portion of
the spinning energy into electricity.
Heat recovery system captures exhaust:
The HRSG creates steam from the gas turbine exhaust heat
and delivers it to the steam turbine and thus some additional
electricity generates.
THE
END

14. Thank
You