2. THERMAL POWER PLANT
The basic energy cycle
A thermal power station is a power plant in involved in the plant is
as follows :
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 Chemical Energy
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 Mechanical Energy
fuel sources. Some thermal power plants also
deliver heat energy for industrial purposes, for
district heating, or for desalination of water as
well as delivering electrical power. Electrical Energy
5. LAWS OF
THERMODYNAMICS
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. The internal energy obeys the principle of
conservation of energy but work and heat are not defined as
separately conserved quantities. ∆Q= ∆U + p.dv
Equivalently, the first law of thermodynamics states that perpetual
motion machines of the first kind are impossible.
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.
Equivalently, perpetual motion machines of the second kind are
impossible.
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.
7. CARNOT CYCLE
The Carnot cycle can be thought of as the most efficient heat engine cycle allowed by
physical laws. The most efficient heat engine cycle is the Carnot cycle, consisting 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 1-2 isothermal heat
processes involved in the addition in a boiler
heat engine cycle must be 2-3 isentropic expansion
reversible and involve no in a turbine
change in entropy. This 3-4 isothermal heat
means that the Carnot rejection in a condenser
cycle is an idealization 4-1 isentropic
compression in a
compressor
T-s diagram of Carnot vapor cycles. 7
8. CARNOT CYCLE EFFICIENCY
If W= net work output of the system in Carnot cycle, and as the system is carried out
through a cycle then there is no change in the internal energy of the
system, therefore QH – Qc = W
QH= TH (S2- S1)
The efficiency η is defined to be: (Work output)/(Heat input)
η= W/QH = (QH-Qc)/QH
also,
Where,
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.
9. CARNOT CYCLE FEASIBILTY
Carnot's theorem: No engine operating between two heat reservoirs can be more
efficient than a Carnot engine operating between those same reservoirs.
The Carnot cycle is the most efficient cycle operating between two specified
temperature limits but it is not a suitable model for power cycles. Because:
Process 1-2 Limiting the heat transfer processes to two-phase systems severely
limits the maximum temperature that can be used in the cycle (374 C for water)
Process 2-3 The turbine cannot handle steam
with a high moisture content because of the
impingement of liquid droplets on the turbine
blades causing erosion and wear.
Process 4-1 It is not practical to design a
compressor that handles two phases.
10. RANKINE TERMINOLOGY
The Rankine cycle most closely describes the process by which steam-
operated heat engines most commonly found in power generation plants to
generate power.
11. RANKINE CURVE
Process 1-2: The working fluid is
pumped from low to high
pressure, as the fluid is a liquid at
this stage the pump requires little
input energy.
Process 2-3: The high
pressure liquid enters
a boiler where it is
heated at constant
pressure by an
external heat source to
become a dry
saturated vapor.
Process 4-1: The wet vapor then
Process 3-4: The dry saturated vapor
enters a condenser where it is
expands through a turbine, generating power.
condensed at a constant
This decreases the temperature and pressure
temperature to become a saturated
of the vapor
liquid.
12.
13. Thermal Efficiency of Rankine
Cycle:
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
η= =
Heat Input Q23
“the rankine cycle has a lower efficiency compared to corresponding
Carnot cycle 2‟-3-4-1‟ with the same maximum and minimum
temperatures.”
14. Reasons for Considering Rankine Cycle as an Ideal Cycle For Steam
Power Plants:
1) It is very difficult to build a pump that will handle a mixture of liquid and vapor
at state 1’ (refer T-s diagram) and deliver saturated liquid at state 2’. It is
much easier to completely condense the vapor and handle only liquid in the
pump.
2) In the rankine cycle, the vapor may be superheated at constant pressure from
3 to 3” without difficulty. In a Carnot cycle using superheated steam, the
superheating will have to be done at constant temperature along path 3-5.
During this process, the pressure has to be dropped. This means that heat is
transferred to the vapor as it undergoes expansion doing work. This is difficult
to achieve in practice.
15. Second law analysis of Rankine cycle
The Rankine cycle is not a totally reversible cycle, it is only internally
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 cycle occur.
It will be possible, therefore, to act on these regions to reduce the
irreversibilities.
To do this we must compute the exergy destruction for each component of
the cycle.
16. MEAN TEMPERATURE
METHOD
In rankine cycle heat is added at a
constant pressure but at infinite
temperatures
If TM1 is the mean temperature of
the heat addition as shown in the
6 Tm1 figure so that the area under the
7 curve 2 to 3” is equal to the area
under 6 and 7 then the heat added
T2
is
Q23” = Tm1 (S3”- S2)
Tm1 = (H3”- H2)/(S3” – S2)
Heat rejected, Q4”1 = H4” – H1
= T2 (S4” – S1)
“The higher the mean temperature
of heat addition, higher will be the η = 1 – Q23”/Q4”1
Rankine cycle efficiency.”
η = [1 – Tm1/T2]
17. DEVIATION OF ACTUAL VAPOUR
POWER CYCLES FROM IDEALIZED
CYCLE
The actual vapor power cycle differs from the ideal Rankine cycle as a
result of irreversibilities in various components.
Fluid friction and heat loss to the surroundings are the two common
sources of irreversibilities.
Isentropic efficiencies
(a) Deviation of actual vapor power cycle from the ideal Rankine cycle.
(b) The effect of pump and turbine irreversibilities on the ideal Rankine cycle.
18. HOW TO IMPROVE
EFFICIENCY
The basic idea behind all the modifications to increase the thermal efficiency
of a power cycle is the same: 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.
Lowering the Condenser Pressure (Lowers Tlow,avg)
To take advantage of the increased
efficiencies at low pressures, the
condensers of steam power plants
usually operate well below the
atmospheric pressure. There is a lower
limit to this pressure depending on the
temperature of the cooling medium
Side effect: Lowering the condenser
pressure increases the moisture content
of the steam at the final stages of the
turbine.
The effect of lowering the condenser pressure on the ideal Rankine cycle.18
19. 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.
Constraint: The temperature is limited by
The effect of superheating the metallurgical considerations. Presently the
steam to higher temperatures highest steam temperature allowed at the
on the ideal Rankine cycle. turbine inlet is about 620 C.
19
20. Increasing the Boiler Pressure (Increases Thigh,avg)
Today many modern steam power
plants operate at supercritical
pressures (P > 22.06 MPa) and
For a fixed turbine inlet have thermal efficiencies of about
temperature, the cycle shifts to the 40% for fossil-fuel plants and 34%
left and the moisture content of for nuclear plants.
steam at the turbine exit increases.
This side effect can be corrected by
reheating the steam.
The effect of increasing the boiler
A supercritical Rankine cycle.
pressure on the ideal Rankine cycle. 20
21. THE IDEAL REHEAT 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. It is limited metallurgically.
2. Expand the steam in the turbine in two stages, and reheat it in between (reheat)
The ideal reheat Rankine cycle.
21
22. The single reheat in a modern power plant
improves the cycle efficiency by 4 to 5% 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. As
the number of stages is increased, the
expansion and reheat processes approach an
isothermal process at the maximum
temperature. The use of more than two reheat
stages is not practical. The theoretical
improvement in efficiency from the second
reheat is about half of that which results from
a single reheat. The average temperature at
which heat is transferred during
The reheat temperatures are very close or
reheating increases as the
equal to the turbine inlet temperature.
number of reheat stages is
The optimum reheat pressure is about one- increased.
