Basic Thermodynamics Cycles used in Power Plant

1. POWER PLANT
ENGINEERING
Submitted By
Rohit Raj(UE124053)
Rishi Kumar(UE124050)

2. WHAT IS THERMODYNAMICS ?
Thermodynamics is a branch of physics concerned
with heat and temperature and their relation to energy
and work. It defines macroscopic variables, such as
internal energy, entropy, and pressure, that partly
describe a body of matter or radiation.
Processes of thermodynamics :
.) Isobaric process
.) Isochoric process
.) Isothermal process
.) Adiabatic process

3.  An isobaric process occurs at constant pressure. An
example would be to have a movable piston in a
cylinder, so that the pressure inside the cylinder is
always at atmospheric pressure, although it is isolated
from the atmosphere. In other words, the system
is dynamically connected, by a movable boundary, to
a constant-pressure reservoir.
 An isochoric process is one in which the volume is
held constant, meaning that the work done by the
system will be zero. It follows that, for the simple
system of two dimensions, any heat energy transferred
to the system externally will be absorbed as internal
energy.

4.  An isothermal process occurs at a constant
temperature. An example would be to have a system
immersed in a large constant-temperature bath. Any
work energy performed by the system will be lost to
the bath, but its temperature will remain constant. In
other words, the system is thermally connected, by a
thermally conductive boundary to a constant-
temperature reservoir.
 An adiabatic process is a process in which there is no
energy added or subtracted from the system by
heating or cooling.

5. Thermodynamic Cycle :
A thermodynamic cycle consists of a linked sequence
of thermodynamic processes that involve transfer of heat
and work into and out of the system, while varying
pressure, temperature, and other state variables within the
system, and that eventually returns the system to its initial
state. In the process of passing through a cycle, the
working fluid (system) may convert heat from a warm
source into useful work, and dispose of the remaining heat
to a cold sink, thereby acting as a heat engine. Conversely,
the cycle may be reversed and use work to move heat
from a cold source and transfer it to a warm sink thereby
acting as a heat pump.

6. Thermodynamics cycles used in the
generation of electricity are :
1.) Rankine Cycle
2.) Brayton Cycle
3.) Diesel Cycle
4.) Otto Cycle

7. Rankine Cycle
All large steam based power plants operate on the
Rankine cycle. Basically, the Rankine cycle utilizes a
steam boiler to produce high pressure, high temperature
steam. The steam, leaving the boiler at temperatures and
pressures as high as 1000 F and 4500 psig, is routed
through a steam turbine where it is expanded to produce
shaft work that drives an electric generator.

8. In order to increase the efficiency of the overall process,
the expansion of the steam is generally performed in
stages. After passing through a high pressure turbine
stage, the steam is returned to the steam generator to be
reheated. After the final expansion stage, the steam is
routed to a condenser, where it is returned fully to liquid
form and pumped back to the steam generator. By using
this method of power production, electric facilities are
able to approach 40% efficiency.

9. Physical layout of the four main devices used in
the Rankine cycle :

10. While many substances could be used as the working
fluid in the Rankine cycle, water is usually the fluid of
choice due to its favorable properties, such as its non-
toxic and unreactive chemistry, abundance, and low
cost, as well as its thermodynamic properties. By
condensing the working steam vapor to a liquid the
pressure at the turbine outlet is lowered and the
energy required by the feed pump consumes only 1%
to 3% of the turbine output power and these factors
contribute to a higher efficiency for the cycle.

11. Brayton Cycle (Carnot Cycle)
The Brayton cycle is the gas turbine power cycle. It is an
open cycle where ambient air is compressed to a high
temperature and pressure before it is fed to the combustion
chamber. In the combustion chamber, the air-fuel mixture
is ignited, dramatically increasing the temperature of the
mixture. These hot gases are then expanded in a turbine,
which can be coupled to an electric generator or some
other load.


12. The term Brayton cycle has more recently been given to
the gas turbine engine. This also has three components:
 A gas compressor
 A burner (or combustion chamber)
 An expansion turbine

13. Otto and Diesel Cycles
The Otto and Diesel cycles are used to power
reciprocating engine generators. They are treated as a
closed, 4 stroke cycles for the purpose of analysis. The
two cycles are similar in theory, with one major
difference; the Otto cycle is a spark-ignition cycle,
whereas the Diesel cycle is a compression ignition
cycle. In the Otto cycle, the piston begins in the top of
the cylinder. As the piston moves down into the
cylinder, the air-fuel mixture is fed into the
combustion chamber.

14. The piston then rises to the top of the cylinder,
compressing the mixture. A spark is ignited inside the
cylinder, burning the air-fuel mixture and forcing the
piston to the bottom of the cylinder. As the piston is
forced down, it turns a crankshaft, generating work.
The piston then rises to the top of the cylinder,
compressing the mixture. A spark is ignited inside the
cylinder, burning the air-fuel mixture and forcing the
piston to the bottom of the cylinder. As the piston is
forced down, it turns a crankshaft, generating work.
The piston returns to the top position, forcing out the
exhaust and completing the cycle. The Diesel cycle
operates similarly.

15. The main difference is that only air is fed into the
combustion chamber during the first process. The fuel
is injected after the air is compressed to near top-dead-
center (TDC). At this point, the air inside the cylinder
is hot enough to cause the fuel to burn without a
spark. The remaining two processes are identical to
the Otto cycle. In the Otto cycle, the air-fuel mixture
does not get hot enough to burn before the spark is
ignited. The compression of the air-fuel (or air) is the
physical process which controls this. Compression
ratios for the Diesel cycle are higher than that for the
Otto cycle, causing the compressed temperature inside
a Diesel cycle cylinder to be higher than the Otto cycle
cylinder.

16. In fact, the auto-ignition temperature of the air-fuel
mixture limits the compression ratio of the Otto cycle.
If this temperature is exceeded, spontaneous
combustion (known as knocking) takes place at the
wrong time in the cycle, robbing the engine of power
and efficiency.
Diesel and Otto cycles are used in smaller applications,
ranging from 10's of kW to 2-3 MW. Boosting the
output of reciprocating engines is possible through
either turbo charging or supercharging the engine.
Both processes increase the input pressure of air-fuel
fed to the motor, allowing it to produce more power.
This lowers the cost on a kW basis, but it also
decreases the efficiency and the life span of the engine.

17. The four strokes of the cycle are intake,
compression, power, and exhaust. Each
corresponds to one full stroke of the piston;
therefore, the complete cycle requires two
revolutions of the crankshaft to complete.
Intake stroke
During the intake stroke, the piston moves downward,
drawing a fresh charge of vaporized fuel/air mixture.
The illustrated engine features a poppet intake valve
which is drawn open by the vacuum produced by the
intake stroke.

18. Compression stroke
As the piston rises, the poppet valve is forced shut by the
increased cylinder pressure. Flywheel momentum drives
the piston upward, compressing the fuel/air mixture.
Power stroke
At the top of the compression stroke, the spark plug fires,
igniting the compressed fuel. As the fuel burns it expands,
driving the piston downward.
Exhaust stroke
At the bottom of the power stroke, the exhaust valve is
opened by the cam/lifter mechanism. The upward stroke of
the piston drives the exhausted fuel out of the cylinder.

19. Example
Intake Stroke Compression Stroke

20. Power Stroke Exhaust Stroke

21. THANK YOU