Week 6. Vapor And Combined Power Cycles I
GENESYS Laboratory
Objectives
1. Analyze vapor power cycles in which the working fluid is alternately vaporized and condensed.
2. Investigate ways to modify the basic Rankine vapor power cycle to increase the cycle thermal efficiency
3. Analyze the reheat and regenerative vapor power cycles
4. Analyze power cycles that consist of two separate cycles known as combined cycles and binary cycles
5. Analyze power generation coupled with process heating called
cogeneration
The Carnot Vapor Cycle
• Impracticalities of Carnot vapor cycle
1. Limiting the heat transfer process to two-phase systems severely limits the maximum temperature that can be used in the cycle. Limiting the maximum temperature in the cycle also limits the thermal efficiency.
2. The quality of the steam decrease during this process→steam with a high moisture content. The impingement of liquid droplets on the turbine blades causes erosion and is a major source of wear.
3. First, it is not easy to control the condensation process so precisely as to end up with the desired quality at state 4. Second, it is not practical to design a
compressor that handles two phases.
T-s diagram of two Carnot vapor cycles
Issues: Isentropic compression to extremely high pressures& isothermal
heat transfer at variable pressures
GENESYS Laboratory
Easy to achieve, but..
hard to accomplish 374
Rankine Cycle: The Ideal Cycle for Vapor Power Cycles
The simple ideal Rankine cycle
• Four process
1→2 : Isentropic compression in a pump
2→3 : Constant pressure heat addition in a boiler 3→4 : Isentropic expansion in a turbine
4→1 : Constant pressure heat rejection in a condenser
Energy Analysis of the Ideal Rankine Cycle
All four components of the Rankine cycle (pump, boiler, turbine ,condenser) are steady-flow device
The steady-flow energy equation per unit mass is
1in out in out e i
pump,in 2 1 2 1 1 1
(kJ/kg)
The boiler & condenser do not involve any work,
and the pump and the turbine are assumed to be isentropic
( 0) : or f @ P &
pump q w h h v P P h h
q q w w h h
v v
1
net ou
in 3 2
turbine,o
net in out turbine,out pump,in
ut 3 4
out 4
t th
in i
1
n
The thermal efficiency of the Rankine cycle is 0 :
0
1 : ( 0) :
f @ P
v boiler w q h h
turbine q w h h
condens
w
er w q
q w q q w w
h
q
h
q
GENESYS Laboratory
Ex 1) The Simple Ideal Rankine Cycle
Ex 1-1) The Simple Ideal Rankine Cycle
Determine the efficiency of a Rankine cycle using steam as the working fluid in which the condenser pressure is 10 kPa. The boiler pressure is 2 MPa. The steam leaves the boiler as saturated vapor.
Deviation of Actual Vapor Power Cycles from Idealized Ones
• Due to irreversibilities: Fluid friction and heat loss to the surroundings
• Fluid friction causes pressure drops in the boiler, the condenser, and the piping between various components
• A pump requires a greater work input, and a turbine produces a smaller work output
• Under ideal conditions, the flow through these devices is isentropic
• The deviation of actual pumps and turbines from the isentropic ones can be accounted for by utilizing isentropic efficiencies
3 4
3 4
a a
T
s s
w h h
w h h
2 1
2 1
s s
P
a a
w h h
w h h
Ex 2) An Actual Steam Power Cycle
GENESYS Laboratory
