Week 7. Vapor And Combined Power Cycles II
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
How Can We increase The Efficiency of The Rankine Cycle?
• The average fluid temperature should be as high as possible during heat addition and as low as possible during heat rejection
• Three ways
1. Lowering the condenser pressure (lowers Tlow,avg)
- Side effect
•It increases the moisture content of the steam at the final stages of the turbine.
• The presence of large quantities of moisture is highly undesirable in turbines because it
decreases the turbine efficiency and erodes the turbine blades.
• Fortunately, this problem can be corrected, as discussed next
GENESYS Laboratory
How Can We increase The Efficiency of The Rankine Cycle?
2. Superheating the steam to high temperatures (increases Thigh,avg)
3. Increasing the boiler pressure (increases Thigh,avg) - Good effect
• It increases the net work
• It decreases the moisture content of the steam at the turbine exit
-Limitation
• The temperature to which steam can be superheated
• Presently the highest steam temperature allowed at the turbine inlet is about 620oC.
New materials are needed. (e.g. ceramics)
Increasing the average temperature during the heat addition process
- Side effect
• The moisture content of steam at the turbine exit increase
• It can be corrected by reheating the steam
How Can We increase The Efficiency of The Rankine Cycle?
• Operating pressures of boiler have gradually increased, today many modern steam power plants operate at supercritical pressures.
A supercritical Rankine cycle (e.g. CO2 Rankine)
GENESYS Laboratory
Ex 3) Effect of Boiler Pressure and Temperature on Efficiency
The Ideal 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?
Two possibilities
1. Superheat the steam to very high temperatures before it enters the turbine 2. Expand the steam in the turbine in two stages, and reheat it in between
Commonly used in modern steam power plants
5
4
2 3 reheat
primary in
h h h
h W
W W
h h h
h q
q q
The total heat input and the total turbine work output for a reheat cycle become
The Ideal Reheat Rankine Cycle
• The cycle efficiency can be improved by 4 to 5 percents.
• The average temperature during the reheat process can be increased by increasing the number of expansion and reheat stages
• Remember that the sole purpose of the reheat cycle is to reduce the moisture content of the steam at the final stages of the expansion process. If we had materials that could withstand sufficiently high temperatures, there would be no need for the reheat cycle.
GENESYS Laboratory
Ex 4) The Ideal Reheat Rankine Cycle
Ex 4-1) The Ideal Reheat Rankine Cycle
Consider a reheat cycle utilizing steam. Steam leaves the boiler and enters the turbine at 4 Mpa, 400oC. After expansion in the turbine to 400 kPa, the steam is reheated to 400oC And then expanded in the low-pressure turbine to 10 kPa. Determine the cycle efficiency.
