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 cycles4. Analyze power cycles that consist of two separate cycles known as

combined cycles and binary cycles5. Analyze power generation coupled with process heating called

cogeneration

GENESYS Laboratory

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 heataddition process- Side effect• The moisture content of steam at the turbine exit increase• It can be corrected by reheating the steam

GENESYS Laboratory

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

GENESYS Laboratory

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 possibilities1. Superheat the steam to very high temperatures before it enters the turbine2. Expand the steam in the turbine in two stages, and reheat it in between Commonly used in modern steam power plants

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The total heat input and the total turbine work output for a reheat cycle become

GENESYS Laboratory

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 contentof the steam at the final stages of the expansion process. If we had materials that couldwithstand sufficiently high temperatures, there would be no need for the reheat cycle.

GENESYS Laboratory

Ex 4) The Ideal Reheat Rankine Cycle

GENESYS Laboratory

Ex 4-1) The Ideal Reheat Rankine Cycle

Consider a reheat cycle utilizing steam. Steam leaves the boiler and enters the turbine at4 Mpa, 400oC. After expansion in the turbine to 400 kPa, the steam is reheated to 400oCAnd then expanded in the low-pressure turbine to 10 kPa. Determine the cycle efficiency.

GENESYS Laboratory