1. Introduction

A vapour power cycle is a thermodynamic cycle in which the working fluid undergoes phase change between liquid and vapour to produce mechanical power.

In most practical applications, the working fluid is water/steam. Vapour power cycles are mainly used in thermal power plants to generate electricity.

The most common vapour power cycle is the Rankine Cycle, which is the ideal cycle for steam power plants.

Basic Principle

In a vapour power cycle:

1. Water is heated in a boiler to produce steam.
2. Steam expands in a turbine producing mechanical work.
3. Steam is condensed back into water.
4. Water is pumped back to the boiler.

This cycle repeats continuously to produce power.

Main Components of Vapour Power Cycle

1. Boiler

The boiler converts water into high-pressure steam by adding heat from fuel.

Functions:

• Heat addition
• Steam generation
• Increase temperature and pressure of steam

2. Steam Turbine

The steam turbine converts the thermal energy of steam into mechanical energy.

Functions:

• Expansion of steam
• Production of shaft work
• Driving an electric generator

3. Condenser

The condenser converts exhaust steam from the turbine into water.

Functions:

• Heat rejection
• Maintain low turbine back pressure
• Improve efficiency

4. Feed Pump

The pump increases the pressure of water and sends it back to the boiler.

Functions:

• Maintain continuous circulation
• Increase pressure of feed water

Rankine Cycle (Ideal Vapour Power Cycle)

The Rankine cycle consists of four thermodynamic processes.

Process 1–2: Isentropic Compression (Pump)

Water from the condenser is pumped to the boiler pressure.

Characteristics:

• Pressure increases
• Temperature slightly increases
• Work is done on the fluid

$$W_p = v(P_2 – P_1)$$Wp​=v(P2​−P1​)

Process 2–3: Constant Pressure Heat Addition (Boiler)

Water is heated in the boiler and converted into high-pressure steam.

Stages:

1. Water heating
2. Evaporation
3. Superheating (in many plants)

Heat added:$$Q_{in} = h_3 – h_2$$

Process 3–4: Isentropic Expansion (Turbine)

High-pressure steam expands in the turbine producing work.

Characteristics:

• Pressure decreases
• Temperature decreases
• Work output is produced

$$W_t = h_3 – h_4$$Wt​=h3​−h4​

Process 4–1: Constant Pressure Heat Rejection (Condenser)

Steam leaving the turbine is condensed into water.

Characteristics:

• Heat rejected
• Phase change occurs (vapour → liquid)

$$Q_{out} = h_4 – h_1$$

Thermal Efficiency of Rankine Cycle

Thermal efficiency is the ratio of net work output to heat supplied.$$\eta = \frac{W_{net}}{Q_{in}}$$η=Qin​Wnet​​

Where$$W_{net} = W_t – W_p$$Wnet​=Wt​−Wp​

Thus,$$\eta = \frac{(h_3 – h_4) – (h_2 – h_1)}{h_3 – h_2}$$

Modified Vapour Power Cycles

To improve efficiency, several modifications of the Rankine cycle are used.

1. Reheat Cycle

In the reheat cycle, steam expands in two stages.

Process:

1. Steam expands in high-pressure turbine
2. Steam returns to boiler for reheating
3. Steam expands again in low-pressure turbine

Advantages

• Reduces turbine blade erosion
• Improves efficiency
• Increases turbine work output

2. Regenerative Cycle

In the regenerative cycle, some steam is extracted from the turbine to heat the feed water before entering the boiler.

Advantages

• Improves cycle efficiency
• Reduces fuel consumption
• Increases average temperature of heat addition

Comparison: Gas Power Cycle vs Vapour Power Cycle

Feature	Gas Power Cycle	Vapour Power Cycle
Working Fluid	Gas (air)	Water/steam
Phase Change	No phase change	Phase change occurs
Main Cycle	Otto, Diesel, Brayton	Rankine
Application	IC engines, gas turbines	Steam power plants

Applications of Vapour Power Cycle

1. Thermal power plants
2. Nuclear power plants
3. Marine propulsion systems
4. Industrial power generation
5. Geothermal power plants

Advantages of Vapour Power Cycle

• High power output
• Suitable for large-scale electricity generation
• Continuous operation possible
• Reliable technology

Limitations

• Large equipment size
• High initial cost
• Water treatment required
• Lower efficiency compared to combined cycles