Short Answer:

The efficiency of the Rankine cycle is the ratio of the net work output of the cycle to the heat energy supplied in the boiler. It measures how effectively the cycle converts heat into useful work. The efficiency mainly depends on boiler pressure, condenser pressure, and the temperature at which heat is added and rejected.

In simple words, Rankine cycle efficiency tells how much of the supplied heat energy is converted into mechanical or electrical energy. Higher steam temperature and pressure increase the efficiency, while high condenser pressure decreases it.

Detailed Explanation :

Efficiency of the Rankine Cycle

The efficiency of the Rankine cycle defines how effectively the thermal energy supplied to the working fluid (steam) in a power plant is converted into mechanical work and then into electricity. It is one of the most important parameters that determine the performance and economy of a thermal power plant.

In the Rankine cycle, water is used as the working fluid. The cycle consists of four main processes — pumping, heating, expansion, and condensation. During these processes, heat is added in the boiler, converted into work in the turbine, and then rejected in the condenser. The difference between the heat added and the heat rejected gives the useful work output, which determines the cycle efficiency.

1. Definition of Efficiency

The thermal efficiency (η) of the Rankine cycle is defined as the ratio of the net work output of the cycle to the heat supplied to the working fluid in the boiler.

Mathematically,

Or,

Where:

•  = Work done by the turbine
•  = Work required by the pump
•  = Heat added in the boiler

Since the pump work is very small compared to the turbine work, it is often neglected for simplicity.

2. Expression for Efficiency

The Rankine cycle consists of four major energy exchanges between the system and its surroundings. These are represented by enthalpy differences at various points in the cycle.

Let:

•  = Enthalpy of water entering the pump
•  = Enthalpy of water leaving the pump (entering the boiler)
•  = Enthalpy of steam leaving the boiler (entering the turbine)
•  = Enthalpy of steam leaving the turbine (entering the condenser)

Now,

1. Work done by the turbine:

1. Work done by the pump:

1. Heat supplied in the boiler:

1. Heat rejected in the condenser:

Thus, the thermal efficiency becomes:

Substituting the above equations:

This is the actual expression for the efficiency of the Rankine cycle in terms of enthalpy.

3. Factors Affecting Efficiency

The efficiency of the Rankine cycle depends on several operating and design factors. The main factors are:

1. Boiler Pressure (High-Pressure Side)

• Increasing the boiler pressure increases the steam temperature and the average temperature of heat addition.
• This leads to an improvement in the cycle efficiency.
• However, higher boiler pressure requires stronger materials and increases costs.

1. Condenser Pressure (Low-Pressure Side)

• Lowering the condenser pressure increases the enthalpy drop across the turbine, which improves efficiency.
• But extremely low pressures can cause condensation inside the turbine and reduce its life.

1. Superheating of Steam

• Superheating increases the temperature of the steam before it enters the turbine.
• This increases the average temperature at which heat is added, thereby improving efficiency.
• It also prevents moisture formation in the turbine.

1. Reheating

• In large power plants, the steam after partial expansion in the turbine is reheated and sent back for further expansion.
• This improves efficiency and reduces turbine blade erosion.

1. Regeneration

• In regenerative cycles, a portion of steam is used to preheat the feedwater before it enters the boiler.
• This reduces the amount of heat required in the boiler and increases the overall efficiency.

4. Typical Efficiency Values

The efficiency of a simple Rankine cycle is usually 30% to 35%.
With modifications such as superheating, reheating, and regeneration, the efficiency can be increased to 40%–45% or even higher in modern power plants.

Although the Rankine cycle has a lower efficiency than the ideal Carnot cycle, it is more practical for real-world applications because it avoids condensation at high temperatures and pressures inside the turbine.

5. Comparison with Carnot Efficiency

The Carnot efficiency represents the maximum possible efficiency between two temperature limits and is given by:

Where:

•  = Absolute temperature at which heat is supplied (in Kelvin)
•  = Absolute temperature at which heat is rejected

The Rankine cycle efficiency is always lower than Carnot efficiency because in the Rankine cycle, heat is added and rejected over a range of temperatures rather than at constant temperatures.

However, by increasing the average temperature of heat addition (through superheating and regeneration) and reducing the temperature of heat rejection, the Rankine cycle efficiency can be made close to the Carnot efficiency.

6. Methods to Improve Rankine Cycle Efficiency

• Superheating the steam before entering the turbine.
• Increasing boiler pressure to raise steam temperature.
• Lowering condenser pressure to improve turbine expansion.
• Using regenerative feedwater heating to preheat water.
• Employing reheating systems to reduce moisture content in turbines.

These modifications help power plants achieve higher efficiency and reduced fuel consumption.

7. Importance of Efficiency

The efficiency of the Rankine cycle is very important for the following reasons:

• It determines the overall performance of a thermal power plant.
• Higher efficiency means less fuel consumption, resulting in lower operating costs.
• Improved efficiency reduces environmental pollution by minimizing fuel usage.
• It ensures better utilization of energy resources and reduces waste heat losses.

Thus, enhancing efficiency is the key objective in modern power plant design.

Conclusion

The efficiency of the Rankine cycle represents the effectiveness of converting heat energy into mechanical or electrical energy. It depends on the operating conditions such as boiler pressure, condenser pressure, and steam temperature. The efficiency can be expressed as the ratio of net work output to heat input, and it generally ranges between 30% and 40% for practical systems. By applying methods such as superheating, reheating, and regeneration, the Rankine cycle efficiency can be significantly improved, leading to better performance and fuel economy in thermal power plants.