Short Answer:

Increasing the boiler pressure in the Rankine cycle improves its thermal efficiency by raising the average temperature at which heat is added to the working fluid. This reduces the amount of heat wasted and increases the net work output. As a result, more electricity is generated from the same amount of fuel.

When boiler pressure is increased, steam enters the turbine at a higher enthalpy, leading to greater energy extraction during expansion. This change helps the Rankine cycle move closer to the ideal Carnot cycle, resulting in better overall efficiency of thermal power plants.

Detailed Explanation:

Increasing boiler pressure in Rankine cycle

The Rankine cycle is the basic operating cycle for steam power plants, where water is heated in a boiler, converted to steam, expanded in a turbine, and then condensed back to water. The efficiency of this cycle depends on the temperature difference between the heat source (boiler) and heat sink (condenser).

By increasing the boiler pressure, the saturation temperature of steam also increases. This allows the cycle to absorb heat at a higher average temperature, which directly improves the thermal efficiency as per the second law of thermodynamics.

Effect of Boiler Pressure on Cycle Efficiency

1. Higher Saturation Temperature
   At higher boiler pressures, the boiling point of water rises. This means steam can be generated at higher temperatures, which increases the mean temperature of heat addition.
2. Increased Enthalpy at Turbine Inlet
   High-pressure steam contains more energy (enthalpy). When this steam expands in the turbine, it delivers more mechanical work.
3. Approaching Carnot Efficiency
   Thermal efficiency is higher when heat is added at a higher temperature range. Increasing boiler pressure helps the Rankine cycle move closer to the Carnot cycle, which is the most efficient theoretical cycle.
4. Lower Heat Rejection
   For a given condenser pressure, increasing boiler pressure reduces the relative heat rejection, thus increasing the net work done.
5. Reduced Steam Flow Rate
   Since high-pressure steam carries more energy, less steam is needed to produce the same amount of work, reducing the size of equipment and fuel consumption.

T-s Diagram Explanation

In a Temperature-Entropy (T-s) diagram:

• The boiler pressure increase shifts the boiler line upward (higher temperature).
• The area under the turbine expansion curve (representing work output) becomes larger.
• The cycle appears taller and more stretched, indicating greater work and efficiency.

Practical Considerations

While increasing boiler pressure improves efficiency, it also introduces challenges:

• Material strength: High pressure requires stronger and more expensive materials to withstand stress.
• Safety: Higher pressures need better control systems and safety valves.
• Design complexity: Components like boilers, turbines, and piping must be designed for higher operating pressures.

Despite these challenges, the benefits outweigh the costs in large-scale power plants, making high-pressure boilers a common choice.

Real-World Examples

1. Superheated Steam Power Plants
   Operate at pressures above 100 bar with supercritical steam to extract maximum efficiency.
2. Ultra-Supercritical Power Plants
   Use steam at pressures beyond the critical point (around 221 bar) and reach efficiencies above 45%.
3. Combined Cycle Power Plants
   Use high-pressure Rankine cycle as the bottoming cycle for even better fuel utilization.

Conclusion

Increasing boiler pressure in the Rankine cycle improves thermal efficiency by raising the average heat addition temperature, which allows more energy extraction during steam expansion. This makes the power plant more efficient, fuel-saving, and environmentally friendly. Although it demands stronger equipment and careful design, it is widely applied in modern thermal plants for higher performance and energy output.