Rankine Cycle

Definition

The Rankine cycle is a thermodynamic cycle that describes the operation of steam-powered turbine power plants. It is named after the Scottish engineer William John Macquorn Rankine, who was a pioneer in the field of thermodynamics and proposed this model for the efficient conversion of heat into work.

5 Must Know Facts For Your Next Test

1. The Rankine cycle consists of four key processes: isentropic expansion in a turbine, constant-pressure heat addition in a boiler, isentropic compression in a pump, and constant-pressure heat rejection in a condenser.
2. The efficiency of the Rankine cycle is limited by the maximum temperature and pressure that can be achieved in the boiler, as well as the minimum temperature in the condenser.
3. Improvements to the basic Rankine cycle, such as reheating and regenerative feedwater heating, can increase the overall efficiency of the power plant.
4. The Rankine cycle is widely used in steam-powered power plants, including coal-fired, nuclear, and some geothermal power plants, to generate electricity.
5. The performance of the Rankine cycle can be enhanced by using higher-pressure and higher-temperature steam, which requires the use of more advanced materials and engineering techniques.

Review Questions

• Explain the key processes that make up the Rankine cycle and how they contribute to the conversion of heat into work.
  • The Rankine cycle consists of four main processes: isentropic expansion in a turbine, constant-pressure heat addition in a boiler, isentropic compression in a pump, and constant-pressure heat rejection in a condenser. The isentropic expansion in the turbine converts the thermal energy of the high-pressure, high-temperature steam into mechanical work, which can then be used to generate electricity. The constant-pressure heat addition in the boiler increases the temperature and pressure of the working fluid, while the isentropic compression in the pump increases the pressure of the working fluid before it enters the boiler. Finally, the constant-pressure heat rejection in the condenser removes the waste heat from the system, completing the cycle. The efficient coordination of these processes is what allows the Rankine cycle to effectively convert heat into useful work.
• Describe how the Rankine cycle is related to the First Law of Thermodynamics and how it can be used to analyze the efficiency of a steam-powered power plant.
  • The Rankine cycle is directly related to the First Law of Thermodynamics, which states that energy can be converted from one form to another, but it cannot be created or destroyed. In the Rankine cycle, the heat added to the system in the boiler is converted into mechanical work in the turbine, in accordance with the First Law. The efficiency of the Rankine cycle can be analyzed by comparing the net work output of the cycle to the heat input, which represents the maximum theoretical efficiency. However, in real-world power plants, various irreversibilities and losses, such as friction, heat transfer limitations, and incomplete combustion, reduce the actual efficiency of the Rankine cycle below the theoretical maximum. Understanding the relationship between the Rankine cycle and the First Law of Thermodynamics is crucial for evaluating and improving the performance of steam-powered power plants.
• Discuss how modifications to the basic Rankine cycle, such as reheating and regenerative feedwater heating, can be used to improve the overall efficiency of a steam power plant.
  • The basic Rankine cycle can be modified in several ways to improve its overall efficiency. One such modification is reheating, where the steam is expanded partially in the turbine, then returned to the boiler to be reheated before further expansion. This increases the average temperature at which heat is added to the cycle, leading to higher thermal efficiency. Another modification is regenerative feedwater heating, where steam is extracted from the turbine at various stages and used to preheat the feedwater entering the boiler. This reduces the amount of heat that needs to be added in the boiler, thereby increasing the cycle's efficiency. These and other refinements to the Rankine cycle, such as the use of higher-pressure and higher-temperature steam, are crucial for improving the performance and competitiveness of steam-powered power plants in the modern energy landscape.

Related terms

Carnot Cycle: An idealized thermodynamic cycle that represents the maximum theoretical efficiency for converting heat into work. The Rankine cycle is based on the Carnot cycle but with modifications to make it more practical for real-world power plant applications.

Boiler: A device used in the Rankine cycle to convert water into high-pressure, high-temperature steam by applying heat, typically from the combustion of fuels.

Turbine: A rotary mechanical device that converts the kinetic energy of a moving fluid, such as steam, into useful work by causing a shaft to rotate.