The Rankine cycle is commonly used in power plants rather than the Carnot cycle for several practical reasons. Although the Carnot cycle is the most efficient thermodynamic cycle for converting heat into work, it is not directly applicable in real-world power plants due to certain limitations. Here are a few reasons why the Rankine cycle is preferred:
Practical limitations: The Carnot cycle assumes reversible processes, which are idealized and do not occur in real systems. Achieving reversible processes in practice is challenging and often not economically viable. The Rankine cycle, on the other hand, allows for more realistic and achievable processes.
Working fluid considerations: The Carnot cycle requires an ideal gas as the working fluid, which is not suitable for power generation applications. In power plants, liquids such as water or steam are used as working fluids due to their higher energy density and better heat transfer properties. The Rankine cycle accommodates the use of liquids as working fluids.
Condensation and heat rejection: The Carnot cycle assumes complete condensation of the working fluid during the isothermal heat rejection process, which is not practical. In the Rankine cycle, the working fluid undergoes partial condensation, allowing for easier heat rejection through cooling systems such as condensers.
Practicality of components: The Carnot cycle assumes ideal components, such as isentropic expansion and compression processes. However, real-world components, such as turbines and pumps, are not perfectly efficient. The Rankine cycle accounts for these losses and provides a more realistic representation of the performance of actual power plant components.
Flexibility and control: The Rankine cycle offers greater flexibility in terms of controlling power output. By adjusting parameters such as the operating pressure and temperature, power plant operators can optimize the system for different operating conditions, load demands, and efficiency requirements. This level of control is not easily achievable with the Carnot cycle.
Overall, while the Carnot cycle represents an idealized theoretical limit, the Rankine cycle is a practical and efficient approximation that takes into account real-world considerations, making it the preferred choice for power generation applications.