Question

Why is the Carnot cycle not a realistic model for steam power plants?

Short answer

Answer: The Carnot cycle is not a realistic model for steam power plants due to its assumptions of reversible processes, ideal working medium, requirement of isothermal expansion and compression, impractical design requirements, and its focus on maximum efficiency that cannot be achieved in real-world power plants.

Step-by-step solution

Reason 1: Reversible Processes

The Carnot cycle is based on reversible processes, meaning that they can proceed in the reverse direction while the system remains in equilibrium. In reality, real processes are irreversible, and energy losses occur due to friction, turbulence, and other factors. A real steam power plant can't operate with completely reversible processes, so it can't achieve the efficiency of a Carnot cycle.

Reason 2: Ideal Working Medium

The Carnot cycle assumes an ideal working medium that has no internal friction, does not conduct heat during adiabatic processes, and has an infinite heat capacity. However, in a steam power plant, water and steam are the working medium which exhibit real properties and can't be considered ideal. Thus, the behavior of water/steam will deviate from that of an ideal working medium, affecting the overall efficiency of the power plant.

Reason 3: Isothermal Expansion and Compression

The Carnot cycle involves isothermal expansion and compression processes, which require the working medium to maintain a constant temperature while it absorbs or rejects heat. However, in steam power plants, these processes occur at varying temperatures, making it impossible to achieve perfect isothermal conditions in real-life systems.

Reason 4: Impractical Design Requirements

A real steam power plant using the Carnot cycle would require complex heat exchangers to maintain constant temperatures during the isothermal processes and infinite heat capacities. It would also require ideal components with no energy losses (e.g., frictionless turbines and pumps). Designing and building such a power plant is practically impossible due to physical limitations and economic constraints.

Reason 5: Lower Actual Efficiency

While the Carnot cycle represents the maximum possible efficiency for a heat engine, real steam power plants operate at significantly lower efficiencies. This is due to various factors such as energy losses in the system, non-ideal behavior of the working medium, and the practical limitations of the components. Consequently, steam power plants cannot achieve the efficiency predicted by the Carnot cycle. In conclusion, the Carnot cycle is not a realistic model for steam power plants due to its assumptions of reversible processes, its ideal working medium, impractical design requirements, and its focus on maximum efficiency that cannot be achieved in real-world power plants.