Question

Consider an ideal Rankine cycle as shown in the figure, where \( T \) and \( S \) represent the temperature and entropy respectively. The overall efficiency of the cycle can be improved by: 

 

• A. Increasing the pressure at which heat is added
• B. Decreasing the pressure at which heat is rejected
• C. Employing an intercooler
• D. Superheating the steam

Hint:

To improve the efficiency of the Rankine cycle, focus on increasing the temperature and pressure of heat addition and reducing the temperature and pressure of heat rejection.

Answer 1

The Correct Option is A

Step 1: Efficiency of the Rankine cycle.
The thermal efficiency of the Rankine cycle is given by: \[ \eta = 1 - \frac{Q_{\text{out}}}{Q_{\text{in}}}, \] where: - \( Q_{\text{out}} \) is the heat rejected in the condenser, - \( Q_{\text{in}} \) is the heat added in the boiler. The efficiency can be improved by increasing \( Q_{\text{in}} \) or decreasing \( Q_{\text{out}} \). Step 2: Analyze the options.
(A) Increasing the pressure at which heat is added: Increasing the boiler pressure increases the mean temperature at which heat is added, reducing irreversibilities and improving efficiency. (B) Decreasing the pressure at which heat is rejected: Lowering the condenser pressure reduces the temperature at which heat is rejected, thereby improving the cycle efficiency.
(C) Employing an intercooler: Intercoolers are typically used in gas cycles (like the Brayton cycle) to improve efficiency, but they are not applicable to the Rankine cycle. (D) Superheating the steam: Superheating increases the temperature at which heat is added without increasing the pressure, thereby improving the efficiency by increasing \( Q_{\text{in}} \).
Conclusion: The overall efficiency of the Rankine cycle can be improved by: (A) Increasing the pressure at which heat is added.
(B) Decreasing the pressure at which heat is rejected.
(D) Superheating the steam.