Reheat Rankine Cycle and Methods of Increasing the Efficiency of the Rankine Cycle

Methods to Enhance Efficiency of Rankine Cycle

• Reducing condenser pressure:
• Decreases steam temperature and heat rejection temperature
• Increases net work output (blue area on T-s diagram)
• Steam exits as saturated mixture at lower pressure's saturation temperature

Superheating Steam to Higher Temperature

• Increases average temperature during heat addition
• Increases net work output (blue area on T-s diagram)
• Illustrated by superheating steam from state 3 to state 3'

Increasing Boiler Pressure

• Increases steam boiling temperature in Rankine cycle
• Shifts process from 2-3 to 2'-3' on T-s diagram
• Increases net work output (blue area) but decreases net work output (gray area)
• Increases moisture content of steam from state 4 to state 4', a negative effect

Reheat Rankine Cycle

• Increasing boiler pressure can increase thermal efficiency, but it also increases moisture content at the turbine exit, making it unacceptable.
• To correct this, the simple Rankine cycle is modified with a reheat process.

Ideal Reheat Rankine Cycle

• Steam is expanded isentropically to an intermediate pressure in a high-pressure turbine (stage I).
• Steam is then sent back to the boiler, where it is reheated at constant pressure to the inlet temperature of the high-pressure turbine.
• The steam is then sent to a low-pressure turbine and expands to the condenser pressure (stage II).

Formulas

• Heat Added (QA): (h1 – h6) + (h3 – h2), kJ/kg
• Turbine Work (WT): (h1 – h2) + (h3 – h4), kJ/kg
• Pump Work (WP): (h6 – h5), kJ/kg or v5 (P6 – P5), kJ/kg
• Enthalpy (h6): v5 (P6 – P5) + h5
• Heat Rejected (QR): (h4 – h5), kJ/kg
• Thermal Efficiency (e): WT - WP / QA