Regenerative Rankine Cycle

Regenerative Rankine Cycle

• Steam power plants generate most of the electric power in the world, and efforts are constantly made to improve the efficiency of the cycle.
• The ideal regenerative Rankine cycle increases the fluid average temperature during the heat addition process.

Limitations of Simple Rankine Cycle

• In a simple Rankine cycle, heat is added to the cycle during process 4-4'-1, but the temperature of the water is low during this stage.
• This reduces the average temperature during heat addition (process 4-4'-1).

Regeneration Process

• Regeneration increases the temperature of the feedwater (water leaving the pump and entering the boiler) by extracting steam from the turbine to heat the feedwater.
• The heat exchanger where heat is transferred from steam to feedwater is called a regenerator, or a feedwater heater.

Types of Feedwater Heaters

• There are two main types of feedwater heaters: open and closed feedwater heaters.
• Open feedwater heaters mix steam with compressed water from the pump, while closed feedwater heaters do not mix the steam with the compressed water.

Open Feedwater Heater

• An open feedwater heater is a mixing chamber where the steam extracted from the turbine mixes with the water exiting the pump.
• In an ideal condition, the water leaves the heater as a saturated liquid at the heater pressure.

Ideal Regenerative Rankine Cycle

• Steam from the boiler expands in the turbine to an intermediate pressure, and some of the steam is extracted and sent to the feedwater heater.
• The remaining steam in the turbine continues to expand to the condenser pressure.
• Saturated water from the condenser is pumped to the feedwater pressure and sent to the feedwater heater.
• At the feedwater heater, the compressed water is mixed with the steam extracted from the turbine and exits the feedwater heater as saturated water at the heater pressure.
• The saturated water is pumped to the boiler pressure by a second pump, then heated to a higher temperature in the boiler, and the cycle repeats again.
• The mass flow rate at each component is different, with y kg of steam extracted to the feedwater heater and (1-y) kg continuing to expand to the condenser pressure.