One method of increasing the efficiency of the Rankine cycle is by regeneration. Regeneration describes the process of transferring heat energy from fluid bled off the turbine (state 6) to the low-temperature liquid emerging from the main pump (state 2). This process is also known as extraction or feedwater heater. In the equipment schematic shown on the left, regeneration is shown using a feedwater heater (FWH).
Superheated steam enters the turbine (state 5). A proportion of the steam is extracted before full expansion (state 6) while the remaining steam expands completely and is fed into the condensor (state 7). At the condensor, the expanded steam is condensed to liquid before being pumped into the feedwater heater (state 2).
At the feedwater heater, heat energy is transferred from extracted turbine steam (state 6) to the condensed liquid. Heat transfer at the feedwater heater can take place by either direct mixing or by a heat exchanger. By "pre-heating" the working fluid, less energy at the boiler is required to superheat the working fluid, therefore increasing overall efficiency of the system.
Pump 1 is used to increase the pressure of the liquid from the condensor (state 1) to the feedwater heater (state 2).
Pump 2 is used to increase the pressure of the "pre-heated" liquid from the feedwater heater (state 3) to the boiler (state 4).
For analysis purporses, we will assume the boiler outlet is saturated vapour, and a superheater is used to add further heat energy. Boiler analysis is identical to the boiler analysis of a simple Rankine cycle.
The purpose of the turbine in a Rankine cycle with regeneration is identical to the purpose of the turbine in a simple Rankine cycle. For analysis purposes, the flowrates have to be determined.
For steady state steady flow conditions, neglible change in potential and kinetic energy, and adiabatic conditions:
Conservation of Mass:
Condenser analysis is identical to the condenser analysis of a simple Rankine cycle.
