Rankine Cycle

Rankine cycle is a vapor power cycle (a type of thermodynamic cycle) on which a steam based thermal power plant works.

It consists of four processes

1. Isobaric heat addition at boiler
2. Isentropic expansion at turbine
3. Isobaric heat rejection at condenser
4. Isentropic compression at pump

Now let’s understand Rankine cycle by relating it to steam based thermal power plant. Below is the diagram showing different components of the plant.

Here is corresponding T-S diagram (Rankine Cycle)

To understand the below mentioned processes, you have to undertake both the above shown diagrams into consideration at the same time.

Process 4-1

It is isobaric heat (Q_B) addition process. Here water is converted into steam with the help of a boiler.

Process 1-2

This is isentropic expansion process. Here steam gets expended in turbine. This exchange of energy result into work output (W_T).

Process 2-3

This is isobaric heat rejection process. Here the excess heat (Q_C) of steam rejected to the environment with the help of a condenser. Condenser converts steam and water mixture to water.

Process 3-4

It is isentropic compression process. Here water (condenser output) is pumped to the boiler. In this process pump does some work (W_P) on the system.

Efficiency of a Rankine cycle

Efficiency of any heat engine is the ratio of net-work output to heat input.

η = Net-Work Output / Heat Input

Work output is the net-work by the system i.e. turbine work minus pump work.

W_NET = W_T – W_P

Also net-heat of the system is heat added at boiler minus heat rejected at condenser.

Q_NET = Q_B – Q_C

According to the first law of thermodynamics

Q_NET = W_NET

Hence

W_NET = Q_B – Q_C

and

η = W_NET / Q_B

Which means

η = (Q_B – Q_C) / Q_B

η = 1 – (Q_C / Q_B)