It consists of four processes
- Isobaric heat addition at boiler
- Isentropic expansion at turbine
- Isobaric heat rejection at condenser
- 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.
It is isobaric heat (QB) addition process. Here water is converted into steam with the help of a boiler.
This is isentropic expansion process. Here steam gets expended in turbine. This exchange of energy result into work output (WT).
This is isobaric heat rejection process. Here the excess heat (QC) of steam rejected to the environment with the help of a condenser. Condenser converts steam and water mixture to water.
It is isentropic compression process. Here water (condenser output) is pumped to the boiler. In this process pump does some work (WP) on the system.
Efficiency of a Rankine cycle
η = Net-Work Output / Heat Input
Work output is the net-work by the system i.e. turbine work minus pump work.
WNET = WT – WP
Also net-heat of the system is heat added at boiler minus heat rejected at condenser.
QNET = QB – QC
According to the first law of thermodynamics
QNET = WNET
WNET = QB – QC
η = WNET / QB
η = (QB – QC) / QB
η = 1 – (QC / QB)