Rankine Cycle: Know Definition, Types, Working Principle & More (2024)

Thermal power plants contribute to a major source of electricity we consume every day. The Rankine cycle is the fundamental operating cycle used by all power plants today. It is a cycle in which all the processes to produce work are reversible and the operating fluid undergoes a continuous cycle of evaporation & condensation.

In this article, we will be learning the working principles, the components commonly found in the cycle, the efficiency, its various types, and applications. We shall make use of phase diagrams and schematic diagrams to understand the concepts.

This thermodynamics concept is as important as the SSC JE ME or GATE ME exam itself. It is advised that you digest the concept well. Are you still reading? Then, let’s start learning!

Types of Rankine Cycle

The thermodynamic efficiency of the Rankine cycle can be increased by increasing the heat input to the cycle. This can be done by increasing the temperature to change the phase of steam to superheated steam. There are numerous variations like this that attempt to increase the thermodynamic efficiency of the cycle.

Below are some of the cycles designed to increase the thermal efficiency of the cycle.

Rankine Cycle with Reheat

Reheating removes the moisture carried by the steam during the final stages of the expansion process in the cycle. With this cycle design, the steam turbines are kept in series to perform the work. Here is a brief on how this cycle works.

1. The high-pressure steam from the boiler enters the first turbine
2. The steam is passed to the boiler and reheated again.
3. The second turbine is at low pressure to which this reheated steam flows.

The purpose of the Rankine cycle with reheat is to increase the average temperature of the steam in the cycle. Reheating the steam through another stage improves the efficiency of the cycle only half as much as the preceding stage.

Advantages of Reheat Rankine Cycle

• Improved the thermal efficiency of the cycle as steam condenses during expansion reducing the damage to turbine blades
• Increases the total work output of the turbine considering the total work input.

Disadvantages of Reheat Rankine Cycle

• These cycles need a long piping setup. Hence, high initial installation costs are followed by high maintenance costs
• As there is reheating, the size of the condenser may increase.

Regenerative Rankine Cycle

In this cycle, the subcooled liquid from the condenser is heated by the steam trapped in the hot portion of the cycle using a regenerator. This process is mixed with a fluid to convert it to a saturated liquid. Thus, the process is known as ‘Regeneration’ and the cycle is called the ‘regenerative Rankine cycle’.

The regeneration process increases the heat input temperature of the cycle by eliminating the heat addition from the boiler that usually exists in a normal Rankine cycle. As the heat flow into the cycle occurs at a high temperature, the efficiency of the cycle is increased.

Advantages of Regenerative Cycle

• As the range of working temperatures of steam in this cycle is decreased, the temperature stresses are also decreased,
• The erosion of the turbine is reduced due to the nature of the working fluid.

Disadvantages of the Regenerative Cycle

• Rate of steam is increased due to reduced work done in the boiler due to the presence of a regenerator.
• High power plant cost due to larger components following the nature of the process of the cycle.

Organic Rankine Cycle

The organic Rankine cycle, also known as ORC, is a thermodynamic cycle which uses organic fluids like toluene or pentane instead of water and steam. These fluids have a high molecular mass with a higher vaporization temperature than water. It allows heat extraction from a low-temperature source like a solar pond, biomass combustion, or geothermal heat.

Advantages of Organic Rankine Cycle

• There is no need for superheating, hence no water-treatment system of fluid
• The inlet temperature and pressure of the turbine will be low.

Disadvantages of Organic Rankine cycle

• ORCs generate less power than a regular cycle at the same operating conditions.
• The organic fluids used for this cycle are combustible. A small leakage can lead to environmental hazards.

Supercritical Rankine Cycle

In a supercritical Rankine cycle, the heat is transferred to a high-pressure supercritical fluid, which is then converted to a supercritical phase. Fluid in this phase is sent to a turbine where it expands and helps in generating power. The steam utilised is sent back to be condensed into a liquid and recycled back through the cycle.

A supercritical fluid, commonly known as SCF, is a fluid at a temperature and pressure above its critical point. This is the point where liquid and gas do not co-exist. These fluids are found on Earth by black smokers (a type of hydrothermal vent).

Advantages of Supercritical Rankine Cycle

• The efficiency of the cycle is greater than all other cycles.
• The outflow of the turbine is also of high quality.

Disadvantages of Supercritical Rankine Cycle

• Regular boilers may not withstand high temperatures and pressure. Hence, a special boiler must be manufactured.
• Due to this, there is an overall cost bump.