Demystifying the Organic Rankine Cycle (ORC)

Waste heat recovery (WHR) has emerged as a cornerstone of industrial sustainability. Among various thermodynamic technologies, the Organic Rankine Cycle (ORC) stands out for its high efficiency in converting low-to-medium temperature thermal resources (80°C to 350°C) into electrical energy.

How the Organic Rankine Cycle Works

The ORC operates on the same basic principles as the traditional steam Rankine cycle, but employs an organic working fluid (such as refrigerants or hydrocarbons) instead of water. The cycle consists of four primary processes:

• Process 1-2 (Pumping): Liquid working fluid is pressurized adiabatically from the condenser to the evaporator.
• Process 2-3 (Evaporation): The high-pressure liquid absorbs heat from the waste heat source, vaporizing into a superheated gas.
• Process 3-4 (Expansion): Vapor expands through a turbine or expander, generating shaft work and electrical power.
• Process 4-1 (Condensation): Exhaust vapor is cooled and condensed back into liquid inside a condenser.

Selecting the Ideal Working Fluid

Fluid selection is the most critical design decision in an ORC system, directly affecting thermal efficiency: \(\eta_{th} = \frac{W_{net}}{Q_{in}}\). Unlike water, organic fluids can be dry or isentropic, meaning they do not condense during expansion, avoiding turbine blade erosion.