Why the Fluid Decides the System
The organic working fluid sets the cycle's temperature window, pressure, and efficiency. Pick the wrong one and the ORC either cannot use the available heat or runs at poor efficiency.
Common ORC Fluids
- R245fa and R134a: good for low-temperature heat, about 100 to 150 degrees C, low pressure, simple design.
- n-Pentane and other hydrocarbons: efficient in the 150 to 300 degrees C range, flammable so needs sealed design.
- Siloxanes: stable at high temperatures, 300 to 400 degrees C, common for exhaust from cement or glass plants.
- Zeotropic mixtures: better match to a varying heat source, lifting output.
Matching Fluid to Source
Clean, moderate heat suits R245fa or pentane. Dirty, high-temperature exhaust from cement or steel plants favors siloxanes behind an isolated heat-transfer loop. Before the ORC, an air-to-air preheater can lift the stream temperature and improve payback.
Safety and Rules
Flammable fluids demand sealed machinery rooms, and some legacy refrigerants face phase-out, so check local rules. The fluid choice also drives evaporator material, covered next.