In search of the ideal Organic Rankine Cycle working fluid

The Organic Rankine Cycle is an evolution of the Rankine Cycle, a heat recovery system, named after the Scottish scientist who discovered it in the 19th century.

In an age where most of the energy is still produced from non-renewable polluting sources, the ORC is a clever way to recover energy that would otherwise be dispersed, by taking advantage of its principle of transformation.

In the Rankine cycle, the residual heat from the production of energy heats a fluid, called "working fluid." Such liquid is brought to gaseous state, pressurized, and then expanded towards a turbine, which spins and produces additional energy.

After transferring part of its energy to the turbine, the working fluid cools down and goes back to the initial stage, ready to start a new cycle.

In its original version, the Rankine cycle used water as working fluid, but soon the search started for alternative solutions that would allow generating energy at lower temperature, therefore making the whole process more efficient.

At the end of the 19th century, naphtha started to be used instead of water, and then other organic fluids followed (such as fluorocarbons), hence the new denomination of Organic Rankine Cycle.

Working fluids are still a subject for research. Ideally, they should:

  • require as little energy as possible to transform into gas
  • have a high molecular density, to transfer more energy to the turbine
  • not be polluting
  • be safe to use
  • be cost-effective.

In the learning project Energy Creating Energy, that we launched during Expo Milan, the ORC is treated within the specific context of a biomass power plant. There are however several possible applications, such as industrial production, solar energy or geothermic power plants.

The ORC is utilised mostly in North America and Asia. In Europe it's less common, although Italy is one of the countries to the forefront. For any type of ORC application, Cividac plays a crucial part by supplying essential components like pressure vessels and heat exchangers.