Organic Rankine Cycles ORC

Organic Rankine Cycles (ORC) are variants of steam-water cycles, which are used when the hot source from which mechanical power is to be generated is at low or medium temperature.

In such conditions, the performance of steam cycles deteriorates, and it becomes preferable to use other thermodynamic fluids.

As many of these are organic in nature, we usually refer to these cycles as organic, but other types of fluid, such as ammonia or carbon dioxide, can also be used.

These cycles have been attracting growing interest since the 1980s, and are the subject of numerous projects, particularly in the English-speaking world. They are well-suited to small power plants, as steam power plants are only economically viable for high power plants, typically above ten MW.

This article presents the various cycle architectures encountered, then develops their main applications:

• geothermal power plants ;

• biomass combustion plants ;

• effluent heat recovery plants ;

• electro-solar thermal power plants.

It then looks at the choice of thermodynamic fluids and the optimization of ORC cycles.

It gives a number of examples of cycles simulated and calculated with the Thermoptim software package.