Organic Rankine Cycle History

Noun – Thermodynamics. The Organic Rankine cycle (ORC) is named for its use of an organic, high molecular mass fluid with a liquid-vapor phase change, or boiling point, occurring at a lower temperature than the water-steam phase change

 

Rankine Cycle:

A thermodynamic cycle in engines by which a fluid is pumped into a boiler at one end and the vapor is condensed at the other. Heat energy is converted into work during this process.

A Rankine cycle is most commonly implemented in power generation plants – the combustion of coal, natural gases and oil act as a heat source for this cycle. Approximately 85% of the world electricity is produced by employing water based Rankine Cycle systems.

History of Rankine:

A Scottish physicist and Engineer by profession, William John Macquorn Rankine (July 5, 1820 – December 24, 1872) developed a comprehensive theory of the steam engine. His published engineering science and practice manuals (1850 – 1860) were used for many decades. His interests were not only in engineering and science, where he published several hundred papers and notes, but also in botany, music theory and number theory.

Read more at http://en.wikipedia.org/wiki/William_John_Macquorn_Rankine

How the Rankine Cycle works:

There are four main processes that define a Rankine cycle.

  1. A working liquid is heated in a boiler at constant pressure by external heat source to become a dry saturated vapor.
  2. The dry saturated steam expands in a turbine, moving the turbine blades to generate power. This expansion of steam reduces the pressure and temperature and also may condense some of the fluid.
  3. The wet vapor then enters a condenser where it is cooled back into a saturated liquid form.
  4. This saturated liquid (working fluid) is pumped from low pressure to high pressure (boiler) for the process to begin again.

Organic fluid instead of water = Organic Rankine Cycle (ORC):

Rankine cycles use water as the working fluid and water boils and vaporizes into steam at 100°C (212°F). By using a working fluid that has a boiling point of less than 100°C (212°F), less heat is required to vaporize the working fluid. In other words, the same amount of heat energy required to vaporize water would produce more work with the organic fluid as compared to steam.

Fluids used in ORCs are organic, high molecular mass fluids with boiling points lower than that of water.

Rankine Cycle Thermodynamics 101:

A thermodynamic state can be presented in a two-axis diagram called a thermodynamic diagram. Various combinations of thermodynamic variables can be used but the most common ones are temperature versus entropy (“T-S”), pressure versus enthalpy (P-V) and pressure versus volume (P-V).

The T-S thermodynamic diagrams for refrigerant and organic working fluid R245fa are provided below along with descriptions of the thermodynamic states before and after the main components of the ORC system.

For an ideal cycle, the thermodynamics of the various components of the ORC may be described as follows:

  1. Constant pressure evaporation in the boiler: The boiler can be divided into three zones for preheating, evaporation and superheating. Preheating is the process of adding sensible heat to the working fluid, so that it begins to vaporize. The heat that is added after it begins to vaporize contributes to a change of state from liquid to gas during evaporation. Finally, once the working fluid is completely converted into a gas, its temperature is further increased during the superheating section.
  2. Expansion in the turbine: The expansion ideally occurs without exchange of heat with the environment and is ideally a reversible process involving no friction losses, no pressure drops and no leakage. The shaft work done by the turbine is converted into electricity though a generator.
  3. Constant pressure condensation: Performing the reverse function of a boiler, a condenser can be divided into functional zones of de-superheating, condensation, and sub-cooling, in order to ready the working fluid for beginning its closed-loop cycle through the ORC again.
  4. Ideal pumping: Finally, the condensed working fluid is pumped back into the boiler at a constant entropy and constant temperature process.

Naphtha Engine: The First Organic Rankine Cycle (ORC) implementation

In 1883 Frank W Ofeldt took out a patent on a naphtha engine, which used naptha instead of water to replace steam on boats. These engines vaporized naphtha instead of water as the working fluid.

Read more at http://gasengine.farmcollector.com/Gas-Engines/The-Naphtha-Engine.aspx

ORC Today:

ORC is used today extensively in waste heat recovery, biomass and geothermal applications for energy production. ORC is a clean and environmentally friendly technique used to generate energy.