The ORC Technology


The Rankine Cycle is a thermodynamic cycle that converts heat into work. The heat is supplied to a closed loop, which typically uses water as working fluid. The Rankine Cycle based on water provides approximately 85% of worldwide electricity production.

The Organic Rankine Cycle's principle is based on a turbogenerator working as a conventional steam turbine to transform thermal energy into mechanical energy and finally into electric energy through an electrical generator. Instead of generating steam from water, the ORC system vaporizes an organic fluid, characterized by a molecular mass higher than that of water, which leads to a slower rotation of the turbine, lower pressures and no erosion of the metal parts and blades.

The ORC turbogenerator uses medium-to-high-temperature thermal oil to preheat and vaporize a suitable organic working fluid in the evaporator (4>5). The organic fluid vapor rotates the turbine (5>6), which is directly coupled to the electric generator, resulting in clean, reliable electric power. 

The exhaust vapor flows through the regenerator (6>7), where it heats the organic liquid (2>3) and is then condensed in the condenser and cooled by the cooling circuit (7>8>1). The organic working fluid is then pumped (1>2) into the regenerator and evaporator, thus completing the closed-cycle operation.



The Rankine cycle is named after William John Macquorn Rankine (July 5, 1820 - December 24, 1872), a Scottish engineer and physicist. He was a founding contributor to the science of thermodynamics. Rankine developed a complete theory of the steam engine and indeed of all heat engines. His manuals of engineering science and practice were used for many decades after their publication in the 1850s and 1860s. He published several hundred papers and notes on science and engineering topics, from 1840 onwards, and his interests were extremely varied, including, in his youth, botany, music theory and number theory, and most major branches of science, mathematics and engineering. He was an enthusiastic amateur singer, pianist and cellist who composed his own humorous songs.


Organic Rankine Cycle is a well-known and widely spread form of energy production, mostly in biomass and geothermal applications, but great rises in solar and heat recovery applications are also expected. Environmental concern over climate change and floating oil and gas prices are powerful reasons supporting the growth of this efficient, clean and reliable way of producing electricity


The Rankine cycle is usually based on water as a working fluid. Applications of the cycle with different working fluids started to appear soon, such as the Naphtha boats. In 1883 Frank Ofeldt developed a unique power system which he hoped would have replaced steam. His naphtha engines are steam engines that boil naphtha (a form of gasoline) instead of water to drive the pistons. At the time, the government required a license to boil water in steam engines, but did not require one to boil gasoline. Therefore, for the first time a gentleman boater could operate his own power boat without the assistance of an engineer. One would have to be of "gentleman's" means to own one of these vessels. In the 1880s, a 21 foot boat with a Naphtha engine cost $750 - one and a half times the annual wage of craftsmen who built them (from the Mystic Seaport Museum website).


The Organic Rankine Cycle technology was seriously developed only during the XX century. In Italy, an experiment was carried out during the Thirties on the Island of Ischia. Important studies were conducted after the Second World War in Russia, USA and Israel. Back to Italy, during the Seventies the Italian ORC School was born at the Politecnico di Milano, Italy's most important engineering University. Its founder was Prof. Gianfranco Angelino, with his colleagues Prof. Ennio Macchi and Prof. Mario Gaia, the founder of Turboden.

Turboden ORC turbogenerator
Turboden ORC