The Rankine Cycle
The Rankine Cycle is a thermodynamic cycle which converts heat into work. The heat is supplied externally to a closed loop, which usually uses water as working fluid. The Rankine Cycle based on water provides approximately 85% of worldwide electricity production.
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.
The first Organic Rankine Cycles: the Naphtha Launch
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).
Organic Rankine Cycle Today
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 rising oil prices are powerful reasons supporting the explosive growth of this efficient, clean and reliable way of producing electricity
The Organic Rankine Cycle’s principle is based on a turbogenerator working as a normal steam turbine to transform thermal energy into mechanical energy and finally into electric energy through an electric generator. Instead of the water steam, the ORC system vaporizes an organic fluid, characterized by a molecular mass higher than water, which leads to a slower rotation of the turbine and lower pressure and erosion of the metallic parts and blades.
How it works
with reference to a standard biomass Combined Heat and Power plant the process is based on the following thermodynamic cycle:
- A heat source heats thermal oil to a high temperature, typically about 300°C, in a closed circuit;
- The hot thermal oil is drawn to and from the ORC module in closed circuit. In the ORC it evaporates the organic working fluid of the ORC in a suitable heat exchanger system (pre-heater and evaporator);
- Organic vapour expands in the turbine, producing mechanical energy, further transformed into electric energy through a generator;
- The vapour is then cooled by a fluid in a closed circuit and condensed. The water warms up at about 80 – 90°C and it is used for different applications requiring heat;
- The condensed organic fluid is pumped back into the regenerator to close the circuit and restart the cycle.
The ORC cycle has a high overall energy efficiency: 98% of incoming thermal power in the thermal oil is transformed into electric energy (around 20%) and heat (78%), with extremely limited thermal leaks, only 2 % due to thermal isolation, radiance and losses in the generator; the electric efficiency obtained in non cogenerative cases is much higher (around 24% and more).
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