The Organic Rankine Engine 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 may be used for different applications requiring heat e.g. space heating and domestic hot water.
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).