Organic Rankine Cycle (ORC) is a technology that can convert thermal energy at relatively low temperatures (80 to 350°C or 175 to 660°F) to actual work that can be further converted into electricity.
It is basically a thermodynamic cycle according to the Rankine principle but specifically uses organic fluids in order to have a boiling point at relatively low temperatures.
The heat is used to make the liquid boil and generate high pressure gases that will then drive equipment able to transmit torque to the shaft and create electrical power.
There are two main types of machines that are able to do this
• Turbine-based system
• Reciprocating piston-based system
The ORC cycle is based off of the simple Rankine Cycle, which is the fundamental operating cycle (thermodynamic cycle) for all power plants where an operating fluid is continuously evaporated and condensed in a closed loop. Both the ORC cycle and the simple Rankine cycle involve four basic processes.
Let’s consider its stages:
1-2 – In condensing and feed pumps, the feedwater pressure grows from Pc to P0 in isentropic process.
2-3 – In the boiler, water is heated until it is a saturated liquid with constant pressure P0. From there, it is compressed into steam, which vaporizes in the process.
4-5 – Dry steam is superheated in this isobaric process after the boiler. The boiler along with the superheater is often called a steam generator.
5-6 – Superheated steam with temperature T0 and pressure P0 is transferred to the turbine where it expands to pressure Pc in an ideal isentropic process. This process spins the turbine which ultimately produces electricity.
6-1 – The low pressure steam is condensed back to a liquid. This process is performed in the condenser, which is basically a large heat exchanger which is cooled by water. There are a lot of circulating and co-current condenser designs depending on application.
Thus, ORC brings opportunities for exploiting available low-grade heat that otherwise would be wasted resulting in improvement in energy efficiency of new or existing energy-intensive applications. During the processes in chemical industries, production of paper, cement, food or other process industries, the heat carrier cools down to a temperature level where the residual heat cannot be used for the primary process. To make use of that residual heat, ORC technology is a viable option which can improve the efficiency of the system or be helpful for electricity generation. ORC units can be employed to alternative heat sources such as solar and geothermal energy as well as biomass besides industrial waste heat sources.