Organic Rankine Cycle – ORC

Like we mentioned that the principle of geothermal power plant lies in thermodynamic cycles, we would like to write briefly about one of them, so called Organic Rankine Cycle (ORC).

Firstly, what is a Rankine Cycle? In general, it is a thermodynamic cycle used in turbines in order to produce electricity. It is one of practical approaches to Carnot Cycle, which refers to ideal type of turning heat into work without any energy losses. Rankine Cycle is used in steam turbines exploiting so called waste heat. In other words, once the “primary” heat is used for some purpose and the part of it was consumed, the waste heat can be a source of energy for Rankine Cycle.

“Organic” means using high molecular mass fluid, which has lower boiling point than water. Very simplified, geothermal water stream heats closed high molecular mass fluid loop, turned into steam and fueling steam turbine. In practice, ORC uses chemical fluid consisting of butane, propane, ammonia or some of new environmental friendly refrigerants. The main advantage of refrigerant is that it boils at a temperature below the temperature of frozen ice. Moreover, additional heating, although by low temperature geothermal stream, increases the pressure in closed loop.

Organic Ranking Cycle is therefore suitable to be used in geothermal power plants established on wells producing water streams of lower temperature, which means especially the range from 50 to some 100 Celsius degrees, or in combination with other thermodynamic cycles producing the waste heat. On the other hand, the efficiency of ORC is low in general.