Organic Rankine cycle (ORC) is a power generation cycle that converts the available heat energy to mechanical work via turbine that drives the generator to produce electricity. Unlike typical Rankine cycles that use steam as the working fluid, Organic Rankine cycles (ORC) use an organic working fluid for instance, isooctane and isopentane, isobutane amongst others. The boiling points of these organic fluids are less than that of water. Thus, ORC makes it possible to extract power from waste heat in industrial processes. They are also used in most geothermal power plants of low entropy .
A typical Organic Rankine cycle used in low temperature geothermal power plant is shown in Fig. 1. The organic Rankine cycle starts at point 1 with the organic working fluid at the liquid state at low pressure. Through the action of the pump, the working fluid moves to preheat and then to the evaporator. The working fluid is converted to high-pressure and temperature vapor after passing through the vaporizer. The high temperature and pressure vapor expands through the turbine, which is coupled to a generator. Because of the expansion, the high temperature and pressure steam loses energy and is converted to low pressure vapor at the turbine exit (point 3). The low-pressure vapor then passes through the condenser where it further cooled and finally pumped back to the vaporizer. The condenser is a very important element that creates a partial vacuum or lowers the pressure at the turbine exit hence increasing the efficiency of the turbine and consequently the turbine work or power generated .
Figure 1: Typical Organic Rankine Cycle used in geothermal power plants
Works Cited
Bronicki, Lucien. Innovative Geothermal Power Plants, Fifteen Years Experience. Florence, Italy: World Geothermal Conference, 1995.
DiPippo, Ronald. Second law assessment of binary plants generating power from low-temperature geothermal fluid. Elsevier Ltd, 2004.
Ormat. Advanced Organic Rankine Cycles in Binary Geothermal Power Plants. Retrieved from : http://www.ormat.com/research/papers/papers3, 2007.
