Study of Development Scenarios for Bottoming Unit Binary Cycle to Utilize Exhaust Steam from Back Pressure Turbine Geothermal Power Plant

Abstract

Binary cycle using organic working fluid or ORC system has been applied in geothermal and some other industrial processes to recover low grade and waste energy to generate electricity. The conventional system to utilize geothermal energy is a condensing system or back pressure system which depend on a turbine used in the system. Two units of the power plant in Flores, Indonesia are using back pressure turbine, which means that there is still a chance to increase the electrical power of the steam which is released through the turbine. The amount of exhaust steam available from these two units power plant is more than 62,000 kg per hour with 99C of temperature and around 2,430 kJ/kg enthalpy. This research is trying to get the optimum power that can be generated by the ORC regarding other consideration parameters such as ambient temperature, thermodynamics condition of resource steam, and energy conversion of each apparatus. It is the aim of this paper to present a thermodynamic study on the utilization of ORC as the bottoming cycle with various types of working fluids to produce additional electricity. Several working fluids are chosen to find the optimum ORC system to utilize this exhaust steam such as isobutane, butane, isobutene, isopentane, propane, propyne, neopentane, R245fa, R236fa, and R134a. ORC system used in this research is simple ORC with basic components such as Pre-Heater, Evaporator, Expander, Condenser, and Pump. The properties of working fluids are calculated by REFPROP. The results show that this binary cycle can generate up to 2.25 MW with the thermal efficiency around 10% depend on the working fluid used. Two working fluids in this scenario that provide the best power generated are Propyne and Isopentane. The type of working fluid must also be considered, propyne is a wet type while isopentane is a dry type working fluid.