Thermodynamic and Economic Comparison of Organic Rankine Cycle and Kalina Cycle as Bottoming Unit to Utilize Exhaust Steam from Back Pressure Turbine Geothermal Power Plant

Abstract

Binary cycle using Organic Rankine Cycle or Kalina Cycle System has been applied in geothermal and some other industrial processes to recover waste energy to generate electricity. Two units of the power plants in Flores Island, Indonesia, used back pressure turbines. The amount of exhaust stream available from these two units of the power plant is more than 62 tones per hour with enthalpy ranging from 2,430 to 2,435 kJ/kg. Instead of releasing the steam to the environment, it would be better to utilize the heat of the brine through a binary cycle, i.e., Kalina Cycle System (KCS) and Organic Rankine Cycle (ORC). The modeling and validation of KCS and ORC were carried out using the Aspen-HYSYS V12.1 software. Ekalina is chosen as KCS meanwhile several working fluids are chosen to find the optimum ORC system, such as butane, isobutane, pentane, isopentane, R-113, R-141b, and R-142b. The EKalina and simple ORC with basic components such as preheater, evaporator, expander, condenser, and pump is used in this research. The validation process of EKalina and ORC modeling resulted in a maximum error of 7.9% and 4.02 %, respectively. The result of optimization of EKalina has maximum net power and thermal efficiency is found at 99% ammonia fraction with the value of 4700.11 kW and 12.01%, respectively. But based on economic assessment 97% ammonia fraction has a better economic value than 99% with a Levelized Cost of Electricity (LCOE) of 2.024 cents/kWh. Meanwhile, the ORC resulted that R-141b is founded to have a maximum thermal efficiency of 10.65% with LCOE of 1.841 cents/kWh and R-113 has a minimum LCOE of 1.681 cents/kWh with thermal efficiency of 10.26%. For the geothermal back pressure turbine unit, the EKalina has better thermal efficiency than ORC but has higher LCOE than ORC.