A single unit of geothermal power plant (GPP) with an installed capacity of 1x55 MW was commissioned in Lumut Balai geothermal field in 2019. The field is a liquid-dominated geothermal system with a reservoir temperature of around 240 degrees Celsius. Lumut Balai GPP disposed 2204.6 tonne/h of separated brine at a temperature of 164.9 degrees Celsius into the hot brine reinjection wells without further utilization. In this study, a bottoming power plant using Organic Rankine Cycle (ORC) is proposed to recover heat from the separated brine to generate more electricity. The proposed bottoming ORC is analyzed using the techno-economic approach, consisting of technical and economic analyses. The technical analysis is conducted using energy analysis with the aid of Engineering Equation Solver (EES) to study the system’s technical performance (net power output and thermal efficiency). The optimization is performed by varying the working fluid and the turbine inlet pressure. Three working fluids are used in this study, namely n-pentane, n-butane, and isobutene. The measured silica concentration in the separated brine is 600 ppm. Using the silica saturation index (SSI) of 1.0 as a silica scaling potential indicator, the temperature of the separated brine is maintained above 146oC to prevent scaling. The energy analysis result shows that the bottoming ORC can yield the highest net power output and thermal efficiency of 13,215 kW and 26.1%, respectively, using n-pentane as the working fluid. Furthermore, economic analysis is performed to study the feasibility of the system using Internal rate of return (IRR), net present value (NPV), and payback period as the indicators. The result shows that the system is economically feasible with an IRR of 18%, an NPV of 66.3 million USD, and a payback period of 7.3 years.

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