Exergy analysis and optimization of a single-flash geothermal power plant are conducted by developing a mathematical model that is applied to the Dieng geothermal power plant in Indonesia. Calculations are conducted by using the Engineering Equation Solver (EES) code using methods based on the laws of thermodynamics. The exergy flow and efficiency are computed at several plant components, including the separator, turbine, condenser, and for the whole power plant. The exergy of the geothermal fluid that is discharged from the production wells is estimated to be 59.52 MW. This amount of fluid produces 21.71 MW of electricity from the power plant overall, with second law efficiency to be 36.48%. There is a considerable amount of waste brine, amounting to 17.98% (10.70 MW) of the total available exergy, which is disposed of in the plant's reservoir. The optimization of the plant is carried out by adjusting the separator pressure. The results show that a slight increase of 20 kW in the output power can be attained by lowering the separator pressure to 9 bar from 10 bar. The Grassmann diagram shows the exergy losses at each component in the power plant. The turbine and separator losses are 7.51 MW (12.62%) and 8.04 MW (13.5%), respectively, while the cooling tower has an exergy loss of 2.62 MW (4.40%). The total condenser loss is 5.8 MW (9.75%). (C) 2013 Elsevier Ltd. All rights reserved.