This paper presents the thermodynamic modeling of an organic Rankine cycle (ORC) that uses waste exhaust energy of a 1000 kW diesel generator on a naval ship. Seven different working fluids have been selected as the ORC fluids. The commercial software (EES) has been used to predict the thermodynamic properties of the selected fluids. The efficiency of the ORC goes up to 32% with toluene. For the needed generator power of 500 kW on cruising, the ideal ORC can produce 118 kW power with the working fluid toluene. Assuming an isentropic efficiency for the turbine and the pump of the case ORC to be 0.75 and 0.20, respectively, and neglecting the losses at the ORC electric generator, the electric power output of the ORC cycle becomes 92 kW. The power of the diesel-ORC system becomes 592 kW while the combined efficiency is calculated as 0.349. The ORC saves 25,500 L of diesel fuel (US$24,870) and reduces 67.2 tons of CO2 emissions at the end of 1000 operating hours. ORC working fluids may result different efficiencies at different temperatures. Therefore, a combined ORC system is proposed to get higher efficiencies at different thermal loads. The exergy efficiencies and irreversibilities were calculated. (C) 2017 Elsevier Ltd. All rights reserved.