Comparative Assessment of Organic Rankine Cogeneration Cycle Efficiency with Secondary Heat Sources from Marine Diesel

Abstract

Research focuses on secondary heat sources utilization of medium speed Wartsila power plants in 25–100% load range due to the new restrictions of the marine transport CO2 emissions by IMO Energy Efficiency Design Index requirement. The uncertainty of cogeneration cycle implementation to marine transport arises in the configuration of the Organic Rankine Cycle (ORC) rational operation due to the energy potential change accordingly to high and low temperature heat sources in the power plant load modes. The research involves numerical programming energy performance studies of the ORC system by Klaipeda University, identifying and comparing rational ORC configuration options, alternatively connecting exhaust gas, engine cooling system and scavenge air heat sources in cycle, to achieve maximum efficiency and performance with respect to the outboard water conditions, load modes of power plant and selected working fluid. First phase of the study the implementation strategy of the cogeneration cycle by regulating working fluid flow in the wide load range of the power plant. Second phase is main emphasis of the study, where alternative heat source variations such as scavenge air and cooling jacket and their by-pass options were evaluated to the cycle at the nominal load of the power plant for the rational implementation for marine transport. The plant efficiency increases by 4% when only scavenge air heat source is used in cogeneration cycle. Significant increase of generated power and efficiency up to 11% is noticed when all heat sources are included in the cycle. Generated power with all heat sources reaches 1,676 kW, while using only exhaust gas heat it reaches 830 kW which is significant. Though lower temperature heat sources can decrease efficiency of the cycle depending of the ambient conditions of outboard water temperatures and working fluid selection, to evaluate impact rational and alternative heat source options for the specific load modes further researches are required. The main diesel engine coefficient of performance with cogeneration systems improved from 4% using only scavenge air and by 11% using all three heat sources. Considering the received results, further studies of cogeneration with secondary heat sources are needed for more rational results in specific load modes and various ambient conditions.