Energy and exergy analysis of the combined cycle power plant recovering waste heat from the marine two-stroke engine under design and off-design conditions

The waste heat recovery (WHR) system consisting of a steam Rankine cycle (RC) and a power turbine is assumed to be the state-of-the-art configuration on board ships. However, interactions and trade-offs between the main propulsion engine and the WHR system are still not fully understood. This paper aims at providing a comprehensive thermodynamic study on this combined cycle power plant which is simplified as the combination of a topping in-cylinder Diesel cycle, a middle scavenging Brayton cycle and a bottoming steam RC. Firstly, a sophisticated thermodynamic model of this combined cycle is established and validated in MATLAB, followed by detailed parametric studies on energy and exergy distributions in each sub-cycle. Trade-offs among those sub-cycles are then investigated under design and off-design conditions. It is concluded that the exhaust energy and exergy can be redistributed by adjusting the turbocharger matching and the exhaust bypass ratio. The heat source temperature plays an important role in determining the optimal high-pressure evaporation pressure and the efficiency of the dual-pressure steam RC. Performance of the combined cycle power plant is load-dependent, and the improvement of fuel economy reaches as high as 7.3% under the rated condition and drops down to 3.2% at 40% load point. (C) 2020 Elsevier Ltd. All rights reserved.