Exhaust Waste Heat Recovery from a Marine Diesel Engine via an Organic Rankine Cycle: System Analysis at Varied Loads

In line with the International Marine Organisation (IMO) Net Zero 2050 strategy and its emphasis on improving shipboard energy efficiency, this project conducts a system analysis of a marine ICE exhaust gas, waste heat recovery system using ORC technology to quantify exergy efficiency and SFOC at varied ICE loads. A Wärtsilä 6L25 2 MW marine ICE in the diesel generation configuration is used for the case study and a commercial ORC system, selected based on calculated engine exhaust gas thermal energy. A high temperature heat exchanger design is calculated from first principles using the analytical NTU method, to give base geometry requirements to assess feasibility. Finally, exergy efficiency and combined ORC-DG system SFOC is quantified. Key results indicated exergy efficiency ranges from 24.5% to 19.5%, with highest exergy efficiency being at 50% ICE load and lowest at 100% ICE load. Combined SFOC was shown to be lowest at 85% ICE load, representing a ≈7 g/kWh reduction from DG SFOC. The analysis indicated a usable ORC power output, ranging from ≈50 kW to 80 kW. The main conclusions drawn are that research and ORC systems suitable for ICEs similar to the Wärtsilä 6L25 2 MW in the DG configuration are underreported, partially owing to a lack of suitable real-world data for research. Exergy efficiency and combined SFOC support the presented ORC-DG waste heat recovery system as an effective way to utilise exhaust gas waste heat that would otherwise be lost. However, when selecting and optimising an ORC system for waste heat recovery, designers should consider both real vessel operational profiles and lowest DG SFOC to maximise exergy recovery and efficiency with a high degree of accuracy. Similarly, optimising heat exchangers for actual DG operational profiles rather than theoretical 100% would avoid underutilisation and a larger physical heat exchanger footprint than necessary. This research should be considered as a first step system analysis of exhaust gas, waste heat recovery systems using ORC, as well as a contribution to critical research surrounding emissions reductions from shipping. Future studies should validate and expand upon the study by software modelling utilising full operational data from a suitable case study of vessels and manufacturers’ equipment data.