Payback Potential and Carbon Savings from Shipboard Waste Heat Recovery Systems

International shipping is indispensable to global commerce, yet it remains a significant contributor to greenhouse gas emissions. Although waste heat recovery has been applied in other industries, its performance and economic viability in shipping are not yet fully understood, particularly across different vessel sizes and engine loads. This study evaluates the technical, economic, and environmental potential of waste heat recovery (WHR) systems onboard ships with main engine power above and below 25,000 kW. Thermodynamic analysis and computational simulations were employed to estimate electricity generation, fuel savings, and emission reductions under optimistic and pessimistic scenarios, using operational data from four representative vessels. The results indicate that larger ships achieve the most significant benefits, with power ratios up to 10%, substantial CO2 reductions, and viable payback periods. Smaller vessels, constrained by thermal and spatial limitations, show reduced efficiency and less favorable financial performance, although they still achieve meaningful environmental gains. The findings confirm that waste heat recovery is a mature and effective technology for improving ship energy efficiency and reducing emissions. The study contributes to scientific knowledge by quantifying performance differences between vessel types and providing a structured framework to support maritime decarbonization strategies.