Reducing shipping carbon emissions under real operative conditions : a study of alternative marine waste heat recovery systems based on the organic rankine cycle

The biggest source of energy loss in shipping is found in the propulsion system. This study focuses on analysing, and working with, the concept of heat management for waste heat energy from the exhaust gas and scavenge air. Using waste heat recovery systems (WHRS) to make shipping more efficient represent a good area of opportunity. On board ships, a water-based Rankine cycle (RC) is typically installed; this has the task of providing steam and power. This work explores alternative waste heat technologies to assess the development and suitability, but also to find better solutions to the traditional RC. Different models coupled with advance optimisation processes were created to understand the marine WHRS. The results show that WHRS are sensitive to environmental and operational factors which must be considered at design stage. While water offers the possibility of producing both steam and power; organic Rankine cycles (ORC) produce larger power outputs at temperatures between 90˚C and 230˚C which translate to lower CO2 emissions. Organic WHRS will play an important role in the future as regulations push for tighter emission controls, and waste energy availability for power production reduces due to an increase in prime mover efficiency and waste heat utilisation for other processes (e.g. ballast treatment). The ORC technology can be applied to any kind of vessel type and size, keeping in mind that the ORC benefits depend on the waste heat temperature and availability, the ship's heat requirements and operational profile. It is also then important to bear in mind some of the drawbacks, such as larger mass flow rates and flammability of some of the organic fluids studied which will introduce additional safety equipment and costs.