Organic Rankine cycle as waste heat recovery system for marine application

Abstract

Carbon dioxide emission into the earth’s atmosphere by maritime activities are a concern for the people within and outside the industry. This is because of the environmental impacts that are caused by these greenhouse gas emissions which changes the very chemistry of this planet. These impacts can be mitigated by reducing the CO2 emissions which can be achieved by several design and/or operational means. Waste heat recovery (WHR) technology is one such means that is capable of reducing emissions. This is achieved by improving the overall fuel efficiency of marine engines which reduces the fuel consumption of the vessel. This improvement is realised by harnessing the heat energy that is expelled by the engine through waste heat sources such as exhaust gas, etc.
 Organic Rankine cycle (ORC) is one of many WHR technologies that is capable of harnessing that waste heat energy from the fuel which cannot be utilised by the engine operation alone. ORC as a WHR system (WHRS) is widely implemented in land based applications due to its fluid choice flexibility, plant simplicity and net efficiency. However, it is rather new to the maritime industry because WHRS on-board ships are predominantly based on steam Rankine cycle or turbo-compounding. These systems have their own advantages but ORC-WHRS may outweigh them in certain applications on-board ships. This is especially for electrical power generation from low and medium temperature waste heat sources. However, ORC in marine application encounters challenges unlike seen in land based applications. These challenges are caused by the physical & geometrical constraints, operational profile of the vessel or uncertainties caused at sea.
 In this thesis, the implementation of ORC-WHRS to marine engines for exhaust gas is investigated and studied to understand how such an application can be beneficial. Unlike steam Rankine cycle, an ORC system has flexibility in choosing an organic fluid that is suitable based on the application. This flexibility in fluid choices are confronted by the above mentioned maritime related challenges. Hence, a screening methodology is devised in this thesis that finds a suitable fluid based on the waste heat source profile and selection parameters. These selection parameters are necessary to filter out functioning organic fluids that can be limited due to the mentioned challenges. In this thesis, the power density of the ORC plant is the selection parameter used.As mentioned earlier, the ORC-WHRS may often be subjected to off-design conditions due to the operational profile of the vessel or by uncertainties at sea. Hence, off-design performance is analysed to study the ORC system when designed at several discrete engine load points. These analysis are carried out in plant models modified from an existing steam based dynamic model and developed into a simple-ORC and a recuperative-ORC dynamic plant models. Sensitivity analysis of these models are also performed to understand uncertainties in the model output corresponding to uncertainties in model input parameter. This analysis is followed by analysis of the dynamic behaviour of the ORC plant model to varying load step functions and step duration for plants designed at discrete engine load points.
 This thesis can be extended, not only to study how ORC can be used to meet future regulations on CO2 emissions, but also on operability limitations imposed on ships. The fluid screening approach used here can also be modified based on parameters, such as toxicity, flammability, cost, specific power etc. This can present a realistic approach to an end product that can be safely and economically operated on board.