Modelling energy efficiency of complex ship propulsion systems, considering sludge recycling, exhaust gas recovery and Flettner rotors.

Energy efficiency has become increasingly important during the recent years due to the negative effects of the anthropogenic activities on the environment; in the maritime field IMO is leading a steep slope down policy on Greenhouse gas emission reduction by enacting stricter rules at a fast pace. In addition to the environmental and legal aspects, a key driver for increasing energy efficiency is operational cost savings due to fuel consumption reduction. In this framework, the thesis focuses on the improvement of the energy efficiency of the ship propulsion plant by analysing different kind of innovative technologies. Modelling and simulation techniques and appropriate key parameter indicators have been extensively used to provide a suitable metric for bench-marking the different solutions. The reason behind the choice to analyse different technologies is because a well defined solution does not exist. Different ship types and different operational conditions may trigger different options. The first way investigated to improve energy efficiency was by means of an innovative technology to transform sludge into new recycled marine fuel oil through a pyrolysis process carried out in a small reactor onboard. A passenger ship was chosen as a case study due to the advantageous large amounts of waste oil involved and the space availability. The feasibility study and the analysis of fuel consumption reduction and EEOI criticalities are reported: the results showed an easy integration of the system inside the incinerator and a reduction of time and costs; about the environmental aspect, the EEOI formula is not suitable for this innovative technologies and the attempt made to calculate it was unsatisfactory. The results are interesting but not good enough to justify the necessary expenditure investment, also in lieu of the noncalculable impact on the efficiency index. The second investigated solution to further improve the ship efficiency was an innovative flexible propulsion and power system with recovery technologies, studied in collaboration with an Italian shipping company. In the propulsion plant there are dual fuel engines coupled with waste heat recovery systems, innovative hybrid turbocharger and electric power shaft motor/generators. In addition to the description of the propulsion plant and its various modes of use, different configurations are analysed in terms of efficiency and costs and the plant was tested with and without the various recovery systems and with natural gas and HFO, referring to the routes currently travelled by the model ship (a Ro-Ro ferry). The results are expressed as a function of the ship speed: for a given speed and a chosen plant configuration (which recovery systems to consider), the best plant mode of use, among the main three described, is chosen, which means the one associated with the minimum fuel expense. The results showed that the Normal Navigation scenario is the one associated with the lowest fuel costs and highest plant efficiency, for all considered recovery systems. Moreover, they all allow considerable cost savings; in particular, the hybrid turbocharger is the more interesting because the low initial investment is paid off by considerable annual savings. On the other hand, the calculation of the EEDI for each plant configuration shows that only by combining WHRS and hybrid turbocharger together it is possible to respect the IMO limit. After the analysis of said hybrid propulsion system, there was a need to go further to try to integrate renewable sources on board. Therefore the research moved to the wind assisted propulsion, which is gaining in popularity due to the expected benefit in emission reduction. A study was performed about the proper integration between the conventional diesel engine with controllable pitch propeller propulsion plant and the wind assisted plant with Flettner rotor. A mathematical model describing the behaviour of the rotor in terms of propulsive thrust and power is proposed; the rotor model was then integrated into a diesel propulsion model in order to evaluate the ship net fuel consumption for a given wind condition. The integrated propulsion model was written in parametric form. The methodology is intended to support the ship designer during the choice of the best possible propulsion diesel engine for a given rotor-propeller configuration, in addition it can be used to optimize the fuel consumption during the ship operation. A 3000 tons Ro-Ro/Pax ferry has been selected as case study; the results showed that a bigger rotor is always beneficial, that the best directions of incoming wind are from side to astern while the worst case is head wind, that the stronger the wind, the wider the range of suitable angle and that wind angle has a greater influence on the fuel consumption than the wind speed. With the optimized propulsion plant, remarkable double digit power savings can be observed in the whole range of ship speeds, while a 20% of fuel saving was achieved at the design ship speed. The three developed numerical models allow to reduce the environmental impact of the ship and these simulators can be used as a tool to design or operate ships able to meet the present and future energy efficiency requirements. Decarbonisation and environmentally friendly innovations are the real challenges of our century. Therefore, the future of research is strongly linked to the improvement of the energy efficiency and the reduction of environmental impact.