Dynamic modelling, simulation based analysis and optimization of hybrid ship propulsion systems

Abstract

Today, air pollution, harmful emissions, global warming, and climate change issues are of great importance both for the future of our world and for human health. It is aimed to keep the emission levels under control and to reduce the emission levels with international agreements such as the Kyoto Protocol and the Paris Agreement, which are regulated on a global scale but are not binding. However, exceeding the levels targeted in these agreements raises concerns for the future of both our world and human life. Greenhouse gases emitted into the atmosphere cause global warming and climate change. The most important indicators of climate change are shifts in seasonal cycles, excessive precipitation or drought, melting of glaciers, and severe natural events such as hurricanes and floods. Carbon dioxide is one of the greenhouse gases. At the same time, emissions such as nitrogen oxides, sulfur oxides, and carbon monoxide, which are other harmful emissions, cause acid rain and are extremely harmful to human health. Therefore, it is vital to control and, if possible, eliminate carbon dioxide emissions in all areas of life. 90% of world trade is carried out by sea transportation. Although it is an efficient transportation network on the basis of freight transported, it is predicted that, together with the increasing trade volume and the number of ships, ship-related emissions will increase between 90% and 130% until 2050. For all these reasons, emissions from maritime transport must also be brought under control and reduced. The International Maritime Organization (IMO) is working to reduce carbon dioxide emissions from ships. In this context, with the MARPOL Annex-VI contract, Energy Efficiency Design Index (EEDI), Ship Energy Efficiency Management Plan (SEEMP) and Energy Efficiency Operational Indicator (EEOI) and regulations that measure design, voyage and operational efficiency have been introduced. Not just limited to these, IMO has set limits based on engine speed for certain emission control areas with the NOx Code and standardized the production and use of machinery in accordance with the limits set for both machinery manufacturers and ships operating in this region. For sulfur oxide and particulate matter emissions, limits are set on the fuel content used. Diesel engines are used to a great extent for the propulsion of ships. Ship diesel engines use heavy fuels, which are fossil fuel derivatives. The chemical component of these fuels contains 85-87% carbon. For this reason, carbon dioxide gas comes out as a result of these heavy fuels burned in diesel machines. According to studies, the maritime sector causes 2.2% of global carbon dioxide emissions. As a result of the continued use of fossil fuels, it is not possible to significantly reduce the amount of emissions and reach the targets. Therefore, it is possible to reduce these emissions with alternative fuels, electric or hybrid propulsion systems, emission reduction technologies, speed reduction or operational improvements. All operations based on energy efficiency on ships, aiming to reduce fuel consumed and therefore emissions, should be specific for each ship. Many variables need to be examined separately according to the ship's usage area, ship size, ship type, and operation profile. The measures to be taken for a large crude oil ship operating in the ocean and for a ferry operating in the city are not the same. Likewise, there may be differences between ships with similar usage areas in terms of volume, area, and operation profile. Therefore, each study remains specific to the ship on which it is based. The efficiency of diesel engines, which act as the ship's main power generator and ensure the movement of the ship, is around 50%, and only half of the chemical energy of the fuel can be obtained from the propeller. Especially in ships operating short distances and making many take-offs and berthing maneuvers during the cruise, the diesel engine load changes many times and may move away from the efficient operating point. In these cases, it is inevitable that the system efficiency will drop below half. For this type of ship, the hybrid propulsion system can give efficient results. In general, when the load fluctuation is high, it is aimed to provide the high-power demand with electricity by keeping the load of the diesel engine constant. In this way, non-ideal fluctuations in the diesel engine operation map can be prevented. Within the scope of this thesis, operational and shipbuilding data were collected from a ferry connected to the City Lines in Istanbul, and a hybrid propulsion system was proposed and analyzed. The prepared thesis consists of five chapters. The second, third and fourth chapters have been published as articles, and the last chapter has been approved for publication as a book chapter. The first part states the introduction and the aim and scope of the study. In the same section, the introduction of the case ship and the data collection process are mentioned. The data of the ship was taken by personally participating in the voyages on the ship and the average of the data collected for the same voyage was taken as the subject of a case study. The electrical power generated from four diesel engines on the ship is transferred to the propellers by means of two electric motors. Classified as diesel-electric as its operating principle, this ship has a bow thruster to facilitate take-off and berthing maneuvers. The ship voyage was made between Kadıköy - Karaköy city lines piers. Many different power generators and energy storage technologies can be used in hybrid systems. Putting an alternative power generator next to the existing diesel engines on the ship will reduce fossil fuel consumption, thus positively affecting emissions. At this point, fuel cells come to the fore due to their zero-emission power generation potential and their use in land vehicles. However, today, there are five different fuel cells that are commercially available and industrially used. Which of these types is suitable for use on the ship and which should be selected are the research subjects. The second part of the thesis is an article prepared and published to seek an answer to this research question. In this study, alkaline, proton exchange membrane, molten carbonate, solid oxide and phosphoric acid fuel cells were compared with the analytical hierarchy method, which is one of the multi-criteria decision-making methods. Eight different criteria determined; safety, emissions, efficiency, cost, life, power output, fuel type and size. Fourteen experts from different fields of the maritime industry, received their opinions through questionnaires and the criteria are ranked in order of importance. While safety is the most important evaluation criterion in this ranking, size comes last. In this context, the diesel oil using molten carbonate fuel cell and the hydrogen using proton exchange membrane fuel cell took the first two places. Following the determination of the fuel cell to be used in the hybrid system, it is also necessary to determine the type of fuel that should be used in the main target of marine decarbonisation. There are two types of alternative fuels that do not contain carbon; these are ammonia and hydrogen. As a result of the use of these two fuels in fuel cells, no carbon emissions occur. However, these two fuels have been examined from many aspects by using the analytical hierarchy method to determine the appropriate fuel. While ammonia stands out as a short-term solution due to the convenience of storage conditions, in parallel with the development of storage technology in the medium and long term, hydrogen stands out with its harmless structure compared to ammonia. Following studies continued by considering the hydrogen as fuel. After determining the fuel cell type and fuel, in the fourth part of the thesis, hybrid propulsion systems, their components, management styles, and architectural structures on ships are emphasized. In the published article, hybrid elements for ships are on power generators, energy storage systems, energy management systems, and hybrid system architecture, respectively. The title of power generators, diesel machines, fuel cells, and renewable energy sources (wind turbines and solar cells) are explained and studies in the literature are explained. In energy storage systems, batteries, flywheels, and supercapacitors that can be used on ships are examined. In the energy management system, the most used approaches and algorithms of rule-based and optimized management systems are introduced. The hybrid system architecture is shown in serial, parallel, and serial-parallel configurations. In the last part of the article, a generic table has been prepared in which hybrid system compatibility is established depending on the ship types. According to this table, equipment compatibility for energy storage, system architecture, and power generation for eight different ship types is shown. In the last part of the thesis before the conclusion, the publication prepared as a book chapter is included. After reviewing hybrid systems in this publication, a case study is made. In the case study, four 100 kW proton exchange membrane fuel cells in serial architecture and only two of the four 450 kW marine diesel engine were used as power generators, and two 100 kWh lithium-ion batteries were used as energy storage devices. Mathematical models of all equipment in the system are shown, these are mechanical and electrical equipment such as shaft, electric motors, converters and speed reducers. Ship hull resistance was calculated parametrically with the Holtrop-Mennen method depending on the speed. The energy management system, on the other hand, is prepared according to the fully charged state of the battery on a rule-based basis. Depending on the power demand, the conditions for the battery to switch on and off are determined. According to the results of the analysis, fuel consumption decreased by 35% and the ship's total emissions decreased by 45% at take-off, 79% at berthing and 100% at port. On the other hand, a 31% reduction was observed in the emissions made in the steady-state sailing.