The modern marine power generation system mainly consists of internal combustion engines, that use different grades of fuel oils derived from crude oil as their primary fuel. These liquids include the basic categories of residual fuel oils and distillate fuel oils. The former are residues from the oil refining process, while the latter are the desired end-result from said process.
Residual fuel oils include heavy, intermediate, and light fuel oils (HFO, IFO, LFO) while marine gas and diesel oils (MGO, MDO) are categorized as distillate fuels. These include many sub-categories based on their chemical and physical properties, such as chemical composition, viscosity, and density. (ISO 8217-2017) Currently these fuel-types hold almost the entire market as can be seen from figure (2.2) showing a prediction of the marine fuel mix until 2050.
Figure 2.2. Predicted fuel mix for the maritime sector until 2050 in [EJ/year]. Source: DNV-GL 2018, 13.
Fuel oils and internal combustion engines have many technical and economic benefits that have made them the best solution for marine applications for a long time. The first main advantage is their relatively low price. HFO being a residual from the oil refining process makes it affordable for the industry and therefore is the most used fuel (Babicz 2015, 374.). The price-aspect of marine fuels is discussed further in chapter (3). The second benefit of fuel oils is their relatively high energy density. In figure (2.3) different fuels are mapped both by their
volumetric and gravimetric energy densities. An optimal fuel would be located on the top right-hand corner. Liquid fossil fuels have a great volumetric energy density but fail to match the gravimetric density of gaseous fuels. This means an addition in the mass of the fuel but a smaller occupied volume, which is an important factor in the maritime industry because of the associated space limitations.
Figure 2.3. Gravimetric and volumetric energy densities of different liquid and gaseous fuels. Prefix C = compressed and L = liquefied. Source: DNV-GL 2018, 70.
Another advantage of internal combustion engines and oils is the fact that they have dominated energy production especially in transportation for decades. In 2012, fossil-based fuels were essentially powering the entire transportation sector according to data provided by the U.S.
Energy Administration (The Maritime Executive, 2015). Mature technologies often have – or at least are perceived to have – higher overall reliability, and availability of resources for research and maintenance.
The number and size of engines varies widely between designs, but generally there are two types of engines onboard: main and auxiliary engines (ME, AE). Main engines, or more generally prime movers, provide the propulsive power that moves the vessel forwards and are often the largest power generation related components onboard both by output power and weight. (Babicz 2015, 477.) This is why they are often located as close to the bottom of the vessel as possible to lower the center of gravity. Auxiliary engines are responsible for
generating electrical power for systems onboard and have a smaller capacity and weight. They can also be called the main sources of electric power (Ibid, 366.). A schematic picture of power generation related components on a containership, resembling the layout on a cruise ship, is shown in figure (2.4).
Figure 2.4. Schematic illustration of the locations of a ship’s: 1. Main engine(s), 2. Auxiliary engine(s), 3.
Auxiliary boiler(s), 4. Auxiliary engine EGB(s), 5. Main engine EGB(s). Source: Alfa Laval 2019.
Internal combustion engines used onboard cruise ships are generally slow (speed up to 400 rpm) or medium speed (400-1200 rpm) diesel engines. These engines can function either on two- or four stroke principles. (Babicz 2015, 176.)
In terms of efficiency, internal combustion engines are far from perfect – even though compared with other mature technologies they can be considered efficient. Generally, the thermal efficiency of slow- and medium-speed marine engines hovers around 40 % (Takaishi et al.
2018, 21.) meaning that 60 % of the energy in the fuel ends up elsewhere than the engines designed output. This 60 % is lost primarily as waste heat, but also as noise and vibrations that exit the engine. The utilization of waste heat streams is a relatively easy way to improve the overall efficiency of the engines and the ship. Waste heat from the engines exits in three main flows: exhaust gases (EG), and high (HT) and low temperature cooling (LT). High temperature heat is generally collected from the engine’s jacket water cooling, and low temperature heat from lubricating oil cooling. Both of these can also utilize different stages of charge air cooling.
(Babicz 2015, 103. & 229-230.) The collected heat can be utilized as heat sources for a variety of processes. An illustration of the energy flows within the case vessel defined in chapter (4) of this thesis is presented in appendix (1). To demonstrate the temperature ranges associated with each heat source, the values for HT- and LT -heat in two dual-fuel Wärtsilä marine engines of different capacities are presented in table (2.1).
Table 2.1. Average temperatures associated with two different engines of different outputs. Sources: Wärtsilä 2019a (3-13), Wärtsilä 2019b (3-6).
Engine W10V31DF W6L34DF
Rated output [kW] 5 500 2 880
HT cooling system [°C] 96 96
LT cooling system [°C] 40 38
The largest engine waste heat stream is the exhaust gases, that often have temperatures of 300…400 °C after the engines (Wärtsilä 2019a (3-11); Wärtsilä 2019b (3-3). The heat from them is recovered with exhaust gas boilers (EGB), that generate steam or hot water for consumption (Babicz 2015, 52. & 229-230.). The limiting factor in exhaust gas waste heat recovery is the temperature at the boiler outlet: Fuels that contain sulphur, such as HFO, are at risk of causing sulphuric acid corrosion. At a low enough temperature the acidic compounds containing sulphur condense on surfaces and cause corrosion. This limits the degree of heat recovery in EGB’s, as the temperature must be kept above the sulphuric acid dew point. (Raiko et al. 2002, 348-349.) Auxiliary boilers are often installed onboard for steam generation (Babicz 2015, 35.), for example in case the supply from the EGB’s is not sufficient for the demand. One potential time for this is staying in port when the main engines are generally shut down. These use similar fuels as the installed engines such as HFO, MGO, or LNG.