Full electric vessels sail using only electrical energy stored in batteries/power packs. By definition a full electric vessel does not use a fuelled power source. This may be called ‘Battery electric’. Another definition is that a full electric vessel does not use an internal combustion engine (ICE). In that case fuel cells may be added to the system to generate electric power for sailing (Fuel cell electric).
A full electric propulsion system without an on-board fuel cell is charged by cold ironing, meaning the vessel needs regular re-charging time at shore. Due to the limited capacity of the battery pack, only short sailing distances are possible. This may not be an issue for ferries, but it is a hindrance for inland waterway transport vessels that are intended to cover large distances without the interruption of regular breaks to re-charge batteries.
Natural gas emits lower levels of nitrogen oxide (NOx) and also emits lower levels of particulate matter (PM). Compressed natural gas (CNG) is made by compressing natural to less than 1 percent of the volume it occupies at standard atmospheric pressure. It is stored and distributed in hard containers at a pressure of 200–250 bar usually in cylindrical or spherical shapes.
Liquefied natural gas or LNG is natural gas that has been converted to a liquid form for the ease of storage or transport by cooling natural gas to approximately −162 °C. Afterwards, it is stored at essentially atmospheric pressure. Liquefied natural gas takes up about one six hundredth the volume of natural gas in the gaseous state at atmospheric pressure or about 2.5 times less volume than CNG at 250 bar pressure.
Dual fuel engine: 80% LNG and 20% diesel:
These Dual-Fuel engines are based on diesel engines. The engines have been converted so they can also be powered by LNG fuel. The fuel is a mix of 80% LNG and 20% diesel.
Dual fuel / pilot diesel engine: 99% LNG and 1% diesel:
In this case the engine is fully optimized for natural gas combustion. This LNG Dual-Fuel system has already been in use for more than 10 years in coastal and ocean shipping. The engines are now also supplied for inland shipping. The LNG Dual-Fuel engines are specifically designed as Dual-Fuel systems so only a limited quantity of pilot fuel is required. The Dual-Fuel engine can nevertheless run fully on diesel. This involves proportions of 1% diesel and 99% LNG.
With the concept of fuel-water emulsion an add-on system emulsifies and mixes fuel with water, resulting in a homogeneous emulsion. This emulsion consists of oil droplets with a water nucleus, which is injected into the engine. A ‘micro-explosion’ divides each fuel oil droplet in numerous smaller fuel oil droplets, as these smaller fuel oil droplets ignite and burn easier than the bigger droplets. This results in the reduction of potential particle matter and soot creation zones, a cooling down of the combustion chamber temperature and a reduction of fuel consumption.
The share of water in diesel fuel usually ranged from 10% to 20%. The water used for the emulsion can be obtained from the conventional water tank in the vessel, but it is preferable to install equipment to obtain desalinated and demineralised water. The effectiveness of FWE is due to the quality of the emulsion, most efficient is an electronic control determining the required share of water. The fuel injection settings should be adjusted, to avoid a reduced power output. Although FWE has already been used in road vehicles and other mobile machinery, there is currently just one supplier (Exomission) installing this technology on inland vessels and there are only a few inland vessels with this system on board.
Hybrid vessels may sail with the use of two or more energy sources. Main engines and generators using a fuelled power source are combined with an integrated electrical energy storage in the form of batteries/power packs, to hybridize either the energy production to ease up the main engine optimization. A way forward to full electric sailing (future) the electrical energy may be generated from fuel cells.
Sailing may be achieved by direct propulsion and/or electric propulsion.
Direct propulsion: The fuelled power source is the only source for propulsion and is directly connected to the propeller through the clutch.
Electric propulsion: Electricity is the only power source for propulsion. The electric energy may come from the battery/power packs and/or the generator sets. The power packs may be used for load levelling or peak shaving or for full electric propulsion, e.g. in areas where zero emissions is required.
The benefits of hybrid propulsion:
• improved vessel flexibility and performance;
• lower fuel consumption, optimise engine performance;
• reduced emissions due to lower fuel consumption;
• lower operating costs due to lower fuel consumption;
• lower maintenance costs;
• reduced noise levels.
Hybrid propulsion makes it possible to use the fuelled power sources more efficiently, by switching the source on only when needed. The fuel power will perform in a higher fuel efficiency area. This is also very beneficial to a proper operation of engine emission control systems such as an SCR catalyst and a DPF. In case the fuel power source is used to generate electricity, energy losses occur due to conversion from mechanical to electric power and from electric power back to mechanical power to generate the propulsion. For that reason it is very important to know the sailing profile in detail before fitting a hybrid system into a vessel.
Gas-To-Liquids (GTL) technology converts natural gas – the cleanest-burning fossil fuel – into high-quality liquid products that would otherwise be made from crude oil. These liquid products include transportation fuels, motor oils and the ingredients for everyday necessities like plastics, detergents and cosmetics. GTL products are colourless, odourless and contain almost none of the impurities – sulphur, aromatics and nitrogen – that are found in crude oil.
GTL Fuel is an alternative fuel for use in diesel engines. Fuel may be used in existing diesel engines for inland vessels without modifications of the engine, allowing for easy switchover from conventional diesel fuel to GTL Fuel. GTL Fuel is already in daily commercial use in Germany and the Netherlands.
No reduction in overall fuel consumption is established. Taking the GTL fuel supply chain into account, the energy consumption and GHG emissions of GTL fuel are 42% and 12% higher than that of diesel, primarily because of the low efficiency of the GTL synthesis process. [Applied Energy; 2010] ; [Institute of Energy, Environment and Economy, 2013] . The application of GTL shows an instantaneous significant reduction in regulated pollutant emissions NOx (3% to 13%), PM (up to 49%); [RSC Advances 2014].
Selective Catalytic Reduction of NOx (SCR deNOx) is a technology applied on diesel engines to reduce the NOx emissions, by injecting a urea-water solution (AdBlue) into the exhaust gas upstream of the SCR catalyst, generating ammonia (NH3). This is absorbed onto the catalyst, converting NOx in diatomic nitrogen (N2) and water (H2O).
A Diesel Particulate Filter reduces the particulate matter emissions from the engine exhaust gases. The most efficient DPF is the wall flow DPF, commonly made from ceramic materials with a honeycomb structure with alternate channels plugged at opposite ends. According to the Manufacturers of Emission Controls Association (MECA), particulate matter is captured by interception and impaction of the solid particles across the porous wall. The filter is designed to hold a certain quantity of soot. During the course of its operational hours, it gets loaded due to the high deposition of soot. This can result in increased back pressure on the engine and when not properly acted upon may lead to clogging of the filter. Therefore, it is important to maintain a sufficiently high average temperature such that the stored particle matter is regenerated (converted to CO2) and the filter is kept clean. This is needed to prevent it from becoming blocked and its function thereby being affected. Alternatively a special active regeneration system can be installed, which increases the filter temperature periodically to high temperature for fast filter regeneration.
SCR and DPF are often combined because then all gaseous as well as particulate emissions are reduced (by 70% or more) and usually the most stringent (future) emission legislation can be met. SCR and DPF often work together nicely leading to an increased SCR efficiency. One of the technical options is the “SCR on DPF technology”, where the DPF part acts as an SCR catalyst as well. This can lead to a more compact configuration.