As things stand there are remarkable few alternatives to two and four-stroke internal combustion engines. Some LNG carriers have made use of boil off gas from cargoes powering steam turbines and there are still new vessels under construction that will be similarly powered.
The list of existing vessel types with a steam turbine is not limited to LNG carriers as there are several installations on types as diverse as tankers, Great Lakes bulk carriers, general cargo vessels and ice breakers.
In the latter case, the vessels are mostly Russian owned and the heat for the turbine comes from nuclear reactors. Nuclear power has been considered a possible alternative for the future quite recently but understandable there appears little enthusiasm amongst shipowners for a variety of reasons.
When even military vessels with nuclear power plants are unwelcome in many commercial ports, the likelihood of a merchant vessel being accepted is virtually nil. That may change of course but only if environmental concerns about climate cannot be allayed by use of other technologies and that will be well into the future.
With more than 300 vessels in existence powered by steam turbines, the list of system makers is quite long but for the nine vessels under construction – seven LNG carriers and two nuclear icebreakers – the list comprises just two names for commercial vessels and whichever option is chosen by the Russian operators for the icebreakers. The two commercial makers are Kawasaki and Mitsubishi both Japanese companies with long histories of dominance in this area.
Batteries are being promoted as a power source for vessel types such as ferries and tugs and assuming that rapid charging facilities are provided in ports this is a development that will accelerate. For cargo ships, batteries are not considered viable although short coastal routes may be able to be served using them. The problem with that arrangement is that the ships will be virtually unsaleable on the second-hand market to owners operating where recharging facilities are not available.
Fuel cells were heavily promoted as being a coming technology until around five years ago when most of the companies involved in development began to withdraw from research or at least stopped from making announcements as to progress. A very small number of prototype fuel systems were produced but with very low power outputs for a large footprint and used only to power some auxiliary equipment.
There are several different fuel cell technologies but until a system that is capable of producing sufficient power to propel a vessel is produced, the technology and the claimed advantages of each type is academic.
Aside from the conventional propulsion systems the only other type of engine in use in the merchant ship sector is the gas turbine. These are common in naval vessels and also in some private luxury yachts but limited to passenger vessels in the commercial field. All of the gas turbines currently in commercial use are variants of General electric’s LM2500 type of aero-derived turbine. Most were installed between 2000 and 2003 on cruise vessels. The most recent use is onboard the fast ferry Francisco built in 2013 by Incat Tasmania.
Capable of running on distillates or LNG, gas turbines are claimed to have the advantage of being more compact and 60% lighter than a conventional diesel engine of similar power output. On the downside, they are not easy to maintain at sea in the event of a failure and have struggled to find favour with shipowners.
Over the last five years, several concepts have been developed and some such as an LNG carrier designed by Hyundai in conjunction with GE have received approval in principal from class societies.