In the early days of mechanically propelled ships, the steam or motor engine was often complemented by older sailing technology to provide ‘free’ power whenever possible. This was both as a means of saving fuel and a necessary boost to engines that were not as efficient as those available today.
Sails and their associated rigging were an expensive item to install and maintain and of course required additional manpower. That was less of a problem for steam ships which carried plenty of crew for stoking boilers and ferrying coal from the bunkers to the boiler but for motor ships with smaller crews, sails became more of a problem as engine efficiency increased and they were eventually abandoned to the point where today only a few specialist sailing cruise ships exist in the commercial arena.
Most modern ships can rely only on the power of their main engines and any auxiliaries onboard for propulsion and other essential services but hard times and regulations have led to searches for gaining more of the energy available from the fuel burned and taking advantage of any ‘free’ energy that can be recovered.
The oil crises of the 1970s were a spur to an earlier generation of efficiency measures well before the IMO had even conceived the need for reducing fuel use and the idea of EEDI. One of the measures that has become commonplace on ships is the shaft generator which uses the
rotary motion of the engine in a mechanical drive configuration to also generate electricity. During a voyage when the main engine is burning HFO, operating a shaft generator can give savings compared to an auxiliary operating on MDO.
The mechanical drive systems of both two and four stroke engines can run a shaft generator which can be positioned at either end of the engine. When placed forward, an extension of the crankshaft at the front of the engine is needed. Where gearboxes are incorporated, the shaft generator might be turned by a take out from the gearbox. Under such circumstances, the generator can be run in reverse and act as a power take in boosting the output for more speed or as a ‘get you home’ device.
Shaft generators come in several varieties ranging from simple systems suited to ships operating with a constant shaft speed that are taken offline when the shaft speed falls below a certain level to more complex versions with gearing and speed management to ensure constant supply of electricity.
On the downside, they are an additional expense and require maintenance. There are also operational issues such as the fact that they increase the fuel consumption of the main engine, they cannot be used in port when the main engine is not running. Under conditions where the main engine speed may be constantly changing – such as when entering into a port – the shaft generator may not be providing sufficient power for navigation systems and a generator will need to be running in any case.
Waste heat recovery
In most types of ships where space is available and engine operation allows, waste heat recovery systems can allow for further fuel saving by reducing the running time of the ship’s auxiliary engines. Even the most efficient engines waste around 50% of the energy
in the fuel through heat. Some may be recovered by the exhaust gas boiler or economiser which provides the hot water demand of the ship but unless a further means of recovery is employed the rest is wasted.
A waste heat recovery system (WHRS) addresses this issue but is not suitable for all vessels only those with a high electricity demand such as container vessels with large numbers of reefer boxes. There are various configurations of WHRSs and to obtain the most benefit they should be vessel specific taking into account the equipment on board and the operating profile of the vessel.
A WHRS can harvest the heat from the exhausts and cooling systems of main and auxiliary engines and the exhaust from oil-fired boilers. Gathering the heat from exhaust systems can be complicated by the need to provide energy for the turbochargers and by exhaust gas cleaning systems or NOx reduction techniques that require lower combustion temperatures to
Early WHRSs were designed for working with exhaust systems but the potential for recovery of waste heat in the engine cooling water is now being explored. Early in 2016 Mitsubishi Heavy Industries Marine Machinery & Engine announced that its link up with US-based Calnetix Technologies had resulted in the first commercial Hydrocurrent Organic Rankine Cycle (ORC) waste heat recovery system being installed on Maersk Line’s 2003-built Arnold Maersk. An ORC system employs very similar principles to a steam turbine but instead of water uses an organic fluid such as n-pentane or toluene. Because these liquids have a lower boiling point than water the system can operate at lower temperatures and make use of lower grade heat.
There have been numerous attempts in the last fifty years to revive the use of sails and other means of harvesting wind power for ship propulsion. A small number of sailing cruise ships exist but these are very niche and have limited application. Harnessing wind for cargo ships has been tried using both kites and more traditional variations of sails.
The latest project to re-introduce sails is run by Hamburg-based Sailing Cargo, which aims to build the world’s biggest sailing cargo ship. The project outlines a plan to build a 170m car carrier, capable of carrying between 1,700 and 2,000 cars, which will be equipped with four DynaRig masts and will operate on hybrid propulsion with sails and diesel-electric engines, and an optional battery system for peak loads. The proposed vessel would be capable of sailing at 10-12 knots with the aim of reaching 14-16 knots in the next few years through combined expertise. Lloyd’s register which is involved in the project believes wind-assisted propulsion offers a realistic option for introducing renewable power into shipping.
The DynaRig is based on traditional square-rigged layout and was initially designed almost 60 years ago in Germany. The masts are freestanding and have rigidly attached curved yards. To adjust the angle of the sails, the entire mast rotates in place. When fully deployed, the sails on each mast have no gaps between them, creating a single panel to capture the wind. It is estimated to have twice the efficiency of a traditional square rig. A few examples have been installed on yachts.
Reviving a concept first employed in the 1920s but soon forgotten as improvements in engine and propeller technology appeared to remove the advantage they gave, Flettner rotors are now enjoying renewed interest. The first vessel to be equipped with them in the modern era was the 2008-built E Ship 1 owned by renewable energy firm Enercon. More recently a Finnish organisation Norse Power has fitted them to and has orders for other ships.
The concept is often described as making use of wind power but this is somewhat misleading as the motive power that Flettner rotors can provide is not wind power in the conventional understanding. Instead it is exploitation of a phenomenon known as the Magnus effect named after German scientist Gustav Magnus who first described the concept in 1852. To take advantage of the effect the rotors must be powered and capable of moving in forward and reverse directions depending on the wind direction. Thus they will consume power but the effect can outweigh this.
The principal behind the effect is that rotating any round object such as a sphere or a cylinder that is moving through a fluid or air affects its trajectory and speed in several ways, in the case of a rotor ship, the wind passing over the surface creates suction. Suction is greatest on any part of the surface that does not move with the wind. If the forward surface of a rotor ship’s cylinder is made to move into the wind – ie clockwise into a starboard wind, counter-clockwise to a port wind – the suction will be strongest on that forward surface and the ship is drawn ahead.
Under some wind conditions, the rotors will not confer any advantage and will need to be stopped. The fact that wind speed and direction is constantly changing also means that a much more sophisticated operating system than was possible in 1924 is needed to gain the best results. Modern sensors and computer technology can adapt the speed of the rotor almost instantly when in use but even so under some circumstances, the wind will be of a strength and direction that the system will be of little use.
US-based Magnuss Corp is another proponent of the Flettner rotor but as yet has not entered the market to the same degree as Norse Power but has an interesting project aimed at using ‘big data’ to gather information on global wind patterns that could help identify the ebst candidates for installing Flettner rotors.