Power and Propulsion

Adding something extra


Malcolm Latarche
Malcolm Latarche
ShipInsight

05 December 2018

Adding something extra

Partly to meet EEDI requirements but mostly in a genuine desire to reduce fuel use, shipowners are enthusiastically embracing novel technologies, although not all of the projects taking place are likely to be successful.

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. In modern parlance such ships might even be considered as hybrids.

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.

Shaft generators

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 supplies 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 and 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 operate.

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.

Wind Power

There have been numerous attempts in the last 50 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.

One 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 using sails and diesel-electric engines with 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 is involved in the project and 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 and a few examples have been installed on yachts.

Another project is taking place in Japan where Eco Marine Power (EMP) has been working towards commercial release of the EnergySail. The company announced this year that it has begun collaboration with ClassNK to confirm design requirements and details related to sea trials. The goal is to pave the way to full commercial production of the EnergySail and thus complete the preparation of technologies required for the release of EMP’s Aquarius Marine Renewable Energy (MRE) solution.

The patented Aquarius MRE is an advanced integrated system of rigid sails, marine-grade solar panels, energy storage modules, a charging system and marine computers that will enable ships to tap into renewable energy by harnessing the power provided by the wind and sun. The rigid sails developed by Eco Marine Power would be automatically positioned by a computer system to best suit the prevailing weather conditions and could be lowered and stored when not in use or in bad weather.

Flettner Rotor

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 Norsepower has fitted them to a number of ships to the point that there are now more vessels using rotors than when they were first invented.

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 spinning in forward and reverse directions depending on the wind direction. Thus, they will consume power but the effect can outweigh this.

The principle behind the effect is that by rotating any round object such as a sphere or a cylinder that is moving through a fluid affects its trajectory and speed in several ways because the friction from the spinning surface affects the fluid’s flow. In the case of a rotor ship, the wind passing over the rotor’s surface creates suction, which 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 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 Norsepower. But it has an interesting project aimed at using ‘big data’ to gather information on global wind patterns that could help identify the best candidates for installing Flettner rotors.