Turning the screw on shaft energy

11 Sep 2017
Permanent magnet-driven shaft generators offer an alternative to trraditiona asynchronous motors

Permanent magnet-driven shaft generators offer an alternative to trraditiona asynchronous motors

The drive for energy efficiency is encouraging more ship owners to harness the latent energy in their propeller shaftlines. Charlie Bartlett examines recent applications and technology improvements.

In the large tanker, container ship and bulk carrier segments where two-stroke engines dominate, low shaft speed allows many ships to circumvent the need for a gearbox, and a requirement to burn expensive fuels like marine diesel. This allows the largest ships to run in an extremely cost-efficient fashion by burning thick, tar-like bunker fuel, suffering very few mechanical losses along the way thanks to the directly-driven propeller.

But vessels must generate electrical power, requiring smaller, typically four-stroke auxiliary engines, linked via gearbox to a generator. While it is possible to burn bunker fuel in such engines, without energy-intensive pre-heating the thick fuel would congeal and cause blockages in these smaller engines, leading to high maintenance costs. As a result, auxiliary engines usually run on  comparatively expensive marine diesel or distillate fuels.

A shaft generator has therefore been ideal for shipowners as a way to generate power cheaply from the most cost-efficient engine on the ship. Traditionally this has been done using a tunnel gear to link the shaft and the generator, stepping-up the generator speed. But with gearboxes come efficiency losses – 2-3% of energy is lost to friction, and a corresponding rise in cooling power requirement. It also limits the speed range at which the generator can derive power from the rotation of the shaft.

But more recently, the development of permanent magnet (PM) technology - also known as brushless AC and used extensively in wind turbines - has been implemented for shaft generators. Containing magnets manufactured from rare-earth metals with up to twice the magnetism of ferrites, PM generators need no separate excitation unlike induction motors, and because they are synchronous, do not suffer from rotor slip. This cuts 30% of total generating losses.

Motor alternatives

“The biggest performance difference between asynchronous machines and PMs is that they don’t have to utilise their energy for exciting the rotor,” explains Mika Koli, Business Development Manager at The Switch.

Further, the technology circumvents the need for a gearbox, and allows permanent magnet shaft generators to derive power whatever the speed of the propeller. “If you think about the operational profiles, how you really run engines – you usually have some reserve,” Koli continues.

“You are not driving them on 100% - usually, it’s 60 or 70%,”. “When you are driving on partial loads that’s good for PM technology – the efficiency is at its highest at 60-70% load. Permanent magnet machines allow you to draw energy at the full torque range of the engine, from zero RPM upwards.”

The fact that vessels could operate a shaft generator while burning low-grade fuel is a major selling point today, but with the upcoming IMO 0.5% sulphur cap set to enter force in 2020, owners will be compelled either to fit scrubbers to their vessels, or to burn expensive, refined fuel in their engines. In the latter case, it might seem as though the main value proposition of the shaft generator will be eroded.

But there are other factors at play. Energy generated using low-sulphur heavy fuel oil (LSHFO) using a low-speed engine and shaft generator would still be cheaper than running a genset on diesel, though not by as wide a margin. But, Koli explains, thanks to its greater efficiency, a shaft generator retains its advantage over auxiliaries simply in terms of specific-fuel-consumption (SFOC).

“I think the market will utilise [the sulphur cap] to increase the price for the fuel,” he says. “Whenever the fuel price increases, the shipowner can make the biggest difference using a shaft generator compared with their competitors.

“Those operators who are really putting effort into having efficient ships will always have the competitive advantage, and when the fuel price is high, you can make a bigger difference in savings using your most efficient engine, and a shaft generator.”
Batteries included

But a shaft generator can only produce power while the vessel is underway, and this has necessitated auxiliary generators to run the hotel load while the vessel is in port. But with recent improvements in battery technology and the anticipated onset of large shipboard energy storage installations, it may not be long before this limitation can be overcome, ensuring compliance with port-based emissions regulations is achieved with minimal additional expenditure.

Wärtsilä’s Hy 2 generator-and-battery system features two configurations – a diesel-electric version comprising a four-stroke genset, and a diesel-mechanical system, featuring a four-stroke engine, a propeller shaft, and, if desired, a shaft generator. Speaking to The Motorship, Fritz-Werner Bergmann, senior manager of drives systems at Wärtsilä Marine Solutions, anticipates that shaft generator technology would perform a pivotal role in a vessel-wide hybrid arrangement.

“The design is ready, the components are available and we are searching for a customer,” he says. “The most critical point for the customers are the investment cost and the question of battery lifetime.”

In August, Wärtsilä won a contract to supply a wide range of propulsion and energy management equipment to six 1,160 TEU feeder newbuilds under construction at China’s Fujian Mawei shipyard, for Germany’s MarLink Project Management. Among the kit to be installed on the vessels was a custom-designed shaft generator, specially engineered to work with a tunnel gear which steps up the speed of the propeller. Energy derived from the shaft generator will likely contribute to the running of a variable frequency drive (VFD) tunnel thruster, near the bow of each ship.

For the newest Wärtsilä systems, a shaft generator working in tandem with a waste heat recovery turbine is often considered, as the steady power output of the former compliments the efficiency of the latter, Bergmann explains. “The basic idea is to use the waste heat energy from the main engine exhaust gas to operate a turbine generator, which is connected to the 450V mains. But formerly, pressure pulsations of the exhaust gas resulted in frequency pulsations in the 450V mains, which resulted in the valves ahead of the turbine opening at the wrong time.

“When it is integrated with a shaft generator, however, the turbine generator is connected to the intermediate voltage circuit of the frequency converter of the shaft generator. Therefore, the valves can be more open without the frequency fluctuating effect. The frequency converter of the shaft generator system keeps the 450 V mains frequency stable,” Bergmann comments.

Wärtsilä-SAM electronics acknowledges that permanent magnets are a huge step forward for shaft generator technology, but Bergmann also points out that there are some drawbacks. “From a technical point of view, the permanent magnet machine needs a much more complex frequency converter designed in so called AFE (Active Front End) technology to control the voltage characteristic of the permanent magnet machines.”

In fact, Bergmann holds that a PM installation is not always the superior choice. “By comparing the efficiency of both systems, we find the difference is approximately 1% in favour of the permanent magnet excited system for high-speed installations, and equal-to or 1.3%-in favour of the conventionally-excited systems for slow speed installations with a power range above 2,000 KW.

“From a technical point of view, the specific rules from the major classification societies has to take permanent magnet machines into consideration, which is another technical burden for the rotor design for slow-speed machines.”

Bridging the gap

Emissions regulations inside ECA zones and near ports are being tightened all the time – particularly in China, where currently, more stringent sulphur regulations are entering force every year. Shaft generators’ power take-in mode (PTI) - or sometimes power take-home (PTH) – is growing in importance as a key differentiator for the installation, allowing any installed ship to become a fully low-sulphur diesel, or LNG-electric for short periods.

“Fuel efficiency will become a primary concern for vessel owners and operators when the oil price goes up again, and the shaft generator is thus likely to be more widely adopted,” says GE’s Loic Leclere, Merchant Marine Solution Leader at GE’s Marine Solutions. “Such a system can also integrate with an energy storage package, which meets stringent marine environment requirements.”

Last year, GE won a contract to supply its power take-off/power take-in (PTO/PTI) shaft generator solution to nine 19,630teu Maersk containerships. The company’s methodology differs somewhat from that of other players in the market, favouring asynchronous induction motors over synchronous designs.

“The induction motors enjoy a simpler design,” Leclere explains. “By removing transformers, the sets are more reliable and require less maintenance. Combined with the solution’s fuel saving feature, this further reduces opex for customers.”

Leclere describes an installation his company undertook on an undisclosed boxship. “We recently installed our innovative power take off/power take in (PTO/PTI) solution on a container vessel,” he says. “The solution consists of two GE motor-generator sets that sit on the two propeller shafts while connecting to the GE MV7000 drives.”

Shaft generators can be used in two possible configurations to add to the vessel’s propulsion power. One is to de-clutch the prime mover and allow the shaft generator to operate as an electric motor driving the ship’s propeller by itself, taking in power from other sources in what is essentially a diesel-electric setup (or, as is likely in the future, battery-driven). On the other hand, in this case GE’s installation prioritises the other possible PTI configuration, sometimes referred to by manufacturers as “boost” mode.

“During the PTI mode, the motor-generator sets play the role of electric propulsion system that provides additional motor power—beyond that of the main diesel engines—to propel the vessel,” Leclere explains. “When the need for propulsion power is reduced, the motor-generator sets switch to the PTO mode, harnessing the mechanical energy from the shaft and converting this into electricity to generate power for electrical equipment on board the vessel.”