Using shaft generators instead of separate generators

Nov 26 2020

Shipping companies can generate power onboard from the main propeller shaft, rather than having a separate generator. This can be more efficient because you draw exactly what you need, a two-stroke main engine has better fuel economy and you can use cheaper fuel. We asked Yaskawa Environmental Energy / The Switch how their system works.

Conventionally, ships have an engine to drive the propeller, and a separate generator, running on diesel, to generate power for the ship.


But a more efficient way to do it can be to use the shaft itself to generate power as well as drive the propeller. Instead of having a separate generator running continuously, generating the same amount of power, you draw the energy from the shaft that you need. Plus, the two-stroke main engine has better fuel economy and allows you to use cheaper fuel.


Yaskawa Environmental Energy / The Switch, a company based in Helsinki and a subsidiary of Yaskawa Electric Corporation in Japan, provides permanent magnet (PM) shaft generators and associated electrical equipment.


The payback time is usually only few years, with a reduction in fuel consumption for power and a reduction in maintenance costs on power generation systems of 10–20 per cent.


Systems are from 700 kW to 20 MW, although about 2 MW is more typical. One of the biggest systems delivered was for a large container ship with a big need for power for its cooling containers. Tankers typically need most electrical power for pumps, and they are not running all the time, so can be served with a much smaller generator plus a battery.


By comparison, a large vessel may have propulsion power of 80 MW.


In the tanker sector, Yaskawa Environmental Energy has PM shaft generators installed on vessels including Saga LNG, Terntank, Rederiet Stenersen and Ektank. Over 40 of its PM shaft generators are currently sailing on vessels.


The company has a contract with Caterpillar, which is integrating PM machines for propulsion and auxiliary gensets together with a state-of-the-art DC-Hub solution for bulk carriers. “We’re also in the process of signing contracts with several LNG carriers,” says Ville Parpala, Director of Product Marketing, Marine Business at Yaskawa Environmental Energy / The Switch.


Another recent installation was on the dynamically positioned supply vessel North Sea Giant. It had 6 separate generators onboard, three of which were for back-up. One genset was replaced with batteries and The Switch DC-Hub system. After installation, the vessel now uses only one diesel generator with batteries instead of running all three generators at the same time. Fuel savings have been enormous.


The PM shaft generators are made in Finland, and the electrical DC-Hubs are built in Norway.


How it works

The permanent magnet shaft generator is fitted in-line on the shaft – so as the shaft rotates, it rotates the generator. The shaft generator requires that the shaft has two sections, so one can be bolted to each side of the generator.


The generator draws off a small amount of the shaft’s power in doing so – but an amount so small the propulsion system will barely notice, because the amount of energy needed to propel the vessel is so many times more than the power demands from shipboard equipment and services.


The power delivered by the shaft generator can be adjusted electronically with the frequency converter, and reduced to zero if necessary. In contrast, a standalone generator will normally have little adjustment potential, being either on or off.


This means that no gearbox is required, which would be additional equipment to purchase and maintain.


The generator power can charge up batteries or be used directly onboard.


If it charges up batteries, then it can be run in reverse, with the batteries turning the shaft. This would not provide a similar level of power to the main engine, but could offer enough power for vessel manoeuvres in port, for example. This may offer further benefits in a future where vessels are asked not to burn any fuel at all in port. 


It can also provide a limited amount of propulsion power if the main engine fails, something the company calls “Take Me Home” operational mode. You can achieve around 30 per cent of normal speed this way, for a limited time period.


The systems are custom built by Yaskawa Environmental Energy / The Switch, so can be designed to fit into the space which the vessel has available for it.


The machines use ‘permanent magnets’ for generation, which are like fridge magnets, rather than electromagnets. These do not need any electricity to start them up, which simplifies the system requirements.


The magnet material, called Neodymium-Iron-Boron, is the strongest type of permanent magnet available commercially.


Permanent magnet generators are typically 2–4 per cent more efficient at full load, and 10 per cent more efficient at part loads, than conventional electromagnet machines, due to avoiding current losses in the rotor.


The company provides a range of power electronics to go with the system, including frequency converters for controlling the speed of rotating machines or adjusting the power to ship consumers and allowing the use of green power, such as batteries, solar panels or other sources.


Dual windings increase redundancy and allow the use of two separate converters.


If there is any malfunction in the shaft generator, mechanically disconnecting the shaft generator from the propeller shaft is a fairly simple task, which takes about 15 minutes.


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