Most ships today use a mechanical propulsion system. These vessels have a shaftline and sometimes a gearbox between the main engine and the propeller. At constant speeds this kind of arrangement is highly efficient with minimal energy losses. A shaft generator can electrify your vessel’s mechanical propulsion – here are four successful ways to achieve the biggest benefits.
There are four ways to reduce fuel consumption and lower emissions by electrifying your vessel’s mechanical propulsion, from the basic to the brilliant. How many have you considered?
The easiest way to start electrifying a mechanical propulsion system is to install a shaft generator and run it in power take-off (PTO) mode. Shaft generators are a proven technology that have been used on board ships for over 50 years. They work by generating electrical power from the rotation of the main engine’s propeller shaft, which can then be used to supply the vessel’s electrical load, significantly reducing the need for auxiliary engines during transit. Because the main engine is more efficient, this cuts both fuel costs and emissions.
Shaft generator systems include a frequency converter to ensure stable electrical output even when the propeller speed varies. This gives them many more running hours than old-style shaft generators – up to six times more on deep-sea vessels like LNG carriers, for example. The system will work at low speeds, in heavy seas and when your propeller speed is frequently changing. If your ship has a controllable pitch propeller (CPP) system, you can continuously optimise your combinator curves during operation and the shaft generator system will continue to work as normal.
PTO mode has become the standard in recent years on all merchant vessels, from fast-turning units on small chemical tankers to slow-turning systems on large mega-container vessels. The biggest systems currently deliver up to 5 MW of electrical power at medium voltage (6.6 kV). PTO mode is especially beneficial for vessels such as bulk carriers, container ships, LNG carriers and RoRo vessels that have long periods of steady cruising.
If you operate LNG carriers, a shaft generator system in PTO mode is the perfect way to handle your boil-off gas. When the main engine is running on gas, the shaft generator allows you to generate all the electrical power needed onboard with very good efficiency and almost no maintenance costs for the whole shaft generator system. Many gas carriers are built for long distances and sailing in deep waters, so they want to focus on pure power generation. For this setup the Wärtsilä shaft generator system has the highest efficiency on the market as it uses two optimised components that are specially designed for this application.
The shaft generator can also be used as an electric motor – this is called PTI (power take-in) mode. In PTI mode, the shaft generator works in reverse – instead of generating power it uses electrical energy to assist or replace mechanical propulsion.
PTI mode is useful in two situations:
PTI mode is often used by superyachts when cruising, with the main engine only used for transits. It is also increasingly common on merchant ships looking to balance optimising performance with improving fuel economy and maintaining emissions targets.
Adding batteries to a shaft generator system creates a hybrid power system for your vessel. Hybrid shaft generator systems contribute to decarbonisation by greatly reducing or even eliminating the need for auxiliary engines and their associated emissions. Hybrid systems also contribute to reduced fuel costs and maintenance requirements.
Batteries decouple energy production from consumption, meaning that energy-producing assets on board your ship can operate at maximum efficiency, providing instant power as needed and improving redundancy, load response and power availability. With batteries available you benefit from:
Large ferries, especially RoPax vessels, employ powerful thrusters to provide the high levels of manoeuvrability they often need in harbours. Traditionally these are powered by dedicated auxiliary engines that are only used for a few minutes a day. In a hybrid system, the battery is charged by the shaft generator during the voyage and then discharges energy to the thrusters for use in port. The result is quieter operation, reduced visible smoke emissions and no need to power up auxiliary gensets in harbours for loading, unloading and manoeuvring.
Depending on the size of the vessel and the length of stay in the harbour, the batteries can handle the full hotel load – the electrical power needed for onboard systems other than propulsion – so less energy needs to be consumed via the onshore connection and auxiliary engine operation can be further reduced.
A PTO/PTI hybrid system, especially on twin-shaft ships such as RoPax and RoRo vessels, provides a similar level of flexibility to fully electric propulsion systems, but it is less complex. You can choose the optimal way to operate your vessel at different speeds while ensuring minimal losses when the main engine is running. Ferries, with their higher hotel load and significant time spent in and around harbours, are especially well-placed to benefit from hybrid shaft generator systems.
To succeed with a hybrid setup you need smart integration of the various components. And this is where Wärtsilä can help you, with a highly sophisticated and fully integrated energy management system (EMS). This acts as the ‘brain’ of the hybrid system, optimising the energy flow between power producers and consumers. The smart EMS means ships in harbour can operate on batteries alone.
Electrifying your vessel’s mechanical propulsion system isn’t finished when the vessel is built. Even in PTO systems there is a lot of optimisation potential, all of which adds up to greater fuel savings. Too often we see systems running sub-optimally. Consider these typical scenarios:
The answer to all these scenarios is proper crew training and continuous monitoring. Ensuring your crew know how to use the system effectively – and you having the knowledge that it’s always being used optimally – can unlock significant additional savings.
If you’re interested in discovering the benefits of electrifying your vessel’s mechanical propulsion system, it’s best to talk to the experts. Wärtsilä has over 60 years’ experience with shaft generators and has delivered over 1,000 systems. In fact, every second shaft generator system in the megawatt range comes from Wärtsilä – more than all our competitors combined. We currently deliver 80–100 systems a year, allowing owners and operators to reap significant operations benefits. Will your vessel be next?
Come and meet us at https://www.wartsila.com/marine/events/electric-hybrid-marine-expo-europe
