Introduction

Today the large majority of inland and coastal vessels is equipped with internal combustion engines, the so-called diesel-direct propulsion. The engine torque most commonly is transmitted to the propeller via a gearbox or directly for slow-speed engines which can be reversed. These systems are technically mature, durable and cost-effective. Even if combustion engines with alternative fuels like methanol or hydrogen can be integrated analogue, “x-electric” drives (the “x” stands for any energy source) are increasingly being used in inland and coastal shipping, even despite the additional energy conversion brings additional losses. On the one hand, this is due to the complex operating profiles with very different power requirements, for example when sailing up and down rivers with fluctuation water depths. On the other hand, not all fuels and combustion processes are suitable for meeting the characteristics of the propeller in terms of rpm-dependent torque. In addition, an appropriately designed x-electric drive system can be adapted more flexibly to future developments in clean drives.

For the deployment of electric drives on board, products from numerous manufacturers are available in the required power range of 500 kW – 1200 kW for inland vessels and up to 10 MW for coastal vessels. If the individual technical requirements and boundary conditions of a ship cannot be covered by the manufacturer's portfolio, many manufacturers offer the option of having a customised drive built. In principle, DC motors or three-phase synchronous motors can be considered. The differences, advantages and disadvantages of the various motors and grid types are explained below.

Regulations

For inland waterway vessels, the ES-TRIN 2025 Chapter 11 Special provision applicable to electric vessel propulsion deals with regulations for electric propulsion:

One electrical power source required for electric propulsion systems with a single main propulsor.
Two electrical power sources required for electric propulsion systems with more than one main propulsors.
Redundancy not generally required, however, independence of two electric propulsion systems (where required by other provisions of the ES-TRIN, e.g. for passenger vessels) only to be assumed if.
Power supply circuits of the electric propulsion engines completely separate from one another, OR
A FMEA-S safety study demonstrates that no failure of one electric propulsion system impairs the operation of the other, OR
The electric propulsion systems can be separated from each other by a separating device automatically activated in case of malfunction or failure of one of the electric propulsion systems.
Manually operated emergency shut-down device for each electric engine.
Electrical power sources must be capable of absorbing power feedback occurring during reversing manoeuvres.

Load changes, including brief overloads, and manoeuvres must not impair safe operation of electric propulsion engines
Externally cooled power electronics must have emergency running characteristics ensuring "steerageway under the vessel’s own power” for at least 30 minutes. This requirement does not apply in the presence of a second independent propulsion system (see above).
Control, indicating and monitoring requirements harmonised with other propulsion systems and set up – as far as possible – in a technologically neutral way (Article 7.04)
For integrated power management systems / energy supply systems priority is required for systems necessary for the safe operation of the vessel.
Test concept for electric propulsion systems to be developed by the manufacturer or the system integrator, based on minimum requirements defined in ES-TRIN.

For coastal vessels the IMO rules in Part 6, chapter 2, section 16 are to be considered.

Technical Concept

AC vs DC grid

In an AC grid both voltage and frequency are required to be monitored and controlled for maintaining the power stability. In a DC grid, however, there are no reactive power interactions, and then the system control is oriented to the voltage only. This is advantageous for keeping the power stability. Moreover it is mean less effort to integrate DC power sources (e.g., fuel cells, lithium–ion batteries, supercapacitors, etc.) into a DC-bus.

If a DC network is to be installed on board, this does not necessarily mean that all consumers must be operated with direct current. Instead, the DC grid can be set up as a main or sub-distribution grid. There is the main-bus type and the sub-bus type.

Main-bus type: The electricity generated on board is converted to the same voltage of direct current. The main drive is connected directly to the DC circuit rail. An AC grid is supplied by converters for common AC consumers.

Sub-bus type: The electricity generated on board is converted to the same voltage and frequency alternating current. A DC grid for the main engine is established by rectifiers from the AC grid.

Fuel cells and batteries provide their energy as direct current. This means that the type of mains does not have to be changed to supply the motor. Every form of energy conversion (e.g. from AC to DC) is subject to losses. Avoiding additional conversions increases the overall efficiency of the drive. In addition, DC motors offer a very high torque in the starting phase and can cover a wide speed range. The choice of motor, in terms of torque and speed, can be matched to the propeller shaft either as a direct drive or for use with a gearbox. In both cases, there is no need to provide a reversing gearbox, as the direction of rotation of the machines can be changed electrically and without delay.

Generation Concept

In the AC-grid system the genset must run constantly at the given frequency of 50 or 60 Hz. In a DC-grid the genset can be operated with variable speeds. This leads to a wider operational range with high fuel efficiency. It was stated that an efficiency gain of up to 20 % is possible. In addition, it is comparatively easy to integrate DC power sources such as batteries or fuel cells into the DC bus system.

Example of a generator fuel consumption graph.

Electric Motor Types

DC Motors

DC motors essentially consist of a rotor and a stator, both of which must be supplied with power. As the rotor rotates, the transmission can only be realised with slip rings. These are subject to minimal wear during operation. As a result, DC motors are not maintenance-free and incur some costs over their service life. However, these machines are made of ordinary metals such as copper and steel. It is therefore possible to completely recycle such a motor at the end of its service life. In DC motors, there are various ways of interconnecting the excitation windings of the stator and the armature windings of the rotor. This allows different machine characteristics to be achieved. Only separately excited DC machines are suitable for the main drive on board inland waterway vessels. The excitation windings and armature windings are fed by two different control circuits. Both are based on pulse width modulation of the main voltage. This separation means that the machine can be optimised for the applied load. As a result, a high degree of efficiency can be achieved over the entire operating range of the machine.

Three-Phase Synchronous Motors

Like DC motors, synchronous motors are available in different speed and torque ranges. They can also change their direction of rotation without delay and can be connected to the propeller shaft either directly without a gearbox or via a gearbox.

The characteristics required in marine applications can be provided most efficiently by permanently excited synchronous motors. These high-pole motors with a good torque-to-mass ratio are often referred to as torque motors. They are characterised by a constant torque curve over the entire speed range. The speed is varied, and the machine characteristic curve is adapted to the load characteristic curve only by supplying the excitation windings. No complex tuning of various resistors and the magnetic flux is necessary here, as is the case with DC machines. The excitation windings are supplied with alternating current. The motor speed is directly proportional to the frequency of the excitation voltage. The torque is adjusted via the current through the excitation windings, which in turn is approximately proportional to the level of the supply voltage. A frequency converter is usually used to convert the energy from the mains into a frequency and voltage-variable network for a synchronous motor. When the rated speed of the machine is reached, the torque decreases because the power remains constant. A DC machine is recommended for ships characterised by frequent speed changes and partial load ranges. For ships where a highly efficient drive is paramount, the synchronous machine is the first choice. For the control of motors, a distinction is essentially only made between direct current and three phase machines. Pulse width modulation is normally used to control DC motors. A frequency converter is generally used to control three-phase machines.

Operational Aspects

All electric motors have in common that they require less maintenance which results in lower operational costs.

Infrastructure Aspects / Charging

Depending on the chosen energy source for the electric motor, bunkering or charging charging infrastructure is often the bottleneck. More on batteries and different energy sources can be found in the other fact sheets.

Economics & Environmental Sustainability

The investment and operating costs are highly dependent on the individual ship and its operational profile. Nevertheless, there are key figures for some components. These are linked to a database so that they are updated as soon as new information is available.

Investment Costs

The electrification of the propulsion system can cost, dependent on the vessel size and system architecture, between 250 000 € and 2 000 000 €.

Operational Costs

A significant reduction of maintenance costs is possible.

Deployment Examples

Vessel type	Inland container vessel
ENI	02323207
Vessel Size	110.0 m × 11.45 m
Build Year	2009
Propulsion	Mitsubishi S 16 R-C2 MPTK, 1250 kW Hybrid Propulsion system by Koedood Dieselservice BV and Hybrid Ship Propulsion BV (HSP)

Vessel type	Inland dry cargo vessel
ENI	02335529
Vessel Size	135.0 m × 11.45 m
Build Year	2013
Propulsion	2 x Mitsubishi S 12 A2-MPTA 630 kW and 2 x Baumüller DS T 2-400 electric motor 286 kW

Project Coordinator

DST - Development Centre for Ship Technology and Transport Systems

Partners

SPB – Stichting Projecten Binnenvaart (NL)

Scandinaos AB (SE)

MARIN – Maritime Research Institute Netherlands (NL)

viadonau – Österreichische Wasserstraßen-GmbH (AT)

TTS – Transport Trade Services GmbH (AT)

ZT Büro Anzböck Richard (AT)

EUFRAK – Euroconsults Berlin GmbH (DE)

CRS – Hrvatski Registar Brodova (HR)

OST – Ostschweizer Fachhochschule (CH)

Argo-Anleg GmbH (DE)

FPS – Future Proof Shipping (NL)

Mercurius Shipbuilding BV (NL)

ZES – Zero Emission Services (NL)

Compagnie Fluviale de Transport (FR)

Sogestran (FR)

Koedood Diesel Service BV (NL)

CMB – Revolve Technologies Ltd. (UK)

Contact:	BF	IB
	BF	IB
	BF	IB

Disclaimer

The content of the publication herein is the sole responsibility of the publishers and it does not necessarily represent the views expressed by the European Commission or its services. While the information contained in the document is believed to be accurate, the author(s) or any other participant in the SYNERGETICS consortium make no warranty of any kind with regard to this material including, but not limited to the implied warranties of merchantability and fitness for a particular purpose. Neither the SYNERGETICS Consortium nor any of its members, their officers, employees or agents shall be responsible or liable in negligence or otherwise howsoever in respect of any inaccuracy or omission herein.

Key Facts