E-ferry set to electrify the passenger ship sector

Danish operator Ærø Kommune’s all-electric ferry Ellen breaks several barriers: with its route covering a 22-nautical mile crossing, it will travel a greater distance than any other all-electric ferry and will have the largest battery pack installed at sea. It is also likely to be the first electric ferry to have no emergency back-up generator.

Creating this E-ferry prototype, due for delivery in May, involved building a fully electric-powered ‘green’ ferry which can sail without CO₂ emissions.

Trine Heinemann (Ærø Kommune): The charging time is 20 minutes, not enough for a return trip but this is compensated for by having a larger battery capacity

The project is supported by the European Commission’s €80Bn (US$90.5Bn) research and innovation initiative Horizon 2020 (H2020).

Due to the lack of an emergency generator, extra redundancy was created by splitting the 4.3-MWh battery system into 20 units connected to separate converters. If there is an issue with one, the ferry only loses one-twentieth of its available power – a distinct advantage over the two-battery unit configuration used on other vessels.

“The charging arm is a first-of-its-kind and developed especially for the E-ferry”

Ærø Kommune E-ferry project co-ordinator Trine Heinemann explains: “The batteries themselves serve as emergency generators for each other. Each battery unit has a control unit that keeps track of the temperature and voltage of the batteries, making sure it is within limits and if not, then in principle it will shut down.”

Light as possible

Making the vessel as light as possible was a priority. “We designed it as lightweight and energy efficient as possible, as the less power you need the less batteries,” says Ms Heinemann. “Due to having a longer journey, steel was a requirement. But we have tried to design a ship that does not use more steel than necessary.”

The ferry design saves weight by cutting one deck from the superstructure, bringing the passenger areas to the same level as the car deck. The deck furniture is made of recycled paper, so is much lighter than a traditional choice, such as wood.

Battery supplier Leclanché joined the project in June 2015. Vice president system engineering Mika Lehmusto says: “[We used] a new marine-optimised model of a previous version of the batteries, which have been used in one vessel and in ground transport applications. One of the important motivations for us was to get funding to develop technology for this marine-optimised module and marine rack system.”

The EU partially funded the marine battery module’s development. The project was highly significant as the battery was type-approved by DNV GL.

Leclanché reduced the weight of the modules by 15-20%. The new module structure was the result of several months of careful 3D mechanical designing to create a light and strong, but also configurable and economical design.

The battery system has been split into 20 separate units to create extra redundancy, due to the lack of an emergency generator

There are 20 battery strings split between two battery rooms, with 42 battery modules in each string. The batteries on board weigh a combined 50 tonnes.

Battery challenges

An important consideration was fire prevention, especially stopping fire spreading to neighbouring battery modules.

“We changed our battery cabinet structure in many ways, including introducing an IP65 battery casing, which means encapsulating each battery module in a metallic enclosure,” says Mr Lehmusto. “We introduced a fire gas exhaust channel so that if there was a fire, the hot smoke is directed outside the vessel and is controlled, so [it] does not create hot air and high temperatures in the battery room.”

The third element of fire control involved introducing solutions to put the fire out. A high-pressure water mist-based solution was chosen, alongside a cold foam-based solution.

Charging the batteries also offered challenges. The batteries will run on direct current (DC) power, but all shore power is via alternating current (AC). “We need to have power that is accessible to the batteries. The converters are another system with a lot of components and units. They take up space and weight and we prefer to keep that shoreside, because it saves weight and space on the vessel.”

The ferry operator created its own ’house’ on shore which holds this equipment and converts the electricity. It includes a separate line from the main supply so that the electricity needed to charge the ferry batteries does not tap the town’s supply. In the house are four transformers to ensure the ferry operator has its own supply and conversion from AC to DC. The charging cables go to the ramp of the vessel where there is a large plug attached on an arm that can move and connect to the plug on the vessel’s side. Those two connect and are charged through inverters into batteries.

The battery charger is on the ramp of the ferry rather than shoreside to address challenges such as high tide, that would otherwise make charging difficult

The charging arm is an innovative solution. It is a first-of-its-kind and developed especially for the E-ferry. “Most ferries have chargers on the shoreside rather than on the ramp. When there are changes in tide, the arm can be too high or low in relation to where the plug is. But the ramp and vessel move together and hopefully there will be fewer situations where charging cannot take place,” says Ms Heinemann.

The time allowed for charging will be 20 minutes. Ms Heinemann explains that this is not enough power for one return trip, but the gap in power is topped up by having a larger battery capacity than needed. Every night the ship will also be plugged in and slowly charged.

“The ferry design saves weight by cutting one deck from the superstructure, bringing the passenger areas to the same level as the car deck”

Danfoss Editron supplied the electric propulsion engines, the bow thruster engines and the frequency converter. A major benefit of the engines is they are half the size and weight compared to other electric components, weighing in at 950 kg each.

Each battery room contains 10 strings. Danfoss Editron operations manager marine Siebe de Vries says: “Each voltage string is different. To make a general solution we have made our converter a DC converter between the battery strings and DC Bus, which powers up the voltage to 750 kW voltage.”

Danfoss’ frequency converter is especially lightweight and space saving. In contrast to other converters, which are housed in 2-m long steel cabinets, Danfoss’ converter is the size of a shoebox. Mr de Vries explains: “Instead of having a complete electrical room full of cabinets with drives, we can mount drives close to the propulsion and bow thruster, which saves weight and space.”

Elsewhere, manoeuvrability was also important. “Our home harbour is a bit difficult to manoeuvre, especially if there is wind. That is why we have bow thrusters. These give better manoeuvrability when docking. It is good for energy efficiency, as higher thruster power is only needed when manoeuvring in harbour. That takes some of the requirements off the main propulsion engines, so we didn’t need to over-dimension them,” says Ms Heinemann. The Danfoss thrusters consist of 250 kw of power.

As part of its EU funding, Ærø Kommune is required to log all data to provide concrete numbers on the operational costs of running a ferry. This is significant in promoting the concept to other operators because the upfront costs of building this type of ferry are higher than building a diesel alternative. Building a follow-up ferry to Ellen (as a prototype, Ellen’s development will lower costs for vessels that follow), is expected to cost €18M (US$20M), compared to €13.5M (US$15.3M) for a conventional diesel ferry.

The capex differential is overcome by savings on opex. Operational costs of the E-ferry are forecast to be €500,000 (US$565,000) a year less than a diesel alternative – and the savings may well exceed the estimate.