Danish operator Ærø Kommune's prototype electric ferry project could lead to widespread use of fully-electric power in the ferry 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 Ellen, due for delivery in May, involved designing, building and demonstrating a fully electric-powered ‘green’ ferry which can sail without CO2 emissions.
The project is supported by the European Commission's €80Bn (US$90.5Bn) research and innovation initiative Horizon 2020 (H2020).
A major consideration behind the design of the ferry was to create extra redundancy due to the lack of an emergency generator. This was created by splitting the battery system into 20 separate units, each connected to separate converters which control the energy output. 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.
Ærø Kommune E-ferry project co-ordinator Trine Heinemann explains “Redundancy is a luxury, but we do not have an emergency generator, so we are dependent on being able to operate on just the batteries. 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.”
The batteries used have a capacity of 4.3 megawatt hours. “This is quite a lot for a relatively small ferry,” says Ms Heinemann, explaining that the lack of emergency generator accounted for the large capacity.
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, therefore a lot of thinking has gone into making sure we have the most energy efficient propulsion,” says Ms Heinemann. “We could not build the hull in lightweight material as 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 vessel has used innovative ways to reduce weight. Ms Heinemann singles out that many passenger ships have ramps attached to them. “But we do not. Instead, we have a very big ramp on shore, so the weight is on shore rather than the ship.”
Another weight-saving innovation is that the ferry design has cut out one deck from the superstructure by bringing the passenger areas to the same level as the car deck, therefore saving a lot of steel. And rather than use steel, the bridge is constructed from aluminium.
Another innovation is that deck furniture is made of recycled paper, so is much lighter than a traditional choice such as wood.
Leclanché joined the project in June 2015 and supplied the batteries. This was a breakthrough as this particular battery solution has not been used before in a marine application. Leclanché vice president system engineering Mika Lehmusto says “It is 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.
Mr Lehmusto said “We had a lot of discussions about the weight of the batteries and there was quite significant mass, with 4.3 megawatt hours of battery.”
Leclanché reduced the weight of the modules by 15-20%. “We reduced the material thickness to be more optimal for this type of application. In previous projects aluminium or plastic structures were used, which is a bit thicker.” 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.
There are 20 battery strings split between two battery rooms, with 42 battery modules in each string. There are a total of 840 battery modules throughout the vessel, weighing a combined 50 tonnes. Mr Lehmusto says that energy density was very large, with 1 kg of material in a battery cell.
A very important consideration was fire prevention, including stopping fire spreading to neighbouring battery modules. Mr Lehmusto said “We did some fire testing in a laboratory and found that a burning battery module meant quite an aggressive fire was possible. Therefore, we changed our battery cabinet structure in many ways, including introducing a IP65 battery casing which means encapsulating each battery module in a metallic enclosure. 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 does not create hot air and high temperatures in the battery room.”
The third element was to introduce solutions to put the fire out. A high pressure water mist-based solution was chosen as well as a cold foam-based solution. “When foam is close to fire it is able to cool it,” says Mr Lehmusto. Both of these were type-approved.
When it comes to charging the batteries, there were also challenges to overcome. The batteries will run on DC power, but all shore power is via ACDC. “We need to have power that is accessible to the batteries, so we need to convert from ACDC to DC. The converters are another whole 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.” Therefore, the ferry operator has 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 doesn’t tap the town’s supply.
The battery charger is on the ramp of the ferry rather than shoreside (like most other vessels)
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 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.
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.”
Explaining the AC and DC status of charging the batteries on the ship, he said “Batteries are DC and if you transfer to AC, you have to convert and have losses, so [within the ship] we stay with DC. It is only with the propulsion engines and bow thrusters that we convert to AC, as this is needed to run the engines.”
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.”
In addition, all the Danfoss components are water-cooled. All electric products that create heat must be water or air-cooled. The advantages of using a water system are that the converters can be smaller. “The water-cooling system is also completely closed, with a closed loop running through the engine and through our drives. This means it is less affected by air, temperature, moisture or other environmental surroundings.”
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 – that was a choice we made. 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) versus €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.
Trine Heinemann (Ærø Kommune)
Trine Heinemann is project co-ordinator for the EU Horizon 2020 funded project, E-ferry. Before joining the E-ferry project, by appointment with the Municipality of Ærø, she worked as a university researcher and lecturer in communication, design and innovation at various universities in the UK, Sweden, Denmark, Finland and the Netherlands.
Electric propulsion engines, bow thruster engines and frequency converter: Danfoss Editron
Naval architect: Jens Kristensen Consulting
Class society: DNV GL
Lightweight composite structure: Tuco Marine
Shipyard: Soby Vaerft