The GIFT project address energy flexibility on islands. Energy flexibility depends ultimately on knowledge and predictability of energy consumption, capacity and quality. In the case of the e-ferry crossing between Grytøya and Hinnøya at Harstad, the timetables and distance provide guidelines, but assumptions still have to be made.
A source of concern is that Grytøya receives electricity through two radial submarine cables. They are both 2,9 km long and only carry 22 kV. Based on the findings and vulnerability of the submarine cable, the only charger will be set up at Storneset port. Whenever the electric ferry recharges, spikes will hit the power grid. These spikes will be reduced by a 50 kW battery substation placed on Grytøya
Different models for charging have undergone evaluation. Indirect charging and battery-as-a-service has been considered, but direct charging provides several advantages. Direct charging is more standardized, it supports higher kWh than indirect charging, and it is in general easier to do maintenance on cables and plugs where smart meters must be installed and predictability is an issue.
Estimates indicated that the electric ferry will spend between 130 and 140 kWh traversing between the ports of Storneset at Hinnøya and Bjørnerå at Grytøya. The estimates vary based on the amount of hybridisation of the ferry. A 60% hybridisation indicates the onboard battery will be recharged by an aggregator powered by biofuel. If diesel is being used, the generator will typically has a specific fuel oil consumption on auxiliary engines of 250 g/kWh – taking performance degradation and low load factor into consideration. With an emission factor of 3,17 g CO2/g fuel, the CO2 emission for on board generation of electricity is 790 g CO2/kWh. The fuel economy of the ferry in MGO (marine gas oil)- and full electric mode are assumed to be 2.8 km/GJ and 6.3 km/GJ, respectively.
To make the electric ferry independent of the power grid, the battery needs to have a capacity for 4 crossings. The total battery capacity up to a minimum of 540 kWh with the remaining kWh being provided by the aggregator. Based on the battery technology being used, it is not recommended with a charge of less than 10% and more than 80% (EV batteries well beyond 70–80% remaining capacity EOL threshold) [source] of total capacity. Values outside this range can seriously impact the life cycle and effect of the battery. Energy capacity fade is a more limiting factor affected battery life cycle than power fade. For the time being a 10 year cycle is defined. It is worth noting that battery retirement metric can also be affected by charging in more locations and when daily driver needs are not met. The calculations are therefore based on assumptions and are subject changes.
The electric ferry spends about 10 minutes for loading and unloading. During this period, the ferry recharges directly from a cable carrying 200 kWh. In the calculations made for the e-ferry, the required minimum charging infrastructure at each quay will be considered equivalent to 0.8 MW. The exact need for charge will depend on the actual energy consumption during the crossing. This may be affected by weather conditions, load and other properties with the ferry. The amount of energy being consumed during each charge may also depend on the current load in the grid, the price and potential availability of the charger.
The ferry will not support Vehicle-to-grid technology (V2G). The risk of accelerated electric vehicle battery degradation has been a concern expressed by many operators. To what degree this degradation affects the battery depends on technology, as expressed in a lithium-ion battery capacity fade model. However, examples from several other vehicles and vessels demonstrate that several other factors also influence the results such as current, internal resistance and state-of-charge (SOC).
A study (2016) was conducted on V2G with frequency regulation and peak load shaving by Lawrence Berkeley National Laboratory. The findings indicate that battery wear remains minimal with V2G grid services offered 2 hours each day – even if this grid service is offered every day over the vehicle lifetime.
As a side note, batteries from ferries that are put out of commission can instead be serve as a secondary battery storage and contribute to shave power peaks and offer Ancillary services: frequency regulation and voltage support as suggested in the report “Energy Storage: A Key Enabler for the Decarbonisation of the Transport Sector” provided by EASE (European Association for Storage of Energy) [source].