Hafenstrom has provided new content and calculation models for electric ferries in the GIFT project. The models will assist ferry owners and ferry operators in planning of new routes, onshore and onboard configuration, as well as management of the vessels.
Several new methodologies have been developed for identifying and visualizing consumption and charging profiles related to energy market flexibility offers and demands. Having a detailed description of the schedule and times for arrival, departure and the available time for charging per hour is of paramount importance to evaluate the flexibility available in each period.
Previous articles in this series can be found here:
• Electric Ferry GIFT project (hafenstrom.com/electric-ferry-gift-project/)
• E-ferry battery and charging (GIFT project) (hafenstrom.com/e-ferry-battery-and-charging-gift-project/)
• Hafenstrom engagements in H2020 projects (hafenstrom.com/hafenstrom-engagements-in-h2020-projects/)
Read more about the GIFT project: gift-h2020.eu
The technical details for the ferries will affect the consumption and the emission profile, which the formulas take into account. The amount of hybridisation has a direct impact on the capacity of the onboard battery – but may also influence the setup of the onshore substations and battery buffers.
Typical considerations are the state of discharge of the battery, the onshore voltage, sufficient charging time for the necessary amounts of crossings before battery recharge can be initiated, consumption rate vs. on-board state of charge.
The energy consumption can be described like:
Where E = Energy, N = fixed number, P = Power. The power represents in this case the consumption profile, and is in this case directly related to the ferry propulsion.
The calculation above is based on a general approach. A more detailed view is based on the actual propulsion effect and hence state of charge of the battery on the different stages of the crossing of the ferry. A crossing typically consists of five stages: manoeuvring (from quay), acceleration, transit, deceleration, manoeuvring (to quay). In addition to this, the ferry will typically also run at low power while being docked. The ferry will also have to take into account the amount of time necessary to connect to the grid and disconnect before leaving the quay. The remaining time is how long the ferry actually is able to charge.
Some of the lessons learned was that depending on the shore-side charger and available effect, fully charging the battery for a crossing may take between 30 minutes and 1 hour. If the ferry has a layover time less than this, a fully electric configuration may not be feasible. This is even more notable if the layover time is so short that the ferry for all intents and purposes may conduct several crossings before it actually can start charging. A hybridisation of 65% or more may therefore the most relevant alternative – at least with current battery technology.
This brings up to the following considerations that needs to serve as a basis for further calculations:
- A ferry will cross between two or more ports
- The ferry has a battery with a given capacity
- The battery is charged to a given level on departure from port
- The ferry has a given energy consumption per minute (this will be variable based on speed, climatic conditions, load, etc. – but a fixed value can be used)
- The ferry has a fuel tank with a given capacity (depends on the hybridisation)
- The ferry has a fuel consumption of a certain number of litres per minute (depends on the configuration of the ferry)
- The fuel delivers a certain amount of energy in addition to the battery per minute (is based on the technology used)
- One or more ports support charging
- The ports may have chargers with different capacities
- Refuelling is a manual process. We assume that when the tank is empty, the ferry is operated until the battery is also empty.
- Data access has approved, data quality is acceptable, data granularity < 1 hour (in order to receive, calculate and present models)
Furthermore, the travel distance can be described as:
- Every time the ferry docks and connects with a charger, the distance is extended before a new charge is needed.
- As long as there is enough fuel, the distance a ferry can sail before charging is required will also be extended
The outcome of the calculations depends on these decisions to present a model that fits the expectations. The formulas will receive adjustments if the final values are too far from what is actually measured under similar conditions. However, it should be noted that 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.
Other considerations when selecting models is what battery technology will be used. It is for instance not recommended with a charge of less than 10% and more than 80% (EV batteries well beyond 70–80% remaining capacity EOL threshold) 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.
Concluding, we learn that charging electric ferries can affect the power system on land to varying degrees, especially with the use of direct charging. The models that have been developed will be refined in order to provide very concrete measurements of the battery level at any given point of the trip. The models offer the opportunity to present monitoring for certain threshold values that can be set in advance, and also provide an accurate calculation of the necessary number of different kinds of fuels and emissions that would be necessary for the same distance.
Photo by Andrea Piacquadio from: Pexels.com