Batteries from the world's longest-sailing electric ferry get a new life: From waves to sunshine

On the Danish island of Ærø, the world's longest-sailing electric ferry, Ellen, has been running on clean electricity for over five years – carving out a green chapter in the history of transportation. But, like all batteries, capacity fades over time.

And for a ferry that must sail 40 kilometres several times a day without recharging, the requirement is unforgiving: once battery capacity drops to 80%, the packs must be replaced.

So what happens next? Are they scrapped? Not at all. A groundbreaking project shows that these batteries can be reborn on land – as stable solar and wind power storage systems.

"It's almost the perfect example of reuse," says Henrik Andersen, Associate Professor at the University of Southern Denmark in Sønderborg.

He is leading a project with fellow researcher Kun Qian that gives Ellen's batteries a 'second life' as a cornerstone of future energy storage solutions.

A Massive Hidden Resource
Most electric car batteries can continue operating for years even after their driving range declines. Ferry batteries are different. They must deliver full power every day, multiple times a day. This is why they are replaced long before they are truly worn out.

And the potential is enormous. Ellen operated with a battery capacity of 4.3 megawatt-hours – the equivalent of 26,000 individual battery cells. That's enough to store electricity from several large solar farms or wind turbines.

"They're still excellent batteries," explains Kun Qian.

"We've tested them in the lab, and their capacity and resistance remain stable. They only need minimal sorting to be reused in stationary storage systems."

New technologies make the dream feasible
The new project, UniversalBatteryPowerStorage, supported by Denmark's EUDP fund, is making this reuse possible. Researchers and companies are developing a flexible container solution that can house new and used batteries – from electric cars, buses, and ferries.

Thanks to advanced electronics, the system can handle batteries of different ages, chemistries, and capacities – something that has long been a major technical challenge.

The result? A mobile energy storage unit that can store green electricity from sun and wind, reduce waste, and stabilize the power grid – at a significantly lower cost than using brand-new batteries.

Sailing into a circular green future
For Henrik Andersen and Kun Qian, the project is not just about technology – it's about a vision for a circular energy sector.

"Instead of throwing away something that still works, we can create a chain reaction of green innovation," says Andersen.

"And really, how green is an electric ferry if its batteries can't be reused?"

"It's about using what we already have in smarter ways. Otherwise, we're just shifting the environmental problem from marine emissions to battery waste on land," Andersen emphasizes.

The two researchers have received advanced testing equipment to analyze the batteries in detail. With a climate chamber, battery testers, and precision instruments, the goal is now to prove that sustainable transport doesn't end at the harbour – but continues into the recycling loop.

Fact Box: The Ellen Ferry

• Ellen entered service in August 2019 and sails between Søby on Ærø and Fynshav on the island of Als in southern Denmark.
• The daily route is 22 nautical miles (approx. 40 km). Still, Ellen has demonstrated the ability to sail up to 92 km on a single charge – a world record for electric ferries carrying passengers and vehicles.
• Ellen is equipped with a 4.3 MWh battery system – the world's largest class-approved battery pack for maritime use.
• The ferry can carry up to 200 passengers and 30 cars and sails at a speed of approximately 13 knots.