Hybrid propulsion lowers emissions, maintenance and costs

Port authorities are placing more emphasis on lowering emissions in harbours and terminals. They see tugs as one of the main contributors to emissions, and are increasingly asking owners to reduce their output. This has been a feature in recent port towage concession and extension negotiations.

Owners are turning to hybrid propulsion systems, whether this involves energy storage or various shaft generators and motors, to reduce greenhouse gas and smoke emissions.

Wärtsilä Marine Power director of business development Giulio Tirelli said hybrid propulsion balances the need for lower emissions without impacting towage performance.

“It is about finding the balance between everyday needs of low power with the occasional need for high bollard pull for heavy assistance,” he said during Riviera Maritime Media’s Hybrid and electric tug viability: the future’s bright webinar. This Wärtsilä-sponsored webinar on 3 September was during Tug Technology Webinar Week.

Mr Tirelli acknowledged high bollard pull could be needed just 1% of a tug’s operating time. “We need to retain this capacity, but finding the balance is challenging,” he said.

It costs more to build a tug with hybrid systems compared with conventional diesel-only propulsion. These additional costs depend on whether power take-in or take-out (PTI/PTO) devices are used or batteries are installed.

But there are considerable reductions in operating expenditure. In one recent newbuild delivery, maintenance costs were cut by 50%. “Hybrid helps find this balance between opex and capex,” said Mr Tirelli.

In some newbuildings, owners have reduced the size and power from their engines, mitigating some of the higher capital costs. The rest of the higher capex can be recuperated in lower fuel consumption by using motors or batteries during the lifetime operations.

Tugs spend up to 75% of their time idle or transiting at slow speeds. For the rest of the time they are working, towing ships and assisting them into berths, or carrying out undocking operations.

Therefore, tugs often do not need to operate both engines at full or half load. Some tugs use batteries, if installed, for transit and other power requirements instead of engines, others use motors to supplement engine power, said Mr Tirelli.

“PTO/PTI electric motors are important links in hybrid-electric and mechanical systems,” he said. “The size of the tug, safety requirements and bollard pull are important drivers in selecting hybrid propulsion and batteries,” he added. “There is interest in the flexibility of operations.”

Tug operators will have various bollard pull needs in different modes of operation, which means hybrid propulsion needs to be “tuned for the best fuel consumption and least extra capex. There are a lot of possibilities” said Mr Tirelli.

Some hybrid propulsion systems provide tugs with a bollard pull boost. Others enable owners to operate tugs in green mode. “There are no emissions or noise when they are driven by electric-only,” he explained.

Wärtsilä has patented a new start-up procedure that does not produce any black smoke from the funnel. This uses batteries instead of diesel injection. “This is useful for vessel operations close to the public,” said Mr Tirelli.

Its hybrid mode produces instant responses with no smoke while maintaining power output from engines and batteries. This results in “50% lower fuel costs, lower maintenance, zero black smoke and better environmental performance” said Tirelli.

Wärtsilä supplied hybrid propulsion to Port of Luleå’s newbuild ice-breaking tug Vilja, which entered service in 2019. This was built by Gondan Shipyard in Spain to a Robert Allan TundRA 3600 design with Wärtsilä integrated power systems.

During the webinar, Port of Luleå’s technical manager John Sundvall provided insight into the decision process and from more than one year of operating Vilja.

“We evaluated several options before going for hybrid,” he said. “We needed a lot of power, but full power was seldom used.”

He said the reasons for investing in a hybrid tug was to reduce emissions during transits to towage jobs and to use clean energy at Port of Luleå.

“A key benefit with our hybrid is low emissions during missions, when we may only need to use one engine,” said Mr Sundvall. “We have clean energy available with the possibility of charging at the quayside, which is suitable for hybrid.”

Another benefit is the peak shaving function when power loads are uneven as Vilja operates through ice in the Port of Luleå. “We can run on one main engine and a motor,” Mr Sundvall said. “We can start the engine using electric and there are different modes of operation.”

Batteries provide a boost to Vilja’s power for heavy towage in ice.

Vilja’s bollard pull is:

Using just the electric propulsion: 30 tonnes.

In hybrid mode: 40 tonnes.

In mechanical-only mode: 90 tonnes,

Mechanical with battery boost: 100 tonnes.

This makes Vilja the most powerful hybrid tug in the sector.

“We selected Wärtsilä because we wanted one supplier for the whole system,” said Mr Sundvall. This included the main engines, generators, batteries, electric motor and the power management system. Power from this system feeds a pair of Kongsberg US355 thrusters.

After Vilja was completed in Spain it underwent sea trials, but it needed trials in ice in northern Sweden. Mr Sundvall said the operator was pleasantly surprised from the result of these trials and a full year of operations.

“We are getting better consumption savings and lower costs than anticipated,” he said. “We are only using diesel during ship assistance and we are getting a better range during transits, when on electric mode, than we expected.”

Hybrid propulsion technology and alternative fuels will be discussed in depth during Riviera Maritime Media’s webinar weeks and virtual conferences, starting with Marine Propulsion Webinar Week, from 29 September - use this link for more details and to register for these webinars