Conventional generators are not the only option for shipowners looking to boost their electricity supply
As demands for energy efficiency increase, shipowners are increasingly looking beyond their diesel gensets to find ways of generating electricity. The result is that waste energy – whether in the form of engine heat, shaftline torque, turbocharger exhaust or even shipboard waste – is being harnessed on vessels like never before.
One example is the well-established power take-out (PTO) capability of shaft generators, as exemplified recently by Stena Line’s new E-Flexer ropax vessels, under construction at Avic Wehai shipyard in China. The first of a series of eight vessels, Stena Estrid, was floated on 16 January, the same day that the third E-Flexer, yard project number W0267, had its keel laid.
A variable frequency drive from WE Tech enables efficient power distribution. In PTO mode, this drive allows the propulsion machinery to operate at variable speeds while the shaft generator provides the vessel’s grid with electrical power.
A common DC-link is used to distribute power, offering a direct link to large electrical consumers, such as thruster or large compressors through dedicated inverter units. This enables efficient power distribution and controlled short-circuit currents, which allows for lighter switchboards.
Shaft generators are becoming a mainstay of electric generation on modern vessels. Canada’s Canal Marine & Industrial recently retrofitted an electric generator to Western Towboat’s Ocean Titan. The installation means the twin 200 kW diesel generators are dormant for days at a time during towing voyages to and from Alaska.
Using a motor coupled to the front of one of the two Caterpillar main engines, a power conversion system housed in Canal’s electronic generator cabinet applies a controlled negative torque through the motor. This turns it into a generator which is used to maintain a DC bus that acts as a grid converter, providing power to support vessel services.
Canal lead engineer Chris Wright says: “The system acts as a variable speed shaft generator, able to provide ample power at regulated voltage and frequency, while the main engine speed varies from idle to maximum rpm. It is no longer necessary to run lightly loaded diesel generators and the resulting increase in load on the large main engine is hardly noticeable.”
“A cruise ship carrying 5,000 passengers can reduce fossil fuel consumption by 1,800 tonnes a year, eliminating 6,500 tonnes of greenhouse gas emissions”
But shaft generators are not the only option. A consortium, led by Bowman Power Group in the UK, has created an electricity generation system that is able to harvest waste heat. After a four-year, £1.5m (US$1.9m) project in collaboration with Rolls-Royce, UCL and Lloyd’s Register, Bowman’s system indicated the potential for achieving fuel savings of up to 7.8% on a frigate vessel test case.
The company used electric turbo compounding (ETC) technology to harness energy from the engine exhaust. Applied in-line or parallel to the engine’s turbocharger, the system produces electricity typically at around 1,000 Hz. To maintain engine performance, the turbocharger is re-matched to work with the turbo generator. Through the power electronics element of the system, the high frequency electricity is converted into grid-quality electricity at 50/60Hz, as suited to a shipboard net.
Bowman's electro-turbo compounding technology enables electricity generation from the engine exhaust
Bowman CEO Paul Dowman-Tucker says: “In the past year we have explored and successfully entered a number of new markets, including active discussions with two large marine engine OEMs and a major ferry operator.”
Waste-to-energy
Passenger vessel operators are particularly interested in alternative sources of electricity generation, given their high demand for power from hotel functions. As a result – and as a result of the large volumes of waste generated on these ships – cruise lines are looking with greater interest at emerging waste-to-energy technologies. More suppliers are entering the market for converting on-board waste into heat and power.
Expedition cruise company Mystic Cruises will install Terragon Environmental Technologies’ micro auto gasification systems on two ships ordered last year, to be built at West Sea Viana Shipyard in Portugal.
“Cruise lines are looking at emerging waste-to-energy technologies as a result of the large volumes of waste generated on these ships”
The systems heat carbon-based waste, creating a gas comprised of hydrogen and carbon monoxide that is used to fuel the process and provide up to 150 kW in heat energy for other on-board consumers. This avoids the formation of harmful substances associated with conventional incineration, while emissions to air are further reduced by the addition of an exhaust gas cleaning unit.
The dual order is part of an overall waste treatment package to be executed by Terragon, Delitek and Marship Engineering. It follows a similar arrangement for Mystic’s previous newbuild, World Explorer, due to enter service this year. The sister ships will follow in 2020 and 2021.
Another gasification technology is close to commercialisation. Norwegian company Teco Tech uses recaptured heat generated in the gasification process through proprietary sodium heat pipe economisers, making the system even more efficient.
“Looking at the different vessel segments, our gasification unit would offer the greatest benefits to the cruise industry,” says Teco Tech CEO Henrik Brixen. “It would not only cut down their vessels’ energy consumption, it could also generate large amount of syngas, which could be used to power other systems on board.”
The company is running a technology qualification with class society DNV GL and cruise companies Royal Caribbean Cruises, Carnival Corp and Norwegian Cruise Line. The project involves risk analysis down to component level to identify potential weaknesses in a newly developed system.
Other technologies are emerging, including pyrolysis, which is like gasification, but also creates a solid or liquid (char) that can be used as fuel. Microwave-assisted pyrolysis uses microwaves to reduce the energy required to turn waste into usable gases. Norwegian company Scanship is developing this technology for shipboard use and has already signed an agreement with new cruise company Virgin Voyages to provide systems. The line’s first ship, Scarlet Lady, is due for delivery next year and is expected to use the technology.
Scanship’s process turns all carbon-based waste – including food, sewage, paper, wood, plastics and oils – into flammable gas, bio-oil and charcoal, which can be used to satisfy other energy demands on the ship. The company estimates that a cruise ship carrying 5,000 passengers can reduce fossil fuel consumption by 1,800 tonnes a year, eliminating 6,500 tonnes of greenhouse gas emissions.
Shipping’s big technology suppliers are also beginning to look at the potential of waste-to-energy. Wärtsilä will launch a gasification system this year. The technology will initially be tested on one of Carnival Corp’s existing vessels, with a view to rolling it out on its newbuilds, starting with those due for delivery in 2021.
Conventional electricity generation using engine-based gensets is also evolving to accommodate new technologies. Bakker Sliedrecht has developed a generator protection system for dual-fuel vessels which operate with a dynamic positioning (DP) system. Generator protection is used to prevent a blackout and to stop malfunctioning generators from affecting the performance of other generators.
Protection is even more crucial with dual-fuel generators, which feature more complex controls and operating modes. Without generator protection, a complex DP vessel would not receive a certificate to operate in DP2 or DP3 closed bus mode.
Bakker Sliedrecht’s system uses software to detect and disable an ‘unhealthy’ generator before it can affect the ‘healthy’ generators, making the entire power plant unstable. It has been extensively tested in a test set-up of four generators and has received approval from DNV GL.
The generator protection system will be deployed for the first time on an offshore heavy-lift vessel currently under construction. The ship is equipped with a power plant of four generators of 10 MW each, which run on both diesel and LNG.
“Our technology will prevent the ship from getting out of position,” says Bakker Sliedrecht technology manager Paul Bracké. “That requires not only the first dual-fuel generator protection system, but also a good power management and zone security system, including an experienced operator."
© 2023 Riviera Maritime Media Ltd.