Electric drives, batteries, DC hubs and frequency converters improve tug power flexibility and fuel efficiency
Electric drive technology will change tug and workboat operations by introducing more flexibility and fuel efficiency by helping to regulate power demand on engines and minimise the need for burning fuel during periods of low energy consumption.
Tug operators can lower operating costs by using – either singularly or in combination – batteries, direct current (DC) hubs, frequency converters and permanent magnets. According to electric drive specialist The Switch, this technology is the most flexible method of arranging the power transmission in tugs or large workboats.
This is because tugs “spend an extremely high proportion of their time with very low power consumption since power capacity is dimensioned for bollard pull,” The Switch business development manager Mika Koli explained to Tug Technology & Business. Tugs only need high bollard pull during large ship manoeuvring and towing and when operating in difficult weather and ocean conditions. This is when full power is needed while the rest of the time tugs only need a small proportion of their power.
“Low-load operations can be performed with zero or low emissions when using batteries” such as when moving slowly in harbour areas, said Mr Koli. This can be combined with a DC hub and frequency converter technology to provide quick responses to changes in load demands.
“Low-load operations can be performed with zero or low emissions when using batteries”
Mr Koli said these technologies also reduce the amount of space needed in the engineroom for components and to enable shore power connections if these are available. “The use of a DC hub provides several exceptional advantages,” he said including providing more redundancy.
It would also help making a vessel’s operations future-proof, he added, because frequency converter technology with a DC link would give tug operators the flexibility to operate tugs with different hybrid technologies and link to shore power connections in the future.
Machines using permanent magnets can also be added as a companion to batteries to minimise wastage of energy stored in these power cells. “Permanent magnet technology is efficient and transforms the electrical energy to mechanical energy almost entirely instead of creating unnecessary heat waste,” Mr Koli explained.
The Switch’s electric drive technology can be installed on tugboats that have power requirements of more than 700 kW. It supplies shaft generators, permanent magnet motors, frequency converters and other electric propulsion components. So far, most of its equipment has been installed on ships owned by Scandinavian operators. However, electric drive technology could be installed on more vessels, workboats and tugs built in China following a deal announced in October.
The Switch signed an agreement with China Shipbuilding Industry Corp (CSIC) to jointly develop profitable power generation and drive train solutions for vessels built in China. Initial training sessions had been prepared and technical teams were being organised in October.
Hybrid tug designs
Wärtsilä has launched a new portfolio of hybrid propulsion tug designs to lower fuel costs and increase operational flexibility for owners. The Wärtsilä HY Tug series feature battery technology along with diesel engines for harbour towage and escort operations.
Three new designs were unveiled at Riviera’s Asian Tug Technology and Salvage Conference, which was held in Singapore on 18-19 September. Wärtsilä has produced a design for a 28 m harbour tug with 50 tonnes of bollard pull. Another one in the series is the Wärtsilä HY Tug 75 HT tug, which is a 29.5 m harbour tug with 75 tonnes of bollard pull. A third design is for a 35 m escort tug with 75 tonnes of bollard pull.
These designs come with an option for tug operators to select either diesel-mechanical hybrid, or diesel-electric hybrid propulsion before construction begins. The quoted bollard pull figures are also flexible and the designs can be adjusted to offer bollard pulls in the range of 40 to 90 tonnes.
Wärtsilä explained that energy storage technology enables the total installed main engine power to be lower than with conventional tug designs. Using less engine power decreases exhaust emission levels and the environmental impact of a tug. It also reduces the fuel bill and lessens the amount of engine maintenance needed.
Wärtsilä said the tug designs were optimised for low hull resistance, high towing and escort performance, seakeeping, crew safety and comfort. They are designed with thrusters from the Wärtsilä WST series, featuring large propeller diameters for efficient and high performance propulsion. Classification societies ABS, Bureau Veritas and Lloyd’s Register have provided approval-in-principle certification for these designs.
Battery technology took a step closer to maturity in October after class society DNV GL type-approved two types of energy storage system.
One was Corvus Energy’s Orca Energy batteries for hybrid-powered vessels, which the class society confirmed were compliant with international standards and regulations and with DNV GL’s rules. This approval is valid to August 2022 for all vessels classed by DNV GL.
The other was Plan B Energy Storage (PBES)’s devices for commercial marine applications. This includes its patented CellCool cell level liquid cooling, ThermalStop anti-propagation barrier, E-Vent gas venting system and CellSwap battery recoring. PBES said these systems were designed to integrate seamlessly with all types of power generation in a variety of commercial marine applications.
So far, DNV GL and Bureau Veritas have released rules for energy storage on ships and Lloyd’s Register has produced guidance on large battery installations.
Caterpillar Marine is developing a proprietary propulsion system for tugs that uses the Cat advanced variable drive (AVD) patented system. This combines Caterpillar’s experience with continuously variable transmission technology, advanced controls and integrated power systems.
Its innovation and technology development division is working to validate a fully integrated marine propulsion system, from bridge interfaces to tug propellers. Caterpillar Marine product definition engineer Nathan Kelly, told Riviera Maritime Media that the new concept would be introduced for tugs in 2018 and would be applicable to other workboats.
This will be an integrated hybrid propulsion solution that reduces maintenance costs and has conventional service requirements. The AVD will modulate and optimise the speed of the tug’s engines independently from the speed of the fixed pitch propellers, which can also be managed and varied.
Power from the main and auxiliary engines can be channelled independently or jointly to propel the vessel, which Mr Kelly said improves vessel performance and manoeuvrability, while enabling improvements in fuel and operational efficiency.