Electric Ferries – the future
Submarines have always worked on electric motors, since they can hardly be emitting a cloud of diesel smoke while under the water – but battery-operated surface ships, are a novel idea. CalMac is perhaps eyeing the future with some concern, realising that oil will go on rising in price as world supplies near extinction – or else it has had a sudden pang of conscience about emissions. Whatever the reason, it has commissioned two battery-driven ferries, to be constructed at Ferguson’s shipyard in Port Glasgow at a cost of £22m.
The basic difficulty about electric power is the problem of storing it. Battery technology still struggles for a solution to the problems of weight and bulk. It’s the stumbling block in electric car design, and marine engineering grapples with the same dilemma. For this reason CalMac has commissioned relatively small ferries of 900-tonne ships, designed to carry up to 150 passengers and 23 cars on short routes, powered by two lithium-ion battery banks with a total of 700kWh. Lithium-ion is the most efficient electricity storage system known at present, but even so, the batteries weigh up to six tonnes each.
The ships will recharge these batteries from the national grid overnight, drawing on renewable energy, but in fact they work on a hybrid system rather than a fully electrical one. Small diesel generator sets feed power to a 400 volt switchboard that supplies power to electric propulsion motors. The charged batteries supply a minimum of 20% of the energy consumed, so they are useful, but not adequate on their own. All the same, calculations show that the design will reduce lifetime costs and lead to considerable long-term savings. Exactly the cost of diesel continues to rise.
In ecological terms, the electric ferries are obviously a good thing, with a marked reduction in Carbon Dioxide, Sulphur Oxide and Nitrous Oxide emissions. Design-wise, the hull shape and power configuration will realise an impressive 19-24% savings of power input to the propulsion units over a conventional diesel mechanical set-up. Fergusons are also looking at the possibility of using energy from local wind, wave or solar systems to charge the batteries, making the process even more environmentally friendly. The first vessel is anticipated to enter service in the spring 2013, the first ever sea-going Roll On Roll Off diesel electric hybrid ferry.
Would such a system work for Arran? At the moment, probably not, because of the much greater size of the vessels needed and the longer crossings involved. It all depends on battery technology. The experimental ‘flow’ batteries that work on constant electrical transfer could evolve into a system powered by the ship’s own movement and if that happens, we will be into a whole new world of propulsion methods and power generation. For the present, alas, that’s no more than a gleam in a technical dreamer’s eye.
http://www.cmassets.co.uk/en/our-work/projects/current-projects/hybrid-ferries-project.html
