The true hybrid platform supply vessel becomes reality

01 May 2012
‘Viking Lady’, which has already demonstrated ‘green’ ship power technology through the use of a gas-diesel electric power system supplemented by a fuel cell, is to be fitted with a battery pack, of about 0.5MWh capacity, as a demonstration of the feasibility of a true hybrid propulsion system

‘Viking Lady’, which has already demonstrated ‘green’ ship power technology through the use of a gas-diesel electric power system supplemented by a fuel cell, is to be fitted with a battery pack, of about 0.5MWh capacity, as a demonstration of the feasibility of a true hybrid propulsion system

Det Norske Veritas has announced the latest, and third, phase of the FellowShip project, in which energy storage capability in the form of a battery pack is introduced in the energy system of the offshore supply ship ‘Viking Lady’. This allows the benefits of a true hybrid energy system to be explored.

According to DNV, the primary potential environmental benefits of a hybrid energy system for ships are a 20% to 30% reduction in fuel consumption and CO2 emissions, and potentially greater savings in other exhaust emission components. Ship operators will be able to enjoy corresponding potential savings in fuel costs.

But the advantages of a hybrid ship propulsion system for an offshore supply vessel don’t stop there, according to DNV project manager Bjørn-Johan Vartdal. “We know that the hybrid system will reduce the energy consumption. When operating, for example, on dynamic positioning, there will be a major fuel saving potential. When in harbour, too, the ship should be able to operate on the fuel cell and its battery power alone, which will reduce emissions significantly. For environmentally sensitive areas, this will be an essential benefit. Additional benefits are related to reductions in machinery maintenance costs and in noise and vibrations.”

DNV draws some understandable comparisons between a hybrid ship and a hybrid road vehicle such as the Toyota Prius. Surprisingly, the ship system stacks up very well in financial terms. Unlike road vehicles, ship engines are designed to operate most efficiently at optimum load, showing a significant increase in specific fuel oil consumption below 20% to 40% of MCR. When redundancy requirements are taken into account, even with a diesel-electric power system, two engines operating at low load just to keep the ship on station is a very inefficient mode of operation.

Vehicle engines have to meet certain standards for transient load operation, but because of traditional ship operating patterns, there is no defined transient effect and little if any information is available about transient operation. However, the ability to run an engine at constant load and to use the battery to cope with transients, the effect can be minimised.

Gas fuelled engines, like the Wärtsilä 34 duel-fuel engines powering Viking Lady, may solve some of the emissions compliance problems, but in terms of greenhouse gases the story is less clear cut. Dual fuel engines operating at similarly low power levels emit comparatively high levels of methane, so there are similar pressures to keep gas and duel fuel engines operating at higher power levels.

The use of an energy storage device can change that. With battery power, just one genset – or no genset at all – can provide sufficient power for a significant amount of the ship’s operation. In the case of Viking Lady, for example, for about 60% of the time the ship is running below 30% of maximum power, with minimal use of full power (i.e. 80%-plus). DNV says it is producing new class rules covering ships with energy storage devices – including running on battery power alone – which will allow redundancy requirements to be met with a combination of batteries, gensets and fuel cells.

One obvious benefit of battery power is that auxiliary systems in harbour can run on battery power alone, with no need to run the engines. However, according to Mr Vartdal, if shore power (cold ironing) is available, the ship can hook up to the supply which will keep the batteries charged, and could mean that engines are only needed outside port limits or outside ECAs, the ship using battery power within the low-emission zone to approach and leave port, and using shore power while moored. This could be a particularly important consideration in ports like Bergen.

A further claimed benefit for the hybrid system is a reduction in noise and vibration. This, says DNV, could prove interesting for fishery research vessels.

One much-heralded benefit for hybrid vehicle technology is that it can harness, and feed back into the battery, energy from regenerative braking. Such a system is somewhat less applicable to ships – though the FellowShip presentation does include a diagram of a turbine arrangement below the waterline, and unused motors connected to redundant propellers could be used as shaft generators. The system should work well with other renewable energy sources, such as wind turbines and solar cells, though current technology does not really provide high enough efficiency to make it worthwhile.

RETURN ON INVESTMENT

So just how worthwhile could hybrid technology prove to be at sea? DNV and its FellowShip partners, Eidesvik Offshore and Wärtsilä, have calculated a typical return on investment for a Viking Lady type application, though they do stress that actual figures will depend on variables such as the size of battery required, which is determined by the operating profile and power requirement of the individual ship. For Viking Lady, the optimum battery capacity is estimated at 2MWh – for the initial test phase a much smaller, about 0.5MWh, battery will be employed. Even a 2MWh battery should be contained within a standard 20ft container that can be safely stowed on deck, allowing simple replacement when necessary. However, Mr Vartdal says that a lithium polymer battery, similar to that to be used in the trials, will have a service life of 10-15 years.

The additional cost of the hybrid system, taken up mainly by the battery itself and the necessary system integration, will be around $2million. Annual fuel costs, at current prices, for Viking Lady are running at $5million. A 20% saving amounts to $1milliion, equating to a two-year payback period. Actual fuel savings are likely to be somewhat higher, even without taking into account the likely relative increases in bunker costs, or the effect of financial incentives like the Norwegian NOx fund. So it seems perfectly feasible that a hybrid PSV could pay for itself well within the two-year period. This, says DNV, compares with a four-year payback period for the hybrid tugs currently in operation, and the 15-year period for a typical hybrid car.

The next stage is verification of the technology and the figures through a comprehensive measurement programme within the FellowShip project. These measurements will be used to facilitate optimisation of future hybrid systems. As a follow-up DNV will update its classification rules to allow for installation of hybrid power systems with energy storage for this and other ship types. Such rules, says DNV, will accommodate pure electric ships operated on stored electric power alone as well as hybrids.

OWNERS' PERSPECTIVE

So, what is the perspective of the other participants in the FellowShip project? Shipowner Eidesvik values its involvement highly, but admits that it has seen no actual profit so far from ‘being green’. The way that charter agreements work at present, mean that the owner pays the premium for the environmental and fuel-saving benefits while the charterer reaps the benefits of the lower fuel costs. A greener ship like Viking Lady can attract higher rates, and although actual figures in such matters are confidential, it is no secret that the owner sees little financial return. Where Eidesvig benefits currently, according to Eidesvik Offshore CEO Jan Fredrik Meling, is in its own company culture, where being seen as a positive, environmentally friendly operator pays dividends in staff morale and retention. More tangible benefits are expected later, as the industry as a whole wakes up more to fuel-saving and emissions-reduction measures.

Wärtsilä’s involvement is primarily in systems integration, as well as supply of the dual fuel gensets in Viking Lady. Ingve Sørfonn, director R&D Wärtsilä ship power technology, says that the FellowShip project fits perfectly with the company’s focus on smart grid systems, which lend themselves well to the hybrid concept, combining diesel electric gensets with batter banks and, in the case of Viking Lady, the fuel cell. “Such projects all help to bring environmental solutions to the market, enabling vessels to comply with strict local rules as well as IMO emission limits”, he says.

Although the first generation of hybrid ships are likely to use diesel (or gas/diesel) and batteries alone, without the fuel cell fitted to Viking Lady, work is continuing on refining and enlarging fuel cell technology for maritime use within the FellowShip project, following the successful demonstration so far.

The operational pattern of OSVs like Viking Lady lends itself particularly well to a hybrid gas/diesel/battery solution, but DNV stresses that the FellowShip project is concerned with all ship types. Although bulk cargo vessels spending long periods steaming at constant speed may not draw the same benefits from hybrid propulsion, there can be tangible benefits from battery operation in ports or in ECAs, particularly when renewable resources can be harnessed at sea, and various other ship types including passenger vessels could find such solutions highly attractive.

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