In an unusual role reversal, the military trails in the wake of the commercial sector in the move towards the all-electric ship. George Coupe reports.
When the Royal Navy launched Daring, the first of its new Type 45 Class destroyers, last February, it was inevitable that much attention would focus on its integrated electric propulsion system.
This was a first for a front-line warship, giving Daringbig benefits in areas such as range and operational capability.
But while the ship was a landmark for the Royal Navy, the military’s maritime cousins in the commercial shipping sector could be forgiven for wondering what all the fuss was about.
In the commercial arena, truly ‘all-electric’ vessels have become ubiquitous, and while Daring marks a big step down the road, the ‘all-electric’ warship remains elusive to the world’s navies.
To merit that description, for example, systems such as the rudder and stabilisers would have to be operated by electric actuator instead of the conventional hydraulic systems. This is still a way off in the military sphere, with many challenges to overcome.
It is a different story in the commercial shipbuilding sector, where the move towards the all-electric ship began. Here, different degrees of electric ship technology have spread across most classes of new vessel, from cruise liner to ultra large cruise carriers, pleasure boats and private yachts.
One example is the world’s largest cruise ship, The Liberty of the Seas (see Insight). Launched last month, this huge vessel is driven by giant 16MW electric motors.
The success of all-electric ship technology in cruise liners and large yachts has shown that electric ships provide significant advantages in speed, manoeuvrability and hull-space use. But all of these have not, as yet, been successfully transferred to waships such as Daring, and in an unusual role-reversal, the military finds itself playing catch-up with its commercial cousins.
Despite this the system on Daring will bring big advantages to the new warship, including greater range and operability, and reduced maintenance, and hence the vessel’s through-life cost.
The system replaces the traditional, heavily-engineered shaft line, gearboxes and diesel engines. It not only turns the vessel’s propellers but supplies all the other auxiliary electrical loads aboard, in a fully integrated and managed system.
Generally, the modern electric propulsion system consists of a centralised generating system, which also supplies services requirements, and a variable speed drive system connected to fixed pitch propellers.
A comprehensive power management system ensures that each prime mover operates at its optimal load, while systems such as direct torque control of the motors allow immediate and smooth drive control with minimum wear and tear on the system.
Advantages of the technology over traditional propulsion systems include better safety and reliability, improved manoeuvrability, greater efficiency and enhanced passenger and crew comfort through reduced noise and vibration levels.
The concept also allows for the optimisation of a vessel’s architecture, so that priority can be given to other requirements, with the new components of propulsion, (prime movers, transformers and converters sited conveniently around the ship).
Most radically, ABB’s Azipod electric motors are placed outside the hull in a pod that can rotate through 360º. This has released space and offered better manoeuvrability.
ABB has also developed variants such as the compact Azipod for smaller vessels and the contra-rotating Azipod. The CRP Azipod is installed in place of a normal rudder, downstream of the main propeller that is turning in the opposite direction. This obviates the need for the large, hydraulic rudder system.
ABB says this configuration is highly efficient because the pod’s propeller uses the remaining energy from the rotating water leaving the forward propeller.
According to ABB the system encompasses unique advantages, resulting in the best hydrodynamic efficiency in the industry. Gains in efficiency are achieved by applying the CRP principle, dividing the load over two propellers.
ABB’s Azipod electric motors are placed outside the ship’s hull in a pod that can rotate through 360o which releases space throughout the vessel and offers better manoeuvrability
In the civil marine propulsion industry there has been a significant increase in the development and installation of electric ship technology, with new standards for power, size and efficiency regularly being set.
Earlier this year, Converteam, formerly Alstom’s marine propulsion division that supplied the equipment for the Type 45, was awarded a multi-million euro contract by shipbuilding group Aker Yards to supply electric power and propulsion systems on two new vessels for Norwegian Cruise Line.
The vessels, with a gross tonnage of about 150,000 and 4,200 passenger berths, will be the first cruise ships fully equipped with Converteam’s new electric propulsion solution, based on high-torque density induction motors and advanced digital propulsion control.
The company claims its high-torque density induction motors offer efficiency of more than 97 per cent and are low-noise and low-vibration. Meanwhile, press-pack insulated gate bipolar transistor technology enables the design of more efficient converters with higher powers than previously possible.
Converteam claims these converters, with the HDT motors, allow significantly enhanced electric propulsion. The converters are also relatively compact, saving space in the ship.
Another company set to enter this market is Germany’s Siemens, which has announced it is to build a highly efficient propulsion motor for all-electric ships. The synchronous machine is part of an electric propulsion system based on high-temperature superconducting technology (HTS).
Siemens says the superconductors of the rotor windings carry a current density 100 times greater than that in conventional copper windings. As a result, significant weight and volume reductions are possible. In addition, there are no electrical losses with HTS, which means greater efficiency. The enclosed, self-regulating system, which cools the superconducting rotor windings of the motor to a temperature of 27K, also promises cheaper cooling with low maintenance. In 2009, the developers plan to run the machine on a test-bed for large-scale propulsion units.
Meanwhile, on the power generation side, Rolls-Royce, which has supplied the gas turbines for the Type 45 programme, has been making advances in turbine design and technology.
With so much technical innovation taking place, it would be expected that the navies of the world would be launching all-electric ships by the dozen.
In fact at BAE Systems’yard in Glasgow, where the Type 45’s propulsion system is being tested, the project’s chief electrical systems engineer said there is no such thing as an all-electric warship — even in the planning stage.
‘The Type 45 certainly isn’t an all-electric ship… where you have almost got rid of all your hydraulics and your stabilisers your rudders are all controlled electrically,’ he said. ‘What we have with the T45 is an integrated, full-electric propulsion vessel,’ he said.
The vessel’s power system is able to satisfy both those dedicated to propelling it through the water and a second group comprising the ship’s weapon and sensor systems and the service systems providing ventilation, fuel, cooling water and domestic or ‘hotel’ needs.
The power system, supplied by Alstom, meets military shock standards, but also achieves a high power density compared with equivalent commercial designs.
Switchgear distributes the power, either at 4160V AC to the propulsion power converters, or via transformers at 440V and 115V AC to the ship’s weapon and service systems.
The converters provide variable frequency supplies to the 100-tonne, 20MY, reversible propulsion motors. These are directly coupled to the propeller shafts, obviating the need for traditional gearboxes or controllable pitch propeller systems. Harmonic filters remove unwanted distortion in the electrical supplies.
On board are two Rolls-Royce WR-21 gas turbines and two diesels. The WR21 is the first large marine gas turbine to incorporate compressor intercooling and exhaust heat recuperation technologies, which allows it to deliver high-efficiency down to low loads. This means it can be run across a much wider power range, and is well suited to meeting a ship’s fluctuating power demands.
Crucially, a single gas turbine can power both propulsion and domestic loads. Each turbine can drive either or both propellers, offering a revolutionary degree of operational flexibility. A platform management system, accessible from 80 ship-wide plug-in points facilitates remote machinery control and surveillance.
Reid said: ‘The real advantage of integrating propulsion with auxiliary power is in the running hours of engines and the maintenance costs associated with that. In a traditional vessel with separate propulsion and auxiliary engines you would need perhaps to run two auxiliary engines to provide the auxiliary power, and depending on your speed, one, two or more propulsion engines. You would rack up the number of hours, and hours are proportional to maintenance.
‘The T45 is aiming at single generator operation for cruising so we run around the seas for the majority of our time on a single engine.’
Reid credits these advantages to BAE’s suppliers, Converteam and Rolls-Royce. His task has been to perform the actual integration to produce the first large electrically driven warship.
‘A classic mechanical propulsion scheme would almost work of its own accord without any auxiliary supplies. It would pump its own cooling water and lubricating oil, and it would turn the propeller. It could almost run independently from your electrical systems,’ he said. In the T45, everything becomes interdependent.’
Reid said that as electric technology advances and the equipment gets smaller, the see-saw gets more tilted to the all-electric solution. He said in the UK we are probably at a situation where the old technology, such as the big gearboxes and a big heavy engineering base, is diminishing. This means electric technology, which we can still do, is growing.
‘If you think about the number of power conversions that you have, from the prime mover, through its alternator, the converters, and into the motors, these all carry the total shaft power and there is the addition of the switchboards, which are not small. Each conversion represents an amount of loss, cooling and space which adds up to more than the gearbox.’
Reid added that although the T45 would easily fit inside a large cruise liner the total power and propulsion power are roughly the same. ‘The cruise liner has lots of hotel load, so it makes sense to have a power generation scheme, which can either put power into the hotel load or into propulsion.
‘In the T45 we have only got 10 per cent auxiliary power and 90 per cent propulsion power. The size of this kit is not smaller than a gearbox and its auxiliary. When you have added chilled water plant and cooling pumps you do not have a more compact scheme.’
The need for ‘survivability’ has also affected the vesssel’s ability to take advantage of better hull-space offered by electric technology, said Reid.
‘To put the motors outside the hull in pods is a great way of getting rid of the shaft. But in the T45, because of vulnerability, we essentially have a very similar shafting arrangement to a conventional ship. We have to put one motor forward and one motor aft so if a single compartment floods it doesn’t take out all capability.
‘So, yes there is a flexibility. But the uptakes and downtakes for the gas turbines occupy a great deal of volume. You have to place them in locations where they can integrate with the hull form and the topside design to be able to ingest the air and get rid of the exhaust. If you are looking for invulnerability, of having separate machine spaces, you tend to find yourself in a rather similar situation to the one you had before.
‘The destroyer is a very power-dense vessel, compared with the commercial world, and until these devices get more power dense we have difficulty taking the advantages, which they definitely see in the commercial world, into the small warship environment.’
The Type 45 is the first major warship that is approaching the all-electric benchmark, and so far, said Reid, the project is meeting all its programme targets ‘very well’. However, he said, the economics of the all-electric ship have still to be proven.
Many of the developments achieved on the Type 45 will no doubt prove to be valuable lessons for military shipping in general. However, it remains to be seen whether this project will move the industry closer to the concept of the truly all-electric ship.