The future is electric

The last decade has been exciting for electric propulsion with around 2% of vessels over 500t being propelled by it. This represents about 1,200 ships and the annual market for marine electric motors and propulsion drives is estimated to grow from $800m to between $4 and $5 billion by the year 2013.

Many new classes of ships including ferries (Fig 1), cruise ships, FPSOs, dynamically positioned drill-ships, cable and pipe layers, shuttle tankers, dredgers, ice-breakers and offshore supply vessels have benefited from advances in power electronics, electro-magnetics and innovative hydro-dynamic and propulsion designs.

All future NATO major warships will have full electric propulsion with the UK?s Type 45 Daring class destroyers, each having two 20MW AC variable speed drive propulsion systems, set to be the world?s first ?electric warships?.

The concept of electric propulsion, however, is not new. The idea is over 100 years old with battery powered vessels recorded in Russia and Germany around 1870. In 1893, one of the first British ?electric boats?, aptly namely Electricity, carried passengers on the River Thames in England. With the start of the First World War however, the development of electric propulsion, with the exception of submarines, waned significantly as steam turbines and diesel engines, both capable of producing more power, took precedence.

Synchronous motor

Interest in electric propulsion was rekindled during the inter-war years with the most famous example being the 1936 built trans-Atlantic liner Normandie, which utilised electric propulsion via four 29MW synchronous motors each powered by a dedicated turbo-generator and driving one of the four propellers. DC motors were also used for main propulsion during the Second War World, most famously in the US-built T2 tankers, six of which are still in service today; four in commercial use and two in US maritime reserve.

More recently, there was the 1980?s conversion of the liner Queen Elizabeth 2 to diesel-electric at a cost of £100m. This involved replacing the steam-driven plant with diesel generators and two 44MW synchronous motors, the largest marine motors ever built at that time. The result was a top speed in excess of 32 knots and significantly reduced fuel

consumption.

Benefits of electric propulsion

The benefit of increased manoeuvrability provided by electrically-driven thrusters, whether variable speed FPP or constant speed CPP, are widely acknowledged. However, as far as bulk carriers, tankers and containerships are concerned, the most important considerations are those of economy and reliability, which is why these vessel types still retain slow speed diesel engines, acknowledged as having high efficiency and high reliability. The decision to install electrically-driven main propulsion however, is significantly more complex and usually depends on the type of vessel and its operational profile. Large ocean-going vessels would undoubtedly benefit from electric propulsion from the redundancy aspect but the significantly higher fuel costs could be prohibitive.

The many benefits attributable to electric propulsion include:

l Better space utilisation permitting more cargo to be carried (see Fig 2)

l Less machine space is required by the electrical propulsion systems. This is especially true where externally-mounted podded propulsors are used

l When electric propulsion was introduced into the Stolt ?Innovation? class of chemical parcel tankers, the engine room space was reduced by six metres, compared to a conventionally diesel engined ship of the same class, adding some 1,800m3 of cargo carrying capacity

l More efficient use of fuel resulting in reduced fuel consumption where the operational speeds are variable

l Diesel engines operate at their maximum efficiency at between 60-100% load. At less than 50% load the efficiency drops off quickly. In contrast, ?electric ships? have multiple generators which are optimised according to the ?required power for speed? profile. However, since electric propulsion requires more components (diesel-generators, transformers, drive converters and electric motors) it has a lower overall efficiency

l Improved manoeuvrability where azimuth or podded propulsors installed

l These types of propulsion systems have gained popularity over the past decade, especially in the cruise ship sector. (See Fig 3)

Redundancy

Currently, the marine classification societies do not permit electrically driven vessels to operate with only a single electrical propulsion unit. The minimum permitted is two and, therefore, if the motor (or drive converter) fails, the vessel retains the ability to ?get you home? which is not the case with single diesel-engined ships.

Lower emissions

A conventional slow speed diesel is acknowledged, when running at or near full speed, to have higher efficiency, use less fuel and, since the combustion process is more efficient compared to that at slow speeds, have emissions which although acceptable in the open sea may not be so acceptable when close to land. Medium speed diesels, on the other hand, are not always as efficient as slow speed engines although the emissions depend on the number of medium speed generators operational at any one time. At less than full speed there would be fewer generators running, and arguably fewer emissions.

Limits for NOx, SOx, CO2 and other emissions are set by a host of international and national regulations such as MARPOL Annex VI, the monitoring of the US Federal standards promulgated under CAA 123 but aligned under MARPOL Annex IV which imposes emission limits within 250 miles of the US coastline. In the EU, serious consideration is being given to limit the maximum sulphur content for ships operating within EU waters. This would require ships to have two separate fuel storage systems, ie one for expensive, low sulphur fuel for EU waters, the other for standard fuel for the rest of the world.

Future limits for pollutants from marine diesel engines, whether slow or medium speed, are more likely to be lowered than raised. Pressure, through the promotion, for example, of cleaner and more expensive fuels, may force ship-owners and operators of all vessel types to seriously consider electric propulsion.

Advances in power electronics the key

Suffice to say that there would not be electric propulsion as we know it without the continuing advances in power electronics. We have come a long way from the SCRs (thyristors) of the early 1970s. Fast, reliable and low loss semi-conductor devices are now available which permit drive converters to be connected directly up to 6.6kV, resulting in more compact motors and converters compared to low voltage units.

There is a range of drive technologies available depending on the thrust and other operational requirements but the prime factor in any decision as to which type to install is usually one of cost:

DC motors and SCR (thyristor) converters ? DC systems, once popular for research vessels where quiet operation was required, have lost significant market share to AC voltage fed drives in recent years. Simple converter but maintenance of dc motor brush gear can be an issue. Power limit is <2-3MW at voltages up to 750V dc.

Voltage source AC converters ? Utilising Pulse Width Modulation (PWM) or Direct Torque Control (DTC) (Fig 3) techniques ? are used with squirrel cage induction, synchronous and permanent magnet motors. They are popular in LV and MV form to around 7-8MW although the power range is increased up to approximately 30MW using new types of medium voltage devices.

Load commutated AC converters can be used for both synchronous and asynchronous (ie squirrel cage) motors up to around 100MW. On these applications the motor can be three-phase or six-phase (ie two three-phase windings displaced by 30 electrical deg to reduce torque ripple). Each three-phase winding would be supplied by a separate converter. Motors have to be specially designed for operation with this type of converter, for example, a high level of motor magnetising current for commutation purposes is required.

Cyclo-converters emanated from the coal mining industry. From the late 1920s this type of converter, obviously using different power conversion technologies, was used to drive mine winders. These drives are popular for vessels where high torque at low speeds is required such as, for example, icebreakers. The maximum output frequency is one third of input frequency so, at 60hz, 20hz is the maximum output frequency which is quite suitable for direct drive to the shaft using high pole motors. Applications to date include podded propulsion and the power range is around 2 to 22MW. However, it is anticipated that as AC voltage source converters, which have distinct technical advantages, extend their power range, the use of cyclo-converters will diminish.

All motors used with the AC drives mentioned above may require special rotor designs, higher levels of winding insulation and insulated bearings due to the levels of harmonics and dv/dt in the converter output waveforms. In addition, forced ventilation or water cooling may be used to minimise motor frame size and, hence, weight.

However, more advanced designs of electric motors, specifically designed for AC converters, are becoming available including ?advance inductions motors? (AIM), of which two 15 phase, 20MW versions are due for installation in each of the UK?s proposed Type 45 destroyers. AC permanent magnet motors which are much smaller and claim to produce, at least in theory, significantly more torque and power than conventional motors have already been installed in podded drives and submarines. The US ?high temperature superconductor? (HTS) ?supermotors?, claiming to be 60% smaller in size than conventional AC motors, are already undergoing tests with the US Navy with advanced converters.

Voltage distortion can be a problem

All these marine drives have one thing in common. With the exception of DC drives which convert AC voltage to variable DC, they all convert the vessel?s fixed voltage and frequency to a variable output voltage and frequency. During the power conversion processes in all the drive types, non-sinusoidal currents are drawn, resulting in harmonic distortion of the ship?s voltage supplies.

Harmonic distortion is widely recognised as being a cause of damage to, and malfunctioning of, electrical equipment. On ships, the effects of harmonics are significantly more pronounced. Generators are ?weak? sources where impedances can be as high as 15-20% compared to ?stiff? sources (4-6%), more commonly found in shore-based utility applications. The ?weaker? the source, the higher the voltage distortion for a given harmonic current. Due to the serious concerns regarding harmonics, including those relating to safety, classification societies now stipulate strict limits on voltage distortion with a 5% Vthd (total harmonic voltage distortion) limit being the most common. However, it is the author?s opinion that the policing of marine harmonic distortion limits is not yet being enforced to any significant degree, citing at least one vessel where the Vthd was in excess of 20% on a common power system and still remained in class. It can be argued that, on dedicated systems where the harmonic generating, ?non-linear? loads are electrically isolated from other loads, higher levels of Vthd may be permissible, subject to the connected equipment being designed for those higher levels of distortion.

Drive manufacturers use a range of techniques to attenuate harmonics. Multipulse drives and phase shift transformers are very common, although the additional size and weight requirements may be problematic. Twelve-pulse drives however, are still universally popular reducing ?total harmonic current distortion? (Ithd) to between around 9%-15% depending on the type of transformer used, albeit at the loss of 3-4% in overall drive-train efficiency terms. However, the use of 12-pulse drives no longer guarantees compliance with current marine harmonic limits, especially on multiple 12-pulse installations. On IT networks (ungrounded neutrals), where standard converter EMC filters cannot be used, double-wound phase shift transformers are manufactured with special copper screens between windings to provide a high impedance path for ?common mode noise?.

For higher levels of harmonic mitigation, active filters (which inject ?cancellation current? into the load) or active ?front ends? (which synthesize sinusoidal input waveforms) are occasionally fitted to AC voltage fed converters but are very complex and expensive.

However, there is a new harmonic mitigation option for marine AC drives which does purport to offer excellent performance for relatively low cost. Called LineatorTM , this patented passive filter was developed by Mirus International Inc and provides six-pulse and 12-pulse PWM and DTC AC voltage source drives with harmonic performance akin to 18 pulse (5%-8% Ithd) and 24 pulse (2%-4% Ithd) drives respectively without the high cost and space requirements of these multipulse solutions. By installing LineatorTM in lieu of multi-pulse AC drives, overall drive efficiencies are increased by 3%-4%, reducing fuel costs. Its compact size, rugged design, high performance, high efficiency and low comparative cost should make Lineator technology a very attractive candidate for the future marine AC voltage fed drive harmonic mitigation requirements.

The future

There is a range of exciting technologies looming on the horizon, most emanating from warship designs which will eventually permeate through to commercial vessels. These include so-called ?harmonic-free? compact AC drive technologies such as matrix converters, resonant conversion converters and the US SAIC ?multi-port? PFM (pulse frequency modulation) drives, all potentially capable of driving propulsion motors up to 80-100MW.

The military conversion to ?electric ships? will provide navies with additional war-fighting capabilities. When not at ?flank? (full) speed, electrical power can be diverted to operate a new generation of electric weapons including high energy ?coil guns? and rail guns?, both electromagnetic launch systems, pulsed lasers and other advanced weapon platforms.

Conclusion

What is clear is that electric propulsion is here to stay. The benefits are widely acknowledged and the higher initial capital costs will reduce as sales of marine electric drive systems grow. Technological advances in electric motors, power electronics, hydro-dynamics and propulsion engineering will continue to drive prices down further in the future.

In 50 years? time one may wonder just how many ships will not be driven by some form of electric propulsion. There may be very few.

The author would to express his thanks to ABB Marine, Bakker-Sliedrecht and the American Bureau of Shipping for their assistance in the preparation of this article.