All-Electric Propulsion – Does it make sense?<?xml:namespace prefix = o ns = “urn:schemas-microsoft-com:office:office” />
By Edward Lundquist
All-Electric Propulsion is a promising technology for both naval and commercial marine applications.
Diesels and gas turbines have been replacing steam plants in naval vessel new construction for many years. Now all-electric ships – with diesel or gas-turbine engines – or a combination of both – along with electric generators and motors – are replacing direct drive propulsion and auxiliary steam systems in naval and maritime applications. These motor-generators can provide either AC or DC power that can be adapted for propulsion, hotel services and combat systems. This provides a more efficient and flexible electrical propulsion and distribution system throughout the ship.
In both commercial and military ships, using electric motors instead of a mechanical drive system reduces noise and vibration. Ships with high power demands can achieve better fuel efficiency, whether the requirement is for the hotel load on a cruise ship or combat systems on a warship.
Both commercial and naval ships have a growing demand for hotel loads. All-electric ships provide this capability through power management and making available all of the ship’s installed power to electric loads throughout the ship.
A major advantage of electric drive for naval ships is that the prime movers can be located anywhere in the ship. Prime movers, whether gas turbines or diesels, do not need to be located in a central machinery space and mechanically connected to the propeller shaft as with traditional propulsion systems. Instead, engines can be distributed throughout the hull and connected to generators to supply power. This power can be fed to a central bus that can be used for propulsion.
“With an all-electric integrated propulsion system, you could put the engines in the bow, or in the stern, or even place smaller engines up in the superstructure,” says Mike Worley, vice president of naval marine programs for Rolls-Royce North America. “Distributed power is desirable for a warship because of survivability. If one of the engines gets knocked out in one part of the ship, you can isolate that part of the distribution system and still make and distribute power throughout the rest of the system.”
On the other hand, distributed power does not hold much appeal for commercial ships. Centralized engines are easier to monitor for merchant ships, and cruise ships want to insulate their passengers from engine noise as much as possible. However, arrangeability of spaces can be advantageous for cruise ships. As pods replace in hull motors and large shaft spaces there is space for additional cabins. Unlike cruise ships, combatants must reduce radiated noise in the water as much as possible to avoid detection. Warships need greater power for sustained high speeds while maneuvering in battle.
Both commercial and naval ships have a growing demand for hotel loads. Sailors and passengers have more personal computers and electronic devices than ever before, and higher demands for air conditioning. Having said this, new naval vessels are being designed to operate with much smaller crews to reduce life cycle costs.
Smaller, lighter, better.
The emphasis today is to develop smaller, lighter and more efficient engines, generators and motors.
The U.S. Navy's DD(X) destroyer will be an all-electric ship. The current Arleigh Burke-class guided missile destroyer (DDG) uses four gas turbines for propulsion (two per shaft) and three smaller ship service gas turbine generators for the combat systems and hotel services. The DDG propulsion power cannot be shared or distributed with these other systems. About 90% of the total installed power is used for propulsion and cannot be transferred for other mission requirements, such as combat systems or ship services. DD(X), however, will have two large gas turbine generators and two smaller ones, all providing power that can be used for propulsion or ship services. The electric drive and power distribution system makes all power available to all systems.
The combat value of an electric ship goes well beyond efficiencies and signature reduction. Once on station, a warship cruises at a reduced power. When not needed, one or more engines can be taken offline to save fuel. At lower speeds, DD(X) has a surplus of power that can be made available as needed. The Navy wants to harness this power availability for new weapons including directed energy weapons or rail guns that may eventually find their way aboard DD(X) or similar combatants. The power previously trapped in the propulsion train can now be directed to enhance combat capability and mission flexibility.
At full power, DD(X) will achieve speeds in excess of 30 knots. If one of the main turbines is lost, the plant can be isolated and still achieve 27 knots. Normal station-keeping can be accommodated with the two small turbines.
The DD(X) will employ fixed pitch propellers. Controllable Pitch Propellers (and their associated hydraulics) are not required since the motor, and thus the shaft, can be electrically reversed.
The DD(X) Engineering Development Model (EDM) at the Land-Based Test Site (LBTS) in Philadelphia is currently configured with one Main Turbine Generator Set (MTGS) powered by a Rolls Royce MT30 and two auxiliary turbine generator sets, one each powered by a Rolls Royce MT5S and a GE LM500.
“We did lots of studies of different combinations of gas turbines,” says Mike Collins, program manager for Integrated Power Systems with Program Executive Office – Ships (PEO Ships). “We didn’t want to develop any new gas turbines that weren’t qualified and in service or being brought into service.”
The DD(X) electric propulsion EDM originally encompassed the development of a new permanent magnet motor (PMM) being developed by DRS Technologies of Parsippany, New Jersey. However, technical issues, primarily with insulation, are causing delays with development. As a result the motor will not likely be ready in time for the first DD(X).
Instead, the Navy is now planning to use a proven advanced induction motor (AIM) as the baseline. A team of Alstom Power Conversion and Curtis-Wright, both of Pittsburgh, PA, will produce the AIM. While heavier, the AIM is available now and is essentially the same system being installed on the Royal Navy's Type 45 destroyer. The combination of the MT 30 and LM500 are baselined in the DD(X), but there will be a full and open competition to select the engines for the lead ship.
“(The AIM) is the baseline for the Type-45 that will go to sea in a year,” says DD(X) Program Manager Capt. Chuck Goddard. “It’s good technology.”
Before the PMM can be installed on ships, the motor will have to pass the critical technical parameter test, running at full power, torque and speed in a land-based test site operating with the rest of the plant. Although the PMM will not be on the lead DD(X), the Navy remains very interested in using PMM technology later, perhaps as one of the spiral insertions in a future ship.
“We like permanent magnet motor technology,” says Capt. Goddard. “We like it because of the power density it has. What does that mean? It means it's smaller for the same amount of power and less weight.”
“We acknowledge that problems occurred in our original insulation system in January, but are pleased to report that these problems are now well behind us. Our new insulation system has been successfully applied to thousands of machines, including some onboard Virginia-class submarines,” says Edward Bartlett, President of DRS Power Systems. “Using the new insulation system we have successfully rebuilt one of two stator rings in the DD(X) PMM Engineering Development Model, giving it ½ power capability. This machine has successfully completed both unloaded and loaded full speed and full voltage factory testing during June and we look forward over the next couple of months to completing the rebuild of the second stator ring and to shipping a full scale full power 36 MW machine to the Philadelphia LBTS for testing in early Fall. While we understand that we have an uphill path to convincing the Navy that it is ready for the lead ship of the DD(X) program, we are convinced that the significant size, weight, acoustic performance, fuel economy and arrangement benefits that this system offers over any available alternative make a compelling case for immediate deployment of the technology.”
Collins says the DD(X) will utilize a pulse width modulated motor drive for better control and reduced signature.
Although there are advantages to distributing the power system throughout a warship hull, the size and weight of the various components has usually necessitated keeping the propulsion equipment low in the ship for stability reasons.
The DD(X) engineering plant layout is relatively conventional because of the air intake, exhaust, and drive arrangement, according to Collins.
The Rolls-Royce MT 30 marine gas turbine is based on the RR Trent 800 engine that powers the Boeing 777 twin-engine airliner. The aviation engine has a design and demonstrated reliability of 99.98% and the 777 aircraft can operate on one engine. The marinized MT 30 version has 80% commonality with the Trent 800, but has different blade coatings for operation in a salt-water environment and engineering changes to support shock requirements.
General Electric’s candidate for DD(X) is the LM6000, delivering 36 MW and the LM500, with 4 MW. The LM-6000 has been in industrial service for more than a decade. The LM500 provides power on a variety of ships today, and is the prime mover on the Danish Flyvefisken patrol craft class and the Japanese PG01 and 11PG patrol craft. The new Korean PKX patrol craft will be powered by the LM500, as are the Foilcat fast ferries in use by Far East Hydrofoil in Hong Kong.
While work continues to create small, lighter engines with greater power density, more promising is simplifying or improving the ancillary equipment that accompanies a gas turbine installed on a ship. Engines are housed inside sound-proof enclosures that include the fuel, lube oil and air management systems, fire suppression, as well as sound and heat insulation, all upon a heavy shock-mounted base. “There's potential to reduce the size and weight of the enclosure,” says Rolls-Royce's Worley. “But you can't make your garage smaller than your car.”
The Next-Generation Aircraft Carrier
The U.S. Navy’s next aircraft carrier, designated as the CVN-21 program, is increasing the size of its electrical generation system to accommodate the new Electromagnetic Aircraft Launch (EMALS) and the electric Advanced Arresting Gear, both supplied by General Atomics (GA) Electromagnetic Systems Division of San Diego. GA has experience in very high-energy pulsed power applications, including significant research in fusion power that has translated to naval applications such as EMALS and the electromagnetic rail gun.
The CVN-21’s reactors will make steam to generate electricity and propulsion. The ship will be driven by the same steam turbine/reduction gear system on current carriers, but will have larger generators. The availability of this additional electric power will allow for the installation of an electromagnetic launcher to replace the current steam catapult system and a turboelectric arresting gear to replace the current hydraulic arresting gear. While these systems have not radically changed the power plant configuration of the next generation carriers, they do point out the continued importance of electric power technologies and the ability to generate and manage high levels of power efficiently throughout the ship
Electric drive is not new to the U.S. Navy. Numerous ships built during World War II used diesel engines coupled to generator sets to power a large motor. The USS Langley, a converted collier and the Navy’s first aircraft carrier had electric drive. The submarine tender USS Fulton (AS-11), commissioned in 1941, had electric drive and served for 50 years. Dozens of Navajo-class ocean-going fleet tugs were equipped with electric motors that provided precise, responsive ship control when maneuvering for salvage and towing operations.The nuclear attack submarine USS Tullibee (SSN 597) had a turbo-electric drive that reduced the submarine’s acoustic signature and reduced ambient noise for its sonar system. The Navy’s 41,000-tondry cargo/ammunition ship, USNS Lewis and Clark (T-AKE ), being constructed for the Military Sealift Command, was christened in May of 2005, and will have a single-shaft diesel-electric propulsion system to deliver speeds up to 20 knots.
Since some materials are much better conductors at very cold temperatures, with virtually no electrical resistance, supercooled conductors make for much more efficient motors. Superconducting wire can carry more current and generate higher magnetic fields in very small areas, and can result in a significantly smaller motor.
General Atomics is currently developing a Superconducting DC Homopolar motor for propulsion applications. The superconducting DC motor system is smaller and lighter than comparable traditional and superconducting AC motor systems, according to Michael Reed, Vice President of GA’s Electromagnetic Systems Division.
GA’s motor uses low-temperature supercooling that employs gaseous helium to maintain the superconducting wire for the magnets within the motor at 5 degrees Kelvin, which is almost absolute zero. A comparable high-temperature supercooled system operates between 40 and 75 degrees Kelvin, depending upon the technology chosen. Refrigeration at higher temperatures is easier, but the high temperature superconducting material is not as easy to produce and is much more expensive than the superconducting niobium-titanium wire in the low-temperature motor, says Reed. He adds, Niobium-titanium wire is the most widely used and available superconducting wire in world-wide commercial applications.
GA has built a 5,000 HP motor that is just 4.5 feet in diameter, says Reed. “Our superconducting DC motor is longer, rather than fatter,” he says, meaning that this technology can be more readily adapted to propulsion pod applications and can be more slender, lighter and with less drag and, therefore, more fuel-efficient.
Additionally, Reed says that while superconducting AC motors have similar costs to the superconducting DC motor, there is no need for power inverters and the associated electronics to switch it to AC. “Power electronics are a size and weight driver,” Reed adds.
Direct Current for DD(X)
DD(X) power generators produce 4,160 volts AC, which is rectified to DC and allows for ship service power distribution to be tailored to the ships needs. “There are three advantages to DC,” says PEO Ship’s Mike Collins. “First, DC uses solid state power conversion that feeds loads, with conversion at the end back to AC. It’s cleaner power. Second, a lot of our combat systems loads are DC. Finally, you can share and auction power. DC allows you to provide uninterrupted power even if we have a casualty.”
Electric power can be used for propellers or waterjets. Most marine motor applications are located within the hull and coupled to the shaft. But pods are becoming increasingly popular. The “Mermaid,” from RRAB (a joint venture with Rolls-Royce AB and Alstom), and ABB’s “Azipod” systems can rotate 360-degree. These pod systems on cruise ships eliminate the need for rudder assemblies, and keep much of the propulsion gear outside the hull, freeing up that volume for other purposes. The pods provide increased maneuverability, especially for cruise ships routinely entering and leaving port. In naval applications, reliability and shock resistance are critical.
Pods and azimuthing thrusters have military applications, too. With a pod, the motor is in the pod, while an azimuthing thruster has the motor located in the hull. The Royal Navy’s Echo-class of survey vessels uses electric azimuthing thrusters.
Pods were considered for DD(X), but ruled out because of their size. Also, the signatures could be mitigated if the propulsion system was isolated inside the hull.
Long, slender motors are required for SWATH (Small Water Plane Area Twin Hull) ships, which have propulsion equipment located in the submerged cylindrical buoyant hull sections in a catamaran configuration. SWATH ships can mount prime movers above the waterline
ThyssenKrupp’s Nordseewerke has built the SWATH research vessel Planet for the German Federal Office of Defense Technology and Procurement (BWB). Planet will assess new propulsion technologies and evaluate the sea keeping characteristics of the SWATH hull form. Its electric propulsion enables it to test mine detection and undersea warfare systems and countermeasures.
Siemens in Germany is evaluating High Temperature Superconductors, finding improved power availability and system responsiveness in podded waterjets. Siemens is also developing fuel cell technology for ship propulsion.
The Office of Naval Research is developing an 130-foot-long craft called the Advanced Electric Ship Demonstrator (AESD) with waterjet-based propulsion, at the Navy's Acoustic Research Detachment at Lake Pend Oreille, Idaho. ONR engineers expect to achieve improved efficiency and maneuverability with a smaller, lighter propulsion system while reducing noise at the same time. The tests will be conducted with the battery-powered craft and uses a one-quarter scale Rolls-Royce AWJ21 water jet propulsion system. The craft resembles the DD(X) hull form.
ONR has also developed an Advanced Hull Form Inshore Demonstrator (APHID) which is testing a complete electric podded propulsion system. The Rim-Driven Propulsor Pod (RPD) uses a Pulse-Width Modulated (PWM) motor drive system mounted on the Hybrid Small Waterplane Area Craft (HYSWAC). The HYSWAC is built from a modified Navy Surface Effect Ship and uses a Vericor TF-40 gas turbine prime mover.
There is a variety of electric propulsion and power distribution systems in service or in the development process that provide a number of power generation and propulsion options for ship designers. The state of development of the required supporting technologies makes electric drive a viable choice for ship designers.
Overall, integrated electric drive offers ship designers and operators a plant flexibility that does not exist with mechanical drive systems. “However,” says General Electric’s Read Tuddenham, “electric drive is not a panacea. The ship designers still need to do the trade studies to determine the appropriate choice of power and propulsion system for their ship.”
There are not a lot of electric drive warships in service, says Tuddenham, who is GE’s manager of integrated propulsion systems and new applications. .“DD(X), Type 45 and T-AKE are the only all-electric warships I'm aware of and they are still in the design phase or under construction.”
There are some ships with partial electric drive, hybrid electric drive mechanical drive systems, he says. These include the operational Type 23 frigates; the European Multi-Mission Frigates (FREMM), a joint program between France and Italy, which is still in design; and the amphibious assault ship USS Makin Island (LHD 8) (under construction). “Interest in electric drive systems is far higher than it used to be, but the number of ships actually under construction and in operation is still relatively small,” Tuddenham says.
There are advantages to a mechanical drive system. “Mechanical drive systems transmit energy from the prime mover to the propulsor more efficiently than electric drive systems do,” Tuddenham says. “Approximately 98% of the energy at the prime mover output shaft makes it to the propulsor in mechanical drive. The same number for electric drive is on the order of 91% to 93%.”
One design is not optimum for all situations, says Tuddenham. Cruise ships with large portions of their itineraries at low power benefit from electric drive. Fast ferries which go to full throttle as soon as they clear the breakwater and remain at full throttle until they reach the next port would be at a disadvantage with electric drive. Electric drive is not the solution for all cases.
Edward Lundquist is a senior technical director for Anteon Corporation. He is the director of corporate communication for Anteon’s Center for Security Strategies and Operations and supports the U.S. Navy’s Surface Warfare Directorate. He is a retired U.S. Navy captain.