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Electricity Shifts The Currents of Ship Propulsion

December 2002
By Sharon Berry

Engineers work to unlock power shipwide.

Over the years, ship propulsion has evolved from sail to steam to diesel and gas turbine engines. The U.S. Navy now is transitioning to all-electric ships, which will increase available power throughout a vessel. The benefits will be enhanced ship survivability, improved combat capability, reduced crew size—sending fewer sailors into harm’s way—and lowered ship life-cycle costs.

Because today’s warship propulsion systems are separate from their auxiliary systems, weapons and sensors, 90 percent of more than 80 megawatts of power is locked into the propulsion system and is not available for high-energy weapons and sensors. All-electric ships will unlock this power by applying an integrated power system (IPS) that provides electric power to the entire ship.

More than just electric drive, the IPS enables a ship’s electrical loads, such as pumps and lighting, to be powered from the same electrical source as the propulsion system. This eliminates the need for separate power generation capabilities and uses available power more efficiently through reconfiguration. When a ship is hit in battle, power distribution can be reconfigured to support surviving sections. IPS also enables the number of prime movers, for example gas turbines, to be reduced—allowing greater flexibility for their placement—and frees up space for more fuel or weapon systems. It also reduces maintenance and manpower demands with a savings of approximately 20 percent in manpower.

In an address to the U.S. House of Representatives Armed Services Committee earlier this year, Rear Adm. Jay M. Cohen, USN, chief of naval research, Office of Naval Research, Arlington, Virginia, emphasized that the electric warship is an essential future naval capability and is key to naval transformation. The industrial base already is evolving to support electricity-powered technologies employed on commercial cruise lines, and the service must adapt to this trend. “Make no mistake.” Adm. Cohen said, “There is risk in going where no one has gone before. There is zero certainty that every research investment will pay the dividends we desire. What is certain: If we do not invest in promising research today, we guarantee that options and opportu nities will be severely curtailed in future years.”

The United Kingdom is at the forefront of warship development. The Defence Evaluation and Research Agency fielded a trimaran warship demonstrator, the research vessel Triton, which represents a significant change in warship design. The ship development effort includes refitting the main propulsion system with electric propulsion, testing permanent magnetic motors and employing integrated technology masts, 8-megawatt gas turbines, battery systems, composite shafts and electric rudders.

In the United States, the Office of Naval Research, the Naval Sea Systems Command (NAVSEA) and the Program Executive Office for Surface Strike (PEO(S)) are tasked with achieving the U.S. electric warship capability. The PEO(S) is taking a leadership role, and the Electric Warship Strategy Task Force, which includes flag officers from each organization, is helping to coordinate the work. Additionally, the United States and the United Kingdom will share information under a number of information exchange agreements, both bilateral and multilateral.

Navy leaders have identified and are already working on several priorities for the all-electric ship’s design: improve tactical endurance, increase survivability and support high-energy weapons and systems.

Confronting priority one—improving tactical endurance—requires an increase in power to mission-critical systems. The Navy is taking a progressive approach to developing alternative power sources. “For near-term efforts, we’re going to be using conventional electricity generators powered by gas turbines,” says Scott G. Littlefield, deputy director, Naval Ship Science and Technology Office, Office of Naval Research, Arlington, Virginia. “That’s essentially the same way we’re generating power on ships today, but we’ll be scaling that up so we can generate enough power for propulsion as well as for all of the other loads. And that’s the baseline for DD(X).” DD(X) is the next generation of destroyers.

Using a spiral development process, the DD(X) program will focus on technology development and maturation, including robust land-based and at-sea testing of transformational technologies that could be leveraged across multiple ship classes. The DD(X) Flight I is envisioned to have electric drive and will feature an integrated power system. The ship is one of several in progress that will serve as a baseline for the electric warship concept.

To generate power in the near term, electric warship developers are using a concept similar to the hybrid electric power employed in certain automobile models by Honda and Toyota in which a tank is still filled with gasoline, but the fuel is used to generate electricity. “In our case, we’re still filling the ship with a heavy distillate fuel, but instead of using the fuel to drive a mechanical drivetrain, we’re using it to generate electricity, which is used to power the ship,” Littlefield says. “From a logistics point of view, we’re still operating on the same fuels that the Navy already operates on. But we’ll use less fuel, be more efficient and have more flexibility to use the power for either propulsion or other applications.”

As the electric warship future naval capability evolves, the Navy will employ more advanced power sources. One prospect is high-speed generators, which are similar to traditional generators but run at higher revolutions per minute and provide more power from a smaller generator. A second prospect is the fuel cell, a chemical engine that produces electricity. Unlike a battery, a fuel cell will produce power as long as fuel, typically in the form of gaseous hydrogen and oxygen, is provided to the cell. Researchers are undertaking challenges to adapt commercial fuel cell development to a Navy environment. These challenges include reforming Navy-standard diesel fuel into a hydrogen-rich sulfur-free gas and increasing power density to meet shipboard operating requirements. However, the Navy is making progress and preparing for a land-based demonstration of a 625-kilowatt fuel cell in the near future.

Increasing survivability is another Navy-identified priority and is a tough problem, Littlefield shares. “If you start with the premise that electrical power is critical to your ability to fight the ship, then you really need to make sure that the power is still there if you get hit,” he says. “For example, in the USS Cole, the electrical power went out for some time. When the power goes out, that means you’ve got no pumping, no ventilation, no light. So fighting the ship and saving the ship become difficult. As we go to smaller and smaller crews and more automation, we need to count on the ship’s system being reliable and the ship’s power staying on even if it is attacked.”

To address this challenge, engineers are developing a system called integrated fight-through power. This reconfigurable power distribution architecture ensures that power stays on even when a ship is attacked. The system uses zonal ship-electric-power-distribution technology to provide redundancy of power distribution. Hardware and software are included to allow for reconfiguration in the event of a change in load requirements among the zones or in the event of component failure. These building-block technologies can be adapted to the voltages and types of power required by each system application, and the hardware and software can be continuously upgraded.

The reconfigurable power grid also is an initial step toward the third development priority—having enough power to direct high-energy weapon pulse loads. “We require a substantial fraction of the ship’s power for new kinds of weapons and sensors,” Littlefield says.

In Roadmap to an Electric Naval Force, the Naval Research Advisory Committee reports that “electric weapons and advanced sensors will provide the superior warfighting capability for the electric warship.” These include high-energy lasers, electromagnetic guns, high-resolution target tracking and high-power microwave radar. According to the committee’s report, many of these advanced weapons and sensors are not available today and have very large power requirements. “The tens of megawatts required by these systems will be provided by diversion of propulsion power via the common electric power system. The increased firepower, range and resolution capabilities of these high-power electric weapons and systems would be unaffordable if each required its own power source.”

According to Rear Adm. Paul E. Sullivan, USN, deputy commander for integrated warfare systems, Naval Sea Systems Command, Washington, D.C., ships can generate a large amount of raw power to travel at high speed. However, over the life of a ship, the ship does not run at maximum speed all of the time. A destroyer that can cruise at more than 25 knots spends most of its life traveling at between 10 and 15 knots. This excess power also could be used for weapons, he says.

Naval researchers anticipate that by meeting these development priorities, including reducing the number of prime movers and using an IPS for propulsion and nonpropulsion electrical loads, life-cycle cost savings will result. “By employing an IPS, a destroyer can transition from seven gas turbines down to four,” Littlefield relates. “You can run those gas turbines closer to their peak efficiency points, which saves a lot of fuel, starting at 15 to 19 percent savings. There’s also a combat payoff from the reduced fuel savings. It’s not just money, it’s more time on station, extended endurance and reduced dependence on a logistics train to get the fuel to the fleet.”

Because future ship systems will be more efficient and redundant, fewer sailors will be needed. Adm. Sullivan admits this has been controversial because some people argue that past catastrophes sustained by frigates and cruisers have required the full attention of every crew member to save the ship. Today’s destroyers have more than 300 people, and submarines have more than 100 people. The DD(X) will require a crew of fewer than 100 people.

Reduced crew sizes also will bring about life-cycle cost benefits. “The electric warship is a piece of the pie on reducing crew size, but the effort to cut crew size by two-thirds or more is pervasive and ongoing,” Adm. Sullivan says. “As you know, it costs a lot to buy a ship. But that is only the first cost. Once you’ve bought the ship, the most expensive part of keeping the ship up is paying the sailors. Depending on the ship class, personnel expenses could make up 35 to 60 percent of the cost of operating that ship for a 30-year service life. If you apply the Parado Principle where you go after the heaviest hitters first—reducing the number of sailors on our ships—it’s a number one priority for reducing life-cycle costs. It isn’t just the pay and the benefits but the training pipeline and keeping them current. Overall, we’ll have to operate these ships differently. And we’ll have to train the crews differently.”

The commander of the Naval Sea Systems Command, Vice Adm. Phillip M. Balisle, USN, is standing up a human interface integration office that will deal with issues of a crew’s complement and educating sailors on new systems. “‘What is the technology needed to operate our ships with fewer people? What is the training needed?’ These are the types of questions he will need to answer,” Adm. Sullivan says. To provide answers, Adm. Balisle will work with the Task Force for Excellence through Commitment to Education and Learning (EXCEL)—an initiative that oversees the implementation of pilot programs designed to enhance and strengthen the Navy’s training and education structure.

Electrical power sources, which includes the electric warship effort, have been designated as one of the Navy’s Grand Challenges. It will be a long-term research venture that will take place over a 20- to 50-year time frame. By emphasizing ongoing research and planning for long-term research, the service will realize capabilities required for the future. In the immediate future, the Navy plans to work with the U.K. Ministry of Defence and test IPS modules onboard the Triton.

What Industry Sows, the U.S. Navy Will Reap

Since 1980, commercial ship designers have made significant advances in electric marine systems, and the U.S. Navy is taking advantage of them. Carnival’s Elation was the world’s first cruise ship to feature a unique electric propulsion system called the Azipod. In the Azipod propulsion system, the conventional ship propellers, propeller shafts, reduction gear, rudders and steering gear are replaced by the diesel-electric Azipod units.

Azipod units on cruise vessels improve maneuverability, improve fuel economy, and enhance passenger and crew comfort as propeller-induced hull girder vibration and noise are reduced. Recently, a compact Azipod propulsion system was developed for smaller vessels.

According to experts, the global market for electric motors and systems for electric propulsion in commercial cruise ships is $400 million per year. The market is expected to grow to $2 billion annually by 2010 as electric drive becomes the propulsion system standard for both commercial and Navy ships.

In its report, Roadmap to an Electric Naval Force, the Naval Research Advisory Committee notes that advances with electricity-powered systems are making their way into the design of several Navy vessels. “This transition has been made largely due to the availability of high-power, variable frequency converter drives enabled by advances in power electronics technologies and by the resulting evolution of external, podded electric propulsors,” the committee states.

“As you know, a lot is going on in the commercial world right now,” says Rear Adm. Paul E. Sullivan, USN, deputy commander for integrated warfare systems, Naval Sea Systems Command, Washington, D.C. “It’s pretty exciting. Many other cruise ships are turning to electric systems because they are easily operable. However, there are items in the Navy that are specific to warships that make it so we can’t just adopt what’s out there in the commercial market.”

First, the Navy’s focus on reduced sound signatures will diminish a vessel’s susceptibility to detection, classification and targeting while improving survivability. “Our ships have to have a reduced signature, particularly submarines,” the admiral says. “But what is on the commercial market is [still] far too noisy for military applications. That doesn’t mean we can’t take advantage of the market place. It does mean we’ll have to work pretty hard to tailor the components to make them very quiet.”

The Navy is conducting research in two key areas on this issue, including reduced underwater signatures and reduced above-water signatures. The work on reduced signatures is being coordinated with programs in several Office of Naval Research thrusts—advanced electrical power systems, hull-life assurance and hydromechanics, which ensures that all vessel elements will operate in unison.

Torque is a second Navy-specific area that researchers are addressing. “Cruise ships start out at port and transit across to their next port at a specific speed, and they don’t have to go to war,” the admiral explains. “In the Navy, we’re not only trying to produce a lot of electric power, but we also move quickly. For the aircraft carriers, submarines and destroyers, we need a lot of power. We’re always looking for high power in a small size. Not only do we want to put more power into the water, but we also want to do it with a slower turning motor. That’s a challenge.”

American Superconductor is developing a 6,500 horsepower motor that will operate at 230 revolutions per minute (RPM) and represents a ten-fold increase in torque over the 5,000-horsepower, 1,800 RPM high-temperature superconductor (HTS) motor previously built. The low-speed, high-torque 6,500 horsepower HTS motor is a critical development milestone on the path to 20,000- and 35,000-horsepower motors.

Finally, shock hardening for every component must be addressed. “This includes the generator, the power distribution system, the circuit breaker—all have to be looked at,” Adm. Sullivan notes. “When we send the DD(X) to sea, all of these components will have been shock tested.” DD(X) is the next generation of destroyers. The program focuses on technology development and land-based and at-sea testing of transformational technologies that could be leveraged across multiple ship classes.

“We have ships going to sea soon,” he adds. “The LHD-8 has about a 5,000 to 6,000 horsepower electric motor. That’s not a full integrated electric drive system. But we’re making progress.” Navy-standardizing the electric technologies will lead to greater progress in the future.