The United States Navy’s interest in electric propulsion dates back to the launching of the 19,360-ton USS Jupiter (Figure 1) in August 1912. The Jupiter had a prototype electric propulsion system that was later widely used in Navy capital ships built during the late 1910s and 1920s. For example, the 36,000-ton USS New Mexico, commissioned in 1918, was the first of three battleships that used electric motors to drive their propellers. The New Mexico’s propulsion system—four turbo-electric steam turbines driven by six 300-psi boilers—produced 40,000 hp, giving the battleship a top speed of 21.5 knots. During the same period, several classes of destroyer escorts and diesel electric submarines were fitted with DC generators that powered electric motors coupled directly to the propeller screws. A manual rheostat varied the amount of current going to the motors to control speed. Later escort-class ships, fitted with steam turbine–driven generators, saw service through World War II.

 1. First electric ship. The USS Jupiter (later the USS Langley), was the first U.S. Navy ship propelled by electric motors (1912).
Courtesy: U.S. Navy

At the time, electric drive was seen as a natural evolution from the direct-connected, low-rpm steam engines found in the turn-of-the-century "Great White Fleet." The introduction of high-rpm steam turbines early in the 20th century meant that either a geared system or an electric drive system was needed to transmit power to the shaft. Advances in gear manufacturing pushed ship design to direct-connected systems, because early electric-drive systems were larger, heavier, less efficient, and more maintenance-intensive than geared mechanical drive. However, during WWII limitations on the gear manufacturing capabilities of U.S. industry necessitated the use of electric drive to maintain the rate of ship production.

Interest in electric-drive ships declined after the war with the advent of nuclear and then gas turbine propulsion. Today, other than nuclear-powered aircraft carriers, Navy ships are typically powered by two propulsion modules, each containing a pair of marinized General Electric LM2500 gas turbines that are connected to a common double-reduction gear box via an SSS Clutch Company Inc. (Newcastle, Del.) overrunning clutch that puts 50,000 shaft horsepower (SHP) to the water through a reversible-pitch propeller. Some 100,000 SHP is needed to push 10,000 tons through the water at 30+ knots, although most Navy warships spend 90% of their life cruising on only one or two engines. Because power requirements rise with the cube of speed, each ship carries a few extra engines for those times when a little acceleration is necessary to stay out of harm’s way.

The typical warship’s electric load (including lights and weapons) is supplied by three marinized Allison 501 gas turbine packages (called ship service turbine- generators or SSTGs), each augmented by a heat-recovery hot water generator.

Electric redux

Beginning in the 1970s, the availability of much faster solid-state power electronics devices and switches, and synchronous AC motors with higher power densities, renewed the Navy’s interest in all-electric warships. In the past, ship designers were constrained by the required location of the propulsion modules: The location of the propellers defines the shaft centerline and therefore the vertical location of the gas turbines in the ship. For survivability reasons, one module must be located forward and the other aft. Significantly, an all-electric propulsion system frees the ship designer to locate the gas turbine-generators wherever they will optimize the weight and balance of the ship. The gas turbine-generators are connected to the propulsion motors by distribution cables and fast-acting solid-state switches.

A fully implemented all-electric warship design would serve as the "wall plug" for other future technologies such as electromagnetic guns, high-energy lasers, high-power sensors, and organic surveillance drones (Figure 2). Such a design would dramatically improve the overall combat capability and mission flexibility of the ship. In addition to having lower manning and operating costs, the ship also would have greater survivability because generation would be distributed throughout the ship. An integrated power system also would allow the efficient diversion power as necessary, better matching the typical combat operating profile: a high-speed transit followed by a low-speed loiter.

2. Future shock (and awe). A fully implemented all-electric warship design.
Source: U.S. Navy

On the horizon

The first two members of the coming generation of all-electric warships will be spawned by two recent U.S. Navy contracts. One is for the DD(X), which will be powered by an advanced integrated electric propulsion system and built by Northrop Grumman Ship Systems (the prime contractor) and Raytheon (the weapons and electronic systems integrator). The Navy plans to award detail design and construction for the lead ship of the class early this year. The lead ship is scheduled for fleet delivery in 2011 and to enter service in 2013. The second contract is for the T-AKE Combat Logistics Force ship, which will be designed and built by San Diego–based National Steel and Shipbuilding Co. (a General Dynamics company). Powered by a commercial-derivative integrated power system for propulsion and ship’s service systems, the first T-AKE delivery is scheduled for mid-2005. The electric systems of both ships will be based on current technology gas turbine-generators and SSTGs that, when hooked up to an electrical bus, will provide power to conventional electric motors connected to the power shafts.

Chief of Naval Research, Rear Admiral Jay Cohen has recognized the importance of taking the next step from an electric-drive ship to the fully implemented electric warship envisioned in the "Navy after Next" program by focusing the required investment in science and technology development. Looking forward, Rear Admiral Cohen says, "Electric [propulsion] will be as revolutionary as sail to steam, oil to nuclear, or guns to missiles." But to make the electric warship a reality, the Navy will have to do plenty to advance the state of the art in a number of technologies, including power generation and distribution, energy storage, power control and conditioning, and propulsors (Figure 3).

3. Plug and play. The size of future electric weapons and the multiplicity of onboard sensors demand an integrated power system.
Source: U.S. Navy

Advanced electronics. Advanced power electronics and control research is directed at incorporating advanced switching devices into power modules to work at higher voltage levels and to prevent voltage breakdown within modules. The technology is available, although much work remains to develop manufacturing techniques to reduce cost, size, and weight. Managing short-duration power surges is the job of "power electronic building blocks" that combine state-of-the-art modular high-power switches into generic, low-cost, flexible, reprogrammable power control devices. A 9-MVA prototype is scheduled for testing later this year, and a 13-MVA unit is under active development.

Ship service fuel cell. The ship service fuel cell (SSFC) will demonstrate that a fuel cell, with diesel reforming, can serve as an improved shipboard power source and replace the SSTGs. Fuel cells can produce DC power with no need for an internal combustion engine, at higher cycle efficiencies (up to 50%), and with no environmental discharge other than water—at a lower lifecycle cost to boot. The key to success will be developing a reformer that can separate hydrogen/methane from Navy distillate fuel, as the sulfur contaminates reformer catalyst. But because the startup time of a high-temperature fuel cell is longer than that of a low-temperature cell, speed is another major objective. It might not make much difference in a commercial application, but aboard a warship the extra time could be vitally important.

Sea cells

In the integrated power system envisioned for the all-electric warship, fuel cells will be key contributors to the overall power available. Currently, two different fuel cell prototypes—a proton exchange membrane (PEM) and a molten carbonate fuel cell module—are under development; which one is finally chosen will depend on developments in the commercial fuel cell market, cost-sharing development contracts, and prototype test results.

Development of a 500-kW PEM fuel cell continues to leverage work completed by the U.S. DOE. This project aims to produce an effective integrated fuel processor (with a sulfur-tolerant auto-thermal fuel reformer, regenerable sulfur adsorption beds, and catalyst guard beds) for converting Navy standard distillate into a fuel usable by a commercial PEM fuel cell. The design (Figure 4) has already been completed and was scheduled for demonstration in late 2004. The final design will package the system into a single module (Figure 4) with almost three times the power density. Full-scale testing is slated to begin later this year. Operating at a cell temperature of 180F, the system efficiency is predicted to be approximately 40% with a 40,000-hour life. SOFCo-EFS Holdings LLC (Alliance, Ohio) is supplying the reformer package in a 50/50 cost-sharing arrangement with ONR.

  4. Ship fuel cell. The ACES architecture features high-efficiency fuel cells and energy storage technologies. The design of the fuel reforming system (left) for a PEM fuel cell is complete. The final design will package the system into a single module (right) as part of the transition to production. Testing will begin later this year.
Courtesy: U.S. Navy

Technology developments

Work on the first two technology thrust areas (Table 1) is already well under way at the Office of Naval Research (ONR), to the extent that funding is available. Modest research programs involving understanding the basic science of the remaining technologies are also ongoing, although the electromagnetic gun has gotten the lion’s share of attention of late. The following projects are expected to have operating prototypes during the next year or two.

  Table 1. The U.S. Navy’s all-electric warship roadmap
Source: U.S. Navy

Also under development is a 50%- efficient, 625-kW molten carbonate fuel cell module that will include (in addition to the cell stacks): a fuel reformer capable of converting Navy standard distillate with up to 10,000 ppm of sulfur into a hydrogen-rich gas, a desulfurizer, steam and condensate systems, heat exchangers, an air handling system, a power conversion system, and an automated control and safety system (Figure 5). With an electrolyte of potassium-lithium-carbonate, the cell operates at 1,200F with a life expectancy of 40,000 hours and generates power at 450 VAC. This module (Figure 6), which was factory-tested last year, is scheduled for land-based testing this year. FuelCell Energy Inc. (Danbury, Conn.) is supplying the complete package in a cost-sharing arrangement with ONR.

  5. Molten option. One design option under active development is the 50%-efficient molten carbonate ship service fuel cell. FuelCell Energy is supplying the complete package in a cost-sharing contract with the Office of Naval Research. The fuel processor module is designed as a stand-alone unit. The overall size is 26.5 by 8.3 by 11 feet.
Source: U.S. Navy

  6. Under test. The 625-kW molten carbonate fuel cell being assembled in May 2004. Testing is scheduled to begin this year.
Courtesy: U.S. Navy

Although the current SSFC has a capacity of 500 to 600 kW, the challenge will be to develop and deploy higher-power (>20 MW) fuel cells for propulsion and weapons systems. These designs will necessarily require much higher power density and the ability to be connected in parallel, making them truly "plug and play."


The "Navy after Next"

As descriptions of the ongoing R&D into advanced power and control electronics and fuel cells hint, much basic work remains to be done before the dream of an all-electric warship becomes a reality. Among the fields with technical challenges that face ONR engineers and scientists seeking to build an integrated shipboard power system are the following.

Advanced main propulsion motor controllers will make possible quieter, more fault-tolerant and power-dense, higher-bandwidth, and higher-efficiency controllers rated at 25 MW or higher.

Kinetic energy weapons and advanced launchers, in concept, will be capable of delivering 6 to 12 rounds per minute of sabot-type projectiles with 20 megajoules of kinetic energy at a distance of more than 200 nautical miles. The key advantage of the "rail gun" is the reduced weight of the projectiles; others are elimination of the need for gunpowder, rocket motors, and high-explosive warheads, and the possibility of increasing the number of projectiles ten-fold.

Directed energy weapons, including high-energy lasers, have the potential to deliver lethal force literally at the speed of light. But both solid-state and free-electron lasers will require large amounts of electrical power that only an all-electric warship can provide.

Compact energy storage is critical to the shipboard integration of kinetic energy weapons, high-energy lasers, electromagnetic launchers, high-power sensors, and ship ride-through/fight-through capability. Depending on the specific shipboard application, capacitors, batteries, flywheels, or even a superconducting magnetic energy storage device may end up filling the bill.

Advanced power generation systems could make use of superconducting and/or high-speed generator technologies to increase power density while reducing weight and volume by 50% or more (Figure 7). Shipboard installations would be in the 10 to 30 MW range.

7. Super motor. A 5-MW superconducting generator developed by American Superconductor and Alstom uses 30 K helium gas for cooling. The 2.4-kV, three-phase motor turns at 230 rpm. Operational testing at full power is scheduled for early 2005.
Courtesy: U.S. Navy. 

Electric actuators promise to remove all hydraulic and pneumatic equipment on an electric warship, significantly reducing maintenance and manning. Electric actuation systems will capitalize on, and be fully integrated into, future shipwide reconfigurable electric power and damage control systems.