As part of an ambitious technology plan for the Navy fleet of the future, the Office of Naval Research is exploring ways to power all-electric ships.
The Navy wants to develop these ships to make more efficient use of on-board power and to cut fuel use. The technology will also help meet future requirements for high-power weapons such as the electromagnetic gun, high power microwave and high energy lasers, said John Pazik, director of the ship systems and engineering division at ONR.
The all-electric ship effort is still in its infancy. ONR plans to roll out new power systems by fiscal year 2012, said Richard Carlin, head of ONR’s sea warfare and weapons division. “At this point there’s no acquisition, so it’s more of a technology push.”
The program is known as the next-generation integrated power system (NGIPS). It takes electric propulsion technology and combines it with other energy efficient power systems throughout a ship. “We’re starting this process of thinking how we map out what the future electric Naval force is going to be,” Pazik told National Defense.
For example, some ships use auxiliary systems that are steam powered, hydraulically powered, or pneumatically powered. Converting those systems to electrical power and combining them with electric drive propulsion would produce an all-electric ship.
Analysts predict that electric technology will have many benefits.
“Electric drive offers significant anticipated benefits for U.S. Navy ships in terms of reducing ship life-cycle cost, increasing ship stealthiness, payload, survivability and power,” wrote Congressional Research Service analyst Ronald O’Rourke in a report, “Electric-Drive Propulsion for U.S. Navy Ships.”
Today, most military vessels use mechanical drive. These systems convert the engine’s high-speed revolutions per minute (RPMs) to low speed RPMs using a set of gears, O’Rourke said. Ships with mechanical drive systems actually have two sets of engines. One set is used for ship propulsion. A second and separate set, connected to generators, is used to create electricity for all of the electrically powered equipment on the ship.
With an electric drive, a generator converts the engine’s high speed RPMs into electricity. Ships with such a system can be designed so that a single set of engines produces a common pool of electricity for use by the ship’s propulsion and non-propulsion systems.
The Navy first broached the technology in January 2000 when it selected electric-drive propulsion for use on its next-generation DD-21 — now DDG-1000 — destroyer, O’Rourke said.
Using this technology for Navy surface ships is not a new concept, he noted. It was first used on some Navy ships in the early 20th century, and again during World War II.
O’Rourke said that electric-drive technology was adopted by the cruise ship industry beginning in the late 1980s, and that today, most if not all cruise ships under construction are being built with electric drives. Electric propulsion also is found in icebreakers and floating offshore oil platforms, and is becoming more common in passenger and car ferries.
Other commercial ships now being built with electric drive, O’Rourke said, include shuttle tankers, pipe and cable-laying ships, and research ships.
The technology has been reintroduced into the U.S. military surface fleet by the Navy’s Lewis and Clark TAKE-1 dry cargo ships, which employ a modified version of a commercial integrated electric drive system. The first TAKE was delivered in 2006.
A 2001 study found that fitting a Navy cruiser with more energy-efficient electrical equipment could reduce the ship’s fuel use by 10 to 25 percent, O’Rourke said in another CRS report, “Navy Ship Propulsion Technologies: Options for Reducing Oil Use.” In the 2007 Defense Authorization Act, Congress directed the Navy to use integrated power systems, fuel cells and nuclear power as possible propulsion alternatives for future surface combatant ships.
ONR has created five “focus areas” for researching advanced naval power for the all-electric ship. They are power generation; distribution and control; energy storage; heat transfer and thermal management; and motors and actuators, Pazik said.
For power generation, the agency is interested in fuels cells, advanced generators, solar power and alternative fuels, said Pazik.
The Navy’s existing gas turbine engines operate at 16 to 18 percent efficiency because ships typically sail at low to medium speeds, O’Rourke said. In contrast, the fuel cell system that ONR is developing will be able to operate between 37 and 52 percent efficiency. This means the Navy could see substantial fuel savings.
“The Navy has estimated, using past fuel prices, that shifting to fuel cell technology could save more than $1 million per ship per year in ship-service fuel costs,” O’Rourke said.
ONR scientists are also studying power distribution and control. They are interested in creating an electrical system architecture that is based on a main bus, and distributes DC power through the ship. Such a system would allow energy to move around in a more efficient way.
The third focus area involves storage technologies such as batteries, capacitors and flywheels. In future electric ships, short pulses of power will run weapons such as the rail gun. “We might want to have or be able to release a huge amount of energy really quickly,” said Pazik.
Capacitor technology will also provide the ability to release energy slowly. “So if you have power surges, or spikes, you can commission out that power through a capacitor bank that gives you a constant power, that stores and releases very slowly,” Pazik said.
High-speed generators are another priority at ONR. Navy ships currently run off of two diesel engines so that the ship will never go “dark” or lose power, Pazik said. A high-speed generator/diesel engine combination would save fuel and run more efficiently. One diesel engine would be running, the second one would be turned off, and the generator would be constantly moving. If the first engine failed, the stored energy in the generator could keep the ship going while operators start up the second engine, he explained.
Another research effort involves motors and actuators. ONR is evaluating three variants: the advanced induction motor, permanent magnet motor or high temperature superconducting motor. The Navy plans to use the advanced induction motor in the DDG-1000, said Carlin.
ONR recently tested a 36.5-megawatt high temperature superconducting motor, Pazik said. Program engineers previously built a smaller 5-megawatt motor, which was studied and further developed by the electric ship research and development consortium, a group of university engineers and scientists.
Researchers are also studying high power density electromagnetic actuators. These systems use electric drive to perform mechanical motions to move various ship components, Pazik said. This technology has high torque density and could readily move large systems on board an electric ship. It could replace hydraulic actuators used on existing ships.
Heat transfer and thermal management projects focus on technologies that discard, reuse and store waste heat, Pazik said. Thermal management is critical to an electric ship design because as power density increases, heat generation is confined to smaller and smaller volumes. “Overheating can become a problem,” said Pazik. Technologies in development include thermal electric cooling, spray cooling, microchannel cooling and advanced heat pipes.
Navy scientists hope that the next generation integrated power system will decrease fuel use. However, electric ships will probably be more expensive that current vessels.
Today’s electric-drive propulsion equipment, O’Rourke said, might be bigger and heavier than equivalent mechanical-drive devices, but the size and weight of electric-drive systems is expected to come down over time, and might eventually be comparable to that of mechanical-drive equipment. Meanwhile, he said, the fuel savings and other advantages of integrated electric-drive propulsion can offset the size and weight penalty.
Development of an all-electric ship, O’Rourke said, is a feasible goal, and is also being pursued by the United Kingdom’s Royal Navy.