Navy to Test Hybrid-Propulsion on Destroyers
http://defensetech.org : by KRIS OSBORN on AUGUST 6, 2013
The U.S. Navy plans to conduct a series of tests on its hybrid-electric propulsion system for destroyers in order to assess its potential for future application on one of the ships’ two propellers, service officials said.
The technology, now in use on the USS Makin Island and being engineered into the next-generation America-class, big-deck Amphibious Assault Ships; the USS America (LHA-6) and the USS Tripoli (LHA-7) are engineered with a hybrid-drive propulsion system, meaning the ships can use both diesel electric propulsion as well as gas-turbine engines.
“We’re beginning to explore the possibility on some other surface combatants such as DDGs (destroyers) and we’ll be looking to do more of those tests over the next few years. If those prove out as they have aboard the amphibs, that is something that we’ll look to take out to more and more DDGs during their upgrade cycle,” said Tom Hicks, Deputy Assistant Secretary of the Navy, Energy. “Based on the success of the Makin Island, we’ll look to begin to identify ships that are in their mid-life upgrade where we could bring that type of technology aboard.”
When it comes to ship propulsion, hybrid-electric propulsion involves a gas turbine engine as well as an electric motor and diesel generator. The electric motors can help propel the ship at speeds up to around 12 knots, and the generator can generate electricity for the ship.
When traveling at speeds greater than 12 knots, the ship can then rely upon its gas turbine engine. At the same time, the generators can also provide on-board power for many of the ships systems such as sensors, weapons and other electronics, according to Navy officials.
Much like their amphibious counterparts, the DDGs are equipped for missions likely to require moving at slower speeds, potentially closer to shore, Hicks explained.
“We’d be looking to potentially put hybrid-electric drive aboard one of the two propellers on a DDG. We think that is all that is going to be necessary to get the maximized impact. Those types of surface combatants do spend a fair amount of time not operating at high speeds, so that seems to be a perfect sweet spot for hybrid electric drive,” Hicks said.
Another energy-efficiency technique being utilized by the Navy is the addition of what’s called a “stern flap,” essentially an additional piece of the ship which changes the flow characteristics under the boat, impacting how water flows under and around the hull, Navy engineers explained.
Hicks also talked about anti-corrosion hull coatings and paints which make the surface of the hull more slippery and therefore able to more smoothly glide through the water.
“Hull coatings or propeller coatings are things which make the ship more resistant to the turbulent effects of the water,” Hicks said.
Each of these innovations wind up reducing the amount of fuel needed to propel the ship, Hicks added.
Another energy-efficiency increasing innovation is something called “Smart Voyage Planning Software,” a software program able to maximize route efficiency by calculating and integrating a wide swath of weather conditions and environmental factors likely to impact ship propulsion.
“We’re getting more fidelity in terms of the data that exists out in the ocean. All of the conditions that exist such as water conditions, current, temperature and wind all really have an impact on a ship’s ability to reach certain points. The idea is to look at all the environmental factors that impact the ship’s ability to get from one point to another,” Hicks explained.
US Destroyers Get Hybrid Electric Drive (HED)
The US Navy’s DDG-51 Arleigh Burke Class destroyers are the backbone of their present and future fleet. Unfortunately, they have a problem. Two problems, actually. One problem involves keeping the fleet’s operating costs low, especially fuel use. These ships aren’t as efficient as more modern designs, especially at the low cruising speeds that make up so much of a ship’s operating life. The other problem is that DDG-51 destroyers are having trouble generating enough power, especially if they want to receive new radars like the proposed AMDR. Since this destroyer class includes the majority of American ballistic missile-defense ships, their ability to remain relevant, and to field an upgraded “Flight III” variant, is critical to maintaining US sea power.
Low Speed Problem
Right now, DDG-51 destroyers are fielded with a ship service electrical system, and an independent main propulsion system of LM2500 gas turbines that are tied to a mechanical drive through the Main Reduction Gear assembly. Each shaft is tied to 2 LM2500 gas turbines (GTMs), which have just 2 speeds: off, and on. Another 3 ship service turbine-generators (GTGs) provide electrical power, with the 3rd designed as a redundant back-up. Using this mechanical arrangement, current DDG-51 Flight IIA ships have a reported total power output of 7.5 MW, and end up using too much effort from their LM2500 gas turbines for propulsion at low speeds.
During underway operations under 15 knots, in low-threat areas, 2 engines are typically on line: a GTM with a trail shaft, and a smaller GTG for basic power to the ship, navigation radars etc. Speed changes up to 15-18 knots are controlled by varying propeller pitch, and are independent of the LM2500 GTM. For more electricity, another GTG generator can be brought online to power the main SPY-1 radar if needed.
At low speeds, Hybrid Electric Drives would allow ships to take the GTM offline, and rely on 1-2 smaller GTGs for both propulsion and power, using less fuel and offering more power flexibility. Ships could also be designed with fixed-pitch propellers, which are quieter than variable-pitch blades.
During July 31/08 testimony before a House Armed Services Committee panel, Congressional Research Service naval analyst Ron O’Rourke said that adding hybrid electric drive to the DDG-51′s traditional mechanical-drive propulsion could reduce fuel use by about 16%, cutting each ship’s annual steaming cost by abut $2.5 million (at $112.14 per barrel).
That 7.5 MW power output is also a problem. Advanced radars and their cooling systems need a lot of power, especially if they’re slated to perform long-range missile defense roles. That has emerged as a problem in studies to create a more modern DDG-51 Flight III design.
In contrast, the new DDG-1000 Zumwalt Class’ integrated all-electric system can divide up to 78.0 MW of power between propulsion and power to the ship’s electronic systems, offering more efficiency at low speeds, and more power for ship systems at both low and high speeds. Unsurprisingly, all-electric drive systems are a growing trend in military ships. They’re almost the rule in new European warship designs, and can be found in new American designs like the DDG-1000 Zumwalts, LHD 8 Makin Island, the new America Class escort carriers, and T-AKE support ships.
As a bonus, Hybrid Electric Drives (HED) propulsion is less noisy, which is useful when a ship is trying to deal with enemy submarines.
A hybrid-electric drive could theoretically be backfit into existing DDG-51 destroyers, to give them many of the same advantages as counterparts like the DDG-1000s. The catch is that refits dealing with propulsion systems tend to be expensive, because the builder usually has to cut into the hull to move engines around, etc. Still, the class is already going through a major modernization program, and with estimates of a mission-ready, BMD-capable DDG-51 hovering around $2 billion, an HED backfit could still be seen as a bargain. Much depends on how much of a ship’s electrical distribution system, which runs throughout the ship, would need to be replaced.