Electrifying the US Navy: meeting future demand
From meeting the need to improve fuel economy and enhance operational flexibility to preparing for the power-hungry radar, communications and energy weapons systems of the future, the US Navy has been steadily surging ahead with electrification. Dr Gareth Evans examines the push towards electrification over the last decade.
According to the US Navy's Electric Ships Office (ESO), established in 2007 to develop and provide the new kinds of power capabilities required, integrated power and energy systems (IPES) will play an increasingly important role in tomorrow's fleet.
"The navy's shift to IPES is driven by a number of factors, the most important being warfighting capability," says ESO Director, Steve Markle. "The selection of ship power architectures depends upon the mission systems that have been selected for the platform based fundamentally on the threats that the ship is designed to combat."
Markle explains that the process involves first decomposing those threats into required capabilities, which are then devolved into technologies, and finally the power system architectures are designed to meet these technology considerations.
"Recent advances in technologies associated with pulsed high power sensor and weapons systems and their inclusions in future ship designs are driving us toward the IPES," he says.
Big steps forward
It is a fast moving arena; even within the two years between the release of the 2013 Naval Power and Energy Systems Technology Development Roadmap (NPES TDR) and its most recent biennial update in 2015, there have been a number of significant developments.
In October 2014, the amphibious assault ship USS America (LHA 6) was commissioned - the first of its class - with a hybrid electric propulsion system coupling two 35,290 shaft horsepower gas turbines and two 5,000 horsepower auxiliary propulsion motors. The DDG 51 hybrid electric drive system successfully completed its factory acceptance test and the hardware was duly delivered to the Naval Ship Systems Engineering Station in January 2015 and at the end of the same year, the USS Zumwalt (DDG 1000) - the US Navy's first modern all-electric warship - began at-sea trials in the Atlantic.
Markle says that there are other benefits to be gained from an IPES, including increased operational efficiencies throughout the ship's operating profile, because electrical equipment is operated nearer to its peak efficiency.
His predecessor at the ESO, Dr Timothy McCoy, observed that historically on-board generation capacity has grown exponentially, and at the same time, the percentage of ship real-estate involved in electric power production and distribution had itself also grown in relation to everything else. While acknowledging that forecasting the future is seldom a straightforward task, Markle suggest that this trend may now be about to end.
He says that as the navy moves away from mechanical drive ship architectures to electrical ones, there will inevitably be a growth in the volume and weight of electrical equipment, but there will also be a corresponding decrease in the non-electrical ship propulsion elements.
"This transition has already occurred, so I would expect the net increase in overall percentage of the ships machinery plant devoted to electrical to stay constant or actually decline," he adds.
Increasing power density
The ESO is working to simplify electric power components and make them more power dense in line with the time-phased goals set out in the NPES TDR document, and Markle thinks that this could see the demand for ship real-estate, on a per megawatt basis, fall significantly.
He points to industry's shift towards wide-band gap materials such as silicon carbide which enable power densities to be increased two- or three-fold, with the additional benefit of operating at a higher temperature, which means a lower need for cooling.
"If we can achieve advances in power density that are affordable to meet our needs we will be able to reduce the percentage of space and weight devoted to electrical power systems as we move forward," Markle says.
The road ahead
The 2015 NPES TDR lays out the goals and challenges along the way. High on the list for the years to 2025 is the full-scale demonstration of the energy magazine concept introduced in the previous roadmap - a common, modular and scalable intermediate power system for use across multiple mission systems and ship installations.
Development goals over the period include advanced medium voltage DC circuit protection and power distribution systems, energy storage and recovery technologies, and advanced controls able to interface with ship combat systems and integrate multiple generation and storage devices with dynamic power demands.
In the mid term, from 2026 to 2035, the emphasis is expected to be on the engineering development and integration of actual ship hardware for future platforms, focussing on building and testing a variety of engineering development models and first article production components for the critical technologies. Additionally, the IPES electric and control systems will undergo detailed testing to validate their performance capabilities, and, assuming the underlying technology has sufficiently matured, shipboard energy recovery systems will also be demonstrated at full scale.
Looking to the long term
Looking further ahead still, to 2036 and beyond, the navy anticipates that several significant developments will be likely to have taken place which will have implications for the power demands in the long term.
Platforms will operate a number of high powered mission systems simultaneously, a range of vessels will have been introduced that are flexible and modular in design and able to accommodate multiple payload packages and advances in energy storage and computing will allow ships to become increasingly 'smart' and autonomous.
Progress towards meeting these objectives is already well underway. Markle says that the navy has issued a request for information, looking for power dense, energy-efficient 20-30MW gas turbine generators in an attempt to increase its power density and efficiency. Work is also underway to realise the energy magazine concept for both future designs and to retro-fit on existing vessels, and active power and energy control systems that meet emerging cyber-security requirements to ensure continuity of operations in a hostile environment are receiving priority, too.
"The future is bright for naval integrated power and energy systems," Markle says. "Warfighting need for power will drive the electrification of warships. The opportunities we have are to operate in a more power efficient manner than in the past."