Hybrid propulsion has been used in naval propulsion for some time and classes of ship such as the Royal Navy’s Type 23 Duke-class frigates, designed in the mid-1980s, were fitted with
a hybrid system to boost anti-submarine warfare (ASW) capability. The hybrid systems available now are more advanced, but Richard Partridge, chief of naval systems at Rolls-Royce, told E&H Marine that not all warship types will become hybrid electric.
“There will be some new ship programs that select mechanical, as maybe there are not the anti-submarine weapons [ASW] requirements for exceptionally low underwater signatures that would drive them down the hybrid route. Hybrid can have a slightly higher initial cost than a mechanical arrangement, and the challenge is that some navies still select the propulsion arrangement based on initial cost and often don’t factor in the whole-life cost or even the first five years,” he says.
Partridge believes that for most capital warship programs, building frigates and destroyers with displacements from about 3,500-8,000 metric tons, a hybrid solution can be the best option – and not just in terms of lifetime costs and ship signatures. “Most navies see the operational benefit of hybrid and given the reference of the past 10 to 15 years, when for FF/DD [frigates/ destroyers], most navies have gone down that route, it is not just a flash in the pan. There is a discernible trend away from mechanical to hybrid and full electric,” he says.
The potential of hybrid
Since the 1980s, the hybrid system design has progressed and the new RN Type 26 frigate uses a direct motor driveshaft, with one motor per shaft, but using AC motors instead of DC and employing pulse width modulators (PWM) power converters and similar technologies that have become the mainstay of hybrid propulsion systems.
“One other interesting trend is that the earlier T23s use twin-gas turbines (Rolls-Royce Spey, 12.75MW per engine) configuration, and we now see that all the navies that went hybrid in the 1990s selected single GT propulsion: a hybrid shaft-mounted motor and main reduction gearbox. Rather than twin GTs, they adopted a single higher-power-density GT with a cross-connect gearbox, to split the power from the turbine equally to both propulsion shafts,” Partridge adds.
According to Partridge, the Rolls-Royce MT30 GT has enabled this arrangement and Rolls-Royce has worked with shipyards to address survivability, vulnerability and redundancy concerns. Its analysis has found that with a hybrid system using a single gas turbine, there is as good a level of survivability as a conventional twin GT mechanical arrangement, and in some cases even better. There must, however, be adequate physical and functional separation of other components, such as cable runs, and juxtaposition of generators, switchboards, transformers and power converters.
“Most affordable platforms below 7,000 metric-ton displacement, even in a twin-GT mechanical arrangement, don’t have the luxury of the GTs being configured in different machinery spaces, but they will be co-located in the same machinery space, keeping the ship affordable in terms of length and displacement. With fire and action damage in the gearbox or GT room you would lose either the single or twin GTs. So it comes down to serviceability, reliability and the ability to achieve adequate speed of advance on the electric motors,” he explains.
In addition, a single GT means less ducting, which releases space on board for mission systems. Most ships now have a multifunctional role, unlike the single-role ships of the Cold War, and are fitted with ASW, land attack and possibly anti-aircraft weapons (AAW), so having a single GT has strong platform benefits.
The reliability of the MT30 comes from a series of developments undertaken in the Aero Trent 800 series since the 1990s, and has allowed navies to be more confident in relying on a single GT. The MT30 also has a higher power density when rated at 40MW, so ships can have a combination of diesel electric or GT (CODLOG) arrangement rather than a combined diesel electric and GT (CODLAG) arrangement. Partridge explained that CODLOG means the electric propulsion motors (rated at up to 3.5MW each) can handle propulsion up to about 17-18kts, with the GT taking over for fast or maximum speeds. With a less power-dense GT under a CODLAG arrangement, to attain those top speeds the electric motor continues to run alongside the gas turbine. This means that less of that approximately 12MW installed electric power generation (typically spread across four generator sets) can be devoted to the hotel and mission systems load, reducing the ability to incrementally add more sensors and weapons in the future.
“In CODLAG, you have to supplement the power from the GT with that from the motors,” Partridge says. “What that can do is increase the complexity, cost, size and mass of the power converter, and there will be different additional machinery states. So the control system becomes more complex for normal machinery states and machinery damage, and crew training is more complex too. The adage ‘keep it simple, stupid’ definitely applies to designing propulsion for warships and the MT30 provides the opportunity to do that.”
CODLOG future-proofs ships to a greater extent, as it will enable the addition of systems without many costly modifications to the power generation and distribution system. With the advances in hybrid electric propulsion and the substantial lifetime cost savings realized so far, it is not surprising that navies want to extend the electrical plants to a full electric propulsion system and include all ship speeds. However, full electric propulsion is not suitable for all ships.
“One of the things about full electric is that because of the power needed from the power converters and the size of the electric motors that are required for top ship speeds of 29-30kts in a typical frigate or destroyer, the system needs high voltage rather than low voltage, so you have a significant amount of electrical equipment installed, as well as the large power generation system,” Partridge says.
This means using GTs rather than diesel gensets due to their power density, but because of the size of the electrical components, the system requires more volume, therefore only lending itself to larger ships. Partridge highlights the RN Type 45 destroyer, in excess of 8,000 metric ton displacement, the USS Zumwalt-class destroyer over 16,000 metric tons, and the RN Queen Elizabeth-class [QEC] carriers at over 70,000 metric tons. “Unlike smaller ships, these have room for the sizeable motors, power converters, electrics and GT gensets,” he says.
“Companies in the industry, the electrical system suppliers and GT genset suppliers like us, are investing in making the technology more compact and power dense, or in the case of motors more torque dense. But I can’t see full electric propulsion where there is a need
for 28-29kts, even 30kts, becoming the mainstay any time soon, mainly because of the physical fit and insulation factor in smaller frigates and destroyers.”
Full electric propulsion, however, means fewer engines per ships. This is important for platform design and lifetime costs – not just fuel savings but also maintenance – as it means potentially smaller engineering departments and a leaner crew – something that is critical to warship whole-life cost reduction.
“The RN QEC at 70,000 metric tons has just six engines installed per platform, compared with 12 on the smaller 20,000 metric ton RN Invincible-class carriers,” Partridge says. “You can see the difference there, going from mechanical to full electric from one generation of carrier to the next. Also with QEC, by virtue of the power density of the MT30, we install them below the two islands in the sponsons, the outboard part of the ship’s hangar, which minimizes the impact of GT ducting, reduces the ducting losses – which is good for efficiency – and also massively increases survivability by introducing not just longitudinal but also vertical separation.”
Volume and mass
The use of hydrogen fuel cells is unlikely to have much impact on naval propulsion in the short or medium term. Partridge says that he examines progress in fuel cell technology every few years to see if there are advances applicable to navies, but the main problem is the size and lack of power density.
“To my mind one of the critical aspects of generators on a warship is power density,” he says. “It’s about getting maximum power from a compact unit. Fuel cells seem to be big; they are not power dense at all at the moment. And it’s not just the fuel cell units – it’s the fuel conditioning, processing and hydrogen storage too. The power density currently available means that fuel cells don’t yet lend themselves to warships.
“The other key aspect is the load-following capability of fuel cells, which like steady-state operations with a near-constant power demand. A warship is an island power generation system, with a lot of substantial electrical loads, not just propulsion. There are also air-conditioning, aircraft lifts, fire pumps and so on. When you start and stop those, there is a step change in the electrical load demand and thermal transients do not suit hydrogen fuel cells at the moment.”
While hydrogen is probably only suited to shore-side power, the future of hybrid propulsion is strong and full-electric propulsion will develop further. However, Partridge believes that the overriding factor should be the operational profile of the ship – everything depends on what the ship is tasked with doing and how it is intended to perform. This, coupled with ship design aspects, is what should dictate whether the selected propulsion system remains mechanical, or goes for hybrid or full electric in future warship programs.