Sea Power 2010: The future of batteries | ADM Apr 2010
Gregor Ferguson | Sydney
The inescapable truth about submarine design is that the battery is an absolutely fundamental part of the design because it affects and shapes every single aspect of the platform and combat system: the main propulsion motor, generator capacity, the electrical distribution system, performance and submerged endurance, the boat's magnetic signature, its stability and trim, combat system capacity and cooling, hotel services and, of course, whichever Air Independent Propulsion (AIP) system the boat incorporates.
The Design Stage of the Navy's Future Submarine Project - Sea 1000 Phase 1 - is looming.
The Initial Design Stage currently under way has identified some of the likely risk and technology boundaries within which the Future Submarine design effort will need to work.
One of the boundaries is likely to be a requirement that all equipment proposed for the Future Submarine be at a Technology Readiness Level (TRL) of seven or greater (system prototype in operational environment) at the time of First Pass Approval.
This milestone is anticipated around late-2011 or 2012, which means that any technology which is developmental today may - just - be mature enough for consideration.
This in turn suggests that the initial submarine design will be relatively conservative, embodying incremental improvements of current proven technologies, rather than any disruptive technological breakthrough.
However, bearing in mind that the Future Submarine is likely to be built in successive batches or ‘flights' of three or four boats each, this could enable propulsion and energy storage technology upgrades with each successive batch.
A typical lead-acid battery can occupy 6-8 per cent of the hull volume, but account for 12-15 per cent of the boat's displacement and is laborious to install, test and remove - on the Collins class the Main Storage Battery (MSB) consist of 400 pairs of cells, each weighing over a tonne, which must be painstakingly installed along the keel of the submarines.
This fact alone makes battery type and size a fundamental design determinant, so the early selection of a battery and propulsion system lies squarely on the critical path for Project Sea 1000.
The starting point for the design process is, of course, the mission.
A conservative assumption is that a typical RAN patrol could start and finish with a 3,000nm transit to the patrol area.
Once on station the boat is likely to spend a considerable amount of time at ‘patrol quiet' speeds, with the occasional fast transit from one task or patrol area to the next.
An AIP system comes into its own in the patrol quiet state, but is less likely to make a useful contribution to mission endurance and battery longevity during a long transit.
It's these lengthy transits, snorting from time to time to recharge the batteries, which distinguishes Australia's submarine operations from those of most European nations.
This is one of the determinants of power train characteristics, including the diesels, generators and battery type and capacity.
New technology
There is a range of alternative battery technologies under development.
Thanks in part to the cellular telephone industry, Lithium-Ion (Li-Ion) battery technology has developed apace in recent years, though not yet at the 20 megawatt-hour scale required for submarine propulsion.
This battery type offers very good energy storage and, thanks to its internal chemistry, needs much less maintenance.
It also has high charge acceptance enabling rapid and near-complete recharging while snorting with powerful diesel generators, and also allows very high submarine speeds to be maintained for much longer than existing lead-acids.
And it offers a long service life - up to 10 years, by some estimates.
The design challenges Li-Ion batteries pose are a function of their maximum cell size and need for individual monitoring and control which means a boat the likely size of the Future Submarine will need some 15,000 battery cells, a complete new battery and electrical distribution system architecture, as well as a different design approach to the battery compartment itself.
The inherent risk of thermal runaway under certain fault conditions and the consequent risk of fire (as happened recently with a US Navy mini-submarine) means the cells themselves may need some separation and a protective casing which could affect volume also.
However, variants such as Li-Polymer cells developed by German firm GAIA and Li-Titanate from US company Altair-nano show considerable promise.
The former out-perform lead-acid batteries quite significantly in terms of capacity, and especially available capacity at high discharge rates.
It's a characteristic of different battery types that their ability to deliver their full capacity depends on the rate at which they're being discharged.
A fully charged lead-acid battery may deliver only a proportion of its nominal charge when the boat is sprinting away from a contact, but will deliver every last kilowatt-hour when it is being discharged gently at patrol quiet state.
Lithium-Polymer batteries can out-perform lead-acid ones by as much as 300 per cent at high discharge rates, and by as much as 20 per cent at patrol speeds.
For all their advantages in certain areas, Li-Ion batteries could cost two or three times as much as a conventional lead-acid battery over the life of the boat.
More importantly, no other major submarine program currently plans to adopt Li-Ion or Li-polymer MSBs; would the RAN wish to be a pioneer in this critical area?
Similarly, the Sodium-Nickel-Chloride or ZEBRA batteries fitted to the NATO Submarine Rescue Vehicle offer significant advantages, but a couple of significant disadvantages also.
On the credit side, this technology offers exceptional energy storage, benign failure modes, a life of up to 15 years and thousands of recharge cycles and very high reliability.
Through-life cost would be the same as a lead-acid battery, according to industry estimates.
On the debit side, they need to be kept at 350o Celsius to operate, and the IP behind them hasn't really been extended as yet to embrace submarine propulsion.
Considerable development work would be required for a submarine application.
AIP - only part of the solution
What does AIP technology mean for submarine design?
The energy density of a typical AIP installation can be two or three times that of a battery, but AIP systems deliver lower peak power.
Given the volume and weight constraints on a submarine design, the limiting factor in AIP power delivery is actually the storage required for the liquid oxygen most AIP systems use.
So for all its energy storage capability, the biggest difference an AIP system can make is at patrol quiet speeds.
Those speeds are classified but within a representative speed spectrum, an AIP system could increase the submarine's submerged patrol quiet endurance by a factor of five or more, reducing as speed increases.
If the concept of operations allows, this means that the submarine's main battery could be optimised for long transits, snorting en-route, and for short-range sprints.
New technology could also reduce the battery maintenance requirements considerably.
However, one of the greatest determinants of submerged endurance at a given dived speed is the power demand of the boat's hotel services: halving a typical hotel load could double submerged endurance of modern lead-acid batteries at patrol quiet speeds.
This is a fundamental design issue which needs to be addressed in detail at the same time the battery and propulsion requirements are being hammered out.
Back to the basics
One battery technology not discussed so far is improved lead-acid.
Relatively simple mechanical design and battery chemistry improvements can deliver incremental improvements in existing lead acid battery capacity, energy density and, importantly, sprint discharge rate capacity.
A 15 per cent capacity increase at patrol speeds is useful; a near-100 per cent increase at sprint speeds is potentially transformational as far as traditional submarines are concerned.
However, the recent emphasis in northern hemisphere submarine research has shifted to AIP technologies.
Development has largely not been possible with submarine batteries, because of design limitations of submarine systems to accept performance upgrades and a customer focus on delivering reliability at low cost.
But it would be unwise of the Commonwealth to ignore the advantages of an improved lead-acid battery, especially given its likely very conservative approach to technology risk in the Future Submarine project.
Lead-acid batteries have good energy density at low discharge rates; they are scaleable to any size the platform design dictates, which means that architectures can be relatively flexible while the installation and removal route can be optimized (remember each cell in a Collins-class boat is approximately the size and weight of an upright piano); their charging system can be designed to suit a range of diesel and generator configurations; and they have a proven safe life: over eight years has been demonstrated.
From the engineering management point of view, the RAN and the industry are used to lead acid batteries, and especially the handling of acid and hydrogen.
And the batteries themselves can be made very tough - sufficient to withstand easily the sort of shocks that submarines can be subjected to.
There are disadvantages, however: if you try to recharge a lead acid battery too quickly - when snorting, for example - you may not get more than 70 per cent of a full charge: batteries prefer being recharged slowly.
They also have a reduced apparent capacity at high discharge rates.
Nevertheless, given the schedule for 1st and 2nd Pass for at least the first batch of three or four boats, a propulsion system based on low-risk improved lead-acid battery technology augmented by a proven AIP system might deliver the initial capabilities the RAN seeks.
If, as most observers anticipate, the Future Submarine will actually be a family of submarines built in successive batches with incremental technology insertions, then a path exists for a research program designed to capture technology improvements and incorporate these in successive batches of the Future Submarine.
This path could run parallel to the development program for the batteries and propulsion system for the first batch of submarines, and deliver new or enhanced propulsion and energy storage capabilities into the second and subsequent batches.
This in turn would satisfy two potentially conflicting requirements: a low-risk solution with minimal impact on project schedule, on the one hand, and a technically advanced solution which delivers a significant capability edge, on the other.
The right approach could allow Australia to have its cake and eat it, albeit in successive courses rather than all at once.
Pacific Marine Batteries eyes Future Submarine challenges
Gregor Ferguson | Sydney
One of the Australian companies at the core of the Collins-class submarine project has been studying technology options for its successor for several years already.
Pacific Marine Batteries Pty Ltd, whose factory is right across the road from ASC, in 2008 received a contract to support Defence's Sea 1000 preliminary studies by examining the battery and propulsion issues confronting the project.
Its research has highlighted the promise of new battery technologies, as well as the enduring strengths of existing lead-acid battery technology and the benefits to be gained in the short-medium term from incremental improvements to this technology.
The company was established at Osborne, SA, in the late-1980s to build the 420-tonne main storage batteries for the Collins-class submarines.
Its founding shareholders were Swedish battery manufacturer Varta and local company Pacific Dunlop.
However, PMB has undergone a change of ownership and a restructuring that has transformed it from being simply a battery manufacturer to a somewhat broader energy business.
That broader interest reflects the company's recent R&D investment in stored energy and Air-Independent Propulsion (AIP) systems and underscores the fact that it is the only battery manufacturer in the world whose exclusive focus is on submarine propulsion.
Three years ago a new team of Australian, US and Swedish owners led by Dr Chris Abell (of Vision Abell fame) and chairman David Hills set PMB on its new direction.
Former GD Land Systems (Australia) head Paul Merrow was appointed CEO and expanded the local engineering team as well as the company's global network of technology and business links.