The Power of Persistence
“Infantry wins battles. Logistics wins wars.”This quote, attributed to U.S. Army General John Pershing in the context of World War I expeditionary operations, is equally applicable today as we move further into the era of autonomous undersea systems. From the age of sail to the nuclear age, maritime power has been underpinned by how far and how long forces could operate forward, and while logistics has traditionally meant sustaining crews and platforms with fuel, food, and materiel, the logistics burden of autonomous underwater operations increasingly centers on one fundamental requirement: energy. It is energy that powers every onboard system – propulsion, navigation, communication, and autonomy – and ultimately enables the mission-critical endurance necessary to maintain a persistent subsea presence.For years, the dominant underwater power paradigm has been stored energy. With their high energy density, scalability, and mature manufacturing base, lithium ion (Li-ion) batteries have been the standard for underwater systems for defense and commercial customers alike. But Li-ion’s long-standing dominance could be giving way as onboard energy production, in the form of hydrogen fuel cells, is steadily gaining traction.While a relatively new innovation for autonomous underwater operations, hydrogen fuel cells have been employed by crewed submarines for years to provide air-independent propulsion (AIP; also referred to as air-independent power). Conventional diesel-electric submarines must periodically snorkel to draw oxygen into their diesel engines, which in turn drive generators that recharge the batteries. But snorkeling makes a submarine vulnerable to detection by surface or air platforms that could be prowling above. Fuel cells solve this problem by enabling a submarine to generate electricity without combustion and without atmospheric oxygen. Stored hydrogen and liquid oxygen feed proton exchange membrane (PEM) fuel cells, producing electricity silently, with water and heat as the byproducts. The result is stealthy, fully submerged power production, and an endurance measured in weeks rather than days. The same characteristics that make fuel cells attractive for conventional submarines- quiet operation, high efficiency, and extended submerged endurance - are now driving their adoption in autonomous underwater vehicles.Unlike Li-ion batteries, which store a finite amount of electrical energy that is gradually depleted during a mission, fuel cells generate electricity for as long as reactants remain available. Although they are generally larger, more complex, and more expensive than battery-only solutions, fuel cells can provide several times the usable energy of a comparably sized and weighted battery system, and can extend underwater endurance from hours or days to weeks, or even months. In addition to higher energy density, fuel cells do not self-discharge during periods of inactivity, which could enable missions involving prepositioned systems in forward areas. And the higher endurance offered by such systems could reduce the total cost of sustaining an autonomous undersea presence. Maintaining that presence would require operating a fleet of platforms on a rotational basis - deploying, patrolling, recovering, replenishing onboard energy stores, and performing maintenance. If vehicles can remain on station for longer periods, and their energy systems can be replenished in minutes rather than hours, operators may require fewer platforms to sustain the same level of persistence and area coverage.The endurance benefits of fuel cells for conventionally propelled AUVs are substantial, but combining them with an unconventional AUV whose unique design already provides extended range and endurance would be a significant achievement. General Atomics has done just that with the G-Ray, a blended-wing underwater glider that minimizes energy consumption through its high lift-to-drag ratio and buoyancy-driven propulsion. Recently unveiled at the Navy League’s Sea-Air-Space conference in April of this year, the G-Ray incorporates a hybrid battery-hydrogen fuel cell system that enables extended endurance and range on the order of months, with potential for even longer durations. Rather than storing compressed hydrogen, the system reacts a lightweight aluminum alloy with water to generate hydrogen, resulting in an innovative on-demand energy system that shifts the endurance constraint away from stored energy to onboard reactant capacity.General Atomics’ G-Ray. Credit: General AtomicsHydrogen fuel cells are no longer confined to underwater vehicles; they are becoming part of the subsea infrastructure that supports persistent autonomous operations. The Teledyne Marine Subsea Supercharger (SSC) is a fuel cell seabed energy depot enabling in-situ power production for sensors, effectors, or vehicle docking systems to allow AUVs to recharge and redeploy without surface recovery. Conceptually, an SSC functions much like a forward logistics node, providing the sustained energy that allows a regional autonomous undersea force to remain on station. The idea is not entirely new. Nearly a decade ago, the U.S. Navy proposed the Forward-Deployed Energy and Communication Outpost (FDECO) concept to support persistent anti-submarine warfare (ASW) and intelligence, surveillance, and reconnaissance (ISR) operations.Teledyne Marine’s Subsea Supercharger. Credit: Teledyne MarineFor some companies, subsea hydrogen fuel cell power represents not only a game-changing product differentiator, but also an emerging market opportunity. In June of this year, Cellula Robotics and Infinity Fuel Cell and Hydrogen Inc. unveiled NautiGEN, a joint venture focusing exclusively on providing fuel cell power solutions for maritime autonomous systems. Windsor, Connecticut-based Infinity is a developer of fuel cell systems for aerospace, defense, and maritime applications, while British Columbia-based Cellula has been at the forefront of commercializing fuel cells through its family of long-endurance AUVs. Cellula and Infinity have previously partnered on fuel cell technology; in recent trials of Cellula's Envoy LD-AUV, the vehicle completed a submerged transit exceeding 2000 km over a mission duration of 385 hours, during which it executed over 4000 energy-consuming turns and maneuvers. The vehicle was powered by Infinity's Mystic 2000 AUV power system, which leverages fuel cell technology originally developed for aerospace applications before being adapted for maritime use. The creation of NautiGEN reflects growing confidence that fuel cell power has matured into a viable commercial market rather than remaining a specialized enabling technology.NautiGEN is a joint venture between Cellula Robotics and Infinity Fuel Cell and Hydrogen Inc. focused on providing fuel cell power solutions for maritime autonomous systems. Credit: Cellula RoboticsEndurance is the lynchpin of autonomous subsea operations. It determines how long a platform can remain on station and the size of the area it can effectively monitor or influence. Whether trailing enemy submarines, conducting long-range seabed strike missions, or blockading a strategic chokepoint, AUVs will need access to high-density onboard energy to ensure mission success. Hydrogen fuel cells represent a shift from underwater platforms that store electrical energy to platforms capable of generating electrical power throughout a mission. While they may not come to replace Li-ion batteries in subsea operations, fuel cells could be integrated into hybrid power architectures like that of the G-Ray, with fuel cells providing high-energy, long-duration baseline power, and batteries supplying rapid bursts of electrical power for maneuvering, buoyancy control, communications, and payload operation. Whether fuel cells become the dominant energy solution for the broader autonomous maritime sector remains to be seen, but they appear poised to play an increasingly important role in establishing and sustaining a credible, persistent subsea force.