As the military prepares for future conflicts with near-peer adversaries, it’s starting to explore a new generation of weapon systems and platforms that will create a tactical edge over a pacing enemy. It’s also exploring alternatives to the traditional fossil fuels that it has historically relied on to power these platforms.

The need to replace diesel fuel and other traditional energy sources is driven largely by the fact that fossil fuels cannot be generated on the battlefield. Instead, they need to be extracted, refined, and then shipped to the frontlines where they’re needed. This can create logistical challenges and bottlenecks that slow operations.

As fuel requirements increase and the cost of fuel rises, it’s becoming increasingly difficult and expensive for the DoD to continue supplying fuel to the front lines. It’s also incredibly dangerous. According to an article featured on the Modern War Institute’s Website, “Service members often pay in blood to transport fuel in combat zones. Fuel convoys have been one of the most frequent targets of ambushes and improvised explosive device attacks…”

The weapon systems and platforms of the future will need a more sophisticated fuel source that can generate tremendous amounts of energy over long periods. They also need a fuel source that can be generated in theater. This is why fuel cell systems and technologies are a hot topic of discussion and attention across the U.S. Department of Defense (DoD).

Not only are fuel cell systems more sustainable and efficient, but fuel cell vehicles and platforms can also offer substantial performance advantages over other Li-ion batteries and other propulsion technologies. This can extend the range and reliability of unmanned underwater vehicles (UUVs), unmanned surface vehicles (USVs), and other similar weapon systems.

To learn more about how fuel cell systems function, and how they can revolutionize military operations, The Modern Battlespace spoke with Robert Roy, Program Engineering Manager at Collins Aerospace.

The Modern Battlespace (TMB): Why would the military want to evolve unmanned systems from using Li-ion batteries for power to using fuel cells for power?

Robert Roy: There has been a lot of investment in battery systems to improve their performance, especially lithium-ion batteries. However, our maritime customers are finding that energy storage solutions based on advanced batteries are falling short of mission requirements. They need energy systems with increased endurance and rapid recharge times.

“fuel cell power systems provide positive separation of the energetic materials from the power generator, making hazard mitigation more manageable.” — Robert Roy

Efforts to make these highly energetic battery systems more efficient have resulted in significant system failures – one leading to the loss of a swimmer delivery vehicle for the U.S. Navy in 2008.

TMB: What benefits would fuel cells enable? Why would they be considered superior to batteries for these use cases?

Robert Roy: Interest in fuel cell power systems as an alternative to rechargeable batteries continues to grow and has been primarily driven by three factors.

First, fuel cell power systems provide positive separation of the energetic materials from the power generator, making hazard mitigation more manageable. That’s not to say there still isn’t risk with fuel cell power systems – but well-understood pathways and engineering solutions are readily available to mitigate the risk associated with high energy density reactants.

Second, you can design fuel cell power systems that have higher specific energy and energy density than battery systems. This improvement in the efficiency of the power and energy system allows the vehicle integrator more space for valuable payloads relevant to the mission.

“other benefits that may not be as obvious include quieter propulsion systems and reduced maintenance requirements.” — Robert Roy

Finally, the recharge rate of a fuel cell-based energy storage system is only limited by the rate of reactant transfer, allowing a more rapid vehicle turnaround time than a battery power system that must limit its recharge rate due to thermal considerations. Spare battery systems can address this shortcoming, but at the price of increased logistical support.

TMB: Do you anticipate the use of fuel cells increasing in unmanned vehicles?

Robert Roy: Yes, I anticipate increasing fuel cell use in unmanned vehicles. But other benefits that may not be as obvious include quieter propulsion systems and reduced maintenance requirements.

For example, historically, unmanned aerial vehicles, or drones, have relied on an internal combustion engine (ICE) for their propulsion system. However, these engines are noisy and require frequent maintenance. There is no noise signature for a fuel cell power system providing propulsion for these drones – and maintenance requirements are minimal compared to an ICE.

“Battery systems are typically negatively buoyant, meaning that the vehicle designer must build additional system volume, or design the remainder of the UUV to be positively buoyant as a counter.” — Robert Roy

In UUVs, another often overlooked benefit is that fuel cell power systems can be either neutrally or positively buoyant, allowing efficient use of the energy section hull of the UUV. Battery systems are typically negatively buoyant, meaning that the vehicle designer must build additional system volume, or design the remainder of the UUV to be positively buoyant as a counter.

TMB: What is the “fuel” that fuel cells use to deliver power to an unmanned vehicle? How readily available is this fuel on the battlefield today?

Robert Roy: There are many types of fuel cells that are designated by the electrolyte that conducts charge. Our fuel cell is based on proton exchange membrane (PEM) technology and operates at relatively low temperatures. Hydrogen is the fuel used in PEM fuel cells.

Hydrogen is not readily available on the battlefield today, and hydrogen storage can be complex. Hydrogen can be stored as a pure substance or as part of a compound in multiple ways. Pure hydrogen can be stored as a compressed gas, cryogenic liquid, or supercritical fluid.

The fuel cell power systems employed by automotive original equipment manufacturers (OEMs) store hydrogen in composite pressure vessels charged to 10,000 pounds per square inch gauge (psig) to achieve similar distances between refueling as gasoline-powered cars. Liquid hydrogen has also been evaluated in fuel cell power systems; however, the energy to liquefy and maintain hydrogen as a liquid can be significant.

“system-level trade studies must be conducted to guide selection for hydrogen and oxygen storage; these typically include safety, weight, power, and volume factors, but also logistical factors such as speed of recharge and availability of the resource weigh heavily in these trades.” — Robert Roy

Hydrogen can be stored within the lattice of metals in what are known as reversible metal hydrides. These systems require thermal management during the charge or discharge processes and can be quite heavy. German air-independent propulsion (AIP) submarines utilize metal hydride storage.

Hydrogen can be stored as a chemical, where hydrogen can be released via the input of energy. Examples include water electrolysis, where water can be split into its constituents of hydrogen and oxygen; reformation of hydrocarbon sources to produce a hydrogen-rich gas stream; and hydrolysis and thermolysis of certain hydrogen-containing salts.

Oxygen is required for air-independent applications. As with hydrogen, oxygen can be stored in a variety of ways:

Pure oxygen can be stored as a compressed gas or a cryogenic liquid. The military has significant experience with liquid oxygen for aviators. Compressed oxygen can be stored safely in cylinders with proper precautions.

Oxygen can be stored chemically and subsequently evolved via either thermolysis or catalytic decomposition. Emergency oxygen systems deployed to commercial aircraft and submarines use oxygen candles that produce oxygen from thermolysis.

Usually, system-level trade studies must be conducted to guide selection for hydrogen and oxygen storage; these typically include safety, weight, power, and volume factors, but also logistical factors such as speed of recharge and availability of the resource weigh heavily in these trades.