Open Water Power is developing a new aluminium-based battery that uses water itself as a power source. It believes the technology could offer a tenfold increase in AUV range
Underwater vehicles are taking on more tasks than ever, but power facilities and mission requirements still tend to separate them into two broad camps: tethered remotely operated vehicles (ROVs) and battery-powered autonomous underwater vehicles (AUVs).
Much of this has to do with battery capacities. Drones capable of travelling great distances unassisted will typically require low-power components, while available power can also restrict the amount of tasks it can perform. While large AUVs have impressive endurance – Kongsberg Maritime’s HUGIN, for example, can travel for 100 hours at 4 knots – bigger batteries will be needed as more tasks are handed to robots.
It is here that li-ion technology begins to slow down. At these size requirements, it often is not safe or practical to transport large battery packs. They cannot be transported by air, for example, and to work at any kind of depth they also require housing in some form of pressure vessels.
The solution, as MIT spin-off Open Water Power sees it, is to use a battery that runs on seawater itself. Using patent-pending aluminium alloy cell chemistry, the company says it can offer a tenfold increase in the range of so-called unpiloted underwater vehicles (UUVs), compared to the use of traditional li-ion batteries.
Water, water everywhere The system consists of an activated aluminium anode – an alloy that also contains small amounts of other non-toxic metals – an aqueous alkaline electrolyte and a hydrogen evolving cathode. Sea water is pulled into the battery, and is split at the cathode into hydroxide anions and hydrogen gas. The hydroxide anions interact with the aluminium anode, creating aluminium hydroxide and releasing electrons. Those electrons travel back toward the cathode, transferring energy to a circuit along the way – essentially splitting the process into two half-reactions, creating a fuel cell.
OWP says the chemistry achieves an optimal total efficiency of about 33%, and produces only aluminium hydroxide and hydrogen gas as harmless waste. According to calculations provide by the company, around 20mg per minute of aluminium and 40mg per minute of water will produce 1W of power, along with 2W of heat, 2 mg hydrogen and 58 mg of aluminium hydroxide.
In terms of energy density, the system offers somewhere between 2.1-5.5kWh per litre, far above more hazardous li-ion options.
On dry land, the system is inert and the battery only activates when flooded with water, meaning it could be flown out to offshore missions if necessary. In addition, once the interior aluminium corrodes, it can be replaced at a fractional cost.
OWP notes that the anode only reacts significantly in the presence of its electrolyte, and does not risk run-away even under the most severe conditions, including a system puncture or an internal short circuit. Moreover, depending on the mission, the alloy itself can be tailored to fit the specific power and lifetime requirements. An internal pump also circulates the electrolyte, pushing aluminium hydroxide from the anode and onto a custom precipitation trap. When saturated, the waste traps are ejected and replaced automatically. Meanwhile, the electrolyte itself prevents marine organisms from forming inside the power system. Another reason for its effectiveness is the systems’ ability to use produced hydrogen as a means of maintaining neutrally buoyancy. This, OWP says, offers a lower and therefore better density at all depths, compared with syntactic foams and alternative gases. “Furthermore, our chemistry’s continuous production of hydrogen at negligible gauge pressure allows us to control buoyancy dynamically using only lightweight bladders and plumbing, turning UUVs equipped with our power systems into powerful hybrid gliders,” OWP says.
Naval gazing OWP is currently working with the US Navy to replace batteries in acoustic sensors designed to detect enemy submarines. In vehicle applications, it hopes that the battery technology will allow AUVs to run for considerably greater times and distances. Where current systems can patrol pipelines and assets a few dozen miles from base before returning, the company believes that its chemistry will enable missions hundreds of miles out to sea. This summer, the company plans to launch a pilot with Riptide Autonomous Solutions, which will use the battery systems in AUVs performing underwater surveys. Riptide’s current models can travel roughly 100 nautical miles (185 km) in one go, but OWP is looking to increase that distance to 1,000 nautical miles (1,850 km). Where typical AUV battery life might be 24 hours, or less in deepwater missions, OWP’s chemistry could extend that to a month. From an oil and gas perspective this is significant: the ability to monitor pipelines or subsea assets externally, or collect environmental data, for example, without a manned mission offshore represents an immediate saving of thousands of dollars. With greater power output and lifespan also comes greater flexibility – UUVs could carry some form of manipulator function, enabling them to perform automatic or guided interventions when necessary. Speaking with MIT News, co-inventor Ian Salmon McKay drew on the example posed by Malaysia Airlines’ missing airliner: “In looking for the debris, a sizeable amount of the power budget for missions like that is used descending to depth and ascending back to the surface, so their working time on the sea floor is very limited.” he said. “Our power system will improve on that.”
OWP holds a contract with the Department of Defense to develop power systems for portable AUVs and a separate signing for sea-floor based power systems. However, it is upfront about some military and energy applications remaining off-limits: “We are not a good fit for extremely high sustained power-density systems like torpedoes, nor are we a good fit, because of our water-based chemistry, for high temperature applications like downhole drilling. But if your application requires long-duration, moderate power underwater, our chemistry’s performance is the best in class.”
With trials ongoing, interest in the battery system is sure to be high. So far, it has run larger cells for a week, and smaller, low-power cells for a month, but the next step will be integrating the technology into a fully functioning system. OWP itself pegs its cell technology at TRL level 6, and battery systems at level 4, but there already look to be plenty of applications where further refinements could help develop a final product – in particular UUVs, ocean-floor sensors and sonobuoys, it says.
That scale-up could come quickly. In May, OWP was acquired by L3 Technologies, a technology provider for comms and sensor systems for military applications. Oil and gas will perhaps be slower to follow the military’s lead, but marine engineering firms should be quick to jump on the possibilities, if OWP’s technology can be incorporated into their own.
“We have great confidence – and the data to back it up – that this technology is the future of underwater power,” OWP says on its website – and it might just be right.