MUCH is made of the potential of flying drones. But drones are useful at sea, too. Unmanned underwater vehicles (UUVs), as they are known technically, are employed for things ranging from prospecting for oil and gas to naval warfare. Like their aerial cousins, though, ocean-going drones have limited ranges—limits that are often imposed by their batteries.
At the moment those batteries are usually either alkaline or lead-acid. Lithium-ion batteries, fashionable elsewhere, have not conquered the UUV world. Their tendency to catch fire counts against them. And they are sensitive to pressure, which is undesirable in devices that operate underwater. But a firm in Massachusetts, called Open Water Power (OWP), is offering an alternative: batteries based on aluminium. With these, its engineers hope to extend the ranges of underwater drones tenfold.
Each of OWP’s battery cells has a block of aluminium as its anode. The cell’s cathode is made of nickel. In a working battery, these anodes and cathodes alternate, and are bathed in an electrolyte made of seawater with some potassium hydroxide dissolved in it. This chemical keeps the battery free from marine organisms that might otherwise grow within it. It also plays two other roles. These are in the battery’s chemical operation.
One of these roles lies in the reaction that drives the battery, between the aluminium of the anode and the hydroxide ions in the electrolyte. A hydroxide ion is a negatively charged combination of a single hydrogen atom and a single oxygen atom (OH- in chemical shorthand). Unadulterated water contains some hydroxide ions (its molecules, H2O, sometimes disintegrate spontaneously into OH- and positively charged hydrogen ions, H+) but adding potassium hydroxide boosts their number.The result of the reaction is aluminium hydroxide, which is electrically neutral, and electrons, which carry away the hydroxide ion’s negative charge. These electrons then travel towards the cathode via a circuit that can, for example, power a motor. To complete the circuit, electrons at the cathode combine with hydrogen ions from the electrolyte’s water to produce hydrogen gas, which is vented from the battery, leaving those ions’ hydroxide partners behind to replenish the store of OH-.Previous attempts to make a commercial aluminium battery have failed because their anodes have got clogged up with aluminium hydroxide, which is insoluble in water. This is where the added potassium hydroxide does its third job, for an aqueous solution of potassium hydroxide will dissolve aluminium hydroxide in a way that pure water cannot.A pump circulates the potassium-hydroxide-bearing electrolyte through the battery, where it picks up aluminium hydroxide from the anodes. The resulting solution then passes through a chamber filled with a plug made of foam rubber. This is a material that packs an enormous amount of surface area into a tiny volume and whose chemistry encourages the aluminium hydroxide to precipitate on that surface. A small piece of foam rubber can thus hold a lot of aluminium hydroxide. When a plug is saturated with the stuff the battery ejects it and replaces it with a fresh one that has been kept, compressed, in an adjacent plug store. Each battery carries enough plugs to keep it going until its supply of aluminium has run out.One test of OWP’s technology will come this summer, when the firm will fit its batteries into UUVs built by Riptide Autonomous Solutions, which is also in Massachusetts. Riptide’s products are used by oil and gas companies to undertake underwater surveys. At the moment, they have a range of about 100 nautical miles (185km). Riptide reckons that OWP’s batteries could increase that to 1,000 nautical miles.The armed forces are interested, too. Though OWP is coy about the details, records in the public domain show that OWP is working with America’s navy and also with the country’s Special Operations Command, which carries out clandestine missions. The navy contract asks for something to replace the existing batteries on its Shallow Water Surveillance System, a series of acoustic sensors designed to detect enemy submarines. The Special Operations contract is light on detail, but is for “man-portable UUVs”.One other use for aluminium batteries might be to power crewed deep-diving submersibles such as Alvin, which found fame in 1986 when it was used to explore the wreck ofTitanic, a British liner sunk by an iceberg in 1912. At the moment Alvin still relies on lead-acid batteries. This limits its dives to eight hours and means it cannot go as far down into the ocean as its titanium shell would otherwise permit. Aluminium batteries would let it and its kind dive longer and deeper, letting researchers visit the abyss more easily in person.Correction: This article has been corrected. A previous version misnamed Riptide Autonomous Solutions, and gave the wrong actual and hoped-for ranges for its UUVs.