Underwater robots could get their batteries recharged by munching the sea floor. A device created by the US Navy extracts electrical energy from layers of fish faeces and other organic matter to provide an endless source of power.
All underwater devices have a fundamental limitation 鈥� battery life. They are incredibly useful for exploring and monitoring the depths, but once their power reserves start to run low there鈥檚 no choice but to bring them to the surface or abandon them.
One approach to solving this is microbial fuel cells (MFCs) that use the sea floor to produce electrical energy. The MFCs use naturally-occurring bacteria which feed on organic matter, mainly faeces from fish, in the sediment. As they digest, these bacteria shed electrons causing a flow of current, which can be used to power electronics by channeling it through the MFC. It is simply a matter of putting one electrode in the sediment and the other in the water above it.
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鈥淚t鈥檚 like a battery made by biology,鈥� says Meriah Arias-Thode at the Naval Information Warfare Center Pacific in San Diego, which is an organisation within the US Navy.
Arias-Thode鈥檚 team has already used MFCs for powering basic sensors, such as for monitoring magnetic fields of acidity levels. Previously these devices could last a few weeks under water, one powered by 聽MFCs聽 lasted eight months.
In principle, the devices can provide power indefinitely as the sediment is constantly replenished by the 聽slow fall of fish poo, dead creatures, and general organic debris, and they don鈥檛 produce pollution when in use.
The main stumbling block is their low power density. It would take them around a day to charge an iPhone X and around a year to charge a small unmanned underwater vehicle. Hence, an array of lots of MFCs will be needed, or lower-powered underwater vehicles.
The US Navy is also experimenting with positioning giant battery packs on the sea bed as recharging stations for unmanned submarines. In the long run, it may be possible to trickle charge these from a large number of MFCs, allowing underwater robots to operate anywhere for prolonged periods.
