What's good for fish may be good for robots, too, as researchers from the Helmholtz-Zentrum Hereon have developed an artificial gill that can extract oxygen from seawater to run fuel cells to power robotic sea gliders on long missions.
Underwater gliders have become an increasingly valuable tool for oceanographic research. Ditching traditional propellers and thrusters, they move about by means of variable buoyancy propulsion, which is a cumbersome way of saying that they propel themselves by rising in the water and then using hydrofoils to control their direction as they descend.
It's not very fast, but it is economical and allows the gliders to carry out long missions across thousands of miles to monitor ocean conditions, seek out pollution, and conduct military reconnaissance as they dive to depths of up to 1,000 m (3,300 ft). They are also much cheaper to operate than research vessels, so what's the problem?
The fly in the deep sea ointment is powering the gliders. Batteries are needed to run the sensors, recorders, and telemetry systems but the go-to lithium batteries are classified as containing hazardous materials that are subject to strict safety and environmental regulations.
In addition, lithium batteries have their technical limitations. They are sensitive to pressure, are in danger of leaking if seals are compromised, can be severely damaged by seawater, do not handle cold temperatures well, and can release dangerous chemicals.
As a safer, less restricted alternative, Hereon engineers Dr. Lucas Merckelbach and Dr. Prokopios Georgopanos have been looking at fuel cells, which convert hydrogen and oxygen into electricity. The hydrogen is easy enough to store until ready. Just store it in a container with metal hydrides that absorb the hydrogen until needed. The oxygen is another matter. It weighs eight times per unit more than hydrogen in water and is very difficult to store even in cryogenic conditions.
Hereon's answer is not to even bother. Instead, the non-profit research institution has come up with an advanced silicone polymer membrane that has high oxygen permeability, yet is hydrophobic to keep water from seeping through. In the sea, the higher oxygen concentration on the wet side allows the oxygen atoms to migrate through the membrane to be collected by an internal recirculating airflow and fed to a Proton Exchange Membrane Fuel Cell (PEMFC) where it combines with hydrogen to produce electricity, with water the only waste product.
According to the team, the modular system's design can handle various underwater conditions, including changes in water temperature, salinity, and pressure to ensure consistent oxygen supply. In addition, computational fluid dynamics (CFD) simulations optimize the flow of water around the membrane.
Along with the membrane, there is a thermal management system to carry away the heat generated by the fuel cell and, ironically, a lithium battery to store power for peak demand periods. Laboratory tests indicate a conversion factor of 50% or equivalent to stored oxygen systems in underwater conditions.
“This system eliminates the need for onboard oxygen storage," said Georgopanos. "The weight and volume saved can be used for additional hydrogen storage, enabling higher energy density and lower operating costs compared to current battery solutions."
The research was published in Advanced Science.
Source: Helmholtz-Zentrum Hereon
