…and plankton could provide their power
(New Scientist Via Thomson Dialog NewsEdge) BATTERY life always limits the usefulness of mobile gadgets and robotic submarines are no exception. If such subs are ever going to undertake lengthy missions such as those envisaged by the US National Oceanographic and Atmospheric Administration (see left) new power sources will be needed.
Roboticists Chris Melhuish, Ioannis Ieropoulos and John Greenman at the Bristol Robotics Laboratory in the UK might have the answer. They are developing a power source that harvests what the ocean has to offer: plankton and dissolved oxygen. The Bristol team famously built Ecobot, the robot powered by dead flies (New Scientist
, 11 September 2004, p 19). Now they have shown that the microbial fuel cell that powered Ecobot can work underwater.
In Ecobot's fuel cell, anaerobic bacteria were used to break down the flies' exoskeletons into sugars. This time they will break down plankton. The sugars are then metabolised by the bacteria, releasing electrons that can be collected by a positive electrode, as well as hydrogen ions that are drawn across the fuel cell's ion-exchange membrane . To create a current, the electrons need to be drawn out of the negative electrode on the other side of the cell.
EcoBot used oxygen from the air to draw out the electrons. The underwater fuel cell uses the oxygen dissolved in water, in a similar way to the gills of a fish. A constant source of dissolved oxygen is supplied by water flowing over the negative electrode. In gills, a protein in the fish's blood binds to the oxygen, but in the artificial gill the oxygen combines with electrons from the circuit and the protons released by the bacteria, producing water.
The Bristol team have yet to develop a way to collect the plankton, so they used vinegar to test the fuel cell. It produced a current of just over 100 microamps. "We've shown it is possible for a class of self-sustaining robots to exist underwater," says Melhuish. The gill produced a stronger current at higher water flow rates, because this made more oxygen available. A robot using the cells could seek out faster-flowing currents to boost its power, says Melhuish.
The system won't provide enough power for continuous motion, but would fuel a robot with sporadic energy needs. "It would be good for event monitoring," Melhuish says. "A robot could just sit at the bottom of a river or ocean until something happened. It could watch for seismic activity or pollution."
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