AN ARTIFICIAL eel that can harvest energy from the currents of the ocean, a river or even a sewer is being developed by engineers in the US. The eel's tail thrashes around in turbulent underwater flows, flexing to produce a current that charges a battery.
The idea is to provide long-term power for remote-sensing and surveillance devices that the US Navy drops into the sea. Currently, these use small dynamos to generate power, but their turbines clog quickly, rendering them useless.
"We have built a prototype that works well and we're now developing a larger-scale version," says Sean Kammenn, chief engineer on the project at Ocean Power Technologies in Pennington, New Jersey.
The prototype eel is essentially an underwater flag the size of a football scarf. It's made of a piezoelectric polymer called polyvinylidene fluoride (PVDF), which produces a small current when flexed. The flag is only a couple of millimetres thick and so flutters easily when it is anchored in a turbulent flow. The energy this produces trickle-charges a battery. "The prototype produces about 10 milliwatts, but the eel we are developing for the US Navy will produce about 1 watt of usable power," says Kammenn.
The navy plans to use the eel to power sensors dropped in remote ocean locations. These will measure factors such as sea temperature and salinity for weather forecasts and broadcast the data via a satellite to the mainland. The eel may also power underwater microphones for listening to underwater traffic. "The Navy drops lots of sensors that are only able to broadcast data for about an hour because of battery limitations. However, the eel should produce power for at least a year," says Kammenn.
Alexander Smits, a mechanical engineer at Princeton University who has been working on the fluid dynamics of the eel, says it should be far more robust than other power generators. "The Navy has not had a happy time experimenting with small turbines," he says. "They tend to clog up and break down quickly. But since the eel is constantly flexing and has no mechanical parts it should last a lot longer." The team has already carried out trials in the lab and is hoping to begin ocean testing in a tidal estuary sometime during 2001. "We're looking for a suitable site right now," says Smits.
The eventual objective, however, is to make an artificial eel that can swim. By reversing the flow of current to the piezoelectric material, the researchers believe it should be possible to make it flex in a way that drives it through the water like a real eel. The idea is that an eel carrying surveillance equipment and a fully charged battery would be dropped into an enemy sewer or storm drain, stopping to recharge whenever necessary. "That's the long-term goal," says Kammenn, "but there are plenty of challenges ahead." Most difficult will be developing a control system that can monitor the eel's attitude and control its movement through the water, says Kammenn.
Author: Justin Mullins
New Scientist issue: 23/30 December 2000
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