The well-known answer to this question is that it stops being a motor and becomes a generator. Instead of using electricity to turn a propeller and drive the vehicle along, the flow of water turns the propeller, generating electricity.

What's new about the Southampton design is its simplicity. "This is a compact design that does away with many of the moving parts found in current marine turbines. It's a new take on tidal energy generation," says Turnock.

Most current tidal stream generators are essentially wind turbines turned upside down and made to work underwater. They often include complex gearboxes and move the entire assembly to face the flow of the water. For example, they turn a half a circle as the tidal current reverses direction. Gears and moving parts require expensive maintenance, especially when they are used underwater.

This pushes up the cost of running the turbines, a cost that is passed on to the consumers of the generated electricity. The Southampton design does not need to turn around because the design of its turbine blades means that they turn equally well, regardless of which way the water flows past them. The blades are also placed in a specially shaped housing that helps channel the water smoothly through the turbine.

Another beauty of the Southampton design is that everything is wrapped in a single package that can be prefabricated so there will be few on-site construction costs. "Just drop it into flowing water and it will start generating electricity. It will work best in fast flowing, shallow water," says Turnock, who foresees rows of these devices secured to sea floors and riverbeds.

The present prototype is just twenty-five centimetres across and the research team now plan to design a larger model with improved propeller blades that will further increase the efficiency of generating electricity. All being well, the team envisage the generator becoming commercially available within five years.

Using funding from EPSRC and industry in the early 2000s, Abu-Sharkh, Turnock and their team created a novel tethered underwater vehicle thruster that used electricity to turn a ducted propeller, providing thrust to control the vehicle's position and speed. Tethered underwater vehicles are extensively used in the offshore industry for conducting underwater inspections and robotic manipulation.

An overall propulsion system based on electrical thrusters is much smaller and lighter than the traditional hydraulic thrusters used in tethered underwater vehicles. So using the new ones reduces the weight of the vehicles, meaning that they require less power to move them and so are cheaper to run.

The concept of an electricity generator sprang out of the fundamental research involved in the hydrodynamic and electrical design of the integrated electric thruster. These thrusters, manufactured under licence by the local Hampshire company TSL, are already in use around the world for a variety of underwater vehicle applications.

Funding for these tests (which build on the original EPSRC-funded work) was provided as part of the University of Southampton's School of Engineering Sciences' MSc programme in Maritime Engineering Science.

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University of Southampton
Engineering and Physical Sciences Research Council