TorqSense torque transducer selected for tidal turbine

Practically infinite reliability was the defining requirement when researchers wanted to run a TorqSense torque transducer under the sea as part of extensive trials of a tidal turbine. Dependable, affordable energy from tidal streams and ocean currents could soon be a reality, with scale models of the innovative Evopod demonstrating the viability of the oceans as an energy source.

Developed by Ocean Flow Energy, Evopod is a semi-submerged, floating, tethered tidal energy capture device. It uses a simple but effective mooring system that allows the free-floating device to maintain optimum heading into the tidal stream. Installed as an individual device or as a tidal farm, the technology offers clean, 'green' energy.

Evopod is claimed to overcome the key concerns that have previously been expressed for tidal stream turbine installations. As a floating tethered device it imposes minimal disturbance on sensitive seabed ecosystems and its single turbine rotates at such low speeds (10-20rpm) that they are likely to be only a low threat to marine wildlife. Furthermore, Evopod's novel mooring employs a tight envelope to reduce the size of the exclusion zone for shipping. A seabed region of one square kilometre can support enough Evopods to supply the energy needs of up to 40,000 homes. This would reduce carbon dioxide emissions by 110,000 tonnes per annum if replacing power from a coal-fired power station.

An important milestone in the development of Oceanflow's Evopod technology was reached on 6 March 2011 with the demonstration of grid connectivity by the company's 1:10th scale trial unit in Strangford Narrows, Northern Ireland. The output from the Evopod can now be fed into the domestic mains circuit of the Queen's University Marine Laboratory.

The Evopod employs a fixed-pitch turbine driving a permanent-magnet generator through a gearbox. Power control and data capture are essential for reliable energy generation. For effective sensing to measure the torque and rotational speed of the turbine, Ocean Flow Energy turned to Sensor Technology and its TorqSense strain transducer. TorqSense is a surface acoustic wave (SAW) technology device.

Torque is a critical measurement, as it indicates the power that can be derived from the system, as well as giving an indication of the stresses on the turbine. But the marine environment and the nature of the turbine's operation places unique performance requirements on the sensing equipment.

Ocean Flow Energy design engineer Roger Cox comments: "We had used TorqSense transducers before and had good experiences with them. We knew they were reliable in challenging applications, and would give us the quality data we needed as part of our proof of concept of the Evopod."

"Data is logged on board, but also transmitted back to shore so we can remotely monitor the operations," says Cox.

"We used TorqSense devices on the very first Evopod design to go into the sea, and they've been working reliably on our 1/10 scale test unit for five years," he continues. "They are now being considered for our larger scale mono-turbine and twin turbine units. A 1/40th scale model of our twin turbine unit has been tested in Newcastle University's flume tank with the support of a NEEIC grant. At full scale the unit would be fitted with twin 1.2MW rated generators, each coupled to a 16m diameter three-bladed turbine. The unit would generate its combined rated output of 2.4MW in flow speeds of 3.2m/s or above."