Tidal power remains a nascent technology, but offers several advantages over relatively more advanced renewable energy sources such as solar and wind: the Sun doesn’t always shine; the wind doesn’t always blow. On the contrary, the sea tides rise and fall at times that are predictable to the minute, years in advance. This means tidal currents are a reliable, predictable source of clean energy, as dependable as the Moon’s phases. Tidal turbines are placed on the seabed at locations with high tidal current velocities, where they extract energy from the flowing water and convert it into electricity. As tidal turbine technologies are relatively new, they have been expensive compared to other renewable and fossil fuel sources of energy. But real progress is being made to drive down costs. The EU-funded D2T2 project is slashing the costs of operating a tidal turbine. “Our new 100-kW turbine (Nova M100-D) is poised to make a positive impact on coastal areas across the globe. It is more efficient, reducing the cost of producing tidal energy by 30 %. This enables it to displace diesel generation,” notes Seumas MacKenzie, D2T2 project manager at Nova Innovation.
Direct-drive versus gearbox technology
The new turbine expands on the success of the world-first fully operational offshore array of tidal power turbines in Shetland, Scotland. The turbines have been generating clean electricity and exporting it to the grid for more than four years. Drawing on experience from the wind industry, the new turbine design eliminates the need for a gearbox. A direct-drive mechanism replaces the gearbox between the turbine’s rotor and generator, leading to higher power output, and lower operations and maintenance costs. Fewer moving parts in the turbine increase its reliability and lengthen the time between service intervals. “Tidal turbines are very much like underwater windmills; blades rotate in a similar way, except the rotors are driven by tidal currents. The rotational motion is converted into electrical energy by a direct-drive generator,” notes MacKenzie. He continues: “A conventional generator requires a gearbox to increase the slow rotation of the blades in the water to a fast spinning motion in the turbine to produce electricity. The direct-drive mechanism differs in that it enables electricity to be generated at low rotational speeds: the same shaft that turns with the blades also turns the generator’s rotor at the same low speed. This clean predictable electricity is then exported to the grid via a subsea power cable.” Unlike other technologies that require high rotational speeds, tide currents contain a tremendous amount of energy that can be harnessed more efficiently by a low-speed, high-torque direct-drive generator. Developing a robust technology that can withstand the harsh ocean environment has been challenging. Nova Innovation’s engineers were prepared for this thanks to experience gathered from previous research and development projects.
Looking at the future with confidence
The Nova M100-D underwent extensive testing at the ORE's National Renewable Energy Centre in Blyth, UK. This provided valuable feedback on the expected lifetime of the turbine, helping to de-risk operations before going to the sea. The turbine was then tested in the ocean, fulfilling expectations. The success of the project resulted in D2T2 reaching the finals of the Sustainable Energy Week Awards for contributing to Europe’s clean energy future. “Our innovative gearless turbine will rapidly drive down tidal energy costs, transforming it in a mainstream energy source,” concludes MacKenzie.
D2T2, tidal turbine, Nova Innovation, tidal energy, rotor, direct-drive generator, gearbox