Star-shaped concrete structures called tetrapods are often used to protect coastlines from eroding away under the constant barrage of waves. Now a project from the Okinawa Institute of Science and Technology (OIST) has outlined plans for turbines that would sit alongside tetrapods, helping to not only dissipate wave energy, but harvest it.

The constant crashing of the waves represents an essentially limitless amount of energy. Current techniques to tap into it include; Azura in Hawaii, which uses a 360-degree rotating float mechanism; a system of buoys attached to a jetty in Gibraltar that rise and fall with the waves; a proposed system from UC Berkeley that would absorb that energy by carpeting the seafloor; and an "artificial blowhole" in development in Australia that captures energy from air displaced by waves.

The OIST system is designed to not only harvest wave energy but dissipate it, working with tetrapods to reduce the impact on Japanese coastlines. Turbines would be placed in the direct line of fire, such as in front of tetrapods or around natural structures like coral reefs, to take advantage of fast-moving jet flows of water created where waves break.

These Wave Energy Converter (WEC) turbines would be anchored to the sea floor with mooring cables, and peek up just above sea level where the waves can wash over them. Each turbine would feature five blades with a diameter of 70 cm (27.6 in) attached to a permanent magnet electric generator encased in ceramic to keep the damaging seawater out. The electrical energy created would be channeled through a cable in the support stem and back to shore to feed into the grid.

Electrical energy is created through a permanent magnet electric generator, and fed back to land through a cable in the support column OIST

While spinning blades might not sound safe, the team says there are a few precautions built in to protect sea life and the turbines themselves from harm. The speed of the blades' rotation has been carefully calculated so that any animals that get swept up into them can escape. To protect the turbines during rough swells or extreme events like typhoons, the team took a cue from dolphin fins and made the blades flexible so they release stress. The supporting structure is also designed to bend under pressure, like the stems of flowers.

According to the team, the turbines have a working life of 10 years, and maintenance can be undertaken by the same crews who inspect tetrapods. While the researchers haven't given any figures for how much energy each turbine could produce, they have given a rough estimate for the amount produced by a fleet of them over a wide area.

"Surprisingly, 30% of the seashore in mainland Japan is covered with tetrapods and wave breakers," says Professor Tsumoru Shintake, lead researcher on the project. "Using just 1% of the seashore of mainland Japan can [generate] about 10 gigawats [of energy], which is equivalent to 10 nuclear power plants. That's huge."

The next step for the project is to install two half-scale model turbines, with blade diameters of 35 cm (13.8 in), which would power LEDs to demonstrate the technology.

"I'm imagining the planet two hundred years later," says Shintake. "I hope these [turbines] will be working hard quietly, and nicely, on each beach on which they have been installed."

Source: OIST