Back in 2004, the Pelamis Wave Energy Converter became the world’s first offshore wave machine to successfully generate electricity and deliver it to the grid. Unlike many other wave energy harvesters, the Pelamis ‘sea-snake’ responded to the shape and frequency of waves, rather than their height. 120 metres long, it comprised of a series of semi-submerged cylindrical sections that flexed in the waves, moving hydraulic rams that, in turn, drove electric generators. The company who developed the Pelamis went into administration in 2014, but its unique design has continued to inspire researchers ever since.

The latest effort, published in Nano Energy [DOI: 10.1016/j.nanoen.2018.03.062] comes from Georgia Tech, but rather than relying on hydraulics, these engineers propose using triboelectric nanogenerators (TENGs) to havest electricity from waves. TENGs are particularly effective at harvesting low-frequency energy, which ocean waves are – they have a typical frequency of much less than 1 Hz – but they have historically struggled in wet, high salinity environments.

The proposed design makes use of a type of contact electrification called ‘rolling electrification’. It works by collecting the charges generated by PTFE balls rolling across a nylon substrate with interdigited copper electrodes. The TENGs are stacked and housed in rectangular acrylic boxes, which are, in turn, arranged into a chain connected by springs. Under an external trigger (e.g. an ocean wave passing by), the snake-like structure bends in response, inclining the boxes, causing the balls to roll down the nylon film. This generates an electric current that flows through the electrodes.

These TENGs are relatively small and lightweight – a single segment measures 5.1 x 6.4 x 2.54 cm – but with four segments under horizontal motion (at 1 Hz), a maximum voltage of 300 V was. By introducing an airgap into the structure, the researchers overcame the common issue of water ions screening the output potential, without compromising the system’s mechanical performance. To demonstrate its behaviour, a three segment TENG-snake was tested in simulated ocean conditions. The team found that under a periodic motion, it could power 152 LED lights connected in series, had a maximum power density of 3 W/m3. According to the authors, this is “… the first TENG to harvest energy from the wave's curvature.” Their work on the system is ongoing.

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Steven L. Zhang, Minyi Xu, Chunli Zhang, Yi-Cheng Wang, Haiyang Zou, Xu He, Zhengjun Wang, Zhong Lin Wang. “Rationally designed sea snake structure based triboelectric nanogenerators for effectively and efficiently harvesting ocean wave energy with minimized water screening effect” Nano Energy 48 (2018) 421–429. DOI: 10.1016/j.nanoen.2018.03.062