Design of an S-shaped point-absorber wave energy converter with a non-linear PTO to power the satellite-respondent buoys in the East China Sea

An S-shaped wave energy converter (SB-WEC) was designed to assist near-zero-energy oceanography. The SB-WEC comprises a non-linear mechanical power take-off (PTO) and a novel S-shaped buoy (SB) for operation in the wave climate of the National Shallow Sea Comprehensive Test Site in China. The PTO used a ball screw -based transmission system to harvest wave energy during upward and downward strokes, and its operational characteristics were validated through simulations and experimental testing. A comparative analysis was per-formed to select an optimal buoy shape, and tests were conducted in a lab-scale wave flume on the 3D-printed scaled buoys. Using an ultramodern algorithm, buoy scaling was performed to identify the pure shape-change effects on the heave response irrespective of their size. The optimal buoy size was evaluated, and an algo-rithm was developed in MATLAB Simscape to study its feasibility of running the non-linear PTO in realistic waves. A study on the tunability of the PTO using a rotary motion rectifier (RMR) suggested that the application of the proposed RMR is not suitable in waves below 0.3 Hz. During experimental tests, the PTO generated 13.5W at 0.3 Hz (realistic wave frequency) with an energy conversion efficiency above 50%. The experiments showed that under identical design and test conditions, the S-shaped buoy (SB) could achieve a 34% greater RMS heave response than the cylindrical-hemispherical buoy. A feasibility study based on MATLAB-AQWA showed that the RMS power absorption of the full-scale SB buoy was 17% higher than the input power of the PTO in irregular waves, validating the proposed buoy shape and size. The SB-WEC can reasonably power the sensors in the satellite-respondent buoys.