Electromagnetic spring for Sliding Wave Energy Converter

We propose a Linear Sliding Wave Energy Converter (LS-WEC) to power ocean buoys by wave energy harvesting. The concept involves a linear translator and a rotary generator that roll and pitch in ocean waves. The translator is a steel mass that is supported by wheels on rails with low-friction bearings. The translator linear motion is converted to rotary motion to spin an electric generator using a set of rack and pinion gears. The rack is fixed to the translator; the pinion is coupled to the generator shaft. An "electromagnetic spring (eSpring)" is implemented by using the generator as a motor and performing feedback control of the translator displacement. With variable stiffness, the eSpring tunes the system in resonance with changing ocean wave motions in order to maximize the power harvested. We derive the dynamic system fundamentals, which indicate a large "effective mass" associated with the polar moment of inertia of the generator rotor. We calculate power-harvesting performance by numerical simulations in sinusoidal and random waves, with and without an eSpring. The results show that without an eSpring the power decreases proportionally with the effective mass, but with the eSpring, the system uses the effective mass to store energy that is extracted by the generator. For a 3m long LS-WEC with a 30-kg translator mass in swell conditions (1.25-m wave-height and 5-s period), the average power is 2.3w and 17.8w without and with the eSpring, respectively.