Applying double-mass pendulum oscillator with tunable ultra-low frequency in wave energy converters

Ocean waves are an ultra-low-frequency renewable energy source. Various point absorbers have been developed over the past decades as small-sized wave energy converters (WECs). However, realizing tunable and ultra-low frequencies in WECs remains an extremely challenging technological issue. This paper proposes a novel and simple design of a double-mass pendulum (DMP) oscillator whose natural frequency can be conveniently tuned by simply adjusting the positions of two independent masses. Shake table test results successfully illustrated that the tunable ultra-low natural frequency range (0.2–1.4 Hz) can be achieved even in a small-size DMP prototype, which can be hardly achieved in conventional oscillator designs. Subsequently, a small prototype of a point absorber enclosing the DMP-based energy harvester was fabricated and tested in a wave flume under different wave heights and periods. Average output power of nearly 100 mW was captured in the experimental case when the wave period and height were 0.7 s and 0.1 m, respectively. The corresponding power extraction in a large-scale case is predicted to be up to 3.5 kW under wave height of 2 m and wave period of 6 s. The experimental results demonstrated that the proposed DMP oscillator is a promising power extraction device with an appealing tunable ultra-low frequency, which is extremely suitable for WECs, as well as other energy harvesters designed for ultra-low-frequency vibration sources.