Broadband rotary hybrid generator for wide-flow-rate fluid energy harvesting and bubble power generation

•A broadband rotary piezoelectric-electromagnetic hybrid generator (BRHG) is proposed.•The BRHG effectively collects extremely low-speed rotation energy as low as 0.01 Hz.•The energy conversion efficiency and speed are 8.2 and 60 times that of a cantilever.•Wide-flow-rate fluid energy is harvested, including wind, water, and two-phase flow.•This work contributes significantly to in situ power supply of underwater devices. The ocean contains copious available natural resources and energy. Ocean in situ power generation technology, directly harvesting energy from ocean fluids, is the crux of solving the problem of power supply for eupelagic or deep-sea exploration equipment. However, the ultralow frequency and wide-flow-rate characteristics of ocean fluids have brought severe challenges to in situ power generation in the ocean. In this paper, we develop a broadband rotary piezoelectric-electromagnetic hybrid generator to effectively harvest the energy of wide-flow-rate and ultralow frequency fluids. By using rotational torque to directly drive deformation of piezoelectric films, the hybrid generator effectively collects extremely low-speed rotational energy (as low as 0.01 Hz), with the efficiency and speed of energy conversion of the piezoelectric component of the generator being respectively 8.2 times and 60 times higher than that of a traditional piezoelectric cantilever beam harvester. The integration of the electromagnetic component further improves the energy harvesting performance of the generator over a wide flow velocity range. Compared with existing energy harvesters, the water-driven power generation per unit flow rate is enhanced by a factor of 75.5. The output power density of the proposed underwater bubble power generation system using the hybrid generator is 287 mW/m3, which is 275 times higher than previous devices. This work provides a viable approach to blue energy exploitation, especially ocean in situ power generation, and opens up many prospects for underwater cable-free power supply to eupelagic or deep-sea equipment.