Scalable pendulum energy harvester for unmanned surface vehicles

Abstract This paper proposes a novel pendulum energy harvester design for converting energy of low frequency ambient vibration, such as that found in unmanned surface vehicles (USVs) due to ocean waves, into usable electrical energy. The primary novelty of this design is the mechanical rotation rectifier (MRR) system, which is able to improve on existing designs through the use of spur gears and sprag clutches capable of handling significant torque, in an arrangement which is easily scalable to larger devices. Using a minimal number of offset gears, this system is designed to maintain high efficiency and minimal backlash. The energy harvester was proven to produce a maximum normalised average power output of 20.62 W/g2, corresponding to an average power of 0.72 W and a power density of 0.43 W/kg, at 0.186 g rms acceleration. The resonant frequency of the system is designed at 1 Hz, within the range of the expected natural frequencies of USV motion. The energy conversion efficiency at resonance was 43.5 %. Furthermore, a mathematical model was developed and verified through experimental testing, and was used to simulate the effects of various pendulum lengths, masses and flywheels on the system. This demonstrates the specific negative relationship between pendulum arm length and natural frequency of the system, as well as the positive correlation between pendulum mass and maximum power. Furthermore, higher inertia flywheels are shown to produce significantly smoothed voltages and increased output power, while extending the time it takes for the harvester to reach steady state. It is concluded that this device would be viable for assisting in powering USV communications and extending the exploration life of these vessels.