Battery-Free Hybrid Ambient RF and Wind Energy Harvester for Outdoor IoTs

The paper proposes a hybrid RF and wind energy harvester. It is constructed by structural and functional integration of the two dissimilar energy harvesting techniques, constituting a conformal design. The rectifying efficiency can be boosted by the hybrid power source excitation, thereby increasing DC output power compared to standalone power source. A fan-shaped omnidirectional antenna and a hybrid single shunt-diode rectifier are designed to realize energy receiving and rectifying, respectively. A prototype is implemented and measured. The receiving part can achieve 2.29-dBi peak gain and 0.92-dB non-roundness at 1.85 GHz. It can also work smoothly when the wind speed varies from 0 to 12 m/s. The RF-DC conversion efficiency at -20 dBm and AC-DC output voltage at 12 m/s are measured as 20% and 79 mV, respectively. When both RF and wind power sources are accessible, the hybrid DC output power of 1.0 uW can be obtained with -30-dBm RF power and 10m/s wind speed. Moreover, the output power at hybrid rectifying mode is higher than that of simply superimposed RF and wind energy. Efficiency gain of up to 182% can be achieved. The hybrid energy harvester is a good candidate to power the sensors in battery-free IoTs.