Challenges for Energy Harvesting Systems Under Intermittent Excitation

Energy harvesting is showing great promise for powering wireless sensors. However, under intermittent environmental power, low-power harvesting systems designed for stable conditions suffer reduced effectiveness or fail entirely. This work aims to improve a harvester´s capability to extract useful power from low and intermittent vibration sources, by addressing the power-conditioning interface circuitry between the harvester and load. In view of this, two specific challenges are analyzed. The first challenge is that of start-up, where the goal is to make as short as possible the transition from completely depleted energy storage to the first powering-up of a load. The second challenge is to improve the energy transmission to a load after its first powering-up, under intermittent excitation. The investigation uses an ultra-low-power and fully-autonomous kinetic energy harvesting system under intermittent excitation. A number of solutions are presented. Decoupling filters between parallel converters and the harvester are used to demonstrate the importance of maintaining the optimal harvester loading, even during short transients. Input-power-dependent power gating of the power conditioning is also demonstrated. Both methods demonstrated experimentally using discrete circuit implementations, and shown to successfully increase the start-up speed and operational frequency of the load. The achieved reduction in start-up time is ~ 67% at a maximum harvestable power of 135 μW, under a predefined profile of pulsed excitation at 3 m·s^-2. The experimental results provide insight into complex transient interactions of the harvester and power conditioning.