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Electricity is the basic and essential need for every single person and guarantees the development of society, industry, and the economy. The limited conventional fossil fuel meets the challenge of the sharply increasing demand, while may lead to severe pollution. Emerging green energy harvesting systems can convert ambient kinetic, thermal, and solar energy to electricity effectivity. In this review paper, the authors provide an overview of the new and emerging methods, droplet‐based electricity generators, that convert energy from the ubiquitously existing water droplets. The authors begin with a brief introduction of the working principle for droplet‐based power generators. Existing configurations of the droplet‐based power generator are also summarized, including 1) single electrode, 2) transistor‐based, and 3) direct current generator. Then, a detailed discussion of the design and operation factors that may impact the energy harvesting efficiency of droplet‐based power generators is provided. Hybrid energy harvester that integrates droplet‐based power generators with other triboelectric, pyroelectric, and photovoltaic‐based devices are summarized. Finally, challenges and perspectives regarding the future development of droplet‐based power generators are described. The reviews demonstrate a highly efficient method for energy harvesting from water droplets to meet the vital needs of the energy crisis. [ABSTRACT FROM AUTHOR]

In this study, we present an adaptive capacitor charging circuit for efficient piezoelectric (PE) energy harvesting (EH), which simplifies the configuration for powering wireless sensor network (WSN) nodes. In the circuit, the PE harvester directly charges the storage capacitor through the rectifier, eliminating the energy loss in the dc–dc converter of conventional EH circuits. An adaptive control circuit is designed to optimize the average charging power. Theoretical analysis shows that the optimal average power of the novel circuit varies from 81.4% to 86.5% of the maximum power of the conventional EH circuit, without considering the power dissipation in the dc–dc converter. Moreover, the power losses of both the proposed and conventional circuits are estimated. When the excitation level is low, the net harvested power of the proposed circuit can be enhanced to as high as 180% of that of the conventional circuit. The experimental results validate that the power transferred to the load is increased by adopting the proposed interface. It is also proved that the circuit is adaptive to different harvesters and excitation conditions. In addition, the area of the proposed circuit is reduced, allowing it to be easily integrated into the WSN nodes. [ABSTRACT FROM AUTHOR]

Using the piezoelectric effect to harvest energy from surrounding vibrations is a promising alternative solution for powering small electronic devices such as wireless sensors and portable devices. A conventional piezoelectric energy harvester (PEH) can only efficiently collect energy within a small range around the resonance frequency. To realize broadband vibration energy harvesting, the idea of multiple-degrees-of-freedom (DOF) PEH to realize multiple resonant frequencies within a certain range has been recently proposed and some preliminary research has validated its feasibility. Therefore, this paper proposed a multi-DOF wideband PEH based on the frequency interval shortening mechanism to realize five resonance frequencies close enough to each other. The PEH consists of five tip masses, two U-shaped cantilever beams and a straight beam, and tuning of the resonance frequencies is realized by specific parameter design. The electrical characteristics of the PEH are analyzed by simulation and experiment, validating that the PEH can effectively expand the operating bandwidth and collect vibration energy in the low frequency. Experimental results show that the PEH has five low-frequency resonant frequencies, which are 13, 15, 18, 21 and 24 Hz; under the action of 0.5 g acceleration, the maximum output power is 52.2, 49.4, 61.3, 39.2 and 32.1 μW, respectively. In view of the difference between the simulation and the experimental results, this paper conducted an error analysis and revealed that the material parameters and parasitic capacitance are important factors that affect the simulation results. Based on the analysis, the simulation is improved for better agreement with experiments. [ABSTRACT FROM AUTHOR]

• A new two-port hybrid diode topology is proposed for solar-powered WSN. • An adaptive supercapacitor buffer energy management strategy is designed. • The system cost, size, control complexity, and energy loss are decreased. • The battery lifetime is increased by at least two times that of the existing HESSs. Solar energy harvesting is promising to provide long-term power autonomy for wireless sensor networks. Energy storage devices like lithium-ion batteries are usually integrated to solar-powered sensor nodes to overcome the intermittency of solar power. However, the cycle life of lithium-ion batteries is short, which limits the lifetime of the nodes. Therefore, supercapacitor-battery hybrid energy storage system has been used to extend the cycle life of battery. The design of hybrid energy storage systems significantly affects the performance of wireless sensor network nodes in many ways. However, hybrid energy storage system optimization is a comprehensive problem because the configuration and power-split strategy need to be considered concurrently, which has not been done in the existing studies of solar-powered sensor nodes. In this study, a two-port hybrid diode topology is proposed, accompanied by an adaptive supercapacitor buffer energy management strategy to achieve integrated optimization. The hybrid diode semi-active topology uses a unidirectional DC/DC converter accompanied by two diodes to replace the bidirectional DC/DC converter in the existing semi-active topology. Accordingly, the energy lose is decreased by 15.5% while the cost, size, and controlling complexity are also decreased. Moreover, an adaptive supercapacitor buffer energy management strategy is designed based on the new topology to decrease the battery degradation. Simulation and experimental results indicate that the proposed hybrid energy storage system increases the battery lifetime to at least 3 times that of existing hybrid energy storage systems. Finally, a long-life solar-powered wireless sensor network node with a relatively small size and cost is achievable. [ABSTRACT FROM AUTHOR]