mechanical energy exists extensively in our daily life, such as vibration, wind, and waves. [3] However, most of this energy is wasted and long-neglected. For one thing, environmental mechanical energy is random in amplitude, low in frequently, widely distributed, unstable, and random. Therefore, the design of effective and environmental-friendly energy harvesters still has lots of technical and economic challenges. [4] Besides, IoT has made a great progress with the development of sensor, wireless communication, and micro/nanotechnology. [5] IoT enables collecting and processing data among the whole network, which consists of huge numbers of widely distributed sensors or smart devices. But how to supply adaptive and sustainable energy for nodes of IoT is a big task even though each node only needs a tiny power. According to the characteristic of wide distribution, harvesting environmental mechanical energy is an attractive approach to powering the nodes of IoT. [6]

In a DC microgrid (DC-MG), the dc bus voltage is vulnerable to power fluctuation derived from the intermittent distributed energy or local loads variation. In this paper, a virtual inertia control strategy for DC-MG through bidirectional grid-connected converters (BGCs) analogized with virtual synchronous machine (VSM) is proposed to enhance the inertia of the DC-MG, and to restrain the dc bus voltage fluctuation. The small-signal model of the BGC system is established, and the smallsignal transfer function between the dc bus voltage and the dc output current of the BGC is deduced. The dynamic characteristic of the dc bus voltage with power fluctuation in the DC-MG is analyzed in detail. As a result, the dc output current of the BGC is equivalent to a disturbance, which affects the dynamic response of the dc bus voltage. For this reason, a dc output current feed-forward disturbance suppressing method for the BGC is introduced to smooth the dynamic response of the dc bus voltage. By analyzing the control system stability, the appropriate virtual inertia control parameters are selected. Finally, simulations and experiments verified the validity of the proposed control strategy.

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A rotating-disk-based hybridized electromagnetic–triboelectric nanogenerator exhibits a much better charging performance and provides a better platform for WPT.

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Traditional grid-connected inverters (TGCI) could suffer from small-signal instability owing to the dynamic interactions among inverters and a weak grid. In this letter, the small-signal sequence impedance model of the virtual synchronous generator (VSG) is built, and the sequence impedance characteristics of the VSG and the TGCI are compared and analyzed. The sequence impedance of the TGCI is mainly capacitive in the middle-frequency area, and the impedance amplitude is quite high. By contrast, the sequence impedance of the VSG, being consistent with the grid impedance characteristics, is generally inductive, and the impedance amplitude is quite low. Based on the sequence impedance model and the Nyquist stability criterion, the influence of the grid stiffness, number of paralleled inverters, and phase-locked loop (PLL) bandwidth on the stability of the VSG and the TGCI grid-connected system is analyzed. The stability analysis results show that the TGCI loses stability easily whereas the VSG still works well without PLL restrictions under an ultraweak grid or with a large number of inverters connected to the grid. Therefore, the VSG is more suitable than the TGCI for achieving high penetration of renewable energy generation in an ultra-weak grid from a system stability viewpoint. Finally, experimental results validate the sequence impedance model and the stability analysis.