A self-sustained energy storage system with an electrostatic automatic switch and a buck converter for triboelectric nanogenerators

In this paper, we present for the first time a complete energy harvesting system fo triboelectric nanogenerators (TENGs) that includes as a first stage a half-wave rectifier, and as a second stage an electrostatic automatic switch combined with a buck converter. This simple two-stage system allows to deal with the very high output voltages of TENGs: the system can power a commercial low-voltage output regulator, which cannot be realized by directly charging the storage capacitor only with diode rectifiers. Different from the previous works using dissipating transformers for voltage stepping-down or switches integrated with the TENG for impedance matching, this work demonstrates the properties of high power pumping up, potential size reduction and low power dissipations.


Introduction
During the past two decades, there has been a great increasing interest in Kinetic Energy Harvesters (KEHs) that scavenge vibration energy from the ambient environments, with the development of the wireless sensing nodes, wearable electronics and Internet of Tings (IOT). There have been many kinds of KEHs like electrostatic electret harvesters [1], piezoelectric harvesters [2], thermoelectric harvesters [3] and triboelectric nanogenerators [4]. Among them, triboelectric nanogenerators (TENGs) attract more and more attentions. The operation of the TENGs are based on the coupling principles of contact electrification and electrostatic induction. The classical contact-separate mode of TENGs consists in the physical contact of two materials with different abilities to attract electrons. Opposite charges are left on the surfaces of the two materials and a current will take place from one electrode to another to rebalance the electrostatic field, when the transducer's capacitance changes with each other with external mechanical forces. TENGs usually generate a high voltage impulse output with a peak above 100 V, however the generated energy per cycle (or power) is not always high enough to power an electronic device. Efficiently management and storage of energy under such a high voltage is one of the main challenges in the TENGs design.
Simplest, the impulse voltage is rectified using a half-wave or a full-wave rectifier to charge a capacitor that drives the electronics. However, there is a great energy loss on the energy transfer from the TENG to the storage capacitor, as the internal capacitance of the TENG is much smaller than that of the external storage capacitor. In order to improve the efficiency and suppress the energy transferring loss, Niu et al. [5] has realized a two-stage charging system that firstly charges to the optimized voltage a small buffer capacitor matching the impedance of the TENG, and then transfers the energy to a larger reservoir using two electronic switches and a transformer. However, the electronic switches and the transformer are quite power-consuming, weakening the efficiency improvement. Furthermore, the transformer is large in volume and weight. In this paper, we propose for the first time a two-stage charging system with an electrostatic automatic switch and a DC-DC buck converter, which considerably increases the charging efficiency and has the potential to be fully integrated.

Schematic and experimental setup
As shown in Figure 1, the 1 st stage of the proposed energy storage system is a half-wave rectifying module. A small buffer capacitor (Cbuf = 5nF) is quickly charged by the TENG to a high-voltage (~200 V). The TENG used here was firstly reported in [6]. It is constructed by a triangular-shaped conductive polyurethane foam (C-PUF), a 50 µm-thick polytetrafluoroethylene (PTFE) film and an aluminium film tape. The C-PUF plays the roles of electrode, space, spring, and friction material. A fabricated device with dimensions of 6×6×1 cm 3 can be seen in Figure 1b. The TENG was placed under a vibration shaker where the vibration frequency and amplitude/force can be controlled with a signal generator. A peak open-circuit voltage of ~102 V and a peak short-circuit current of 2.2 μA were obtained when the TENG was periodically pressed with a 5 Hz, 10 N-peak force. Compared to the full-wave rectifier, the half-wave diode bridge supplies a higher saturated voltage [7], although it requires a longer time to get saturated. A higher voltage leads to a higher energy per cycle (power) [8], therefore in this paper the half-wave rectifier is employed to rectify the impulse voltage generated by the TENG.
The 2 nd stage includes an electrostatic automatic switch (EAS) and an LC buck converter. The schematic of the automatic switch is shown in Figure 1c, which is constructed by a copper wire as the mobile electrode (Elect-1), a fixed aluminium plate (Elect-2), and a micro-positioning platform to control the gap between Elect-1 and Elect-2. This gap is the key parameter defining the pull-in (switching ON) voltage of the EAS. Elect-1 is connecting to Cbuf (Node M), and Elect-2 is connecting to the DC-DC buck converter (Node N). The inductor of the buck converter is 100 mH, and the storage capacitor (Cstore) is 10 μF. All the diodes in Figure 1 are double in series to enhance the breakdown voltage up to 500 V. The voltage across Cstore is used to drive a commercial regulator (LTC3588-1), while the output of the regulator is connected to an oscilloscope via a 1 MΩ probe.

Results and discussions
The measured voltage across Cbuf (VCbuf at Node M) is shown in Figure 2a, with a zoom-in in Figure 2b. VCbuf is slowly increasing to ~190 V when EAS is OFF, reaching the pull-in voltage of the EAS that turns ON. VCbuf quickly drops to ~0 V while charging Cstore through the inductor. The switching ONtime is 0.23 s and the switching OFF-time is 2.1 s, indicating a switching frequency of 0.43Hz ( Figure  2b).
The voltage across Cstore (VCstore) and the output of the regulator are shown in Figure 2c. There are also voltage up-down oscillations for VCstore. This voltage increase is due to the charge accumulation transferring from Cbuf when EAS is ON, while the voltage decrease is caused by the power consumption of the commercial regulator and the output load (the 1 MΩ testing probe). As shown by the red line in Figure 3c, the output of the regulator is initially 0V, and has a short starting-up oscillation when VCstore approaches 4.5V. This phenomenon is attributed to the inherent characteristic of the commercial component. The output finally stands at 3.3V after 35 s.
To evaluate the harvested energy or power quantitatively, the energy per cycle in Cstore is calculated, based on the formula: ∆ = ( − ) 2 ⁄ , where and represent V at the (n+1) th and n th cycle. The calculated energy per cycle in Cstore is shown in Figure 2d   As a comparison, we also did experiments using only a half-wave rectifier with Cstore =10 μF to drive the same regulator, without EAS and buck converter, as shown in Figure 3a. Firstly, we compared to the proposed two-stage method as aforementioned. The voltage across Cstore grows much slower (between Figure 2c and 3b). Secondly, the commercial regulator cannot be sustained by the voltage across Cstore, as the output of the regulator is quickly dropping to 0V once it reaches 3.3 V. This means that the proposed two-stage method can considerably improve the energy storage efficiency.

Conclusion
In summary, we proposed and demonstrated a two-stage energy storage system with electrostatic automatic switch and DC-DC buck converter. A smaller buffer capacitor was firstly charged to a high voltage, leading to the pull-in (switching-ON) of the electrostatic switch. Then the harvested energy in the buffer was transferred to the storage capacitor via a buck converter. The application of the buffer capacitor eliminates the previously-existed huge energy loss caused by the impedance mismatch between the TENG and the storage capacitor. A commercial regulator can be sustained at a constant 3.3V output, using the proposed two-stage method, which cannot be realized with a classical diodebridge rectifier. However, the reported automatic switch is only a proof of concept with imprecise manual control. In the future, we aim at a fully-integrated system using the microelectromechanical system (MEMS) technologies, to push this method forward in the practical applications.