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

Hemin Zhang, Dimitri Galayko, Philippe Basset

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Hemin Zhang, Dimitri Galayko, Philippe Basset. A self-sustained energy storage system with an electrostatic automatic switch and a buck converter for triboelectric nanogenerators. 18th International Conference on Micro and Nanotechnology for Power Generation and Energy Conversion Applications, Dec 2018, Daytona Beach, Florida, United States. pp.012016, 10.1088/1742-6596/1407/1/012016. [hal-04031078](https://hal.science/hal-04031078)

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

To cite this article: Hemin Zhang et al 2019 J. Phys.: Conf. Ser. 1407 012016

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

Hemin Zhang¹, Dimitri Galayko² and Philippe Basset¹

¹Université Paris-Est, ESYCOM, ESIEE Paris, France

²Sorbonne Universités, LIP6, France

E-mail: [1989hemin@gmail.com](mailto:1989hemin@gmail.com)

**Abstract.** In this paper, we present for the first time a complete energy harvesting system for 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.

## 1. 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 Things (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

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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.

## 2 Experimental setup and results discussion

### 2.1 Schematic and experimental setup

As shown in Figure 1, the 1st stage of the proposed energy storage system is a half-wave rectifying module. A small buffer capacitor ($C_{buf}$ = 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 cm³ 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 2nd 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 $C_{buf}$ (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 ($C_{store}$) is 10 µF. All the diodes in Figure 1 are double in series to enhance the break-down voltage up to 500 V. The voltage across $C_{store}$ 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.

<figure><img src="image_6.png" /><figcaption>**Figure 1.** Description of the system. (a) Schematic of the self-sustained energy harvesting system, (b) Photo (side view) of the TENG, (c) Principle and setup of the electrostatic switch.</figcaption></figure>
## 2.2 Results and discussions

The measured voltage across $C_{buf}$ ($V_{Cbuf}$ at Node M) is shown in Figure 2a, with a zoom-in in Figure 2b. $V_{Cbuf}$ is slowly increasing to ~190 V when EAS is OFF, reaching the pull-in voltage of the EAS that turns ON. $V_{Cbuf}$ quickly drops to ~0 V while charging $C_{store}$ through the inductor. The switching ON-time 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 $C_{store}$ ($V_{Cstore}$) and the output of the regulator are shown in Figure 2c. There are also voltage up-down oscillations for $V_{Cstore}$. This voltage increase is due to the charge accumulation transferring from $C_{buf}$ 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 $V_{Cstore}$ 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 $C_{store}$ is calculated, based on the formula: $\Delta E_{cyc} = C_{store}(V_{n+1}^2 - V_n^2)/2$, where $V_{n+1}$ and $V_n$ represent $V_{Cstore}$ at the $(n+1)^{th}$ and $n^{th}$ cycle. The calculated energy per cycle in $C_{store}$ is shown in Figure 2d. Because the energy flows in $C_{store}$ is discontinuous based on the switch state ON or OFF, the calculated curve of the energy per cycle includes a negative component (no energy flows into $C_{store}$ corresponding to the OFF-state duration), and a positive component (energy flows into $C_{store}$ corresponding to the ON-state duration). The positive value in Figure 2d represents the harvested energy in $C_{store}$ ($\Delta E_{harv}$) minus the consumed energy ($\Delta E_{cons}$) by the load (regulator plus probe). The negative value is equal to the consumed energy. We obtained from the figure that during normal operation (when the switching occurs at 200 V): $\Delta E_{harv\_peak} - \Delta E_{cons\_peak} \approx 1\mu J$, and $\Delta E_{cons\_peak} \approx 0.7\mu J$, thus the finally harvested energy per cycle of $C_{store}$ is $1.7\mu J$, corresponding to a power of $8.5\mu W$.

<figure><img src="image_7.png" /><figcaption>**Figure 2.** Electrical performance of the energy storage system. Voltage across $C_{buf}$ (Node M) (a) and its detailed view (b) versus time. Voltage across $C_{store}$ ($V_{Cstore}$) (c) and the calculated energy per cycle in $C_{store}$ (d).</figcaption></figure>
As a comparison, we also did experiments using only a half-wave rectifier with $C_{store}$ =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 $C_{store}$ grows much slower (between Figure 2c and 3b). Secondly, the commercial regulator cannot be sustained by the voltage across $C_{store}$, 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.

<figure><img src="image_8.png" /><figcaption>**Figure 3.** Using a normal half-wave rectifier without switch and buck converter to drive the regulator. Experimental setup (a), and voltage across $C_{store}$ as well as output of the regulator (b).</figcaption></figure>

### 3. 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 diode-bridge 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.

### References

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