Tribo-electric/electret (Nano) Generator TENG)

Grants:  CNRS, Campus France


Modeling
Devices
Conditioning circuit with micro-plasma switch
Publications

Modeling

Simple TENG model

In addition to the model proposed by  Niu et al, we have proposed a lumped model directly inspired from the  traditional electret Kinetic Energy Harvester. This model is suitable for PSpice and can easily take into account practical specifications of the TENG for accurate conditioning circuit simulations.
 
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TENG electrical model. a. Schematic of a TENG. b. Detailed electrical model. c. Compact electrical model.

VTE is a constant voltage source representing the charge trapped in the the triboelectret layer. It is independant from the mobile electrode position and from the load. It can be directly measured with an electrostatic contactless volmeter, or, if the layer is not accessible, using an electrical technique we have developped in purpose.

CTENG is the capacitance measured accross the TENG terminals. It  can be obtained  by measuring the phase shift in an RC circuit.

In this work, we have also modelized the effect of the mechanical contact.
 
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Comparison between PSpice simulation and experimental results for a gap-closing TENG.
  

Optimization of a two-stage power management

We provided a comprehensive general theory that determines the optimal electrical bias conditions for this class of rectifiers. We proposed generic formulas that applie to full-wave and half-wave diode bridges. Key figures have been demonstrated like for instance the optimal bias voltage or the maximum converted energy. It is confirmed that half-wave rectifiers always have a higher saturated voltage, as well as higher maximum energy per cycle, but at the cost of longer start-up time. On the contrary, full-wave rectifiers perform better only when the output voltage is much lower than the internal triboelectric voltage of the TENG. These rectifiers followed by a buck DC-DC converter have also been studied in details, often required to provide a low output voltage. We showed that the optimal buck’ switch activation is between .5 and .7 of the charge-pump saturation voltage, depending on the hysteresis of the switch, and that the charging time of the output capacitor is at least twice as fast with a half-wave rectifier than with a full-wave rectifier. The theoretical results were confirmed by simulations and experiments using a plasma switch.

Optimizatiopn of a 2-stage conditioning circuit
Simulation results of a two-stage power management circuit (2SPMC). (a) Schematic of the 2SPMC. (b) Energy and average energy per cycle in Crect using the HW rectifier. (c) Voltage across Crec with two levels of switch hysteresis.

Optimization for a small number of mechanical actuations

We have presented an optimization framework to maximize the energy conversion in the early stages of transducer operation by tuning the output capacitor (Crec) of the DC rectifier. Through a combination of analytical modeling, SPICE simulations, and experimental validation using a custom test bench, we show that after the first mechanical actuation, tuning Crect to a value close to the minimum value of the TENG capacitance, and choosing the half-wave rather than the full-wave rectifier configuration, can drastically enhance the energy conversion. For the first actuation, half-wave and full-wave are equivalent as long as Crect is minimized and less than the maximum value of the TENG capacitance.

Optimization for a small number of actuation
Harvested energy obtained with SPICE simulations and analytically (identical results) for the first four actuations. (a) Accumulated energy ΔW for the half-wave rectifier, (b) average energy per mechanical cycle for the halfwave rectifier, (c) accumulated energy ΔW for the full wave rectifier, and (d) average energy per mechanical cycle for the full-wave rectifier.

Devices

Soft TENG

We have devloped a simple flexible and progressive triboelectric nanogenerator based on macro-triangle-prism-shaped conductive polyurethane (PU) foam and polytetrafluoroethylene (PTFE) film. The proposed macro-structured conductive PU foam also integrates the functions of spring, spacer and electrode. Thanks to the innovative structures and choice of the materials, an extended current pulse width is obtained. 
 
Caracterization w/o ball
Caracterization w/o ball
Ilustration of the flexible TENG. Diagram (a), top view b(i), side view b(ii), and fabricated TENG with triangle prisms b(iii). SEM image of the C-PUF (c). Measured transient current of the TENG (e), Capacitance variation of the TENG (e). Measurement setup (g).

Road bump TENG

We have also proposed a road bump triboelectric generator with Bluetooth communications:

 Road bump TENG
(a) Road transducer prototype and (b) side view schematic of the transducer.

Road bump TENG circuit with BLE communication
Full circuit for powering a BLE module and transmit sensors’ data.

Conditioning circuit with micro-plasma switch

Rectification with stable charge pumps

We have shown that a half-wave (HW) rectifier usually performs much better than a full-wave (FW) diode bridge after a few conversion cycles. Indeed, during the early cycles, the output voltage of the FW rectifiers increases with twice the slope of the HW, although in that case both powers are far from the optimum of each circuit. However, if the output voltage of the rectifier can be set to half of its saturation voltage, HW outperforms the FW by a factor (CTENG_max/CTENG_min+1)/2.

  Caracterization with ball
Measured and calculated energy delivered to a capacitive load as a function of VTENG for a FW and a HW diode bridge.

Rectification with unstable charge pumps

We have proposed to use the Bennet doubler with TENGs. This is a new class of conditioning circuit inspired from the electrical machines of the 18th century. It is also made of diodes and capacitors only. These circuits have the ability to exponentially increase the charge on the TENG's electrodes during operation and so to increase its bias and conversion efficiency. A minimum value of  CTENG_max/CTENG_min is necessary, which is typically 2 for the most simple architecture, but it can vary depending on the circuit configuration.
 
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Bennet doubler circuit and experimental comparison with diode bridges.
  

High-voltage power management using a plasma switch

Electrostatic energy harvesters need a high bias, which for TENGs is brought by an efficient triboelectric contact and/or thanks to the voltage boost from an unstable charge pumps. However, for most applications it is necessary to convert the output to a low voltage around a few volts. This can be obtained using a Buck DC-DC converter, but its switch needs to be controlled at a voltage close to the high bias value. We proposed to use a MEMS micro-plasma switch to self-control the charge transfer through a Buck circuit at high voltages. By adjusting the MEMS switch design, we can control its hysteresis in order to continuously maintain a high-bias on the transducer and hence maximize the energy conversion.

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Bennet doubler circuit and experimental comparison with diode bridges.
  
This work has been highlighted in the  Electronics Insights Blog of the Electropages website.

Related publications

Modeling of TENGs

Flexible TENG

Conditioning circuits for TENGs and other electrostatic kinetic harvesters

 

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