SPICE modelling of a coupled piezoelectric-bimetal heat engine for autonomous Wireless Sensor Nodes (WSN) power supply

Abstract

Abstract. This paper deals with an electrical modelling and optimization of a thermal energy harvester dedicated to power autonomous systems. Such devices based on bimetal strips and piezoceramics turn thermal gradients into electricity by a two-step conversion mechanism. This work focuses first on a demonstration of a ST-WSN (GreenNet demonstration platform) supplied by the harvester to validate, for the first time, the harvesters viability. That demonstration focuses attention on the need for an optimized power management circuit for piezoelectric generators able to reach output voltages up to 20V. The work deals then with the proposal of an equivalent lumped element model of the piezoelectric transducer with its SPICE implementation to enable the optimization of a dedicated power management circuit based on the Pulsed Synchronous Charge Extractor (PSCE). Simulations using the SPICE model and the power management circuit lead to an increased extracted power by 144%. IntroductionThe field of energy harvesting has become a predominant research area, especially, with the development of wireless sensors and communication node networks. Industrial and health monitoring, smart buildings, internet of things, these are some domains where node networks can be widely used. However, the impractical aspect of wired systems and the problems related to the power supply by short-lifetime batteries are raised for those applications. This brought attention on ambient energy harvesters and explains the great interest in energy scavenging devices over the last years. In this paper, a thermal energy harvester previously presented in [1,2,3] is studied. The harvester is based on coupling a bimetal to a piezoelectric transducer where a two-step conversion mechanism occurs. The heat flowing through the bimetal strip is converted into mechanical oscillations insuring the thermo-mechanical conversion; the second step consists in the conversion of the bimetallic strip impacts into electricity by a piezoelectric transducer. A bimetal strip is characterized by two equilibrium positions as shown in Fig. 1(a). Due to the bimetal thermo-mechanical bi-stability explained in [4], the bimetal snaps up and hits the piezoelectric membrane once its temperature raises and reaches a threshold temperature called the snap-up temperature. At that point, the bimetal is at its upper-position and releases a part of its energy to the piezoelectric. While being in contact with the piezoelectric acting as a cold surface, the temperature of the bimetal decreases until it reaches a certain snapping-back temperature that makes it switch-back to its initial

Coupling of PZT Thin Films with Bimetallic Strip Heat Engines for Thermal Energy Harvesting

“…This study has allowed us to demonstrate the feasibility of this coupling. However, since the amount of harvested power did not exceed 10 nW for the most powerful system, it is important to carry out further studies with regard to film deposition optimization and harvested energy improvement before performing a wireless sensor node demonstration as in [ 25 ].…”

Dynamic model of a novel thermogravity rotary actuator for solar energy harvesting

“…The raw output power ranged up to 13.46 W per device with a 60°C heat energy source and up to 10 W using three devices in parallel. The article 7 presented the electrical modeling and optimization of bimetallic strip and piezoceramics thermal energy harvesters, which were used in two steps to convert thermal gradients into electricity up to 20 V. The developed electrical model based on lumped parameter model was implemented in SPICE for simulation of device performance analysis in different working phases. In Reference 8, the thermal modeling of a coupled piezoelectric and bimetallic‐based heat engine based on equivalent electrical circuit approach was presented.…”

Thermo-mechanical efficiency of the bimetallic strip heat engine at the macro-scale and micro-scale

“…The bimetallic strip heat engine has also been developed with the objective of being an alternative solution to thermoelectric generators [20]. Proofs of concept of these heat engines were presented in [21][22][23][24][25][26]. These energy harvesters are designed to exploit the sudden displacement of a bistable bimetallic membrane switching from a critical equilibrium position to a stable one, in order to harvest part of the thermal energy flowing through the beam, and to convert it into kinetic energy.…”

“…As the bimetallic strip only realizes a thermo-mechanical transduction, a second conversion step is needed to convert heat into electricity. Various coupling methods were reported: in [21,23,25,26], the kinetic energy of a switching bimetallic shell is transferred to a piezoelectric bimorph used as an electro-mechanical transducer and a heat exchanger. Reference [24] reports that a NC4-invar 3 K-hysteresis bimetallic strip (36 × 18 × 0.3 mm 3 ) placed in a thermally-optimized harvester and working between 373 K and the ambient temperature, generates an output electrical power of 2.56 μW with an operation frequency of 0.25 Hz.…”

“…In [22,23], an electret placed inside the device realizes a capacitive transduction of the mechanical energy of the bistable membrane into electrical energy: an output power of 5 μW is generated with a B72M-Invar 3 °C-hysteresis bimetallic strip (20 × 10 × 0.12 mm 3 ) working in a 45 K thermal gradient and switching at 2 Hz. Tests presented in [23,26] have demonstrated the capability of the bimetallic strip heat engines to supply Wireless Sensor Nodes: [26] reports that the output power of 2.56 μW per harvester is sufficient to power a STMicroelectronics GreenNet node sending data every 29 s and [23] demonstrates the possibility of powering the same node every 40 s with ten devices coupled with an electret transducer.…”