Arthur Arnaud

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

Bimetallic strip heat engines are energy harvesters that exploit the thermo-mechanical properties of bistable bimetallic membranes to convert heat into mechanical energy. They thus represent a solution to transform low-grade heat into electrical energy if the bimetallic membrane is coupled with an electro-mechanical transducer. The simplicity of these devices allows us to consider their miniaturization using MEMS fabrication techniques. In order to design and optimize these devices at the macro-scale and micro-scale, this article proposes an explanation of the origin of the thermal snap-through by giving the expressions of the constitutive equations of composite beams. This allows us to evaluate the capability of bimetallic strips to convert heat into mechanical energy whatever their size is, and to give the theoretical thermo-mechanical efficiencies which can be obtained with these harvesters.

Thermal modeling and optimization of a thermally matched energy harvester

The interest in energy harvesting devices has grown with the development of wireless sensors requiring small amounts of energy to function. The present article addresses the thermal investigation of a coupled piezoelectric and bimetal-based heat engine. The thermal energy harvester in question converts low-grade heat flows into electrical charges by achieving a two-step conversion mechanism for which the key point is the ability to maintain a significant thermal gradient without any heat sink. Many studies have previously focused on the electrical properties of this innovative device for energy harvesting but until now, no thermal modeling has been able to describe the device specificities or improve its thermal performances. The research reported in this paper focuses on the modeling of the harvester using an equivalent electrical circuit approach. It is shown that the knowledge of the thermal properties inside the device and a good comprehension of its heat exchange with the surrounding play a key role in the optimization procedure. To validate the thermal modeling, finite element analyses as well as experimental measurements on a hot plate were carried out and the techniques were compared. The proposed model provided a practical guideline for improving the generator design to obtain a thermally matched energy harvester that can function over a wide range of hot source temperatures for the same bimetal. A direct application of this study has been implemented on scaled structures to maintain an important temperature difference between the cold surface and the hot reservoir. Using the equations of the thermal model, predictions of the thermal properties were evaluated depending on the scaling factor and solutions for future thermal improvements are presented.

Coupling of a bimetallic strip heat engine with a piezoelectric transducer for thermal energy harvesting

ABSTRACT The field of energy harvesting became attractive due to the development of wireless sensors networks. Consequently many systems emerged to harvest wasted energies to convert it into usable electrical energy. The goal behind these harvesters is to be able to supply low power sensor nodes. The research work described in this paper deals with a thermal energy harvester based on a double stage conversion to obtain usable electrical energy. More specifically, this paper presents the properties of the materials used in this harvester. It concerns its components, its thermal modeling and optimization, its electrical modeling and finally the piezoelectric benchmark.

Reduced model for the comprehension of the operation of a thermo-mechanical energy harvester

The bimetallic strip heat engines are thermal energy harvesters that have been designed to convert low-grade heat flux coming from local thermal gradients into mechanical energy by using the thermo-mechanical instability of bimetallic membranes. Great efforts must be done on the modeling of these heat engines in order to understand their way of working. This paper is a contribution to these efforts since it proposes approximate analytical expressions of the efficiencies of these heat engines and figures of merit to compare bimetallic beams.

Modeling of the thermo-mechanical efficiency of the bimetal strip heat engines

This paper presents a theoretical demonstration of the bimetal strip heat engine working, based on the study of the thermo-mechanical instability of the pre-buckled bimetallic beams. Starting from the Euler buckling equation, this paper describes the bimetal strips like classical but non-linear thermodynamic systems, and gives the bistability criterion of such beams. Studying the thermodynamic potentials of these beams helps to evaluate the release of the kinetic energy happening during the beam snap-through, to give the Maxwell relations between each partial derivative of the thermodynamic potentials and to show that the thermal snap-through is a first-order transition according to the Ehrenfest theory. The model is then used to draw the temperature-entropy cycle of the bimetal heat engines and to evaluate the performances of these harvesters (available mechanical energy and thermodynamic cycle efficiency).

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

A thermal energy harvester based on a double transduction mechanism and which converts thermal energy into electrical energy by means of piezoelectric membranes and bimetals, has previously been developed and widely presented in the literature In such a device, the thermo-mechanical conversion is ensured by a bimetal whereas the electro-mechanical conversion is generated by a piezoelectric ceramic. However, it has been shown that only 19% of the mechanical energy delivered by the bimetal during its snap is converted into electrical energy. To extract more energy from the bimetallic strip and to increase the transduction efficiency, a new way to couple piezoelectric materials with bimetals has thus been explored through direct deposition of piezoelectric layers on bimetals. This paper consequently presents an alternative way to harvest heat, based on piezoelectric bimetallic strip heat engines and presents a proof of concept of such a system. In this light, different PZT (Lead zirconate titanate) thin films were synthesized directly on aluminium foils and were attached to the bimetals using conductive epoxy. The fabrication process of each sample is presented herein as well as the experimental tests carried out on the devices. Throughout this study, different thicknesses of the piezoelectric layers and substrates were tested to determine the most powerful configuration. Finally, the study also gives some guidelines for future improvements of piezoelectric bimetals.

Stacked Nanowires FETs: Mechanical robustness evaluation for sub-7nm nodes

Stacked Nanowires FETs are proposed to replace FinFET and FDSOI for sub-7nm nodes. While most studies demonstrate the performances gain offered by such structures, mechanical stability of the suspended silicon channels needs to be considered. This paper provides a fully mechanical analytical description of nanowire stacks to explain the occurrence of buckling phenomena of silicon channels.

Dynamic study for performance improvements of a thermo-mechanically bistable heat engine

This paper focuses on a thermal study of a thermal energy harvester based on the coupling of a bimetallic strip heat engine with a piezoelectric membrane for wasted heat scavenging. Such a harvester is dedicated to power autonomous systems such as wireless sensor nodes. For a better understanding of the working principle of the system, it is compulsory to have a good understanding of the thermal specificities and phenomenon taking place inside the harvester. Attention is consequently focused on the thermal modeling of the harvester in static mode using the equivalence between the electrical and thermal quantities. This first modeling step allowed the improvement of the thermal properties inside the system by increasing the thermal gradient across it. However, the bimetal being the active part of the system has not been taken into account in this model and shadow zones persisted regarding the bimetal operation windows as a function of its snapping temperatures and hysteresis. To overcome this, a dynamic model is proposed in this paper taking into account the bimetal as a switched capacitance alternatively in contact with the hot source and the cold surface. This last model completed the static one by predicting the bimetals operation windows in function of its intrinsic properties and the operation range evolution in function of the snapping temperature first and then in function of the bimetal thermal hysteresis. Moreover, experimental measurements enable to validate the proposed model and to point out the most powerful bimetals for scavenging higher amounts of power.

Electrical performances of pyroelectric bimetallic strip heat engines describing a Stirling cycle

This paper deals with the analytical modeling of pyroelectric bimetallic strip heat engines. These devices are designed to exploit the snap-through of a thermo-mechanically bistable membrane to transform a part of the heat flowing through the membrane into mechanical energy and to convert it into electric energy by means of a piezoelectric layer deposited on the surface of the bistable membrane. In this paper, we describe the properties of these heat engines in the case when they complete a Stirling cycle, and we evaluate the performances (available energy, Carnot efficiency...) of these harvesters at the macro- and micro-scale.