Thermal energy harvesting system based on magnetocaloric materials★

Smail Ahmim,Morgan Almanza,Alexandre Pasko,Frédéric Mazaleyrat,Martino Lobue

doi:10.1051/epjap/2019180284

Abstract

We numerically study the design of a thermomagnetic generator aimed to convert a heat flow into electrical energy. The device uses the variation of magnetization of a magnetocaloric material (MCM) along a cyclic transformation between the hot and the cold sources. The magnetic energy is transformed into mechanical energy via the magnetic forces and eventually into electrical energy through an electromechanical transducer. Firstly, we work-out the optimal size of the cantilever in order to achieve the self-oscillation of the MCM between the two heat sources. Eventually, using finite element calculations, we compare the efficiency of a piezoelectric transducer (PZT 5a) with that of a set of coils in order to convert the mechanical into electrical energy. The piezoelectrics and the coils recover 0.025% and 0.018% respectively of the available mechanical energy (116 mJ/cm3). The possible strategies to achieve a better performance are discussed in theconclusion.

Highlights

Nowadays the number of connected systems is constantly increasing
The magnetocaloric effect is used to pump heat through the entropy change of the magnetocaloric material (MCM) associated to application/ removal of a field
Whereas general thermodynamic considerations allow to work-out the efficiency of the thermal cycle, estimating the efficiency of the mechanical to electrical energy conversion can be a hard task, and it often relies on extrapolations from approximated models

Summary

Introduction

Nowadays the number of connected systems is constantly increasing. Some of these systems are autonomous, often small (mm to cm), and require a source of energy. Low grade heat sources (i.e. temperature differences below 60 K) are quite ubiquitous [1] making thermal energy recovery systems the natural candidates for powering small autonomous devices. Thermomagnetic generators (TMGs) use the pyromagnetic effect, namely the variation of magnetization as a function of the temperature, to perform thermodynamic cycles under varying field and eventually convert the magnetic into electrical energy. Ujihara and coworkers [3] realized a TMG showing an estimated electrical power density between 1.85 and 3.61 mW/cm for a temperature difference between the hot and cold source of DTres = 50 K. Whereas general thermodynamic considerations allow to work-out the efficiency of the thermal cycle, estimating the efficiency of the mechanical to electrical energy conversion can be a hard task, and it often relies on extrapolations from approximated models. We discuss the results and foreseen some possible optimization strategies

Operating principle and thermal to mechanical energy conversion

Energy harvesting and mechanical to electrical energy conversion

Estimation of the recoverable mechanical energy using coils

Estimation of the recoverable mechanical energy using piezoelectric elements

Findings

Discussion and conclusions