A Thermal Energy Harvester Using LaFeSi Magnetocaloric Materials

Abstract

A thermomagnetic generator (TMG) using an Halbach structure as a field source and a first order LaFeSi magnetocaloric material (MCM) from Vacuumschmelze as active substance, is presented. The MCM, suspended on a cantilever beam self oscillates between a hot source and a heat sink. The mechanical energy associated with the oscillation is harvested and converted into electricity using piezoelectric materials. This system is working between a 18°C cold end, and a 56°C heat source (i.e. over a temperature difference between reservoirs ∆Tres = 38°C) and shows a power of 0.12µW (6.8 µW per 1 cm 3 of MCM). We present and discuss a detailed analysis of the thermodynamic cycle underpinning the device mechanism, relying on direct measurements on the working prototype and on a full laboratory characterization of the MCM. Although our system shows a state-of-the-art power output, our analysis gives useful clues towards further performance improvements.

Self-oscillation and heat management in a LaFeSi based thermomagnetic generator

“…3) curves measured with a VSM magnetometer are shown in Fig. 3 (see [19] for more details). Defects in the assembly with the magnets (misalignment) or in the magnets themselves lead to a lack-off flatness.…”

“…Recently, we presented a piezoelectric based TMG [19] using a commercial first order MCM (CALORIVAC ® from Vacumschmelze) as active substance. The adopted design is similar to the one presented in [13] with two major differences: the magnetic field source is a Halbach array in order to achieve high field gradients keeping demagnetizing fields as low as possible; the active substance undergoes a first order magnetic transition allowing higher efficiency at maximum power as shown in a recent study [20].…”

“…This design, generally referred as linear [16], is particularly well suited to work over a well controlled thermodynamic four-strokes cycle (viz. two quick adiabats, and two heat exchange transformations) [21], and to reach a self-oscillating regime where no external actuation is needed to drive the device [22,4,19]. The device reaches up to 0.12 µW electric power-output with a density of 6.8 µW.cm −3 per volume of the MCM [19].…”

“…two quick adiabats, and two heat exchange transformations) [21], and to reach a self-oscillating regime where no external actuation is needed to drive the device [22,4,19]. The device reaches up to 0.12 µW electric power-output with a density of 6.8 µW.cm −3 per volume of the MCM [19]. It works over a temperature difference ∆T res = 38 K, with a 0.8 mm linear displacement, at a frequency f = 0.12 Hz.…”

“…Steps 2 and 4 are steep, a signature of the researched bistable behavior. Working over a total displacement d = x 1 − x 0 = 0.48 mm the device reach a frequency f = 0.41 Hz which is three times higher than the frequency of our previous prototype [19]. Position of the MCM (mm) The voltage induced on the piezoelectrics by the deformation is measured using a load resistor which maximizes the output power.…”