Lumped Element Model for Thermomagnetic Generators Based on Magnetic SMA Films

Abstract

This paper presents a lumped element model (LEM) to describe the coupled dynamic properties of thermomagnetic generators (TMGs) based on magnetic shape memory alloy (MSMA) films. The TMG generators make use of the concept of resonant self-actuation of a freely movable cantilever, caused by a large abrupt temperature-dependent change of magnetization and rapid heat transfer inherent to the MSMA films. The LEM is validated for the case of a Ni-Mn-Ga film with Curie temperature TC of 375 K. For a heat source temperature of 443 K, the maximum power generated is 3.1 µW corresponding to a power density with respect to the active material’s volume of 80 mW/cm3. Corresponding LEM simulations allow for a detailed study of the time-resolved temperature change of the MSMA film, the change of magnetic field at the position of the film and of the corresponding film magnetization. Resonant self-actuation is observed at 114 Hz, while rapid temperature changes of about 10 K occur within 1 ms during mechanical contact between heat source and Ni-Mn-Ga film. The LEM is used to estimate the effect of decreasing TC on the lower limit of heat source temperature in order to predict possible routes towards waste heat recovery near room temperature.

Thermal processes of miniature thermomagnetic generators in resonant self-actuation mode

“…Most TMGs either convert the thermal energy directly using the magnetization change of the material or indirectly using a periodic mechanical motion generated from the magnetization change of the material ( Kishore and Priya, 2018 ). The development of TMGs encounters a number of design and engineering challenges due to the strong thermo-magneto-electro-mechanical coupling of system properties and several loss mechanisms due to magnetic stray fields and parasitic heat transfer ( Dzekan et al., 2021 ; Joseph et al., 2020 , 2021 ).…”

“…So far, resonant self-actuation has been investigated for heat source temperatures in the range of 130°C–170°C using Ni-Mn-Ga films with ferromagnetic transition temperature of 102°C. In order to access low-grade waste heat, dedicated thermomagnetic materials with critical temperature near room temperature and large magnetization change Δ M /Δ T will be needed ( Joseph et al., 2020 , 2021 ; Kohl et al., 2018 ). Furthermore, the demands on design and engineering to enable resonant self-actuation become severe requiring detailed understanding of the heat transfer dynamics.…”

“…A lumped element model (LEM) has been developed to describe and understand the heat intake of the TM film and subsequent heat dissipation as well as the resulting magnetization change of the TM film and the magnetic force-induced deflection of the cantilever ( Joseph et al., 2021 ). The unique mechanism of resonant self-actuation enables efficient conversion of thermal energy via magneto-mechanical energy into electrical energy.…”

“…The LEM should enable investigating the conditions for resonant self-actuation as well as identifying the optimal performance parameters for maximum power and efficiency during resonant self-actuation. The LEM is implemented in SIMSCAPE (R-2018b, 2018, MATLAB, Karlsruhe, Germany) to calculate the thermal, magnetic, and mechanical performances ( Joseph et al., 2021 ). The modeling parameters are summarized in Table 1 .…”

Smart Materials and Devices for Energy Harvesting

“…Another new device is the thermomagnetic generator (TMG) based on magnetic shape memory alloy (MSMA) films. The TMG generators make use of the concept of resonant self-actuation of a cantilever, caused by a large abrupt temperature-dependent change of magnetization in the material and the rapid heat transfer inherent to the MSMA films [ 13 ]. A prototype based on Ni-Mn-Ga film, with a Curie temperature TC of 375 K, has shown a relevant power density of 80 mW/cm 3 for a heat source temperature of 443 K. This device is modellable with a lumped element model that can be used to estimate the effect of decreasing TC on the lower limit of the heat source temperature in order to predict the possible routes towards waste heat recovery near room temperature [ 13 ].…”