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.
Highlights
Recovery of low-grade thermal energy is of special interest, which is mostly rejected as waste heat making up a huge portion of energy lost in the environment
We present a lumped element modeling (LEM) approach to investigate the interplay of the involved physical properties, in particular, the effects of heat intake and heat dissipation on the local temperature changes of the active material as well as the resulting changes of magnetization and force dynamics on power output
The output parameters are the magnetic force F, which is a function of temperature and position of the the magnetic force Fmag, which is a function of temperature and position of the magnetic shape memory alloy (MSMA) film, is a function eters arefilm, the and magnetic force F, which the electromagnetic damping force. of temperature and position of the and the electromagnetic damping force
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. One option is to make use of the steep increase in magnetization at the firstorder transformation between non-ferromagnetic martensite and ferromagnetic austenite in metamagnetic alloys, which is highly attractive in the case of small hysteresis [18,19,20] Another option is to utilize the large change of magnetization at the second-order transition, e.g., in the Heusler alloy Ni-Mn-Ga, which occurs without hysteresis [21,22]. Another critical aspect of device performance is the engineering of heat intake and dissipation for optimum energy conversion. The main novelties of this investigation are describing the experimental performance characteristics of a TMG demonstrator at resonant self-actuation with high accuracy and predicting the effect of decreasing Curie temperature on the lower limit of heat source temperature
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