Mathematical modelling and simulation results of a linear thermomagnetic motor with gravity return

Abstract

Thermomagnetic motors are based on the effect of heat on magnetic properties of magnetic or magnetocaloric materials, which undergoes temperature and magnetic field cycles to produce mechanical power. This kind of motor has the potential to be used in the industry to recover low-grade heat, as energy harvesters. In this work, a model has been developed to simulate the time-dependent heat transfer processes in the heat exchanger coupled to the thermomagnetic phenomena. The heat exchanger is composed of prismatic segments of gadolinium magnetocaloric material with a circular channel, establishing an internal flow of working fluid. Gadolinium (Gd), which is usually adopted as reference material for magnetic cooling at room temperature, was considered as the working material because of the availability of properties data, and a mixture of water-glycol (20%) is used as working fluid. This way, the present paper investigated throughout numerical simulations the influences of the demagnetizing effects, magnetocaloric effect (MCE), heat exchanger geometric parameters (segmentation) and total weight of the heat exchanger on the performance of a linear thermomagnetic motor which the operating principle is based on the magnetic and gravitational forces balance (gravity return). Results for magnetic force, cycle period, and produced power are presented as a function of different operating conditions, such as flow rate and cold reservoir temperature, and design constraints. A maximum produced power of 0.3 W was found for a flow rate of 1.5 l/m and 20 working material bodies in the magnetic heat exchanger.