On the power and efficiency of Ni₂MnGa magnetic shape memory alloy power harvesters

The martensite variant reorientation in Ni₂MnGa magnetic shape memory alloys (MSMAs) causes a change in their bulk magnetization, that can be harvested into useful voltage/power by means of a pick-up coil. The coil may be placed directly surrounding an MSMA element or to the side of the MSMA element wrapped around a magnetic core. This paper reports new power harvesting data generated with a bi-axial magnetic field and a surrounding coil and full strain field data for an MSMA subject to load similar to what is seen during power harvesting, then compares the performance of MSMA-based power harvesters with different designs to determine which give the best output. For this comparison, we provide a framework for evaluating the performance of MSMA-based power harvesters reported in the literature. This framework involves normalizing the results to the design characteristics of the respective harvesters, i.e. number of turns of the pickup coil, cross-sectional area of the pickup coil, frequency of excitation, and sample size, to allow for a direct comparison of power harvesters’ output. Results show that power harvesting with the bi-axial field and a surrounding coil does not generate as much power as previously thought. The strain maps reveal the potential for perpendicular twin boundaries that block each other’s motion limiting variant reorientation and correspondingly the harvester’s power output. The paper concludes that the largest change in magnetic flux density, which is the driver for power harvesting, occurs in the side coil setup with an optimized magnetic circuit and it recommends using this configuration for future MSMA-based power harvester designs for maximum power output.