First vs second order magnetocaloric material for thermomagnetic energy conversion

We estimate the power and efficiency of a thermal energy harvesting thermodynamic Brayton cycle using a first and second order magnetocaloric materials as active substance. The thermodynamic cycle was computed using a simple thermal exchange model and an equation of state deduced from a phenomenological Landau model. For the first and second order materials, narrow and high frequency cycles are optimum and give similar performances. Considering technological issues hindering the increase of frequency, we introduced a more detailed approach where we take into account the time needed to switch the material between two heat reservoirs. We show that the first order material equation of state leads thermodynamic cycle shape keeping it closer to the optimum cycle. Conditions to improve the performance of second order materials are discussed. In addition, we infer key remarks for prototype design regarding the power density and efficiency reachable in different configurations.