Understanding energy flow and inefficiency of a thermomagnetic generator by transient multi-physics modelling

Waste heat recovery improves energy efficiency and reduces greenhouse gas emissions, yet much industrial and environmental heat is currently wasted at low temperature. Thermomagnetic generators (TMG) convert such low-grade heat to electricity through a solid-state cycle, but existing prototypes suffer from low system efficiency and low cycle frequency. Here we develop a transient 3D digital twin of a TMG from fully coupled multi-physics simulations that use only measured material and geometry data, and apply it to the device with the highest reported power density, which has a genus-3 flux topology. We show why three-dimensional transient simulations are needed to capture the coupled magnetic, thermal, fluid-flow, and electrical domains. The model reproduces the measured open-circuit voltage within 4% and the output power within 5%, and reveals the origins of both shortcomings. A Sankey analysis shows that only 11% of the input heat reaches the thermomagnetic plates, with inlet mixing and unused outlet enthalpy as the two dominant losses; the plates themselves experience only one third of the 30 K inlet temperature span. Tracking the transient heat flow identifies the mixing chamber and the plate-channel transit as the main limits on cycle frequency. Removing the mixing chamber and interrupting flow between half-cycles are concrete design levers for higher-frequency, more efficient TMGs. These findings provide a validated and practical thermal-design basis for developing the next generation of TMG prototypes with higher operating frequencies and improved efficiency for low-grade waste-heat recovery.