Waste heat to power conversion by means of thermomagnetic hydrodynamic energy harvester

Energy harvesting from extremely low enthalpy sources can play an important role in increasing the sustainability of future energy applications: low temperature differences are common and offer an abundant source, available both in the natural environment and as the result of a many industrial process. This paper presents the first closed-loop thermomagnetic hydrodynamic energy harvester, based on thermomagnetic advection and exploiting a commercial ferrofluid. The lab-scale prototype has a toroidal geometry adopted from the well-known tokamak inertial machines. Peltier modules are used to control the thermal gradient that is harvested and converted directly to electric energy, while permanent magnets trigger the advection. Temperature sensors are installed along the toroidal walls (thermistors) and are placed in contact with the rotating fluid (thermocouples). To extract and ensure the electrical energy output, the structure is wrapped-up with induction coils. Two coil configurations (purely poloidal and mixed poloidal/toroidal windings) are tested, in a heterogeneous two-phase flow from the combination of water carrier and ferrofluid packets, reaching a maximum extracted electrical power per unit of temperature difference of 10.4 μW/K. This positions the device close to 20% of the ideal Carnot efficiency of a thermal machine working on the same temperature drop. Numerical analysis of the system has been performed developing a Fortran™ code in a Eulerian framework, using a mixed Fourier-Galerkin/finite difference spatial discretization. The harvester is suitable for producing electricity from running engines, appliances, warm gas exhausts, exothermic processes.