A HEAT TRANSFER STUDY AIMING OPTIMIZATION OF MAGNETIC HEAT EXCHANGERS OF THERMOMAGNETIC MOTORS

摘要

The utilization of renewable energy sources is becoming ever more important to our society, because of the serious environmental problems associated with the use of fossil fuels to satisfy its energy needs. So, it is important to have devices able to efficiently use low quality energy sources, as the ones coming from solar energy, geothermal energy and industrial or vehicular waste heat. Prominent in this respect is the use of thermomagnetic motors, once it is expected that their efficiency can be better than the ones of conventional devices for energy conversion (Egolf et al, 2009). We developed two prototypes of thermomagnetic motors, one linear reciprocating and the other a variable reluctance rotating type, to work with hot water obtained from plate solar collectors as a hot source (70 - 80°C), and water at room temperature (25 - 30°C) as cold source. In both models the force or torque element is constituted of a Halbach arrangement of permanent magnets (Halbach, 1980) and a plate of magnetic material that has a Curie temperature midway between the temperatures of the hot and cold sources. The Halbach arrangement provides the field gradients necessary to generate strong forces and torques, and the magnetic material is shaped in form of a parallelepiped plate in the case of the reciprocating motor, and in shape of a cylindrical plate in the case of the rotating motor. Both types of plates work also as heat exchangers, once their magnetic states must be periodically switched from ferromagnetic to paramagnetic in order for the motors to work appropriately. As the power to be obtained from the motors is a direct function of their operating frequency, it is important to have a design of the heat exchangers channels optimized to enhance the heat exchange, so that higher frequencies can be obtained. We report in this work the optimization procedures done using CFD software aiming to have shapes of the channels in the magnetic plates of the motors maximizing the heat exchange between the magnetic materials and the hot and cold fluids, in order to have the maximum possible operation frequency of the motors.