The thermodynamics of devices with solid layered-bed magnetocaloric refrigerants

In conventional refrigeration the use of the T–S diagram to determine thermodynamic cycle performance is a well accepted tool. This method is based on a single state description of the gaseous refrigerant during its cycling. Devices filled with a magnetocaloric refrigerant, due to slow diffusion and inhomogeneous temperature distributions, usually do not allow such a simple description. If the layered bed technique is applied, the situation is even more complex. The related problems are addressed and some new solutions are presented, e.g. a generalized T∗–S∗ diagram containing weights w∗. Furthermore, an analysis shows that an implementation of a layered bed of numerous weak magnetocaloric alloys does not substantially decrease the coefficient of performance, COP, compared to a single correspondingly better performing magnetocaloric material, but leads to an increase of the magnet size, the volume and the related price of a device. With devices operating with higher frequencies this can be counterbalanced and magnetic heating, refrigeration and energy conversion – with present available magnetocaloric materials – suddenly show a realistic chance for certain market penetrations