A novel rotary state of the art active magnetic regenerator (AMR) refrigeration prototype was used in an experimental investigation with special focus on efficiency. Based on an applied cooling load, measured shaft power and pumping power applied to the AMR, a maximum second-law efficiency of 18% was obtained at a cooling load of 81.5 W, resulting in a temperature span of 15.5 K and a COP of 3.6. A loss analysis is given, based on measured pumping power and shaft power together with

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... y estimated regenerator presssure drop. It is shown that, especially for the pressure drop, significant improvements can be made to the machine. However, a large part of the losses may be attributed to regenerator irreversibilities. Considering these unchanged, an estimated upper limit to the 2 nd -law efficiency of 30% is given by eliminating parasitic losses and replacing the packed spheres with a theoretical parallel plate regenerator. Furthermore, significant potential efficiency improvements through optimized regenerator geometries are estimated and discussed.