Ironless Dual-Rotor Permanent-Magnet Machine for Flywheel Batteries

Energy storage is an emerging technology that can enable the transition towards renewable-energy-based distributed generation, reducing peak power demand and the time difference between production and use. The energy storage could be implemented both at grid level or at user level and nowadays different technologies are available for this purpose. In particular chemical batteries represent the de facto standard of storage systems for performance and maturity, however batteries feature a quite large environmental footprint and use precious raw materials. As an alternative to chemical batteries, mechanical storage technologies could be evaluated because of their reduced impacts on the environment and on the exploitation of raw materials. This paper presents the design, the prototyping and the experimental results of an ironless surface permanent-magnet (SPM) synchronous machine for a flywheel energy storage at household level. The electrical machine can work either as a motor or as a generator and it is the main item to fulfill the major challenges of a household application: high discharge duration and compatible size/dimension. The performance of the proposed ironless SPM machine are also compared with traditional iron core SPM synchronous machine by means of Finite Element Analysis (FEA).