A Utility-Scale Flywheel Energy Storage System with a Shaftless, Hubless, High-Strength Steel Rotor

Energy storage is crucial for both smart grids and renewable energy sources such as wind or solar, which are intermittent in nature. Compared to electrochemical batteries, flywheel energy storage systems (ESSs) offer many unique benefits such as low environmental impact, high power quality, and larger life cycles. This paper presents a novel utility-scale flywheel ESS that features a shaftless, hubless flywheel. The unique shaftless design gives it the potential of doubled energy density and a compact form factor. Its energy and power capacities are 100 kWh and 100 kW, respectively. The flywheel is made of high-strength steel, which makes it much easier to manufacture, assemble, and recycle. Steels also cost much less than composite materials. Design and analysis of the shaftless flywheel are presented first. In addition, the system incorporates a new combination active magnetic bearing. Its working principle and levitation control for the flywheel are discussed. The design of an integrated coreless permanent-magnet (PM) motor/generator for the flywheel is given as well. Initial test results show that the magnetic bearing provides stable levitation for the 5443-kg flywheel with small current consumption.