Lubricant Free Foil Bearings Pave Way to Highly Efficient and Reliable Flywheel Energy Storage System

Advanced compliant foil bearings capable of operating in low ambient pressures associated with soft vacuum are now paving the way to a new type of flywheel energy storage system. Many conventional flywheel energy storage system design approaches use active magnetic bearings with backup bearing technologies to meet the need for high speed operation in a low ambient pressure environment. Low ambient pressures are needed to overcome the power loss limitations associated with windage at high surface speeds. However, bearing technologies that rely on active control tend to be large, are dynamically soft which necessitates backup bearings and require a power supply which consumes some of the stored power to maintain rotor levitation. In this paper the authors will demonstrate both theoretically and experimentally the ability of advanced 5th generation compliant foil bearings to support large flywheel rotors weighing in excess of 900 N and which can operate to speeds in excess of 40,000 rpm. Testing conducted at pressures as low as 7 kPa demonstrates the ability of foil bearings to operate in low ambient pressures consistent with flywheel energy storage system needs for low windage loss. The authors will also present a hypothesis and the mechanisms involved in a hydrodynamic phenomenon that allows a foil bearing to operate successfully when the mean free path of the air molecules is exceedingly large due to low ambient pressures.