Sealing system for flywheel in a vacuum

An apparatus comprising:

• a flywheel;
• a shaft;
• a bearing arrangement for supporting the shaft;
• a sealing arrangement;
• and
• a housing for housing the flywheel, the shaft, the bearing arrangement and the sealing arrangement, wherein the flywheel is mounted on the shaft, the sealing arrangement is mounted between the shaft and the housing, and the bearing arrangement is mounted both to the shaft, between the flywheel and the sealing arrangement, and to the housing;
• and
• wherein the sealing arrangement comprises a pair of seals that are configured to extend circumferentially around the shaft, the seals being arranged to form a hermetic seal against the shaft to hermetically seal the housing,
• wherein a cavity is defined between the seals, the cavity also extending circumferentially around the shaft;
• and
• wherein, in use, the sealing arrangement is arranged to maintain a first pressure within the housing that is lower than a second pressure outside of the housing, the seals and pressure control circuit being arranged to retain a sealing fluid within the cavity, and to maintain the fluid at an intermediate pressure between the first and the second pressure.