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.

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.