Novel applications of the flywheel energy storage system

Flywheel energy storage system is focused as an uninterruptible power supplies (UPS) from the view point of a clean ecological energy storage system. However, in high speed rotating machines, e.g. motor, generator and flywheel, the windage loss amounts to a large ratio of the total losses. The reason is that windage loss is proportional to the cube of its angular velocity; a windage loss may lead to the reduction of total system efficiency. To cope with this problem, Ajisman et al. proposed the use of helium–air mixture gas into the housing and indicated that the helium (50 vol%)–air (50 vol%) mixture gas can reduce the windage loss to 42% of that in the air (100 vol%) case. Helium is the second lightest and smallest monatomic molecule gas. Its molecular weight and gas density are about 1/7 those of air, thermal conductivity is 10 times as large as that of air. Then, enclosing helium–air mixture gas into the housing of rotating machine, a large amount of windage loss can be reduced.