At present, we recognize four basic methods of electric energy storage: pumped hydroelectric energy storage, chemical cell (battery), compressed air energy storage and flywheel.
Competitive use of flywheel as energy storage device is hampered by the high investment costs related to the flywheel construction materials being heavy steel or light, though durable, carbon-fiber composite. The amount of accumulated energy is proportional to the flywheel mass and its velocity squared, so doubling its mass increases the energy twice but doubling the velocity increases the energy fourfold. High mass represents enormous demands on the flywheel bearing. With declining prices of permanent magnets, transition from ball bearings to magnetic levitation has been observed. High rotation of flywheel causes losses due to air friction, therefore rotary flywheel must be enclosed in vacuum container. Such casing must be sufficiently resistant against possible destruction of the flywheel because of extreme centrifugal forces acting on the rotary flywheel.
Moreover, flywheel rotating for prolonged period of time is affected by gyroscopic effect caused by rotation of Earth around its axis while the flywheel tends to maintain its own orientation of its axis of rotation, which give rise to undesired forces acting on the bearing. Deflection of the flywheel's axis of rotation represents 360° per 24 hours, i.e. 15° per one hour, which cannot be neglected as these forces cause undesired friction at the axis of the flywheel and thereby cause losses in energy storage efficiency.
The amount of accumulated energy is proportional to the mass of the flywheel, therefore it is desired that the flywheel has variable mass and thus also variable angular momentum. Flywheels solving the problem of the variable angular momentum by filling hollow flywheel with a liquid are known, e.g. from WO/2012/127194 or GB2463534.
However, construction of energy storage device with variable capacity significantly exceeding 1 MWh (3.6 GJ) still faces the following technological barriers:                high cost of flywheel composite materials;        required durability of the flywheel protective casing;        required drive shaft strength; and        undesired gyroscopic effect.        
These technological barriers cause that the possible increase in capacity does not lead to the required reduction of energy density and, more importantly, to reduction of costs per stored energy unit required to allow this method of energy storage become competitive.