Conceptual design of a liquid-based variable inertia flywheel for microgrid applications

For a rural microgrid requiring simplicity and robustness, a flywheel-generator combination directly coupled to the grid offers both energy storage and inherent stability. The tradeoff of using a directly-coupled flywheel with a constant moment of inertia is that the amount of energy extractable from within the narrow band of synchronous speeds is limited. Furthermore, a large amount of unusable energy remains within the flywheel below the grid's minimum frequency. By using a flywheel with a variable moment of inertia, although the nett energy may be less, the extractability can be increased by straightforward design. This paper presents the concept of a liquid-based flywheel where the moment of inertia is engineered to be a function of speed by appropriately sizing and shaping the liquid's spinning container.