Design and implementation of a flywheel-battery hybrid energy storage system for microgrids

Abstract

The goal of the flywheel-battery hybrid energy storage system (HESS) is to enhance the performance of the flywheel energy storage system (FESS) by increasing its energy density to enable it to discharge for long periods in a smart grid, specifically a microgrid. This system aims to supplement the power generated by renewable energy sources (RESs) and balance the load demand in the power grid. The user requirements for the flywheel-battery HESS include ease of use, and capacity to store large amounts of energy. The general system design of the flywheel-battery HESS includes an AC power source to operate the motor/generator (M/G) unit, which rotates the flywheel. The FESS is designed to supplement the power grid when there is a drop in the system's frequency, and the BESS kicks in to continue to stabilize the frequency back to its original 50 Hz when the FESS has exhausted its discharge. The switching between the two storage systems is controlled by a programmable logic controller (PLC) that gives way to the storage system with higher power output. The FESS is given priority over the BESS due to its high-power density and fast response. Additionally, a pulley system with varying diameters is used for speed conversion, particularly during the generation mode, to increase the speed produced by the FESS. The FESS operates at low speeds (<10,000 rpm), and the selection of journal bearings influences the other design specifications around the FESS, such as the flywheel shaft's.