## 1 Introduction  
Cleaner production of energy is urgently needed in today's world due to the concerns about global warming and growing population. This need has allowed widespread use of distributed generation (DG) and renewable energy sources (RES) integrated to electricity networks [1]. However, due to their dependency on seasonal variations, RES are not completely reliable and stable and cannot be used as the only primary source of energy. Additionally, if renewable sources of energy are added into microgrids powered previously only by fossil fired engines, this intermittency combined with fluctuation in demand leads to further stability challenges. Energy storage systems (ESS) offer a solution, which can mitigate the effects of RES intermittency by providing a balance between electrical supply and demand [2; 3]. ESS can also reduce the power rating of the generating engine to meet peak demand and enables power production to meet average demand and reducing generation cost [4]. Also,there are various solutions proposed to ensure network stability and reliability with RES including demand management, interconnection with external grids, and ESS [5]. There are ranges of systems involved in energy storage process, which can convert, store and deliver energy on demand. Performance of these systems depends on the amount of energy they can store, the storage time and delivery of energy with minimum losses [6]. Therefore, high power capability, high efficiency, low capital cost and environmentally friendly attributes improve the value of ESS [7]. A FESS is an assembly of a rotating mass, electrical machine, power electronics converter, bearing system and containment. The design of the rotor used in FESS depends on the materials from which they are made. Solid disk or solid cylinder are made from isotropic materials like steel. Rim type or hollow cylinder flywheel rotors are constructed from non-isotropic material like composite carbon fiber. Solid disk or solid cylinder flywheel rotors have simple construction and are commonly used [8]. Solid rotor flywheels exhibit less displacement from axes due to centrifugal forces hence simplifying flywheel attachment to the shaft and with electrical machine [9]. Schematic diagram of solid rotor FESS is shown in Figure 1.  
Carbon fibre-reinforced composite rim type or hollow cylinder flywheel rotor assembly contains the hub and the flywheel rim, which are mounted together on the metallic shaft [10]. Due to mismatching of strains of hub and the flywheel rim material, attachment of the composite flywheel to the shaft is an issue, due to this issue, integrated hollow flywheel design is introduced recently, in which electric machine and bearing system are integrated in the hub of the flywheel [11]. In this design, the rotor of electric machine is outside of the stator which enables very high-speed design [9]. Integrated hollow flywheels attain high speeds, have more specific energy, less cost, occupy less space and contain less windage losses. However, shaftless hollow flywheel design faces challenges such as heating issues, strength of the rotor materials and suspension of the rotor [9; 11]. Schematic diagram of integrated hollow flywheel is shown in Figure 2. This configuration holds an axial type PMSM with two PM rotors and a single stator. In addition, there are some other configurations including the type in which the flywheel and the rotor of the electric machine are combined together as addressed in [12]. Figure 1: Structure of solid disk FESS. With fast response time, relatively high-energy density and high-power capability, flywheel energy storage systems (FESS) offer an attractive energy storage solution [13]. FESS is highly suitable for applications where a large number of daily charge-discharge cycles as required for power smoothing, renewable energy integration, stability and power quality improvement [14].