## 1 Introduction
FESS and battery energy storage systems (BESS) can be strong competitors against each other for a number of applications [15], with FESS being best suited and leading the competition for short term and high-power applications. Unlike FESS, performance of BESS is affected by many factors and importantly the temperature dependency [15]. Hence, the BESS is less favorable in countries where temperatures are high throughout the year. BESS cannot withstand high charge-discharge rates and requires quite a large volume to store a larger amount of energy [16]. Moreover, the research findings in [17] show that the probability of a battery storage failure is seven times more likely than that of a flywheel storage which makes BESS less reliable than FESS. However, costs of BESS have reduced significantly in recent years and have the added advantage of durations of around one to two hours so may provide power for longer periods. On the other hand, the cost of flywheels needs to reduce significantly and this is possible with lower cost design and volume manufacture [18]. Recently, FESS technology has been implemented in many applications such as UPS, aerospace, transport, power levelling, wind power and frequency regulations. FESS has been successfully used to secure critical loads during power outages in the microgrid environment [17]. In [19], a low speed FESS has been used as a hybrid UPS composed of an engine generator and a flywheel to improve voltage fluctuations of less than 2%. To support emergency loads when main grid supply is disconnected, a successful utilisation of a FESS used as a UPS instead of BESS is reported in [20]. As an alternative to batteries, the flywheel technology has been used for removing DC voltage ripples [21], short-term ride through [22], fast balancing of power in microgrids [23], and islanded solar PV systems [24]. The electrical machine is a key part of a FESS which performs the charge-discharge cycles and can be responsible for major losses in the process. The choice of electrical machine must be carefully made according to the application requirements. Machines commonly used in FESS are induction machines (IM), permanent magnet synchronous machines (PMSM), switched reluctance machines (SRM) and homopolar synchronous machines (HSM). SRM have robust design, fault tolerant capability and ability to operate in high temperature environments; however, it is less commonly used because of its complex torque current and high current ripples [25]. Due to the use of electromagnets, SRM can be completely demagnetised [6] that would lead to no idling losses at zero torque (no load) [26]. Highly nonlinear behavior of SRM makes selection of suitable control gains difficult and this makes design of control system challenging and problematic [27]. In the generator mode, SRM require excitation to produce power [25] produced by means of electronic converter [28], but it will result in extra losses. PMSM is used in high-speed applications due to its very high efficiency while the IM,  
Figure 2: Structure of Integrated hollow FESS. due to its rugged construction, is used for high power applications [29]. A challenge for IM is the mechanical design in high-speed application, which can be overcome by using appropriate solid-rotor topologies such as slotted solid rotor, coated solid rotor, caged solid rotor and smooth solid rotor [25]. Homopolar synchronous machines (HSM) are used for long-term energy storage in high speed and high efficiency applications. HSM have robust rotor design and due to their lower standby losses compared to PMSM, they can be used an alternative to PMSM in high-speed applications [25; 30]. However, the main disadvantage is the difficult design of HSM due to its complex magnetic flux paths and geometry [31]. Many researchers have studied induction machine-based FESS, but a thorough analysis of the performance of IM in FESS is lacking [32; 33; 34; 35; 36].