1 INTRODUCTION

These days, the power system is evolving rapidly with the increased number of transmission lines and generation units and has become an interesting area for research. The increase in energy demand by industries, households, and developing nations has boosted energy prices.¹ As well, conventional methods of generating energy are arousing the problems of power quality, higher emission of carbon dioxide, and market deregulation.^{2, 3} Due to this fact, the management, control, and protection of the electrical network had become more complicated. Thus, distributed generations (DGs) and renewable energy sources (RESs) are viable alternatives to the traditional methods to scale down the issues.³ However, due to the periodic nature of the RESs, challenges occur while supplying energy over a period of time.⁴ During the energy supply from RESs, the energy demand might be less, but at the time of peak energy demand, RESs may exceed its limit of production. Also, supply from RESs fluctuates monthly, seasonally, and annually as they are exposed to weather conditions, thus creating an issue of reliability.^{3, 4}

To overcome the drawbacks of RESs, energy storage systems (ESSs) are introduced so that they can be used for enhancing the system quality in every aspect.^{5, 6} Currently, ESSs plays a significant role in the electrical network by storing electrical energy, converting it into various forms, and supplying it whenever necessary, in the form of electricity.^7-9 Many authors have described various kinds of ESSs such as battery ESS (BESS),¹⁰ compressed air ESS (CAESS),¹¹ supercapacitor ESS (SCESS),¹² thermal ESS (TESS),¹³ superconducting magnetic ESS (SMESS),^{14, 15} hydrogen ESS (HESS),¹⁶ pumped hydro ESS (PHESS),¹⁷ and flywheel ESS (FESS).^18-20 A comparative study of different ESSs and their advantages and disadvantages are presented in Table 1.

At present, demands are higher for an eco-friendly, cost-effective, reliable, and durable ESSs.^{21, 22} FESS can fulfill the demands under high energy and power density, higher efficiency, and rapid response.²³ Advancement in its materials, power electronics, and bearings have developed the technology of FESS to compete with other available ESSs and their applications.^{24, 25} With the potential of 500 MJ storage and power range of kW to GW, FESS operates many applications, among which most common are for power quality enhancement, uninterruptible power supplies (UPS), and flexible AC transmission system (FACTS).^26-29 For such applications, BESS is unlikely to last longer, even for 10 years, due to its short lifecycle since the number of cycles for these applications is frequently too high. However, BESS can achieve the state by keeping its depth of discharge (DoD) low, leading to increased capacity and cost. On the other hand, SCESS has also been analyzed for these applications, but its practical lifetime is low compared to the FESS at a higher capital.³ Thus, FESS introduces such a minimized system in the capacity that drives to reduced cost and can feed the grid many times during high demand in a day. It can be concluded from the tabular comparison and above discussion that the FESS acts as the best ESS in comparison to other storage devices and can replace other ESSs.³⁰

In the literature, authors have presented distinct reviews on flywheel-based ESSs.¹⁸ A comparison between different ESSs has been made where FESS problems and improvements are testified through graphical and numerical reviews.³¹ Researchers have discussed the four types of ESSs: mechanical, chemical, electromagnetic, and thermal storage. A thorough comparative study based on energy density, specific power, efficiency lifespan, life-cycle, self-discharge rates, cost of investment, scale, application, technical enhancement, and environment impact among all ESSs has been carried out. FESS, along with PHESS and CAESS are categorized under mechanical ESS.³² A meticulous study of the literature indicates that the authors have brought out a detailed comparison between different ESSs, issues faced by FESS, and the possible ways of the improvement using graphical illustrations and numerical data. Table 2 shows a comparative study among all types of mechanical ESS, proving FESS to be a more viable alternative than other mechanical ESSs.

From Table 2, it can be inferred that the FESS technology proves to be the best with maximum efficiency, low impact on the environment, high specific power and energy, high power and energy density, longer life cycle, faster in response, and requires very low maintenance.^{31, 33} However, the primary shortcomings involved are extremely high self-discharge rates and safety.¹⁸ To overcome the issues following improvements are suggested such as (1) enhancement of bearing for lowering the self-discharge rate; (2) use of advanced materials for low-speed FESS; (3) for high-speed FESS use of electromagnetic and super conducting variants; (4) use of a permanent magnet for lifting the flywheel mass and (5) implementation of superconductor impregnated nanotube yarns.

The authors have conducted a survey on power system applications based on FESS and have discussed high power applications of energy storage technologies.^34-36 Authors have also explained the high-speed FESS control of space applications.³⁷ Many authors have focused on the evolutionary part of the motor and generator for FESS. Many have reported the use of various electrical machines in FESS and their controls.^{38, 39} For a segregated wind power system, authors have presented a review of FESS and its simulation.^{28, 40} Authors have also discussed the enhancement in battery lifespan in the renewable energy system by incorporating FESS.^{41, 42} A summary highlighting the review methodology of the present article under study has been illustrated in Figure 1 for easy understanding of the readers.

The entire research article has been divided into the following sections. Background theory and structure of the FESS have been described in detail in Sections 2 and 3, respectively. Section 4 discusses the characteristics gained by the FESS. Various applications that use the FESS technology are illustrated and reviewed in Section 5. The cost model of FESS has been highlighted in Section 6. The control approach, stability enhancement, and maintenance of FESSs have been discussed in Section 7. Section 8 covers the future work recommendations that would help in amplifying the FESS technology. Section 9 presents the conclusion from the entire study.

2 BACKGROUND THEORY

A flywheel is classified among mechanical ESS that has been endured since thousands of years back. The flywheel works under the effect of maintaining its energy by its inertia.⁴³ Potter's wheel is an example used as a rotatory object that undergoes the effect. More of it, such as hand mills, lathe, water wheel, and other manually operated rotary objects, performs the flywheel applications. The same spinning wheels have been used over centuries with different modifications. Major developments in the 18th century were the adoption of flywheels in steam engines and the replacement of wood from metal in the construction of the machines. The production of flywheels in an entire piece is possible due to the enhancement in iron and cast iron production, which increases the moment of inertia.⁴³ The word “flywheel” came across during the industrial revolution in 1784. In factories, flywheels function as energy accumulators and are also used on steam engines and boats.⁴⁴ During the 19th century, advancement in cast steel and cast iron led to the production of larger flywheels and curved spokes. Many changes have been introduced with few designs and shapes, but in the 20th century, significant improvements came up with rotational stresses and rotor shapes when they were intensively investigated and treated as potential ESSs.⁴⁵ Gyrobus, an example of the flywheel system in which a 1500 kg flywheel was manufactured in Switzerland and was used for transportation purposes during the 1950s.⁴⁶ FESS was suggested for static power back-ups, electric vehicles, and space operations during the 1960s and 1970s. Continuous progress had appeared in the 1980s with fiber composite rotors and low-speed magnetic bearings.^{47, 48} Despite the flywheel developments and their effective use, it has been refused for further development with the electric grid. Still, FESS stands as a substantial option for energy storage applications after installing high-speed motors and advancement in magnetic bearings, materials, and power electronic devices.^{49, 50}

Figure 2 illustrates the single line diagram of FESS in a grid-connected mode of operation. The principle of rotating mass causes energy to store in a flywheel by converting electrical energy into mechanical energy in the form of rotational kinetic energy.³⁹ The energy fed to an FESS is mostly dragged from an electrical energy source, which may or may not be connected to the grid. The speed of the flywheel increases and slows down as it stores energy and gets discharged, respectively. An M/G is responsible for exchanging energy in the two different forms, which drive the rotating flywheel.⁵¹ Both M/G and flywheel are connected coaxially, which indicates controlling M/G can empower flywheel control.^{52, 53}

3 STRUCTURE WITH ITS ASSOCIATED COMPONENTS

Figure 3 depicts the detailed structure of the FESS and its associated components that are mostly used for ground-based power. The main units of FESS include rotor, M/G, bearings, power electronics, and housing.⁵⁴ The detailed description of each component has been described in this section.

Figure 4 represents different configurations of FESS. In Figure 4A, the “conventional” or “lid” type of FESS has been shown in which the M/G has the option to be installed either inside or outside the containment housing. To lower the rotor losses due to aero-dynamical drag, it is preferably maintained at low pressure. A shaft or a mechanical coupling connects the M/G and rotor without physical contact. The overall diameter of the rotor is made larger to store kinetic energy.⁵⁵ In several flywheel systems, M/G outside configuration is usually employed. Here the M/G works at a comparatively lower speed, and the heat generated from it should be eliminated rapidly. A configuration termed “barrel” or “hollow” type has been projected in Figure 4B. This structure provides the advantages for composite rotor flywheel as it is compact and can reserve energy in a ring having a lightweight hub in the middle.

Nevertheless, this design is not efficient when employed for isotropic steel rotor flywheels. An “integrated” or “hybrid” type of design is depicted in Figure 4C. This structure is a combination of the rotor's energy storage parts and electromagnetic units.⁷ Here, the overall weight of the containment configuration can be reduced by employing this design. However, some serious issues are as follows: (1) needs safety concern of the containment setup and (2) not applicable for rotors of composite type as it requires electromagnetic material to produce torque.

3.1 Rotor

The rotor is the main component of an FESS and has been used for years in machines to provide smooth running. In recent years, steel is being used for the structure but could not withstand long due to its low speed, up to 10 000 rpm. Further, the composites can undergo high rotational stress due to their lower weight than the steel ones, which are more heaver. The only problem with the composite materials is their cost, which is assumed to decrease in the future.⁴⁸

The kinetic energy stored in the rotating mass of a flywheel is linearly proportional to the square of its angular velocity and the moment of inertia as demonstrated in Equation (1):

()

where “ ” is the kinetic energy stored, “ ” represents the moment of inertia, and “ω” refers to the angular velocity of the flywheel.

The energy of a flywheel can also be obtained within a range of speed having minimum speed “ ” and maximum speed “ ” by Equation (2):

()

Consistently, to limit an M/G's maximum torque and avoid a greater voltage variation for a given power rating, a flywheel is operated between its speed range. The moment of inertia of a rotor depends on its mass and shape factor.⁷ However, their structure may differ by building hollow or solid cylinders in short disc form or long drum form.^{51, 56} The angular mass for both solid and hollow cylinder flywheels can be calculated through Equations (3) and (4) as given below:

()

()

where “ ” represents the rotor mass, “ ” is the radius of a disk type cylinder, “ ” and “ ” are the outer and inner radius of a hollow type cylinder, respectively.

Thus, the moment of inertia and energy stored for a solid cylindrical flywheel can be calculated as a function of flywheel length “ ” and mass density “ ” by Equations (5) and (6):

()

()

The strength of a material used for the rotor is also known as tensile strength “ It determines the maximum speed limit of a flywheel at which it may operate to maintain the stress on the rotor lower than the material strength.^{45, 57} Properties of several flywheel materials are given in Table 3. Equation (7) determines the maximum stress experienced by a rotor as follows:

()

The speed limit also depends on the shape factor “ ,” which decides the flywheel structure.⁵⁷ Figure 5 depicts the various shapes of the flywheel and its corresponding shape factor. Both specific energy and energy density (ie, energy per unit mass “ / ” and energy per unit volume “ / ) are dependent on a flywheel shape which can be expressed in terms of “ as shown in Equations (8) and (9), respectively:

()

()

TABLE 3. Properties of several flywheel materials

Material	Mass density “ ” (kg/m3)	Tensile strength “ ” (MPa)	Specific energy “ / (W/kg)	Energy density “ / ” (kW/m3)
Steel (AISI 4340)	7800	1800	39	303
Alloy (AlMnMg)	2700	600	38	101
Titanium (TiAl6Zr5)	4500	1200	45	202
Fiberglass (60%)	2000	1600	135	269
Carbon fiber (60%)	1500	2400	269	404

4 FESS CHARACTERISTICS

The attractive attributes of a flywheel are quick response, high efficiency, longer lifetime, high charging and discharging capacity, high cycle life, high power and energy density, and lower impact on the environment.^{51, 61, 64} The rotational speed of a flywheel can help in measuring the state of charge (SoC) without affecting its temperature or life.²⁷ However, the major limitation associated with FESS is the higher self-discharge rate than other storage technology.

In contrast with other ESSs, flywheels tend to rapidly charge and discharge themselves without getting influenced by the DoD.⁶⁵ Authors in the literature have concluded that the life cycle of FESS is independent of the DoD. FESS has the capability of operating efficiently on frequent shallow discharges as well as shallow discharges.²⁶ Along with robust performance, a flywheel's lifetime is predicted to be more than 20 years and hundreds of thousands of cycle life as they do not need a longer charging and discharging cycle.^{3, 51, 61} They can be charged and discharged rapidly, depending on the application and necessity; hence is not affected by the DoD, charge depth, and temperature.^{7, 57, 66} Flywheel life cycle cannot be characterized by DoD as its non-dependent on DoD, and thus it is also predicted that it will have a life as long the system, although each cycle is at 100% DoD.⁶⁷

Flywheels can deliver a large amount of power in seconds, with an efficiency of 90%-95%.^{57, 68} It has always been an eco-friendly technology with zero emissions during its operation, as the materials used are non-hazardous.^{57, 61} The energy and power rating of an FESS can be independently optimized based on their applications.⁶⁹ The energy rating of an FESS is decided by the rotor speed and its size, whereas the power rating is deduced by the size of M/G and linked power electronic components.^{3, 26}

High power-based FESS has a large scale of applications for shorter periods; however, an FESS can be designed for longer durations with few developments in the rotor design and material.²⁶ As compared to batteries, the power density of the flywheels is five to ten times greater. Because of the lower volume and longer lifetime of the flywheels, they can mitigate the use of batteries in few applications such as electric vehicles, and so on.²⁶

5 FESS APPLICATIONS

At present, FESSs are used to support very small launching systems to giant electrical power generation and transmission systems, thus proving to be the fittest among all other ESSs.^{7, 30} In various researches, FESS has been implemented for different operations such as UPS, transportation, renewable energy resources integration, marine and space applications, FACTS, smoothing of power in weak grids, and frequency regulation.^70-72 Figure 6 depicts a block diagram of the FESSs application in the electrical power system and microgrids (MGs). In this section, a detailed review of these applications is conferred.

5.2 Transportation

In transportation, hybrid and electric vehicles use flywheels to store energy to assist the vehicles when harsh acceleration is needed.⁷⁶ Hybrid vehicles maintain constant power, which keeps running the vehicle at a constant speed and reduces noise and air pollution, fuel consumption, and maintenance, which increases engine life.^{25, 26} Concurrently, flywheels store energy produced during the slowdown of the vehicle through regenerative braking, which provides a boost in acceleration when needed.⁶² Flywheels rank first among all other ESS due to their key features of high power density, high efficiency, longer lifetime, and faster charging and discharging capability.²⁶

Further, flywheels are being developed for rail applications in both hybrid and electrical systems.⁶⁶ The energy and power ratings can be determined by the speed and weight of the train. Flywheels are fixed at stations in the train system that can restore 30% of the energy through a regenerative braking mechanism.⁷⁷ As well, they solve the voltage sag problem during distribution and transmission in railways without letting the line capacity increase.⁷⁸ Authors have reported a 10% decrease in electricity consumption at stations due to the energy recovery system, which leads to a saving of $90 000 per station due to the incorporation of FESS.⁷⁹ Many such examples are present in literature that elaborates on the use and advantages of FESS in different areas of transportation such as (1) air, (2) road, (3) rail and (4) sea.^{79, 80} Figure 7 depicts a schematic diagram of the advantages of FESS application in various areas of transportation.

5.3 Renewable energy sources integration

Flywheels can penetrate the power systems assisted by solar and wind energy by improving their stability and balancing the grid frequency due to being faster in response.^{80, 82} Figures 8 and 9 outlines the application of FESS for wind and solar integration, respectively. During sunny or windy seasons, storing energy helps in compensating the oscillations in power that occurs because of wind and solar sources.^{27, 28, 83} Flywheels can be used along with batteries to enhance the output power of a solar system, as well as it can alone adjust the wind oscillations, improving the overall frequency.^{27, 84}

Authors have illustrated the use of a hybrid system made by combining photovoltaic panels and wind turbines and have specified the consumption of 40% fuel due to diesel generators, which could not result in preserving fuel.⁵⁷ Diesel generators should be kept mostly shut down and to be evoked only during demand. Thus, FESS can lower the number of periodic cycles for starting and shutting down the generators, which helps in preserving fuel and reducing power fluctuations.⁸⁵ There has been a large scale of flywheel systems established to incorporate with renewable energy systems such as Urenco Power, VYCON technology, ABB's Power Store, and Beacon Power.⁸⁶

5.4 FACTS devices

Some FACTS devices bear a DC link in their system, combined with ESSs constructed with battery, supercapacitor, or flywheel. There are two major types of FACTS devices named dynamic voltage restorer (DVR),⁸⁷ and static synchronous compensator (STATCOM).^{88, 89} Numerous authors have investigated the work of connecting FESS with the DC link of these devices.⁹⁰ Authors have presented the application of FESS in mitigating the issue of voltage sag when the FESS has been coupled with a DVR.⁸⁷ Researchers have used distribution static compensator (DSTATCOM) with FESS to mitigate the problem of fluctuations in the distribution network of the wind generation unit.⁸⁸ To resolve the above issue, DSTATCOM should be operated by a quick response DC voltage source, which makes FESS a proper applicant. A new FACTS device linked with FESS, known as a flexible power conditioner, has been conferred in the literature, compensating the load voltage and unbalance in grid power generation.⁹¹ FESS also enhances the system's stability by rapidly transferring active and reactive powers.⁹² FACTS devices are capable of performing good and improvement over power flow control and long-distance AC transmission line due to the addition of FESS having high speed and fast response. Figure 10 illustrates the application of FESSs to FACTS devices like DSTATCOM and DVRs.⁸⁹

6 COST MODEL OF FESS

ESS plays a significant role in ensuring a constant electricity supply to the power grid, utilities, transport, and so on. However, consumers must pay a considerable amount of cost for storing the energy.¹²⁰ Cost of an ESS depends on the following: (1) application it is employed for; (2) location; (3) method of constructive; (4) size; and (5) source of energy used.¹²¹ The technical, economic, cost-benefit study of ESS requires recent cost data and cost analysis structure.¹²²

The total cost can be broken down into the following categories: (1) ESS cost, which is actually the overnight capital cost of the storage unit and can be divided into two parts, namely cost per unit power output ($/kW) and cost per unit energy stored ($/kWh); (2) power conversion system unit cost which comprises of cost for all equipment installed for working like control unit, transmission system, isolation units, transformers, and so on; (3) balance of plant cost comprising of construction cost, land, taxes, permission, and fees; (4) operation and maintenance fixed cost involving routine maintenance cost annually; and (5) operational maintenance variable cost.

In general, the overall cost can be calculated as follows:

()

where “ represents the cost per unit power output ($/kW), “ is the cost per unit energy stored ($/kWh), and “” is the additional system costs ($).

The cost invested in the storage of energy can be levied off in many ways such as (1) by charging consumers for energy consumed; (2) increased profit from more energy produced; (3) income increased by improved assistance; (4) reduced charge of demand; (5) control over losses, and (6) more revenue to be collected from renewable sources of energy.¹²³ Applications involving regular energy storage and discharge at a high rate need to focus on the cost per unit power output to choose the appropriate ESS. FESS is found to be the cheapest ESS which involves high output power. FESS is mostly employed for transmission and distribution purposes.¹²⁴ Table 4 highlights the overall cost analysis for FESS.

7 CONTROL APPROACH, STABILITY ENHANCEMENT, AND MAINTENANCE OF FESS

Instability in the electrical power grid network basically occurs due to an improper balance between the power generated and the consumer's requirement. In this regard, ESSs play a very vital role in stabilizing the voltage by either storing or consuming the energy, thereby enhancing the voltage level in various parts of the power network. FESS is an alternate device for BESS and other ESS due to its characteristics. Researchers have explored that the FESSs can be implemented for dynamic or transient stability enhancement and thus augments voltage and frequency deviation in the electrical power networks and MGs.^129-135 Authors have reported that the fluctuations in the power of an offshore wind farm were reduced and dynamic stability was improved with the use of FESS.¹²⁹ FESS can inject or absorb a huge amount of energy within a short time span (seconds) and hence can be employed to enhance the transient stability of power grids.¹³⁵ Several authors have presented a significant effect on stability advancement when the FESS is used with a fuzzy controller being tuned by a neural network.^{40, 136} The FESS has also been applied for the regulation of frequency.^{133, 134} For improving the dynamic performance of the diesel generator, hydro, and wind energy-based hybrid islanded power system, FESS has been implemented.^{134, 137} Further, the fuel cost of the diesel generator is reduced to a greater extent due to the use of FESS. The Beacon power company has introduced 200 units of FESS with a net capacity of 20 MW for regulating frequency in an MG. Power can be stored as mechanical energy in the FESS during the low load period, and the above process can be reversed during a high load period.¹³⁸

A major challenge faced by the FESS is to facilitate the control of the flywheel rotor speed, ensuring no change in the DC link voltage.¹³⁹ Traditionally, this control is executed using a proportional-integral (PI) controller based on a linear system model. Authors have studied and conducted experiments on the application of PI controller to a permanent magnet synchronous machine (PMSM) coupled with FESS.¹⁴⁰ A proportional-integral-derivative (PID) controller for controlling FESS coupled with hybrid electric vehicles has been reported in the literature.^{141, 142} Control strategies based on fuzzy logic control (FLC) have been proposed by many researchers to mitigate power quality issues and enhance the quality of power being fed to the power grid.¹¹⁸ The charging and discharging operation of FESS has been reported to be controlled by artificial neural network (ANN).¹⁴³ For eliminating non-linearity of the FESS unit, a feedback linearization (FL) technique has been implemented in the literature.¹⁴⁴ Field oriented control (FOC) and direct torque control (DTC) are employed to regulate the speed of the FESS.¹⁴⁵ The model predictive control (MPC) method is more efficient than the PI controller as it procures better accuracy in response to flywheel speed control and regulation of DC-link voltage.¹⁴⁶ Recently, a new enhanced tube-based MPC (T-MPC) technique has been proposed by authors, which deliberately enhanced the capability of current and amplitude of power ripple.¹⁴⁷

A coordinated control approach consisting of primary and secondary control has been proposed for all micro-sources and ESSs.¹⁴⁸ FESS has a faster response rate, so it usually gets engaged in the primary control, whereas other micro-sources such as microturbine (MT) and fuel cell (FC) are associated with secondary control of frequency. Authors have classified another mode of operation of ESSs in an autonomous MG, such as PQ control, droop control, and V/f control.¹³¹ These control methods have been operated, and the performance of each technique on the overall stability of the MG has been studied. The authors have suggested using the PQ control strategy for FESS only in the grid operated mode, whereas droop control and V/f control during the islanded mode of operation. Figure 11 depicts different types of control strategies.

Hybridizing FESS with one or more ESSs while operating in a power system requires simultaneous charging and discharging phenomenon.¹⁴⁹ In this regard, authors have suggested the control of FESS based on SoC. This method enhances the response of voltage regulation, and the lifespan of the system.¹⁵⁰ SoC control strategy can be adopted for the control of voltage and power in the DC MG, and for AC MG, it ensures frequency, active and reactive power control.¹⁵¹ Authors have also applied the optimization techniques primarily for controlling the hybridization of FESS with other ESSs units to enhance system stability and efficiency.^{55, 142, 152, 153} A detailed comparison for the different FESS control methods highlighting advantages and disadvantages of each control strategy has been listed in Table 5.

8 FUTURE TRENDS

FESS possesses numerous advantages compared to other ESSs in terms of the compact, rapid response, high peak power, long life-cycle, environmentally friendly, high efficiency, and larger energy density, thus making it successfully penetrate as a reliable storage unit the recent electricity market. However, it encounters some serious issues which need to be dealt with for effective use of FESS. In this section, a thorough discussion regarding issues faced by the present-day FESS and its future establishments to be made for the hassle-free operation has been deliberated under various subsections after a thorough review of the literature.

9 CONCLUSION

Modern technology advancement and investigation reveal that the FESS is an assuring candidate for administering ancillary benefits, promoting the incorporation of renewable energy generation, and delivering a promising future in commercialization. FESS holds many merits compared to other ESSs units such as modular, recyclable, quick response, larger peak power, long life-cycle, highly efficient, eco-friendly, and high energy density. In this regard, this research article is an attempt toward highlighting in detail FESS in terms of its background theory, detailed description of the structure and its associated components, characteristics, economics, applications in various fields, control strategy, stability enhancement, maintenance, and future prospects.

A detailed comparison between FESS and other ESSs technologies has been discussed meticulously and presented in a table, giving readers a clear idea of the advantages of the FESS hence concluding the maximum penetration of FESS into the recent electricity market. A summary highlighting the review methodology of the present article understudy has been illustrated a better understanding of the researchers, academicians, industrialists, and users working in FESS. The primary objectives of this research article have been elaborated. A thorough light is thrown toward the background theory of the FESS technology. A detailed overview of various configurations, structures, and associated components such as rotor, M/G set, rotor bearings, power electronic interfaces, and housing has been clearly deliberated. The properties of several flywheel materials and the shape factor of various flywheel shapes have been studied.

A comprehensive idea about the FESS characteristics considering various attributes such as DoD and life cycle has also been outlined in this research article. A study based on associated literature, circuit diagram, and operation of various FESS power system applications such as UPS, transportation, RESs, FACTS, military, spacecraft, frequency regulation, power quality improvement, and marine has been critically summarized. The economics and the cost model associated with FESS implementation are also projected. A clear insight to control approach, stability enhancement, and maintenance of FESS has been analyzed. A rigorous literature review of all available control techniques, including their merits and demerits, is highlighted. In addition to this, the study also brings out the several issues faced by FESS from the literature. The possible viable solutions for hassle-free operation have been conferred in-depth in the future trends section. Thus, the article is believed to serve as a good reference for researchers and academicians working in the area of FESS technology. It also justifies the wide through applications of FESS in the electrical grid network for real-time implementations.