# Critical Review of Flywheel Energy Storage System

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## Abstract

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## 1. Introduction

_{2}capture [11,12,13,14]. In the last decade, the renewable energy sources’ capacity was exponentially increased, resulting in a critical need for energy conversion/storage systems that can effectively use/store such an increase in energy. Regarding energy conversion devices, fuel cells are efficient devices [15,16,17] that are fueled by renewable fuels such as biohydrogen [18], biogas [19], or other biomass resources [20,21]; they have high potential to replace conventional energy conversion sysetms in several applications, such as water desalination [22,23], transportation [24,25,26], aviation [27], and portable applications [28]. The energy storage systems are divided into four categories, i.e., electrical, electrochemical, thermal, and mechanical. Mechanical ones are suitable for large-scale capacities with low environmental impacts compared to the other types. Among the different mechanical energy storage systems, the flywheel energy storage system (FESS) is considered suitable for commercial applications. An FESS, shown in Figure 1, is a spinning mass, composite or steel, secured within a vessel with very low ambient pressure. The reduced pressure within the vessel reduces drag on the spinning mass, thereby maintaining momentum and generating electricity for longer [29]. A flywheel stores energy in a rotating mass, and the kinetic energy produced is stored as rotational energy. The amount of kinetic energy stored depends on the inertia and speed of the rotating mass. In order to eradicate any energy loss due to friction, the flywheel is placed inside a vacuum containment. It is also suspended by bearings so that operation is stable. This results in the flywheel being able to continue spinning without any added power and with very little energy loss. The kinetic energy is transferred in and out of the flywheel by an electrical machine. The electrical device has two modes of operation, namely either a motor or generator. These modes of operation are dependent on the load angle [30,31,32,33]. When the machine is acting as a motor, electrical energy is provided to the stator winding. The stator winding is a wire coil built into the motor, which produces a rotating magnetic field when energised. This energy is then converted to torque and applied to the rotor, resulting in it spinning rapidly and gaining kinetic energy. This is used when an excess of energy is being produced from an external source, and, therefore, the flywheel stores the energy [34]. When this stored energy is required, the electrical machine acts as a generator, and the kinetic energy stored in the rotor applies a torque. This is then converted into electrical energy. This causes the wheel to slow back down [30].

- Pumped hydro storage (PHS);
- Compressed-air energy storage (CAES);
- Battery energy storage (BES);
- Capacitor storage (CS);
- Supercapacitor energy storage (SCES);
- Superconducting magnetic energy storage (SMES);
- Thermal energy storage (TES);
- Hydrogen storage system (HSS);
- Flywheel energy storage system (FESS).

- Machine;
- Bearing;
- Rotating mass;
- Power electronic interface (PEI).

#### Merits and Demerits of FESS

- Low speed, less than 10,000 revolutions per minute (rpm);
- High speed, 10,000 to 100,000 rpm.

## 2. Components of Flywheel Energy Storage System

#### 2.1. Machine

- Permanent magnet synchronous machine (PMSM)

- Induction Machine (IM)
- ○
- The induction machine shown in Figure 5 should be considered the best all-round choice for high-power installations [73]. They are stoutly constructed, with low build cost, they suit high torque applications, and they are highly reliable [67,74]. They do not suit high-speed applications [75], although double-fed induction machines (DFIM) are in development to overcome this limitation [76,77].

- Brushless direct current machine (BLDCM)
- ○
- A BLDCM as shown in Figure 6 is designed to be a synchronous machine containing a permanent magnet within the rotor and operates a self-controlling function, optimising the stator current by the use of an inverter. High efficiency, large operational speed range, compactness, mechanical stablility, low maintenance, and operation without electromagnetic interference are the main advantages of a brushless direct current machine [78,79,80,81,82,83].

- Switched reluctance machine (SRM)
- ○
- The switched reluctance machine shown in Figure 7 with it’s rotor (7(a)) below has a simplified build, and idle losses are low. They can operate in harsh environments, including at temperatures of 400 °C, still with a wide speed range [84]. Switched reluctance machines similarly are operational even when one or two phases are damaged. An SRM is challenging to control at lower speeds but is easier to control at high speeds than an induction machine [85].

- Homopolar machine (HM)
- ○
- The HM shown in Figure 8 is also known as the homopolar inductor alternator and also the homopolar synchronous machine [86]. The homopolar machine is well built with low idle losses and suits long-term high-speed applications due to its reliability [87]. There is a reduction in the self-discharge rate using this technology, hence increasing efficiency as well as energy density [88]. This, therefore, reduces the overall cost of this technology. This motor utilises a single winding to generate torque in the absence of permanent magnets [89,90]. They are also ideal for industrial blowers, hole pumps, etc. [91]. Current excitation on the stator side for this type of machine is used with a flywheel [92,93]. During the idling period, magnetisation can be eliminated through the turning down of the current, hence reducing self-discharge losses.

- Synchronous Reluctance Machine (SRM)
- ○
- An SRM depicted in Figure 9 is capable of controlling varying torques via two thyristor-controlled components coupled to the stator windings. This allows a dampening effect in the rotating components when used in oscillation mode [95]. Table 4 compares the differences between the switched reluctance machine and synchronous reluctance machine.

#### 2.2. Rotor Bearing

- Permanent (Passive) magnetic bearing (PMB).
- ○
- PMBs are permanent magnets (Figure 10) rather than electromagnetic so therefore are used with other bearing types as they are unable to sufficiently dampen movement on all axes [104]. PMBs offer extremely low losses because of the absence of electromagnetic drag and have low construction and installation costs. They are also used as auxiliary bearings as required [105,106].

- Active magnetic bearing (AMB)
- ○
- Active magnetic bearing (Figure 11) accommodates coils that can adjust the amount of electromagnetic force in the system, thereby reducing vibrations in the rotating mass [108,109]. This is achieved via a feedback system monitoring the shaft position and increasing the stability of the FESS. This suspension and stiffness control increase losses from the power output due to the presence of the control system current. When used in conjunction with mechanical bearings, the complications of the control system can be reduced, making this option more cost-effective, stable, and feasible. However, there is still a need for a complex control system and, therefore, this should not be considered in applications susceptible to electromagnetic interference [110].

- Superconducting magnetic bearing (SMB).
- ○
- SMBs are best for high-speed use, as shown in Figure 12 below. The high speeds generated and the comparatively friction-free environment offer long life and stability. The main disadvantage with SMBs is the need for very low operating temperatures. Cryogenic cooling is needed to keep the bearings from failing. High-temperature superconductors (HTS) are in use with SMBs to combat this system requirement, and there have been recent attempts to incorporate cryogenic isolation for the SMBs to reduce costs and keep the operating temperatures low, but, at the moment, SMBs and PMBs need to be used at the same time [111,112].

- Bearingless machine (BM)
- ○
- A bearingless machine is capable of combining the two independent operations of magnetic suspension and generating torque into a single machine. This approach can be applied to the other types of machine mentioned herein, offering a reduced cost and compact design. BM can be implemented in high-speed FESS [122].

#### 2.3. Containment/Housing

#### 2.4. Power Electronic Interface

## 3. Current Application of FESS

#### 3.1. Application of Flywheel in the Transport Sector

#### 3.1.1. Rail Transport

- On-board use in diesel–electric vehicles to store braking energy;
- On-board use in DC systems to raise the recuperation rate;
- Stationary use in DC power supply systems to raise the recuperation rate.

#### 3.1.2. Road Transport

#### 3.2. Aircraft Carrier

#### 3.2.1. Incredible Hulk Roller Coaster

#### 3.2.2. Domestic Applications of Flywheels

#### 3.2.3. Flywheel Application in the Space Industry

#### 3.3. Load Levellers

#### 3.4. Emergency Devices

#### 3.5. Low-Energy Applications

#### 3.6. Utility Grid

#### 3.7. Materials

- The flywheel stores kinetic energy E

- 2.
- J = πρR
^{4}t/2 axial mass moment of inertia of the disc and ρ is density; therefore,

^{2}tρ

- 3.
- Energy per unit of mass is the ratio of the last two equations

- 4.
- Poisson’s ratio, v. Stress must not exceed yield strength (σ
_{y}), factor of safety, S.

- 5.
- v is roughly 1/3 for solids and is treated as a constant

_{s}units is kJ/kg.

#### Commercial Use

## 4. Design

#### 4.1. Rotor Design

#### 4.2. The Flywheel Rotor Design Process

#### 4.3. The Stress Analysis

#### 4.4. The Failure Criteria Selection

#### 4.5. Overview of Flywheel Mounted on a Rotating System

#### 4.5.1. Bearing Load

#### Shock

#### Vibrations

#### Rotating Mass Imbalance

^{−6}m from the centre of rotation. The radial magnetic bearings determine the centre of rotation. Due to this balance level, the bearing loads are often around 89 N or lower, within the flywheel energy storage system’s operating speed range. The presence of the magnetic bearing provides the potential to sustain a larger mass imbalance. Should the flywheel energy storage system flywheel rotor fail in holding its precision balance, the magnetic bearing control algorithm can be employed to rebalance the rotor [155,156].

#### Gyrodynamics

^{−2});

- (i)
- The vehicle turn in-plane, provided that the flywheel is not in its home orientation in relation to the vehicle;
- (ii)
- Vehicle pitch or roll, subject to whether the flywheel energy storage support uses a spring or damper bolted to the car frame;
- (iii)
- Vehicle lateral acceleration, provided that the support uses a pendulum-type spring;
- (iv)
- Torque on the flywheel energy storage emanating from the flywheel energy storage system motor-generator, provided that the stator’s reaction torque vector comes with an element normal to the spin axes of the flywheel;
- (v)
- Torque on the flywheel energy storage systems rotor obtained using bearings, but the beating’s stator reaction torque vectors must be normal to the spin axis.

## 5. Future Development of FESS

#### 5.1. Application of Flywheel Energy Storage Systems in Renewable Energy Sources

#### 5.2. Application of Flywheel Energy Storage Systems in Military

#### 5.3. Application of Flywheel Energy Storage Systems in Spacecraft

## 6. Conclusions

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

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**Figure 1.**An illustration of a typical FESS, reproduced with permission from Elsevier [29].

**Figure 2.**An indication of the increase in renewable energy sources, adapted from [45].

**Figure 3.**Components of flywheel energy storage system, reproduced with permission from Elsevier [47].

**Figure 4.**Diagram of permanent magnet synchronous machine (PMSM) for flywheels, adapted from [72].

**Figure 5.**Flywheel energy storage system with an induction motor adapted from [73].

**Figure 6.**Brushless DC machine with flywheel adopted from [78].

**Figure 7.**Switched reluctance motor for flywheel adapted from [85].

**Figure 8.**AC homopolar motor, adapted from [94].

**Figure 9.**Diagram of a synchronous reluctance motor, adapted from [95].

**Figure 10.**Passive magnetic bearing (PMB), adapted from [107].

**Figure 11.**Active magnetic bearing system, adapted from [110].

**Figure 12.**Superconducting magnetic bearing, adapted from [113].

**Figure 13.**Flywheel containment structure, adapted from [126].

**Figure 14.**Back to back layout, adapted from [128].

**Figure 15.**(

**a**) Direct matrix converter, (

**b**) indirect matrix converter, reproduced with permission from [48].

**Figure 17.**Flywheel energy storage system in rail transport, reproduced with permission from [35].

**Figure 18.**Applications of flywheels in vehicles, adapted from [131].

**Figure 19.**S60 Sedan volvo flywheel, adapted from [132].

**Figure 20.**Electromagnetic aircraft launch system, adapted from [133].

**Figure 21.**NASA flywheel and mounting bracket, adapted from [138].

**Figure 22.**Domestic application of flywheel, adapted from [139].

**Figure 23.**Shapes of FESS composed of composite and metallic materials, reproduced with permission from Elsevier [141].

**Figure 24.**Flywheel rotor design process and its influence factor, reproduced with permission from [143].

**Figure 25.**A schematic diagram of hybrid composite flywheel structure, reproduced with permission from Elsevier [144].

**Figure 26.**Rotor set up for flywheel, adapted from [146].

**Figure 27.**Requirements for vibration spectrum for flywheel at the rear end of a bus, adapted from [153].

**Figure 28.**Mathematical model and coordinate systems used to analyze flywheel gyrodynamics, adapted from [159].

Advantages | Disadvantages | Ref. |
---|---|---|

- High energy-efficiency
- Almost immediate delivery
- Strong power
- Requires little maintenance
- Long service life
- Environmentally friendly
- Simple and safe
- Flexible in the rate of charging and/or discharging
| - The need for permanent magnets in the rotor
- May require costly cryogenic cooling devices
- Cryogenic cooling also reduces the overall energy storage efficiency
- Deep discharging cannot be achieved
- High capital cost, whether due to the materials’ cost for the light rotational mass, i.e., at high rpm, or for the magnetic bearing using heavy rotational masses
- High self-discharge rate and low energy density
| [29] |

[47] | ||

[48] | ||

[49] | ||

[50,51] |

**Table 2.**Comparison between high-speed flywheel energy storage system (HSFESS) and low-speed flywheel energy storage system (LSFESS).

LSFESS | HSFESS | ||
---|---|---|---|

Material for disk | Steel | Composite | Ref. |

Electrical machine | Permanent magnet synchronous machine (PMSM), induction machine | Permanent magnet synchronous machine (PMSM) | [58] |

Bearing | Mechanical | Magnetic | [59] |

Application | Power quality | Traction and aerospace industry | [60] |

Cost | Low | High | [60] |

Characteristics | Flywheel | Battery |
---|---|---|

Cycles | 100,000 to 10 mil | Up to 20,000 (according to the type) |

Energy density (Wh/kg) | 130 | 160 |

Charging/discharging time | 10 s–10 min | Several hours |

Self-discharging time | Few hours | 5–25 months |

Energy conversion | Determined by generator | Chemical process |

Switched Reluctance Machine | Synchronous Reluctance Machine | Ref. | |

Saliency | Double (Stator and Rotor) | Single (Rotor) | [96] |

Sensor | Salient poles (Concentrated coil) | Conventional AC machine | [97] |

Rotor | Salient poles | Arrangement of internal flux guides | [98] |

Winding | Single tooth winding | Poly phase distributed windings | [99,100] |

Excitation | Pulse DC voltage sequence | Balanced sinusoidal currents | [101] |

Waveform | Triangular/Trapezoidal | Sinusoidal | [102] |

Converter | Asymmetric half bridge | Conventional 3-phase inverter | [103] |

Material | Density (kg/m3), ρ | Strength (MPa), σ | Energy Density (MJ/kg) | Cost ($/lb) |
---|---|---|---|---|

Steel (AICI 4340) | 7800 | 1800 | 0.231 | 1 |

Alloy (ALM _{n}M_{g)} | 2700 | 600 | 0.22 | 3 |

Titanium (TiAI _{6}Zr_{5}) | 4500 | 1200 | 0.27 | 9 |

Carbon-fibre composite (S2) | 1920 | 1470 | 0.766 | 24.6 |

Carbon-fibre composite (M30S) | 1553 | 2760 | 1.777 | n/a |

(M30S) 1553 2760 1.777 n/a Carbon-fibre composite (T1000G) | 1664 | 3620 | 2.175 | 101.8 |

Material | kJ/kg | Comments | Ref. |
---|---|---|---|

Ceramics | 200–2000 | High failure rates so rarely used | [142] |

Composites: CFRP | 200–500 | Most popular, used in a wide range of applications | [142] |

Composites: GFRP | 100–400 | Also widely used. Less range than CFRP but cheaper | [139] |

Beryllium | 300 | High costs and challenging to work with | [140] |

High-Strength Steel | 100–200 | These blends are all equal in strength and applications. Steel and aluminium are less expensive than magnesium and titanium | [141] |

High-Strength Al Alloys | 100–200 | ||

High-Strength Mg Alloys | 100–200 | ||

Ti Alloys | 100–200 | ||

Lead Alloys | 3 | Traditionally used when requiring high density and in low-speed applications | [140] |

Cast Alloys | 8–10 |

Material Property | T300/2500 | Glass/Epoxy | Aluminium Alloy 7075 |
---|---|---|---|

Density (Kg/m^{3}) | 1600 | 1800 | 2800 |

Long-Trans. Poisson’s Ratio | 0.3 | 0.26 | 0.3 |

Longitudinal Young’s Modulus (GPa) | 130 | 38.6 | 72.5 |

Transverse Young’s Modulus (GPa) | 9 | 8.27 | - |

Long Tensile Strength (MPa) | 1800 | 1062 | 590 |

Long Compressive Strength (MPa) | 1400 | 610 | - |

Trans. Tensile Strength (MPa) | 80 | 31 | - |

Trans. Compressive Strength (MPa) | 168 | 118 | - |

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**MDPI and ACS Style**

Olabi, A.G.; Wilberforce, T.; Abdelkareem, M.A.; Ramadan, M.
Critical Review of Flywheel Energy Storage System. *Energies* **2021**, *14*, 2159.
https://doi.org/10.3390/en14082159

**AMA Style**

Olabi AG, Wilberforce T, Abdelkareem MA, Ramadan M.
Critical Review of Flywheel Energy Storage System. *Energies*. 2021; 14(8):2159.
https://doi.org/10.3390/en14082159

**Chicago/Turabian Style**

Olabi, Abdul Ghani, Tabbi Wilberforce, Mohammad Ali Abdelkareem, and Mohamad Ramadan.
2021. "Critical Review of Flywheel Energy Storage System" *Energies* 14, no. 8: 2159.
https://doi.org/10.3390/en14082159